FGM of YAG-MgO composites with enhanced thermal conductivity using spark plasma sintering

FGM of YAG-MgO composites with enhanced thermal conductivity using spark plasma sintering | 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 FGM of YAG-MgO composites with enhanced thermal conductivity using spark plasma sintering Michal Sakajio, Leonid Logvin, Gennady E. Shter, Meirav Mann-Lahav, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8327715/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 9 You are reading this latest preprint version Abstract YAG-based materials, widely used as lasing media, offer only moderate thermal conductivity (~10 W/m·K), posing challenges for thermal management in solid-state lasers where pump-induced heat leads to thermal gradients that impair performance. To address this issue, MgO with high thermal conductivity (40–60 W/mK) is proposed as a heat sink material. Direct bonding of MgO to YAG is challenging due to their different thermal expansion coefficients. This study introduces a novel approach using a functionally graded material (FGM) to bond heat conductive MgO/YAG composite to YAG disc via spark plasma sintering (SPS). It was found that a YAG/MGO composite with 50 wt.% MgO led to an improvement of over 50% in thermal conductivity between room temperature and 200 °C. The influence of FGM architecture, layer composition and thickness, and sintering parameters on sample integrity and microstructure was systematically investigated. Through optimization, a dense FGM structure with step-gradient concentrations, and robust interfacial bonding was obtained. SEM/EDS analysis confirm a gradual distribution of phases and seamless bonding at the interfaces. These findings indicate that the proposed FGM approach offers an effective means for fabricating smart structures tailored to advanced technological applications. FGM YAG MgO composites spark plasma sintering thermal conductivity Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 1. Introduction Thermal management is a major challenge in continuous and pulsed solid-state lasers. The heat generated in the laser medium by the absorption of pump radiation leads to temperature gradients, changes in the refractive index of the medium, and other undesirable thermal effects. These accompanying effects limit the beam quality and the output power of the laser system [ 1 , 2 ]. The predominant lasing media are yttrium aluminum garnet (YAG)-based materials, which are characterized with relatively moderate thermal conductivity (~ 10W/mK) [ 3 ]. Magnesium oxide (MgO) is an attractive heat-dissipation ceramic material, due to its excellent thermo-mechanical properties, in particular its high thermal conductivity (40–60 W/mK) [ 4 – 7 ]. Recently, the use of MgO as an alternative substrate material to sapphire and silicon for high-temperature microelectromechanical systems [ 5 ] and as substrates for high-temperature superconductor applications [ 6 , 7 ], has been reported. Coupling MgO to YAG using conventional diffusion bonding or glass bonding methods is challenging due to the significant differences in the thermal expansion coefficients of the two ceramics (~ 12·10 − 6 /ºC and ~ 7·10 − 6 /ºC, respectively). Therefore, in this current work, a novel method is proposed to attach MgO to YAG via a functionally graded material (FGM). FGMs refer to composite materials in which a spatial variation in composition and/or microstructure is applied to optimize functional properties of the resulting structure. FGMs have been the subject of extensive research in recent years for a wide range of applications such as energy, aerospace, medicine, semiconductors, etc. [ 8 ]. The continuous gradation of material properties in FGMs helps reduce thermal stresses evolving from differences in thermal expansion coefficients of the composite constituents [ 9 ]. Spark plasma sintering (SPS) is an advanced sintering method for high-density bulk ceramics. In SPS the sample is directly heated by pulsed electric current while it is pressed under a uniaxial load [ 10 , 11 ]. This promotes an effective densification process at lower temperatures and shorter durations, reducing undesirable grain growth. SPS facilitates sintering of FGMs by varying the mixing ratio of the component powders in the depth direction of the die, in a stepwise or continuous manner [ 8 , 9 , 11 ]. Alternatively, it can be employed for diffusion bonding of sintered layers of graded composition to produce an FGM structure. In this study SPS is utilized to form diffusion bonds between YAG disc and a thermally conductive MgO/YAG composite using a functional gradient multilayer MgO/YAG interface. Here, the YAG to MgO bonding is achieved through a layered interface with a step-gradient concentration of the two components. 2. Experimental 2.1. Materials and Processing Commercial MgO (HEFA Rare Earth Canada Co. Ltd., ≥ 99.99% purity) and YAG (Baikowski France, ≥ 99.99% purity) powders were used as the starting material. The MgO and YAG powders have a specific surface area of 10.67 m 2 /g and 5.19 m 2 /g (BET, Flowsorb II 2300, Micromeritics), respectively. The powders were mixed in the desired composition by ball milling with ZrO 2 medium for 30 min in isopropyl alcohol. The milled mixtures were dried at 80°C for 3 h in air, and then manually ground using a mortar and pestle. Sintering was carried out by an SPS apparatus (FCT Systems GmbH, Rauenstein, Germany) in vacuum (3 − 5 mbar). For the sintering of single-composition samples, the powder was deposited into a cylindrical graphite die with an internal diameter of 20 mm. The samples were sintered under a uniaxial pressure of 70 MPa at 1350°C, with a controlled heating rate of 10–50°C/min and a dwell time ranging between 15 to 60 min. The temperature was measured by an optical pyrometer focused on the upper graphite plunger surface. For the FGM samples, a two-stage process was employed as presented in Fig. 1 . The initial stage of the process consisted SPS sintering of the YAG disc, followed by a polishing procedure. During the second phase of the process the pre-sintered YAG disc was placed within a die. Next, graded powder layers, exhibiting a varying YAG/MgO mixing ratio across the die's depth, were introduced. A subsequent sintering process was implemented to achieve the cohesive integration of all layers into a consolidated FGM sintered sample. 2.2. Characterization Methods Microstructure of the starting powder and ceramic samples was detected by field emission scanning electron microscopy (Zeiss XB340VP, Germany). EDS (Bruker XFlash 6, USA) was used for element mapping. The phases of the starting powder and sintered ceramics were characterized by XRD (SmartLab 9 kW, Rigaku, Japan) with Cu Kα radiation, operating at 150–200 mA and 45 kV. Diffraction patterns were obtained using a step of 0.01° and a scanning rate between 3–8 °/min. Thermal diffusivity and conductivity measurements were conducted with LFA 1000 Laser Flash system (NETZSCH, Germany), in the temperature range of RT -200°C. Both, thermal diffusivity ( α ) and specific heat ( C p ) were determined from the temperature vs. time data, and the thermal conductivity was calculated by the equation: k(T)=α(T)∙C P (T)∙ρ . Where, the geometric density of the polished samples was employed as the density (ρ) for the calculation. 