Batteries: thermoelectric and photoelectric – Thermoelectric – Electric power generator
Reexamination Certificate
2001-12-02
2004-03-23
Ryan, Patrick (Department: 1745)
Batteries: thermoelectric and photoelectric
Thermoelectric
Electric power generator
C136S201000, C136S236100, C136S203000
Reexamination Certificate
active
06710238
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a thermoelectric element used for a cooling system and a temperature controller utilizing a Peltier effect, a power generator for generating electricity using a Seebeck effect of a temperature difference, a thermocouple using thermoelectromotive force, and a variety of sensors and so on, and concerns a thermoelectric material making up the thermoelectric element and a method for manufacturing the thermoelectric material.
BACKGROUND ART
When different types of semiconductors are connected to form an electric circuit and direct current is applied, a junction produces heat and the other junction absorbs heat. This phenomenon is called a Peltier effect. Electronic cooling on a target by using a Peltier effect is called thermoelectric cooling, a device formed for this object is called a thermoelectric cooling element or a Peltier element in general. Further, when a temperature difference appears between two junctions, electromotive force is generated according to the temperature difference. The phenomenon is called a Seebeck effect, and power generation using generated electromotive force is called thermoelectric power generation.
Moreover, a sensor is called a thermocouple, which senses a temperature difference between two bonding parts by bonding different types of metal to form an electric circuit and measuring thermoelectromotive force appearing between two bonding parts. In addition to the thermocouple, a variety of sensors using a Seebeck effect include a device, a module, or a system for obtaining a change in a quantity of intensive property (intensity variable) by sensing the temperature difference by a potential difference, the change having one-to-one correspondence with a temperature difference, and for giving feedback to a variety of functions.
Such a different type of metal or an element having a basic configuration for connecting a semiconductor is generally called a thermoelectric element, and metal or a semiconductor used for the element with a high thermoelectric performance is called a thermoelectric material.
Since thermoelectric cooling is causing a solid element, it is characterized In that no toxic refrigerant gas is necessary, no noise occurs, and partial a cooling is possible. Besides, since heating is possible due to a Pertier effect by switching a direction of current, a temperature can be regulated with accuracy. Applications using such characteristics, include the following: cooling and precise temperature regulation of an electronic component and temperature control of a storage cabinet such as a wine cooler, in which temperature control is important. When a thermoelectric material with high performance is used at room temperature, it is possible to achieve a refrigerator and a freezer without using any toxic gas such as a CFC.
Meanwhile, thermoelectric power generation realizes effective use of energy that includes power generation using waste heat from a heat engine such as a factory, a power plant, and a vehicle, and power generation using abundant solar energy. Further, a metallic thermoelectric material with large thermoelectromotive force and a small resistance has a high usage value as a temperature sensor such as a thermocouple having a high sensitivity.
High performance of a thermoelectric element is generally indicated by a fact that any one of thermoelectromotive force (V), a Seebeck coefficient (&agr;), a Peltier coefficient (&pgr;), a Thomson coefficient (&tgr;), a Nernst coefficient (Q), an Ettingshausen coefficient (P), electrical conductivity (&sgr;), a power factor (PF), a figure of merit (Z), and a dimensionless figure of merit (ZT) is high, or any one of a thermal conductivity (&kgr;), a Lorentz number (L), and a electrical resistivity (&rgr;) is low. The above performances of a thermoelectric element are called a variety of thermoelectric performances. Additionally, a Seebeck coefficient is also called thermoelectric power.
Particularly, a dimensionless figure of merit (ZT) is indicated by ZT=&agr;
2
&sgr;T/&kgr; (here, T indicates an absolute temperature) and is an important element for determining efficiency of thermoelectric conversion energy such as a coefficient of performance in thermoelectric cooling and conversion efficiency in thermoelectric power generation. Therefore, it is possible to increase efficiency of cooling and power generation by using a thermoelectric material with a large figure of merit (Z=&agr;
2
&sgr;T/&kgr;) to form a thermoelectric element.
Namely, as a thermoelectric material, a material having a large Seebeck coefficient (&agr;) is preferable. Further, a material having a large electrical conductivity and a large power factor accordingly (PF=&agr;
2
&sgr;) is particularly preferable. Besides, a material having a low thermal conductivity (&kgr;) is the most preferable. Moreover, in other words, a material is preferable which has a large Seebeck coefficient (&agr;) and a large ratio &sgr;/&kgr; (=1/TL; mainly in the case of metal) of an electrical conductivity and a thermal conductivity.
However, as long as a single thermoelectric material is used by a conventional method, a figure of merit (Z), particularly a power factor (PF) and a Seebeck coefficient (&agr;) are substantially determined by a kind of an element making up the material and a composition ratio thereof. Thus, any material has not been found which remarkably exceeds a conventionally used thermoelectric semiconductor Bi
2
Te
3
, PbTe, or Si—Ge in various thermoelectric capabilities.
Therefore, it has been considered that a dimensionless figure of merit (ZT) is improved by forming a multilayered structure by using a thin-film forming technique such as MBE (Molecular Beam Epitaxy) method and CVD (Chemical Vapor Deposition) method. In recent studies, a two-dimensional quantum well structure (National Publication of the Translated Version of PCT Application No. 8-505736 discloses a theoretical study (L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B, 47, (1993) p12727), in which regarding a Bi
2
Te
3
semiconductor, a dimensionless figure of merit (ZT) is about seven times larger than that of a bulk material by confining a charge carrier (electron and hole) in a two-dimensional manner. The improvement of a figure of merit (Z) is proved in an experiment as well.
Moreover, another proposal (R. Venkatasubramanian and T. Colpitts, Materials Research Society Symposium Proceedings Vol. 478(1997) p73) is made as follows: when a number of interfaces exist like a superlattice structure, in which the interfaces are brought into contact with layers each being made of a different material with a thickness of about several tens nm, since phonon used for thermal conduction is scattered on the interfaces, a thermal conductivity is made smaller than that of a bulk material.
However, in order to manufacture a repeated multilayered structure having a constant quantum size to improve a performance index or a dimensionless performance index by using the above techniques, it is necessary alternately stack films on the order of nm with the MBE method and the CVD method. Thus, a film-forming speed is slow and is disadvantageous in industry. To be specific, generally as shown in
FIG. 1
, a thermoelectric element is configured such that both ends of a P-type semiconductor and an N-type semiconductor are sandwiched by metallic electrodes. A somewhat large thickness is necessary for the semiconductor because heat returns from a high-temperature electrode to a low-temperature electrode. For example, at a film-forming speed of 0.02 &mgr;m/minute, a continuous film-forming process of about a week is required for forming a thermoelectric material having a quantum well structure with a thickness of 200 &mgr;m.
In the application for thermoelectric power generation, a material having a large power factor as well as a large figure of merit may be required. A performance index (Z) is a value obtained by dividing a power factor (PF=&agr;
2
&sgr;) by a thermal conductivity (&kgr;). When &kgr; is small
Imaoka Nobuyoshi
Ishihara Keiichi
Morimoto Isao
Shingu Hideo
Yamanaka Shozo
Asahi Kasei Kabushiki Kaisha
Dickstein Shapiro Morin & Oshinsky LLP.
Parsons Thomas H.
Ryan Patrick
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