U.S. patent application number 13/328495 was filed with the patent office on 2012-06-28 for thermoelectric conversion module and production method therefor.
This patent application is currently assigned to HITACHI POWDERED METALS CO., LTD.. Invention is credited to Zenzo ISHIJIMA, Takahiro JINUSHI.
Application Number | 20120160293 13/328495 |
Document ID | / |
Family ID | 46315222 |
Filed Date | 2012-06-28 |
United States Patent
Application |
20120160293 |
Kind Code |
A1 |
JINUSHI; Takahiro ; et
al. |
June 28, 2012 |
THERMOELECTRIC CONVERSION MODULE AND PRODUCTION METHOD THEREFOR
Abstract
A thermoelectric conversion module has a thermoelectric
conversion element and an electrode, which are metallurgically
bonded together via a porous metal layer. The porous metal layer is
made of nickel or silver and has a density ratio of 50 to 90%.
Inventors: |
JINUSHI; Takahiro;
(Matsudo-shi, JP) ; ISHIJIMA; Zenzo; (Matsudo-shi,
JP) |
Assignee: |
HITACHI POWDERED METALS CO.,
LTD.
Matsudo-shi
JP
|
Family ID: |
46315222 |
Appl. No.: |
13/328495 |
Filed: |
December 16, 2011 |
Current U.S.
Class: |
136/237 ;
136/201 |
Current CPC
Class: |
H01L 35/08 20130101;
H01L 35/34 20130101 |
Class at
Publication: |
136/237 ;
136/201 |
International
Class: |
H01L 35/30 20060101
H01L035/30; H01L 35/34 20060101 H01L035/34; H01L 35/08 20060101
H01L035/08 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 24, 2010 |
JP |
2010-286930 |
Claims
1. A thermoelectric conversion module having a thermoelectric
conversion element and an electrode, which are metallurgically
bonded together via a porous metal layer, the porous metal layer
being made of nickel or silver and having a density ratio of 50 to
90%.
2. The thermoelectric conversion module according to claim 1,
wherein the porous metal layer has a thickness of 10 to 100
.mu.m.
3. The thermoelectric conversion module according to claim 1,
wherein the porous metal layer is made by sintering metal powder
particles having an average particle diameter of 0.1 to 10 .mu.m,
and the metal powder particles are one of nickel powder particles
and silver powder particles.
4. The thermoelectric conversion module according to claim 1,
wherein the thermoelectric conversion element has an end surface
that is covered with a metal, and the end surface is bonded to the
electrode via the porous metal layer.
5. A production method for a thermoelectric conversion module
having a thermoelectric conversion element with an end surface and
having an electrode, the production method comprising: preparing a
paste in which metal powder particles are dispersed, the metal
powder particles having an average particle diameter of 0.1 to 10
.mu.m and being one of nickel powder particles and silver powder
particles; applying the paste to the end surface of the
thermoelectric conversion element; abutting the end surface, to
which the paste is applied, to the electrode so as to connect the
thermoelectric conversion element and the electrode; heating the
paste between the thermoelectric conversion element and the
electrode in an inert gas atmosphere, a reducing gas atmosphere, or
a vacuum atmosphere, so as to remove the paste except for the metal
powder particles and cause the metal powder particles to remain;
sintering the remaining metal powder particles so as to form a
porous metal layer; and diffusion bonding the porous metal layer to
the end surface of the thermoelectric conversion element and to the
electrode so as to metallurgically bond them, wherein the sintering
and the diffusion bonding are simultaneously performed at a
temperature of 650 to 850.degree. C. when the nickel powder
particles are used, or at a temperature of 450 to 750.degree. C.
when the silver powder particles are used.
6. The production method for the thermoelectric conversion module
according to claim 5, wherein the metal powder particles are
dispersed in the paste at 30 to 50 volume %.
7. The production method for the thermoelectric conversion module
according to claim 5, wherein the paste has a viscosity of 10 to
100 Pas.
8. The production method for the thermoelectric conversion module
according to claim 5, wherein the paste has a shear strength of not
less than 0.1 N/cm.sup.2.
9. The production method for the thermoelectric conversion module
according to claim 5, wherein the end surface of the thermoelectric
conversion element is covered with a metal, and the paste is
applied to the end surface covered with the metal.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] The present invention relates to a thermoelectric conversion
module that converts thermal energy to electric energy, and relates
to a production method therefor. The thermoelectric conversion
module has a bonding portion between a thermoelectric conversion
element and an electrode, and thermal stress may occur in the
bonding portion. Therefore, specifically, the present invention
relates to a thermoelectric conversion module with a function to
decrease the thermal stress, and relates to a production technique
therefor.
