DE1106968B - Tellurium and selenium or selenium and sulfur-containing lead-based alloys are suitable as legs of thermocouples - Google Patents

Description

GERMAN

Known thermoelectric power generators mainly consist of metal alloys that only deliver low voltages, have a low thermal efficiency and therefore only in can be used to a limited extent. So commonly used thermocouples develop that consist of metallic components, even with extreme temperature differences between the hot ones and the cold junction, e.g. B. from about 1400 and 980 ° C, at best voltages of about 50 millivolts. Such extreme temperature differences are practically inapplicable for a long time. Even the thermal efficiency is only a fraction of a percent.

Thermocouples made from intermetallic compounds with semiconductor character are already known, the tellurium and selenium or selenium and sulfur-containing lead base alloys described below are peculiar.

With the alloy according to the invention, on the other hand, thermocouples can be produced which have a high Have thermal conversion efficiency and their internal resistance is arbitrary can be changed so that it is the external circuit or the working device that supplies it with electricity should, can be adjusted. The tellurium and selenium or selenium and sulfur-containing ones required for this Lead base alloy is characterized by the fact that in the lead-tellurium-selenium alloys in addition to lead as The remainder is the sum of the tellurium and selenium contents within the linear range for traces Selenium 37.0 to 38.05 percent by weight tellurium and, in the case of traces of tellurium, 26.5 to 27.55 percent by weight selenium and that in the case of lead-selenium-sulfur alloys, in addition to lead, the remainder is the sum of the contents Selenium and sulfur lie within the linear range, which is 26.5 to 27.55 percent by weight for traces of sulfur Selenium and traces of selenium make up 12.9 to 13.37 percent by weight of sulfur and as further stock a small addition to z. B. gallium, zirconium, titanium, tantalum, bismuth, chlorine, bromine or Iodine is added, which affects the thermal voltage and the specific resistance.

The alloys complying with these conditions and the various additives are described below explained in more detail with reference to drawings.

In Fig. 1, several alloys, consisting of lead and tellurium or selenium, are shown graphically, the can serve for legs of thermocouples. On the abscissa are the selenium and tellurium components in Atomic percent indicated, in linear form, based on tellurium with only a trace of selenium the left up to selenium with only a trace of tellurium on the right. The left ordinate gives

suitable tellurium and selenium or selenium and sulfur-containing lead base alloys

Applicant:

Minnesota Mining

and Manufacturing Company,

St. Paul, Minn. (V. St. A.)

Representative: Dr.-Ing. F. Wuesthoff, Dipl.-Ing. G. Pulse

and Dipl.-Chem. Dr. rer. nat. E. Frhr. v. Bad luck man,

Patent Attorneys, Munich 9, Schweigerstr. 2

Claimed priority:
V. St. v. America July 12th and December 15th, 1954

Robert Washburn Fritts, Elm Grove, Wis.,

Sebastian Karrer, Port Republic, Md.,
Proboscis Edgar Fredrick and William Varney Huck,

Milwaukee, Wis. (V. St. A.),
have been named as inventors

in percent by weight is the amount of lead that is alloyed with the tellurium, selenium or tellurium-selenium constituent can be, while the right ordinate the weight percent of the tellurium, selenium or selenium-tellurium constituent in the final composition, with the remainder being lead.

In these alloys, tellurium or selenium or both elements can be present side by side because they are soluble in one another in the entire area shown. Tellurium and selenium can therefore act against each other be replaced in whole or in part, which means that thermocouple components can be manufactured economically is important insofar as selenium no longer has to be separated from tellurium and vice versa. These namely, both components regularly appear as mutual companions. The separation procedures are difficult and expensive and cannot be fully implemented. As a result, the outermost alloys shown in Figure 1 with 37.0 to 38.05 weight percent tellurium, the remainder, i.e. 63.0 up to 61.95 percent by weight lead, at least a trace of selenium. On the other hand, the alloys contain

109 607/367

26.50 to 27.55 weight percent selenium, balance, i.e. H. 73.50 to 72.45 percent by weight lead, traces of tellurium. In the following, “trace” is understood to mean a set that is used in ordinary examinations is not recognized, but one must assume that it is still present. The alloys with only Traces of selenium or tellurium are referred to below as "end" alloys.

