U.S. patent application number 12/938250 was filed with the patent office on 2011-02-24 for thermoelectric compositions and process.
This patent application is currently assigned to BOARD OF TRUSTEES OF MICHIGAN STATE UNIVERSITY. Invention is credited to JOHN ANDROULAKIS, MERCOURI G. KANATZIDIS, JOSEPH R. SOOTSMAN.
Application Number | 20110042607 12/938250 |
Document ID | / |
Family ID | 37498972 |
Filed Date | 2011-02-24 |
United States Patent
Application |
20110042607 |
Kind Code |
A1 |
KANATZIDIS; MERCOURI G. ; et
al. |
February 24, 2011 |
THERMOELECTRIC COMPOSITIONS AND PROCESS
Abstract
A process for producing bulk thermoelectric compositions
containing nanoscale inclusions is described. The thermoelectric
compositions have a higher figure of merit (ZT) than without the
inclusions. The compositions are useful for power generation and in
heat pumps for instance.
Inventors: |
KANATZIDIS; MERCOURI G.;
(Wilmette, IL) ; ANDROULAKIS; JOHN; (Evanston,
IL) ; SOOTSMAN; JOSEPH R.; (Pasadena, CA) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET, FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Assignee: |
BOARD OF TRUSTEES OF MICHIGAN STATE
UNIVERSITY
East Lansing
MI
|
Family ID: |
37498972 |
Appl. No.: |
12/938250 |
Filed: |
November 2, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11445662 |
Jun 2, 2006 |
7847179 |
|
|
12938250 |
|
|
|
|
60687769 |
Jun 6, 2005 |
|
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Current U.S.
Class: |
252/62.3T |
Current CPC
Class: |
H01L 35/16 20130101;
H01L 35/34 20130101; H01L 35/26 20130101 |
Class at
Publication: |
252/62.3T |
International
Class: |
H01L 35/16 20060101
H01L035/16 |
Claims
1. A thermoelectric composition comprising: a matrix comprising a
first chalcogenide; and nanoscale inclusions in the matrix, the
nanoscale inclusions being coherent or semi-coherent with the
matrix, the nanoscale inclusions having a different composition
than the first chalcogenide, so that the nanoscale inclusions
decrease the thermal conductivity of the composition by scattering
phonons in the composition while substantially maintaining or
increasing electrical conductivity and Seebeck coefficient of the
composition.
2. The thermoelectric composition of claim 1, wherein the nanoscale
inclusions are coherent with the matrix.
3. The thermoelectric composition of claim 1, wherein the nanoscale
inclusions have a first melting point, the matrix has a second
melting point, and the first melting point is lower than the second
melting point.
4. The thermoelectric composition of claim 1, wherein the nanoscale
inclusions have a first melting point, the matrix has a second
melting point, and the second melting point is lower than the first
melting point.
5. The thermoelectric composition of claim 1, wherein the nanoscale
inclusions have a first melting point, the matrix has a second
melting point, and the first melting point is different than the
second melting point.
6. The thermoelectric composition of claim 1, wherein at least a
portion of the nanoscale inclusions are a uniform dispersion of
nanoparticles.
7. The thermoelectric composition of claim 1, wherein about 0.1 to
15% of the composition comprises the nanoscale inclusions.
8. The thermoelectric composition of claim 1, wherein at least a
portion of the nanoscale inclusions comprise a material that is
nonreactive, has a lower melting point, and is soluble with the
matrix in a liquid state.
9. The thermoelectric composition of claim 1, wherein the first
chalcogenide comprises a chalcogen selected from the group
consisting of tellurium, sulfur and selenium.
10. The thermoelectric composition of claim 1, wherein at least a
portion of the nanoscale inclusions have a size between about 1 and
200 nanometers.
11. The thermoelectric composition of claim 1, wherein the
nanoscale inclusions comprise multiple types of inclusions, each
type having a different chemistry.
12. The thermoelectric composition of claim 1, wherein the
composition has lattice thermal conductivity which is more than 40%
reduced as compared to lattice thermal conductivity of the
matrix.
13. The thermoelectric composition of claim 1, wherein the matrix
comprises PbTe.
