Method for presenting second phase in bismuth telluride-based thermoelectric semiconductor material

Abstract

The invention relates to a method for presenting a second phase in a bismuth telluride-based thermoelectric semiconductor material, and belongs to the technical field of detection and analysis of a precipitated phase of the bismuth telluride-based thermoelectric semiconductor material. According to the method, the second phase on the surface of the bismuth telluride thermoelectric material is clearly and selectively presented by using different corrosive agents and adopting a step-by-step corrosion method. According to the method, the macroscopic distribution and the particle size of the second phase of the bismuth telluride-based thermoelectric semiconductor material can be clearly observed. The method disclosed by the invention can well solve the problem of vacancy in the research of the second phase of the bismuth telluride-based thermoelectric semiconductor material in the prior art; good technical support and material preparation basis are provided for macroscopic distribution research, particle size statistics, relation with a matrix and other research work of the bismuth telluride-based thermoelectric semiconductor material second phase in the thermoelectric industry and academic circle. The method has the advantages of simplicity, convenience, high efficiency, convenience in operation, high controllability, excellent technical effect and the like.

Description

Technical Field

[0001] This invention relates to the field of precipitation phase detection and analysis technology for bismuth telluride thermoelectric semiconductor materials, and particularly to a method for revealing the second phase in bismuth telluride-based thermoelectric semiconductor materials. Background Technology

[0002] Bismuth telluride-based thermoelectric materials are currently the only commercially available thermoelectric materials on a large scale. The main methods for improving the thermoelectric performance of bismuth telluride are band engineering and phonon engineering. Band engineering primarily involves altering electron movement within the semiconductor through doping and other methods to improve the crystal structure, optimize the energy band, and regulate carrier concentration, thereby enhancing thermoelectric performance. Phonon engineering, also known as nanoengineering, mainly involves controlling phonon transport mechanisms and interfaces through second-phase mechanisms, improving carrier concentration, and reducing thermal conductivity, thus improving thermoelectric performance. When the grain size of the bulk material approaches or reaches the nanoscale, or when nanoparticles are uniformly dispersed as a second phase within the bulk material, the grain boundary or phase density is significantly increased. The periodic potential field within a single grain changes at the grain boundary or phase interface, thus affecting the transport of carriers and phonons at the interface or grain boundary.

[0003] The introduction of a second phase is a key strategy for improving the thermoelectric performance of bismuth telluride-based materials, achieving synergistic optimization through phonon scattering, electrical transport modulation, and mechanical property enhancement. For example, introducing a uniformly dispersed second phase into thermoelectric materials introduces interfaces or dislocations, hindering phonon propagation and thus reducing the overall thermal conductivity of the material. The potential barrier formed at the interface between the second phase and the matrix filters low-energy carriers, increasing the Seebeck coefficient and thereby improving the power factor. Some second phases can optimize carrier concentration, suppress intrinsic excitations, and improve electrical conductivity. In addition, the introduction of the second phase can "pin" dislocations, hindering their movement, or significantly deflect cracks, consuming crack propagation energy, thereby improving the shear properties and compressive strength of the material and enhancing device durability. During the self-doping of bismuth telluride to introduce a second phase, increasing the content of additional tellurium, bismuth, selenium, and antimony elements decreases the carrier concentration and reduces the conductivity of the material. Simultaneously, the scattering of phonons by carriers weakens, but the presence of the second phase enhances phonon scattering. The combined effect causes the lattice thermal efficiency to initially decrease and then increase with tellurium content near room temperature. When a large number of grain boundaries or the second phase are present in the material, an additional potential barrier is formed between it and the matrix material, preferentially scattering low-energy carriers. Low-energy carriers contribute negatively to the Seebeck coefficient; their scattering can increase the Seebeck coefficient, leading to a significant increase in the calculated effective mass of the density of states (ZT) and thus a substantial increase in the ZT value of the material.

