Abstract:
A low-cost filled skutterudite for advanced thermoelectric applications is disclosed. The filled skutterudite uses the relatively low-cost mischmetal, either alone or in addition to rare earth elements, as a starting material for guest or filler atoms.

Description:
TECHNICAL FIELD 
     This application claims the benefit of a provisional application, Ser. No. 60/590,878, filed Jul. 23, 2004. 
    
    
     The present invention relates to thermoelectric devices which utilize a thermal gradient to generate electrical power and can be used in heating and cooling applications. More particularly, the present invention relates to filled skutterudites as thermoelectric materials, the fabrication of which includes use of the low-cost mischmetal alloys and mischmetal-transition metal alloys as starting materials. 
     BACKGROUND OF THE INVENTION 
     Due to an increasing awareness of global energy needs and environmental pollution in recent years, much interest has been devoted to the development and use of thermoelectric (TE) materials for automotive and other applications. TE devices are capable of transforming heat directly into electrical energy and also acting as solid state coolers. Through their energy-generating capability, TE devices are capable of enhancing the ability of internal combustion engines to convert fuel into useful power. The cooling capability of TE devices can contribute to a resolution of the greenhouse concerns associated with refrigerant use, as well as enable new design concepts for heating and air conditioning and improve the reliability of batteries. TE-based waste heat recovery is also applicable to modes of transportation such as diesel-electric locomotives, locomotive diesel engines, automotive diesel engines, diesel-electric hybrid buses, fuel cells, etc. 
     The energy conversion efficiency and cooling coefficient of performance (COP) of a TE device are determined by the dimensionless figure of merit, ZT, defined as ZT=S 2 T/ρK total =S 2 T/ρ(K L +K e ) where S, T, ρ, K total , K L , and K e  are the Seebeck coefficient, absolute temperature, electrical resistivity, total thermal conductivity, lattice thermal conductivity and electronic thermal conductivity, respectively. The larger the ZT values, the higher the efficiency or the Coefficient of Performance (COP). An effective thermoelectric material should possess a large Seebeck coefficient, a low electrical resistivity and a low total thermal conductivity. 
     Binary skutterudites are semiconductors with small band gaps of ˜100 meV, high carrier mobilities, and modest Seebeck coefficients. Binary skutterudite compounds crystallize in a body-centered-cubic structure with space group Im3 and have the form MX 3 , where M is Co, Rh or Ir and X is P, As or Sb. Despite their excellent electronic properties, binary skutterudites have thermal conductivities that are excessively high to compete with state-of-the-art thermoelectric materials. It was found that filled skutterudites have much lower thermal conductivities. Therefore, filled skutterudites are increasingly popular as a thermoelectric material due to their lower thermal conductivities. 
     Filled skutterudites can be formed by inserting rare earth guest atoms interstitially into large voids in the crystal structure of binary skutterudites. The chemical composition for filled skutterudites can be expressed as G y M 4 X 12 , where G represents a guest atom, typically a rare earth atom, and y is its filling fraction. Compared to binary skutterudites, the lattice thermal conductivities of the rare earth filled skutterudites are significantly reduced over a wide temperature range. This property of filled skutterudites is due to the scattering of heat-carrying low-frequency phonons by the heavy rare earth atoms, which rattle inside the interstitial voids in the skutterudite crystal structure. 
     In recent years, both n- and p-type rare earth filled skutterudites have been reported to have superior thermoelectric figure of merit (ZT) values in excess of 1 for temperatures above ˜500 degrees C. For rare earth filled skutterudites, the best p-type materials are La—Fe—Co—Sb and Ce—Fe—Co—Sb skutterudites. The best p-type materials are Yb—Co—Sb and Ba—Ni—Co—Sb.  FIG. 1  shows ZT values of recently-discovered filled skutterudites as compared to those of state-of-the-art thermoelectric materials. 
     The relatively high cost of high-purity starting materials for rare earth filled skutterudites contributes to the overall cost of the fabricated thermoelectric devices. Therefore, filled skutterudites are needed which utilize low-cost starting materials to decrease the overall cost of thermoelectric devices. 
