Grain oriented ceramics, thermoelectric conversion element...

Details Grain oriented ceramics, thermoelectric conversion element... Grain oriented ceramics, thermoelectric conversion element...

2002-04-26

2004-10-19

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Refractory

C501S123000, C423S263000, C423S594120, C252S519150, C252S519130, C252S521100, C252S521200, C136S236100

Reexamination Certificate

active

06806218

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to grain oriented ceramics and a production process thereof, a platelike powder for producing the same, and a thermoelectric conversion element. More specifically, the present invention relates to the following: grain oriented ceramics suitable as a thermoelectric conversion material constituting thermoelectric conversion elements used in a variety of thermoelectric generators (including solar thermoelectric generators, thermoelectric generators using temperature difference in seawater, thermoelectric generator using fossil fuels and regenerators using waste heat from factories or automobiles), accurate temperature control devices (including a photodetector, a laser diode, a field-effect transistor, a photomultiplier, a cell of a spectrophotometer and a chromatographic column), thermostats, air conditioners, refrigerators and electrical power sources for clocks; and a production process thereof; a platelike powder for producing such grain oriented ceramics; and a thermoelectric conversion element using such grain oriented ceramics as the thermoelectric conversion materials.
2. Description of Related Art
Thermoelectric conversion means direct conversion between electric energy and thermal energy, taking advantage of the Seebeck effect or the Peltier effect. The thermoelectric conversion has attracted attention as a technology for high-efficiency energy use since it is characterized as, for example: 1) discharging no excess of waste products during energy conversion; 2) allowing effective use of waste heat; 3) enabling electric power to be generated continuously until the materials deteriorate; and 4) dispensing with a moving part such as a motor or a turbine, thus being maintenance-free.
As an index for evaluating the performance of materials capable of converting between thermal energy and electric energy, namely, thermoelectric conversion materials, it is common to use a figure of merit Z (Z =S
2
&sgr;/K , where S, &sgr; and K are a Seebeck coefficient, electrical conductivity and thermal conductivity, respectively) or a dimensionless figure of merit ZT expressed as a product of the value of the figure of merit Z and the absolute temperature T at which that value is shown. The Seebeck coefficient represents the magnitude of thermoelectric power generated by temperature difference of 1 K. The thermoelectric conversion materials have their specific values of the Seebeck coefficient, and they are classified into those having positive Seebeck coefficients (p-type) and those having negative ones (n-type).
In addition, typically, the thermoelectric conversion materials are used in a state of joining between the p-type and n-type materials. Such a joining pair is commonly called a thermoelectric conversion element. The figure of merit of a thermoelectric conversion element depends on the figure of merit Z
p
of the p-type thermoelectric conversion material, the figure of merit Z
n
of the n-type thermoelectric conversion material, and the forms of the p-type and n-type thermoelectric conversion materials. It is known that, if the forms of those materials are optimized, the figure of merit of the thermoelectric conversion element increases with increasing Z
p
and/or Z
n
. Therefore, to obtain a thermoelectric conversion element having a high figure of merit, it is important to use thermoelectric conversion materials of which figures of merit Z
p
and Z
n
are high.
In these thermoelectric conversion materials, there have been known materials such as Bi—Te, Pb—Te, Si—Ge and oxide-ceramic systems. Among them, a Bi—Te system compound semiconductor exhibits excellent thermoelectric properties (ZT: approx. 0.8) near room temperature, and a Pb—Te system compound semiconductor does so in a middle-temperature range of 300-500° C. However, these compound semiconductors are difficult to use in a high-temperature range. In addition, there is a problem that those materials include expensive rare elements (such as Te, Sb or Se) or highly toxic substances which place a load on the environment (such as Te, Sb, Se or Pb).
On the other hand, a compound semiconductor of a Si—Ge system exhibits excellent thermoelectric properties in the high-temperature range around 1000° C., and its materials contain no environmentally hazardous substances. However, for prolonged use of the compound semiconductor of the Si—Ge system at high temperatures in air, it is required to protect the surfaces of its materials, which deteriorates the performances of the thermoelectric element.
In contrast to this, thermoelectric conversion materials of an oxide-ceramics system do not necessarily contain a rare element or an environmentally hazardous substance. In addition, their thermoelectric properties do not deteriorate greatly even if they are used for prolonged periods of time at high temperatures in air, meaning that they are excellent in heat endurance. Therefore, the thermoelectric conversion materials of oxide-ceramic systems have received attention as materials that can replace compound semiconductors, and there have been various propositions about new materials having excellent thermoelectric properties and about the processes for producing those materials.
For example, A. C. Masset et al. prepared a polycrystalline body and a single crystal of Ca
3
Co
4
O
9
that is a kind of layered oxide containing cobalt (hereinafter referred to as a “layered cobaltite”), and they evaluated the crystal structure and thermoelectric properties (see A. C. Masset et al., Phys. Rev. B, 62(1), pp.166-175, 2000). This literature mentions that Ca
3
Co
4
O
9
is a lattice misfit-layered oxide in which Ca
2
CoO
3
layers having a rock-salt crystal structure and CoO
2
layers having a CdI
2
-type crystal structure are stacked at a predetermined cycle along a c-axis.
In addition, the same literature mentions that specific resistance of Ca
3
Co
4
0
9
is anisotropic; the specific resistance is much smaller within the {001} plane than in the direction perpendicular to the {001} plane (i.e. along the c-axis). Furthermore, it also mentions that the Seebeck coefficient in the direction of the {001} plane of the Ca
3
Co
4
09 single crystal reaches approximately 125 &mgr;V/K in the neighborhood of 300 K and that the Seebeck coefficient has small dependence on temperature.
The “{001} plane” of the layered cobaltite denotes a plane having excellent thermoelectric properties, that is, a plane parallel to the CoO
2
layer. Many kinds of the layered cobaltite have not been clarified concerning their crystal structures. Moreover, their crystallographic axes and crystal planes are defined variously depending on what unit lattice is adopted. Nevertheless, the {001} plane is defined as above in the present invention.
Also, Japanese Patent Application Unexamined Publication No. 2001-19544, for example, discloses a sintered complex oxide of which composition is expressed by such a general formula as Bi
2
Sr
2−x
Ca
x
Co
2
O
w
, Bi
2−y
Pb
y
Sr
2
Co
2
O,
w
or Bi
2
Sr
2−z
La
z
Co
2
O
w
(where 0≦x≦2, 0≦y≦0.5, 0<z≦0.5) and which has a layered crystal structure and electrical conductivity of 1.0×10
4
S/m or higher. This publication also discloses a process of producing a complex oxide, in which to pelletize the powders including sources of Bi, Sr, Ca and Co, to heat the green body in oxygen with uniaxial pressing so as to partially melt part of the materials, and to make it cool slowly.
In addition, Japanese Patent Application Unexamined Publication No. 2000-269560 discloses a complex oxide assembly obtained by die-pressing NaCo
2
O
4
crystals with 5 mm average grain size and 20 &mgr;m average thickness which is synthesized by the flux method, and by hot-pressing the compact body. This publication also discloses a process of producing a thin film of a complex oxide by forming a NaCo
2
O
4
thin film on a substrate using the sputtering method.
The layered cobaltite such as Ca
3
Co

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