JP2005263562A - METHOD FOR MANUFACTURING beta-FeSi2 - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing single-phase β-FeSi<SB>2</SB>in a simpler way at a lower temperature in a shorter period of time as compared with a conventional method. <P>SOLUTION: In the method for manufacturing β-FeSi<SB>2</SB>, powders of simple elements of Fe and Si are mixed in an atomic ratio of 1:2, a reaction aid is added to this mixed powder and mixed, and this mixture is heat-treated. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to a method for producing β-FeSi ₂ by solid phase reaction of Fe and Si powder.

  In recent years, energy and environmental issues have been highlighted. A phenomenon in which electromotive force is generated by giving a temperature difference between materials, that is, Seebeck effect is used, and thermal energy and electric energy are directly and mutually via mechanical elements. Thermoelectric conversion that does not convert is attracting attention. This thermoelectric conversion does not require any mechanical elements, so there is no energy loss, no vibration, noise, and no waste, and it is an energy conversion method with low environmental impact. It is attracting attention as a new power generation method that can realize effective use and does not use fossil fuels, and research and development are underway.

Among materials with high thermoelectric conversion efficiency, various studies have been made on Si-based sintered bodies because of their high temperature characteristics. In particular, β-FeSi ₂ is used as an environmentally conscious high-temperature power generation material because it uses Fe and Si, which are present in large quantities in the crust as raw materials, is inexpensive and harmless, and has excellent oxidation resistance and can be used at high temperatures in the atmosphere. Many efforts are being made in research and development.

Β-FeSi ₂ not only has excellent characteristics as a thermoelectric conversion material, but also has a large Seebeck coefficient and low electrical resistance, and can be either a p-type or n-type semiconductor. It has excellent characteristics as a semiconductor, such as having transition type photoexcitation characteristics, and is attracting attention as a next-generation semiconductor.

In β-FeSi ₂ , since Fe and Si as raw materials are stable at a low temperature, first, a mixed powder of Fe and Si is heated to a high temperature of 1230 ° C. or more to produce α-FeSi ₂ , and then It is fabricated by performing a phase transformation by performing a heat treatment (annealing treatment or the like) for about 100 hours at about 800 ° C. However, this method has a problem that it takes a long time to manufacture and is inefficient, and requires equipment that can handle processing under high temperature conditions, resulting in high manufacturing costs.

In order to solve the above problems, a method for shortening the heat treatment time necessary for the phase transformation from the α phase to the β phase has been studied (for example, see Patent Document 1). However, in the method described in Patent Document 1, pure β-FeSi ₂ cannot be produced because Cu is added. On the other hand, a method for directly producing β-FeSi ₂ without producing α-FeSi ₂ has been studied (for example, see Patent Document 2). Although the method described in Patent Document 2 can produce β-FeSi ₂ at a lower temperature and in a shorter time than the conventional method, it still requires a heat treatment of about 20 hours. In addition, it is necessary to enclose the quartz tube.

Furthermore, for the purpose of shortening the heat treatment time and lowering the heat treatment temperature, the starting material is finely divided by the mechanical alloying method (MA method) or mixed by a discharge plasma sintering method (SPS method) in a short time. A composite process in which raw material powder is sintered or a combination of the MA method and the SPS method has been studied (for example, see Non-Patent Documents 1 to 3). However, in the MA method, in order to make the starting material into fine particles, the raw material and a pulverizing material such as ball-like alumina are placed in an airtight container and the raw material is pulverized into fine particles. There is a problem that the material constituting the inner wall of the container and the pulverizing material are mixed into the raw material powder, and the purity of the raw material powder is lowered. In addition, the SPS method is not practical because expensive starting equipment such as a pressurizing device in a vacuum and a power source for discharge is necessary because the starting raw material powder mixed in a vacuum is discharged and sintered. There is a problem.
JP-A-8-274380 JP 2002-76450 A “Powder and powder metallurgy”, 1994, vol. 41, pp. 560-564 “The Journal of the Japan Institute of Metals”, 1999, vol. 63, pp. 569-572 “Journal of the Japan Society of Powder and Powder Metallurgy”, 2000, vol. 47, no. 4, pp. 369-374

An object of the present invention is to provide a method for producing single-phase β-FeSi ₂ at a low temperature and in a short time more easily than in the prior art.

As a result of intensive studies, the inventors have added a chloride such as KCl and NaCl as a reaction aid and then heated the mixed powder of Fe and Si at a lower temperature and in a shorter time than before. It has been found that single-phase β-FeSi ₂ can be produced. The present invention has been made based on such findings.

