U.S. patent application number 14/517763 was filed with the patent office on 2015-03-12 for methods for high figure-of-merit in nanostructured thermoelectric materials.
The applicant listed for this patent is MASSACHUSETTS INSTITUTE OF TECHNOLOGY, TRUSTEES OF BOSTON COLLEGE. Invention is credited to Gang Chen, Xiaoyuan Chen, Mildred Dresselhaus, Joshi R. Giri, Qing Hao, Yucheng Lan, Yi Ma, Bed Poudel, Zhifeng Ren, Dezhi Wang, Xiaowei Wang, Xiao Yan, Bo Yu.
Application Number | 20150068574 14/517763 |
Document ID | / |
Family ID | 39714507 |
Filed Date | 2015-03-12 |
United States Patent
Application |
20150068574 |
Kind Code |
A1 |
Ren; Zhifeng ; et
al. |
March 12, 2015 |
METHODS FOR HIGH FIGURE-OF-MERIT IN NANOSTRUCTURED THERMOELECTRIC
MATERIALS
Abstract
Thermoelectric materials with high figures of merit, ZT values,
are disclosed. In many instances, such materials include nano-sized
domains (e.g., nanocrystalline), which are hypothesized to help
increase the ZT value of the material (e.g., by increasing phonon
scattering due to interfaces at grain boundaries or grain/inclusion
boundaries). The ZT value of such materials can be greater than
about 1, 1.2, 1.4, 1.5, 1.8, 2 and even higher. Such materials can
be manufactured from a thermoelectric starting material by
generating nanoparticles therefrom, or mechanically alloyed
nanoparticles from elements which can be subsequently consolidated
(e.g., via direct current induced hot press) into a new bulk
material. Non-limiting examples of starting materials include
bismuth, lead, and/or silicon-based materials, which can be
alloyed, elemental, and/or doped. Various compositions and methods
relating to aspects of nanostructured theromoelectric materials
(e.g., modulation doping) are further disclosed.
Inventors: |
Ren; Zhifeng; (Houston,
TX) ; Poudel; Bed; (West Newton, MA) ; Chen;
Gang; (Carlisle, MA) ; Lan; Yucheng; (Newton,
MA) ; Wang; Dezhi; (Wellesley, MA) ; Hao;
Qing; (Tucson, AZ) ; Dresselhaus; Mildred;
(Arlington, MA) ; Ma; Yi; (Somerville, MA)
; Yan; Xiao; (Brighton, MA) ; Chen; Xiaoyuan;
(Acton, MA) ; Wang; Xiaowei; (Newton, MA) ;
Giri; Joshi R.; (Allston, MA) ; Yu; Bo;
(Allston, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
TRUSTEES OF BOSTON COLLEGE |
Cambridge
Chestnut Hill |
MA
MA |
US
US |
|
|
Family ID: |
39714507 |
Appl. No.: |
14/517763 |
Filed: |
October 17, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11949353 |
Dec 3, 2007 |
8865995 |
|
|
14517763 |
|
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|
10977363 |
Oct 29, 2004 |
7465871 |
|
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11949353 |
|
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|
60872242 |
Dec 1, 2006 |
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Current U.S.
Class: |
136/200 ;
264/104 |
Current CPC
Class: |
H01L 35/16 20130101;
H01L 35/20 20130101; H01L 35/22 20130101; H01L 35/34 20130101; H01L
35/18 20130101 |
Class at
Publication: |
136/200 ;
264/104 |
International
Class: |
H01L 35/20 20060101
H01L035/20; H01L 35/18 20060101 H01L035/18; H01L 35/34 20060101
H01L035/34; H01L 35/16 20060101 H01L035/16 |
Goverment Interests
FEDERALLY SPONSORED RESEARCH
[0002] The invention was made with government support under Grant
No. NSF0506830 awarded by the National Science Foundation. The
government has certain rights in this invention.
Claims
1-92. (canceled)
93. A method of forming a semiconductor alloy thermoelectric
material, comprising compacting a powder by hot pressing to
consolidate the powder at a pressure in a range of about 40 MPa to
about 300 MPa and at a temperature in a range of about 400.degree.
C. to about 1200.degree. C. to form the semiconductor alloy
thermoelectric material comprising a plurality of randomly oriented
grains having an average size below 5000 nm, wherein the
semiconductor alloy thermoelectric material exhibits a ZT value
between 0.8 and 4 for at least one temperature in a range of about
room temperature to about 2000.degree. C.
94. The method of claim 1, further comprising in-situ precipitating
during the hot pressing one or more precipitation regions in at
least some of the grains, the one or more precipitation regions
having an average size of 1 nm to 50 nm.
95. The method of claim 94, wherein the precipitation region is
characterized by at least one of a different composition, a
different crystalline direction and a different phase relative to
the rest of the grain in which the precipitation region is
formed.
96. The method of claim 1, further comprising: providing at least
two different elemental powders; and forming the powder by
mechanically alloying the at least two different elemental powders
to form a semiconductor alloy powder comprising semiconductor alloy
nanoparticles.
97. The method of claim 96, wherein: the step of mechanically
alloying comprises milling the at least two different elemental
powders; and the semiconductor alloy nanoparticles have an average
size of about 1 to about 200 nm.
98. The method of claim 1, wherein the powder comprises a mixture
of at least two different elemental powders each comprising
elemental nanoparticles.
99. The method of claim 1, wherein the powder comprises a
semiconductor alloy powder comprising semiconductor alloy
nanoparticles.
100. The method of claim 99, wherein the powder is formed by
breaking up a bulk semiconductor alloy material to form a powder
comprising nanoparticles of the semiconductor alloy material.
101. The method of claim 100, wherein the nanoparticles of the
semiconductor alloy material have an average size of about 1 to 200
nm.
102. The method of claim 1, wherein the powder is consolidated at a
pressure in a range of 40-160 MPa.
103. The method of claim 1, wherein the material exhibits a ZT
value between 1 and 4 for at least one temperature in a range of
room temperature to 2000.degree. C.
104. The method of claim 1, wherein the material exhibits a ZT
value between 1.2 and 4 for at least one temperature in a range of
room temperature to 2000.degree. C.
105. The method of claim 1, wherein the plurality of randomly
oriented grains have an average size below 2000 nm.
106. The method of claim 1, wherein the plurality of randomly
oriented grains have an average size below 500 nm.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S. patent
application bearing Ser. No. 11/949,353, filed Dec. 3, 2007 and
issued as U.S. Pat. No. 8,865,995 on Oct. 21, 2014, entitled
"Methods for high Figure-of-Merit in Nanostructured Thermoelectric
Materials," which itself is a continuation-in-part of a U.S. patent
application bearing Ser. No. 10/977,363, filed Oct. 29, 2004,
entitled "Nanocomposites with High Thermoelectric Figures of
Merit," and which issued as U.S. Pat. No. 7,465,871. U.S. patent
application bearing Ser. No. 11/949,353 also claims the benefit of
a U.S. Provisional Patent Application bearing Ser. No. 60/872,242,
filed Dec. 1, 2006, entitled "Methods for High Figure-of-Merit in
Nanostructured Thermoelectric Materials." The contents of all these
applications are hereby incorporated herein by reference in their
entireties.
FIELD OF THE APPLICATION
[0003] The present application relates generally to thermoelectric
materials and methods for their fabrication, and more particularly,
to such thermoelectric materials that exhibit enhanced
thermoelectric properties.
BACKGROUND
[0004] The thermoelectric properties of any material can be
characterized by a quantity called figure of merit Z (or
dimensionless figure of merit ZT), defined as Z=S.sup.2.sigma./k,
where S is Seebeck coefficient, .sigma. is electrical conductivity,
and k is total thermal conductivity. It is desirable to construct
materials with high ZT values (e.g., having low thermal
conductivity k and/or high power factor S.sup.2.sigma.). By way of
example, such materials can potentially be used to construct high
quality power generation devices and cooling devices.
SUMMARY
[0005] In one aspect, the invention is directed to a method of
fabricating a thermoelectric material by generating a plurality of
nanoparticles from a starting material such as a thermoelectric
bulk material, and consolidating those nanoparticles under pressure
at an elevated temperature to form a thermoelectric material that
exhibits a higher ZT value than the thermoelectric starting
material, e.g., at a temperature below about 2000.degree. C., below
about 1000.degree. C., below about 600.degree. C., below about
200.degree. C., or below about 20.degree. C. In some instances the
peak ZT value of the formed material can be about 25% to about
1000% greater than the peak ZT value of the starting material. In
other instances, the peak ZT of the formed material can be
substantially higher than 1000% of the peak ZT of the starting
material.
[0006] The term "nanoparticle" is generally known in the art, and
it is used herein to refer to a material particle having a size
(e.g., an average or a maximum size) less than about 1 micron such
as in a range from about 1 nm to about 1000 nm. Preferably the size
can be less than about 500 nanometers (nm), preferably in a range
of about 1 to about 200 nm, and more preferably in a range of about
1 to about 100 nm. The nanoparticles can be generated, for
instance, by breaking up a starting material into nano-sized pieces
(e.g., grinding using any of dry milling, wet milling, or other
suitable techniques). In one example, ball milling can be used to
achieve the desired nanoparticles. Optionally, cooling can also be
employed while generating nanoparticles (e.g., cooling a starting
material while grinding it), so as to further reduce the size of
the particles. Some other methods of generating the nanoparticles
can include condensation from a gas phase, wet chemical methods,
and other methods of forming nanoparticles. In some cases,
nanoparticles of different elemental materials (e.g., bismuth or
tellurium) can be generated separately, and subsequently
consolidated into a resultant thermoelectric material, as discussed
further below.