If selenium and tellurium in the selenium-tellurium component correspond in atomic percent, then makes of this from 31.75 to 32.8 percent by weight of the alloy, with the remainder, i. H. 68.25 to 67.2 percent by weight is made of lead.

In Fig. 2, semimetal alloys made of lead and selenium or sulfur are shown. On the abscissa the selenium and sulfur contents are plotted linearly in atomic percent. They extend from selenium with only a trace of sulfur on the left to sulfur with only a trace of selenium on the right Page. The left ordinate shows the amount of sulfur in percent by weight that is associated with the selenium, sulfur or Selenium-sulfur constituents can be alloyed, while the right ordinate shows the percentages by weight of the sulfur, selenium or selenium-sulfur constituent, with the remainder being lead.

Since selenium and sulfur in a lead alloy are soluble in one another within the specified range, the two elements can be completely or partially exchanged for one another.

In Fig. 2 it is also about end alloys, namely those on the left the scale appears with 26.5 to 27.55 percent by weight selenium, the remainder, i.e. H. 72.45 to 73.50 percent by weight Lead. They contain at least a trace of sulfur. The alloy at the other end of the scale at 12.90 up to 13.37 weight percent sulfur and the remainder, d. H. Contain 86.63 to 87.10 percent by weight lead at least a trace of selenium.

If selenium and sulfur in the selenium-sulfur component correspond in atomic percentages, then it does this constituent consists of 19.7 to 20.46 percent by weight of the alloy with the remainder, i. H. 80.3 to 79.54 Weight percent is lead. The final compositions described, consisting essentially only of lead and tellurium, may also contain some sulfur and selenium, i. H. to the extent that Finds sulfur as an impurity in commercially available tellurium. The end alloys that are used in the consist essentially of lead and selenium or of lead and sulfur, some tellurium and sulfur can be used or tellurium and selenium. Contain the alloys located between the final alloys two of the elements tellurium, selenium and sulfur and a trace of the third element in this group.

The lower limit of the lead constituent of the alloys represented by the lower curve of FIGS. 1 and 2 is shown, must be regarded as decisive, since a further reduction in the lead content the desired values for the Seebeck voltage and the specific resistance do not occur and the advantageous electrical and mechanical properties can no longer be reproduced. If the Lead content exceeds the upper limit shown by the upper curve in Figs. 1 and 2, the alloy becomes too metallic and useless for thermocouples.

A further condition for the alloys of this invention is that the impurities in the final alloy of the order of 0.001 ⁰ / o and have less need and that the alloy is to be substantially free of oxygen.

It is therefore advisable to use lead, tellurium, selenium and sulfur, which do not contain any metallic impurities contained above about 0.001%. But you can also use components of lower purity go out when the alloy formed is recrystallized in a manner to be described. Different Impurities like copper that are common

ίο are in the commercial qualities of the four components, reduce the Seebeck tension and must therefore be removed.

The alloys described are two-phase alloys. When dividing and the microscopic examination it is found that they consist of a main phase with crystal grains from Diameter 1 to 10 mm exist and that between these grains thin, relatively darker areas a second phase exist. The primary phase grains are crystals of the metal compounds Lead telluride, lead selenide and lead sulphide or mixtures of these crystals which are about 61.89, 72.41 and Contains 86.60 percent by weight lead. The darker secondary phase that becomes clear at the grain boundaries consists of lead with a small amount of selenium, tellurium or sulfur. The second phase has a threefold function. First, the thermal equilibrium between the two phases creates which is set by the heat treatment to be described, in the primary lead-telluride, Lead selenide or lead sulfide phase has a negative Seebeck voltage and conductivity, with the secondary The electrical properties of the phase because of their high concentration in the alloy Two-phase alloy determined. Second, the thin layers act as an adhesive for the grains of the primary phase, increasing the mechanical strength of the alloy compared to the pure metal compound is improved. Thirdly, this adhesive effect enables the secondary phase to have good conductivity the polycrystalline alloy by increasing the specific resistance between the grains of a negligible size. The concentration of the secondary phase is not decisive when the composition is in the above range.