14. The thermoelectric composition of claim 13, wherein the
nanoscale inclusions comprise PbS.
15. The thermoelectric composition of claim 13, wherein the
nanoscale inclusions comprise at least one element selected from
the group consisting of antimony, bismuth, and arsenic.
16. The thermoelectric composition of claim 13, wherein the
nanoscale inclusions comprise lead and antimony.
17. The thermoelectric composition of claim 13, wherein the
nanoscale inclusions comprise Cd.sub.1-xHg.sub.xTe and
0<x<1.
18. The thermoelectric composition of claim 1, wherein the matrix
comprises PbS and the nanoscale inclusions comprise PbTe.
19. The thermoelectric composition of claim 1, wherein the
nanoscale inclusions comprise a metal.
20. The thermoelectric composition of claim 1, wherein the
nanoscale inclusions comprise a semiconductor.
21. The thermoelectric composition of claim 1, wherein the
nanoscale inclusions comprise a second chalcogenide different from
the first chalcogenide, the second chalcogenide comprising a
chalcogen selected from the group consisting of tellurium, sulfur
and selenium.
22. The thermoelectric composition of claim 1, wherein the matrix
comprises PbQ, and the Q component comprises at least one element
selected from the group consisting of: tellurium, selenium, and
sulfur.
23. The thermoelectric composition of claim 1, wherein the matrix
comprises SnQ, and the Q component comprises at least one element
selected from the group consisting of: tellurium and selenium.
24. The thermoelectric composition of claim 1, wherein the
nanoscale inclusions do not act as a strong scatterer to
electrons.
25. The thermoelectric composition of claim 1, wherein at least a
portion of the inclusions in the matrix are thermally stable to a
temperature higher than 650 K.
26. The thermoelectric composition of claim 1, wherein the
composition comprises a bulk composition.
27. A thermoelectric composition comprising: a matrix comprising a
first chalcogenide; and nanoscale inclusions in the matrix, the
nanoscale inclusions have a first melting point, the matrix has a
second melting point, the first melting point is lower than the
second melting point, the nanoscale inclusions having a different
composition than the first chalcogenide, so that the nanoscale
inclusions decrease the thermal conductivity of the composition by
scattering phonons in the composition while substantially
maintaining or increasing electrical conductivity and Seebeck
coefficient of the composition.
28. A thermoelectric composition comprising: a matrix comprising a
first chalcogenide; and nanoscale inclusions in the matrix, the
nanoscale inclusions comprise a second chalcogenide different from
the first chalcogenide, the second chalcogenide comprising a
chalcogen selected from the group consisting of tellurium, sulfur
and selenium, so that the nano scale inclusions decrease the
thermal conductivity of the composition by scattering phonons in
the composition while substantially maintaining or increasing
electrical conductivity and Seebeck coefficient of the composition.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 11/445,662 filed Jun. 2, 2006 and incorporated in its
entirety by reference herein, which claims the benefit of U.S.
Provisional Patent Application No. 60/687,769 filed Jun. 6,
2005.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a process for producing
novel bulk thermoelectric compositions with nanoscale inclusions
which enhance the figure of merit (ZT). In particular, the present
invention relates to thermoelectric compositions wherein the
nanoscale inclusions are visible by conventional nanoscale imaging
techniques such as transmission electron microscopy (TEM) imaging.
They are useful for power generation and heat pumps.
[0004] 2. Description of the Related Art
[0005] The prior art in thermoelectric materials and devices is
generally described in U.S. Pat. No. 5,448,109 to Cauchy, U.S. Pat.
No. 6,312,617 to Kanatzidis et al., as well as published
application 2004/0200519 A1 to Sterzel et al. and 2005/0076944 A1
to Kanatzidis et al. Each of these references is concerned with
increasing the figure of merit (ZT) which is directly influenced by
the product of electrical conductivity and the square of the
thermopower divided by the thermal conductivity. Generally as the
electrical conductivity of a thermoelectric material is increased,
the thermal conductivity is increased. The efficiency of the
thermoelectric device is less than theoretical and may not be
sufficiently efficient for commercial purposes.
