JP3878341B2 - Method for producing CoSb3-based compound having filled skutterudite structure - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a CoSb ₃ group compound having a filled skutterudite structure.
[0002]
[Prior art]
The basic requirements of thermoelectric conversion materials currently used for the purpose of direct thermoelectric conversion by the Seebeck effect and Peltier effect are as follows:
(1) Material figure of merit Z (Z = S ² σ / κ, where S is the thermoelectric power V / K, σ is the electrical conductivity Ω ⁻¹ cm ⁻¹ , and κ is the thermal conductivity W / cmK) Have a large value in a wide operating temperature range),
(2) The material is abundant in resources and manufacturing of the material is easy.
And so on.
[0003]
At present, the main material of thermoelectric conversion materials is a ternary compound based on Bi ₂ Te ₃ . In general, a compound of Bi ₂ Te ₃ and Sb ₂ Te ₃ is used for the p-type material, and a compound of Bi ₂ Te ₃ and Bi ₂ Sb ₃ is used for the n-type material. The figure of merit Z in the thermoelectric conversion system of these materials is 2.3 × 10 ⁻³ K ⁻¹ at room temperature for both p-type and n-type. This value is the highest among existing thermoelectric conversion materials, and has already been put to practical use as a material for thermoelectric cooling and thermoelectric power generation.
[0004]
However, the Bi ₂ Te ₃ based material described above has a very small reserve on the earth of Te, which is a raw material, and is difficult to supply in terms of resources. In addition, the temperature change of the figure of merit Z is large, and the value of Z is small in the temperature range other than room temperature. Industrially, the figure of merit is the same as that of Bi ₂ Te ₃ series material, or a material rich in resources. Is desired.
[0005]
Thermoelectric property data of a CoSb ₃ compound having a skutterudite-type crystal structure, which has recently attracted attention as a novel thermoelectric conversion material, is disclosed in L.L. D. Dudkin's Soviet Phys Solid State; 1, p126-1a3, 1959. However, in order to use it as a practical thermoelectric conversion material, it has been necessary to reduce the thermal conductivity.
[0006]
In an attempt to reduce the thermal conductivity in this material system, a compound having a filled skutterudite structure filled with a rare earth element was synthesized. C. Salees et al., Science Vo. 1, 272, 1325-1328, 1996.
[0007]
This filled skutterudite structure is a structure in which the skutterudite structure belongs to the cubic space group Im3, and eight TX ₆ octahedrons are connected by vertex sharing in the unit cell, and the gap caused by vertex sharing is empty. The focus is on the child position. The vacancy site is relatively large, and a structure filled with rare earth elements is a filled skutterudite structure in order to reduce thermal conductivity.
By filling the element, it is possible to reduce the thermal conductivity as compared with CoSb ₃ before filling due to the rattling effect of filling element ions such as Ce and La ions.
[0008]
Conventionally, as a method for synthesizing a compound having a filled skutterudite structure, B.I. C. According to Science Vo 1.272, 135-1328, 1996, etc. by Salees et al.
[0009]
The rare earth element, which is a raw material element, was sealed in a quartz tube coated with carbon in a He-dry box so as not to be oxidized using a 99.99% purity rod that was electrically surface-treated, and then at 1000 to 1100 ° C. It had melted over 24-48 hours. That is, a solid phase reaction method or a dissolution method using metal powder or metal ingot as a raw material has been used. At this time, since the metal powder is easily oxidized, it is difficult to handle as described above, and oxygen contamination is a problem.
In addition, a 99.99% purity rod that has been electrically surface-treated has a high material unit price, and handling thereof requires attention. Therefore, the thermoelectric conversion _material, only two examples of CeFe _4-X Co X _{Sb 12} and _{LaFe 4-X} Co X _{Sb 12} have been reported.
[0010]
[Problems to be solved by the invention]
The present invention solves the problems of conventional materials in thermoelectric materials, that is, the high thermal conductivity and impractical thermoelectric performance and the problem of supply of raw material resources, and uses materials rich in raw material resources. Thermoelectric conversion material with excellent thermoelectric performance with reduced thermal conductivity by low-cost process, especially CoSb _3- base material with skutterudite-type crystal structure that is excellent in thermoelectric performance and low-cost thermoelectric conversion material An object of the present invention is to provide a manufacturing method .
[0011]
[Means for Solving the Problems]
The present invention is obtained by mixing a raw material consisting of cobalt powder containing cerium chloride and / or its hydrate and antimony powder so that the raw material is uniformly dispersed, then molding, and subjecting this to a solid phase reaction in a reducing atmosphere. after grinding the filled skutterudite compound powder, a method for producing a CoSb ₃ group compound having a filled skutterudite structure, characterized in that the sintering.
[0012]
That is, Firudosuku' as a raw material of the filling element, CoSb ₃ group compound having a filled skutterudite structure of the present invention produced from the solid-phase reaction using a halogen compound of the fill elements, having a conventional equivalent properties A terdite type thermoelectric material, and the halogen compound used in the present invention has a low material unit price and is easy to handle. Further, unlike the conventional method, the vacuum sealing process in the quartz vacuum and the He-dry box required at that time are not required, so that the manufacturing process is a low-cost process.
