U.S. patent application number 12/643130 was filed with the patent office on 2010-06-24 for filled skutterudites for advanced thermoelectric applications.
This patent application is currently assigned to GM Global Technology Operations, Inc.. Invention is credited to Gregory P. Meisner, Jihui Yang.
Application Number | 20100155675 12/643130 |
Document ID | / |
Family ID | 35655840 |
Filed Date | 2010-06-24 |
United States Patent
Application |
20100155675 |
Kind Code |
A1 |
Yang; Jihui ; et
al. |
June 24, 2010 |
Filled Skutterudites for Advanced Thermoelectric Applications
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.
Inventors: |
Yang; Jihui; (Lakeshore,
CA) ; 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
|
Family ID: |
35655840 |
Appl. No.: |
12/643130 |
Filed: |
December 21, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10979005 |
Nov 1, 2004 |
7648552 |
|
|
12643130 |
|
|
|
|
60590878 |
Jul 23, 2004 |
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Current U.S.
Class: |
252/519.13 ;
252/519.1; 252/519.14 |
Current CPC
Class: |
H01L 35/34 20130101;
C22B 1/2413 20130101; C22C 1/0491 20130101; H01L 35/18 20130101;
C22B 59/00 20130101 |
Class at
Publication: |
252/519.13 ;
252/519.1; 252/519.14 |
International
Class: |
H01B 1/02 20060101
H01B001/02 |
Claims
1. A filled skutterudite comprising a chemical composition of
G.sub.yM.sub.4X.sub.12, where G comprises mischmetal as a source of
guest atoms; wherein mischmetal comprises at least two rare earth
elements and one or more non-rare earth impurities, y is a filling
fraction of said guest atoms, M represents transition metal atoms,
and X represents atoms from groups IVA-VIA of the periodic
table.
2. The filled skutterudite of claim 1 wherein said chemical
composition is Mm.sub.yCo.sub.4Sb.sub.12 (0<y.ltoreq.1), where
Mm is mischmetal.
3. The filled skutterudite of claim 1 wherein M is a transition
metal selected from the group consisting of Mn, Tc, Re, Fe, Ru, Os,
Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag and Au.
4. The filled skutterudite of claim 3 wherein M is Co, Rh or
Ir.
5. The filled skutterudite of claim 4 wherein M is Co.
6. The filled skutterudite of claim 1 wherein X is an atom selected
from the group consisting of C, Si, Ge, Sn, Pb, N, P, As, Sb, Bi,
O, S, Se, Te and Po.
7. The filled skutterudite of claim 6 wherein X is P, As or Sb.
8. The filled skutterudite of claim 7 wherein X is Sb.
9. A filled skutterudite comprising a chemical composition of
G.sub.yM.sub.4X.sub.12, where G represents mischmetal alone or in
combination with at least one rare earth element, as a source of
guest atoms; wherein mischmetal comprises at least two rare earth
elements and one or more non-rare earth impurities, y is a filling
fraction of said guest atoms, M represents transition metal atoms,
and X represents atoms from groups IVA-VIA of the periodic
table.
10. The filled skutterudite of claim 9 wherein said rare earth
element is a rare earth element selected from the group consisting
of Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, and combinations
of these atoms.
11. The filled skutterudite of claim 9 wherein M is a transition
metal selected from the group consisting of Mn, Tc, Re, Fe, Ru, Os,
Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag and Au.
12. The filled skutterudite of claim 11 wherein M is Co, Rh or
Ir.
13. The filled skutterudite of claim 12 wherein M is Co.
14. The filled skutterudite of claim 9 wherein X is an atom
selected from the group consisting of C, Si, Ge, Sn, Pb, N, P, As,
Sb, Bi, O, S, Se, Te and Po.
15. The filled skutterudite of claim 14 wherein X is P, As or
Sb.
16. The filled skutterudite of claim 15 wherein X is Sb.
Description
[0001] This application is a divisional of application Ser. No.
10/979,005, filed Nov. 1, 2004.
TECHNICAL FIELD
[0002] 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
[0003] 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.
[0004] 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.sup.2
T/.rho..kappa..sub.total=S.sup.2T/.rho.(.kappa..sub.L+.kappa..sub.e),
where S, T, .rho., .kappa..sub.total, .kappa..sub.L, and
.kappa..sub.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.
[0005] Binary skutterudites are semiconductors with small band gaps
of 18 100 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.sub.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.
[0006] 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.sub.yM.sub.4X.sub.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.
[0007] 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
.about.500 degrees C. For rare earth filled skutterudites, the best
n-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.
[0008] 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
[0009] 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 (.about.50 wt. %) and La
(.about.50 wt. %). In a typical embodiment, the filled skutterudite
has a composition of Mm.sub.yCo.sub.4Sb.sub.12(0<y.ltoreq.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
[0010] The invention will now be described, by way of example, with
reference to the accompanying drawings, in which:
[0011] 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;
[0012] FIG. 2 illustrates a body-centered cubic crystal structure
of a filled skutterudite fabricated according to the present
invention; and
[0013] 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
[0014] 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.sub.yM.sub.4X.sub.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.sub.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.
[0015] 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.
[0016] 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.
[0017] The guest atoms 12 in the filled skutterudite 10 may be 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. %). The
mischmetal may be used alone or in combination with a rare earth
metal or metals, or with Na, K, Ca, Sr, Ba, and combinations of
these 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.sub.yCo.sub.4Sb.sub.12 (0<y.ltoreq.1).
[0018] 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.
[0019] 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>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.about.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.
[0020] 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.
* * * * *