JP2003338641A - Thermoelectric element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To avoid deterioration in an element function, element breakdown and the like due to thermal fatigue, by reducing thermal stress caused by a soldering layer or the like in a thermoelectric element using a thermoelectric semiconductor. <P>SOLUTION: The thermoelectric element 1 is composed by connecting an n-type thermoelectric semiconductor and a p-type thermoelectric semiconductor 5 that are alternately aligned on a support member 2 in series by first and second electrodes 6 and 7 that are joined to ends of the semiconductors via the soldering layers 8 and 9, respectively. The soldering layers 8 and 9 for joining thermoelectric semiconductors 4 and 5 to the electrodes 6 and 7 is composed of composite solder containing a thermal expansion inhibitor, such as metal powder, a metal fiber, ceramics powder, and a ceramics fiber, where average a linear expansion coefficient from 25 to 100°C is smaller than a soldering base material and the inhibitor has a volume ratio of 5 to 80%. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a thermoelectric element using a thermoelectric semiconductor.

[0002]

2. Description of the Related Art Bismuth (Bi) -tellurium (Te) system,
Iron (Fe) -silicide (Si) system, cobalt (Co)
-A thermoelectric element using a thermoelectric semiconductor such as antimony (Sb) is used as a cooling or heating device. Thermoelectric elements are small and thin and can be cooled without using a heat medium (refrigerant, etc.) such as liquid or gas. It is drawing attention as a cooling device. Further, also in applications other than these, it is used as a cooling device and a heating device in various fields.

Such a thermoelectric element has, for example, a plurality of P elements.
Type thermoelectric semiconductors and N-type thermoelectric semiconductors are alternately arranged, and a plurality of these thermoelectric semiconductors are arranged in series by an endothermic side electrode arranged on one end side and a heat radiating side electrode arranged on the other end side. It has a connected structure. In such a thermoelectric element, when a direct current is made to flow from the N-type thermoelectric semiconductor to the P-type thermoelectric semiconductor, heat absorption occurs at one end side of the thermoelectric semiconductor due to the Peltier effect and heat dissipation at the other end side ( Since heat is generated, cooling can be performed by disposing a member to be cooled or a device on the heat absorbing side.

As a concrete structure of the thermoelectric element, for example, a π-type structure as shown below is known (for example, JP-A-7-321379, JP-A-11-340527, and JP-A-2001-).
332773, JP 2001-352107, etc.). That is, a supporting member such as a ceramics substrate on which a first metal electrode group (heat radiation side electrode group) is formed is prepared, and a plurality of P-type thermoelectric semiconductors and N-type thermoelectric semiconductors are respectively provided on the first metal electrode group. Are arranged alternately. A second metal electrode group (heat absorption side electrode group) is provided on the upper end side of the P-type thermoelectric semiconductor and the N-type thermoelectric semiconductor.
And each metal electrode is bonded to the P-type and N-type thermoelectric semiconductors so that finally all the thermoelectric semiconductors are electrically connected in series. Solder is generally used for joining the metal electrodes and the thermoelectric semiconductor.

By the way, during the operation of the π-type thermoelectric element as described above, each component (support member, metal electrode, solder layer, thermoelectric semiconductor, etc.) is repeatedly subjected to a cooling / heating cycle. Since the thermoelectric element has different thermal expansion (elongation) between the heat absorption side and the heat radiation side, the entire thermoelectric element is bent. Further, since there is a difference in the coefficient of thermal expansion between the respective constituent members, thermal stress is locally generated when the cooling / heating cycle is applied. In particular, when a ceramic substrate is used as the supporting member, the difference in thermal expansion from other constituent materials is large, so that the thermal stress caused by repeated cooling and heating cycles becomes large. Due to the thermal stress due to the deflection of the entire element and the difference in thermal expansion between the constituent materials, the thermoelectric element is subjected to thermal fatigue during operation, and cracks and peeling of the bonding interface caused by this thermal fatigue deteriorate element performance. In addition, it is a cause of element destruction.

[0006]

As described above, the conventional thermoelectric elements have various problems due to thermal fatigue caused by the application of the cooling / heating cycle. Among the thermal stresses based on the difference in thermal expansion between the constituent materials described above, since solder has a higher coefficient of thermal expansion than other constituent materials, the thermal stress applied to the members to which the solder is joined and the solder itself is large, When the solder layer is thick, the effect of thermal stress becomes significant.
The thermal stress caused by such a solder layer causes a crack at the bonding interface between the thermoelectric semiconductor and the solder layer, or causes a crack in the solder layer or the thermoelectric semiconductor itself. These are major causes of performance deterioration and element destruction of thermoelectric elements.

Further, among the members to which the solder is joined, the thermoelectric semiconductor is a brittle material and has a low mechanical strength, so that fatigue cracks are likely to occur in the thermoelectric semiconductor. Further, since the solder itself has a low fatigue strength, fatigue cracks easily occur in the solder layer itself.
Since the thermoelectric element is configured by connecting a plurality of thermoelectric semiconductors in series, if the crack occurs even at one location in the thermoelectric semiconductor, the solder layer, or the bonding interface between these, the resistance value of the entire element increases, etc. Then, the function of the thermoelectric element is impaired.

The present invention has been made to address such a problem, and suppresses the deterioration of the element function and the occurrence of element destruction due to thermal fatigue by reducing the thermal stress caused by the solder layer. The object is to provide a thermoelectric element that enables the above.

