Method of producing thermoelectric semiconductor

Details Method of producing thermoelectric semiconductor Method of producing thermoelectric semiconductor

2000-02-25

2001-11-13

Christianson, Keith (Department: 2813)

Semiconductor device manufacturing: process

Making device or circuit emissive of nonelectrical signal

Reexamination Certificate

active

06316279

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method of producing a thermoelectric semiconductor which is used, particularly, as a thermoelectric cooling element.
2. Discussion of the Background
As a conventional thermoelectric semiconductor compound, a crystal is well known which is obtained by solidifying a thermoelectric semiconductor material, such as a bismuth-tellurium series, through one of the Bridgman technique and Zone melting crystallization. However, such a one-way crystallized thermoelectric semiconductor crystal has cleavage along planes determined by bonding surfaces of tellurium-tellurium, which results in it being brittle. Thus, employing such a one-way crystallized thermoelectric semiconductor as a thermoelectric cooling element inevitably causes problems in reliability and/or mechanical strength.
In order to overcome such serious problems from the viewpoint of practical use, especially for improving mechanical strength, Japanese Patent Laid-open Print No Sho.62-264682 (published in 1987 without examination) provides a new thermoelectric semiconductor which is produced by the following steps: powdering a crystalline body of a thermoelectric semiconductor, and sintering the resultant powdered material while applying a force thereto along one direction. Due to the fact that the thermoelectric semiconductor is applied with the force along only one direction, the cleavage planes are oriented in a direction perpendicular to the force application direction. Thus, applying en electric current along each of the resultant cleavage planes utilizes or activates the electrical isotropy of the crystalline body of the thermoelectric semiconductor, in addition to achieving an improved mechanical strength which results from the sintering.
However, in this method, before performing the foregoing sintering under pressure, the powdered thermoelectric semiconductor is placed in a mold device and hot pressed, which results in that orientation of each cleavage plane is restricted or limited, thereby inhibiting the electrical isotropy of the bismuth-tellurium series thermoelectric semiconductor as an original property thereof.
By contrast, Japanese Patent Laid-open Print No. Hei. 10-112558 published in 1998 without examination provides a method of producing a thermoelectric semiconductor having the following steps: powdering a thermoelectric semiconductor material, and sintering such a powdered thermoelectric semiconductor during extrusion. In the resultant sintered thermoelectric semiconductor body, cleavage planes are oriented along the extruding direction, thereby improving the electric conductivity of the thermoelectric semiconductor body.
However, even the foregoing newly published sintering methods as described above fail to provide a remarkable improvement in the electric conductivity of the thermoelectric semiconductor, and the performance index is at most less than 3.5, which is not satisfactory from the view point of practical use.
Thus, there is a need for a further developed method of producing a thermoelectric semiconductor which is excellent in electric conductivity, and correspondingly performance index, and which is mechanically strong.
SUMMARY OF THE INVENTION
The present invention has been developed to satisfy the need noted above, and thus has a primary object to provide a method of producing a thermoelectric semiconductor which, in addition to having a high mechanical strength, has a good performance index resulting from improved electric conductivity.
A first aspect of the present invention provides a method of producing a thermoelectric semiconductor which comprises an extrusion step for obtaining a plurality of bar-shaped thermoelectric semiconductor materials by extruding either a powdered thermoelectric semiconductor crystal or a pressed body of a powdered thermoelectric semiconductor crystal; and a sintering and integration step for obtaining an integrated sintered body by sintering and integrating the plural bar-shaped thermoelectric semiconductor materials after arranging the plural bar-shaped thermoelectric semiconductor materials in side-by-side fashion to constitute a bundle. The sintering is done currently with or prior to the integrating. The integrating is done by applying a force to the bundle of thermoelectric semiconductor materials along a direction which is perpendicular to an axis of each of the bar-shaped thermoelectric semiconductor materials.
In accordance with the first aspect of the present invention, during the extrusion step, in the thermoelectric semiconductor crystal, flows of materials occur along the direction of the force application or the axial direction of the bar-shaped thermoelectric semiconductor material, which causes orientations of the cleavage planes. In addition, during the sintering and integration step, each of the bar-shaped thermoelectric semiconductor materials is applied with the force along a direction perpendicular to its axis, which causes further flows of materials along the direction of the axis, resulting in further orientations of the cleavage planes. Thus, in comparison with a thermoelectric semiconductor obtained by a conventional method, the present invention provides an increased orientation effect as a result of the sintering and integration step, thereby improving the electric conductivity along such a direction. Thus, the first aspect of the present invention enables the thermoelectric semiconductor to increase its performance index.
As a second aspect of the present invention, a deformation step for deforming the integrated sintered body is performed by applying a deforming force thereto which causes an extension thereof in such a manner that a direction of the deforming force is perpendicular to the axis of the each of the thermoelectric semiconductor materials, the direction of the extension of the integrated sintered body being perpendicular to the direction of the deforming force.
In accordance with the second aspect of the present invention, flows of materials occur along a direction, or the extension direction which is perpendicular to the direction of the force application, which increases the orientation effect along such a direction. Due to the fact that the direction of the force application is perpendicular to the axis of each of the bar-shaped thermoelectric semiconductor materials which constitute the sintered body, the direction of the orientation of cleavage planes comes to coincide with the axis, which provides the increased orientation effect, thereby improving the electric conductivity. Thus, the second aspect of the present invention enables the thermoelectric semiconductor to increase its performance index.
As a third aspect of the present invention, the deformation step employs a molding device having cavity whose diameter is larger than a maximum diameter of a plane of the body which is perpendicular to the direction of the deforming force, the body is put in the cavity and is pressed in the cavity along the direction of the deforming force, thereby causing an extension thereof along a direction perpendicular to the direction of the deforming force.
In accordance with the third aspect of the present invention, the body of the thermoelectric semiconductor is placed in the cavity of the mold device so as to define a gap between the body and the cavity, which fills up with gas when the body is pressed. Such pressing allows a uniform extension degree of the body when mass produced. Thus, constant electric anisotropy of the thermoelectric semiconductor can be reserved, thereby stabilizing the quality and/or performance index thereof. In addition, cracks of the thermoelectric semiconductor can be prevented, thereby increasing its mechanical strength.
A fourth aspect of the present invention provides a method of producing a thermoelectric semiconductor by an extension and sintering step for obtaining a plurality of bar-shaped thermoelectric semiconductor materials by extruding and sintering concurrently either a powdered the thermoelectr

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