SUMMARY OF THE INVENTION
[0006] It is therefore an object of the present invention to
provide relatively efficient bulk thermoelectric materials.
Further, it is an object of the present invention to provide a
process for the preparation of these thermoelectric materials.
Further, it is an object of the present invention to provide
thermoelectric materials which are relatively economical to prepare
compared to artificial deposited superlattice thin film
thermoelectric materials. These and other objects will become
increasingly apparent by reference to the following description and
the drawings.
[0007] The present invention relates to a thermoelectric
composition which comprises: a homogenous solid solution or
compound of a first chalcogenide providing a matrix with nanoscale
inclusions of a second phase which has a different composition
wherein a figure of merit (ZT) of the composition is greater than
that without the inclusions. Preferably the inclusion has been
formed by spinodal decomposition as a result of annealing the
composition at an appropriate temperature less than a melting point
of the homogenous solid solution based upon a phase diagram.
Preferably the inclusion has been formed by matrix encapsulation as
a result of doping of a mol ten solution of the matrix. Preferably
the inclusion has been formed by nucleation and growth of the
inclusion by cooling a molten solution of the matrix.
[0008] The present invention also relates to a thermoelectric
composition which comprises a homogenous solid solution or compound
of a chalcogenide comprising a uniform precipitated dispersion of
nano particles of at least two different metal chalcogenides
wherein the chalcogen is selected from the group consisting of
tellurium, sulfur and selenium. Preferably the composition has been
formed by spinodal decomposition of the solid solution.
[0009] The present invention also relates to a thermoelectric
composition which comprises a homogenous solid solution or compound
of a chalcogenide with dispersed nano particles derived from a
metal or a semiconductor which have been added to the
chalcogenide.
[0010] The present invention also relates to a thermoelectric
composition which comprises a homogenous solid solution or compound
of a chalcogenide which has been annealed at a temperature which
allows the formation of nano particles having a different
composition than the solid solution or compound.
[0011] Further, the present invention relates to a composition
wherein the inclusion has been formed by matrix encapsulation as a
result of doping of a molten solution of the matrix.
[0012] Still further, the present invention relates to a
composition wherein the inclusion has been formed by nucleation and
growth of the inclusion by cooling a molten solution of the
matrix.
[0013] The present invention further relates to a process for
preparing a thermoelectric composition which comprises:
[0014] (a) forming a liquid solution or compound of a first
chalcogenide and a second phase which has a different
composition;
[0015] (b) cooling the solution rapidly so that a solid solution of
the first chalcogenide as a matrix and the second phase as a
nanoscale inclusion is formed, so that the figure of merit is
greater than without the inclusions. Preferably the inclusion is
formed by spinodal decomposition as a result of annealing the
composition at an appropriate temperature less than a melting point
of the homogenous solid solution based upon a phase diagram.
Preferably the inclusion is formed by matrix encapsulation as a
result of cooling a molten solution of the matrix. Preferably the
inclusion is formed by nucleation and growth of the inclusion in a
supersaturated solid solution of the matrix. Preferably the
chalcogenides are of a chalcogen selected from the group consisting
of tellurium, sulfur and selenium. Preferably the inclusions are
between about 1 and 200 nanometers.
[0016] The substance and advantages of the present invention will
become increasingly apparent by reference to the following drawings
and the description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a graph showing the lowest thermal conductivity
exhibited by the PbTe--PbS 16% nanocomposite.
[0018] FIG. 2A is a theoretical phase diagram for A and B.
[0019] FIG. 2B is a diagram showing the spatial difference in the
composition of the phases.
[0020] FIG. 3A shows a PbTe--PbS phase diagram. FIG. 3B is a
PbTe--PbS x % nano-composite reaction and post-annealing profile
taking advantage of spinodal decomposition. FIGS. 3C and 3D are
high resolution TEM images of a PbTe--PbS 16% spinodally decomposed
system.
