Thermoelectric performance evaluation method based on in-situ synthesis

Abstract

The invention discloses a thermoelectric performance evaluation method based on in-situ synthesis, which comprises the following steps of: placing a pair of thermocouples on each of the upper bottom surface and the lower bottom surface of a material to be synthesized, ensuring that temperature measuring points of the thermocouples are in electric contact with the material, placing a gradient heat source sheet at the bottom of the material, and separating the gradient heat source sheet from the material by using an electric insulation sheet; two pairs of thermocouples are used for measuring temperature and thermoelectromotive force; the gradient heat source sheet is electrified and heated to generate temperature gradient in the material; the measuring module is assembled in a high-pressure chamber, after the pressure and the temperature are stable, periodic voltage is applied to the gradient heat source sheet, a periodic temperature gradient correspondingly appears in the material in the axial direction, and temperature signals of the two pairs of thermocouples and thermoelectromotive force signals between the positive electrodes and the negative electrodes of the two pairs of thermocouples are synchronously collected; and through linear fitting of the thermoelectromotive force signal, the Seebeck coefficient of the material is finally obtained. And resistivity measurement is synchronously carried out through the function change-over switch. The in-situ measurement difficulty of the synthesis of the Seebeck coefficient and the resistivity is greatly reduced.

Description

Technical Field

[0001] The present invention relates to the field of measurement technology and material science technology, and in particular to a thermoelectric performance evaluation method based on in-situ synthesis. Background Art

[0002] As global energy demand continues to rise, traditional energy reserves are facing an increasingly severe depletion crisis, making the research and application of thermoelectric materials increasingly critical. These materials possess the unique ability to directly convert thermal energy into electricity, while offering significant advantages such as silent operation and zero emissions. They present a promising future for energy technology. Their superior energy conversion efficiency and reliable operating characteristics offer new technological solutions to address global challenges such as energy shortages and ecological pollution.

[0003] The Seebeck coefficient and resistivity are fundamental parameters for measuring a material's thermoelectric conversion efficiency. In materials research, the traditional approach involves synthesizing thermoelectric materials and measuring their thermoelectric properties as two separate steps: first, synthesizing the material using high-temperature equipment, and then measuring its thermoelectric parameters using testing equipment. This approach has significant limitations. The first step can result in excessively high synthesis temperatures or excessively long synthesis times (so-called "over-sintering"), or insufficient temperatures and times, leading to under-sintering. Therefore, optimizing the thermoelectric properties of a new thermoelectric material system primarily relies on repeatedly adjusting process parameters such as synthesis temperature and time. This approach is inefficient and expensive. Furthermore, high-pressure conditions (i.e., extreme pressures far exceeding one atmosphere) are frequently employed in the synthesis of thermoelectric materials. The resulting materials often exhibit thermoelectric properties that cannot be achieved under normal-pressure synthesis. This is because high pressure promotes material densification, accelerating the reaction and sintering process, and can also form unique crystal structures or grain boundary structures. It is worth noting that, like synthesis temperature and time, the optimal synthesis pressure also depends on the material system.

[0004] In summary, if a performance evaluation system can be developed to measure the Seebeck coefficient and resistivity in situ during synthesis, then the optimal synthesis temperature, time and pressure conditions can be accurately found, greatly improving the development efficiency of thermoelectric synthetic materials. This technology will provide strong support for finding more sustainable and environmentally friendly green energy and improving the efficiency of energy conversion, and is of great significance for solving the energy crisis.

[0005] Currently, the Seebeck coefficient and resistivity are typically measured separately using independent equipment, and the measurement conditions are limited to normal pressure and at relatively low temperatures. For example, Chinese patent CN112285153A discloses a high-precision room-temperature Seebeck coefficient test device. This solution can only measure the Seebeck coefficient alone and is applicable only under normal pressure and room temperature conditions. Furthermore, the method's temperature control accuracy is limited, and thermal contact errors are not completely eliminated. In 2013, Zhu Pinwen et al. disclosed an apparatus and method for in-situ testing the transport properties of conductive materials under high temperature and high pressure (Patent Publication No.: CN103399044A). Although this method can simultaneously measure the Seebeck coefficient and resistivity, the maximum temperature allowed is only approximately 500°C. The temperature gradient within the material during measurement is determined by the heating characteristics of the assembly and is uncontrollable, which poses certain limitations for accurately measuring the Seebeck coefficient of the material. Furthermore, the assembly technology involved in the patent is overly complex, hindering the success rate of the measurement. In 2021, Zhu Pinwen and others disclosed a device and method for in-situ precise measurement of electrothermal transport properties at high temperature and high pressure (patent publication number: CN113777404A). The non-contact design of the thermocouple in this method may introduce temperature measurement errors, and the measured temperature is less than 400K, and the stability and repeatability under extreme conditions still need to be verified. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to address the shortcomings of the current technology for measuring the Seebeck coefficient and resistivity in the field of high-pressure synthesis, and to provide a method for evaluating thermoelectric performance based on in-situ synthesis. The method dynamically measures the Seebeck coefficient by setting a gradient heat source, and reuses the test electrodes for measuring the Seebeck coefficient to test the resistivity using a two-probe method, thereby accurately evaluating the Seebeck coefficient and resistivity of the in-situ synthesized material for material performance evaluation.

[0007] The technical solution adopted in the present invention is:

[0008] A method for evaluating thermoelectric performance based on in-situ synthesis comprises the following steps:

[0009] S1. Assemble the six-sided pyrophyllite internal components:

[0010] Hexahedral pyrophyllite and mullite tubes are used as pressure-transmitting sealed media and heat-insulating materials, a heating tube is used as a heater, and an insulating inert material is used as a sample chamber and filler. A semiconductor material is set below the sample chamber as a gradient heat source, and metal wires are used as electrode wires of the gradient heat source. The temperature measurement points of two pairs of thermocouple electrode wires are introduced into the sealed sample chamber and are in close contact with the upper and lower surfaces of the sample, maintaining an electrically conductive state.

