A thermoelectric material of a ternary system and a preparation method thereof

Abstract

The present invention discloses a thermoelectric material of a ternary system and a preparation method thereof, including La, Bi and Te. In terms of mass percentage, the proportion of the La element is 4-5%, the proportion of the Bi element is 10-19%, the proportion of the Te element is 2%-14%, and the rest is the O element, which is synthesized by a solvothermal method. The present invention innovatively uses La to improve the electrical properties of the material and uses Bi to reduce the thermal conductivity, ultimately improving the thermoelectric properties, ensuring that its various performance parameters do not fluctuate greatly with temperature changes, and the preparation of its material has the potential for industrial production, being suitable for miniaturized and large-scale production and manufacturing.

Description

Technical Field

[0001] The invention belongs to the technical field of energy materials, and in particular relates to a ternary system thermoelectric material and a preparation method thereof, and involves a solvent thermal synthesis method and a spark plasma sintering technology. Background Art

[0002] Thermoelectric materials, also known as thermoelectric materials, are functional materials that use the directional migration of carriers inside solids to achieve direct conversion between heat and electricity. Due to their unique advantages such as no mechanical moving parts, no noise operation, no need for refrigerants, wide temperature range (200-1400K), environmental friendliness, compact structure, small footprint, precision and reliability, and long service life, they can better solve the environmental pollution and energy crisis problems facing the world, and are therefore widely used in waste heat recovery, thermoelectric power generation, thermoelectric refrigeration, space exploration, etc.

[0003] The conversion efficiency of thermoelectric materials is measured by the dimensionless thermoelectric figure of merit (ZT), which is expressed as

[0004]

[0005] Where Z is the thermoelectric quality factor; T is the absolute temperature (K); S is the Seebeck coefficient; σ is the electrical conductivity; κ is the thermal conductivity, which is calculated from the phonon thermal conductivity κ L and the electronic thermal conductivity κ e Since the larger the ZT value, the better the thermoelectric performance of the material, but these performance parameters affect each other and are entangled with each other, the main goal of researchers is to reduce or maintain its thermal transport performance as much as possible while improving the electrical transport performance of the material.

[0006] Generally speaking, there are several ways to improve the performance of thermoelectric materials: ① Through doping modification of materials and adjusting energy band processes, donors are doped in the potential barrier, which can make electrons enter the conduction band of the potential well layer from the conduction band of the barrier layer, so that the electrons will not be scattered by the ionized donors, thereby improving the carrier mobility; ② Additional phonon scattering is caused by nanoparticles and nanograin boundaries, and the mean free path of phonons is reduced through solid solution (point defect scattering), resonance scattering and other means to reduce the lattice thermal conductivity of the material. ③ Expand the use of thermoelectric materials through gradient, combine two or more single materials together, and make each material work at the optimal temperature. ④ Low-dimensional thermoelectric materials are conducive to improving the Seebeck coefficient and increasing phonon scattering on the surface of the potential well wall.

[0007] Telluride is a high-temperature thermoelectric material with good performance and wide applications. However, there are also some problems, such as it cannot be applied to medium-temperature or even low-temperature environments, and the thermal conductivity of bismuth telluride thermoelectric materials prepared by conventional methods is 0.5 W / m / K or greater. La can improve the electrical properties of the material, but currently, it is difficult to control the feed ratio of the La-doped telluride nanothermoelectric materials prepared by the only solid-phase synthesis method, and it is not easy to regulate their microtopography and properties. Moreover, the simple hydrothermal method cannot directly synthesize La with Te. Summary of the Invention

[0008] The object of the present invention is to provide a low-temperature thermoelectric material of a ternary system, whose thermoelectric properties such as Seebeck coefficient and power factor are more stable.

[0009] The technical solution to achieve the object of the present invention is: a thermoelectric material of a ternary system, including La, Bi, and Te. In terms of mass percentage, the proportion of La element is 4-5%, the proportion of Bi element is 10-19%, the proportion of Te element is 2%-14%, and the rest is O element.

[0010] Preferably, the thermoelectric material is composed of a solid solution of Bi, Bi 3.20 Te 0.80 O 6.40 and LaTe.

[0011] A preparation method of a thermoelectric material of a ternary system adopts a solvothermal method, including the following steps:

[0012] Step 1: Under the protection of nitrogen, mix LaCl3, BiCl3, and Te solid powders and add them to a hydrothermal autoclave, then add KOH solid thereto, mix well, and dissolve with DMF solvent;

[0013] Step 2: Dropwise add the DMF solution of NaBH4 into the hydrothermal autoclave;

[0014] Step 3: After the system is stable, keep it at a constant temperature of 180±5°C for 24 hours;

[0015] Step 4: Wash, vacuum dry, and grind;

[0016] Step 5: Sinter the powder obtained in Step 4 by plasma discharge to obtain the thermoelectric material.