[0021] FIG. 4A is a schematic of the matrix encapsulation. FIG. 4B
is a graph of a PbTe--Sb heating profile. FIG. 4C is a PbTe--Sb
(2%) Bright Field Image. FIGS. 4D and 4E are corresponding Bright
and Dark Field TEM images of PbTe--InSb (2%). FIGS. 4F to 4K are
transmission electron micrographs showing dispersed nanoparticles
of Sb within a crystalline matrix of PbTe. Similar size, shape, and
volume fraction are observed for (A) PbTe--Sb (2%) (B) PbTe--Sb
(4%) (C) PbTe--Sb (8%) and (D) PbTe--Sb (16%). Because the 8 and
16% samples contain distinct Sb regions the images shown in FIGS.
4C and 4D are from the PbTe rich region. FIG. 4E is a high
resolution transmission electron micrograph showing several
nanoprecipitates of Sb coherently embedded within the matrix of
PbTe. Embedded particles help to maintain high electron mobility
while serving as a site for phonon scattering to reduce the thermal
conductivity. FIG. 4F is a high resolution micrograph of the
PbTe--Bi (4%) system also showing embedded particles in the PbTe
matrix. FIG. 4L is a graph showing thermal conductivity as a
function of temperature.
[0022] FIGS. 5A and 5B are scanning electron micrographs of PbTe+Pb
(2%)+Sb (3%). Large regions or ribbons, several hundred microns in
length, composed of a Pb--Sb eutectic appear throughout the sample.
Similar microstructure is observed for other samples with similar
composition.
[0023] FIGS. 6A and 6B show powder x-ray diffraction clearly
indicating additional phases of Pb and Sb as revealed by the
magnified inset between 25 and 40 degrees. The peak at .about.29
deg. corresponds to elemental Sb while the peaks at 31 and 36 deg.
can be indexed according to elemental Pb.
[0024] FIGS. 7A and 7B show (7A) low magnification transmission
electron micrographs showing dispersed particles of Pb and Sb
within the PbTe matrix. FIG. 7B shows high magnification TEM
micrographs showing the particles appear coherently embedded in the
matrix.
[0025] FIG. 8 is a graph showing lattice thermal conductivity as a
function of Pb/Sb ratio at 350K and 600K. A strong linear
dependence of the lattice thermal conductivity is observed as the
ratio is varied.
[0026] FIG. 9A is a schematic diagram of supersaturated solid
solution produced through quenching. FIG. 9B is a schematic diagram
of a post annealing within the two phase region of the phase
diagram which initiates the coalescence of the second phase into
ordered nano-precipitates. FIG. 9C is a schematic diagram of a
coherent nano-particle which has been formed. FIG. 9D is a
PbTe--CdTe phase diagram. FIG. 9E is a graph of PbTe--CdTe x %
reaction and post annealing profile for 2.ltoreq.x.ltoreq.9. FIGS.
9F and 9G are TEM images of PbS--PbTe6%, and FIGS. 9H and 9I are
TEM images of PbTe--CdTe9%.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0027] The bulk materials containing nanometer-sized inclusions
provide enhanced thermoelectric properties. The thermoelectric
figure of merit is improved by reducing the thermal conductivity
while maintaining or increasing the electrical conductivity and the
Seebeck coefficient. Coherent nanometer sized inclusions in a
matrix can serve as sites for scattering of phonons that
subsequently lower the thermal conductivity. General methods for
preparation of these materials have been developed.
[0028] The thermoelectric heat to electricity converters will play
a key role in future energy conservation, management, and
utilization. Thermoelectric coolers also play an important role in
electronics and other industries. More efficient thermoelectric
materials need to be identified in order to extend their use in
power generation and cooling applications.
[0029] As previously noted, the measure used to determine the
quality of a thermoelectric material is the dimensionless figure of
merit ZT, where ZT=(.sigma.S.sup.2/K)T, where .sigma. is the
electrical conductivity, S the Seebeck coefficient or the absolute
thermopower, T is the temperature and .kappa. is the thermal
conductivity. The quantity .sigma.S.sup.2 is called the power
factor. The goal is then to simultaneously improve the thermopower,
electrical conductivity (i.e. the power factor), and reduce the
thermal conductivity thereby raising ZT. The aforementioned
properties are intimately related.
