Synergistic heating effect via CNT/Fe3O4 nanocomposite aerogels enabling uniform and rapid induction welding with high bond strength in CFRTPs

Synergistic heating effect via CNT/Fe3O4 nanocomposite aerogels enabling uniform and rapid induction welding with high bond strength in CFRTPs | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Synergistic heating effect via CNT/Fe3O4 nanocomposite aerogels enabling uniform and rapid induction welding with high bond strength in CFRTPs Nayeong Kim, Inseok Baek, Byeongho Park, Dayoung Kim, Chaewon Jeong, and 7 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8372467/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 9 You are reading this latest preprint version Abstract Effective induction welding of carbon fiber-reinforced thermoplastic composites (CFRTPs) requires both uniform temperature distribution and rapid heating at the welding interface. However, achieving both is challenging because heat localizes due to non-uniform magnetic-field distribution and the difficulty of forming current loops within the composite structure. In this study, a powdered CNT/Fe 3 O 4 /PA6 aerogel (Nanocomposite) was adopted to enhance induction-heating performance by simultaneously leveraging electrical conductivity and magnetic-loss mechanisms. The nanocomposite contains carbon nanotubes (CNTs) that form continuous conductive pathways, enabling efficient Joule heating and improved heat transfer. The CNTs also act as nanoscale mechanical pins that bridge CFRTPs, thereby enhancing interfacial weld strength. In addition, embedded Fe 3 O 4 particles respond rapidly to the alternating magnetic field and act as localized heat sources via magnetic losses, increasing heating efficiency. As a result, the nanocomposite-enabled system demonstrated a 94.6% higher initial heating rate and a 57% reduction in the time required to reach the target processing temperature at the welding interface. The interfacial shear strength increased by 30.8%, and fracture-surface analysis revealed more uniform melting and more effective impregnation across the weld area. These results indicate that the proposed nanocomposite offers a promising strategy for achieving both rapid and uniform heating, leading to improved mechanical performance in the induction welding of CFRTPs. Induction welding Nanocomposites Aerogels Joule heating Hysteresis loss Carbon nanotubes Magnetic particles Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Highlights ㆍ Proposed a novel nanocomposite aerogel that maximizes induction heating via Joule and magnetic loss mechanisms. ㆍ Incorporated CNTs for Joule heating and FeO for magnetic loss-driven heating enhancement. ㆍ Fabricated nanocomposites with varying ratios and experimentally identified the optimal composition. ㆍ Demonstrated the potential of the proposed material as a high-efficiency nanocomposite for induction welding. 1. Introduction Induction heating has attracted attention as a welding technique for CFRTPs because it offers rapid heat generation, minimal surface contamination via a non-contact process, and selective heating of targeted areas. Owing to these advantages, induction welding has seen increasing adoption in high-performance sectors such as automotive and aerospace, where rapid and precise joining is essential [ 1 – 3 ]. The efficiency of induction heating is governed by key parameters, including the electrical and magnetic properties of the materials and the continuity of induced-current loops [ 4 ]. However, most carbon-fiber-reinforced thermoplastics (CFRTPs) do not satisfy these conditions, leading to induced currents that do not effectively reach the welding interface and difficulty in rapidly attaining the process temperature [ 5 ]. In addition, edge effects can cause insufficient melting at the joint center, resulting in incomplete interfacial fusion and, ultimately, reduced joint strength. Consequently, incorporating a susceptor is essential to maximize the induction-heating effect and to induce selective heat generation at the welding interface [ 6 , 7 ]. Carbon nanotubes (CNTs) have emerged as promising constituents for induction-assisted joining due to their excellent electrical conductivity and ability to form percolated networks through their high aspect ratio [ 8 ]. When exposed to microwaves or high-frequency fields, percolated CNT networks produce Joule heating, thereby supporting more uniform heat generation at the welding interface [ 9 – 11 ]. For example, Cao et al. inserted multiwalled carbon nanotube (MWCNT) sheets at the edges of a carbon fiber heating element to mitigate non-uniform heating, improving overall thermal uniformity [ 12 ]. Importantly, the efficacy of CNTs depends strongly on loading level: below the electrical percolation threshold, conduction pathways are sparse and heating is limited, whereas loadings exceeding this threshold result in a rapid increase in network conductivity and heat delivery [ 13 , 14 ]. However, when the CNT content is increased to excessive levels, dispersion becomes challenging and viscosity increases, which can promote agglomeration, hinder processability, and degrade interfacial uniformity [ 15 ]. Moreover, because CNTs are non-magnetic, their heating under alternating magnetic fields is fundamentally less efficient than that of ferromagnetic susceptors [ 16 , 17 ]. As a result, CNTs are often used as functional reinforcements or auxiliary thermal conductors rather than as primary induction susceptors for welding [ 18 ]. Notably, aligned CNT interlayers can significantly enhance interlaminar strength—e.g., improvements of up to 14% and 40% have been reported—while CNT-film electrothermal elements have enabled uniform, rapid consolidation with substantial energy savings [ 19 – 21 ]. Nevertheless, limited magnetic responsiveness and shallow heat penetration constrain the use of CNTs as standalone susceptors in induction systems [ 22 ]. Fe 3 O 4 is a ferrimagnetic material that heats efficiently via magnetic hysteresis losses when subjected to an alternating magnetic field. Accordingly, Fe 3 O 4 has been dispersed in polymer matrices and used as a susceptor for induction heating [ 23 , 24 ]. For instance, Sha et al. aligned Fe 3 O 4 nanoparticles in a Nylon-12 adhesive under an AC magnetic field, boosting the heating rate by up to 200% and enabling reversible joining [ 25 ], while Baek et al. achieved stable heating and stable interfacial bonding using PA6 films containing 75 wt% Fe 3 O 4 for CF/PA6 induction welding [ 26 ]. Although high Fe 3 O 4 contents can lead to agglomeration and localized overheating, degrading spatial thermal uniformity [ 27 , 28 ], design strategies such as particle alignment, architectural control, and hybridization with conductive phases can mitigate these issues [ 29 ]. This study deliberately integrates these complementary behaviors in a nanocomposite to achieve both thermal efficiency and interfacial robustness in composite joining. Rather than emphasizing electromagnetic interference (EMI) shielding or purely anisotropic constructs, as in prior CNT/Fe 3 O 4 hybrids [ 30 – 32 ], our work focuses on three focal points relevant to induction welding: (i) a powdered CNT/Fe 3 O 4 /PA6 aerogel interlayer that is simple to process yet conformal to the weld line; (ii) synergistic heating that couples CNT-enabled Joule-heating networks with Fe 3 O 4 magnetic hysteresis to flatten thermal gradients across the interface; and (iii) mechanical reinforcement via CNT anchoring within the resin-rich layer, enhancing interfacial weld strength. The concept is illustrated in Fig. 1 and validated experimentally via heating performance and interfacial mechanical testing. We anticipate that this design provides a practical pathway toward rapid, spatially uniform heating with improved mechanical performance in the induction welding of CFRTPs. 2. Materials and Methods 2.1. Materials Single-walled carbon nanotubes (SWCNTs, outer diameter 1.6 ± 0.4 nm, length ≥ 5 µm) were purchased from TUBALL® (OCSiAl, Luxembourg). The SWCNTs were ultrasonically dispersed in DI water with Sodium dodecylbenzenesulfonate (Sigma-Aldrich) as a surfactant and used in solution. Magnetite particles (Fe 3 O 4 , density 4.52 g·cm − 3 , average particle size ≈ 5 µm) were obtained from Shanghai Laiwu Powder Material Co., Ltd. (China). Polyamide 6 (PA6, density 1.17 g·cm − 3 , average particle size 15–20 µm) was supplied by Goodfellow (Huntingdon, UK). All materials were employed without additional treatment. 2.2. Induction Heating Induction heating tests were performed with an induction heater (HF-6K, Taeyang Induction Heater Co., Ltd., Korea). An elliptical, pancake-type copper coil (inner diameter of 3.2 mm) was used; cooling water was circulated through the coil to prevent overheating. The operating frequency and power were set to 300 kHz and 5 kW. The specimen was centered within the coil, with a coil–specimen gap of 2 mm. Specimen temperature was monitored using an infrared thermal imaging camera (FLIR E54, FLIR Systems, Inc., USA). 2.3. Induction Welding Process Induction welding was carried out using CF/PA6 unidirectional laminate specimens. The specimen dimensions were 101.6 × 25.4 mm 2 with a thickness of 1.81 mm, and the laminate exhibited a fiber volume fraction of 47.3% and a void content of 1.57%. The stacking sequence was [0/90/±45] 2s , fabricated to achieve a quasi-isotropic lay-up. Two specimens were joined in a single-lap configuration with an overlap length of 12.7 mm, and a pressure of 40 kg was applied to the upper adherend during welding. The specimens were mounted in an alumina fixture to maintain alignment. The heating time was set to 70 s, and cooling to room temperature was carried out under pressure. 2.4. Characterization 2.4.1. Characterization of Nanocomposites A range of characterization techniques was employed to comprehensively evaluate the structural, electrical, and magnetic properties of the nanocomposites. These measurements were conducted to elucidate how the nanocomposite architecture influences percolation behavior and the electrical and magnetic responsiveness required for induction heating. Thermogravimetric analysis (TGA; TGA55, TA Instruments, USA) was performed in air at a heating rate of 10°C·min − 1 up to 600°C, and the Fe 3 O 4 residual mass was used to assess how uniformly the Fe 3 O 4 particles were distributed along the thickness direction of the aerogel. X-ray micro-computed tomography (micro-CT; vtomex m300, GE, USA) was employed to visualize the three-dimensional dispersion uniformity of Fe 3 O 4 particles within the aerogel structure. Nitrogen adsorption–desorption analysis (BET; iQ 1MP, Quantachrome Instruments, USA) was performed on powdered aerogels to verify their specific surface area and to evaluate nitrogen adsorption and desorption behavior through the isotherm graph. Electrical conductivity was measured using a four-point probe (FPP-2000, DASOL ENG, Korea) on hot-pressed films (25.4 × 12.7 mm 2 ) to examine how Fe 3 O 4 loading affects the formation of CNT percolation networks. Vibrating sample magnetometry (VSM; MicroSense EZ VSM, MicroSense, USA) was conducted under an applied magnetic field of ± 25 kOe to analyze the magnetic responsiveness of the nanocomposites, with particular emphasis on determining how well the Fe 3 O 4 -based magnetic response is retained upon CNT incorporation. Field-emission scanning electron microscopy (FE-SEM; JSM-7800F, JEOL, Japan) was used to observe the porous aerogel microstructure and to verify the formation of interconnected CNT/Fe 3 O 4 networks that define the structural characteristics of the interlayer. 2.4.2. Characterization of Welded joints To evaluate the reliability of the welded joints, a series of non-destructive, mechanical, and microstructural analyses were performed. Non-destructive CT inspection was further conducted to assess the integrity of the welded interface. For this evaluation, a Phoenix Vtomex M240 system equipped with a micro-focus X-ray tube was operated at 60 kV and 180 µA. The voxel resolution was set to 10 µm to enable precise detection of interfacial defects, and a total of 2,200 projection images were collected and reconstructed into a three-dimensional CT volume. In addition, immersion ultrasonic C-scan mapping was performed using a TTU system (GTR4010-ARN-CA) to evaluate the continuity and uniformity of the welded region across the entire bonded area, and to detect internal defects such as lack of fusion through area-based attenuation signals. Single-lap shear (SLS) tests were conducted according to ASTM D5868 at a crosshead speed of 1 mm·min − 1 , using n ≥ 3 specimens per condition to ensure reproducibility. These tests were used to compare the mechanical strength improvement achieved by incorporating the nanocomposite interlayer relative to the CFRP-only joints. Field-emission scanning electron microscopy (FE-SEM; JSM-7800F, JEOL, Japan) was employed to observe CNT pull-out features on the fracture surfaces, enabling analysis of interfacial failure behavior and the contribution of CNT anchoring to the mechanical performance of the welded joints. 