[0009]

A thermoelectric element according to the present invention has a support member and P-type thermoelectric semiconductors and N-type thermoelectric semiconductors alternately arranged on the support member, as described in claim 1. A first electrode provided on the surface of the support member and joined to one end of the P-type thermoelectric semiconductor and the N-type thermoelectric semiconductor via a solder layer; and the P-type thermoelectric semiconductor and the N-type In a thermoelectric element comprising a second electrode joined to the other end of the thermoelectric semiconductor via a solder layer so that the thermoelectric semiconductor is connected in series, the solder layer has a temperature of 25 ° C to 100 ° C.
It is characterized by being composed of a composite solder containing a thermal expansion suppressing material whose average linear expansion coefficient up to ℃ is smaller than that of the solder base material in a volume ratio of 5 to 80%.

In the thermoelectric element of the present invention, the composite solder has an average linear expansion coefficient of 12 × 10 ^{−6 to} 21 × 10 ⁻⁶ / ° C. from 25 ° C. to 100 ° C. as described in claim 2. It is preferable. Further, as the thermal expansion suppressing material having an average linear expansion coefficient smaller than that of the solder base material, as described in claim 3, for example, at least one selected from metal powder, metal fiber, ceramic powder, and ceramic fiber is used.

In the thermoelectric element of the present invention, the solder layer is composed of the composite solder containing the thermal expansion suppressing material having a thermal expansion coefficient smaller than that of the solder base material as described above. According to such a solder layer, the thermal expansion coefficient itself of the solder layer is reduced by including the thermal expansion suppressing material, and the thermal expansion (elongation) of the solder layer is mechanically increased by the thermal expansion suppressing material. Therefore, the thermal deformation of the solder layer due to the cooling operation of the thermoelectric element can be suppressed. Therefore, the thermal stress applied to the thermoelectric semiconductor bonded to the solder layer or the solder layer itself is relieved, and the occurrence of cracks due to thermal fatigue of the thermoelectric semiconductor or the solder layer itself can be suppressed. Further, even if a crack occurs in the solder layer, the thermal expansion suppressing material has a function of preventing the propagation of the crack, so that the reliability of the solder layer can be significantly improved. As a result, it becomes possible to suppress the functional deterioration and element destruction due to the thermal cycle of the thermoelectric element.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Modes for carrying out the present invention will be described below. FIG. 1 is a sectional view showing a schematic structure of a thermoelectric element according to an embodiment of the present invention, and FIG. 2 is an enlarged sectional view showing a main part thereof. The thermoelectric element 1 shown in these figures has upper and lower support members 2 and 3, and the lower support member 2 and the upper support member 3 are arranged to face each other. In the thermoelectric element 1 of this embodiment, the lower support member 2 side is a heat dissipation surface and the upper support member 3 side is a heat absorption surface.

Of these support members 2 and 3, the lower support member (heat radiation side support member) 2 functions as a structural support of the thermoelectric element 1, and is, for example, an alumina substrate, an aluminum nitride substrate, a silicon nitride substrate, or the like. It is preferable to use an insulating ceramic substrate. As the upper support member 3 (heat absorption side support member), a ceramic substrate, which is an insulating substrate, may be used similarly to the lower support member 2, and the lower support member 2 can sufficiently support the entire element structure. For example, the upper support member 3 may be composed of an insulating resin substrate or an insulating resin film.

The above-mentioned lower support member 2 and upper support member 3
A plurality of N-type thermoelectric semiconductors 4 and P-type thermoelectric semiconductors 5 are alternately arranged between and, and these elements are arranged in a matrix as a whole element. Thermoelectric semiconductor 4, 5
Various known materials can be used for, and a typical example thereof is a Bi-Te-based thermoelectric semiconductor. Bi-T
The e-based thermoelectric semiconductor contains at least one element selected from Bi and Sb and at least one element selected from Te and Se as essential elements, and further contains I, Cl, Br, and Hg as necessary. Compound semiconductors containing additive elements such as Au, Au, and Cu are known. The thermoelectric semiconductors 4 and 5 are not limited to Bi-Te based thermoelectric semiconductors, and various thermoelectric semiconductors such as Fe-Si based and Co-Sb based can be applied.

The plurality of N-type thermoelectric semiconductors 4 and P-type thermoelectric semiconductors 5 are arranged in the direction from N-type thermoelectric semiconductor 4 to P-type thermoelectric semiconductor 5, that is, N-type thermoelectric semiconductor 4, P-type thermoelectric semiconductor 5,
A first electrode 6 provided on the lower support member 2 side and a second electrode 7 provided on the upper support member 3 side so that a direct current flows in the order of the N-type thermoelectric semiconductor 4, the P-type thermoelectric semiconductor 5 ... Are electrically connected in series. Each of the first and second electrodes 6 and 7 constitutes a plurality of electrode groups. Each of the electrodes 6 and 7 is made of, for example, a metal plate such as a copper plate or an aluminum plate, or a metal adhesion layer.

That is, a plurality of first electrodes 6 serving as heat dissipation side electrodes are provided on the surface of the lower support member 2. On the other hand, a plurality of second electrodes 7 serving as heat absorption side electrodes are arranged on the upper support member 3 side. The second electrode 7 on the heat absorption side has a shape for electrically connecting the adjacent N-type thermoelectric semiconductor 4 and P-type thermoelectric semiconductor 5 in this order. The first electrode 6 on the heat radiation side has a shape that electrically connects the adjacent P-type thermoelectric semiconductor 5 and N-type thermoelectric semiconductor 4 in this order, except for the electrodes (lead extraction electrodes) at both ends. ing.

