JP2002076450A - Method for manufacturing thermoelectric material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for simply manufacturing a thermoelectric material made of a pure β phase at a lower temperature than that in prior are within a shorter time. SOLUTION: The method for manufacturing a thermoelectric material comprises a step of mixing Fe powder with a single element of Si at an element ratio of 1:2. The method also comprises steps of pressure molding the mixed powder to a molding 2, sealing the molding 2 in a vacuum in a quartz tube 1, and then heat-treating the molding 2. In the heat-treating step, the tube 1 is heated to 875 to 1,025 deg.C near the molding 2. It is preferred to set the temperature to 450 deg.C or higher except the vicinity of the molding 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a Si-based thermoelectric material useful as a thermoelectric material.
_{The present} invention relates to a method for producing a thermoelectric material.

[0002]

The thermoelectric power generation (thermoelectric power generation) using the thermoelectric conversion material is based on the Seebeck effect, that is, a material having different thermoelectric characteristics such as two different metals or a p-type semiconductor and an n-type semiconductor. A technology that converts thermal energy directly into electric power by using the thermoelectric effect of generating a thermoelectromotive force at both ends when a temperature difference is applied between the joints by placing the thermal energy in parallel and connecting them electrically in series. It has these thermoelectric characteristics because it has no mechanical movable elements in its thermoelectric conversion part, so it does not generate vibration or noise, there is no energy loss, and it does not generate waste, and it has a small environmental load. Materials (hereinafter referred to as thermoelectric materials) are being developed in various fields.

[0003] Among them, FeSi ₂ , CrSi ₂ , MnSi
Various researches have been conducted on Si-based sintered bodies such as _2-x because of their good high-temperature characteristics.In particular, β-FeSi ₂ -based thermoelectric materials have excellent oxidation resistance, and iron and silicon as raw materials are The material is abundant and inexpensive, and has little adverse effect on the environment.

Conventionally, such a sintered body of β-FeSi ₂ or the like has been produced by, for example, using a simple substance of Fe and Si as a starting material and an element ratio of Si to Fe of 1: 2 (preferably 1: 2. 1) and heat-treated in a high-frequency melting furnace at a high temperature of about 1900K, melted in a vacuum, solidified, and then pulverized to improve the purity of the pulverized particles and the crystallinity. About 14
Sintered at 00K, once α phase (Fe ₂ Si ₅ ) which is a high temperature phase
Was formed, a heat treatment for the purpose of phase transformation was performed at about 1050 K, which is the transformation temperature to the β phase.

However, this method requires about 100 hours for the heat treatment for the phase transformation from the α phase to the β phase, so that the production efficiency is not good, and the α phase, β phase Although an ε phase (FeSi) is also formed in addition to the phase,
In particular, in the deposition reaction in which a β phase is generated so as to surround the ε phase, there is a problem that it is difficult to generate a pure β phase because the ε phase remains unless heat treatment is performed for a long time. . Further, the raw material powder is melted and solidified at 1900K.
Since it is necessary to perform processing under very high temperature conditions, such as crushing into crushed particles and sintering the crushed particles at 1400K,
There is a problem that the accompanying capital investment increases.

For the purpose of shortening the heat treatment time required for the phase transformation from the α phase to the β phase, Japanese Patent Application Laid-Open No. 8-274380 discloses that the amount of Si to iron is smaller than the element ratio. A method for producing a β-FeSi ₂ -based electrothermal material that suppresses generation of the ε phase by increasing the size and promotes phase transformation by adding a small amount of copper (Cu) is disclosed. This publication describes that the α phase can be transformed into the β phase by heat treatment for about 3 hours. However, since Cu is added in this method, pure β-FeS
It is not possible to produce i _2, and since Si is added in an excessive amount, Si tends to remain after heat treatment for the purpose of phase transformation, and as a result, a material consisting of pure β phase cannot be obtained. There was a problem.

Further, recently, for the purpose of shortening the heat treatment time and lowering the heat treatment temperature, the starting material is made into fine particles by a mechanical alloying method (MA method), or by a discharge plasma sintering method (SPS method). Sintering of the starting material powder mixed in a short time has been considered.
In the method, in order to make the starting material into fine particles, the raw material and a material for grinding such as alumina are put in a closed container and the raw material is crushed into fine particles. However, there is a problem that the material and the material for pulverization are mixed into the raw material powder, and the purity of the raw material powder is reduced. In addition, since the SPS method discharge-processes and sinters the starting material powder mixed in a vacuum, it requires expensive auxiliary equipment such as a pressurizing device and a power supply for a discharge in a vacuum, which is not practical. There is a problem.