3. Results and discussion 3.1. Fabrication and Microstructure of CNT/Fe 3 O 4 /PA6 Nanocomposite aerogel To achieve rapid and spatially uniform induction heating, both conductive pathways and magnetically responsive particles must be homogeneously distributed through the nanocomposite architecture. We therefore prepared a CNT/Fe 3 O 4 /PA6 nanocomposite via an aqueous route followed by rapid freezing and vacuum freeze-drying. The process yielded aerogels that were readily shaped into arbitrary geometries without cracking (Fig. 2 c) and possessed exceptionally low density due to their percolated pore network (Fig. 2 d). This aerogel form factor offered practical advantages for welding: it can be trimmed to a conformal interlayers that match the weld line, handled and transferred with minimal mass loading, and, if needed, gently milled into a powdered aerogel while retaining the underlying CNT–Fe 3 O 4 connectivity. Accordingly, single-walled CNTs were individually dispersed in water with an surfactant and then mixed with Fe 3 O 4 and PA6 powders; the powder-dispersed solution was quenched in liquid nitrogen to grow ice crystals and lyophilized (− 120°C, 24 h), during which solvent sublimation generated the porous aerogel (Fig. 2 a). The freeze-drying route mitigated sedimentation of relatively dense Fe 3 O 4 during solvent removal, which enabled a uniform through-thickness distribution within the CNT scaffold. To quantify compositional uniformity, thermogravimetric analysis (TGA) was performed on sections taken from the top and bottom of the aerogel. The residual masses were 36.4% and 37.2%, respectively, corresponding to a difference of only ~ 0.8%, which confirmed that particle settling was effectively suppressed during processing. Complementary X-ray micro-computed tomography (micro-CT) revealed a homogeneous three-dimensional dispersion of Fe 3 O 4 throughout the aerogel volume, consistent with the TGA results (Fig. 2 e, f). Microstructural observations supported the formation of a dual-loss nanocomposite network. Field-emission SEM images showed an open, vertically porous architecture characteristic of freeze-dried structures (Fig. 2 b). Additional microstructural features of the aerogel monolith are provided (see Fig. S1 in the Supplementary data). Within this porous network, Fe 3 O 4 particles were embedded and wrapped by CNT bundles to form an interconnected percolation network (Fig. 2 g). This architecture was expected to (i) provide continuous electronic pathways for distributed Joule heating via the CNT network and (ii) supply numerous, well-dispersed magnetic nano/microsources for hysteresis-based heat generation via Fe 3 O 4 under an alternating field. The intimate CNT–Fe 3 O 4 contact further facilitated heat sharing across phases and promoted mechanical interlocking (“pinning”) of CNTs within the resin-rich layer after deposition, establishing a structural basis for both uniform thermal fields and enhanced interfacial bond strength during induction welding. The CNT/Fe 3 O 4 /PA6 nanocomposites were prepared with varying compositions, and the samples were designated based on their component combination. In the sample codes, C, F, and P represented CNT, Fe 3 O 4 , and PA6, respectively, and the following number indicated the PA6 content (wt%). The detailed compositions are summarized in Table 1 . Table 1 Composition of CNT/Fe 3 O 4 /PA6 nanocomposite samples. Sample CNT (wt%) Fe 3 O 4 (wt%) PA6 (wt%) FP - 100 100 CFP0 100 100 - CFP10 100 100 10 CFP30 100 100 30 CFP50 100 100 50 Nitrogen adsorption–desorption analysis showed that the powdered aerogel had a specific surface area of 104.24 m 2 ·g − 1 (Fig. 3 a). This value is significantly higher than the specific surface area range of carbon fibers (CF), which is 0.5–5 m 2 ·g − 1 . Therefore, the powdered aerogel provides more pathways for resin infiltration compared to using only carbon fiber composites, and the open pore structure allows for more efficient resin penetration into the internal pores. As a result, it is anticipated that resin impregnation will occur more effectively, leading to an enhancement in interfacial welding strength. Electrical conductivity measurements, performed at a fixed CNT:PA6 ratio of 1:0.5, showed a non-monotonic dependence of bulk conductivity (σ) on Fe 3 O 4 loading (Fig. 3 b). In Regime I, the introduction of Fe 3 O 4 initially reduced σ because the relative CNT content decreased, and some tube–tube junctions were obstructed (dilution/blocked-junction effect). Beyond the percolation threshold of Fe 3 O 4 , Regime II emerged: σ increased again as Fe 3 O 4 participated in assisted percolation, effectively bridging separated CNT domains and restoring long-range connectivity. The composition with equal masses of CNT and Fe 3 O 4 (CFP50) delivered the highest conductivity, 3347.92 S·m − 1 , consistent with an optimally connected hybrid network capable of distributing Joule heat uniformly along the weld line. The two regimes were summarized schematically in Fig. 3 c. Magnetic characterization by VSM, normalized by the Fe 3 O 4 mass, confirmed that ferrimagnetic behavior was retained after hybridization with CNT and PA6 (Fig. 3 d; Table 2 ). Bare Fe 3 O 4 showed M s = 70.59 emu·g − 1 . Mixing Fe 3 O 4 with PA6 at equal mass decreased M s by only 7.77%, indicating minimal magnetic interference from the polymer. In contrast, CNT-containing hybrids displayed a larger reduction in M s —up to 53.93%—which we ascribed to interfacial charge transfer and spin disorder at CNT/Fe 3 O 4 contacts that weakened exchange interactions. Notably, the electrically optimal composition (CFP50) still retained substantial magnetization, yielding a balanced electro-magnetic profile: percolated CNT pathways for heat spreading via Joule heating together with Fe 3 O 4 -driven hysteresis losses for rapid local heat generation. Table 2 Summary of VSM magnetization results Sample Magnetization (emu·g − 1 ) Reduction vs. bare Fe 3 O 4 (%) Bare Fe 3 O 4 70.59 - FP 65.11 7.77 CFP0 34.77 70.74 CFP50 32.53 53.93 Taken together, the BET, conductivity, and magnetization results demonstrated that the powdered nanocomposite aerogel (CFP50) promoted interfacial contact, restored conductive percolation at optimal Fe 3 O 4 loadings, and preserved sufficient magnetic response. This combination is precisely what is required to achieve rapid yet spatially uniform induction heating and high interfacial bond strength in CFRTP welding. 3.2. Induction Heating Performance of the Nanocomposite We assessed the nanocomposite as an active heating material by quantifying heating efficiency and spatial uniformity under a kilohertz alternating magnetic field (Fig. 4 ). To probe whether electronic and magnetic loss channels acted cooperatively, CNT-only aerogels were compared with CNT/Fe 3 O 4 /PA6 hybrids. Over a 60 s induction period, the hybrid consistently reached higher peak temperatures than CNT-only (Fig. 4 a). The best performance occurred when CNT, Fe 3 O 4 , and PA6 were all present at CFP50, indicating that Joule losses in the percolated CNT network and magnetothermal losses (hysteresis/relaxation) in Fe 3 O 4 are mutually reinforcing, with dielectric dissipation in PA6 providing a smaller but beneficial contribution [ 33 , 34 ]. To isolate the role of the polymer, the CNT:Fe 3 O 4 mass ratio was fixed at 1:1 while PA6 content was varied (Fig. 4 b). Increasing PA6 led to a monotonic decrease in peak temperature, consistent with the dilution of the conductive skeleton and a reduced relative contribution from the CNT/Fe 3 O 4 network at high polymer fractions. Thus, although PA6 contributes measurable dielectric loss near processing temperatures, excess polymer penalizes overall heating efficiency by lowering the effective density of current paths and magnetic heat sources. Heating uniformity was examined by infrared thermography (Fig. 4 c,d). At CFP50, the nanocomposite aerogel exhibited a spatially uniform temperature field above the PA6 melting point without obvious hot spots (Fig. 4 c), consistent with balanced power deposition from electronic and magnetic losses and subsequent smoothing by thermal diffusion through a continuous polymer phase. The temperature non-uniformity index, calculated from the IR thermography data, was 10.89%, further confirming the spatially uniform interfacial heating. In contrast, at CFP10, hot spots emerged (Fig. 4 d), attributable to insufficient dielectric participation and heat spreading, which concentrate power in highly conductive or strongly magnetic locales. The uniformity trend aligns with the peak-temperature optimum near CFP50 (Fig. 4 a) and the decline at higher polymer contents (Fig. 4 b). Early-time responsiveness was benchmarked using the initial heating rate (Fig. 4 e). Under identical inputs, CFRP-only showed little rate gain with voltage, whereas the nanocomposite aerogels responded sharply; at 170 V the maximum initial rate reached 148.7°C·s − 1 , which was 94.6% higher than CFRP-only. This contrast reflects the lack of closed induced-current loops in CFRP (dielectric-loss-dominated) versus the percolated, closed-loop network in the nanocomposite aerogel. Finally, to simulate the welding process, the nanocomposite aerogel was deployed as a thin interlayer laminated on CFRP (Fig. 4 f). Relative to CFRP-only, overlaying the laminate with the nanocomposite aerogel increased the initial heating rate from 43.7 to 68.6°C·s − 1 (~ 57%), the time to reach the processing temperature was markedly shorter, and the temperature at t = 20 s was up to 173°C higher under the same input. These results demonstrate that the CNT/Fe 3 O 4 /PA6 nanocomposite accelerates heating while flattening thermal gradients, thereby improving process efficiency and establishing favorable thermal conditions for strong, uniform induction-welded joints. 3.3. Application to Induction Welding The powdered aerogel nanocomposite (CFP50) was inserted as a thin interlayer in single-lap CFRP joints (Fig. 5 a,b) to evaluate whether the improved heating efficiency and spatial uniformity (Fig. 4 ) translated into reliable joints. Welding was performed in an alumina fixture to maintain alignment. To evaluate the reliability of the weld interface, micro-CT analyses were conducted from both cross-sectional and top-view perspectives. In the CFRP-only specimen, the cross-section revealed incomplete joining with noticeable delamination regions, and in the top view, a central defect was observed, indicating the presence of unwelded areas (Fig. S2). These unwelded regions were attributed to non-uniform melting during heating. In contrast, the CFP50 specimen exhibited no delamination at the joint interface, and Fe 3 O 4 particles dispersed within the powdered aerogel were clearly observed (Fig. 5 c). The small voids formed near the welding line appeared to originate from the rapid melting of the CFRP matrix during induction heating; however, no separation or lifting of the welded line was observed after solidification. Furthermore, in the top view, unlike the CFRP-only specimen, the welding interface exhibited uniform melting, and Fe 3 O 4 particles were uniformly distributed throughout the entire welding region (Fig. 5 d). Taken together, the observations demonstrated that the CNT/Fe 3 O 4 /PA6 nanocomposite acted as an effective thermal mediator during induction heating, facilitating the formation of a uniform and stable interface. The transmission ultrasonic inspection results (Table 3 ) revealed a clear distinction between the CFRP-only joints and the joints incorporating nanocomposites with different PA6 contents. In the Figure legend (Fig. 5 g), the red color can be regarded as the reference signal corresponding to 100% transmission, whereas darker colors indicate a progressive decrease in relative ultrasonic transmission. The CFRP-only joint exhibited a transmission ratio of only 6% at 57 dB, indicating that ultrasonic waves were largely attenuated even under relatively high input, and continuous transmission across the interface was not achieved. In the C-scan images, a distinct boundary was observed between the edge and center regions, suggesting that a significant portion of the joint interface was non-uniformly or incompletely welded. In contrast, the joints with nanocomposites (CFP10 and CFP50) exhibited transmission ratios of 41% and 43% at comparable dB levels (55–60 dB), confirming continuous and uniform ultrasonic transmission throughout the overlap region. These results demonstrate that, even when considering differences in dB settings, the nanocomposite-containing joints showed significantly higher transmission ratios than the CFRP-only joints, suggesting that the incorporation of nanocomposites improved interfacial uniformity and