U.S. patent application number 09/790509 was filed with the patent office on 2002-04-18 for process for producing thermoelectric material.
Invention is credited to Anno, Yuichi, Kajino, Hitoshi, Karino, Shinji, Tsukuda, Ryouma, Yashima, Isamu.
Application Number | 20020044877 09/790509 |
Document ID | / |
Family ID | 26585904 |
Filed Date | 2002-04-18 |
United States Patent
Application |
20020044877 |
Kind Code |
A1 |
Karino, Shinji ; et
al. |
April 18, 2002 |
Process for producing thermoelectric material
Abstract
A process for producing a thermoelectric material comprising
mixing at least two of bismuth, tellurium, selenium, and antimony
and, if desired, a dopant, melting the mixture, grinding the
resulting alloy ingot, forming the powder, and sintering the green
body under normal pressure, or hot pressing the powder, wherein the
grinding and the normal sintering or hot pressing are carried out
in the presence of a solvent represented by C.sub.nH.sub.2n+1OH or
C.sub.nH.sub.2n+2CO (wherein n is 1, 2 or 3).
Inventors: |
Karino, Shinji; (Saitama,
JP) ; Tsukuda, Ryouma; (Ageo-shi, JP) ; Anno,
Yuichi; (Ageo-shi, JP) ; Yashima, Isamu;
(Ageo-shi, JP) ; Kajino, Hitoshi; (Ageo-shi,
JP) |
Correspondence
Address: |
YOUNG & THOMPSON
745 SOUTH 23RD STREET 2ND FLOOR
ARLINGTON
VA
22202
|
Family ID: |
26585904 |
Appl. No.: |
09/790509 |
Filed: |
February 23, 2001 |
Current U.S.
Class: |
419/36 ;
75/351 |
Current CPC
Class: |
H01L 35/34 20130101 |
Class at
Publication: |
419/36 ;
75/351 |
International
Class: |
B22F 001/00; B22F
009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 23, 2000 |
JP |
2000-45897 |
Feb 23, 2000 |
JP |
2000-45898 |
Claims
What is claimed is:
1. A process for producing a thermoelectric material comprising
mixing at least two members selected from bismuth, tellurium,
selenium, and antimony and, if desired, a dopant, melting the
mixture, grinding the resulting alloy ingot, forming the powder,
and sintering the green body under normal pressure, which is
characterized in that the grinding and the normal sintering are
carried out in the presence of a solvent represented by formula:
C.sub.nH.sub.2n+1OH or C.sub.nH.sub.2n+2CO (wherein n is 1, 2 or
3).
2. The process for producing a thermoelectric material as claimed
in claim 1, wherein the normal sintering is carried out by raising
temperature in a hydrogen gas atmosphere, maintaining at a raised
temperature, further raising temperature in an inert gas
atmosphere, and maintaining at a raised temperature.
3. A process for producing a thermoelectric material comprising
mixing at least two members selected from bismuth, tellurium,
selenium, and antimony and, if desired, a dopant, melting the
mixture, grinding the resulting alloy ingot, and hot pressing the
powder, which is characterized in that the grinding and the hot
pressing are carried out in the presence of a solvent represented
by formula: C.sub.nH.sub.2n+1OH or C.sub.nH.sub.2n+2CO (wherein n
is 1, 2 or 3).
4. The process for producing a thermoelectric material as claimed
in claim 3, wherein the hot pressing is carried out in a
non-oxidizing gas atmosphere.
5. A thermoelectric material obtained by the process as claimed in
any one of claims 1 to 4, which is used as an n-type thermoelectric
device.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a process for producing a
thermoelectric material which can be used as a material of
thermoelectric devices utilizing Peltier effect or Seebeck effect.
More particularly, it relates to a process of producing a
thermoelectric material, in which grinding and normal sintering or
hot pressing are carried out in the presence of a specific solvent
to bring about a markedly improved figure of merit.
[0003] 2. Description of Related Art
[0004] Thermoelectric devices utilizing Peltier effect have found
wide application as thermoelectric electricity generators,
thermosensors, etc.
