High-performance calcium manganate thermoelectric ceramic block and preparation method thereof

Abstract

The invention relates to a high-performance calcium manganate thermoelectric ceramic block and a preparation method thereof, and the high-performance calcium manganate thermoelectric ceramic block comprises the following raw materials: pure CaCO3 and pure MnO2 in a molar ratio of (1-0.96): 1; the preparation method comprises the following steps: firstly, implementing a low-stress forming process to effectively reduce residual stress in a sintering process; secondly, regulating and controlling a sintering temperature field by stages, and implementing gradient slow cooling treatment in a critical phase change temperature interval, so as to inhibit the generation of phase change micro-cracks; through the process combination, the relative density of the finished product is 98% or above, and the finished product has no macroscopic crack defect; the thermoelectric performance is excellent, the power factor is increased to 336 [mu] W / cm / K2, and the comprehensive thermoelectric performance reaches the leading level of a pure CaMnO3 material; on the premise of keeping a high Seebeck coefficient (greater than or equal to 300 [mu] V / K) of the material, the conductivity is greatly improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation of thermoelectric materials, and particularly relates to a high-performance calcium manganite thermoelectric ceramic bulk and a preparation method thereof. Background Art

[0002] The present invention relates to the technical field of thermoelectric materials, and particularly relates to a method for optimizing the performance of calcium manganite (CaMnO3)-based thermoelectric ceramics. Thermoelectric materials directly convert thermal energy into electrical energy through the Seebeck effect, and their conversion efficiency is quantitatively characterized by the thermoelectric figure of merit zT, and its expression is: zT = S2σT / (κ L +κ e ), where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, κ L is the lattice thermal conductivity, and κ e is the electronic thermal conductivity. An ideal thermoelectric material needs to meet the co-optimization requirements of high S value, high σ and low κ L and κ e .

[0003] Oxide thermoelectric materials have important application values in the field of thermoelectric power generation due to their advantages of high-temperature stability, low cost and environmental compatibility. Among them, as a typical n-type oxide thermoelectric body, CaMnO3 has a relatively high Seebeck coefficient (about -300 μV / K), but its intrinsic resistivity (>1000 mΩ·cm) is too high and its lattice thermal conductivity (>3 W / m / K) is relatively high, resulting in the zT value (<0.1) being difficult to meet the practical requirements, and it is urgent to achieve performance breakthroughs through material design and process innovation.

[0004] The existing CaMnO3 preparation technologies mainly include solid-phase sintering method, co-precipitation method, hydrothermal method and sol-gel method. The solid-phase sintering method has a simple process but has problems such as a wide powder particle size distribution (5 - 20 μm) and composition segregation; although the wet chemical method (co-precipitation / hydrothermal / sol-gel) can obtain sub-micron-sized uniform powders (0.2 - 1 μm), the process complexity is high and the reaction parameters (pH value ±0.2, temperature ±5 °C) need to be precisely controlled. No matter which powder preparation process is adopted, subsequent cold pressing (pressure 100 - 300 MPa) and high-temperature sintering (1200 - 1400 °C) are required to obtain bulk materials. Although the traditional sintering process has a low equipment cost and is suitable for large-scale production, the obtained bulk materials often have defects such as microcracks (crack density >5 pieces / mm 2 ) and insufficient density (<92%).

[0005] Currently, the performance optimization of CaMnO3 mostly adopts the element doping strategy (such as donor doping with Nb, Mo, W, etc.). For example, in the paper published in June 2017, paper number 201701030734, "Research on Doped CaMnO3-Based Thermoelectric Ceramics", by increasing the carrier concentration (to 1×1019 cm -3 Improve the electrical conductivity above). However, the doping concentration is limited by the solid solubility limit (<3 at%), and excessive doping will trigger the carrier scattering effect, resulting in a 40-60% attenuation of the Seebeck coefficient. In addition, the existing technologies generally ignore the regulation effect of the material's intrinsic point defects (such as anti-site defects and cation vacancies) on the thermoelectric parameters, restricting the performance improvement space.

[0006] In terms of the bulk densification process, traditional technologies attempt to improve the density by increasing the forming pressure (>250 MPa) and sintering temperature (>1350 °C). However, experiments show that CaMnO3 is prone to residual stress due to lattice distortion during the high-temperature sintering stage. An excessive forming pressure (>200 MPa) will instead exacerbate the crack propagation after sintering (crack width >5 μm), resulting in a 20-30% decrease in electrical conductivity. This phenomenon reveals the compatibility contradiction between the traditional densification process and the material's intrinsic properties, and a new type of coordinated control technology for forming and sintering needs to be developed to solve this technical bottleneck. Summary of the Invention

[0007] In order to overcome the above-mentioned disadvantages of the prior art, the purpose of the present invention is to provide a high-performance calcium manganite (CaMnO3) thermoelectric ceramic bulk and its preparation method, which can improve the material performance by synergistically optimizing the forming process and the phase transition regulation strategy; first, implement a low-stress forming process to effectively reduce the residual stress during the sintering process; second, adopt a staged regulation of the sintering temperature field and perform a gradient slow cooling treatment in the critical phase transition temperature range to suppress the generation of phase transition microcracks; the above process combination enables the finished product to obtain a relative density of more than 98% and no macroscopic crack defects.

