U.S. patent application number 11/456887 was filed with the patent office on 2010-05-06 for potassium and sodium filled skutterudites.
This patent application is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS, INC.. Invention is credited to Lidong Chen, Gregory P. Meisner, Jihui Yang, Wenqing Zhang.
Application Number | 20100111754 11/456887 |
Document ID | / |
Family ID | 38924018 |
Filed Date | 2010-05-06 |
United States Patent
Application |
20100111754 |
Kind Code |
A1 |
Yang; Jihui ; et
al. |
May 6, 2010 |
Potassium and Sodium Filled Skutterudites
Abstract
Interstitial voids of the cubic CoSb.sub.3 type skutterudite
structure can be filled with sodium and/or potassium atoms. Such
filled skutterudites have the general formulas,
K.sub.yCo.sub.4Sb.sub.12 and Na.sub.yCo.sub.4Sb.sub.12, where y
indicates the filling fraction of potassium and sodium,
respectively, in the CoSb.sub.3 cubic crystal structure, and has a
value greater than zero and less than one. Also sodium-filled
and/or potassium-filled skutterudites of the general formula, (K,
Na).sub.yT.sub.4Pn.sub.12 are made, 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.
Inventors: |
Yang; Jihui; (Lakeshore,
CA) ; Zhang; Wenqing; (Shanghai, CN) ; Chen;
Lidong; (Shanghai, CN) ; Meisner; Gregory P.;
(Ann Arbor, MI) |
Correspondence
Address: |
General Motors Corporation;c/o REISING, ETHINGTON, BARNES, KISSELLE, P.C.
P.O. BOX 4390
TROY
MI
48099-4390
US
|
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS,
INC.
Detroit
MI
Shanghai Institute of Ceramics Chinese Academy of
Sciences
Shanghai
|
Family ID: |
38924018 |
Appl. No.: |
11/456887 |
Filed: |
July 12, 2006 |
Current U.S.
Class: |
420/576 |
Current CPC
Class: |
C01P 2002/77 20130101;
C01P 2004/80 20130101; C01G 51/00 20130101; C01P 2002/54 20130101;
Y02P 20/129 20151101; C01G 51/006 20130101 |
Class at
Publication: |
420/576 |
International
Class: |
C22C 12/00 20060101
C22C012/00 |
Claims
1. Filled CoSb.sub.3 type skutterudites in which interstitial voids
in the cubic CoSb.sub.3 structure contain only potassium or sodium
atoms, and in which the filling fraction of potassium or sodium
atoms is 0.2 to 0.6.
2. A filled CoSb.sub.3 type skutterudite as recited in claim 1
having the formula K.sub.yCo.sub.4Sb.sub.12, where y has a value in
the range of 0.5 to 0.6.
3. A filled CoSb.sub.3 type skutterudite as recited in claim 2
having the formula K.sub.yCo.sub.4Sb.sub.12, where y has a value of
0.6.
4. A filled CoSb.sub.3 type skutterudite as recited in claim 1
having the formula Na.sub.yCo.sub.4Sb.sub.12, where y has a value
in the range of 0.5 to 0.6.
5. A filled CoSb.sub.3 type skutterudite as recited in claim 4
having the formula Na.sub.yCo.sub.4Sb.sub.12, where y has a value
of 0.6.
6. (canceled)
7. (canceled)
8. (canceled)
Description
TECHNICAL FIELD
[0001] 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
[0002] Skutterudite is the name of a CoAs.sub.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.
[0003] One group of filled skutterudites are represented by the
formula LnT.sub.4Pn.sub.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.sub.4Pn.sub.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.sub.3 have been made with alkaline earth elements, calcium,
strontium, and barium.
[0004] 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.degree. C. to about 700.degree. C.
Both p-type and n-type conductivities have been obtained and
thermoelectric devices comprising materials of both types have been
made.
