1. Field of the Invention
This invention relates to the design and preparation of semiconductor materials having skutterudite type crystal lattice structures with voids or cavities that may be selectively filled to enhance various thermoelectric properties of the semiconductor materials.
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, for cooling applications, 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=S2"sgr"T/xcexaxe2x80x83xe2x80x83(1)
where S, "sgr", xcexa, 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 charge carriers (electrons or holes) can transfer energy as they move through a thermoelectric element which is subjected to a temperature gradient. The type of carrier (electron or hole) is a function of the dopants in the semiconductor materials selected to form each thermoelectric element.
The electrical properties (sometimes referred to as power factor, electrical characteristics, electronic properties, or electronic characteristics) associated with materials used to form thermoelectric elements may be represented by S2"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 (ZT) is also related to the strength of interactions between the carriers and vibrational modes 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 (xcexa) is a function of both an electronic component (xcexae) primarily associated with the respective carriers and a lattice component (xcexag) 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:
xcexa=xcexae+xcexagxe2x80x83xe2x80x83(2)
The thermoelectric figure of merit (ZT) may also be stated by the equation:                     ZT        =                                            S              2                        ⁢            T                                ρ            ⁢                          xe2x80x83                        ⁢            κ                                              (        3        )            
xcfx81=electrical resistivity
"sgr"=electrical conductivity       electrical    ⁢          xe2x80x83        ⁢    conductivity    =                    1                  electrical          ⁢                      xe2x80x83                    ⁢          resistivity                    ⁢              xe2x80x83            ⁢      or      ⁢              xe2x80x83            ⁢      σ        =          1      ρ      
Thermoelectric materials for cooling and power generation applications such as alloys of Bi2Te3, 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 power generation devices. Commercially available thermoelectric materials are generally limited to use in a temperature range between 200xc2x0 K and 1300xc2x0 K with a maximum ZT value of approximately one. The efficiency of such thermoelectric devices remains relatively low at approximately five to eight percent (5-8%) energy conversion efficiency. For the temperature range of xe2x88x92100xc2x0 C. to 1000xc2x0 C., maximum ZT for many state of the art thermoelectric materials also remains limited to values of approximately 1, except for Texe2x80x94Agxe2x80x94Gexe2x80x94Sb alloys (TAGS) which may achieve a ZT of 1.2 to 1.4 in a relatively narrow temperature range. Recently developed materials such as Si80Ge20 alloys used in thermoelectric generators to power spacecrafts for deep space missions have an average thermoelectric figure of merit (ZT) of approximately equal to 0.5 from 300xc2x0 C. to 1,000xc2x0 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 skutterudite type crystal lattice structure of such semiconductor compounds is generally relatively large which often results in a thermoelectric figure of merit (ZT) which is less than desired.
Research and development has previously been conducted on fabricating thermoelectric devices with thermoelectric elements formed from materials having skutterudite type crystal lattice structures. Examples of such developments are shown in U.S. Pat. No. 5,610,366 entitled High Performance Thermoelectric Materials and Methods of Preparation issued Mar. 11, 1997. Patent application Ser. No. 08/101,901 filed Aug. 3, 1993, entitled A Semiconductor Apparatus Utilizing Gradient Freeze and Liquid-Solid Techniques, now U.S. Pat. No. 5,769,943, and patent application Ser. No. 08/412,700 filed Mar. 29, 1995, entitled Advanced Thermoelectric Materials with Enhanced Crystal Lattice Structure and Methods of Preparation, now U.S. Pat. No. 5,747,728, show additional examples of skutterudite type crystal lattice structures which may be used to fabricate thermoelectric devices.
One reference presents X-ray analysis of synthesized samples with the major phase being partially filled phosphorous based skutterudite type crystal lattice structures. See, xe2x80x9cSynthesis and Crystal Structure of a New Series of Ternary Phosphides in the System TR-Co-P (TR: Rare Earth)xe2x80x9d by S. Zemni, D. Tranqui, P. Chaudouet, R. Madar, and J. P. Senateur, Journal Solid State Chemistry, 65, 1 (1986).
In accordance with teachings of the present invention, the design and preparation of semiconductor materials used in fabrication of thermoelectric devices have been substantially improved to provide significantly enhanced operating efficiencies. The present invention provides the ability to obtain increased efficiency from a thermoelectric device having one or more thermoelectric elements fabricated from materials with skutterudite type crystal lattice structures modified in accordance with the teachings of the present invention to optimize selected thermoelectric characteristics of the resulting thermoelectric device. A significant reduction in thermal conductivity (xcexa) may be achieved by filling a portion of the voids associated with skutterudite type crystal lattice structures as compared to materials having skutterudite type crystal lattice structures with either essentially no filling of the associated voids or approximately one hundred percent (100%) filling of the associated voids. By selecting atoms and/or molecules in accordance with teachings of the present invention to fill a portion of such void spaces, phonon propagation through the resulting skutterudite type crystal lattice structure may be significantly affected to reduced thermal conductivity (xcexa) while at the same time minimizing any reduction in electrical properties (S2"sgr") of the associated materials. Examples of semiconductor materials and compounds which may be satisfactory for use with the present invention include, but are not limited to, CoP3, CoAs3, CoSb3, RhP3, RhAs3, RhSb3, IrP3, IrAs3 and IrSb3 and alloys of these compounds.