The present invention has been made in view of the above problems, and has as its object to provide a method for easily and simply producing a pure β-phase thermoelectric material at a lower temperature and in a shorter time. In particular, an object of the present invention is to provide a method for producing a thermoelectric material composed of a high-purity β phase that reflects the purity of a raw material powder.

[0009]

Means for Solving the Problems As a result of intensive studies in view of the above object, the present inventors have found that Fe, Co, Ni, Cr, T, which form a compound exhibiting thermoelectric properties with a simple powder of Si.
i, Cu, Mn, Al, V, Pt, Pd, Nb, Ta,
A simple powder of a metal such as W, Mo or Zr is mixed at the same element ratio as the desired elemental composition of the silicon compound, and the mixed powder is molded under pressure. If necessary, heat-treat while evacuating and then encapsulating in a vacuum, or encapsulating under an inert gas atmosphere, and then forming the desired silicon compound exhibiting thermoelectric properties. If heat treatment is performed at a temperature, first, a silicon compound generated at a lower temperature than the desired silicon compound (hereinafter referred to as a low-temperature generated silicon compound) is generated in the initial stage, and then the desired silicon compound is generated. As a result, a compact having a composition composed of the low-temperature-generated silicon compound, the desired silicon compound, and the remaining Si is formed. By continuing the heat treatment on the compact, the silicon compound formed at a low temperature reacts with the remaining Si in a short time to be transformed into the desired silicon compound. It has been found that it can be manufactured in a shorter time than that. In particular, in the case of Fe and Si, powders of the elemental elements of Fe and Si are mixed at an element ratio of 1: 2 and sealed as a compact in a sealed container.
75 to 1025 ° C;
If heat treatment is performed at 50 ° C. or more, the β phase is formed well,
By continuing the heat treatment for about 20 hours, the β phase can be generated by reacting the ε phase with Si, and the high purity β-Fe
It has been found that Si ₂ can be produced. Based on these, it corresponded to the present invention.

The method for producing a thermoelectric material according to claim 1 of the present invention is characterized in that Fe, Co, Ni, Cr, Ti, Cu, Mn, A
a step of mixing a powder of one or more metals selected from l, V, Pt, Pd, Nb, Ta, W, Mo or Zr and a simple elemental powder of Si at a predetermined element ratio, and the mixed powder Is sealed in a sealing container made of a high melting point material in a vacuum state or an inert gas atmosphere, and a heat treatment step subsequent thereto.

Further, the method for producing a thermoelectric material according to claim 2 of the present invention is characterized in that Fe, Co, Ni, Cr, Ti, Cu, M
mixing a powder of one or more metals selected from n, Al, V, Pt, Pd, Nb, Ta, W, Mo or Zr and a simple element powder of Si at a predetermined element ratio; This is a method that includes a step of placing the mixed powder in a sealed container made of a high melting point material and performing a heat treatment while evacuating the vacuum, a step of subsequently filling the mixed powder in a vacuum state or an inert gas atmosphere, and a heat treatment step.

According to a third aspect of the present invention, in the method for producing a thermoelectric material according to the first or second aspect, the metal is Fe,
This is a method in which powders of e and Si simple elements are mixed at an element ratio of 1: 2.

According to a fourth aspect of the present invention, in the method for producing a thermoelectric material according to the third aspect, the heat treatment step after the step of enclosing the mixed powder in the vacuum state or under an inert gas atmosphere includes the step of: Is set to 875 to 1025 ° C. in the vicinity of the molded body.

Further, in the method for producing a thermoelectric material according to claim 5, the heat treatment step after the step of encapsulating the mixed powder in the vacuum state or under an inert gas atmosphere according to claim 3 or 4, comprises: In this method, the temperature of the enclosure is set to 450 ° C. or higher except for the vicinity of the molded body.

[0015]

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to Fe, Co, Ni, Cr, Ti, which forms a compound exhibiting thermoelectric properties with silicon.
Cu, Mn, Al, V, Pt, Pd, Nb, Ta, W,
Although it can be widely applied to Mo or Zr, β-FeSi ₂ is the most practical among these metal silicon compounds in terms of practicality and the like. Therefore, the case of Fe will be described below as an example.

