Method for measuring performance anisotropy of Bi2Te3 thermoelectric material by using EBSD technology
EBSD technology measures the anisotropy of Bi2Te3 thermoelectric materials, solves the problem of brittle fracture during the deformation process, and achieves accurate prediction and optimization of material properties.
Patent Information
- Application Number
- CN202510540210.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-27
- Publication Date
- 2025-08-01
AI Technical Summary
The prior art is difficult to effectively measure the mechanical and thermoelectric properties anisotropy of Bi2Te3 thermoelectric materials during deformation, resulting in brittle fracture of the material under external loads, which brings difficulties to production and application.
The sample coordinate system was established using EBSD technology, and the orientation and orientation differences of each grain were analyzed by crystal diffraction, the Schmitt factor value was calculated, the deformation state of the material was simulated in different directions, and the performance anisotropy was measured in combination with the thermoelectric superiority calculation formula.
It provides a simple and effective method that can measure the performance anisotropy of Bi2Te3 thermoelectric materials at room temperature and high temperature conditions, predict its deformation difficulty in different directions, and support preparation and processing in actual production.
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Figure CN120404813A_ABST
Abstract
Description
Technical Field
[0001] The present invention belongs to the field of research on material deformation, and particularly provides a method for measuring the anisotropy of the performance of Bi2Te3 thermoelectric materials by using EBSD technology. Background Art
[0002] Materials will undergo deformation and bear various loadings during the processes of preparation, processing, testing, analysis, and later service. When the stress magnitude exceeds the elastic limit, the component or sample will undergo plastic deformation, and the internal grains mainly coordinate the deformation by activating slip systems or twin systems, and relative sliding between crystal planes will also occur in the crystal. Most of the materials actually used are composed of polycrystals, and the deformation mode of each grain in the polycrystal is similar to that of a single crystal. However, the sizes of individual grains in the polycrystal are different, and the orientations of each grain are also different, so the grains in the polycrystalline material need to coordinate with each other during deformation. During deformation, grains with different orientations will bear different stress actions due to inconsistent slip planes and slip directions. Grains in favorable orientations will slip first, and grains in unfavorable orientations will slip later.
[0003] Bi2Te3-based thermoelectric materials belong to the first-generation thermoelectric materials, and due to their excellent thermoelectric properties such as fast response, precise temperature control, small size, light weight, and high thermoelectric conversion efficiency in the room temperature range, they have obtained large-scale commercial applications in the fields of solid-state refrigeration / temperature control, etc. The crystal structure of Bi2Te3 is a rhombohedral structure and belongs to the R-3m space group. During plastic deformation, the deformation modes of Bi2Te3 materials include basal planes Slip, <a+c> slip and {0001} twinning deformation, where the basal plane Displacement slip is considered to be the most important deformation mechanism in thermoelectric materials. During the material preparation and processing, bulk Bi2Te3 needs to undergo deformation such as ball milling, rocking, hot pressing, zone melting or hot extrusion to regulate the microstructure, grain size, crystal orientation, microdefects and carrier concentration, etc., in order to obtain the best electro-acoustic transport properties and mechanical properties. After deformation, the bismuth telluride alloy is prone to form texture, and its strength and plasticity are poor. If the coordination between slip systems in each grain cannot be maintained under external loads, the material is prone to brittle fracture, bringing many difficulties to actual production and application.
[0004] In the prior art, the research on the deformation behavior and deformation mechanism of polycrystalline metal materials is mainly divided into macroscopic research and microscopic research. Among them, the main research methods for microscopic research are electron backscatter diffraction technology (EBSD) and transmission electron diffraction technology (TEM). Among them, the TEM technology can penetrate to the nanoscale and can give the specific dislocation and twin structure in the sample. However, the information of these dislocations and twins is too local to be combined with the overall deformation mechanism of the material and the external force. However, the EBSD technology can reach the sub-micron order of magnitude, can study the orientation, size and orientation difference between grains in the material from a microscopic perspective, can give the statistical results of a large number of grains, and can better combine macroscopic deformation with microscopic deformation.