[0007] The nanoparticles can be consolidated under a selected
temperature and selected pressure so as to induce electrical
coupling between the nanoparticles sufficient to form the resultant
thermoelectric material. By way of examples, hot pressing including
current induced hot press, unidirectional hot press, plasma
pressure compaction (P.sup.2C) or spark plasma sintering (SPS), and
isostatic hot press processes can be used to achieve the
consolidation of the nanoparticles. The selected pressure can be,
for example, in a range of about 10 MPa to about 900 MPa, or in a
range of about 40 MPa to about 300 MPa, and preferably in a range
of about 60 MPa to about 200 MPa. The selected temperature can be,
for example, in a range between about 200.degree. C. to about the
melting point of the thermoelectric material (e.g., 200.degree. C.
to about 2000.degree. C.), or in a range of about 400.degree. C. to
about 1200.degree. C., or in a range of about 400.degree. C. to
about 600.degree. C., or in a range of about 400.degree. C. to
about 550.degree. C. in the case of Bi.sub.2Te.sub.3 based
materials.
[0008] In a related aspect, in the above method, consolidating the
nanoparticles refers to compactifying the nanoparticles so as to
provide a material exhibiting a density in a range of about 90% to
about 100% of the respective theoretical density (e.g., a porosity
less than about 10% or less than about 1%).
[0009] In a related aspect, a thermoelectric material generated by
the methods of the invention, such as those discussed above,
exhibits a ZT value (e.g., a peak ZT value) greater than about 1,
or greater than about 1.2, or greater than about 1.4, and
preferably greater than about 1.5, and most preferably greater than
about 2. Further, in many embodiments, the thermoelectric materials
exhibit high ZT values at certain operating temperatures, which can
depend, e.g., on the materials' melting point, e.g., at a
temperature below about 300.degree. C. for Bi.sub.2Te.sub.3-based
materials. The elevated ZT values can also depend on the doping
levels and/or the material's microstructure
[0010] In many cases, the starting thermoelectric material (e.g., a
starting bulk material, or a fluid phase material for synthesizing
particles) exhibits a ZT value less than about 1, and optionally
greater than about 0.1, and the final thermoelectric material,
obtained by generating nanoparticles from the starting material
(e.g., breaking up the starting material by grinding or other
suitable technique) and consolidating those nanoparticles, exhibits
a ZT value greater than about 1, 1.1, 1.2, 1.3, 1.4, 1.5, or 2.
[0011] A variety of thermoelectric materials can be used as the
starting material in the practice of the invention. The starting
thermoelectric material can be p-doped or n-doped. Exemplary
starting thermoelectric materials include, without limitation,
bismuth-based, lead-based, or silicon-based materials. For example,
the starting thermoelectric material can comprise a
bismuth-antimony-tellurium alloy, a bismuth-selenium-tellurium
alloy, a lead-tellurium alloy, a lead-selenium alloy, or a
silicon-germanium (e.g., SiGe) alloy. By way of example, in some
embodiments, the thermoelectric material can be
Bi.sub.2Te.sub.3-xSe.sub.x alloy, wherein x is in a range of about
0 to about 0.8. Alternatively, in some other embodiments, the
thermoelectric material can be Bi.sub.xSb.sub.2-xTe.sub.3 alloy,
wherein x is in a range of about 0 to about 0.8. In some
embodiments, starting thermoelectric materials can be used that
have a polycrystalline structure, which can optionally include an
average crystalline grain size (e.g., greater than about 1
micron).
[0012] In another aspect, nanoparticles can be generated from a
starting thermoelectric material such that the generated
nanoparticles have sizes (e.g., average or maximum sizes) less than
about 1000 nm, or less than about 500 nm, or less than about 200
nm, and preferably less than about 100 nm, e.g., in a range of
about 1 nm to about 200 nm, or in a range of about 1 nm to about
100 nm, and preferably in a range of about 1 nm to about 50 nm.
Such particle sizes can be generated by any of the techniques
discussed herein, such as grinding a starting material by ball
milling, or other suitable techniques.
[0013] In a related aspect, in the above method, the nanoparticles
are held at the elevated temperature under pressure for a time
period, e.g., in a range of about 1 second to about 10 hours, so as
to generate a resultant thermoelectric material with enhanced
thermoelectric properties. In other aspects, the nanoparticles are
subjected to a selected temperature while being held at low or
ambient pressure for a time sufficient to allow the resultant
thermoelectric material to be formed. In another aspect,
nanoparticles can be consolidated under high pressure at room
temperature to form a sample with high theoretical density (e.g.,
about 100%), and then annealed at high temperature to form the
final thermoelectric material.
[0014] Another aspect is directed to a method of forming a
thermoelectric material that includes generating a plurality of
nanoparticles. By way of example, the particles can be generated by
grinding one or more bulk elemental materials. For example, the
nanoparticles can be generated by grinding at least two different
bulk elemental materials such as bismuth and tellurium; bismuth and
selenium; antimony and tellurium; antimony and selenium; and
silicon and germanium in any workable proportion. In such an
instance, at least two types of nanoparticles can be formed. If the
different types of particles are generated separately, the
particles can be mixed and further grinded (e.g., ball milled) to
form mechanically alloyed particles. Alternatively, the various
bulk materials can all be grinded simultaneously to form the
mechanically alloyed particles. A mixture of the nanoparticles,
formed using mechanical alloying or separately generated
nanoparticles from elements, compounds, or alloys, can be
compactified under pressure and at an elevated temperature to
generate a resultant thermoelectric material exhibiting a ZT value
greater than about 1. A dopant can optionally be added to the
mixture. In other embodiments, the nanoparticles can be
compactified with other types of particles such as particles from a
source material having a good ZT value (e.g., greater than about
0.5), and/or micron-sized particles (e.g., particles having an
average size from about 1 micron to about 10, 50, 100, or 500
microns).
[0015] In another aspect, a thermoelectric material is provided
that includes a material structure comprising a plurality of
inclusions having an average size in a range of about 1 nm to about
500 nm, wherein the structure exhibits a ZT value (e.g., a peak ZT
value) greater than about 1, and preferably a ZT value greater than
about 1.2, or greater than about 1.5, or even greater than about
2.
[0016] In a related aspect, the thermoelectric material can exhibit
the above ZT values at a temperature below about 2000.degree. C. or
below about 1000.degree. C. or below about 600.degree. C. or below
about 200.degree. C. or below about 20.degree. C. Further, the
average grain size can be in a range of about 1 to about 500 nm.
The structure can be substantially free of grains larger than about
500 nm (e.g., it is substantially free of grains having an average
and/or a maximum dimension greater than about 500 nm), or can
include some larger size grains (e.g., larger than about 1
.mu.m).
[0017] In another aspect, one or more of the grains include one or
more precipitation regions or other inclusions therein, where the
precipitation region or other inclusion can have, e.g., a size in a
range of about 1 to about 50 nm, or in a range of about 1 nm to
about 20 nm. A precipitation region can be characterized by a
different composition, and/or the same composition but different
crystalline direction, and/or different phase relative to the rest
of the grain.
[0018] In another aspect, the thermoelectric material can have a
density in a range of about 90% to about 100% of a respective
theoretical density. By way of example, the thermoelectric material
can exhibit a porosity less than about 10%, and preferably less
than about 1%.
[0019] In a related aspect, the thermoelectric material exhibits a
polycrystalline structure formed of small crystalline grains (e.g.,
having average sizes less than about 500 nm, or less than about 200
nm, and preferably in a range of about 1 nm to about 100 nm)
randomly oriented relative to one another.
[0020] One aspect of the invention is directed to a thermoelectric
material, which can include a material structure having a plurality
of grains. The grains can have an average size in the range of
about 1 micron to about 10 microns, or a range of about 1 micron to
about 5 microns, or a range of about 1 micron to about 2 microns.
At least some of the grains can include one or more precipitation
regions or other types of inclusions. Such regions can have an
average size from about 1 nm to about 100 nm, or about 1 nm to
about 50 nm. The thermoelectric material can have a ZT value
greater than about 1, 1.2, 1.5, or 2. For example, the ZT value can
also be in a range from about 1 to about 5. The thermoelectric
material can exhibit such ZT values at an operating temperature
less than about 2000.degree. C., or less than about 1000.degree.
C., or less than about 600.degree. C., or less than about
200.degree. C., or less than about 20.degree. C. The grains can be
formed from a variety of materials such as any combination of a
bismuth-based alloy, a lead-based alloy, and a silicon-based
alloy.
[0021] Another aspect of the invention is drawn to a thermoelectric
material that includes a host material having a plurality of
inclusions or particles dispersed throughout the host. The
particles or inclusions can have a size less than a threshold
value, e.g., less than about 20 microns. Host materials can include
one or more grains, where at least some of the grains have a size
(e.g., a maximum size in any dimension and/or an average size)
greater than about 1 micron, or less than about 1 micron. In some
embodiments, the host materials are not heavily doped as in typical
thermoelectric materials because a large portion of charge
carriers, e.g., more than 50%, 80%, 90%, and preferably 99% in the
host material, are due to the presence of these inclusions. In some
embodiments, the particles can be more highly doped than the host
material. The thermoelectric material can exhibit a carrier
concentration and/or charge carrier mobility greater than a
respective carrier concentration and/or charge carrier mobility of
the host material in the absence of the particles or inclusions,
and consequently a higher power factor (S.sup.2.sigma.). Also, or
alternatively, the thermoelectric material can be characterized by
the inclusions having an energy band (e.g., conduction or valence)
for the charge carrier type that has a higher energy relative to
the associated energy band of the host material for the
corresponding charge carrier type. The thermoelectric material can
optionally include any number of the properties discussed herein
with respect to thermoelectric materials. For example, the
thermoelectric material can exhibit a ZT value greater than about
1, 1.1, 1.2, 1.3, 1.4, 1.5, or 2.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] Some embodiments of the present invention can be better
understood with reference to the following drawings, which are not
necessarily drawn to scale.