[0008] In order to achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0009] A high-performance calcium manganite thermoelectric ceramic, the raw materials include pure CaCO3 and pure MnO2 with a molar ratio of (1~0.96):1.

[0010] A preparation method for a high-performance calcium manganite thermoelectric ceramic bulk, specifically including the following steps:

[0011] Step 1: Mix the CaCO3 and MnO2 powder raw materials in a molar ratio of (1~0.96):1 and ball mill them in anhydrous ethanol as the medium for 10~12 h to make the powder raw materials evenly mixed;

[0012] Step 2: Dry the ball-milled powder raw materials at a temperature above 80 °C;

[0013] Step 3: Pre-sinter the dried powder raw materials at 1050~1200 °C in an air atmosphere for 4~6 h to obtain the CaMnO3 thermoelectric ceramic matrix powder;

[0014] Step 4: Subject the CaMnO3 thermoelectric ceramic matrix powder obtained in Step 3 to secondary ball milling and drying with the same parameters as those for primary ball milling and drying.

[0015] Step 5: Granulate the powder obtained after secondary ball milling and drying in Step 4.

[0016] Step 6: Take the granulated powder in Step 5 and press it under a pressure of 100 - 150 MPa to obtain a green body.

[0017] Step 7: Place the green body obtained in Step 6 into a muffle furnace for water drainage and binder removal to obtain a CaMnO3 ceramic block.

[0018] The specific method of granulation in Step 5 is as follows: Add 5 wt% of polyvinyl alcohol (PVA) to the powder, and the addition amount is 7.5 - 8 wt% of the powder mass; perform grinding granulation, then screen, and take the powder with a particle size between 60 - 100 mesh.

[0019] The specific method of water drainage and binder removal in Step 7 is as follows: Place the green body obtained in Step 6 into a muffle furnace, heat it from room temperature to 100 - 120 °C at a rate of 2 - 4 °C / min, hold for 2 - 4 h, then heat it to 550 - 600 °C at a rate of 2 - 4 °C / min, hold for 6 - 10 h, and then heat it to 1200 - 1300 °C at a rate of 3 - 5 °C / min, hold for 2 - 10 h to obtain a CaMnO3 ceramic block; adopt gradient cooling, cool it to 900 - 950 °C at a rate of 3 - 5 °C / min, then cool it to 750 - 800 °C at a rate of 1 - 1.5 °C / min, cool it to 400 - 450 °C at a rate of 2 - 4 °C / min, and then cool it with the furnace.

[0020] A high-performance calcium manganite thermoelectric ceramic block is prepared from the raw materials described above or according to the preparation method described in Steps 1 to 8.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] 1. Gradient densification synergistic regulation technology (improving electrical properties)

[0023] The present invention innovatively combines the optimization of pre-pressing pressure and the slow cooling process of the phase transition temperature to form a gradient densification control system. By using a relatively small pre-pressing pressure (100 - 150 MPa), the particles are uniformly distributed, which not only reduces the pores / crack defects in the forming process but also provides a uniform stress field basis for subsequent sintering. During the sintering stage, a slow cooling process of 1 - 1.5 °C / min is implemented at the phase transition temperature (850 - 900 °C), and the directional growth of grains is achieved through thermodynamic equilibrium control. This combined technology enables the material density to reach more than 98.5%, eliminates the phenomenon of local stress concentration, and significantly improves the conductivity (an increase of 239%).

[0024] 2. Intrinsic defect regulation technology (decoupling of electron-phonon transport)

[0025] The present invention constructs an intrinsic defect regulation system by precisely controlling the Ca / Mn stoichiometric ratio (0.96 - 0.99). Mn anti-site defects are innovatively introduced, increasing the carrier concentration (up to the order of 10 Ca + cm 20 cm -3 magnitude) without introducing external defects. This design realizes the synergistic improvement of carrier concentration and mobility, increasing the Seebeck coefficient from -273.4 μV / K (323 K) to -307.5 μV / K (323 K), and at the same time reducing the lattice thermal conductivity to 2.43 W / mK through point defect scattering. In particular, cation vacancies and anti-site defects are formed by Ca-site deficiency, generating multi-scale phonon scattering centers, and increasing the ZT value to 0.23 (about 2 times higher) while maintaining a high power factor.

[0026] 3. In terms of material design, the present invention realizes the non-stoichiometric ratio of Ca and Mn elements through stoichiometric ratio optimization, and constructs high-concentration electron-phonon scattering centers by combining the strategy of intrinsic point defect engineering. This composition regulation method breaks through the solid solubility limit of traditional hetero-element doping. On the premise of maintaining a high Seebeck coefficient (≥300 μV / K) of the material, the electrical conductivity is increased to 220 - 250% of the original level, and finally the thermoelectric figure of merit ZT value is increased from below the reference value of 0.1 to the range of 0.18 - 0.23.

