Synthesis method of bismuth telluride-based semiconductor thermoelectric material

Abstract

The present invention adopts 1) an induction-coupling-assisted zone melting-directional solidification vertical zone melting method; 2) a high-density crystal nucleus rapid formation method; and 3) an argon pressure modulation method for controlling the solid, liquid, and vapor volume ratios. By modulating and controlling the temperature gradient at the growth front, the supercooled molten liquid temperature, and the volume ratios of the solid, liquid, and vapor in the three aggregation states at the crystallization front, the nucleation and growth processes of the crystal are obtained, and a bismuth telluride-based rod-shaped thermoelectric material with nano-grain inlays having a ZT value of up to 1.2 in the temperature range near room temperature and a physical property non-uniformity of ≤5% along the crystal growth direction is obtained. The thermoelectric material synthesized by the present invention greatly promotes the performance of semiconductor thermoelectric devices and can be widely applied to fields such as efficient waste heat recovery and active point temperature management.

Description

Technical Field

[0001] The present invention relates to a method for synthesizing materials, in particular to a method for synthesizing semiconductor thermoelectric materials. Specifically, it is a method for synthesizing bismuth telluride-based rod-shaped thermoelectric materials with nano-grains embedded therein, having a ZT value of 1.2 in the temperature range near room temperature and uniform physical properties along the crystal growth direction. Background Art

[0002] The thermoelectric (TE) phenomenon is also known as the thermoelectric effect. In 1822, Thomas Seebeck discovered the thermoelectric emf effect (the principle of TE material power generation); in 1834, Jean Peltier discovered the cooling effect at the interface of two different material conductors in a current loop (the principle of TE material refrigeration). In the 1950s, some good semiconductor TE materials were discovered. Materials with ZT≥0.5 are usually called TE materials. The larger the ZT, the higher the efficiency of the TE device. To overcome the obstacle of the lack of high-ZT TE materials, people turned to the structural design of natural TE materials and the research and development of artificially synthesized TE materials - low-dimensional thermoelectric materials. Mesoscopic physical theory research shows that under the same working conditions, low-dimensional thin-film structure TE materials have higher ZT values than other materials. So far, there are three types of typical low-dimensional thin-film structure TE materials: (1) Quantum dot structures, which improve the density of states near the Fermi level by means of the quantum confinement effect, thus increasing the conductivity of the material; (2) Phonon-blocking / electron-transmitting superlattices, which reduce the lattice thermal conductivity (kL) of the material by introducing the so-called "acoustic mismatch" between the superlattice components. Different from conventional TE alloy materials, materials of this type usually have a significant reduction in the carrier scattering rate, that is, high conductivity; (3) Thin-film structure materials that use the thermionic effects in semiconductor heterostructures to increase the ZT value of the material. Hicks and Dresslhaus proposed that quantum well superlattices can greatly increase the ZT value of the material, and quantum wire superlattices can even bring a greater increase. Physically speaking, the reason why these systems can increase the ZT value of the material is that the scale under the low-dimensional geometry of quantum wells and nanowires increases the density of electron states per unit volume. In terms of related principle materials, in February 2006, Kim et al. reported in Phys. Rev. Lett. a CVD-grown semi-metal nanocrystal random mosaic superlattice structure material - ErAs (nanoparticles) @ In0.53Ga0.47As (alloy) - which is very innovative. They claim that: (1) The effective suppression of short-wave phonons by a large number of point defects in the alloy material and the suppression of medium- and long-wave phonons by ErAs nanocrystals lead to a significant decrease in the thermal conductivity of the structure; (2) The doping-like effect of semi-metal ErAs nanocrystals slightly increases the conductivity of the structural material.The combined effect is ZT > 2 at room temperature! This work has paved the way for the "industrialization" application of thermoelectric nanomaterials. The ideal thermoelectric conversion material is one with a dimensionless power generation performance index ZT reaching 2 or higher. So far, the main materials include bismuth intermetallic compound bismuth telluride (Bi2Te3), lead telluride (PtTe), zinc antimonide (ZnSb), germanium, iron silicide (FeSi2), etc. Among them, especially the Bi2Te3-based compound has a relatively large ZT value at relatively low temperatures, rising continuously from room temperature to about 450K, and is currently the most widely used thermoelectric conversion material. The research on new low-dimensional TE structure materials has great theoretical and application value. Discovering materials with high ZT values (ZT > 4) will trigger technological revolutions in the refrigeration industry, energy industry, and semiconductor microelectronics industry. Although quantum dot or superlattice materials can obtain thermoelectric materials with a dimensionless figure of merit factor above 2, the application of such structural materials is limited by factors such as complex device fabrication processes, high costs, and difficulty in mass production. In addition to being difficult to obtain high-quality-factor thermoelectric materials, traditional thermoelectric material preparation methods such as zone melting, hot pressing, hot extrusion, and spark plasma sintering (SPS) are also difficult to obtain high-yield thermoelectric ingots. The available part of the ingot generally does not exceed 2 / 3 of the entire ingot. Therefore, developing high-quality bulk thermoelectric materials with nano-microstructures and good radial and axial uniformity may be a more realistic approach for the industrial application of thermoelectric materials. The present invention adopts 1) an induction coupling-assisted zone melting-directional solidification vertical zone melting method; 2) a high-density crystal nucleus rapid formation method; 3) an argon pressure modulation method for controlling the solid, liquid, and vapor volume ratios. By modulating and controlling the temperature gradient at the growth front, the supercooled molten liquid temperature, and the volume ratios of the solid, liquid, and vapor states at the crystallization front, the nucleation and growth processes of the crystal are controlled, and a bismuth telluride-based rod-shaped thermoelectric material with nano-crystals embedded therein, having a ZT value of 1.2 in the temperature range near room temperature and a physical property non-uniformity degree ≤ 5% along the crystal growth direction, is obtained. Summary of the Invention

