JPH1140861A - Manufacture of cobalt antimonide thermoelectric material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate a grinding process, where impurities are liable to be mixed and to realize a manufacturing method where processes are simple and suitable for mass production by a method, wherein a material is melted into a solution of prescribed composition, the solution is solidified in a mold, and the solidified body is subjected to a thermal treatment where it is kept to stand for a prescribed time at a specific temperature. SOLUTION: A mixture of cobalt(Co) of purity 99.9985% and antimony(Sb) of purity 99.999% compounded at a mol ratio 1:3 is introduced into an alumina crucible, heated through high-frequency heating, and raised up to a temperature of 1100 deg.C into an alloy melt. The alloy melt is kept at a temperature of 1100 deg.C for five minutes, then poured into a copper mold kept at a room temperature in an argon atmosphere, and solidified into about a 150×200×10 mm<3> alloy cast piece. Furthermore, the alloy cast piece is subjected to a thermal treatment which is carried out in an argon atmosphere of atmospheric pressure at temperatures of 400 to 850 deg.C, especially at a temperature of 650 deg.C for fifty hours, whereby all the cast piece is turned into a CoSb3 body of single phase. Through this setup, manufacturing cost can be reduced.

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 thermoelectric material which can be used for thermoelectric power generation and thermoelectric cooling, and more particularly to a method for producing a thermoelectric material comprising a cobalt antimonide (CoSb ₃ ) -based compound. .

[0002]

2. Description of the Related Art Thermoelectric materials are materials that can be used for thermoelectric generation in which heat is directly converted into electricity by the Seebeck effect and thermoelectric cooling by the Peltier effect. Research is being actively conducted.

Conventionally, materials such as bismuth / tellurium, lead tellurium, germanium / silicon, and iron / silicon have been used as thermoelectric materials, but further improvement is desired in terms of thermoelectric conversion efficiency. Therefore, as a new thermoelectric material, cobalt antimonide (hereinafter referred to as Co
Sb ₃ ) -based compounds are attracting attention.

This is because CoSb ₃ having a skutterudite type crystal structure, or its constituent element Co and / or Sb
Is partially replaced by another element.
No. 186,294 discloses that a part of Co (composition ratio X is 0.1%).
(001-0.2) is substituted by one or more of Pd, Rh, and Ru, and a thermoelectric material comprising a substituted compound Co _1-x M _x Sb ₃ is disclosed. There is also a report on a thermoelectric material in which a part of Co is replaced with Pd and Pt (Preprints of the 44th Federation of Applied Physics-related lectures No. 1, p. 82, 19).
97).

Conventionally, as a method for producing such a thermoelectric material, a so-called powder metallurgy method in which element powders having a predetermined composition are mixed and pulverized, and then sintered under pressure to produce a sintered body is generally used. Has been used for In the powder metallurgy method, there is a case where an ingot of a predetermined composition obtained by pre-melting is pulverized into a powder, and the powder is sintered under pressure. Further, a single crystal growing method of growing a single crystal from a raw material melt having a predetermined composition by a unidirectional solidification method, such as the Czochralski method, is also employed.

[0006]

However, in the powder metallurgy method described above, there are many impurities mixed in the pulverizing and mixing steps, and these impurities rarely lower the thermoelectric properties. Especially Co
Sb ₃ -based thermoelectric materials are not preferred because the effect of trace impurities on thermoelectric performance is large. In addition, there is a problem that the process is complicated because powder is handled.

On the other hand, the single crystal growing method has a problem that the growth rate of the single crystal is limited and the productivity cannot be increased. In particular, the CoSb ₃ -based thermoelectric material has a problem that the product yield is low because Sb can grow a single crystal only from an excess of Co—Sb melt. Furthermore, if the moving speed of the solid-liquid interface is increased, the Sb phase is mixed,
Another problem is that the crystal growth rate, that is, the productivity cannot be increased.