[0004] Due to the size limitation (approximately 5 nm to 100 μm) of the second phase in bismuth telluride thermoelectric semiconductor materials, previous research has primarily employed X-ray diffraction (XRD) and transmission electron microscopy (TEM) to study this second phase. However, the detection limit of XRD is highly dependent on the type and crystallinity of the analyte, and varies significantly depending on the phase and conditions. Generally, phases with a content below 5% may not be detectable. The second phase in materials is typically present in low concentrations; if the detection limit is met, it may be detected, otherwise it will be masked or ignored. Furthermore, TEM can only observe a very small area of ​​the second phase, making it impossible to determine its macroscopic distribution and statistical characteristics. Moreover, sample preparation for TEM observation is difficult and complex. In addition, the TEM sample preparation process is highly complex and requires skilled operators. More importantly, due to the limitations of TEM observation, obtaining the macroscopic distribution characteristics of the second phase in bismuth telluride thermoelectric materials requires extensive experimental observation.

[0005] Currently, existing etching methods for bismuth telluride thermoelectric materials have certain limitations, and obtaining clearly visible bulk samples of the second phase remains a pressing problem. Therefore, finding a suitable etchant and etching method that can reveal the precipitated phase in bulk bismuth telluride thermoelectric materials at room temperature or near room temperature would simplify and streamline the study of the second phase, significantly advancing research and performance optimization of bismuth telluride thermoelectric materials. Researchers in the bismuth telluride thermoelectric industry are eager to find a method to reveal the second phase in bismuth telluride-based thermoelectric semiconductor materials. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for presenting a second phase in a bismuth telluride-based thermoelectric semiconductor material.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention provides an etchant suitable for bismuth telluride-based materials, said etchant being: a ferric sulfate solution with a concentration of 0.3~1.2 mol / L; or, composed of 5~10 mL lactic acid, 2~5 mL 65% nitric acid solution, 0.1~0.5 mL benzotriazole solution, 0.1~0.5 g sodium dodecyl sulfate, 0.1~1 mL ethoxylated nonionic fluorocarbon surfactant, and 100~200 mL water.

[0008] In a preferred embodiment of the first aspect, the ethoxylated nonionic fluorocarbon surfactant is Zonyl® FSN-100, with CAS number 65545-80-4 and linear molecular formula (C2H4O). x (CF2) y C2H5FO.

[0009] Alternatively, the concentration of the ferric sulfate solution may also be 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.7 mol / L, 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, or 1.2 mol / L.

[0010] Optionally, the amount of lactic acid added can be 5 mL, 7 mL, 8 mL, 9 mL or 10 mL; the amount of benzotriazole added can be 0.1 mL, 0.3 mL, 0.4 mL or 0.5 mL; the amount of 65% nitric acid added can be 2 mL, 3 mL, 4 mL or 5 mL; the amount of sodium dodecyl sulfate added can be 0.1 g, 0.2 g, 0.3 g or 0.5 g; the amount of Zonyl FSN-100 surfactant added can be 0.1 mL, 0.2 mL, 0.3 mL, 0.4 mL, 0.5 mL, 0.7 mL, 0.8 mL, 0.9 mL or 1 mL; and the amount of water added can be 110 mL, 120 mL, 150 mL, 180 mL, 190 mL or 200 mL.

[0011] This invention prepares two different etchants for etching the surface of bismuth telluride-based materials to reveal their second phase. The macroscopic distribution and particle size of the second phase in the bismuth telluride-based thermoelectric semiconductor material can be clearly observed. Using the above-mentioned ratio, uniform and controllable etching of the complex microstructure of bismuth telluride-based thermoelectric materials can be achieved, better highlighting the height difference and interface between the second phase and the substrate.

[0012] Lactic acid is a mild organic acid, far less corrosive than sulfuric acid or hydrochloric acid. It has a slow chelating corrosion effect on bismuth telluride substrates, forming soluble complexes with metal ions. This prevents excessively rapid corrosion that could lead to grain boundary over-etching or surface roughness, thus achieving slow, controlled chemical corrosion of the substrate and obtaining a clearer contrast. Nitric acid provides the necessary oxidizing properties to promote the initiation of the corrosion reaction, effectively dissolving or etching the substrate, but its dosage is relatively small, and overall corrosion is controllable. Benzotriazole (BTA) is an effective metal corrosion inhibitor that enhances the selective corrosion of different phases, resulting in a more distinct contrast between the exposed second phase and the protected substrate. Deionized water is used as the solvent to dilute each component to an appropriate working concentration. Its content directly affects the ionic strength and reactivity of the etchant, and is crucial for controlling the corrosion rate. Sodium dodecyl sulfate (SDS) and ethoxylated nonionic fluorocarbon surfactants can reduce surface tension, improve wettability, and promote the removal of corrosion products, enabling the corrosive agent to spread and penetrate evenly onto the sample surface. This inhibits over-corrosion and under-corrosion and improves surface smoothness, which can greatly enhance the uniformity of corrosion and the ability to reveal the second phase.

