JPH11112037A - Thermionic element and method for forming electrode of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the effective ratio resistance of a thermoelectric element and to improve ability as the thermionic element by forming the electrodes of low contact resistance on FeSi2 semiconductors. SOLUTION: In the thermionic element where one end 1a and 2a-side in the P-type FeSi2 semiconductor 1 and the N-type FeSi2 semiconductor 2 are directly connected or through a conductor, the electrodes 3 which contain the doping materials of Fe, Ni, Co or Mn and Co and which is constituted of Fe are directly formed on the other end 1b and 2b-side of the P-type FeSi2 semiconductor 1 and the N-type FeSi2 semiconductor 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermoelectric element made of a FeSi ₂ semiconductor, and more particularly, to an electrode forming technique for lowering the resistance of a thermoelectric element.

[0002]

2. Description of the Related Art In recent years, with the effective use of heat energy and diversification of heat sources, expectations for high-efficiency thermoelectric materials have been increasing, and various materials have been researched and developed. Above all, Fe
Si ₂ semiconductors can be used at relatively high temperatures,
Since it has excellent oxidation resistance, it can be used in the air. By appropriately selecting an impurity element to be added, it is possible to control the P-type or N-type conductivity type as a semiconductor, and It is attracting attention because of its many advantages that it is rich in iron and silicon and its raw material cost is low. However, such a FeSi ₂ semiconductor also has a lower performance of a thermoelectric element described later than other thermoelectric materials, and improvement of this performance has been desired for practical use as a thermoelectric element. Generally, the performance of a thermoelectric element is evaluated using a figure of merit z (z = α ² / ρκ) as an index. Here, α is the thermoelectric power, ρ is the specific resistance, κ is the thermal conductivity, and the figure of merit z is a value specific to a substance that depends on temperature. From this viewpoint, the thermoelectric power α is large and the specific resistance ρ
And a substance having a small thermal conductivity κ is desirable.

Here, the FeSi ₂ semiconductor is β-Fe
An iron silicide having a Si ₂ phase as a main component.In an alloy having such a composition, a β-FeSi ₂ phase having a very narrow single-phase region of a stoichiometric composition exists as a stable phase, and at a predetermined temperature. A β phase having semiconductor characteristics was obtained by heat treatment. In addition, since the ingot of β-FeSi ₂ is porous and has many cracks and is brittle, powder metallurgy is generally used for its production, and a typical sintering method in the powder metallurgy is as follows. Pressure sintering (PLS), hot press sintering (HP), hot isostatic sintering (HIP) and the like are used.

[0004] This β-FeSi ₂ is composed of P such as Mn or Al.
Doping with a type impurity can provide a P-type FeSi ₂ semiconductor having a P-type conductivity, while doping with an N-type impurity such as Co can provide an N-type FeSi ₂ semiconductor having an N-type conductivity.

Meanwhile, by using FeSi ₂ semiconductor if specifically configure the thermoelectric element, joining the N-type FeSi ₂ semiconductor one ends doped with P-type FeSi ₂ semiconductor and N-type impurity doped with P-type impurities , P-type and N-type FeSi ₂ semiconductors are generally provided with an electrode portion on the other end side. Here, when the semiconductor is used as a thermoelectric generator for generating a thermoelectromotive force between both electrodes by giving a temperature difference to both ends of the semiconductor, the junction between the P-type and N-type semiconductors is arranged on the high temperature side. The junction of the P-type and N-type semiconductors uses a high-temperature brazing material such as silver brazing because the melting point of the FeSi ₂ semiconductor is high, or is directly joined by various methods, and via a conductor in the same manner. It was joined and formed. On the other hand, the electrode portion formed on the low-temperature side does not form a special electrode with a special conductive electrode material. In general, an electrical connection with an external circuit has been formed by the connection.