[0003] To solve the problems of low efficiency of existing semiconductor thermoelectric devices, difficulty in achieving large temperature differences and high-energy-density heat sources, etc., the present invention provides a synthesis method for a bismuth telluride-based rod-shaped thermoelectric material with nano-crystals embedded therein, having a ZT value of 1.2 in the temperature range near room temperature and a physical property non-uniformity degree ≤ 5% along the crystal growth direction, so as to overcome the deficiencies of the prior art.

[0004] The nucleation principle in crystal growth is based on the reasonable maintenance of the quasi-equilibrium relationship between the chemical potential of the species crystal phase and the chemical potential of the species in the relevant phase. For example, in crystal growth from a solution, it is required that the solute has a certain appropriate supersaturation (supercooling in terms of temperature) near the equilibrium solubility. In this way, in the traditional zone melting state, it is relatively easy to nucleate at the cylindrical surface position of the crystal rod cylinder, the nucleation density of the overall growth front is not high, and at the same time, the radial uniformity obtained by zone melting is also poor. The use of induction heating assisted by the bottom of the heating stage can bring two improvements. One is to effectively reduce the temperature gradient of the bottom growth front during initial growth, control the appearance of the amorphous structure and component segregation. The other is that uniform nucleation and a high density of crystal nuclei can be obtained on the growth front according to the following principle.

[0005] The formula for generating an electric field by a magnetic field:

[0006] The electric field generated by a changing magnetic field is:

[0007] The current density of the conductive substrate in the quartz tube increases with the radius, obtaining more Joule heat J = σE generated by a larger induced current, which improves the temperature uniformity of the plane perpendicular to the growth direction and generates a higher density of crystal nuclei on the surface.

[0008] In addition, by frequently turning on and off the induction-assisted heating, a driving force for rapid nucleation and inhibition of the rapid growth of grains can be generated, forming a higher density of nanocrystals.

[0009] The technical solution for achieving the object of the present invention is: a method for synthesizing a semiconductor thermoelectric material, including the following steps:

[0010] Step 1: Place the bismuth telluride-based thermoelectric material sintered by the melt mixing method in a vacuum-sealed quartz tube into an induction-coupling-assisted zone melting-directional solidification vertical zone furnace, move the columnar heating coil to the bottom of the quartz tube, set the heating temperature, which can be continuously adjusted in the range from room temperature to 1650K, and start the heating element to heat.

