Bi2Te3 / PbTe segmented thermoelectric device based on TiTe2 contact layer and preparation method of Bi2Te3 / PbTe segmented thermoelectric device

Abstract

The invention relates to the technical field of PbTe / Bi2Te3 segmented thermoelectric device design, and discloses a TiTe2 contact layer-based PbTe / Bi2Te3 segmented thermoelectric device and a preparation method thereof, the TiTe2 contact layer-based PbTe / Bi2Te3 segmented thermoelectric device comprises a three-layer structure connected in sequence, the three-layer structure comprises a Bi2Te3 layer, a TiTe2 contact layer and a PbTe layer, the TiTe2 contact layer is arranged between the Bi2Te3 layer and the PbTe layer, and the PbTe layer is arranged between the Bi2Te3 layer and the PbTe layer. The TiTe2 contact layer forms buffering among all interfaces, interface thermal stress, element diffusion and contact resistance are reduced, and meanwhile connection of PbTe and Bi2Te3 materials is achieved.

Description

Technical Field

[0001] This invention relates to the field of PbTe / Bi2Te3 segmented thermoelectric device design technology, and particularly to a Bi2Te3 / PbTe segmented thermoelectric device based on a TiTe2 contact layer and its fabrication method. Background Technology

[0002] PbTe-based materials are thermoelectric materials with excellent thermoelectric properties and high reliability in the mid-temperature range. They have been validated in isotope thermoelectric generators used in aerospace applications for decades. Due to the low near-room temperature thermoelectric properties of PbTe materials, they are often combined with near-room temperature thermoelectric materials Bi2Te3 to form segmented devices to achieve high thermoelectric performance and high device output efficiency over a wide temperature range.

[0003] Currently, segmented devices are mainly fabricated through multi-step welding, which is a relatively complex process. There are problems such as element diffusion, thermal stress concentration, and interfacial contact resistance between different segmented materials and between materials and electrodes. These problems are more serious at high operating temperatures. Summary of the Invention

[0004] Therefore, it is necessary to address the material screening problem of existing Bi2Te3 / PbTe segmented thermoelectric devices. A Bi2Te3 / PbTe segmented thermoelectric device material and preparation method based on TiTe2 contact layer are proposed.

[0005] The first aspect of this application provides a segmented Bi2Te3 / PbTe thermoelectric device based on a TiTe2 contact layer, comprising: a three-layer structure connected in sequence, the three-layer structure including a Bi2Te3 layer, a TiTe2 contact layer and a PbTe layer, wherein the TiTe2 contact layer is disposed between the Bi2Te3 layer and the PbTe layer.

[0006] Preferably, the thickness of the TiTe2 layer is 0.2-0.3 mm.

[0007] Preferably, the height of the thermoelectric device is 5-15 mm.

[0008] Preferably, the height ratio of the Bi2Te3 layer to the PbTe layer is Bi2Te3 layer / PbTe layer = 0.25-0.66.

[0009] A second aspect of this application provides a method for fabricating a Bi2Te3 / PbTe segmented thermoelectric device based on a TiTe2 contact layer, used to fabricate the thermoelectric device described in any of the above claims, comprising: PbTe powder, TiTe2 powder, and Bi2Te3 powder are sequentially laid in a pre-designed mold; After sintering the above-prepared material in a mold, it is cooled to room temperature to obtain a Bi2Te3 / PbTe segmented thermoelectric device material based on a TiTe2 contact layer.

[0010] Preferably, the amount of TiTe2 powder laid is not less than 0.15 g / cm³. 2 .

[0011] Preferably, the sintering method is spark plasma sintering.

[0012] Preferably, the sintering temperature is no more than 520℃ and the sintering time is 5-30 min.

[0013] Preferably, the method for preparing the TiTe2 powder is as follows: The powdered elements with an atomic ratio of Ti:Te of 1:2 are mixed evenly and then cold-pressed into blocks. Sintering is carried out at 800-1000℃ under vacuum or protective atmosphere to obtain bulk TiTe2; The bulk TiTe2 was prepared into powdered TiTe2.

[0014] The beneficial effects of this invention are as follows: By introducing TiTe2 as an interface layer, a three-layer composite structure of BizTes / TiTe2 / PbTe is constructed. The TiTe2 layer also serves as the contact layer for Bi2Te3 and PbTe, solving the problems of interface element diffusion, thermal stress concentration, and high contact resistance caused by inherent differences in thermal expansion coefficients and chemical activity in segmented devices. TiTe2 has a relatively small difference in thermal expansion coefficients with Bi2Te3 and PbTe, and also possesses a low Young's modulus, effectively buffering and absorbing interfacial thermal stress, ensuring the structural stability and reliability of the device at high temperatures. Furthermore, thanks to the thermal stability and compatibility of TiTe2 with the materials on both sides, the entire segmented device can be fabricated in one step using sintering technology. The unified contact layer design greatly simplifies the device structure. Compared to the complex interface structure and multi-step welding process of traditional segmented devices, this significantly improves the interface bonding quality and fabrication efficiency, providing an innovative and universally applicable solution for the simplified fabrication of high-performance, high-reliability segmented thermoelectric devices. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] in: Figure 1 (a) A schematic diagram of the temperature distribution of a thermoelectric device in one embodiment; Figure 1 (b) A schematic diagram showing the change in the width ratio of the PbTe layer to the TiTe2 layer of the thermoelectric device in one embodiment and the temperature. Figure 1 (c) A schematic diagram showing the change in open-circuit voltage between the width ratio of the PbTe layer and the TiTe2 layer in a thermoelectric device in one embodiment; Figure 1 (d) is a schematic diagram showing the change in energy conversion efficiency between the width ratio of the PbTe layer and the TiTe2 layer in a thermoelectric device in one embodiment; Figure 2 This is a schematic diagram of the junction temperature of a thermoelectric device in one embodiment; Figure 3 This is a schematic diagram showing the width ratio of the PbTe layer to the TiTe2 layer of a thermoelectric device in one embodiment, the energy conversion efficiency, and the junction temperature variation. Figure 4 XRD pattern of TiTe2 prepared by solid-state reaction; Figure 5 Schematic diagram of the thermal expansion properties of PbTe, Bi2Te3 and TiTe2; Figure 6 Schematic diagram of Young's modulus and hardness for PbTe, Bi2Te3 and TiTe2; Figure 7 Schematic diagrams of interface morphology and elemental distribution of PbTe and TiTe2 before and after aging; Figure 8 Schematic diagram of interface morphology and elemental distribution of Bi2Te3 and TiTe2 before and after aging; Figure 9 A schematic diagram showing the interfacial contact resistance of PbTe and TiTe2 before and after aging; Figure 10 A schematic diagram showing the interfacial contact resistance of Bi2Te3 and TiTe2 before and after aging; Figure 11 A schematic diagram illustrating the performance changes of a segmented Bi2Te3 / PbTe thermoelectric device during long-term service. Figure 12 Schematic diagram of the thermoelectric properties of PbTe and Bi2Te3 materials; Figure 13 A schematic diagram of the output performance of a segmented Bi2Te3 / PbTe thermoelectric device; Figure 14 This is a schematic diagram of a segmented thermoelectric device based on Bi2Te3 / PbTe. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] This invention provides a segmented Bi2Te3 / PbTe thermoelectric device based on a TiTe2 contact layer, comprising a three-layer structure connected in sequence, the three layers being a Bi2Te3 layer, a TiTe2 contact layer, and a PbTe layer, wherein the TiTe2 contact layer is disposed between the Bi2Te3 layer and the PbTe layer. The specific structure is as follows: Figure 14 As shown.

