US3510364A - Contact structure for a thermoelectric device - Google Patents

Description

y 1970 e. OESTERHELT E L 3,510,364

CONTACT STRUCTURE FOR A THERMOELECTRIC DEVICE Filed March 20, 1968 United States Patent US. Cl. 136239 5 Claims ABSTRACT OF THE DISCLOSURE A thermoelectric device whose thermocouple legs consist of silicon alloys has these legs contacted and interconnected by structure made of a hafnium-silicon alloy with about 1 to atom percent hafnium and about 99 to 85 atom percent silicon. Preferably the composition contains 7.5 atom percent hafnium and 92.5 percent silicon.

Our invention relates to contact structures for use as contact pieces and contact bridges in thermoelectric devices, preferably thermogenerators, in which these structures are bonded to thermocouple legs made of semicon ducting silicon alloy such as silicon-germanium alloy. In this respect, our invention is related to those described in the copending applications Ser. No. 599,934, filed Dec. 7, 1966, Pat. No. 3,441,812 and Ser. No. 556,211, filed June 8, 1966, both assigned to the assignee of the present invention.

In such thermoelectric devices, the contact structure may serve as a terminal for attaching an electric lead to a thermocouple leg, or it may form a contact bridge between the two thermoelectrically different legs of each couple. Since, as a rule, many such couples are combined to a battery, the contact structure should have approximately the same thermal coefficient of expansion as the semiconducting silicon alloy of which the legs are made so that a variation in temperature does not cause excessive mechanical tension. The contact structure is also supposed to have good mechanical qualities, particularly great strength. It should be resistant to chemically aggressive atmospheres. When using the contact structure as a contact bridge within a thermogenerator, it is also desirable that the bridge structure have good electrical and thermal conductivity since the thermogenerator efficiency depends upon these qualities.

It is an object of our invention to devise a thermoelectric device, particularly a thermogenerator, whose contact structures, used in the above-described manner conjointly satisfy the various desiderata just mentioned.

We have discovered that this is achieved if the contact structure to be bonded to a thermocouple leg of semiconducting silicon alloy is formed of a hafnium-silicon alloy in which the mixing ratio of hafnium and silicon corresponds at least approximately to a eutectic or dystectic, composition. A dystectic composition has those proportions of two or more metals which produce the highest constant melting point, and a eutectic composition has those proportions of two or more metals which produce the lowest constant melting point.

A mixing ratio of this type is particularly favorable for the purpose of the invention because it affords obtaining and using the material of the contact structure either in form of a microscopic mixture of the pure crystals of the respective two components or as a stoichiometric compound. Dystectic compositions are additionally of advantage if the contact structure is to have "ice a particularly high melting point. As a rule, the electrical and thermal conductivity of the alloy is decisive in the selection of the mixing ratio preferred for a particular contact piece or contact bridge. That is, both the electrical and the thermal conductivity should be as high as feasible. From these viewpoints and in accordance with our invention, the hafnium-silicon alloy of which the contact structure is constituted consists of the composition Hf Si with 0 x 0.l5. That is, the composition is formed substantially of hafnium in an amount ranging from small but effective traces, such as about 1 atom percent, up to 15 atom percent, the balance being 99 to atom percent silicon. Particularly well suitable within this range is the alloy composition Hf ,,,si which has been found to exhibit particularly good electrical and thermal conductivity values.

A contact piece or contact bridge according to the invention not only satisfies all of the above-mentioned desiderata but also offers other advantageous properties. It has a high stability with respect to temperature and changes in temperature. Due to its metallic component, it also exhibits great hardness and great breaking strength. This makes the contact structures particularly Well suitable for thermogenerators, i.e. thermocouple batteries which directly convert heat into electricity. Tests have shown that the contact junctions can readily be operated at temperatures up to more than 1000 C. so that thermogenerators equipped with such contact structures attain a better efliciency than those having contact structures of materials as heretofore available. It has also been found that the hafnium contained in the contact structure according to the invention does not have a detrimental doping effect upon the germanium-silicon alloy of which the thermocouple legs are constituted.