stabilized the ultrasonic propagation path, thereby promoting spatially uniform welding. Although ultrasonic transmission characteristics do not directly correspond to mechanical strength, combined interpretation with mechanical tests is required to draw comprehensive conclusions. Nevertheless, the increased transmission can be interpreted as evidence of improved interfacial uniformity. Furthermore, it should be noted that a decrease in ultrasonic transmission cannot always be solely attributed to interfacial defects, but may also arise from structural factors such as increased joint thickness or the fiber stacking sequence of the CFRP. Table 3 Ultrasonic transmission results of CFRP-only and nanocomposite joints. Sample dB Transmission ratio (%) CFRP-only 57 6 With CFP0 62 25 With CFP10 60 41 With CFP50 55 43 To verify the mechanical reinforcement effect of the nanocomposite (CFP50), fracture surfaces were examined after single-lap shear testing. Relative to the CFRP-only, the nanocomposite afforded faster interfacial heating and more uniform melting across the weldline, as indicated by the absence of visually unmelted central regions after welding (Fig. 6 b). Correspondingly, the single-lap shear strength increased from 19.21 MPa (CFRP-only) to 25.12 MPa with the nanocomposite (+ 30.8%; Fig. 6 a). We attribute this gain to (i) balanced power deposition from Joule and hysteresis losses that flattens the thermal gradient and promotes uniform fusion, and (ii) CNT-mediated load transfer, whereby nanotube bundles embedded in the resin-rich layer act as “pins,” improving interfacial traction under shear. To elucidate the microstructural origins of strength improvement, we examined fracture surfaces by FE-SEM (Fig. 7 ). Prior to induction heating, the powdered aerogel nanocomposite showed a distinct granular/layered morphology at the interface (Fig. 7 a). After welding, PA6 infiltration was evident along the CNT–Fe 3 O 4 network (Fig. 7 b), indicating intimate wetting and co-melting between the interlayer and the CFRTP resin. High-magnification images (Fig. 7 c–e) all showed CNT pull-outs. Figure 7 c revealed that CNTs were uniformly pulled out over a wide area of the joint interface, while Fig. 7 d provided a magnified view of this region for more detailed observation. Figure 7 e presented an even higher magnification, clearly showing individual CNT strands fully pulled out. These CNT pull-outs and bridging ligaments are indicative of effective mechanical interlocking and energy dissipation during loading and, together with the uniform thermal field established during welding, rationalize the observed enhancement in lap-shear strength. Overall, the CNT/Fe 3 O 4 /PA6 nanocomposite functions as an electromagnetically balanced heating material that accelerates heating, equalizes temperature across the overlap, and reinforces the interface at multiple length scales. This combination yields stronger and more reliable induction-welded CFRP joints. 3.4. Proposed Heating and welding Mechanism In an alternating (kHz) magnetic field, the nanocomposite dissipates energy via three coupled channels (Fig. 8 a): (i) Joule heating in the percolated CNT network that establishes low-impedance pathways; (ii) magnetic hysteresis/relaxation losses in Fe 3 O 4 that provide volumetric magnetothermal heating in the RF/kHz regime; and (iii) dielectric loss in PA6 that becomes non-negligible near processing temperatures. The coexistence of conductive and magnetic pathways increases the effective loss factor and explains the higher initial heating propensity and shorter time-to-melt relative to single-mode heating materials. This interpretation agrees with polymer induction-processing literature (magnetite susceptors operated near ~ 100 kHz for welding) [ 35 ] and with reports that CNT networks enable efficient electrothermal self-heating [ 36 ], while Fe 3 O 4 particles heat through hysteresis/relaxation under comparable frequencies [ 37 – 40 ]. The freeze-dried then powdered aerogel morphology increases accessible surface area and packability at the weldline, improving thermal/physical contact and enabling conformal placement in local gaps (our measurements) (Fig. 8 b). In parallel, the uniform Fe 3 O 4 distribution within the CNT network mitigates current hot-spots and spreads magnetic losses, flattening temperature gradients—consistent with the IR thermography uniformity observed here and with studies showing heating efficiency/uniformity are dispersion-controlled in magnetically susceptible nanocomposites. During welding, uniformly heated PA6 melts and infiltrates the overlap, while interfacial CNT filaments act as nanoscale pins/bridges, enhancing load transfer and resisting interfacial crack growth via pull-out/bridging—mechanisms widely reported to raise interlaminar toughness in CNT-modified joints and laminates [ 41 ] (Fig. 8 c). The combination of magnetothermal efficiency (Fe 3 O 4 ) and interfacial toughening (CNT) is consistent with induction-welded thermoplastic systems employing magnetite-based susceptors. 4. Conclusions We demonstrated that a powdered CNT/Fe 3 O 4 /PA6 nanocomposite enables uniform and rapid induction heating for polymer–composite welding. When subjected to an alternating magnetic field, the percolated CNT network provides efficient Joule heating, Fe 3 O 4 contributes magnetothermal losses (hysteresis/relaxation), and PA6 adds dielectric dissipation; together these loss channels produce spatially uniform interfacial temperatures and robust weld quality. The powdered aerogel morphology increases accessible surface area and packability at the overlap, improving thermal/physical contact and facilitating conformal placement, which results in more homogeneous melting and infiltration across the joint. Quantitatively, the nanocomposite increased the initial heating rate to 148.7°C·s − 1 , and shortened the time-to-Tₘₑₗₜ by 57%. IR thermography indicated that the temperature non-uniformity index was 10.89%, consistent with the uniform interfacial temperature field. In terms of mechanical performance, the lap-shear strength increased from 19.21 MPa to 25.12 MPa. Fractography revealed CNT pull-out and bridging, supporting an interfacial pinning mechanism for the observed strength gains. The proposed nanocomposite offers a drop-in route to uniform, fast induction welding using off-the-shelf kHz systems and gram-scale material usage, enabling scalable and selective repairs or assembly of thermoplastic-rich interfaces. By engineering loss partitioning and leveraging IR-guided digital twins for temperature-field control—then by extending the concept across materials and lay-ups—the approach can evolve into a robust, deployable joining platform. Declarations Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (No. NRF- 2022M3H4A1A0407637213, RS-2024-00448445), and Korea Institute of Materials Science (PNKA400). Author Contribution Nayeong Kim: Writing – original draft, Methodology, Visualization. Inseok Baek: Writing – original draft, Methodology, Visualization. Byeongho Park: Investigation, Resources. Dayoung Kim: Investigation, Resources. Jungwan Lee: Data curation, Review manuscript. Jin Woo Yi: Data curation, Review manuscript. Jung-soo Kim: Investigation, Data curation. Geunsu Joo: Investigation, Data curation. Seokpum Kim: Review manuscript. Vipin Kumar: Review manuscript. Youngseok Oh: Supervision, Project administration, Conceptualization, Funding acquisition, Methodology. Jinsu Kim: Spervision, Resources, Review & editing manuscript. Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (No. NRF- 2022M3H4A1A0407637213, RS-2024-00448445), and Korea Institute of Materials Science (PNKA400). References Ahmed TJ, Stavrov D, Bersee HEN, Beukers A. Induction welding of thermoplastic composites—an overview. Compos. Part A: Appl. Sci. Manuf. 2006;37:1638–1651. Liu J, Quan D, Scarselli G, Alderliesten R, Wang H, Zhao G. Developments and future prospects of welding technology for carbon fiber thermoplastic composites. Compos. Part B: Eng. 2025;297:112314. Bayerl T, Duhovic M, Mitschang P, Bhattacharyya D. The heating of polymer composites by electromagnetic induction – A review. Compos. Part A: Appl. Sci. Manuf . 2014;57:27–40. Farahani RD, Dubé M. Novel heating elements for induction welding of carbon fiber/polyphenylene sulfide thermoplastic composites. Adv. Eng. Mater. 2017;19:1700196. Gouin O'Shaughnessey P, Dubé M, Fernandez Villegas I. Modeling and experimental investigation of induction welding of thermoplastic composites and comparison with other welding processes. J. Compos. Mater. 2016;50:2895–2910. Vattathurvalappil SH, Haq M. Thermomechanical characterization of Nano-Fe₃O₄ reinforced thermoplastic adhesives and single lap-joints. Compos. Part B: Eng. 2019;175:107128. Roman CL, da Silva Moura N, Wicker S, Dooley KM, Dorman JA. Induction heating of magnetically susceptible nanoparticles for enhanced hydrogenation of oleic acid. ACS Appl. Nano Mater. 2022;5:3676–3685. Bryning MB, Milkie DE, Islam MF, Hough LA, Kikkawa JM, Yodh AG. Carbon nanotube aerogels. Adv. Mater. 2007;19:661–664. Sweeney CB, Lackey BA, Pospisil MJ, Achee TC, Hicks VK, Moran AG, Teipel BR, Saed MA, Green MJ. Welding of 3D-printed carbon nanotube–polymer composites by locally induced microwave heating. Sci. Adv. 2017;3:e1700262. Wang F, Zhang P, Luo J, Li B, Liu G, Zhan X. Effect of heat input on temperature characteristics and fusion behavior at bonding interface of CFRTP induction welded joint with carbon fiber susceptor. Polym. Compos. 2022;44:1586–1602. Poyraz S, Zhang L, Schroder A, Zhang X. Ultrafast microwave welding/reinforcing approach at the interface of thermoplastic materials. ACS Appl. Mater. Interfaces 2015;7:22469–22477. Cao D. Fusion joining of thermoplastic composites with a carbon fabric heating element modified by multiwalled carbon nanotube sheets. Int. J. Adv. Manuf. Technol. 2023;128:4443–4453. Marconnet AM, Yamamoto N, Panzer MA, Wardle BL, Goodson KE. Thermal conduction in aligned carbon nanotube–polymer nanocomposites with high packing density. ACS Nano 2011;5:4818–4825. Karalis G, Tzounis L, Dimos E, Mytafides CK, Liebscher M, Karydis-Messinis A, et al. Printed single-wall carbon nanotube-based Joule heating devices integrated as functional laminae in advanced composites. ACS Appl. Mater. Interfaces 2021;13:39880–39893. Islam MF, Rojas E, Bergey DM, Johnson AT, Yodh AG. High weight fraction surfactant solubilization of single-wall carbon nanotubes in water. Nano Lett . 2003;3:269–273. Ma M, Wu Y, Zhou J, Sun Y, Zhang Y, Gu N. Size dependence of specific power absorption of Fe₃O₄ particles in AC magnetic field. J. Magn. Magn. Mater. 2004;268:33–39. Martin RG, Johansson C, Tavares JR, Dubé M. Material selection methodology for an induction welding magnetic susceptor based on hysteresis losses . Adv. Eng. Mater. 2021;24:2100739. Li X, Li M, Peng W, Li C, Yi J, Zhao L, et al. Orientation adaptive heterogeneous CNT/Fe₃O₄ composite membrane with excellent Joule heating and enhanced electromagnetic shielding properties . J. Mater. Sci. 2024;59:16583–16603. Guzman de Villoria R, Hallander P, Ydrefors L, Nordin P, Wardle BL. In-plane strength enhancement of laminated composites via aligned carbon nanotube interlaminar reinforcement. Compos. Sci. Technol. 2016;133:33–39. Ni X, Furtado C, Kalfon-Cohen E, Zhou Y, Valdes GA, Hank TJ, et al. Static and fatigue interlaminar shear reinforcement in aligned carbon nanotube-reinforced hierarchical advanced composites. Compos. Part A: Appl. Sci. Manuf. 2019;120:106–115. Li X, Daso F, Lee J, Spangler J, Canart J-P, Kinsella M, et al. Consolidation of aerospace-grade high-temperature thermoplastic carbon fiber composites via nano-engineered electrothermal heating. Compos. Part B: Eng. 2023;262:110781. Rudolf R, Mitschang P, Neitzel M. Induction heating of continuous carbon-fibre-reinforced thermoplastics. Compos. Part A: Appl. Sci. Manuf. 2000;31:1191–1202. Cheng X, Zhou Y, Charles ADM, Yu Y, Islam MS, Peng S, et al. Enabling contactless rapid on-demand debonding and rebonding using hysteresis heating of ferrimagnetic nanoparticles. Mater. Des. 2021;210:110071. Sha Z, Cheng X, Zhou Y, Rider AN, Charles ADM, Chang W, et al. Simultaneous improvement of heating efficiency and mechanical strength of a self-healing thermoplastic polymer by hybridizing magnetic particles with conductive fibres. Compos. Part A: Appl. Sci. Manuf. 2023;172:107257. Sha Z, Cheng X, Charles ADM, Zhou Y, Islam MS, Rider AN, et al. In-situ aligning magnetic nanoparticles in thermoplastic adhesives for contactless rapid joining of composite structures. Compos. Struct. 