[0003] 2. Background Art
[0004] A power generation system provided with a thermoelectric
conversion module using thermoelectric conversion elements directly
generates electricity. This power generation system has a simple
structure and does not have a movable part, thereby having high
reliability and facilitating maintenance thereof. However, the
power generation system has low power density and low energy
conversion efficiency. Due to this, this power generation system
has been developed only for special uses at low output scale, such
as for use in space. However, in view of recent environmental
issues, this power generation system is expected to be useful as an
environmental protection measure. For example, this power
generation system is anticipated to be useful as a small-scale
distributed power generation system that uses an exhaust heat
source of a waste incinerator, a cogeneration system, etc. This
power generation system is also anticipated to be useful as an
automobile power generation system that uses heat in exhaust gas of
automobiles. Consequently, reduction in the cost of this power
generation and improvement in durability of a thermoelectric
conversion module system are required in this power generation
system.
[0005] A thermoelectric conversion module is shown in FIG. 1. As
shown in FIG. 1, the thermoelectric conversion module is
constructed by stacking an electrode 2 on each side of a
thermoelectric conversion element 1. Moreover, each of a cooling
duct 4 and a heating duct 5 is stacked on the rest side of each
electrode 2 via an electric insulating layer 3. The electrode 2 may
be made of copper, and the electric insulating layer 3 may be made
of mica. In this thermoelectric conversion module, by sending air
to the cooling duct 4 and by supplying high-temperature exhaust gas
to the heating duct 5, a temperature difference is generated
between the two ends of the thermoelectric conversion element 1.
The temperature difference generates thermoelectric power in the
thermoelectric conversion element 1, whereby direct current is
obtained from the electrode 2. Such a thermoelectric conversion
module is disclosed in Japanese Patent Application of Laid-open No.
9-293906, for example.
[0006] In general, the thermoelectric conversion module is produced
by pressing and bonding the thermoelectric conversion element and
the electrodes, or by bonding them with a soldering material. As
described above, the thermoelectric conversion module generates
power based on the thermoelectric power. The thermoelectric power
occurs by the temperature difference between the two ends of the
thermoelectric conversion element. Therefore, when the temperature
difference between the two ends of the thermoelectric conversion
element is larger, the thermoelectric power is increased, and a
greater amount of electricity is generated. In order to increase
the temperature difference between the two ends of the
thermoelectric conversion element, the temperature of the cooling
side (cooling duct 4) may be decreased, but a special device is
required, which is not preferable. Accordingly, usually the
temperature of the heating side (heating duct 5) is increased to
not more than an upper temperature limit of the thermoelectric
conversion element.
[0007] The thermoelectric conversion element and the electrode at
the heating side of the thermoelectric conversion module have a
bonding portion therebetween. The thermoelectric conversion element
does not greatly expand with the heat, but the electrode greatly
expands with the heat. Therefore, in the bonding portion, there is
a difference in the amounts of the thermal expansion of the
thermoelectric conversion element and the electrode. As a result,
the bonding portion receives stress due to the difference in the
amounts of the thermal expansion. Accordingly, when the temperature
of the heating side of the thermoelectric conversion element is
increased so as to increase the amount of power generation, large
thermal stress occurs in the bonding portion by the difference in
the amounts of the thermal expansion. The large thermal stress
easily causes fracture at the bonding portion between the
thermoelectric conversion element and the electrode and its
vicinity.
[0008] The thermoelectric conversion element and the electrode may
be bonded with a soft soldering material. In this case, if the
temperature of the heating side is set to be not less than the melt
temperature of the soft soldering material, the soft soldering
material is melted and leaks. Therefore, in this kind of
thermoelectric conversion module, the temperature of the heating
side is limited, and the amount of the power generation is
limited.
SUMMARY OF THE INVENTION
[0009] In view of these circumstances, an object of the present
invention is to provide a thermoelectric conversion module and a
production method therefor. In the thermoelectric conversion
module, thermal stress occurs when it is used, but the degree of
the thermal stress is decreased. Therefore, in the thermoelectric
conversion module, the bonding portion between the thermoelectric
conversion element and the electrode and its vicinity are prevented
from fracturing. Moreover, the thermoelectric conversion module can
be used at high temperatures.
[0010] The present invention provides a thermoelectric conversion
module having a thermoelectric conversion element and an electrode,
which are metallurgically bonded together via a porous metal layer.
The porous metal layer is made of nickel or silver and has a
density ratio of 50 to 90%.
[0011] In the thermoelectric conversion module of the present
invention, the porous metal layer preferably has a thickness of 10
to 100 .mu.m. In addition, the porous metal layer is preferably
made by sintering metal powder particles which have an average
particle diameter of 0.1 to 10 .mu.m and are one of nickel powder
particles and silver powder particles. Moreover, the thermoelectric
conversion element may have an end surface that is covered with a
metal, and this end surface may be bonded to the electrode via the
porous metal layer.