[0013]
Further, CoSb ₃ group compound having a filled skutterudite structure of the present invention, as the filling element and is Ba well rare earth elements, Sr, Y, even filling of various elements such as Zr possible.
[0014]
Examples of the halogen compound used in the present invention include cerium chloride or its hydrate CeCl ₃ · nH ₂ O, lanthanum chloride or its hydrate LaC1 ₃ · nH ₂ O, barium chloride or its hydrate BaC1 ₂ · nH ₂ O, yttrium chloride or its hydrate YCl ₃ · nH ₂ O, zirconium chloride or its hydrate ZrCl ₄ · nH ₂ O, and strontium chloride or its hydrate SrCl ₂ · nH ₂ O And at least one halogenated compound or a hydrate thereof. The halogen compound may be not only one kind of halogen compound but also a mixture of two or more kinds of halogen compounds.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail by embodiments.
A CoSb ₃ compound having a filled skutterudite structure was prepared by the following procedure.
Cobalt Co powder (99.99%), antimony Sb powder (99.9999%), cerium chloride hydrate CeCl ₃ .nH ₂ O (99.99%) as a raw material, the composition ratio of CeCo ₄ Sb ₁₂ Ce: Co : Sb = 1: 4: 12. In addition, chlorides usually become hydrates because they absorb moisture in the atmosphere. Since it becomes a problem when weighing if the moisture absorption amount is not known, hydrate (nH ₂ O) is used from the beginning as a raw material, and in this example, n = 7 chloride is used.
[0016]
After mixing so that the powder raw material was uniformly dispersed, it was pressure-molded with a force of 150 to 200 kg / cm ^{2 by} a uniaxial press. The prepared pellets had a size of about 15 mm × 5 φmm. FIG. 1 shows a high-temperature atmosphere furnace used for the solid-phase reaction.
First, a pellet formed in a double carbon container 9 was placed in the furnace body 10. In the double container, only one small hole was made so that the inside of the container was evacuated. In addition, carbon powder that does not enter the inner container is placed in the outer container. The furnace body 10 can be evacuated to a vacuum, and the reducing atmosphere gas can be replaced and kept flowing at an arbitrary flow rate.
[0017]
Inside the furnace, the valves 3, 4 and 5 were closed, the valve 2 was opened, the vacuum pump 1 was exhausted, and the vacuum was exhausted to about 10-2 Pa. Thereafter, the valve 2 was closed, the valves 5, 3 and 4 were opened, and the reducing atmosphere gas H ₂ + Ar was introduced into the furnace from the cylinder 6 to normal pressure while adjusting the flow rate using the flow rate regulator 7. After evacuating the furnace, the operation of introducing the reducing gas was repeated several times.
Next, the valves 3, 4 and 5 are tightened, the valve 2 is opened, and the vacuum pump 1 is evacuated. Then, the valve 2 is tightened and the valves 5, 3, and 4 are opened to adjust the flow rate of the flow rate regulator 7. From the cylinder 6, the reducing atmosphere gas H ₂ + Ar was introduced into the furnace up to normal pressure, the valve 4 was opened, and the gas was allowed to flow through the excluding device 8. At this time, the flow rate regulator 7 was set to a flow rate of several ccm. While monitoring the temperature in the furnace with the temperature sensor 11 provided in the furnace, the heater output was controlled so that the temperature in the furnace was 650 ° C. The temperature raising time until the temperature in the furnace reached 650 ° C. was 2 hours, and the solid phase reaction time was 24 hours. By setting the temperature rising time to 2 hours, the hydrate (nH ₂ O) absorbed by the chlorides was evaporated and carried to the outside of the furnace by the reducing gas atmosphere.
[0018]
The problem of material oxidation was solved as follows.
First, the oxygen partial pressure in the furnace was reduced by repeating the exhaust in the furnace and the filling of the reducing atmosphere gas H ₂ + Ar. Thereafter, the oxygen partial pressure in the furnace was maintained in a reduced state by continuing to flow the reducing atmosphere gas H ₂ + Ar. Still, the remaining oxygen reacted with the carbon black powder placed in the surface of the carbon container 9 and the outer container to prevent oxidation of the material itself to be reacted. The solid-phase reaction method is not limited to the above method, and any method that prevents oxidation of the material may be used.
[0019]
During the solid phase reaction, chlorine Cl in the cerium chloride hydrate is mainly discharged as hydrogen chloride HCl by hydrogen H2 in the reducing atmosphere gas and water H ₂ O as water vapor.