[0027] In summary, the present invention adopts: ① gradient densification process to achieve cross-scale regulation of the microstructure, breaking through the problem of the inversion of density-uniformity in traditional processes; ② intrinsic defect regulation to solve the mutual restriction problem between carrier concentration and Seebeck coefficient. The technical effect is verified by experiments, and the power factor is increased to 336 μW / cm / K2, and the comprehensive thermoelectric performance reaches the leading level of pure CaMnO3 materials; on the material, the non-stoichiometric ratio of Ca and Mn elements is realized through stoichiometric ratio optimization, and high-concentration electron-phonon scattering centers are constructed by combining the strategy of intrinsic point defect engineering, breaking through the solid solubility limit of traditional hetero-element doping. On the premise of maintaining a high Seebeck coefficient (≥300 μV / K) of the material, the electrical conductivity is greatly increased. Compared with the existing doping modification technology, this method effectively avoids the deterioration effect of the Seebeck coefficient caused by doping (the reduction amplitude of the traditional method reaches 40 - 60%), providing a new technical path for the performance optimization of perovskite oxide thermoelectric materials. It provides a brand-new and effective method for the electron-phonon decoupling of perovskite thermoelectric oxide materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1Comparison of the resistivity between CaMnO3 bulk compacts that are pressed at 200 MPa, sintered at 1200 °C for 2 h and cooled in the furnace, and CaMnO3 bulk compacts that are pressed at 100 MPa, sintered at 1200 °C for 2 h and cooled with a gradient.

[0029] Figure 2 Figure showing the performance comparison between the pure CaMnO3 obtained in this invention and the pure CaMnO3 in other literatures. Among them, (a) is the comparison chart of the power factor PF of CaMnO3 and CaMnO3 in other literatures; (b) is the comparison chart of the thermal conductivity κ of CaMnO3 and CaMnO3 in other literatures; (c) is the comparison chart of the thermoelectric figure of merit zT of CaMnO3 and CaMnO3 in other literatures; (d) is the comparison chart of the average power factor and the average thermoelectric figure of merit zT of CaMnO3 with different Ca / Mn obtained in this invention and CaMnO3 in other literatures. 0.98 MnO3 and the comparison chart of the power factor PF of CaMnO3 in other literatures; (b) is the comparison chart of the thermal conductivity κ of Ca 0.98 MnO3 and the comparison chart of the thermal conductivity κ of CaMnO3 in other literatures; (c) is the comparison chart of the thermoelectric figure of merit zT of Ca 0.98 MnO3 and the comparison chart of the thermoelectric figure of merit zT of CaMnO3 in other literatures; (d) is the comparison chart of the average power factor and the average thermoelectric figure of merit zT of CaMnO3 with different Ca / Mn obtained in this invention and CaMnO3 in other literatures.

[0030] Figure 3 Figure showing the performance comparison of CaMnO3 with different Ca / Mn in this invention at 973.15 K (CaMnO3, x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06). Among them, (a) is the variation diagram of the Seebeck coefficient, conductivity σ and power factor PF of CaMnO3 under different x; (b) is the variation diagram of the thermoelectric figure of merit zT of CaMnO3 under different x. 1-x MnO3, x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06). Among them, (a) is the variation diagram of the Seebeck coefficient, conductivity σ and power factor PF of CaMnO3 under different x; (b) is the variation diagram of the thermoelectric figure of merit zT of CaMnO3 under different x. Detailed implementation mode

[0031] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0032] Embodiment 1:

[0033] A preparation method of a high-performance calcium manganite thermoelectric ceramic bulk, specifically including the following steps:

[0034] Step 1: Mix the CaCO3 and MnO2 powder raw materials in a molar ratio of 1:1 and ball mill them in anhydrous ethanol as the medium for 10 h to make the powder raw materials evenly mixed;

[0035] Step 2: Dry the mixed powder raw materials at 80 °C;

[0036] Step 3: Pre-sinter the dried powder raw materials at 1050 °C in an air atmosphere for 4 h to obtain CaMnO3 thermoelectric ceramic matrix powder;

[0037] Step 4: Ball mill and dry the CaMnO3 thermoelectric ceramic matrix powder obtained in Step 3 with the same parameters as the primary ball milling and drying;

[0038] Step 5: Take the CaMnO3 powder prepared in Step 4, add 5 wt% of polyvinyl alcohol (PVA) to it, with the addition amount being 7.5 wt% of the powder mass, carry out grinding and granulation, and then screen through a 60-mesh sieve to obtain a powder with a particle size of 60 mesh.

[0039] Step 6: Take the granulated powder obtained in Step 5, press it under a pressure of 100 MPa to obtain a green body.