[0011] S2. Place the assembled six-sided pyrophyllite into the pressure chamber of the six-sided large-cavity press, lead the thermocouple electrode wire out from the gap between the tungsten carbide anvils of the press, and perform insulation treatment;

[0012] S3. Connect the four thermocouple electrode wires to a multi-channel voltmeter, and connect the top hammers corresponding to the metal wires to a signal source, setting the process curve of the six-sided top large cavity press to achieve high pressure and high temperature conditions;

[0013] S4. Measure the Seebeck coefficient: Set the signal source to output a sinusoidal signal. After passing through a power amplifier, the signal is connected to the corresponding anvil and powered by a gradient heat source to generate a controllable temperature gradient within the sample. Simultaneously, a multi-channel voltmeter is used to monitor the voltage drop between each of the four thermocouple electrode wires. Combined with the Seebeck coefficient of the thermocouple electrode wires themselves, the in-situ Seebeck coefficient of the synthesized material during synthesis at high temperature and high pressure is fitted and evaluated in real time.

[0014] S5. Measure resistivity: Turn off the signal source and power amplifier, stop powering the gradient heat source, and after the sample temperature stabilizes, pass current through the positive poles of the two pairs of thermocouple electrode wires, record the voltage drop between the negative poles of the two pairs of thermocouple electrode wires, then change the direction of the current, and record the voltage drop between the negative poles of the two pairs of thermocouple electrode wires again. Combined with the cross-sectional area and height of the sample, the in-situ resistivity of the material during high-temperature and high-pressure synthesis is evaluated in real time using the formula.

[0015] In the above scheme, S1 specifically includes the following steps:

[0016] Press the lower pyrophyllite ring into the bottom of the through hole of the six-sided pyrophyllite, install the lower steel plug inside the lower pyrophyllite ring, then press the lower titanium sheet into the through hole of the six-sided pyrophyllite, and insert the mullite tube into the through hole of the six-sided pyrophyllite;

[0017] Insert the heating tube into the through hole of the mullite tube, press the lower heating plate into the bottom of the heating tube, then insert the insulating inert material tube into the heating tube, and fill the bottom of the insulating inert material tube with the lower insulating inert material column;

[0018] Pass the two metal wires through the insulating inert material tube along the through holes below the center points of the two opposite surfaces, fold them rightward 90 degrees to fit tightly against the insulating inert material tube wall, cut off the outer ends of the metal wires, fold them 90 degrees, and apply copper tape.

[0019] The gradient heat source sheet is pressed into the insulating inert material tube, and a thrust is applied from top to bottom on the upper surface of the gradient heat source sheet to achieve an interference fit between the sheet and the two metal wires, and then an insulating inert material sheet is filled on the upper part of the gradient heat source sheet;

[0020] Pass a pair of thermocouple electrode wires through the insulating inert material tube along the through-hole below the center point of the edge, and then bend the thermocouple electrode wires upward 90 degrees on the inner wall opposite to the insulating inert material tube, so that they are close to the inner wall of the insulating inert material tube;

[0021] Place the sample to be tested into an insulating inert material ring and then into an insulating inert material tube. Apply downward thrust to the upper surface of the sample to achieve an interference fit between the sample and the thermocouple electrode wire, so that the node of the thermocouple electrode wire is directly below the sample and in close contact.

[0022] Pass the other pair of thermocouple electrode wires through the insulating inert material tube along the through-hole above the center point of the edge. Bend the thermocouple electrode wires upward 90 degrees on the inner wall opposite the insulating inert material tube, keeping them close to the inner wall of the insulating inert material tube. Fill the insulating inert material tube with the upper insulating inert material column so that the nodes of the thermocouple electrode wires are directly above the sample and in close contact.

[0023] The upper heating plate is pressed into the heating tube, and then the upper titanium plate, the upper pyrophyllite ring and the upper steel plug are pressed into the six-sided pyrophyllite through-hole in sequence.

[0024] In the above solution, the heating tube, the lower heating plate and the upper heating plate are made of graphite, tantalum or molybdenum.

[0025] In the above solution, the insulating inert material is magnesium oxide or boron nitride.

[0026] In the above solution, the gradient heat source is a lanthanum chromate sheet or a zirconium diboride ceramic sheet.

[0027] In the above solution, the metal wire is made of iron wire, nickel wire, copper wire, niobium wire, tungsten wire or platinum wire; the thermocouple electrode wire is made of tungsten-rhenium 3 / 25 or NiCr / NiSi.

[0028] In the above solution, in step S4, the Seebeck coefficient of the thermocouple electrode wire itself and the Seebeck coefficient of the negative electrode of the thermocouple electrode wire are combined, and according to the measurement principle of the dynamic measurement method, the Seebeck coefficient of the material is calculated according to the following formula:

[0029]

[0030] Where, Indicates that the sample material is at temperature Seebeck coefficient under ; Thermocouple electrode wire at temperature Seebeck coefficient under ; The negative pole of the thermocouple electrode wire is at the temperature Seebeck coefficient under pos Indicates the voltage drop between the positive electrodes of two pairs of thermocouple electrodes; U neg It represents the voltage drop between the negative electrodes of two pairs of thermocouple electrodes.

[0031] In the above scheme, in step S5, after stopping the power supply of the gradient heat source, the function conversion switch is used to switch to the resistivity measurement system; during the resistivity measurement process, the current direction of the constant current source is changed by the current reversal switch.

[0032] In the above scheme, in step S5, a constant current source is used to pass a constant current I between the positive electrodes of the thermocouple electrode wires, and the voltage drop between the negative electrodes of the test electrodes is recorded as U + After changing the direction of the current, the voltage drop between the negative electrodes of the test electrodes is recorded as U - , the error introduced by the temperature difference electromotive force of the test electrode can be eliminated by making the difference between the two, that is, fitting can be performed using the following formula:

[0033]

[0034] Where ρ is the resistivity of the sample material, A is the cross-sectional area of the sample material, and L is the height of the sample material.

[0035] In the above scheme, the experimental pressure ranges from atmospheric pressure to 5 GPa, and the experimental temperature ranges from room temperature to a maximum of 1950 K; steps S4 and S5 are repeated as the set ambient temperature increases.