[0011] Step 2: After the temperature reaches the set value, start the induction-coupling-assisted heating system after the bottom material is completely melted.

[0012] Step 3: Start the heating element lifting device and slowly lift the heating element.

[0013] Step 4: When the upper edge of the zone melting resistance heating element reaches the upper edge of the material inside the quartz tube, introduce high-purity argon through the mass flow controller, and stop heating and the heating element lifting device when the lower edge of the zone melting resistance heating element reaches the upper edge of the material inside the quartz tube.

[0014] As a further improvement of the present invention, in the step 1, the induction-coupling-assisted zone melting - directional solidification furnace is a vertical zone melting furnace, and the induction coupling coil can be made into a continuously variable diameter structure, which can obtain better temperature gradient control.

[0015] As a further improvement of the present invention, in the step 2, the frequency adjustment range of the induction coupling heating system is from 400 KHz to 1000 KHz, which is mainly used to adjust the mass transfer and heat transfer during the growth process, so as to control the growth stripe structure during the material growth process. The on / off frequency of the induction-assisted heating system The time step is 0.1 s - 5 s, and the relevant strategy is to interrupt the cooling after the crystallization starts, and then heat at an ultra-fast rate to "up-quench" the previously frozen structure.

[0016] Among them, T and T melt are the temperature at the center position of the heating platform and the melting point of the zone-melted material respectively, is the rising rate of the heating platform, and k is a proportionality constant. As a further improvement of the present invention, in the step 3, the lifting speed of the heating body lifting device is adjusted by a frequency converter controller, and the lifting speed range is from 0 mm / min to 10 mm / min. The lifting device is used to adjust the temperature gradient and control the growth driving force, monomer concentration, nucleation density and Ostwald ripening during the growth process.

[0017] As a further improvement of the present invention, in the step 4, the mass flow controller is used to introduce high-purity argon gas, and the flow rate of the introduced argon gas can be controlled by the numerical feedback of the heating body lifting speed, so as to improve the axial and radial uniformity of the synthetic material in the quartz tube.

[0018] As a further improvement of the present invention, the coordinated control among the parameters in the step 1, step 2, step 3 and step 4 can modulate two growth paths for the formation of nanocrystals: one is through monomer growth, and this growth mode is controlled by diffusion and surface reaction; the other is through the direct fusion between particles to achieve growth. For a specific nanocrystal material, which way to achieve it needs to be analyzed specifically according to the specific situation.

[0019] In the present invention, the induction-coupling-assisted zone melting - directional solidification vertical zone melting method can be applied to the preparation, growth and purification of metal and semiconductor nanoparticle-embedded polycrystalline materials with a melting point below 1650 K.

[0020] The cleavage plane of the thermoelectric element material must be mainly oriented along the direction of the expected current path through the thermoelectric elements integrated in the device. The proportion of grains with cleavage planes facing this direction must be minimized. Preferably, the material prepared by the design method of the present invention for invention patents contains nanostructured units that impede the propagation of cleavage in the thermoelectric element material, resulting in the thermoelectric element breaking along the current direction. For example, such structural elements can be the boundaries of adjacent grains, whose cleavage planes have a misorientation angle only in the plane perpendicular to the current direction. The ingots grown by conventional zone melting are usually stressed due to the non-linear axial temperature gradient, and even when the crystallization front is flat, stress is generated due to the anisotropic thermal expansion coefficients of adjacent grains with different orientations. These ingot stresses lead to the formation of microcracks during the growth stage. In addition, special measures must be taken to prevent the growth of grains with large orientation differences, such as dendrites. The method of the present invention can greatly reduce the probability of dendrite growth.

[0021] The increase in the maximum strain of the thermoelectric material obtained by the growth method designed by this patent before being damaged is about 0.6%, which is significantly higher than 0.25% of the materials produced by traditional methods, improving the toughness, resistance, and thermal shock resistance of the material.

[0022] The beneficial effects of the method and process of the present invention are as follows:

[0023] 1. The equipment preparation process is simple, the performance adjustment range is wide, the growth process is visible, the energy efficiency is high, there is no pollution, and the cost is low.

[0024] 2. There are many adjustable parameters during the synthesis process, which is convenient for quickly obtaining optimized process parameters.

