Patent Publication Number: US-2010111754-A1

Title: Potassium and Sodium Filled Skutterudites

Description:
TECHNICAL FIELD 
     This invention pertains to filled skutterudites for thermoelectric applications. More specifically, this invention pertains to sodium-filled and potassium-filled skutterudites. 
     BACKGROUND OF THE INVENTION 
     Skutterudite is the name of a CoAs 3  containing mineral mined in the region of Skutterud, Norway to obtain cobalt and nickel. The mineral has a cubic crystal structure, and compounds with the same crystal structure are called skutterudites. The skutterudite crystal structure has two interstitial voids in each unit cell that are large enough to accommodate different atoms. When skutterudite type compositions are synthesized with atoms that are introduced into such voids, the products are called filled-skutterudites. Thus, filled skutterudites are derived from the skutterudite crystal structure. 
     One group of filled skutterudites are represented by the formula LnT 4 Pn 12 ; where “Ln” demotes one or more of the rare earth elements La, Ce, Pr, Nd, Sm, Eu, Gd, Th, or U; “T” denotes Fe, Ru, Os, Co, Rh, or Ir; and “Pn” denotes one of the pnicogen elements P, As, or Sb. A skutterudite is said to be filled when empty octants in the skutterudite structure of T 4 Pn 12  are filled with rare earth atoms. Since the synthesis of rare earth element filled skutterudites other suitable filler atoms have been discovered. For example, filled compounds of CoSb 3  have been made with alkaline earth elements, calcium, strontium, and barium. 
     Some of the filled skutterudites of various compositions prepared by a combination of melting and powder metallurgy techniques have shown exceptional thermoelectric properties in the temperature range of about 350° C. to about 700° C. Both p-type and n-type conductivities have been obtained and thermoelectric devices comprising materials of both types have been made. 
     Thermoelectric materials can be tested and characterized by a “figure of merit.” The thermoelectric figure of merit, ZT, is given by ZT=S 2 T/ρκ, where S is the Seebeck coefficient, T is the absolute temperature, ρ is the electrical resistivity, and κ is the thermal conductivity. ZT values at 650° C. in the range of, for example, 1.2 to 1.8 have been obtained from measurements on several filled skutterudites and on other, state-of-the-art thermoelectric materials. But higher values are desired for many applications of these materials. High-performance thermoelectric materials could be used to make thermoelectric power generators, coolers, and detectors that would operate with efficiencies greater than those of the corresponding devices now in use and could thus be useful in a greater variety of applications. 
     There is a further need of filled skutterudite thermoelectric materials for adaptation in thermoelectric material applications. 
     SUMMARY OF THE INVENTION 
     In a first embodiment, this invention provides potassium-filled and sodium-filled cobalt triantimonide filled skutterudites. These ternary-filled materials are suitably prepared as the K y Co 4 Sb 12  phase and the Na y Co 4 Sb 12  phase, where y indicates the filling fraction of potassium and sodium, respectively, in the CoSb 3  cubic crystal structure. Thus “y” can have values greater than zero and up to 1 depending on the proportion of the interstitial voids that are filled in the CoSb 3  structure. 
     Filled skutterudites are a class of recently discovered materials which show exceptional thermoelectric properties for automotive waste heat recovery and other thermoelectric applications. One of the challenges to further improve the thermoelectric performance of these materials is the existence of a so-called “Filling Fraction Limit (FFL)” for ternary filled skutterudites. The inventors have developed some first principles methods to understand the mechanisms controlling FFL for ternary filled skutterudites. Based on these tools and understanding, a very high FFL for K-filled and Na-filled ternary skutterudites was predicted even though these materials had not been made. For example, calculations showed that K can have an ultra-high filling fraction up to more than 60% in CoSb 3 , as compared with those previously reported fillers for CoSb 3 , such as Sr, Ba, Ca, La, Ce, and Yb. 
     Synthesis of potassium filled cobalt triantimonide yielded the composition K 0.5 Co 4 Sb 12 , a 50% filling fraction for K in CoSb 3 . Sodium filled CoSb 3  can also be prepared. These materials offer utility in thermoelectric applications. 
     In a second and broader embodiment, the invention provides sodium-filled and/or potassium-filled skutterudites of the general formula, (K, Na) y T 4 Pn 12 , where T denotes Fe, Ru, Os, Co, Rh, or Ir; and Pn denotes one of the pnicogen elements P, As, or Sb. Again, y represents the filling fraction of sodium and/potassium in the T 4 Pn 12  structure. 
