US2366881A - Thermoelectric alloys - Google Patents

Description

Jan. 9, 1945. M, IELKES 2,366,881

TERMOELECTRLC ALLOY Filed March 51, 1943 2 Sheets-Sheet l 5 FL' .1 J7?, .2 I9 ,.9

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I I I I 2 4- 6 8 ADDED TIN PER CENT I l l l 2 4 6 8 AUDED TIN PER CENT Inv into?" l/lf A v.

SPECIFIC HEAT oNDpcTIvITY IN vyATTTs/ Jan. 9, 1945. M. TELKES THERMOELECTRIC ALLOY 2 Sheets-Sheet 2 Filed March 3l, 1943 n n s .W n 2. o I 1D S n Z U m 7 6 5 4 3 2 i 0 og w. w w D. M w. w .Zuzro z Czfw Q zbmmm n n S S i 2 1 1J n O .0 5. n 1 Z D l 3 2 J1 0 ADDED SILVER PER CENT Fly. 7

ADDED SILVER PER CENT iSn ADDED SILVER PER CENT Patented Jan. 9, 1945 THERMOELECTRIC ALLOYS Maria Telkes, Cambridge, Mass., assignor to Research Corporation, New York, N. Y., a corporation of New York Application March 31, 1943, Serial No. 481,243

(ci. 'z5- 149) 6 Claims.

Thepresent invention relates by thermoelectric alloys for use in thermocouples by whi'chthermal energy may be. converted into electrical energy.

When thermocouples are operated under identical conditions, with xed hot and cold junction temperature, their efficiency is in first approximation proportional to Pac. Ka a.

Where e is the thermoelectric power in microvolts per degree centigrade; psv is the average specic resistance at `the operating temperature interval (ohm/cm); and Kev. is the average specic heat conductivity at the operating temperature interval (watt./ C./cm. Thus, while it is desirable for high conversion eciency that the thermoelectric power (e) be as high as possible, it is necessary also to take account of both the Specific resistance and the heat conductivity of an alloy before a true estimate of its relative efficiency can be made.

It is likewise necessary, as a practical matter, to take into account the physical properties of an alloy, since many thermoelectric alloys are incapable of utilization for various reasons, such as physical instability, low melting point, and especially excessive brittleness. For certain applications it may be more important that the alloy have low brittleness than that the conversion eflciency be as high as possible.

It has long been known that alloys of zinc and antimony possesses relatively high thermoelectric power. The' composition generally consists of 33% to 38% zinc and the balance antimony, the intermetallic compounds ZnSb containing 35% zinc, balance antimony. Such alloy is, however, exceedingly brittle, so as to have but limited usefulness.

Other zinc-antimony alloys, such as ZmSba and ZnaSbz, exhibit high thermoelectric power. but these, in addition to extreme brittleness, are physically unstable, due to a thermolytic migration of excess zinc which gradually takes place in the direction of the temperature gradient and which brings about a substantial decline in the eillciency of the alloy.

Numerous attempts have been made in the past to improve the zinc-antimony alloys by additional agents, but without -apparent success. In several instances, the benecial results of various added agents have been mentioned, but

higher conversion eiciencies than alloys heretofore available.

It is likewise an object to provide thermoelectric alloys of high conversion eiciency, characterized by low brittleness and physical stability, so as to permit utilization in special applications such as require thermocouples ofminute size and relative durability. y

I have attained these objects through the addition to a zincantimony alloy of the approximate composition 35% Zn, 65% antimony, of certain elements in accurately controlled' amounts. Not only has it become possible to attain useful operating efficiencies of the order of 5 per cent., but, in addition, high efficiency alloys can now be made having high physical stability and low brittleness. Y

The elements which have been found to effect the greatest improvement in a Zn-Sb alloyare' tin and silver. The tin alone is highly effective' when used in the. Zn-Sb alloy, both to improve the conversion eciency and to diminish the brittleness, while tin and silver together in suitable small amounts bring about an even greater increase in eiciency over the basis alloy of zinc and antimony, along with diminished brittleness.