N-type thermoelectric semiconductor 4 and P-type thermoelectric semiconductor 5
The lower end portion (end portion on the heat radiation side) is joined to the first electrode 6 via the solder layer 8 as shown in an enlarged view in FIG. Further, the upper end portions (end portions on the heat absorption side) of the N-type thermoelectric semiconductor 4 and the P-type thermoelectric semiconductor 5 are similarly joined to the second electrode 7 via the solder layer 9. In this way, when the N-type thermoelectric semiconductor 4 and the P-type thermoelectric semiconductor 5 which are adjacent to each other are sequentially connected by the first electrode 6 and the second electrode 7, respectively, a plurality of thermoelectric elements 1 can be obtained. The N-type thermoelectric semiconductors 4 and the plurality of P-type thermoelectric semiconductors 5 are alternately connected in series.

When a direct current is passed through the thermoelectric element 1 from the DC power source 10 in the direction from the N-type thermoelectric semiconductor 4 to the P-type thermoelectric semiconductor 5, heat absorption occurs on the upper end side of the thermoelectric semiconductors 4 and 5 due to the Peltier effect. Heat dissipation occurs on the lower end side. Therefore, by bringing the object to be cooled (member or device for cooling) into contact with the upper support member 3 corresponding to the heat absorbing side of the thermoelectric element 1, heat is taken from the object to be cooled for cooling.
The heat taken from the object to be cooled is radiated from the lower support member 2 side corresponding to the heat radiation side of the thermoelectric element 1.

In the thermoelectric element 1 having the above-mentioned structure, the solder layers 8 and 9 for bonding the thermoelectric semiconductors 4 and 5 to the first and second electrodes 6 and 7 are 25 ° C. to 100 ° C., respectively.
Is composed of a composite solder containing a thermal expansion suppressing material having an average linear expansion coefficient lower than that of the solder base material in a volume ratio of 5 to 80%. Here, as the solder material (solder base material) that is the base material of the solder layers 8 and 9, various kinds of solder generally used for joining electronic components can be applied. Specifically, Sn-Pb system, Pb-Sn system, Sn-Pb-Bi
System, Sn-Pb-Ag system, Sn-Ag system, Sn-Sb
A variety of solders such as a Pb-Ag system, a Pb-Ag system, a Bi-Sn system, and a Bi-Pb-Sn system can be used, and the solder material itself is not particularly limited.

The composite solder constituting the solder layers 8 and 9 comprises various kinds of solder base materials as described above, and a thermal expansion suppressing material having a smaller average linear expansion coefficient (25 to 100 ° C.) than that of the above-mentioned solder base material with respect to the entire composite solder. It is added so that the volume ratio is in the range of 5 to 80%. The average linear expansion coefficient of the composite solder itself (25 to 100
C.) is preferably in the range of 12.times.10.sup.- ^{6 to} 21.times.10.sup.- ⁶ / .degree. By forming the solder layers 8 and 9 using such a composite solder, thermal expansion (expansion due to heat) of the solder layers 8 and 9 during the cold heat operation of the thermoelectric element 1 is suppressed, whereby the thermoelectric element 1 is suppressed. This makes it possible to prevent functional deterioration and element destruction due to the thermal cycle of.

That is, in the thermoelectric element, generally, the amount of thermal expansion (elongation) at the time of carrying out the cooling operation by energizing is different between the heat absorbing side and the heat radiating side, and in particular, is interposed between the heat radiating side electrode and the thermoelectric semiconductor. A large stress is applied to the solder layer in the substrate surface direction. For example, as shown in FIG. 3, in a thermoelectric element 1'having solder layers 8'and 9'formed by conventional ordinary solder, the amount of thermal expansion (elongation) during cooling operation is on the heat absorption side (arrow B in the figure). Since the heat radiation side (shown by the arrow A in the figure) is larger than the heat radiation side electrode 6), the solder layer 8 ′ interposed between the heat radiation side electrode 6 and the thermoelectric semiconductors 4 and 5 becomes larger. Further, the solder layers 8'and 9'formed of normal solder also have a large difference in thermal expansion between the thermoelectric semiconductors 4 and 5.

As a result, a large thermal stress (indicated by an arrow C in the figure) acts on the thermoelectric semiconductors 4 and 5 (especially on the heat dissipation electrode 6 side) in the conventional thermoelectric element 1 '. Such thermal stress causes cracks X ₁ in the thermoelectric semiconductors 4 and 5, and cracks X ₂ in the bonding interfaces between the thermoelectric semiconductors 4 and 5 and the solder layers 8 ′ and 9 ′. Further, conventional solder layer 8 was formed in solder ', 9' is lower fatigue strength itself also, the solder layer 8 ', 9' cracks X ₃ is likely to occur in itself. The cracks X in the thermoelectric semiconductors 4 and 5 and the solder layers 8'and 9'cause a rise in the resistance value of the thermoelectric element 1'and the like, which leads to deterioration in element performance. Further, if the crack X propagates, the thermoelectric element 1'is destroyed.

In contrast to the conventional element structure as described above,
In the thermoelectric element 1 of this embodiment, since the solder layers 8 and 9 are composed of the composite solder containing the thermal expansion suppressing material having an average linear expansion coefficient (25 to 100 ° C.) smaller than that of the solder base material, the cracks as described above are generated. It is possible to suppress the occurrence of cracks and prevent the cracks from developing even when cracks occur. That is,
Since the thermal expansion coefficient itself of the solder layers 8 and 9 is lowered by adding the thermal expansion suppressing material, the difference in thermal expansion from the thermoelectric semiconductors 4 and 5 is smaller than that in the conventional case. This contributes to relaxation of thermal stress caused by the difference in thermal expansion.