In the ranges of the various alloys given above, it should be noted that in each In the case of a lead excess over the stoichiometric stoichiometric required for the connection with tellurium, selenium or sulfur Amount of lead is present. In the final alloy described above, which consists essentially of lead and Tellurium and contains 61.95 to 63.0 percent by weight lead, an excess of 0.06 to 1.11 percent by weight is Lead over the amount of lead of 61.89 percent by weight required for the connection with tellurium available. The final alloy consisting essentially of lead and selenium contains an excess of 0.04 to 1.09 weight percent lead above that stoichiometrically required for a compound with selenium Lead amount of 72.41 * / ». The same is of course true for the final alloy, which is essentially lead and sulfur and in which the excess lead is 0.03 to 0.5 percent by weight of lead, based on the Total alloy.

The lead-tellurium, lead-selenium, lead-tellurium-selenium, lead-sulfur and lead-selenium-sulfur alloys, in which the components are in a somewhat imperfect stoichiometric ratio to one another, will hereinafter referred to as "base alloys".

For the sake of simplicity, the manufacture of thermocouple legs !! subsequently using described by "end" alloys.

In the case of the lead-tellurium alloys described above, the thermoelectric power and the specific electrical resistance depends greatly on the heat treatment that the alloy is subjected to during manufacture is subjected, and these properties can be controlled by the heat treatment. So shows z. B. a lead-tellurium alloy that is annealed for a few hours at 540 to 815 ° C and then quenched, a lower thermoelectric power and a lower specific electrical resistance than the same alloys that annealed similarly, but then slowly cooled have been.

The following Table I gives typical electrical properties of the lead-tellurium alloys at room temperature as a function of the quenching temperature. The data in Table I apply to lead-tellurium alloys, the annealed at 540 to 815 ° C and then slowly, z. B. 50 ° C per hour, on the in Column 1 indicated temperature and then cooled

Table III

Deterrent Thermoelectric More specific temperature power ■ resistance ⁰ C Microvolt per ° C Ohm-cm 815 -144 0.00094 704 -162 0.0011 538 -198 0.0017 427 -225 0.0029 316 -280 0.0048 204 -28Q 0.0048

have been deterred.

Table I.

Deterrent Thermoelectric More specific temperature power resistance ⁰ C Microvolt per ° C Ohm-cm 815 -316 0.0023 704 -352 0.0034 538 -396 0.0074 427 -414 0.0150 316 -414 0.0173 204 -414 0.0175

Also with the lead-selenium and lead-sulfur alloys, the thermoelectric performance and the specific electrical resistance strongly depends on the heat treatment. A lead-selenium and lead-sulfur alloy, which was annealed for several hours at 540 to 815 ° C and then quenched, shows. a lower thermoelectric performance and lower electrical resistivity than that same alloys that were annealed in a similar manner but then slowly cooled. In the tables II and III are some of the electrical properties of lead-selenium and lead-sulfur alloys Room temperature given as a function of the quenching temperature. The information relates to alloys, those annealed at 540 to 815 ° C and slowly, d. H. 50 ° C per hour to that specified in column 1 Temperature and then quenched.

Table II

Deterrent Thermoelectric More specific temperature power resistance ⁰ C Microvolt per ° C Ohm-cm 815 - 155 0.00068 704 -166 0.00071 538 -220 0.0013 427 -267 0.0023 316 - 288 0.0030 204 -288 0.0030

Of course, the use of the alloys must remain limited to such a temperature which does not affect their electrical properties. One therefore wants when using higher Temperature to maintain the electrical properties unchanged, this must be done by others Adjust factors.

The electrical properties of the base alloys that are desired for thermocouple legs can can be substantially and advantageously changed in a reproducible manner by adding small amounts of others Adding substances. In the following, they are referred to as "additions".

All of the base alloys described show a negative thermoelectric performance and a negative one Conductivity. With the additives, these negative properties can be intensified, as well the polarity of the electrical properties can be reversed. They are therefore called "positive" or "Negative" additions and the resulting alloy as a "positive" or "negative" alloy.