[0005] Therefore, it is necessary to find a simple and effective method to measure the mechanical and thermoelectric property anisotropy of Bi2Te3 materials using EBSD technology. Summary of the Invention
[0006] The object of the present invention is to provide a method for measuring the anisotropy of the performance of Bi2Te3 thermoelectric materials by using EBSD technology. Through this method, representative samples can be selected in the Bi2Te3 thermoelectric materials, and a sample coordinate system can be established. After stress relief treatment of the sample, crystal diffraction is carried out by means of EBSD technology, and the sample coordinate system is kept consistent with the sample stage coordinate system. According to the orientation differences between grains in the analyzed area, the crystal coordinate systems (represented by Euler angles) of each grain can be established. Subsequently, information such as Euler angles, cell orientations, Cohen coefficients, and pole figures is analyzed and calculated to determine the grain orientations or sample textures in the studied area. According to the slip systems and deformation direction conditions that need to be input, the Schmid factor surface distribution map is analyzed, and the distribution curve of Schmid factor values and the magnitude of the average Schmid factor value are calculated to simulate the deformation state of the material along a specific direction. According to Schmid's law, the difficulty of subsequent deformation of the whole material or internal grains along different directions is determined, and the anisotropy difference of its thermoelectric performance is measured by combining the thermoelectric figure of merit calculation formula. In particular, this method can also analyze the states of each grain in the studied area (such as grain size, crystal orientation, Cohen coefficient, Schmid factor values of deformation along different directions, etc.), predict the thermoelectric performance of different grains in bismuth telluride along each direction, and the difficulty of subsequent deformation. This method is applicable to the cold deformation process at room temperature and also applicable to the hot deformation process at high temperature.
[0007] The technical solution of the present invention is as follows: A method for measuring the anisotropy of the performance of Bi2Te3 thermoelectric materials by using EBSD technology, which specifically includes the following steps:
[0008] S1) Establish a sample coordinate system: Mark according to the type of bismuth telluride material to be analyzed;
[0009] S2) Sample preparation: The bismuth telluride material marked in S1) is sampled by a wire cutting device, and the length, width, and height of the obtained sample are perpendicular to each other and respectively correspond to the X-axis, Y-axis, and Z-axis of the sample coordinate system; then polishing is carried out to obtain the sample;
[0010] S3) Remove stress: Remove the surface residual stress of the sample processed in S2);
[0011] S4) Crystal diffraction: Keep the X-axis, Y-axis, and Z-axis of the sample processed in S3) parallel to the X-axis, Y-axis, and Z-axis of the test equipment sample stage respectively, and use the backscattered electron diffraction technology to scan the area of interest on the sample surface to obtain the crystallographic data of all grains in this area;
[0012] S5) Grain state calculation: Based on the measured and calculated Euler angles, unit cell orientations, Koen coefficients, and pole figure information, determine the grain orientations or sample textures in the area under study. According to the slip systems and deformation direction conditions that need to be input, analyze the Schmid factor contour map, calculate the distribution curve of Schmid factor values and the magnitude of the average Schmid factor value, and simulate the deformation state of the material along a specific direction; or mark the individual Schmid factor values corresponding to each grain of the analyzed sample and calculate the grain state within each individual grain.
[0013] S6) Mechanical and thermoelectric property analysis: Determine the ease of subsequent deformation of the overall material or internal grains along different directions according to Schmid's law, and measure the anisotropy differences in its thermoelectric properties in combination with the thermoelectric figure of merit calculation formula.
[0014] Furthermore, the Bi2Te3 material in S1) is a single crystal or polycrystalline material.
[0015] Furthermore, if the Bi2Te3 material in S1) is a bulk sample, the X-axis, Y-axis, and Z-axis can be correspondingly marked as the longitudinal direction LD, the transverse direction TD, and the normal direction ND.
[0016] If the Bi2Te3 material is a rod-shaped sample, the X-axis, Y-axis, and Z-axis can be correspondingly marked as the axial direction AD, the tangential direction TD, and the radial direction RD.
