Thermoelectric generators having partitioned self-segmenting thermoelectric legs

[57] ABSTRACT

Thermoelectric generators comprising thermoelectric legs that are partitioned into at least two longitudinally spaced sections that each consist essentially of thermoelectrically useful self-segmenting alloy compositions having the same ingredients. A barrier member is disposed between the two sections in electrical and thermal contact with the sections and prevents movement between sections of a metal element that does migrate within the sections.

Claims, 5 Drawing Figures

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3,873,370

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3,873,370

THERMOELECTRIC GENERATORS HAVING PARTITIONED SELF-SEGMENTING THERMOELECTRIC LEGS

The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

Hampl, Ser. No. 36,131, now abandoned in favor of Ser. No. 293,862, filed Oct. 2, 1972, describes thermoelectrically useful alloy compositions of metal and nonmetal elements in which atoms of the major metal element undergo a significant beneficial movement in the presence of thermal and electrical gradients. For example, in a P-type thermoelectric leg consisting of about 65.5 atomic percent copper, 1 atomic percent silver, and 33.5 atomic percent selenium, copper atoms migrate from the hot end of the leg toward the cold end of the leg when the thermoelectric generator including the leg is operated to apply thermal and electrical gradients to the leg. A gradation of different amounts of copper develops in the leg as a result of this migration, with the most copper being at the cold end and the least at the hot end of the leg. This gradation is beneficial for the thermoelectric conversion properties of the leg, since the lower the amount of copper present the higher will be the doping level—or the level of currentcarrying holes—in the composition, and since more current carriers are needed in the thermoelectric leg at positions closer to the hot end of the leg. A thermoelectric leg exhibiting this beneficial migration and gradation of atoms is said to be self-segmenting, since it automatically achieves the variation in level of current carriers that was previously obtained only by mechanically assembling discrete thermoelectric leg segments that included different levels of doping agent.

In general, the self-segmenting alloy compositions described in the above-identified application, which is incorporated herein by reference, are substantially single-phase compositions that consist essentially of metal and nonmetal elements united in a distinct crystal lattice structure that is nonstroichiometric because of an excess or deficiency of atoms of at least the major metal element of the crystal lattice structure. This excess or deficiency of atoms provides the current carriers needed for the composition. The compositions further have the property, discussed above, the atoms of the major metal element migrate from a first end of the leg toward the second end under the influence of combined thermal and electrical gradients applied to the leg, forming a gradation of inherently stable current carrier concentrations that is beneficial for thermoelectric conversion.

While the described gradation of current carriers provides improved thermoelectric conversion properties as described above, such a gradation has also been found, through the present invention, to be responsible for certain problems that limit usefulness of thermoelectric legs experiencing the gradation. By the present invention new thermoelectric structures have been developed that control the migration and as a result increase the utility for thermoelectric conversion purpose of the compositions that exhibit the migration.

One problem overcome by the present invention was a tendency for the hot end of a copper-silver-selenium leg as described above to undergo creep-deformation after a period of sustained power-generating operation, with the leg under longitudinal pressure in a pressurecontact arrangement and with the hot end of the leg heated to a temperature on the order of 800°C or greater. For example, after 100 hours of such operation, the diameter of the hot end of the leg might increase by as much as 15 percent. The problem could be avoided by not heating the hot end of the leg to the described temperature, but the result of such a procedure would be a reduced efficiency of power-generation by the leg.

It has been found that this creep-deformation occurs because the described migration of copper toward the cold end of the leg causes the hot end of the leg to have a higher proportion of selenium than its overall proportion in the leg. And it has been found that the greater the selenium in the composition, the more susceptible is the composition to a creep-deformation. The present invention overcomes this creep-deformation problem by reducing the amount of selenium at the hot end.

Another problem that has been found to arise as a result of the described gradation is loss of material from the hot end of a leg experiencing the gradation. While a copper-silver-selenium thermoelectric leg as described above has a very low vapor pressure in isothermal tests, surprisingly, when the leg is operated at matched load in a thermoelectric generator with the hot end heated to 800°C or higher, there is a significant loss of selenium from the hot end of the leg, which causes the operation of the leg to be unstable. Again, it has now been found that this loss of selenium can be traced to the migration of copper atoms in the composition which increases the proportion of selenium at the hot end, and the problem is again overcome by reducing the amount of selenium at the hot end of the leg.

