DE1533328B1 - ALLOY MADE OF GE-TE AND AG-SB-TE TIEF 2 - Google Patents

Description

The present invention relates to an alloy consisting of _{GeTe and AgSbTe 2.}

A limited number of compositions or alloys covered by the formula

(AgSbTe ₂ ) ^ _x + (GeTe) _x .

are previously known and have been proposed for use as thermocouples in thermoelectric devices. In the formula, χ is equal to the parts with a molar fraction. However, as far as is known, none of these alloys have been used in commercial establishments. In technical reports it is customary to attribute the commercial non-application to the fact that the known alloys of the type described above are not superior or even inferior in their thermoelectric performance and their mechanical properties to the known lead telluride thermocouples, which are used in approximately the same temperature range.

Thermoelectric converters have had numerous applications on the in recent years Caused a stir on earth and in space. As this is according to the invention by means of an alloy reached that out

_ao 85 mole percent GeTe and

15 mol percent AgSbTe ₂ or 80 mol percent GeTe and 20 mol percent AgSbTe ₂ .

Advantageous uses of this alloy are characterized in the subclaims.

The alloys obtained by this invention belong to the same kind of almost degenerate Semiconductors, such as the related known alloy rods, namely to the p-conductors.

The properties of the alloys according to the invention were with the most analogous, so far known alloys compared to the substantial advance in the prior art by the to illustrate the present invention. In each case, special care was taken to ensure that that only recognized and established assessment procedures and standards were used and that all Samples were examined by the same procedure.

The values given here were determined in all cases by determining the mean value from at least four samples and in those cases in which GeTe represented with a proportion of 80 to 85% of the alloy was determined by determining the mean value from 12 or 38 samples. The schematic representation a thermoelectric device and the comparative assessment of the invention in relation to on known devices is included in the drawings. It shows

Fig. 1 the schematic representation (sectional view) a thermoelectric device for converting thermal energy into electrical energy by using the Seebeck effect,

2 shows the graphic representation of a comparison of the Seebeck coefficient (<x) for the most closely related known alloys and the alloys produced according to this invention at temperatures above the normal temperature gradient for thermoelectric devices and using

The consequences of this are government agencies and industry substantial funds for research and development of the alloys of the invention for thermal winding purposes expended with the aim of creating suitable elements,

Neten replacement for the lead telluride thermocouples to F i g. 3 is a graphic representation analogous to FIG

find, and in the hope of finding new elements with better- Fig. 2, but based on the specific cons

to discover their thermoelectric and mechanical properties and for the same temperature range. Despite these intensive efforts in the field of thermoelectrics, lead telluride remains the substance with the best electrical properties (ρ),

F i g. 4 shows a graphic representation analogous to FIG. 2, but based on the thermal conductivity (K),

F i g. 5 is a graphic representation analogous to FIG

Fig. 2, which relates to the efficiency ZT . Here T = ⁰ K and

Z =

K is equal to the thermal conductivity in watt / ° K cm, FIG. 6 shows a graph of the efficiency (E) of the energy conversion of thermoelectric devices when using an alloy according to the present invention or conventional lead telluride alloys with sodium as the p-conductor. In all cases, commercially available lead telluride with lead iodide was used as the η conductor. The calculations were carried out in each case according to the known formula for the efficiency, namely

E =

Tn

yi + ζτ - -ι

^: τ7

] / l + ZT +

Tn

In the above formula, T _n = temperature on the hot side, T _c = temperature on the cold side and ~ ZT - average efficiency of the tested thermocouple, determined from the entire temperature range.

The ZT values given in FIG. 5 for 100% GeTe are known and, given the current state of the art, are well founded. You were the book "Thermoelectricity: Science and Technology" by R. R. Heikes and R. W. Ure jr. (P. 435), extract, which was published in 1961 by Interscience Publishers.

Supplementing given in the graph in Fig. 2 to 5 data, Table I shows the characteristics of above figures, on the basis of a temperature of 75O ⁰ K. Table I also contains information on the thermoelectric properties of hypothetical substances of stoichiometric composition, while not in FIG. 2 to 5 were recorded, but are consistent with the characteristics of the graphical representations shown there.