[0017] Furthermore, the length of the sample in S2) is not greater than 15 mm, the width is not greater than 10 mm, and the height is not greater than 8 mm.
[0018] The process of mechanical polishing is as follows: successively use 6-#00, 1200#, 2000#, 3000#, and 5000# SiC sandpapers to grind the sample until the test surface is flat, then use a flocked polishing cloth and polishing paste with W1.0 - W2.5 for rough polishing, and use a silk velvet polishing cloth and polishing paste with W0.5 for polishing.
[0019] Furthermore, the method for removing surface stress in S3) is argon ion bombardment or electrolytic polishing.
[0020] After stress removal, the surface of the sample should be in a mirror state, and the EBSD resolution rate should be above 95%.
[0021] Furthermore, the process of electrolytic polishing is as follows: the electrolyte used is a mixed solution of 10% perchloric acid by total mass and 90% alcohol by total mass, the electrolytic cathode is made of titanium alloy, the voltage is 50 V, the current is 0.08 A, the electrolytic time is 5 min, and the initial electrolytic temperature is -60°C.
[0022] Furthermore, the crystallographic data of the grains in S4) includes grain boundaries, grain size, grain orientation, pole figure, inverse pole figure, and Schmid factor.
[0023] Furthermore, the slip system in S5) is {0001}<-12-10>.
[0024] The beneficial effects of the present invention are as follows: Due to the adoption of the above technical solution, the EBSD test process used in the method of the present invention is relatively common in the field of material analysis. The sample preparation process consumes less materials. The method of measuring the anisotropy of the thermoelectric material properties of the Bi2Te3 material by the EBSD technology is simple and an easy-to-operate method. This method is applicable to a relatively wide temperature range, applicable to both the cold deformation process at room temperature and the hot deformation process at high temperature, and applicable to the opening conditions of different slip systems in any deformation direction during the deformation process. According to the sample coordinate system and the crystal coordinate system set in the EBSD process, all grain information of the representative area of the sample is calculated, and then according to the slip system and deformation direction conditions input according to the analysis needs, the distribution curve of the Schmid factor value and the magnitude of the average Schmid factor value are calculated to predict the anisotropy of the mechanical and thermoelectric properties of the Bi2Te3 thermoelectric material. In particular, this method can also determine the deformation state and the anisotropy of the thermoelectric properties inside a single grain. Using this method to measure the mechanical and thermoelectric properties of the Bi2Te3 thermoelectric material in different directions can provide technical support for the cold deformation or hot deformation process during the preparation and processing in the actual production of the Bi2Te3 thermoelectric material, and it is an effective research method for measuring the anisotropy of the Bi2Te3 thermoelectric material properties by using the EBSD technology. Description of the Drawings
[0025] Figure 1 is the orientation distribution map and phase distribution map of the Bi2Te3 material in Example 1 of the present invention;
[0026] Figure 2 is the schematic diagram for the selection of slip systems and deformation directions in Example 1 of the present invention;
[0027] Figure 3 is the Schmidt factor plane distribution map of the Bi2Te3 material in Example 1 of the present invention along the LD, TD, and ND directions;
[0028] Figure 4 is the Schmidt factor distribution curve of the Bi2Te3 material in Example 1 of the present invention along the LD, TD, and ND directions;
[0029] Figure 5 is the orientation distribution map and unit cell schematic diagram of the Bi2Te3 material in Example 2 of the present invention;
[0030] Figure 6This is the Schmidt factor surface distribution diagram of the Bi2Te3 material deformed along the RD, TD, and AD directions in Embodiment 2 of the present invention;
[0031] Figure 7 This is the Schmidt factor distribution curve of the Bi2Te3 material deformed along the RD, TD, and AD directions in Embodiment 2 of the present invention. Detailed implementation manners
[0032] Please ensure that the content of the specification is fully disclosed, that is, those skilled in the art can repeat the solution without creative labor according to the description in the specification and obtain the corresponding effects.