[0030] PbTe and Si/Ge alloys are the current thermoelectric
materials used for power generation. These compounds once doped
possess a maximum ZT of approximately 0.8 at 600 K and 1200 K
respectively. By lowering the thermal conductivity of these
materials the ZT can be improved without sacrificing the properties
already known. Currently Bi.sub.2Te.sub.3 and its alloys with
Bi.sub.2Se.sub.3 and Sb.sub.2Te.sub.3, along with alloys of Bi and
Sb, are considered the state of the art in terms of thermoelectric
cooling materials. These materials have been modified in many ways
chemically in order to optimize their performance, however
significant improvements can be made to enhance the properties of
the currently used thermoelectric materials.
[0031] Increasing the efficiency of a thermoelectric material
usually involves raising the scattering rate of phonons while at
the same time maintaining high carrier mobility. In this respect,
it has been demonstrated that thin-film superlattice materials have
enhanced ZT that can be explained by the decrease in the thermal
conductivity. A supperlattice structure creates de facto a complex
arrangement of structural interfaces which in effect raises the
thermal resistance of propagating phonons. On the other hand,
lattice-matching and coherence of the interfaces ensures
undisturbed electron flow thus maintaining a high mobility. This
decoupling of electrical and lattice thermal conductivity is
necessary to reduce the total thermal conductivity without
sacrificing the electrical conductivity.
[0032] The drawbacks of superlattice thin films are that they are
expensive to prepare, difficult to grow, and will not easily
support a large temperature difference across the material. It is
thus desirable to incorporate inclusions on the nanometer length
scale into a bulk material that is low cost, easy to manufacture,
and can support a temperature gradient easily.
[0033] In the present invention, three methods have been employed
in the production of the desired nanocomposite material for
thermoelectric material fabrication. Each of these methods are
discussed in detail in the following sections along with an
example, Transmission Electron Microscope (TEM) images, and a table
of materials systems that can be produced from each general method.
The first, spinodal decomposition, has been used to create a
material with compositional fluctuations on the nanometer length
scale. The other two methods, matrix encapsulation and nucleation
and growth, have shown the ability to produce inclusions of various
materials inside a host matrix.
[0034] Phonon mean free paths, .sub.ph, in semiconducting crystals
are in the range 1.ltoreq..sub.ph.ltoreq.100 nm with a tendency to
decrease with increasing temperatures. Realization of
nano-composite thermoelectric materials offer a way of introducing
nano-meter sized scatterers that can greatly suppress the lattice
thermal conductivity through phonon scattering. The existence of a
wide particle size distribution offers the possibility of
scattering a wider range of the phononic spectrum.
[0035] Experimental confirmation of the above at room temperature
and below comes from the PbTe--PbS 16% at. system as the plot of
the lattice thermal conductivity shows in FIG. 1, a >40%
reduction of the lattice thermal conductivity is observed in the
case of the nano-precipitate specimen with respect to the perfect
mixture of the same composition at room temperature.
[0036] It has been suggested that band gap or electron energy
states engineering offer an alternative route to further enhancing
the power factor of thermoelectric materials. Essentially the idea
consists in the mixing of parabolic bands (bulk semiconductors)
with reduced dimensionality structures (e.g. nano-dots exhibit a
comb-like density of states) to produce a ripple effect on the
resulting density of states of the composite.
[0037] Experimental confirmation of enhanced power factors come
from the following systems shown in Table 1:
TABLE-US-00001 TABLE 1 Power factor Temperature Sample composition
(.mu.W/cmK.sup.2) (K) PbTe--PbS 16%: PbI.sub.2 28 400 0.05%
PbTe--CdTe 5%: PbI.sub.2 30 300 0.05% PbTe--CdTe 9% 26 300 PbTe--Sb
4% 20 300 PbTe--InSb 2% 21 300 PbTe--Pb (0.5%)--Sb (2%) 28 300
PbTe--Pb (2%)--Sb 3%) 19 300
Method 1: Spinodal Decomposition
[0038] Spinodal decomposition refers to the way a stable
single-phase mixture of two phases can be made unstable.