Table I Thermoelectric properties at 750 ° K

VoGeTe AgSbTe ₂ α (μν / ° Κ) f (mQon) K (mW / cm- ° K) ZT 50 50 228 4.00 24.3 0.39 75 25th 204 1.72 21.5 0.85 80 20th 219 1.76 13.4 1.52 83 * 17th 197 1.57 17.7 1.04 83Va 16Vs 184 1,3.5 85 * 15th 198 , 1.19 17.3 1.43 85V ₇ 14Vt 194 1.12 21.9 1.16 87 * 13th 167 0.87 20.0 1.25 87V ₂ 12V » 168 0.95 90 10 177 0.82 23.0 1.24

* non-stoichiometric composition

Since the above-mentioned related alloys are not available in order to make a comparison of the alloys according to the invention with the already known alloys, all samples had to be prepared. Initially, attempts were made to manufacture thermocouples by cold processing and subsequent sintering. It was found, however, that this method gave completely unsatisfactory results in terms of mechanical consistency, ie the samples cracked and crumbled during the pressing process. So was z. B. a series of samples of finely divided particles of (AgSbTe ₂ ) _{0> 15} (GeTe) ₀₈₅ and in a size conventionally used for cold pressing and sintering, first ^{pressed with a series of pressures between 4000 and 9000 kp / cm 2} and then for a period of Sintered for 4 hours in an argon atmosphere at 582, 604 or 627 ° C. The sintered material showed the above-mentioned defects in each case.

During the tests it turned out that this problem, especially in the case of the present Invention composed new alloys can be solved by using a Gußverf ahrens can. For this purpose, the alloys were prepared in such a way that the four elements to be alloyed were in the desired correct quantities were placed in a Vycor tube, which was then evacuated and melted shut became. The tube was then heated in a shaker oven at 750 ° C for 1 hour; H. until a Melt forms. The tubes were allowed to cool slowly to about 600 ° C. and held at that temperature for a period of 16 hours. The tubes were then allowed to cool to room temperature.

In addition, part of this casting material was further treated by crushing it into finely divided particles, such as by grinding and sieving with a sieve size of -35, cold pressing at 4000 kp / cm ² and subsequent hot pressing at 525 ° C. and 350 kp / cm ² for the duration of about 30 minutes to make a p-type thermocouple. Essentially, the cast samples showed the same thermoelectric properties as the samples that were further treated.

The thermoelectric materials that are the subject of the present invention were called "alloys" denotes, because they represent solutions in a solid aggregate state, no indications of a chemical bond can be determined, and because stoichiometric compositions of not stoichiometric seem to differ only by the concentration of the constituents. Basically the microanalysis shows a grid of

Ag — Sb — Ge — Te with a small one over the whole of Fig. 6 is characteristic because it has the

Sample distributed AgTe portion. In the case of samples with an exceptionally high thermoelectric conversion of germanium content, a lower Ge phase development of these alloys can be present in conventional operating conditions, which expresses temperatures. So z. B. One would assume that such alloys, 5 at a temperature of 428 ° C (900 ⁰ F) to the other than by the formation of an actual molten hot junction (measuring point) and a temperature be not feasible, because otherwise a Suspen - from 38 ⁰ C (100 ⁰ F) at the cold soldering point (version of the higher melting metals or alloy non-equality) the thermoelectric power in the lower melting metals or approximately 10%. The efficiency is so significant alloys would be obtained. X-ray diffraction examinations showed that in conventional thermoelectric devices, substances with a GeTe content of 90% or more achieved 4 to 8%.

a rhombohedral crystal structure with a lattice- In view of the known shortcomings

have an angle of almost 90 °, while substances of thermocouples conventionally together with with a GeTe content of 75% or less, 15 settlements of Ag, Sb, Te and Ge were samples of the have face-centered cubic crystal structure. Alloys according to the invention produced after Both crystal structures were comparative tests in substances with a specified method GeTe content between 75 and 90% observed. So samples of p-type lead telluride of the same size with there is reason to believe that subjected to attainment and shape. The lead telluride was from the presence of both criminals for optimal results stall structure types are required. Nevertheless, the cold pressing and soft manufacturing of the thermocouples applies the optimal ZT values from the mean observation sintering process.