That is, the present invention
(1) A powder of elemental elements of Fe and Si is mixed at an atomic ratio of 1: 2, a solid reaction aid is added to and mixed with the mixed powder, and the mixture is heated. A method of producing FeSi ₂ ,
(2) The method for producing β-FeSi _{2 according to} item (1), wherein the solid reaction aid is a chloride-based reaction aid.
(3) The method for producing β-FeSi _{2 according} to (1) or (2), wherein the solid reaction aid is KCl or NaCl,
(4) In any one of (1) to (3), the step of adding and mixing a solid reaction aid to the mixed powder of Fe and Si is performed in an inert gas atmosphere. The method for producing β-FeSi ₂ described above, and (5) the step of adding and mixing a solid reaction aid to the mixed powder of Fe and Si is performed in an argon atmosphere (1) to there is provided a method for producing a beta-FeSi ₂ according to any one of (4).

The production method of β-FeSi _{2 according to} the present invention does not require the conventional two-stage heat treatment, only the one-stage heat treatment, and the overall reaction time is short. Further, β-FeSi ₂ can be easily produced at low cost without requiring a special apparatus. Moreover, since the solid reaction aid used in the present invention is water-soluble, it can be easily removed by washing with water and does not remain, so that pure β-FeSi ₂ can be produced. In addition, since the method of the present invention is a solid-phase reaction of powder, additives can be added simultaneously.

Hereinafter, the present invention will be described in detail.
The production method of the present invention includes the following steps (1) to (5).
(1) Mixing step of Fe powder and Si powder (2) Adding / mixing step of solid reaction aid (3) Heat treatment step of mixed powder (4) Removal step of reaction aid (5) Drying step

(1) Mixing step of Fe powder and Si powder First, powders of simple elements of Fe and Si are mixed at an atomic ratio (molar ratio) of 1: 2. From the viewpoint of producing pure β-FeSi ₂ , the Fe and Si simple element powders used in the present invention preferably have a purity of 99.9% or more. Moreover, 0.5-40 micrometers is preferable and the particle size of each powder has more preferable 1-several micrometers.

(2) Adding / mixing step of solid reaction aid Next, the solid reaction aid is added to and mixed with the mixed powder of Fe and Si.
The solid reaction aid used in the present invention is preferably a chloride reaction aid, more preferably an alkali halide, and particularly preferably KCl and NaCl. The addition amount of the solid reaction aid is preferably 6% by mass or more, more preferably 8% by mass or more with respect to the mixed powder of Fe and Si from the viewpoint of forming a liquid phase and improving the movement of Fe and Si. Further, the upper limit of the addition amount of the solid reaction aid is preferably 15% by mass or less, more preferably 13% by mass or less, and particularly preferably 10% by mass or less.

The mixing method of the mixed powder of Fe and Si and the solid reaction aid is not particularly limited, and any mixing method can be applied. For example, the mixing can be performed by a ball mill or the like. The mixing speed is not particularly limited. However, if the mixing speed is too high, oxidation may occur due to heat generated by frictional heat, which is not preferable.
The step of adding and mixing the solid reaction aid to the mixed powder of Fe and Si is preferably performed in an inert gas (preferably argon) atmosphere from the viewpoint of preventing oxidation of Fe. The mixing time is preferably as short as possible from the viewpoint of prevention of oxidation, specifically about 10 to 30 minutes, more preferably about 15 to 20 minutes.

  After adding and mixing the solid reaction aid to the mixed powder of Fe and Si so that Fe and Si are present close to the reaction aid, the mixed powder is formed into a certain shape by press molding or the like. It is preferable to mold.

(3) Heat treatment process of mixed powder Heat treatment is performed after mixing with the solid reaction aid. The heat treatment is preferably performed in an inert gas (preferably argon) atmosphere from the viewpoint of preventing oxidation of Fe and Si.
The heating temperature is preferably as low as possible from the viewpoint of efficient production, but is preferably 820 to 910 ° C, more preferably 850 to 900 ° C. Since the reaction temperature is too low at about 700 to 800 ° C., Fe or Si that cannot be reacted causes an oxidation reaction during a reaction auxiliary agent removal step and a drying step described later, and an oxide of Fe or Si is generated. It is not preferable.
The heating time is preferably as short as possible from the viewpoint of efficient production, but is preferably 2.5 to 10 hours, and more preferably 3.5 to 7 hours.
Note that if the heating temperature is raised, the heating time can be shortened, and if the heating time is lengthened, the heating temperature can be lowered, and preferable reaction conditions can be appropriately determined.