A thermoelectric leg of the present invention that overcomes these problems may be briefly described as (1) consisting essentially of at least two longitudinally separated full-transverse-area sections that each consist essentially of a self-segmenting alloy composition as described above that includes the same metal and nonmetal elements, and (2) a barrier member that is disposed between the two sections, extends over at least the whole transverse area of the leg, is in compatible, low-resistance, electrical and thermal contact with each of the two sections, has good electrical conductivity, and prevents migration of said migrating metal element between the sections.

For purposes of brevity, thermoelectric legs having this structure are described herein as “partitioned” thermoelectric legs. It should be noted that such “partitioned” thermoelectric legs are distinguished from thermoelectric legs known in the art as “segmented” legs in that in partitioned legs the sections consist essentially of the same elemnts. Whenever barrier members have been used for segmented legs, there has been some difference in composition between the segments, and the barrier members were used only because of that difference, partitioning is used for a different purpose than segmenting and is used only on the special class of legs that are self-segmenting to prevent migration between sections of the migrating element that provides the self-segmenting.

For a thermoelectric self-segmenting leg that is not partitioned, the carrier concentration varies continuously from cold end to hot end when the leg is operated to generate electric power, with the net increase in carrier concentration being determined by the operating conditions (that is, temperature interval, current, and geometry of the leg). The variation in carrier concen3,873,370 tration over the length of a partitioned self-segmenting thermoelectric leg is interrupted at the barrier member, so that the net increase in carrier concentration from the cold end of the leg to the hot end is less than it is for an unpartitioned thermoelectric leg under the same operating conditions. To illustrate, the carrier concentrations in unpartitioned and partitioned selfsegmenting thermoelectric legs are shown schematically in FIGS. 1 and 2 respectively.

In these FIGS, values on the ordinate represent positions in the leg, a representing the cold end of the leg, b representing the hot end, and c representing a position in the middle of the leg. Values on the abscissa represent carrier concentration within the leg. The dotted lines indicate the carrier concentration that would exist over the whole length of the leg when no thermal and electrical gradients are applied to the leg. The solid lines represent the carrier concentration that exists over the length of the legs when the legs are operated under identical operating conditions. Because of the interruption produced by a barrier member in the leg of FIG. 2, the amount of the increase in carrier concentration at the hot end of the leg above the average current carrier concentration in the leg (that is, the distance between the dotted line and the end of the solid line at b) is about one-half as great as the amount of the increase in carrier concentration at the point b of the leg represented in FIG. 1.

A second situation is illustrated in FIGS. 3 and 4, which show schematically the carrier concentration of unpartitioned and partitioned self-segmenting thermoelectric legs that are operated with the cold end of the unpartitioned leg, and the cold end of each of the two sections of the partitioned leg, fixed at a two-phase boundary as described in the above-identified application, Ser. No. 36,131. The dashed lines in these figures represent the carrier concentration of material in which the migrating metal element is at its maximum solubility limit in the composition, and the solid lines again represent the carrier concentration that exists over the length of the legs when the legs are operated under identical operating conditions. During operation, only the cold end of the unpartitioned leg of FIG. 3 and the cold ends of the two sections of the partitioned leg of FIG. 4 retain a composition in which the migrating metal element is at its maximum.solubility limit in the composition. And, because of the interruptionn produced by the barrier member, the increase in carrier concentration at the hot end of the partitioned leg above the carrier concentration represented by the dashed line is about one-half as great as the increase in carrier concentration for the unpartitioned leg.

Thus, for the copper-silver-selenium composition discussed above, there is a greater proportion of copper at the hot end of a partitioned leg than there would be if the leg were not partitioned. As described above, it is this greater porportion of copper at the hot end of the partitioned copper-silver-selenium leg, and correspondingly the lesser proportion of selenium, that accounts for the improvement in creep-deformation properties and reduction in loss of selenium at the hot end of the leg.

DETAILED DESCRIPTION

FIG. 5 of the drawing shows an illustrative partitioned thermoelectric leg 10 of the invention. As shown, the leg 10 consists of two logitudinally separated, full-transverse-area sections 11 and 12 (meaning that the sections have the same transverse area as the leg), and a barrier member 13 located between the two sections and in compatible, low-resistance, electrical 5 and thermal contact with the two sections. The barrier member extends at least over the whole transverse area of the leg between the sections. While the drawing shows a thermoelectric leg partitioned into two sections, thermoelectric legs of the invention may also be partitioned into a greater number of sections to provide greater control over the distribution of carriers in the leg when operated.