Thermodynamically, the necessary condition for the stability or
metastability of a heterogeneous phase is that the chemical
potential of a component must increase with increasing density of
that component. For two components this reduces to
.differential. 2 G .differential. 2 X T , P > 0 ,
##EQU00001##
where X is the concentration. If this condition is not met, the
mixture is unstable with respect to continuous compositional
variations and the limit of this metastability is called the
spinodal defined as
.differential. 2 G .differential. 2 X = 0 , ##EQU00002##
where X is the concentration. Spinodal fluctuations do not involve
any crystalline transformation, since both components of the mixed
phase system are sharing the same lattice, but involves a spatial
modulation of the local composition at the nanoscale. This spatial
modulation was exploited to create coherently embedded
nano-particles of a phase into a thermoelectric matrix and thus
create nanostructured thermoelectric materials on a large reaction
scale.
[0039] Consider a phase diagram with a miscibility gap, i.e. an
area within the coexistence curve of an isobaric or an isothermal
phase diagram where there are at least two phases coexisting (see
FIG. 2A). If a mixture of phases A and B and of composition X.sub.o
is solution treated at a high temperature T.sub.1 and then quenched
at to a lower temperature T.sub.2 the composition instantly will be
the same everywhere (ideal solid solution) and hence the system's
free energy will be G.sub.o on the G(X) curve. However,
infinitesimal compositional fluctuations cause the system to
locally produce A-rich and B-rich regions. The system now has
become unstable since the total free energy has decreased. In time,
the system decomposes until the equilibrium compositions X.sub.1
and X.sub.2 are reached throughout the system (compare FIG. 2A and
FIG. 2B).
[0040] There are two major advantages in the application of the
spinodal decomposition process in order to produce thermoelectric
nanocomposites; (a) thermodynamic principles define the spatial
modulation wavelength .lamda. to be in the range nm which is a very
desirable phonon-scattering length scale and (b) the nano-structure
is thermodynamically stable. Therefore, spinodally decomposed
thermoelectric materials are naturally produced bulk nanocomposites
which can be perpetually stable when used within a specified
temperature region defined by the phase diagram.
[0041] The aforementioned procedure was applied extensively in the
PbTe--PbS system where PbTe serves as the matrix.
Example
PbTe--PbS x % Preparation Example
[0042] Spinodal decomposition in the two components system
PbTe--PbS x % occurs for .about.4.ltoreq.x.ltoreq.96% for
temperatures roughly below 700.degree. C. (see accompanying phase
diagram FIG. 3A). The high purity starting materials are mixed in
aqua regia cleaned fused silica tubes and fired according to the
reaction profile shown in FIG. 3B.
[0043] The TEM images of spinodally decomposed system PbTe--PbS 16%
are shown in FIGS. 3C, 3D.
[0044] The following Table 2 show systems that can be produced to
exist in a nanostructured state via the Spinodal Decomposition
mechanism. The listing is a set of materials composed of component
A and B in a A.sub.1-XB.sub.x stoichiometry (0<x<1).
TABLE-US-00002 TABLE 2 Spinodal Decomposition A-B PbTe--PbS
AgSbTe.sub.2--SnTe PbS--PbTe SnTe/SnSe SnTe/PbS SnTe/PbSe SnTe/SnSe
SnSe/PbS SnSe/PbSe SnSe/PbTe AgSbSe.sub.2--SnTe AgSbTe.sub.2--SnSe
AgSbSe.sub.2--PbTe AgSbS.sub.2--PbTe SnTe--REPn (RE = rare earth
element, Pn = P, As, Sb, Bi) PbTe--REPn (RE = rare earth element,
Pn = P, As, Sb, Bi) PbSe--REPn (RE = rare earth element, Pn = P,
As, Sb, Bi) SnSe--REPn (RE = rare earth element, Pn = P, As, Sb,
Bi)
Method 2: Matrix Encapsulation
[0045] These systems, as described by a phase diagram, should have
a solid solution in the composition range of approximately 0.1-15%
of the minor phase. However, it has been observed that, when
quenched from a melt, these systems exhibit inclusions on the
nanometer scale of the minor phase material. This phenomenon can be
extended to other systems of thermoelectric interest where the
matrix is a good thermoelectric and the minor phase is a material
that is nonreactive, has a lower melting point, and is soluble with
the matrix in the liquid state. The minor phase may also be a
mixture of two or more of these non-reactive materials which mayor
may not form a compound themselves. These materials must be
quenched quickly through the melting point of the matrix in order
to freeze the minor phase. After quenching the samples must be post
annealed to improve crystallinity and thermoelectric
properties.