[0036] The beneficial effects produced by the present invention are:

[0037] 1. This invention, for the first time, introduces a method for dynamically measuring the Seebeck coefficient into the measurement process of the Seebeck coefficient under high pressure and high temperature. By placing an internal heat source at the bottom of the sample, a controllable temperature gradient is generated inside the sample, enabling rapid and accurate in-situ measurement of the Seebeck coefficient of thermoelectric materials under high temperature and high pressure conditions, effectively resolving the technical bottleneck of Seebeck coefficient measurement under high temperature and high pressure environments. This dynamic measurement system has multiple technical advantages and can effectively reduce the interference of system errors on the measurement results, including the contact thermal resistance between the sample and the temperature measuring element, stray heat flux in the temperature measurement circuit, and other potential error sources. Compared with existing technologies, this method exhibits superior measurement performance under extreme working conditions and can obtain more reliable material thermoelectric characteristic parameters.

[0038] 2. The present invention simultaneously performs the two steps of high-pressure material synthesis and thermoelectric performance measurement, which can obtain in-situ thermoelectric performance during sample sintering, reducing time costs while making it more likely to obtain thermoelectric materials with excellent performance.

[0039] 3. This invention reuses the test electrodes used to measure the Seebeck coefficient for resistivity measurements. A function conversion switch allows for rapid switching between Seebeck coefficient and resistivity measurements. During resistivity measurements, a current reversal switch changes the direction of the constant current to eliminate errors caused by the test electrode's thermoelectromotive force. Compared to most current measurement schemes, this significantly reduces wiring pressure and device complexity. Furthermore, by using steel plugs at the top and bottom, routing the gradient heat source from the side to the surface and securing it with copper tape, and fully utilizing the power supply from the synthesis equipment's top hammer, this simplifies the power supply circuit.

[0040] 4. The present invention uses holes opened on the side edges of hexahedral pyrophyllite to pass through the test thermocouple electrode wire, and ensures insulation between the test electrode and the graphite heating tube through a single-hole alumina tube. At the same time, deformation space is reserved to prevent mechanical fracture, solving the problem of building a measurement system in a confined space under high-voltage environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0042] Figure 1 Schematic diagram of the top hammer of the six-sided large cavity press and the assembled six-sided pyrophyllite in accordance with the embodiment of the present invention;

[0043] Figure 2 This is a cross-sectional view of a six-sided pyrophyllite assembly according to an embodiment of the present invention;

[0044] Figure 3 Schematic diagram of the installation of metal wire and thermocouple electrode wire in an embodiment of the present invention;

[0045] Figure 4 is a schematic diagram of a measurement scheme in an embodiment of the present invention;

[0046] Figure 5 is the high-temperature Seebeck coefficient of iron-silicon at 5 GPa measured using the method of the present invention;

[0047] Figure 6 It is the high temperature resistivity coefficient of iron silicon at 5 GPa measured by the method of the present invention.

[0048] In the figure: 1. Hexagonal pyrophyllite; 2. Steel plug; 3. Mullite tube; 4. Graphite tube heating element; 5. Magnesium oxide tube; 6. Upper magnesium oxide cylinder; 7. Magnesium oxide ring; 8. Magnesium oxide sheet; 9. Lanthanum chromate sheet; 10. Lower magnesium oxide cylinder; 11. Titanium sheet; 12. Sample to be tested; 13. Graphite sheet; 14. Pyrophyllite ring; 15. Thermocouple electrode wire hole; 16. Metal wire hole. DETAILED DESCRIPTION

[0049] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0050] It should be noted that the illustrations provided in the embodiments of the present invention are only schematic illustrations of the basic concept of the present invention. Therefore, the drawings only show components related to the present invention and are not drawn according to the number, shape and size of components in actual implementation. In actual implementation, the type, quantity and proportion of each component can be changed at will, and the component layout type may also be more complicated.

[0051] In the present invention, it should also be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer" and the like are used to indicate positions or locations based on those shown in the accompanying drawings. These terms are intended solely to facilitate the description of the present application and to simplify the description. They are not intended to indicate or imply that the devices or components referred to must have a specific orientation, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limiting the present application. Furthermore, the terms "first" and "second" are used solely for descriptive and distinguishing purposes and should not be construed as indicating or implying relative importance.

[0052] The present invention proposes a method for evaluating thermoelectric performance based on in-situ synthesis. By placing a gradient heat source at the bottom of the sample, a controllable temperature gradient is generated inside the sample, and the Seebeck coefficient is dynamically measured. The test electrode used for measuring Seebeck is reused to measure the resistivity using a two-probe method to eliminate the error introduced by the thermoelectric electromotive force of the test electrode. The two measurements are switched through a simple function conversion switch, thereby accurately evaluating the Seebeck coefficient and resistivity of the in-situ synthesized material for performance evaluation.

[0053] This embodiment uses a six-sided large-cavity press as the equipment platform for generating high temperature and high pressure, uses six-sided pyrophyllite and mullite tubes as pressure transmission media and thermal insulation materials, graphite tubes and graphite sheets as heaters, lanthanum chromate sheets as gradient heat sources, pure magnesium oxide tubes as sample chambers, pure magnesium oxide cylinders as fillers, metal wires as gradient heat source power lines, and tungsten-rhenium 3 / 25 thermocouple wires as test electrodes. The thermocouple wire nodes are introduced into a sealed sample chamber and in close contact with the upper and lower surfaces of the sample to meet high-pressure experimental conditions. The method specifically includes the following steps:

[0054] 1. Processing of each component:

[0055] (1) Select six-sided pyrophyllite 1 material that matches the hammer head of the six-sided large cavity press to transmit pressure and seal the sample. Its shape is a cubic block with a side length of 37.5 mm. A through hole is processed in the middle for installing the graphite heating element, sample chamber and other components. The through hole diameter is 25.1 mm;

[0056] (2) Processing a mullite tube 3 as a thermal insulation material, whose outer diameter matches the inner diameter of the through hole of the hexagonal pyrophyllite 1, and whose inner diameter matches the graphite tube 4, with an outer diameter of 25.0 mm and an inner diameter of 12.3 mm, respectively. It is located between the titanium sheets 11 on both sides and has a height of 22.5 mm;