3. Results and discussion Secondary electron (SE) images and X-ray diffraction (XRD) patterns of the MgO and YAG initial powders are shown in Fig. 2 and Fig. 3 , respectively. The powders exhibit spherical and uniform-sized particles, possessing a submicron particle size, with the MgO powder being a bit finer. The initial MgO powder mostly comprises the MgO phase, with a minor presence of magnesium hydroxide as a secondary phase. The YAG powder consists mostly YAG (Y 3 Al 5 O 12 ) phase, with diffraction peaks at 26.7⁰ and 30.6⁰ indicating the presence of the minority YAM phase (Y 4 Al 2 O 9 ). To optimize the design of the FGM structure, an initial investigation was conducted on discrete compositional samples. The objective was to determine the influence of YAG/MgO composition on the thermal conductivity, aiming to benefit from the high thermal conductivity of MgO while minimizing the development of thermal stresses at the interfaces between layers. The effective thermal conductivity of composite materials can be simply predicted by the analytical models of Maxwell-Garnett and Bruggeman [ 12 – 14 ]. These approximations are suitable for two phase composites where one of the elements functions as a host medium and the other as an inclusion [ 15 , 16 ]. The Maxwell-Garnet model considers a dilute dispersion of spherical particles embedded in a continuous matrix, where thermal interactions between dispersed particles are negligible. The Bruggeman model is a differential effective medium theory, which assumes a random dispersion of the two components and considers the geometric anisotropy of the particles. The effective thermal conductivity, k e according to the Maxwell-Garnet model is given by Eq. (1) and the Bruggeman model by Eq. (2): $$\:{k}_{e}={k}_{1}\frac{{k}_{2}(1+{V}_{2})-{k}_{1}(2{V}_{2}-2)}{{k}_{1}(2+{V}_{2})+{k}_{2}(1-{V}_{2})}$$ (1) $$\:{V}_{1}\frac{{k}_{1}-{k}_{e}}{{k}_{1}+{2k}_{e}}+{V}_{2}\frac{{k}_{2}-{k}_{e}}{{k}_{2}+2{k}_{e}}=0$$ (2) Where, k 1 and k 2 are the thermal conductivity of the host medium and the dispersed phase, respectively; and V 1 and V 2 are the volume fraction of the host medium and dispersed phase, respectively. The calculated effective thermal conductivities of MgO-YAG composites as a function of YAG content at room temperature (RT) are presented in Fig. 4 (a). The experimental results are also indicated in blue squares. The experimental data demonstrate a consistent trend with the calculated effective k , showing decreasing thermal conductivity with increasing YAG content in the mixture. However, the measured values are slightly lower than those predicted theoretically. This finding can be ascribed to the existence of grain boundaries in the ceramics, which function as interfacial heat resistance [ 13 ]. The measured thermal conductivities as a function of temperature (RT − 200°C) for different MgO-YAG composites are presented in Fig. 4 (b). The reduction in thermal conductivity with increasing temperature arises from intrinsic phonon-phonon interactions, which contribute to enhanced thermal resistance within the ceramic [ 17 ]. For a composite ceramic containing 50 wt.% MgO, an enhancement of over 50% in k is observed across the whole tested temperature range. This outcome indicates that employing a composite material with adequate MgO content may function as an efficient heat sink for YAG. The composite samples were characterized for their microstructure and phase composition by HRSEM/EDS (Fig. 5 ) and XRD (Fig. 6 ), respectively. In Fig. 5 , the bright phase corresponds to YAG, whereas the dark phase represents MgO. At a low YAG content of 10 wt.% (Fig. 5 b), YAG predominantly accumulates along grain boundaries and at triple junctions. In contrast, higher YAG concentrations result in a more uniform phase distribution throughout the microstructure (Fig. 5 c,d). Figure 6 presents XRD patterns of sintered MgO, YAG and MgO/YAG composite ceramics. XRD analysis of the MgO/YAG composites reveals secondary phases of YAlO 3 (YAP) and MgAl 2 O 4 (marked with a triangle and an asterisk, respectively), alongside the expected phases of MgO and YAG (PDF# 00-004-0829 and PDF# 01-073-1370). This finding suggests that a solid-state reaction has taken place, as proposed below: $$\:{Y}_{3}{Al}_{5}{O}_{12}+MgO\:\to\:3YAl{O}_{3}+Mg{Al}_{2}{O}_{4}$$ (3) The EDS data obtained from the MgO 50/YAG 50 wt.% sample also corroborates this finding, indicating the existence of a Y-rich phase which can be attributed to YAP phase, as shown in Fig. 7 . We suggest that the relatively comparable and fine morphology of the raw powders, as shown in Fig. 2 (b) and Fig. 3 (b), promotes the significant solid-state reaction. To address this issue, we examined the effect of dwell time at maximum temperature during the sintering process. Figure 8 shows XRD patterns of MgO 10/YAG 90 wt.% composite samples that were sintered with different dwell time at 1350 ºC (15, 30 and 60 min). Reducing the dwell duration from 60 to 15 minutes markedly diminishes the intensity of the secondary phase peaks, indicating a reduction in the extent of the solid-state reaction. In the next stage, a series of experiments was performed to fabricate FGM assemblies through diffusion bonding of a pre-sintered YAG disc to a five-layered powder structure with the following composition: 1-2-5-7.5-10 wt.% MgO in YAG. The purpose of this series of experiments was to perform an initial optimization of the SPS profile applied in the diffusion bonding process. Details of the experimental setup for sample preparation, the sintering conditions and the obtained results are summarized in Table 1 . Based on the obtained results, the following conclusions can be drawn: (1) a slow cooling rate offers advantages in reducing thermal stress development and crack formation during the cooling stage. This is evident from the first two samples, in which a rapid cooling rate was applied and cracks were observed, attributed to differences in thermal expansion coefficients between the two constituent materials (~ 12·10 − 6 /ºC of MgO vs. ~7·10 − 6 /ºC of YAG). (2) The application of high pressure is beneficial as it improves the contact between layers and facilitates diffusion mechanisms, resulting in improved bonding. This effect is evident in sample 4, where a uniform and defect-free interface was obtained, as illustrated in the cross-sectional images of the sample in Fig. 9 . Although sample 5 was structurally intact, microscopic cross-sectional analysis showed a gap between the YAG disc and the layered assembly, indicating imperfect bonding, presumably caused by insufficient dwell time. Table 1 Summary