[0006]

The performance of the thermoelectric element is evaluated using the performance index z as an index as described above. Even if the performance index z of the thermoelectric material itself is high, the performance of the P-type and N-type semiconductors can be improved. If the contact resistance at the joint and the electrode is high,
The effective specific resistance ρ of the thermoelectric element as a whole becomes apparently large, resulting in a substantial decrease in performance. That is, when functioning as a thermoelectric generator, each of the contact resistances acts as an internal resistance of the thermoelectric generator, and a voltage drop occurs in each of the contact resistances according to an output current. As a result, the voltage value of the thermoelectromotive force generated due to the temperature difference between the bonding portion and each of the electrode portions is effectively reduced. In the above-described ultrasonic soldering or the like used for forming the electrode portion of the thermoelectric element using the FeSi ₂ semiconductor, an alloy or metal such as solder and FeSi are used.
₍₂ ) An energy barrier (Schottky barrier) is generated at the contact surface with the semiconductor, and unless a voltage exceeding the energy barrier applied to both ends is applied, electrical conduction cannot be obtained, and good ohmic contact is not achieved without substantial ohmic contact. As a result, the contact resistance is remarkably increased, and particularly at a low temperature, which is one of the hindrance factors for improving the performance of the thermoelectric element.

In general, when a metal and a semiconductor are brought into contact with each other, the carrier recombination speed at the interface is very fast, the Schottky barrier is sufficiently low, or the barrier is sufficiently large so that the carrier can tunnel. Since the ohmic contact is obtained when the thickness is too small, a good ohmic contact can be obtained at a higher temperature, and the contact resistance at the junction and the electrode decreases. Therefore, the reduction in the contact resistance of the electrode portion provided on the low-temperature side contributes to the high performance of the thermoelectric element together with the high performance of the thermoelectric material itself.

Further, when used as a thermoelectric generator,
If the temperature difference between the junction of the P-type and N-type semiconductors on the high-temperature side and the electrode on the low-temperature side is increased, a larger electromotive force is obtained and the power generation capacity is increased, but the contact resistance of the electrode is reduced. If the temperature of the electrode part is raised to lower the temperature, the generated electromotive force is rather lowered, and the power generation capacity is hindered. For this reason,
The electrode portion provided on the low temperature side is usually cooled by water cooling or the like. Therefore, it is necessary to reduce the contact resistance of the electrode section by another means.

In addition, if the electrode portion is formed directly on the surface of each semiconductor by using a conductive electrode material capable of ohmic contact with the FeSi ₂ semiconductor without using ultrasonic soldering or the like, the above problem may be apparently caused. Although it seems to be able to solve the problem, the difference in the thermal expansion coefficient and the shrinkage rate during firing between the electrode formed of a different material and the FeSi ₂ semiconductor causes a large temperature difference in the process including the electrode formation. May occur. Further, such a problem is solved and FeS
No suitable electrode material capable of making good ohmic contact with the i ₂ semiconductor has been known.

The present invention has been made in view of the above circumstances, and an object of the present invention is to form an electrode having a low contact resistance to lower the effective specific resistance ρ and improve the performance as a thermoelectric element. Another object of the present invention is to provide a high-performance thermoelectric element suitable for high-temperature use by effectively utilizing the advantages inherent in FeSi ₂ semiconductors.

[0011]

A first feature of a thermoelectric element according to the present invention for achieving this object is that a FeSi ₂ semiconductor has a FeSi ₂ semiconductor as described in claim 1 of the claims. , Ni, Co, or an electrode containing Fe as a component containing a doping material such as Mn or Co is directly formed.

[0012] The second characteristic configuration is that, as described in claim 2 of the claims, a P-type FeSi ₂ semiconductor and an N-type
The other ends of the P-type FeSi ₂ semiconductor and the N-type FeSi ₂ semiconductor are connected to one end of the type FeSi ₂ semiconductor directly or via a conductor.
The point is that an electrode containing o or Fe containing a doping material such as Mn or Co as a component is directly formed. Here, the expression that the electrode is directly formed means that the FeSi ₂ semiconductor and the electrode material are electrically and physically connected via a medium having a composition different from that of the FeSi ₂ semiconductor such as solder or silver brazing and the electrode material. It refers to a state in which they are not connected but are directly connected without passing through such a medium. For example, a coating of the electrode material is formed by vacuum deposition, thermal spraying, coating, or the like on a predetermined portion of an already manufactured FeSi ₂ semiconductor, and the film is fired, or the FeSi ₂ semiconductor and the electrode are sintered by a sintering method. At the same time, by integrally molding, an electrode can be directly formed on the FeSi ₂ semiconductor.

