U.S. patent application number 12/809041 was filed with the patent office on 2011-02-17 for method for producing a thermoelectric intermetallic compound.
Invention is credited to Mazhar Ali Bari.
Application Number | 20110036099 12/809041 |
Document ID | / |
Family ID | 39048357 |
Filed Date | 2011-02-17 |
United States Patent
Application |
20110036099 |
Kind Code |
A1 |
Bari; Mazhar Ali |
February 17, 2011 |
METHOD FOR PRODUCING A THERMOELECTRIC INTERMETALLIC COMPOUND
Abstract
A method for producing an intermetallic compound, the method
comprising: (1) providing components A, B and X; and forming by
solid state reaction of components A, B and X an intermetallic
compound having a filled skutterudite structure and formula of
A.sub.aB.sub.bX.sub.c; (2) melting the intermetallic compound
having a filled skutterudite structure produced in step (1) in the
presence of additional X; and (3) annealing the intermetallic
compound of step (2) in the presence of additional X at a
temperature equal to, or greater than the phase formation
temperature of the intermetallic compound.
Inventors: |
Bari; Mazhar Ali; (Dublin,
IE) |
Correspondence
Address: |
MANNAVA & KANG, P.C.
11240 WAPLES MILL ROAD, Suite 300
FAIRFAX
VA
22030
US
|
Family ID: |
39048357 |
Appl. No.: |
12/809041 |
Filed: |
December 19, 2008 |
PCT Filed: |
December 19, 2008 |
PCT NO: |
PCT/GB2008/004209 |
371 Date: |
October 4, 2010 |
Current U.S.
Class: |
62/3.1 ; 420/576;
75/10.14; 75/10.65; 75/357; 75/703 |
Current CPC
Class: |
H01L 35/18 20130101;
C22C 1/00 20130101; C22C 12/00 20130101 |
Class at
Publication: |
62/3.1 ; 75/703;
75/10.14; 75/10.65; 75/357; 420/576 |
International
Class: |
F25B 21/00 20060101
F25B021/00; C22B 30/02 20060101 C22B030/02; C22B 4/04 20060101
C22B004/04; B22F 9/02 20060101 B22F009/02; C22C 12/00 20060101
C22C012/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 19, 2007 |
GB |
0724752.1 |
Claims
1. A method for producing an intermetallic compound, the method
comprising: (1) providing components A, B and X, wherein A, B and X
are: A is selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Yb, In, Ca, Sr,
Ba and Tl, and mixtures of two or more thereof; B is selected from
one or more transition metal atoms; and X is selected from one or
more Group IIIA-VIIA atoms; and forming by solid state reaction of
components A, B and X an intermetallic compound having a filled
skutterudite structure and formula of A.sub.aB.sub.bX.sub.c wherein
A, B and X are as defined above and a is the sum of all A atoms
wherein 0<a.ltoreq.1 b is the sum of all B atoms wherein
3.5.ltoreq.b.ltoreq.4; and c is the sum of all X atoms wherein
8.ltoreq.c.ltoreq.12; (2) melting the intermetallic compound having
a filled skutterudite structure produced in step (1) in the
presence of additional X; and (3) annealing the intermetallic
compound of step (2) in the presence of additional X at a
temperature equal to, or greater than the phase formation
temperature of the intermetallic compound.
2. The method according to claim 1 wherein A, B and X are provided
in stoichiometric amounts.
3. The method according to claim 1 wherein the solid state reaction
of A, B and X to form the intermetallic compound having a filled
skutterudite structure and a formula of A.sub.aB.sub.bX.sub.c is
carried out by heating at a temperature range of from 500.degree.
C. to 900.degree. C. for from 1 to 7 days.
4. The method according to claim 1 wherein the intermetallic
compound is melted using an arc and/or induction melter.
5. The method according to claim 1, further comprising a step of
pelletising the intermetallic compound provided in step (1) before
the melting step (2).
6. The method according to claim 1, wherein the annealing step (3)
is performed for a duration greater than 24 hours.
7. (canceled)
8. (canceled)
9. The method according to claim 1, wherein A is selected from the
rare earth elements and mixtures of two or more thereof.
10. (canceled)
11. The method according to claim 1 wherein B is selected Fe, Co,
Rh, Ru, Os and Ir.
12. (canceled)
13. The method according to claim 1 wherein X is selected from C,
Si, Ge, Sn, Pb, N, P, As, Sb, Bi, S, Se and Te.
14. (canceled)
15. (canceled)
16. The method according to claim 1 wherein a is in the range of
from 0.5 to 1.
17. (canceled)
18. The method according to claim 1 wherein b is 4 and/or wherein c
is 12.
19. (canceled)
20. An intermetallic compound obtainable by the method of claim
1.
21. An intermetallic compound exhibiting a ZT.sub.600 k value of
.gtoreq.0.9 obtainable by the method of claim 1.
22. (canceled)
23. (canceled)
24. An intermetallic compound exhibiting a ZT.sub.600 k value of
.gtoreq.0.9 and having the formula: A'.sub.a'B'.sub.b'X'.sub.c'
wherein, A' is selected from at least two of La, Ce, Pr, Nd, Pm,
Sm, Eu, Yb, Ca, Sr, Ba and Tl, and mixtures of three or more
thereof; B' is selected from one or more transition metal atoms; X'
is selected from one or more Group IIIA-VIIA atoms; a' is the sum
of all A' atoms wherein 0<a'.ltoreq.1 b' is the sum of all B'
atoms wherein 3.5.ltoreq.b'.ltoreq.4; and c' is the sum of all X'
atoms wherein 10.ltoreq.c'.ltoreq.12.