Negative or positive alloys are to be understood as meaning those that are used in measurements of the Hall effect or the thermoelectric effect in both cases at room temperature a negative or show positive conductivity.

Negative additives are substances which, when added, change the electrical conductivity of the base alloys, without the polarity of their conductivity or thermoelectric performance, which according to the previous one Definition negative is to change. Positive additives are those in which the base alloys, if you add in very small amounts, reduce their conductivity to a minimum and with their further addition the conductivity of the alloy increases, which is a reversal polarity, conductivity and thermoelectric power, d. H. from negative to positive is accompanied.

The effect of the negative and positive additives is compared in the following for clarification.

1. An increasing concentration of the negative additives increases the conductivity and decreases the thermoelectric performance of the alloy compared to the base alloy while being the negative Maintains polarity of conductivity and thermoelectric power.

2. An increasing concentration of the positive additives initially causes a reduction in conductivity and an increase in the Seebeck voltage of the base alloy to a minimum conductivity is reached, whereupon the thermoelectric power and the conductivity their polarity in the positive sense reverse and with a further increase in the

centration of the positive addition an increase in conductivity and a decrease in thermoelectric Causes performance in the alloy.

For the sake of simplicity, the additives are discussed below using the final alloys, with it goes without saying that these additives can also be added to an intermediate alloy. In this case, the type and amount of additives should depend on the concentration of the final alloys straighten in the intermediate alloy.

The effects mentioned above are negative and positive in the case of a final alloy of lead and tellurium Additions of different concentrations are shown graphically in FIGS. 3 and 3A. With both figures the middle vertical axis shows the properties of the lead-tellurium base alloy. the the left half indicates the change in the electrical properties of the lead-tellurium base alloys at room temperature and with the addition of the negative additives. Own the two drawings no scales, as the quantity ranges change with each of the following additions as a result of the change the atomic weights and the concentration limits changes. The right halves of Figs. 3 and 3A show the change in electrical properties when positive additives are added to the base alloys. For a certain addition, the maximum specific resistance and the polarity reversal result the thermoelectric power at the same concentration. This concentration is shown in FIG. 3 and 3 A is denoted by "α".

In the first column of Table IV elements are given as negative additions to lead-tellurium base alloys are effective. The quantities given in the second column represent the respective The upper limit of the content at which the electrical properties of the base alloy are still changed will. Larger amounts have no discernible effect within the meaning of the invention. The third and fourth column of Table IV gives the electrical properties of the lead-tellurium alloys at room temperature with the maximum usable concentrations of negative additives after annealing at high Temperature.

Table IV

5
Negatives
additions
IO Maximum
effective
Con
centering
border
Weight
percent Thermo
elec
tric
power
in micro
volt
⁰ C each More specific
resistance
Ohm-cm bismuth
Tantalum
manganese
15 zirconium
titanium
Aluminum ....
gallium
chlorine
20 bromine
iodine
uranium 0.60 to 20 *
0.50
0.25
0.25
0.15
0.10
0.25
0.10
0.20
0.25
0.80 -72
-121
-113
-23
-45
-59
-36
-45
-47
-45
-72 0.00031
0.00032
0.00036
0.00012
0.00020
0.00016
0.00015
0.00019
0.00015
0.00015
0.00020

The specified range is discussed below.

In FIGS. 4, 4 A, 5 and 5 A, the effect of the negative additives according to Table IV is shown graphically the specific resistance and the thermoelectric performance of the lead-tellurium base alloy at room temperature explained. Column 1 of Table V shows positive additions for lead-tellurium alloys. Column 3 of Table V gives the positive additives in percent by weight, when added the polarity of the conductivity and the thermoelectric performance of the alloy containing the additive reverses. For the respective additives, these are the corresponding quantities indicated by the Point "α" in Fig. 3 and 3A are designated.

Columns 4 and 5 give the thermoelectric power and the resistivity of the alloy after adding the positive additives at room temperature after adding them at high temperature was annealed and then slowly cooled.