[0033] A method for measuring the anisotropy of the performance of Bi2Te3 thermoelectric materials using the EBSD technique according to the present invention, the method comprising the following steps:
[0034] 1) Establish a sample coordinate system: Mark according to the type of bismuth telluride material to be analyzed. If it is a bulk sample, the X-axis, Y-axis, and Z-axis can be marked as the longitudinal direction (LD), the transverse direction (TD), and the normal direction (ND), respectively; if it is a rod-shaped sample, the X-axis, Y-axis, and Z-axis can be marked as the axial direction (AD), the tangential direction (TD), and the radial direction (RD), respectively;
[0035] 2) Sample preparation: Use a wire cutting device to sample the Bi2Te3 thermoelectric material. The length, width, and height of the sample are perpendicular to each other and correspond to the X-axis, Y-axis, and Z-axis of the sample coordinate system respectively; for easy distinction, the values of the length, width, and height of the sample should not be equal; successively use 600#, 1200#, 2000#, 3000#, and 5000# SiC sandpapers to grind the sample until the test surface is flat, then use a flocked polishing cloth and a polishing paste of W1.0 - W2.5 for rough polishing, and use a silk velvet polishing cloth and a polishing paste of W 0.5 for fine polishing; after fine polishing, the surface of the sample is bright and there are no obvious visible scratches; then clean and dry the sample with alcohol to reduce the influence of impurities on the surface state of the sample during the stress relief treatment;
[0036] 3) Remove stress: Treat the mechanically polished sample by methods such as argon ion bombardment or electrolytic polishing to remove the surface residual stress of the sample to be tested; if electrolytic polishing is used, the electrolytic products on the surface of the sample will be enriched and turn black, and it is necessary to clean and remove the remaining black substances on the surface of the sample; after the stress relief treatment, the surface of the sample should be in a mirror state and the EBSD resolution should be above 95%;
[0037] The regulation of the use of mixed acid, refrigeration, electrolytic voltage and current density is conducive to accurately controlling the corrosion rate and surface quality of pure rhenium samples, avoiding the problems of corrosion pits caused by too fast initial corrosion and polishing treatment rate and uneven surface quality of the whole sample in the later stage.
[0038] 4) Crystal diffraction: Keep the X-axis, Y-axis and Z-axis of the sample parallel to the X-axis, Y-axis and Z-axis of the sample stage of the testing equipment respectively, that is, the LD, TD and ND (or AD, TD and RD) of the sample are all parallel to one of the X-axis, Y-axis and Z-axis of the sample stage. Use the electron backscatter diffraction (EBSD) technique to collect information on the area of interest on the sample surface and obtain the crystallographic data of all grains in the analyzed area, specifically including data such as grain boundaries, grain size, grain orientation, pole figure, inverse pole figure and Schmidt factor;
[0039] 5) Grain state calculation: Determine the grain orientation or sample texture of the area under study according to the measured and calculated Euler angles, unit cell orientation, Koen coefficients and pole figures, etc. Select and input the deformation direction, and the deformation direction can be selected in any direction along the material sample coordinate system. The main slip system of the Bi2Te3 thermoelectric material is {0001}<-12-10>. Analyze the distribution map of the Schmidt factor on the specific deformation direction in the area under study, calculate the distribution curve of the Schmidt factor value and the magnitude of the average Schmidt factor value, and simulate the deformation state of the material along a specific direction; This method can also mark the single Schmidt factor value corresponding to each grain of the analyzed sample and calculate the grain state inside a single grain;
[0040] 6) Mechanical and thermoelectric property analysis: Determine the difficulty of subsequent deformation of the whole material or internal grains along different directions according to Schmid's law, and measure the anisotropy difference of its thermoelectric properties in combination with the thermoelectric figure of merit calculation formula.
