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[Susumu Meguro](https://orcid.org/0000-0001-9247-4091), Yoshiki Takagiwa, Takashi Kimura, [Takanobu Hiroto](https://orcid.org/0000-0002-6176-5782), Terumi Nakamura

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[Elemental distribution at diffusion bonding interface of FeAlSi thermoelectric material and Cu](https://mdr.nims.go.jp/datasets/982f556f-64e2-43d6-b7aa-23a13ae48766)

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Elemental distribution at diffusion bonding interface of FeAlSi thermoelectric material and CuVol.:(0123456789)Welding in the World https://doi.org/10.1007/s40194-025-02220-1RESEARCH PAPERElemental distribution at diffusion bonding interface of FeAlSi thermoelectric material and CuSusumu Meguro1   · Yoshiki Takagiwa1 · Takashi Kimura1 · Takanobu Hiroto1 · Terumi Nakamura1Received: 25 April 2024 / Accepted: 17 October 2025 © The Author(s) 2025AbstractThermoelectric generation is a promising technology for the realization of a carbon–neutral society. Many kinds of semi-conductors are developed as high-efficiency thermoelectric materials. For example, some types of silicide semiconductors consisting of widely available low-cost elements are known to be promising materials. Among them, FeAlSi thermoelectric (FAST) material is a newly developed derivative of iron-silicide material that consists of commonly available non-toxic elements. This material has an excellent Seebeck coefficient up to temperatures of 500 K. With thermoelectric device fab-rication, soldering is mainly used to connect FAST material and metal electrodes. However, this connection is unstable in a medium temperature range because the solder melts. Therefore, we investigated the diffusion bonding of FAST material and copper plate, which is stable in a medium temperature range. In this study, we determine the bonding temperature range that allows successful bonding. Furthermore, the elemental distribution at the bonding interface was investigated using EPMA. The mutual diffusion of Al and Cu is dominant in the diffusion bonding process. Furthermore, the crystalline phases of the bonded joint area are investigated using X-ray diffraction. The spectra show that the Fe-Si and Cu-Al phases were precipitated in the Cu-diffused area of the FAST material. These results show that the mutual diffusion and eutectoid reaction of Cu and Al contribute to the diffusion bonding process.Keywords  FeAlSi · Cu · Diffusion bonding · EPMA · X-ray diffraction · Eutectoid reaction1  IntroductionThermoelectric generation is a promising technology for the realization of a carbon–neutral society. It is a phenomenon whereby thermal energy is directly exchanged for electrical energy. In addition, thermoelectric generation occurs when a temperature gradient is applied between two connections in an electric circuit consisting of different kinds of material. In this case, the efficiency of the thermoelectric generation is evaluated by a figure of merit ZT, which is given by the following equation: ZT = S2σ∆T/κ. Here, S is the relative Seebeck coefficient, � is the electrical conductivity, ∆T is the temperature gradient and κ is the thermal conductivity.This equation shows that a high electrical conductivity and a low thermal conductivity are necessary if we are to increase the ZT. As materials that satisfy these conditions, bismuth telluride (Bi2Te3) semiconductor and derivative materials have been continuously developed since the  1950 s [1, 2]. In contrast, Bi2Te3 has several serious issues, such as high toxicity and a decrease in the Seebeck coefficient at certain temperatures. As a result, many materials have been developed to overcome these issues. For example, some kinds of silicide semiconductors that consist of low-cost and nontoxic ubiquitous elements have been developed [3]. Iron silicides and derivative semiconductor materials have been investigated in terms of their various properties [4, 5]. In this study, we focused on an iron aluminum silicide (Fe-Al-Si) thermoelectric material, called “FAST” material [6–11]. This material is a derivative of iron silicide. Furthermore, the Seebeck coefficient remains at a certain high value up to 500 K in the medium temperature range, and the