JPH1093147A - Thermo-element material and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable preliminary formation and pressure sintering of powder under air atmosphere by making oxygen in air which entered inside a preliminary molded item to react selectively with addition metal having large affinity with oxygen and forming oxide of addition metal in a formation process of a preliminary molded item. SOLUTION: Raw powder of a single metal, constituting a thermo-element material and powder of a metal whose affinity with oxygen is larger than any other raw metal are mixed in an oxygen-free atmosphere. Then, mixed powder is put in a die in the oxygen-free atmosphere or in air atmosphere. A die charged with mixed powder is set in a press machine, pressurized under air atmosphere of a room temperature, and mixed powder is formed into a preliminary molded item. Then, a preliminary molded item is sintered under air atmosphere, and a thermoelectric material molded item is formed. Oxygen entering inside a preliminary molded item reacts with the addition metal and is removed. Thereafter, an oxide film formed in a surface is removed by grinding.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a thermoelectric element material and a method for manufacturing the same.

[0002]

2. Description of the Related Art Bi-Te-based thermoelectric elements containing Bi and Te, for example, have excellent characteristics in the low temperature range from room temperature to 300 ° C. It is used for various applications such as heat utilization power generation.

[0003] In the state of an ingot obtained by mixing and melting a metal raw material and then gradually cooling and crystallizing the thermoelectric material, the thermoelectric material is liable to crack in a direction perpendicular to the crystal c-axis and has a problem of poor yield. For this reason, the method of powder metallurgy is introduced, the ingot is once crushed into a raw material powder, put into a press die of a hot press device, heated, and simultaneously pressed and sintered to have a predetermined strength. A method for forming a thermoelectric material molded body is known.

[0004]

However, in the above method, when the preformed body is formed in the atmosphere, the surface of the preformed body is oxidized, and oxygen enters the inside.
The amount of oxygen that has penetrated into the inside of the preform is extremely small.
Raw metals such as i and Te have a weak affinity for oxygen and are therefore enclosed in the preform. For an n-type element,
Oxygen functions as a dopant, so that the carrier concentration of the obtained thermoelectric element material becomes unnecessarily high. As a result, the electric resistance becomes too small, and the semiconductor becomes unsuitable. On the other hand, in a p-type device, the opposite is the case, and oxygen causes a decrease in carrier concentration.

If the formation of the powder preform and the pressure sintering of the preform by hot pressing are performed in an oxygen-free atmosphere such as a rare gas or a nitrogen gas, the carrier concentration can be increased or decreased by oxygen. Although the problem can be solved, when mass-producing the thermoelectric element material, large-scale equipment is required to make the entire press apparatus an oxygen-free atmosphere, and the working efficiency is poor. Therefore, a manufacturing method that can be mass-produced at low cost is desired.

SUMMARY OF THE INVENTION An object of the present invention is to provide a thermoelectric element material in which the carrier concentration is adjusted to an optimum range even when the preforming of the powder and the pressure sintering of the preforming are performed in the atmosphere, and a method for producing the same. It is to provide.

[0007]

In order to achieve the above object, in the method for producing a thermoelectric element material according to the present invention, the raw material powder of the metal constituting the thermoelectric element is mixed with oxygen more than any of the raw metal. A powder of a metal having a high affinity is added, and these powders are mixed in an oxygen-free atmosphere. The mixed powder is formed into a preformed body in an air atmosphere, and the preformed body is pressure-sintered in an air atmosphere. In the step of forming a preformed body, oxygen in the air that has entered the inside of the preformed body selectively reacts with an additional metal having a high affinity for oxygen, This is to be an oxide of the additive metal.

Further, the thermoelectric element material according to the present invention is obtained by adding a metal powder having a higher affinity for oxygen than any metal of the raw metal to a raw material powder of a metal constituting the thermoelectric element and mixing the resultant. A thermoelectric element material obtained by sintering a mixed powder, in which an oxide of an additional metal having a high affinity for oxygen is substantially uniformly dispersed in a structure of a raw metal.