[0023] FIG. 1A is a schematic of a plurality of grains in a
thermoelectric material, where some of the grains include one or
more precipitation regions, consistent with some embodiments of the
present invention;
[0024] FIG. 1B is a schematic diagram of a host material with
embedded inclusions therein, consistent with some embodiments of
the present invention;
[0025] FIG. 1C is a schematic of conduction energy diagram for the
material depicted in FIG. 1B;
[0026] FIG. 2 is an XRD pattern of p-type BiSbTe nanoparticles
prepared by ball milling, consistent with some embodiments;
[0027] FIG. 3A is a SEM image of the p-type BiSbTe nanoparticles of
FIG. 2;
[0028] FIG. 3B is a lower resolution TEM micrograph of the BiSbTe
nanoparticles of FIG. 2;
[0029] FIG. 3C is a higher resolution TEM micrograph of the BiSbTe
nanoparticles shown in FIG. 3B;
[0030] FIG. 4A is a diagram of a DC hot press device which can be
utilized with some embodiments of the present invention;
[0031] FIG. 4B is a photograph of a DC hot press device which can
be utilized with some embodiments of the present invention;
[0032] FIG. 5 is a graph depicting the temperature dependence of
the electrical conductivity for a thermoelectric material prepared
from the particles of FIG. 2 and a state-of-the-art bulk material
of p-type BiSbTe alloy, consistent with some embodiments;
[0033] FIG. 6 is a graph depicting the temperature dependence of
the Seebeck Coefficient for a thermoelectric material prepared from
the particles of FIG. 2 and a state-of-the-art bulk material of
p-type BiSbTe alloy, consistent with some embodiments;
[0034] FIG. 7 is a graph depicting the temperature dependence of
the power factor for a thermoelectric material prepared from the
particles of FIG. 2 and a state-of-the-art bulk material of p-type
BiSbTe alloy, consistent with some embodiments;
[0035] FIG. 8 is a graph depicting the temperature dependence of
the thermal conductivity for a thermoelectric material prepared
from the particles of FIG. 2 and a state-of-the-art bulk material
of p-type BiSbTe alloy, consistent with some embodiments;
[0036] FIG. 9 is a graph depicting the temperature dependence of
the figure of merit, ZT, for a thermoelectric material prepared
from the particles of FIG. 2 and a state-of-the-art bulk material
of p-type BiSbTe alloy, consistent with some embodiments;
[0037] FIG. 10 is a TEM micrograph of a thermoelectric material
prepared from the particles of FIG. 2;
[0038] FIG. 11 is a magnified TEM micrograph of a thermoelectric
material prepared from the particles of FIG. 2 showing the nano
size of the closely packed nanograins;
[0039] FIG. 12 is a TEM micrograph of a thermoelectric material
prepared from the particles of FIG. 2 showing the presence of a
grain larger than the nanograins shown in FIG. 11;
[0040] FIG. 13 is a TEM micrograph of a thermoelectric material
prepared from the particles of FIG. 2 showing the presence of
nanodots;
[0041] FIG. 14 is a TEM micrograph of a thermoelectric material
prepared from the particles of FIG. 2 showing the presence of a
nanodot with small angle boundaries;
[0042] FIG. 15 is a TEM micrograph of a thermoelectric material
prepared from the particles of FIG. 2 showing a Te precipitate, the
inset of the figure depicting an electron diffraction pattern of
the Te precipitate;
[0043] FIG. 16 is a graph depicting the temperature dependence of
the electrical conductivity for a thermoelectric material prepared
from a p-type SiGe bulk starting material, consistent with some
embodiments;
[0044] FIG. 17 is a graph depicting the temperature dependence of
the Seebeck Coefficient for a thermoelectric material prepared from
a p-type SiGe bulk starting material, consistent with some
embodiments;
[0045] FIG. 18 is a graph depicting the temperature dependence of
the thermal conductivity for a thermoelectric material prepared
from a p-type SiGe bulk starting material, consistent with some
embodiments;
[0046] FIG. 19 is a graph depicting the temperature dependence of
the figure-of-merit, ZT, for a thermoelectric material prepared
from a p-type SiGe bulk starting material, consistent with some
embodiments;
[0047] FIG. 20 is a graph depicting the temperature dependence of
the electrical conductivity for a thermoelectric material prepared
from a n-type SiGe bulk starting material, consistent with some
embodiments;
[0048] FIG. 21 is a graph depicting the temperature dependence of
the Seebeck Coefficient for a thermoelectric material prepared from
a n-type SiGe bulk starting material, consistent with some
embodiments;
[0049] FIG. 22 is a graph depicting the temperature dependence of
the thermal conductivity for a thermoelectric material prepared
from a n-type SiGe bulk starting material, consistent with some
embodiments;
[0050] FIG. 23 is a graph depicting the temperature dependence of
the figure-of-merit, ZT, for a thermoelectric material prepared
from a n-type SiGe bulk starting material, consistent with some
embodiments;
[0051] FIG. 24 is a TEM micrograph of a ball milled sample of SiGe
bulk starting material, consistent with some embodiments of the
present invention;
[0052] FIG. 25 is a TEM micrograph of the particles of FIG. 24
after hot pressing, the inset showing a corresponding electron
diffraction pattern on the sample;
[0053] FIG. 26 is a high-resolution TEM micrograph of the hot
pressed sample shown in FIG. 25;
[0054] FIG. 27 is a graph depicting the temperature dependence of
the electrical conductivity for a thermoelectric material prepared
from a p-type Bi.sub.0.3Sb.sub.1.7Te.sub.3 bulk starting material,
consistent with some embodiments;
[0055] FIG. 28 is a graph depicting the temperature dependence of
the Seebeck Coefficient for a thermoelectric material prepared from
a p-type Bi.sub.0.3Sb.sub.1.7Te.sub.3 bulk starting material,
consistent with some embodiments;
[0056] FIG. 29 is a graph depicting the temperature dependence of
the thermal conductivity for a thermoelectric material prepared
from a p-type Bi.sub.0.3Sb.sub.1.7Te.sub.3 bulk starting material,
consistent with some embodiments;
[0057] FIG. 30 is a graph depicting the temperature dependence of
the figure of merit, ZT, for a thermoelectric material prepared
from a p-type Bi.sub.0.3Sb.sub.1.7Te.sub.3 bulk starting material,
consistent with some embodiments;
[0058] FIG. 31 is a graph depicting the temperature dependence of
the electrical conductivity for a thermoelectric material prepared
from a p-type Bi.sub.0.5Sb.sub.1.5Te.sub.3 bulk starting material,
consistent with some embodiments;
[0059] FIG. 32 is a graph depicting the temperature dependence of
the Seebeck Coefficient for a thermoelectric material prepared from
a p-type Bi.sub.0.5Sb.sub.1.5Te.sub.3 bulk starting material,
consistent with some embodiments;
[0060] FIG. 33 is a graph depicting the temperature dependence of
the thermal conductivity for a thermoelectric material prepared
from a p-type Bi.sub.0.5Sb.sub.1.5Te.sub.3 bulk starting material,
consistent with some embodiments; and
[0061] FIG. 34 is a graph depicting the temperature dependence of
the figure of merit, ZT, for a thermoelectric material prepared
from a p-type Bi.sub.0.5Sb.sub.1.5Te.sub.3 bulk starting material,
consistent with some embodiments.
DETAILED DESCRIPTION
[0062] In one aspect, the invention is directed to thermoelectric
materials that have high ZT values, and methods of producing such
materials. In general, such thermoelectric materials typically
comprise a plurality of grains. Such grains can be, e.g., in the
form of nano-sized grains that can be obtained from a bulk material
such as a starting thermoelectric material. In general,
thermoelectric materials consistent with embodiments of the
invention can include a variety of sizes of grains. For example,
the thermoelectric material can have some grains larger than 1
.mu.m and some grains smaller than 1 .mu.m. In some embodiments,
thermoelectric materials can be substantially-free of grains that
can adversely affect the ZT value of the material (e.g., being
substantially free of adverse grains that can decrease the ZT value
of the entire material by more than about 5%, 10%, 15%, 20%, 25%,
30%, 40%, or 50%). Some embodiments are directed to thermoelectric
materials with a plurality of grains having an average grain size
on the order of microns (e.g., greater than about 1 micron). In
some instances, the material can be substantially-free of large
grains. Non-limiting examples include being substantially free of
grains larger than about 5000 nm, 1000 nm, 300 nm, 100 nm, 50 nm,
20 nm, or 10 nm. In many cases, such grains can optionally include
one or more precipitation regions or other types of inclusions
having average sizes, e.g., in a range of about 1 nm to about 50
nm. In some preferred embodiments, at least some, and preferably
substantially all, of the grains include precipitation regions,
nanoparticles, and/or other types of inclusions; these various
inclusions can be formed in-situ by chemical reaction and/or by
insertion of such inclusions. Further embodiments are directed to a
material having a plurality of grain sizes (e.g., at least some
nanosized grains and some grains larger than 1 micron), wherein
some of the grains can optionally include precipitation regions or
other types of inclusions. In other words, the thermoelectric
materials of the invention can include any combination of
sub-micron sized grains with or without precipitation regions,
grains larger than 1 micron with or without precipitation regions
(e.g., using modulation doping), or a mixture of sub-micron grains
and grains larger than 1 micron with or without precipitation
regions. Any of these grains can be formed by a plurality of
mechanisms including, but not limited to, precipitation region
formation during material compaction, particle insertion into a
host matrix, and/or formation by solid-state chemical reaction.