25. An intermetallic compound according to claim 24 having a filled
skutterudite structure.
26. An intermetallic compound according to claim 24 wherein A' is
selected at least two rare earth elements and mixtures of three or
more thereof.
27. (canceled)
28. An intermetallic compound according to claim 24 wherein B' is
selected Fe, Co, Rh, Ru, Os and Ir.
29. (canceled)
30. An intermetallic compound according to claim 24 wherein X' is
selected from C, Si, Ge, Sn, Pb, N, P, As, Sb, Bi, S, Se and
Te.
31. (canceled)
32. (canceled)
33. An intermetallic compound according to claim 24 wherein a' is
in the range of from 0.5 to 1.
34. (canceled)
35. An intermetallic compound according to claim 24 wherein b' is 4
and/or wherein c' is 12.
36. (canceled)
37. An intermetallic compound according to claim 24 exhibiting a
ZT.sub.600 k value of .gtoreq.1.
38. (canceled)
39. (canceled)
40. A peltier cooler, a thermoelectric generator or a
magnetoresistor comprising an intermetallic compound as defined in
claim 24.
41. (canceled)
42. (canceled)
43. The use of an intermetallic compound as defined in claim 24 as
a thermoelectric material.
44. A multilayer comprising a first electrode, a second electrode,
and an interlayer there between, said interlayer comprising an
intermetallic compound as defined in claim 24.
Description
[0001] The present invention relates to the field of
thermoelectrics, specifically an intermetallic compound and a
method of manufacture thereof. These materials can utilize a
thermal gradient to generate electrical power or can be
electrically powered for heating/cooling applications.
BACKGROUND TO THE INVENTION
[0002] Increasing consciousness of environmental issues has lead to
a desire for increased energy efficiency in a range of energy
production methods. Considerable interest has been invested in the
production, development and use of thermoelectric materials.
Thermoelectric materials convert heat flow into electrical current
and vice versa. Therefore these materials have been sought for the
production of electricity from the surroundings, for example, in
power station cooling towers to extract energy from the hot exhaust
gases. Other practical uses which have been envisioned include
nuclear-heated power generating applications for powering
spacecraft where solar power is not feasible. The thermoelectric
power and, hence, the figure of merit of conventional bulk
thermoelectric materials are not currently sufficient for power or
space applications.
[0003] In the automobile industry thermoelectric materials could be
used to transform heat directly into electrical energy, allowing
fuel to be used for power generation more efficiently. Power
generators in cars could be made obsolete by utilizing heat from
exhaust gases. Thermo-electric 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 cooling capability of thermoelectric devices has also
been viewed as a solution to other environmental problems.
Specifically the use of a thermoelectric material to extract heat
from a container, for example a refrigerator, removes the
requirement for potentially damaging refrigerants. Furthermore,
such devices enable new designs of coolers/heaters and air
conditioning.
[0005] Thermoelectric materials are particularly favoured for the
above applications due to the simplicity of their design. The solid
state components are robust and highly reliable with low failure
rates. In the use of a thermoelectric material, either in the
generation of electrical power, or the creation of a temperature
gradient, there are no pollutants released into the atmosphere.
[0006] Furthermore, thermoelectric materials are scalable and hence
ideal for miniature power generation, for example, in
thermoelectric micro devices. A thermoelectric cooler will enable
temperature control for bio-medical lab-on-chip applications and
optoelectronics hundreds of times faster and more precisely than
existing technology. Micro thermoelectronic generators will enable
self powering microelectonics such as a thermoelectronic wristwatch
or, combined with a microcombustor could replace Li-ion batteries
in portable electronics. It is hoped that the use of such materials
could allow improvements in the reliability of batteries.
[0007] The present invention aims to overcome or at least mitigate
at least some of the problems associated with the prior art.
[0008] The suitability of a material for thermoelectric
applications is determined by its dimensionless figure of merit,
ZT. ZT=(S.sup.2.alpha.T)/(k) where S, .alpha., k and T are the
thermo-power (Seebeck coefficient), electrical conductivity,
thermal conductivity and temperature respectively. Thermal
conductivity `k` is sum of two contributions--electronic (k.sub.e)
and lattice (k.sub.l) contributions. A good thermoelectric material
has a large ZT value. This large value may result from a large
Seebeck coefficient, a high electrical and/or low thermal
conductivity. The electronic properties are determined by the power
factor, S.sup.2.alpha.T, which can be optimized by tuning the
carrier concentration.
[0009] ZT may be specified at the temperature at which it is
measured. For example, ZT.sub.600K is the thermoelectric
dimensionless figure of merit of a material at 600 degrees
Kelvin.
[0010] Binary skutterudites are semiconductors with small band gaps
of 100 meV, high carrier mobility, and modest Seebeck coefficients.