[0019] Bi₂Te₃ is a semiconductor compound and a high-performance low-temperature thermoelectric material. It is typically obtained by solid solution treatment with Sb₂Te₃, which allows for the control of hole carriers and introduces point defects to reduce the lattice thermal conductivity, thus optimizing the thermoelectric properties. Sb₂Te₃ and Bi₂Te₃ can be continuously solid-solidified, and its chemical formula is Bi₂Te₃. 2-x Sb x Te3, where 0 ≤ x ≤ 2, preferably, in this embodiment x = 1.52, and the specific chemical formula is Bi. 0.48 Sb 1.52 Te3, hereinafter referred to as Bi2Te3.

[0020] The PbTe layer is not a simple binary PbTe; its chemical formula is A. x Pb 1-x Te, where A is a doping element, including one or more of Na, Eu, Cd, K, Mn, Sr, Zn, Ag, Cu, and Mg; 0 ≤ x ≤ 0.3. Preferably, in this embodiment, Na, Eu, and Cd doping are used as experimental materials, with the specific chemical formula Na. 0.03 Eu 0.03 Cd 0.03 Pb 0.91 Te, Na, Eu, and Cd are introduced into the PbTe lattice as dopants (or activators). In practical applications, other PbTe-based materials can achieve the same effect. The multi-element co-doping strategy can optimize carrier concentration by precisely controlling the hole concentration to reach an optimal value, thereby maximizing the material's power factor, i.e., optimizing electrical conductivity. The doped atoms, acting as point defects, can strongly scatter heat-transferring phonons, significantly reducing the lattice's thermal conductivity. By synergistically optimizing electrical and thermal conductivity, a more ideal thermoelectric figure of merit is obtained.

[0021] The TiTe2 contact layer serves as a crucial interface layer connecting Bi2Te3 and PbTe materials. Its core function is to address challenges such as high thermal stress, high contact resistance, and element interdiffusion in traditional segmented thermoelectric device fabrication. At high temperatures, the TiTe2 layer prevents the interdiffusion of elements (such as Bi and Sb) in Bi2Te3 with elements (such as Pb) in PbTe, avoiding the formation of harmful second phases, thereby reducing interface resistance and improving stability. The initial interface morphology of each segment in a Bi2Te3 / PbTe segmented thermoelectric device, and the interface morphology after aging treatment at different times, are shown in the figure. Figure 7 and Figure 8 As shown, the aging temperature of the PbTe / TiTe2 interface is 450℃, and the aging temperature of the Bi2Te3 / TiTe2 interface is 200℃. Figure 7 The images show the scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) images of the interface between the PbTe and TiTe2 layers. Figure 8 The images show scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) images of the interface between the Bi2Te3 and TiTe2 layers. The SEM images reveal the actual microstructure at the interface, while the EDS images use different colors to indicate the distribution of elements. Figure 7 and Figure 8 The scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) images show that both types of contact surfaces remained intact before and after aging, with no obvious interfacial diffusion behavior, indicating high thermodynamic stability at both interfaces. The purpose of aging is to simulate the environment of thermoelectric devices under actual use. After 800 hours of aging, the contact interfaces still showed no obvious interfacial diffusion behavior, indicating their long-term stability in practical applications.

[0022] Due to the different coefficients of thermal expansion of Bi₂Te₃ and PbTe, stress will be generated at the interface when the temperature changes. The TiTe₂ layer, acting as a buffer layer, can effectively absorb and release these stresses, preventing interface cracking. Simultaneously, it provides excellent electrical connectivity, ensuring that charge carriers (holes) can efficiently pass through the interface, reducing energy loss. Figure 5 and Figure 6 As shown, the coefficients of thermal expansion and Young's modulus of Bi2Te3 and TiTe2 layers, as well as PbTe and TiTe2 layers, were tested respectively.