Table V

Positive additions Limit of the maximum
effective concentration
Weight percent Concentration,
Weight percent,
at which the polarity
reverses
(Point y> a «
in Fig. 3 and 3 A) Thermoelectric
power
Microvolt per ° C More specific
resistance
Ohm-cm l \ * atrium
potassium
Thallium
arsenic 0.06
0.10
0.25 to 1.00 *
0.07 to 0.25 * 0.0002
0.0004
0.005 to 0.02 *
0.0008 to 0.002 * + 173
+ 198
+256
+270 0.00074
0.00076
0.00290
0.00450

The areas indicated are discussed below.

When the additives are added to the two-phase alloy, they are distributed between the two phases. The nature of this distribution has a negligible effect on the electrical in all cases Properties of the alloy, except for bismuth, thallium and arsenic. It depends on these three elements the effective maximum concentration depends on the lead content of the lead-tellurium base alloy, within the range given in Tables IV and V. A content of 1.20 percent by weight bismuth is the highest effective concentration for lead-tellurium base alloys with a content of 63.0% lead. With a lower lead content, the bismuth content must be lower. It goes up to 0.60 percent by weight when the lead content is only 61.95%. For thallium, the range is from 1.00 percent by weight for the lead-tellurium base alloys with 63.0 percent lead up to 0.25 percent by weight with a lead content of 61.95 "/ O. For arsenic they are corresponding values 0.25% for base alloys with 63.0% lead to 0.07% for base alloys with 61.95% lead.

9 10

As indicated in Table V, the Konbeck voltage and the electrical conductivity of the centrations at which the polarity is reversed in the electrical conductor made of lead and tellurium alloys Range from 0.0002 to 0.02 weight percent in the way that thermocouple components are controlled The extent to which the lead content of the base alloy can be made to match such alloys, changes in the range from 61.95 to 63.0 "/ O. The behavior, which results in thermal voltages that are many of bismuth, thallium and arsenic is likely to occur through the formation of conventional thermocouples a bismuth-lead-tellurium-, a thallium- the voltages are. The thermocouples have Lead-tellurium or an arsenic-lead-tellurium complex also has a higher thermal effectiveness. the in the intermediate phase to be explained to the an alloy according to the invention of lead and tellurium Part of the addition falls. All other additions can be made with a second electrical conductor with the intermediate phase, basic materials known to a much lesser extent, such as stainless steel, Complexes, which within the meaning of the invention are of no importance. You can also use any of the negatives tion is. electrical conductor with each of the positive electrical

In Tables IV and V, the views refer to conductors combined to form a thermocouple about the theoretical performance and the special 15 whose performance is the sum of the Seebeck fish Resistance to the alloy with 61.95% stresses and the power output of each Lead, the remainder essentially tellurium, where in the case of is conductor.

Bismuth, thallium and arsenic are the lower in terms of quantity. In Figs. 8, 8A and 9 the Seebeck voltages

Limit has been complied with. conditions for a temperature gradient of 555 ° C (10 and

In Fig. 6 and 7, the effect of the positive to 20 565 ° C) is indicated, which one with lead-tellurium alloy

sets of table V on the specific resistance can be obtained with various additives. In

and the thermoelectric performance at room temperatures. Figs. 10, 10A and 11 are the additives in

temperature specified. weight percentage against the values of the short-circuit

4 to 7 show the percentages by weight and power in watts, which are in a cubic meter logarithmic resistivity and the 25 centimeter cube of lead-tellurium containing additives

thermoelectric power plotted linearly. Alloys with a uniform temperature gradient

From Figs. 3 to 7, for. B. from Figs. 4 A and 5 A, between the two sides of 10 and 565 ° C

it can be seen that z. B. wrap by adding zirconium to which electrodes have been applied,

the specific resistance of the lead-tellurium-basic To the maximum power output with appropriate To calculate the alloy to more than about a hundredth of a load, the values shown in Fig. 10,

can be changed, with the thermoelectric 10A and 11 apparent power values being 4

Power reduced to about a twentieth. be divided, since the load resistance is the

The alloys according to the invention are provided for half of the total resistance of the circuit, by carrying the very pure alloy constituents.

parts within the desired concentration limits. From FIG. 8A it can be seen that a lead merges. A selenium content in the handeis tellurium alloy with a content of up to 0.25 usual The order of magnitude of 1% in tellurium causes weight percent zirconium versus stainless steel no significant changes in the electrical properties at a temperature gradient of 555 ° C, i.e. 10 ° C shafts of the alloys. at the cold junction and 565 ° C at the