[0012] The present invention provides a production method for a
thermoelectric conversion module having a thermoelectric conversion
element with an end surface and having an electrode. The production
method includes preparing a paste in which metal powder particles
are dispersed. The metal powder particles have an average particle
diameter of 0.1 to 10 .mu.m and are one of nickel powder particles
and silver powder particles. The production method also includes
applying the paste to the end surface of the thermoelectric
conversion element and abutting the end surface, which is applied
with the paste, to the electrode so as to connect the
thermoelectric conversion element and the electrode. The production
method further includes heating the paste between the
thermoelectric conversion element and the electrode in an inert gas
atmosphere, a reducing gas atmosphere, or a vacuum atmosphere, so
as to remove the paste except for the metal powder particles and
cause the metal powder particles to remain. Moreover, the
production method includes sintering the remaining metal powder
particles so as to form a porous metal layer and diffusion bonding
the porous metal layer to the end surface of the thermoelectric
conversion element and to the electrode so as to metallurgically
bond them. The sintering and the diffusion bonding are
simultaneously performed at a temperature of 650 to 850.degree. C.
when the nickel powder particles are used, or at a temperature of
450 to 750.degree. C. when the silver powder particles are
used.
[0013] In the production method for the thermoelectric conversion
module of the present invention, the metal powder particles are
preferably dispersed in the paste at 30 to 50 volume %. The paste
preferably has viscosity of 10 to 100 Pas and preferably has a
shear strength of not less than 0.1 N/cm.sup.2. In addition, the
end surface of the thermoelectric conversion element may be covered
with a metal, and the paste may be applied to the end surface
covered with the metal.
[0014] According to the thermoelectric conversion module of the
present invention, the porous metal layer, which is made of nickel
or silver and has a density ratio of 50 to 90%, is provided between
the thermoelectric conversion element and the electrode. The porous
metal layer decreases the difference in the amounts of the thermal
expansion of the thermoelectric conversion element and the
electrode. Therefore, the bonding portion between the
thermoelectric conversion element and the electrode and its
vicinity are prevented from fracturing. Since the porous metal
layer is metallurgically bonded to both the thermoelectric
conversion element and the electrode, heat and electricity are
efficiently conducted between the thermoelectric conversion element
and the electrode. Moreover, the porous metal layer made of nickel
or silver has a high melting point, thereby providing a
thermoelectric conversion module in which melting and leaking of
the bonding portion do not occur at high temperatures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic view showing an example of a
thermoelectric conversion module.
[0016] FIG. 2 is a schematic view for illustrating a thermoelectric
conversion module of the present invention.
[0017] FIG. 3 is a SEM image of a bonding portion of a
thermoelectric conversion module of the present invention.
PREFERRED EMBODIMENTS OF THE INVENTION
[0018] A thermoelectric conversion module of the present invention
is shown in FIG. 2. As shown in FIG. 2, the thermoelectric
conversion module has a thermoelectric conversion element 1, an
electrode 2, and a porous metal layer 6 made of nickel or silver.
The thermoelectric conversion element 1 and the electrode 2 are
bonded together via the porous metal layer 6. The porous metal
layer 6 includes pores 61 and is thereby easily elastically
deformed. Therefore, the porous metal layer 6 decreases the
difference in amounts of thermal expansion of the thermoelectric
conversion element 1 and the electrode 2 by deforming. In order to
obtain this effect of the porous metal layer 6, the porous metal
layer 6 is made to have a density ratio of not more than 90%
(porosity of not less than 10%). When the density ratio is less,
the deformability of the porous metal layer 6 is increased, but the
thermal conductivity and the electric conductivity are decreased.
Therefore, the porous metal layer 6 is made to have a density ratio
of not less than 50% (porosity of not more than 50%). Accordingly,
the porous metal layer 6 is made to have a density ratio of 50 to
90%.
[0019] The composition of the porous metal layer 6 has a high
melting point, that is, nickel has a melting point of 1455.degree.
C., and silver has a melting point of 962.degree. C. Since the
porous metal layer 6 is made of nickel or silver having such a high
melting point, the thermoelectric conversion module can be used
even at a temperature near the upper temperature limit of the
thermoelectric conversion element 1. In this case, melting and
leaking of the bonding material do not occur, unlike in the case of
using a soft soldering material with a low melting point. The
thermoelectric conversion module can be used at a temperature that
does not exceed the upper temperature limit of the thermoelectric
conversion element 1, whereby electricity is efficiently
generated.
[0020] The porous metal layer 6 is metallurgically bonded to both
the thermoelectric conversion element 1 and the electrode 2.
Therefore, heat and electricity are efficiently conducted between
the thermoelectric conversion element 1 and the porous metal layer
6 and between the porous metal layer 6 and the electrode 2.