     Other objects and advantages of the invention will become apparent from a description of preferred embodiments which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The single drawing FIGURE, is a schematic diagram of a unit cell of the cubic crystal structure of the skutterudite, CoSb 3 . The cobalt atoms are represented by the dark filled circles and the antimony atoms are the unfilled circles. The arrangement of the twenty-seven cobalt atoms divides the unit-cell cube into eight smaller cubes (octants). The twenty-four antimony atoms are grouped in four-member rings, shown connected by gray-filled squares for easier visualization. The four member rings of antimony atoms occupy six of the octants defined by the cobalt atoms. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Many physical properties of crystalline solids, such as the electrical or thermal transport, the luminescence, and the magnetic susceptibility, depend pivotally on the presence of impurities. Materials that possess the skutterudite structure are typical examples of narrow-gap semiconductors with relatively high impurity solubilities for the interstitial voids. In the past decade, filled skutterudites with different filler atoms (Ce, La, Nd, Eu, Yb, Tl, Ca, and Ba) have been intensively studied in an effort to search for better thermoelectric materials. In connection with this effort, a group of researchers, including an inventor in the subject of this application, studied the doping limit or FFL of various impurities for the intrinsic voids in the lattice of CoSb 3  using the density functional method. This work is published as “Filling Fraction Limit for Intrinsic voids in Crystals: Doping in Skutterudites,” X. Shi, W. Zhang, L. Chen, and J. Yang, Phys. Rev. Lett., 95, 185503 (2005). 
     In that study, the FFL of skutterudites was shown to be determined not only by the interaction between the impurity and host atoms but also by the formation of secondary phases between the impurity atoms and one of the host atoms. The predicted FFLs for Ca, Sr, Ba, La, Ce, and Yb in CoSb 3  were in excellent agreement with reported experimental data. A like study using the density functional method by the inventors herein has predicted high FFL values for the incorporation of potassium and sodium in CoSb 3 . These materials are now candidates as small-gap semi-conductors for use in thermoelectric applications. 
     The drawing FIGURE is a schematic illustration of a unit cell of the cubic crystal structure of CoSb 3 . Twenty seven cobalt atoms (dark filled circles) are illustrated as occupying corners, edges, and faces of a cubic unit cell. A body-centered cobalt atom divides the unit cell cube into eight smaller cubes, sometimes called octants. Six of the octants are seen filled with four square rings of antimony atoms (unfilled circles), where the ring are arbitrarily highlighted by square grey-filled areas. The highlighted rings help to visualize the like spatial attitudes of the rings of antimony atoms in diagonally opposing octants of the unit cell. 
     Thus, 24 antimony atoms occupy the unit cell. The cobalt atoms in the faces of the unit cell are shared with adjoining cells and there are only a total of eight cobalt atoms attributable to the single illustrated unit cell. The illustrated unit cell consists of two primitive cells that contain the minimum number of cobalt and antimony atoms representative of the structure. Accordingly, this skutterudite structure is sometimes referred to as a CoSb 3  structure because of the ratio of the atoms in the structure, or as a Co 4 Sb 12  cubic structure based on the numbers of respective atoms in a single primitive cell. 
     In accordance with this invention, CoSb 3  structures are synthesized in which sodium atoms and/or potassium atoms are introduced into the intrinsic voids in the CoSb 3  structure. These voids are illustrated schematically in the FIGURE by the vacant octants at the lower right rear and upper left front cubes of the unit cell. 
     Preparation of Potassium-Filled Cobalt Triantimonide. 
     Tripotassium antimonide, K 3 Sb, was prepared by heating Sb and K in a steel crucible to ˜300 C on a hotplate in an inert atmosphere glove box. This material was ground and reheated to ˜340° C. The final product was a greenish grey powder that could be ground and sieved to remove traces of free K. X-ray diffraction showed the material to be K 3 Sb. 
     This powder of K 3 Sb was added to pieces of CoSb 2.828  and Sb to give a nominal stoichiometry for the precursor mixture of K y Co 4 Sb 12  with y˜1. This mixture was loaded into a carbon-coated quartz tube and heated slowly to 900° C., and the molten alloy was soaked for 1 hour. Then the temperature was reduced to 700° C. and held for 6 days in order to form and anneal the K y Co 4 Sb 12  skutterudite phase. 
     Finally the sample was removed from the furnace and air cooled to room temperature. The quartz tube was broken open and the sample was in the form of agglomerated chunks of fine-grained crystalline powder that stuck slightly to the quartz. X-ray diffraction showed two sets of peaks indicating a mixture of two skutterudite phases having slightly different lattice constants. Electron microprobe analysis showed that these two phases have different amounts of K. About 80% of the sample have y=0.5 and the remaining 20% have y=0.20. There were also trace amounts of CoSb 3  and CoSb 2 . 
     Na y CoSb 3  compounds can be prepared by an analogous procedure. Alternatively, K y CoSb 3  and Na y CoSb 3  compounds can be made by methods described in J. Yang, M. G. Endres, and G. P. Meisner, Phys. Rev B 66, 014436 (2002) and J. Yang, D. T. Morelli, G. P. Meisner, W. Chen, J. S. Dyck, and C. Uher, Phys. Rev. B 67, 165207 (2003). 
     Thus, this invention provides new sodium-filled and potassium-filled CoSb 3  or Co 4 Sb 12  skutterudites of the general formulas Na y Co 4 Sb 12  and K y Co 4 Sb 12 . Here y indicates the filling fraction of potassium and sodium, respectively, in the CoSb 3  cubic crystal structure, and may have a value greater than zero and less than one. Generally y has a value in the range of 0.2 to 0.6. 
     In a broader aspect, the invention provides sodium-filled and/or potassium-filled skutterudites of the general formula, (K, Na) y T 4 Pn 12 , where T denotes Fe, Ru, Os, Co, Rh, or Ir; and “Pn” denotes one of the pnicogen elements P, As, or Sb. Again, y has values less than one.