[0013] In a second aspect, the present invention provides the application of the etchant described in the first aspect in the preparation of a second phase of a bismuth telluride-based material.

[0014] Thirdly, the present invention provides a method for presenting a second phase of a bismuth telluride-based material, the method comprising the following steps: Step 1: Grind the bismuth telluride-based material sample; Step 2: Polish the ground sample to achieve a surface roughness Ra≤0.05μm; Step 3: Perform the first etching on the polished sample using etchant 1; Step 4: Use etchant 2 to perform a second etching on the sample from step 3; The corrosive agent 1 is a ferric sulfate solution; the corrosive agent 2 is composed of 5-10 mL lactic acid, 2-5 mL 65% nitric acid solution, 0.1-0.5 mL benzotriazole solution, 0.1-0.5 g sodium dodecyl sulfate, 0.1-1 mL ethoxylated nonionic fluorocarbon surfactant, and 100-200 mL water.

[0015] This invention utilizes a step-by-step etching strategy to achieve clear and selective exposure of the second phase (such as precipitates, inclusions, or grain boundaries) on the surface of bismuth telluride thermoelectric materials. Etching agent 1 and etchant 2 have different chemical properties and etching mechanisms.

[0016] Etching agent 1 focuses on oxidative corrosion. It is used to perform preliminary and controllable oxidative corrosion to remove the surface layer and micro-scratches of the bismuth telluride material and initially expose the second phase. By setting the concentration of ferric sulfate, the corrosion rate and depth are controlled to avoid excessive corrosion that could damage the second phase.

[0017] Etching agent 2 focuses on acidic etching and surface modification, further refining the etching process and enhancing the contrast of the second phase. The lactic acid and nitric acid in etchant 2 provide an acidic environment, further etching the material surface, selectively etching the matrix or second phase, and enhancing the contrast of the phase interface. Nitric acid, with its strong oxidizing properties, works synergistically with lactic acid to regulate the etching rate, avoiding over-etching or under-etching caused by a single etchant. Benzotriazole (BTA) is a corrosion inhibitor that adsorbs onto the surface of specific phases (such as the metallic phase), inhibiting over-etching and protecting the morphological integrity of the second phase. Sodium dodecyl sulfate (SDS) and ethoxylated nonionic fluorocarbon surfactants act as surfactants, reducing surface tension, improving the wettability and uniformity of the etchant, ensuring a uniform etching reaction, and helping to remove etching products. Simultaneously, combined with ultrasonic vibration and rinsing, it helps remove etching products and contaminants, ensuring a clean sample surface for subsequent microscopic observation.

[0018] Therefore, in this invention, the pretreatment of etchant 1 lays the foundation for the subsequent treatment with etchant 2. Preliminary corrosion reduces surface irregularities, allowing etchant 2 to penetrate and act more uniformly. The order in which the two are used is also crucial, affecting the presentation of the second phase.

[0019] The two are chemically interdependent: the oxidative corrosion of etchant 1 creates a suitable surface condition for the acidic corrosion of etchant 2. If etchant 2 is used first, its acidic components may react directly with the material surface, leading to disordered or excessive corrosion and damaging the structure of the second phase. The oxidative properties of etchant 1 may not be able to effectively repair this damage. There is a sequential requirement for surface pretreatment: etchant 1 removes the surface and strain layers through controlled oxidation, making the surface more uniform, thus allowing etchant 2 to selectively corrode. If the order is reversed, etchant 2 may not penetrate evenly, resulting in uneven corrosion and affecting the presentation of the second phase. Functionally, they are complementary: etchant 1 focuses on "coarse corrosion," while etchant 2 focuses on "fine corrosion." The logic of oxidation followed by acid etching is a common sequence in metallography, ensuring that the corrosion process is gradually refined from macroscopic to microscopic. Reversing the order may cause the oxidative effect of etchant 1 to fail on the already acid-etched surface or cause unnecessary chemical reactions.