[0041] When the Bi2Te3 material undergoes plastic deformation, the activation of the slip system is related to the applied external force and crystal orientation. Under the action of an external force, regardless of the magnitude, direction and action mode of the external force, it can be decomposed into a normal stress perpendicular to a certain crystal plane and a shear stress along this plane. For the slip under the action of shear stress, each grain does not participate in deformation at the same time, and various slip systems inside a single grain do not participate in slip at the same time either. Only when the resolved shear stress along the slip direction on the slip plane reaches a certain critical value, the slip process can start. This resolved shear stress is called the critical resolved shear stress (CRSS) of slip on a specific crystal plane and specific slip direction, and its value is related to factors such as temperature, crystal structure, bond energy, purity, etc. The possibility of the opening of a specific slip system in the Bi2Te3 crystal is described by Schmid's law, as shown in formula (1):
[0042] τCRSS = σ·m (1)
[0044] where τ CRSS , σ, and m represent the critical resolved shear stress, the applied stress, and the Schmid factor, respectively. The activation conditions of slip systems with different orientations are different, that is, the strengths of the corresponding crystals are also different. In addition, when the value of m is larger, the shear stress for activating the slip system is smaller, and the grains are more likely to undergo plastic deformation; when the slip system is more difficult to activate, the corresponding value of m is smaller, that is, the activation condition of the deformation system can be predicted theoretically.
[0045] Under the same reference observation direction, different colors in the Schmid factor distribution map of Bi2Te3 thermoelectric materials correspond to different Schmid factor values, where blue to red represents the distribution of m values from low to high (0 - 0.5). When the Schmid factor value is between 0 - 0.4, the grains are in a hard orientation and are more likely to deform; when the Schmid factor value is between 0.4 - 0.5, the grains are more difficult to deform. Since grains with different Schmid factor values correspond to different degrees of difficulty in activating different slip systems, and m represents the average Schmid factor value of all grains, it is necessary to quantitatively calculate m, and the specific calculation method is shown in formula (2):
[0046]
[0047] where f is the area percentage of grains with a specific Schmid factor m in all grains.
[0048] Generally speaking, the critical shear stresses of different slip systems are different, and the slip system with the smallest critical shear stress will be activated first. Therefore, the slip system preferentially activated by polycrystalline materials at a specific temperature is certain. Then, the degree of difficulty of a certain grain or the whole material during the deformation process can be determined according to the deformation state of different grains and the Schmid factor.
[0049] Due to the existence of grain structure or texture in Bi2Te3 materials, the thermoelectric properties also show obvious anisotropy. The characteristic of the thermoelectric figure of merit is dimensionless, and its calculation method is shown in formula (3):
[0050]
[0051] where α, σ, κ, and T represent the Seebeck coefficient, the electrical conductivity, the thermal conductivity, and the temperature, respectively. Bi2Te3 materials have the characteristics of a layered structure and weak van der Waals forces between Te - Te layers, and they have different electrical and thermal conductivity properties in different directions along the plane and between the layers. For example, the electrical conductivity along the plane direction is 3 - 7 times that between the layers, and at the same time, the thermal conductivity along the plane is 2 - 2.5 times that between the layers. The Seebeck coefficients of the same material are relatively close, so the thermoelectric properties of the plane are usually better than those between the layers.
[0052] Furthermore, the method has a wide range of applications. The Bi2Te3 material can be a single crystal or polycrystalline material, and it is applicable to both the cold deformation process of the material at room temperature and the hot deformation process at high temperature. This method is applicable to the opening situation of different slip systems in any deformation direction during the deformation process, and the slip systems and deformation direction conditions need to be selected and established according to crystal characteristics and human needs.
[0053] Example 1
[0054] A method for measuring the anisotropy of the performance of Bi2Te3 thermoelectric materials using EBSD technology, which is applied to P-type Bi2Te3 materials prepared by hot pressing process, and is used to determine the cold deformation ability of the materials at room temperature, so as to reduce the breakage and waste of samples during dicing and granulation in the subsequent device development process.