The manner in which the various characteristics of the ZnSb alloy are modified by the addition of tin, and by tin and silver together, is j illustrated in the accompanying drawings, in which Figures 1-4 show the effect on efciency, specic resistance, thermoelectric power and specic heat conductivity, respectively, of adding tin to a ZnSb alloy, while Figures 5-8 are plots ofy said characteristics in terms of added silver in ZnSb alloys containing 1% and 2% tin, respectively. The values giyen for thermoelectric power are with reference to Copel (an alloy consisting of 45% Ni and 55% Cu), while the conversion eliiciencies are based on the use of Copel as the negative element of the couple.

In all cases the basis alloy to which the tin, or tin and silver, is added is the intermetallic compound ZnSb, composed of zinc and antimony in the approximate ratio 35% zinc and 65% antimony. Where the term basis alloy is ernployed, it is an alloy consisting of these components in substantially the stated proportions that is comprehended.

From an examination of Figure 1, it is apparent that vonly slight amounts of added tin are improvement is due to the increase in thermoelectric power and decrease in specic resistance which result when up to 2% tin is added. Beyond this point, there is no further increase in thermoelectric power, and the eiciency falls oil, due to the sharp increase in .heat conductivity. However, for applications requiring still lower brittleness of alloy than one having 2% added tin, such as thermocouples for temperature measurement, and particularly radiometers requiring couples of extremely minute size and consequent physical delicacy, substantially greater amounts of tin may be added while still providing relatively high efficiency, since for such applications low brittleness is essential, and in addition, the resulting decrease in specific resistance is generally beneficial.

Starting with an eciency of slightly over 2.5 for ZnSb alone, it is seen that by the addition of 1% to 2% tin the efciency may be raised to values over 4%. Still higher efficiencies are obtainable if small amounts, less than 0.5%, of silver are added to the basis alloy containing in addition one or two per cent. tin. Figure 5 indicates that 1% tin and 0.1% silver give the highest efficiencies, nearly 5%, but again requirements of fabrication, ruggedness, and the like may dictate somewhat larger amounts of tin. Figures 6, 7

and 8 indicate that the effect of the added silver is to decrease the specific resistance in the case ofthe basis alloy plus 1% added tin, though at the 4 expense of somewhat decreased thermoelectric power. With 0.1% to 0.2% added silver, there is but little diierence between alloys which contain 1% tin and those containing 2% tin, the alloy containing 2% tin being less brittle, however.

Attainment of high conversion efiiciencies requires that certain precautionsbe observed. For example, the formation of oxides during melting and `casting of the alloy must be avoided as far as possible. Where vacuum casting is impractical, the use of a graphite crucible, with the alloys prepared under a. layer of charcoal, is desirable. To avoid contact diiliculties resulting from oxidation at the hot junction, it is essential that an absolute minimum of solder be employed. Any excess solder will result in diiusion of tin into the alloy, and unless the amount of tin is small, the thermoelectric power will be changed.

I claim:

1. A thermoelectric alloy comprising zinc and antimony in substantially the ratio 35% zinc. antimony, the zinc and antimony constituting at least approximately of the alloy, and the balance consisting substantially of tin.

2. A thermoelectric alloy comprising zinc and antimony in substantially the` ratio 35% zinc, 65% antimony, the zinc and antimony constituting the major portion of the alloy, and the balance of the alloy consisting substantially of tin in an amount between approximately 1% and 4%.

3. A thermoelectric alloy comprising zinc and antimony in substantially the ratio 35% zinc, 65% antimony, the zinc and antimony constituting approximately 98% of the alloy, and including tin in the amount of approximately 2%.

4. A thermoelectric alloy consisting of zinc, an-

timony and tin, the zinc and antimony being pres.

ent in the approximate ratio 35% zinc, 65% antimony, and together constituting between 99% and 96% of the alloy.

5. A thermoelectric alloy comprising zinc and antimony in substantially the ratio 35% zinc, 65% antimony, the zinc and antimony constituting between 99% and 96% of the alloy, the balance comprising tin in an amount between 1 and 4% and silver in an amount less than 0.5%.

6. A thermoelectric alloy consisting of zinc, antimony, tin and silver, in the following proportions: tin approximately 2%, silver approximately 0.2%, and the balance zinc and antimony in the approximate ratio 35% zinc, 65% antimony.

MARIA TELKES.

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