In accordance with one aspect of the present invention, a selected portion of the voids or cavities typically associated with a skutterudite type crystal lattice structure may be filled to optimize the reduction in thermal conductivity (xcexa) of the associated semiconductor material while at the same time minimizing any reduction in electrical properties which results in maximizing the thermoelectric figure of merit (ZT) for the resulting thermoelectric device. Semiconductor materials having a skutterudite crystal lattice structure with a portion of the associated voids filled in accordance with teachings of the present invention have demonstrated an order of magnitude decrease in thermal conductivity (xcexa) measured at room temperature and a much larger decrease at lower temperatures. Thermoelectric devices with thermoelectric elements fabricated from materials with skutterudite type crystal lattice structures partially filled in accordance with teachings of the present invention may have substantially enhanced thermoelectric operating characteristics and improved efficiencies as compared to previous thermoelectric devices.
In accordance with another aspect of the present invention, both N-type and P-type semiconductor materials with skutterudite type crystal lattice structures may have a portion of their associated void spaces filled. Thermoelectric elements fabricated from such semiconductor materials may be used for manufacturing thermoelectric devices with substantially enhanced operating characteristics and improved efficiencies as compared to previous thermoelectric devices.
Technical advantages of the present invention include the use of semiconductor materials having skutterudite type crystal lattice structures in which a portion of the voids associated with such crystal structures have been filled with atoms and/or molecules selected in accordance with teachings of the present invention. Fabricating a thermoelectric device from such semiconductor materials may substantially enhance the associated operating efficiency. Such thermoelectric devices may be used for cooling, heating, electrical power generation and temperature sensing.
Various embodiments of the present invention have been reduced to practice and selected tests have been performed to prove the principles and concepts associated with the present invention. The xe2x80x9cdisorderxe2x80x9d resulting from atoms, selected in accordance with teachings of the present invention, to fill voids or cavities in skutterudite type crystal lattice structures associated with various semiconductor compounds have a substantial influence on reducing thermal conductivity (xcexa) of the resulting lattice structure. Filling only a portion of the voids or cavities in accordance with teachings of the present invention may result in less scattering of the carriers when subjected to both a temperature gradient and a difference in DC voltage as compared to skutterudite type crystal lattice structures with approximately one hundred percent (100%) filling of such voids or cavities. Some of these embodiments and selected test results will be discussed later in more detail.
Depending upon the desired thermoelectric device, various semiconductor materials may be formed in accordance with the teachings of the present invention include, but are not limited to, the following described general formulas. One variation of the present invention may be used to form a semiconductor compound with the general formula
X41By1A3xX212xe2x88x923x
with y selected to be in the range of greater than zero and less than one to fill the selected portions of normally void subcells to optimize the thermoelectric figure of merit. Another variation of the present invention is a semiconductor compound having a partially filled skutterudite type crystal lattice structure with the general formula:
DyExX14xe2x88x92xX212
with D selected from the group consisting of La, Ce, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, K, Th, Pa, U, Sn, Pb, Bi, Br and I; E selected from the group consisting of Co, Rh and Ir; X1 selected from the group consisting of Ni, Pd, Ru, Os, Pt and Fe; X2 selected from the group consisting of Sb, P, As, Bi, Sn, In, Ga, Ge, Si or a combination thereof; and, y selected to be in the range of greater than zero and less than one to fill the selected portion of normally void subcells to optimize the thermoelectric figure of merit. Another embodiment of the present invention includes the following variation to form a semiconductor compound having the general formula:
X14YbyA3xX212xe2x88x923x
with a generally stoichiometric ratio of elements X1 and X2 which normally form a binary semiconductor compound having the skutterudite crystal lattice structure; A selected from the group consisting of Bi, Sn, In, Ga, Ge, and Si; and y selected to be in the range of greater than zero and less than one to fill the portion of normally void subcells to optimize the thermoelectric figure of merit. Lastly, the present invention may utilize the following variation to form a semiconductor compound having the general formula:
YbyExX14xe2x88x92xX212
with E selected from the group consisting of Co, Rh and Ir; X1 selected from the group consisting of Sn, Ge, In, Ga and Si; X2 selected from the group consisting of Sb, P, As, Bi, Sn, In, Ga, Ge and Si, or a combination thereof; and y selected to be in the range of greater than zero and less than one to fill the selected portion of normally void subcells to optimize the thermoelectric figure of merit.