Batteries: thermoelectric and photoelectric – Thermoelectric – Electric power generator
Utility Patent
1999-05-04
2001-01-02
Gorgos, Kathryn (Department: 1741)
Batteries: thermoelectric and photoelectric
Thermoelectric
Electric power generator
C136S203000, C136S238000, C136S239000, C136S241000
Utility Patent
active
06169245
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
This invention relates to semiconductor materials having enhanced thermoelectric properties for use in fabricating thermoelectric devices.
2. Description of the Related Art
The basic theory and operation of thermoelectric devices has been developed for many years. Presently available thermoelectric devices used for cooling typically include an array of thermocouples which operate in accordance with the Peltier effect. Thermoelectric devices may also be used for heating, power generation and temperature sensing.
Thermoelectric devices may be described as essentially small heat pumps which follow the laws of thermodynamics in the same manner as mechanical heat pumps, refrigerators, or any other apparatus used to transfer heat energy. A principal difference is that thermoelectric devices function with solid state electrical components (thermoelectric elements or thermocouples) as compared to more traditional mechanical/fluid heating and cooling components. The efficiency of a thermoelectric device is generally limited to its associated Carnot cycle efficiency reduced by a factor which is dependent upon the thermoelectric figure of merit (ZT) of materials used in fabrication of the associated thermoelectric elements. Materials used to fabricate other components such as electrical connections, hot plates and cold plates may also affect the overall efficiency of the resulting thermoelectric device.
The thermoelectric figure of merit (ZT) is a dimensionless measure of the effectiveness of a thermoelectric device and is related to material properties by the following equation:
ZT=S
2
&sgr;T/&kgr; (1)
where S, &sgr;, &kgr;, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively. The Seebeck coefficient (S) is a measure of how readily the respective carriers (electrons or holes) can transfer energy as they move through a thermoelectric element which is subjected to a temperature and electric potential gradient. The type of carrier (electron or hole) is a function of the materials selected to form each thermoelectric element.
The electrical properties (sometimes referred to as electrical characteristics, electronic properties, or electronic characteristics) associated with materials used to form thermoelectric elements may be represented by S
2
&sgr;. Many of the materials which are used to form thermoelectric elements may be generally described as semiconductor compounds or semiconductor materials. Examples of such materials will be discussed later in more detail.
The thermoelectric figure of merit is also related to the strength of interactions between the carriers and vibrations of the crystal lattice structure (phonons) and available carrier energy states. Both the crystal lattice structure and the carrier energy states are a function of the materials selected to form each thermoelectric device. As a result, thermal conductivity (&kgr;) is a function of both an electronic component (&kgr;
e
) primarily associated with the respective carriers and a lattice component (&kgr;
g
) primarily associated with the respective crystal lattice structure and propagation of phonons through the respective crystal lattice structure. In the most general sense, thermal conductivity may be stated by the equation:
&kgr;=&kgr;
e
+&kgr;
g
(2)
The thermoelectric figure of merit (ZT) may also be stated by the equation:
ZT
=
S
2
T
ρ
κ
ρ
=
electrical resistivity
σ
=
electrical conductivity
electrical conductivity
=
1
electrical resistivity
or
σ
=
1
ρ
(
3
)
Thermoelectric materials such as alloys of Bi
2
Te
3
, PbTe and BiSb were developed thirty to forty years ago. More recently, semiconductor alloys such as SiGe have been used in the fabrication of thermoelectric devices. Commercially available thermoelectric materials are generally limited to use in a temperature range between 200K and 1300K with a maximum ZT value of approximately one. The coefficient of performance of such thermoelectric devices remains relatively low at approximately one, compared to approximately three for a mechanical device. For the temperature range of −100° C. to 1000° C., maximum ZT for many state of the art thermoelectric materials also remains limited to values of approximately 1, except for Te—Ag—Ge—Sb alloys (TAGS) which may achieve a ZT of 1.2 to 1.4 in a relatively narrow temperature range. Materials such as Si
80
Ge
20
alloys used in thermoelectric generators to power spacecrafts for deep space missions have an average thermoelectric figure of merit of approximately equal to 0.5 from 300° C. to 1,000° C.
Many crystalline materials with low thermal conductivity do not have good electrical conductivity and many crystalline materials with good electrical conductivity often have relatively high values of thermal conductivity. For example, many binary semiconductor compounds which have skutterudite type crystal lattice structures have relatively good electrical properties. However, the value of thermal conductivity associated with the crystal lattice structures of such semiconductor compounds is generally relatively large which often results in a thermoelectric figure of merit which is less than desired.
The terms “Zintl phase” and “Zintl compound” are often used to describe intermetallic compounds having metal polyanions which have no exopolyhedral ligands at the respective vertices. As a result, it is relatively easy for such polyanions to form metal to metal bonds with atoms of the main metal group and transition metal group. U.S. Pat. No. 5,368,701 entitled Process for Forming Zintl Phases And The Products Thereof provides additional information concerning such materials and their electrical characteristics.
Alternatively, the terms “Zintl phase” and “Zintl compound” may be used to describe a binary compound formed between the alkali or alkaline-earth elements and the main-group elements from group 14 to the right of the “Zintl boundary.” F. Laves,
Naturwissenschaften
29 (1941), p. 244. Some of the features that typify Zintl phases began to be introduced in E. Zintl, W. Z. Dullenkopf,
Z. Phys. Chem., Abt
. B 16 (1932), p. 183. The definition and both references are taken from John D. Corbett,
Chem. Rev
. 85 (1985), p. 383-397.
K
2
SnTe
5
compounds are described in Eisenmann et al., Materials Research Bulletin, vol. 18 (1983), pp. 383-387. Tl
2
GeTe
5
compounds are described in Abba Toure et al., Journal of Solid State Chemistry, vol. 84 (1990), pp. 245-252; Marsh, Journal of Solid State Chemistry, vol. 87 (1990), pp. 467-468. Tl
2
SnTe
5
compounds are described in Agafonov et al., Acta Crystallographica C, vol. 47 (1991), pp. 850-852. Zintl phases have been proposed as a place to look for advanced thermoelectric materials. See Sharp, Materials Research Society Symposium Proceedings, vol. 478, pp. 15-24.
Two other researchers have suggested possibly using Zintl phase compounds as thermoelectric materials.
SrSi
2
—Bruce Cook—Ames National Laboratory
BaSbTe
3
and CsSb
x
Te
4
—Mercouri Kanatzidis—Michigan State
Some Zintl compounds may be described as clathrate compounds and some clathrate compounds may be described as Zintl compounds. However, many clathrate compounds are not Zintl compounds and many Zintl compounds are not clathrate compounds.
SUMMARY OF THE INVENTION
In accordance with teachings of the present invention, the design and preparation of semiconductor materials for fabrication of thermoelectric devices has been substantially improved to provide enhanced operating efficiencies. Examples of such semiconductor materials include, but are not limited to, Tl
2
SnTe
5
, Tl
2
GeTe
5
, K
2
SnTe
5
, Rb
2
SnTe
5
and alloys or mixtures of these compounds.
The present invention provides the ability to obtain increased efficiency from a thermoelectric device having one or more thermoelectric elements fabr
Baker & Botts L.L.P.
Gorgos Kathryn
Marlow Industries, Inc.
Parsons Thomas H
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