2023;321:117324. Baek I, Lee S. A study of films incorporating magnetite nanoparticles as susceptors for induction welding of carbon fiber reinforced thermoplastic. Mater. (Basel) 2020;13:457. Bae D, Shin P, Kwak S, Moon M, Shon M, Oh S, et al. Heating behavior of ferromagnetic Fe particle-embedded thermoplastic polyurethane adhesive film by induction heating. J. Ind. Eng. Chem. 2015;30:92–97. Vattathurvalappil SH, Kundurthi S, Drzal LT, Haq M. Thermo-mechanical degradation in ABS-Fe₃O₄ polymer nanocomposite due to repeated electromagnetic heating. Compos. Part B: Eng. 2020;201:108364. Liu K, Han L, Tang P, Yang K, Gan D, Wang X, et al. An anisotropic hydrogel based on mussel-inspired conductive ferrofluid composed of electromagnetic nanohybrids. Nano Lett. 2019;19:8343–8356. Zhang B, Qu Z, Ruiz-Agudo C, Yang L, Chi B, Kong Y, et al. Lightweight magnetic carbon nanotube/cellulose nanofibre aerogels with microstructure engineering for enhanced microwave absorption. Carbon 2025;234:119652. Wang X, Zhao Z, Qu J, Wang Z, Qiu J. Fabrication and characterization of magnetic Fe₃O₄–CNT composites. J. Phys. Chem. Solids 2010;71:673–676. Rajagukguk J, Simamora P, Saragih CS, Abdullah H, Gultom NS, Imaduddin A, et al. Superparamagnetic behaviour and surface analysis of Fe₃O₄/PPY/CNT nanocomposites. J. Nanomater. 2020;2020:8896243. Li M, Wen L, Wang S, Liang J, Hou X. Heat generation mechanisms during induction heating of cross-ply carbon fiber reinforced thermoplastic laminates. Polym. Test. 2024;130:108318. Becker S, Michel M, Mitschang P, Duhovic M. Influence of polymer matrix on the induction heating behavior of CFRPC laminates. Compos. Part B: Eng. 2022;231:109538. Miller SG, et al. Assessment of thermoplastic composite joining by induction welding (NASA TM-20250006527). NASA 2025. Rosensweig RE. Heating magnetic fluid with alternating magnetic field. J. Magn. Magn. Mater. 2002;252:370–374. Dennis CL, Ivkov R. Physics of heat generation using magnetic nanoparticles for hyperthermia. Int. J. Hyperthermia 2013;29:715–729. Shah RR, Davis TP, Glover AL, Nikles DE, Brazel CS. Impact of magnetic field parameters and iron oxide nanoparticle properties on heat generation for use in magnetic hyperthermia. J. Magn. Magn. Mater. 2015;387:96–106. Sun Y, Davis E. Nanoplatforms for targeted stimuli-responsive drug delivery: A review of platform materials and stimuli-responsive release and targeting mechanisms. Nanomaterials (Basel) 2021;11:746. Obaidat IM, Issa B, Haik Y. Magnetic properties of magnetic nanoparticles for efficient hyperthermia. Nanomaterials (Basel) 2015;5:63–89. Gude MR, Prolongo SG, Ureña A. Toughening effect of carbon nanotubes and carbon nanofibres in epoxy adhesives for joining carbon fibre laminates. Int. J. Adhes. Adhes. 2015;62:139–145. Additional Declarations No competing interests reported. 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10:57:38","extension":"html","order_by":21,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":117909,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8372467/v1/f623a7688ce79a5ef9044774.html"},{"id":100674555,"identity":"a79b9a18-d4c5-4c26-9f2f-213bd178a0c0","added_by":"auto","created_at":"2026-01-20 11:00:46","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":86199,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of the CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanocomposite aerogel for CFRTP induction welding: (a) synergistic heating effect; (b) uniform heat distribution; (c) CNT pinning effect enhancing interfacial bonding.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8372467/v1/077b2e5502291c3d1fe4c3c6.jpg"},{"id":100674584,"identity":"cbd3ef4e-6f3d-467b-9297-dd8f1dfe3ac4","added_by":"auto","created_at":"2026-01-20 11:01:26","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":102442,"visible":true,"origin":"","legend":"\u003cp\u003eFreeze-dried CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/PA6 aerogel: (a) schematic of freeze drying process; (b) porous aerogel morphology; (c) fabricated shapes; (d) lightweight property; (e) Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e distribution through the thickness (TGA); (f) Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e distribution (3D micro-CT); (g) formation of CNT–Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e network (FE-SEM).\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8372467/v1/96e35980a84787f3c7ceb5c1.jpg"},{"id":100674439,"identity":"deba29b8-e016-4ade-aea0-8c01c7c124c4","added_by":"auto","created_at":"2026-01-20 10:59:47","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":93147,"visible":true,"origin":"","legend":"\u003cp\u003eCharacterization of CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/PA6 nanocomposites: (a) Nitrogen adsorption–desorption isotherm of the powdered aerogel; (b) electrical conductivity as a function of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e content; (c) schematic of Regime I and II conduction mechanisms; (d) VSM M–H curves.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8372467/v1/0752448d28a5109d575b2d3e.jpg"},{"id":100674206,"identity":"b43a3b40-d68e-43be-92a1-9dee211b78f9","added_by":"auto","created_at":"2026-01-20 10:57:34","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":110132,"visible":true,"origin":"","legend":"\u003cp\u003eInduction heating behavior of CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/PA6 nanocomposites: (a) heating performance of the nanocomposite vs CNT-only; (b) as a function of PA6 content; (c) uniform heat distribution (CFP50); (d) non-uniform heat distribution (CFP10); (e) faster initial heating rate vs CFRP-only; (f) shorter time to PA6 melting temperature vs CFRP-only.\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8372467/v1/5a53a2303b6208a804bb2397.jpg"},{"id":100674202,"identity":"7f2b6aa1-ed1a-4ce0-841e-447619672fe9","added_by":"auto","created_at":"2026-01-20 10:57:31","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":88781,"visible":true,"origin":"","legend":"\u003cp\u003eApplication of CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/PA6 nanocomposites to induction welding: (a) assembly of CFRP; (b) schematic illustration of induction-welding setup; Micro-CT images of the CFP50 showing complete interfacial welding (c) the cross-section; (d) top view; (g) C-scan images of overlap area vs CFRP-only.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8372467/v1/b91c694c7c5290a4d0db303c.jpg"},{"id":100674056,"identity":"eacc7e3f-6918-4b47-91d9-ce37cf0a5b01","added_by":"auto","created_at":"2026-01-20 10:56:13","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":35196,"visible":true,"origin":"","legend":"\u003cp\u003eMechanical strength test of fracture surface observations: (a) single lap shear strength vs CFRP-only; (b) fracture surfaces.\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8372467/v1/49717f7ff4e9a3abdc917c04.jpg"},{"id":100674579,"identity":"288b3b7e-bfda-4329-990e-27995d8d6c75","added_by":"auto","created_at":"2026-01-20 11:01:23","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":100345,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of fractured interfaces: (a) unwelded interlayer morphology; (b) welded interface with PA6 infiltration and residual polymer; (c,d) high-magnification views showing CNT pull-out; (e) pulled-out individual CNT.\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8372467/v1/5e6cfdbab5cf640e7e6a55a0.jpg"},{"id":100674557,"identity":"119d0738-90c7-4e88-ab51-7e86a2533668","added_by":"auto","created_at":"2026-01-20 11:00:49","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":106340,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of proposed heating and welding mechanisms: (a) synergistic heating from CNT, Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, and PA6 losses; (b) comparison of interfacial contact in monolith and powder; (c) CNT pinning effect reinforcing the welded region.\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8372467/v1/3c7701b478eae976d8c8f887.jpg"},{"id":100680190,"identity":"43e0b35b-a795-4835-b337-eec092381b23","added_by":"auto","created_at":"2026-01-20 11:55:53","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1588524,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8372467/v1/082cba42-8ac3-4102-8a92-b77d4eea33f0.pdf"},{"id":100674220,"identity":"4d60cf69-41ce-42c7-be34-47b751e4ab11","added_by":"auto","created_at":"2026-01-20 10:57:43","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":1631552,"visible":true,"origin":"","legend":"","description":"","filename":"supplementarydata.docx","url":"https://assets-eu.researchsquare.com/files/rs-8372467/v1/d1212abf42a924d235a22348.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Synergistic heating effect via CNT/Fe3O4 nanocomposite aerogels enabling uniform and rapid induction welding with high bond strength in CFRTPs","fulltext":[{"header":"Highlights","content":"\u003cp\u003eㆍ Proposed a novel nanocomposite aerogel that maximizes induction heating via Joule and magnetic loss mechanisms.\u003c/p\u003e\u003cp\u003eㆍ Incorporated CNTs for Joule heating and FeO for magnetic loss-driven heating enhancement.\u003c/p\u003e\u003cp\u003eㆍ Fabricated nanocomposites with varying ratios and experimentally identified the optimal composition.\u003c/p\u003e\u003cp\u003eㆍ Demonstrated the potential of the proposed material as a high-efficiency nanocomposite for induction welding.\u003c/p\u003e"},{"header":"1. Introduction","content":"\u003cp\u003eInduction heating has attracted attention as a welding technique for CFRTPs because it offers rapid heat generation, minimal surface contamination via a non-contact process, and selective heating of targeted areas. Owing to these advantages, induction welding has seen increasing adoption in high-performance sectors such as automotive and aerospace, where rapid and precise joining is essential [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. The efficiency of induction heating is governed by key parameters, including the electrical and magnetic properties of the materials and the continuity of induced-current loops [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. However, most carbon-fiber-reinforced thermoplastics (CFRTPs) do not satisfy these conditions, leading to induced currents that do not effectively reach the welding interface and difficulty in rapidly attaining the process temperature [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. In addition, edge effects can cause insufficient melting at the joint center, resulting in incomplete interfacial fusion and, ultimately, reduced joint strength. Consequently, incorporating a susceptor is essential to maximize the induction-heating effect and to induce selective heat generation at the welding interface [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eCarbon nanotubes (CNTs) have emerged as promising constituents for induction-assisted joining due to their excellent electrical conductivity and ability to form percolated networks through their high aspect ratio [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. When exposed to microwaves or high-frequency fields, percolated CNT networks produce Joule heating, thereby supporting more uniform heat generation at the welding interface [\u003cspan additionalcitationids=\"CR10\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. For example, Cao et al. inserted multiwalled carbon nanotube (MWCNT) sheets at the edges of a carbon fiber heating element to mitigate non-uniform heating, improving overall thermal uniformity [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Importantly, the efficacy of CNTs depends strongly on loading level: below the electrical percolation threshold, conduction pathways are sparse and heating is limited, whereas loadings exceeding this threshold result in a rapid increase in network conductivity and heat delivery [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. However, when the CNT content is increased to excessive levels, dispersion becomes challenging and viscosity increases, which can promote agglomeration, hinder processability, and degrade interfacial uniformity [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Moreover, because CNTs are non-magnetic, their heating under alternating magnetic fields is fundamentally less efficient than that of ferromagnetic susceptors [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. As a result, CNTs are often used as functional reinforcements or auxiliary thermal conductors rather than as primary induction susceptors for welding [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Notably, aligned CNT interlayers can significantly enhance interlaminar strength\u0026mdash;e.g., improvements of up to 14% and 40% have been reported\u0026mdash;while CNT-film electrothermal elements have enabled uniform, rapid consolidation with substantial energy savings [\u003cspan additionalcitationids=\"CR20\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Nevertheless, limited magnetic responsiveness and shallow heat penetration constrain the use of CNTs as standalone susceptors in induction systems [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e is a ferrimagnetic material that heats efficiently via