p-n conduc-tion type is controllable by controlling the chemical compo-sition of Fe, Al, and Si without dopants [7].Recommended for publication by Commission IX - Behaviour of Metals Subjected to Welding *  Susumu Meguro   MEGURO.Susumu@nims.go.jp1  National Institute for Materials Science, Tsukuba, Ibaraki 305‑0047, Japanhttp://crossmark.crossref.org/dialog/?doi=10.1007/s40194-025-02220-1&domain=pdfhttp://orcid.org/0000-0001-9247-4091  Welding in the WorldWith thermoelectric device fabrication using FAST mate-rial, soldering is used to connect the FAST material with a metal electrode [10]. However, this connection is not stable in the 500 K medium temperature range because the solder melts. Therefore, the aim of this study is to investigate the diffusion bonding of the FAST material and copper electrode plate to overcome this issue. Diffusion bonding is a kind of solid-state bonding that connects materials directly without solder. In recent research into the diffusion bonding of dis-similar materials containing Cu and Al, some interesting characterization studies have been reported. These studies include the enhancement of functional properties using the reaction between elements [12], a detailed investigation of an Al-Cu composite microstructure [13], the improvement of performance by using brazing [14], and the improvement of diamond/Cu diffusion bonding with a Ti interlayer [15]. Therefore, when FAST material and copper are combined, we can expect a complicated structure at the bonding inter-face. Therefore, we investigated the element distribution and undertook a quantitative analysis at the bonding interface using EPMA. Furthermore, X-ray diffraction is carried out to determine the kinds of crystalline phases around the bonding interface. Using these results, we can expect to determine the behavior of the elements and the diffusion bonding mecha-nism in detail.2 � Experimental procedure2.1 � Test materialsIn this study, FeAiSi (FAST) semiconductor thermoelec-tric material and oxygen-free copper plate were prepared as test materials. The composition of the FAST material is n-type Fe0.38Al0.24Si0.38, which was obtained by machine learning [9]. In addition, this material is chemically stable up to at least 973 K [8]. A powder consisting of a mixture of iron, aluminum and silicon was hot-pressed and then cut to a diameter of 10 mm and a length of 10 mm. Moreover, the circular surface of the FAST material was mirror-pol-ished and then subjected to diffusion bonding. Additionally, 99.96% grade oxygen-free and 0.3-mm-thick copper plates were cut to 15 × 12 mm and then also subjected to diffusion bonding.2.2 � Diffusion bondingIn this study, we selected diffusion bonding to obtain a direct bonded joint between the FAST material and a cop-per plate. This method involves solid-state bonding that depends on the diffusion of atoms and creep deformation of materials at the bonding interface. Therefore, no solder is needed to obtain joints. Figure 1 shows a schematic view of the diffusion bonding experiment. The FAST material was heated by thermal conduction from stainless steel holding metal chips because the electrical resistivity of the semiconductor material is too high for the use of RF induction heating. The RF frequency is 40 kHz, which is expected to increase the temperature uniformity in the horizontal direction of the figure. Furthermore, the bond-ing temperatures were 973 K, 1023 K, 1073 K, 1123 K, and 1173 K. The bonding period was 1800s. After bond-ing, the specimen including a bonded joint was cooled in the furnace to room temperature without temperature con-trol. In addition, the loading pressure on the materials was 5.1 MPa, which we realized by using a 3-kg weight and lever with a 10:1 ratio. The vacuum in the chamber was kept at less than 10−4 Pa during the bonding and cooling process to avoid any oxidization of the materials. Further-more, the diffusion bonded joint specimens were cut nor-mal to the bonding interface and subjected to macroscale optical observation and microscale elemental analysis.Fig. 1   Schematic view of diffusion bondingWelding in the World 2.3 � Elemental analysisLine profile, element mapping and quantitative analy-ses were carried out using EPMA. From these analyses, we