First, a method of manufacturing a thermoelectric material according to a first embodiment of the present invention will be described. FIG. 1 is a flowchart showing the manufacturing process of the first embodiment.
i is a simple powder having a molar ratio (element ratio) of substantially Fe: Si =
Weigh to 1: 2. At this time, the smaller the particle size of the single powder is, the more the reactivity is improved, especially when the pressure molding described below is performed. Therefore, the particle size is usually 125 μm or less and Fe and Si are It is preferable that the particle diameters are substantially the same, and it is better to avoid that the particle diameter of Fe is much smaller than the particle diameter of Si. Also, use F
The purity of the e and Si powders is preferably higher because it is reflected in the purity of the obtained β phase, but may be about 98% or less when the purity of the powder after heat treatment is not considered.

Then, powders of Fe and Si are mixed. In this mixing, in order to maintain the purity of the raw material powder, for example, the measured Fe and Si powders are put into a container such as a mortar made of a material harder than Fe and Si such as agate, and mixed with a stirring rod. It may be done by doing. At this time, it is desirable that the stirring bar is also made of a material harder than Fe and Si. The main purpose of this mixing is to uniformly mix the raw material powders, and it is not for the purpose of forming fine particles, so that the mixing may be performed manually for about 15 minutes.

Subsequently, the mixed powder is pressed at room temperature. The pressure during the pressure molding may be about 500 kg / cm ² . Pressure molding is intended to facilitate handling in subsequent steps such as vacuum encapsulation, and it is expected that reactivity can be improved by increasing the contact area of crystal grains. Since the β-phase generation reaction proceeds by performing heat treatment under appropriate heat treatment conditions without forming a molded body, the pressure molding step is not an essential requirement.

Next, the molded body is inserted into an enclosure made of a high melting point material. Specifically, as an enclosure made of a high melting point material, as shown in FIG. 2, one side of a tube material mainly composed of quartz is sealed by melting quartz with a burner using a mixed gas of oxygen and hydrogen. A tube (hereinafter simply referred to as a quartz tube) 1 is used.
Insert At this time, it is desirable that the quartz tube 1 be previously cleaned using an organic solvent, an acid, or the like in order to remove organic substances and metals attached to the inside of the tube. Subsequently, while the air in the quartz tube 1 is exhausted from the open tube side 1A by a vacuum exhaust device (not shown), the open tube side 1A of the quartz tube 1 is melted and sealed using a burner or the like. Thus, FIG.
As shown in the figure, the compact 2 can be vacuum-sealed in the quartz tube 1. In this case, the vacuum exhaust device does not need to completely exhaust the inside of the quartz tube 1 but may be any device that can achieve a final vacuum degree of about 3 × 10 ⁻² Torr such as a rotary pump. As a method for sealing the quartz tube 1, a method of heating the quartz tube to a temperature at which the quartz tube becomes soft, welding it, and threading the quartz tube 1 may be used. In this example, the quartz tube 1 was used as an enclosure for enclosing the compact 2 in a vacuum. However, any high-melting-point material that does not react with Fe, Si, and the silicide phase at a heat treatment temperature in a heat treatment step described later may be used. Those made of materials other than quartz can also be used.

Then, the quartz tube 1 enclosing the compact 2 is placed in a heating furnace such as a tubular electric furnace and heated (heat treatment step).
At this time, the quartz tube 1 enclosing the molded body 2 has a region (a) near the molded body 2 and another region (b) as shown in FIG.
And the heating conditions are set separately. Specifically, the temperature of the region (a) is 875 to 1025 ° C., preferably 900 ° C.
Set to ~ 950 ° C. When the temperature of the region (a) is lower than 875 ° C., the surface diffusion between the crystal grains of Fe and Si does not progress, and as a result, the rate of the reaction of generating β-FeSi ₂ (hereinafter sometimes simply referred to as β phase) Or the activation energy required for the formation of the β phase is not obtained, and β
While the phase formation reaction does not proceed, if it exceeds 1025 ° C.,
Α-Fe ₂ Si _{5 as} a high-temperature phase (hereinafter, sometimes simply referred to as α-phase) is generated. Further, the temperature of the region (b) is preferably set to 450 ° C. or higher. When the temperature in the region (b) is lower than 450 ° C., the reactivity of the molded body 2 itself decreases. By this heat treatment, in the initial stage,
The crystal grains constituting the compact 2 are composed of residual Si and ε-FeSi
(Hereinafter sometimes simply referred to as ε phase) and β phase.

Therefore, the above-mentioned heat treatment time is set to 20 times.
By maintaining for about an hour, or about 10 hours depending on conditions, Si and the ε phase can be reacted to form a β phase, and as a result, a β phase with high purity can be obtained. At this time, if the heat treatment time is short, the unreacted Si and the ε phase formed in the initial stage of the reaction remain mainly because the β phase is being generated, and the purity of the β phase is reduced. It is efficient that this heat treatment is performed continuously,
Once the heat treatment is stopped at the stage when the phase and the β phase are formed, the heat treatment is restarted, and the
It may be a phase. After the heat treatment, the sample is cooled to room temperature. This cooling may be performed by standing cooling or rapid cooling.

