JP2010147283A - Thermoelectric material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thermoelectric material which is reduced in oxygen content and consists essentially of a Heusler compound having small electric resistivity while maintaining a Seebeck coefficient. <P>SOLUTION: The thermoelectric material consists essentially of the Fe-V-Al-based Heusler compound, and the oxygen concentration in the material is <2,000 ppm. It is preferred that the thermoelectric material has a nitrogen concentration of ≤50 ppm and a sulfur concentration of ≤50 ppm. An embodiment of a method of manufacturing the thermoelectric material includes a dissolution process, a deoxidation processing process, and a solidification process, wherein the oxygen concentration in the material is ≤2,000 ppm in the deoxidation processing process. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a thermoelectric material, particularly a thermoelectric material mainly composed of a Fe-V-Al-based Heusler compound.

  In recent years, thermoelectric conversion technology has attracted attention as an energy conversion technology. By using this thermoelectric conversion technology, heat energy and electric energy can be directly converted into each other by the temperature difference of the material. This utilizes the Seebeck effect, in which when p-type and n-type thermoelectric materials are joined and both ends are kept at different temperatures, holes move and electrons move, and an electromotive force is generated between both terminals. To do. Features of this thermoelectric conversion technology include high reliability because there are no moving parts, maintenance-free and long life, and low efficiency even at a small scale.

As an index for evaluating the thermoelectric conversion performance of such a thermoelectric material, the figure of merit Z or a dimensionless figure of merit ZT that is the product of the figure of merit Z and the absolute temperature T is used. The dimensionless figure of merit ZT is expressed by the following equation.
ZT = S ² T / ρκ (1)
Here, S is the Seebeck coefficient, ρ is the electrical resistivity, κ is the thermal conductivity, and T is the absolute temperature. As can be seen from this equation, in order to improve the thermoelectric conversion performance, a material having a high Seebeck coefficient S and a low electrical resistivity ρ and thermal conductivity κ is desirable.

As conventional thermoelectric materials,
Bi-Te-based, Pb-Te-based, Si-Ge-based compound semiconductors Co-based oxides Zn-Sb-based, Co-Sb-based, Fe-Sb-based skutterudite compounds Half-Heusler compounds such as ZrNiSn Fe- V-Al-based Heusler compounds and the like are known.

  Among these, Bi-Te-based, Pb-Te-based, and Si-Ge-based compound semiconductors exhibit a high dimensionless figure of merit ZT at low temperatures, but contain a large amount of environmental load elements such as Pb, Te, and Sb. Yes. On the other hand, the Fe-V-Al-based Heusler compound does not contain an environmental load element, and a method of replacing any of the Fe site, V site, or Al site with another element, or a method of shifting from the stoichiometric composition. As a result, a high ZT is obtained in a low temperature range. For this reason, research on Fe-V-Al-based Heusler compounds is being conducted as a substitute for Bi-Te-based and Pb-Te-based thermoelectric materials. Known techniques for increasing the thermoelectric conversion efficiency of Fe-V-Al-based Heusler compounds include controlling lattice defects (see Patent Document 1) and controlling the total number of valence electrons (see Patent Document 2). ing.

Japanese Patent Laid-Open No. 2008-021982 International Publication No. WO2003 / 019681 Pamphlet

The thermoelectric material is evaluated by a dimensionless figure of merit represented by ZT = S ² T / ρκ. In metal, Wiedemann Franz rule
ρκ / T = L (2)
Therefore, since the Lorentz number L is constant, the electrical resistivity ρ and the thermal conductivity κ cannot be controlled independently. For this reason, it is considered difficult to control the thermal conductivity and the electrical resistivity independently with a normal metal.

  Accordingly, an object of the present invention is to solve the above-described problems and to provide a thermoelectric material made of a Heusler compound having a low electric resistivity while maintaining the Seebeck coefficient and the thermal conductivity.

  In order to achieve the above object, a thermoelectric material according to the present invention is a thermoelectric material mainly composed of a Fe-V-Al-based Heusler compound, wherein the oxygen concentration in the material is 2000 ppm or less. To do.