obtained information about the diffusion length, two-dimensional distribution of elements and composition of precipitated phases. For the element mapping analysis, we employed the EPMA scatter diagram method. This method determines the composition of precipitates at the bonding interface using the X-ray intensity profile of each element [16]. The accelerating voltage was 15 kV in the line analysis, mapping and most of the quantitative analysis. In addition, for quantitative analysis in a detailed structure, we used an accelerating voltage of 10 kV to improve spatial resolution.2.4 � X‑ray diffractionX-ray diffraction was carried out to identify crystalline phases of precipitates around a bonded joint. This method determines composites and alloys that are precipitated dur-ing diffusion bonding.A micro-focus X-ray diffraction experiment with Cu Kα radiation, which was generated by a Cu-rotator anode with an operating voltage and current of 45 kV and 200 mA, respectively, was performed using an X-ray diffractometer, SmartLab (Rigaku Co. Ltd., Tokyo, Japan), equipped with a HyPix-3000-pixel array detector. A micro-X-ray beam with a 200-µm diameter, which is comparable to the diffusion area, was shaped using a collimator, and the camera was set at a distance of 150 mm.3 � Results and discussion3.1 � Diffusion bonding of FeAlSi (FAST) semiconductor material and copper plateThe diffusion bonding of the FeAlSi (FAST) semiconduc-tor material and copper plate was carried out at bonding temperatures of 973, 1023, 1073, 1123, and 1173 K. Table 1 shows the shapes of the joints and optical images of the cross sections. While cutting cross sections of the joints, the FAST material peeled from the copper plate in the specimen fabricated with a 973 K bonding temperature. On the other hand, at bonding temperatures of 1023 K and 1073 K, diffu-sion bonding appeared to be successful. Furthermore, cracks were observed in the FAST material. The large crack at the interface of copper and FAST material in the specimen of 973 K bonding temperature is expected to be an unbonded area. On the other hand, the cracks in the FAST material area of the specimens are expected to be generated after diffusion bonding by the difference between the thermal expansion of copper and the FAST material. However, the optical images of the cross-section show that some of the elements in the FAST material or copper can be expected to diffuse across the bonding interface.Table 1   Overview of specimen and optical image of bonding interface  Welding in the WorldMoreover, the copper plate disappeared at bonding tem-peratures of 1123 K and 1173 K. In these cases, the bonding temperature was above the eutectic point of 1076 K for cop-per and silicon [17]. Therefore, the copper was expected to fuse with the silicon and disappear. These results indicate that the diffusion bonding temperature of FAST material and the copper plate should be below the eutectic point of copper and silicon.3.2 � Line analysis at the bonding interfaceDiffusion bonding experiments revealed diffusion layers at the cross section of the optical images below the 1073 K bonding temperature. However, information about the dif-fused elements across the bonding interface remains unclear. Therefore, we carried out a line analysis using EPMA to determine the kinds of elements and the diffusion length of each element.Figure 2 shows a composition image and the line analysis scanning position at the bonding interface of the 1073 K bonding temperature specimen. This image is divided into four areas according to brightness. The topmost and bot-tommost areas are copper plate base material and FAST material, respectively. Furthermore, the middle two areas both seem to be diffused across the bonding interface. In this study, we must identify the diffusion elements if we are to understand the joining process. For these reasons, the line analysis position is set across all areas in the image.Figure 3 shows the results of the line analysis of the position that is shown in Fig. 2. In this case, Al in the FAST material and Cu diffused across the bonding interface. However, the diffusion of Fe and Si into the copper plate was not confirmed. Moreover, similar line profiles were observed at the interface of the specimens with bonding temperatures of 973 K and 1023 K. Figures 4 and 5 show the Al and Cu intensity profiles across the bonding interface of the specimens with bonding temperatures of 973 K, 1023 K, and 1073 K. The diffusion length of Al into the copper plate increased as the bonding temperature increased. Table 2 shows the diffusion lengths. In this study, we performed the line analysis on three lines at different places on the specimen. These values are the averages of the measured lengths. The relationships between the diffu-sion length L, diffusion coefficient D, and activation energy Q are described by the following equations.