of experiments of the 5-layered FGM sample, sintering conditions and results: Sample No. Layer thickness [mm] Cooling rate [⁰C/min] Dwell time @ 1350 ⁰C [min] SPS Pressure [MPa] Results 1 0.4 50 15 70 Cracks formed in the YAG disc 2 0.6 50 15 70 Cracks formed across the sample 3 0.4 30 10 70 First layer was delaminated during polishing 4 0.6 30 15 100 Fully intact, defect-free 5 0.4 23 down to 1000 ⁰C, 30 down to 300 ⁰C 8 100 Fully intact, an interfacial gap was detected between the YAG disc and adjacent layers Figure 10 presents SEM images of various regions within the FGM structure of sample 4. The bright phase corresponds to YAG, while the dark phase represents MgO. A uniform distribution of the phases is observed, indicating consistent mixing and a continuous, homogeneous layer composition. Figure 10 (e) displays the interfacial region at the joint with the YAG disc, where a defect-free and seamless bond is observed. The absence of defects or voids implies on high interfacial integrity, indicating a successful diffusion bonding process. In the final stage, a pre-sintered YAG disc was diffusion bonded to 13 sequential powder layers with the following composition: 1-2-5-7.5-10-15-20-25-30-35-40-45-50 wt.% MgO in YAG. Diffusion bonding via SPS was performed at 1350 ºC under 100MPa, with a dwell time of 15 min and a cooling rate of 30 ºC/min. Figure 11 (a) presents a schematic illustration of the sample architecture. Figure 11 (b) displays an image of the sintered sample after SPS, demonstrating its structural integrity. An optical image of the sample cross-section alongside a position-matched EDS line-scan are presented in Fig. 12 . The continuous bonding between the layers and the YAG disc is clearly visible, as well as the gradual variation in elemental composition throughout the sample, as anticipated in FGM structure [ 9 , 18 ]. Figure 13 depicts SE image and corresponding EDS mapping of the FGM sample at the MgO-rich region. At least six distinct layers are visible with comparable composition of 25, 30, 35, 40, 45 and 50 wt.% MgO in YAG. The gradual transition of Mg, Al and Y elements is prominently shown. The smooth compositional gradient implies strong interfacial bonding, further supported by the absence of visible flaws in the interfacial regions. The use of compositionally graded FGMs by SPS processing is shown to be effective for fabricating multilayered structures suited to sophisticated applications. 4. Conclusions In this study, we successfully demonstrated a diffusion bonding process between YAG disc and a multilayered heat conductive FGM structure, with a step-gradient concentration of YAG and MgO, using SPS. The results confirm that SPS enables effective interfacial bonding, characterized by structural integrity, continuity across layers and absence of defects. A composite ceramic containing 50 wt.% MgO in YAG demonstrated a thermal conductivity enhancement exceeding 50% between room temperature and 200°C, suggesting that a ceramic composite with a sufficient MgO content may function as an effective heat sink. Adjusting the sintering profile, particularly by employing a short dwell time, proved effective in minimizing solid-state reactions and preventing the formation of secondary phases of YAlO 3 and MgAl 2 O 4 . The application of high pressure during SPS significantly improves interlayer contact and promotes diffusion, resulting in defect-free interface. A slow cooling rate contributes to reduced thermal stress and minimizes crack formation, as evidenced by the improved mechanical integrity of samples processed under these conditions. Our findings highlight the potential of the proposed approach for integrating multilayered structures in the realization of complex architectures for advanced applications, such as laser systems. Declarations Funding This work was supported by the PAZY Foundation and by the Nancy and Stephan Grand Technion Energy Program. Author Contribution Conceptualization: M.S., L.L., G.S.Methodology: M.S., L.L., G.S.Formal analysis and investigation: M.S., L.L., S.Z. Visualization: M.S., M.M.L. Writing—original draft preparation: M.S.Writing—review and editing: L.L., M.M.L., G.S., G.G Funding acquisition and Resources: M.M.L., G.G., S.Z.Project Administration- M.M.L. Supervision: G.G. References W.A. Clarkson, Thermal effects and their mitigation in end-pumped solid-state lasers, J. Phys. D: Appl. Phys 34 (2001) 2381–2395. J. Sanghera, W. Kim, G. Villalobos, B. Shaw, C. Baker, J. Frantz, B. Sadowski, I. Aggarwal, Ceramic laser materials: Past and present, Optical Materials 35 (2013) 693–699. https://doi.org/10.1016/j.optmat.2012.04.021. Y. Sato, J. Akiyama, T. 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Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 17 Mar, 2026 Reviews received at journal 16 Mar, 2026 Reviews received at journal 11 Mar, 2026 Reviewers agreed at journal 02 Mar, 2026 Reviewers agreed at journal 25 Feb, 2026 Reviewers invited by journal 25 Feb, 2026 Editor assigned by journal 29 Jan, 2026 Submission checks completed at journal 06 Jan, 2026 First submitted to journal 10 Dec, 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8327715","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":597388124,"identity":"ef71a5ab-c962-4734-af6a-8c16599f999e","order_by":0,"name":"Michal Sakajio","email":"","orcid":"","institution":"Technion- Israel Institute of Technology","correspondingAuthor":false,"prefix":"","firstName":"Michal","middleName":"","lastName":"Sakajio","suffix":""},{"id":597388126,"identity":"755331a1-dc6b-49cd-8eac-c0c928144565","order_by":1,"name":"Leonid Logvin","email":"","orcid":"","institution":"Technion- Israel Institute of Technology","correspondingAuthor":false,"prefix":"","firstName":"Leonid","middleName":"","lastName":"Logvin","suffix":""},{"id":597388129,"identity":"c1fbda1d-d00a-4307-8d37-16097008c723","order_by":2,"name":"Gennady E. 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The red and green curves are the standard reflections of MgO (PDF card no. 00-004-0829) and Mg(OH)\u003csub\u003e2\u003c/sub\u003e\u0026nbsp;(PDF card no. 01-071-5972), respectively.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8327715/v1/49ecb82eca80206cf628d5b2.png"},{"id":103749855,"identity":"bf37ed1b-6943-4a8f-9085-18d37066c4b7","added_by":"auto","created_at":"2026-03-02 12:49:20","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":293992,"visible":true,"origin":"","legend":"\u003cp\u003eSE image (a) and XRD pattern (b) of the YAG initial powder. The red curve is the standard reflections of Y\u003csub\u003e3\u003c/sub\u003eAl\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e12 \u003c/sub\u003e(PDF card no. 01-073-1370).