[0055] This example specifically includes the following steps:
[0056] (1) Mark the cuboid block of P-type Bi2Te3 thermoelectric material prepared by hot pressing process, and mark its length, width and height directions as longitudinal direction (LD), transverse direction (TD) and normal direction (ND) respectively;
[0057] (2) Use wire electrical discharge machining equipment to sample the Bi2Te3 thermoelectric material. On the length, width and height (i.e., LD, TD and ND) directions of the hot-pressed block, cut samples with lengths, widths and heights of 10 mm, 8 mm and 3 mm respectively to distinguish each direction. The three directions of the sample length, width and height are perpendicular to each other, and LD, TD and ND correspond to the X-axis, Y-axis and Z-axis of the sample coordinate system respectively; for the wire-cut cuboid thin-film sample, successively select 600#, 1200#, 2000#, 3000#, 5000# SiC sandpaper for grinding until the test surface is flat, then select a flocked polishing cloth and polishing paste with W 2.0 for rough polishing, and select a silk velvet polishing cloth and polishing paste with W 0.5 for fine polishing; after fine polishing, the sample surface is bright and there are no obvious visible scratches; only use alcohol to clean and dry the polished sample, and there should be no contaminants visible to the naked eye on the sample surface;
[0058] (3) Remove the stress of the mechanically polished sample by electrolytic polishing. The electrolyte used during electrolytic polishing is a mixed solution of 10% perchloric acid + 90% alcohol. The electrolytic cathode is made of titanium alloy, the voltage is 50 V, the current is 0.08 A, the electrolytic time is 5 min, and the initial electrolytic temperature is -60 °C. After the electrolytic products are enriched on the sample surface and turn black, clean and remove the remaining black substances on the sample surface; after stress removal treatment, the sample surface should be in a mirror state;
[0059] (4) Keep the X-axis, Y-axis, and Z-axis of the sample parallel to the X-axis, Y-axis, and Z-axis of the sample stage of the testing equipment respectively, that is, the LD, TD, and ND of the sample correspond to the X-axis, Y-axis, and Z-axis of the sample stage. Use the EBSD technique to perform crystal diffraction and information collection on the region of interest on the sample surface, and obtain the crystallographic data of all grains in the analyzed region, specifically including data such as grain boundaries, grain sizes, grain orientations, pole figures, inverse pole figures, and Schmidt factors. The EBSD resolution is above 95%;
[0060] (5) Analyze the information measured in the EBSD experiment using HKL Channel 5 software. From Figure 1 it can be seen that the analyzed material phase is Bi2Te3, and along the observation direction, the orientations of most grains are {10-10}⊥ND and {11-20}⊥ND. Select and input the deformation direction and slip system. The deformation direction can be selected in any direction along the material sample coordinate system, such as Figure 2 shown. Here, the deformation is selected to be along the LD, TD, and ND directions of the sample. The main slip system of the Bi2Te3 thermoelectric material is {0001}<-12-10>. Analyze the Schmidt factors in the specific deformation direction of the studied region to obtain the Schmidt factor surface distribution map as shown in Figure 3 shown. Figure 4 is Figure 3 the distribution curves of the Schmidt factors of all grains in the Schmidt factor surface distribution map in Figure 3-4 . It can be observed that when deforming along the TD direction, the number of grains with Schmidt factors of 0.4 - 0.5 is significantly higher than that when deforming along the LD and ND directions. Statistically calculate the Schmidt factors of all grains in this region according to formula (2) to obtain the average Schmidt factor values corresponding to the {0001}<-12-10> slip system when deforming along the LD, TD, and ND directions, which are 0.27, 0.33, and 0.27 respectively, as shown in Table 1;
[0061]
[0062] (6) Combining with Schmidt's law in formula (1), it can be known that at the same temperature, the critical shear stress corresponding to a specific slip system is the same. When deforming along the LD and ND directions, the Schmidt factor is relatively low, and the deformation resistance is relatively large when performing subsequent hot working or cold working on the material along these two directions. Compared with the TD direction, cracks are more likely to be initiated or the material is more likely to be damaged in this P-type Bi2Te3 thermoelectric material. Combining the calculation and prediction with the sample texture strength and the results of formula (3), the thermoelectric properties of this material along LD and ND are better than those along the TD direction; the test results of the compression properties and the thermoelectric figure of merit at room temperature show that: when deforming along the LD and ND directions, the compression strengths are 129 MPa and 132 MPa respectively, and when deforming along the TD direction, the compression strength is 77 MPa; the ZT values along the LD and ND directions are 1.05 and 1.06 respectively, and the ZT value along the TD direction is 0.61; the analysis and prediction results of the mechanical and thermoelectric properties are consistent.