[0040] Step 7: Put the green body obtained in Step 6 into a muffle furnace for water and binder removal: Heat from room temperature to 100 °C at a rate of 2 °C / min, hold for 2 h, then heat to 550 °C at a rate of 2 °C / min, hold for 6 h, and then heat to 1200 °C at a rate of 3 °C / min, hold for 2 h to obtain a CaMnO3 ceramic block; Adopt gradient cooling, cool to 900 °C at a rate of 3 °C / min, then cool to 750 °C at a rate of 1 °C / min, cool to 400 °C at a rate of 2 °C / min, and then cool with the furnace.

[0041] The high-performance calcium manganite thermoelectric ceramic block prepared in this example has a composition of CaMnO3, its conductivity at 50 °C is 14.61 S / cm, and the power factor PF at 700 °C is 2.23 μW / cm / K 2 , the thermal conductivity κ is 1.68 W / m / K, and the thermoelectric figure of merit zT is 0.13.

[0042] Example 2:

[0043] A preparation method of a high-performance calcium manganite thermoelectric ceramic block specifically includes the following steps:

[0044] Step 1: Mix the CaCO3 and MnO2 powder raw materials at a molar ratio of 0.99:1 and ball-mill them in anhydrous ethanol as a medium for 10.5 h to make the powder raw materials evenly mixed.

[0045] Step 2: Dry the mixed powder raw materials at 85 °C.

[0046] Step 3: Pre-sinter the dried powder raw materials at 1100 °C in an air atmosphere for 4.5 h to obtain CaMnO3 thermoelectric ceramic matrix powder.

[0047] Step 4: Ball-mill and dry the CaMnO3 thermoelectric ceramic matrix powder obtained in Step 3 with the same parameters as the primary ball-milling and drying.

[0048] Step 5: Take the CaMnO3 powder prepared in Step 4, add 5 wt% of polyvinyl alcohol (PVA) to it, with the addition amount being 7.6 wt% of the powder mass, carry out grinding and granulation, and then screen through an 80-mesh sieve to obtain a powder with a particle size of 80 mesh.

[0049] Step 6: Take the powder after granulation in Step 5 and press it under a pressure of 120 MPa to obtain a green body;

[0050] Step 7: Put the green body obtained in Step 6 into a muffle furnace for water and binder removal: Heat from room temperature to 105 °C at a rate of 2.5 °C / min, hold for 2.5 h, then heat to 570 °C at a rate of 2.5 °C / min, hold for 7 h, and then heat to 1225 °C at a rate of 3.5 °C / min, hold for 4 h to obtain a CaMnO3 ceramic block; Adopt gradient cooling, cool to 920 °C at a rate of 3.5 °C / min, then cool to 770 °C at a rate of 1.2 °C / min, cool to 420 °C at a rate of 2.5 °C / min, and then cool with the furnace.

[0051] The high-performance calcium manganite thermoelectric ceramic block prepared in this example has a composition of Ca 0.99 MnO3, its conductivity is 23.16 S / cm at 50 °C, and the power factor PF is 3.18 μW / cm / K at 700 °C 2 , the thermal conductivity κ is 1.63 W / m / K, and the thermoelectric figure of merit zT is 0.19.

[0052] Example 3:

[0053] A preparation method of a high-performance calcium manganite thermoelectric ceramic block specifically includes the following steps:

[0054] Step 1: Mix the CaCO3 and MnO2 powder raw materials at a molar ratio of 0.98:1 and ball-mill them in anhydrous ethanol as the medium for 11 h to make the powder raw materials evenly mixed;

[0055] Step 2: Dry the mixed powder raw materials at 90 °C;

[0056] Step 3: Pre-sinter the dried powder raw materials at 1150 °C in an air atmosphere for 5 h to obtain CaMnO3 thermoelectric ceramic matrix powder;

[0057] Step 4: Ball-mill and dry the CaMnO3 thermoelectric ceramic matrix powder obtained in Step 3 with the same parameters as the first ball-milling and drying;

[0058] Step 5: Take the CaMnO3 powder prepared in Step 4, add 5 wt% of polyvinyl alcohol (PVA) to it, and the addition amount is 7.8 wt% of the powder mass, grind and granulate it, and then screen it through a 100-mesh sieve to obtain a powder with a particle size of 100 mesh;

[0059] Step 6: Take the powder after granulation in Step 5 and press it under a pressure of 130 MPa to obtain a green body;

[0060] Step 7: Put the green body obtained in Step 6 into a muffle furnace for water and binder removal: Heat from room temperature to 110 °C at a rate of 3 °C / min, hold for 3 h, then heat to 580 °C at a rate of 3 °C / min, hold for 8 h, and then heat to 1250 °C at a rate of 4 °C / min, hold for 4 h to obtain a CaMnO3 ceramic block; Adopt gradient cooling, cool to 930 °C at a rate of 4 °C / min, then cool to 780 °C at a rate of 1.3 °C / min, cool to 430 °C at a rate of 3 °C / min, and then cool with the furnace.

[0061] The high-performance calcium manganate thermoelectric ceramic block prepared in this example has a composition of Ca 0.98 MnO3, its conductivity at 50 °C is 23.96 S / cm, and the power factor PF at 700 °C is 3.37 μW / cm / K 2 , the thermal conductivity κ is 1.44 W / m / K, and the thermoelectric figure of merit zT is 0.23.

