Batteries: thermoelectric and photoelectric – Thermoelectric – Processes
Reexamination Certificate
1999-03-15
2001-08-14
Gorgos, Kathryn (Department: 1741)
Batteries: thermoelectric and photoelectric
Thermoelectric
Processes
C136S205000, C136S238000, C136S240000
Reexamination Certificate
active
06274802
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thermoelectric semiconductor material, its method of manufacture and a thermoelectric module using this, and, in addition, to a method of hot forging.
2. Description of the Related Art
Application of electronic cooling elements utilizing the Peltier effect or Ettinghausen effect, or thermoelectric power generating elements utilizing the Seebeck effect over a wide range is noted on account of their simple construction, ease of handling, and ability to maintain stable characteristics. In particular regarding electronic cooling elements, research is being conducted in many places aimed at temperature stabilization etc. of optoelectronics or semiconductor lasers etc., on account of the capability that they possess for precise temperature control of local cooling and in the vicinity of room temperature.
As shown in
FIG. 12
, a thermoelectric module employed in such electronic cooling and thermoelectric power generation is constructed such that a pn element pair is formed by joining a p-type semiconductor
5
and n-type semiconductor
6
through a metallic electrode
7
, a plurality of such pn elements being arranged in series, heat being generated at one end while cooling occurs at the other end, depending on the direction of the current flowing through the junctions. A material of large figure of merit Z(=&agr;
2
/&rgr;K) expressed by the Seebeck coefficient &agr;, resistivity &rgr;and thermal conductivity K, which are constants characteristic of the material, is employed as the material of such a thermoelectric element.
Most thermoelectric semiconductor materials have anisotropy of thermoelectric performance depending on their crystal structure i.e. the figure of merit Z is different depending on crystal orientation. A single crystal material is therefore employed with current being passed in a crystal orientation of large thermoelectric performance. In general, anisotropic crystals are subject to cleavage and are brittle, so, as a practical material, instead of employing single crystals, a material is employed in which alignment is effected in a crystal orientation of large thermoelectric performance by unidirectional solidification achieved by the Bridgman method etc.
However, albeit not to the same degree as a single crystal, such a unidirectional solidified material is still brittle, and problems are experienced during element working regarding cracking and/or chipping of the elements. In contrast to such crystalline material, powder sintered material has no cleavage and the material strength is enormously better, but the alignments of the crystal orientations are random or, if there is crystal alignment, this shows a gently sloping distribution, so there was the problem that the thermoelectric performance was inferior to that of crystalline materials. Thermoelectric semiconductor materials having both satisfactory strength and performance were thus hitherto unavailable. Specifically, the crystalline materials that were typically employed as electronic cooling elements were mixed crystals of bismuth telluride (Bi
2
Te
3
), antimony telluride (Sb
2
Te
3
) and bismuth selenide (Bi
2
Se
3
), but these crystals have the problems of being subject to severe cleavage and that the yield was very considerably lowered owing to cracking and chipping on undergoing slicing and dicing steps etc. to obtain the thermoelectric element from the ingot.
Formation of powder sintered elements has therefore been tried in order to improve mechanical strength. When the material is employed in the form of a powder sintered body instead of crystals, the problem of cleavage is eliminated, but, as mentioned above, performance is inferior due to the poor alignment. In other words, there was the problem of a low figure of merit Z.
In view of the foregoing, it is an object of the present invention to provide a thermoelectric semiconductor material exhibiting satisfactory strength and performance and of high manufacturing yield.
SUMMARY OF THE INVENTION
Definitions of terms used in this specification and in the claims will now be given.
A “crystal grain” is a unit constituted by crystalline structure, its circumference being enclosed by a grain boundary. For example, in the case of a powder sintered material, these originate in the particles of the powder when this sintered material is formed.
Also, “a subcrystal grain” is a constituent unit of a crystal grain; in this specification, it means a unit crystal from the crystallographic point of view.
Accordingly, although both the case where the crystal grains consist of single subcrystal grains and the case where they consist of a plurality of subcrystal grains may be considered, both of these cases are included in this specification.
Consequently, if the subcrystal grains are aligned, the crystal grains will be aligned.
A first aspect of the present invention comprises: a heating step in which material powder is mixed so as to have a desired composition, and melted by heating; a solidification step, in which a solid solution ingot of thermoelectric semiconductor material having a rhombohedral structure (hexagonal crystal structure) is formed; a crushing step in which the solid solution ingot is crushed to form a solid solution powder; a screening step in which the grain size of the solid solution powder is made uniform; a sintering step in which the solid solution powder whose grain size has been made uniform is subjected to pressing and sintering; and a hot upset forging step in which hot upset forging is performed by subjecting the powder sintered body to plastic deformation while hot, and extending it, thereby aligning crystal grains of the powder sintered structure in a crystal orientation of excellent figure of merit, such that a density ratio of the powder sintered body is finally at least 97%.
Preferably the hot upset forging step is an upset forging step wherein the powder sintered body is extended in one axial direction only, while hot.
Also, preferably, the hot upset forging step is a step in which extension is performed whilst pressing in a direction coinciding with the direction of pressing in the sintering step.
Further, preferably, a heat treatment step in which heat treatment is performed is further included after the hot upset forging step.
A second aspect of the present invention comprises: an ingot forming step in which a mixture whose chief constituents are bismuth, antimony, tellurium, and selenium of prescribed composition is melted by heating, and a solid solution ingot of Bi
2
Te
3
-based thermoelectric semiconductor material is formed; a crushing step in which the solid solution ingot is crushed to form a solid solution powder; a screening step in which the grain size of the solid solution powder is made uniform; a sintering step in which the solid solution powder whose grain size has been made uniform is subjected to pressing and sintering; and a hot upset forging step in which hot upset forging is performed by subjecting the powder sintered body to plastic deformation while hot, and extending it, thereby aligning crystal grains of the powder sintered structure in a crystal orientation of excellent figure of merit, such that a density ratio of the powder sintered body is finally at least 97% .
Preferably, the hot upset forging step is an upset forging step in which the powder sintered body is extended in one axial direction only, while hot.
Preferably, the hot upset forging step is a step in which extension is effected whilst pressing in a direction coinciding with the direction of application of pressure in the sintering step.
Also preferably, a heat treatment step in which heat treatment is performed is further included after the hot upset forging step.
Further, preferably, after the screening step and before the sintering step there is included a hydrogen reduction step in which the solid solution powder is subjected to heat treatment in a hydrogen atmosphere.
A third aspect of the present invention comprises an ingot formation step in
Fukuda Katsushi
Kajihara Takeshi
Sato Yasunori
Gorgos Kathryn
Komatsu Ltd.
Parsons Thomas H
Varndell & Varndell PLLC
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