In view of the above-mentioned problems of the prior art, the present invention does not include a pulverizing step in which impurities such as oxygen and container material are easily mixed when producing a CoSb ₃ -based thermoelectric material.
It is another object of the present invention to provide a new manufacturing method which is simple in process and suitable for mass production. It is another object of the present invention to increase the productivity and yield during the production of a CoSb ₃ -based thermoelectric material and contribute to the reduction of the production cost.

[0009]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have made various studies focusing on producing CoSb ₃ -based thermoelectric materials by a melting / solidification method. As a result, it was found that a material having good thermoelectric properties could not be obtained simply by dissolving and solidifying a raw material mixed in a predetermined composition in a mold.

[0010] In the process because the solidification melt composition of CoSb _3, CoSb _2, CoSb other CoSb ₃ phase,
This is due to the formation of the Sb phase. However, when the coagulated piece composed of this mixed phase is heat-treated under a predetermined condition, the whole becomes CoS
b It has been found that the phase becomes a _three- phase and shows good thermoelectric properties.

Also, during the above heat treatment, voids are formed inside the coagulated pieces and the porosity increases. By reducing the porosity by hot pressing, the thermoelectric properties are improved. I found that.

The present invention has been made on the basis of the above findings, and its gist is a method for producing a thermoelectric material comprising a CoSb ₃ -based compound, comprising a step of dissolving a raw material to form a melt having a predetermined composition. a step of solidifying the melt in the mold, a method for producing a CoSb ₃ based thermoelectric material characterized by comprising a heat treatment step of holding the solidified strip to a predetermined time 400 to 850 ° C..

Here, the CoSb ₃ -based compound is represented by the general formula Co _1-x M _x (Sb _1-y N _y ) ₃ (where M is a substitution element with Co for Pd, Rh, Ru, Pt, X = 0 to 0.3 including at least one of Ir, Ni, Fe, and N is a substitution element for Sb, Se, Te, Sn, Ge, Pb, P, As, Bi.
And y = 0 to 0.3).

The CoSb ₃ -based material is characterized in that the material produced by the above method is subjected to a pressure treatment at 300 to 850 ° C. at a pressure of 5 MPa or more to reduce the porosity in the material to 10% or less. This is a method for producing a thermoelectric material.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The CoSb ₃ -based thermoelectric material according to the present invention generally comprises the above-mentioned CoSb ₃ -based compound having a skutterudite-type crystal structure. Is included.

The present invention is a method for producing a CoSb ₃ -based thermoelectric material as described above, comprising the steps of: dissolving a raw material to obtain a melt having a predetermined composition; and solidifying the melt in a mold. A heat treatment step of maintaining the solidified pieces at 400 to 850 ° C. for a predetermined time.

The raw material to be dissolved may be a powder or an ingot. Each element may be blended into a predetermined composition, or a pre-melted alloy may be used. As can be seen from the Co-Sb phase diagram, the raw material is heated to 1050 ° C or higher (preferably 1100 ° C or higher) slightly higher than the melting point of CoSb ₃ in order to completely dissolve and homogenize the raw material. Should be kept for 5 minutes or more.

The reason for solidifying the melt in the mold is to adjust the cooling rate during solidification and to form the melt into a shape suitable for heat treatment. There is no particular limitation on the material of the mold, but since the structure of the solidified piece (hereinafter, referred to as an alloy slab) changes depending on the cooling rate during solidification, the material of the mold is selected so that a desired cooling rate is obtained. . It is also effective to cool or heat the mold to adjust the cooling rate.

Although there is no particular limitation on the internal shape of the mold, a plate-shaped or rod-shaped alloy slab is usually prepared in order to shorten the soaking time in the subsequent heat treatment step.

Conventionally, a method of pulverizing a melted and solidified ingot of a predetermined composition to produce a thermoelectric material by a powder metallurgy method is generally performed. On the other hand, the method of the present invention is to produce a thermoelectric material having desired properties only by heat treatment without pulverizing the obtained alloy slab into powder, and for that purpose, heat treatment conditions The choice is important.