It should be recognized that contact pieces and bridges according to the invention contain a component of the semiconductor material of which the legs are made. For that reason, the semiconductor body of a leg can be directly alloyed onto a contact structure according to the invention without any discernible impairment of the electrical semiconductor properties.

For contacting the semiconductor body of the leg with a contact piece or bridge according to the invention, the semiconductor body of the leg need only be placed upon the contact area which for this purpose is provided on' the contact structure, and can then be joined with the structure by fusing the leg onto the structure.

The invention will be further described with reference to an embodiment of a thermogenerator illustrated in section and by way of example on the accompanying drawing.

The illustrated thermogenerator is equipped with a heat exchanger 1 for a gaseous medium at the hot contact junction side of the thermocouple battery, and with a heat exchanger 2 to be traversed by a liquid medium at the cold junction side of the battery. The visible portion of the thermogenerator battery comprises two thermocouple legs 3 and 4 which are both made of germanium-silicon alloy. One of the legs is doped for p-type conductivity by having a corresponding dopant content of boron, gallium or indium, for example. The other leg is doped for n-type conductivity, for example with phosphorus, arsenic or antimony.

While we prefer forming the thermocouple legs of semiconducting germanium-silicon, other semiconducting silicon alloys are also applicable, for example an ironsilicon alloy.

Two contact pieces 6, 7 and a bridge member 5 which interconnects the two legs 3 and 4 are made of the abovedescribed hafnium-silicon alloy. They are isolated from the respective heat exchangers by an electrically insulating but thermally conducting layer 9 or 10, such as a coating of aluminum oxide. The contact pieces 6 and 7 are joined with electrical cables 11 and 12. For a suitable method of producing the silicon-oxide coatings 9, 10 reference may be had to the above-mentioned copending application Ser. No. 556,211.

The contact bonding is effected in the following manner. The contact pieces 6, 7 and the bridge 5 of hafniumsilicon alloy are first separately produced in the appropriate shapes. Then the thermocouple legs are placed upon the contact areas 8 which for this purpose are provided on the contact structures, these areas being properly prepared, for example by grinding. Thereafter, the legs 3' and 4 are fused onto the contact structures or are soldered thereto.

To those skilled in the art, it Will be obvious upon a study of this disclosure that our invention permits of different variants with respect to the compositions of the thermocouple legs and contact structures as well as with respect to the particular design and shape of the thermoelectric device in which the contact structures of hafniumsilicon alloy are to be used, thus being amenable to embodiments other than particularly illustrated and described herein, without departing from the essential features of our invention and within the scope of the claims annexed hereto.

We claim:

1. In a thermoelectric device having thermocouple legs of semiconducting silicon alloy and an electrical contact bonded to said legs, said contact being formed of a hafnium-silicon alloy whose hafnium-silicon mixing ratio corresponds substantially to a eutectic or dystectic composition.

2. An electrical contact for a thermoelectric device according to claim 1, containing from about 1 atom percent to 15 atom percent hafnium, the remainder being substantially all of silicon.

3. In a thermoelectric device having thermocouple legs of semiconducting silicon alloy and an electrical contact bonded to said legs, said contact being formed of a hafnium-silicon alloy having substantially the composition Hf Si wherein x is greater than zero and less than 0.15.

4. An electrical contact for a thermoelectIic device according to claim 3 having substantially the compoistion rJ.0'I5 o.925-

5. In a thermoelectric device according to claim 3, including said silicon-hafnium contact having an area fusion-bonded with one of said thermocouple legs.

References Cited UNITED STATES PATENTS 2,458,013 l/1949 Malmberg 317-241 2,831,787 4/1958 Emeis 317235 3,279,954 10/1966 Cody et a1 l36205 2,826,725 3/1958 Roberts 3l7--238 JAMES D. KALLAM, Primary Examiner US. Cl. X.R.

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