[0023] Figure 5This displays the relative change in material dimensions with increasing temperature. The X-axis represents T (K) – absolute temperature, ranging from room temperature (approximately 300K) to near the device's highest operating temperature (approximately 800K), covering the actual operating temperature range of the device. The Y-axis represents dL / L0 (%) – the percentage change in relative length. This indicates the amount of thermal expansion of the material when heated from its initial length (L0) to a certain temperature, and is direct data for calculating the coefficient of thermal expansion. The solid red line represents the high-temperature material (PbTe), which has the highest coefficient of thermal expansion and the steepest curve slope, indicating that it is most sensitive to temperature and most prone to thermal expansion and contraction. The dashed blue line represents the low-temperature material (Bi2Te3), which has the lowest coefficient of thermal expansion and the flattest curve, indicating that its dimensions change the least with temperature. The dashed green line represents the interface layer TiTe2, whose thermal expansion curve lies precisely between that of PbTe and Bi2Te3. Their coefficients of thermal expansion differ by only 9.7% (Bi₂Te₃) and 10.4% (PbTe), respectively. This high degree of thermal expansion matching indicates that when the device operates under temperature variations, the internal stress (thermal stress) caused by the different deformation amounts between the three layers will be greatly reduced due to the "synchronous" expansion and contraction of the three materials. Minimal thermal mismatch is a prerequisite for ensuring that the interface does not peel or crack after high-temperature operation and multiple thermal cycles, and is directly related to the long-term reliability of the device.

[0024] Figure 6 Comparing the elastic modulus and hardness of the three materials. The left Y-axis and red bars represent Modulus (GPa) – elastic modulus (usually Young's modulus), which measures a material's resistance to elastic deformation, i.e., "stiffness." The higher the modulus, the "harder" the material, and the less it deforms under the same stress. The right Y-axis and blue bars represent Hardness (GPa), which measures a material's resistance to localized plastic deformation (such as indentation). Bi2Te3 has the lowest modulus and hardness, making it the relatively "softest." TiTe2 has the highest modulus and hardness, making it the most "rigid" of the three. Although TiTe2 has the highest absolute modulus, its key advantage lies in the significant modulus difference between it and Bi2Te3. Between the "hard" TiTe2 and the "soft" Bi2Te3, stress is more easily released through plastic deformation on the relatively softer Bi2Te3 side, rather than accumulating at the brittle interface and leading to brittle fracture. The high hardness and modulus mean that the TiTe2 layer itself has good mechanical strength, which can maintain structural integrity during preparation and service, and is not easily crushed or damaged. It creates a favorable stress buffer structure (hard-soft-hard gradient), which allows thermal stress to be dissipated in the "softest" Bi2Te3 layer, protecting the brittle interfacial bond.

[0025] Therefore, TiTe2 is not only a good bonding layer chemically and electrically (low contact resistance, diffusion blocking), but also an ideal "bridge" connecting PbTe and Bi2Te3 in terms of physical properties. By matching thermo-mechanical properties, the problem of thermal stress failure caused by material differences in segmented thermoelectric devices is solved, which is a key design for achieving high efficiency and long lifespan of the device.

[0026] TiTe2 material has a Young's modulus of only 22.2 GPa. The matching coefficient of thermal expansion and low Young's modulus can effectively reduce interfacial thermal stress. Therefore, TiTe2 can be used as an interfacial material to form a buffer between the interfaces, thereby reducing interfacial thermal stress, element diffusion and contact resistance.

[0027] The working principle of thermoelectric devices is based on the Seebeck effect. When there is a temperature difference between the two ends of the device, an electromotive force (voltage) is generated inside, which can directly convert heat energy into electrical energy. Any thermoelectric material has optimal performance (i.e., the highest thermoelectric figure of merit) only within a specific temperature range. Bi₂Te₃ exhibits excellent performance in the low-temperature range (300-450K), but its performance degrades at high temperatures; while PbTe performs excellently in the medium-to-high-temperature range (450-800K). Connecting Bi₂Te₃ and PbTe in series to form a segmented structure allows each material to operate within its "comfort zone," thereby maximizing the average thermoelectric performance across the entire operating temperature range (from room temperature to high temperatures). If Bi₂Te₃ and PbTe are in direct contact, due to lattice mismatch and bandgap issues, a high contact resistance will be generated at the interface, resulting in significant energy loss at the interface, converted into useless Joule heat, and a substantial reduction in output efficiency. TiTe₂, as a semiconductor compound, has a band structure that allows it to form good ohmic contacts with Bi₂Te₃ and PbTe. This means that charge carriers (holes) can pass through the two interfaces almost without hindrance, i.e., the interface resistance is low. For example... Figure 9 and Figure 10 As shown, the contact interface resistance of Bi2Te3 and TiTe2 layers, as well as PbTe and TiTe2 layers, was tested before and after aging. To verify their long-term service performance, aging treatment was performed, and the interface contact resistivity was measured at various time points during the aging process. Specific aging parameters were: aging temperature of 450℃ for the PbTe / TiTe2 interface; and aging temperature of 200℃ for the Bi2Te3 / TiTe2 interface. Figure 9 and Figure 10 It can be seen that both contact interfaces have extremely low interfacial resistivity, and both show only a slight increase during aging, indicating extremely low energy loss at the interface. The freshly sintered joint has low interfacial resistivity, and the contact resistance increases slowly during aging, eventually stabilizing at 2.8 μΩ·cm. 2 and 1.2 μΩ·cm2 The superior interface matching properties not only reduce interface losses but also improve interface stability, enabling high performance and high reliability of Bi2Te3 / PbTe segmented thermoelectric devices. The contact interface morphology before and after aging is shown in the figure. Figure 7 and Figure 8 As shown, there is no obvious interfacial diffusion behavior, indicating high thermodynamic stability at both interfaces. It should be noted that the shape of the boundary line in the SEM image is different each time the SEM is performed because the observed position is different. Energy dispersive spectroscopy imaging shows that there is no obvious diffusion of elements near the contact interface during the aging process.

[0028] In some embodiments, the thickness of the TiTe2 layer is 0.2-0.3 mm. Because the thermoelectric device is segmented by sintering each layer through powder spreading, there are slight variations in thickness at different locations. An overall TiTe2 layer thickness of 0.2-0.3 mm is optimal, and more precisely, 0.23-0.25 mm is ideal. The output performance (voltage, power, efficiency) of the thermoelectric device directly depends on the effective temperature difference between its two ends. Heat flowing from the hot end (PbTe side) to the cold end (Bi2Te3 side) must pass through the entire device, including the TiTe2 layer. The thermal conductivity of TiTe2 itself is fixed; the thicker the layer, the greater the "resistance" (i.e., thermal resistance) required for heat to pass through it. This results in some temperature difference (i.e., temperature drop) being wasted on the TiTe2 layer itself, reducing the effective temperature difference actually applied to the two active thermoelectric material segments of Bi2Te3 and PbTe. If the TiTe2 layer is too thin (e.g., less than the characteristic length for element diffusion at high temperatures), it may not form a continuous, dense barrier layer. Elements may diffuse through grain boundaries or directly penetrate defects (such as pores) in the thin layer, causing blocking failure.