[0062] Example 4:

[0063] A preparation method of a high-performance calcium manganate thermoelectric ceramic block specifically includes the following steps:

[0064] Step 1: Mix the CaCO3 and MnO2 powder raw materials at a molar ratio of 0.97:1 and ball-mill them for 11.5 h at once with anhydrous ethanol as the medium to make the powder raw materials evenly mixed;

[0065] Step 2: Dry the mixed powder raw materials at 95 °C;

[0066] Step 3: Pre-burn the dried powder raw materials at 1175 °C in an air atmosphere for 5.5 h to obtain CaMnO3 thermoelectric ceramic matrix powder;

[0067] Step 4: Re-ball-mill and dry the CaMnO3 thermoelectric ceramic matrix powder obtained in Step 3 with the same parameters as the first ball-milling and drying;

[0068] Step 5: Take the CaMnO3 powder prepared in Step 4, add 5 wt% of polyvinyl alcohol (PVA) to it, and the addition amount is 7.9 wt% of the powder mass, carry out grinding and granulation, and then screen through a 60-mesh sieve to obtain a powder with a 60-mesh particle size;

[0069] Step 6: Take the granulated powder in Step 5 and press it under a pressure of 140 MPa to obtain a green body;

[0070] Step 7: Put the green body obtained in Step 6 into a muffle furnace for water and binder removal: Heat from room temperature to 115°C at a rate of 3.5°C / min, hold for 3.5 h, then heat to 590°C at a rate of 3.5°C / min, hold for 9 h, then heat to 1275°C at a rate of 4.5°C / min, hold for 8 h to obtain a CaMnO3 ceramic block; Adopt gradient cooling, cool to 940°C at a rate of 4.5°C / min, then cool to 790°C at a rate of 1.4°C / min, cool to 440°C at a rate of 3.5°C / min, and then cool with the furnace.

[0071] The high-performance calcium manganite thermoelectric ceramic block prepared in this example has a composition of Ca 0.97 MnO3, its conductivity at 50°C is 23.54 S / cm, and the power factor PF at 700°C is 3.03 μW / cm / K 2 , the thermal conductivity κ is 1.68 W / m / K, and the thermoelectric figure of merit zT is 0.18.

[0072] Example 5:

[0073] A preparation method of a high-performance calcium manganite thermoelectric ceramic block specifically includes the following steps:

[0074] Step 1: Mix the CaCO3 and MnO2 powder raw materials at a molar ratio of 0.96:1 in absolute ethanol as a medium and ball mill for 12 h to make the powder raw materials evenly mixed;

[0075] Step 3: Dry the mixed powder raw materials at 100°C;

[0076] Step 4: Pre-sinter the dried powder raw materials at 1200°C in an air atmosphere for 6 h to obtain CaMnO3 thermoelectric ceramic matrix powder;

[0077] Step 4: Re-ball mill and dry the CaMnO3 thermoelectric ceramic matrix powder obtained in Step 3 with the same parameters as the primary ball milling and drying;

[0078] Step 5: Take the CaMnO3 powder prepared in Step 4, add 5 wt% of polyvinyl alcohol (PVA), the addition amount is 8 wt% of the powder mass, carry out grinding and granulation, and then screen through 80 meshes to obtain a powder with a particle size of 80 meshes;

[0079] Step 6: Take the granulated powder in Step 5 and press it under a pressure of 150 MPa to obtain a green body;

[0080] Step 7: Put the green body obtained in Step 6 into a muffle furnace for water and binder removal: Heat from room temperature to 120°C at a rate of 4°C / min, hold for 4 h, then heat to 600°C at a rate of 4°C / min, hold for 10 h, and then heat to 1300°C at a rate of 5°C / min, hold for 4 h to obtain a CaMnO3 ceramic block; Adopt gradient cooling, cool to 950°C at a rate of 5°C / min, then cool to 800°C at a rate of 1.5°C / min, cool to 450°C at a rate of 4°C / min, and then cool with the furnace.

[0081] The high-performance calcium manganate thermoelectric ceramic block prepared in this example has a composition of Ca 0.96 MnO3, its conductivity at 50°C is 22.86 S / cm, and the power factor PF at 700°C is 2.88 μW / cm / K 2 , the thermal conductivity κ is 1.53 W / m / K, and the thermoelectric figure of merit zT is 0.18.

[0082] Cut the sintered wafer into a cuboid of 3 mm × 4 mm × 10 mm for thermoelectric performance testing.

[0083] Performance analysis:

[0084] Figure 1 It is a comparison of the resistivity of the CaMnO3 block prepared by traditional pressing at 200 MPa, sintering at 1200°C for 2 h, and cooling with the furnace and the CaMnO3 block prepared in the present invention by pressing at 100 MPa, sintering at 1200°C for 2 h, and gradient cooling.