From such a point, the average linear expansion coefficient (25 to 100 ° C.) of the composite solder containing the thermal expansion suppressing material is 12 × 10 ^{−6 to} 21 × 10.
It is preferably in the range of ^-6 / ° C. If the average coefficient of linear expansion of the composite solder exceeds 21 × 10 ⁻⁶ / ° C., the effect of suppressing the difference in thermal expansion cannot be sufficiently obtained. On the other hand, when the average coefficient of linear expansion of the composite solder is less than 12 × 10 ⁻⁶ / ° C., the thermoelectric semiconductors 4 and 5 are likely to be cracked during soldering, which may deteriorate fatigue characteristics. The average linear expansion coefficient (25 to 100 ° C.) of the composite solder is more preferably in the range of 13 × 10 ^{−6 to} 19 × 10 ⁻⁶ / ° C., further preferably 15 × 10 ⁻⁶ to
The range is 19 × 10 ^-6 / ° C.

The thermal expansion suppressing material not only lowers the coefficient of thermal expansion of the solder layers 8 and 9, but also mechanically restrains the thermal expansion (elongation) of the solder layers 8 and 9. Also effective in improving the physical strength. That is, the thermoelectric element 1
Since the expansion of the solder layers 8 and 9 due to the cold heat operation is restrained by the thermal expansion suppressing material, the amount of thermal deformation of the solder layers 8 and 9 can be reduced. Therefore, the stress applied to the thermoelectric semiconductors 4 and 5 in direct contact with the solder layers 8 and 9 and the stress applied to the solder layers 8 and 9 themselves can be relaxed. By relaxing the thermal stress in this way, the thermoelectric semiconductors 4, 5 and the solder layer 8,
It is possible to suppress the occurrence of cracks due to thermal fatigue of 9 itself.

Further, even if a crack is generated in the solder layers 8 and 9, the thermal expansion suppressing material has a function of preventing the propagation of the crack, so that the deterioration of the element function and the occurrence of the element destruction due to the progress of the crack are prevented. Therefore, the reliability of the solder layers 8 and 9 can be significantly improved. The thermal expansion suppressing material is the solder layers 8 and 9
Since it is also effective in improving the mechanical strength of, the generation and development of cracks can be prevented from this point as well. As described above, in the thermoelectric element 1, the generation and development of cracks due to thermal fatigue of the thermoelectric semiconductors 4 and 5 and the solder layers 8 and 9 themselves are suppressed. It becomes possible to suppress. That is, the long-term reliability of the thermoelectric element 1, particularly the thermoelectric element 1 used under the condition that a cooling / heating cycle is frequently added.
The long-term reliability of can be significantly increased.

The solder layers 8 and 9 made of the composite solder as described above are prepared by sufficiently kneading a thermal expansion suppressing material with a normal solder paste to prepare a composite solder paste, and using such a composite solder paste, an electrode is formed. 6 and 7 and the thermoelectric semiconductors 4 and 5 are soldered to form solder layers 8 and 9 in which a thermal expansion suppressing material is dispersed. The thermal expansion suppressing material is added so as to be in a volume ratio of 5 to 80% with respect to the entire composite solder. The volume ratio of the thermal expansion suppressing material is a value calculated from the mass ratio of the metal component of the solder to the thermal expansion suppressing material and the true density of each component.

If the addition amount of the thermal expansion suppressing material is less than 5% by volume, the effect of reducing the thermal expansion coefficient of the solder layers 8 and 9 and the effect of suppressing thermal deformation cannot be sufficiently obtained. On the other hand, when the addition amount of the thermal expansion suppressing material exceeds 80% by volume, voids and the like are likely to be formed in the solder layers 8 and 9, and conversely the strength is decreased.
In addition, problems such as a decrease in thermal conductivity and conductivity occur. Furthermore, since the excessive addition of the thermal expansion suppressing material causes a decrease in the fluidity of the solder paste, workability during soldering also deteriorates.

The addition amount of the thermal expansion suppressing material is more preferably in the range of 10 to 60% (more preferably in the range of 20 to 60%) by volume ratio with respect to the entire composite solder. Further, the addition amount of the thermal expansion suppressing material is an average linear expansion coefficient (25 to 100 ° C.) of the composite solder in consideration of the values of the average linear expansion coefficient of the lower support member 2 as a structural support and the thermoelectric semiconductors 4 and 5. Is preferably controlled to be an appropriate value. Here, the average coefficient of linear expansion of the composite solder is approximately the value calculated by the compound rule based on the respective volume ratios of the coefficient of linear expansion of the solder base material and the coefficient of linear expansion and coefficient of thermal expansion of the thermal expansion suppressing material. It is desirable to optimize the addition amount of the thermal expansion suppressing material in consideration of the value of the average linear expansion coefficient of the composite solder and the values of the average linear expansion coefficient of the lower support member 2 and the thermoelectric semiconductors 4 and 5.

As the thermal expansion suppressing material as described above, various powders and fibers can be applied as long as the material has an average linear expansion coefficient (25 to 100 ° C.) smaller than that of the solder base material. At least selected from metal fiber, ceramic powder, ceramic fiber (including glass fiber), etc.
One kind can be used. Further, as the thermal expansion suppressing material, it is preferable to use a material which is hard to be alloyed or react with the solder base material and its bonding temperature or lower in addition to having a small average linear expansion coefficient. If the thermal expansion suppressing material itself alloys or reacts with the solder base material, the function as the thermal expansion suppressing material may be impaired, or the characteristics of the solder itself may deteriorate.