The electrical properties of the lead alloys 40 are called junction depending on the zirconium

With the additions, a negative Seebeck voltage of 200 to

determined by their heat treatment. It shows 28 millivolts. Thermocouple components of the same

but the fluctuations in the electrical properties of the base alloy with a content of 0.01 to

ten considerably less than the lead tellurium 0.10 percent by weight potassium show among equals

Base alloy without additives. One can find positive Seebeck tensions in the area in all conditions

say in common that the stabilization of electrical from 100 to 153 millivolts (see Fig. 9). If you have this

Properties with the concentration of additives up to both conductors as components of a thermocouple in

increases to a certain amount so that the series connects, they develop voltages of 128

Valuability of the alloys at high temperatures up to 353 millivolts with a temperature gradient of

for thermoelectric power sources significantly increased 50 only 555 ° C. These Seebeck voltages from thermal

will. In the case of alloys with positive additives as well as elements with components,

at application temperatures around 570 ° C, settlement should be a multiple of the stresses

Use the positive additives in amounts that are known to metal thermocouple components, too

approach the maximum effective limit, if only a temperature gradient of half the

due to the positive polarity of the alloy obtained 55 previously specified size prevails,

remain. In addition, the positive additions, from FIGS. 8 to 11 it can be seen that low

if a positive electrical conductor is desired, concentrations of negative additives are high negative

will cause Seebeck stresses in the lead-tellurium base alloy in quantities, but as a result

are present that are not below the value in column 3 of the specific resistance of the alloy in the case of the

The values given in Table V are (point »α« in 60-like concentrations) no high power output

3 and 3A). enable. On the other hand, high concentrations

As shown graphically in FIGS. 9 and 11, negative additions occur in low lake basins practically usable Seebeck tensions and tensions a maximum power output. Dem performance levies in the case of alloys with a positive allowance, the concentration of the negative ones is changed add only when the positive addition in 65 additions depending on the requirements of the actual The following minimum quantities, based on the basic application of the negative components, d. H. the contraction, present: Sodium 0.002 percent by weight, Ka- stand of the working circuit.

lium 0.003 percent by weight, thallium 0.01 to 0.04 Ge The Seebeck voltage and the short-circuit power

percent by weight, arsenic 0.005 to 0.015 percent by weight. the alloys with positive additives, measured

As already mentioned, the polarity, the sea under the conditions mentioned above, can also depend on

if it depends on the concentration of the additives concerned. In contrast to the negative alloys it causes the increase in the concentration of positive additives in the lead-tellurium alloys increases both the Seebeck voltage and the short-circuit power. This is illustrated in Figures 8 to 11, in which the information relates to the unit cube made of lead-tellurium alloys and the Seebeck voltage when using stainless steel as the second component of the thermoelectric Power source was measured.

The values for the Seebeck voltages and for the short-circuit power of the alloys with low Amounts of positive additives are shown in FIGS. 9 and 11 as dashed lines around the amounts in which the polarity of the Seebeck voltages is reversed. The ones actually occurring Performance values in this area depend largely on the type of remaining contamination in the Lead-tellurium alloy that is and is not in the order of 0.001 percent by weight and less so much from the heat treatment of the alloys.

Table VI shows the power output and thermal efficiency of a thermocouple compared with the alloy according to the invention with those of a known thermocouple. The information refer to thermocouples made up of two components, each component having the dimensions of a cubic centimeter cube and has a uniform temperature gradient λόπ 565 to 10 ° C. The numbers for the thermal Efficiencies are based on the total heat flow through the cube and the developed thermal current calculated.

Table VI

Thermocouple tension
mV At ent
speaking
load
submitted
power
watt Thermal
Effect
Degree
° / o Iron constantan ..
Pb-Te alloy
with 0.03 ° / o Zr ...
Pb-Te alloy
with 0.06 ° / o K .... 30.8
• 250.0 2.7
1.6 0.5 to 0.6
5

The data show that the voltage and the efficiency of the thermocouples according to the invention are almost an order of magnitude higher than the same properties of the known thermocouple. Thermocouples with the lead-tellurium alloys described require due to their larger size thermal efficiency significantly smaller flames or heat sources for the same power output.

