3D-Printed Bentonite-Graphite Composite Materials Inspired by Pencil Lead

3D-Printed Bentonite-Graphite Composite Materials Inspired by Pencil Lead | 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 3D-Printed Bentonite-Graphite Composite Materials Inspired by Pencil Lead Yumei Gong, Wei Wang, Xiaohang Tuo, Yujie Duan, Ji Jia, Qianhui Qin This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7392366/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 11 You are reading this latest preprint version Abstract Traditional 3D-printed conductive composites face limitations in widespread application due to the weak heat resistance, poor corrosion resistance, and high cost of their matrix material. Inspired by pencil lead, 3D-printable bentonite-graphite (HB) composite inks have been developed. Ammonium sulfate (AS) solution is selected to increase graphite content and achieve uniform drying shrinkage. The 3D-printed greenware is sintered at a high temperature of 1000°C to obtain a composite clay with shape stability and functional diversity. Notably, the 3D-printed H8B2 clay with 60% infill density exhibits a compressive strength of 3.5 MPa, resistivity of 3.77 Ω·cm and a surface resistance of 108.63 Ω. Based on the formability and high conductivity of HB composites, multifunctional 3D-printed lightweight products with high specific surface areas and complex shape designs play a crucial role in fields such as thermal management, electromagnetic protection, electrolytic hydrogen, adsorption filtration, and energy batteries. Bentonite Graphite 3D-printing Conductivity Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction In the field of materials science, multifunctional composite materials have attracted extensive attention in recent years due to their unique performance advantages [ 1 – 3 ]. The functions of these materials mainly focus on electrical and thermal conduction, and play an important role in many application scenarios such as aerospace [ 4 , 5 ], automotive manufacturing [ 6 , 7 ], construction facilities [ 8 , 9 ], energy conversion [ 10 , 11 ], and marine engineering [ 12 , 13 ]. Currently, the common reinforcing materials are mostly carbon-based inorganic materials, which have characteristics such as good electrical [ 14 – 16 ], thermal conductivity [ 17 – 19 ], high temperature [ 20 , 21 ], corrosion resistance [ 22 , 23 ], and large specific surface area [ 24 , 25 ]. Among them, graphite has the highest cost-performance [ 26 , 27 ]. On the other hand, the matrix materials are mainly divided into three categories: polymers [ 28 , 29 ], metals [ 30 , 31 ], and inorganics [ 5 , 6 ]. They play the roles of bonding and shaping in composite materials. However, the insufficient heat resistance of polymer matrices and the poor corrosion resistance of metal matrices limit the application of composite materials under extreme working conditions [ 32 – 35 ]. In addition, there are significant differences in physical properties such as thermal expansion coefficient and elastic modulus between the above two matrices and carbon-based inorganic materials. The component characteristic mismatch leads to weak interface contact, poor modulus adaptation, and local stress concentration, and ultimately reduces the shape stability of composite materials. Inspired by pencil leads, when selecting inorganic matrices such as cement [ 35 , 37 ], ceramics [ 6 , 13 ], and carbon materials [ 4 , 38 ], we have noticed a water absorbing expansive clay, namely sodium bentonite (NaBent) [ 39 , 40 ]. Pencil leads are mainly composed of graphite and clay, and their common specifications are HB and 2B. H represents hardness (Hard), and B represents softness (Black). The graphite contents of HB and 2B pencil leads are 60 ~ 70 wt% and 70 ~ 80 wt% respectively. This high graphite content endows pencil leads with good conduction properties. Therefore, we plan to combine NaBent with graphite, and prepare a multifunctional composite material similar to pencil lead through the clay sintering process (800 ~ 1100°C) [ 41 ]. In this way, the composite materials have not only good thermostability and corrosion resistance, but also adjustable conductivity and mechanical properties. To further enhance the performance of bentonite-graphite (HB) composites, we employ 3D printing technology (direct ink writing, DIW) to increase the macroscopic contact surface. The HB composite inks are required to have good fluidity [ 42 , 43 ]. Hence, water is used as a solvent to disperse the particulate Nabent for 3D-printing. However, there are complex and reversible interactions between polar water molecules and high negatively charged NaBent.This leads to the lack of reproducibility in the preparation and shaping of ink [ 3 , 4 ]. To solve the problems of unstable consistency and dry fracture, ammonium sulfate (AS) solution is selected to modify NaBent through cation exchange. The modified sodium bentonite (NaBent-AS) is beneficial for colloidal stability and greenware formability [ 3 ]. On this basis, the component content, printing design, and sintering condition are further adjusted to evaluate the printability, physical properties, and functional performance of HB composites. This study is expected to develop a series of 3D-printable composite inks to meet flexible structural design and multifunctional application scenarios. 2. Experimental 2.1. Materials Flake graphite (1250 mesh, thermal conductivity 151 W·m − 1 ·K − 1 ) was provided by Qinghe Canon Metal Materials Co., Ltd. Sodium bentonite (NaBent, 1250 mesh) was purchased by Lingshou Dehang Mineral Products Co., Ltd. The composition of NaBent included Al 2 O 3 (16.64%), SiO 2 (64.36%), Fe 2 O 3 (4.93%), K 2 O (1.57%), MgO (1.69%), Na 2 O (1.02%), TiO 2 (0.66%), and CaO (0.88%). (NH 4 ) 2 SO 4 (AS), CuSO 4 , and NaOH, were purchased from Shandong Keyuan Biochemical Co., Ltd. Ammonium sulfate (AS), copper sulfate, and sodium hydroxide were purchased from Shandong Keyuan Chemical Co., Ltd. 2.2. Sample preparation NaBent and graphite were mixed in weight ratios of 10:0, 9:1, 8:2, 7:3, 6:4, 5:5, and 4:6. AS solution was added to the solid mixtures and stirred to prepare 3D-printable inks. These inks were printed using a DIW printer (LB-T180, Shenzhen Luobubu Technology Co., Ltd., China) with a nozzle inner diameter of 0.86 mm, and an printing speed of 20 mm·s − 1 . The prepared 3D-printed samples were dried for 24 h in a 37°C oven (DHG-9053A, Shanghai Jing Hong Laboratory Instrument Co.,Ltd., China). The dried samples were sintered in a tube furnace (CTF 12/100/900, Verder Shanghai Instruments and Equipment Co., Ltd., China) from 25 to 1000°C, with a heating rate of 5°C·min − 1 , and a holding time of 2 h. The preparation process of 3D-printed HB composites is shown in Fig. S1 . 2.3. Sample characterization The microstructure of NaBent before and after AS modification, and the microstructure and elemental analysis of HB composites after sintering were observed by a scanning electron microscopy (FE-SEM, JSM-7800F, Japan) at 1000× magnification. The surface elements of above materials were analyzed by energy dispersive X-ray spectrometer (EDS, XMax50 spectrometer, Oxford Instruments, UK). The water contact angle was measured by a contact angle goniometer (JYSP-360, Beijing Quality Precision Instrument Co., Ltd., China) through the sessile drop method. The Zeta potential of NaBent before and after AS modification was tested using a Zeta potential analyzer (Zetasizer 3000HSA, Malvern Instruments Ltd., UK). The rheological properties of HB composite inks were tested using a rotational rheometer (ARES-G2, McMurry Tekt Instruments (Shanghai) Co., Ltd., China), and the temperature of 25°C, the angular velocity range from 0.01 to 1000 rad·s − 1 . The N 2 adsorption-desorption curves of NaBent-AS, H8B2, and H6B4 samples were tested using a pore and specific surface area analyzer (TriStar 3020, Micromeritics Insrument Corporation, USA). The pore size distribution of the powders was obtained using the BJH (Barrett-Joyner-Halenda) method. The thermal weight loss of HB composites and their components was tested using a thermogravimetric analyzer (Q500, TA Instruments, USA) with the heating range from 50 to 800°C at a heating rate of 10°C·min − 1 under N 2 atmosphere. The functional group characteristic peaks of NaBent before and after AS modification, as well as HB composites at sintering temperatures, were tested using an Fourier transform infrared spectroscopy (FT-IR, Spectrum II, Japan) with the scanning range from 400 to 4000 cm − 1 . Then the modified solution collected was coated on a KBr window (HW-7A, Φ30×3 mm 3 , Hench). The crystal structure of NaBent before and after AS modification, as well as HB composites with sintering temperatures and graphite contents, were tested using an X-ray diffractometer (XRD-700S, Rigaku Corporation, Japan) with a Cu target (36 kV, 25 mA), scanned at 2 θ from 5° to 80°. The 60%-infilled 3D-printed samples (30×30×13 mm 3 , after sintering) with three graphite contents were tested using a mechanical material testing machine (CMT4503, Shandong Wanchen Testing Machine Co., Ltd., China) with 5-kN sensor at room temperature. The compression speed was 2 mm·min − 1 . The Yeoh model was adopted as a mathematical model for the compression stress-strain curves by the finite element analysis system (ABAQUS software) [ 61 ]. The heat transfer performance of 3D-printed HB samples was tested using an infrared thermal imager (UTi320E, Uni Trend Technologies Co., Ltd., China) at intervals of 20 s. The electrical resistivity of HB composites were tested by using a digital four-point probe resistivity tester (ST2253, Suzhou Jingge Electronic Co., Ltd., China). The surface resistance of samples were tested by using a multimeter (34461A, SCHDEVS Technology (Shenzhen) Co., Ltd., China), and the average data of 20 tests was calculated. 3D-printed H8B2 sample was electroplated through a 75 g·L − 1 CuSO 4 solution at a voltage of 15 V. The resistance values of the sample surfaces before and after electroplating were tested by the same multimeter. The electromagnetic shielding function of a 3D-printed H8B2 Ferrari-like cage was showed by preventing handheld Tesla coils from lighting up a LED light bulb. The Joule heating performance of H8B2 composites at different voltages was generated by a DC power supply system (SPPS-C1203, Shenzhen Kuaiqu Electronics Co., Ltd., China). The electrolytic water function of 3D-printed H8B2 electrodes was also showed by applying a voltage of 6 V. The conductive liquid was a 30wt% sodium hydroxide solution. The gas generation states of hydrogen and oxygen were observed. 