As described above, in the present embodiment, the ε phase is generated before the β phase is generated, and this ε phase is generated.
The phase becomes β phase by reacting with Si, but β phase becomes ε
Since the ε phase is generated so as to surround the phase (encapsulation reaction), when the crystal grains of the ε phase are large, a long heat treatment time is required to generate a pure β phase. Therefore, in order to suppress the growth of the crystal grains of the ε phase and shorten the heat treatment time, it is effective to raise the compact 2 to a temperature of 875 to 1025 ° C. in which the β phase is generated in a short time. It is.

As described in detail above, in the method of the first embodiment of the present invention for producing a thermoelectric material, a simple powder of Fe and Si is mixed, and the mixed powder is subjected to pressure molding if necessary. The obtained molded body 2 is vacuum-enclosed in a quartz tube 1 which is an enclosure made of a high melting point material, and is subjected to a heat treatment.
The reactivity is improved by heat treatment under vacuum encapsulation, and the pure β phase can be reduced by a short heat treatment of about 20 hours or less without performing the phase transformation step by a long heat treatment from the α phase as in the past. Powder can be manufactured directly. The obtained β-phase powder reflects the purity of the raw materials Fe and Si, and since Fe and Si are mixed at a molar ratio of 1: 2, there is no residual Si.

In the method of the first embodiment of the present invention, it is important to perform a heat treatment under vacuum sealing.
For example, when heat treatment is performed while vacuum evacuation is performed even at the same processing temperature of 875 to 1025 ° C., unlike the case where the heat treatment is performed by vacuum sealing in the quartz tube 1 as in the present embodiment, no β phase is formed and no Si or ε phase is formed. Is formed. This is considered for the following reasons. That is, the pressure in the closed tube (quartz tube) increases in the heat treatment step from the time of evacuation in vacuum because the tube is closed when vacuum-sealed. Also, Fe
In the heat treatment temperature of 875 to 1025 ° C. in the case of
The reaction form of e and Si is considered to be mainly a solid phase reaction. In this solid-phase reaction, the reaction proceeds by firstly causing surface diffusion to proceed through the contact surface of the crystal grains. Therefore,
There is a correlation between the contact area of the crystal grains and the reactivity.
However, in the heat treatment step in this embodiment, a powder mainly composed of the β phase can be produced even if the powder is not press-formed by adjusting the heat treatment conditions. From these facts, it is considered that when vacuum sealing is performed, the contact surface of the crystal grains of the molded body is activated by the vapor of Fe or Si, the surface diffusion between the crystal grains is promoted, and the reaction proceeds. Therefore, when vacuum sealing is performed, the reactivity increases due to an increase in the vapor pressure of Fe or Si in the vicinity of the molded body, whereas F increases under vacuum evacuation conditions.
Since the vapor pressures of e and Si do not increase, it is considered that the β phase is not formed even under the same temperature condition. Further, the fact that the β phase is not generated unless the temperature of the region (b) is set to 450 ° C. or higher can also be explained in relation to the vapor pressure of Fe or Si. That is, even if the quartz tube 1 has a low-temperature portion lower than 450 ° C. even in the closed tube structure, the vaporized Fe or Si agglomerates in the low-temperature portion, and accordingly, the Fe or Si in the vicinity of the formed body is condensed. It is considered that the vapor pressure of Fe decreases as in the heat treatment step under vacuum evacuation, thereby reducing the reactivity of the crystal grains of Fe and Si, thereby suppressing the generation of β phase.

In the above embodiment, in order to produce a closed tube structure, the compact 2 is placed in a quartz tube 1 and the quartz tube is melted by a burner such as a mixed gas of oxygen and hydrogen under vacuum evacuation. Although the exhaust port is closed, the vapor pressures of Fe and Si in the vicinity of the compact 2 do not depend on the partial pressures of other gases, so that it is not necessary to perform heat treatment under reduced pressure unless the compact 2 is oxidized. . Therefore, the heat treatment may be performed in a closed tube filled with an inert gas such as argon or helium under an inert gas atmosphere.

By the way, since the crystal grains of Fe and Si used in the first embodiment are stored in the air, their surfaces are usually slightly oxidized. When the surface of the crystal grain is oxidized as described above, Fe and Si
A problem arises that the reaction between is suppressed. However, by performing the heat treatment as in this embodiment,
Since a part of the crystal grain surface becomes vapor, the surface oxide layer is removed from the crystal grain surface, and the surface diffusion between the Fe and Si crystal grains is promoted. As a result, the reactivity is improved. . Further, for the same reason, after performing a reduction treatment on the single powder of Fe and Si before the heat treatment step,
By immediately enclosing the inside of the quartz tube 1, the heat treatment step can be further shortened.