(4) Reaction auxiliary agent removal step After completion of the heat treatment, the solid reaction auxiliary agent is removed. The sintered body cooled to room temperature is pulverized using an agate mortar or the like and washed several times with pure water. Since the solid reaction aid used in the present invention is water-soluble, it can be easily removed by washing with water.

(5) Drying process The powder washed with water is dried.

Through the above steps, the method of the present invention can produce single-phase β-FeSi ₂ by only one-step heat treatment without producing α-FeSi ₂ . Further, the method of the present invention does not require a special apparatus such as a discharge plasma sintering apparatus or a long-time mechanical alloying.
The β-FeSi ₂ powder obtained by the method of the present invention can be formed into a sintered body by hot pressing in argon. Further, β-FeSi ₂ produced by the method of the present invention can be used in various fields as a thermoelectric conversion material or a semiconductor material.

  EXAMPLES Next, although this invention is demonstrated further in detail based on an Example, this invention is not limited to these Examples.

Example 1
Fe powder (˜300 mesh, purity 3N; manufactured by Furuuchi Chemical Co., trade name: FEM-30005A), Si powder (˜300 mesh, purity 4N; manufactured by Furuuchi Chemical Co., trade name: SIM-65005A), KCl (purity 99 .50%, manufactured by Koizumi Chemical Co., Ltd.), and Fe powder and Si powder were weighed with Fe: Si = 1: 2 (atomic ratio) (Fe = 4.985549 g, Si = 5.1451 g). To this, 10% by mass (1.00000 g) of KCl was added and dry-mixed in a planetary ball mill for 20 minutes under an argon atmosphere.

2 g of the mixed powder sample was weighed and pressed at 300 kgf / cm ² (about 30 MPa) for 10 minutes to produce a tablet-type green compact.
The produced green compact was placed on an alumina board and inserted into the core tube. Note that both ends of the core tube were sealed with rubber stoppers, and vacuum grease was applied for the purpose of preventing oxygen contamination from the gaps. Further, Ar gas was allowed to flow for 10 minutes at room temperature for the purpose of removing residual oxygen. Further, in consideration of the possibility that an extremely small amount of oxygen is mixed in the Ar gas, the Ar gas was installed so as not to directly hit the sample. The sample was heated to 900 ° C. at a heating rate of 800 ° C./h and kept for 3.5 hours to sinter while circulating high purity Ar gas in the furnace core tube. After completion of the heat treatment, it was cooled by natural cooling.

The sintered sample was taken out and pulverized using an agate mortar and pestle, and washed several times with pure water to remove KCl. Thereafter, the sample was dried with a dryer (drying temperature 100 ° C., drying time 30 minutes).
After drying, the sintered pattern was examined for a diffraction pattern by an X-ray diffractometer (trade name: CN4037A1, manufactured by Rigaku Corporation). The results are shown in FIG. As is apparent from FIG. 1, single-phase β-FeSi ₂ could be obtained.

Comparative Example A test was conducted in the same manner as in Example 1 except that KCl was not added. As a result, Fe and Si did not react, and the X-ray diffraction merely detected Fe and Si peaks.

Example 2
A test was conducted in the same manner as in Example 1 except that the amount of KCl added was 7% by mass (0.700 g) with respect to the mixed powder of Fe and Si. β-FeSi ₂ could be obtained.

Example 3
When the test was conducted in the same manner as in Example 1 except that the reaction temperature was 850 ° C. and the reaction time (retention time) was 7 hours, single-phase β-FeSi ₂ was obtained in the same manner as in Example 1. I was able to.

The diffraction pattern by the X-ray-diffraction apparatus of the sintered sample in Example 1 is shown.

Claims (5)

The powder for each element of Fe and Si 1: mixed with 2 atomic ratio, and mixed by adding solid reaction aid to the powder mixture, the beta-FeSi _2, characterized in that the heat treatment the mixture Production method. The method for producing β-FeSi _{2 according} to claim 1, wherein the solid reaction aid is a chloride-based reaction aid. The method for producing β-FeSi _{2 according} to claim 1 or 2, wherein the solid reaction aid is KCl or NaCl. The β-FeSi according to any one of claims 1 to 3, wherein the step of adding and mixing a solid reaction aid to the mixed powder of Fe and Si is performed in an inert gas atmosphere. _2. Manufacturing method. The β-FeSi _{2 according} to any one of claims 1 to 4, wherein the step of adding and mixing a solid reaction aid to the mixed powder of Fe and Si is performed in an argon atmosphere. Production method.

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