3. Results and discussion The main component of NaBent is montmorillonite. Its interlayer structure is composed of two tetrahedral layers sandwiching an octahedral layer, as shown in Fig. 1 a. This layer structure endows montmorillonite with strong water absorption and high expansibility [ 44 ]. After being immersed in AS solution, the layer space provides sufficient sites for the cation exchange between sodium ions (Na + ) and ammonium ions (NH 4 + ) [ 45 , 46 ]. Since NH 4 + radius (148 pm) is larger than Na + radius (101 pm), the interlayer distance of montmorillonite was increased, resulting in improved particle dispersion [ 3 ]. From a microscopic perspective, the particle agglomeration phenomenon of NaBent was reduced, as shown in Fig. 1 b. NaBent and H8B2 samples were prepared independently. The NaBent was mixed with either water (NaBent-H 2 O) or 8 wt% AS solution (NaBent-AS), and the H8B2 was prepared in the same manner, as shown in Fig. 1 d-g. It was found that the AS solution facilitated uniform shrinkage of the HB samples. The shrinkage behavior of NaBent could be influenced by physicochemical effects. A key influencing factor of the physicochemical effects was solution concentration. The 8 wt% AS solution increased the pore water salinity, while pore water is distributed to smaller pores [ 44 ]. Upon water evaporation, (NH 4 ) 2 SO 4 precipitated, causing NaBent particles to aggregate more closely and promoting stronger adhesion among them than in the presence of water (Fig S2 a, b) [ 44 ]. The interaction force between the particle surface and water molecules was also weakened [ 47 ]. From a macroscopic perspective, the surface hydrophilicity of NaBent-AS became weaker. The stable water contact angle of NaBent-AS was 57.5°, and the contact angle of NaBent-H 2 O decreased from 63.6° to 0° (Fig. S2 c). Therefore, the NaBent-AS and H8B2 disc greenware exhibited a uniform shrinkage phenomenon during water evaporation process, avoiding cracking and fragmentation. This provided feasibility for the 3D-printing of NaBent-based composites. In detail, due to the ion exchange between NH 4 + and Na + , an 8 wt% AS solution decreased the Zeta potential value of NaBent from 35.44 to 13.04 mV, as shown in Fig. 1 h. After comparing the viscous state (Fig. S3 a-d), the ink prepared with a mass ratio of NaBent to AS solution of 1:1.2 was most suitable for 3D-printing extrusion. In addition, the printing performance of NaBent with different concentrations of AS solution were shown in Fig. S3 e-g. According to the above mass ratio, the extrusion amounts of the three inks (NaBent-H 2 O, NaBent-AS and H4B6) were controllable, as shown in Fig. 2 a. Remarkably, the AS solution proved more suitable for printing compared to water. Furthermore, the extrusion continuity and shape stability during the 3D-printing process were recorded (Videos S1 and S2). This stable colloidal state indicated the printability of composite inks, and also reflected the modification effect of AS on NaBent. As shown in Fig. 2 b, the rotational rheological viscosity of composite inks decreased with increasing graphite content. But the composite ink still maintained the characteristic that the viscosity decreased as the shear rate increased within a higher graphite content range (~ 60 wt%). 3D-printing technology not only achieved lightweight of finished products, but also provided a larger specific surface area for functional components. The physical adsorption performance on the HB composite powder surfaces depended on NaBent. However, the graphite addition reduced the adsorption performance. The pore size distribution and N 2 adsorption-desorption isotherms of the powders with different graphite contents were shown in Fig. 2 c. These isotherms showed non-smooth protrusion characteristics during relative pressure change, and belonged to Type IV isotherms [ 48 ]. This H3-type hysteresis phenomenon indicated that the material surface has a typical mesoporous structure [ 49 ]. Moreover, the pore size distribution of the tested powders was concentrated in the range from 2 to 50 nm, which is also a typical mesoporous scale. The sintering temperature are crucial indicators for post-processing of the dried 3D-printed composites. AS was the only raw material that is rapidly thermal decomposed at high temperature. For this reason, its thermogravimetric curve was presented separately on the secondary ordinate, as shown in Fig. 3 a. AS started to thermally decompose at 280°C, and the decomposition was most significant at 400°C (Fig. S4 a) [ 50 ]. During the composite preparing and sintering processes, AS not only helped with the early forming of 3D-printing, but also did not leave any residue in the later sintered products. However, most other salt solutions such as NaCl do not have this advantage [ 51 ]. Regarding other component raw materials, flake graphite has the best thermal stability [ 52 ], followed by NaBent. NaBent showed obvious thermal decomposition at the temperature of 100°C and 650°C, with weight losses of 5% and 10%, which correspond to the adsorbed and bound water lost, respectively [ 19 , 53 ]. The continuous heating influence on the chemical structure, crystal structure, and microstructure of the components in the composite materials was analyzed by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray spectroscopy (EDS). Firstly, NaBent-AS exhibited a variety of chemical bonds (Fig. 3 b). The infrared absorption characteristics of AS included: the NH 4 + asymmetric angular deformation vibration with a sharp peak at 1400 cm − 1 [ 54 ], the NH 4 + stretching and asymmetric stretching vibrations, and the hydroxyl group with a strong and broad peak at 3000 ~ 3300 cm − 1 [ 55 ]. The infrared absorption characteristics of NaBent were as follows: the Si-O-Mg absorption peak at 521 cm − 1 , the Si-O-Si sheasymmetric stretching vibration peak at 1029 cm − 1 , the Si-O-Al bending vibration peak at 790 cm − 1 , the H-O-H bending vibration peak of water molecules at 1642 cm − 1 , the H-O-H stretching vibration peak of interlayer water molecules at 3406 cm − 1 , and the hydroxyl stretching vibration peak at 3617 cm − 1 [ 56 , 57 ]. By comparing the characteristic peaks before and after the modification of NaBent, it was found that NaBent-AS only occurred in the form of ion exchange [ 58 ]. In Fig. 3 c, the NaBent crystal structure had the four diffraction peaks (6.78°, 19.8°, 34.7°, and 61.8°) [ 59 ] of montmorillonite. The peakes of AS were observed at 22.8° and 38.7° [ 68 ]. AS had no effect on the NaBent crystal lattice. Moreover, NaBent did not undergo lattice changes in HB composites as the amount of graphite increased, as shown in Fig. 3 d. We set the sintering temperature for NaBent-based composites by referring to the roasting temperature range of pencil lead (800 ~ 1100°C). The crystal lattice and morphology of graphite were not affected at 1000°C (Fig. S4 b-d) [ 52 , 60 ]. After the high-temperature sintering, the infrared absorption characteristic peaks of NH 4 + significantly weakened at 400°C and completely disappeared at 1000°C, as shown in Fig. 3 e. This phenomenon was consistent with the thermogravimetric analysis results of AS. We also found that the O-H and Si-O absorption peaks of NaBent showed signs of weakening at 1000°C. Secondly, the XRD peaks of the HB composites with different sintering temperatures and graphite contents were compared and analyzed to verify the crystal structure changes of silicon oxygen tetrahedra and aluminum oxygen octahedra between NaBent layers. The NaBent crystal lattice changed from 400°C as shown in Fig. 3 f [ 41 ]. Before 800°C, the diffraction peaks (19.8° and 64.7°) of montmorillonite still existed, which demonstrated that the structure of bentonite was not completely destroyed. After sintering at 1000°C, all peaks of montmorillonite disappeared, which demonstrated that the structure of bentonite was completely destroyed [ 41 ]. Therefore, 1000°C was set as the sintering temperature for these NaBent-based composites, promoting the compactness and ceramization of HB composites. During the sintering process, changes in the diffraction peaks between 25.14° and 32.72° indicated that NaBent underwent complex phase transformations (Fig. S5 ) [ 69 ]. Thirdly, the water absorption and expansion characteristics of NaBent contributed to the graphite dispersion in 3D-printable composite inks. The SEM images of sintered HB samples with different graphite content are shown in Fig. 4 a. The drying and sintering processes caused NaBent to agglomerate. This dense agglomeration state is beneficial to the mechanical properties of HB composites. But the mechanical properties were considered to weaken as graphite content. In addition, the proportion of carbon element (graphite) in sintered composites was higher than that of 3D-printable inks, as shown in Fig. 4 b. These two proportions showed a linear relationship ( R 2 = 0.93) (Fig. S6 a). Taking H8B2 composites as an example, NaBent particles agglomerated, graphite distributed around the aggregated particles, and the sintered H8B2 composites showed no obvious microporosity (Fig. 4 a2). However, the dried H8B2 composites (H8B2-d) exhibited pronounced microporosity, as shown in Fig. 4 c. The C and Si element contents of the sintered H8B2 composites were 61.11% and 9.11%, respectively, while the contents of H8B2-d composites were 55.55% and 10.26%, demonstrating that sintering could promote the ceramization of HB composites. During the sintering process, only NaBent’s structure was destroyed, while graphite’s structure remained stable (Fig. S4 c, d). So the SEM images of sintered NaBent-AS (NaBent-AS-s) and dried NaBent-AS (NaBent-AS-d) were shown in Fig. 4 d, e. Due to ceramization, the most particles in NaBent-AS-s were in a molten state and bond together, while NaBent-AS-d consisted mainly of irregular, layered aggregates with rough particle surfaces and blurred boundaries[ 49 , 58 ]. The infill density and graphite content, affected the function performance of HB composites to match various application scenarios. The mechanical stability was an important indicator of the durability of 3D-printed composites. Hollow models with three infill densities (40%, 60%, and 80%) were