[0057] (3) Processing the graphite tube heating element 4, wherein the outer diameter of the graphite tube column matches the mounting hole diameter of the mullite tube 3, the inner diameter matches the magnesium oxide tube 5, and the height matches the mullite tube 3, respectively having an outer diameter of 12.2 mm, an inner diameter of 10.1 mm, and a height of 22.5 mm. The upper and lower graphite sheets 13 have a diameter of 10.0 mm and a thickness of 1.5 mm;

[0058] (4) Processing the magnesium oxide tube 5, whose outer diameter matches the inner diameter of the graphite tube heating element 4, and whose inner diameter matches the magnesium oxide ring 7, with an outer diameter of 10.0 mm;

[0059] (5) Processing the upper magnesium oxide cylinder 6 and the lower magnesium oxide cylinder 10 and the magnesium oxide sheet 8, whose diameters match the inner diameter of the magnesium oxide tube 5 and whose heights vary from 1.0 to 11.0 mm, for filling both ends of the sample;

[0060] (6) Processing the magnesium oxide ring 7, whose outer diameter matches the inner diameter of the magnesium oxide tube 5, and the inner diameter matches the sample to be tested;

[0061] (7) Processing lanthanum chromate sheet 9, the diameter of which matches the inner diameter of magnesium oxide tube 5 and the thickness of which is 1.0-2.0 mm;

[0062] (8) Processing talc ring 14 and steel plug 2 for introducing heating current into graphite tube heating element 4.

[0063] The tolerance of the above-mentioned parts is ±0.10mm, and the components are matched according to the basic hole system H8.

[0064] 2. Heat treatment of each component:

[0065] The processed hexahedral pyrophyllite 1 and mullite tube 3 were baked at 300-600℃ for 3 hours; the graphite tube and graphite sheet were kept in a vacuum drying oven at 100-200℃ for 10-24 hours; the magnesium oxide tube and magnesium oxide cylinder, pyrophyllite ring and steel plug were dried in a vacuum drying oven at 200℃ for at least 24 hours.

[0066] 3. Processing the thermocouple electrode wire hole 15 and the metal wire hole 16:

[0067] The baked hexahedral pyrophyllite 1, mullite tube 3, graphite tube 4, and magnesium oxide tube 5 are drilled using a drill rig on two vertical edges on one side, along an angle perpendicular to the edges, with one through-hole drilled above and below the center point, for a total of four through-holes. The two through-holes below the center point are used to install the first pair of thermocouple electrode wires, and the two through-holes above the center point are used to install the other pair of thermocouple electrode wires. Take the two adjacent side surfaces of the side and drill one through-hole below the center point along a direction perpendicular to the surface, for a total of two through-holes, for installing the gradient heat source power supply line (i.e., metal wire). The diameter of all through-holes is 1.25mm.

[0068] 4. Processing of samples to be tested:

[0069] If the measured object is initially in the form of a powder, it is pressed into a cylinder with a diameter of 3 mm and a height of 2.5 mm to 3.5 mm using a powder tablet press. If the measured object is initially in the form of a sintered block, it is machined into a cylindrical shape with a diameter of 3 mm and a height of 2.5 mm to 3.5 mm.

[0070] 5. Assemble the components, see Figure 2 :

[0071] (1) Assemble the lower pyrophyllite ring and the steel plug 2 and press them into the bottom of the six-sided pyrophyllite through-hole, press in the lower titanium sheet 11, and insert the mullite tube 3 into the six-sided pyrophyllite 1 through-hole;

[0072] (2) Insert the graphite tube column in the graphite tube heating element 4 into the through hole of the mullite tube 3, press the graphite sheet below into it, and then insert the pure magnesium oxide tube 5 into the graphite tube and fill it with the pure magnesium oxide column 10;

[0073] (3) Insert a 0.2 mm metal wire into an alumina tube with an inner diameter of 0.7 mm and an outer diameter of 1.2 mm, and then pass through the six-sided pyrophyllite 1, mullite tube 3, graphite tube heating element 4 column and magnesium oxide tube 5 along the lower through-holes of the two opposite surfaces in sequence, and then fold it rightward 90 degrees to fit closely against the inner wall of the magnesium oxide tube 5. At the same time, cut the metal wire at the outer end and fold it 90 degrees so that it is completely immersed in the groove on the side of the six-sided pyrophyllite 1, and then stick copper tape on it;

[0074] (4) Press the lanthanum chromate sheet 9 into the pure magnesium oxide tube 5, apply a thrust from top to bottom on the upper surface of the lanthanum chromate sheet 9 to achieve an interference fit between it and the metal wire, and fill the tube with pure magnesium oxide sheet 8, so that the metal wire is on the outermost side of the lanthanum chromate sheet 9 and in close contact;

[0075] (5) A pair of tungsten-rhenium 3 / 25 thermocouple electrode wires with a diameter of 0.25 mm are inserted into an alumina tube with an inner diameter of 0.7 mm and an outer diameter of 1.2 mm, and are passed through the pyrophyllite 1, the graphite tube heating element 4 column, and the pure magnesium oxide tube 5 in sequence along the through-holes on the lower side of the edge center point. After passing through the through-holes, the thermocouple electrode wires are bent 90 degrees on the inner wall opposite to the magnesium oxide tube 5, close to the inner wall of the magnesium oxide tube 5, and the two thermocouple electrode wires intersect at the center;

[0076] (6) The processed sample 12 to be tested is placed into the pure magnesium oxide 7, and then the whole is placed into the pure magnesium oxide tube 5. By applying a thrust from top to bottom on the upper surface of the sample, an interference fit is achieved between the sample and the electrode, so that the thermocouple wire node is at the lower end of the sample and in close contact;

[0077] (7) Another pair of tungsten-rhenium 3 / 25 thermocouple electrode wires with a diameter of 0.25 mm are inserted into an alumina tube with an inner diameter of 0.7 mm and an outer diameter of 1.2 mm. They are then passed through the pyrophyllite 1, the graphite tube heating element 4 column, and the pure magnesium oxide tube 5 in sequence along the through-holes on the upper side of the edge center point. After passing through the through-holes, the electrodes are bent 90 degrees on the inner wall opposite the magnesium oxide tube 5, closely attached to the inner wall of the magnesium oxide tube 5, and filled with pure magnesium oxide columns, so that the nodes of the thermocouple electrode wires are at the upper end of the sample and in close contact;

[0078] For the installation of two metal wires and four thermocouple electrode wires, refer to Figure 3 .

