Thermoelectric material

Abstract

To provide a thermoelectric material that achieves high power generation output and also has excellent mechanical strength.SOLUTION: The thermoelectric material comprises a half-Heusler compound represented by the general formula: VA(Fe1-xMx)BSbC (where M is at least one of Ni and Cu; and 0.80≤A≤1.20, 0.80≤B≤1.20, and 0.80≤C≤1.20). In the general formula, x is greater than 0 and less than or equal to 0.02, and is preferably greater than or equal to 0.004 and less than or equal to 0.007. In addition, the Vickers hardness is 6.0 GPa or more.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to thermoelectric materials. [Background technology]

[0002] As carbon neutral policies move towards reality, expectations are growing for thermoelectric power generation as an innovative technology for making effective use of unused waste heat in our surroundings. Thermoelectric power generation is a power generation technology in which electromotive force is generated from temperature differences through the Seebeck effect of solids. When a thermoelectric power generation element is in contact with a waste heat source, it continues to generate electricity as long as there is a temperature difference. It is said that the total amount of unused waste heat in Japan at around room temperature (0°C to 100°C) is equivalent to the annual power generation of 100 typical thermal power plants. Research is being conducted into recovering as much of this enormous amount of waste heat as possible in the form of electricity.

[0003] The material that shows the highest thermoelectric power generation performance near room temperature is bismuth telluride (Bi 2 Te 3 )-based materials are known (see, for example, Patent Document 1). The reason why bismuth-tellurium-based materials have not yet been widely used is thought to be due to the fact that their power generation output is insufficient for practical use, as well as the mechanical fragility due to their structure. Generally, for thermoelectric power generation, a π-type module is used in which about 100 pairs of p-type and n-type elements are electrically connected in series. Therefore, if one part of the π-type module breaks, the entire π-type module will be unable to generate power. Therefore, the thermoelectric material that constitutes the p-type or n-type element must have not only power generation performance but also mechanical strength that can withstand repeated thermal stress. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2006-108418 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, previous research into thermoelectric materials has focused on the pursuit of high power output, with little emphasis on mechanical strength.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a thermoelectric material which is capable of obtaining high power generation output and has excellent mechanical strength. [Means for solving the problem]

[0007] The present invention has the following aspects. [1] General formula: V A (Fe 1-x M x ) B Sb C A thermoelectric material consisting of a half-Heusler compound represented by the formula (1) (M is at least one of Ni and Cu; 0.80≦A≦1.20, 0.80≦B≦1.20, 0.80≦C≦1.20). [2] The thermoelectric material according to [1], wherein in the general formula, x is greater than 0 and not greater than 0.02. [3] The thermoelectric material according to [1] or [2], wherein in the general formula, x is 0.004 or more and 0.007 or less. [4] The thermoelectric material according to any one of [1] to [3], which has a Vickers hardness of 6.0 GPa or more. Effect of the Invention

[0008] According to the present invention, it is possible to provide a thermoelectric material that is capable of obtaining high power generation output and has excellent mechanical strength. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing the results of crystal structure analysis of the V(Fe0.994Ni0.006)Sb solid solution phase by the Rietveld method. [Diagram 2] FIG. 13 is a diagram showing the results of measuring the temperature change of the Seebeck coefficient of a thermoelectric material. [Diagram 3]FIG. 1 is a diagram showing the results of measuring the temperature change in electrical resistivity of a thermoelectric material. [Figure 4] FIG. 13 is a diagram showing the results of measuring the temperature change in thermal conductivity of a thermoelectric material. [Diagram 5] FIG. 13 is a diagram showing the results of measuring the temperature change of the power factor of a thermoelectric material. [Figure 6] FIG. 13 is a diagram showing the results of measuring the temperature change of a dimensionless figure of merit of a thermoelectric material. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] An embodiment of the thermoelectric material of the present invention will be described. It should be noted that the present embodiment is specifically described to allow a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified.