[0005] Performance properties of thermoelectric materials which can
provide thermoelectric devices are evaluated by the thermoelectric
figure of merit Z which is obtained from Seebeck coefficient
.alpha., thermal conductivity .kappa., and electrical resistivity
.rho. (or electrical conductivity .sigma.) as represented by
equation:
Z=.alpha..sup.2/(.rho..multidot..kappa.)=.alpha..sup.2.multidot..sigma./.k-
appa.
[0006] That is, in order to improve the figure of merit (to
increase the figure of merit) of a thermoelectric material, it is
required that the Seebeck coefficient be high, and both the
specific resistance and the thermal conductivity be low.
[0007] The following process is adopted to produce a thermoelectric
material for obtaining a p-type or n-type thermoelectric device,
the material comprising an appropriately doped alloy comprising at
least two elements selected from the group consisting of Bi, Te, Se
and Sb elements. The process comprises mixing weighed amounts of
Bi, Te, Se or Sb powder and a dopant, melting the mixture, grinding
the resulting alloy ingot to powder, and sintering the powder to
obtain a thermoelectric material as a sintered body.
[0008] Sintering techniques used here include normal sintering, hot
pressing, vacuum sintering, gas-pressure sintering, plasma
sintering, and hot isostatic press sintering (HIP). Normal
sintering and hot pressing are particularly preferred. Normal
sintering is practical for ease in producing a sintered body of
complicated configuration and also from the economical standpoint.
Hot pressing provides a sintered body with few cracks and excellent
mechanical strength.
[0009] However, the figure of merit Z of thermoelectric materials
obtained by normal sintering, hot pressing, etc. is lower than the
desired range. It has been a demand to produce a thermoelectric
material having a high figure of merit Z. In particular, considered
that the figure of merit Z of available p-type thermoelectric
devices exceeds 3.0.times.10.sup.-3 K.sup.-1, whereas those of
available n-type ones are around 2.5.times.10.sup.-3 K.sup.-1,
there has been a keen demand to further improve the figure of merit
of n-type thermoelectric devices.
SUMMARY OF THE INVENTION
[0010] An object of the present invention is to provide an
economically excellent process for producing a thermoelectric
device having a low carrier density, a high mobility, and a greatly
improved figure of merit.
[0011] As a result of investigation, the present inventors have
found that the above object is accomplished by the presence of a
specific solvent in grinding an alloy ingot obtained by mixing and
melting and also in normal sintering or hot pressing the resulting
powder.
[0012] The present invention has been completed based on the above
finding. The present invention provides a process for producing a
thermoelectric material comprising mixing at least two members
selected from bismuth, tellurium, selenium, and antimony and, if
desired, a dopant, melting the mixture, grinding the resulting
alloy ingot, forming the powder, and sintering the green body under
normal pressure, which is characterized in that the grinding and
the normal sintering are carried out in the presence of a solvent
represented by formula: C.sub.nH.sub.2n+1OH or C.sub.nH.sub.2n+2CO
(wherein n is 1, 2 or 3).
[0013] The present invention also provides a process for producing
a thermoelectric material comprising mixing at least two members
selected from bismuth, tellurium, selenium, and antimony and, if
desired, a dopant, melting the mixture, grinding the resulting
alloy ingot, and hot pressing the powder, which is characterized in
that the grinding and the hot pressing are carried out in the
presence of a solvent represented by formula: C.sub.nH.sub.2n+1OH
or C.sub.nH.sub.2n+2CO (wherein n is 1, 2 or 3).
[0014] The thermoelectric material obtained by the process of the
invention has a low carrier density, a high mobility, and a
satisfactory figure of merit. Where normal sintering is adopted,
the process is economically excellent. The thermoelectric material
thus obtained are applicable to various fields as a thermoelectric
device with its Peltier effect being taken into advantage of
DETAILED DESCRIPTION OF THE INVENTION
[0015] The process for production according to the present
invention will hereinafter be described in detail.
[0016] At least two members selected from bismuth, tellurium,
selenium and antimony are used in the present invention as
constituent elements of the thermoelectric material.
[0017] A dopant is used in addition if necessary to obtain a
desired n-type thermoelectric device or a desired p-type
thermoelectric device. Such a dopant includes BiF.sub.3,
BiCl.sub.3, BiBr.sub.3, BiI.sub.3, TeCl.sub.4, TeI.sub.2,
TeI.sub.4, TeBr.sub.4, SeCl.sub.4, SeBr.sub.4, SeI.sub.4,
SbF.sub.3, SbCl.sub.3, SbBr.sub.3, Se, and Te.