[0021] The porous metal layer 6 preferably has a thickness of not
less than 10 .mu.m because the porous metal layer 6 having too
small a thickness is difficult to deform. On the other hand, the
porous metal layer 6 is porous, and thereby the heat conductivity
and the electric conductivity are low compared with a metal layer
which does not include the pores 61 and has a high density near the
true density. Therefore, when the porous metal layer 6 has a large
thickness, the heat conductivity and the electric conductivity
between the thermoelectric conversion element 1 and the electrode 2
are decreased. Accordingly, the porous metal layer 6 preferably has
a thickness of not more than 100 .mu.m.
[0022] The porous metal layer 6 is formed by sintering the metal
powder particles of one of the nickel powder particles and the
silver powder particles. In this case, if the metal powder
particles are singly dispersed and form one layer, the porous metal
layer 6 is difficult to elastically deform. Therefore, the porous
metal layer 6 is preferably formed by sintering the metal powder
particles that are laminated. From this point of view, the porous
metal layer 6 is preferably formed by sintering fine metal powder
particles with an average particle diameter of not more than 10
.mu.m. The average particle diameter is a median diameter (particle
diameter at 50% of a cumulative distribution). If extremely fine
metal powder particles are used, the porous metal layer 6 is
greatly densified in the sintering, and the density ratio tends to
exceed 90%. Therefore, it is preferable to use metal powder
particles having an average particle diameter of not less than 0.1
.mu.m. In order to make the porous metal layer 6 have a thickness
of 10 .mu.m, the average particle diameter of the metal powder
particles is preferably set so that the metal powder particles form
plural layers. Although the average particle diameter is in the
range of 0.1 to 10 .mu.m, the thickness of the porous metal layer 6
is difficult to control if the metal powder particles include large
powder particles. Therefore, it is preferable to use metal powder
particles having a maximum particle diameter of not more than 30
.mu.m.
[0023] In order to decrease the thermal stress, the pores 61
dispersed in the porous metal layer 6 preferably have approximately
spherical shapes. In the present invention, since the fine metal
powder particles are used as described above, the sintering
actively proceeds, and the pores 61 are formed into approximately
spherical shapes.
[0024] The porous metal layer 6 may be formed as follows. First, a
sintered compact is prepared by sintering the metal powder
particles of one of the nickel powder particles and the silver
powder particles. The sintered compact is held and is arranged
between the thermoelectric conversion element 1 and the electrode
2, and they are diffusion bonded by pressing and heating. In this
case, since the metal powder particles are once sintered into a
sintered compact, the metal powder particles are difficult to be
dispersed to the thermoelectric conversion element 1 and the
electrode 6. Accordingly, the porous metal layer 6 is preferably
formed by the following manner. One of the nickel powder particles
and the silver particle powders in the form of metal powder
particles are arranged to an end surface of the thermoelectric
conversion element 1 or to a portion of the electrode 2, which is
to be connected to the thermoelectric conversion element 1. Then,
the metal powder particles are sintered and are simultaneously
diffusion bonded to both the thermoelectric conversion element 1
and the electrode 2.
[0025] In order to arrange one of the nickel powder particles and
the silver powder particles to the end surface of the
thermoelectric conversion element 1 or to the portion of the
electrode 2, a paste is preferably used. In the paste, the metal
powder particles having the above particle diameter are dispersed.
The paste may include at least one of a dispersing agent, an
adhesive agent, a viscosity modifier, and the like. In addition,
the paste may also include resin of monomer. By using the paste,
the metal powder particles are easily arranged to the end portion
of the thermoelectric conversion element 1 or to the portion of the
electrode 2. In this case, first, a paste, in which the metal
powder particles with the above particle diameter are dispersed, is
prepared. This paste is applied to an end surface of the
thermoelectric conversion element 1, and then the end surface is
abutted to the electrode 2. Accordingly, the paste is held and is
arranged between the thermoelectric conversion element 1 and the
electrode 2. Then, the set of the thermoelectric conversion element
1, the paste, and the electrode 2, is heated in an inert gas
atmosphere, a reducing gas atmosphere, or a vacuum atmosphere, so
as to remove the paste except for the metal powder particles. Thus,
the metal powder particles of one of the nickel powder particles
and the silver powder particles are arranged between the
thermoelectric conversion element 1 and the electrode 2.
[0026] The paste may be applied to the portion of the electrode 2,
and the thermoelectric conversion element 1 may be abutted to this
portion. Then, the set of the thermoelectric conversion element 1,
the paste, and the electrode 2, is heated so as to remove the paste
except for the metal powder particles, as described above.
[0027] After the paste is arranged between the thermoelectric
conversion element 1 and the electrode 2, the paste may be
volatilized or be dried so as to solidify. Then, the set of the
thermoelectric conversion element 1, the paste, and the electrode
2, is heated so as to remove the paste except for the metal powder
particles.