[0020]
The reacted powder was pulverized and evaluated by X-ray diffraction. When the lattice constant is estimated by extrapolation from X-ray diffraction, as shown in FIG. 2, the produced CeCo ₄ Sb ₁₂ has a larger lattice constant than CoSb _{3 as} Ce enters the lattice. Further, from the diffraction intensity pattern, it was confirmed that Ce had a filled skutterudite structure contained in the skutterudite structure, and that the other phase was not confirmed and that it was a CeCo ₄ Sb ₁₂ single-phase material.
[0021]
The filled skutterudite compound powder thus obtained by the solid phase reaction is further finely pulverized to an average particle size of 40 mm or less, and fired using a spark plasma sintering machine (SPS1050 manufactured by Sumitomo Coal Mining Co., Ltd.) as a raw material. A bonded body was prepared (hereinafter, discharge plasma sintering is referred to as SPS).
SPS sintering conditions were set to a pressure of 20 MPa during sintering, a sintering temperature of 640 ° C., and a sintering time of 5 minutes.
[0022]
The density of the sintered body was 6.81 g / cm ³ . Similar to the powder subjected to the solid phase reaction, the produced sintered body was confirmed by X-ray diffraction method to have a filled skutterudite structure from the diffraction pattern. The method for producing the sintered body is not limited to spark plasma sintering, and may be a HIP method, a hot press method, or the like.
[0023]
The electrical conductivity and thermal conductivity of the sintered body of CoSb ₃ produced by the same production method as the CeCo ₄ Sb ₁₂ sintered body produced by the above method are shown in FIG. 3 and FIG.
The electric conductivity was almost the same when CoSb ₃ and CeCo ₄ Sb ₁₂ filled with Ce were compared. On the lower temperature side, the electric conductivity of CeCo ₄ Sb ₁₂ was higher.
[0024]
In the temperature range from room temperature to 700 K, CeCo ₄ Sb ₁₂ filled with Ce had a thermal conductivity reduced by 40% compared to CoSb ₃ . The fact that the thermal conductivity could be reduced despite the same electrical conductivity was a very favorable result as a thermoelectric material.
Moreover, this time prepared sintered body, measuring the Seebeck coefficient, it was confirmed that the thermoelectric properties of the equivalent CeCo ₄ Sb ₁₂ that previously reported.
[0025]
Therefore, by filling the Ce cerium chloride hydrate CeCl ₃ · 7H ₂ O in CoSb _3, been shown to be capable of synthesis of CeCo ₄ Sb ₁₂ material with CoSb ₃ equal or more thermoelectric performance, further Unlike conventional methods, it was shown that raw materials can be synthesized at low cost and with a simple production method.
[0026]
Also, lanthanum chloride hydrate LaC1 ₃ · 7H ₂ O, barium chloride hydrate BaC1 ₂ · 2H ₂ O, yttrium chloride hydrate YCl ₃ · 6H ₂ O, zirconium chloride ZrCl ₄ and strontium chloride hydrate SrCl ₂ the · 6H 2 _O was used as raw material to prepare a thermoelectric conversion material in the same manner as above. When the thermoelectric properties of each of the prepared materials were evaluated, they showed the same properties as the thermoelectric conversion material prepared from cerium chloride hydrate CeCl ₃ · 7H ₂ O.
Further, in this example, a CoSb ₃ compound was used, but a CoSb ₃ group compound (Co _X Fe _1-X Sb ₃ , Co _X Ni _{1- X} Sb ₃ , Fe _X Ni _1-X Sb _3, etc.) can also be prepared by a similar method using powder raw materials such as Fe, Ni and the like.
[0027]
【effect】
The present invention solves the problems of conventional materials in thermoelectric materials, that is, the high thermal conductivity and impractical thermoelectric performance and the problem of supply of raw material resources, and uses materials rich in raw material resources. Thermoelectric conversion material with excellent thermoelectric performance with reduced thermal conductivity by low-cost process, especially CoSb _3- base material with skutterudite-type crystal structure that is excellent in thermoelectric performance and low-cost thermoelectric conversion material We were able to provide a manufacturing method.
[Brief description of the drawings]
FIG. 1 is a view showing a high-temperature atmosphere furnace used for a solid-phase reaction used in an embodiment.
FIG. 2 is a graph showing a lattice constant estimated by an extrapolation method.
FIG. 3 is a graph showing electrical conductivities of CoSb ₃ and CeCo ₄ Sb ₁₂ ;
FIG. 4 is a graph showing the thermal conductivity of CoSb ₃ and CeCo ₄ Sb ₁₂ ;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Vacuum pump 2 Valve 3 Valve 4 Valve 5 Valve 6 Cylinder 7 Flow rate regulator 8 Exclusion device 9 Double carbon container 10 Furnace body 11 Temperature sensor 12 Heater

Claims (1)

The raw material consisting of cobalt powder containing cerium chloride and / or its hydrate and antimony powder is mixed so as to be uniformly dispersed, then molded, and this is subjected to solid phase reaction in a reducing atmosphere, and the obtained skutter is obtained. CoSb having a filled skutterudite structure, characterized in that it is sintered after pulverizing the dite compound powder ₃ A method for producing a base compound.

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