[0017] Preferably, the molar ratio of LaCl3, BiCl3, and Te is 1-2:1-3:1-6, preferably 1:1:1, 1:2:3, 1:2:6, 2:3:3.

[0018] Preferably, the molar ratio of KOH to NaBH4 is 1:24-48.

[0019] Preferably, the molar amount of NaBH4 is at least 1.4 times the total molar amount of LaCl3, BiCl3, and Te.

[0020] Preferably, the solvent can be ethylene glycol or DMF.

[0021] Preferably, the plasma discharge sintering temperature is 400 ± 10 °C, the heating time is 20 min, the holding time is 20 min, the pressure is 50 Mpa, and the heating rate is 20 °C·min -1 。

[0022] The method provided by the present invention has the following advantages compared with the prior art:

[0023] (1) By adjusting the carrier concentration in the ternary system with La, and at the same time, the melting of elemental bismuth can introduce dislocations in the system, which strongly scatters phonons, thus greatly reducing the lattice thermal conductivity of the thermoelectric material.

[0024] (2) The process of the present invention is relatively simple, and it can be completely mass-produced on an industrial assembly line. Moreover, the performance parameters are relatively stable within the operating temperature range and are all in the low-temperature region, so it can play a role in the field of micro-devices.

[0025] (3) The maximum Seebeck coefficient of the ternary system thermoelectric material of the present invention exceeds 130 μV / K between 300 K and 600 K. Compared with other existing ternary system thermoelectric materials, the performance is improved by about 44.4%, and there is no large fluctuation.

[0026] (4) The required reaction temperature is low, the consumption time period is short, the product purity is high, and the material particle size is small (mostly nanoscale). The microscopic morphology of the material can be simply adjusted by changing the reaction conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The drawings are used to further illustrate and explain the present invention, and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention, and do not constitute a limitation to the present invention.

[0028] Figure 1 is the XRD diffraction pattern of experimental sample 3 of the present invention.

[0029] Figure 2 is the XRD diffraction pattern of experimental sample 5 of the present invention.

[0030] Figure 3 is the XRD diffraction pattern of experimental sample 6 of the present invention.

[0031] Figure 4 is the TEM image of experimental sample 6 of the present invention.

[0032] Figure 5 These are the EDS diagrams of experimental samples 3, 5, and 6 of the present invention.

[0033] Figure 6 These are the Seebeck coefficients of experimental samples 3, 5, and 6 of the present invention.

[0034] Figure 7 These are the resistivities of experimental samples 3, 5, and 6 of the present invention.

[0035] Figure 8 These are the thermal conductivities of experimental samples 3, 5, and 6 of the present invention.

[0036] Figure 9 These are the power factors of experimental samples 3, 5, and 6 of the present invention.

[0037] Figure 10 These are the thermoelectric figure of merit (ZT) of experimental samples 3, 5, and 6 of the present invention. Detailed implementation manners

[0038] To more intuitively and clearly demonstrate the purpose and advantages of the present invention, the technical solutions in the embodiments of the present invention will be further described in detail below in conjunction with the accompanying drawings in the embodiments of the present invention. However, the protection scope of the present invention is not limited to the content described. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative efforts fall within the protection scope of the present invention.

[0039] A low-temperature thermoelectric material of a ternary system described in the present invention is different from traditional lanthanide metal telluride nano-thermoelectric materials, which are all high-temperature types (above 1000K). It can be used in fields such as automotive exhaust waste heat recovery, small device refrigeration, and the functions of wearable devices.

[0040] The concept of the present invention is:

[0041] In the present invention, the electrical properties of the material are improved by doping La, and the thermal conductivity of the material is reduced by generating dislocations with Bi to improve the thermoelectric properties of the material. By constructing the La-Te-Bi system, when sintering the ternary system thermoelectric material by spark plasma sintering, elemental bismuth melts, leaving a large number of dislocations at the grain boundaries, resulting in strong scattering of phonons, greatly reducing the lattice thermal conductivity, and improving the thermoelectric properties.

[0042] In the present invention, by doping La elements into the Bi-Te system to make it a ternary system, its unique f-layer electron energy band can hybridize with the d electrons of other elements, increasing the band gap width and the electron energy state density near the Fermi level, resulting in an increase in the Seebeck coefficient of the material.

[0043] The present invention obtains an n-type semiconductor material through a solvothermal method. The reason is that the electronegativity of La (1.1 eV) is significantly smaller than that of Bi and Te (1.9 and 2.3 eV). Therefore, in the ternary system, La will lose some outer electrons to act as a donor impurity.

[0044] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods.

[0045] Unless otherwise specified, the materials, reagents, etc. used in the following examples can all be obtained through commercial channels.

