Batteries: thermoelectric and photoelectric – Thermoelectric – Processes
Reexamination Certificate
2001-11-29
2004-06-01
Ryan, Patrick (Department: 1745)
Batteries: thermoelectric and photoelectric
Thermoelectric
Processes
C136S203000, C136S205000, C136S238000, C136S240000, C148S513000, C419S041000, C075S228000
Reexamination Certificate
active
06743973
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to thermoelectric material and, more particularly, to high-efficiency thermoelectric material, a process for producing thereof and Peltier module using the thermoelectric material.
DESCRIPTION OF THE RELATED ART
The figure of merit Z is convenient for evaluating the thermoelectric material, and is expressed as follows.
Z=&agr;
2
/(
&rgr;×&kgr;)
equation 1
where &agr; is the Seebeck coefficient in &mgr;·V/K, &rgr; is the electric resistivity in &OHgr;·m, &kgr; is the thermal conductivity in W/m·K. The greater the figure of merit is, the more preferable the thermoelectric material is. From equation 1, it is desirable for the thermoelectric material to have a small electric resistivity and a small thermal conductivity. In general, it is known to persons skilled in the art that the thermal conductivity is reduced together with the grain size. It is also the known fact that the electric resistivity is reduced together with the number of crystal grains through which the electric current flows. Thus, the figure of merit is improved by controlling the growth of crystal.
One of the crystal structure controlling technologies is carried out by using a hot pressing. A sintered body is a typical example of the solidified thermoelectric material in Bi
2
Te
3
system. A thermoelectric element is made from the thermoelectric material as follows. The thermoelectric material is pulverized, and the resultant powder is shaped into a sintered product through a hot pressing. While the powder is being sintered in the hot pressing, the crystal grains tend to be solidified in such a manner that a-axes of the crystal grains, which are the low-resistive direction of the crystal, are oriented in the perpendicular direction to the direction of the pressure. When the electric current flows in the low-resistive direction, the sintered product exhibits a large figure of merit. For this reason, the manufacturer spaces electrodes in the low-resistive direction on a piece of sintered product. The electric current flows through the crystal grains in the direction parallel to the a-axes, and the piece of the sintered product exhibits a large figure of merit. The piece of sintered product is used as an essential part of the thermoelectric element, and plural thermoelectric elements are assembled into a thermoelectric module.
Another crystal structure controlling technology is disclosed in Japanese Patent Application laid-open No. 11-163422. The crystal structure controlling technology disclosed in the Japanese Patent Application laid-open No. 11-163422 is carried out through an extrusion.
FIGS. 1A and 1B
show the prior art extrusion process. The prior art extrusion process starts with preparation of a bulk
101
of thermoelectric material as shown in FIG.
1
A. The thermoelectric material has the composition containing at least one element selected from the group consisting of Bi and Sb and another element selected from the group consisting of Te and Se.
A die unit
102
is heated with a heater
104
, and the bulk
101
of the thermoelectric material is pressed to the die unit
102
as indicated by an arrow in FIG.
1
B. The bulk
101
is softened, and a rod
103
of the thermoelectric material is extruded from the die unit
102
. While the soft thermoelectric material is passing through the die unit
102
, the soft thermoelectric material is subjected to the slit orientation, and a large amount of crystal grains are oriented so as to have (001) crystal plane, i.e., c-plane in a certain direction. After the extrusion, the thermoelectric material forming the rod
103
is solidified to have fine crystal grains without changing the orientation. Although the electric resistivity &rgr; is not varied between the bulk
101
and the rod
103
, the thermal conductivity &kgr; is lowered.
Yet another crystal structure controlling technology is disclosed in the Proceedings of 2000 Spring Conference of Japan Society of Powder and Powder Metallurgy. According to the proceedings, a bulk of thermoelectric material is forced to pass through an elbow passage. The bulk is pressed against the inner surface, and a sharing force is exerted on the bulk of thermoelectric material for orienting the crystal grains.
FIG. 2
shows an extruder used in the prior art crystal structure controlling technique disclosed in the proceedings. Reference numeral
110
designates the die unit
110
, and a passage
110
a
is formed in the die unit
110
. The passage
110
a
has an elbow-like shape. A green compact
112
is formed from powder of p-type thermoelectric material expressed as (Bi
2
Te
3
)
0.2
(Sb
2
Te
3
)
0.8
. The green compact
112
is put into the passage
110
a
, and a punch
111
is inserted into the passage
110
a
. The punch
111
presses the green compact
112
against the inner surface of the die unit
110
, and a sharing force is exerted on the green compact
112
. The green compact
112
is bent, and a plate
113
of the thermoelectric material is extruded from the die unit
110
. While the sharing force is being exerted on the green compact
112
, the crystal planes are oriented in a certain direction.
Still another crystal structure controlling technology is disclosed in Japanese Patent Application laid-open No. 178218.
FIGS. 3A and 3B
show the process of the hot upset forging disclosed in the Japanese Patent Application laid-open. The process starts with preparation of an ingot of solid solution of thermoelectric material. The ingot is pulverized, and the resultant powder is subjected to a pressure sintering.
The sintered product
124
is placed in an inner space of the upset forging machine as shown in FIG.
3
A. The upset forging machine has a base plate
121
and column-shaped sleeves
122
. The base plate
121
and the sleeves
122
are assembled together so as to define the rectangular parallelepiped inner space. A punch
123
is movable in the rectangular parallelepiped inner space.
The sintered product
124
is heated, and the punch
123
is downwardly moved. A compressive force is exerted on the sintered product
124
. The plastic deformation takes place in the sintered product
124
, and the sintered product
124
is stretched on the base plate
121
as shown in FIG.
3
B. The crystal grains of the sintered product
124
are oriented in a direction at which the figure of merit is improved. Thus, the thermoelectric semiconductor material
125
is improved in the figure of merit through the hot upset forging.
A problem is encountered in the prior art crystal structure controlling technologies described with reference to
FIGS. 1A
,
1
B,
2
,
3
A and
3
B in that the products
103
/
113
/
125
are different in thermoelectric properties between the p-type thermoelectric material and the n-type thermoelectric material. In detail, it has been known to the persons skilled in the art that the p-type thermoelectric material is superior in thermoelectric properties to the n-type thermoelectric material. When the manufacturer designs the p-type thermoelectric material and the corresponding n-type thermoelectric material to have the Seebeck coefficient equal therebetween, the n-type thermoelectric material obtained through any one of the prior art crystal structure controlling technologies is higher in electric resistivity than the p-type thermoelectric material also obtained through the same prior art crystal structure controlling technology. If the manufacturer designs the p-type thermoelectric material and the corresponding n-type thermoelectric material to have the electric resistivity equal therebetween, the n-type thermoelectric material obtained through any one of the prior art crystal structure controlling technologies is higher in the Seebeck coefficient than the p-type thermoelectric material also obtained through the same prior art crystal structure controlling technology. In fact, the manufacturer thinks it impossible to produce n-type thermoelectric material in the (Bi, Sb)
2
(Te, Se)
3
system with the figure of merit greater than 3.00&tim
Hayashi Takahiro
Horio Yuma
Hoshi Toshiharu
Dickstein Shapiro Morin & Oshinsky LLP.
Parsons Thomas H.
Ryan Patrick
Yamaha Corporation
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