[0020] As a preferred embodiment of the third side, the grinding process in step 1 involves grinding the sample sequentially with sandpaper of grits of 600#, 1000#, 2500#, and 5000#.

[0021] As a preferred embodiment of the third aspect, the polishing process in step 2 includes coarse polishing and fine polishing; the coarse polishing is performed using a flocked polishing cloth and polishing paste with a particle size of W1.5~W2.5; the fine polishing is performed using a silk velvet polishing cloth and polishing paste with a particle size of W0.5~W1.0.

[0022] Because conventional smoothness produces numerous corrosion artifacts after etching, resulting in uneven etching, the image becomes blurred, obscuring or distorting the true morphology of the second phase. To reveal the internal microstructure of the material (such as the second phase and grain boundaries) more realistically and clearly, this invention employs a process ranging from coarse to fine sandpaper polishing (600# to 5000#) to mechanical polishing with ultra-fine polishing paste (W0.5~W1.0). This is to avoid corrosion artifacts, create conditions for uniform etching, and ensure the true manifestation of the second phase. High-precision polishing ensures macroscopic and microscopic surface smoothness, allowing the etchant to act uniformly on the entire observation surface, thus revealing all second phases under the same conditions, resulting in a clear image with high contrast. If scratches or deformed layers exist on the surface, the etchant will preferentially react in these defective, high-energy areas. This leads to over-etching, deepening, and widening of scratches, which may even be misjudged as grain boundaries or second phases under a microscope.

[0023] Before etching, the present invention performs fine polishing on the bismuth telluride thermoelectric material sample to make its surface roughness Ra≤0.05μm, which can more realistically and clearly show the internal microstructure of the material (such as the second phase).

[0024] In a preferred embodiment of the third side, the corrosion temperature in step 3 is 30~80℃ and the corrosion time is 4~12min.

[0025] As a preferred embodiment of the third aspect, the corrosion in step 3 is treated by ultrasonic oscillation.

[0026] In a preferred embodiment of the third aspect, the frequency of the ultrasonic wave is 20kHz to 40kHz.

[0027] As a preferred embodiment of the third side, the corrosion temperature in step 4 is 15~30℃ and the corrosion time is 0.5~3min.

[0028] As a preferred embodiment of the third aspect, the bismuth telluride-based material includes, but is not limited to, Bi2Te3, Bi... 0.5 Sb 1.5 Te3, Bi2Te 2.7 Se 0.3 .

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention provides a method for revealing the second phase of bismuth telluride-based thermoelectric semiconductor materials. After processing using this method, the macroscopic distribution and particle size of the second phase of the bismuth telluride-based thermoelectric semiconductor material can be clearly observed. This invention effectively addresses the gaps in existing research on the second phase of bismuth telluride-based thermoelectric semiconductor materials, providing excellent technical support and a solid foundation for research in the thermoelectric industry and academia on the macroscopic distribution, particle size statistics, and relationship with the matrix of the second phase of bismuth telluride-based thermoelectric semiconductor materials. This method also has the advantages of being simple, efficient, easy to operate, highly controllable, and producing excellent technical results. Attached Figure Description

[0030] Figure 1 This is a flowchart of the method for preparing the second phase of a bismuth telluride-based thermoelectric semiconductor material according to the present invention; Figure 2 The image shows the morphology of the bismuth telluride hot-pressed sample from Example 3. Figure 3 This is an orientation diagram of the hot-pressed sample after annealing in Example 3; Figure 4 Metallographic diagram of the bismuth telluride second phase presented in Example 3 using the method of the present invention; Figure 5 This is a schematic diagram showing the SEM morphology and EDS analysis results of the bismuth telluride second phase presented using the method of the present invention in Example 3. Figure 6 This is a morphology image of the bismuth telluride hot-extruded sample from Example 4; Figure 7 This is a texture distribution diagram of the hot-extruded sample in Example 4; Figure 8 Metallographic diagram of the bismuth telluride second phase presented in Example 4 using the method of the present invention; Figure 9 This is a schematic diagram showing the SEM morphology and EDS analysis results of the bismuth telluride second phase presented using the method of the present invention in Example 4.