Example
[0046] This method has been applied to PbTe--Sb, PbTe--Bi,
PbTe--InSb and PbTe--Pb--Sb showing promise in each of the
cases.
PbTe--Sb 4% Preparation Example
[0047] Lead telluride and antimony were combined in the appropriate
molar ratio and sealed in an evacuated fused silica tube and heated
according to the profile shown in FIG. 4B. The bright field and
dark field images are shown in FIGS. 4C to 4E. The TEM images of
encapsulated nanoparticles are shown in FIGS. 4F to 4K. FIG. 4L
shows the lattice thermal conductivity.
[0048] Table 3 shows systems for Matrix Encapsulation listing the
matrix and precipitate.
Matrix Encapsulation Using Two or More Types of Nanophase
Particles:
[0049] It is possible to produce samples via the matrix
encapsulation method which have multiple nanoscale inclusions (two
or more from those listed in Table 3).
[0050] These inclusions may be used to combine the favorable
properties of each to produce a superior thermoelectric material.
The additional phases must also be soluble with the matrix in the
liquid state, mayor may not be reactive with the matrix, and mayor
may not form a compound between each other. This method has been
applied to PbTe with inclusions of both Sb and Pb with interesting
behavior in terms of both the reduction of the thermal
conductivity, and modification of the behavior of the electrical
transport as well. The ratio of Pb to Sb can modify the
conductivity such that a higher electrical conductivity may be
maintained through the desired temperature range. The mass
fluctuations associated with the additional phase reduce the
thermal conductivity as seen in the previously discussed
examples.
PbTe--Pb--Sb Preparation Example
[0051] Pb, Sb, and Te were sealed in an evacuated fused silica tube
and heated to the molten state. The tube was then removed from the
high temperature furnace for rapid cooling of the melt. This
procedure is similar to those discussed above, however multiple
nanoprecipitate inclusion phases are used rather than a single
component inclusion. Many different possible inclusion combinations
are possible and one example, the PbTe--Pb--Sb case, is given
below.
[0052] SEM micrographs (FIGS. 5A and 5B), Powder X-ray diffraction
(FIGS. 6A and 6B), TEM micrographs (FIGS. 7A and 7B), and
experimental power factors and thermal conductivity values (FIG.
8). These systems represent an interesting set of materials in
which the transport properties can be tuned by several variables
such as total concentration, ratio of various inclusion phases, and
the properties of the inclusions themselves. Optimization is still
underway and ZT values of over 1 have been obtained in the
as-prepared systems.
TABLE-US-00003 TABLE 3 Matrix Encapsulation
A(matrix)-B(precipitate) Pb(Te,Se,S)--Sb,Bi,As)
Pb(Te,Se,S--(InSb,GaSb) Pb(Te,Se,S)--Yb Pb(Te,Se,S)--(InAs,GaAs)
Pb(Te,Se,S)--Eu Pb(Te,Se,S)--In Pb(Te,Se,S)--Ga Pb(Te,Se,S)--Al
Pb(Te,Se,S)--Zn Pb(Te,Se,S)--Cd Pb(Te,Se,S)--Sn
Pb(Te,Se,S)--T1InQ2* Pb(Te,Se,S)--ZnxSby Pb(Te,Se,S)--AgYb
Pb(Te,Se,S)--CdPd Pb(Te,Ae,S)--REPb3 (RE = rare earth element,Y)
Pb(Te,Se,S)--M (M = Ge, Sn, Pb) Pb(Te,Se,S)--Ag4Eu
Pb(Te,Se,S)--AgEu Pb(Te,Se,S)--AgCe Pb(Te,Se,S)--Mg2Cu
Pb(Te,Se,S)--Cu2La Pb(Te,Se,S)--Cu6Eu Pb(Te,Se,S)Eu3Pd2
Pb(Te,Se,S)--Mg2Eu Pb(Te,Se,S)--PdTe2 Pb(Te,Se,S)--Mg2Sn
Pb(Te,Se,S)--MgSm Pb(Te,Se,S)--MgPr Pb(Te,Se,S)--Mg2Pb
Pb(Te,Se,S)--Ca84Ni16 Pb(Te,Se,S)--RE2Pb (RE = rare earth element)