[0011] [Thermoelectric materials] The thermoelectric material according to one embodiment of the present invention has the general formula: A (Fe 1-x M x ) B Sb C (M is at least one of Ni and Cu. 0.80≦A≦1.20, 0.80≦B≦1.20, 0.80≦C≦1.20). That is, the thermoelectric material of this embodiment is a full Heusler compound represented by VFeSb (V: vanadium, Fe: iron, Sb: antimony) in which part of Fe is substituted with at least one of nickel (Ni) and copper (Cu). That is, the thermoelectric material of this embodiment may contain only Ni, only Cu, or both Ni and Cu as the substitution element in the general formula.

[0012] In the general formula, x is preferably more than 0 and not more than 0.02. When x is within the above range, the absolute value of the Seebeck coefficient tends to decrease, the electrical resistivity tends to decrease, and the thermal conductivity tends to increase as x increases. When x exceeds the upper limit, the absolute value of the Seebeck coefficient is too small and the electrical resistivity is too low to obtain a sufficient power factor.

[0013] In the general formula, it is more preferable that x is 0.004 or more and 0.007 or less. When x is within the above range, the power factor is 5 mWK -2 m -1 and the dimensionless figure of merit is 0.3 or more. If x is less than the lower limit, the power factor is too low and the power output is insufficient. If x is more than the upper limit, the power factor is too low and the power output is insufficient.

[0014] In the general formula, A is 0.80 to 1.20, preferably 0.85 to 1.05. If A is less than the lower limit, a second phase is generated and the electrical resistivity increases. If A exceeds the upper limit, a second phase is generated and the electrical resistivity increases.

[0015] In the general formula, B is 0.80 to 1.20, preferably 0.85 to 1.15. If B is less than the lower limit, a second phase is generated and the electrical resistivity increases. If B exceeds the upper limit, a second phase is generated and the electrical resistivity increases.

[0016] In the general formula, C is 0.80 to 1.20, preferably 0.85 to 1.05. If C is less than the lower limit, a second phase is generated and the electrical resistivity increases. If C exceeds the upper limit, a second phase is generated and the electrical resistivity increases.

[0017] The thermoelectric material of this embodiment preferably has a Vickers hardness of 6.0 GPa or more, more preferably 7.0 GPa or more. When the Vickers hardness is equal to or higher than the lower limit value, a thermoelectric power generation module having mechanical strength capable of withstanding repeated thermal stress can be realized.

[0018] The Vickers hardness of the thermoelectric material can be obtained from the relationship between the contact area of the indenter and the sample, which is calculated from the diagonal length of the indentation when an appropriate test force is applied to the diamond square pyramid and pressed into the sample and then the indenter is removed.

[0019] The performance of the thermoelectric material is represented by the material performance index z and the output factor P. The material performance index z is defined by the following formula (1). The output factor P is defined by the following formula (2). z = S 2 / ρκ = S 2 σ / κ (K -1 ) (1) P = S 2 / ρ = S 2 σ (W / K 2 m) (2) In the above formulas (1) and (2), S is the Seebeck coefficient (V / K) representing the electromotive force per unit temperature difference, ρ is the electrical resistivity (Ωm) representing the internal resistance, κ is the thermal conductivity (W / Km) representing the ease of temperature difference, and σ is the conductivity (S / cm).

[0020] In order to improve the thermoelectric performance of the thermoelectric material, it is required to achieve both a high output factor P (a physical quantity corresponding to the power generation output per 1 °C temperature difference, which is the product of the square of the Seebeck coefficient S and the conductivity σ: S 2 σ) and a low thermal conductivity. Further, the thermal conductivity κ of the thermoelectric material is represented by the sum of the carrier thermal conductivity κ c (c stands for carrier) and the lattice thermal conductivity κ p (p stands for phonon).

[0021] An index indicating the thermoelectric conversion efficiency of the thermoelectric material is the dimensionless performance index zT. The dimensionless performance index zT is defined by the following formula (3). zT = S 2 σT / κ = S2 σT / (κ c +κ p )=(S 2 σT / κ c )×(1 / (1+κ p / κ c )) (3) From the above formula (3), in order to improve the thermoelectric performance of a thermoelectric material, it is necessary to achieve both high electrical conductivity and low thermal conductivity. That is, in the above formula (3), S 2 σT / κ c Maximize 1 / (1+κ p / κ c ) can improve the thermoelectric performance of the thermoelectric material.