Binary skutterudite compounds crystallize in a body-centred-cubic
structure with space group Im3 and have the form MX.sub.3, where M
is commonly Fe, Co, Rh or Ir and X is P, As or Sb. Despite having
excellent electronic properties, binary skutterudites have thermal
conductivities too high to compete with state-of-the-art
thermoelectric materials. However, filled skutterudites have much
lower thermal conductivities. Therefore, filled skutterudites are
increasingly popular as a thermoelectric material due to their
lower thermal conductivities.
[0011] 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 Z.sub.yM.sub.4X.sub.12,
where Z 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. As the thermal conductivity due to lattice (k.sub.l) are
minimized, ZT is maximized. 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. The filled
skutterudites possess attractive electrical transport properties
and serve as potential candidates for achieving figure of merit
significantly larger than conventional thermoelectric
materials.
[0012] The thermoelectric power of these materials can be
understood in terms of the phonon glass-electron model. The filler
atoms can `rattle` inside the oversized voids in the skutterudite
structure. The generation of low frequency phonon modes increases
the phonon-phonon scattering which in turn decreases the magnitude
of k.sub.l.
[0013] The value ZT=1 can be viewed as a bench mark of quality of a
thermoelectric material. In recent years, both n- and p-type rare
earth filled skutterudites have been reported to have ZT values
around 1 above 500.degree. C. One aim of the present invention is
to provide a thermoelectric material having a value greater than
1.4 or 1.5 at 600K and to search for novel materials for both
cooling and power applications.
[0014] For power and generator applications there is a need for
materials and manufacturing techniques which yield high figure of
merit, with a large electrical conductivity and low thermal
conductivity. It is also desirable that the magnitude of the
thermo-power increases with increasing temperature.
SUMMARY OF THE INVENTION
[0015] In the first aspect, the present invention provides a method
for producing an intermetallic compound, the method comprising:
[0016] (1) providing components A, B and X, wherein A, B and X are:
[0017] A is selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Yb, In, Ca,
Sr, Ba and Tl, and mixtures of two or more thereof; [0018] B is
selected from one or more transition metal atoms; and [0019] X is
selected from one or more Group IIIA-VIIA atoms; [0020] and forming
by solid state reaction of components A, B and X an intermetallic
compound having a filled skutterudite structure and formula of
[0020] A.sub.aB.sub.bX.sub.c [0021] wherein A, B and X are as
defined above and [0022] a is the sum of all A atoms wherein
0<a.ltoreq.1 [0023] b is the sum of all B atoms wherein
3.5.ltoreq.b.ltoreq.4; and [0024] c is the sum of all X atoms
wherein 8.ltoreq.c.ltoreq.12; [0025] (2) melting the intermetallic
compound having a filled skutterudite structure produced in step
(1) in the presence of additional X; and [0026] (3) annealing the
intermetallic compound of step (2) in the presence of additional X
at a temperature equal to, or greater than the phase formation
temperature of the intermetallic compound.
[0027] In the second aspect, the present invention provides an
intermetallic compound exhibiting a ZT.sub.600 k value of
.gtoreq.0.9 and having the formula:
A'.sub.a', B'.sub.b', X'.sub.c'
wherein, [0028] A' is selected from at least two of La, Ce, Pr, Nd,
Pm, Sm, Eu, Yb, Ca, Sr, Ba and Tl, and mixtures of three or more
thereof; [0029] B' is selected from one or more transition metal
atoms; [0030] X' is selected from one or more Group IIIA-VIIA
atoms; [0031] a' is the sum of all A atoms wherein 0<a'.ltoreq.1
[0032] a' is the sum of all B atoms wherein 3.5.ltoreq.b'.ltoreq.4;
and [0033] c' is the sum of all X atoms wherein
10.ltoreq.c'.ltoreq.12.
[0034] In a further aspect, the present invention provides an
intermetallic compound obtainable by the process as described
herein.
DESCRIPTION OF THE FIGURES
[0035] FIG. 1 shows the variation of thermoelectric power for (a)
Ce.sub.0.85Fe.sub.4Sb.sub.12 (b)
Ce.sub.0.85Fe.sub.2.8Co.sub.1.2Sb.sub.12 (c)
Ce.sub.0.4Yb.sub.0.53Fe.sub.2.8Co.sub.1.2Sb.sub.12 and (d) annealed
Ce.sub.0.4Yb.sub.0.53Fe.sub.2.8Co.sub.1.2Sb.sub.12 in the
temperature range 10-600 K.
[0036] FIG. 2 shows the variation of thermoelectric power for (a)
La.sub.0.85Fe.sub.4Sb.sub.12 (b)
La.sub.0.85Fe.sub.2.8Co.sub.1.2Sb.sub.12 (c)
La.sub.0.4Yb.sub.0.53Fe.sub.2.8Co.sub.1.2Sb.sub.12 and (d) annealed
Ce.sub.0.4Yb.sub.0.53Fe.sub.2.8Co.sub.1.2Sb.sub.12 in the
temperature range 10-600 K.
[0037] FIG. 3 shows the resistivity against temperature plots of
(a) Ce.sub.0.4Yb.sub.0.53Fe.sub.4Sb.sub.12 (b) annealed
Ce.sub.0.4Yb.sub.0.53Fe.sub.4Sb.sub.12 (c)
Ce.sub.0.4Yb.sub.0.53Fe.sub.2.8Co.sub.1.2Sb.sub.12 (d) annealed
Ce.sub.0.4Yb.sub.0.53Fe.sub.2.8Co.sub.1.2Sb.sub.12 and (e) annealed
La.sub.0.4M.sub.0.53Fe.sub.2.8Co.sub.1.2Sb.sub.n in the temperature
range 10-600 K.