[0085] The resistivity of the sample pressed at 200 MPa, sintered at 1200°C for 2 h, and cooled with the furnace at room temperature is 627.45 mΩ·cm, while the resistivity of the sample obtained in the present invention pressed at 100 MPa, sintered at 1200°C for 2 h, and gradient cooled at room temperature is 262.47 mΩ·cm, which is 41.83% of the former. The electrical performance has been greatly improved.

[0086] Figure 2 It is a performance comparison diagram of the pure CaMnO3 obtained in the present invention and the pure CaMnO3 in other literatures. From Figure 2 (a), it can be seen that at 373.15 K, the power factor PF of the Ca 0.98 MnO3 sample obtained in the present invention is 2.53 μW / cm / K 2 , compared with the highest value of 1.36 μW / cm / K of CaMnO3 in other literatures 2 it is increased by 86%; at 973.15 K, the power factor PF of the Ca 0.98 MnO3 sample obtained in the present invention is 3.37 μW / cm / K 2, compared with the highest value of 2.56 μW / cm / K of CaMnO3 in other literatures 2 it is increased by 32%, and the electrical properties are greatly improved in the whole temperature range.

[0087] Figure 2 (b), at 323.15 K, the thermal conductivity κ of the CaMnO3 sample obtained in the present invention 0.98 is 2.45 W / m / K, which is 88% of the lowest value of 2.78 W / m / K of CaMnO3 in other literatures; at 973.15 K, the thermal conductivity κ of the Ca 0.98 MnO3 sample obtained in the present invention is 1.44 W / m / K, which is 81% of the lowest value of 1.77 W / m / K of CaMnO3 in other literatures. The thermal conductivity is greatly reduced in the whole temperature range.

[0088] Figure 2 (c), at 973.15 K, the thermoelectric figure of merit zT of the CaMnO3 sample obtained in the present invention 0.98 is 0.23, which is 2.1 times of the highest value of 0.11 of CaMnO3 in other literatures. The thermoelectric performance is greatly improved.

[0089] Figure 2 (d), the average power factor PF of the CaMnO3 sample obtained in the present invention 0.98 is ave 3.09 μW / cm / K 2 , which is 1.5 times of the highest value of 2.11 μW / cm / K of CaMnO3 in other literatures; the average thermoelectric figure of merit zT of the Ca 2 MnO3 sample obtained in the present invention 0.98 is ave 0.12, which is 2.4 times of the highest value of 0.05 of CaMnO3 in other literatures. The average thermoelectric performance is greatly improved, which is beneficial to improving the maximum conversion efficiency of thermoelectric devices.

[0090] Figure 3 is the performance comparison of CaMnO3 under different Ca / Mn in the present invention (Ca 1-x MnO3, x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06).

[0091] Figure 3As can be seen from (a), at 973.15 K, with the decrease of Ca content, the absolute value of the Seebeck coefficient of the sample shows a trend of first increasing and then decreasing, indicating that a proper decrease in Ca content can improve the Seebeck coefficient of CaMnO₃. The Seebeck coefficients of the samples with x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06 are -192.71, -201.95, -207.67, -199.10, -198.06, -198.06, -193.11 μV / K respectively; the electrical conductivity σ of the sample shows a trend of first increasing and then decreasing, indicating that a proper decrease in Ca content can improve the electrical conductivity of CaMnO₃. The electrical conductivities of the samples with x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06 are 59.97, 77.91, 78.05, 76.41, 73.31, 71.66, 71.07 S / cm respectively; the power factor PF of the sample shows a trend of first increasing and then decreasing, indicating that a proper decrease in Ca content can improve the power factor of CaMnO₃. The power factors of the samples with x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06 are 2.23, 3.18, 3.37, 3.03, 2.88, 2.81, 2.65 μW / cm / K 2 .

[0092] Figure 3 In (b), at 973.15 K, with the decrease of Ca content, the thermoelectric figure of merit zT of the sample shows a trend of first increasing and then decreasing, indicating that a proper decrease in Ca content can improve the comprehensive thermoelectric performance of CaMnO₃. The power factors of the samples with x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06 are 0.13, 0.19, 0.23, 0.18, 0.18, 0.16, 0.14 respectively.

Claims

1. A high performance calcium manganate thermoelectric ceramic, characterized in that: The raw materials include pure CaCO3 and pure MnO2 in a molar ratio of (1-0.96):

1.

2. A method for preparing a high-performance calcium manganate thermoelectric ceramic block, characterized in that: The specific steps include: Step 1, mixing CaCO3 and MnO2 powder raw materials at a molar ratio of (1-0.96):1 with anhydrous ethanol as a medium and ball milling for 10-12 hours to make the powder raw materials uniform; Step 2, drying the ball-milled powder raw material at a temperature above 80°C; Step 3, pre-calcining the dried powder raw material at 1050-1200° C. in an air atmosphere for 4-6 hours to obtain a CaMnO3 thermoelectric ceramic matrix powder; Step 4, ball-milling and drying the CaMnO3 thermoelectric ceramic matrix powder obtained in step 3 for a second time with the same parameters as the first ball-milling and drying; Step 5, granulating the powder after the second ball milling and drying in step 4; Step 6: Take the powder granulated in step 5 and press it at a pressure of 100-150 MPa to obtain a green body; Step 7: Place the green body obtained in step 6 into a muffle furnace for drainage and debinding to obtain a CaMnO3 ceramic block.