[0031] Figure 10 The metallographic structure of bismuth telluride second phase in Comparative Example 12, which was etched without the use of etchant 1; Figure 11 The metallographic structure of bismuth telluride second phase was obtained by etching without using etchant 2 in Comparative Example 13. Figure 12 The metallographic structure of the bismuth telluride second phase after corrosion in Comparative Example 14 is shown. Detailed Implementation

[0032] To better illustrate the purpose, technical solution, and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0033] Example 1 This embodiment provides an etchant suitable for bismuth telluride-based thermoelectric semiconductor materials, wherein the etchant is a 0.6 mol / L ferric sulfate solution.

[0034] Preparation method: Prepare a 0.6 mol / L ferric sulfate solution by stirring with a glass rod and letting it stand for 30 minutes to avoid the heat released after preparation causing the solution temperature to rise.

[0035] Example 2 This embodiment provides an etchant suitable for bismuth telluride-based thermoelectric semiconductor materials. The etchant is composed of 6 mL of lactic acid, 0.2 mL of benzotriazole, 2.5 mL of 65% nitric acid, 0.4 g of sodium dodecyl sulfate, 0.6 mL of ethoxylated nonionic fluorocarbon surfactant, and 100 mL of water.

[0036] Preparation method: Add 6 mL of lactic acid, 0.2 mL of benzotriazole, 2.5 mL of 65% nitric acid, 0.4 g of sodium dodecyl sulfate, and ethoxylated nonionic fluorocarbon surfactant to 100 mL of deionized water in sequence, and stir to mix well.

[0037] The ethoxylated nonionic fluorocarbon surfactant used in this embodiment is Zonyl® FSN-100, with CAS number 65545-80-4 and linear molecular formula (C2H4O). x (CF2) y C2H5FO.

[0038] Example 3 This embodiment provides a method for presenting a second phase of a bismuth telluride-based material, specifically including the following steps: Step 1: Grind the bismuth telluride material sample; the specific operation is as follows: From hot-pressed samples (e.g.) Figure 2 A bismuth telluride material block sample was selected from the sample shown, with a composition of Bi. 0.5 Sb 1.5 Te3, sample structure and orientation distribution as follows Figure 3 As shown, the bismuth telluride material sample was ground sequentially using sandpaper with mesh sizes of 600#, 1000#, 2500#, and 5000#. Step 2: Polish the bismuth telluride thermoelectric material sample; the specific operation is as follows: The grinding sample processed in step 1 was coarsely polished using flocking material and W1.5 polishing paste 1, and finely polished using silk velvet material and W0.5 polishing paste 2 to remove the residual micro-grind marks on the sample surface. The sample was then observed under a microscope until it achieved a bright mirror finish with a surface roughness Ra≤0.05μm. Step 3: Perform the first etching on the polished sample; the specific operation is as follows: The polished sample treated in step 2 was completely immersed in etchant 1 (the etchant in Example 1) with the polished surface facing upwards, and ultrasonic etching was performed. The temperature of etchant 1 was 50°C, the etching time was 10 min, and the frequency of the ultrasonic cleaner was 30 kHz. Subsequently, the etched sample was removed, rinsed with water, then rinsed with anhydrous ethanol, and dried with a hair dryer. Step 4: Perform a second etching on the polished sample; the specific operation is as follows: Take out the dried sample treated in step 3, immerse it completely in etchant 2 (the etchant in Example 2), and etch it at 20°C for 2 minutes; then take out the etched sample, rinse it with water, then rinse it with anhydrous ethanol and dry it.

[0039] The parameters for Examples 3-5 and Comparative Examples 1-2 are shown in the table below: Table 1 The difference between Comparative Example 3 and Example 3 is that ferric chloride is used instead of ferric sulfate in corrosive agent 1, with the same concentration, while the rest is the same as in Example 3.

[0040] The difference between Comparative Example 4 and Example 3 is that the corrosive agent 2 lacks lactic acid, and the missing part is filled with water; otherwise, it is the same as Example 3.

[0041] The difference between Comparative Example 5 and Example 3 is that benzotriazole is missing in corrosive agent 2, and the missing part is filled with water; otherwise, it is the same as in Example 3.

[0042] The difference between Comparative Example 6 and Example 3 is that the corrosive agent 2 lacks nitric acid, and the missing part is filled with water; otherwise, it is the same as Example 3.