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8327715/v1/97d8fa1a8b17784364d284a8.png"},{"id":103749863,"identity":"7f754841-40f2-4b7f-8d66-72e8c8fd0469","added_by":"auto","created_at":"2026-03-02 12:49:20","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":129609,"visible":true,"origin":"","legend":"\u003cp\u003eCalculated thermal conductivity as a function of YAG content in MgO/YAG mixtures obtained by the Maxwell-Garnet (black) and Bruggeman (red) models; Experimental results are indicated in blue squares (a). Measured thermal conductivity as a function of temperature for different MgO/YAG composites (b).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8327715/v1/73905d5a8bd55d49a5895fef.png"},{"id":103749859,"identity":"f62fc952-3faf-4ca3-9957-ace84246e44f","added_by":"auto","created_at":"2026-03-02 12:49:20","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1573991,"visible":true,"origin":"","legend":"\u003cp\u003eSE micrographs of sintered and polished MgO/YAG composites with different concentrations: 100% MgO (a); MgO; MgO 90/YAG 10 wt.% (b); MgO 50/YAG 50 wt.% (c); MgO 10/YAG 90 wt.% (d).\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8327715/v1/6bec22e6740c1e9e778092c0.png"},{"id":104400332,"identity":"48767e84-be0b-497e-aa8b-6b24d5a73897","added_by":"auto","created_at":"2026-03-11 12:09:39","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":106658,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of\u0026nbsp;sintered MgO, YAG and MgO/YAG composites with different concentrations. The standard XRD patterns of YAG (PDF card no. 01-073-1370) and MgO\u0026nbsp;(PDF card no. 00-004-0829) are shown in black at the bottom. Diffraction peaks of MgAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and YAlO\u003csub\u003e3\u003c/sub\u003e secondary phases are indicated by black asterisk and black triangle, respectively.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8327715/v1/c5f8257b585b9d6e5ca6bfef.png"},{"id":104779357,"identity":"b33473ac-c9bd-4f5b-bbf8-7e04b8b4117e","added_by":"auto","created_at":"2026-03-17 07:39:13","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":940964,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructure analysis of MgO 50/YAG 50 wt.% composite ceramic sample: SE micrograph (a), EDS elemental map (b)\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8327715/v1/8f61c51209e210ace27ec59d.png"},{"id":103749858,"identity":"85baaf09-f380-4e67-8259-43cf83f97311","added_by":"auto","created_at":"2026-03-02 12:49:20","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":100748,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of MgO 10/YAG 90 wt.% composite samples sintered with different dwell time at 1350 ⁰C. The standard XRD patterns of YAG (PDF card no. 01-073-1370) and MgO\u0026nbsp;(PDF card no. 00-004-0829) are shown in black at the bottom. Diffraction peaks of MgAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and YAlO\u003csub\u003e3\u003c/sub\u003e secondary phases are indicated by black asterisk and black triangle, respectively.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-8327715/v1/e81a704dc00b7358e323647e.png"},{"id":104399833,"identity":"b4fcc2cd-5abf-481b-9050-e18b60f6109f","added_by":"auto","created_at":"2026-03-11 12:07:48","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":538985,"visible":true,"origin":"","legend":"\u003cp\u003eOptical images showing the cross-sectional structure of FGM sample No. 4, YAG bonded to 5 layers (1-2-5-7.5-10 wt.% MgO in YAG) structure, sintered at 1350 ºC under 100 MPa for 15 min.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-8327715/v1/28de9aa1108e8ddbff659532.png"},{"id":103749867,"identity":"32bc00c9-1292-4aac-b568-171408a4100a","added_by":"auto","created_at":"2026-03-02 12:49:20","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":1523271,"visible":true,"origin":"","legend":"\u003cp\u003eSE micrographs of the FGM sample No. 4 cross-section demonstrating the composition of Individual layers (a-d). Image (e) presents the interfacial region at the bonding site between the layered structure and the YAG disk.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-8327715/v1/a0e313f3b3383aa258bbf101.png"},{"id":103749860,"identity":"2ec889af-930a-49c8-baf1-caa849a78182","added_by":"auto","created_at":"2026-03-02 12:49:20","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":168233,"visible":true,"origin":"","legend":"\u003cp\u003eAn Illustration of the 13-layers FGM architecture, up to 50 wt.% MgO, attached to a pre-sintered YAG disc (a); an image of the sintered sample before cutting and polishing for cross-sectional examination (b).\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-8327715/v1/e79854915b4dbf5e1a622758.png"},{"id":103749866,"identity":"b5bb2e88-dd73-420a-8b59-06f654af9c5b","added_by":"auto","created_at":"2026-03-02 12:49:20","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":305191,"visible":true,"origin":"","legend":"\u003cp\u003eCross sectional image of the FGM sample (up) showing the YAG disc bonded to a 13-layers FGM structure; EDS line-scan across the sample cross-section (bottom) showing the gradual transition in composition.\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-8327715/v1/09833db17059330b2b100bad.png"},{"id":104400210,"identity":"68075ffb-d6ef-439a-a3f3-07c8147ac32a","added_by":"auto","created_at":"2026-03-11 12:09:14","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":1301653,"visible":true,"origin":"","legend":"\u003cp\u003eSE micrograph (a) and EDS elemental mapping (b-f) of the FGM sample cross-section focusing on the MgO-rich region. The percentage of MgO in YAG changes from 50% in layer 1 to 25% in layer 6.\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-8327715/v1/cf66aab7286549c12cfe9c9f.png"},{"id":104835389,"identity":"fdf95e2f-5d8e-43f7-9910-b8a0ad011b00","added_by":"auto","created_at":"2026-03-17 17:44:27","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8112375,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8327715/v1/9e93c1e9-594c-4a13-a5ee-f8f583f68dee.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"FGM of YAG-MgO composites with enhanced thermal conductivity using spark plasma sintering","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThermal management is a major challenge in continuous and pulsed solid-state lasers. The heat generated in the laser medium by the absorption of pump radiation leads to temperature gradients, changes in the refractive index of the medium, and other undesirable thermal effects. These accompanying effects limit the beam quality and the output power of the laser system [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. The predominant lasing media are yttrium aluminum garnet (YAG)-based materials, which are characterized with relatively moderate thermal conductivity (~\u0026thinsp;10W/mK) [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eMagnesium oxide (MgO) is an attractive heat-dissipation ceramic material, due to its excellent thermo-mechanical properties, in particular its high thermal conductivity (40\u0026ndash;60 W/mK) [\u003cspan additionalcitationids=\"CR5 CR6\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Recently, the use of MgO as an alternative substrate material to sapphire and silicon for high-temperature microelectromechanical systems [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] and as substrates for high-temperature superconductor applications [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], has been reported. Coupling MgO to YAG using conventional diffusion bonding or glass bonding methods is challenging due to the significant differences in the thermal expansion coefficients of the two ceramics (~\u0026thinsp;12\u0026middot;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e/\u0026ordm;C and ~\u0026thinsp;7\u0026middot;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e/\u0026ordm;C, respectively). Therefore, in this current work, a novel method is proposed to attach MgO to YAG via a functionally graded material (FGM).