[0038] FIG. 4 shows the figure of merit for several filled
skutterudite compositions before and after annealing in Sb
vapour.
DETAILED DESCRIPTION OF THE INVENTION
[0039] The present invention will now be further described. In the
following passages different aspects of the invention are defined
in more detail. Each aspect so defined may be combined with any
other aspect or aspects unless clearly indicated to the contrary.
In particular, any feature indicated as being preferred or
advantageous may be combined with any other feature or features
indicated as being preferred or advantageous.
[0040] Advantageously, the present inventors have found that by
using the method of the present invention, intermetallic compounds
may be provided which have improved thermoelectric properties.
[0041] Preferably the method of the present invention produces an
intermetallic compound exhibiting a ZT.sub.600 k value of 0.9, more
preferably .gtoreq.1, .gtoreq.1.1, or .gtoreq.1.2.
[0042] The method of the present invention comprises at least three
steps. It will, however, be understood that further steps may be
incorporated before, between and/or after these three steps.
[0043] The first step involves providing components A, B and X,
wherein A, B and X are: [0044] A is selected from La, Ce, Pr, Nd,
Pm, Sm, Eu, Yb, In, Ca, Sr, Ba and Tl, and mixtures of two or more
thereof; [0045] B is selected from one or more transition metal
atoms; and [0046] X is selected from one or more Group IIIA-VIIA
atoms.
[0047] Solid state reaction of components A, B and X provides an
intermetallic compound having a filled skutterudite structure and
formula of
A.sub.aB.sub.bX.sub.c [0048] wherein A, B and X are as defined
above and [0049] a is the sum of all A atoms wherein
0<a.ltoreq.1 [0050] b is the sum of all B atoms wherein
3.5.ltoreq.b.ltoreq.4; and [0051] c is the sum of all X atoms
wherein 8.ltoreq.c.ltoreq.12.
[0052] Preferably, stoichiometric mixtures of the reactants A, B
and X are reacted together to form the intermetallic compound of
formula A.sub.aB.sub.bX.sub.c.
[0053] Typically the components A, B and X will be provided as
ingots of the elements. Such ingots are readily available
commercially. Conventional solid state reaction techniques are well
known. The filled skutterudite structure of formula of
A.sub.aB.sub.bX.sub.c may be produced, for example, by baking
ingots of the elements together in an evacuated container at a
suitable time for a suitable duration. The baking is preferably
carried out under vacuum or under an inert atmosphere according to
conventional methods such as sealing in evacuated glass tubes. The
inert atmosphere prevents contamination, oxidation or decomposition
of the produced intermetallic. Alternatively, or additionally,
unreactive metal tubes may be used.
[0054] Preferably the solid state reaction of A, B and X to form
the intermetallic compound having a filled skutterudite structure
and a formula of A.sub.aB.sub.bX.sub.c is carried out by heating at
a temperature range of from 500.degree. C. to 900.degree. C. for
from 1 to 7 days.
[0055] Preferably in the method of the present invention A is
selected from rare earth elements and mixtures of two or more
thereof. Preferably, A is selected from one or more of La, Ce and
Yb. Most preferably A is Ce and Yb, or La and Yb.
[0056] Preferably in the method of the present invention B is
selected from one or more of Fe, Co, Rh, Ru, Os and Ir. More
preferably B is Fe and Co. B may be Fe.
[0057] Preferably X is selected from C, Si, Ge, Sn, Pb, N, P, As,
Sb, Bi, S, Se and Te. More preferably still X is P, As or Sb. Most
preferably X is Sb.
[0058] "a" is the sum of all A atoms in the intermetallic compound
A.sub.aB.sub.bX.sub.c. Advantageously "a" is in the range of from
0.5 to 1, more preferably "a" is in the range of from 0.8 to 1,
most preferably "a" is 1. "a" may be in the range 0.81-0.87.
[0059] "b" is the sum of all B atoms in the intermetallic compound
A.sub.aB.sub.bX.sub.c. Preferably "b" is in the range of from 3.8
to 4, most preferably "b" is 4.
[0060] "c" is the sum of all X atoms in the intermetallic compound
A.sub.aB.sub.bX.sub.c. Preferably "c" is in the range of from 11 to
12.
[0061] The second step comprises melting the intermetallic compound
having a filled skutterudite structure produced in step (1) in the
presence of additional X. Melting may be carried using an arc
and/or an induction melter. The melting step may be vacuum melting
in, for example, a quartz tube.
[0062] The term "additional X" is used herein to mean that the
filled skutterudite is melted in the presence of at least some X
which has not originated from the filled skutterudite structure.
The additional X may optionally be added to the filled skutterudite
in the reaction chamber before melting.
[0063] The third step comprises annealing the filled skutterudite
in an X-rich environment at a temperature equal to, or greater than
the phase formation temperature of the filled skutterudite. In
particular, in the method of the present invention the
intermetallic compound is annealed in the presence of additional
X.
[0064] The term "additional X" is used herein to mean that the
filled skutterudite is annealed in the presence of at least some X
which has not originated from the filled skutterudite structure.