[0028] After the paste, except for the metal powder particles, is
removed by heating, the metal powder particles may be cooled and be
sintered later. In this condition, the metal powder particles are
not sintered and thereby easily fall apart. Therefore, the metal
powder particles are preferably continuously heated and sintered so
as to form the porous metal layer 6. Simultaneously, the porous
metal layer 6 is diffusion bonded to the end surface of the
thermoelectric conversion element 1 and to the electrode 2 so as to
metallurgically bond them. In this case, the metal powder particles
do not fall apart, and energy for reheating is saved.
[0029] The sintering and the diffusion bonding are performed at a
temperature of 650 to 850.degree. C. when the nickel powder
particles are used as the metal powder particles. On the other
hand, the sintering and the diffusion bonding are performed at a
temperature of 450 to 750.degree. C. when the silver powder
particles are used as the metal powder particles. The sintering and
the diffusion bonding are performed without applying pressure. The
metal powder particles of one of the nickel powder particles and
the silver powder particles having an average particle diameter of
0.1 to 10 .mu.m are used. Such a fine powder has a large surface
area and is easily sintered. Therefore, even when the pressure is
not applied, the metal powder particles are sintered and are
diffusion bonded to the thermoelectric conversion element 1 and to
the electrode 2. By performing the sintering and the diffusion
bonding at not less than 650.degree. C. in the case of using the
nickel powder particles and at not less than 450.degree. C. in the
case of using the silver powder particles, the density ratio of the
metal powder 6 becomes not less than 50%. Thus, a porous metal
layer 6 is obtained, and the porous metal layer 6 sufficiently
decreases the thermal stress when used in the thermoelectric
conversion module. In contrast, since the fine powder is easily
sintered, if the sintering is performed at more than 850.degree. C.
in the case of using the nickel powder particles and at more than
750.degree. C. in the case of using the silver powder particles,
the porous metal layer 6 is greatly densified, and the density
ratio becomes greater than 90%.
[0030] As described above, the sintering and the diffusion bonding
are performed without applying pressure, but may be performed by
applying pressure of not more than 1 MPa. When the sintering is
performed by applying pressure, the thermoelectric conversion
element 1, the electrode 2, and the metal powder particles arranged
therebetween, are closely contacted, whereby the diffusion bonding
is easily performed. The thermoelectric conversion module includes
plural thermoelectric conversion elements 1 that have variation in
height. However, by sintering and applying pressure, the metal
powder particles absorb the variations of the heights of the
thermoelectric conversion elements 1 and form the porous metal
layer 6. Therefore, a thermoelectric conversion module, in which
the distances between the electrodes 2 are the same, is produced.
In this case, the metal powder particles are brought into close
contact with each other and are easily sintered, whereby the
density ratio of the porous metal layer 6 is increased.
Accordingly, in a case of performing the sintering by applying
pressure, the pressure should be not greater than 1 MPa.
[0031] If a paste including a small amount of the metal powder
particles is used, the amount of the metal powder particles applied
at one time is small. Therefore, in order to arrange a necessary
amount of the metal powder particles between the thermoelectric
conversion element 1 and the electrode 2, the paste must be applied
several times. In contrast, if the paste includes an excessive
amount of the metal powder particles, components other than the
metal powder particles in the paste are relatively decreased.
Therefore, the fluidity of the paste is decreased, and the paste is
difficult to adhere, and the paste is therefore difficult to use.
In view of this, the nickel powder particles or the silver powder
particles are preferably dispersed in the paste at 30 to 50 volume
%. By using such a paste, the necessary amount of the metal powder
particles is easily applied at one time.
[0032] If a paste having low viscosity is used, the amount of the
paste applied at one time is small, whereby the paste must be
applied several times in order to make the porous metal layer 6
with a predetermined thickness. Moreover, the paste may leak from
the predetermined position and may hang down. On the other hand, if
the paste has very high viscosity, the paste is difficult to use.
Moreover, the amount of the paste applied at one time is increased,
which may cause a removing process of the extra amount of the paste
after the application. From this point of view, the paste is
preferably adjusted so as to have a viscosity of 10 to 100 Pas.
[0033] The paste preferably adheres in order to prevent slip of the
thermoelectric conversion element 1 and the electrode 2 after they
are assembled together. Accordingly, the assembled thermoelectric
conversion element 1 and the electrode 2 are easily handled until
the sintering. In this case, the paste has only to temporarily
adhere the electrode 2 to the thermoelectric conversion element 1
until the diffusion bonding in the sintering. Therefore, a paste
with a shear strength of approximately not less than 0.1 N/cm.sup.2
is sufficient to prevent the slip in the handling.
[0034] The paste may include an adhesive material and may be
hardened after the paste is applied. In this case, the electrode 2
is securely fixed (adhered) to the thermoelectric conversion
element 1 by the hardened paste, whereby they are easily
handled.