\u003c/p\u003e \u003cp\u003eFGMs refer to composite materials in which a spatial variation in composition and/or microstructure is applied to optimize functional properties of the resulting structure. FGMs have been the subject of extensive research in recent years for a wide range of applications such as energy, aerospace, medicine, semiconductors, etc. [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. The continuous gradation of material properties in FGMs helps reduce thermal stresses evolving from differences in thermal expansion coefficients of the composite constituents [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSpark plasma sintering (SPS) is an advanced sintering method for high-density bulk ceramics. In SPS the sample is directly heated by pulsed electric current while it is pressed under a uniaxial load [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. This promotes an effective densification process at lower temperatures and shorter durations, reducing undesirable grain growth. SPS facilitates sintering of FGMs by varying the mixing ratio of the component powders in the depth direction of the die, in a stepwise or continuous manner [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Alternatively, it can be employed for diffusion bonding of sintered layers of graded composition to produce an FGM structure.\u003c/p\u003e \u003cp\u003eIn this study SPS is utilized to form diffusion bonds between YAG disc and a thermally conductive MgO/YAG composite using a functional gradient multilayer MgO/YAG interface. Here, the YAG to MgO bonding is achieved through a layered interface with a step-gradient concentration of the two components.\u003c/p\u003e"},{"header":"2. Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Materials and Processing\u003c/h2\u003e \u003cp\u003eCommercial MgO (HEFA Rare Earth Canada Co. Ltd., \u0026ge;\u0026thinsp;99.99% purity) and YAG (Baikowski France, \u0026ge;\u0026thinsp;99.99% purity) powders were used as the starting material. The MgO and YAG powders have a specific surface area of 10.67 m\u003csup\u003e2\u003c/sup\u003e/g and 5.19 m\u003csup\u003e2\u003c/sup\u003e/g (BET, Flowsorb II 2300, Micromeritics), respectively. The powders were mixed in the desired composition by ball milling with ZrO\u003csub\u003e2\u003c/sub\u003e medium for 30 min in isopropyl alcohol. The milled mixtures were dried at 80\u0026deg;C for 3 h in air, and then manually ground using a mortar and pestle. Sintering was carried out by an SPS apparatus (FCT Systems GmbH, Rauenstein, Germany) in vacuum (3\u0026thinsp;\u0026minus;\u0026thinsp;5 mbar). For the sintering of single-composition samples, the powder was deposited into a cylindrical graphite die with an internal diameter of 20 mm. The samples were sintered under a uniaxial pressure of 70 MPa at 1350\u0026deg;C, with a controlled heating rate of 10\u0026ndash;50\u0026deg;C/min and a dwell time ranging between 15 to 60 min. The temperature was measured by an optical pyrometer focused on the upper graphite plunger surface. For the FGM samples, a two-stage process was employed as presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The initial stage of the process consisted SPS sintering of the YAG disc, followed by a polishing procedure. During the second phase of the process the pre-sintered YAG disc was placed within a die. Next, graded powder layers, exhibiting a varying YAG/MgO mixing ratio across the die's depth, were introduced. A subsequent sintering process was implemented to achieve the cohesive integration of all layers into a consolidated FGM sintered sample.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Characterization Methods\u003c/h2\u003e \u003cp\u003eMicrostructure of the starting powder and ceramic samples was detected by field emission scanning electron microscopy (Zeiss XB340VP, Germany). EDS (Bruker XFlash 6, USA) was used for element mapping. The phases of the starting powder and sintered ceramics were characterized by XRD (SmartLab 9 kW, Rigaku, Japan) with Cu Kα radiation, operating at 150\u0026ndash;200 mA and 45 kV. Diffraction patterns were obtained using a step of 0.01\u0026deg; and a scanning rate between 3\u0026ndash;8 \u0026deg;/min. Thermal diffusivity and conductivity measurements were conducted with LFA 1000 Laser Flash system (NETZSCH, Germany), in the temperature range of RT -200\u0026deg;C. Both, thermal diffusivity (\u003cem\u003eα\u003c/em\u003e) and specific heat (\u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e) were determined from the temperature vs. time data, and the thermal conductivity was calculated by the equation: \u003cem\u003ek(T)=α(T)∙C\u003c/em\u003e\u003csub\u003e\u003cem\u003eP\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e(T)∙ρ\u003c/em\u003e. Where, the geometric density of the polished samples was employed as the density (ρ) for the calculation.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003eSecondary electron (SE) images and X-ray diffraction (XRD) patterns of the MgO and YAG initial powders are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, respectively. The powders exhibit spherical and uniform-sized particles, possessing a submicron particle size, with the MgO powder being a bit finer. The initial MgO powder mostly comprises the MgO phase, with a minor presence of magnesium hydroxide as a secondary phase. The YAG powder consists mostly YAG (Y\u003csub\u003e3\u003c/sub\u003eAl\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e12\u003c/sub\u003e) phase, with diffraction peaks at 26.7⁰ and 30.6⁰ indicating the presence of the minority YAM phase (Y\u003csub\u003e4\u003c/sub\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e9\u003c/sub\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo optimize the design of the FGM structure, an initial investigation was conducted on discrete compositional samples. The objective was to determine the influence of YAG/MgO composition on the thermal conductivity, aiming to benefit from the high thermal conductivity of MgO while minimizing the development of thermal stresses at the interfaces between layers.