The additional X may optionally be added to the filled skutterudite
in the reaction chamber before annealing.
[0065] Preferably the additional X is the same as the X present in
the filled skutterudite structure of formula A.sub.aB.sub.bX.sub.C.
Preferably X is P, As or Sb. Most preferably X is Sb.
[0066] By annealing, it is to be understood that the intermetallic
is heated to relieve stresses in the structure. Annealing involves
heating to a temperature where diffusion can occur. Maintaining a
metal at elevated temperatures reduces dislocation, vacancies,
frozen-in stress and other metastable conditions. There are several
stages in the annealing process, with the first being the recovery
phase, which results in softening of the metal through removal of
crystal defects and the internal stresses which they cause. The
second phase is recrystallization, where new grains nucleate and
grow to replace those deformed by internal stresses. Finally a very
slow cooling phase is used to induce softness, relieve internal
stresses, refine the structure and improve cold working properties.
The slow cooling minimises thermal gradients which could
re-introduce stress by differential thermal contraction.
[0067] The process of annealing can allow guest atoms to be
incorporated into substitutional positions in the crystal lattice,
resulting in drastic changes in the electrical properties of the
material. Annealing occurs by the diffusion of atoms within a solid
material, so that the material progresses towards its equilibrium
state. Heat is needed to increase the rate of diffusion by
providing the energy needed to break and form new bonds. The
movement of atoms has the effect of redistributing and destroying
the dislocations in metals.
[0068] The amount of process-initiating Gibbs free energy in a
deformed metal is also reduced by the annealing process. In
practice and industry, this reduction of Gibbs free energy is
termed "stress relief."
[0069] Thus, the relief of internal stresses is a thermodynamically
spontaneous process, however, at room temperatures, it is a very
slow process. Therefore, the high temperatures at which the
annealing process occurs serve to accelerate this
slow-albeit-spontaneous process.
[0070] The step of annealing in an atmosphere rich in X (preferably
an atmosphere comprising Sb and optionally Yb) allows the compound
to settle in a high temperature phase structure with very few
lattice vacancies, these being filled by the excess vapours
provided. Whilst not wishing to be bound by particular theory, it
appears that this vacancy filing serves to improve the
thermoelectric properties of the material. This is surprising as a
number of electronic properties of n- and p-type doped
semiconductors arise from having vacancies in the lattice
structure.
[0071] The annealing step (3) may also be carried out in the
presence of additional A, wherein A is as defined above.
[0072] The method of the present invention may further comprise a
step of pelletising the intermetallic compound provided in step (1)
before the melting step (2).
[0073] Advantageously the annealing step is performed for a
duration greater than 6 hours, 12 hours, 24 hours, more preferably
for longer than 72 hours and most preferably for a duration greater
than 1 week. Annealing is preferably performed at no less than
50.degree. C. less than the phase formation temperature of the
target compound. The phase formation temperature of
YbFe.sub.4Sb.sub.12, for example, is about 650.degree. C.
[0074] In one embodiment of the present invention provides a method
for producing an intermetallic compound, the method comprising:
[0075] (1) providing components A, B and X, wherein A, B and X are:
[0076] A is selected from at least two of La, Ce, Pr, Nd, Pm, Sm,
Eu, Yb, Ca, Sr, Ba and Tl, and mixtures of three or more thereof;
[0077] B is selected from one or more transition metal atoms;
[0078] X is selected from one or more Group IIIA-VIIA atoms and
forming by solid state reaction of components A, B and X an
intermetallic compound having a filled skutterudite structure and
formula of
[0078] A.sub.aB.sub.bX.sub.c [0079] wherein A, B and X are as
defined above and [0080] a is the sum of all A atoms wherein
0<a.ltoreq.1 [0081] b is the sum of all B atoms wherein
3.5.ltoreq.b.ltoreq.4; and [0082] c is the sum of all X atoms
wherein 8.ltoreq.c.ltoreq.12; [0083] (2) melting the intermetallic
compound having a filled skutterudite structure produced in step
(1) in the presence of additional X; and [0084] (3) annealing the
intermetallic compound of step (2) in the presence of additional X
at a temperature equal to, or greater than the phase formation
temperature of the intermetallic compound.
[0085] The present inventors have prepared intermetallic compounds
exhibiting a ZT.sub.600 k value of 0.9 and having the having the
formula:
A'.sub.a'B'.sub.b'X'.sub.c'
wherein, [0086] A' is selected from at least two of La, Ce, Pr, Nd,
Pm, Sm, Eu, Yb, Ca, Sr, Ba and Tl, and mixtures of three or more
thereof; [0087] B' is selected from one or more transition metal
atoms; [0088] X' is selected from one or more Group IIIA-VIIA
atoms; [0089] a' is the sum of all A' atoms wherein
0<a'.ltoreq.1 [0090] b' is the sum of all B' atoms wherein
3.5.ltoreq.b'.ltoreq.4; and [0091] c' is the sum of all X' atoms
wherein 10.ltoreq.c'.ltoreq.12.
[0092] Preferably the intermetallic compound has the formula
A'B'.sub.4X'.sub.12, where A', B' and X' are as defined above. It
will, however, be understood that the compound may not be of this
exact stoichiometric formula, there may be some deficiencies in the
number of A', B' and/or X' atoms in the structure of the
compound.