[0022] The thermoelectric material of the present embodiment is a compound represented by the general formula: V(Fe 1-x M x A half-Heusler compound represented by the formula (I) is used, where M is at least one of Ni and Cu.

[0023] According to the thermoelectric material of this embodiment, a high power generation output can be obtained, and a thermoelectric material having excellent mechanical strength can be obtained.

[0024] [Method of manufacturing thermoelectric materials] A method for producing the thermoelectric material of this embodiment will be described. First, reagents (with a purity of 99.9% or more, granular) of the constituent elements (Fe, V, Sb, Ni, Cu) are weighed according to the target composition formula, and an ingot sample is prepared by the arc melting method. The obtained ingot is crushed into powder in an alumina mortar, then placed in a graphite mold and subjected to spark plasma sintering (SPS) under the conditions of 500°C to 900°C, 10 MPa to 100 MPa, and 1 minute to 30 minutes. The sample after the spark plasma sintering is vacuum sealed in a quartz tube and subjected to homogenization treatment (heat treatment) at 500°C to 900°C for 10 hours to 100 hours. The order of the spark plasma sintering and homogenization treatment may be reversed. The time for the spark plasma sintering and homogenization treatment may be longer than the upper limit value. Through the above operations, the thermoelectric material of this embodiment is obtained. EXAMPLES

[0025] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0026] [Sample synthesis] First, the constituent elements (Fe, V, Sb, Ni, Cu) were weighed out according to the desired composition formula (purity 99.9% or more, granular) and an ingot sample was prepared by the arc melting method. The obtained ingot was crushed into powder in an alumina mortar, then placed in a graphite mold and subjected to spark plasma sintering (SPS) at 600°C and 50 MPa for 20 minutes. The sample after the spark plasma sintering was vacuum sealed in a quartz tube and subjected to homogenization treatment (heat treatment) at 600°C for 100 hours or less. The obtained cylindrical samples were cut into sizes suitable for each measurement.

[0027] [Evaluation of crystal structure] The obtained samples were powdered and subjected to X-ray diffraction (XRD) measurements to identify the product phase and refine the lattice constants by the Rietveld method.

[0028] V(Fe 1-x M x The crystal structure of the )Sb (M=Ni, Cu) solid solution phase was analyzed by the Rietveld method. As an example of crystal structure analysis, the analysis result of a sample with M=Ni and x=0.006 is shown in Figure 1. In Figure 1, the green lines (dashed lines in the clean copy) are calculated XRD patterns simulating the structural model, and the points below the green lines (dashed lines in the clean copy) indicate the measurement points. The blue lines (solid lines in the clean copy) below these lines indicate the difference between the measured and calculated patterns. In the analysis, a model was adopted in which the space group is F-43m (minus sign is above 4), V occupies the 4a (0,0,0) site, Sb occupies the 4b (1 / 2,1 / 2,1 / 2) site, Fe is present at 90% in the Fe1:4c (1 / 4,1 / 4,1 / 4) site, and Fe2:4d (3 / 4,3 / 4,3 / 4) site at a ratio of 10%. From the results shown in Figure 1, it was found that Ni substitutes for Fe.

[0029] Tables 1 to 3 show the lattice constants of the respective solid solution phases refined by crystal structure analysis using the Rietveld method, together with the R-factors.

[0030] [Table 1]

[0031] [Table 2]

[0032] [Table 3]

[0033] Since the target substance is a cubic crystal, only the a-axis length is defined as the lattice constant in Tables 1 to 3. In addition, in Tables 1 to 3, the numbers in parentheses after the refined values ​​indicate the standard deviation of the last digit. The R-factor is the weighted pattern R-factor (R WP ) and the pattern R factor (R P ) and R WP and R eThe ratio S of (the lowest statistically expected R value) was adopted. WP and R P It can be concluded that the lower the better, and that an analysis with sufficient accuracy was performed when S was about 2 or less. From the results shown in Tables 1 to 3, it can be determined that the results of this experiment all had sufficient accuracy.