(1)L = 2√DtFig. 2   EPMA composition image and line analysis position of FAST/Cu diffusion bonding interface (bonding temperature 1073 K)Fig. 3   Line profile across FAST/Cu diffusion bonding interface (bonding temperature 1073 K)Fig. 4   Al intensity profile across bonding interfaceWelding in the World Here, the t value was fixed at 1800s. D0 is the diffusion constant, Q is the activation energy, R is the gas constant, and T is the absolute temperature.Figure 6 shows an Arrhenius plot for the diffusion coef-ficient of Al from the FAST material to the copper plate. From the plots, D0 and Q in Eq. (2) were approximated by the following values.(2)D = L2∕4t = D0Exp(−Q∕RT)Furthermore, Fig. 7 shows an Arrhenius plot for the dif-fusion coefficient of Cu from the copper plate to the FAST material. D0 and Q in Eq. (2) were approximated by the following values.3.3 � Element mapping at the bonding interfaceAccording to the results of the line analysis reported in the previous section, Cu and Al can be expected to contribute to diffusion bonding. Table 3 shows EPMA mapping images of the composition (CP), Cu, Al, Fe, Si, and O at the bonding interface. The accelerating voltage was 15 kV. The diffusion area of Cu and Al across the bonding interface increased as the bonding temperature increased. These images show the precipitation of some elements. Therefore, we carried out the EPMA scatter diagram method for more detailed phase mapping. In this study, the composition of each phase was estimated using the X-ray intensity correlation of each ele-ment. Table 4 shows phase mapping images of the FAST/copper plate bonding interface obtained using the EPMA scatter diagram method. Different compositions of Cu-Al intermetallic compounds were observed in the Al diffusion area of the copper plate. In addition, the structure of the Cu diffusion area in the FAST material was complicated. Fe-Si precipitation occurred not only in the Cu diffused area but also in the base material. Moreover, multiple phases were D0 = 3.97 × 10−5m2∕sQ = 150 kJ∕molD0 = 6.52 × 10−7m2∕sQ = 113 kJ∕molFig. 5   Cu intensity profile across bonding interfaceTable 2   Diffusion length of Al into copper plate and Cu into FAST materialBonding temperature/K 973 1023 1073Al diffusion length L/µm 51 75 121Cu diffusion length L/µm 63 87 122Fig. 6   Arrhenius plot for diffusion coefficient of Al from FAST mate-rial to copper plateFig. 7   Arrhenius plot for diffusion coefficient of Cu from copper plate to FAST material  Welding in the Worldexpected to be precipitated in the Cu diffusion area in the FAST material.To determine the composition of the phases, we under-took a quantitative analysis at several typical points at the bonding interface. Figure 8 shows the quantitative analy-sis points at the bonding interface of the specimen with a 1073 K bonding temperature. The accelerating voltage was 15 kV. The quantitative values are shown in Table 5. The possible phase was determined using X-ray diffraction, which is described in a subsequent section.In the Al diffusion area of the copper plate, the Cu-Al phase precipitates at points 3 and 4. Moreover, the Al concentrations at points 1 and 2 in the surrounding area were approximately within the solid solution limit [17]. From the results, the structure of the Al diffusion area was estimated to be a eutectoid microstructure including the Al solid solution phase of Cu and a precipitate of Cu-Al composite.In the Cu diffusion area in the FAST material, rela-tively large amounts of precipitation of up to 20 µm were observed. From a quantitative analysis at points 5 and 6 in this area, the precipitation