[0079] (8) Press the upper graphite sheet in, then press the titanium sheet 11 in sequence, assemble the upper pyrophyllite ring and the steel plug 2 and press them into the top of the six-sided pyrophyllite.

[0080] 6. Place the assembled six-sided pyrophyllite 1 as a whole into the pressure chamber of the six-sided large cavity press, and lead the thermocouple electrode wire out from the gap between the tungsten carbide top hammers of the press. At this time, use a soft silicone protective cover as an insulating layer on the outside of the pressure chamber, and use a heat shrink tubing to fix the connection at the connection to achieve electrical insulation between it and the top hammer.

[0081] 7. If Figure 4 As shown, Figure 3 The four thermocouple electrode wires shown are respectively connected to the four terminals in the middle of the function conversion switch. When the four terminals move upward, the four thermocouple electrode wires will be respectively connected to a high-precision multi-channel voltmeter to measure the voltage drop between them. Among them, channels 1 and 2 are used to measure the temperature signals given by thermocouples 1 and 2 respectively, and the average of these two temperature signals represents the actual temperature of the sample; channels 3 and 4 are used to measure the voltage drop between the positive and negative poles of the two pairs of thermocouples respectively; these four measured values will be used to calculate the Seebeck coefficient of the sample. When the four terminals move downward, the positive wires of the four thermocouple electrode wires will be respectively connected to the positive and negative poles of the constant current source to pass a constant current, and the negative wire will be connected to channel 5 of the voltmeter to measure the voltage drop between the upper and lower surfaces of the sample under constant current excitation; this measured value is combined with the geometric size data of the sample to calculate the resistivity of the sample. In order to eliminate the influence of the thermoelectromotive force of the thermocouple electrode wires on the measurement, a current reversing switch is installed in the middle of the constant current circuit. As shown Figure 4 As shown, when the middle terminal moves upward, the constant current will flow from the positive wire of thermocouple 1 into the sample and from the positive wire of thermocouple 2 back to the power supply; when the middle terminal moves downward, the constant current will flow from the positive wire of thermocouple 2 into the sample and from the positive wire of thermocouple 1 back to the power supply. Figure 4 The circuit connection method shown is designed to reduce the workload of repeated wiring during measurement and to achieve one-touch conversion of measurement function and current flow direction.

[0082] The front and rear hammers are connected to a power amplifier, and the process curve of the hexagonal large-cavity press is set to achieve high pressure and high temperature conditions. The gradient heat source power supply wire is recessed into a groove on the side of the hexagonal pyrophyllite. Copper tape is applied to ensure conductivity between the front and rear hammers. The hammers serve as a medium for connection to the signal source. After the temperature of the sample under test reaches and stabilizes at the preset value, a controllable signal source generates a sinusoidal current input, producing a stable temperature gradient within a small range.

[0083] 8. Measure the Seebeck coefficient: Output a sinusoidal signal through a signal source, pass the signal through a power amplifier, and connect it to the front and rear hammers. Power the gradient heat source to generate a periodic temperature gradient along the axial direction inside the sample. At the same time, as described in step 7, move the four middle terminals of the function conversion switch upward, and use a multi-channel voltmeter to monitor the voltage drop between each of the four thermocouple electrode wires. Combined with the Seebeck coefficient of the thermocouple electrode wire itself, fit the in-situ Seebeck coefficient of the synthetic material during synthesis under high temperature and high pressure for real-time evaluation.

[0084] Specifically, a periodic voltage is applied to the lanthanum chromate sheet 9, which generates periodic heat. The heat is conducted upward into the cylindrical sample material, generating a periodic temperature gradient along the axial direction inside the cylindrical sample material. When the pressure and temperature of the sample material are in equilibrium, the signals within a heating cycle are synchronously collected, including the temperature signals of the two pairs of thermocouples and the voltage drop U between the positive electrodes of the two pairs of thermocouples. pos The voltage drop between the negative pole and neg .

[0085] The average temperature collected by the two pairs of thermocouples is recorded as Will U pos and U neg The data were fitted linearly, and the slope was recorded as Seebeck coefficient of the final material Determine as follows:

[0086]

[0087] Where, Indicates that the sample material is at temperature Seebeck coefficient under ; Thermocouple electrode wire at temperature Seebeck coefficient under ; The negative pole of the thermocouple electrode wire is at the temperature Seebeck coefficient under pos Indicates the voltage drop between the positive electrodes of two pairs of thermocouple electrodes; U neg It represents the voltage drop between the negative electrodes of two pairs of thermocouple electrodes.

[0088] 9. Measure resistivity: After the Seebeck coefficient measurement is completed, turn off the power amplifier and signal source, stop the gradient heat source power supply, and after the sample temperature stabilizes, move the four terminals in the middle of the function conversion switch downward, pass current through the positive poles of the two pairs of thermocouple electrode wires, and record the voltage drop between the negative poles of the two pairs of thermocouple electrode wires. Then, change the direction of the current through the current reversal switch, and record the voltage drop between the negative poles of the two pairs of thermocouple electrode wires again. Combined with the cross-sectional area and height of the sample, the in-situ resistivity of the material during high-temperature and high-pressure synthesis is evaluated in real time using the formula.

[0089] Specifically, a constant current source is used to pass a constant current I between the positive electrodes of the thermocouple electrode wires, and the voltage drop between the negative electrodes of the test electrodes is recorded as U + After changing direction, the voltage drop between the negative electrodes of the test electrodes is recorded as U - , the error introduced by the temperature difference electromotive force of the test electrode can be eliminated by making the difference between the two, that is, fitting can be performed using the following formula:

[0090]

[0091] Where ρ is the resistivity of the sample material, A is the cross-sectional area of the sample material, and L is the height of the sample material.

[0092] The experimental pressure is 5 GPa, and the experimental temperature ranges from room temperature to a maximum of 1950 K. Repeat steps 8 and 9 above as the set ambient temperature increases.