[0025] 3. It is suitable for the synthesis and preparation of thermoelectric materials on various substrates.

[0026] 4. By controlling the nucleation density and temperature gradient, a bismuth telluride-based thermoelectric material with adjustable grain size and grain distribution density and embedded with nanograins can be obtained to obtain different electron state densities and phonon scattering rates near the Fermi surface, adapting to the material selection of various thermoelectric devices.

[0027] In addition, the method of the present invention is also applicable to the preparation of high-quality thermoelectric materials such as bismuth telluride-based and lead telluride-based with different microstructures and bismuth telluride-based topological insulator materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic structural diagram of an inductively coupled assisted zone melting and directional solidification furnace; wherein, 1 - inlet pipe; 2 - needle valve; 3 - quartz glass tube; 4 - mass flow controller; 5 - heating element; 6 - induction coil; 7 - reducer; 8 - motor; 9 - positioning draw bar; 10 - screw.

[0029] Figure 2Microstructure (SEM@2μm) of the N-type bismuth telluride-based thermoelectric material with embedded nanocrystals synthesized by the method of the present invention.

[0030] Figure 3 Microstructure (SEM@2μm) of the P-type bismuth telluride-based thermoelectric material with embedded nanocrystals synthesized by the method of the present invention.

[0031] Figure 4 Test results of the axial uniformity of the resistivity of P / N crystal rods. Detailed implementation manners

[0032] Example 1

[0033] Synthesis method of an N-type bismuth telluride-based thermoelectric material with embedded nanocrystals, comprising the following steps:

[0034] 1. Place the N bismuth telluride-based thermoelectric material sintered by the melt mixing method in a vacuum-sealed quartz tube into an induction coupling-assisted zone melting-directional solidification vertical zone furnace, move the columnar heating coil to the bottom of the quartz tube, and set the heating temperature to 973K;

[0035] 2. After the temperature reaches 973K, stabilize for 30 minutes, start the induction coupling-assisted heating system, and set the frequency to 600KHz;

[0036] 3. Set the lifting speed of the heating body to 0.2mm / min, start the heating body lifting device, and slowly lift the heating body;

[0037] 4. When the upper edge of the zone melting resistance heating body reaches the upper edge of the material inside the top of the quartz tube, introduce high-purity argon gas at a flow rate of 2SCCM through a mass flow controller;

[0038] 5. When the lower edge of the zone melting resistance heating body reaches the upper edge of the material inside the top of the quartz tube, stop heating and the heating body lifting device, the limit switch acts to stop the movement of the heating body, and then turn off the heating power supply.

[0039] The above synthesis method is suitable for the preparation of high-quality thermoelectric materials such as N-type bismuth telluride-based and lead telluride-based with different microstructures, and bismuth telluride-based topological insulator materials. The microstructure of the N-type bismuth telluride-based thermoelectric material with embedded nanocrystals synthesized by the method of the present invention is as Figure 2 shown, and the test results of the axial uniformity of the resistivity of its crystal rod are as Figure 4 shown.

[0040] Example 2

[0041] Synthesis method of a P-type bismuth telluride-based thermoelectric material with embedded nanocrystals, comprising the following steps:

[0042] 1. Place the P-bismuth telluride-based thermoelectric material sintered by the melt blending method in a vacuum-sealed quartz tube into an induction-coupled assisted zone melting-directional solidification vertical zone furnace. Move the columnar heating coil to the bottom of the quartz tube and set the heating temperature to 893K;

[0043] 2. After the temperature reaches 893K, stabilize for 30 minutes, start the induction-coupled assisted heating system, and set the frequency to 800KHz;

[0044] 3. Set the lifting speed of the heating body to 0.5mm / min, start the heating body lifting device, and slowly lift the heating body;

[0045] 4. When the upper edge of the zone melting resistance heating body reaches the upper edge of the material inside the top of the quartz tube, introduce high-purity argon gas at a flow rate of 5 SCCM through the mass flow controller;

[0046] 5. When the lower edge of the zone melting resistance heating body reaches the upper edge of the material inside the top of the quartz tube, stop the heating and the heating body lifting device. The limit switch operates to stop the movement of the heating body, and then turn off the heating power supply.

