Compositions – Electrically conductive or emissive compositions – Elemental carbon containing
Reexamination Certificate
2002-08-08
2004-09-28
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Elemental carbon containing
C136S236100, C423S324000, C252S521300
Reexamination Certificate
active
06797199
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to clathrate compounds, high efficiency thermoelectric materials and thermoelectric modules utilizing the clathrate compound, the semiconductor materials or hard materials utilizing the clathrate compounds, as well as manufacturing methods thereof.
2. Description of the Related Art
Recently, in high-tech fields such as electronics, the development of new high performance materials which differ greatly from conventional materials has received much attention.
For example, various methods for Using thermoelectric materials are under investigation, but conventional thermoelectric materials display poor thermoelectric conversion efficiency, and are limited to certain uses where reliability is not particularly important. Consequently, it has been deemed problematic to propose the use of thermoelectric materials for typical uses such as-waste heat power generation.
Furthermore, in order to improve the efficiency of these type of thermoelectric materials and enable their use as high efficiency thermoelectric materials, the following types of conditions need to be satisfied.
(1) a low thermal conductivity
(2) a high Seebeck coefficient
(3) a high electrical conductivity
However, the technique employed for developing conventional thermoelectric materials has involved selecting a composition based on experience, and then pursuing development of that material. As a result, the only example of a thermoelectric material currently being developed, for which the value of the dimensionless figure of merit (ZT) is greater than 1 at temperatures above 700K, is the p-type thermoelectric material GeTe—AgSbTe
2
.
Furthermore in the semiconductor field, laser devices, which are essential to optical communication technology, use silicon (Si), germanium (Ge), or group III-V compound semiconductors such as gallium arsenide (GaAs). Because the temperature range for stable operation of this type of compound semiconductors is low, and ensuring good heat dissipation is a large problem, the development of the semiconductors which will also operate at higher temperatures has been greatly needed.
Furthermore, in the case of short wavelength laser emission devices required for use in high density recordings such as optical disks and digital video disks (DVD), semiconductors with a wide forbidden bandwidth are used. Examples of this type of wide forbidden bandwidth (wide gap) semiconductors include ZnS, ZnSe, GaN, SiC and diamond.
The emission wavelength of a semiconductor laser device is determined by the inherent forbidden bandwidth of the semiconductor materials, and if the emission wavelength and the forbidden bandwidth are termed &lgr; (nm) and Eg (eV) respectively, then the relationship is described by the equation (1) below.
&lgr;(
nm
)=1240/
Eg
(
eV
) (1)
The visible light region is between wavelengths of 380~760 nm, and the corresponding forbidden bandwidth is 1.63~3.26 eV. Conventionally, emission devices emitting green light and light further towards the red end of the spectrum, with wavelengths of at least 550 nm, have used group II-V compound semiconductors with a forbidden bandwidth of no more than 2 eV, such as GaP, GaAs, or GaAlAs.
However, in order to generate a blue light emission device with a wavelength of less than 500 nm, then from the relationship shown in equation (1) it is clear that a wide forbidden bandwidth (wide gap) semiconductor with a forbidden bandwidth of at least 2.5 eV is required. Examples of this type of wide gap semiconductor include group II-VI compound semiconductors such as ZnS (forbidden bandwidth: 3.39 eV) and ZnSe (forbidden bandwidth: 3.39 eV), group III-V compound semiconductors such as GaN (forbidden bandwidth: 3.39 eV), and SiC (forbidden bandwidth: 3.39 eV).
Furthermore in the field of hard materials, although diamond is widely used, because of the associated high cost, an alternative hard material has been sought after. Although cubic boron nitride (CBN) is able to be synthesized, it remains limited to applications such as abrasive grits, and a material which can be used for members in mechanical components and sliding components which require low friction and good abrasion resistance has been keenly sought.
If conventional materials are considered within the background described above, then first it is true to say that a thermoelectric material which satisfies all the requirements for a high efficiency thermoelectric material has not yet appeared. For example in the case of metals, although offering the benefits of a large electrical conductivity, they suffer from having a large thermal conductivity and a small Seebeck coefficient. In the case of semiconductors, although offering the advantages of a small thermal conductivity and a large Seebeck coefficient, because the electrical conductivity is small, they can not be considered a high efficiency thermoelectric materials. Furthermore it is known that BiTe is used as a thermoelectric material at around room temperature. However, the efficiency thereof at 100° C. or higher is low, and it is unable to withstand practical use.
Furthermore, in order to use a thermoelectric material in typical power generation, a power generation system must be constructed by combining a p-type thermoelectric material and an u-type thermoelectric material. However, in the case of the aforementioned conventional thermoelectric material of GeTe—AgSbTe
2
, an n-type material does not exist.
In contrast, with conventional semiconductor materials, the temperature range for stable operation is low, meaning the operating environment is limited to a thermal environment close to room temperature.
Conventionally, heat generation has been suppressed in order to achieve stable operation of a semiconductor device, and so a large heat radiator has been necessary. For example, widely used silicon devices typically have a stable operating temperature range below 125° C., and so electronic equipment utilizing silicon devices has required large heat sinks. Even with the use of heat sinks, the stable operating temperature range for a silicon device is, at the most, no more than 200° C., and currently semiconductor devices do not exist which are capable of withstanding use in fields such as automobile components, high temperature gas sensors, engine control sections of space rockets, underground detection measuring apparatus, and nuclear power applications. The circumstances are the same for compound semiconductors.
Furthermore, in order to use a compound as a semiconductor, a doping atom must be introduced to make the conductivity either p-type or n-type. However, in order to introduce a p-type or n-type doping atom into GaN or SiC, a new artificial superlattice structuring is necessary, which makes the crystal growth process difficult. Furthermore, ZnS suffers from an additional problem in that the crystals cannot be obtained cheaply. In addition, in the case of diamond, control of the doping atoms is problematic.
As a result, the reality is that a semiconductor with a wide forbidden bandwidth which can be operated stably under conditions of high temperature or high pressure is not currently available.
In addition, conventional methods of manufacturing clathrate compounds include a method disclosed in Japanese Unexamined Patent Application, First Publications No. Hei-9-183607, wherein a monoclinic system crystal is produced by heating a mixture of an element from group 4B of the periodic table and an alkali metal under an atmosphere of argon, while a cubic system crystal is produced by heating a mixture of an element from group 4B of the periodic table and an alkali earth metal under an atmosphere of argon, and the following mixing of the monoclinic system crystal and the cubic system crystal and subsequent heating to form a precursor comprising a ternary solid solution, this precursor was heated under reduced pressure to effect an alkali metal element distillation and produce the clathrate compound. According to this method, the production process
Eguchi Haruki
Kihara Shigemitsu
Miyahara Kaoru
Suzuki Akihiko
Takahashi Satoshi
Ishikawajima-Harima Heavy Industries Co. Ltd.
Kopec Mark
Pearne & Gordon LLP
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