designed and printed, as shown in Fig. 5 a, b. The 3D model and optical photograph of the printed sample with 60% infill density were presented (Fig. S7). The infill density on the macroscopic structure balanced the compressive performance and conduction function of HB composites. The 60%-infilled 3D-printed samples with three graphite contents were compressed, and the force-displacement curves of their representative samples were compared, as shown in Fig. 5 c. It is evident that the compressive strength of HB composites decreased with increasing graphite content. The inset shows an optical image of the mechanical test of the HB composites. The sample exhibited severe damage at the edges after compression. The data of the mechanical test was the average of five times in each group, and the compression strength of them were fully presented in Fig. 5 d. The H8B2 retains a relatively high strength of 3.08 ± 0.40 MPa, while the strength of H7B3 drops to 1.53 ± 0.30 MPa, and the strength of H6B4 is only 0.44 ± 0.04 MPa. The reason of the strength sharp declination from H8B2 3.08 ± 0.40 MPa to H7B3 1.53 ± 0.30 MPa is as follows. The graphite exhibits a flake-like morphology, with layers primarily bound by van der Waals forces rather than chemical bonds[ 12 , 27 ], making it prone to interlayer sliding. Based on the engineering stress-strain curve segments before stress failure (Fig. S6 b), the true stress-strain curves of each sample (Fig. S6 c) were converted [ 61 ]. In the elastic region of the true stress-strain tensile curve, the fitting curve and material parameters were calculated and obtained, as shown in Fig. 5 e. Among them, C10 was represented as the overall stiffness of the samples. As its value increases, the fitting curve moves up as a whole, while its value decreases and the fitting curve moves down as a whole. C20 was represented as the position of the inflection point on the curve, with an increase in its value causing the inflection point to move forward, a decrease in its value causing the inflection point to move backward. C30 was represented as the slope of the second half of the curve, which increases and steepens, while decreases and flattens. In Fig. 5 e, the C10 and C20 values expressed a stiffness of H8B2 higher than the other two low infill density. The C10 and C20 values of H7B3 and H6B4 were similar, while the C30 values of h8b2 are higher than those of H7B3 and H6B4. The C10, C20, and C30 values from the fitting curves were the material parameters of the Yeoh constitutive model. These parameter information were loaded into each network unit of the compressed product, which is of great value in practical engineering applications. For instance, the re-design of 3D-printed components, the new part development with identical materials, the adjustment of experimental parameters, and the expansion of testing methodologies. Von Mises stress, as an important material mechanics index, was represented by contour lines to indicate stress distribution, enabling analysts to quickly identify the most dangerous areas in the model. In the color stress nephogram as shown in Fig. 5 f, red or yellow represented high pressure, while blue or green represented low pressure. The eight edge points of a compression sample was the most easily damaged area during the compression process. So in many cases, the fracture of the sample was not in the middle. According to the ceramic types and structural designs, the maximum values of Mises equivalent stress in 3D-printed materials were different and had regularity. The Mises value of HB composites varied regularly with the infill density, corresponding to the experimental measurement value. This verified that samples with high infill densitiy have stronger compressive strength under the same compression ratio. According to the experimental data and theoretical prediction, NaBent matrix played a leading role in compressive performance. In terms of the functional performance of HB composites, the thermal conduction effect of graphite was compared and analyzed by switching between the 200°C heating platform and the room-temperature environment. The heating performance of the 60%-infilled 3D-printed samples with three graphite contents and sintered NaBent-AS (NaBent-AS-s) were synchronous, and the heating curves converged at 175°C and 153°C, as shown in Fig. 6 a. There was a “percolation threshold” between graphite content and thermal conductivity [ 62 ]. From Fig. 6 a, it is evident that NaBent-AS-s had rapid heating performance. Graphite enhanced the overall heating performance of the HB composites. After the graphite component was tightly connected in a 3D-network in composites, the thermal conduction performance no longer improved by the additive amount under the condition of a fixed heat source [ 63 ]. This provided the possibility for the HB composites to balance mechanical stability and high-efficiency functionality. From the perspective of thermal management, a higher graphite content was beneficial for rapid cooling, as shown in Fig. 6 b. This is due to the significantly lower thermal conductivity of NaBent (0.15 ~ 0.3 W·m − 1 ·K − 1 ) compared to graphite [ 63 ]. The NaBent-AS-s also had rapid cooling performance. Under high graphite content, the thermal conductivity of the HB composites showed minimal variation. To investigate the reason behind this, we tested the heating and cooling performance of the dried HB composites (NaBent-AS-d, HB-d). In Fig. 6 c, the stable temperatures of H6B4-d and NaBent-AS-d were 153.3°C and 105.7°C, respectively, which were lower than those of H6B4 and NaBent-AS. The reason was that NaBent had not been sintered, and thus it did not transform into ceramics. In Fig. 6 d, during the cooling stage, the temperatures of H6B4-d, H7B3-d, and H8B2-d dropped rapidly from 151.5°C, 138.6°C, and 121.6°C, respectively, to 57.5°C within 120 s, whereas that of NaBent-AS-d decreased from 105.7°C to 50.7°C. These results demonstrated that higher graphite content enhances the heat dissipation of HB-d composites. For the same graphite content, the infill density also affected the heat transfer behavior of the 3D-printed composites in the air, as shown in Fig. 6 e, f. The infill density was independent of the “percolation threshold” and affected the temperature change rate [ 64 ]. Among the three infill density settings, the initial heating rate of the 40%-infilled HB composite was slightly lower, and the final cooling rate was slightly higher. The electrical conductivity of 3D-printed HB composites was evaluated for diverse application scenarios. Figure 7 a shows the resistivities of three samples H8B2, H7B3, and H6B4. As is shown, the resistivities are 3.77, 0.94, and 0.82 Ω·cm, respectively, indicating that the electroconductivity of the composites increases with the graphite content increase. The electric resistance of three composites showed a non-linearly accelerated enhancement as graphite content, rising from 15.98 Ω (H6B4) to 34.39 Ω (H7B3) and then to 108.63 Ω (H8B2), as shown in Fig. 7 b. The graphite content of 20 wt% was close to the stable conductive 3D-network and met the deformation stability of composites. After the H8B2 sample was subjected to an increasing voltage, the measured current value and surface temperature showed a stable linear upward trend (Fig. 7 c-e), with correlation coefficients of 0.9833 (for electric current) and 0.9822 (for surface temperature) respectively. The results demonstrated that the heating/cooling speeds and stable temperatures of H8B2 remain consistent across the four cycles. The stable values were 43.19, 109.85, 195.68, and 305.03°C under 5, 10, 15 and 20 V for 1400 s, respectively, and remain steady. The heat distribution of H8B2 in the heating process at 15 V was observed via thermal imaging. The heat distribution of H8B2 was distributed more evenly across the entire composite. This was primarily attributed to the homogeneous distribution of the graphite in the composite. Compared with other carbon-containing electrothermal materials, the H8B2 composite exhibited superior electric heating performance [ 70 – 78 ], and its rapid heating and cooling capability made it a promising candidate for indoor heating applications [ 74 ]. Moreover, the time of rising to the highest temperature and falling to room temperature was similar. Compared with the heat conduction, the Joule heating had a wider regulation range but longer time [ 39 , 40 ]. Due to the excellent electrical conductivity of these HB composites and the high durability of inorganic material themselves, many existing products could be replaced. For example, a highly adherent copper layer was successfully electroplated on the surface of the H8B2 sample (Fig. S8a, Video S3), and the resulting product with a macroscopic hollow structure and a microscopic mesoporous surface could be used as a ternary catalytic filter element in automobiles [ 65 ]. The 3D-printed lightweight structure construction of the products, mimicking the Faraday cage, could provide electromagnetic shielding (Fig. S8b, Videos S4, S5) and was suitable for high-frequency circuit boards [ 66 ]. In addition, the high conductivity, high specific surface area, and corrosion resistance of composites offered new manufactured parts for battery cathode materials and water electrolysis [ 67 ], as shown in Fig. S8c (Video S6). 4. Conclusion In this paper, a novel multifunctional 3D-printed composite clay was successfully developed for large-scale application under different working conditions. We found that NaBent with an 8% AS solution in a weight ratio of 1:1.2 is the optimal ratio for the matrix of 3D-printable inks. AS improved the drying shrinkage problem of NaBent greenware through ion exchange, and disappeared from the finished clay after high-temperature sintering. NaBent-AS and graphite played their respective roles in maintaining shape stability and enhancing functional efficiency. In addition, the flexible design of 3D-printing further expanded the application potential of HB composites, such as lightweight and high specific surface area. This study provided a basic idea for the manufacturing and application of clay-based 3D-printed products through systematic collaborative innovation of materials, processes, and design. Declarations Conflict of Interest The authors declare no conflict of interest. Funding This work was supported by Basic Research Funds for Undergraduate Universities in Liaoning Province (LJ212410152013), Dalian Science and Technology Innovation Fund (2022JJ12GX030), and State Key Laboratory of Bio-Fibers and EcoTextiles (Qingdao University) (KF2020106). Author Contribution Yumei Gong: Conceptualization, Lead; Project administration, Writing–original draftWei Wang: Data curation, Formal analysis, Methodology, ValidationYujie Duan: Data curation, Formal analysis, InvestigationJi Jia : Formal analysis; MethodologyQianhui Qin: Softwar, VisualizationXiaohang Tuo: Software, Writing–original draft, Writing–review & editing, Lead Acknowledgements The authors acknowledge support from Basic Research Funds for Undergraduate Universities in Liaoning Province (LJ212410152013), Dalian Science and Technology Innovation Fund (2022JJ12GX030), and State Key Laboratory of Bio-Fibers and EcoTextiles (Qingdao University) (KF2020106). Data Availability Statement The data that support the findings of this study are available from the corresponding author upon reasonable request. 