magnetic hysteresis losses when subjected to an alternating magnetic field. Accordingly, Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e has been dispersed in polymer matrices and used as a susceptor for induction heating [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. For instance, Sha et al. aligned Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanoparticles in a Nylon-12 adhesive under an AC magnetic field, boosting the heating rate by up to 200% and enabling reversible joining [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e], while Baek et al. achieved stable heating and stable interfacial bonding using PA6 films containing 75 wt% Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e for CF/PA6 induction welding [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Although high Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e contents can lead to agglomeration and localized overheating, degrading spatial thermal uniformity [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e], design strategies such as particle alignment, architectural control, and hybridization with conductive phases can mitigate these issues [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThis study deliberately integrates these complementary behaviors in a nanocomposite to achieve both thermal efficiency and interfacial robustness in composite joining. Rather than emphasizing electromagnetic interference (EMI) shielding or purely anisotropic constructs, as in prior CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e hybrids [\u003cspan additionalcitationids=\"CR31\" citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e], our work focuses on three focal points relevant to induction welding: (i) a powdered CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/PA6 aerogel interlayer that is simple to process yet conformal to the weld line; (ii) synergistic heating that couples CNT-enabled Joule-heating networks with Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e magnetic hysteresis to flatten thermal gradients across the interface; and (iii) mechanical reinforcement via CNT anchoring within the resin-rich layer, enhancing interfacial weld strength. The concept is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and validated experimentally via heating performance and interfacial mechanical testing. We anticipate that this design provides a practical pathway toward rapid, spatially uniform heating with improved mechanical performance in the induction welding of CFRTPs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Materials\u003c/h2\u003e \u003cp\u003eSingle-walled carbon nanotubes (SWCNTs, outer diameter 1.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 nm, length\u0026thinsp;\u0026ge;\u0026thinsp;5 \u0026micro;m) were purchased from TUBALL\u0026reg; (OCSiAl, Luxembourg). The SWCNTs were ultrasonically dispersed in DI water with Sodium dodecylbenzenesulfonate (Sigma-Aldrich) as a surfactant and used in solution. Magnetite particles (Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, density 4.52 g\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e, average particle size\u0026thinsp;\u0026asymp;\u0026thinsp;5 \u0026micro;m) were obtained from Shanghai Laiwu Powder Material Co., Ltd. (China). Polyamide 6 (PA6, density 1.17 g\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e, average particle size 15\u0026ndash;20 \u0026micro;m) was supplied by Goodfellow (Huntingdon, UK). All materials were employed without additional treatment.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Induction Heating\u003c/h2\u003e \u003cp\u003eInduction heating tests were performed with an induction heater (HF-6K, Taeyang Induction Heater Co., Ltd., Korea). An elliptical, pancake-type copper coil (inner diameter of 3.2 mm) was used; cooling water was circulated through the coil to prevent overheating. The operating frequency and power were set to 300 kHz and 5 kW. The specimen was centered within the coil, with a coil\u0026ndash;specimen gap of 2 mm. Specimen temperature was monitored using an infrared thermal imaging camera (FLIR E54, FLIR Systems, Inc., USA).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Induction Welding Process\u003c/h2\u003e \u003cp\u003eInduction welding was carried out using CF/PA6 unidirectional laminate specimens. The specimen dimensions were 101.6 \u0026times; 25.4 mm\u003csup\u003e2\u003c/sup\u003e with a thickness of 1.81 mm, and the laminate exhibited a fiber volume fraction of 47.3% and a void content of 1.57%. The stacking sequence was [0/90/\u0026plusmn;45]\u003csub\u003e2s\u003c/sub\u003e, fabricated to achieve a quasi-isotropic lay-up. Two specimens were joined in a single-lap configuration with an overlap length of 12.7 mm, and a pressure of 40 kg was applied to the upper adherend during welding. The specimens were mounted in an alumina fixture to maintain alignment. The heating time was set to 70 s, and cooling to room temperature was carried out under pressure.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4. Characterization\u003c/h2\u003e \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e \u003ch2\u003e2.4.1. Characterization of Nanocomposites\u003c/h2\u003e \u003cp\u003eA range of characterization techniques was employed to comprehensively evaluate the structural, electrical, and magnetic properties of the nanocomposites. These measurements were conducted to elucidate how the nanocomposite architecture influences percolation behavior and the electrical and magnetic responsiveness required for induction heating.\u003c/p\u003e \u003cp\u003eThermogravimetric analysis (TGA; TGA55, TA Instruments, USA) was performed in air at a heating rate of 10\u0026deg;C\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e up to 600\u0026deg;C, and the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e residual mass was used to assess how uniformly the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e particles were distributed along the thickness direction of the aerogel. X-ray micro-computed tomography (micro-CT; vtomex m300, GE, USA) was employed to visualize the three-dimensional dispersion uniformity of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e particles within the aerogel structure.\u003c/p\u003e \u003cp\u003eNitrogen adsorption\u0026ndash;desorption analysis (BET; iQ 1MP, Quantachrome Instruments, USA) was performed on powdered aerogels to verify their specific surface area and to evaluate nitrogen adsorption and desorption behavior through the isotherm graph.\u003c/p\u003e \u003cp\u003eElectrical conductivity was measured using a four-point probe (FPP-2000, DASOL ENG, Korea) on hot-pressed films (25.4 \u0026times; 12.7 mm\u003csup\u003e2\u003c/sup\u003e) to examine how Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e loading affects the formation of CNT percolation networks. Vibrating sample magnetometry (VSM; MicroSense EZ VSM, MicroSense, USA) was conducted under an applied magnetic field of \u0026plusmn;\u0026thinsp;25 kOe to analyze the magnetic responsiveness of the nanocomposites, with particular emphasis on determining how well the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e-based magnetic response is retained upon CNT incorporation.\u003c/p\u003e \u003cp\u003eField-emission scanning electron microscopy (FE-SEM; JSM-7800F, JEOL, Japan) was used to observe the porous aerogel microstructure and to verify the formation of interconnected CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e networks that define the structural characteristics of the interlayer.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e \u003ch2\u003e2.4.2. Characterization of Welded joints\u003c/h2\u003e \u003cp\u003eTo evaluate the reliability of the welded joints, a series of non-destructive, mechanical, and microstructural analyses were performed. Non-destructive CT inspection was further conducted to assess the integrity of the welded interface. For this evaluation, a Phoenix Vtomex M240 system equipped with a micro-focus X-ray tube was operated at 60 kV and 180 \u0026micro;A. The voxel resolution was set to 10 \u0026micro;m to enable precise detection of interfacial defects, and a total of 2,200 projection images were collected and reconstructed into a three-dimensional CT volume. In addition, immersion ultrasonic C-scan mapping was performed using a TTU system (GTR4010-ARN-CA) to evaluate the continuity and uniformity of the welded region across the entire bonded area, and to detect internal defects such as lack of fusion through area-based attenuation signals.\u003c/p\u003e \u003cp\u003eSingle-lap shear (SLS) tests were conducted according to ASTM D5868 at a crosshead speed of 1 mm\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, using n\u0026thinsp;\u0026ge;\u0026thinsp;3 specimens per condition to ensure reproducibility. These tests were used to compare the mechanical strength improvement achieved by incorporating the nanocomposite interlayer relative to the CFRP-only joints.\u003c/p\u003e \u003cp\u003eField-emission scanning electron microscopy (FE-SEM; JSM-7800F, JEOL, Japan) was employed to observe CNT pull-out features on the fracture surfaces, enabling analysis of interfacial failure behavior and the contribution of CNT anchoring to the mechanical performance of the welded joints.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Fabrication and Microstructure of CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/PA6 Nanocomposite aerogel\u003c/h2\u003e \u003cp\u003eTo achieve rapid and spatially uniform induction heating, both conductive pathways and magnetically responsive particles must be homogeneously distributed through the nanocomposite architecture. We therefore prepared a CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/PA6 nanocomposite via an aqueous route followed by rapid freezing and vacuum freeze-drying. The process yielded aerogels that were readily shaped into arbitrary geometries without cracking (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec) and possessed exceptionally low density due to their percolated pore network (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed). This aerogel form factor offered practical advantages for welding: it can be trimmed to a conformal interlayers that match the weld line, handled and transferred with minimal mass loading, and, if needed, gently milled into a powdered aerogel while retaining the underlying CNT\u0026ndash;Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e connectivity. Accordingly, single-walled CNTs were individually dispersed in water with an surfactant and then mixed with Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and PA6 powders; the powder-dispersed solution was quenched in liquid nitrogen to grow ice crystals and lyophilized (\u0026minus;\u0026thinsp;120\u0026deg;C, 24 h), during which solvent sublimation generated the porous aerogel (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe freeze-drying route mitigated sedimentation of relatively dense Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e during solvent removal, which enabled a uniform through-thickness distribution within the CNT scaffold. To quantify compositional uniformity, thermogravimetric analysis (TGA) was performed on sections taken from the top and bottom of the aerogel. The residual masses were 36.4% and 37.2%, respectively, corresponding to a difference of only\u0026thinsp;~\u0026thinsp;0.8%, which confirmed that particle settling was effectively suppressed during processing. Complementary X-ray micro-computed tomography (micro-CT) revealed a homogeneous three-dimensional dispersion of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e throughout the aerogel volume, consistent with the TGA results (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee, f).\u003c/p\u003e \u003cp\u003eMicrostructural observations supported the formation of a dual-loss nanocomposite network. Field-emission SEM images showed an open, vertically porous architecture characteristic of freeze-dried structures (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). Additional microstructural features of the aerogel monolith are provided (see Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e in the Supplementary data). Within this porous network, Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e particles were embedded and wrapped by CNT bundles to form an interconnected percolation network (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg). This architecture was expected to (i) provide continuous electronic pathways for distributed Joule heating via the CNT network and (ii) supply numerous, well-dispersed magnetic nano/microsources for hysteresis-based heat generation via Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e under an alternating field. The intimate CNT\u0026ndash;Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e contact further facilitated heat sharing across phases and promoted mechanical interlocking (\u0026ldquo;pinning\u0026rdquo;) of CNTs within the resin-rich layer after deposition, establishing a structural basis for both uniform thermal fields and enhanced interfacial bond strength during induction welding.