[0093] Preferably the intermetallic compound has a filled
skutterudite structure (for example, LaFe.sub.4P.sub.12).
Antonomides are an example of the filled skutterudite structure.
They have a body centred cubic structure with square planar rings
of Sb atoms. Metal atoms, for example, iron, form a simple cubic
sub-lattice and the guest atoms occupy the two remaining holes in
the unit cell.
[0094] Preferably A' is selected from at least two rare earth
elements and mixtures of three or more thereof. More preferably, A'
is selected from two or more of La, Ce and Yb. Most preferably A'
is Ce and Yb, or La and Yb.
[0095] Preferably B' is preferably one or more of Fe, Co, Rh, Ru,
Os and Ir. More preferably, A' is Fe and Co. More preferably still,
A' is Fe.
[0096] Preferably X' is selected from C, Si, Ge, Sn, Pb, N, P, As,
Sb, Bi, S, Se, and Te. More preferably X is P, As or Sb. Most
preferably X' is Sb.
[0097] "a'" is the sum of all A' atoms in the intermetallic
compound. Advantageously "a'" is in the range of from 0.5 to 1,
more preferably "a'" is in the range of from 0.8 to 1, most
preferably "a'" is 1. "a'" may be in the range 0.81-0.87.
[0098] "b'" is the sum of all B atoms in the intermetallic
compound. Preferably "b'" is in the range of from 3.8 to 4, most
preferably "b'" is 4.
[0099] "c'" is the sum of all X' atoms in the intermetallic
compound. Preferably "c'" is in the range of from 11 to 12.
[0100] The intermetallic compound as described herein shows
improved thermoelectric properties over prior art compounds. In
particular, the intermetallic compounds have high dimensionless
figure of merit (ZT) values. Preferably the intermetallic compound
as described herein exhibits a ZT.sub.600 k value of .gtoreq.1,
more preferably ZT.sub.600 k value of .gtoreq.1.1, more preferably
still ZT.sub.600 k value of .gtoreq.1.2.
[0101] In a preferred embodiment of the present invention, A' is Ce
and Yb, B' is Fe and X' is Sb. For example, the intermetallic
compound may have the formula
Ce.sub.0.4Yb.sub.0.53Fe.sub.4Sb.sub.12.
[0102] In another embodiment of the present invention A' is La and
Yb, B' is Fe and X' is Sb. For example, the intermetallic compound
may have the formula La.sub.0.4Yb.sub.0.53Fe.sub.4Sb.sub.12.
[0103] In another embodiment of the present invention A' is La and
Yb, B' is Fe and Co and X' is Sb. For example, the intermetallic
compound may have the formula
La.sub.0.4Yb.sub.0.53Fe.sub.2.8Co.sub.1.2Sb.sub.12.
[0104] In an alternative embodiment, A' is Ce and Yb, or A' is Ce
and La, B' is Fe and Co and X' is Sb. In particular, intermetallic
compounds having the formulas
Ce.sub.0.4Yb.sub.0.53Fe.sub.2.8Co.sub.1.2Sb.sub.12 and
Ce.sub.0.4Yb.sub.0.53Fe.sub.2.8Co.sub.1.2Sb.sub.12 have been shown
to have particularly high dimensionless figure of merit values,
which may be up to or greater than 1.2.
[0105] The present inventors have found that substitution of Co at
Fe sites in the filled skutterudite structure has been found to
advantageously increase the figure of merit value. Cobalt also has
the advantage that it is a cheap material for doping the filled
skutterudite. In a preferred embodiment of the present invention B'
is Fe and Co.
[0106] In one aspect of the present invention there is provided a
film comprising the intermetallic compound as described herein. The
film may be a thin film, having a diameter of from 10 nm to 10
micron, or from 10 nm to 1 micron. The film may be made using
standard techniques known in the art, for example sputtering
techniques or pulse laser deposition. In one embodiment of the
present invention the intermetallic compound as described herein
may be deposited onto a substrate. The substrate may be, for
example a metal, alloy, and/or a ceramic.
[0107] In another aspect of the present invention there is provided
a wafer comprising the intermetallic compound as described
herein.
[0108] In another aspect of the present invention there is provided
a peltier cooler comprising an intermetallic compound as described
herein.
[0109] In another aspect of the present invention there is provided
a thermoelectric generator comprising an intermetallic compound as
described herein.
[0110] In a further aspect of the present invention there is
provided the use of an intermetallic compound as described herein
as a thermoelectric material.
[0111] A peltier cooler/heater device comprising an intermetallic
filled skutterudite compound as described above will have a high
efficiency in converting inputted electricity to creating a large
temperature gradient within the device. A thermoelectric generator
comprising a filled skutterudite compound according to the present
invention will have a high efficiency in producing electricity from
a provided temperature gradient.
[0112] In another aspect of the present invention there is provided
a magnetoresistor comprising an intermetallic compound as described
herein.
[0113] When the thermoelectric materials of the present invention
are employed in magnetoresistors they are found to exhibit improved
temperature stability of the magnetoresistance over an extended
temperature range.
[0114] In a further aspect of the present invention there is
provided a multilayer comprising a first electrode, a second
electrode, and an interlayer there between, said interlayer
comprising an intermetallic compound as described herein.
[0115] The present invention will now be described further with
reference to the following non-limiting examples.