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Supplementary Files VideoS1.mp4 VideoS2.mp4 VideoS3.mp4 VideoS4.mp4 VideoS5.mp4 VideoS6.mp4 floatimage1.jpeg Graphical Abstract SupplementaryInformation.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 29 Sep, 2025 Reviews received at journal 28 Sep, 2025 Reviewers agreed at journal 27 Sep, 2025 Reviewers agreed at journal 24 Sep, 2025 Reviewers agreed at journal 23 Sep, 2025 Reviews received at journal 09 Sep, 2025 Reviewers agreed at journal 04 Sep, 2025 Reviewers invited by journal 02 Sep, 2025 Editor assigned by journal 22 Aug, 2025 Submission checks completed at journal 19 Aug, 2025 First submitted to journal 17 Aug, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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(b, c) SEM microstructure of NaBent and NaBent-AS, (d-g) dehydration shrinkage of NaBent and H8B2, and (h) Zeta potential of NaBent before and after modification.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7392366/v1/d840e19524a97e4da3e7f63f.png"},{"id":90832881,"identity":"e44361b4-dfef-4d92-aa7b-1126406e97d6","added_by":"auto","created_at":"2025-09-08 16:59:14","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1377695,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Extrusion state of printable inks within 0, 10, 20, and 30 s. (b) Rheological properties of printable inks with different graphite contents. (c) Pore distribution of composites after drying.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7392366/v1/a6f1328e7fd3b12e9ae8e922.png"},{"id":90833271,"identity":"d504f32b-a9ab-4865-b24d-cd48757ef871","added_by":"auto","created_at":"2025-09-08 17:07:14","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":660785,"visible":true,"origin":"","legend":"\u003cp\u003e(a) TG analysis of raw materials and their composites. (b) FT-IR spectra and (c) XRD profiles of NaBent before and after modification. (d) XRD profiles and (e) FT-IR spectra of high graphite-filled NaBent composites at room temperature different sintering temperatures. (f) NaBent-based composites with different graphite contents sintered at 1000 °C.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7392366/v1/3420b79ba2ffa32db84e67ca.png"},{"id":90833272,"identity":"be374db2-c611-417e-abfb-494d75db4922","added_by":"auto","created_at":"2025-09-08 17:07:14","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":2419914,"visible":true,"origin":"","legend":"\u003cp\u003e(a, b) Microscopic morphology and elemental analysis of HB composites with different contents after sintering at 1000 °C, and (c) dried H8B2 composites (H8B2-d). (d) SEM images of dried and (e) sintered NaBent-AS composites (NaBent-AS-d, NaBent-AS-s ).\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7392366/v1/10166689f03ef6a62f626123.png"},{"id":90832885,"identity":"76a7f1c0-52e4-4a36-981b-4f4f0e89adb6","added_by":"auto","created_at":"2025-09-08 16:59:14","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":885038,"visible":true,"origin":"","legend":"\u003cp\u003e(a) 3D models, (b) Test samples, (c) Force-displacement curves, (d) the compressive strength of the three samples, and the inset is stress nephograms. (e) Stress-strain fitting curves, and (f) Force model of 3D-printed HB composites.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-7392366/v1/aab642fae770549ed696bd8f.png"},{"id":90832884,"identity":"870a9b41-498f-4eb4-a394-6b93abaebbbe","added_by":"auto","created_at":"2025-09-08 16:59:14","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":619416,"visible":true,"origin":"","legend":"\u003cp\u003eTemperature changes of 3D printed HB composites by switching the 200 °C heating platform. (a-d) Graphite contents, (e) and (f) Infill densities.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-7392366/v1/f11d13d9f70465a5a3c53e0e.png"},{"id":90832901,"identity":"6a598b08-2e70-4868-ac06-fca37a850c4a","added_by":"auto","created_at":"2025-09-08 16:59:14","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1147040,"visible":true,"origin":"","legend":"\u003cp\u003eElectrical conductivity of 3D-printed HB composites. (a) Resistivity, (b) Surface resistance, (c) Electric current, (d) Joule heating, (e) Quantitative relationship of temperature and current with applied voltage, and (f) Comparison between stable temperatures of carbon-containing composites under applied voltages (g) Thermal images of H8B2 under 15 V.\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-7392366/v1/0ce4872f8bd7400ecf599e22.png"},{"id":90834770,"identity":"34bde1c3-afc6-4922-9d6b-856663c2cdf3","added_by":"auto","created_at":"2025-09-08 17:31:20","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":11049182,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7392366/v1/698579c7-4ac0-48be-bc6f-836d636ae0a0.pdf"},{"id":90833861,"identity":"de530750-16c9-4914-b66a-55d9f3fa288d","added_by":"auto","created_at":"2025-09-08 17:15:14","extension":"mp4","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":5292166,"visible":true,"origin":"","legend":"","description":"","filename":"VideoS1.mp4","url":"https://assets-eu.researchsquare.com/files/rs-7392366/v1/c2112635fec89e6a03f2d0c5.mp4"},{"id":90833276,"identity":"976bbd98-0087-45bd-8db6-eed078ca4baa","added_by":"auto","created_at":"2025-09-08 17:07:14","extension":"mp4","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":3650652,"visible":true,"origin":"","legend":"","description":"","filename":"VideoS2.mp4","url":"https://assets-eu.researchsquare.com/files/rs-7392366/v1/12052132ae9cf4c9693b3fe3.mp4"},{"id":90833859,"identity":"16148017-451a-41b3-8488-4ea23eeab820","added_by":"auto","created_at":"2025-09-08 17:15:14","extension":"mp4","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":4249086,"visible":true,"origin":"","legend":"","description":"","filename":"VideoS3.mp4","url":"https://assets-eu.researchsquare.com/files/rs-7392366/v1/f6771766726fa017e2c5a582.mp4"},{"id":90832898,"identity":"399178ff-b7d5-4b01-b8e4-84775e64964e","added_by":"auto","created_at":"2025-09-08 16:59:14","extension":"mp4","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":721867,"visible":true,"origin":"","legend":"","description":"","filename":"VideoS4.mp4","url":"https://assets-eu.researchsquare.com/files/rs-7392366/v1/1326c8bd134e7f4a476a54ae.mp4"},{"id":90834373,"identity":"7d8759d2-e694-440d-84b9-c9d0b545f955","added_by":"auto","created_at":"2025-09-08 17:23:15","extension":"mp4","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":781711,"visible":true,"origin":"","legend":"","description":"","filename":"VideoS5.mp4","url":"https://assets-eu.researchsquare.com/files/rs-7392366/v1/51607b15bda8c4f3b14d7b7e.mp4"},{"id":90832907,"identity":"7f32edb6-4fb5-4864-81a6-fe3229598042","added_by":"auto","created_at":"2025-09-08 16:59:14","extension":"mp4","order_by":6,"title":"","display":"","copyAsset":false,"role":"supplement","size":637213,"visible":true,"origin":"","legend":"","description":"","filename":"VideoS6.mp4","url":"https://assets-eu.researchsquare.com/files/rs-7392366/v1/886f66d85f7446bfb4783d0b.mp4"},{"id":90833865,"identity":"8712170f-5e40-4705-940e-c8b4c90695fa","added_by":"auto","created_at":"2025-09-08 17:15:14","extension":"jpeg","order_by":7,"title":"","display":"","copyAsset":false,"role":"supplement","size":603967,"visible":true,"origin":"","legend":"\u003cp\u003eGraphical Abstract\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7392366/v1/ad6a3b2060bdd126b7b07b1e.jpeg"},{"id":90832926,"identity":"47efa3f8-56d1-4dce-b590-49236f25cf3a","added_by":"auto","created_at":"2025-09-08 16:59:15","extension":"docx","order_by":8,"title":"","display":"","copyAsset":false,"role":"supplement","size":5225578,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-7392366/v1/01b82b255b57284f48a39aa0.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"3D-Printed Bentonite-Graphite Composite Materials Inspired by Pencil Lead","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eIn the field of materials science, multifunctional composite materials have attracted extensive attention in recent years due to their unique performance advantages [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. The functions of these materials mainly focus on electrical and thermal conduction, and play an important role in many application scenarios such as aerospace [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], automotive manufacturing [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], construction facilities [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], energy conversion [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], and marine engineering [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Currently, the common reinforcing materials are mostly carbon-based inorganic materials, which have characteristics such as good electrical [\u003cspan additionalcitationids=\"CR15\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], thermal conductivity [\u003cspan additionalcitationids=\"CR18\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e], high temperature [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], corrosion resistance [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], and large specific surface area [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Among them, graphite has the highest cost-performance [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. On the other hand, the matrix materials are mainly divided into three categories: polymers [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e], metals [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e], and inorganics [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. They play the roles of bonding and shaping in composite materials. However, the insufficient heat resistance of polymer matrices and the poor corrosion resistance of metal matrices limit the application of composite materials under extreme working conditions [\u003cspan additionalcitationids=\"CR33 CR34\" citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. In addition, there are significant differences in physical properties such as thermal expansion coefficient and elastic modulus between the above two matrices and carbon-based inorganic materials. The component characteristic mismatch leads to weak interface contact, poor modulus adaptation, and local stress concentration, and ultimately reduces the shape stability of composite materials.