A first feature of the method for forming an electrode of a thermoelectric element according to the present invention for achieving this object is as described in claim 3 of the claims. Item 3. The method for forming an electrode of a thermoelectric element according to Item 1 or 2, wherein the powder of the electrode material is disposed at a position where the electrode is formed, and the powder of the semiconductor material of a predetermined conductivity type of the FeSi ₂ semiconductor is disposed at a position where the semiconductor is formed. Then, a mixed powder of the electrode material and the semiconductor material is disposed between the electrode formation location and the semiconductor formation location, and the FeSi ₂ semiconductor and the electrode are simultaneously fired by a sintering method to be integrally molded. .

According to a second feature of the present invention, in addition to the first feature of the method for forming an electrode of a thermoelectric element, a mixing ratio of the mixed powder is set as described in claim 4 of the claims. From the boundary with the semiconductor formation location to the boundary with the electrode formation location, the mixing ratio of the electrode material powder changes stepwise or continuously.

The operation and effect will be described below. According to the first or second characteristic configuration of the thermoelectric element according to the present invention, even at a low temperature, the electrode material and the FeSi ₂ semiconductor form a good ohmic contact, and the contact resistance can be reduced. As a result, the effective specific resistance of the entire thermoelectric element can be reduced, and the performance of the thermoelectric element can be improved. In particular, according to the second characteristic configuration, since the polarity of the electrical characteristics of the thermoelectric effect of the P-type FeSi ₂ semiconductor and the N-type FeSi ₂ semiconductor is inverted, even if the thermoelectric elements are connected in series, the generated electromotive force and the temperature difference are reduced. It is not canceled out and high performance can be achieved. Further, the same configuration can be connected in series or in parallel to expand the configuration.

By the way, Fe, Ni, Co, or M
Conventionally, Fe containing a doping material such as n or Co has conventionally been FeS
Although it was considered that good ohmic contact was difficult in contact with i ₂ semiconductor, the present inventor newly confirmed by experiments that good ohmic contact is possible,
Such new findings have been obtained. For example, when the electrode material is Fe or Fe containing a doping material such as Mn or Co, FeS ₂ semiconductor and FeS
It is presumed that a thin film layer of i was formed and a good ohmic contact was formed via this FeSi. Here, even if the FeSi layer is actually formed at the interface between the FeSi ₂ semiconductor and the electrode, such an intermediate layer is formed directly from the FeSi ₂ semiconductor material and the electrode material as a result of the electrode forming step. It cannot be said that the connection medium has a different composition from that of the connection medium.
This does not deny the state of being directly formed on the i ₂ semiconductor.

Electrode formation of the above-mentioned thermoelectric element according to the present invention
According to a first feature of the method, the electrode and the FeSi_TwoSemiconduct
Since the body is manufactured at the same time, the electrode formation process is
There is no need to provide a way, and the process can be simplified,
Due to the large temperature difference in the sintering process including electrode formation,
The mixed powder of the electrode material and the semiconductor material is sintered.
In forming the intermediate layer, the coefficient of thermal expansion and the rate of shrinkage during firing are flat.
The electrode and FeSi_TwoIt is an intermediate value between semiconductors.
The difference between the thermal expansion coefficient and the contraction rate _Twosemiconductor
Crack does not occur because it does not change rapidly between
You can do it. In addition, without providing such mixed powder
Electrode without cracking at a constant yield
It is also possible to adjust the process conditions appropriately
However, it is relatively difficult to extract the appropriate value by preliminary experiments, etc.
It is difficult and it is a problem for mass production at high yield.
According to the feature configuration, stable high yield can be expected and mass production
It is highly adaptable.

According to the second characteristic configuration, the electrode and the FeS
Since the coefficient of thermal expansion and the coefficient of contraction can be changed stepwise or continuously between the i ₂ semiconductors, the occurrence of the crack can be more reliably prevented. In addition, the application of a specific sintering method in which cracks easily occur is facilitated, and the degree of freedom in selecting the sintering method is increased.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a thermoelectric element and an electrode forming method according to the present invention will be described with reference to the drawings. As shown in FIG. 1, in the thermoelectric element according to the present invention, a rod-shaped P-type FeSi ₂ semiconductor 1 and a rod-shaped N-type FeSi ₂ semiconductor 2 are directly connected to each other at one end 1a, 2a in a V-shape in plan view. And the P-type FeSi ₂ semiconductor 1
An electrode 3 containing Fe as a component is formed directly on the upper surface of each of the other ends 1b and 2b of the N-type FeSi ₂ semiconductor 2.