\u003c/p\u003e \u003cp\u003eThe CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/PA6 nanocomposites were prepared with varying compositions, and the samples were designated based on their component combination. In the sample codes, C, F, and P represented CNT, Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, and PA6, respectively, and the following number indicated the PA6 content (wt%). The detailed compositions are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComposition of CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/PA6 nanocomposite samples.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCNT (wt%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eFe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (wt%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ePA6 (wt%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCFP0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCFP10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCFP30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCFP50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eNitrogen adsorption\u0026ndash;desorption analysis showed that the powdered aerogel had a specific surface area of 104.24 m\u003csup\u003e2\u003c/sup\u003e\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). This value is significantly higher than the specific surface area range of carbon fibers (CF), which is 0.5\u0026ndash;5 m\u003csup\u003e2\u003c/sup\u003e\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Therefore, the powdered aerogel provides more pathways for resin infiltration compared to using only carbon fiber composites, and the open pore structure allows for more efficient resin penetration into the internal pores. As a result, it is anticipated that resin impregnation will occur more effectively, leading to an enhancement in interfacial welding strength.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eElectrical conductivity measurements, performed at a fixed CNT:PA6 ratio of 1:0.5, showed a non-monotonic dependence of bulk conductivity (σ) on Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e loading (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). In Regime I, the introduction of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e initially reduced σ because the relative CNT content decreased, and some tube\u0026ndash;tube junctions were obstructed (dilution/blocked-junction effect). Beyond the percolation threshold of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, Regime II emerged: σ increased again as Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e participated in assisted percolation, effectively bridging separated CNT domains and restoring long-range connectivity. The composition with equal masses of CNT and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (CFP50) delivered the highest conductivity, 3347.92 S\u0026middot;m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, consistent with an optimally connected hybrid network capable of distributing Joule heat uniformly along the weld line. The two regimes were summarized schematically in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec.\u003c/p\u003e \u003cp\u003eMagnetic characterization by VSM, normalized by the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e mass, confirmed that ferrimagnetic behavior was retained after hybridization with CNT and PA6 (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed; Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Bare Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e showed M\u003csub\u003es\u003c/sub\u003e = 70.59 emu\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Mixing Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e with PA6 at equal mass decreased M\u003csub\u003es\u003c/sub\u003e by only 7.77%, indicating minimal magnetic interference from the polymer. In contrast, CNT-containing hybrids displayed a larger reduction in M\u003csub\u003es\u003c/sub\u003e\u0026mdash;up to 53.93%\u0026mdash;which we ascribed to interfacial charge transfer and spin disorder at CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e contacts that weakened exchange interactions. Notably, the electrically optimal composition (CFP50) still retained substantial magnetization, yielding a balanced electro-magnetic profile: percolated CNT pathways for heat spreading via Joule heating together with Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e-driven hysteresis losses for rapid local heat generation.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSummary of VSM magnetization results\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMagnetization (emu\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eReduction vs. bare Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBare Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e70.59\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e65.11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e7.77\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCFP0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e34.77\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e70.74\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCFP50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e32.53\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e53.93\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eTaken together, the BET, conductivity, and magnetization results demonstrated that the powdered nanocomposite aerogel (CFP50) promoted interfacial contact, restored conductive percolation at optimal Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e loadings, and preserved sufficient magnetic response. This combination is precisely what is required to achieve rapid yet spatially uniform induction heating and high interfacial bond strength in CFRTP welding.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Induction Heating Performance of the Nanocomposite\u003c/h2\u003e \u003cp\u003eWe assessed the nanocomposite as an active heating material by quantifying heating efficiency and spatial uniformity under a kilohertz alternating magnetic field (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). To probe whether electronic and magnetic loss channels acted cooperatively, CNT-only aerogels were compared with CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/PA6 hybrids. Over a 60 s induction period, the hybrid consistently reached higher peak temperatures than CNT-only (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). The best performance occurred when CNT, Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, and PA6 were all present at CFP50, indicating that Joule losses in the percolated CNT network and magnetothermal losses (hysteresis/relaxation) in Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e are mutually reinforcing, with dielectric dissipation in PA6 providing a smaller but beneficial contribution [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo isolate the role of the polymer, the CNT:Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e mass ratio was fixed at 1:1 while PA6 content was varied (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). Increasing PA6 led to a monotonic decrease in peak temperature, consistent with the dilution of the conductive skeleton and a reduced relative contribution from the CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e network at high polymer fractions. Thus, although PA6 contributes measurable dielectric loss near processing temperatures, excess polymer penalizes overall heating efficiency by lowering the effective density of current paths and magnetic heat sources.\u003c/p\u003e \u003cp\u003eHeating uniformity was examined by infrared thermography (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec,d). At CFP50, the nanocomposite aerogel exhibited a spatially uniform temperature field above the PA6 melting point without obvious hot spots (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec), consistent with balanced power deposition from electronic and magnetic losses and subsequent smoothing by thermal diffusion through a continuous polymer phase. The temperature non-uniformity index, calculated from the IR thermography data, was 10.89%, further confirming the spatially uniform interfacial heating. In contrast, at CFP10, hot spots emerged (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed), attributable to insufficient dielectric participation and heat spreading, which concentrate power in highly conductive or strongly magnetic locales. The uniformity trend aligns with the peak-temperature optimum near CFP50 (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea) and the decline at higher polymer contents (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003eEarly-time responsiveness was benchmarked using the initial heating rate (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee). Under identical inputs, CFRP-only showed little rate gain with voltage, whereas the nanocomposite aerogels responded sharply; at 170 V the maximum initial rate reached 148.7\u0026deg;C\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which was 94.6% higher than CFRP-only. This contrast reflects the lack of closed induced-current loops in CFRP (dielectric-loss-dominated) versus the percolated, closed-loop network in the nanocomposite aerogel.\u003c/p\u003e \u003cp\u003eFinally, to simulate the welding process, the nanocomposite aerogel was deployed as a thin interlayer laminated on CFRP (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef). Relative to CFRP-only, overlaying the laminate with the nanocomposite aerogel increased the initial heating rate from 43.7 to 68.6\u0026deg;C\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (~\u0026thinsp;57%), the time to reach the processing temperature was markedly shorter, and the temperature at t\u0026thinsp;=\u0026thinsp;20 s was up to 173\u0026deg;C higher under the same input. These results demonstrate that the CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/PA6 nanocomposite accelerates heating while flattening thermal gradients, thereby improving process efficiency and establishing favorable thermal conditions for strong, uniform induction-welded joints.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Application to Induction Welding\u003c/h2\u003e \u003cp\u003eThe powdered aerogel nanocomposite (CFP50) was inserted as a thin interlayer in single-lap CFRP joints (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea,b) to evaluate whether the improved heating efficiency and spatial uniformity (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) translated into reliable joints. Welding was performed in an alumina fixture to maintain alignment.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo evaluate the reliability of the weld interface, micro-CT analyses were conducted from both cross-sectional and top-view perspectives. In the CFRP-only specimen, the cross-section revealed incomplete joining with noticeable delamination regions, and in the top view, a central defect was observed, indicating the presence of unwelded areas (Fig. S2). These unwelded regions were attributed to non-uniform melting during heating. In contrast, the CFP50 specimen exhibited no delamination at the joint interface, and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e particles dispersed within the powdered aerogel were clearly observed (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). The small voids formed near the welding line appeared to originate from the rapid melting of the CFRP matrix during induction heating; however, no separation or lifting of the welded line was observed after solidification. Furthermore, in the top view, unlike the CFRP-only specimen, the welding interface exhibited uniform melting, and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e particles were uniformly distributed throughout the entire welding region (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed). Taken together, the observations demonstrated that the CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/PA6 nanocomposite acted as an effective thermal mediator during induction heating, facilitating the formation of a uniform and stable interface.