[0018] Weighed amounts of the constituent elements and the dopant
for a thermoelectric material are mixed and melted. After mixing in
a molten state, the melt is cooled to obtain an alloy ingot.
[0019] The resulting alloy ingot is ground in a solvent represented
by formula: C.sub.nH.sub.2n+1OH or C.sub.nH.sub.2n+2CO (wherein n
is 1, 2 or 3) by means of a vibration mill, etc. to obtain an alloy
powder having an average particle size of 0.5 to 50 .mu.m.
[0020] The solvent represented by formula: C.sub.nH.sub.2n+1OH or
C.sub.nH.sub.2n+2CO (wherein n is 1, 2 or 3) is methanol, ethanol,
propanol, acetaldehyde, acetone or methyl ethyl ketone, with
methanol or acetone being preferred. Use of such a solvent brings
about an improvement on the figure of merit Z of the resulting
thermoelectric material.
[0021] It is preferred for the solvent to have a dielectric
constant of 21.4% or higher at 20.degree. C., a dipole moment of
1.68.times.10.sup.-16 c.s.u or higher, a molecular weight of 58.1
or lower, and a boiling point of 55 to 79.degree. C.
[0022] Then the alloy powder is formed and sintered under normal
pressure, or the alloy powder is hot pressed.
[0023] In the case of powder forming followed by normal sintering,
forming is preferably conducted by uniaxial pressing or cold
isostatic pressing (CIP). Pressing provides a green body with an
increased density.
[0024] The green body is subjected to normal sintering in the
presence of the solvent at 300 to 600.degree. C., desirably for 0.1
to 10 hours. Normal sintering in the presence of the solvent brings
about improvement on the figure of merit Z of the thermoelectric
material.
[0025] The normal sintering is preferably effected in two stages.
Specifically, the green body of the alloy powder is heated in
hydrogen gas and maintained at 300 to 600.degree. C. for 1 to 60
minutes and then further heated in an inert gas, such as argon gas,
and held at 400 to 600.degree. C. for 1 to 60 minutes, followed by
cooling.
[0026] In the case where the allow powder is hot pressed, hot
pressing is performed in the presence of the solvent. Hot pressing
is preferably conducted in a non-oxidizing gas atmosphere, e.g., an
argon atmosphere, at 400 to 600.degree. C. for 0.1 to 10 hours. Hot
pressing in the presence of the solvent produces an improvement in
the figure of merit Z of the thermoelectric material.
[0027] There is thus obtained a thermoelectric material comprising
a sintered body. Specific examples of the thermoelectric material
include bismuth telluride, bismuth selenide, antimony telluride,
antimony selenide, bismuth sulfide, antimony sulfide, etc., which
can be used either individually or as a combination thereof.
Polycrystalline materials made up of a single compound such as
bismuth telluride, bismuth selenide, etc. or a solid solution of
these compounds are used as a material of thermoelectric devices
for cooling, heat generation or electricity generation, such as a
Peltier element. Use of these thermoelectric materials provides
thermoelectric devices having satisfactory characteristics,
especially n-type thermoelectric devices.
[0028] The thermoelectric device is joined with a metal electrode
to make a thermoelectric module, the Peltier effect of which is
taken advantage of in thermoelectric conversion of waste heat from
various thermal engines or factories, small-scale electricity
generators, simple-structured air conditioning systems, and
refrigerators. The module is particularly useful as a CPU cooling
module.
[0029] The present invention will now be illustrated in greater
detail with reference to Examples.
EXAMPLES 1 TO 4 AND COMPARATIVE EXAMPLE 1
[0030] Tellurium, bismuth and antimony flakes were weighed out to
give a bismuth selenide (Bi.sub.2Se3) to bismuth telluride
(Bi.sub.2Te.sub.3) alloy ratio of 15:85 by mole. A predetermined
amount of tellurium iodide was weighed out as a dopant. These
weighed materials were melted in a graphite crucible in an argon
gas at 750.degree. C. for 2 hours to obtain an alloy ingot having a
desired composition.