was estimated to be FeSi. Moreover, a similar precipitation was observed in the base material area (points 9 and 10). Here, the FeSi in the base material area was estimated to be generated by a eutectoid reaction during FAST material fabrication before diffusion bonding [11].Another precipitation was observed in the Cu diffusion area of the FAST material. This is part of the lamellar structure and has a similar composition to Cu-Al (points 11 and 12). In this study, all the bonding temperatures were above the eutectoid point of Cu and Cu3Al (840 K) [18]. Therefore, we estimated that the structure consists of pro-eutectoid FeSi and a lamellar structure that con-tains Cu-Al precipitation. Moreover, this lamellar structure was observed only in the Cu diffusion area of the FAST material. It is possible that Fe and Si caused the eutectoid reaction in the area during the diffusion bonding process.To determine the quantitative values in the lamellar structure in detail, a further analysis was carried out by reducing the accelerating voltage to 10 kV. Figure 9 shows a magnified view of the quantitative analysis points at the bonding interface of the specimen with a 1073 K bonding temperature. The island-like area was expected to be the FeSi phase that exists in the base material. Moreover, in the precipitate phase in the lamellar structure (point 14), we can expect to observe the Cu3Al phase, which is gen-erated by a eutectoid reaction of Al in the FAST material and Cu. The matrix of the lamellar structure is expected to exhibit the FeSi phase (points 13, 15, and 16).Table 3   EPMA mapping images of the composition (CP), Cu, Al, Fe, Si, and O in FAST/copper plate bonding interface (bonding tempera-ture 973 K, 1023 K, and 1073 K)Table 4   EPMA phase mapping images at FAST/copper plate bonding interface (bonding temperatures 973 K, 1023 K, and 1073 K)Welding in the World 3.4 � X‑ray diffraction analysis (Supplementary material)In the previous section, the chemical composition of each pre-cipitate around the bonded joint was determined. Furthermore, X-ray diffraction (XRD) analysis was carried out to obtain information about the crystal structure and determine the possible crystalline phase of the precipitates. Figure 10 shows the XRD spectrum of the FAST area, the Cu diffusion area in FAST, and the Al diffusion area in copper. The diffraction spectrum of the FAST material shows peaks of Fe3Al2Si3 (τ1). This result agrees with those of previous studies [11, 19]. In the Cu diffusion area in FAST, this spectrum shows that the ε-FeSi phase was generated in addition to Fe3Al2Si3. No other peaks were identified in this spectrum because the peaks of them were expected to appear extremely close to the peaks of Fe3Al2Si3 and ε-FeSi. Moreover, the spectrum obtained from the Al diffusion copper area shows the Cu, Cu3Al, and γ-Cu9Al4 base material of the plate. Here, the Cu3Al phase appears at high temperature. However, the bonding process is non-equilibrium. Therefore, the high-temperature phase remained until room temperature was reached. These results and estimations are reflected in Table 5. Here, some peaks could not be identified using only diffraction spectra because the peaks from FAST appeared extremely close in diffraction angles to the peaks from other composites. Therefore, the results by EPMA were used to identify them.These results show that the mutual diffusion of Al and Cu triggered a eutectoid reaction in the diffusion area of the bonded joint and enhanced the bonding process.4 � ConclusionsThe diffusion bonding of FeAlSi (FAST) thermoelectric material and copper plate was successful. Moreover, the fol-lowing results were obtained with elemental analysis using EPMA at the diffusion bonding interface.(a) Overall image(b) Magnified image near point 5Fig. 8   Quantitative analysis points at bonding interface of 1073  K bonding temperature specimen. a Overall image; b magnified image near point 5Table 5   Quantitative analysis value near FAST/Cu bonding interface and estimated phasePoint Cu Fe Al Si Possible phase1 81.0 0.2 17.5 1.3 Cu2 80.3 0.1 18.2 1.4 Cu3 72.3 0.0 25.9 1.8 Cu3Al + Cu9Al44 71.4 0.1 26.6 1.9 Cu3Al + Cu9Al45 0.2 49.5 5.2 45.1 e-FeSi6 0.2 49.5 5.1 45.1 e-FeSi7  < 0.1 37.4 26.1 36.5 FAST8  < 0.1 37.2 26.8 35.9 FAST9  < 0.1 49.4 5.2 45.4 e-FeSi10  < 0.1 49.6 5.1 45.2 e-FeSi11 74.0 4.1 20.9 1.0 Cu3Al12 75.3 5.0 18.5 1.2 Cu3Al13 1.3 45.4 2.3 50.9 e-FeSi14 72.0 3.0 24.1 0.8 Cu3Al15 0.9 49.5 3.2 46.4 e-FeSi16 0.9 48.3 2.9 47.9 e-FeSi  Welding in the World1.  