[0029] like Figure 1 As shown, the effects of different TiTe2 thicknesses (tc) on the effective temperature difference, open-circuit voltage, and output efficiency of the device were investigated using COMSOL software. Figure 1 As shown in (a), this is a schematic diagram of the simulated temperature distribution. Figure 1 As shown in (b), the effective temperature difference of the device under different tc values ​​increases. As the thickness tc increases, the temperature difference between the interface between Bi2Te3 material and TiTe2 layer (low-temperature side interface) and the interface between TiTe2 layer and PbTe material (high-temperature side interface) both increase rapidly, which is not conducive to heat conduction. Therefore, under the premise of ensuring the blocking performance of TiTe2 layer, the thinner the better.

[0030] In some implementations, the height of the Bi2Te3 / PbTe segmented thermoelectric device (hereinafter referred to as the Bi2Te3 / PbTe segmented thermoelectric device) based on the TiTe2 contact layer is 5-15 mm. In this embodiment, the height refers to the vertical dimension of the Bi2Te3 / PbTe segmented thermoelectric device. This range is based on practical application requirements, such as: In modular thermoelectric generators or refrigerators, this size facilitates matching with other components (such as heat exchangers or electrodes). Sufficient height ensures that the material is not easily broken under thermal cycling or stress. Height can affect the distribution of heat flow and current, thus affecting thermoelectric conversion efficiency. Furthermore, 5-8 mm is preferred.

[0031] In some implementations, the height ratio of the Bi2Te3 layer to the PbTe layer is 0.25-0.66. Modeling and simulation were performed for different height ratios, using the Bi2Te3 / PbTe segmented thermoelectric device as a thermoelectric arm to construct the segmented device. The thermoelectric arm dimensions are 4*4*6 mm. 3 The external temperature of the cold end of the device is set to 293 K (Tc = 293 K), and the temperature of the hot end is set to 743 K (Th = 743 K), corresponding to a temperature difference of 450 K, with a segment height ratio H. Bi2Te3 / H PbTe This is the height ratio. (Refer to...) Figure 2 The device is composed of two layers of different thermoelectric materials (PbTe and Bi2Te3) stacked together. Heat (Q) flows from the top hot end (T). h = 743 K) flows in, passes through the PbTe segment, and crosses the middle junction region (temperature T) j ), then flows through the Bi2Te3 section, and finally from the bottom cold end (T) c = 293 K) outflow. The thickness (or height) H of the two material sections. PbTe and H Bi2Te3 It is a designable variable, and its ratio determines the distribution of heat between the two material sections, thereby determining the intermediate junction temperature T. j The location. Figure 3 To quantitatively reveal how the height ratio affects junction temperature and efficiency through COMSOL simulations, and to determine the optimal value, the maximum power conversion efficiency η of the device at each height ratio was obtained using COMSOL's built-in optimization algorithm. max and the corresponding junction temperature T j The simulation data is as follows Figure 3 As shown, the simulation results are listed in Table 1. Efficiency (η) max The curve showing the change in height ratio (blue curve) exhibits a camel-hump shape, initially rising rapidly and then slowly declining. At H... Bi2Te3 / H PbTeAround ≈ 0.42, the efficiency reaches its peak at approximately 16.5%. Therefore, from Figure 3 It can be seen that when H Bi2Te3 / H PbTe At a height ratio of approximately 0.42, the device's energy conversion efficiency is at its maximum, and the junction temperature (actual operating temperature) is also close to the target temperature of 450 K. A height ratio in the range of 0.25-0.66 can meet the requirements of practical applications. More preferably, a height ratio between 0.38-0.48 allows the device's energy conversion efficiency to be near its maximum, while the junction temperature is also close to the target temperature of 450 K.

[0032] In some embodiments, silver-plated copper foil is also provided at both ends of the Bi2Te3, TiTe2 and PbTe three-layer structure.

[0033] Specifically, the sintered Bi2Te3, TiTe2, and PbTe three-layer structure was wire-cut to obtain a thermoelectric arm with dimensions of 3.5*3.5*7.5 mm3. After polishing with sandpaper and cleaning off oil, the two ends of the thermoelectric arm were welded to silver-plated copper foil as electrodes using nano-silver solder. The welding temperature was 250-350℃, the pressure was 5-8 MPa, and the welding time was 5-20 min, resulting in a Bi2Te3 / PbTe segmented thermoelectric device including copper foil electrodes.

[0034] The performance of the above-mentioned Bi2Te3 / PbTe segmented thermoelectric device was tested, such as... Figure 13 As shown, where, Figure 13 (a) shows the output current-voltage relationship (IU curve). The Bi2Te3 / PbTe segmented device can generate a significant open-circuit voltage under all temperature differences (ΔT), and the voltage increases linearly with ΔT, perfectly following the Seebeck effect. When a load is connected, the output voltage does not drop sharply due to the current output, and the device can operate stably over a wide current range, exhibiting good load-carrying capacity.

[0035] Figure 13 (b) Output current-power relationship (IP curve). The Bi2Te3 / PbTe segmented thermoelectric device can output considerable electrical power and has a clear maximum power point, which facilitates system optimization. Each PI curve exhibits a clear parabolic shape and reaches a peak value (P0). max P max The efficiency increases significantly with increasing ΔT, reaching a global maximum at ΔT=450K. This demonstrates that the Bi2Te3 / PbTe segmented thermoelectric device can achieve powerful electrical energy output when utilizing a large temperature difference heat source.