  Further, the nitrogen concentration in the thermoelectric material is 50 ppm or less.

  Furthermore, the sulfur concentration in the thermoelectric material is 50 ppm or less.

  The electrical resistivity can be reduced by reducing the oxygen contained in the Heusler compound at the manufacturing stage. Thereby, a thermoelectric material with high thermoelectric conversion efficiency can be obtained.

  Hereinafter, embodiments of the present invention will be described in detail.

  The thermoelectric material according to the present invention is a thermoelectric material mainly composed of an Fe-V-Al-based Heusler compound and having an oxygen concentration of 2000 ppm or less in the material. In addition, the thermoelectric material which concerns on this Embodiment means the thing which has a Fe-V-Al type Heusler compound as a main component and contains a small amount of other additives.

  In the thermoelectric material according to the present embodiment, when the oxygen concentration in the thermoelectric material is 2000 ppm or less in the manufacturing stage, the electric resistivity of the thermoelectric material is reduced, and highly efficient thermoelectric conversion is possible. More specifically, in a thermoelectric material having a Fe—V—Al-based Heusler compound as a main component, the oxygen concentration in the thermoelectric material shows a large value of ZT at a boundary of 2000 ppm. Furthermore, if the oxygen concentration is 1500 ppm or less, the ZT value is substantially the same even if the oxygen concentration is lower than this, and therefore it is more preferably 1500 ppm or less.

  Further, by setting the nitrogen concentration in the thermoelectric material to 50 ppm or less and the sulfur concentration to 50 ppm or less, high-efficiency thermoelectric conversion can be similarly performed.

  Next, a method for manufacturing a thermoelectric material according to the present embodiment will be described. The manufacturing method of the thermoelectric material which concerns on embodiment of this invention is equipped with the melt | dissolution process, the deoxygenation process, and the coagulation process.

The dissolution step is a step of dissolving the raw material mixture formulated so that an Fe ₂ VAl-based Heusler compound can be obtained.
The raw material mixture may contain only a single element, or may be an alloy or compound containing two or more elements. Moreover, each raw material mixture is mix | blended so that a predetermined | prescribed composition may be obtained. The method for dissolving the raw material mixture is not particularly limited, and various methods can be used. For example, it may be melted by arc melting, high frequency melting, a vacuum induction device using an alumina crucible, a cold crucible melting device or the like. The raw material mixture is preferably dissolved in an inert atmosphere in order to prevent oxidation.

The deoxygenation process is a process of deoxygenating the molten metal melted in the melting process to reduce the oxygen concentration in the material. The method of deoxygenation treatment is not particularly limited, and various methods can be used. For example, when the melting step is performed using high frequency melting, vacuum is drawn until the pressure in the high frequency furnace becomes 1 × 10 ⁻¹ Pa or lower, and argon gas with a purity of 99.999% or higher is 1 × 10 ² to Pour until 1 × 10 ⁶ Pa. This process is repeated, and when the air in the furnace is exhausted, a vacuum is drawn again. Next, the temperature in the furnace is kept at 1500 ° C. or higher, and hydrogen gas is kept flowing. Thereby, oxygen in the material reacts with hydrogen, and the oxygen concentration is reduced. Note that the gas used for the deoxygenation treatment is not limited to hydrogen, and any gas having a strong reducing property may be used. For example, ammonia, hydrogen iodide, carbon monoxide and the like.

  The desulfurization treatment step is a step of performing a desulfurization treatment on the molten metal melted in the melting step to reduce the sulfur concentration in the material. The method for the desulfurization treatment is not particularly limited, and various methods can be used. For example, there is a method of reducing the sulfur concentration in a reducing atmosphere by keeping the inside of a high-frequency furnace at a high temperature of 1500 to 2000 ° C. and pouring ammonia decomposition gas into the furnace at a flow rate of 1 to 100 liters / minute. An inert gas atmosphere such as Ar may be used.