[0063] Example 2
[0064] A method for measuring the anisotropy of the performance of Bi2Te3 thermoelectric materials using the EBSD technique, which is applied to the N-type Bi2Te3 cylindrical bulk prepared by the spark plasma sintering (SPS) process. The content of this example is basically the same as that of Examples 1 and 2, and the difference is that: the N-type Bi2Te3 bulk prepared by the SPS process is analyzed by the method described in steps (1) to (6) in Example 1.
[0065] The N-type Bi2Te3 cylindrical bulk prepared by SPS process was marked, and the sample coordinate axes were established in the radial direction (RD), tangential direction (TD), and axial direction (AD). The X-axis, Y-axis, and Z-axis of the sample coordinate system corresponded to RD, TD, and AD. A wire electrical discharge machining equipment was used to sample the Bi2Te3 thermoelectric material. The length, width, and height directions of the sample were parallel to the RD, TD, and AD directions on the Bi2Te3 bulk, respectively. The size of the cut sample was 8×6×2 mm to distinguish each direction. For the sliced sample after wire cutting, SiC sandpapers of 600#, 1200#, 2000#, 3000#, and 5000# were successively selected for grinding. The surface to be tested was coarsely polished with a flocked polishing cloth and W1.5 polishing paste, and finely polished with a silk velvet polishing cloth and W0.5 polishing paste. Subsequently, the polished sample was cleaned with alcohol and dried. The sample surface was in a bright state and had no obvious visible scratches. The stress relief treatment of the mechanically polished sample was carried out by ion milling. The area of interest was selected, the milling angle was 85°, and it was milled for 1 h at 4 kv voltage and 20 min at 2 kv voltage. Then, the EBSD sample mounting was carried out. The ion-milled sample was kept parallel to the X-axis, Y-axis, and Z-axis of the test equipment sample stage, that is, the RD, TD, and AD of the sample corresponded to the X-axis, Y-axis, and Z-axis of the sample stage. The EBSD technology was used to perform crystal diffraction and information acquisition on the area of interest on the sample surface, and the crystallographic data of all grains in the analyzed area were obtained, specifically including data such as grain boundaries, grain sizes, grain orientations, and Schmidt factors. The EBSD resolution was above 95%.
[0066] The data information measured in the EBSD experiment was analyzed using HKL Channel 5 software. Figure 5 Figure 5 is the orientation distribution map and the corresponding unit cell schematic diagram of the sample prepared by the SPS process. It can be seen from the figure that the orientations of most grains are {0001}⊥AD along the observation direction. The deformation direction and slip system were selected and input. The deformation direction was selected to deform along the RD, TD, and AD of the sample, and the slip system was selected as {0001}<-12-10>. The Schmidt factors in the specific deformation direction of the studied area were analyzed, and the Schmidt factor surface distribution map was obtained as shown in Figure 6 shown. Figure 7 Figure Figure 6 is the distribution curve of the Schmidt factors corresponding to all grains in Figure 6-7It can be observed that the number of grains with a Schmid factor of 0.4 - 0.5 during deformation along the AD direction is significantly higher than that during deformation along the RD and TD directions. That is, when deforming along the AD direction, the Schmid factor corresponding to the {0001}<-12-10> slip system is the largest, and the deformation resistance is relatively small. Combining the texture strength of the sample and the results of formula (3) for calculation and prediction, the thermoelectric properties of this material along the RD and TD directions are better than those along the AD direction; in addition, in order to compare the deformation difficulty of grains with different orientations, Table 2 gives the single Schmid factor values for deformation along the RD, TD, and AD directions in regions 1 to 5.
[0067]
[0068] As can be seen from the table, region 1 has a hard orientation and is difficult to deform when deforming along the RD direction, and is relatively easy to deform when deforming along the TD and AD directions; region 2 has similar high Schmid factor values when deforming along the RD, TD, and AD directions, so it is easy to deform; region 3 has similar extremely low Schmid factor values regardless of whether it is deformed along the RD, TD, or AD directions, so the deformation resistance is high; region 4 has a soft orientation when deforming along the AD direction, and the deformation resistance is low; region 5 has a hard orientation when deforming along the TD direction, and the deformation resistance is high.