In this way, a pure β-phase powder can be produced in a short time, but generally, when the heat treatment temperature is lowered, the crystal growth rate is reduced, and the crystallinity is deteriorated. In the present embodiment, there is a concern that crystallinity may be degraded because a high-temperature heat treatment step for generating an α-phase, which is a high-temperature phase, is not employed as in the prior art. When compared by the value width, the X-ray half-value width of the β-FeSi ₂ powder obtained by the method of the present embodiment is determined by a conventional method of heat-treating an α-phase and transforming the β-phase into a β-phase (hereinafter, referred to as
This is almost the same as β-FeSi ₂ powder obtained by the conventional method, and the method of this embodiment does not deteriorate the crystallinity.

Also, in the conventional method, excess Si (F
e: Si = 1: 2.1), there is a case where β-FeSi ₂ single phase is formed due to mixing of Fe contained in the pulverization container. Rate is greater than when the ε phase is included,
Is known to be about 15 to 25 (Ω · cm) (for example, Tani et. Al .: Jpn.JA).
ppl. Phys. Vol.39 (2000) pp10
54-1057). On the other hand, in the sintered body of β-FeSi ₂ powder produced by the method of the present embodiment, the starting composition is Fe: Si = 1: 2, which is different from the conventional method, but the electric resistivity is 300K and 15%. ２５25 (Ω · cm), and both have almost the same value. From this electric resistance value, it can be said that the powder obtained by the method of this embodiment is a pure β phase. Further, the β-FeS prepared by the method of the present embodiment
sintered body was produced by performing heat treatment after pressing at room temperature powder i ₂ indicates substantially the same electrical resistivity in a conventional pure β-phase sintered body of the same temperature as produced by the process. Therefore, when this sintered body is used as a thermoelectric material, in practical terms, in terms of electric resistance in addition to crystallinity, it is of the same quality as that produced by the conventional technology and has no problem. -It can be used as it is in the field where the sintered body of FeSi ₂ was used.

The β-FeSi ₂ powder produced by the method of the present invention as described above can be used as a thermoelectric material as a sintered body. At this time, in order to produce a sintered body having a high degree of sintering from the powder, it is necessary to further apply a conventional technique such as a heat treatment step for phase transformation or an SPS method. When used as a starting material for preparing a single crystal such as a phase growth method, this powder can be used as it is. In particular, the raw materials Fe and S have not passed through a long-time fine-grain manufacturing process such as the MA method.
A single crystal reflecting the purity of the simple substance powder of i can be produced. Note that the chemical transport method is a type of crystal growth method in which a raw material and a material having a high vapor pressure such as iodine are simultaneously placed in a closed tube having a high melting point such as a quartz tube and heat-treated to perform vapor phase growth. Since this is a crystal growth method in a closed tube, the purity of the raw material reflects the purity of the crystal. Therefore, a high-purity single crystal can be produced by using a high-purity raw material. Therefore, the one produced by the method of this embodiment can be said to be particularly suitable in this respect.

Incidentally, the β phase is composed of Si (100) plane and S
Since lattice formation with the i (111) plane is good and a high-quality thin film reflecting crystal information of the substrate is expected, application to not only thermoelectric elements but also solar cells and optical elements is being studied. As one of the thin film production methods, a sputtering method, a laser ablation method, and a method combining them are known. In these methods, a sintered body target may be used as a raw material in some cases. In this case, the purity of the thin film reflects the purity of the target, and in order to produce a high-purity thin film, it is necessary to use a high-purity target. However, the β-FeSi ₂ powder produced by the method of this embodiment is required. Is suitable for use as a raw material for a high-purity target because it can reflect the purity of the simple substance powder as described above. As a result, a high-purity thin film can be manufactured.

Next, a method of manufacturing a thermoelectric material according to a second embodiment of the present invention will be described with reference to the flowchart of FIG. The method of the second embodiment is basically the same as that of the first embodiment described above.
Since the method is the same as that of the embodiment, detailed description of the same steps will be omitted.

First, simple powders of Fe and Si are weighed such that the molar ratio (element ratio) is substantially 1: 2. At this time, the particle size of the single powder is preferably 125 μm or less for the same reason as in the method of the first embodiment described above. The purity of the powder of Fe and Si used is 98.
% Or less.