  The denitrogenation process is a process for reducing the nitrogen concentration in the material by performing denitrification on the molten metal melted in the melting process. The method for the denitrification treatment is not particularly limited, and various methods can be used. For example, the inside of a high-frequency furnace is kept at 1500 ° C. or higher, and a mixed gas of ammonia decomposition gas and hydrogen gas is flowed into the furnace at a flow rate of 1 to 100 liters / minute so that 0 <hydrogen gas / mixed gas ≦ 1. There is a method of reducing the crowded and nitrogen concentration.

  The solidification step is a step of solidifying the molten metal that has been subjected to the deoxidation treatment step. As a method for solidifying molten metal, there are a method of slow cooling in a melted furnace and a method of rapid cooling by another method. In particular, when the molten metal is rapidly solidified, a uniform solid solution with little segregation can be obtained. The method of rapid solidification is not particularly limited, and various methods can be used. For example, a crucible method may be used in which a raw material is dissolved in a crucible and the whole crucible is placed in a refrigerant.

  The thermoelectric material manufactured in such a process can reduce the content of impurities by controlling the concentration of oxygen, sulfur, and nitrogen. In addition, said process may be only a dissolution process, a deoxygenation process, and a coagulation process. After a deoxygenation process, either a desulfurization process or a denitrogenation process may be performed first, or only one of them. Good.

  Next, a description will be given of a characteristic investigation conducted for calculating the dimensionless figure of merit ZT for the thermoelectric material according to the present invention manufactured by the above method. The characteristic investigation items were Seebeck coefficient Z, electrical resistivity ρ, thermal conductivity κ, oxygen concentration, sulfur concentration, and nitrogen concentration.

  The Seebeck coefficient and the electrical resistivity were measured by using a TEM3 manufactured by ULVAC-RIKO Co., Ltd. by a four-terminal method.

Thermal conductivity κ is
κ = cδα (3)
It was calculated by the following formula. Here, c is specific heat, δ is density, and α is thermal diffusivity. Specific heat is DSC method, using EXSTAR6000DSC manufactured by SII Nanotechnology, density is Archimedes method using precision electronic balance, thermal diffusivity is laser flash method, using ULTC Riko Co., Ltd. TC-7000, Each was measured.

  From this measurement result, a dimensionless figure of merit ZT, which is an index for evaluating the thermoelectric conversion performance, was calculated using equation (1).

  The thermoelectric material manufactured by the above-described manufacturing method based on the above measurement results has a low content of impurities such as oxygen, sulfur, and nitrogen, and a low electrical resistivity. Thereby, thermoelectric conversion performance becomes high and it is possible to obtain a thermoelectric material having high thermoelectric conversion efficiency while maintaining the Seebeck coefficient.

  EXAMPLES Hereinafter, although this invention is demonstrated further in detail based on an Example, this invention is not limited to them.

(Example)
Table 1 shows high purity Fe of purity 3N (= 0.999, the same applies hereinafter), high purity V of purity 3N or higher, high purity Al of purity 3N or higher, and one other element of purity 3N or higher. A raw material mixture is prepared by weighing at the composition ratio shown. This raw material mixture was arc-melted in an Ar atmosphere, and then cooled to obtain an ingot.

After putting the ingot in the high frequency furnace, a vacuum is drawn until the pressure in the furnace becomes 1 × 10 ⁻¹ Pa or less. Then, Ar gas having a purity of 99.999% or more is poured into the furnace until it reaches 1 × 10 ² to 1 × 10 ⁶ Pa, and then the vacuum is pulled again. Thereafter, similarly, Ar gas is allowed to flow into the furnace, and the process of drawing a vacuum is repeated twice to discharge the air in the furnace. In addition, the process of flowing Ar gas in the furnace and drawing the vacuum may be performed once or more as long as the air in the furnace can be discharged.

  Next, ammonia decomposition gas is flowed into the furnace at a flow rate of 1 to 100 liters / minute, and when the atmospheric pressure is reached, the temperature in the furnace is raised to 1500 ° C. at a pace of 10 ° C./minute. Then, deoxygenation treatment is performed in which oxygen in the material is reduced by hydrogen by keeping the state of 1500 ° C. or higher and continuously flowing ammonia decomposition gas.