EXAMPLES
[0116] The compounds Z.sub.1-xYb.sub.xFe.sub.4Sb.sub.12 and
Z.sub.1-xYb.sub.xFe.sub.3.9Co.sub.0.01Sb.sub.12 (Z.dbd.Ce, La) were
synthesized from high purity starting ingots of La, Ce, Yb, Fe, Co
and Sb purchased from Sigma Aldrich. Stoichiometric mixtures of
these reactants were then sealed in evacuated (<10.sup.-6 mbar)
quartz tubes. The samples were then heated very slowly with a ramp
of 0.5.degree./min to 400.degree. C. The samples were kept at this
temperature for 48 hours. The temperature was then raised to
700.degree. C. for one week. Finally the samples were quenched to
room temperature.
[0117] Each resulting sample was then pressed into pellets and
induction melted. An excess of Yb and Sb were added prior to
melting. The resulting dense pellets were then annealed at
700.degree. C. for 24 hours.
[0118] Phase formation was confirmed by powder X-ray diffraction
method and microscopy/composition analysis is carried out using
SEM/EDX technique. Measurements of electrical resistance as a
function of temperature were performed using a DC four-probe
technique using spring contacts. Thermopower (S) measurements were
performed in the temperature range by the differential method. Low
temperature measurements were made with the help of close circuit
refrigerator. High temperature measurements were carried out using
a furnace and Pt thermocouples soldered to Platinum foils and
temperature controlled by Eurotherm temperature controller. The
Figure of Merit, ZT, is then computed by ZT=(S.sup.2.alpha.T)/(k).
The thermal conductivity (k) was measured in a custom designed
system using a steady state technique.
[0119] For thermoelectric power measurements, the samples were cut
into bar-shaped samples by a diamond wheel. These bar-shaped sample
were clamped between a heater and a copper block which acted as a
heat sink and incorporated the thermometers and connections. The
heater was pressed against the sample by springs. Discs of gold
(foils) were fixed with electrically insulating and thermally
conducting epoxy on top of the heater and on the bottom of the
copper block to ensure a homogeneous lateral temperature and an
electrical insulation of the sample. Two Chromel/Constantan
thermocouples measured the steady thermal gradient (.DELTA.T,
typically 1 K) established across the sample and the voltage
(thermoelectric voltage) was measured. Resistivity measurements
were carried by standard van der Pauw four probe method.
[0120] FIGS. 1&2 illustrate the Seebeck coefficient of
compounds (Ce,Yb)Fe.sub.4Sb.sub.12 and (La,Yb)Fe.sub.4Sb.sub.12 in
the temperature range 10-600 K. It can be noted that Seebeck
coefficient increases with temperature up to 600 K for all
compositions.
[0121] Resistivity plots of (Ce,Yb)Fe.sub.4Sb.sub.12 and
(La,Yb)Fe.sub.4Sb.sub.12 are shown in FIG. 3. The resistivity of
all of the compounds indicated metallic behaviour. Room temperature
resistivity decreased with Yb doping in both series, as shown in
Table I and FIG. 3. The linear variation of resistivity as a
function of temperature can be explained by the semi-metallic
behaviour found in Fe-rich filled skutterudites.
[0122] FIG. 4 shows the calculated dimensionless figure of merit
for Ce.sub.0.4Yb.sub.0.53Fe.sub.4Sb.sub.12 and
Ce.sub.0.4Yb.sub.0.53Fe.sub.2.8Co.sub.1.2Sb.sub.12 before and after
annealing. The composition Ce.sub.0.4Yb.sub.0.53Fe.sub.4Sb.sub.12
has ZT=0.27 at room temperature which increases with increasing
temperature to ZT=0.61. Annealing in Sb atmosphere has profound
effect on thermo-power and hence the figure of merit. The figure of
merit of annealed Ce.sub.0.4Yb.sub.0.53Fe.sub.4Sb.sub.12 reaches
0.48 and 1.1 at 300 K and 600 K respectively.
[0123] Table I summarizes the values of resistivity values (.rho.)
(values in brackets correspond to 600 K), Seebeck coefficient (S),
thermal conductivity (.lamda.) and figure of merit ZT at 300 K and
600 K for (Ce,Yb)Fe.sub.4Sb.sub.12 and (La,Yb)Fe.sub.4Sb.sub.12 and
related compounds.