Diffusion bonding occurs at bonding temperatures rang-ing from 973 to 1073 K. Moreover, the diffusion bond-ing temperature should be below the eutectic point of Cu and Si.2.  Line analysis and element mapping using EPMA revealed that the mutual diffusion of Al and Cu is domi-nant in the diffusion bonding process. Moreover, the dif-fusion constant and activation energy of these elements were obtained.3.  EPMA quantitative analysis, scatter diagram method, and X-ray diffraction revealed Cu3Al-based precipitation and an ε-FeSi matrix in the lamellar microstructure of the Cu diffusion area of the FAST material. Further-more, the Cu-Al solid solution, γ-Cu9Al4 and Cu3Al phases were observed in copper material. It is possible that the eutectoid reaction of Cu and Al enhances the diffusion bonding process of FAST material and copper.Acknowledgements  The authors thank Koji Nakazato of the Materials Forming Unit in the National Institute for Materials Science (NIMS) for preparing the EPMA and X-ray diffraction specimens.Author contributions  S. Meguro performed the diffusion bonding, evaluated the specimens and wrote the manuscript. Y. Takagawa pro-posed the chemical composition of FAST material and distributed the material. T. Kimura performed the characterizations of specimens using EPMA. T. Hiroto performed the characterizations of specimens using X-ray diffraction. T. Nakamura advised on the whole manuscript. All authors have read and agreed to the manuscript.Funding  A part of this study was supported by Mitsuaki Nishio of the Surface and Bulk Analysis Unit in the National Institute for Materials Science (NIMS) for EPMA analysis.Data availability  Not applicable.(a) Near bonding interface(b) Near base materialFig. 9   Quantitative analysis points at bonding interface of 1073  K bonding temperature specimen (10 kV). a Near bonding interface; b near base materialFig. 10   X-ray diffraction spectra in FAST/Cu diffusion bonded intnt (1073K bonding temperature)Welding in the World Declarations Competing interests  The authors declare no competing interests.Open Access  This article is licensed under a Creative Commons Attri-bution 4.0 International License, which permits use, sharing, adapta-tion, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.References  1.  Goldsmid HJ (2014) Bismuth telluride and its alloys as materials for thermoelectric generation. Materials 7:2577–2592. https://​doi.​org/​10.​3390/​ma704​2577  2.  Wei J, Yang L, Ma Z, Song P, Zhang M, Ma J, Yang F, Wang X (2020) Review of current high-ZT thermoelectric materi-als. J Mater Sci 55:12642–12704. https://​doi.​org/​10.​1007/​s10853-​020-​04949-0  3.  Nozariasbmarz A et al (2017) Thermoelectric silicides: a review. Jpn J Appl Phys 56:05DA04. https://​doi.​org/​10.​7567/​JJAP.​56.​05DA04  4.  Heinrich A et al (2001) Thermoelectric properties of β-FeSi2 sin-gle crystals and polycrystalline β-FeSi2+x thin films. Thin Solid Films 381:287–295. https://​doi.​org/​10.​1016/​S0040-​6090(00)​01758-2  5.  Nishida I (1973) Study of semiconductor-to-metal transition in Mn-doped FeSi2. Phys Rev B 7:2710–2713. https://​doi.​org/​10.​1103/​PhysR​evB.7.​2710  6.  Takagiwa Y, Isoda Y, Goto M, Shinohara Y (2018) Electronic structure and thermoelectric properties of narrow-band-gap inter-metallic compound Al2Fe3Si3. J Therm Anal Calorim 131:281–287. https://​doi.​org/​10.​1007/​s10973-​017-​6621-9  7.  Takagiwa Y, Isoda Y, Goto M, Shinohara Y (2018) Conduction type control and power factor enhancement of the thermoelectric material Al2Fe3Si3. J Phys Chem Solids 118:95–98. https://​doi.​org/​10.​1016/j.​jpcs.​2018.​03.​003  8.  Takagiwa Y, Shinohara Y (2019) A practical appraisal of ther-moelectric materials for use in an autonomous power supply. Scr Mater 172:98–104. https://​doi.​org/​10.​1016/j.​scrip​tamat.​2019.​07.​022  9.  Hou Z, Takagiwa Y, Shinohara Y, Xu Y, Tsuda K (2019) Machine-learning-assisted development and theoretical consideration for the Al2Fe3Si3 thermoelectric material. ACS Appl Mater Inter-faces 11:11545–11554. https://​doi.