[0005] Thermoelectric materials can be tested and characterized by
a "figure of merit." The thermoelectric figure of merit, ZT, is
given by ZT=S.sup.2T/.rho..kappa., where S is the Seebeck
coefficient, T is the absolute temperature, .rho. is the electrical
resistivity, and .kappa. is the thermal conductivity. ZT values at
650.degree. 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.
[0006] There is a further need of filled skutterudite
thermoelectric materials for adaptation in thermoelectric material
applications.
SUMMARY OF THE INVENTION
[0007] 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.sub.yCo.sub.4Sb.sub.12 phase and the
Na.sub.yCo.sub.4Sb.sub.12 phase, where y indicates the filling
fraction of potassium and sodium, respectively, in the CoSb.sub.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.sub.3 structure.
[0008] 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.sub.3, as compared with those
previously reported fillers for CoSb.sub.3, such as Sr, Ba, Ca, La,
Ce, and Yb.
[0009] Synthesis of potassium filled cobalt triantimonide yielded
the composition K.sub.0.5Co.sub.4Sb.sub.12, a 50% filling fraction
for K in CoSb.sub.3. Sodium filled CoSb.sub.3 can also be prepared.
These materials offer utility in thermoelectric applications.
[0010] In a second and broader embodiment, the invention provides
sodium-filled and/or potassium-filled skutterudites of the general
formula, (K, Na).sub.yT.sub.4Pn.sub.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.sub.4Pn.sub.12 structure.
[0011] Other objects and advantages of the invention will become
apparent from a description of preferred embodiments which
follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The single drawing FIGURE, is a schematic diagram of a unit
cell of the cubic crystal structure of the skutterudite,
CoSb.sub.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
[0013] 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.sub.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).
[0014] 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.sub.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.sub.3. These materials are now candidates as small-gap
semi-conductors for use in thermoelectric applications.
[0015] The drawing FIGURE is a schematic illustration of a unit
cell of the cubic crystal structure of CoSb.sub.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.
[0016] 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.sub.3 structure because of the ratio of the atoms in the
structure, or as a Co.sub.4Sb.sub.12 cubic structure based on the
numbers of respective atoms in a single primitive cell.
[0017] In accordance with this invention, CoSb.sub.3 structures are
synthesized in which sodium atoms and/or potassium atoms are
introduced into the intrinsic voids in the CoSb.sub.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.
[0018] Tripotassium antimonide, K.sub.3Sb, was prepared by heating
Sb and K in a steel crucible to .about.300 C on a hotplate in an
inert atmosphere glove box. This material was ground and reheated
to .about.340.degree. 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.sub.3Sb.
[0019] This powder of K.sub.3Sb was added to pieces of
CoSb.sub.2.828 and Sb to give a nominal stoichiometry for the
precursor mixture of K.sub.yCo.sub.4Sb.sub.12 with y.about.1. This
mixture was loaded into a carbon-coated quartz tube and heated
slowly to 900.degree. C., and the molten alloy was soaked for 1
hour. Then the temperature was reduced to 700.degree. C. and held
for 6 days in order to form and anneal the K.sub.yCo.sub.4Sb.sub.12
skutterudite phase.
[0020] 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.sub.3 and CoSb.sub.2.
[0021] Na.sub.yCoSb.sub.3 compounds can be prepared by an analogous
procedure. Alternatively, K.sub.yCoSb.sub.3 and Na.sub.yCoSb.sub.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).
[0022] Thus, this invention provides new sodium-filled and
potassium-filled CoSb.sub.3 or Co.sub.4Sb.sub.12 skutterudites of
the general formulas Na.sub.yCo.sub.4Sb.sub.12 and
K.sub.yCo.sub.4Sb.sub.12. Here y indicates the filling fraction of
potassium and sodium, respectively, in the CoSb.sub.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.
[0023] In a broader aspect, the invention provides sodium-filled
and/or potassium-filled skutterudites of the general formula, (K,
Na).sub.yT.sub.4Pn.sub.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.
* * * * *