[0043] The difference between Comparative Example 7 and Example 3 is that the corrosive agent 2 lacks SDS, and the missing part is filled with water; otherwise, it is the same as Example 3.

[0044] The difference between Comparative Example 8 and Example 3 is that Corroder 2 lacks ZonylFSN-100, and the missing part is filled with water; otherwise, it is the same as Example 3.

[0045] The difference between Comparative Example 9 and Example 3 is that thiourea is used instead of benzotriazole in corrosive agent 2, otherwise the same as in Example 3.

[0046] The difference between Comparative Example 10 and Example 3 is that glacial acetic acid was used instead of lactic acid in corrosive agent 2, otherwise the same as in Example 3.

[0047] The difference between Comparative Example 11 and Example 3 is that FSN-100 was replaced with the non-fluorocarbon surfactant Triton X-10 in corrosive agent 2, and the rest is the same as in Example 3.

[0048] The difference between Comparative Example 12 and Example 3 is that corrosive agent 1 is not used for corrosion, otherwise it is the same as Example 3.

[0049] The difference between Comparative Example 13 and Example 3 is that corrosive agent 2 is not used for corrosion, otherwise it is the same as Example 3.

[0050] The difference between Comparative Example 14 and Example 3 is that the order in which corrosive agents 1 and 2 are used is reversed; otherwise, they are the same as in Example 3. Specifically: Step 3: Immerse the polished sample treated in Step 2 completely in etchant 2 for the first etching; Step 4: Immerse the polished sample treated in Step 3 completely in Etching Agent 1 for a second etching.

[0051] Observation of the second phase of the test sample: The treated corrosion samples were placed under a metallographic microscope and a scanning electron microscope to observe and analyze the precipitated phases on the corrosion surface of the bismuth telluride thermoelectric material: Results of sample treatment in Example 3: Metallurgical microscopy observation results are as follows Figure 4 As shown, the dark brown areas represent the specific distribution locations of the second phase in the bismuth telluride-based thermoelectric semiconductor material, and the size of these areas represents the size of the second phase in the alloy. Scanning electron microscopy observations are as follows... Figure 5 As shown, the secondary electron morphology, backscattering morphology, and EDS energy dispersive spectroscopy analysis were performed on the precipitated phase on the surface of the bismuth telluride thermoelectric material. Figure 5 As shown, by combining secondary electron morphology and backscattering morphology, the distribution, morphology and size of the second phase in the corroded material can be clearly distinguished. EDS point scan energy dispersive spectroscopy analysis of the second phase shows that the second phase in the hot-pressed sample after annealing is a Te-rich second phase. The Te content in the second phase (marked in purple) is much higher than the Te content in the matrix (marked in green).

[0052] Results of sample treatment in Example 4: Metallurgical microscopy observation results are as follows Figure 8 As shown, the bright white area represents the size, morphology, and distribution of the second phase in the bismuth telluride-based thermoelectric semiconductor material within the matrix. Scanning electron microscopy observations are as follows: Figure 9As shown, by combining secondary electron morphology and backscattering morphology, the distribution, morphology and size of the second phase in the corroded material can be clearly distinguished. EDS point scan energy dispersive spectroscopy analysis of the second phase shows that the second phase in the hot-pressed sample after annealing is a Te-rich second phase (green mark) and a Sb-rich second phase (purple mark). The comparison shows that the Te-rich second phase, the Sb-rich second phase and the Sb and Te element contents in the matrix are quite different.

[0053] Results of sample treatment in Example 5: Under a metallographic microscope, the macroscopic distribution of the second phase in the bismuth telluride-based thermoelectric semiconductor material can be observed, with clarity comparable to that of Examples 3 and 4. The boundary between the second phase and the matrix is ​​distinct, allowing for effective particle size analysis. Secondary electron and backscattered electron morphology images obtained by SEM clearly distinguish the size, morphology, and distribution of the second phase within the matrix. EDS spot scan energy dispersive spectroscopy analysis of the second phase reveals the presence of a Te-rich second phase in the sample, with a significantly different elemental composition compared to the matrix. This demonstrates that the method of this invention can effectively and reliably reveal second phase information under different process parameters.