​org/​10.​1021/​acsami.​9b023​81 10.  Takagiwa Y, Hou Z, Tsuda K, Ikeda T, Kojima H (2021) Fe−Al−Si thermoelectric (FAST) materials and modules: diffusion couple and machine-learning-assisted materials development. ACS Appl Mater Interfaces 13:53346–53354. https://​doi.​org/​10.​1021/​acsami.​1c045​83 11.  Srinithia AK, Sepehri-Amina H, Takagiwa Y, Hono K (2022) Effect of microstructure on the electrical conductivity of p-type Fe–Al–Si thermoelectric materials. J Alloy Compd 903:163835. https://​doi.​org/​10.​1016/j.​jallc​om.​2022.​163835 12.  Yao F, You G, Zeng S, Lu D, Ming Y (2023) Reaction-tunable dif-fusion bonding to multilayered Cu mesh/ZK61 Mg foil composites with thermal conductivity and lightweight synergy. J Mater Sci Technol 139:10–22. https://​doi.​org/​10.​1016/j.​jmst.​2022.​07.​060 13.  Wang C, Chen J, Liang S, Shao W, Yang XY (2023) Microstruc-ture evolution and mechanical properties of CuW/Al interface fabricated by infiltration and vacuum hot-pressing diffusion bond-ing. Vacuum 210:111882. https://​doi.​org/​10.​1016/j.​vacuum.​2023.​111882 14.  Zhang Y, Guo W, Yin CH, Xu YQ, Mei H, Shao TW, Zhu Y, Zhang HQ (2024) The interlayer self-reinforcement and filler re-optimization achieving multiple collaborative enhancement of the brazed superalloy heterogeneous joint. Mater Sci Eng A 903:146694. https://​doi.​org/​10.​1016/j.​msea.​2024.​146694 15.  Hu DC, Wang L, Chen L, Feng S, Wang N, Zhou Y (2023) Thermal conduction and strength of diamond-copper composite sandwich obtained by SPS diffusion bonding with Ti interlayer. J Mater Res Technol 26:8806–8812. https://​doi.​org/​10.​1016/j.​jmrt.​2023.​09.​188 16.  Kimura T, Sugisaki T, Nishida K, Ishikawa N, Tanuma S (2004) Analysis of joining boundary between Ni-P electroless plate and solder by EPMA scatter diagram method (I). J Japan Inst Metals 68:8–13 17.  Okamoto H (2002) Cu-Si (copper-silicon). J Phase Equilib 23:281–282. https://​doi.​org/​10.​1361/​10549​71027​70331​857 18.  Murray J. L, Al-Cu Phase Diagram (1990). ASM alloy phase dia-grams database, P. Villars, editor-in-chief; H. Okamoto and K. Cenzual, section editors; http://​www.​asmin​terna​tional.​org, ASM International, Materials Park, OH, 2016. 19.  Marker MCJ, Skolyszewska-Kühberger B, Effenberger HS, Schmetterer C, Richter KW (2011) Phase equilibria and structural investigations in the system Al-Fe-Si. Intermetallics 19:1919–1929. https://​doi.​org/​10.​1016/j.​inter​met.​2011.​05.​003Publisher's Note  Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.http://creativecommons.org/licenses/by/4.0/https://doi.org/10.3390/ma7042577https://doi.org/10.3390/ma7042577https://doi.org/10.1007/s10853-020-04949-0https://doi.org/10.1007/s10853-020-04949-0https://doi.org/10.7567/JJAP.56.05DA04https://doi.org/10.7567/JJAP.56.05DA04https://doi.org/10.1016/S0040-6090(00)01758-2https://doi.org/10.1016/S0040-6090(00)01758-2https://doi.org/10.1103/PhysRevB.7.2710https://doi.org/10.1103/PhysRevB.7.2710https://doi.org/10.1007/s10973-017-6621-9https://doi.org/10.1016/j.jpcs.2018.03.003https://doi.org/10.1016/j.jpcs.2018.03.003https://doi.org/10.1016/j.scriptamat.2019.07.022https://doi.org/10.1016/j.scriptamat.2019.07.022https://doi.org/10.1021/acsami.9b02381https://doi.org/10.1021/acsami.1c04583https://doi.org/10.1021/acsami.1c04583https://doi.org/10.1016/j.jallcom.2022.163835https://doi.org/10.1016/j.jmst.2022.07.060https://doi.org/10.1016/j.vacuum.2023.111882https://doi.org/10.1016/j.vacuum.2023.111882https://doi.org/10.1016/j.msea.2024.146694https://doi.org/10.1016/j.jmrt.2023.09.188https://doi.org/10.1016/j.jmrt.2023.09.188https://doi.org/10.1361/105497102770331857http://www.asminternational.orghttps://doi.org/10.1016/j.intermet.2011.05.003 Elemental distribution at diffusion bonding interface of FeAlSi thermoelectric material and Cu Abstract 1 Introduction 2 Experimental procedure 2.1 Test materials 2.2 Diffusion bonding 2.3 Elemental analysis 2.4 X-ray diffraction 3 Results and discussion 3.1 Diffusion bonding of FeAlSi (FAST) semiconductor material and copper plate 3.2 Line analysis at the bonding interface 3.3 Element mapping at the bonding interface 3.4 X-ray diffraction analysis (Supplementary material) 4 Conclusions Acknowledgements  References