[0093] 10. When the sample temperature drops to room temperature, unload the applied high pressure and recover the sample.

[0094] In the above scheme, the large-cavity press includes six tungsten carbide anvils to provide the required sample pressure. The upper and lower anvils can be powered by electricity and work with the graphite tube heater 4 to maintain the desired sample temperature. The front and rear anvils can also be powered by electricity and work with the power supply wire, lanthanum chromate sheet 9, and constant current source to produce a stable temperature gradient within a small range.

[0095] In the above scheme, the role of the titanium sheet 11 and the steel plug 2 is to conduct the top hammer current to the graphite tube heating element 4. Pyrophyllite has multiple advantages as a pressure transmission medium: it has outstanding high temperature resistance, low shear strength, good electrical insulation and thermal insulation properties, and is cost-effective. Graphite material is an ideal heating element material because of its excellent electrical and thermal conductivity and extremely low thermal expansion coefficient, which can maintain structural stability in high temperature environments. The use of high-purity magnesium oxide to prepare the sample chamber can effectively ensure the electrical insulation performance between the sample and the graphite heating element.

[0096] In the above scheme, a single-hole alumina tube is used inside the pressure chamber as an insulating protective layer for the electrode and thermocouple wire. This material has excellent high-temperature resistance and mechanical strength. A soft silicone protective cover is used outside the pressure chamber as an insulating layer. This flexible material has good environmental adaptability but limited heat resistance, so it is only suitable for normal temperature areas. Heat shrink tubing is used to fix the connection at the joint.

[0097] In the above scheme, four thermocouple electrode wires are introduced into the sample chamber horizontally from the edge of the six-sided pyrophyllite 1, wherein each pair of thermocouple electrode wires is located at the center point of the upper and lower surfaces of the sample by bending 90 degrees on the inner wall opposite to the magnesium oxide tube 5 and intersecting at the center, and maintaining electrical connection with the sample. Introducing the test electrode into the center of the upper and lower surfaces of the sample in this way can ensure that the electrode always maintains good contact with the sample during the pressurization and heating process and avoid electrode breakage.

[0098] In the above solution, the thermocouple electrode wire and the power supply metal wire are bent with small waves in the pressure chamber, leaving space for deformation to avoid mechanical fracture caused by the pressure application process.

[0099] Example 1

[0100] This embodiment takes the high-temperature and high-pressure in-situ Seebeck coefficient and resistivity measurement of powder-pressed ferrous silicide (FeSi) as an example to describe the technical details and key points of the present invention.

[0101] (1) According to step 1, pure magnesium oxide is selected as a highly insulating inert material and a lanthanum chromate sheet is used as a gradient heat source to process each component. The inner diameter of the processed magnesium oxide tube 5 is 6.1 mm, which matches the diameters of the upper magnesium oxide cylinder 6 and the lower magnesium oxide cylinder 10, the magnesium oxide sheet 8, and the lanthanum chromate sheet 9; the outer diameter of the magnesium oxide ring 5 is 6.0 mm and the inner diameter is 3.1 mm, which matches the iron sample to be tested and the magnesium oxide tube 5. The diameters of the processed upper magnesium oxide cylinder 6 and the lower magnesium oxide cylinder 10, the magnesium oxide sheet 8, and the lanthanum chromate sheet 9 are all 6.0 mm, which matches the inner diameter of the magnesium oxide tube 5. The heights of the magnesium oxide cylinders 6 and 10 and the magnesium oxide sheet 8 are 8.5 mm, 4.5 mm, and 2 mm, respectively, which are used to fill the two ends of the sample. The thickness of the lanthanum chromate sheet 9 is 2 mm.

[0102] The metal wire used was iron wire, and the thermocouple electrode wire used was tungsten-rhenium 3 / 25 thermocouple wire.

[0103] (2) heat treating each component as described in step 2;

[0104] (3) Processing the electrode wire and power supply wire apertures as described in step 3;

[0105] (4) According to step 4, powdered FeSi was selected as the component to be tested. The initial form of the component in this example was a powder with a particle size of 2-10 μm. 2.5 g of iron powder was weighed and placed into a forming mold. The powder was formed using a powder tablet press under 20 MPa oil pressure. The diameter of the formed iron sample was 3 mm and the initial height was 2.5 mm.

[0106] (5) Complete the assembly process of each component as described in step 5;

[0107] (6) Complete the pre-test assembly and equipment connections as described in steps 6 and 7. Set the process curve for the hexagonal large cavity press, and set the sample pressure at 5 GPa. After the pressure reaches the preset value, slowly increase the heating current flowing through the graphite tube heating element to gradually increase the sample temperature. After the sample temperature reaches the preset value, maintain it for 15 minutes before preparing to begin measurement. The maximum temperature set for the measurement is approximately 1950K.

[0108] (7) As described in step 8, the four thermocouple electrode wires are connected to a multi-channel voltmeter to measure the voltage drop between the four electrodes. The voltage drop between the positive and negative electrodes of each pair of thermocouple electrode wires is used to display the specific temperature at both ends of the sample. The voltage drop between the positive electrodes of the two pairs of thermocouple electrode wires is recorded as U pos , the voltage drop between the negative electrodes is recorded as Uneg .

[0109] Specific manifestations such as Figure 3 and Figure 4 As shown, two pairs of thermocouples are placed at both ends of the material to be tested, and their temperature measuring ends maintain good electrical contact with the sample. If there is a temperature difference ΔT between the upper and lower ends of the sample to be tested, a thermoelectric potential will exist at both ends, which is the Seebeck effect. At this time, if the voltage signals between the positive and negative electrodes of the two pairs of thermocouples are measured simultaneously (represented as U pos and U neg ), combined with the Seebeck coefficient of the thermocouple itself (denoted as ), the Seebeck coefficient of the material to be tested can be derived.

[0110] According to the measurement principle of this dynamic measurement method, the Seebeck coefficient of the material iron silicon is calculated numerically according to the following formula:

[0111]

[0112] (8) As described in step 8, the gradient heat source stops supplying power. After the sample temperature stabilizes, the function conversion switch is used to switch to resistivity measurement, and the voltage across the sample when current in different directions is applied is recorded.