[0063] The ZT value of a thermoelectric material of the invention
can take on a variety of values. For example, the peak ZT value, or
the average ZT value, of the material can be greater than the peak
ZT value, or the average ZT value, of a corresponding starting
material from which the thermoelectric material is formed by
converting the starting material into nanoparticles and
compactifying the nanoparticles under pressure and at an elevated
temperature. For example, the ZT value of the material can be about
25% to about 1000% greater than the ZT value of the starting
material. In other examples, the ZT value of the material can be
substantially greater than 1000% of the ZT value of the starting
material. Starting materials can have a range of ZT values. In some
embodiments, the ZT values of the formed material can be greater
than about 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8,
1.9, or 2. In some embodiments, the thermoelectric material can
exhibit a ZT value in a range whose lower limit is one of the above
ZT values and whose upper limit reaches to a value of about 4, 5,
or 6.
[0064] While these elevated ZT values can be identified without a
limitation in temperature, in some embodiments the thermoelectric
materials can exhibit the elevated ZT value at a particular
temperature or within a temperature range. For example, the
thermoelectric material can exhibit an elevated ZT value at a
temperature below about 2000.degree. C., below about 1000.degree.
C., below about 800.degree. C., below about 600.degree. C., or
below about 400.degree. C. In other examples, the thermoelectric
material can exhibit an elevated ZT value at a temperature range
that begins to approach, or includes, room temperature (e.g., a
temperature below about 200.degree. C., below about 150.degree. C.,
below about 100.degree. C., below about 60.degree. C., below about
40.degree. C., below about 30.degree. C., or below about 20.degree.
C.). In still other examples, the thermoelectric material can
exhibit an elevated ZT value at a temperature range that
approaches, or includes, cryogenic temperatures (e.g., a
temperature below about 0.degree. C., below about -50.degree. C.,
or below about -100.degree. C.); such materials can be useful for
particular cooling applications such as air conditioners,
refrigerators, or superconductors. In some embodiments, the
temperature range in which an elevated ZT value is exhibited can
depend upon the composition of a thermoelectric material. In some
non-limiting examples, a boron-carbide based composition can
exhibit, in some embodiments, an elevated ZT value below about
2000.degree. C., a SiGe based composition can exhibit, in some
embodiments, an elevated ZT value below about 1000.degree. C.; a
PbTe based composition can exhibit, in some embodiments, an
elevated ZT value below about 600.degree. C.; and/or a
Bi.sub.2Te.sub.3 based composition can exhibit, in some
embodiments, an elevated ZT value below about 200.degree. C. In
another non-limiting example, the thermoelectric material comprises
Bi.sub.xSb.sub.1-x and exhibits an elevated ZT below room
temperature (e.g., below about 20.degree. C.).
[0065] Without necessarily being bound by any particular theory, it
is believed that the high ZT values of such thermoelectric
materials can be the result of variations in any combination of the
thermal conductivity, the Seebeck coefficient, and the electrical
conductivity. Thermal conductivity has two contributions: lattice
and electron contributions. In single crystals or polycrystalline
samples with large grains, lattice thermal conductivity is fixed
for a specific material. However, if the bulk material is composed
of nanosized grains, and/or nanoparticles embedded in a grain
larger than the nanoparticle, we can think of three effects
resulting from the nanograins and/or embedded nanoparticles. First,
the lattice part of thermal conductivity drops due to interface
scattering of phonons. Second, the Seebeck coefficient can increase
because of carrier filtering effect (usually low energy
electrons/holes are scattered thereby increasing Seebeck
coefficient), and third, the electrical conductivity can increase
because of a modulation doping effect--the particles serve as a
carrier (electron and hole) contributors, and hence reduce impurity
scattering in comparison to conventional materials that are
homogeneously doped. The electronic contribution to thermal
conductivity can potentially be reduced by interfacial barrier
scattering of electrons, especially the bi-polar contribution to
thermal conductivity since the barrier can preferentially scatter
one-type of charge (electrons or holes) without substantially
affecting another type of carrier. Additionally, quantum size
effects can further affect the Seebeck coefficient and electrical
conductivity so that S.sup.2.sigma. increases. Accordingly, some
embodiments of the present invention can utilize nanoparticles
prepared, e.g., by ball milling a starting material, to prepare
dense samples (e.g., about 90% to about 100% of theoretical
density) by hot press including P.sup.2C, unidirectional hot press,
isostatic hot press. These hot pressed samples typically show lower
thermal conductivity compared to the bulk counterpart, thereby
enhancing the ZT value; the power factor is usually maintained or
enhanced, though it can also be lowered if the gain in ZT from the
drop in thermal conductivity is sufficient.
[0066] In some embodiments, a thermoelectric material can comprise
grains generated from a bulk starting material, such as a bulk
thermoelectric material. Examples include bulk starting materials
with a large power factor and/or starting materials with a good ZT
value (e.g., a ZT value above about 0.1). For instance, the ZT of
the starting material can be greater than about 0.05, 0.1, 0.2,
0.3, 0.4, 0.5, or higher. In some non-limiting instances, the
starting material can have a ZT value lower than about 0.8, 0.9, 1,
1.1, 1.2, 1.3, 1.4, 1.5, or 2. In other examples, the starting
thermoelectric materials can have a high power factor (e.g.,
S.sup.2 larger than 20 .mu.W/cm-K.sup.2 and preferably larger than
40 .mu.W/cm-K.sup.2) but a large thermal conductivity (e.g., larger
than 2 W/mK). Such bulk thermoelectric materials can be
specifically prepared, or commercially available materials can be
utilized. Though many bulk starting materials are solids that can
be broken apart to generate grains, bulk starting materials can
also be generated from other thermodynamic states such as gases,
when generating grains from gas phase condensation, or liquids,
when generating grains from wet chemical methods. It is also
understood that the grains can be generated from more than one type
of bulk starting material, or a mixture of materials having
different thermodynamic phases (e.g., a mixture of liquid and
gas).
[0067] Though any number of starting materials can be utilized, in
some embodiments the bulk starting material can be chosen from any
combination of a bismuth-based material, a lead-based material,
and/or a silicon-based material. In some embodiments, the bulk
starting materials can be derived from various alloys such as
bismuth-antimony-tellurium alloys, bismuth-selenium-tellurium
alloys, bismuth-antimony-tellurium-selenium alloys, lead-tellurium
alloys, lead-selenium alloys, silicon-germanium alloys, or any
combination thereof. Particular embodiments can be drawn to using
bulk starting materials that are either p-type or n-type materials.
For example, such starting materials can be compositionally
modified forms of a parent composition such as Bi.sub.2Te.sub.3. By
way of example, n-type materials can be obtained by substituting
tellurium in Bi.sub.2Te.sub.3 with selenium such that the
stoichiometry of the bulk material has a formula
Bi.sub.2Te.sub.3-xSe.sub.x, where x is in a range of about 0 to
about 0.8. For p-type materials, antimony can be, for example, used
to replace bismuth such that the stoichiometry of the bulk material
has a formula Bi.sub.xSb.sub.2-xTe.sub.3, where x is in a range of
about 0 to about 0.8. In a particular embodiment, the bulk starting
material utilized is Bi.sub.0.5Sb.sub.1.5Te.sub.3. In general, the
bulk starting materials can be a crystalline material or a
polycrystalline material (e.g., polycrystalline with an average
crystal grain size greater than about a micron). Other examples of
starting materials include MgSi.sub.2, InSb, GaAs CoSb.sub.3,
Zn.sub.4Sb.sub.3, etc. In some instances, the bulk starting
material can be a material with a threshold power factor value,
S.sup.2.sigma., e.g., larger than about 20 .mu.V/cm K.sup.2. In
such instances, the bulk starting material can have a reasonable ZT
value (e.g., greater than about 0.1) due to the bulk starting
material's low thermal conductivity, or the power factor can be at
or above a threshold value but the ZT value of the starting
material can be low because of the material's relatively high
thermal conductivity.
[0068] In some embodiments, particles (e.g., nanoparticles) of a
thermoelectric material can be generated from a bulk starting
material, or elemental materials, by methods beyond
grinding/milling one or more starting materials. Particles can be
generated by a number of methods, including those methodologies
known to the skilled artisan. Non-limiting examples include gas
phase condensation, laser ablation, chemical synthesis (e.g., wet
or dry methods), rapid cooling of sprays, etc. Accordingly, the
scope of the present application is not limited to the specific
particle production methodologies discussed herein. It is
understood that particle generation techniques can be combined in
any fashion to create materials for consolidation. For example,
some particles can be generated by ball milling (e.g., to create a
host material), while other particles can be generated by one or
more other techniques (e.g., gas phase condensation, laser
ablation, etc.).
[0069] Grains that form a thermoelectric material can have a
variety of characteristics. In some embodiments, each grain has a
crystalline structure. In such an instance, the thermoelectric
material can comprise a polycrystalline-like structure in which the
grains generally lack a preferred orientation (e.g., randomly
distributed). In some instances, the grains can also exhibit some
type of preferred orientation due to grain shapes, where the
general crystalline direction of the grains can either be random or
exhibit some preferred direction relative to one another.