[0035] According to the above production method, a thermoelectric
conversion module can be easily obtained by a few steps using a
small amount of energy. The thermoelectric conversion module has
the thermoelectric conversion element 1 and the electrode 2 that
are metallurgically bonded together via the porous metal layer 6
with a density ratio of 50 to 90%.
[0036] The porous metal layer 6 is provided at least between the
thermoelectric conversion element 1 and the electrode 2 at the side
of the heating duct 5. The porous metal layer 6 may be provided to
both end portions of the thermoelectric conversion element 1.
Nevertheless, when the temperature of the end portion of the
thermoelectric conversion element 1 at the side of the cooling duct
4 is low so that it does not cause leak of a soldering material,
the thermoelectric conversion element 1 and the electrode 2 can be
bonded together by a conventional soldering material.
[0037] In the thermoelectric conversion module of the present
invention, a thermoelectric conversion element having a high upper
temperature limit is suitably used. For example, a thermoelectric
conversion element made of an alloy such as of the
silicon-germanium type, magnesium-silicon type, manganese-silicon
type, or iron-silicide type, may be used. On the other hand, a
thermoelectric conversion element having a low upper temperature
limit is not suitably used because the upper temperature limit of
the element is lower than the temperatures at the sintering and at
the diffusion bonding. The thermoelectric conversion element having
a low upper temperature limit is made of an alloy such as of the
bismuth-tellurium type, lead-tellurium type, or
iron-vanadium-aluminum type, for example.
[0038] One of the thermoelectric conversion elements having a high
upper temperature limit, for example, a thermoelectric conversion
element made of iron silicide is known to have the following
negative influence. When this thermoelectric conversion element is
brought into direct contact with a copper electrode, copper is
diffused from the copper electrode into the thermoelectric
conversion element. As a result, erosion of the copper electrode
occurs, and the thermoelectric conversion element is deteriorated,
whereby electricity is not generated. However, by arranging the
porous metal layer 6 between the thermoelectric conversion element
and the electrode, the metal layer of the nickel or the silver
prevents the copper from diffusing from the copper electrode.
[0039] In the thermoelectric conversion module of the present
invention, the end surface of the thermoelectric conversion element
1 may be covered with a metal beforehand, and the paste may be
applied to the end surface covered with the metal. Then, the paste,
except for the metal powder particles, is removed, and sintering of
the metal powder particles and diffusion bonding of the porous
metal layer 6 to the thermoelectric conversion element 1 and to the
electrode 2 are performed.
[0040] The end surface of the thermoelectric conversion element 1
may be covered with the metal by plating, vapor deposition,
sputtering, or thermal spraying. When the end surface of the
thermoelectric conversion element 1 is covered by one of these
methods, the end surface of the thermoelectric conversion element 1
is smoothed. Therefore, the area of the metal powder particles
contacting the end surface of the thermoelectric conversion element
1 is increased. Accordingly, when the metal powder particles are
sintered and are diffusion bonded to the thermoelectric conversion
element 1, the porous metal layer 6 is easily diffusion bonded to
the end surface of the thermoelectric conversion element 1.
[0041] A metal such as nickel, iron, silver, or cobalt, which can
be suitably diffusion bonded to the nickel or the silver, is
preferably used as the metal for covering the end surface of the
thermoelectric conversion element 1. In this case, the porous metal
layer 6 and the thermoelectric conversion element 1 are strongly
bonded together by diffusion bonding.
[0042] In a case of using a thermoelectric conversion element 1
that actively reacts with nickel, a metal such as iron, silver, or
cobalt, may be used as the metal for covering the end surface of
the thermoelectric conversion element 1. This metal functions as a
barrier layer and prevents the nickel from diffusing from the
porous metal layer 6 to the thermoelectric conversion element 1.
Therefore, deterioration of the thermoelectric conversion element 1
is prevented. Accordingly, the material of the thermoelectric
conversion element 1 can be selected from various materials.
EXAMPLES
First Example
[0043] Nickel powder particles having an average particle diameter
of 1 .mu.m and a maximum particle diameter of not more than 10
.mu.m were prepared. Then, 35 volume % of the nickel powder
particles were dispersed in normal methyl pyrolidone including 8
volume % of hydroxyl propyl cellulose, whereby a paste was
prepared. The paste had a viscosity of approximately 40 Pas. The
paste was applied to both end surfaces of a thermoelectric
conversion element made of silicon germanium. The both end surfaces
of the thermoelectric conversion element were abutted with
electrodes made of molybdenum. Then, the paste was arranged between
the thermoelectric conversion element and the electrode, whereby a
thermoelectric conversion module was assembled. A weight of 50 g
(corresponding to 1 kPa) was applied to the thermoelectric
conversion module and it was placed in a sintering furnace. The
thermoelectric conversion module was heated at 500.degree. C. in a
hydrogen gas atmosphere so as to remove the paste except for the
metal powder particles. Moreover, the thermoelectric conversion
module was heated to a temperature shown in Table 1 so as to sinter
and diffusion bond the metal powder particles. Accordingly, two
sets of samples of thermoelectric conversion modules of sample Nos.