From the above, when metal powder or metal fiber is applied to the thermal expansion suppressing material, W, Mo, Cr,
It is preferable to use Zr, Ti, V, Nb, a low thermal expansion Fe-based alloy, or the like. All of these metals and alloys have a small coefficient of thermal expansion and low reactivity with the solder material. As the low thermal expansion Fe-based alloy, Invar alloy (for example, Fe-36 mass% Ni), Super Invar alloy (for example, Fe-31 mass% Ni-5 mass% Co), Kovar alloy (for example, Fe-29 mass% Ni). -17% by mass C
o), 42 alloy (Fe-42 mass% Ni) and the like.

On the other hand, a ceramic material generally has a small coefficient of thermal expansion, and since it does not easily react with the metal material due to its small wetting, it is a material suitable as a thermal expansion suppressing material to be dispersed in the solder layers 8 and 9. is there. The constituent materials of the ceramic powder and the ceramic fiber are not particularly limited,
For example, alumina, silica, silicon carbide, carbon, a composite compound thereof, or the like may be used, and glass fiber or the like may be used. Various fibrous substances such as whiskers, short fibers and long fibers can be used as the ceramic fibers.

As described above, it is preferable to use, as the thermal expansion suppressing material, a material that is less likely to be alloyed or react with the solder base material and the bonding temperature thereof, but the thermal expansion suppressing material (particles or fibers) and the solder matrix. The binding force with (base material) is too weak,
For example, in the state where particles or fibers easily fall out of the solder matrix, the force cannot be sufficiently transmitted between them, and the effect of restraining the thermal expansion of the solder layers 8 and 9 is impaired. Become. Therefore, when using a material having low reactivity with the solder base material as the thermal expansion suppressing material, a material having good wettability with solder is previously coated on the surface of the powder or fiber by, for example, a plating method or a vapor deposition method. Preferably. For the surface coating material of the thermal expansion suppressing material,
It is preferable to apply Ni, Cu, Ag, Au, and alloys containing these elements.

When metal powder or ceramic powder is used as the thermal expansion suppressing material, it is preferable to use powder having an average particle size of 32 μm or less. The average particle size of the powder is
If it exceeds 32 μm, the bonding force between the thermal expansion suppressing material (particles) and the solder matrix (base material) is weakened and the force is not sufficiently transmitted, and the effect of restraining the thermal expansion of the solder layers 8 and 9 is impaired. May be The lower limit of the average particle size of the metal powder or the ceramic powder is not particularly limited, but the average particle size is set in consideration of the ease of handling (suppression of spontaneous combustion).
It is preferable to use a powder of 0.1 μm or more.

When a metal fiber or a ceramic fiber is used as the thermal expansion suppressing material, the average aspect ratio (fiber length (major axis) / fiber It is preferable to use fibers having a diameter (short diameter) of 2 or more. By dispersing such fibers so that the length direction thereof is arranged in the plane direction of the solder layers 8 and 9, the extension (thermal expansion) of the solder layers 8 and 9 in the plane direction is more effectively suppressed. be able to. The fiber diameter (minor axis) is the same as the average particle size of the powder.
It is preferably 30 μm or less. That is, if the fiber has a too large diameter (thickness), the force is not sufficiently transmitted to the solder base (base material), and the effect of restraining the thermal expansion of the solder layers 8 and 9 may be impaired. .

Thermoelectric element 1 having the structure as described above
Is suitable for use in a cooling device in various fields, including a cooling device for a high heat generation semiconductor device such as an ultra-high integrated circuit device such as a CPU of a computer or a laser device. The heat absorbing surface of the thermoelectric element 1 and the object to be cooled (various devices or parts) may be directly contacted with each other, but it is preferable to contact them via silicone grease or the like.
In this case, it is preferable to mix powder of high thermal conductivity such as graphite powder or boron nitride powder in the silicone grease, which can further improve heat transfer efficiency. Furthermore, since the silicone grease containing graphite powder, boron nitride powder, etc. has the effect of reducing the shear stress due to wear, it becomes possible to reduce the thermal cycle fatigue of the module using the thermoelectric element.

[0037]

EXAMPLES Next, specific examples of the present invention will be described.

Examples 1 and 2, Comparative Example 1 First, carbon powder having an average particle diameter of 1.5 μm (average linear expansion coefficient (25
^{~100 ℃): 3~4 × 10 -} 6 / ℃) using, those subjected to surface previously coated with Ni plating on the surface (Example 1) and those not subjected to the surface coating (Example 2) And prepared. Each of these carbon powders was added to a Pb-Sn solder paste at a volume ratio of the solder metal to the carbon powder of 70 /.
They were blended so as to be 30, and these were sufficiently kneaded to prepare composite solder pastes.

Using each of the above-mentioned composite solder pastes, a test thermoelectric element shown in FIG. 4 was produced as follows.
First, a 10 mm × 15 mm × 1.5 mm thick alumina ceramic substrate 11 and a 4 mm × 12 mm × 0.5 mm thick electrolytic copper electrode material 12 were treated by the DBC method (above the eutectic temperature of copper and copper oxide and below the melting point of copper). After heating at a temperature of (1) and directly bonding without using a brazing material), the copper electrode material was patterned by etching to form the heat radiation side electrodes 12a, 12b. 4mm x 8mm with these heat radiation side electrodes 12a, 12b
B. A Bi-Te based N-type thermoelectric semiconductor 14 and a P-type thermoelectric semiconductor 15, which were processed into a cube of 3 mm square, were joined with the heat absorption side electrode 13 made of an electrolytic copper electrode material having a thickness of 0.5 mm so as to be sandwiched therebetween.