\u003c/p\u003e \u003cp\u003eThe effective thermal conductivity of composite materials can be simply predicted by the analytical models of Maxwell-Garnett and Bruggeman [\u003cspan additionalcitationids=\"CR13\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. These approximations are suitable for two phase composites where one of the elements functions as a host medium and the other as an inclusion [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. The Maxwell-Garnet model considers a dilute dispersion of spherical particles embedded in a continuous matrix, where thermal interactions between dispersed particles are negligible. The Bruggeman model is a differential effective medium theory, which assumes a random dispersion of the two components and considers the geometric anisotropy of the particles. The effective thermal conductivity, \u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003ee\u003c/em\u003e\u003c/sub\u003e according to the Maxwell-Garnet model is given by Eq.\u0026nbsp;(1) and the Bruggeman model by Eq.\u0026nbsp;(2):\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:{k}_{e}={k}_{1}\\frac{{k}_{2}(1+{V}_{2})-{k}_{1}(2{V}_{2}-2)}{{k}_{1}(2+{V}_{2})+{k}_{2}(1-{V}_{2})}$$\u003c/div\u003e\u003c/div\u003e(1) \u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:{V}_{1}\\frac{{k}_{1}-{k}_{e}}{{k}_{1}+{2k}_{e}}+{V}_{2}\\frac{{k}_{2}-{k}_{e}}{{k}_{2}+2{k}_{e}}=0$$\u003c/div\u003e\u003c/div\u003e(2) \u003c/p\u003e \u003cp\u003eWhere, \u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e are the thermal conductivity of the host medium and the dispersed phase, respectively; and \u003cem\u003eV\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eV\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e are the volume fraction of the host medium and dispersed phase, respectively. The calculated effective thermal conductivities of MgO-YAG composites as a function of YAG content at room temperature (RT) are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (a). The experimental results are also indicated in blue squares. The experimental data demonstrate a consistent trend with the calculated effective \u003cem\u003ek\u003c/em\u003e, showing decreasing thermal conductivity with increasing YAG content in the mixture. However, the measured values are slightly lower than those predicted theoretically. This finding can be ascribed to the existence of grain boundaries in the ceramics, which function as interfacial heat resistance [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. The measured thermal conductivities as a function of temperature (RT \u0026minus;\u0026thinsp;200\u0026deg;C) for different MgO-YAG composites are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (b). The reduction in thermal conductivity with increasing temperature arises from intrinsic phonon-phonon interactions, which contribute to enhanced thermal resistance within the ceramic [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. For a composite ceramic containing 50 wt.% MgO, an enhancement of over 50% in \u003cem\u003ek\u003c/em\u003e is observed across the whole tested temperature range. This outcome indicates that employing a composite material with adequate MgO content may function as an efficient heat sink for YAG.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe composite samples were characterized for their microstructure and phase composition by HRSEM/EDS (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) and XRD (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e), respectively. In Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, the bright phase corresponds to YAG, whereas the dark phase represents MgO. At a low YAG content of 10 wt.% (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb), YAG predominantly accumulates along grain boundaries and at triple junctions. In contrast, higher YAG concentrations result in a more uniform phase distribution throughout the microstructure (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec,d). Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e presents XRD patterns of sintered MgO, YAG and MgO/YAG composite ceramics. XRD analysis of the MgO/YAG composites reveals secondary phases of YAlO\u003csub\u003e3\u003c/sub\u003e (YAP) and MgAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (marked with a triangle and an asterisk, respectively), alongside the expected phases of MgO and YAG (PDF# 00-004-0829 and PDF# 01-073-1370). This finding suggests that a solid-state reaction has taken place, as proposed below:\u003cdiv id=\"Equc\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equc\" name=\"EquationSource\"\u003e\n$$\\:{Y}_{3}{Al}_{5}{O}_{12}+MgO\\:\\to\\:3YAl{O}_{3}+Mg{Al}_{2}{O}_{4}$$\u003c/div\u003e\u003c/div\u003e(3) \u003c/p\u003e \u003cp\u003eThe EDS data obtained from the MgO 50/YAG 50 wt.% sample also corroborates this finding, indicating the existence of a Y-rich phase which can be attributed to YAP phase, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. We suggest that the relatively comparable and fine morphology of the raw powders, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b) and Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b), promotes the significant solid-state reaction. To address this issue, we examined the effect of dwell time at maximum temperature during the sintering process. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e shows XRD patterns of MgO 10/YAG 90 wt.% composite samples that were sintered with different dwell time at 1350 \u0026ordm;C (15, 30 and 60 min). Reducing the dwell duration from 60 to 15 minutes markedly diminishes the intensity of the secondary phase peaks, indicating a reduction in the extent of the solid-state reaction.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn the next stage, a series of experiments was performed to fabricate FGM assemblies through diffusion bonding of a pre-sintered YAG disc to a five-layered powder structure with the following composition: 1-2-5-7.5-10 wt.% MgO in YAG. The purpose of this series of experiments was to perform an initial optimization of the SPS profile applied in the diffusion bonding process. Details of the experimental setup for sample preparation, the sintering conditions and the obtained results are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Based on the obtained results, the following conclusions can be drawn: (1) a slow cooling rate offers advantages in reducing thermal stress development and crack formation during the cooling stage. This is evident from the first two samples, in which a rapid cooling rate was applied and cracks were observed, attributed to differences in thermal expansion coefficients between the two constituent materials (~\u0026thinsp;12\u0026middot;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e/\u0026ordm;C of MgO vs. ~7\u0026middot;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e/\u0026ordm;C of YAG). (2) The application of high pressure is beneficial as it improves the contact between layers and facilitates diffusion mechanisms, resulting in improved bonding. This effect is evident in sample 4, where a uniform and defect-free interface was obtained, as illustrated in the cross-sectional images of the sample in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. Although sample 5 was structurally intact, microscopic cross-sectional analysis showed a gap between the YAG disc and the layered assembly, indicating imperfect bonding, presumably caused by insufficient dwell time.