[0036] Figure 13(c) is the output current-heat flow relationship (IQ) out (Curve). Bi2Te3 / PbTe segmented thermoelectric devices can actively adjust the heat absorption from the heat source according to the operating current, Q out The current changes significantly, rising from I=0, reflecting the enhanced Peltier effect and the influence of Joule heating within the device. This indicates that the device is effectively converting thermal energy into electrical energy, but at the same time, it is accompanied by an increase in internal energy loss.

[0037] Figure 13 (d) shows the current-to-energy conversion efficiency relationship (I-η curve). Bi₂Te₃ / PbTe segmented thermoelectric devices exhibit high energy conversion efficiency and possess an optimal efficiency operating point, which is a core indicator for evaluating their performance-economic efficiency. Each η-I curve has a prominent efficiency peak (η). max ). η max It increases significantly with the increase of ΔT, reaching its highest value at the maximum temperature difference. That is, the energy conversion efficiency increases significantly with the temperature difference, which has significant practical application value.

[0038] Figure 13 This application comprehensively demonstrates the superior performance of the Bi2Te3 / PbTe segmented thermoelectric device, a thermoelectric power generation device. Its high voltage output, high power density, and high conversion efficiency over a wide temperature range collectively constitute its strong competitiveness in fields such as waste heat recovery.

[0039] This application tested the performance of a segmented Bi2Te3 / PbTe thermoelectric device during a 50-hour long-term service period, such as... Figure 11 As shown, it exhibits only 2% performance degradation during a long service period of 50 hours, demonstrating its high performance and high stability.

[0040] By introducing TiTe2 as an interface layer, a three-layer composite structure of BizTes / TiTe2 / PbTe was constructed. The TiTe2 layer also serves as the contact layer for Bi2Te3 and PbTe, solving the problems of interfacial element diffusion, thermal stress concentration, and high contact resistance caused by inherent differences in thermal expansion coefficients and chemical reactivity in segmented devices. TiTe2 has a relatively small difference in thermal expansion coefficients with Bi2Te3 and PbTe, and also possesses a low Young's modulus, which can effectively buffer and absorb interfacial thermal stress, ensuring the structural stability and reliability of the device at high temperatures. Furthermore, thanks to the thermal stability and compatibility of TiTe2 with the materials on both sides, the entire segmented device can be fabricated in one step using sintering technology. At the same time, the unified contact layer design greatly simplifies the device structure. Compared with the complex interface structure and multi-step welding process of traditional segmented devices, it significantly improves the interface bonding quality and fabrication efficiency, providing an innovative and universal solution for the simplified fabrication of high-performance, high-reliability segmented thermoelectric devices.

[0041] This invention also provides a method for fabricating a segmented Bi2Te3 / PbTe thermoelectric device based on a TiTe2 contact layer, specifically including: S1: PbTe powder, TiTe2 powder and Bi2Te3 powder are sequentially laid in a preset mold.

[0042] Specifically, the mold has a cavity of a specific shape and size, capable of withstanding thermal shocks far exceeding the sintering temperature (500°C in specific applications). In this embodiment, a graphite mold is used, which is resistant to high temperatures (>2500°C), has good electrical / thermal conductivity, sufficient strength and self-lubricating properties under high pressure, is relatively low in cost, and is easy to process. First, a predetermined amount of PbTe powder is poured into the mold, and the powder is compacted using a flat-headed metal rod (or alumina rod), ensuring that the powder surface is as level and flat as possible so that subsequent layers can cover the surface evenly. Then, TiTe2 powder and Bi2Te3 powder are laid down in the same manner.

[0043] Among them, PbTe powder refers to Na 0.03 Eu 0.03 Cd 0.03 Pb 0.91 Te powder, with Na, Eu, and Cd as doping elements. Bi2Te3 powder refers to Bi... 0.48 Sb 1.52 Te3 powder, commonly abbreviated as Bi2Te3.

[0044] S2: After sintering the material laid in step S1 in a mold, cool it to room temperature to obtain a Bi2Te3 / PbTe segmented thermoelectric device material based on a TiTe2 contact layer.

[0045] Specifically, during the sintering process, under pressure and high temperature, the three layers of powder sinter together densely, mechanically interlocking to form a robust structure. This achieves the high-performance Bi2Te3 / PbTe segmented thermoelectric device described in this application.

[0046] In some embodiments, the amount of TiTe2 powder laid is not less than 0.15 g / cm³. 2 .

[0047] Specifically, the amount of TiTe2 powder laid determines the thickness of the TiTe2 layer after sintering. If the amount laid is too low, it will not be able to effectively cover the underlying PbTe powder, or the TiTe2 layer will be too thin and will not meet the performance requirements of the interface layer.

[0048] This application simplifies the traditional lengthy process of "material synthesis → bulk sintering → precision cutting → surface treatment → multiple welding" into two main steps: "powder spreading → sintering", thus streamlining the preparation process.

[0049] In some embodiments, the sintering method is spark plasma sintering.

[0050] Specifically, spark plasma sintering (SPS) is an advanced rapid powder densification sintering technology. By simultaneously applying uniaxial pressure and low-temperature activation with pulsed direct current, powder particles achieve densification and strong bonding in an extremely short time. Simultaneously, the pulsed current generates instantaneous, high-energy plasma between the powder particles. This plasma cleans the particle surface, breaks down the oxide film, greatly improves atomic diffusion, and significantly reduces the sintering temperature.

[0051] In some embodiments, the graphite mold is placed in a spark plasma sintering furnace, the heating rate is controlled at 80-120℃ / min, the sintering temperature does not exceed 520℃, the sintering time is 5-30min, and after sintering, it is cooled at a cooling rate of 6-15℃ / min.

[0052] In this embodiment, the heating rate is 100℃ / min, the sintering temperature is 500℃, the sintering time is 10min, and the cooling rate is 10℃ / min.