Pb(Te,Se,S)--Mg2Pb Pb(Te,Se,S)--REPb (RE = rare earth element) *Q =
S,Se,Te
Method 3: Nucleation and Growth Mechanism:
[0053] The method of nucleation and growth of nanoparticles within
the matrix of a thermoelectric material consists of three distinct
thermal treatments that depend crucially on the phase diagram of
the composite:
[0054] a) The starting materials (mixed in appropriate
stoichiometry) are heated from the two-phase region to the
single-phase region of the phase diagram to dissolve all
precipitates. The mixture is held there for several hours to ensure
complete homogeneity;
[0055] b) The melt or solid solution is quenched to room
temperature using different methods: air quenching, water
quenching, ice water quenching. This freezes the high temperature
homogenous phase into a supersaturated solid solution; and
[0056] c) Depending on the kinetics of the specific system the
specimen is post annealed at an elevated temperature within the
two-phase region of the phase diagram and is held there for several
hours to allow the nanoprecipitates to form and grow. Annealing
time and temperature is proportional to the size growth of the
precipitates. Therefore, the size of the nanoprecipitates can be
controlled through careful selection of annealing time and
temperature.
[0057] The following schematic shows in FIGS. 9A, 9B and 9C roughly
how the nano-precipitation of the second phase is taking place.
[0058] As a general rule this kind of nanostructured thermoelectric
materials should meet two conditions: (1) The two phases should
contain elements that enter a solid solution phase at a specific
temperature and separate into a mixture at another lower
temperature. (2) The phase that precipitates out must create a
coherent or at best semi-coherent precipitate. Coherency is
important since it ensures bonding with the lattice of the matrix
and hence the precipitate does not act as a strong scatterer to the
electrons.
[0059] The above procedure has been extensively applied to the
PbTe--CdTe system with excellent results.
Example
PbTe--CdTe x % Preparation Example
[0060] Stoichiometric quantities of Pb, Te and Cd are weighed
targeting x % values in the range 2.ltoreq.x.ltoreq.9. The starting
materials are placed in graphite crucibles, which are subsequently
sealed under high vacuum in fused silica tubes and fired according
to the reaction profile shown below (FIG. 9E). The reaction profile
is decided based on the phase diagram of the PbTe--CdTe system
(FIG. 9D). FIGS. 9F and 9G show the TEM images for precipitation
and growth of PbS--PbTe 6%. FIGS. 9H and 9I show the system
PbTe--CdTe 9%.
[0061] The following Table 4 shows systems for nucleation and
growth listing the matrix and precipitate.
TABLE-US-00004 TABLE 4 Nucleation and Growth
A(matrix)-B(precipitate) PbSe--Sb.sub.2Se.sub.3 PbSe--SnSe.sub.2
PbSe--Zn PbTe--Hg.sub.1-xCd.sub.xTe (0 < x < 1) PbTe--ZnTe
PbTe--Sb.sub.2Se.sub.3 PbTe--Zn PbTe--In.sub.2Se.sub.3
PbTe--In.sub.2Te.sub.3 PbTe--Ga.sub.2Te.sub.3 PbTe--AgInTe.sub.2
PbTe--CuInTe.sub.2 PbTe--CuInSe.sub.2 PbTe--CuInTe.sub.2
PbSe--Hg.sub.1-xCd.sub.xQ (0 < x < 1, Q = S, Se, Te)
PbSe--ZnTe PbSe--In.sub.2Se.sub.3 PbSe--In.sub.2Te.sub.3
PbSe--Ga.sub.2Te.sub.3 PbSe--AgInSe.sub.2 PbSe--CuInTe.sub.2
PbSe--CuInSe.sub.2 PbSe--CuInTe.sub.2
[0062] It is intended that the foregoing description be only
illustrative of the present invention and that the present
invention be limited only by the hereinafter appended claims.
* * * * *