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSummary of experiments of the 5-layered FGM sample, sintering conditions and results:\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample No.\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLayer thickness [mm]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCooling rate [⁰C/min]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eDwell time @ 1350 ⁰C [min]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eSPS Pressure [MPa]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eResults\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e70\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCracks formed in the YAG disc\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e70\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCracks formed across the sample\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e70\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eFirst layer was delaminated during polishing\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eFully intact, defect-free\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e23 down to 1000 ⁰C,\u003c/p\u003e \u003cp\u003e30 down to 300 ⁰C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eFully intact, an interfacial gap was detected between the YAG disc and adjacent layers\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e presents SEM images of various regions within the FGM structure of sample 4. The bright phase corresponds to YAG, while the dark phase represents MgO. A uniform distribution of the phases is observed, indicating consistent mixing and a continuous, homogeneous layer composition. Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e (e) displays the interfacial region at the joint with the YAG disc, where a defect-free and seamless bond is observed. The absence of defects or voids implies on high interfacial integrity, indicating a successful diffusion bonding process.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn the final stage, a pre-sintered YAG disc was diffusion bonded to 13 sequential powder layers with the following composition: 1-2-5-7.5-10-15-20-25-30-35-40-45-50 wt.% MgO in YAG. Diffusion bonding via SPS was performed at 1350 \u0026ordm;C under 100MPa, with a dwell time of 15 min and a cooling rate of 30 \u0026ordm;C/min. Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e (a) presents a schematic illustration of the sample architecture. Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e (b) displays an image of the sintered sample after SPS, demonstrating its structural integrity.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAn optical image of the sample cross-section alongside a position-matched EDS line-scan are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e. The continuous bonding between the layers and the YAG disc is clearly visible, as well as the gradual variation in elemental composition throughout the sample, as anticipated in FGM structure [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e depicts SE image and corresponding EDS mapping of the FGM sample at the MgO-rich region. At least six distinct layers are visible with comparable composition of 25, 30, 35, 40, 45 and 50 wt.% MgO in YAG. The gradual transition of Mg, Al and Y elements is prominently shown. The smooth compositional gradient implies strong interfacial bonding, further supported by the absence of visible flaws in the interfacial regions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe use of compositionally graded FGMs by SPS processing is shown to be effective for fabricating multilayered structures suited to sophisticated applications.\u003c/p\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn this study, we successfully demonstrated a diffusion bonding process between YAG disc and a multilayered heat conductive FGM structure, with a step-gradient concentration of YAG and MgO, using SPS. The results confirm that SPS enables effective interfacial bonding, characterized by structural integrity, continuity across layers and absence of defects. A composite ceramic containing 50 wt.% MgO in YAG demonstrated a thermal conductivity enhancement exceeding 50% between room temperature and 200\u0026deg;C, suggesting that a ceramic composite with a sufficient MgO content may function as an effective heat sink. Adjusting the sintering profile, particularly by employing a short dwell time, proved effective in minimizing solid-state reactions and preventing the formation of secondary phases of YAlO\u003csub\u003e3\u003c/sub\u003e and MgAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e. The application of high pressure during SPS significantly improves interlayer contact and promotes diffusion, resulting in defect-free interface. A slow cooling rate contributes to reduced thermal stress and minimizes crack formation, as evidenced by the improved mechanical integrity of samples processed under these conditions. Our findings highlight the potential of the proposed approach for integrating multilayered structures in the realization of complex architectures for advanced applications, such as laser systems.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThis work was supported by the PAZY Foundation and by the Nancy and Stephan Grand Technion Energy Program.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eConceptualization: M.S., L.L., G.S.Methodology: M.S., L.L., G.S.Formal analysis and investigation: M.S., L.L., S.Z. Visualization: M.S., M.M.L. Writing\u0026mdash;original draft preparation: M.S.Writing\u0026mdash;review and editing: L.L., M.M.L., G.S., G.G Funding acquisition and Resources: M.M.L., G.G., S.Z.Project Administration- M.M.L. Supervision: G.G.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eW.A. Clarkson, Thermal effects and their mitigation in end-pumped solid-state lasers, J. Phys. D: Appl. Phys 34 (2001) 2381\u0026ndash;2395.\u003c/li\u003e\n \u003cli\u003eJ. Sanghera, W. Kim, G. Villalobos, B. Shaw, C. Baker, J. Frantz, B. Sadowski, I. Aggarwal, Ceramic laser materials: Past and present, Optical Materials 35 (2013) 693\u0026ndash;699. https://doi.org/10.1016/j.optmat.2012.04.021.\u003c/li\u003e\n \u003cli\u003eY. Sato, J. Akiyama, T. Taira, Effects of rare-earth doping on thermal conductivity in Y3Al5O12 crystals, Optical Materials 31 (2009) 720\u0026ndash;724. https://doi.org/10.1016/j.optmat.2008.10.040.