The electrodes 12a, 12b, 13 and the N-type thermoelectric semiconductor 14 and the P-type thermoelectric semiconductor 15 are bonded to each other using the above-mentioned two kinds of composite solder pastes, respectively, and the composite solder pastes are applied to the bonding surfaces and then raised. It was carried out by heating to melt the solder. The solder layers 16 and 17 formed after the solder bonding contain carbon powder, respectively.
The thickness of each solder layer 16 and 17 was 0.1 mm. Carbon powder 3
The average linear expansion coefficient (25 to 100 ° C) of composite solder containing 0% by volume is 14
× 10 ⁻⁶ / ° C. The average coefficient of linear expansion of this composite solder is 5 mm diameter graphite type hot pressed composite solder,
A round bar sample was prepared and measured.

Further, the Bi-Te system N-type thermoelectric semiconductor 14
Bi28at.%-Te57at.%-Sb12at.%-Se3a
The composition of t.% is used, and Bi-Te based P-type thermoelectric semiconductor 15 has Bi10at.%-Te57at.%-Sb10at.%.
-Se3 at.% Composition was used. These Bi-T
Average linear expansion coefficient of e-type thermoelectric semiconductors 14 and 15 (25 to 100
℃) is about 13 × 10 ^-6 / ℃. Each thermoelectric semiconductor 14, 15
Has a nickel-plated joint surface in advance. After that, the fluorine-based insulating resin film 18 is formed on the heat absorption side electrode 13.
Was attached and lead wires 19 for energization were attached to make test thermoelectric elements 20 respectively.

On the other hand, as a comparative example 1 with the present invention, a test thermoelectric element 20 was prepared in the same manner as in the above-mentioned example 1 except that a Pb-Sn solder paste (normal solder paste) containing no carbon powder was used. It was made. By the way, the average linear expansion coefficient of the solder layer (25 to 10
0 ° C.) is 23 × 10 ⁻⁶ / ° C.

A thermal fatigue test was carried out by repeatedly energizing the test thermoelectric elements 20 of Examples 1, 2 and Comparative Example 1 thus obtained. The specific conditions of the thermal fatigue test are as follows. The thermoelectric element 20 for test has a heat radiating side on the side of the ceramic substrate 11, and a water-cooled copper block is pressed against it forcibly cooling it to 25 ° C. at all times. The temperature of the central part of the insulating resin film 18 on the heat absorption side is 10 ° C /
The heating rate was adjusted to a minute, and the current was set to zero when the maximum temperature reached 80 ° C. When the temperature of the central portion of the heat absorbing side insulating resin film 18 dropped to 30 ° C., the power was supplied again. A thermal fatigue test was carried out using this energization pattern as one cycle. Regarding the progress of fatigue, the heat dissipation side electrode 12
It was judged by measuring the change in the resistance value between a and 12b. Figure 5
The relationship between the number of thermal cycles (the number of energization cycles) and the change in resistance value of each test thermoelectric element 20 is shown in FIG.

As is apparent from FIG. 5, the test thermoelectric element of Comparative Example 1 using the Pb-Sn solder paste containing no carbon powder had a rapid increase in resistance value, resulting in poor energization in about 100 cycles. On the other hand, the test thermoelectric elements of Example 1 and Example 2 were both kept in a sufficiently electrified state even after the thermal cycles of 300 cycles or more,
It can be seen that the thermal fatigue resistance is excellent. In particular, in Example 1 using the Ni-plated carbon powder, 50
It exhibits a heat-resistant cycle property of 0 or more cycles, which makes it possible to significantly improve the long-term reliability of the thermoelectric element.

When the state of each element after the thermal fatigue test was observed in detail, the test thermoelectric element of Comparative Example 1 was found to have cracks in the solder layer and cleavage cleavage in the thermoelectric semiconductor. On the other hand, in the test thermoelectric element of Example 1, no abnormality was found in either the solder layer or the thermoelectric semiconductor. However, regarding the test thermoelectric element of Example 2, a slight cleavage crack was observed in the thermoelectric semiconductor. From this, it can be seen that it is effective to coat the surface of the carbon powder with a metal having high solder wettability.

Example 3 and Comparative Example 2 First, Mo powder having an average particle diameter of 3 μm (average linear expansion coefficient (25 to
100 ° C.): 5.1 × 10 ⁻⁶ / ° C.) was prepared. This Mo powder was mixed in a Pb-Sn solder paste so that the volume ratio of the solder metal and the Mo powder would be the kneading ratios shown in Table 1, and these were sufficiently kneaded to produce composite solder pastes. did. Test thermoelectric elements were produced in the same manner as in Example 1 except that these composite solder pastes were used. The average linear expansion coefficient (25 to 100 ° C.) of each composite solder containing Mo powder is as shown in Table 1. The average coefficient of linear expansion of these composite solders was measured by hot pressing the composite solders with a graphite mold having a diameter of 5 mm and producing round bar-shaped samples. The same applies to the following examples.

A thermal fatigue test of each test thermoelectric element thus obtained was carried out in the same manner as in Example 1. The fatigue life of each element is as shown in Table 1. The fatigue life was defined as the time when the resistance change between the heat radiation side electrodes 12a and 12b reached 70% of the initial value, and the fatigue life was indicated by the number of heat cycles at that time.