     SUMMARY OF THE INVENTION 
     The present invention is generally directed to filled skutterudites which are low-cost and suitable for use as a thermoelectric material. According to the invention, mischmetal (Mm), in addition to a transition metal alloy or both rare earth and transition metal alloys, is used as a starting material for the fabrication of both n-type and p-type filled skutterudites. Mischmetal is an alloy of both Ce (˜50 wt. %) and La (˜50 wt. %). In a typical embodiment, the filled skutterudite has a composition of Mm y Co 4 Sb 12  (0&lt;y≦1). Use of mischmetal as a starting material for fabrication of the skutterudite provides a low-cost alternative to high-purity rare earth starting materials which characterize conventional skutterudite fabrication processes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a line graph which compares the thermoelectric figure of merit (ZT) for recently-discovered filled skutterudites with the thermoelectric figure of merit for state-of-the-art thermoelectric materials; 
         FIG. 2  illustrates a body-centered cubic crystal structure of a filled skutterudite fabricated according to the present invention; and 
         FIG. 3  is a flow diagram which illustrates sequential process steps carried out in a typical method of fabricating a filled skutterudite according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 2 , a portion of a filled skutterudite according to the present invention is generally indicated by reference numeral  10 . The filled skutterudite  10  is shown in the form of a body-centered cubic crystal structure having the composition G y M 4 X 12 , where G represents guest (filling) atoms  12 ; y is the filling fraction of the guest atoms  12 ; M represents transition metal atoms  14 ; and X represents atoms from groups IVA-VIA of the periodic table  16 . In the filled skutterudite  10 , a transition metal atom  14  is enclosed in each X 6  tetrahedron formed by the X atoms  16 . The guest atoms  12  are enclosed in the irregular dodecahedral cages formed by the adjacent tetrahedra of X atoms  16 . 
     In the filled skutterudite  10 , the X atom sites  16  may be C, Si, Ge, Sn, Pb, N, P, As, Sb, Bi, O, S, Se, Te or Po, or combinations of these atoms. Preferably, the X atoms  16  are P, As or Sb. Most preferably, the X atoms  16  are Sb. 
     The transition metal atoms  14  may be Mn, Tc, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag or Au, or combinations of these atoms. Preferably, the transition metal atoms  14  are Co, Rh or Ir. Most preferably, the transition metal atoms  14  are Co. 
     The guest atoms  12  in the filled skutterudite  10  maybe rare earth atoms, Na, K, Ca, Sr, Ba, and combinations of these atoms. According to the present invention, a source or starting material of the guest atoms  12  is mischmetal (Mm) alloy, which is an alloy of mostly Ce (about 50 wt.%) and La (about 50 wt.%) Mischmetal is a naturally occurring material. It is commercially available and typically comprises varying amounts of at least four rare earth elements such as lanthanum, cerium, praseodymium and neodymium, and also non-rare earth impurities such as iron, magnesium, silicon, sulfur, and phosphorus. The mischmetal may be used alone or in combination with a rare earth metal or metals as the source or starting material for the guest atoms  12 . These rare-earth metals include those from the lanthanide series, such as Ce, Pr, Nd, Sm, Eu and Gd, as well as those from the actinides series, such as Th and U. In a typical embodiment, the composition of the filled skutterudite  10  is Mm y Co 4 Sb 12 (0&lt;y≦1). 
     In the filled skutterudite  10 , “rattling” of the guest atoms  12  in the irregular dodecahedral cages formed by the adjacent tetrahedra of X atoms  16  reduces the lattice thermal conductivity of the material while minimally affecting carrier mobility by scattering phonons. Use of mischmetal as a source or starting material for the guest atoms  12  reduces the overall cost of the filled skutterudite  10 , since mischmetal is relatively low in cost compared to high-purity rare earth elements which serve as the starting material for the filler or guest atoms in conventional filled skutterudites. 
     An illustrative method of fabricating the filled skutterudite  10  according to the present invention is shown in the flow diagram of  FIG. 3 . In step  1 , mischmetal, transition metal and X powders are provided. These powders are available from commercial vendors. In step  2 , a precursor pellet is prepared using the mischmetal and transition metal powders, or the mischmetal and other guest element powders in combination with the transition metal powder, using the proper stoichiometric ratios. This step may be carried out by using an induction furnace process or any other process which is capable of producing high temperatures (typically &gt;1,200 degrees C.) followed by rapid cooling or quenching. In step  3 , the precursor pellet formed in step  2  is mixed with the X powder. In step  4 , the mixture containing the precursor pellet and X powder is sintered. This is followed by annealing of the mixture (step  5 ) at a temperature of typically about 500˜1000 degrees C. for at least about 24 hours. Finally, the annealed mixture is hot-pressed into the filled skutterudite material at a pressure of typically at least about 57,200 psi. 
     While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.