Then, powders of Fe and Si are mixed. This mixing may be performed in the same manner as in the method of the first embodiment. Subsequently, the mixed powder thus mixed is subjected to pressure molding at room temperature. The pressure during this pressure molding is 500 kg /
cm ² may be sufficient.

Next, it is inserted into an enclosure made of a high melting point material, specifically, a quartz tube 1 as shown in FIG. At this time, it is desirable that the quartz tube 1 be previously cleaned using an organic solvent, an acid, or the like in order to remove organic substances and metals attached to the inside of the tube. Subsequently, while evacuating from the open tube side 1A of the quartz tube 1 using a vacuum evacuation device (not shown),
The quartz tube 1 is placed in a heating furnace such as a tubular electric furnace and heated.
At this time, the temperature of the quartz tube 1 near the molded body 2 is set to about 500 ° C.

The time of the heat treatment may be about 30 minutes. Thereafter, the open tube side 1A of the quartz tube 1 is melted and sealed using a burner or the like using a mixed gas of oxygen and hydrogen. The step of sealing in the quartz tube 1 may be the same as the method of the first embodiment described above. In this example, the quartz tube 1 was used as an enclosure for enclosing the compact 2 in a vacuum. However, any high-melting-point material that does not react with Fe, Si, and the silicide phase at a heat treatment temperature in a heat treatment step described later may be used. Those made of materials other than quartz can also be used.

Then, the quartz tube 1 in which the molded body 2 is sealed is placed in a heating furnace such as a tubular electric furnace, and heat treatment may be performed under the same conditions as in the above-described first embodiment.

Thus, the method of the second embodiment of the present invention
In the method of the first embodiment described above, when the compact 2 is set in the quartz tube 1, the air in the quartz tube 1 is evacuated from the open tube side 1A using a vacuum exhaust device (not shown). In addition, a step of installing and heating the quartz tube 1 in a heating furnace such as a tubular electric furnace is added, and the following effects can be obtained by this step. In other words, the surface of the raw material powder adsorbs moisture and gas in the air to a considerable extent,
When an adsorbed substance is present on the surface of the raw material powder, the adsorbed substance may be reattached or mixed with the powder after the heat treatment step to become an impurity and reduce purity. In such adsorbed materials, water and light elements are separated from the raw material powder by heat treatment under reduced pressure.However, if the heat treatment is too high, vapors of Fe and Si are generated, so if the structure is not a closed tube structure, There is a problem that generation of the β phase is suppressed. Therefore, by maintaining the temperature of the quartz tube 1 near the molded body 2 at about 500 ° C., adsorbed substances such as moisture are removed by vacuum elimination without generating vapors of Fe or Si, and then closed. By performing the same heat treatment as in the method of the first embodiment described above, a β-phase powder with higher purity can be produced. After the evacuation, heat treatment may be performed in an inert gas atmosphere in a closed tube filled with an inert gas such as argon or helium.

The β-phase powder thus produced is
It has the same characteristics as the powder obtained by the method of the first embodiment, and can be used in the same application and manner.

As described above, the thermoelectric material of the present invention contains Fe and S
Although the case of the compound with i has been described in detail as an example, the methods of the first and second embodiments of the present invention as described above may be applied to Co, Ni, Cr, Ti, Cu, M other than Fe.
The present invention can be similarly applied to a metal which forms a compound exhibiting thermoelectric properties with silicon such as n, Al, V, Pt, Pd, Nb, Ta, W, Mo or Zr. In particular, Mn, Al, V,
By adding a metal such as Pt or Pd to β-FeSi ₂ , it becomes possible to control the P-type and N-type of the obtained semiconductor.

[0040]

The present invention will be described in more detail with reference to the following specific examples. Example 1 and Comparative Example 1 As raw materials, simple powders of Fe and Si were mixed in a molar ratio of 1: 2 in an agate mortar, and this mixed powder was mixed with 500 powder.
It was molded under pressure at kg / cm ² to obtain a molded body.

The compact thus obtained was placed in a quartz tube as shown in FIG.
After evacuating to 0 ⁻² Torr or less, the quartz tube was sealed to obtain a test sample (Example 1) as shown in FIG.

The test sample was put into a tubular electric furnace, and the whole was heat-treated at 750 ° C., 850 ° C., 950 ° C., 1050 ° C. and 1150 ° C. for 2 hours, and the powder was analyzed by X-ray diffraction measurement. The integrated intensity of the diffraction peak of Si, ε phase, and β phase was calculated. Table 1 shows the results. In addition, for comparison, Si and ε were used when a molded body was placed on a quartz tube and the tube was not closed (comparative sample: Comparative Example 1) and similarly heat-treated for 2 hours while evacuating with a rotary pump. Table 1 also shows the results of calculating the integrated intensities of the diffraction peaks of the phase and the β phase.