[0048]

[Table 1]

As is clear from Table 1, the volume ratio of Mo powder (thermal expansion suppressing material) is 5%, and the average linear expansion coefficient (25 to 100 ° C.) of the composite solder is 21 × 10 ⁻⁶ / ° C. Then, a remarkable improvement effect of the fatigue life of the thermoelectric element was recognized. Also, Mo
When the volume ratio of the powder (thermal expansion inhibitor) exceeds 80%, the average linear expansion coefficient becomes a sufficiently small value, but the effect of improving fatigue life is drastically reduced. This is because the fluidity of the solder paste was significantly deteriorated and the bonding force with the thermoelectric semiconductor was reduced, and the bonding between the solder metal and the Mo particles in the solder layer was insufficient. In the element of Sample No. 9, it was confirmed that cracks thought to be caused by these were generated in the solder layer and the thermoelectric semiconductor.

Example 4 Various metal powders and alloy powders were used as the thermal expansion suppressing material, and their effects were confirmed. Specifically, each metal powder having the average particle size shown in Table 2 is prepared, and the volume ratio of the solder metal to the metal powder is shown in Table 1 in the Pb-Sn solder paste. They were mixed so as to have a kneading ratio, and these were sufficiently kneaded to prepare composite solder pastes. Test thermoelectric elements were produced in the same manner as in Example 1 except that these composite solder pastes were used. Average coefficient of linear expansion of each composite solder (25 to 10
(0 ° C.) is as shown in Table 2. A thermal fatigue test of each of these test thermoelectric elements was carried out in the same manner as in Example 3, and the fatigue life was measured. The fatigue life of each element is shown in Table 2.

[0051]

[Table 2]

As is clear from Table 2, metal powders such as W, Cr, Zr, Ti, V, and Nb having a small coefficient of thermal expansion, or powders of a low thermal expansion Fe-based alloy (Invar alloy, 42 alloy, etc.), It can be seen that the thermal fatigue of the thermoelectric element can be reduced and the fatigue life can be improved by using it as a thermal expansion suppressing material and dispersing it in the solder layer. The average linear expansion coefficient of 25 to 100 ° C. of each metal powder used in Example 4 was W 4.5.
× 10 ^-6 / ° C, Cr 6.5 × 10 ^-6 / ° C, Zr 5.0 × 10 ^-6 /
C, Ti is 8.9 × 10 ^-6 / ° C, V is 8.3 × 10 ^-6 / ° C, Nb is
7.2 × 10 ^-6 / ° C, Invar alloy 1.2 × 10 ^-6 / ° C, 42 alloy 5.3 × 10 ^-6 / ° C.

Example 5 and Comparative Example 3 First, silicon carbide whiskers having an average aspect ratio of 10 and an average minor axis of 0.5 μm (average linear expansion coefficient (25 to 100 ° C.): 5.0 × 10 5)
^-6 / ° C.) was prepared, and the surface was pre-coated with Ni plating. This silicon carbide whisker is Pb-
The Sn-based solder paste was mixed so that the volume ratio of the solder metal to the silicon carbide whiskers was 25/75 (= silicon carbide whiskers / solder metal), and these were sufficiently kneaded to prepare a composite solder paste. . A test thermoelectric element was produced in the same manner as in Example 1 except that this composite solder paste was used. The average linear expansion coefficient (25 to 100 ° C) of the composite solder containing 25% by volume of silicon carbide whiskers is 14 × 10 ^-6 / ° C.

On the other hand, as Comparative Example 3 with the present invention, a test thermoelectric element was prepared in the same manner as in Example 5 except that a Pb-Sn solder paste (normal solder paste) containing no silicon carbide whiskers was used. It was made. By the way, Pb-
The average linear expansion coefficient (25 to 100 ° C.) of the solder layer composed of only the Sn-based solder paste is 23 × 10 ⁻⁶ / ° C.

A thermal fatigue test was conducted on each of the test thermoelectric elements of Example 5 and Comparative Example 3 thus obtained in the same manner as in Example 1. FIG. 6 shows the relationship between the number of heat cycles (number of energization cycles) and the change in resistance value of each test thermoelectric element. As is clear from FIG. 6, the test thermoelectric element of Example 5 using the solder paste containing silicon carbide whiskers maintained a sufficient energized state even after a thermal cycle of 500 cycles or more, and had a thermal fatigue resistance characteristic. It turns out to be excellent. That is, the long-term reliability of the thermoelectric element can be significantly improved.

Example 6 and Comparative Example 4 First, short carbon fibers plated with Ni (average linear expansion coefficient (25 to 100 ° C.): 3 to 4 × 10 ⁻⁶ / ° C., average aspect ratio: 2
0, average short diameter: 5 μm) and prepare this carbon short fiber with Pb
-Sn solder paste was mixed so that the volume ratio of the solder metal and the short carbon fibers would be the kneading ratio shown in Table 3, and these were sufficiently kneaded to produce composite solder pastes. Test thermoelectric elements were produced in the same manner as in Example 1 except that these composite solder pastes were used. Average linear expansion coefficient of each composite solder containing short carbon fibers (25
(~ 100 ° C) is as shown in Table 3. A thermal fatigue test of each of these test thermoelectric elements was carried out in the same manner as in Example 3, and the fatigue life was measured. The fatigue life of each element is shown in Table 3.

[0057]

[Table 3]

As is apparent from Table 3, the volume ratio of the short carbon fibers (thermal expansion suppressing material) was 5%, and the average linear expansion coefficient (25 to 100 ° C.) of the composite solder was 21 × 10 ⁻⁶ / ° C. At the boundary, a remarkable improvement effect on the fatigue life of the thermoelectric element was recognized.