3. The method for preparing a high-performance calcium manganate thermoelectric ceramic block according to claim 2, characterized in that: The specific method of granulation in step 5 is: adding 5wt% polyvinyl alcohol (PVA) to the powder, the added amount being 7.5-8wt% of the mass of the powder; grinding and granulating, and then sieving to obtain powder with a particle size between 60-100 meshes.

4. The method for preparing a high-performance calcium manganate thermoelectric ceramic block according to claim 2, characterized in that: The specific method of draining and debinding in step 7 is: putting the green body obtained in step 6 into a muffle furnace, heating it from room temperature to 100-120°C at a rate of 2-4°C / min, keeping it warm for 2-4h, then heating it to 550-600°C at a rate of 2-4°C / min, keeping it warm for 6-10h, then heating it to 1200-1300°C at a rate of 3-5°C / min, keeping it warm for 2-10h to obtain a CaMnO3 ceramic block; adopting gradient cooling, cooling it to 900-950°C at a rate of 3-5°C / min, then cooling it to 750-800°C at a rate of 1-1.5°C / min, cooling it to 400-450°C at a rate of 2-4°C / min, and then cooling it with the furnace.

5. The method for preparing a high-performance calcium manganate thermoelectric ceramic block according to claim 2, characterized in that: The specific steps include: Step 1, mixing CaCO3 and MnO2 powder raw materials at a molar ratio of 1:1 with anhydrous ethanol as a medium and ball milling for 10 hours to make the powder raw materials uniform; Step 2: Dry the mixed powder raw materials at 80°C; Step 3, pre-calcining the dried powder raw material at 1050° C. in an air atmosphere for 4 hours to obtain a CaMnO3 thermoelectric ceramic matrix powder; Step 4, ball-milling and drying the CaMnO3 thermoelectric ceramic matrix powder obtained in step 3 for a second time with the same parameters as the first ball-milling and drying; Step 5: Take the CaMnO3 powder obtained in step 4, add 5wt% polyvinyl alcohol (PVA) thereto, the added amount is 7.5wt% of the powder mass, grind and granulate, and then sieve through 60 mesh to obtain a powder with a particle size of 60 mesh; Step 6: Take the powder granulated in step 5 and press it at a pressure of 100 MPa to obtain a green body; Step 7. Place the green body obtained in step 6 into a muffle furnace for drainage and debinding: heat from room temperature to 100°C at a rate of 2°C / min, keep warm for 2 hours, then heat to 550°C at a rate of 2°C / min, keep warm for 6 hours, then heat to 1200°C at a rate of 3°C / min, keep warm for 2 hours to obtain a CaMnO3 ceramic block; adopt gradient cooling, cool to 900°C at a rate of 3°C / min, then cool to 750°C at 1°C / min, cool to 400°C at 2°C / min, and then cool with the furnace.

6. The method for preparing a high-performance calcium manganate thermoelectric ceramic block according to claim 2, characterized in that: The specific steps include: Step 1, mixing CaCO3 and MnO2 powder raw materials at a molar ratio of 0.99:1 with anhydrous ethanol as a medium and ball milling for 10.5 hours to make the powder raw materials mixed evenly; Step 2: Dry the mixed powder raw materials at 85°C; Step 3, pre-calcining the dried powder raw material at 1100° C. in an air atmosphere for 4.5 hours to obtain a CaMnO3 thermoelectric ceramic matrix powder; Step 4, ball-milling and drying the CaMnO3 thermoelectric ceramic matrix powder obtained in step 3 for a second time with the same parameters as the first ball-milling and drying; Step 5: Take the CaMnO3 powder obtained in step 4, add 5wt% polyvinyl alcohol (PVA) thereto, the added amount is 7.6wt% of the powder mass, grind and granulate, and then sieve through 80 mesh to obtain a powder with a particle size of 80 mesh; Step 6: Take the powder granulated in step 5 and press it at a pressure of 120 MPa to obtain a green body; Step 7. Place the green body obtained in step 6 into a muffle furnace for drainage and debinding: heat from room temperature to 105°C at a rate of 2.5°C / min, keep warm for 2.5 hours, then heat to 570°C at a rate of 2.5°C / min, keep warm for 7 hours, then heat to 1225°C at 3.5°C / min, keep warm for 4 hours to obtain a CaMnO3 ceramic block; adopt gradient cooling, cool to 920°C at a rate of 3.5°C / min, then cool to 770°C at 1.2°C / min, cool to 420°C at 2.5°C / min, and then cool with the furnace.