[0053] Rapid heating can quickly bypass the medium-low temperature zone, reducing the diffusion time on the powder surface and facilitating the formation of fine-grained structures. Fine grains enhance phonon scattering, potentially reducing thermal conductivity and benefiting thermoelectric properties. It also shortens the process cycle. However, excessively low heating rates (<80°C / min) negate these advantages; excessively high rates (>120°C / min) may lead to inaccurate temperature control, excessive temperature differences between the inside and outside of the mold or between the upper and lower powder layers, resulting in thermal stress.

[0054] The sintering temperature must be high enough to ensure adequate densification of all materials, especially PbTe. Too low a temperature leads to incomplete sintering, resulting in porous materials with low electrical conductivity and poor interfacial bonding. The solidus (temperature at which Bi2Te3 begins to melt) is around 600°C. Although 520°C is far below its melting point, rapid heating and pressure under SPS can cause localized, transient liquefaction at particle contact points, which is beneficial for densification. However, if the temperature is too high (e.g., 550°C), this localized liquefaction intensifies, causing the Bi2Te3 powder to completely lose its strength and be extruded from the mold under pressure, resulting in device structural damage and fabrication failure. In the study of the Bi2Te3 / PbTe segmented thermoelectric device of this application, when the discharge plasma sintering temperature was 550°C, because this sintering temperature is close to the melting point of Bi2Te3 (approximately 600°C), Bi2Te3 was extruded from the graphite mold in liquid form during sintering, leading to material fabrication failure.

[0055] Sintering time refers to the holding time of the material at the sintering temperature, used to complete densification and interfacial diffusion bonding. Sufficient time (≥5 min) allows for sufficient atomic diffusion, enabling strong metallurgical bonding between powder particles and between the Bi2Te3 / TiTe2 / PbTe layers, resulting in low interfacial resistivity. Too short a time (<5 min) may lead to incomplete densification or insufficient interfacial bonding strength; too long a time (>30 min) leads to grain coarsening (detrimental to mechanical and thermoelectric properties) and excessive element diffusion at the interface. Although TiTe2 is a barrier layer, excessive holding time may still increase the risk of interdiffusion.

[0056] In some embodiments, the method for preparing the TiTe2 powder is as follows: S11: Mix the powdered elements with an atomic ratio of Ti:Te of 1:2 evenly and then cold-press them into blocks; S12: Sintering is carried out at 800-1000℃ under vacuum or protective atmosphere to obtain bulk TiTe2; S13: Prepare TiTe2 from bulk TiTe2 into powdered TiTe2.

[0057] Specifically, the above-mentioned method for preparing TiTe2 powder is a solid-state preparation method, that is, Ti and Te undergo a solid-state reaction. By mixing the elements and reacting them at high temperature, elemental TiTe2 is formed. The XRD pattern of the material synthesized by the above method is shown below. Figure 4 As shown, this is the pure-phase intermetallic compound TiTe2.

[0058] In exploring methods for preparing TiTe2, the melt method was attempted, but the process failed because the boiling point of Te (1390℃) is lower than the melting point of Ti (1670℃), preventing the two from reacting during melting. However, when TiTe2 was prepared by ball milling, the resulting blocky TiTe2, as determined by XRD diffraction, showed that the material was a mixture of Ti and Te, with no TiTe2 formed.

[0059] In some embodiments, Ti and Te powdered elements can be ball-milled from block, granular, or powdered Ti and Te elements into a homogeneous powder. The ball milling must be carried out under an inert gas atmosphere, such as argon or nitrogen. The ball milling time is at least 20 hours to ensure a fine powder and uniform mixing, guaranteeing the adequacy of subsequent reactions.

[0060] Cold pressing refers to placing a uniformly mixed powdered substance into a mold and die-casting it at room temperature. In some embodiments, cold pressing is performed for 10-15 minutes at a pressure of 5-10 MPa. In this embodiment, cold pressing is performed for 10 minutes at a pressure of 10 MPa, resulting in a block shape.

[0061] Specifically, sintering is a powder metallurgy process that involves heating metal or ceramic powders (or powder compacts) at a temperature below the melting point of their main components. Through atomic diffusion, flow, and migration between powder particles, adhesion occurs, ultimately resulting in a dense material or product with the desired strength, density, and microstructure. When the cold-pressed mixed powder is heated to a high temperature of 800-1000℃, the kinetic energy of Ti and Te atoms increases significantly, causing them to diffuse across the interface into each other's regions. Through sufficient atomic diffusion, Ti and Te atoms combine in a 1:2 stoichiometric ratio, forming new chemical bonds and generating the target compound—the intermetallic compound TiTe2.

[0062] In some implementations, the sintering time is no less than 48 hours. If the sintering time is too short, the reaction may only occur on the particle surface, leaving the particle interior as unreacted Ti and Te. The final XRD pattern will show a mixed phase of TiTe2, Ti, and Te, rather than... Figure 4 The diagram shows a single-phase TiTe2 spectrum. This impure product, acting as an interface layer, has uncontrollable and unstable physical properties (such as electrical conductivity and coefficient of thermal expansion), directly leading to the degradation of device interface performance.

[0063] In some implementations, PbTe-based materials (Na) 0.03 Eu 0.03 Cd 0.03 Pb 0.91 The detailed preparation steps for Te are as follows: S31: Raw material weighing and preparation According to the chemical formula Na 0.03 Eu 0.03 Cd 0.03 Pb 0.91 The atomic ratio of Te is determined by accurately weighing high-purity (usually ≥99.99%) elemental raw materials of Na, Eu, Cd, Pb, and Te.

[0064] Specifically, precise stoichiometry is fundamental to obtaining the target thermoelectric properties. Na, Eu, and Cd are used as dopants (activators) to optimize carrier concentration and phonon scattering.

[0065] S32 vacuum package The weighed raw materials are loaded into a quartz tube. The quartz tube is connected to a high vacuum system, and the vacuum is evacuated to 10. - A vacuum of 3 Pa (0.001 Pa) or higher is applied, and then the quartz tube is melted and sealed using tools such as an oxyhydrogen flame.