[0021] As already mentioned, the alloy cast piece obtained by melting and solidifying a composition composed mainly of CoSb ₃ comprises CoSb _2, CoSb, Sb phase in addition to the CoSb ₃ phase, do not show good thermoelectric properties . However, according to the knowledge of the present inventors, an alloy slab composed of this mixed phase is kept for 400 to 8 hours for a predetermined time.
By performing a heat treatment at 50 ° C., Sb becomes CoSb ₂ ,
By thermally diffusing into the CoSb phase, it can be relatively easily changed to a uniform phase of CoSb ₃ . It has been confirmed by X-ray diffraction that this CoSb ₃ phase has a skutterudite-type crystal structure, and the alloy slab after the heat treatment exhibits good thermoelectric properties.

The lower limit of the heat treatment temperature is set to 400 ° C.
If the heat treatment time is less than this, the heat treatment time becomes too long to be practical, and the upper limit is set to 850 ° C.
oSb ₃ phase may be decomposed.
Is easy to evaporate and the composition tends to fluctuate.

A more preferable range of the heat treatment temperature is 550 to 550.
The temperature is 700 ° C., and within this range, a phase of only CoSb ₃ (hereinafter, sometimes referred to as a single phase) can be reliably formed by a relatively short heat treatment.

In the above-mentioned CoSb ₃ -based compound, the element M replaces a part of Co and the element N replaces a part of Sb to form a skutterudite crystal structure. When the composition ratio y of the ratio x and N is in the range of 0 to 0.3, the solidified alloy slab is composed of a mixed phase, which is heat-treated to obtain Co _1-x M _x (Sb _1-y N _y ). _The formation of type ₃ single phase is the same as above.

The heat treatment time required to form a single phase is:
The higher the heat treatment temperature and the finer the solidification structure, the shorter it becomes. For example, in the case of an alloy slab solidified in a copper mold,
A heat treatment at 50 ° C. for about 25 hours or 700 ° C. for about 10 hours can provide homogeneous CoSb ₃ or a substituted compound thereof.

The above heat treatment may be performed on the obtained alloy slab as it is (unprocessed), or a part thereof may be cut out or cut. The heat treatment is desirably performed in an inert atmosphere, for example, an Ar atmosphere or under vacuum to prevent oxidation.
However, since the method of the present invention heat-treats the slab without pulverizing it, it is one of the features that the restriction on the oxygen concentration in the atmosphere is relaxed as compared with the case of the powder metallurgy method.

[0027] By the above heat treatment, the slab was a single phase of CoSb ₃ or a substituted compound thereof was found to micro voids therein occurs. It is assumed that the vacancies are generated due to the difference in the diffusion rate between Sb and Co or the substitution element M. The volume ratio (porosity) of the holes depends on the heat treatment conditions, but ranges from 20 to 35%, which is undesirable because it lowers the mechanical strength and electrical conductivity of the material.

However, if the slab after the heat treatment is hot-pressed, the porosity is greatly reduced, and if the porosity is 10% or less, the above-mentioned mechanical strength and electric conductivity are obtained. It has been found that the decrease in the amount does not pose a practical problem.

The present invention according to claim 2 is based on the above findings, and the material produced by the method according to claim 1 is subjected to pressure treatment at 300 to 850 ° C. at a pressure of 5 MPa or more, and Has a porosity of 10% or less.

Although there is no particular limitation on the method of the pressure treatment, it is preferable to use, for example, hot isostatic pressure (HIP) treatment. The material to be subjected to the pressure treatment may be a slab as it is after the heat treatment, or may be a cut slab or a crushed slab to a certain size.

These materials are vacuum sealed in, for example, a stainless steel foil having a thickness of about 0.1 mm and placed in a HIP device to perform HIP processing. However, at this time, in order to avoid a reaction between the stainless steel and the material to be treated, it is preferable to sandwich a tungsten foil having low reactivity with the material to be treated.