[0009]

In the stage before the formation of the preform, oxygen in the air comes into contact with the mixed powder of the raw material metal and the added metal, and penetrates between the powders. For this reason, when the pre-formed body is formed, a small amount of oxygen has penetrated into the pre-formed body, and this oxygen selectively reacts with the added metal having a high affinity for oxygen to form an oxide of the added metal. I do. If the content of the additional metal is appropriate, almost all of the oxygen that has entered the inside of the preform reacts with the additional metal and changes into an oxide form, thereby eliminating the oxygen that increases the carrier concentration. In addition, since the oxide film is formed on the surface of the preform and the oxide film acts as a barrier against air, after the preform is formed, oxygen in the atmosphere is prevented from entering the inside of the preform. . In the subsequent pressure sintering step, the raw material metal is formed into a dense compact, and the oxide inside the preform is granulated. As a result, a thermoelectric element material in which the particulate oxide is substantially uniformly dispersed in the structure of the raw metal can be obtained. In addition, the particulate oxide does not adversely affect the thermoelectric performance, and the oxide film formed on the outermost surface of the molded product is slight, so that it can be finally easily removed by machining. it can.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION A raw material powder of a simple metal constituting a thermoelectric element material and a metal powder having a higher affinity for oxygen than any of the raw material metals are prepared. Examples of the thermoelectric element material include Bi-Te based thermoelectric element materials such as Bi-Te and Bi-Te-Se. As an element having a higher affinity for oxygen than a general metal constituting a thermoelectric material, for example, La, Ce, a misch metal (La, Ce)
And metals having a high affinity such as Pr, Nd, Al, Si, and Mn.

The raw material metal powders are weighed substantially in stoichiometric ratio, and a predetermined amount of a metal powder having a higher affinity for oxygen than any of the raw material metals is added to the metal raw material powder. Mix in atmosphere. Here, the oxygen-free atmosphere refers to an atmosphere in which the raw material metal of the thermoelectric element material is not oxidized, such as a vacuum, an inert atmosphere such as a rare gas (Ar gas or the like) or a nitrogen gas. In the subsequent step of forming the preform, if the powder is handled in the air, the powder contains about 1000 to 10000 ppm of oxygen. After grasping the amount, the amount of the additional metal to be reacted with the amount is determined and added to the raw material metal powder.

The average particle diameter of the raw metal powder is about 100 μm
In the following cases, it is sufficient to simply mix in an inert atmosphere,
When the average particle size is large, they are mixed while being pulverized in a high energy ball mill or a rolling ball mill in an inert atmosphere to obtain a mixed powder having an average particle size of about 100 μm or less.

Next, the mixed powder is loaded into a mold. The loading here need not necessarily be performed in an oxygen-free atmosphere, but may be performed in an air atmosphere.

Next, the mold loaded with the mixed powder is set in a press machine, and the mixed powder is preformed under an atmosphere at normal temperature, at a pressure of about 1000 kgf / cm ² or more and for a time of about 2 minutes or less. Form on the body. The molding temperature is not limited to room temperature, and may be slightly increased.

The preform taken out of the mold is 300
After holding for an appropriate time (for example, about 1 hour) in a heating apparatus maintained at a temperature in the range of ５550 ° C., put it in a mold again, and apply a pressure of about 1000 kgf / cm ² or more and a time of about 2 minutes or less Sintering in the atmosphere under the conditions described above to form a thermoelectric material molded body. The preformed body may be pressed and molded by heating the preformed body together with the mold in a heating device.

When the preformed body is formed, when the preformed body is heated, and when the thermoelectric material molded body is formed, the surface of the molded body comes into contact with air and an oxide film adheres. The oxide film on the surface of the compact is removed by mechanical cutting or grinding.

[0017]

EXAMPLE will be described below with reference to Examples of the present invention. In an embodiment of the present invention were prepared _{(Bi 2 Te 3) 0.85 (} Bi 2 Se 3) n -type thermoelectric element material consisting of Bi-Te-Se represented by _0.15. First, the raw material powder is mixed in a molar ratio of Bi: Te: Se =
The weight was weighed so that 2: 2.55: 0.45 = 40: 51: 9. Example 1 was obtained by adding 0.5 parts by weight of La as an additive metal to 100 parts by weight of this raw material powder, and 1 part of La was added.
The powder to which parts by weight were added was referred to as Example 2, and the powder to which La was not added at all was referred to as Comparative Example 1.