01 to 07 were made.
[0044] One set of the thermoelectric conversion modules was cut in
a direction perpendicular to the bonding surface, and each of the
sectional metallic structures was observed by microscope. Thus,
bonding condition of the interface between the porous metal layer
and the thermoelectric conversion element and bonding condition of
the interface between the porous metal layer and the electrode were
investigated. The metal structure was photographed at 500-times
magnification, and this image was analyzed by using image analyzing
software ("Win ROOF" produced by Mitani Corporation), whereby
density ratio of the porous metal layer was measured. These results
are also shown in Table 1. Table 1 shows results of the evaluation
of the bonding conditions before the heat test. In Table 1, the
mark "O" indicates a sample which had not less than 50% of a
metallurgically bonded portion at the interface, and the mark "x"
indicates a sample which had less than 50% of the metallurgically
bonded portion at the interface.
[0045] In the samples having the mark "O" in the microscope
observation, a heat test was performed by using the other set of
the samples. The heat test was performed such that one of the
electrodes was maintained at 550.degree. C. and the other electrode
was heated to 20.degree. C. for 24 hours. Then, the sample was cut
in the direction perpendicular to the bonding surface, and the
sectional metallic structure was observed by a microscope. Thus,
the bonding portion of the porous metal layer and the
thermoelectric conversion element and the bonding portion of the
porous metal layer and the electrode were investigated. These
results are also shown in Table 1. The bonding conditions after the
heat test were evaluated as in the case of the evaluation of the
bonding conditions before the heat test.
TABLE-US-00001 TABLE 1 Density ratio of Sintering porous metal
Bonding condition Sample temperature layer Before After No.
.degree. C. % heat test heat test 01 600 46 x -- 02 650 50
.smallcircle. .smallcircle. 03 700 58 .smallcircle. .smallcircle.
04 750 76 .smallcircle. .smallcircle. 05 800 86 .smallcircle.
.smallcircle. 06 850 90 .smallcircle. .smallcircle. 07 1000 97
.smallcircle. x
[0046] As shown in Table 1, in the sample of the sample No. 01 that
was sintered at less than 650.degree. C., the sintering did not
proceed sufficiently. Therefore, the density ratio of the porous
metal layer was less than 50%, and the thermoelectric conversion
element and the porous metal layer, and the electrode and the
porous metal layer, were insufficiently bonded. According to the
increase of the sintering temperature, the sintering was
accelerated, whereby the density ratio of the porous metal layer
was increased. In the sample of the sample No. 02 that was sintered
at 650.degree. C., the sintering proceeded sufficiently. Therefore,
the density ratio of the porous metal layer was 50%, and the
thermoelectric conversion element and the porous metal layer, and
the electrode and the porous metal layer, were sufficiently bonded
together. Moreover, the pores of the porous metal layer decreased
the thermal stress, whereby the bonding condition of the
thermoelectric conversion element and the porous metal layer and
the bonding condition of the electrode and the porous metal layer
were maintained after the heat test.
[0047] FIG. 3 is a SEM image of the bonding portion of the sample
of the sample No. 04. As shown in FIG. 3, the nickel powder
particles in the form of the paste arranged between the
thermoelectric conversion element and the electrode were sintered
and formed a porous metal layer. The porous metal layer was
metallurgically bonded to the thermoelectric conversion element and
to the electrode. By forming such a porous metal layer, even when
the bonding surface of the thermoelectric conversion element and
the electrode is applied with thermal stress, the pores of the
porous metal layer decrease the thermal stress, and good bonding
condition is maintained.
[0048] On the other hand, in the sample of the sample No. 07 that
was sintered at more than 850.degree. C., the sintering proceeded
excessively, whereby the density ratio of the porous metal layer
was greater than 90%. In this sample, the bonding condition after
the sintering was good, but the bonding surface was fractured by
the heat test. That is, since the amount of the pores was small,
the thermal stress was not sufficiently decreased.
[0049] According to these results, when the density ratio of the
porous metal layer was 50 to 90%, the thermal stress was
sufficiently decreased, and good bonding condition was maintained.
In addition, by performing the sintering at 650 to 850.degree. C.,
the above density ratio was obtained.
Second Example
[0050] The thermoelectric conversion element used in the First
Example was changed to a thermoelectric conversion element made of
Mg.sub.2Si, in which both end surfaces were covered with nickel.
This thermoelectric conversion element was prepared as follows.