The reason why the lower limit of the temperature during the pressure treatment is 300 ° C. and the lower limit of the pressure is 5 MPa is that if either of them is less than this, the rate of decrease in the porosity becomes extremely small and it is not practical. , The upper limit of the temperature is 850 ° C.
Beyond this, CoSb ₃ or Co _1-x M _x (Sb _1-y N _y ) ₃
This is because the phase may be decomposed.

A more preferable pressure treatment condition is a temperature of 55
Within a range of 0 to 700 ° C. and a pressure of 100 MPa or more, the porosity of the material can be reliably reduced to 10% or less within a pressure treatment time of about 2 hours or less in this range.

In the present invention, the heat treatment for converting the alloy slab of the mixed phase into a single phase of CoSb ₃ or its substitutional compound may be performed partially or entirely under pressure. The porosity in the material can be reduced to 10% or less without separately performing a pressure treatment.

[0035]

【Example】

(Example 1) As a raw material, a mixture of cobalt (Co) having a purity of 99.9985% and antimony (Sb) having a purity of 99.999% in a molar ratio of 1: 3 was put into an alumina crucible. The mixture was heated to 1100 ° C. by high frequency heating and dissolved to obtain a combined liquid. After holding this combined liquid at 1100 ° C. for 5 minutes, it was cooled and solidified by pouring it into a copper mold at room temperature in an argon gas atmosphere to obtain an alloy slab of about 150 × 200 × 10 mm.

Further, the obtained alloy slab was heat-treated at 650 ° C. for 50 hours in an argon gas atmosphere at atmospheric pressure to form a single phase of CoSb _{3 on} the entire slab.

The obtained CoSb ₃ thermoelectric material has a stoichiometric composition, in which the content of metallic impurities is Fe: 10p
pm, Ni: 3 ppm, Cu: 2 ppm, Cr: 2 pp
m, Al: 10 ppm or less. Further, the oxygen content was 300 ppm. 600 in raw material cobalt
Considering that 0 ppm and 100 ppm of oxygen are contained in the raw material Sb, it can be seen that the oxygen content could be reduced in the above series of steps.

As a result of measuring the thermoelectromotive force (Seebeck coefficient S) of this CoSb ₃ thermoelectric material, the value of S was 30
300 ° V / K at 150 ° C., 340 μV / K at 150 ° C., 30
It was 230 μV / K at 0 ° C., indicating the properties of a p-type semiconductor. The Seebeck coefficient S was generated by attaching a Pt-PtRh thermocouple wire to both ends of a sample piece cut into 2 × 2 × 15 mm and giving a temperature difference of 5 to 6 ° C. to the sample piece in a heating furnace. The measured thermoelectromotive force was measured and divided by the temperature difference between the test pieces.

According to the method of the present invention, a material having good thermoelectric properties can be stably obtained because impurities are less mixed than in the conventional powder metallurgy method.

(Example 2) The same raw material as in Example 1 was purified by zone refining, placed in an alumina crucible, melted in the same manner as in Example 1, and then cooled and solidified. Further, the obtained alloy slab was used in Example 1
By performing a heat treatment under the same conditions as described above, a thermoelectric material of CoSb ₃ was obtained.

The obtained CoSb ₃ material has a stoichiometric composition, in which the metal impurity content is Fe: 2 ppm or less, Ni: 1 ppm or less, Cu: 1 ppm or less, Cr:
The content was 2 ppm or less, Al: 10 ppm or less, and the oxygen content was 50 ppm. As described above, it was confirmed that the method of the present invention can easily produce a thermoelectric material having extremely high purity because impurities are little mixed.

The obtained CoSb ₃ material was obtained in Example 1.
It has the same properties of thermoelectromotive force and p-type semiconductor as described above, and is highly purified, so that the electrical conductivity is improved as compared with Example 1.

(Example 3) C obtained by Example 2
A 5 × 20 × 50 mm test piece cut out of oSb ₃ material was vacuum-sealed in a metal foil,
A hot isostatic pressing (HIP) treatment was performed at 00 ° C. and 100 MPa for 1 hour. The porosity of the obtained material is 2
From 5% to 5%, the mechanical strength and electrical conductivity improved about twice.