In the powder X-ray diffraction measurement, since the diffraction peak having a large integrated intensity of each phase is used as an index, Si has a <111> peak, ε phase has a <210> peak, and β iron silicide has a <202> peak. 〉 Peak. And Si <1
In the case of <11>, the integrated intensity before heat treatment was normalized to 1 and represented by the ratio, and in the case of ε <210> and β <202>, the maximum intensity was normalized to 1.

[0044]

[Table 1]

As is clear from Table 1, in the case of Example 1 in which the molded body 2 was vacuum-sealed, a β phase was formed at 950 ° C.
It can be seen that at 1150 ° C., there is no residual Si. In particular, at 950 ° C., the β phase showed the highest strength. And this 9
After heat treatment at 50 ° C. for 2 hours, Si, ε phase and β phase were confirmed. However, when heat treatment was continued for 20 hours, Si
And the ε phase disappeared, leaving only the β phase. On the other hand, in the case of Comparative Example 1 which was heat-treated while evacuating,
It can be seen that even if the heat treatment temperature rises, Si remains and an ε phase is generated, but no β phase is generated. Therefore, even if this state is maintained for a long time, β-phase formation does not proceed. This seems to be related to the vapor pressures of Fe and Si as described above. From Table 1, when β-FeSi ₂ , which is a β phase with high purity, is obtained from Fe and Si, 8
It is understood that the heat treatment may be performed at 75 to 1025C, preferably 900 to 950C. Example 2 The test sample used in Example 1 was put into a tubular electric furnace, and the area (a) was set to 950 ° C., and the minimum temperature of the area (b) was set to 25 ° C., 170 ° C., 480 ° C., 860 ° C. After each heat treatment at 910 ° C. for 20 hours, the presence or absence of β phase formation was confirmed. Table 2 shows the results.

[0046]

[Table 2]

As is clear from Table 2, when the temperature of the region (b) is 170 ° C. or 25 ° C., which is lower than 450 ° C., even if the temperature of the region (a) is 950 ° C., Although no generation was confirmed, the temperature in the region (b) was 45
In the case of 480 ° C., 860 ° C., and 910 ° C., which is 0 ° C. or higher, generation of a β phase was confirmed. If the temperature is lower than 450 ° C., the vapors of Fe and Si are aggregated in the region (b), and the vapor pressures of Fe and Si in the vicinity of the formed body are reduced. It is considered that the reactivity of the crystal grains of Fe and Si decreases as in the heat treatment step, and the generation of the β phase is suppressed. Example 3 and Comparative Example 2 The test sample used in Example 1 was put into a tubular electric furnace, and the region (a) was heat-treated at 950 ° C. for 20 hours while the minimum temperature of the region (b) was 910 ° C. High-purity β-FeSi ₂ was obtained. The pulverized β-FeSi ₂ (Example 3) was analyzed by an X-ray diffractometer to obtain 2θ /
The half width of the <202> peak of the β phase due to θ was measured.
FeKα radiation was used as the X-ray source. Table 3 shows the results. For comparison, the raw material powders of Fe and Si were
e: Si = 1: 2.1 was mixed at a molar ratio to form an α-phase at a high temperature, and then transformed into a β-phase by a heat treatment for 100 hours to give β-FeSi ₂ (Comparative Example 2) according to a conventional method. On the other hand,
Similarly, the half width of the β phase was measured by an X-ray diffraction measurement device. The results are shown in Table 3.

[0048]

[Table 3]

As is clear from Table 3, the half-value width of the β-phase of β-FeSi ₂ of Example 3 according to the method of the present invention was 0.1 mm.
Comparative Example 2 according to the conventional method, whereas 35 (deg)
Of the β-phase of β-FeSi ₂ of 0.33 (de
g). The half width decreases as the crystallinity improves, but the difference of about 0.02 (deg) in the half width is an error range that can vary depending on the particle size of the powder at the time of measurement. It turns out there is no. This confirmed that the method of the present invention did not impair the crystallinity even when the heat treatment temperature was lowered.