\u003c/p\u003e\u003cp\u003eInspired by pencil leads, when selecting inorganic matrices such as cement [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e], ceramics [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], and carbon materials [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e], we have noticed a water absorbing expansive clay, namely sodium bentonite (NaBent) [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. Pencil leads are mainly composed of graphite and clay, and their common specifications are HB and 2B. H represents hardness (Hard), and B represents softness (Black). The graphite contents of HB and 2B pencil leads are 60\u0026thinsp;~\u0026thinsp;70 wt% and 70\u0026thinsp;~\u0026thinsp;80 wt% respectively. This high graphite content endows pencil leads with good conduction properties. Therefore, we plan to combine NaBent with graphite, and prepare a multifunctional composite material similar to pencil lead through the clay sintering process (800\u0026thinsp;~\u0026thinsp;1100\u0026deg;C) [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. In this way, the composite materials have not only good thermostability and corrosion resistance, but also adjustable conductivity and mechanical properties.\u003c/p\u003e\u003cp\u003eTo further enhance the performance of bentonite-graphite (HB) composites, we employ 3D printing technology (direct ink writing, DIW) to increase the macroscopic contact surface. The HB composite inks are required to have good fluidity [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. Hence, water is used as a solvent to disperse the particulate Nabent for 3D-printing. However, there are complex and reversible interactions between polar water molecules and high negatively charged NaBent.This leads to the lack of reproducibility in the preparation and shaping of ink [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. To solve the problems of unstable consistency and dry fracture, ammonium sulfate (AS) solution is selected to modify NaBent through cation exchange. The modified sodium bentonite (NaBent-AS) is beneficial for colloidal stability and greenware formability [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. On this basis, the component content, printing design, and sintering condition are further adjusted to evaluate the printability, physical properties, and functional performance of HB composites. This study is expected to develop a series of 3D-printable composite inks to meet flexible structural design and multifunctional application scenarios.\u003c/p\u003e"},{"header":"2. Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1. Materials\u003c/h2\u003e\u003cp\u003eFlake graphite (1250 mesh, thermal conductivity 151 W\u0026middot;m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) was provided by Qinghe Canon Metal Materials Co., Ltd. Sodium bentonite (NaBent, 1250 mesh) was purchased by Lingshou Dehang Mineral Products Co., Ltd. The composition of NaBent included Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (16.64%), SiO\u003csub\u003e2\u003c/sub\u003e (64.36%), Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (4.93%), K\u003csub\u003e2\u003c/sub\u003eO (1.57%), MgO (1.69%), Na\u003csub\u003e2\u003c/sub\u003eO (1.02%), TiO\u003csub\u003e2\u003c/sub\u003e (0.66%), and CaO (0.88%). (NH\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e (AS), CuSO\u003csub\u003e4\u003c/sub\u003e, and NaOH, were purchased from Shandong Keyuan Biochemical Co., Ltd. Ammonium sulfate (AS), copper sulfate, and sodium hydroxide were purchased from Shandong Keyuan Chemical Co., Ltd.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2. Sample preparation\u003c/h2\u003e\u003cp\u003eNaBent and graphite were mixed in weight ratios of 10:0, 9:1, 8:2, 7:3, 6:4, 5:5, and 4:6. AS solution was added to the solid mixtures and stirred to prepare 3D-printable inks. These inks were printed using a DIW printer (LB-T180, Shenzhen Luobubu Technology Co., Ltd., China) with a nozzle inner diameter of 0.86 mm, and an printing speed of 20 mm\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The prepared 3D-printed samples were dried for 24 h in a 37\u0026deg;C oven (DHG-9053A, Shanghai Jing Hong Laboratory Instrument Co.,Ltd., China). The dried samples were sintered in a tube furnace (CTF 12/100/900, Verder Shanghai Instruments and Equipment Co., Ltd., China) from 25 to 1000\u0026deg;C, with a heating rate of 5\u0026deg;C\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and a holding time of 2 h. The preparation process of 3D-printed HB composites is shown in Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3. Sample characterization\u003c/h2\u003e\u003cp\u003eThe microstructure of NaBent before and after AS modification, and the microstructure and elemental analysis of HB composites after sintering were observed by a scanning electron microscopy (FE-SEM, JSM-7800F, Japan) at 1000\u0026times; magnification. The surface elements of above materials were analyzed by energy dispersive X-ray spectrometer (EDS, XMax50 spectrometer, Oxford Instruments, UK). The water contact angle was measured by a contact angle goniometer (JYSP-360, Beijing Quality Precision Instrument Co., Ltd., China) through the sessile drop method. The Zeta potential of NaBent before and after AS modification was tested using a Zeta potential analyzer (Zetasizer 3000HSA, Malvern Instruments Ltd., UK). The rheological properties of HB composite inks were tested using a rotational rheometer (ARES-G2, McMurry Tekt Instruments (Shanghai) Co., Ltd., China), and the temperature of 25\u0026deg;C, the angular velocity range from 0.01 to 1000 rad\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption curves of NaBent-AS, H8B2, and H6B4 samples were tested using a pore and specific surface area analyzer (TriStar 3020, Micromeritics Insrument Corporation, USA). The pore size distribution of the powders was obtained using the BJH (Barrett-Joyner-Halenda) method. The thermal weight loss of HB composites and their components was tested using a thermogravimetric analyzer (Q500, TA Instruments, USA) with the heating range from 50 to 800\u0026deg;C at a heating rate of 10\u0026deg;C\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e under N\u003csub\u003e2\u003c/sub\u003e atmosphere. The functional group characteristic peaks of NaBent before and after AS modification, as well as HB composites at sintering temperatures, were tested using an Fourier transform infrared spectroscopy (FT-IR, Spectrum II, Japan) with the scanning range from 400 to 4000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Then the modified solution collected was coated on a KBr window (HW-7A, Φ30\u0026times;3 mm\u003csup\u003e3\u003c/sup\u003e, Hench). The crystal structure of NaBent before and after AS modification, as well as HB composites with sintering temperatures and graphite contents, were tested using an X-ray diffractometer (XRD-700S, Rigaku Corporation, Japan) with a Cu target (36 kV, 25 mA), scanned at 2\u003cem\u003eθ\u003c/em\u003e from 5\u0026deg; to 80\u0026deg;. The 60%-infilled 3D-printed samples (30\u0026times;30\u0026times;13 mm\u003csup\u003e3\u003c/sup\u003e, after sintering) with three graphite contents were tested using a mechanical material testing machine (CMT4503, Shandong Wanchen Testing Machine Co., Ltd., China) with 5-kN sensor at room temperature. The compression speed was 2 mm\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The Yeoh model was adopted as a mathematical model for the compression stress-strain curves by the finite element analysis system (ABAQUS software) [\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e]. The heat transfer performance of 3D-printed HB samples was tested using an infrared thermal imager (UTi320E, Uni Trend Technologies Co., Ltd., China) at intervals of 20 s. The electrical resistivity of HB composites were tested by using a digital four-point probe resistivity tester (ST2253, Suzhou Jingge Electronic Co., Ltd., China). The surface resistance of samples were tested by using a multimeter (34461A, SCHDEVS Technology (Shenzhen) Co., Ltd., China), and the average data of 20 tests was calculated. 3D-printed H8B2 sample was electroplated through a 75 g\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e CuSO\u003csub\u003e4\u003c/sub\u003e solution at a voltage of 15 V. The resistance values of the sample surfaces before and after electroplating were tested by the same multimeter. The electromagnetic shielding function of a 3D-printed H8B2 Ferrari-like cage was showed by preventing handheld Tesla coils from lighting up a LED light bulb. The Joule heating performance of H8B2 composites at different voltages was generated by a DC power supply system (SPPS-C1203, Shenzhen Kuaiqu Electronics Co., Ltd., China). The electrolytic water function of 3D-printed H8B2 electrodes was also showed by applying a voltage of 6 V. The conductive liquid was a 30wt% sodium hydroxide solution. The gas generation states of hydrogen and oxygen were observed.