Batteries: thermoelectric and photoelectric – Thermoelectric – Processes
Reexamination Certificate
1999-10-08
2001-11-06
Bell, Bruce F. (Department: 1741)
Batteries: thermoelectric and photoelectric
Thermoelectric
Processes
C136S203000, C136S205000
Reexamination Certificate
active
06313392
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thermoelectric semiconductor material, a thermoelectric element, a method of manufacturing these and a method of manufacturing a thermoelectric module and a device for manufacturing a thermoelectric semiconductor material, and, in particular, relates to a material, a method of manufacture, and a manufacturing device that effectively contribute to improvement of thermoelectric performance.
2. Description of the Related Art
Thermoelectric elements making use of the thermoelectric phenomenon have conventionally been utilized in heat exchangers and/or temperature sensors. The thermoelectric phenomenon is a general term for the Peltier effect, Thomson effect and Seebeck effect. These will be described as follows.
The Peltier effect is the phenomenon whereby, when current flows to a junction of different metals, heat is generated or absorbed at this junction; the Thomson effect is the phenomenon whereby when current is passed to metal having a temperature gradient, generation or absorption of heat occurs within this metal. A Peltier element which is used as an electronic cooler is a thermoelectric element utilizing the above Peltier effect.
The Seebeck effect is a phenomenon whereby an electromagnetic force is generated at the high-temperature side and low-temperature side of a sample when a junction of different metals is maintained at different temperatures; thermocouples that are employed as temperature sensors are thermoelectric elements utilizing this Seebeck effect. Since such thermoelectric elements are easy to handle and have a simple construction and stable characteristics, research and development is proceeding in many places aimed at applying these to temperature regulation of semiconductor lasers and small freezers.
As the material for forming such thermoelectric elements, alloys are currently employed comprising one or two selected from the group consisting of bismuth (Bi) and antimony (Sb) and one or more selected from the group consisting of tellurium (Te) and selenium (Se). These compounds are laminar structure compounds and constitute semiconductor materials having anisotropy in their thermoelectric characteristics produced by their crystal structure.
Various techniques such as unidirectional solidification, hot pressing, or extrusion are known in order to process the semiconductor material consisting of such a laminar structure compound in order to increase fineness of the crystal grains and the degree of their alignment.
Unidirectional solidification is a method of forming an ingot in which the direction of crystal growth is controlled; by this method, polycrystalline material of excellent alignment is obtained. As a specific example of the uniaxial solidification method, the Bridgeman method is known. However, polycrystalline materials produced by such uniaxial solidification methods are subject to the problem of poor material strength. The polycrystalline materials obtained by this method are therefore undesirable for use as thermoelectric semiconductor elements without modification.
Hot pressing is a method of producing polycrystalline material wherein improvement in the material strength is sought to be achieved by uniaxial compression of powder etc. of an ingot. The reason for applying uniaxial pressure is to forcibly align the crystal orientations by external pressure. By means of such methods, the problem referred to above of the material strength of the uniaxial method being weak is solved, and polycrystalline material of excellent alignment is obtained.
The extrusion method is a method wherein powder or material formed of this powder is introduced into a die and pressure molding is performed whilst extruding the material in this die using a punch. Prior art references disclosing this method of extrusion include Japanese patent application laid-open No. 138789/1988, Japanese patent application laid-open No. 186299/1996, and Japanese patent application laid-open No. 56210/1998. By means of this method, since a strong force is applied to the material as a whole, finer crystal grains can be obtained and material strength is also improved.
Consequently, hot pressing, cold pressing, and the extrusion method are currently widely employed as methods of manufacturing thermoelectric semiconductor elements, for reasons of alignment of the crystals and material strength.
However, in recent years, thermoelectric elements having even better thermoelectric properties are being demanded, and a novel technique in which the prior art described above is further developed is therefore sought.
A first object of the present invention is therefore to provide a thermoelectric semiconductor material or method of manufacturing an element and method of manufacturing a thermoelectric module which should be effective for improving thermoelectric performance.
Also, a thermoelectric element (thermoelectric module) employed in electronic cooling and thermoelectric power generation, as shown in
FIG. 29
, is constituted by forming PN element pairs by joining P-type semiconductors
110
and N-type semiconductors
120
through metallic electrodes
130
, and arranging a plurality of these PN element pairs in straight rows; depending on the direction of the current flowing through the junction, heat is generated at one end while the other is cooled. For the material of such thermoelectric elements, materials are employed of large figure of merit Z (=&agr;
2
/&rgr;&kgr;) expressed by the Seebeck coefficient &agr;, resistivity &rgr; and thermal conductivity &kgr;, which are specific characteristics of the substance, in the temperature range of its utilization.
Most thermoelectric semiconductor materials possess anisotropy of thermoelectric performance due to their crystalline structure. Specifically, the figure of merit Z is different depending on crystal orientation. Single-crystal materials are therefore employed by passing current in a crystal orientation which gives large thermoelectric performance. In general, anisotropic crystals possess a tendency to cleavage and are of low material strength, so, as members for practical use, rather than single-crystal materials, polycrystalline materials wherein the crystal orientations are aligned to give large thermoelectric performance by uniaxial solidification using the Bridgeman method etc.
However, polycrystalline materials are still brittle in material strength albeit not to the degree of single-crystals, and so suffer from the problems of cracking or chipping of the elements during element processing.
Specifically, a polycrystalline material that is typically used for thermoelectric cooling elements is Bi
2
Te
3
based thermoelectric material, which consists of a mixed crystal system of bismuth telluride (Bi
2
Te
3
), antimony telluride (Sb
2
Te
3
), and bismuth selenide (Bi
2
Se
3
). This Bi
2
Te
3
based thermoelectric material is of hexagonal crystal structure, having a structure in which layers of Bi and layers of Te are laminated perpendicular to the hexagonal crystal C axis. Due to this crystalline structure, it possesses electrical and thermal isotropy, the thermoelectric performance being better in the direction of the C plane than in the direction of the C axis. Thermoelectric elements are therefore employed produced by ingots in which the direction of crystal growth is controlled to be the orientation which gives best thermoelectric performance, by a uniaxial solidification method. However, since, in regions where adjacent Te layers are laminated in the crystalline structure, the Te atoms are mutually coupled by Van der Waals forces, they are subject to severe cleavage. There were therefore the problems that yield was very poor due to occurrence of cracking or chipping in the-cutting step etc. to obtain the thermoelectric elements from this brittle crystalline material and that the thermoelectric elements (thermoelectric modules) did not possess durability.
There have therefore previously been attempts to obtain elements of improved mater
Fukuda Katsushi
Ikeda Keisuke
Kajihara Takeshi
Konishi Akio
Sasaki Kiyoharu
Bell Bruce F.
Komatsu Ltd.
Parsons Thomas H
Varndell & Varndell, P
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