[0046] Example 1:

[0047] Accurately weigh 1.2264 g of LaCl3, 3.1534 g of BiCl3, and 3.8280 g of Te solid powders, add them to a 50 mL hydrothermal reactor under the protection of nitrogen, then add 0.14 g of KOH solid thereto, and dissolve with 16 mL of DMF. At the same time, dissolve 2.27 g of NaBH4 with 10 mL of DMF and disperse it. Slowly add the dispersed liquid into the hydrothermal reactor. After the system is stable, put the hydrothermal reactor into a constant temperature oven and keep it at 180 °C for 24 hours. The precipitate obtained after centrifugation is washed and centrifuged with water once, washed and centrifuged with alcohol once, then washed and centrifuged with water once, and washed and centrifuged with alcohol twice. The prepared sample is 3, and its XRD is as Figure 1 shown, and its EDS corresponds to that as Figure 5 shown, and its Seebeck coefficient corresponds to that as Figure 6 shown, and its resistivity corresponds to that as Figure 7 shown, and its thermal conductivity corresponds to that as Figure 8 shown, and its power factor corresponds to that as Figure 9 shown, and its thermoelectric figure of merit corresponds to that as Figure 10 shown.

[0048] Example 2:

[0049] Accurately weigh 2.4527 g of LaCl3, 4.7301 g of BiCl3, and 1.9140 g of Te solid powders, add them to a 50 mL hydrothermal reactor under the protection of nitrogen, then add 0.14 g of KOH solid thereto, and dissolve with 16 mL of DMF. At the same time, dissolve 2.27 g of NaBH4 with 10 mL of DMF and disperse it. Slowly add the dispersed liquid into the hydrothermal reactor. After the system is stable, put the hydrothermal reactor into a constant temperature oven and keep it at 180 °C for 24 hours. The precipitate obtained after centrifugation is washed and centrifuged with water once, washed and centrifuged with alcohol once, then washed and centrifuged with water once, and washed and centrifuged with alcohol twice. The prepared sample is 5, and its XRD is as Figure 2 shown, and its EDS corresponds to that as Figure 5 shown, and its Seebeck coefficient corresponds to that asFigure 6 As shown, its resistivity corresponds to Figure 7 As shown, its thermal conductivity corresponds to Figure 8 As shown, its power factor corresponds to Figure 9 As shown, its figure of merit corresponds to Figure 10 As shown.

[0050] Example 3:

[0051] After accurately weighing 2.4527 g of LaCl3, 4.7301 g of BiCl3, and 1.9140 g of Te three solid powders, they were added into a 50 mL hydrothermal reactor under the protection of nitrogen. Then, 0.14 g of KOH solid was added and dissolved with 16 mL of DMF. At the same time, 4.54 g of NaBH4 was dissolved and dispersed with 10 mL of DMF, and the dispersed liquid was slowly added into the hydrothermal reactor. After the system was stable, the hydrothermal reactor was placed in a constant temperature oven and kept at 180 °C for 24 hours. The precipitate obtained by centrifugation was washed and centrifuged with water once, washed and centrifuged with alcohol once, then washed and centrifuged with water once, and washed and centrifuged with alcohol twice. The prepared sample was 6, and its XRD is as Figure 3 As shown, TEM is as Figure 4 , and its EDS corresponds to Figure 5 As shown, its Seebeck coefficient corresponds to Figure 6 As shown, its resistivity corresponds to Figure 7 As shown, its thermal conductivity corresponds to Figure 8 As shown, its power factor corresponds to Figure 9 As shown, its figure of merit corresponds to Figure 10 As shown.

[0052] From Figures 1 - 3 the XRD test spectrum, it can be seen that this thermoelectric material is composed of three solid solutions combined by Bi, Bi 3.20 Te 0.80 O 6.40 and LaTe.

[0053] Figure 4 From the TEM test of

[0054] it can be seen that the microscopic morphology of the sample is nanospheres with a diameter not exceeding 100 nm. Since DMF has a tetrahedral structure and the molecules are relatively independent in the aggregated state, the nanospheres induced by the crystal molecular structure are also relatively dispersed. Figure 5 As shown in the EDS tests of the above samples 3, 5, and 6, it can be seen that: the atomic percentage of La element fluctuates little, indicating that this synthesis method can stably dope La element in the ternary system; Bi element can account for at least more than 10%, up to 19% at most; Te element accounts for at least more than 2%, up to 14% at most. The solid solution composed of the above three elements becomes

[0055] After subjecting the above samples 3, 5, and 6 to spark plasma sintering, their thermoelectric properties were tested. The steps are as follows:

[0056] 1) The dry powder obtained after liquid-phase synthesis was carefully ground into a powder with uniform particles using an agate mortar. The grinding time was 30 minutes.

[0057] 2) The obtained powder was loaded into a cylindrical mold with an inner diameter of 10.4 mm and subjected to spark plasma sintering.