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003eThe main component of NaBent is montmorillonite. Its interlayer structure is composed of two tetrahedral layers sandwiching an octahedral layer, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. This layer structure endows montmorillonite with strong water absorption and high expansibility [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. After being immersed in AS solution, the layer space provides sufficient sites for the cation exchange between sodium ions (Na\u003csup\u003e+\u003c/sup\u003e) and ammonium ions (NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e) [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]. Since NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e radius (148 pm) is larger than Na\u003csup\u003e+\u003c/sup\u003e radius (101 pm), the interlayer distance of montmorillonite was increased, resulting in improved particle dispersion [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. From a microscopic perspective, the particle agglomeration phenomenon of NaBent was reduced, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e1\u003c/span\u003eb. NaBent and H8B2 samples were prepared independently. The NaBent was mixed with either water (NaBent-H\u003csub\u003e2\u003c/sub\u003eO) or 8 wt% AS solution (NaBent-AS), and the H8B2 was prepared in the same manner, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e1\u003c/span\u003ed-g. It was found that the AS solution facilitated uniform shrinkage of the HB samples. The shrinkage behavior of NaBent could be influenced by physicochemical effects. A key influencing factor of the physicochemical effects was solution concentration. The 8 wt% AS solution increased the pore water salinity, while pore water is distributed to smaller pores [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. Upon water evaporation, (NH\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e precipitated, causing NaBent particles to aggregate more closely and promoting stronger adhesion among them than in the presence of water (Fig \u003cspan refid=\"MOESM2\" class=\"InternalRef\"\u003eS2\u003c/span\u003ea, b) [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. The interaction force between the particle surface and water molecules was also weakened [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. From a macroscopic perspective, the surface hydrophilicity of NaBent-AS became weaker. The stable water contact angle of NaBent-AS was 57.5\u0026deg;, and the contact angle of NaBent-H\u003csub\u003e2\u003c/sub\u003eO decreased from 63.6\u0026deg; to 0\u0026deg; (Fig. \u003cspan refid=\"MOESM2\" class=\"InternalRef\"\u003eS2\u003c/span\u003ec). Therefore, the NaBent-AS and H8B2 disc greenware exhibited a uniform shrinkage phenomenon during water evaporation process, avoiding cracking and fragmentation. This provided feasibility for the 3D-printing of NaBent-based composites. In detail, due to the ion exchange between NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e and Na\u003csup\u003e+\u003c/sup\u003e, an 8 wt% AS solution decreased the Zeta potential value of NaBent from 35.44 to 13.04 mV, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e1\u003c/span\u003eh.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAfter comparing the viscous state (Fig. \u003cspan refid=\"MOESM3\" class=\"InternalRef\"\u003eS3\u003c/span\u003e a-d), the ink prepared with a mass ratio of NaBent to AS solution of 1:1.2 was most suitable for 3D-printing extrusion. In addition, the printing performance of NaBent with different concentrations of AS solution were shown in Fig. \u003cspan refid=\"MOESM3\" class=\"InternalRef\"\u003eS3\u003c/span\u003e e-g. According to the above mass ratio, the extrusion amounts of the three inks (NaBent-H\u003csub\u003e2\u003c/sub\u003eO, NaBent-AS and H4B6) were controllable, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e2\u003c/span\u003ea. Remarkably, the AS solution proved more suitable for printing compared to water. Furthermore, the extrusion continuity and shape stability during the 3D-printing process were recorded (Videos S1 and S2). This stable colloidal state indicated the printability of composite inks, and also reflected the modification effect of AS on NaBent. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e2\u003c/span\u003eb, the rotational rheological viscosity of composite inks decreased with increasing graphite content. But the composite ink still maintained the characteristic that the viscosity decreased as the shear rate increased within a higher graphite content range (~\u0026thinsp;60 wt%). 3D-printing technology not only achieved lightweight of finished products, but also provided a larger specific surface area for functional components. The physical adsorption performance on the HB composite powder surfaces depended on NaBent. However, the graphite addition reduced the adsorption performance. The pore size distribution and N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherms of the powders with different graphite contents were shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e2\u003c/span\u003ec. These isotherms showed non-smooth protrusion characteristics during relative pressure change, and belonged to Type IV isotherms [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]. This H3-type hysteresis phenomenon indicated that the material surface has a typical mesoporous structure [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. Moreover, the pore size distribution of the tested powders was concentrated in the range from 2 to 50 nm, which is also a typical mesoporous scale.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe sintering temperature are crucial indicators for post-processing of the dried 3D-printed composites. AS was the only raw material that is rapidly thermal decomposed at high temperature. For this reason, its thermogravimetric curve was presented separately on the secondary ordinate, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e3\u003c/span\u003ea. AS started to thermally decompose at 280\u0026deg;C, and the decomposition was most significant at 400\u0026deg;C (Fig. \u003cspan refid=\"MOESM4\" class=\"InternalRef\"\u003eS4\u003c/span\u003ea) [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e]. During the composite preparing and sintering processes, AS not only helped with the early forming of 3D-printing, but also did not leave any residue in the later sintered products. However, most other salt solutions such as NaCl do not have this advantage [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]. Regarding other component raw materials, flake graphite has the best thermal stability [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e], followed by NaBent. NaBent showed obvious thermal decomposition at the temperature of 100\u0026deg;C and 650\u0026deg;C, with weight losses of 5% and 10%, which correspond to the adsorbed and bound water lost, respectively [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe continuous heating influence on the chemical structure, crystal structure, and microstructure of the components in the composite materials was analyzed by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray spectroscopy (EDS). Firstly, NaBent-AS exhibited a variety of chemical bonds (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). The infrared absorption characteristics of AS included: the NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e asymmetric angular deformation vibration with a sharp peak at 1400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e], the NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e stretching and asymmetric stretching vibrations, and the hydroxyl group with a strong and broad peak at 3000\u0026thinsp;~\u0026thinsp;3300 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]. The infrared absorption characteristics of NaBent were as follows: the Si-O-Mg absorption peak at 521 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the Si-O-Si sheasymmetric stretching vibration peak at 1029 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the Si-O-Al bending vibration peak at 790 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the H-O-H bending vibration peak of water molecules at 1642 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the H-O-H stretching vibration peak of interlayer water molecules at 3406 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and the hydroxyl stretching vibration peak at 3617 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e [\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e, \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e]. By comparing the characteristic peaks before and after the modification of NaBent, it was found that NaBent-AS only occurred in the form of ion exchange [\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e]. In Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e3\u003c/span\u003ec, the NaBent crystal structure had the four diffraction peaks (6.78\u0026deg;, 19.8\u0026deg;, 34.7\u0026deg;, and 61.8\u0026deg;) [\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e] of montmorillonite. The peakes of AS were observed at 22.8\u0026deg; and 38.7\u0026deg; [\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e68\u003c/span\u003e]. AS had no effect on the NaBent crystal lattice. Moreover, NaBent did not undergo lattice changes in HB composites as the amount of graphite increased, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e3\u003c/span\u003ed.