The powder was pulverized and mixed for about 15 hours in a rolling ball mill in an Ar gas atmosphere to obtain a powder in which each powder was sufficiently mixed. The average particle size of the obtained mixed powder is about 10 μm. After loading the mixed powder into a mold under an air atmosphere, the mold is set in a press machine, and the mixed powder is preliminarily prepared under the atmosphere at a normal temperature, a pressure of 2800 kgf / cm ² and a press time of 20 seconds. A compact was formed. The obtained preform was heated for 1 hour in a heating device maintained at 500 ° C. in an air atmosphere. Next, the preformed body was taken out of the heating device, and was pressed and formed under the conditions of a pressure of 3150 kgf / cm ² and a pressing time of 20 seconds in an air atmosphere. The size of the obtained thermoelectric element material compact is 45 mm in diameter × 10 in thickness.
mm.

The carrier concentration of the thermoelectric element material compact obtained from each of the powders of Example 1, Example 2 and Comparative Example 1 was measured by the Hall measurement method, and the results are shown in Table 1.

◎

[Table 1]

As shown in the results of Table 1, in the n-type thermoelectric element material, it can be seen that the carrier concentration is reduced by adding La. As described above, the carrier concentration can be controlled even when the formation of the preformed body and the pressure sintering of the preformed body are performed in the air atmosphere.

FIG. 1 shows the metallographic microstructure (× 100) of the compact obtained in Example 2. In FIG. 1, it can be seen that the black particles are La oxides and are dispersed almost uniformly in the structure in the form of fine particles of about 100 μm or less.

The molded product obtained in Example 2 was observed by SEM and also by electron probe micro analysis (EPMA). FIG. 2 shows the structure observed by SEM and the distribution of each element of O (oxygen), Bi, Te, Se and La by EPMA. In the distribution of each element, whitish portions indicate portions where the concentration of each element is high, as shown in the figure. From FIG. 2, the position of the high concentration of O is L
a, which substantially corresponds to the position of high concentration of Bi, Te, Se
It can be seen that this does not correspond to the high density position. This indicates that O exists only as an oxide of La and hardly forms a solid solution in the crystal lattice.

[0024]

According to the method for producing a thermoelectric element material of the present invention,
Even if the formation of the pre-formed body and the pressure sintering of the pre-formed body are performed in an air atmosphere, the oxygen in the air that has entered the inside of the pre-formed body selectively reacts with the added metal having a high affinity for oxygen. React to form an oxide of the additional metal. In the thermoelectric element material of the present invention, an oxide of an additional metal having a high affinity for oxygen is substantially uniformly dispersed in a tissue, and an increase in carrier concentration is suppressed with respect to an n-type element. , A decrease in carrier concentration can be suppressed.
As described above, even if the formation of the powder pre-formed body and the pressure sintering of the obtained pre-formed body are performed in an air atmosphere, a thermoelectric element material having a predetermined carrier concentration can be obtained. Therefore, it is possible to achieve a reduction in manufacturing costs associated with maintaining a vacuum or consuming an inert gas. INDUSTRIAL APPLICABILITY The thermoelectric element and the method for manufacturing the thermoelectric element of the present invention have extremely large industrial applicability as a thermoelectric material used in a wide range of fields, such as thermoelectric power generation and thermoelectric cooling, temperature sensors and constant temperature devices in semiconductor processes, and cooling of electronic devices. It can be said that.

[Brief description of the drawings]

FIG. 1 is a drawing-substituting micrograph (× 100) showing a metal structure of a thermoelectric material molded body of the present invention.

FIG. 2 is a drawing substitute photograph showing a metal structure of a thermoelectric material molded body of the present invention by SEM, and showing each component of O, Bi, Te, Se and La by EPMA.

Claims (2)

[Claims] 1. A raw material powder of a metal constituting a thermoelectric element is weighed, the weighed metal raw material powder is mixed, the mixed powder is pressed to form a preform, and the preform is pressure-fired. In the method of manufacturing a thermoelectric element material by sintering, to the metal raw material powder, a metal powder having a higher affinity for oxygen than any of the raw material metals is added, and these powders are mixed in an oxygen-free atmosphere, The mixed powder is formed into a pre-formed body in an air atmosphere, and the pre-formed body is pressure-sintered in an air atmosphere. The method according to claim 1, wherein the oxygen in the atmosphere selectively reacts with an additional metal having a high affinity for oxygen to form an oxide of the additional metal. 2. A raw material powder of a metal constituting a thermoelectric element,
A thermoelectric element material obtained by adding and mixing a metal powder having a higher affinity for oxygen than any of the raw material metals, and sintering the resulting mixed powder. A thermoelectric element material, characterized in that an oxide of an additive metal having a large value is substantially uniformly dispersed.