7. The method for preparing a high-performance calcium manganate thermoelectric ceramic block according to claim 2, characterized in that: The specific steps include: Step 1, mixing CaCO3 and MnO2 powder raw materials at a molar ratio of 0.98:1 with anhydrous ethanol as a medium and ball milling for 11 hours to make the powder raw materials mixed evenly; Step 2: Dry the mixed powder raw materials at 90°C; Step 3, pre-calcining the dried powder raw material at 1150° C. in an air atmosphere for 5 hours to obtain a CaMnO3 thermoelectric ceramic matrix powder; Step 4, ball-milling and drying the CaMnO3 thermoelectric ceramic matrix powder obtained in step 3 for a second time with the same parameters as the first ball-milling and drying; Step 5: Take the CaMnO3 powder obtained in step 4, add 5wt% polyvinyl alcohol (PVA) thereto, the added amount is 7.8wt% of the powder mass, grind and granulate, and then sieve through 100 mesh to obtain a powder with a particle size of 100 mesh; Step 6: Take the powder granulated in step 5 and press it at a pressure of 130 MPa to obtain a green body; Step 7. Place the green body obtained in step 6 into a muffle furnace for drainage and debinding: heat from room temperature to 110°C at a rate of 3°C / min, keep warm for 3 hours, then heat to 580°C at a rate of 3°C / min, keep warm for 8 hours, then heat to 1250°C at 4°C / min, keep warm for 4 hours to obtain a CaMnO3 ceramic block; adopt gradient cooling, cool to 930°C at a rate of 4°C / min, then cool to 780°C at 1.3°C / min, cool to 430°C at 3°C / min, and then cool with the furnace.

8. The method for preparing a high-performance calcium manganate thermoelectric ceramic block according to claim 2, characterized in that: The specific steps include: Step 1, mixing CaCO3 and MnO2 powder raw materials at a molar ratio of 0.97:1 with anhydrous ethanol as a medium and ball milling for 11.5 hours to make the powder raw materials uniform; Step 2: Dry the mixed powder raw materials at 95°C; Step 3, pre-calcining the dried powder raw material at 1175° C. in an air atmosphere for 5.5 hours to obtain a CaMnO3 thermoelectric ceramic matrix powder; Step 4, ball-milling and drying the CaMnO3 thermoelectric ceramic matrix powder obtained in step 3 for a second time with the same parameters as the first ball-milling and drying; Step 5: Take the CaMnO3 powder obtained in step 4, add 5wt% polyvinyl alcohol (PVA) thereto, the added amount is 7.9wt% of the powder mass, grind and granulate, and then sieve through 60 mesh to obtain a powder with a particle size of 60 mesh; Step 6: Take the powder granulated in step 5 and press it at a pressure of 140 MPa to obtain a green body; Step 7. Place the green body obtained in step 6 into a muffle furnace for drainage and debinding: heat from room temperature to 115°C at a rate of 3.5°C / min, keep warm for 3.5 hours, then heat to 590°C at a rate of 3.5°C / min, keep warm for 9 hours, then heat to 1275°C at 4.5°C / min, keep warm for 8 hours to obtain a CaMnO3 ceramic block; adopt gradient cooling, cool to 940°C at a rate of 4.5°C / min, then cool to 790°C at 1.4°C / min, cool to 440°C at 3.5°C / min, and then cool with the furnace.

9. The method for preparing a high-performance calcium manganate thermoelectric ceramic block according to claim 2, characterized in that: The specific steps include: Step 1, mixing CaCO3 and MnO2 powder raw materials at a molar ratio of 0.96:1 with anhydrous ethanol as a medium and ball milling for 12 hours to make the powder raw materials uniform; Step 3: Dry the mixed powder raw materials at 100°C; Step 4, pre-sintering the dried powder raw material at 1200° C. in an air atmosphere for 6 hours to obtain a CaMnO3 thermoelectric ceramic matrix powder; Step 4, ball-milling and drying the CaMnO3 thermoelectric ceramic matrix powder obtained in step 3 for a second time with the same parameters as the first ball-milling and drying; Step 5: Take the CaMnO3 powder obtained in step 4, add 5wt% polyvinyl alcohol (PVA) thereto, the added amount is 8wt% of the powder mass, grind and granulate, and then sieve through 80 mesh to obtain a powder with a particle size of 80 mesh; Step 6: Take the powder granulated in step 5 and press it at a pressure of 150 MPa to obtain a green body; Step 7. Place the green body obtained in step 6 into a muffle furnace for drainage and debinding: heat from room temperature to 120°C at a rate of 4°C / min, keep warm for 4 hours, then heat to 600°C at a rate of 4°C / min, keep warm for 10 hours, then heat to 1300°C at a rate of 5°C / min, keep warm for 4 hours to obtain a CaMnO3 ceramic block; adopt gradient cooling, cool to 950°C at a rate of 5°C / min, then cool to 800°C at 1.5°C / min, cool to 450°C at 4°C / min, and then cool with the furnace.

10. A high-performance calcium manganate thermoelectric ceramic block, made from the raw material according to claim 1 or made according to the preparation method according to any one of claims 2 to 9.

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