worth ZT from and refer to the fact that the samples were

For optimal results, the two types of crystal, even when subjected to compressive stress tests, must be present in unequal amounts, as can be seen from these test results, peaks at around 80 and 85% GeTe, respectively, which illustrate a considerable difference. It should be noted that there are ZT values between the compressive strength of lead telluride on the one hand and 80 and 85 mol percent GeTe, which are below the ZT new alloy with a GeTe proportion of 85% values based on the previous status on the other . All four lead telluride samples broke, the technique can be derived. The double 3 ° when a pressure between 850 and peak formation of the ΖΓ-curve is an extremely un- 1000 kp / cm ² acted on the samples. The one from the inventive appearance. Examination of the alloy according Legie- manufactured by hot pressing with stanchions 85 and 80%, GeTe-share among the samples held on the other hand this Druckbeanspruchun electron microscope show that the former was Legie- gene showed only tion at about 1000 to 2000 kgf / cm ² a less ordered crystal structure than 35 has minor chipping on the edges and the latter. broke only when loaded with about 1400 bis

Preliminary studies indicate that a 2500 kp / cm ² . Cast samples of the inventive shrinkage of the crystal lattice in direct alloy showed flaking at about 1300 to near the area with a GeTe content of 1500 kp / cm ^2. Complete destruction occurred 85%, which to some extent the 4 ° occurs at around 1400 to 1600 kp / cm ² . This explains the extraordinary mechanical stability of the alloy's compressive strength of the new alloy about twice as much. as large as that of lead telluride.

The graphs in Figures 2, 3 and 4 samples of the two fabrics have also been made after

are mainly used for a better understanding of the known methods of impact resistance test graphs in FIGS. 5 and 6. Therefore 45 subjected. The new, formed by hot pressing, Figs. 2, 3 and 4 are not explained in more detail here. Alloy showed after ten blows on each test piece. The graph of the efficiency ling no effect, while the cast element. H. the generally accepted evaluation of thermometers made of the same material after six blows of electrical power in FIG. 5, illustrated were destroyed. In contrast, all clearly the peaks of the thermoelectric effect 5 ° samples made of lead telluride showed cracks in the first degree in the two areas of 80% GeTe to 20% impact stress and broke in AgSbTe ₂ and 85% GeTe up to 15% AgSbTe ₂ . The second completely apart. The peak impact strength in the efficiency of these two tests thus again demonstrated a point clearly showing a clear and unexpected notable superiority of the new alloy .

Proportions of 50, 75, 90 and 100% were extrapolated. Attempts with other related groups

In addition, it should be noted that in the higher settlements from Ag, Sb, Ge and Te, considerable preferred operating range around 750 ° K resulted in the greatest mechanical damage to the elements in the case of peri-deviation in materials with a GeTe proportion of 60 or alternating thermal stress . In addition, 80 and 85 ^fl / o are recorded. It should be noted from the measurement results that damage due to thermal from FIG. 5 was taken as the basis for determining the values in FIG. 6, alternating stress in materials with a high level. This figure illustrates the linear coefficients of thermal expansion that the new alloys are likely to be superior to. Using dilatometric rods according to the present invention compared to thermodard methods, measurements were carried out on samples of both p-type elements made of lead telluride, the known alloys generally used hitherto in the inventive as well as in the corresponding thermoelectric devices. That were grounded. Targeted results are shown in Table II.

Table II

Average coefficient of the linear
Thermal expansion from room temperature to 300 ° C

Mole percent GeTe Expansion coefficient in the alloy 10- ° cm / cm ° C 75 21.3 80 18.5 83 15.6 85 14.1 87 16.1 90 17.9

IO

It can be seen that the new alloy with a GeTe content of 80% has a somewhat higher efficiency ZT at certain operating temperatures than the other new alloy with ao a GeTe content of 85%. The better cons

• The latter material's ability to withstand periodic thermal loads makes its use for numerous purposes more appropriate. So were z. B. Samples with a GeTe content of 85 mole percent and an AgSbTe ₂ content of 15 mole percent were heated and cooled more than 25 times at a rate of 50 ° C. per minute without any mechanical defects being revealed.

The hot and cold pole pieces of the thermocouple (Fig. 1) are to be selected from the substances which have good electrical and thermal conductivity and which are both chemically and physically with the p-conducting thermocouple described here and the one to be used in conjunction with it η-conductive thermocouple are compatible. As pole shoe material, materials are used that are in the based on lead telluride working thermocouples find use, such. B. iron, molybdenum, tungsten and tantalum.