[0069] The above embodiments are only used to illustrate the technical concept and characteristics of the present invention, and their purpose is to enable those familiar with this technology to understand the content of the present invention and implement it accordingly, and it cannot be used to limit the protection scope of the present invention. All equivalent changes or modifications made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.
Claims
1. A method for measuring the anisotropy of the performance of Bi2Te3 thermoelectric materials using EBSD technology, characterized in that, The method specifically includes the following steps: S1) Establish a sample coordinate system: Mark according to the type of bismuth telluride material to be analyzed; S2) Sample preparation: Take samples of the bismuth telluride material marked in S1) using a wire cutting device. The length, width, and height of the obtained samples are perpendicular to each other and respectively correspond to the X-axis, Y-axis, and Z-axis of the sample coordinate system; then perform polishing to obtain the samples; S3) Remove stress: Remove the surface residual stress of the samples processed in S2); S4) Crystal diffraction: Keep the X-axis, Y-axis, and Z-axis of the samples processed in S3) parallel to the X-axis, Y-axis, and Z-axis of the sample stage of the testing equipment respectively. Use the backscattered electron diffraction technique to scan the region of interest on the sample surface to obtain the crystallographic data of all grains in this region; S5) Grain state calculation: Determine the grain orientation or sample texture of the region under study according to the measured and calculated Euler angles, unit cell orientation, Cohen coefficients, and pole figure information. Analyze the Schmidt factor surface distribution map according to the slip system and deformation direction conditions that need to be input, calculate the distribution curve of the Schmidt factor value and the magnitude of the average Schmidt factor value, and simulate the deformation state of the material along a specific direction; or mark the single Schmidt factor value corresponding to each grain of the analyzed sample and calculate the grain state inside a single grain; S6) Mechanical and thermoelectric property analysis: Determine the difficulty of subsequent deformation of the whole material or internal grains along different directions according to Schmid's law, and measure the anisotropy difference of its thermoelectric properties in combination with the thermoelectric figure of merit calculation formula.
2. The method according to claim 1, wherein: The Bi2Te3 material in S1) is a single crystal or polycrystalline material.
3. The method according to claim 1, wherein: If the Bi2Te3 material in S1) is a bulk sample, the X-axis, Y-axis, and Z-axis can be correspondingly marked as longitudinal LD, transverse TD, and normal ND; If the Bi2Te3 material is a rod-shaped sample, the X-axis, Y-axis, and Z-axis can be correspondingly marked as axial AD, tangential TD), and radial RD.
4. The method according to claim 1, characterized in that: The length of the samples in S2) is not greater than 15 mm, the width is not greater than 10 mm, and the height is not greater than 8 mm; The process of mechanical polishing is as follows: First, use 600#, 1200#, 2000#, 3000#, and 5000# SiC sandpapers to grind the samples until the test surface is flat. Then, use a flocked polishing cloth and polishing paste with W1.0 - W2.5 for rough polishing, and use a silk velvet polishing cloth and polishing paste with W 0.5 for polishing.
5. The method according to claim 1, characterized in that: The method for removing surface stress in S3) is argon ion bombardment or electrolytic polishing; After removing stress, the surface of the sample should be in a mirror state, and the EBSD resolution rate is above 95%.
6. The method according to claim 5, wherein: The process of electrolytic polishing is as follows: The electrolyte used is a mixed solution of 10% perchloric acid by total mass and 90% alcohol by total mass. The electrolytic cathode is made of titanium alloy, the voltage is 50 V, the current is 0.08 A, the electrolytic time is 5 min, and the initial electrolytic temperature is -60 °C.
7. The method according to claim 1, wherein: The crystallographic data of the grains in S4) include grain boundaries, grain size, grain orientation, pole figure, inverse pole figure, and Schmidt factor value.
8. The method according to claim 1, characterized in that: The slip system in S5) is {0001}<-12-10>.