Batteries: thermoelectric and photoelectric – Thermoelectric – Having particular thermoelectric composition
Reexamination Certificate
2002-03-18
2004-08-17
Chaney, Carol (Department: 1745)
Batteries: thermoelectric and photoelectric
Thermoelectric
Having particular thermoelectric composition
C252S521100, C423S594600
Reexamination Certificate
active
06777609
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thermoelectric conversion material to be employed at high temperature for so-called thermoelectric conversion (i.e., direct energy conversion without use of any movable parts), including power generation on the basis of Seebeck effect and electronic freezing on the basis of Peltier effect. The invention also relates to a thermoelectric conversion device containing the material.
2. Background Art
Thermoelectric conversion by use of a thermoelectric conversion material; e.g., thermoelectric power generation or electronic freezing, finds utility in a simplified direct-energy-conversion apparatus having no movable parts that generate vibration, noise, wear, etc.; having a simple, reliable structure; having a long service life; and facilitating maintenance. Thus, thermoelectric conversion is suitable for direct generation of DC power without combustion of a variety of fossil fuels or other sources and for temperature control without use of a cooling medium.
Characteristics of thermoelectric conversion materials are evaluated on the bases of power factor (Q) and figure of merit (Z), which are represented by the following formulas:
Q=&sgr;&agr;
2
[Formula 1]
Z
=
σ
α
2
κ
[Formula 2]
wherein &agr; represents Seebeck coefficient; &sgr; represents electric conductivity; and &kgr; represents thermal conductivity. Thermoelectric conversion materials are desired to have a high figure of merit (Z); i.e., a high Seebeck coefficient (&agr;), high electric conductivity (&sgr;), and low thermal conductivity (&kgr;).
For example, when employed for thermoelectric conversion such as thermoelectric power generation, a thermoelectric conversion material is desired to have a figure of merit as high as Z=3×10
−3
1/K or higher and to operate without variation for a long period of time under varying operation conditions. Mass production of thermoelectric power generators for use in vehicles or employing discharged heat gives rise to demand for a thermoelectric conversion material which has sufficiently high heat resistance and strength, particularly at high temperature, and resistance to deterioration in characteristics, as well as a method for producing the material at high efficiency and low cost.
Conventionally, PbTe or silicide materials including silicide compounds such as MSi
2
(M: Cr, Mn, Fe, or Co) and mixtures thereof have been used to serve as the aforementioned thermoelectric conversion materials.
Sb compounds such as TSb
3
(T: Co, Ir, or Ru) have also been used. For example, there has been disclosed a thermoelectric material which comprises a material containing CoSb
3
as a predominant component and an impurity added for determination of conduction type (L. D. Dudkin and N. Kh. Abriko Sov, Soviet Physics Solid State Physics (1959) p. 126; B. N. Zobrinaand, L. D. Dudkin, Soviet Physics Solid State Physics (1960) p. 1668; and K. Matsubara, T. Iyanaga, T. Tsubouchi, K. Kishimoto, and T. Koyanagi, American Institute of Physics (1995) p. 226-229).
Thermoelectric conversion materials formed of PbTe exhibit a high figure of merit (Z)—an index of thermoelectric properties—of approximately 1×10
−3
1/K at about 400° C. However, the materials have a low melting point and poor chemical stability attributed to Te, a volatile component contained in the materials, and cannot be used at high temperature of 500° C. or higher. In addition, since cumbersome production steps are required due to presence of a volatile Te component in the materials, variation in product characteristics tends to be caused by variation in composition of the materials, failing to attain effective mass-production. Another disadvantage is that the raw materials for producing the thermoelectric conversion materials are expensive and highly toxic.
Silicide materials including silicide compounds such as MSi
2
(M: Cr, Mn, Fe, or Co) and mixtures thereof can be produced from inexpensive raw materials; contain no toxic components; are chemically stable; and can be used at temperatures of about 800° C. “
Netsuden Handotai To Sono Oyo
,” authored by Kunio NISHIDA and Kin-ichi UEMURA, (1983) p. 176-180 (published by Nikkan Kogyo Shimbun) discloses a comparatively inexpensive method of producing these silicide materials. However, these silicide materials exhibit a thermoelectric property (e.g., a figure of merit (Z) of approximately 1-2×10
−4
1/K) of about one-tenth that of PbTe and cannot provide sufficient thermoelectric properties comparable to those of PbTe.
Thermoelectric materials containing an Sb compound such as TSb
3
(T: Co, Ir, or Ru) as a predominant component (e.g., CoSb
3
) are produced from non-toxic, comparatively inexpensive raw materials and are known to exhibit a comparatively high figure of merit (<1×10
−3
1/K).
In the production of a conventionally known thermoelectric conversion material having a chemical composition of CoSb
3
, it is desirable that cubic CoSb
3
crystal phase is exclusively formed in the material to serve as a constitutional crystal phase, with other crystal phases (CoSb, CoSb
2
, and Sb), which are detrimental to thermoelectric properties, being removed. However, in reality, when a production method involving melting CoSb
3
is employed, undesired phases (CoSb, CoSb
2
, and Sb) other than CoSb
3
phase are precipitated during the course of solidification. In order to generate a crystal phase formed only of CoSb
3
from such a molten material, heat treatment at approximately 600° C. for about 200 hours is required, and such treatment disadvantageously prolongs the time required for production steps.
In addition, when a production method in which a solidified CoSb
3
melt is pulverized and sintered is applied, undesired phases (CoSb, CoSb
2
) precipitated during solidification and having a higher density than that of CoSb
3
are transformed into CoSb
3
phase during firing. This phase transformation causes volume expansion, thereby disadvantageously inhibiting sintering. Specifically, sufficiently densified material has never been produced, even when pulverized CoSb
3
is hot-pressed at 5×10
3
kg/cm
2
and 600° C. (Reference: K. Matsubara, T. Iyanaga, T. Tsubouchi, K. Kishimoto, and T. Koyanagi, American Institute of Physics (1995) p. 226-229). The maximum density of the thus-sintered CoSb
3
, reported in the reference, is 5.25 g/cm
3
, whereas the theoretical density of cubic CoSb
3
is 7.64 g/cm
3
. Thus, the sintered CoSb
3
is a considerably fragile material, and has poor strength at high temperature.
In order to attain satisfactory durability of a material formed of heavy elements such as Bi, Te, Se, and Pb against contact with industrial process discharge gas and to prevent vaporization of constitutional components in a high-temperature reaction atmosphere and contamination with the vaporized components, there has been desired a new material which can be produced at low cost; causes less environmental pollution; and can be used without causing variation even at high temperature.
In view of the foregoing, a strong tendency to use an oxide as a thermoelectric material has rapidly arisen. Generally, an oxide has low mobility and a typical carrier concentration of about 10
19
cm
−3
, exhibiting no conductivity, unlike a metal. Thus, it has been commonly accepted in the art that an oxide cannot serve as thermoelectric conversion material. However, in 1997, an oxide of layer structure, NaCo
2
O
4
, was surprisingly found to exhibit strong thermoelectromotive force despite its low resistivity (Japanese Patent Application Laid-Open (kokai) No. 2000-211971). Thermoelectric properties of this class of oxide are remarkably superior to those of other oxides, and approach those of existing thermoelectric material used in practice.
However, this oxide also has a drawback in that thermoelectric properties of products vary greatly in accordance with production conditions, due to sublima
Arita Shinsuke
Ichinose Noboru
Ueno Toshiaki
Urano Yuji
Chaney Carol
Hokushin Corporation
Huntley & Associates LLC
Parsons Thomas H.
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