Accordingly, such embodiments differ substantially from many known
thermoelectric materials that exhibit an average crystalline
structure (including superlattice structures formed, e.g., as a
stack of a plurality of semiconducting layers), albeit with small
defect or compositional inhomogeneities in the average crystalline
structure.
[0070] Grains of which the various thermoelectric materials
discussed herein are composed can have a variety of sizes. In some
embodiments, the sizes are generally nanometer-scale, and generally
smaller than a micron. For example, the grains can have an average
grain size less than about 500 nm, or less than about 200 nm, or
less than about 100 nm, or less than about 50 nm, or less than
about 20 nm. In such embodiments, the average grain size can be
greater than some lower threshold value (e.g., about 1 nm). In some
cases, the average grain size can be determined using a variety of
methodologies, including methods understood by those skilled in the
art. For example, transmission electron micrographs (herein "TEMs")
can be used to image the grains whose sizes can then be determined
and averaged. Since grains are typically irregularly shaped, the
measured size of a grain can be determined using any number of
techniques, including ones known to the skilled artisan. For
example, the largest dimension of the grain can be used from an
image (e.g., a SEM and/or TEM image), or an effective diameter can
be calculated based on surface area measurements or the
effective-cross sectional area of grains from an image.
[0071] In many embodiments of the invention, the grains of a
thermoelectric material can be compacted such that the end-product
exhibits desired properties such as an elevated ZT value. In some
embodiments, the thermoelectric material comprises compacted grains
in a structure that exhibits a low porosity (e.g., the actual
density of the end-product can approach the theoretical density of
the composition, for instance a bulk starting material used to make
nanoparticles in some embodiments), which can aid in obtaining an
elevated ZT value. Porosity is defined as the difference between
the theoretical density and the actual density of the material
divided by the theoretical density. In general, the phrase
"theoretical density" is known to those skilled in the art. The
porosity in the material can be less than about 10%, 5%, or 4%, or
3%, or 2%, or 1%, or 0.5%, or 0.1%. In some embodiments, a
thermoelectric material exhibits a density approaching 100% of a
theoretical density. In some embodiments, the density of a
thermoelectric material can be between 100% and 90%, 95%, 96%, 97%,
98%, 99%, 99.5%, or 99.9% of a respective theoretical density.
Without necessarily being bound by theory, it is believed that
densification can help maintain contact between grains, which can
help maintain the electrical conductivity of the material.
[0072] Some embodiments are directed to a thermoelectric material
formed from a plurality of grains, in which one or more of the
grains can include one or more precipitation regions. By way of
example, FIG. 1 schematically depicts such a thermoelectric
material that exhibits a polycrystalline structure including a
plurality of grains 110. The grains can further include one or more
precipitation regions 120, which can enhance the thermoelectric
properties of the material. A precipitation region can be
characterized by a compositional inhomogeneity such as having a
different composition and/or phase than the rest of the grain. A
precipitation region can also be characterized as having a similar
crystalline structure to the matrix in which it is embedded, though
oriented in a different crystalline direction. In some embodiments,
one or more precipitation regions can be embodied as a discrete
particle (e.g., a nanoparticle) embedded in a grain, or the whole
grain can be embodied as a crystal, albeit with defects due to the
presence of a precipitation region. In some embodiments, the
thermoelectric material can include other grains that do not have
precipitation regions. In an alternative embodiment, substantially
all of the grains that comprise a thermoelectric material include
precipitation regions. The precipitation regions typically have
sizes (e.g., maximum average size) less than about 10 nm, or less
than about 50 nm (e.g., in a range of about 1 nm to about 50 nm).
Formation of precipitation regions can be achieved in a variety of
manners including the techniques discussed in U.S. Patent
Application Publication No. US 2006/0102224, bearing Ser. No.
10/977,363, filed Oct. 29, 2004, entitled "Nanocomposites with High
Thermoelectric Figures of Merit," which is incorporated herein by
reference in its entirety.
[0073] In some cases, the precipitation regions are generated
spontaneously through the formation of the thermoelectric material,
e.g., via the methods discussed herein. In other cases, the
precipitation regions are generated by mixing two types of
nanoparticles having different melting temperatures. For example,
one type can have a lower melting point than the other. By mixing
the nanoparticles and heating/consolidating them (e.g., at a
temperature close to the melting point of one type of the
nanoparticles but below the melting point of the other type), the
nanoparticles having the lower melting temperature can form grains
around the other type of nanoparticles. In other words, grains
formed of one type of the nanoparticles can embed the nanoparticles
of the other type Examples of ensemble materials that can be used
to form such embedded nanoparticles include bismuth-telluride
material systems, lead-telluride material systems,
silicon-germanium material systems, etc.
[0074] It should be understood that though the aforementioned
discussion is explicitly directed to precipitate formation in
thermoelectric materials, other materials are formed by utilizing
other types of inclusions into a matrix (e.g., use of nanoparticles
in a host). For example, two or more types of nanoparticles can be
mixed together to form a thermoelectric material may not include
precipitates but can still have advantageous properties (e.g., use
of modulation doping). Accordingly, the disclosure herein regarding
precipitates can also be utilized with respect to other types of
inclusions where appropriate. For example, the precipitation or
inclusion regions can be formed via solid-state chemical reaction
of a particle with the host, such as Mo, Fe, Mn, Mg, Ag, Cr, W, Ta,
Ti, Cu, Ni, or V metallic particle reacting with Si in a SiGe host
to form MoSi.sub.2, FeSi.sub.2, MgSi.sub.2, etc. particles.
[0075] Without being limited to any particular theory, it is
believed that the precipitation regions or other types of
inclusions can enhance phonon scattering in a thermoelectric
material, which can lead to lowering of the thermal conductivity of
the material. In addition, n-doped or p-doped regions can enhance
electrical conductivity of the material, e.g., via a
modulation-doping mechanism. In such an instance, some or all of
the charge carriers (electrons and holes) can be donated by
precipitation regions or other inclusions embedded in larger
grains. Because the distance between inclusion regions can be
larger than the distance between atomic dopants in a homogeneously
doped material, the impurity scattering of the charge carriers is
reduced compared to that in homogeneously doped materials. Such a
modulation-doping like mechanism can increase the electrical
conductivity through improving carrier mobility. In some instances,
these precipitation regions or other inclusions can also improve
the Seebeck coefficient by scattering low energy carriers more than
higher energy ones. As such, the precipitation regions or other
inclusions can improve the ZT of the thermoelectric material.
[0076] In other embodiments, precipitation regions, or grain
regions or other inclusions, may be preferentially doped. In such
circumstances, the carriers of these regions can fall into the
surrounding host medium when they are at a higher potential energy.
For example, in the case of modulation doping, doping in the host
material can be correspondingly reduced or completely eliminated,
thus enhancing the electron mobility in the host by reducing
ionized impurity scattering.
[0077] Embodiments that include precipitation regions or other
inclusions in grains can exhibit any number of grain sizes. In some
embodiments, the grain sizes are consistent with any of the sizes
described herein for grains that are generally smaller than a
micron. For example, the average grain size can be less than about
500 nm, about 200 nm, about 100 nm, about 50 nm, or about 20 nm.
Alternatively or in addition, the average grain size can be greater
than about 1 nm. In other embodiments with one or more inclusions,
the grain sizes can be larger than a micron. For example, a
plurality of grains can have average sizes up to about 2 microns, 5
microns, or 10 microns. In particular embodiments, the plurality of
grains have an average size in a range of about 1 micron to about
10 microns, or in a range of about 1 micron to about 5 microns, or
in a range of about 1 micron to about 2 microns.
[0078] The size of the precipitation regions or inclusions can also
vary. For example, the size of the precipitation region can be
bound by the size of the grain in which it is embedded. In many
embodiments, a precipitation region or inclusion can preferably
have an average size in a range of about 1 nm to about 50 nm, or a
range from about 1 nm to about 20 nm. In other instances, for
example when a modulation doping mechanism is used to increase the
electron performance, the precipitation region or inclusion can
have a larger size, e.g., from 1 nm to 10 microns, while the phonon
thermal conductivity reduction in the surrounding region is
achieved by alloying or nanograining.
[0079] Some embodiments are directed to manufactured thermoelectric
materials that exhibit modulation doping to achieve enhanced
figures of merit. In some embodiments, a thermoelectric material
can include particles (e.g., nanoparticles) or other inclusions
embedded in a host material, where the inclusions donate charge
carriers (e.g., electrons or holes) to the host, thereby increasing
the carrier mobility in the host. This can advantageously enhance
the electrical conductivity of the entire material, and hence
improve its thermoelectric performance, e.g., characterized by the
material's ZT value. In many such cases, the host is selected to be
initially undoped or to have an n-type or p-type doping level
(typically a doping level that is spatially substantially uniform)
that is less than typical doping values for thermoelectric
materials. For example, the initial doping level of the host can be
a factor of 1.5, 2, 5, 10, 100, or 1000 less than a conventional
thermoelectric material. Further, the embedded inclusions (e.g.,
precipitation sites or distinct particles) can be formed of doped
or undoped materials.