[0047] The above synthesis method is suitable for the preparation of high-quality thermoelectric materials such as P-type bismuth telluride-based and lead telluride-based with different microstructures, and bismuth telluride-based topological insulator materials. The microstructure of the P-type bismuth telluride-based thermoelectric material with embedded nanograins synthesized by the method of the present invention is as Figure 3 shown, and the test result of the axial uniformity of the resistivity of the crystal rod is as Figure 4 shown.

Claims

1. A method for synthesizing bismuth telluride-based thermoelectric materials, characterized in that, It includes: 1) An induction-coupling-assisted zone melting-directional solidification vertical zone melting method to improve the uniformity of the lower part of the prepared bismuth telluride-based thermoelectric material and generate a driving force distribution that increases the nucleation density; 2) An argon pressure modulation method for controlling the solid, liquid, and vapor volume ratios to improve the uniformity of the upper part of the prepared bismuth telluride-based thermoelectric material; The induction-coupling-assisted zone melting-directional solidification vertical zone melting method mentioned above refers to setting an induction coil with a diameter slightly larger than the outer diameter of the quartz tube and a length of 3-5 cm at the bottom of the heating body at the lower edge of the molten zone. At the initial stage of zone melting-directional solidification crystallization, induction heating is applied to the bottom of the quartz tube to obtain a small temperature gradient growth state for the crystals at the bottom during the initial growth stage. The temperature gradient field is established jointly by the moving speed of the heating body, the heating temperature, and the inductive coupling-assisted heating; The argon pressure modulation method for controlling the solid, liquid, and vapor volume ratios mentioned above refers to enabling argon to assist in modulating the liquid-solid temperature gradient at the tail of the crystal rod during the vertical zone melting of induction-coupling-assisted zone melting-directional solidification to the tail of the crystal rod. By adjusting the argon, the speed of melt mass transfer and heat transfer is adjusted to obtain the same temperature gradient and crystallization front driving force as during the growth of the middle part of the crystal. Here, the tail of the crystal rod refers to the crystal growth front at the upper end.

2. The synthesis method of the bismuth telluride-based thermoelectric material according to claim 1, characterized in that, Auxiliary induction heating at the bottom of the heating body is used to effectively reduce the temperature gradient of the bottom growth front surface during initial growth, control the appearance of amorphous structures and component segregation, and obtain uniform nucleation on the growth front surface and increase the nucleation density. The principle of obtaining uniform nucleation on the growth front surface and increasing the nucleation density is as follows: Formula for generating an electric field by a magnetic field: The electric field generated by a changing magnetic field is: The current density of the conductive substrate in the quartz tube increases with the radius, generating more Joule heat j = σE from the larger induced current, improving the temperature uniformity of the plane perpendicular to the growth direction, and generating a higher nucleation surface density; In addition, by frequently turning on and off the auxiliary induction heating, a driving force for rapid nucleation is generated and the rapid growth of grains is inhibited, forming a higher density of nanocrystals.

3. The synthesis method of the bismuth telluride-based thermoelectric material according to claim 1, characterized in that, The rapid formation of nuclei and the increase in nucleation density mentioned above refer to when directional solidification is carried out in the direction of the pre-established temperature gradient, inductive coupling pulse heating / stopping switching control is applied to the supercooled molten liquid region to rapidly form a higher density of nuclei. Directly, polycrystals with nanocrystals embedded can be obtained in a closed quartz tube. Its essence is to rapidly fine-tune the melt mass transfer and heat transfer to obtain a higher density of nuclei. The pulse modulation frequency adjustment range is from 400 KHz to 1000 KHz.

4. The synthesis method of the bismuth telluride-based thermoelectric material according to claim 1, characterized in that, The method mentioned above controls the nucleation and growth process of the crystal by modulating the temperature gradient of the crystal growth front, the temperature of the supercooled molten liquid, and the volume ratios of the solid, liquid, and vapor states at the crystallization front, and obtains a bismuth telluride-based rod-shaped thermoelectric material with nanocrystals embedded, with a ZT value reaching 1.2 in the temperature range near room temperature and a physical property non-uniformity along the crystal growth direction ≤ 5%.

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