First, a bulk sintered compact made of Mg.sub.2Si was prepared. The
bulk sintered compact was plated with nickel and was cut into the
shape of the thermoelectric conversion element. Then, a
thermoelectric conversion module was made and was evaluated as in
the case of the First Example. As a result, although the
thermoelectric conversion element was changed, when the density
ratio of the porous metal layer was 50 to 90%, the thermal stress
was sufficiently decreased, and good bonding condition was
maintained. In addition, by performing sintering at 650 to
850.degree. C., the above density ratio was obtained.
Third Example
[0051] Several kinds of nickel powder particles having an average
particle diameter shown in Table 2 were prepared, and pastes were
made as in the case of the First Example. Then, thermoelectric
conversion modules were assembled by using the pastes as in the
case of the First Example. The thermoelectric conversion modules
were sintered at 800.degree. C. so as to remove the paste except
for the metal powder particles, and to sinter and diffusion bond
the metal powder particles as in the case of the First Example.
Accordingly, two sets of samples of the thermoelectric conversion
modules of the sample Nos. 08 to 13 were prepared. In these
samples, the density ratio of the porous metal layer was measured,
and the bonding portion before and after the heat test was
evaluated. These results are also shown in Table 2.
TABLE-US-00002 TABLE 2 Density ratio of Metal powder particles
.mu.m porous metal Bonding condition Sample Average Maximum layer
Before After No. particle size particle size % heat test heat test
08 0.01 10.0 96 .smallcircle. x 09 0.1 10.0 90 .smallcircle.
.smallcircle. 10 0.5 10.0 88 .smallcircle. .smallcircle. 05 1.0
10.0 86 .smallcircle. .smallcircle. 11 5.0 20.0 66 .smallcircle.
.smallcircle. 12 10.0 30.0 50 .smallcircle. .smallcircle. 13 20.0
30.0 44 x --
[0052] As shown in Table 2, when the average particle diameter of
the metal powder particles was smaller, the surface area was
greater, and thereby the sintering proceeded actively, whereby the
porous metal layer was densified, and the density ratio was
increased. In other words, when the average particle diameter of
the metal powder particles was greater, the porous metal layer was
difficult to be densified in the sintering, whereby the density
ratio of the porous metal layer was smaller.
[0053] In the sample of the sample No. 08, the metal powder
particles had an average particle diameter of less than 0.1 .mu.m,
and the amount of the fine powder particles was excessive, whereby
the surface area was too great. Therefore, the sintering proceeded
extremely actively, and the density ratio of the porous metal layer
was greater than 90%, and the porosity was less than 10% and was
small. Accordingly, the interface was fractured by the thermal
stress due to the thermal expansions of the thermoelectric
conversion element and the electrode in the heat test, and the
bonding condition was not good after the heat test. In contrast, in
the sample of the sample No. 09, the metal powder particles had an
average particle diameter of 0.1 .mu.m, whereby the density ratio
of the porous metal layer was 90%, and a sufficient amount of the
pores were dispersed. Therefore, the thermal stress was decreased
by the pores in the heat test, and good bonding condition was
maintained after the heat test. On the other hand, in the sample of
the sample No. 13, the metal powder particles had an average
particle diameter of greater than 10 .mu.m, and the sintering did
not proceed sufficiently. Therefore, the density ratio of the
porous metal layer was less than 50%, and the thermoelectric
conversion element and the porous metal layer, and the electrode
and the porous metal layer, were not sufficiently bonded
together.
[0054] According to these results, by using the metal powder
particles having an average particle diameter of 0.1 to 10 .mu.m,
the density of the porous metal layer was made to be 50 to 90%, and
good bonding condition was obtained.
Fourth Example
[0055] The thermoelectric conversion element used in the Third
Example was changed to the thermoelectric conversion element made
of Mg.sub.2Si, which was covered with nickel and was used in the
Second Example. Then, a thermoelectric conversion module was made
and was evaluated as in the case of the Third Example. As a result,
although the kind of the thermoelectric conversion element was
changed, by using the metal powder particles having an average
particle diameter of 0.1 to 10 .mu.m, the density ratio of the
porous metal layer was made to be 50 to 90%, and good bonding
condition was obtained.
[0056] The thermoelectric conversion module of the present
invention has the porous metal layer that decreases the thermal
stress occurring at high temperatures. Therefore, the
thermoelectric conversion module can be used at high temperatures
at which the function of the thermoelectric conversion element is
most effectively used, whereby a greater amount of electricity is
generated. Accordingly, the thermoelectric conversion module of the
present invention is suitably used for a small-scale distributed
power generation system using an exhaust source of a waste
incinerator, a cogeneration system, etc. Moreover, the
thermoelectric conversion module of the present invention is
suitably used for an automobile power generation system using the
heat in the exhaust gas of automobiles.
* * * * *