[0113] Specific manifestations such as Figure 4 As shown, a positive constant current is first passed between the positive electrodes of two pairs of thermocouples, and the corresponding voltage data between the negative electrodes of the two pairs of thermocouples is collected, recorded as U_+. The direction of this current is then reversed, and the corresponding voltage data is collected again, recorded as U_-. Letting the true value of the voltage be U, the difference between the two can eliminate the error introduced by the thermoelectric electromotive force of the thermocouple. Combined with the cross-sectional area and height of the sample, the resistivity of the sample can be derived.

[0114] According to the measurement principle of the two-probe method, the resistivity of the material iron silicon is calculated according to the following formula:

[0115]

[0116] Keeping the pressure of the sample constant, gradually change the temperature of the sample by changing the heating current; repeat the above eight and nine measurement steps at different temperatures to obtain the Seebeck coefficient and resistivity of the material at different temperatures.

[0117] (9) As described in step 10, when the sample temperature drops to room temperature, release the pressure of the equipment so that the sample is at normal pressure and recover the sample.

[0118] Example 2

[0119] This embodiment takes the high-temperature and high-pressure in-situ Seebeck coefficient and resistivity measurement of bulk ferrous silicide (FeSi) as an example to describe the technical details and key points of the present invention.

[0120] (1) According to step 1, pure boron nitride is selected as a highly insulating inert material and a zirconium diboride ceramic sheet is selected as a gradient heat source to process each component. The inner diameter of the processed boron nitride tube 5 is 6.1 mm, which matches the diameters of the boron nitride cylinders 6 and 10, the boron nitride sheet 8, and the zirconium diboride ceramic sheet 9. The outer diameter of the boron nitride ring 5 is 6.0 mm and the inner diameter is 3.1 mm, which matches the iron sample to be tested and the magnesium oxide tube 5. The diameters of the processed boron nitride cylinders 6 and 10, the boron nitride sheet 8, and the zirconium diboride ceramic sheet 9 are all 6.0 mm, which matches the inner diameter of the boron nitride tube 5. The heights of the boron nitride cylinders 6 and 10 and the boron nitride sheet 8 are 8 mm, 4 mm, and 2.5 mm, respectively, which are used to fill the two ends of the sample. The thickness of the zirconium diboride ceramic sheet 9 is 1.5 mm.

[0121] The metal wire uses niobium wire, and the thermocouple electrode wire uses tungsten-rhenium 3 / 25 thermocouple wire.

[0122] (2) heat treating each component as described in step 2;

[0123] (3) Processing the electrode wire and power supply wire apertures as described in step 3;

[0124] (4) As described in step 4, select a block of FeSi as the component to be tested. In this example, the component is a sintered block, which is cut into cylinders with a diameter of 3 mm and a height of 3.5 mm using an electric spark cutter. After cleaning in an alcohol liquid using an ultrasonic cleaner for 15 minutes, it is kept in a vacuum drying oven at 100°C for 3 hours to completely dry it.

[0125] (5) Complete the assembly process of each component as described in step 5;

[0126] (6) Complete the pre-test assembly and equipment connection as described in steps 6 and 7, set the process curve of the six-sided large cavity press, and set the sample pressure at 5 GPa; after the pressure reaches the preset value, slowly increase the heating current flowing through the graphite heating element to gradually increase the sample temperature; after the sample temperature reaches the preset value, maintain it for 15 minutes and prepare to start the measurement. The maximum temperature set for the measurement is approximately 1850K;

[0127] (7) As described in step 8, the four electrodes are connected to a multi-channel voltmeter to measure the voltage drop between each of the four electrodes. The specific measurement process is as described in Example 1.

[0128] (8) As described in step 9, the gradient heat source stops supplying power. After the sample temperature stabilizes, the function conversion switch is used to switch to resistivity measurement. The voltage across the sample when current in different directions is applied is recorded. The specific measurement process is as described in Example 1.

[0129] (9) As described in step 10, when the sample temperature drops to room temperature, release the pressure of the equipment so that the sample is at normal pressure and recover the sample.

[0130] Figure 5 and Figure 6 They are the Seebeck coefficient and resistivity of iron silicon measured in this embodiment under a pressure of 5 GPa and a temperature of about 400-1850K, respectively. The hollow circles and solid circles are the measurement data of the Seebeck coefficient and resistivity of Examples 1 and 2, respectively. The Seebeck coefficient and resistivity remain stable after multiple measurements, and the data trend is consistent with that of similar materials. The measurement results show very good repeatability, which proves the reliability of the measurement technology of the present invention.

[0131] It should be pointed out that, according to the needs of implementation, the various steps / components described in this application can be split into more steps / components, or two or more steps / components or partial operations of steps / components can be combined into new steps / components to achieve the purpose of the present invention.

[0132] The size of the serial numbers of the steps in the above embodiments does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0133] It should be understood that those skilled in the art can make improvements or changes based on the above description, and all such improvements and changes should fall within the scope of protection of the appended claims of the present invention.

Claims

1. A method for evaluating thermoelectric performance based on in-situ synthesis, characterized in that: The following steps are involved: S1. Assemble the six-sided pyrophyllite internal components: Hexahedral pyrophyllite and mullite tubes are used as pressure-transmitting sealed media and heat-insulating materials, a heating tube is used as a heater, and an insulating inert material is used as a sample chamber and filler. A semiconductor material is set below the sample chamber as a gradient heat source, and metal wires are used as electrode wires of the gradient heat source. The temperature measurement points of two pairs of thermocouple electrode wires are introduced into the sealed sample chamber and are in close contact with the upper and lower surfaces of the sample, maintaining an electrically conductive state. S2. Place the assembled six-sided pyrophyllite into the pressure chamber of the six-sided large-cavity press, lead the thermocouple electrode wire out from the gap between the tungsten carbide anvils of the press, and perform insulation treatment; S3. Connect the four thermocouple electrode wires to a multi-channel voltmeter, and connect the top hammers corresponding to the metal wires to a signal source, setting the process curve of the six-sided top large cavity press to achieve high pressure and high temperature conditions; S4. Measure the Seebeck coefficient: Set the signal source to output a sinusoidal signal. After passing through a power amplifier, the signal is connected to the corresponding anvil and powered by a gradient heat source to generate a controllable temperature gradient within the sample. Simultaneously, a multi-channel voltmeter is used to monitor the voltage drop between each of the four thermocouple electrode wires. Combined with the Seebeck coefficient of the thermocouple electrode wires themselves, the in-situ Seebeck coefficient of the synthesized material during synthesis at high temperature and high pressure is fitted and evaluated in real time. S5. Measure resistivity: Turn off the signal source and power amplifier, stop powering the gradient heat source, and after the sample temperature stabilizes, pass current through the positive poles of the two pairs of thermocouple electrode wires, record the voltage drop between the negative poles of the two pairs of thermocouple electrode wires, then change the direction of the current, and record the voltage drop between the negative poles of the two pairs of thermocouple electrode wires again. Combined with the cross-sectional area and height of the sample, the in-situ resistivity of the material during high-temperature and high-pressure synthesis is evaluated in real time using the formula.