Generally, the self-segmenting compositions useful in the present invention are alloy compositions of a metal and chalcogen (tellurium, selenium, sulfur, and oxygen), with the metal generally being selected from copper, silver, rare-earth metals, and transition metals. In practice, the invention will be utilized only with compositions that have good values for such thermoelectric conversion parameters as Seebeck coefficient, resistivity, and thermal conductivity. As determined by traditional temperature-dependent measurements of the Seebeck coefficient, resistivity, and thermal conductivity (which do not reflect the beneficial results of selfsegmenting), compositions useful in the present invention will generally exhibit a figure of merit of at least 0.5 x IO'³.

The most preferred P-type compositions are compositions of copper, silver, tellurium, and selenium as de³⁰ scribed in an earlier application of Hampl, Ser. No. 635,948, which is incorporated herein by reference and which has been abandoned in favor of Ser. No. 321,222 filed Jan. 5, 1973. Briefly summarizing, those compositions include ingredients in the following proportions:

for tellurium compositions,

<td> 32.5</td><td> atomic percent</td><td> Te</td><td> S 33.7</td>

<td rowspan="2"> 40</td><td> atomic percent g</td><td> Cu</td><td> S 67</td>

<td> atomic percent =5</td><td> Ag</td><td> §40</td>

<td colspan="2"> and for selenium compositions,</td><td></td><td rowspan="2"> §33.7</td>

<td> 32.5</td><td> atomic percent s</td><td> Se</td>

<td> 60</td><td> atomic percent</td><td> Cu</td><td> §67</td>

<td> 0</td><td> atomic percent</td><td> Ag</td><td> § 7</td>

atomic percent atomic percent atomic percent, atomic percent atomic percent atomic percent.

⁴⁵ Within these ranges are compositions that may be cast into thermoelectric legs to form dense, uniform, continuous structures that exist in preferred substantially single-phase crystal forms when heated above a temperature that ranges between 95°C and 575°C, depend⁵⁰ ing on the particular composition; especially in these high-temperature crystal forms, the compositions have very excellent thermoelectric conversion properties. The best compositions include 33.2 to 33.5 atomic percent tellurium or selenium, preferably about the latter ⁵⁵ amount. Copper-silver-selenium and copper-silvertellurium compositions that include about one atomic percent silver and copper-silver-tellurium compositions between about 32 and 36 atomic percent silver are also especially preferred.

N-type compositions of the copper-silver-chalcogen family are also useful in the present invention. The best combination of high-temperature utility and good thermoelectric conversion properties are found with com₆₅ positions that principally comprise silver, selenium, and tellurium but also include up to about 5 atomic percent copper and sulfur. The silver and copper generally comprise between about 65.7 and 67.7 atomic percent

3,873,370 of the composition, and the silver, tellurium, and selenium lie within the following ranges;

60.7 atomic percent atomic percent atomic percent = Ag si 67.7 atomic percent

Te £ 30 atomic percent = Se g 24 atomic percent.

Generally, the barrier member in a thermoelectric leg of the invention takes the form of a thin (preferably less than about 20 mils and more preferably less than about 5 mils thick) foil or a laminate of two or more foils or layers. While it should extend over the whole transverse area of the leg, it may also extend beyond the sides of the leg.

The barrier member should be chemically compatible with the materials of the sections against which it is placed. In addition, while the barrier member should prevent migration of the migrating metal element between sections, it should have good electrical conductivity (that is, it should have an electrical resistivity less than about 10 milliohm-centimeters). Also, the barrier member should not disturb the migration within the sections, and it should not have any other significant deleterious effect on the thermoelectric conversion performance characteristics within the section. For example, free copper should not be the sole constituent of a barrier member for a P-type copper-silverselenium leg, since if copper at its free-state chemical potential is available at the hot end of a section, the copper atoms will migrate through the section in the presence of a thermal and electrical gradient and prevent stable operation of the leg.