[0058] 3) After demolding, a cuboid block and a wafer were cut out along the direction perpendicular to the pressure direction (perpendicular to the radial direction)

[0059] at a speed of 1 mm / min. It was required that the height of the cuboid block be greater than 8 mm.

[0060] 4) The obtained materials were polished with 1500-mesh, 1000-mesh, and 800-mesh sandpapers respectively until the surfaces were uniform, flat, and smooth, and then their thermoelectric properties were tested.

[0061] The results are as Figures 6 - 10 shown: The novel ternary system thermoelectric materials of the present invention all exhibit their thermoelectric properties between 300 K and 600 K. The maximum Seebeck coefficient was measured from sample 5 obtained in Example 2, and its value exceeded 130 μV / K. Compared with the maximum value of 90 μV / K for other ternary system thermoelectric materials synthesized in past literature, the performance was improved by approximately 44.4%. Moreover, the Seebeck properties of the three samples were very stable and did not show the characteristic of large fluctuations with temperature as shown in the literature. The best electrical performance of the novel ternary system thermoelectric materials of the present invention between 300 K and 600 K was obtained from sample 6 obtained in Example 3, with an average resistivity of 1600 μΩ·m. Compared with the huge resistivities of the other two groups of samples, the performance was improved by approximately 98%. The resistivities of the three groups of samples decreased with increasing temperature. The best thermal performance of the novel ternary system thermoelectric materials of the present invention between 300 K and 600 K was obtained from sample 6 obtained in Example 3, with an average thermal conductivity of 0.0040 W / cm / K, which was approximately 20% and 30% higher than the performance of the other two groups of samples respectively. The thermal conductivities of the three groups of samples all showed the characteristic of decreasing with increasing temperature, and were much lower than the thermal conductivity of conventionally prepared bismuth telluride, with a smaller change range and fluctuation. The best power factor of the novel ternary system thermoelectric materials of the present invention between 300 K and 600 K was obtained from sample 6 obtained in Example 3, with an average of 0.00135 mW / m / K 2, the performance improvement is very significant compared with the other two groups of samples; the best thermoelectric figure of merit of the novel ternary thermoelectric material of the present invention between 300K and 600K is obtained from sample 6 in Example 3, and its maximum value is 0.0023, which is very significantly improved compared with the other two groups of samples.

[0062] In the best embodiment of the novel ternary thermoelectric material of the present invention, its thermoelectric figure of merit can achieve a performance doubling at 300K temperature at 600K temperature.

[0063] In the novel ternary thermoelectric material of the present invention, as the proportion of La element in the sample increases, the resistivity of the sample decreases accordingly.

[0064] In the novel ternary thermoelectric material of the present invention, regardless of the proportion of the reaction, the performance indexes of the obtained thermoelectric materials are relatively stable and do not fluctuate greatly with the change of temperature.

[0065] The embodiments of the present invention do not limit that the thermoelectric material can only contain materials of La-Bi-Te. According to different requirements or principles, other materials can be further doped in the thermoelectric material in the embodiments of the present invention to improve the thermoelectric performance or other performances of the thermoelectric material. Therefore, the embodiments of the present invention do not limit the addition of other materials except La materials.

Claims

1. A thermoelectric material of a ternary system, characterized in that, It includes La, Bi and Te. By mass percentage, the proportion of La element is 4 - 5%, the proportion of Bi element is 10 - 19%, the proportion of Te element is 2% - 14%, and the rest is O element; Among them, the thermoelectric material is composed of a solid solution of Bi, Bi 3.20 Te 0.80 O 6.40 and LaTe.

2. The preparation method of the thermoelectric material according to claim 1, characterized in that, The solvothermal method is adopted, including the following steps: Step 1: Under the protection of nitrogen, mix three solid powders of LaCl3, BiCl3 and Te, add them into a hydrothermal reactor, then add KOH solid thereto, and dissolve them with DMF solvent after mixing evenly; Step 2: At the same time, dropwise add the DMF solution of NaBH4 into the hydrothermal reactor; Step 3: After the system is stable, keep it at a constant temperature of 180 ± 5 °C for 24 hours; Step 4: Wash, vacuum dry and grind; Step 5: Use plasma discharge sintering for the powder obtained in Step 4 to obtain the thermoelectric material.

3. The method according to claim 2, wherein The molar ratio of LaCl3, BiCl3 and Te is 1 - 2:1 - 3:1 - 6.

4. The method according to claim 2, wherein The molar ratio of LaCl3, BiCl3 and Te is 1:1:1, 1:2:3, 1:2:6 or 2:3:

3.

5. The method according to claim 2, wherein The molar ratio of KOH to NaBH4 is 1:24 - 48.

6. The method according to claim 2, wherein The molar amount of NaBH4 is at least 1.4 times the total molar amount of LaCl3, BiCl3 and Te.

Images