2. The method for evaluating thermoelectric performance based on in-situ synthesis according to claim 1, characterized in that: S1 specifically includes the following steps: Press the lower pyrophyllite ring into the bottom of the through hole of the six-sided pyrophyllite, install the lower steel plug inside the lower pyrophyllite ring, then press the lower titanium sheet into the through hole of the six-sided pyrophyllite, and insert the mullite tube into the through hole of the six-sided pyrophyllite; Insert the heating tube into the through hole of the mullite tube, press the lower heating plate into the bottom of the heating tube, then insert the insulating inert material tube into the heating tube, and fill the bottom of the insulating inert material tube with the lower insulating inert material column; Pass the two metal wires through the insulating inert material tube along the through holes below the center points of the two opposite surfaces, fold them rightward 90 degrees to fit tightly against the insulating inert material tube wall, cut off the outer ends of the metal wires, fold them 90 degrees, and apply copper tape. The gradient heat source sheet is pressed into the insulating inert material tube, and a thrust is applied from top to bottom on the upper surface of the gradient heat source sheet to achieve an interference fit between the sheet and the two metal wires, and then an insulating inert material sheet is filled on the upper part of the gradient heat source sheet; Pass a pair of thermocouple electrode wires through the insulating inert material tube along the through-hole below the center point of the edge, and then bend the thermocouple electrode wires upward 90 degrees on the inner wall opposite to the insulating inert material tube, so that they are close to the inner wall of the insulating inert material tube; Place the sample to be tested into an insulating inert material ring and then into an insulating inert material tube. Apply downward thrust to the upper surface of the sample to achieve an interference fit between the sample and the thermocouple electrode wire, so that the node of the thermocouple electrode wire is directly below the sample and in close contact. Pass the other pair of thermocouple electrode wires through the insulating inert material tube along the through-hole above the center point of the edge. Bend the thermocouple electrode wires upward 90 degrees on the inner wall opposite the insulating inert material tube, keeping them close to the inner wall of the insulating inert material tube. Fill the insulating inert material tube with the upper insulating inert material column so that the nodes of the thermocouple electrode wires are directly above the sample and in close contact. The upper heating plate is pressed into the heating tube, and then the upper titanium plate, the upper pyrophyllite ring and the upper steel plug are pressed into the six-sided pyrophyllite through-hole in sequence.

3. The method for evaluating thermoelectric performance based on in-situ synthesis according to claim 2, characterized in that: The heating tube, the lower heating plate and the upper heating plate are made of graphite, tantalum or molybdenum.

4. The method for evaluating thermoelectric performance based on in-situ synthesis according to claim 1, characterized in that: The insulating inert material is magnesium oxide or boron nitride.

5. The method for evaluating thermoelectric performance based on in-situ synthesis according to claim 1, characterized in that: The gradient heat source is a lanthanum chromate sheet or a zirconium diboride ceramic sheet.

6. The method for evaluating thermoelectric performance based on in-situ synthesis according to claim 1, characterized in that: The metal wire is made of iron wire, nickel wire, copper wire, niobium wire, tungsten wire or platinum wire; the thermocouple electrode wire is made of tungsten-rhenium 3 / 25 or NiCr / NiSi.

7. The method for evaluating thermoelectric performance based on in-situ synthesis according to claim 1, characterized in that: In step S4, the Seebeck coefficient of the thermocouple electrode wire itself and the Seebeck coefficient of the negative electrode of the thermocouple electrode wire are combined and the Seebeck coefficient of the material is calculated according to the measurement principle of the dynamic measurement method according to the following formula: Where, Indicates that the sample material is at temperature Seebeck coefficient under ; Thermocouple electrode wire at temperature Seebeck coefficient under ; The negative pole of the thermocouple electrode wire is at the temperature Seebeck coefficient under pos Indicates the voltage drop between the positive electrodes of two pairs of thermocouple electrodes; U neg It represents the voltage drop between the negative electrodes of two pairs of thermocouple electrodes.

8. The method for evaluating thermoelectric performance based on in-situ synthesis according to claim 1, characterized in that: In step S5, after the gradient heat source stops supplying power, the resistivity measurement system is switched to through the function conversion switch; during the resistivity measurement process, the current direction of the constant current source is changed through the current reversal switch.

9. The method for evaluating thermoelectric performance based on in-situ synthesis according to claim 1, characterized in that: In step S5, a constant current source is used to pass a constant current I between the positive electrodes of the thermocouple electrode wires, and the voltage drop between the negative electrodes of the test electrodes is recorded as U + After changing the direction of the current, the voltage drop between the negative electrodes of the test electrodes is recorded as U - , the error introduced by the temperature difference electromotive force of the test electrode can be eliminated by making the difference between the two, that is, fitting can be performed using the following formula: Where ρ is the resistivity of the sample material, A is the cross-sectional area of the sample material, and L is the height of the sample material.

10. The method for evaluating thermoelectric performance based on in-situ synthesis according to claim 1, characterized in that: The experimental pressure ranges from atmospheric pressure to 5 GPa, and the experimental temperature ranges from room temperature to a maximum of 1950 K; steps S4 and S5 are repeated as the set ambient temperature increases.

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