[0054] Results of the samples after processing in Comparative Examples 1-14: In Comparative Example 1, the concentration of ferric sulfate in etchant 1 was too low, resulting in insufficient corrosive ability and failure to effectively remove the surface deformation layer. After subsequent treatment with etchant 2, the entire surface showed uneven corrosion, and the outline of the second phase was blurred, making clear observation and statistical analysis impossible. This indicates that a ferric sulfate concentration below the range specified in this invention will lead to failure of the first-step corrosion process.

[0055] In Comparative Example 2, the ferric sulfate concentration in etchant 1 was too high, while etchant 2 contained too little lactic acid and excessive BTA. Etching with etchant 1 was excessively harsh, resulting in numerous pits and over-corrosion on the surface. The unbalanced formulation in etchant 2 further damaged the surface smoothness, and the morphology of the second phase was severely damaged, making it unrecognizable. This demonstrates that exceeding the concentration limits of the components in this invention will compromise the controllability of corrosion.

[0056] In Comparative Example 3, ferric chloride was used instead of ferric sulfate in etchant 1. The sample surface showed pitting corrosion and uneven cloud-like patterns, with chaotic contrast in the backscattered image, making it difficult to distinguish the second phase from the corrosion artifacts. This demonstrates that sulfate ions in ferric sulfate are crucial for achieving uniform and controllable oxidative corrosion, and that replacing them with other iron salts yields poor results.

[0057] In Comparative Example 4, the lack of lactic acid in etchant 2 resulted in a very shallow overall corrosion of the sample surface, weak relief of the second phase, and extremely poor contrast under a microscope, making it almost impossible to observe the distribution of the second phase. This indicates that the chelating and mild corrosive effects of lactic acid are indispensable for selectively highlighting the second phase and forming good contrast.

[0058] In Comparative Example 5, benzotriazole (BTA) was lacking in corrosive agent 2. The second phase, especially the metal-rich phase, was excessively corroded, with rough edges. Some fine second phases were even completely etched through and disappeared, making it impossible to obtain the true morphology and size. This indicates that BTA, as a corrosion inhibitor, is crucial for protecting the morphology of specific second phases and achieving selective corrosion.

[0059] In Comparative Example 6, the lack of nitric acid in corrosive agent 2 resulted in a slow initiation of the corrosion reaction and insufficient overall corrosion. Residual corrosion products covered the surface, the second phase was incompletely visualized, and the image was blurry. This indicates that the oxidizing power provided by nitric acid is a necessary condition for initiating and maintaining an effective corrosion reaction.

[0060] In Comparative Example 7, the absence of sodium dodecyl sulfate (SDS) in etchant 2 resulted in poor wettability of the etchant on the sample surface, leading to noticeable non-wetting areas and uneven corrosion. The visibility of the second phase varied significantly across different regions. This indicates that SDS, as a surfactant, plays a crucial role in ensuring uniform corrosion.

[0061] In Comparative Example 8, the absence of Zonyl FSN-100 in corrosive agent 2 resulted in a significantly reduced clarity of the boundaries of the second phase, especially the submicron-sized particles, despite the general morphology being visible. Fine corrosion residues were also observed on the surface. This indicates that fluorocarbon surfactants, due to their extremely low surface tension, possess an unparalleled advantage over hydrocarbon surfactants in revealing the nano / submicron-scale second-phase interface.

[0062] In Comparative Example 9, thiourea was used instead of BTA in etchant 2. The contrast of the second phase was significantly worse than in Example 3. Some phases were over-etched while others were under-etched, indicating that the corrosion inhibition selectivity of thiourea is different from that of BTA and cannot achieve the best contrast in this system. This shows that BTA has an irreplaceable specific corrosion inhibition function in bismuth telluride-based material systems.

[0063] In Comparative Example 10, glacial acetic acid was used instead of lactic acid in etchant 2. The sample surface showed an "orange peel"-like texture and increased roughness. Although the second phase was visible, its boundary appeared rough due to uneven corrosion of the matrix, and the dimensional measurements were inaccurate. This indicates that the α-hydroxy chelating effect of lactic acid is crucial for achieving a flat and controllable corrosion surface, which is unmatched by other organic acids.