Example 4 As a raw material, the purity was 99.99.
85% Co, 99.999% Sb purity, 99.99% purity.
A raw material in which 99% of palladium (Pd) was blended in a composition of Co _0.97 Pd _0.03 Sb ₃ was dissolved under the same conditions as in Example 1 and then cooled.
Coagulated. Further, the obtained alloy slab was heat-treated under the same conditions as in Example 1 to obtain Co _0.97 Pd
_A thermoelectric material of _0.03 Sb ₃ was obtained.

The obtained Co _0.97 Pd _0.03 Sb ₃ material is:
Shows the properties of n-type semiconductors, and thermoelectromotive force (Seebeck coefficient)
Are −180 μV / K at 30 ° C. and −220 at 150 ° C.
μV / K, which was −280 μV / K at 300 ° C., was confirmed to have a thermoelectric property equal to or better than that of a material manufactured from the same raw material by a conventional powder metallurgy method or a single crystal growing method.

Example 5 As a raw material, the purity was 99.99.
85% cobalt, 99.999% purity antimony,
99.99% pure palladium and 99.99% pure
_Was mixed with Co _0.90 Pd _0.05 Pt _0.05 Sb ₃ composition under the same conditions as in Example 1, and was cooled and solidified. Further, the obtained alloy slab was heat-treated under the same conditions as in Example 1 to obtain Co _0.90 Pd _0.05 Pt _0.05
A thermoelectric material of Sb ₃ was obtained.

The obtained Co _0.90 Pd _0.05 Pt _0.05 Sb ₃
The material is −200 μV / K at 30 ° C. and −23 at 150 ° C.
It exhibited a thermoelectromotive force (Seebeck coefficient) of -280 μV / K at 0 μV / K and 300 ° C., and exhibited the properties of an n-type semiconductor, and was confirmed to have thermoelectric properties equivalent to or better than materials manufactured by a conventional method. .

[0048]

According to the method for producing a CoSb ₃ -based thermoelectric material of the present invention, the production process can be simplified, the productivity and the yield can be improved, the production cost can be reduced, and impurities can be mixed. It has become possible to manufacture a material having good thermoelectric properties with a small amount.

That is, unlike the conventional powder metallurgy method, the steps of pulverizing and sintering the raw materials are not required, so that the steps can be simplified and the mixing of oxygen and container material in the pulverizing step can be prevented. . In addition, the productivity and product yield can be significantly improved compared to the conventional single crystal growth method, and the time for holding the synthetic liquid in the crucible is short, so that the contamination of impurities from the crucible can be reduced, and the thermoelectric power can be reduced. It has become possible to produce a CoSb ₃ -based thermoelectric material having good characteristics at low cost.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI H01L 35/34 H01L 35/34 // C22F 1/00 660 C22F 1/00 660Z 661 661Z 683 683 693 691B 694 694B 694A (72) Inventor Nozomu Taniguchi Inside the Ultra High Temperature Materials Research Laboratories, Ltd.

Claims (2)

[Claims] 1. The general formula Co _1-x M _x (Sb _1-y N _y ) ₃ (where M is a substitution element for Co and is Pd, Rh, Ru, Pt,
It contains at least one of Ir, Ni, and Fe, and x = 0 to 0.
3, N is a substitution element for Sb, Se, Te, Sn, Ge,
Including at least one of Pb, P, As, and Bi, y = 0
0.3) a method for producing a thermoelectric material comprising a compound represented by the formula (3), wherein a step of dissolving a raw material to form a melt having a predetermined composition, a step of solidifying the melt in a mold, A heat treatment step of maintaining the temperature at 400 to 850 ° C. for a predetermined time. 2. A material produced by the method according to claim 1, wherein the material is subjected to a pressure treatment at 300 to 850 ° C. at a pressure of 5 MPa or more to reduce the porosity in the material to 10% or less. A method for producing a cobalt antimonide-based thermoelectric material.