As shown in FIG. 2, a disc 4 having a diameter of 40 mm and a thickness of 7 mm including the P-type FeSi ₂ semiconductor 1, the N-type FeSi ₂ semiconductor 2, and the electrodes 3.
Are simultaneously and integrally formed by a pressureless sintering method, and then cut into a V-shape along a broken line portion in the figure by a micro cutter to manufacture the thermoelectric element. The thickness of the electrode 3 is 2 mm of the total thickness of 7 mm and is located on the upper surface side. All are dimensions after firing. Here, the reason why the thickness of the electrode 3 is 2 mm is that if the thickness is larger than 2 mm,
This is because, when Fe is compared with FeSi ₂ , the shrinkage ratio during firing is larger in Fe, so that cracks easily occur in the electrode 3.

Hereinafter, the production of the disk 4 will be described. The raw material powder of the P-type FeSi ₂ semiconductor 1 is Fe powder
(Purity 99.9% or more), Si (purity 99.9% or more), Mn (purity 99.99% or more)
_{(1 -X)} Mn _x Si ₂ (X = 0.05 to 0.13) The composition was weighed to have a composition of 0.5 to 0.13 and melted in a vacuum melting furnace. 〜8 mm fine powder is used. on the other hand,
The raw material powder of the N-type FeSi ₂ semiconductor 2 includes Fe (purity of 99.9% or more), Si (purity of 99.9% or more), Co
(Purity 99.9% or more) as raw material and Fe _(1-Y) Co _Y
A coarse powder obtained by weighing so as to have a composition of Si ₂ (Y = 0.03 to 0.12) and melting in a vacuum melting furnace is used as a fine powder having an average particle diameter of 5 to 8 mm in the same manner as described above. I do. The raw material powder of the electrode 3 is obtained by subjecting a coarse powder of Fe (purity of 99.9% or more) to a process similar to that described above to obtain an average particle diameter of 1: 1.
A fine powder of 10 to 10 μm is used.

Raw material powders for the P-type FeSi ₂ semiconductor 1 and the N-type FeSi ₂ semiconductor 2 are filled into predetermined cylindrical containers by half cylinders, respectively. The powder was filled and preliminarily formed into a disk shape, and pressureless sintering was performed at a sintering temperature of 1050 ° C. and a heating time of 30 minutes. The sintering temperature of 1050 ° C.
-Type FeSi ₂ semiconductor 1 and N-type FeSi ₂ semiconductor 2
Temperature at which the raw material powder sinters and becomes β-phase
° K or higher. In the pressureless sintering method, since the compactness after firing is 92% to 93%, in order to improve the durability, hot press sintering method, hot isostatic pressure sintering method, or the like is used. Use is preferred. However, when a pressure sintering method such as a hot press sintering method or a hot isostatic pressure sintering method is used as the sintering method, carbon of the graphite punch for pressurization and Fe of the electrode 3 react. However, since the electrode 3 may be peeled off, it is necessary to change the punch to, for example, alumina.

The result of comparing the internal resistance of the thermoelectric element manufactured by the above-described method with the internal resistance of the conventional thermoelectric element having the same shape and the same manufacturing method but not provided with the electrode 3 but provided with an electrode portion by ultrasonic soldering. It was confirmed that what was conventionally 6Ω was reduced to 0.1Ω or less.

Another embodiment will be described below. <1> In the manufacturing process of the disk body 4 in the above embodiment, before the raw material powder for the electrode 3 is filled in the electrode forming portion shown in FIG. -Type FeSi ₂ semiconductor 1 and the N-type FeSi
It is also a preferred embodiment to fill a mixed powder of the conductive type FeSi ₂ semiconductor raw material powder and the raw material powder for the electrode 3 between the filling locations of the _two semiconductor 2 raw material powders.
The mixing ratio of the mixed powder is preferably such that the weight% of the FeSi ₂ semiconductor raw material powder is 10% to 90%.
As described above, an intermediate layer made of the same kind of component is positively formed between the P-type FeSi ₂ semiconductor 1 and the N-type FeSi ₂ semiconductor 2 and the electrode 3, so that the The shock due to shrinkage can be absorbed and alleviated, and the occurrence of cracks can be suppressed.