[0080] By way of example, FIG. 1B schematically depicts such a
thermoelectric material that includes a host 130 in which a
plurality of particles 140 are embedded--the particles acting as
inclusions. In this case, the host includes a plurality of grains
135, e.g., a plurality of crystalline grains, which have in some
cases sizes (e.g., maximum grain size in any dimension) less than
about 1 micron, e.g., in a range of about 500 nm to less than about
1 micron. In other cases, the grain sizes can be larger, e.g., in a
range of about 1 micron to about 20 microns. Further, while in some
cases the particles can have sizes (e.g., a maximum size in any
direction) less than about 1 micron, e.g., in a range of about 1 nm
to about 200 nm or in a range of about 2 nm to about 100 nm, in
other cases the particle sizes can be greater than 1 micron, e.g.,
in a range of about 1 micron to about 10 microns. The inclusions
140 can be formed in a variety of ways. For example, they can be
formed as precipitation regions using any appropriate technique
including those discussed with respect to other embodiments herein.
In other cases, they can be formed of a material different than
that of the host by utilizing, e.g., the techniques discussed in
the above-referenced patent application entitled "Nanocomposites
with High Thermoelectric Figures of Merit." In yet other cases, the
particles can be formed via solid-state chemical reaction, e.g.,
during a consolidation phase.
[0081] Without loss of generality, in this example, the host 130 is
assumed to be a SiGe alloy having a plurality of micron-sizes
and/or nanosized grains 135, and the particles 140 can be
MoSi.sub.2 (molybdenum silicide) particles that are embedded in the
SiGe alloy. Such a thermoelectrical material can be formed, e.g.,
in the following way: adding molybdenum to SiGe, melting the
material, and cooling the material (e.g., in a manner discussed
above) to make ingots, which can be grinded and compacted if
needed. In this approach, the MoSi.sub.2 particles are formed via a
solid-state chemical reaction of Mo with Si, e.g., during the
cooling process. In this example, the SiGe host is not heavily
doped although in other cases it can be, e.g., it can be doped
p-type, but less than in a conventional SiGe thermoelectric
materials by a factor of 2, 5, 10, or 100. Additionally, holes can
be generated by the presence of MoSi.sub.2. Such donation of the
holes to the host can enhance the hole mobility within the material
and hence improve the electrical conduction and consequently
thermoelectric performance of the material. In other instances, the
particles can be formed via solid-state chemical reaction of a Si
in a host (e.g., SiGe) by grinding Si and Ge elements or SiGe
crystalline alloy with Fe, Mn, Mg, Cr, W, Ta, Ti, Cu, Ni, or V to
form FeSi.sub.2, MgSi.sub.2, etc. particles, or grinding the
respective silicides with the Si and Ge or SiGe alloy together.
Some of them may be applicable to n-type while others to p-type
materials. Other nanoparticles (e.g., metallic and/or semiconductor
nanoparticles) that do not react with Si can also be used to create
modulation doping such as Ag as inclusions.
[0082] For further illustration of such donation of charge carriers
from the particles to the host, without being limited to any
particular theory, FIG. 1C schematically depicts a charge carrier
energy diagram corresponding to a hypothetical thermoelectric
material (e.g., the above SiGe-based material having MoSi.sub.2
particles embedded therein) representing portions 151, 152, 153
corresponding to the host material and portions 161, 162
corresponding to a particle embedded in the host. It is understood
that the diagram is schematic and presented only for illustrative
purposes. Charge carriers (e.g., electrons or holes) in a energy
band of the particles 161, 162 (e.g., a conduction or valence band)
can have higher energy than those in an energy band of the host
151, 152, 153, which can be the conduction or valence band.
Accordingly, a plurality of charge carriers in the particles, which
can be either due to additional doping in the particle or due to
its intrinsic large density of electrons (as in metals or
semimetals), can move to the host to lower their energies. This
transfer of charge carriers from the particles to the host can
advantageously increase carrier mobility, e.g., by reducing dopant
in the host material and hence reducing the ionized impurity
scattering. In this manner, a higher electrical conduction can be
achieved. In some cases, even though an overall higher electron
mobility is not achieved, because grain boundaries still scatter
electrons, this modulation doping method can still beneficial by
compensating for the reduction in mobility due to electron grain
boundary scattering. The particles used for modulation doping can
also potentially lead to higher Seebeck coefficient as they can
scatter low energy carriers, and reducing thermal conductivity of
both phonons and electrons. In some other cases, rather than
donating electrons to the host, the particles can donate holes to
the host. Again, without being limited to any particular theory,
the mechanism of such donation of charge carriers from the
particles to the host can be based on some holes moving from higher
energy levels in the valence band of the particles to lower energy
levels in the valence band of the host, or by attracting electrons
in the valence band of the host into the particles, creating more
holes in the host, or attracting electrons in the valence band of
the host into the particles, creating more holes in the host.
[0083] In general, the types of starting materials, the
temperatures at which an elevated ZT is measured, the grain
constituents, the formation methods, and other properties and
processes that can be associated with these embodiments include all
the traits and methods discussed within the present application,
which are consistent with the grain sizes, precipitation regions,
and/or other inclusions described. For example, the grains can be
formed of any suitable thermoelectric material, such as those
discussed above, and can further include n-type or p-type dopants.
In another example, the formed thermoelectric material has a ZT
value greater than about 1.0, greater than about 1.5, greater than
about 2, or in a range from about 1 to about 5. In yet another
example, the formed thermoelectric material has a ZT value (e.g.,
elevated relative to a starting material) at an operating
temperature below about 2000.degree. C., below about 1000.degree.
C., below about 600.degree. C., below about 200.degree. C., or
below about 20.degree. C. In another example, the grains of the
thermoelectric material can include at least one of a bismuth-based
material (e.g., Bi.sub.2Te.sub.3 and/or its associated alloys), a
silicon-based material, and a lead-based material. With regard to
producing such materials, the methods of forming nanoparticles from
bulk starting materials, or elemental materials, can be applied as
discussed herein, albeit by adjusting parameters such as grinding
speeds, duration, and/or temperature (including cryogenic) to
obtain the desired nanoparticle sizes for compaction. Further, such
adjustment of the nanoparticle sizes can be employed to obtain
desired grain sizes in the final thermoelectric material (e.g.,
less than 1 micron, or greater than 1 micron but less than 10
microns). The compaction methods can also be applied as discussed
herein, and as applied by one skilled in the art.
[0084] Other embodiments of the present application are directed
toward methods of fabricating a thermoelectric material. In such a
method, a plurality of nanoparticles is generated from a
thermoelectric material. The nanoparticles can be consolidated
under pressure at an elevated temperature to form the
thermoelectric material. The types of thermoelectric starting
materials that can be utilized to generate the nanoparticles
include, without limitation, any of the bulk materials disclosed
herein, and others known to those skilled in the art. Accordingly,
embodiments can include thermoelectric materials having a ZT value
greater than about 1 (e.g., at a temperature below about
2000.degree. C.). In addition or alternatively, the methods can
utilize starting materials (e.g., bulk thermoelectrics which are
elemental and/or alloys) that are n-doped or p-doped.
[0085] A variety of techniques can be utilized to generate the
nanoparticles from a thermoelectric material. In some embodiments,
the nanoparticles are produced by grinding the thermoelectric
material. Grinding can be performed using a mill, such as a ball
mill using planetary motion, a figure-eight-like motion, or any
other motion. When generating nanoparticles, some techniques, such
as some grinding techniques, produce substantial heat, which may
affect the nanoparticle sizes and properties (e.g., resulting in
particle agglomeration). Thus, in some embodiments, cooling of a
thermoelectric material can be performed while grinding the
material. Such cooling may make a thermoelectric material more
brittle, and ease the creation of nanoparticles. Cooling and
particle generation can be achieved by wet milling and/or
cryomilling (e.g., in the presence of dry-ice or liquid nitrogen
surrounding the mill). Embodiments of the invention can also
utilize other methods for forming nanoparticles. Such methods can
include gas-phase condensation, wet chemical methods, spinning
molten materials at high speed, and other suitable techniques.
[0086] Consolidation of the nanoparticles under pressure and
elevated temperature can be performed in a variety of manners,
under a variety of conditions. Processes such as hot press can be
employed to impose the desired pressure and temperature during
consolidation. A description of this process, and an apparatus for
carrying out this process, is available in U.S. Patent Application
Publication No. US 2006/0102224, bearing Ser. No. 10/977,363, filed
Oct. 29, 2004; which is incorporated by reference in its entirety
herein.
[0087] The pressures utilized are typically super-atmospheric,
which allow for the use of lower temperatures to achieve
consolidation of the nanoparticles. In general, the pressures
utilized can range from about 10 MPa to about 900 MPa. In some
embodiments, the pressure ranges from about 40 MPa to about 300
MPa. In other embodiments, the pressure ranges from about 60 MPa to
about 200 MPa.
[0088] With respect to the elevated temperature, a range of
temperatures can be utilized. In general, the temperature typically
ranges from about 200.degree. C. to about the melting point of the
thermoelectric material. In some exemplary embodiments, the
temperature is in a range from about 400.degree. C. to about
2000.degree. C., from about 400.degree. C. to about 1200.degree.
C., from about 400.degree. C. to about 600.degree. C., from about
400.degree. C. to about 550.degree. C. For some exemplary n-doped
materials, the temperature is in a range from about 450.degree. C.
to about 550.degree. C., while for some exemplary p-doped materials
the range is a few degree higher (e.g., in a range of about
475.degree. C. to about 580.degree. C.). Other temperature ranges
can also be utilized in connection with processing n and p-type
materials. These particular pressure and temperature ranges can be
utilized with any material, though they can preferably be applied
to materials such as BiSbTe alloys and BiSeTe alloys.
[0089] The pressures and temperatures can be maintained for a time
sufficient to allow consolidation of the nanoparticles. In some
embodiments, the time is in the range from about 1 sec to about 10
hours.
[0090] Other consolidation techniques can also be utilized to form
the thermoelectric materials described in the present application.