\u003c/p\u003e\u003cp\u003eWe set the sintering temperature for NaBent-based composites by referring to the roasting temperature range of pencil lead (800\u0026thinsp;~\u0026thinsp;1100\u0026deg;C). The crystal lattice and morphology of graphite were not affected at 1000\u0026deg;C (Fig. \u003cspan refid=\"MOESM4\" class=\"InternalRef\"\u003eS4\u003c/span\u003eb-d) [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e]. After the high-temperature sintering, the infrared absorption characteristic peaks of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e significantly weakened at 400\u0026deg;C and completely disappeared at 1000\u0026deg;C, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e3\u003c/span\u003ee. This phenomenon was consistent with the thermogravimetric analysis results of AS. We also found that the O-H and Si-O absorption peaks of NaBent showed signs of weakening at 1000\u0026deg;C. Secondly, the XRD peaks of the HB composites with different sintering temperatures and graphite contents were compared and analyzed to verify the crystal structure changes of silicon oxygen tetrahedra and aluminum oxygen octahedra between NaBent layers. The NaBent crystal lattice changed from 400\u0026deg;C as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e3\u003c/span\u003ef [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. Before 800\u0026deg;C, the diffraction peaks (19.8\u0026deg; and 64.7\u0026deg;) of montmorillonite still existed, which demonstrated that the structure of bentonite was not completely destroyed. After sintering at 1000\u0026deg;C, all peaks of montmorillonite disappeared, which demonstrated that the structure of bentonite was completely destroyed [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. Therefore, 1000\u0026deg;C was set as the sintering temperature for these NaBent-based composites, promoting the compactness and ceramization of HB composites. During the sintering process, changes in the diffraction peaks between 25.14\u0026deg; and 32.72\u0026deg; indicated that NaBent underwent complex phase transformations (Fig. \u003cspan refid=\"MOESM5\" class=\"InternalRef\"\u003eS5\u003c/span\u003e) [\u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThirdly, the water absorption and expansion characteristics of NaBent contributed to the graphite dispersion in 3D-printable composite inks. The SEM images of sintered HB samples with different graphite content are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e4\u003c/span\u003ea. The drying and sintering processes caused NaBent to agglomerate. This dense agglomeration state is beneficial to the mechanical properties of HB composites. But the mechanical properties were considered to weaken as graphite content. In addition, the proportion of carbon element (graphite) in sintered composites was higher than that of 3D-printable inks, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e4\u003c/span\u003eb. These two proportions showed a linear relationship (\u003cem\u003eR\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.93) (Fig. \u003cspan refid=\"MOESM6\" class=\"InternalRef\"\u003eS6\u003c/span\u003ea). Taking H8B2 composites as an example, NaBent particles agglomerated, graphite distributed around the aggregated particles, and the sintered H8B2 composites showed no obvious microporosity (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e4\u003c/span\u003ea2). However, the dried H8B2 composites (H8B2-d) exhibited pronounced microporosity, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e4\u003c/span\u003ec. The C and Si element contents of the sintered H8B2 composites were 61.11% and 9.11%, respectively, while the contents of H8B2-d composites were 55.55% and 10.26%, demonstrating that sintering could promote the ceramization of HB composites. During the sintering process, only NaBent\u0026rsquo;s structure was destroyed, while graphite\u0026rsquo;s structure remained stable (Fig. \u003cspan refid=\"MOESM4\" class=\"InternalRef\"\u003eS4\u003c/span\u003ec, d). So the SEM images of sintered NaBent-AS (NaBent-AS-s) and dried NaBent-AS (NaBent-AS-d) were shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e4\u003c/span\u003ed, e. Due to ceramization, the most particles in NaBent-AS-s were in a molten state and bond together, while NaBent-AS-d consisted mainly of irregular, layered aggregates with rough particle surfaces and blurred boundaries[\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe infill density and graphite content, affected the function performance of HB composites to match various application scenarios. The mechanical stability was an important indicator of the durability of 3D-printed composites. Hollow models with three infill densities (40%, 60%, and 80%) were designed and printed, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, b. The 3D model and optical photograph of the printed sample with 60% infill density were presented (Fig. S7). The infill density on the macroscopic structure balanced the compressive performance and conduction function of HB composites. The 60%-infilled 3D-printed samples with three graphite contents were compressed, and the force-displacement curves of their representative samples were compared, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e5\u003c/span\u003ec. It is evident that the compressive strength of HB composites decreased with increasing graphite content. The inset shows an optical image of the mechanical test of the HB composites. The sample exhibited severe damage at the edges after compression. The data of the mechanical test was the average of five times in each group, and the compression strength of them were fully presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e5\u003c/span\u003ed. The H8B2 retains a relatively high strength of 3.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.40 MPa, while the strength of H7B3 drops to 1.53\u0026thinsp;\u0026plusmn;\u0026thinsp;0.30 MPa, and the strength of H6B4 is only 0.44\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04 MPa. The reason of the strength sharp declination from H8B2 3.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.40 MPa to H7B3 1.53\u0026thinsp;\u0026plusmn;\u0026thinsp;0.30 MPa is as follows. The graphite exhibits a flake-like morphology, with layers primarily bound by van der Waals forces rather than chemical bonds[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e], making it prone to interlayer sliding.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eBased on the engineering stress-strain curve segments before stress failure (Fig. \u003cspan refid=\"MOESM6\" class=\"InternalRef\"\u003eS6\u003c/span\u003eb), the true stress-strain curves of each sample (Fig. \u003cspan refid=\"MOESM6\" class=\"InternalRef\"\u003eS6\u003c/span\u003ec) were converted [\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e]. In the elastic region of the true stress-strain tensile curve, the fitting curve and material parameters were calculated and obtained, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e5\u003c/span\u003ee. Among them, C10 was represented as the overall stiffness of the samples. As its value increases, the fitting curve moves up as a whole, while its value decreases and the fitting curve moves down as a whole. C20 was represented as the position of the inflection point on the curve, with an increase in its value causing the inflection point to move forward, a decrease in its value causing the inflection point to move backward. C30 was represented as the slope of the second half of the curve, which increases and steepens, while decreases and flattens. In Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e5\u003c/span\u003ee, the C10 and C20 values expressed a stiffness of H8B2 higher than the other two low infill density. The C10 and C20 values of H7B3 and H6B4 were similar, while the C30 values of h8b2 are higher than those of H7B3 and H6B4.\u003c/p\u003e\u003cp\u003eThe C10, C20, and C30 values from the fitting curves were the material parameters of the Yeoh constitutive model. These parameter information were loaded into each network unit of the compressed product, which is of great value in practical engineering applications. For instance, the re-design of 3D-printed components, the new part development with identical materials, the adjustment of experimental parameters, and the expansion of testing methodologies. Von Mises stress, as an important material mechanics index, was represented by contour lines to indicate stress distribution, enabling analysts to quickly identify the most dangerous areas in the model. In the color stress nephogram as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e5\u003c/span\u003ef, red or yellow represented high pressure, while blue or green represented low pressure. The eight edge points of a compression sample was the most easily damaged area during the compression process. So in many cases, the fracture of the sample was not in the middle. According to the ceramic types and structural designs, the maximum values of Mises equivalent stress in 3D-printed materials were different and had regularity. The Mises value of HB composites varied regularly with the infill density, corresponding to the experimental measurement value. This verified that samples with high infill densitiy have stronger compressive strength under the same compression ratio. According to the experimental data and theoretical prediction, NaBent matrix played a leading role in compressive performance.\u003c/p\u003e\u003cp\u003eIn terms of the functional performance of HB composites, the thermal conduction effect of graphite was compared and analyzed by switching between the 200\u0026deg;C heating platform and the room-temperature environment. The heating performance of the 60%-infilled 3D-printed samples with three graphite contents and sintered NaBent-AS (NaBent-AS-s) were synchronous, and the heating curves converged at 175\u0026deg;C and 153\u0026deg;C, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e6\u003c/span\u003ea. There was a \u0026ldquo;percolation threshold\u0026rdquo; between graphite content and thermal conductivity [\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e]. From Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, it is evident that NaBent-AS-s had rapid heating performance. Graphite enhanced the overall heating performance of the HB composites. After the graphite component was tightly connected in a 3D-network in composites, the thermal conduction performance no longer improved by the additive amount under the condition of a fixed heat source [\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e]. This provided the possibility for the HB composites to balance mechanical stability and high-efficiency functionality. From the perspective of thermal management, a higher graphite content was beneficial for rapid cooling, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e6\u003c/span\u003eb. This is due to the significantly lower thermal conductivity of NaBent (0.15\u0026thinsp;~\u0026thinsp;0.3 W\u0026middot;m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) compared to graphite [\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e]. The NaBent-AS-s also had rapid cooling performance. Under high graphite content, the thermal conductivity of the HB composites showed minimal variation. To investigate the reason behind this, we tested the heating and cooling performance of the dried HB composites (NaBent-AS-d, HB-d). In Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e6\u003c/span\u003ec, the stable temperatures of H6B4-d and NaBent-AS-d were 153.3\u0026deg;C and 105.7\u0026deg;C, respectively, which were lower than those of H6B4 and NaBent-AS. The reason was that NaBent had not been sintered, and thus it did not transform into ceramics. In Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e6\u003c/span\u003ed, during the cooling stage, the temperatures of H6B4-d, H7B3-d, and H8B2-d dropped rapidly from 151.5\u0026deg;C, 138.6\u0026deg;C, and 121.6\u0026deg;C, respectively, to 57.5\u0026deg;C within 120 s, whereas that of NaBent-AS-d decreased from 105.7\u0026deg;C to 50.7\u0026deg;C. These results demonstrated that higher graphite content enhances the heat dissipation of HB-d composites. For the same graphite content, the infill density also affected the heat transfer behavior of the 3D-printed composites in the air, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e6\u003c/span\u003ee, f. The infill density was independent of the \u0026ldquo;percolation threshold\u0026rdquo; and affected the temperature change rate [\u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e]. Among the three infill density settings, the initial heating rate of the 40%-infilled HB composite was slightly lower, and the final cooling rate was slightly higher.