\u003c/li\u003e\n \u003cli\u003eH. jeong Jang, H. jin Son, S.J. Ha, Y.K. Moon, J. hwan Kim, H.A. Cha, J.J. Choi, B.D. Hahn, J.W. Lee, S.Y. Yoon, J. Lim, C.W. Ahn, An easy approach to realize high thermal conductivity similar to single crystal and low hygroscopicity in magnesia sintered at low temperature, Ceramics International 50 (2024) 16950\u0026ndash;16955. https://doi.org/10.1016/j.ceramint.2024.02.170.\u003c/li\u003e\n \u003cli\u003eJ. Liu, P. Jia, J. Li, F. Feng, T. Liang, W. Liu, J. Xiong, Hydrophilic direct bonding of MgO/MgO for high-temperature MEMS devices, IEEE Access 8 (2020) 67242\u0026ndash;67249. https://doi.org/10.1109/ACCESS.2020.2985750.\u003c/li\u003e\n \u003cli\u003eS. K\u0026ouml;bel, D. Schneider, L.J. Gauckler, Processing of dense MgO substrates for high-temperature superconductors, International Journal of Materials Research 94 (2003) 200\u0026ndash;207. www.hanser.de/mk.\u003c/li\u003e\n \u003cli\u003eC. Chen, T.J. Baker, R. Synowicki, E.L. Tegtmeier, R.T. Forsyth, A.L. Bissell, A.G. Orlowski, J.M. Christopher, E. Savrun, Tape casting and characterizations of MgO ceramics, Journal of the American Ceramic Society 103 (2020) 6666\u0026ndash;6676.\u003c/li\u003e\n \u003cli\u003eR. Madan, S. Bhowmick, A review on application of FGM fabricated using solid-state processes, Advances in Materials and Processing Technologies 6 (2020) 608\u0026ndash;619. https://doi.org/10.1080/2374068X.2020.1731153.\u003c/li\u003e\n \u003cli\u003eY. Shinohara, Functionally Graded Materials, in: Handbook of Advanced Ceramics: Materials, Applications, Processing, and Properties: Second Edition, Elsevier Inc., 2013: pp. 1179\u0026ndash;1187. https://doi.org/10.1016/B978-0-12-385469-8.00061-7.\u003c/li\u003e\n \u003cli\u003eZ.A. Munir, D.V. Quach, M. Ohyanagi, Electric current activation of sintering: A review of the pulsed electric current sintering process, Journal of the American Ceramic Society 94 (2011) 1\u0026ndash;19. https://doi.org/10.1111/j.1551-2916.2010.04210.x.\u003c/li\u003e\n \u003cli\u003eM. Tokita, Progress of Spark Plasma Sintering (SPS) Method, Systems, Ceramics Applications and Industrialization, Ceramics 4 (2021) 160\u0026ndash;198. https://doi.org/10.3390/ceramics4020014.\u003c/li\u003e\n \u003cli\u003eS. Chen, S. Yang, L. Chen, Z. Ma, J. Chen, W. Guo, MgO-Y2O3:Eu composite ceramics with high quantum yield and excellent thermal performance, Journal of the European Ceramic Society 43 (2023) 3553\u0026ndash;3562. https://doi.org/10.1016/j.jeurceramsoc.2023.01.054.\u003c/li\u003e\n \u003cli\u003eH. Zhao, H. Yu, J. Xu, M. Zhang, X. Li, X. Sun, Novel high-thermal-conductivity composite ceramic phosphors for high-brightness laser-driven lighting, Journal of Materials Chemistry C 9 (2021) 10487\u0026ndash;10496. https://doi.org/10.1039/d1tc02202d.\u003c/li\u003e\n \u003cli\u003eJ.P. Angle, Z. Wang, C. Dames, M.L. Mecartney, Comparison of two-phase thermal conductivity models with experiments on dilute ceramic composites, Journal of the American Ceramic Society 96 (2013) 2935\u0026ndash;2942. https://doi.org/10.1111/jace.12488.\u003c/li\u003e\n \u003cli\u003eK. Pietrak, T.S.W. Wi\u0026acute;sniewski, Journal of Power Technologies 95 (1) (2015) 14-24 A review of models for effective thermal conductivity of composite materials, n.d.\u003c/li\u003e\n \u003cli\u003eX.C. Zeng, D.J. Bergman, P.M. Hui, D. Stroud, Effective-medium theory for weakly nonlinear composites, PHYSICAL REVIEW B 38 (1988) 10970\u0026ndash;10973.\u003c/li\u003e\n \u003cli\u003eJ.-F. Bisson, H. Yagi, T. Yanagitani, A. Kaminskii, Y.N. Barabanenkov, K.-I. Ueda, Influence of the Grain Boundaries on the Heat Transfer in Laser Ceramics, OPT REV 14 (2007) 1\u0026ndash;13. https://doi.org/10.1007/s10043-007-0001-9.\u003c/li\u003e\n \u003cli\u003eZ. Wei, S. Liu, D. Liu, J. Wu, H. Xia, B. Wang, Z. Shi, Fabrication and properties of symmetrical W/Si3N4/W functionally graded materials by spark plasma sintering, Journal of Alloys and Compounds 896 (2022) 163077. https://doi.org/10.1016/j.jallcom.2021.163077.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"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":"FGM, YAG, MgO, composites, spark plasma sintering, thermal conductivity","lastPublishedDoi":"10.21203/rs.3.rs-8327715/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8327715/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eYAG-based materials, widely used as lasing media, offer only moderate thermal conductivity (~10 W/m·K), posing challenges for thermal management in solid-state lasers where pump-induced heat leads to thermal gradients that impair performance. To address this issue, MgO with high thermal conductivity (40–60 W/mK) is proposed as a heat sink material. Direct bonding of MgO to YAG is challenging due to their different thermal expansion coefficients. This study introduces a novel approach using a functionally graded material (FGM) to bond heat conductive MgO/YAG composite to YAG disc via spark plasma sintering (SPS). It was found that a YAG/MGO composite with 50 wt.% MgO led to an improvement of over 50% in thermal conductivity between room temperature and 200 °C. The influence of FGM architecture, layer composition and thickness, and sintering parameters on sample integrity and microstructure was systematically investigated. Through optimization, a dense FGM structure with step-gradient concentrations, and robust interfacial bonding was obtained. SEM/EDS analysis confirm a gradual distribution of phases and seamless bonding at the interfaces. These findings indicate that the proposed FGM approach offers an effective means for fabricating smart structures tailored to advanced technological applications.\u003c/p\u003e","manuscriptTitle":"FGM of YAG-MgO composites with enhanced thermal conductivity using spark plasma sintering","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-02 12:49:15","doi":"10.21203/rs.3.rs-8327715/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-03-17T10:50:10+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-17T01:20:15+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-11T17:32:59+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"283470213596841173632952510164157246606","date":"2026-03-02T16:26:05+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"129699017070029227421752511530998872895","date":"2026-02-25T12:53:24+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-25T12:22:37+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-01-29T17:34:20+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-01-07T02:21:04+00:00","index":"","fulltext":""},{"type":"submitted","content":"Advanced Composites and Hybrid Materials","date":"2025-12-10T12:48:04+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":"85c12619-3a0e-4819-912f-b1ceb051e81b","owner":[],"postedDate":"March 2nd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-11T09:40:00+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-02 12:49:15","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8327715","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8327715","identity":"rs-8327715","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}