Thermoelectric devices that have an element

use of the p-type of the type described here,

Wk could be made with good success using an η-type element of lead-tin-telluride with an iron pole piece at the hot end. The thermocouples made from the new alloy were soldered to the iron shoe with bismuth tellurium solder and tin-tellurium solder, and nickel solder was used to solder the lead-tin telluride element to the iron shoe. Several of these devices were kept in operation for more than 1000 hours and produced output powers that were considerably higher than those that can be achieved from conventional materials with a temperature difference of 400 ° K and the hot side of around 750 ° K. The material for the pole pieces of these designs was supplied by the "Crucible Steel Company of America" in Pittsburgh, Pennsylvania, and bears the trade name "Ferrovac E".

Furthermore, a thermoelectric device consisting of a p-type element composed of 85 mole percent GeTe and 15 mole percent AgSbTe ₂ and an η-type element made of lead-tin-telluride of a commercially available type, with another compatible hot pole piece for the duration of about Operated for 7000 hours without any deterioration in electrical performance.

Although in the investigations shown in FIG a lead-telluride alloy as the η-conductive element was used with lead iodide for the thermoelectric device because it was previously in conjunction with a p-type lead telluride element with sodium had been used, it was found that the p-type thermocouple from the invention Alloy with good success in connection with other known η-conducting thermocouples can be summarized in thermoelectric devices. So the new alloy z. B. as a p-conductive branch of a thermoelectric device without further ado and with good success together with an η-conductive branch made of lead-tin telluride with lead iodide known to be suitable for this application. Typically would have to lead, tin, tellurium and the dopant lead iodide in a molecular ratio of 0.85: 0.15: 1.0: 0.000625 in such an n-type Branch exist.

From the preceding statements it can be seen that so far nobody has made an alloy

Ag-Sb-Ge-Te

with a crystal structure that contains both rhombohedral and face-centered cubic crystals. No alloy has been prepared, the at 75O ⁰ K a ΖΓ value of about 1.3 or higher such. B. 1.4 or 1.5 has. According to the prior art, no such alloys are known whose coefficients of expansion, expressed as 10 ~ ⁶ cm / cm- ⁰ C, are below 15.0 or 16.0, such as e.g. B. the expansion coefficient 14.1 for an alloy with a GeTe proportion of 85%, the expansion coefficient 21.3 for alloys with a GeTe proportion of 75% and the expansion coefficient 17.9 for alloys with a GeTe proportion of 90 % as the most similar known Ag-Sb-Ge-Te alloys of this type can be compared. Thermocouples with such properties have not yet been manufactured.

For this purpose 2 sheets of drawings 209541/97

Claims (4)

Patent claims: 1. From GeTe and AgSbTe ₂ existing alloy, characterized in that it consists of 85 mol percent GeTe and 15 mol percent AgSbTe ₂ or 80 mol percent GeTe and 20 mol percent AgSbTe ₂ . 2. Use of an alloy of the composition according to claim 1 as a p-type leg a thermoelectric element. 3. Use of an alloy composed of has, its application to commercial Purposes in this temperature range is currently known. The present invention is based on the object of creating new _{alloys consisting of GeTe and AgSbTe 2} with superior mechanical and thermoelectric properties, which make them acceptable as superior thermocouples in thermoelectric devices, whose stability in the temperature range of their solid state is guaranteed, and their mechanical properties Strength under physical stress, such as B. thermal shock and periodically changing temperatures normally associated with the An Setting according to claim 1 as a p-conductive leg 15 .Wende thermoelectric devices occur, a thermoelectric element whose n-line is not impaired.
Thigh from one in itself tender leg made of a known alloy of lead-tin-telluride with lead iodide doping or of lead telluride with lead iodide doping. 4. A method of making an alloy of the composition of claim 1, characterized in characterized in that the melt is at 16 hours 600 ° C is annealed. 5, method according to claim 4, characterized in that after the tempering the solidified alloy is crushed and a sintering treatment consisting of cold pressing with a pressure of 4000 kp / cm ² and hot pressing at 525 ° C with a pressure of 350 kp / cm ² over a period of 30 minutes.