  The time at which the temperature reached 1500 ° C. was set to 0 minutes, and the characteristics of the thermoelectric material were investigated after 5 minutes, 7 minutes, 10 minutes, 16 minutes, 20 minutes, 30 minutes, and 40 minutes. The survey items were oxygen concentration, Seebeck coefficient, electrical resistivity, and thermal conductivity.

  The oxygen concentration was measured using EMGA-620W manufactured by Horiba, Ltd. by a non-dispersive infrared absorption method.

  Tables 2 to 4 show the measurement results of the oxygen concentration, Seebeck coefficient, electrical resistivity, and thermal conductivity of each thermoelectric material (samples 1-1 to 1-3), respectively.

  In Samples 1-1 to 1-3, as shown in Tables 2 to 4, ZT tends to increase as the oxygen concentration decreases. This is because the oxygen concentration, which is an impurity in the thermoelectric material, is low and the electrical resistivity is low.

(Example 2)
High purity Fe with a purity of 3N or more, High purity V with a purity of 3N or more, High purity Al with a purity of 3N or more, and two or more other elements with a purity of 3N or more are weighed in the composition ratios shown in Table 5, Prepare a mixture. This raw material mixture is arc-melted in an Ar atmosphere and then cooled to obtain an ingot.

  The thermoelectric material is placed in a high-frequency furnace, a deoxygenation process is performed in the same manner as in Example 1, and the time when the temperature reaches 1500 ° C. is set to 0 minutes, and thereafter, 10 minutes, 15 minutes, 30 After a minute, the characteristics of the thermoelectric material were investigated. The survey items and measurement method were both the same as in Example 1. Tables 6 to 9 show the measurement results of oxygen concentration, Seebeck coefficient, electrical resistivity, and thermal conductivity of each thermoelectric material (samples 2-1 to 2-4), respectively.

  In Samples 2-1 to 2-4, as shown in Tables 6 to 9, ZT tends to increase as the oxygen concentration decreases. This is because, as in Example 1, the oxygen concentration, which is an impurity in the thermoelectric material, is low and the electrical resistivity is low.

(Example 3)
Next, for each sample shown in Table 1 and Table 5, a desulfurization treatment step and a denitrogenation treatment step were performed after the deoxygenation treatment step. The survey items and measurement method were both the same as in Example 1. Compared with Example 1, the case where the value of ZT was large was “◯”, and the case where it was small was “X”. Table 10 shows the results.

1 to 3 show the relationship between the oxygen concentration and ZT for each sample. 1 shows sample 1-1, FIG. 2 shows sample 1-2, sample 1-3 and sample 2-1, and FIG. 3 shows sample 2-2, sample 2-3 and sample 3-1. Yes.
In the graphs of FIGS. 1 to 3, when the oxygen concentration in the material was 2000 ppm or less, the increase rate of ZT increased. In other words, regardless of the composition ratio of the material, if it is a thermoelectric material mainly composed of a Fe-V-Al-based Heusler compound, the thermoelectric conversion efficiency tends to be remarkably improved by setting the oxygen concentration to 2000 ppm or less. It is done. In addition, regarding the sample 1-1 shown in FIG. 1, the ZT shows a large value on the side where the oxygen concentration is low at the boundary of the oxygen concentration of 2000 ppm. Even if it is reduced from 1500 ppm, no significant change is observed.

Graph 1 showing the relationship between oxygen concentration and ZT. Graph 2 showing the relationship between oxygen concentration and ZT. Graph 3 showing the relationship between oxygen concentration and ZT.

Claims (3)

  A thermoelectric material comprising a Fe-V-Al-based Heusler compound as a main component, wherein the oxygen concentration in the material is 2000 ppm or less.   The thermoelectric material according to claim 1, wherein a sulfur concentration in the thermoelectric material is 50 ppm or less.   The thermoelectric material according to claim 1, wherein a nitrogen concentration in the thermoelectric material is 50 ppm or less.

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