[0031] The alloy ingot was ground in a vibrating mill together with
the solvent shown in Table 1 below to obtain an alloy powder having
an average particle size of 8 .mu.m. The powder was pressed into a
prescribed shape, and the green body was subjected to normal
sintering by heating in a hydrogen gas atmosphere in the presence
of the solvent shown in Table 1 at a rate of 15.degree. C./min,
maintaining at 370.degree. C. for 30 minutes, further heating in an
argon atmosphere in the presence of the solvent at a rate of
15.degree. C./min, keeping at 500.degree. C. for 10 minutes, and
slowly cooling.
[0032] The Seebeck coefficient, electrical resistivity, thermal
conductivity, power factor, figure of merit, carrier density, and
mobility of the resulting thermoelectric material are shown in
Table 1. The mobility was obtained by combining Hall measurements
and electrical resistivity.
1 TABLE 1 Seebeck Electrical Thermal Power Factor Figure of Carrier
Coefficient .alpha. Resistivity .rho. Conductivity .kappa.
.alpha..sup.2/.rho. Merit Z Density n Mobility .mu. Solvent
(.mu.V/K) (m.OMEGA. .multidot. cm) (mW/cm .multidot. K) (mW/cm
.multidot. K.sup.2) (.times.10.sup.-3K.sup.-1)
(.times.10.sup.19/cm.sup.3) (cm.sup.2/V .multidot. S) Example 1
methanol -215 1.71 10.0 2.70 1.48 1.48 225 Example 2 acetone -217
1.92 9.12 2.69 1.50 1.50 217 Example 3 ethanol -217 1.95 9.21 2.62
1.51 1.51 212 Example 4 methyl ethyl -217 1.96 9.23 2.61 1.53 1.53
211 ketone Comparative hexane -216 1.93 9.32 2.60 1.57 1.57 206
Example 1
[0033] As is shown in Table 1, the thermoelectric materials of
Examples 1 to 4 have a lower carrier density, a higher mobility,
and a higher figure of merit than those of Comparative Example 1.
These improving effects are conspicuous in Examples 1 and 2 where
methanol and acetone are used as a solvent, respectively. A
difference of figure of merit by 0.01 corresponds to a difference
in cooling ability by about 3.degree. C.
EXAMPLES 5 TO 8 AND COMPARATIVE EXAMPLE 2
[0034] The same alloy ingot as used in Example 1 was ground in a
vibration mill together with the solvent shown in Table 2 to obtain
an alloy powder having an average particle size of 8 .mu.m. The
powder was hot pressed in an argon gas/hydrogen gas (1%) mixture in
the presence of the solvent shown in Table 2 under a pressure of
300 kg/cm.sup.2 at 590.degree. C. for 1 hour.
[0035] The Seebeck coefficient, electrical resistivity, thermal
conductivity, power factor, figure of merit, carrier density, and
mobility of the resulting thermoelectric material are shown in
Table 2.
2 TABLE 2 Seebeck Electrical Thermal Power Factor Figure of Carrier
Coefficient .alpha. Resistivity .rho. Conductivity .kappa.
.alpha..sup.2/.rho. Merit Z Density n Mobility .mu. Solvent
(.mu.V/K) (m.OMEGA. .multidot. cm) (mW/cm .multidot. K) (mW/cm
.multidot. K.sup.2) (.times.10.sup.-3K.sup.-1)
(.times.10.sup.19/cm.sup.3) (cm.sup.2/V .multidot. S) Example 5
methanol -242 1.73 12.3 33.8 2.75 2.21 170 Example 6 acetone -235
1.70 12.4 32.5 2.62 2.42 164 Example 7 acetaldehyde -230 1.61 12.4
32.9 2.66 2.56 159 Example 8 methyl ethyl -238 1.72 12.2 32.9 2.70
2.61 156 ketone Comparative hexane -230 1.53 12.9 34.6 2.66 3.22
132 Example 2
[0036] As is shown in Table 2, the thermoelectric materials of
Examples 5 to 8 have a lower carrier density and a higher mobility
than those of Comparative Example 2. These improving effects are
conspicuous in Example 5 where methanol is used as a solvent. A
difference of the figure of merit by 0.01 corresponds to a
difference in cooling ability by about 3.degree. C.
* * * * *