[0066] Specifically, a high-vacuum environment prevents raw materials from oxidizing at high temperatures and avoids the quartz tube from bursting due to the expansion of residual gas inside during heating. This is crucial for ensuring the purity of the synthesized materials.

[0067] S33: High-temperature melting and homogenization reaction Place the sealed quartz tube in a box furnace. Heat it to 1050°C (between 1000-1100°C) at an appropriate heating rate and hold it at this temperature for 10 hours (between 8-15 hours).

[0068] Specifically, 1050°C is much higher than the melting point of PbTe (approximately 924°C), ensuring that all raw materials are completely melted to form a homogeneous liquid phase. Holding at this temperature for 10 hours provides ample time for the elements in the melt to diffuse and react chemically, ensuring the formation of a homogeneous and accurately composed (Na, Eu, Cd) co-doped PbTe compound.

[0069] S34: Quenching and Long-Term Annealing After the melting and holding period is complete, the quartz tube is quickly removed from the furnace and immersed in water or placed on a metal plate for rapid cooling (quenching) to room temperature. This yields an ingot with a homogeneous composition but which may contain internal stress and compositional segregation.

[0070] The quenched ingot (usually still sealed in a quartz tube) is placed back into the furnace and heated to 630°C (in the range of 600-700°C), and annealed at this temperature for at least 72 hours, allowing the sample to cool slowly with the furnace (furnace cooling) to room temperature.

[0071] In some implementations, Bi2Te3 material Bi 0.48 Sb1.52 The preparation method of Te3 is as follows: S41: Raw material weighing and preparation Press Bi 0.48 Sb 1.52 The stoichiometry of Te3 involves accurately weighing predetermined amounts of elemental Bi, Sb, and Te.

[0072] S42: Vacuum encapsulation Same as PbTe material, high vacuum (<10 - After being sealed in a quartz tube (3 Pa), it was then refrigerated.

[0073] S43: Melting reaction and furnace cooling The quartz tube is heated to 800°C (range between 800-900°C) in a box furnace and held at that temperature for 8 hours (range between 6-10 hours) to allow the raw materials to fully melt and react. After the reaction is complete, the furnace power is turned off directly without quenching, and the sample is allowed to cool slowly with the furnace (furnace cooling) to room temperature.

[0074] Specifically, 800°C is above the melting point of Bi₂Te₃, ensuring complete melting. For the Bi₂Te₃ system, slow cooling helps form its unique layered crystal structure and avoids introducing too many point defects, which is beneficial to its excellent thermoelectric properties (high electrical conductivity) near room temperature. Its properties are less dependent on long-term high-temperature annealing than those of PbTe.

[0075] For details, please refer to Figure 12 ,in Figure 12 (a) Resistivity ρ as a function of temperature. Resistivity ρ measures a material's ability to impede current; lower is better (conductivity σ = 1 / ρ). Bi2Te3 (pink line): In the low-temperature region (~300-450K), the resistivity is low and gradual, indicating good metallic conductivity, suitable for near-room-temperature operation. PbTe (blue line): The resistivity initially increases slowly with increasing temperature, then tends to stabilize or slightly decrease in the medium-high temperature region (>500K). This is typical behavior of heavily doped semiconductors, suitable for medium-high temperature operation. Both materials exhibit low resistivity in their respective target temperature regions, meaning less internal energy loss due to Joule heating (I²R), which is beneficial for improving efficiency.

[0076] Figure 12(b) Seebeck coefficient S as a function of temperature. The Seebeck coefficient S measures a material's ability to generate voltage under temperature differences; a higher absolute value is better. Both materials have positive S values, confirming they are p-type materials (with holes as charge carriers). The S value of Bi₂Te₃ is higher in the low-temperature region and decreases slowly with increasing temperature. The S value of PbTe peaks at approximately 600-700 K, exhibiting excellent mid-to-high-temperature thermoelectric potential characteristics. A high Seebeck coefficient is fundamental to generating high output voltage. The peak S value of PbTe in the mid-to-high-temperature region is a key advantage for its high-efficiency high-temperature performance.

[0077] Figure 12 (c) represents the total thermal conductivity κ. tot The thermal conductivity of a material is measured by its ability to conduct heat as temperature changes, with lower values ​​being better (to maintain a temperature difference). The thermal conductivity of both materials decreases with increasing temperature, due to enhanced phonon scattering. Across the entire temperature range, the thermal conductivity of Bi₂Te₃ is significantly lower than that of PbTe, an inherent advantage of Bi₂Te₃-based materials (the layered structure strongly scatters phonons).

[0078] Figure 12 (d) The thermoelectric figure of merit zT varies with temperature, serving as a dimensionless index for comprehensively evaluating the thermoelectric properties of materials. zT = (S²σ / κ)T. The higher the zT value, the greater the thermoelectric conversion potential of the material. Bi₂Te₃: In the low-temperature region (~350-450K), the zT value is very high (>1.0), then decreases rapidly with increasing temperature. PbTe has a very high zT value (>1.5) in the medium-high temperature region (~450-750K), but a very low zT value in the low-temperature region. The two zT curves intersect at approximately 450K. This means that below 450K, the zT of Bi₂Te₃ is higher than that of PbTe; above 450K, the zT of PbTe is higher than that of Bi₂Te₃.

[0079] Therefore, from Figure 12 It can be seen that Bi2Te3 and PbTe materials prepared by melt-annealing have achieved world-renowned high-performance levels (zT>1) in their respective temperature ranges. The complementarity of the zT curves intuitively demonstrates the limitations of single materials and the enormous potential of segmented design. The intersection of the zT curves (approximately 450K) directly provides the optimization target for the junction temperature inside the device, and all subsequent structural designs (height ratio) and interface engineering (TiTe2) are aimed at achieving and stabilizing this target.