[0064] In Comparative Example 11, Triton X-100 was used instead of Zonyl FSN-100 in etchant 2. After etching, although the macroscopic distribution of the second phase was visible, under high magnification SEM, the edges of the fine second phase were found to be blurred, and the interface clarity was significantly worse than in Example 3. This indicates that the ultra-low surface tension and excellent penetration provided by Zonyl FSN-100 are essential for clearly revealing the phase interface.

[0065] Comparative Example 12: Samples treated directly with etchant 2, without the use of etchant 1, retained scratches from grinding and polishing. These scratches severely interfered with observation. The second phase was easily obscured by these scratches, making it impossible to obtain a complete macroscopic distribution image, such as... Figure 10 As shown, the pretreatment with etchant 1 is necessary to remove the surface damage layer and create a smooth base for the second step of fine etching.

[0066] Comparative Example 13: Without etching agent 2, the sample treated only with etching agent 1 had a relatively flat surface, but the contrast between the second phase and the matrix was very weak, making it impossible to clearly distinguish the accurate shape and particle size of the second phase, and further hindering EDS composition confirmation. Figure 11 As shown, this demonstrates that etchant 2 is decisive in selectively corroding and enhancing the contrast between the second phase and the matrix, thereby clearly revealing the second phase.

[0067] Comparative Example 14, where etchants 1 and 2 were used in reverse order, resulted in extremely poor sample surface condition. The first application of acidic etchant 2 caused random, disordered, and severe corrosion, which the subsequent application of oxidizing etchant 1 could not repair. Figure 12 As shown, the final surface is rough, and the morphology of the second phase is completely destroyed and cannot be observed. This proves that the sequence of steps, "oxidation (corroder 1) followed by acid etching (corroder 2)," is the key to achieving controllable, stepwise etching and ultimately clearly revealing the second phase.

[0068] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A etchant suitable for bismuth telluride-based materials, characterized in that, The corrosive agent is: a ferric sulfate solution with a concentration of 0.3~1.2 mol / L; or, It consists of 5-10 mL lactic acid, 0.1-0.5 mL benzotriazole solution, 2-5 mL 65% nitric acid solution, 0.1-0.5 g sodium dodecyl sulfate, 0.1-1 mL ethoxylated nonionic fluorocarbon surfactant, and 100-200 mL water.

2. The use of the etchant as described in claim 1 in the preparation of a second phase of a bismuth telluride-based material.

3. A method for presenting a second phase of a bismuth telluride-based material, characterized in that, The method includes the following steps: Step 1: Grind the bismuth telluride-based material sample; Step 2: Polish the ground sample to achieve a surface roughness Ra≤0.05μm; Step 3: Perform the first etching on the polished sample using etchant 1; Step 4: Use etchant 2 to perform a second etching on the sample from step 3; The corrosive agent 1 is a 0.3~1.2 mol / L ferric sulfate solution; The corrosive agent 2 is composed of 5-10 mL of lactic acid, 2-5 mL of 65% nitric acid solution, 0.1-0.5 mL of benzotriazole solution, 0.1-0.5 g of sodium dodecyl sulfate, 0.1-1 mL of ethoxy nonionic fluorocarbon surfactant, and 100-200 mL of water.

4. The method as described in claim 3, characterized in that, The grinding process in step 1 involves grinding the sample sequentially with sandpaper of grits of 600#, 1000#, 2500#, and 5000#.

5. The method as described in claim 3, characterized in that, The polishing process in step 2 includes rough polishing and fine polishing; The coarse polishing is performed using a flocked polishing cloth and polishing paste with a particle size of W1.5~W2.5; the fine polishing is performed using a silk velvet polishing cloth and polishing paste with a particle size of W0.5~W1.

0.

6. The method as described in claim 3, characterized in that, In step 3, the corrosion temperature is 30~80℃ and the corrosion time is 4~12min.

7. The method as described in claim 3, characterized in that, In step 3, the corrosion is treated using an ultrasonic vibration method.

8. The method as described in claim 7, characterized in that, The frequency of the ultrasound is 20kHz to 40kHz.

9. The method as described in claim 3, characterized in that, In step 4, the corrosion temperature is 15~30℃ and the corrosion time is 0.5~3min.

10. The method as described in claim 3, characterized in that, The bismuth telluride-based materials include, but are not limited to, Bi2Te3, Bi 0.5 Sb 1.5 Te3, Bi2Te 2.7 Se 0.3 .

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