For example, nanoparticles can be impacted at high speed against
other particles to achieve low temperature compaction. Subsequent
heat treatment can optionally be utilized to form the
thermoelectric material. Other consolidation processes can utilize
annealing of particles (e.g., nanoparticles) using little or no
pressure to consolidate the particles. In such instances, the
temperature can be selected to induce annealing of particles at
whatever pressure the sample is held at during annealing. In other
instances, particles can be consolidated at high pressure at a
relatively low temperature to form a consolidated material, such as
a material with close to 100% theoretical density. The consolidated
material can be subsequently annealed at an elevated temperature to
form the thermoelectric material. Accordingly, consolidation
techniques need not be restricted to P.sup.2C or hot pressing
methods.
[0091] As an exemplary embodiment, nanopowders of various materials
from commercial materials can be prepared by high energy ball
milling to obtain nanoparticles with particle sizes as small as 1
nm. In some cases, dry milling can be combined with wet milling
and/or cryo-milling to inhibit agglomeration of the milled
particles into larger size particles due to heat generated during
the milling. In this manner, more dispersed particles can be
obtained. These powders can be compacted into solid samples by hot
press including a P.sup.2C technique. In many embodiments, about
100% density of the theoretical value can be achieved by this
method within a short period of time (typically about 1 to about 10
minutes per sample). The lattice thermal conductivity of hot
pressed samples prepared by these methods can be reduced to a
fraction of the original value in both n- and p-types while
maintaining a power factor comparable to the bulk counterpart
thereby substantially enhancing the ZT value.
[0092] For example, in a p-type commercial material of
Bi.sub.xSb.sub.2-xTe.sub.3, where x can range from about 0 to about
0.8, the commercial material has a highest ZT value about 1 whereas
after ball milling and hot pressing, it can be as much as 1.4 or
higher. These enhancements are primarily due to reduced thermal
conductivity attributed to the presence of nanostructures in the
samples.
[0093] In some embodiments, rather than converting a thermoelectric
starting material into nanoparticles (or using some other particle
generation method) and compactifying those nanoparticles,
nanoparticles are generated (e.g., by grinding) from at least two
elemental materials (e.g., elemental Bi and elemental Te). The
nanoparticles are then mixed and compactified under pressure and at
elevated temperature (e.g., the pressures and temperatures
discussed above) to generate a resultant thermoelectric material
(e.g., one having a polycrystalline structure with grains having
sizes less than about 500 nm, and preferably in a range of about 1
to about 100 nm) that exhibits a ZT value greater than about 1, and
preferably greater than about 1.2, or about 1.5, or about 2.
[0094] In an alternative embodiment, two or more bulk materials can
be grinded simultaneously to generate a variety of nanoparticles
having different compositions. The grinding process can be used to
"mechanically alloy" the nanoparticles. Mechanical alloying can
also be performed by generating two or more different particles
separately, and subsequently mixing the particles together and
further grinding them to alloy and decrease the size of the
particles to form alloyed nanoparticles. The particles can be
consolidated to form a thermoelectric material having one or more
of the properties discussed in the present application.
[0095] In yet another embodiment, different types of nanoparticles
can be separately generated using any of the techniques discussed
herein (e.g., grinding bulk elemental materials such as bismuth or
tellurium), and then mixed together and consolidated to form a
thermoelectric material. Additional grinding of the mixture can
optionally be applied before consolidation. The end-consolidated
material formed by any of these processes can have any of the
composition characteristics described within the present
application, e.g., Bi.sub.2Te.sub.3-xSe.sub.x where x is in a range
of about 0 to about 0.8 such as Bi.sub.2Te.sub.2.8Se.sub.0.2, or
Bi.sub.xSb.sub.2-xTe.sub.3, wherein x is in a range from about 0 to
about 0.8 such as Bi.sub.0.5Sb.sub.1.5Te.sub.3.
[0096] Other embodiments directed to forming thermoelectric
materials utilize one or more repetitions of steps used to form
thermoelectrics as discussed herein. For example, particles (e.g.,
nanoparticles) can be generated from one or more starting materials
(e.g., bulk starting thermoelectric materials or elemental
materials) and consolidated into a material structure. The
resulting structure can then be used to generate a new plurality of
particles (e.g., by grinding the material structure), which can be
subsequently consolidated to form another material structure. This
process can be repeated any number of times to form an
end-thermoelectric material. Such a process can aid in generating
small grain sizes that are thoroughly mixed.
[0097] For some embodiments, it can be advantageous to protect
particles that are being generated from oxidation (e.g., during a
ball milling process). Non-limiting examples of protection
techniques include exposing the generated particles (e.g., the
environment in which grinding of a material takes place) to an
oxygen-depleted environment such as a relative vacuum or an
environment with low oxygen content relative to atmospheric
pressure. The generated particles can also be exposed to some type
of chemical coating to reduce oxygen exposure to the surface; the
coating can be optionally removed later in the thermoelectric
material manufacturing process. Accordingly, protection schemes can
include any number of adequate techniques, including those known to
one skilled in the art.
[0098] The following experimental section is provided for further
illustration of various aspects of the invention and for
illustrating the feasibility of utilizing the methods of the
invention for generating thermoelectric materials exhibiting
enhanced thermoelectric properties. It should, however, be
understood that the following examples are provided only for
illustrative purposes and are not necessarily indicative of optimal
results achievable by practicing the methods of the invention.
EXPERIMENTAL RESULTS
Example 1
Nanocrystalline Bulk p-type Bi.sub.xSb.sub.2-xTe.sub.3
Materials
[0099] Commercial materials (p-type BiSbTe alloy ingots) were
pulverized and loaded into a zirconia jar inside the glove box in
an argon atmosphere to avoid oxidation. A few zirconia balls (5-15
mm size) were also added and sealed. The sealed jar was placed into
a ball mill and milled for total of about 0.5 to 50 hours at a
speed of 100 to 2000 rpm. The powders were characterized using
scanning electron microscope (SEM), transmission electron
microscope (TEM), and x-ray diffraction (XRD).
[0100] FIG. 3 shows the x-ray diffraction (XRD) pattern of the
nanopowders after ball milling. The XRD pattern verifies that the
powder is a single phase, and is well matched with those of
Bi.sub.0.5Sb.sub.1.5Te.sub.3. The broadened diffraction peaks
indicates that the particles are small. The small size is confirmed
by the scanning electron microscope (SEM) image of the nanopowders
depicted in FIG. 2A, and the lower magnification transmission
electron microscope (TEM) image of the powder presented in FIG. 2B.
The lower resolution TEM image of FIG. 2B clearly shows that the
nanoparticles have sizes of a few to about 50 nm, with an average
size about 20 nm. The high resolution TEM image presented by FIG.
2C confirmed the good crystallinity of the nanoparticles and the
clean particle surfaces, which are desired for good thermoelectric
properties. The inset of FIG. 2C also shows that some of the
nanoparticles are even smaller than 5 nm.
[0101] Once the powders were obtained, powder samples were
processed into bulk disk samples of 1/2'' in diameter and 2-12 mm
thick by hot-pressing of the nanopowders loaded in a 1/2'' diameter
die. The powders after milling, which were stored inside the glove
box to prevent oxidation, were loaded into a graphite die and
pressed into pellets using a DC hot press technique (see FIGS. 4A
and 4B). Parameters for the hot pressing conditions are from 40-160
MPa and 450.degree. C.-600.degree. C. The densities are close to
100% of the theoretical value for all the compositions. Disks of
1/2'' diameter and 2 mm thick and bars of about 2.times.2.times.12
mm.sup.3 were cut and polished from the pressed disks for
measurements of the electrical and thermal conductivities and
Seebeck coefficient using both DC and AC methods.
[0102] Typically in preparing the hot pressed samples, the powder
is exposed to the selected pressure and the device is activated at
a designated heating rate. Upon reaching a selected elevated
temperature, the sample is held at the temperature and pressure for
anywhere between about 0 min and to about 60 min, preferably
between about 0 min to about 30 min, between about 0 min to about
10 min, or between about 0 min and less than 5 min (e.g., for 2
min.). Then cooling is initiated. It is understood, however, that
the pressure can be imposed during or after the sample reaches the
elevated temperature.
[0103] FIGS. 5-9 compare the temperature dependence of various
properties of a hot pressed nanocrystalline material (labeled
BP0572) and a commercial material (labeled com ingot) p-type BiSbTe
alloy ingot. All the properties are measured from the same sample
in FIGS. 5-9. Cylinder-like thick disks were hot-pressed and cut
both along and perpendicular to the press direction and then
measured. To test the temperature stability of the nanocrystalline
bulk samples, the same samples were repeatedly measured up to
250.degree. C. No significant property degradation was
observed.
[0104] FIG. 5 compares the temperature dependence of the electrical
conductivity of the nanocrystalline and commercial samples. The
electrical conductivity was measured by a four-point
current-switching technique. The electrical conductivity of the
nanocrystalline bulk sample is slightly higher than that of the
commercial ingot.
[0105] FIG. 6 presents the temperature dependencies of the Seebeck
coefficient for the nanocrystalline and commercial samples, while
FIG. 7 compares the power factor (S.sup.2.sigma.) temperature
dependencies of the samples. The Seebeck coefficients were measured
by a static DC method based on the slope of a voltage vs.
temperature-difference curve, using commercial equipment (ZEM-3,
Ulvac, Inc.) on the same bar-type sample with a dimension of
2.times.2 mm.sup.2 in cross-section and 12 mm in length, cut along
the disk plane. These properties were also measured on a home-made
system on the same sample. The two sets of measurements are within
5% of each other. The Seebeck coefficient of the nanocrystalline
sample is either slightly lower or higher than that of the ingot
depending on temperature, which makes the power factor of the
nanocrystalline sample comparable to that of the commercial ingot
below 75.degree. C. and higher than that of the commercial ingot
above 75.degree. C.