Example 7 In the same manner as in Sample No. 3 of Example 6 except that the carbon short fibers in which the aspect ratio was adjusted to have the values shown in Table 4 were used in Example 6 described above, respectively. Composite solder pastes were prepared, and test thermoelectric elements were prepared using the composite solder pastes. A thermal fatigue test of each of these test thermoelectric elements was carried out in the same manner as in Example 3, and the fatigue life was measured. The fatigue life of each element is shown in Table 4.

[0060]

[Table 4]

As is clear from Table 4, the aspect ratio is
By using two or more short carbon fibers (thermal expansion suppressing material), the fatigue life of the thermoelectric element can be significantly improved. When carbon short fibers having an aspect ratio of 2 or more were used, it was confirmed that each carbon short fiber had its length direction aligned in the plane direction of the solder layer.

Example 8 Various fibers were used as the thermal expansion suppressing material, and their effects were confirmed. Specifically, first, each fiber shown in Table 5 is prepared, and the volume ratio of the solder metal to the fiber is 20/80 (= fiber / solder metal (however, in the Pb-Sn solder paste. Only E glass fiber was mixed so as to be 40/60)), and these were sufficiently kneaded to prepare composite solder pastes. The aspect ratio of each fiber was adjusted to about 25 to 50. A test thermoelectric element was produced using each of these composite solder pastes, and the fatigue life was measured. The measurement results are shown in Table 5.

[0063]

[Table 5]

As is clear from Table 5, it is understood that the thermal fatigue of the thermoelectric element can be reduced and the fatigue life can be improved by using various ceramic fibers having a small coefficient of thermal expansion as the thermal expansion suppressing material.

Example 9 In order to confirm the effect of the metal coating on the fiber as the thermal expansion suppressing material, the same silicon carbide whiskers as in Example 5 are plated with copper, gold, silver and nickel-phosphorus. A test thermoelectric element was prepared and fatigue life was measured in the same manner as in Example 5 except for the above. As a result, the fatigue life was 105 times when using the silicon carbide whiskers without metal coating, whereas the fatigue life was 105 times or more when using the silicon carbide whiskers with metal coating. It was confirmed to show fatigue life.

[0066]

As described above, according to the thermoelectric element of the present invention, the thermal stress and the like due to the solder layer during the cold heat operation are reduced, which causes the deterioration of the element function and the destruction of the element due to the thermal fatigue. Can be significantly suppressed. That is, it becomes possible to provide a thermoelectric element having excellent long-term reliability.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic structure of a thermoelectric element according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view showing an enlarged main part of the thermoelectric element shown in FIG.

FIG. 3 is a diagram for explaining a crack generation state in a thermoelectric element having a conventional structure.

FIG. 4 is a diagram showing a structure of a thermoelectric element for thermal fatigue test manufactured in an example of the present invention.

FIG. 5 is a diagram showing the relationship between the number of thermal cycles and the change in resistance value of thermoelectric elements according to Examples 1 and 2 of the present invention.

FIG. 6 is a diagram showing the relationship between the number of thermal cycles and the change in resistance value of the thermoelectric element according to Example 5 of the present invention.

[Explanation of symbols]

1 ... Thermoelectric element, 2 ... Lower support member, 3 ... Upper support member, 4 ... N-type thermoelectric semiconductor, 5 ... P-type thermoelectric semiconductor,
6 ... First electrode (discharge side electrode), 7 ... Second electrode (heat absorption side electrode), 8, 9 ... Solder layer, 10 ... DC power supply

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Masami Okamura             8th Shinsugita Town, Isogo Ward, Yokohama City, Kanagawa Prefecture             Ceremony company Toshiba Yokohama office F term (reference) 5F036 AA01 BA23 BA33 BB21 BC06                       BD01 BD13

Claims (7)

[Claims] 1. A support member, P-type thermoelectric semiconductors and N-type thermoelectric semiconductors arranged alternately on the support member, and a P-type thermoelectric semiconductor and an N-type semiconductor provided on the surface of the support member. A first electrode joined to one end of a thermoelectric semiconductor via a solder layer and the other end of the thermoelectric semiconductor via a solder layer so that the P-type thermoelectric semiconductor and the N-type thermoelectric semiconductor are connected in series. In a thermoelectric element comprising a bonded second electrode, the solder layer comprises a thermal expansion suppressing material having an average linear expansion coefficient from 25 ° C. to 100 ° C. smaller than that of the solder base material in a volume ratio of 5 to 80%. A thermoelectric element comprising a composite solder containing in the range of. 2. The thermoelectric element according to claim 1, wherein the composite solder has an average coefficient of linear expansion from 25 ° C. to 100 ° C. of 12.
A thermoelectric element characterized by being in the range of × 10 ^{-6 to} 21 × 10 ^-6 / ° C. 3. The thermoelectric element according to claim 1 or 2, wherein the thermal expansion suppressing material is at least one selected from metal powder, metal fiber, ceramic powder, and ceramic fiber. Thermoelectric element. 4. The thermoelectric element according to claim 3, wherein the metal powder and the metal fiber are W, Mo, Cr, Z.
A thermoelectric element comprising at least one selected from r, Ti, V, Nb, and a low thermal expansion Fe-based alloy. 5. The thermoelectric element according to claim 3, wherein the ceramic powder and the ceramic fiber are made of at least one selected from alumina, silica, silicon carbide, carbon, and composite compounds thereof. element. 6. The thermoelectric element according to claim 1, wherein a surface of the thermal expansion suppressing material is coated with a metal. 7. The thermoelectric element according to claim 6, wherein the metal coating is made of Ni, Cu, Ag, Au, or an alloy containing these elements.