[0034] [Evaluation of thermoelectric performance] The thermoelectric properties of the obtained samples were measured by measuring the Seebeck coefficient, electrical resistivity, and thermal conductivity as a function of temperature. From these measurement results, the power factor and dimensionless figure of merit were calculated. The Seebeck coefficient and electrical resistivity were measured by the temperature difference electromotive force method and the DC four-terminal method, respectively, using a ZEM-3 manufactured by ULVAC-RIKO Co., Ltd. The measurement temperatures for the Seebeck coefficient and electrical resistivity were 50°C, 100°C, and 140°C. The thermal conductivity was measured by a laser flash method using an LFA467HT manufactured by Netzsch Co., Ltd. The thermal conductivity was measured at temperatures of 50°C, 100°C, and 140°C. The results of measuring the Seebeck coefficient as a function of temperature are shown in Figure 2. The results of measuring the electrical resistivity as a function of temperature are shown in Figure 3. The results of measuring the thermal conductivity as a function of temperature are shown in Figure 4. The results of measuring the power factor as a function of temperature are shown in Figure 5. The results of measuring the dimensionless figure of merit as a function of temperature are shown in Figure 6.

[0035] From the results shown in FIG. 2, it was confirmed that the Seebeck coefficient increases as the Ni content increases, regardless of temperature. From the results shown in FIG. 3, it was confirmed that as the Ni content increases, the electrical resistivity decreases, regardless of temperature. From the results shown in FIG. 4, it was confirmed that as the Ni content increases, the thermal conductivity tends to increase, regardless of temperature. From the results shown in Figure 5, if x is between 0.003 and 0.01, the power factor is 4mWK regardless of temperature. -2 m -1 If x is 0.004 or more and 0.007 or less, the power factor is 5mWK regardless of temperature. -2 m -1It was confirmed that this was the case. From the results shown in FIG. 6, it was confirmed that when x is 0.003 or more and 0.01 or less, the dimensionless figure of merit is 0.2 or more regardless of the temperature, and when x is 0.004 or more and 0.007 or less, the dimensionless figure of merit is 0.3 or more regardless of the temperature.

[0036] [Vickers hardness rating] The Vickers hardness of the obtained samples was measured using a Mitutoyo HM-100. The results are shown in Table 4. The average and standard deviation of five measurements for each sample are shown. For reference, Table 4 also shows the Vickers hardness of the Bi 2 Te 3 PbTe-based power generation materials, CoSb 3 , MnSi 1.7 The Vickers hardness of the Si-Ge alloy is shown in Table 4.

[0037] [Table 4]

[0038] From the results shown in Table 4, VFeSb, V(Fe 1-x Ni x )Sb(M=Ni), and V(Fe 1-x Cu x It was found that the hardness of Sb in all compositions was about Hv=700 (7GPa). Furthermore, partial substitution of Ni or Cu did not result in a significant change in Hv. Furthermore, the thermoelectric material made of the half-Heusler compound in the present invention had a hardness of about Hv=700 (7GPa) compared to the practical material Bi. 2 Te 3 It has a Vickers hardness that is more than 10 times that of PbTe-based and PbTe-based power generation materials, and is considered to be sufficiently practical in terms of strength.

Claims

1. General formula: V Ａ (Fe １－ｘ M ｘ ) Ｂ Sb Ｃ (M is at least one of Ni and Cu; 0.80≦A≦1.20, 0.80≦B≦1.20, 0.80≦C≦1.20).

2. 2. The thermoelectric material according to claim 1, wherein in the general formula, x is greater than 0 and is equal to or less than 0.

02.

3. 2. The thermoelectric material according to claim 1, wherein in the general formula, x is 0.004 or more and 0.007 or less.

4. The thermoelectric material according to claim 1 , having a Vickers hardness of 6.0 GPa or more.

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