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe electrical conductivity of 3D-printed HB composites was evaluated for diverse application scenarios. Figure\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e7\u003c/span\u003ea shows the resistivities of three samples H8B2, H7B3, and H6B4. As is shown, the resistivities are 3.77, 0.94, and 0.82 Ω\u0026middot;cm, respectively, indicating that the electroconductivity of the composites increases with the graphite content increase. The electric resistance of three composites showed a non-linearly accelerated enhancement as graphite content, rising from 15.98 Ω (H6B4) to 34.39 Ω (H7B3) and then to 108.63 Ω (H8B2), as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e7\u003c/span\u003eb. The graphite content of 20 wt% was close to the stable conductive 3D-network and met the deformation stability of composites. After the H8B2 sample was subjected to an increasing voltage, the measured current value and surface temperature showed a stable linear upward trend (Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e7\u003c/span\u003ec-e), with correlation coefficients of 0.9833 (for electric current) and 0.9822 (for surface temperature) respectively. The results demonstrated that the heating/cooling speeds and stable temperatures of H8B2 remain consistent across the four cycles. The stable values were 43.19, 109.85, 195.68, and 305.03\u0026deg;C under 5, 10, 15 and 20 V for 1400 s, respectively, and remain steady. The heat distribution of H8B2 in the heating process at 15 V was observed via thermal imaging. The heat distribution of H8B2 was distributed more evenly across the entire composite. This was primarily attributed to the homogeneous distribution of the graphite in the composite. Compared with other carbon-containing electrothermal materials, the H8B2 composite exhibited superior electric heating performance [\u003cspan additionalcitationids=\"CR71 CR72 CR73 CR74 CR75 CR76 CR77\" citationid=\"CR70\" class=\"CitationRef\"\u003e70\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e78\u003c/span\u003e], and its rapid heating and cooling capability made it a promising candidate for indoor heating applications [\u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e74\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eMoreover, the time of rising to the highest temperature and falling to room temperature was similar. Compared with the heat conduction, the \u003cem\u003eJoule\u003c/em\u003e heating had a wider regulation range but longer time [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. Due to the excellent electrical conductivity of these HB composites and the high durability of inorganic material themselves, many existing products could be replaced. For example, a highly adherent copper layer was successfully electroplated on the surface of the H8B2 sample (Fig. S8a, Video S3), and the resulting product with a macroscopic hollow structure and a microscopic mesoporous surface could be used as a ternary catalytic filter element in automobiles [\u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e65\u003c/span\u003e]. The 3D-printed lightweight structure construction of the products, mimicking the Faraday cage, could provide electromagnetic shielding (Fig. S8b, Videos S4, S5) and was suitable for high-frequency circuit boards [\u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e]. In addition, the high conductivity, high specific surface area, and corrosion resistance of composites offered new manufactured parts for battery cathode materials and water electrolysis [\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e67\u003c/span\u003e], as shown in Fig. S8c (Video S6).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn this paper, a novel multifunctional 3D-printed composite clay was successfully developed for large-scale application under different working conditions. We found that NaBent with an 8% AS solution in a weight ratio of 1:1.2 is the optimal ratio for the matrix of 3D-printable inks. AS improved the drying shrinkage problem of NaBent greenware through ion exchange, and disappeared from the finished clay after high-temperature sintering. NaBent-AS and graphite played their respective roles in maintaining shape stability and enhancing functional efficiency. In addition, the flexible design of 3D-printing further expanded the application potential of HB composites, such as lightweight and high specific surface area. This study provided a basic idea for the manufacturing and application of clay-based 3D-printed products through systematic collaborative innovation of materials, processes, and design.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003ch2\u003eConflict of Interest\u003c/h2\u003e\u003cp\u003eThe authors declare no conflict of interest.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e\u003cp\u003eThis work was supported by Basic Research Funds for Undergraduate Universities in Liaoning Province (LJ212410152013), Dalian Science and Technology Innovation Fund (2022JJ12GX030), and State Key Laboratory of Bio-Fibers and EcoTextiles (Qingdao University) (KF2020106).\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eYumei Gong: Conceptualization, Lead; Project administration, Writing\u0026ndash;original draftWei Wang: Data curation, Formal analysis, Methodology, ValidationYujie Duan: Data curation, Formal analysis, InvestigationJi Jia : Formal analysis; MethodologyQianhui Qin: Softwar, VisualizationXiaohang Tuo: Software, Writing\u0026ndash;original draft, Writing\u0026ndash;review \u0026amp; editing, Lead\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e\u003cp\u003eThe authors acknowledge support from Basic Research Funds for Undergraduate Universities in Liaoning Province (LJ212410152013), Dalian Science and Technology Innovation Fund (2022JJ12GX030), and State Key Laboratory of Bio-Fibers and EcoTextiles (Qingdao University) (KF2020106).\u003c/p\u003e\u003ch2\u003eData Availability Statement\u003c/h2\u003e\u003cp\u003eThe data that support the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLi B, Tian H, Li L, Liu W, Zheng Z, Wu N (2024) Graphene\u0026ndash;assisted assembly of electrically and magnetically conductive ceramic nanofibrous aerogels enable multifunctionality. 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Langmuir 38:13584\u0026ndash;13593\u003c/span\u003e\u003c/li\u003e\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":"Bentonite, Graphite, 3D-printing, Conductivity","lastPublishedDoi":"10.21203/rs.3.rs-7392366/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7392366/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTraditional 3D-printed conductive composites face limitations in widespread application due to the weak heat resistance, poor corrosion resistance, and high cost of their matrix material. Inspired by pencil lead, 3D-printable bentonite-graphite (HB) composite inks have been developed. Ammonium sulfate (AS) solution is selected to increase graphite content and achieve uniform drying shrinkage. The 3D-printed greenware is sintered at a high temperature of 1000\u0026deg;C to obtain a composite clay with shape stability and functional diversity. Notably, the 3D-printed H8B2 clay with 60% infill density exhibits a compressive strength of 3.5 MPa, resistivity of 3.77 Ω\u0026middot;cm and a surface resistance of 108.63 Ω. Based on the formability and high conductivity of HB composites, multifunctional 3D-printed lightweight products with high specific surface areas and complex shape designs play a crucial role in fields such as thermal management, electromagnetic protection, electrolytic hydrogen, adsorption filtration, and energy batteries.\u003c/p\u003e","manuscriptTitle":"3D-Printed Bentonite-Graphite Composite Materials Inspired by Pencil Lead","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-08 16:59:09","doi":"10.21203/rs.3.rs-7392366/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-09-29T16:01:23+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-28T18:23:55+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"157010917728361320645541839777265364545","date":"2025-09-28T02:13:50+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"196028550033376999013970023811441610870","date":"2025-09-24T15:32:28+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"71695212836888219327582316980344210892","date":"2025-09-23T06:04:28+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-09T06:11:07+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"241623093402205640824678158140493429465","date":"2025-09-04T04:07:52+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-09-02T22:33:57+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-08-22T16:57:26+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-08-19T07:53:08+00:00","index":"","fulltext":""},{"type":"submitted","content":"Advanced Composites and Hybrid Materials","date":"2025-08-17T12:36:01+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":"1c9ed84b-2bc8-4c2c-82fa-1222342c66ba","owner":[],"postedDate":"September 8th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-01-01T21:38:16+00:00","versionOfRecord":[],"versionCreatedAt":"2025-09-08 16:59:09","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7392366","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7392366","identity":"rs-7392366","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}