On the other hand, a layer of copper can comprise a layer on the hot-end side only of a barrier member (disposed against the cold-end surface of a section) for a P-type copper-silver-selenium leg, if the remaining part of the barrier member comprises a layer that prevents migration of the copper atoms into the cold-end section of the leg. In fact, it is desirable for copper to be present at the cold-end section of P-type copper-silverselenium, since, as described in the earlier application, Ser. No. 36,131, such an arrangement provides for immediate stable operation of the section.

The useful barrier materials will vary widely with the materials of the sections. Some useful barrier materials are tungsten, tungsten-rhenium alloys, molybdenum, columbium, platinum, copper (as described above, for example), and carbon, and liminated assemblies of layers of such materials. Tungsten and tungsten-rhenium are preferred materials for copper-silver-selenium legs, and can be used as the sole material of the barrier member for such legs.

For lowest electrical resistance, a barrier member is bonded to adjacent sections of the leg. One way of bonding adjacent sections to a barrier member is by casting the sections in place against the barrier member Pressure-contacting of a barrier member to adjacent sections may also be used.

EXAMPLE 1

A partitioned thermoelectric leg of the invention was prepared in a casting mold having three sections of slightly different diameter, the smallest at the bottom, a slightly larger section (by approximately 0.01 inch) starting at 0.133 inch from the bottom of the mold, and another larger section (by approximately 0.010 inch) starting at 0.276 inch from the bottom of the mold. The average diameter of the whole mold was about onefourth inch. Tungsten-rhenium discs were placed on the steps in the mold formed by the changes in diameter. A thermoelectric leg having a composition of 65.57 atomic percent copper 1 atomic percent silver, and 33.43 atomic percent selenium was inserted into the reservoir of the mold, and heated to a molten condition by heating the mold to 1,180°C. There were feed holes connecting the reservoir with each mold section to allow the molten thermoelectric composition to flow into the mold. The partitioned element which resulted was removed and a copper-silver eutectic cold-junction electrode was attached.

The total length of the partitioned leg was approximately 0.430 inch. The leg was placed on test in a temperature interval of 800/270°C in an atmosphere of argon at 20 pounds per square inch, a current near matched load, and a longitudinal pressure of 150 pounds per square inch. The leg was operated for 3,200 hours and displayed an average Seebeck Coefficient of 289 microvolts/°C relative to platinum, and a resistivity of 11.99 milliohm-centimeters. The resistivity indicates an extraneous resistance contributed by the barrier member of only about 5.2 percent. No creepdeformation occurred during this test.

EXAMPLE 2

A one-fourth-inch diameter thermoelectric leg hav3Q ing the same composition as Example 1 was sawed into three sections, having lengths, respectively, of 0.230 inch, 0.088 inch, and 0.065 inch. A stack was made, using the longest section at the bottom, then 0.001ir.ch-thick tungsten foil, then the next-longest section, 35 then a Ο.ΟΟΓ-inch-thick tungsten foil, and then the shortest section. The resulting partitioned thermoelectric leg was tested in a vacuum of approximately 10'“ torr, a temperature interval of l,000^o/150°C, and under a longitudinal pressure of 50 psi for 110 hours. The av40 erage Seebeck Coefficient was 250 microvolts/°C relative to platinum and the resistivity was 8.45 milliohmcentimeters. There was a slight creep-deformation, suggesting that a greater number of barrier members than three may be needed for operation at 1,000°C.

EXAMPLE 3

A thermoelectric leg consisting of 65.57 atomic percent copper, 1 atomic percent silver, and 33.43 atomic percent selenium approximately one-fourth inch in di50 ameter was sawed into two sections having lengths 0.10 inch and 0.30 inch respectively. A stack was made composed of, from the bottom, the longer section, a 0.005-inch-thick tungsten disc 0.255 inch in diameter, a 0.005-inch-thick platinum disc 0.25 inch in diameter, 55 and the shorter section. This stack was tested in a vacuum of 10^-6 torr, a temperature interval of 800/200°C, and a longitudinal pressure of 300 psi for 5,200 hours. The average Seebeck Coefficient was 275 microvolts/°C relative to platinum and the average resis⁶⁰ tivity was 10.05 milliohm-centimeters. This represents an extraneous resistance of 0.2 percent. There was a slight creep-deformation, which indicated that additional barrier members might be needed for operation 65 in the described temperature interval.