\u003c/p\u003e \u003cp\u003eThe transmission ultrasonic inspection results (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) revealed a clear distinction between the CFRP-only joints and the joints incorporating nanocomposites with different PA6 contents. In the Figure legend (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eg), the red color can be regarded as the reference signal corresponding to 100% transmission, whereas darker colors indicate a progressive decrease in relative ultrasonic transmission. The CFRP-only joint exhibited a transmission ratio of only 6% at 57 dB, indicating that ultrasonic waves were largely attenuated even under relatively high input, and continuous transmission across the interface was not achieved. In the C-scan images, a distinct boundary was observed between the edge and center regions, suggesting that a significant portion of the joint interface was non-uniformly or incompletely welded. In contrast, the joints with nanocomposites (CFP10 and CFP50) exhibited transmission ratios of 41% and 43% at comparable dB levels (55\u0026ndash;60 dB), confirming continuous and uniform ultrasonic transmission throughout the overlap region. These results demonstrate that, even when considering differences in dB settings, the nanocomposite-containing joints showed significantly higher transmission ratios than the CFRP-only joints, suggesting that the incorporation of nanocomposites improved interfacial uniformity and stabilized the ultrasonic propagation path, thereby promoting spatially uniform welding. Although ultrasonic transmission characteristics do not directly correspond to mechanical strength, combined interpretation with mechanical tests is required to draw comprehensive conclusions. Nevertheless, the increased transmission can be interpreted as evidence of improved interfacial uniformity. Furthermore, it should be noted that a decrease in ultrasonic transmission cannot always be solely attributed to interfacial defects, but may also arise from structural factors such as increased joint thickness or the fiber stacking sequence of the CFRP.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eUltrasonic transmission results of CFRP-only and nanocomposite joints.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003edB\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTransmission ratio (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCFRP-only\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWith CFP0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e62\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWith CFP10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e41\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWith CFP50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e43\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eTo verify the mechanical reinforcement effect of the nanocomposite (CFP50), fracture surfaces were examined after single-lap shear testing. Relative to the CFRP-only, the nanocomposite afforded faster interfacial heating and more uniform melting across the weldline, as indicated by the absence of visually unmelted central regions after welding (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb). Correspondingly, the single-lap shear strength increased from 19.21 MPa (CFRP-only) to 25.12 MPa with the nanocomposite (+\u0026thinsp;30.8%; Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea). We attribute this gain to (i) balanced power deposition from Joule and hysteresis losses that flattens the thermal gradient and promotes uniform fusion, and (ii) CNT-mediated load transfer, whereby nanotube bundles embedded in the resin-rich layer act as \u0026ldquo;pins,\u0026rdquo; improving interfacial traction under shear.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo elucidate the microstructural origins of strength improvement, we examined fracture surfaces by FE-SEM (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). Prior to induction heating, the powdered aerogel nanocomposite showed a distinct granular/layered morphology at the interface (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea). After welding, PA6 infiltration was evident along the CNT\u0026ndash;Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e network (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb), indicating intimate wetting and co-melting between the interlayer and the CFRTP resin. High-magnification images (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec\u0026ndash;e) all showed CNT pull-outs. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec revealed that CNTs were uniformly pulled out over a wide area of the joint interface, while Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed provided a magnified view of this region for more detailed observation. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ee presented an even higher magnification, clearly showing individual CNT strands fully pulled out. These CNT pull-outs and bridging ligaments are indicative of effective mechanical interlocking and energy dissipation during loading and, together with the uniform thermal field established during welding, rationalize the observed enhancement in lap-shear strength.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eOverall, the CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/PA6 nanocomposite functions as an electromagnetically balanced heating material that accelerates heating, equalizes temperature across the overlap, and reinforces the interface at multiple length scales. This combination yields stronger and more reliable induction-welded CFRP joints.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.4. Proposed Heating and welding Mechanism\u003c/h2\u003e \u003cp\u003eIn an alternating (kHz) magnetic field, the nanocomposite dissipates energy via three coupled channels (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea): (i) Joule heating in the percolated CNT network that establishes low-impedance pathways; (ii) magnetic hysteresis/relaxation losses in Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e that provide volumetric magnetothermal heating in the RF/kHz regime; and (iii) dielectric loss in PA6 that becomes non-negligible near processing temperatures. The coexistence of conductive and magnetic pathways increases the effective loss factor and explains the higher initial heating propensity and shorter time-to-melt relative to single-mode heating materials. This interpretation agrees with polymer induction-processing literature (magnetite susceptors operated near ~\u0026thinsp;100 kHz for welding) [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e] and with reports that CNT networks enable efficient electrothermal self-heating [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e], while Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e particles heat through hysteresis/relaxation under comparable frequencies [\u003cspan additionalcitationids=\"CR38 CR39\" citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe freeze-dried then powdered aerogel morphology increases accessible surface area and packability at the weldline, improving thermal/physical contact and enabling conformal placement in local gaps (our measurements) (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb). In parallel, the uniform Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e distribution within the CNT network mitigates current hot-spots and spreads magnetic losses, flattening temperature gradients\u0026mdash;consistent with the IR thermography uniformity observed here and with studies showing heating efficiency/uniformity are dispersion-controlled in magnetically susceptible nanocomposites.\u003c/p\u003e \u003cp\u003eDuring welding, uniformly heated PA6 melts and infiltrates the overlap, while interfacial CNT filaments act as nanoscale pins/bridges, enhancing load transfer and resisting interfacial crack growth via pull-out/bridging\u0026mdash;mechanisms widely reported to raise interlaminar toughness in CNT-modified joints and laminates [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e] (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec). The combination of magnetothermal efficiency (Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) and interfacial toughening (CNT) is consistent with induction-welded thermoplastic systems employing magnetite-based susceptors.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eWe demonstrated that a powdered CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/PA6 nanocomposite enables uniform and rapid induction heating for polymer\u0026ndash;composite welding. When subjected to an alternating magnetic field, the percolated CNT network provides efficient Joule heating, Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e contributes magnetothermal losses (hysteresis/relaxation), and PA6 adds dielectric dissipation; together these loss channels produce spatially uniform interfacial temperatures and robust weld quality. The powdered aerogel morphology increases accessible surface area and packability at the overlap, improving thermal/physical contact and facilitating conformal placement, which results in more homogeneous melting and infiltration across the joint.\u003c/p\u003e \u003cp\u003eQuantitatively, the nanocomposite increased the initial heating rate to 148.7\u0026deg;C\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and shortened the time-to-Tₘₑₗₜ by 57%. IR thermography indicated that the temperature non-uniformity index was 10.89%, consistent with the uniform interfacial temperature field. In terms of mechanical performance, the lap-shear strength increased from 19.21 MPa to 25.12 MPa. Fractography revealed CNT pull-out and bridging, supporting an interfacial pinning mechanism for the observed strength gains.\u003c/p\u003e \u003cp\u003eThe proposed nanocomposite offers a drop-in route to uniform, fast induction welding using off-the-shelf kHz systems and gram-scale material usage, enabling scalable and selective repairs or assembly of thermoplastic-rich interfaces. By engineering loss partitioning and leveraging IR-guided digital twins for temperature-field control\u0026mdash;then by extending the concept across materials and lay-ups\u0026mdash;the approach can evolve into a robust, deployable joining platform.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eDeclaration of competing interest\u003c/h2\u003e \u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThis work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (No. NRF- 2022M3H4A1A0407637213, RS-2024-00448445),\u003c/p\u003e \u003cp\u003eand Korea Institute of Materials Science (PNKA400).\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eNayeong Kim: Writing \u0026ndash; original draft, Methodology, Visualization. Inseok Baek: Writing \u0026ndash; original draft, Methodology, Visualization. Byeongho Park: Investigation, Resources. Dayoung Kim: Investigation, Resources. Jungwan Lee: Data curation, Review manuscript. Jin Woo Yi: Data curation, Review manuscript. Jung-soo Kim: Investigation, Data curation. Geunsu Joo: Investigation, Data curation. Seokpum Kim: Review manuscript. Vipin Kumar: Review manuscript. Youngseok Oh: Supervision, Project administration, Conceptualization, Funding acquisition, Methodology. Jinsu Kim: Spervision, Resources, Review \u0026amp; editing manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (No. NRF- 2022M3H4A1A0407637213, RS-2024-00448445), and Korea Institute of Materials Science (PNKA400).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAhmed TJ, Stavrov D, Bersee HEN, Beukers A. Induction welding of thermoplastic composites\u0026mdash;an overview. \u003cem\u003eCompos. Part A: Appl. Sci. Manuf.\u003c/em\u003e 2006;37:1638\u0026ndash;1651.\u003c/li\u003e\n\u003cli\u003eLiu J, Quan D, Scarselli G, Alderliesten R, Wang H, Zhao G. Developments and future prospects of welding technology for carbon fiber thermoplastic composites. \u003cem\u003eCompos. Part B: Eng.\u003c/em\u003e 2025;297:112314.\u003c/li\u003e\n\u003cli\u003eBayerl T, Duhovic M, Mitschang P, Bhattacharyya D. The heating of polymer composites by electromagnetic induction \u0026ndash; A review. \u003cem\u003eCompos. Part A: Appl. Sci. Manuf\u003c/em\u003e. 2014;57:27\u0026ndash;40.\u003c/li\u003e\n\u003cli\u003eFarahani RD, Dub\u0026eacute; M. Novel heating elements for induction welding of carbon fiber/polyphenylene sulfide thermoplastic composites. \u003cem\u003eAdv. Eng. Mater.\u003c/em\u003e 2017;19:1700196.\u003c/li\u003e\n\u003cli\u003eGouin O\u0026apos;Shaughnessey P, Dub\u0026eacute; M, Fernandez Villegas I. Modeling and experimental investigation of induction welding of thermoplastic composites and comparison with other welding processes. \u003cem\u003eJ. Compos. Mater.\u003c/em\u003e 2016;50:2895\u0026ndash;2910. \u003c/li\u003e\n\u003cli\u003eVattathurvalappil SH, Haq M. Thermomechanical characterization of Nano-Fe₃O₄ reinforced thermoplastic adhesives and single lap-joints. \u003cem\u003eCompos. Part B: Eng.\u003c/em\u003e 2019;175:107128.\u003c/li\u003e\n\u003cli\u003eRoman CL, da Silva Moura N, Wicker S, Dooley KM, Dorman JA. Induction heating of magnetically susceptible nanoparticles for enhanced hydrogenation of oleic acid. \u003cem\u003eACS Appl. Nano Mater.