[0050]

The method for producing a thermoelectric material according to claim 1 of the present invention is characterized in that Fe, Co, Ni, Cr, Ti, Cu, Mn,
Al, V, Pt, Pd, Nb, Ta, W, Mo or Zr
Mixing a powder of one or more metals selected from the group consisting of a single element of Si and a single element of Si at a predetermined element ratio, and placing the mixed powder in an enclosure made of a high melting point material in a vacuum state or an inert gas atmosphere. In this method, a silicon compound having a desired element ratio reflecting the purity of the raw material powder is produced in a short time and by a heat treatment at a lower temperature than before, since the method has a step of encapsulating at a temperature and a subsequent heat treatment step. It becomes possible.

The method for producing a thermoelectric material according to claim 2 of the present invention is characterized in that Fe, Co, Ni, Cr, Ti, Cu, M
mixing a powder of one or more metals selected from n, Al, V, Pt, Pd, Nb, Ta, W, Mo or Zr and a simple element powder of Si at a predetermined element ratio; The method includes a step of heat-treating the mixed powder in an enclosure made of a high-melting-point material and evacuating it under vacuum, a step of subsequently enclosing in a vacuum state or an inert gas atmosphere, and a heat treatment step. It becomes possible to produce a silicon compound having a desired element ratio reflecting the purity of the powder in a short time by a heat treatment at a lower temperature than before.

According to a third aspect of the present invention, in the method for manufacturing a thermoelectric material according to the first or second aspect, the metal is Fe,
Since the method is a method of mixing powders of elemental elements of e and Si at an element ratio of 1: 2, β-FeSi ₂ having a high purity reflecting the purity of the raw material powders Fe and Si can be produced in a short time and in comparison with the conventional method. Can also be manufactured by heat treatment at a low temperature.

In a fourth aspect of the present invention, in the heat treatment step after the step of enclosing the mixed powder in the vacuum state or under an inert gas atmosphere, the temperature of the encapsulation container may be reduced. Is set to 875 to 1025 ° C. in the vicinity of the compact, so that β-FeS having a high purity reflecting the purity of the raw material powders Fe and Si is used.
It is possible to produce i ₂ in a shorter time and by a heat treatment at a lower temperature than before.

Further, in the method for producing a thermoelectric material according to claim 5, the heat treatment step after the step of encapsulating the mixed powder in the vacuum state or under an inert gas atmosphere according to claim 3 or 4, comprises: Since the method is such that the temperature of the sealed container is set to 450 ° C. or higher except for the vicinity of the molded body, β-FeSi ₂ having high purity reflecting the purity of the raw material powders Fe and Si can be produced in a shorter time and in comparison with the conventional method. Can also be manufactured by heat treatment at a low temperature.

[Brief description of the drawings]

FIG. 1 is a flowchart showing a manufacturing process in a method according to a first embodiment of the present invention.

FIG. 2 is a sectional view showing a state in which a molded body is installed in a quartz tube in the method of the first embodiment.

FIG. 3 is a cross-sectional view showing a state where the molded body is sealed in a quartz tube.

FIG. 4 is a flowchart showing a manufacturing process in a method according to a second embodiment of the present invention.

[Explanation of symbols]

 1 Quartz tube 2 Molded body

Claims (5)

[Claims] 1. The method according to claim 1, wherein Fe, Co, Ni, Cr, Ti, Cu,
Mixing a powder of one or more metals selected from Mn, Al, V, Pt, Pd, Nb, Ta, W, Mo or Zr and a single element element of Si at a predetermined element ratio; A method for producing a thermoelectric material, comprising a step of enclosing a mixed powder in an encapsulation container made of a high melting point material in a vacuum state or an inert gas atmosphere, and a subsequent heat treatment step. 2. Fe, Co, Ni, Cr, Ti, Cu,
Mixing a powder of one or more metals selected from Mn, Al, V, Pt, Pd, Nb, Ta, W, Mo or Zr and a single element element of Si at a predetermined element ratio; A thermoelectric step characterized by comprising a step of placing the mixed powder in an enclosure made of a high-melting-point material and performing a heat treatment while evacuating, a step of subsequently enclosing the mixture in a vacuum state or an inert gas atmosphere, and a heat treatment step. Material manufacturing method. 3. The method for producing a thermoelectric material according to claim 1, wherein the metal is Fe, and powders of a single element of Fe and Si are mixed at an element ratio of 1: 2. 4. In a heat treatment step after the step of sealing the mixed powder in the vacuum state or under an inert gas atmosphere, the temperature of the sealing container is set to 875 in the vicinity of the molded body.
The method for producing a thermoelectric material according to claim 3, wherein the temperature is set to -1025C. 5. In a heat treatment step after the step of sealing the mixed powder in a vacuum or in an inert gas atmosphere, the temperature of the sealing container is set to 450 ° C. or higher except for the vicinity of the molded body. The method for producing a thermoelectric material according to claim 3.