[0080] Therefore, the height ratio of each layer of Bi2Te3 and PbTe materials is H. Bi2Te3 / H PbTe When H is set between 0.25 and 0.66, the device can achieve a maximum power conversion efficiency exceeding 16%, meeting the requirements for commercialization. Specifically, when H... Bi2Te3 / H PbTeThe thermoelectric device achieves optimal output performance when the coefficient of performance (COP) is 0.42. To meet practical application requirements, the device height should be in the range of 5-15 mm, meaning the total height of the Bi₂Te₃, PbTe, and TiTe₂ layers should be 5-12 mm. Taking an effective thermoelectric device height of 6 mm as an example, the heights of each component should be as follows: = 0.18 mm, H PbTe = 0.42 mm, and the contact layer thickness is about 0.2-0.3 mm. At this time, the thermoelectric performance of the Bi2Te3 / PbTe segmented thermoelectric device is at its optimal value.

[0081] The Bi2Te3 / PbTe segmented thermoelectric device based on the TiTe2 contact layer, prepared by the above method, was observed by scanning electron microscopy and energy dispersive spectroscopy. Figure 7 and Figure 8 As shown, Figure 7 The images are scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) images of the PbTe and TiTe2 layer interfaces. Figure 8 The images show scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) images of the interface between the Bi2Te3 and TiTe2 layers. The SEM images reveal the actual microstructure at the interface, while the EDS images use different colors to indicate the distribution of elements. Figure 7 and Figure 8 The scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) images show that both types of contact surfaces remained intact before and after aging, with no obvious interfacial diffusion behavior, indicating high thermodynamic stability at both interfaces. The purpose of aging is to simulate the environment of thermoelectric devices under actual use. After 800 hours of aging, the contact interfaces still showed no obvious interfacial diffusion behavior, indicating their long-term stability in practical applications.

[0082] The sintered Bi2Te3 / PbTe segmented thermoelectric device was cut to size 3.5*3.5*7.5 mm by wire cutting. 3 The thermoelectric arm was polished with sandpaper and cleaned of oil. Then, both ends of the thermoelectric arm were soldered to silver-plated copper foil (electrodes) using nano-silver solder. The soldering temperature was 250-350℃, the pressure was 5-8 MPa, and the soldering time was 5-20 minutes, resulting in a usable thermoelectric device. This device was then used to test its output performance and service performance.

[0083] The performance characteristics of the Bi2Te3 / PbTe segmented thermoelectric device prepared by the above method have been described in detail in the preceding structural description.

[0084] By introducing TiTe2 as an interface layer, a three-layer composite structure of Bi2Te3 / TiTe2 / PbTe was constructed. The TiTe2 layer solves the problems of interfacial element diffusion, thermal stress concentration, and high contact resistance caused by inherent differences in thermal expansion coefficients and chemical reactivity in segmented devices. TiTe2 has a relatively small difference in thermal expansion coefficients with Bi2Te3 and PbTe materials, and also has a low Young's modulus, which can effectively buffer and absorb interfacial thermal stress, ensuring the structural stability and reliability of the device at high temperatures. In addition, thanks to the thermal stability and compatibility of TiTe2 with the materials on both sides, the entire segmented device can be fabricated in one step using spark plasma sintering technology, which greatly simplifies the complex process of traditional multi-step welding and cascading, significantly improves the fabrication efficiency and interfacial bonding quality, and provides an innovative and universal solution for the simplified fabrication of high-performance, high-reliability segmented thermoelectric devices.

[0085] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A segmented Bi2Te3 / PbTe thermoelectric device based on a TiTe2 contact layer, characterized in that, include: The three-layer structure is connected in sequence, including a Bi2Te3 layer, a TiTe2 contact layer and a PbTe layer, with the TiTe2 contact layer disposed between the Bi2Te3 layer and the PbTe layer.

2. The Bi2Te3 / PbTe segmented thermoelectric device based on a TiTe2 contact layer according to claim 1, characterized in that, The thickness of the TiTe2 layer is 0.2-0.3 mm.

3. The Bi2Te3 / PbTe segmented thermoelectric device based on a TiTe2 contact layer according to claim 1, characterized in that, The height of the three-layer structure is 5-15 mm.

4. The Bi2Te3 / PbTe segmented thermoelectric device based on a TiTe2 contact layer according to claim 1, characterized in that, The height ratio of the Bi2Te3 layer to the PbTe layer is Bi2Te3 layer / PbTe layer = 0.25-0.

66.

5. A method for fabricating a Bi2Te3 / PbTe segmented thermoelectric device based on a TiTe2 contact layer, used to fabricate the thermoelectric device according to any one of claims 1-4, characterized in that, include: PbTe powder, TiTe2 powder and Bi2Te3 powder are sequentially laid in a pre-set mold; After sintering the above-laid material in a mold, it is cooled to room temperature to obtain a Bi2Te3 / PbTe segmented thermoelectric device material based on a TiTe2 contact layer.

6. The method for fabricating a segmented Bi2Te3 / PbTe thermoelectric device based on a TiTe2 contact layer according to claim 5, characterized in that, The amount of TiTe2 powder laid is not less than 0.15 g / cm³. 2 .

7. The method for preparing Bi2Te3 / PbTe segmented thermoelectric device material based on TiTe2 contact layer according to claim 5, characterized in that, The sintering method is spark plasma sintering.

8. The method for preparing Bi2Te3 / PbTe segmented thermoelectric device material based on TiTe2 contact layer according to claim 5, characterized in that, The sintering temperature is no higher than 520℃, and the sintering time is 5-30 minutes.

9. The method for fabricating a Bi2Te3 / PbTe segmented thermoelectric device based on a TiTe2 contact layer according to claim 5, characterized in that, The method for preparing the TiTe2 powder is as follows: The powdered elements with an atomic ratio of Ti:Te of 1:2 are mixed evenly and then cold-pressed into blocks. Sintering is carried out at 800-1000℃ under vacuum or protective atmosphere to obtain bulk TiTe2; The bulk TiTe2 was prepared into powdered TiTe2.

10. The method for fabricating a Bi2Te3 / PbTe segmented thermoelectric device based on a TiTe2 contact layer according to claim 9, characterized in that, The pressure for cold pressing is 5-10 MPa, and the cold pressing time is 10-15 min; The sintering time shall not be less than 48 hours.

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