TABLE-US-00001 TABLE I S .rho. .lamda. Compound (.mu.V K.sup.-1)
(10.sup.-6 .OMEGA. cm) (W m.sup.-1 K.sup.-1) ZT.sub.300K
ZT.sub.600K Ce.sub.0.85Fe.sub.4Sb.sub.12 61 (95) 405 (470) 2.9 0.10
0.40 Ce.sub.0.4Yb.sub.0.53Fe.sub.4Sb.sub.12 100 (140) 395 (680) 2.8
0.27 0.61 Annealed 125 (175) 370 (640) 2.6 0.48 1.1
Ce.sub.0.4Yb.sub.0.53Fe.sub.4Sb.sub.12
Ce.sub.0.85Fe.sub.2.8Co.sub.1.2Sb.sub.12 90 (144) 446 (875) 2.6
0.21 0.54 Ce.sub.0.4Yb.sub.0.53Fe.sub.2.8Co.sub.1.2Sb.sub.12 102
(160) 402 (830) 2.6 0.30 0.71 Annealed 202 (234) 752 (895) 3.0 0.54
1.2 Ce.sub.0.4Yb.sub.0.53Fe.sub.2.8Co.sub.1.2Sb.sub.12
La.sub.0.85Fe.sub.4Sb.sub.12 77 (123) 443 (610) 3.5 0.11 0.43
La.sub.0.4Yb.sub.0.53Fe.sub.4Sb.sub.12 95 (146) 420 (667) 3.2 0.20
0.59 Annealed 105 (172) 364 (583) 3.2 0.28 0.95
La.sub.0.4Yb.sub.0.53Fe.sub.4Sb.sub.12
La.sub.0.85Fe.sub.2.8Co.sub.1.2Sb.sub.12 100 (160) 435 (852) 3.0
0.23 0.60 La.sub.0.4Yb.sub.0.53Fe.sub.2.8Co.sub.1.2Sb.sub.12 116
(184) 426 (863) 2.9 0.32 0.81 Annealed 160 (239) 696 (886) 3.1 0.40
1.24 La.sub.0.4Yb.sub.0.53Fe.sub.2.8Co.sub.1.2Sb.sub.12
[0124] Thermal conductivity (k) of all the compounds were computed
after subtracting the electronic contribution. Electronic
contribution was estimated from the Wiedeman-Franz law relationship
k.sub.e=L.sub.0.alpha.T (L.sub.0 is the Lorentz
number=2.44.times.10.sup.-8 W.OMEGA.K.sup.-2) from the measured
.rho. values. The temperature dependence of lattice thermal
conductivity was estimated between 50 K<T<100 K. Above RT,
the k.sub.l was computed by extrapolating the temperature
dependence to 600 K. The k.sub.l value at 300 K is considerably
lower than that of CoSb.sub.3 which could probably due to the
ratting motion of the randomly distributed ions in `A` site.
[0125] Thermal conductivity decreases with Yb doping and upon
annealing. Lower thermal conductivity of 2.6 W m.sup.-1K.sup.-1 is
obtained for the composition
Ce.sub.0.85Fe.sub.2.8Co.sub.1.2Sb.sub.12 and
Ce.sub.0.40M.sub.0.53Fe.sub.2.8Co.sub.1.2Sb.sub.12. The relatively
low thermal conductivity can be attributed to the high
concentration of grain and particle boundaries. A similar trend is
observed in the series La.sub.0.9Yb.sub.0.1Fe.sub.4Sb.sub.12 with
the lowest k of 2.9 obtained for
Ce.sub.0.40Yb.sub.0.53Fe.sub.2.8Co.sub.1.2Sb.sub.12.
[0126] Maximum thermal conductivity is measured for the
compositions Ce.sub.0.85Fe.sub.4Sb.sub.12 and
La.sub.0.40Fe.sub.4Sb.sub.12 in (Ce,Yb)Fe.sub.4Sb.sub.12 and
(La,Yb)Fe.sub.4Sb.sub.12 series respectively. Whilst not wishing to
be bound to theory, mixing of these ions might lead to valence
fluctuations and decreases thermal conductivity. Yb is a small and
heavy filler atom. The thermal conductivity of heavier, smaller
filling guest atoms appears to lead to a significant reduction in
lattice thermal conductivity compared to bigger, lighter atoms.
[0127] The reduction in k by doping with Ce and Yb compared to
parent compounds appears to confirm the effective filling of the
dodecahedral voids in the lattice. It may be speculated that
perhaps the phonon scattering due to grain boundary scattering is
dominant at low temperatures where as Umklapp scattering dominates
at high temperature regime. In addition, resonant and point defect
scatterings could determine the behaviour at intermediate
temperatures. In our cases, upon doping and annealing, above. RT
the thermoelectric figure of merit is dominated by point defect and
resonant scattering in reducing the contribution of lattice thermal
conductivity.
[0128] Substitution of Co at Fe site slightly increases the figure
of merit to 1.2, mainly due to the increase in Seebeck coefficient.
Similar behaviour is observed in La--Yb--Fe--Sb series, with ZT of
0.28 and 0.95 for annealed La.sub.0.4Yb.sub.0.53Fe.sub.4Sb.sub.12
at 300 K and 600 K respectively.
[0129] Substitution of Co at Fe site increases the ZT value,
similar to (Ce,Yb) series from 0.40 at 300 K to 1.24 at 600 K. It
is surprising that the influence of annealing on ZT is significant.
The increase in ZT is a consequence of the balance effect of a
increase in S and decrease in p. Both these samples have relative
high power factor above room temperature. Values higher than 1.4
can be achieved using the technique described here, or slight
variations on the technique as would be understood by experts in
the area or by further investigations into these filled
skutterudites.
[0130] The role of the filler atoms and annealing treatment has
been investigated in the thermoelectric power of
(Ce,Yb)Fe.sub.4Sb.sub.12 and (La,Yb)Fe.sub.4Sb.sub.12 filled
skutterudites. Dimensionless figure of merit increases dramatically
upon annealing in Sb atmosphere. Annealing in an Sb-rich atmosphere
increases the thermoelectric power and reduces the thermal
conductivity. This provides a path for enhancing the dimensionless
figure of merit in these materials by simultaneous void filling and
optimized annealing. The results of these materials show promising
features for future microelectronic applications.
* * * * *