[0106] FIG. 8 depicts the temperature dependencies of the thermal
conductivity for the nanocrystalline and commercial samples. The
thermal conductivities are derived from measurements of the thermal
diffusivities and the heat capacities of the samples. The thermal
diffusivity was measured by a laser-flash method on a disk along
the disk axial direction using a commercial system (Netzsch
Instruments, Inc.). After the laser-flash measurements, bars were
diced from the disks and their thermal diffusivities were measured
along the bar (disk-plane) direction using an .ANG.ngstrom method
in the home-built system. The thermal diffusivity values of the bar
and of the disk are in agreement within 5%.
[0107] FIG. 9 documents the variation in the figure of merit, ZT,
as a function of temperature for the nanocrystalline and commercial
samples. Since the thermal conductivity of the nanocrystalline bulk
samples is significantly lower than that of the commercial ingot,
and more importantly the difference increases with increasing
temperature, this leads to significantly enhanced ZTs in the
temperature range of 20-250.degree. C. FIG. 9 also shows that the
peak ZT value is shifted to a higher temperature (100.degree. C.).
The peak ZT of nanocrystalline bulk samples is of about 1.4 at
100.degree. C., which is significantly higher than that of the
commercial Bi.sub.2Te.sub.3-based alloys. The ZT value of the
commercial ingot starts to drop above 75.degree. C., and falls
below 0.25 at 250.degree. C. In comparison, the nanocrystalline
bulk samples exhibit ZTs higher than 0.8 at 250.degree. C. Such ZT
characteristics are very much desired for power generation
applications since there are no good materials presently available
with high ZT in this temperature range.
[0108] A detailed microstructure examination was conducted on the
nanocrystalline bulk samples using a transmission electron
microscope (TEM). The TEM specimens were prepared by dicing,
polishing, and ion milling the bulk nanocrystalline samples.
Hot-pressed nanocrystalline bulk pellets were cut into blocks of
2.times.3.times.1 mm and ground down to 2.times.3.times.0.002 mm
using a mechanical Tripod Polisher. The sample was glued to a
Copper grid, and milled using Precision Ion Polishing System (Gatan
Inc.) for 30 minutes with incident energy of 3.2 kV and a beam
current of 15 .mu.A at an incident angle of 3.5 degrees. FIGS.
10-15 present some representative TEM micrographs, which show the
main structural features observed.
[0109] In general, as depicted in FIGS. 10 and 11, most of the
grains are nanosized.
[0110] Furthermore, the nanograins are highly crystalline,
randomly-oriented (large angles among lattice planes) with very
clean boundaries. As depicted in FIG. 11, the nanograins can be
closely packed, consistent with density measurements that suggest
full dense samples. Some larger grains are also present as shown in
FIG. 12. High-resolution TEM microscopy, as shown in FIG. 13,
reveals that these grains are composed of nanodots 2 to 10 nm in
size without boundaries. These nanodots are typically Sb-rich with
an exemplary composition close to Bi:Sb:Te=8:44:48; the Sb
substituting for Te. Although some of the nanodots are boundaryless
with the matrix as depicted in FIG. 13, other observed nanodots
included small angle boundaries with the matrix as depicted in FIG.
14. Pure Te precipitates of sizes in the range from 5-30 nm were
also observed, as depicted in FIG. 15. The selected-area electron
diffraction pattern, shown in the inset of FIG. 15, confirmed the
Te phase. Generally speaking, nanodots could be found within each
50 nm diameter area.
[0111] Without necessarily being bound by any particular theory, it
can be hypothesized that these nanodots could be formed during the
hot-press heating and cooling processes. The larger-sized grains
containing nanodots, as depicted in FIG. 12, could be the result of
non-uniform milling of the ingot during ball milling. These large
grains may have grown even larger during the hot-press compaction
via Oswald Ripening. Given the large population of nano interfacial
features in our material, such as nanograins, nanodots may not be
the only reason for the strong phonon scattering.
Example 2
Nanocrystalline SiGe Materials
[0112] Silicon and germanium elemental materials, both p and
n-type, were used as starting materials and ground using a ball
mill to form nanoparticles having a size of about 1-about 200 nm.
These elemental materials can have a ZT lower than about 0.01 in
some instances. It is also understood that a SiGe alloy could have
been used to form the particles, perhaps leading to further
improvement in the final manufactured material. Samples were hot
pressed at a pressure of about 40-about 200 MPa and at a
temperature of about 900.degree. C.-1300.degree. C. to form
thermoelectric material samples.
[0113] FIGS. 16-19 depict graphs showing the temperature dependence
of various properties of a hot pressed nanocrystalline material
formed from a p-type SiGe ball milled bulk material. The properties
were measured using the same techniques as described earlier for
FIGS. 5-9. FIG. 16 shows the temperature dependence of the
electrical conductivity of the nanocrystalline p-type SiGe sample.
FIG. 17 presents the temperature dependence of the Seebeck
coefficient for the nanocrystalline p-type SiGe sample. FIG. 18
depicts the temperature dependencies of the thermal conductivity
for the p-type SiGe sample. FIG. 19 documents the variation in the
figure of merit, ZT, as a function of temperature for the
nanocrystalline p-type SiGe sample.
[0114] FIGS. 20-23 depict graphs showing the temperature dependence
of various properties of a hot pressed nanocrystalline material
formed from a n-type SiGe ball milled bulk material. FIG. 20 shows
the temperature dependence of the electrical conductivity of the
nanocrystalline n-type SiGe sample. FIG. 17 presents the
temperature dependence of the Seebeck coefficient for the
nanocrystalline n-type SiGe sample. FIG. 18 depicts the temperature
dependencies of the thermal conductivity for the n-type SiGe
sample. FIG. 19 documents the variation in the figure of merit, ZT,
as a function of temperature for the nanocrystalline n-type SiGe
sample.
[0115] FIGS. 24-26 depict TEM micrographs of p-type SiGe materials
associated with nanocrystalline materials. FIG. 24 presents a TEM
micrograph of a ball-milled powder sample of SiGe bulk material,
showing the nano-sized particulates of the milled particulates.
FIG. 25 presents a TEM micrograph of a SiGe powder sample after hot
pressing. The micrograph shows numerous grains of the hot-pressed
material which are densely packed and in the nano-sized range. The
inset of FIG. 25 presents a selected-area electron diffraction
pattern taken on the sample. FIG. 26 presents a high-resolution TEM
of the hot-pressed SiGe sample, further showing the nano size of
the various grains of the sample, indicating lots of grain
boundaries that are designed for phonon scattering.
Example 3
Temperature Tailoring of Nanocrystalline p-type BiSbTe
Materials
[0116] Samples of nanocrystalline p-type BiSbTe alloyed materials
were prepared to demonstrate how the figure of merit, ZT, can be
tailored to various temperature conditions. In particular,
Bi.sub.xSb.sub.2-xTe.sub.3 type materials can be prepared with
various stoichiometries depending upon the value of x selected. Two
particular example types of samples were prepared: p-type
nanocrystalline, hot pressed materials having a stoichiometry of
Bi.sub.0.3Sb.sub.1.7Te.sub.3, and p-type nanocrystalline, hot
pressed materials having a stoichiometry of
Bi.sub.0.5Sb.sub.1.5Te.sub.3. Appropriate bulk starting materials
were ground up by a ball mill to form nanoparticle samples. The
samples are pressed at 40-160 MPa and 450.degree. C.-600.degree. C.
for up to about 5 minutes.
[0117] FIGS. 27-30 depict the temperature dependence of the
electrical conductivity, Seebeck coefficient, thermal conductivity,
and ZT, respectively, for nanocrystalline
Bi.sub.0.3Sb.sub.1.7Te.sub.3 samples, while FIGS. 31-34 depict the
temperature dependence of the electrical conductivity, Seebeck
coefficient, thermal conductivity, and ZT, respectively, for
nanocrystalline Bi.sub.0.5Sb.sub.1.5Te.sub.3 samples. The
measurements were carried in accord as described in Example 1. As
can be seen in FIGS. 30 and 34, the peak ZT value for a
Bi.sub.0.3Sb.sub.1.7Te.sub.3 sample was measured at about
150.degree. C., while the peak ZT value for a
Bi.sub.0.5Sb.sub.1.5Te.sub.3 sample was measured at about
75.degree. C.
[0118] Accordingly, the results show that a nanocrystalline
material's peak ZT can be tailored for particular temperature range
applications. For example, the lower temperature peak material can
be utilized in applications adapted for closer to room temperature
use, such as cooling--while the higher temperature peak material
can be utilized in applications for high temperature such as power
generation.
[0119] It is understood that the various embodiments discussed
herein, along with the experimental results, describe a variety of
methods and materials that are merely representative of the scope
of the present invention. Indeed, those skilled in the art will
readily appreciate that many other modifications to the methods and
materials disclosed herein can be made. All such modifications
represent related embodiments that are also within the scope of the
present invention. As well, all numbers expressing quantities of
ingredients, reaction conditions, and so forth used in the
specification and claims are to be understood as being modified in
all instances by the term "about." Accordingly, unless indicated to
the contrary, the numerical parameters set forth in this
specification and attached claims are approximations that can vary
depending upon the desired properties.
* * * * *