\u003c/em\u003e 2022;5:3676\u0026ndash;3685. \u003c/li\u003e\n\u003cli\u003eBryning MB, Milkie DE, Islam MF, Hough LA, Kikkawa JM, Yodh AG. Carbon nanotube aerogels. \u003cem\u003eAdv. Mater.\u003c/em\u003e 2007;19:661\u0026ndash;664.\u003c/li\u003e\n\u003cli\u003eSweeney CB, Lackey BA, Pospisil MJ, Achee TC, Hicks VK, Moran AG, Teipel BR, Saed MA, Green MJ. Welding of 3D-printed carbon nanotube\u0026ndash;polymer composites by locally induced microwave heating. \u003cem\u003eSci. Adv.\u003c/em\u003e 2017;3:e1700262.\u003c/li\u003e\n\u003cli\u003eWang F, Zhang P, Luo J, Li B, Liu G, Zhan X. Effect of heat input on temperature characteristics and fusion behavior at bonding interface of CFRTP induction welded joint with carbon fiber susceptor. \u003cem\u003ePolym. Compos.\u003c/em\u003e 2022;44:1586\u0026ndash;1602.\u003c/li\u003e\n\u003cli\u003ePoyraz S, Zhang L, Schroder A, Zhang X. Ultrafast microwave welding/reinforcing approach at the interface of thermoplastic materials. \u003cem\u003eACS Appl. Mater. Interfaces\u003c/em\u003e 2015;7:22469\u0026ndash;22477. \u003c/li\u003e\n\u003cli\u003eCao D. Fusion joining of thermoplastic composites with a carbon fabric heating element modified by multiwalled carbon nanotube sheets. \u003cem\u003eInt. J. Adv. Manuf. Technol.\u003c/em\u003e 2023;128:4443\u0026ndash;4453.\u003c/li\u003e\n\u003cli\u003eMarconnet AM, Yamamoto N, Panzer MA, Wardle BL, Goodson KE. Thermal conduction in aligned carbon nanotube\u0026ndash;polymer nanocomposites with high packing density. \u003cem\u003eACS Nano\u003c/em\u003e 2011;5:4818\u0026ndash;4825. \u003c/li\u003e\n\u003cli\u003eKaralis G, Tzounis L, Dimos E, Mytafides CK, Liebscher M, Karydis-Messinis A, et al. Printed single-wall carbon nanotube-based Joule heating devices integrated as functional laminae in advanced composites. \u003cem\u003eACS Appl. Mater. Interfaces\u003c/em\u003e 2021;13:39880\u0026ndash;39893.\u003c/li\u003e\n\u003cli\u003eIslam MF, Rojas E, Bergey DM, Johnson AT, Yodh AG. High weight fraction surfactant solubilization of single-wall carbon nanotubes in water. \u003cem\u003eNano Lett\u003c/em\u003e. 2003;3:269\u0026ndash;273.\u003c/li\u003e\n\u003cli\u003eMa M, Wu Y, Zhou J, Sun Y, Zhang Y, Gu N. Size dependence of specific power absorption of Fe₃O₄ particles in AC magnetic field. \u003cem\u003eJ. Magn. Magn. Mater.\u003c/em\u003e 2004;268:33\u0026ndash;39.\u003c/li\u003e\n\u003cli\u003eMartin RG, Johansson C, Tavares JR, Dub\u0026eacute; M. Material selection methodology for an induction welding magnetic susceptor based on hysteresis losses\u003cem\u003e. Adv. Eng. Mater.\u003c/em\u003e 2021;24:2100739. \u003c/li\u003e\n\u003cli\u003eLi X, Li M, Peng W, Li C, Yi J, Zhao L, et al. Orientation adaptive heterogeneous CNT/Fe₃O₄ composite membrane with excellent Joule heating and enhanced electromagnetic shielding properties\u003cem\u003e. J. Mater. Sci.\u003c/em\u003e 2024;59:16583\u0026ndash;16603.\u003c/li\u003e\n\u003cli\u003eGuzman de Villoria R, Hallander P, Ydrefors L, Nordin P, Wardle BL. In-plane strength enhancement of laminated composites via aligned carbon nanotube interlaminar reinforcement. \u003cem\u003eCompos. Sci. Technol.\u003c/em\u003e 2016;133:33\u0026ndash;39.\u003c/li\u003e\n\u003cli\u003eNi X, Furtado C, Kalfon-Cohen E, Zhou Y, Valdes GA, Hank TJ, et al. Static and fatigue interlaminar shear reinforcement in aligned carbon nanotube-reinforced hierarchical advanced composites. \u003cem\u003eCompos. Part A: Appl. Sci. Manuf.\u003c/em\u003e 2019;120:106\u0026ndash;115. \u003c/li\u003e\n\u003cli\u003eLi X, Daso F, Lee J, Spangler J, Canart J-P, Kinsella M, et al. Consolidation of aerospace-grade high-temperature thermoplastic carbon fiber composites via nano-engineered electrothermal heating. \u003cem\u003eCompos. Part B: Eng.\u003c/em\u003e 2023;262:110781. \u003c/li\u003e\n\u003cli\u003eRudolf R, Mitschang P, Neitzel M. Induction heating of continuous carbon-fibre-reinforced thermoplastics. \u003cem\u003eCompos. Part A: Appl. Sci. Manuf.\u003c/em\u003e 2000;31:1191\u0026ndash;1202.\u003c/li\u003e\n\u003cli\u003eCheng X, Zhou Y, Charles ADM, Yu Y, Islam MS, Peng S, et al. Enabling contactless rapid on-demand debonding and rebonding using hysteresis heating of ferrimagnetic nanoparticles. \u003cem\u003eMater. Des.\u003c/em\u003e 2021;210:110071.\u003c/li\u003e\n\u003cli\u003eSha Z, Cheng X, Zhou Y, Rider AN, Charles ADM, Chang W, et al. Simultaneous improvement of heating efficiency and mechanical strength of a self-healing thermoplastic polymer by hybridizing magnetic particles with conductive fibres. \u003cem\u003eCompos. Part A: Appl. Sci. Manuf.\u003c/em\u003e 2023;172:107257.\u003c/li\u003e\n\u003cli\u003eSha Z, Cheng X, Charles ADM, Zhou Y, Islam MS, Rider AN, et al. In-situ aligning magnetic nanoparticles in thermoplastic adhesives for contactless rapid joining of composite structures. \u003cem\u003eCompos. Struct.\u003c/em\u003e 2023;321:117324.\u003c/li\u003e\n\u003cli\u003eBaek I, Lee S. A study of films incorporating magnetite nanoparticles as susceptors for induction welding of carbon fiber reinforced thermoplastic. \u003cem\u003eMater. (Basel)\u003c/em\u003e 2020;13:457.\u003c/li\u003e\n\u003cli\u003eBae D, Shin P, Kwak S, Moon M, Shon M, Oh S, et al. Heating behavior of ferromagnetic Fe particle-embedded thermoplastic polyurethane adhesive film by induction heating. \u003cem\u003eJ. Ind. Eng. Chem.\u003c/em\u003e 2015;30:92\u0026ndash;97.\u003c/li\u003e\n\u003cli\u003eVattathurvalappil SH, Kundurthi S, Drzal LT, Haq M. Thermo-mechanical degradation in ABS-Fe₃O₄ polymer nanocomposite due to repeated electromagnetic heating. \u003cem\u003eCompos. Part B: Eng.\u003c/em\u003e 2020;201:108364.\u003c/li\u003e\n\u003cli\u003eLiu K, Han L, Tang P, Yang K, Gan D, Wang X, et al. An anisotropic hydrogel based on mussel-inspired conductive ferrofluid composed of electromagnetic nanohybrids. \u003cem\u003eNano Lett.\u003c/em\u003e 2019;19:8343\u0026ndash;8356.\u003c/li\u003e\n\u003cli\u003eZhang B, Qu Z, Ruiz-Agudo C, Yang L, Chi B, Kong Y, et al. Lightweight magnetic carbon nanotube/cellulose nanofibre aerogels with microstructure engineering for enhanced microwave absorption. \u003cem\u003eCarbon\u003c/em\u003e 2025;234:119652.\u003c/li\u003e\n\u003cli\u003eWang X, Zhao Z, Qu J, Wang Z, Qiu J. Fabrication and characterization of magnetic Fe₃O₄\u0026ndash;CNT composites. \u003cem\u003eJ. Phys. Chem. Solids\u003c/em\u003e 2010;71:673\u0026ndash;676.\u003c/li\u003e\n\u003cli\u003eRajagukguk J, Simamora P, Saragih CS, Abdullah H, Gultom NS, Imaduddin A, et al. Superparamagnetic behaviour and surface analysis of Fe₃O₄/PPY/CNT nanocomposites. \u003cem\u003eJ. Nanomater. \u003c/em\u003e2020;2020:8896243.\u003c/li\u003e\n\u003cli\u003eLi M, Wen L, Wang S, Liang J, Hou X. Heat generation mechanisms during induction heating of cross-ply carbon fiber reinforced thermoplastic laminates. \u003cem\u003ePolym. Test.\u003c/em\u003e 2024;130:108318.\u003c/li\u003e\n\u003cli\u003eBecker S, Michel M, Mitschang P, Duhovic M. Influence of polymer matrix on the induction heating behavior of CFRPC laminates. \u003cem\u003eCompos. Part B: Eng.\u003c/em\u003e 2022;231:109538.\u003c/li\u003e\n\u003cli\u003eMiller SG, et al. Assessment of thermoplastic composite joining by induction welding (NASA TM-20250006527). NASA 2025.\u003c/li\u003e\n\u003cli\u003eRosensweig RE. Heating magnetic fluid with alternating magnetic field. \u003cem\u003eJ. Magn. Magn. Mater.\u003c/em\u003e 2002;252:370\u0026ndash;374.\u003c/li\u003e\n\u003cli\u003eDennis CL, Ivkov R. Physics of heat generation using magnetic nanoparticles for hyperthermia. \u003cem\u003eInt. J. Hyperthermia\u003c/em\u003e 2013;29:715\u0026ndash;729.\u003c/li\u003e\n\u003cli\u003eShah RR, Davis TP, Glover AL, Nikles DE, Brazel CS. Impact of magnetic field parameters and iron oxide nanoparticle properties on heat generation for use in magnetic hyperthermia. \u003cem\u003eJ. Magn. Magn. Mater. \u003c/em\u003e2015;387:96\u0026ndash;106.\u003c/li\u003e\n\u003cli\u003eSun Y, Davis E. Nanoplatforms for targeted stimuli-responsive drug delivery: A review of platform materials and stimuli-responsive release and targeting mechanisms. \u003cem\u003eNanomaterials (Basel)\u003c/em\u003e 2021;11:746.\u003c/li\u003e\n\u003cli\u003eObaidat IM, Issa B, Haik Y. Magnetic properties of magnetic nanoparticles for efficient hyperthermia. \u003cem\u003eNanomaterials (Basel)\u003c/em\u003e 2015;5:63\u0026ndash;89.\u003c/li\u003e\n\u003cli\u003eGude MR, Prolongo SG, Ure\u0026ntilde;a A. Toughening effect of carbon nanotubes and carbon nanofibres in epoxy adhesives for joining carbon fibre laminates. \u003cem\u003eInt. J. Adhes. Adhes.\u003c/em\u003e 2015;62:139\u0026ndash;145.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"advanced-composites-and-hybrid-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"achm","sideBox":"Learn more about [Advanced Composites and Hybrid Materials](https://link.springer.com/journal/42114)","snPcode":"42114","submissionUrl":"https://submission.nature.com/new-submission/42114/3","title":"Advanced Composites and Hybrid Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Induction welding, Nanocomposites, Aerogels, Joule heating, Hysteresis loss, Carbon nanotubes, Magnetic particles","lastPublishedDoi":"10.21203/rs.3.rs-8372467/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8372467/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eEffective induction welding of carbon fiber-reinforced thermoplastic composites (CFRTPs) requires both uniform temperature distribution and rapid heating at the welding interface. However, achieving both is challenging because heat localizes due to non-uniform magnetic-field distribution and the difficulty of forming current loops within the composite structure. In this study, a powdered CNT/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/PA6 aerogel (Nanocomposite) was adopted to enhance induction-heating performance by simultaneously leveraging electrical conductivity and magnetic-loss mechanisms. The nanocomposite contains carbon nanotubes (CNTs) that form continuous conductive pathways, enabling efficient Joule heating and improved heat transfer. The CNTs also act as nanoscale mechanical pins that bridge CFRTPs, thereby enhancing interfacial weld strength. In addition, embedded Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e particles respond rapidly to the alternating magnetic field and act as localized heat sources via magnetic losses, increasing heating efficiency. As a result, the nanocomposite-enabled system demonstrated a 94.6% higher initial heating rate and a 57% reduction in the time required to reach the target processing temperature at the welding interface. The interfacial shear strength increased by 30.8%, and fracture-surface analysis revealed more uniform melting and more effective impregnation across the weld area. These results indicate that the proposed nanocomposite offers a promising strategy for achieving both rapid and uniform heating, leading to improved mechanical performance in the induction welding of CFRTPs.\u003c/p\u003e","manuscriptTitle":"Synergistic heating effect via CNT/Fe3O4 nanocomposite aerogels enabling uniform and rapid induction welding with high bond strength in CFRTPs","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-20 09:30:16","doi":"10.21203/rs.3.rs-8372467/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-04-07T00:12:32+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-06T22:55:15+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"245233000418654971852364588950698982381","date":"2026-03-16T05:15:59+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-30T11:32:51+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"323669008926639007262259967199232573140","date":"2026-01-16T05:41:15+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-01-16T03:22:22+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-01-01T14:39:36+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-12-18T09:31:04+00:00","index":"","fulltext":""},{"type":"submitted","content":"Advanced Composites and Hybrid Materials","date":"2025-12-16T06:13:27+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"advanced-composites-and-hybrid-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"achm","sideBox":"Learn more about [Advanced Composites and Hybrid Materials](https://link.springer.com/journal/42114)","snPcode":"42114","submissionUrl":"https://submission.nature.com/new-submission/42114/3","title":"Advanced Composites and Hybrid Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"bd01fc79-7004-4291-a267-e1aa35176c25","owner":[],"postedDate":"January 20th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-06T00:53:16+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-20 09:30:16","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8372467","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8372467","identity":"rs-8372467","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}