Further, the mixing ratio is determined from the side of each of the FeSi ₂ semiconductors 1 and _{2 at} the filling position of the mixed powder.
The composition may be continuously or intermittently graded toward the side. In this case, since the impact due to the shrinkage of the electrode 3 during firing can be absorbed continuously and intermittently, the above-described effect of suppressing the occurrence of cracks becomes more remarkable, which is more preferable.

<2> The electrode material of the electrode 3 is F
In addition to e, Ni, Co, or Fe containing a doping material such as Mn or Co may be used. Also in these electrode materials, it has been confirmed that good ohmic contact with the FeSi ₂ semiconductor 2 is possible as in the case of Fe.

<3> In this embodiment, the P-type Fe
The Si ₂ semiconductor 1 and the N-type FeSi ₂ semiconductor 2 may be manufactured separately and their ends opposite to the electrode 3 may be connected by an existing connection method such as silver brazing. . Further, the P-type FeSi ₂ semiconductor 1 and the N-type F
The electrodes 3 may be provided at both ends of the eSi ₂ semiconductor 2, and the electrodes at one end may be made common or may be connected separately via a conductor.

<4> In the electrode 3, the P-type FeSi ₂ semiconductor 1 and the N-type FeSi ₂ semiconductor 2 do not necessarily have to be manufactured simultaneously by the sintering method. For example, a film of the electrode material may be formed by vacuum deposition, thermal spraying, coating, or the like on one end of each of the ready-made P-type FeSi ₂ semiconductor and N-type FeSi ₂ semiconductor, and may be formed by firing.
The P-type FeSi ₂ semiconductor and the N-type F
The β-phase conversion of the eSi ₂ semiconductor may be performed simultaneously with the baking of the film of the electrode material. In addition, the ready-made P-type Fe
Si ₂ semiconductor and a manufacturing method of the N-type FeSi ₂ semiconductor may be a sintering method other than pressureless sintering method, also, sputtering a predetermined conductivity type impurity after forming an intrinsic FeSi ₂ semiconductor such It may be made by making.

[0029]

As described above, according to the present invention,
By forming the electrode with low contact resistance on the FeSi ₂ semiconductor, the specific resistance of the effective thermoelectric element can be reduced, and the performance as the thermoelectric element can be improved.
₍₂₎ To provide a high-performance thermoelectric element suitable for high-temperature use, making effective use of many advantages inherent in semiconductors such as high temperature use, oxidation resistance, easy control of conductivity type, low raw material cost, etc. did it.

[Brief description of the drawings]

FIG. 1 is a perspective view showing one embodiment of a thermoelectric element according to the present invention.

FIG. 2 is a plan view illustrating a manufacturing process of one embodiment of the thermoelectric element according to the present invention.

[Explanation of symbols]

1 P-type FeSi ₂ semiconductor 1a P-type FeSi ₂ semiconductor end 1b P-type FeSi ₂ semiconductor of the other end 2 N type FeSi ₂ semiconductor 2a N-type FeSi ₂ semiconductor end 2b N-type FeSi ₂ semiconductor of the other end 3 electrode 4 disc body

Claims (4)

[Claims] 1. An FeSi ₂ semiconductor comprising Fe, Ni, C
A thermoelectric element in which an electrode composed of o or Fe containing a doping material such as Mn or Co is directly formed. 2. A P-type FeSi ₂ semiconductor and an N-type FeSi _2.
A thermoelectric element in which one end of a semiconductor is connected directly or via a conductor, and the other end of the P-type FeSi ₂ semiconductor and the N-type FeSi ₂ semiconductor is Fe, Ni, Co, or Mn, C
A thermoelectric element in which an electrode containing Fe as a component containing a doping material such as o is directly formed. 3. The power supply of the thermoelectric element according to claim 1 or 2.
A method of forming a pole, comprising: placing a powder of the electrode material at an electrode formation location;
The above-mentioned FeSi _TwoSemiconductor of a given conductivity type of semiconductor
A material powder is placed, and the electrode forming portion and the semiconductor
A mixed powder of the electrode material and the semiconductor material in the middle of the formation location
And the FeSi_TwoSintering the semiconductor and the electrode
Characterized by simultaneous firing and integral molding by the method
Method for forming electrodes of thermoelectric element. 4. The mixing ratio of the powder mixture changes stepwise or continuously from the boundary with the semiconductor formation location to the boundary with the electrode formation location so as to increase the blending of the powder of the electrode material. The method for forming an electrode of a thermoelectric element according to claim 3.