Thermocouple legs made from the additional lead-tellurium alloys can be heated in a non-oxidizing atmosphere at a temperature of 565 ° C Connection point can be used. They are even stable up to 815 ° C. The sublimates above about 570 ° C Lead-tellurium alloy to a small extent. Components made from these alloys have a temperature gradient stable. Recrystallization and diffusion have no adverse effects.

End alloys with positive and negative additives, which essentially consist of lead and selenium, are graphed in Figures 12 and 12A. The middle vertical lines show Properties of the lead-selenium base alloys. The left half of each figure indicates the change in electrical properties of the lead-selenium base alloys at room temperature and when the negative additions. -

There are no scales in the two drawings as the concentration ranges for each of the im

ίο the following additions to be described as a result of the change of atomic weights and concentration limits. The right halves of FIG. 12 and 12A show the change in electrical properties of the base alloy upon addition of the positive additions. For a given additive, the maximum resistivity and polarity reversal lies the thermoelectric power at the same concentration. It is shown in FIG. 12 and 12A labeled "α".

Table VII, first column, shows elements that are used in the lead-selenium base alloys as negative additives work. The second column shows the maximum concentration of the additives in percent by weight specified. Additions beyond the specified amounts have no further recognizable effect to the electrical properties with which the present invention is concerned. The third and the fourth column of Table VII gives the electrical properties of the lead-selenium alloys Room temperature again.

Table VII

Negatives
additions

iodine

chlorine

bromine

zirconium

Silicon ..
titanium

Indium ...

Tantalum ....

Gallium ..

aluminum
Copper ...

gold

Bismuth ..

Antimony ..

Fluorine ....
niobium

Limits
the maximum
effective

Concentrations
Weight percent

0.50
0.20
0.60
0.60
0.10
0.10
0.20
0.60
0.15
0.03
0.30
0.35
0.40 to 2.5 *

0.02
0.35

Thermoelectric
power

Microvolt
⁰ C each

-40

-38

-34

-27

-54

-40

-54

-45

-58

-101

-104

-104

-75

-103

-133

-65

Specific resistance

Ohm-cm

0.00016 0.00018 0.00014 0.00016 0.00024 0.00026 0.00024 0.00022 0.00027 0.00037 0.00038 0.00039 0.00040 0.00050 0.00047 0.00023

* The above area is discussed below.

In Figs. 13, 13 A, 13 B, 14, 14A and 14 B is the Effect of the negative additives according to Table VII on the specific resistance and the thermoelectric Performance, measured at room temperature, indicated. The positive additives for the lead-selenium alloys are given in column 1 of Table VIII. The second column gives the limits of the maximum concentration of the additives to the base alloy in percent by weight.

In column 3 of Table VIII, the concentrations of the positive additives are given in percent by weight, at which the polarity of conductivity

and reverse the thermoelectric power. They are designated as "a" in FIGS. 12 and 12A. Columns 4 and 5 show the thermoelectric conductors

performance and the specific resistance after annealing and subsequent slow cooling at room temperature specified.

Table VIII

Positive additions Limit of the maximum
effective concentration
Weight percent Concentration,
Weight percent,
at which the polarity
reverses
(Point "a"
in Fig. 12 and 12 A) Thermoelectric
power
Microvokje ° C More specific
resistance
Ohm-cm sodium
Thallium
potassium
lithium
arsenic 0.08
0.72 to 1.5 *
0.15
0.03
0.10 to 0.30 * 0.002
0.04 to 0.08 *
0.003
0.002
0.02 to 0.06 * + 180
+ 263
+ 250
+288
+ 331 0.0019
0.0108
0.0076
0.0108
0.0110

* The above area is discussed below.

The additives have a negligible effect on the electrical properties of the two-phase alloy except for bismuth, antimony, thallium and arsenic. 2.50 percent by weight of bismuth is the maximum effective concentration for lead-selenium base alloys with 73.50 "/ o lead. In base alloys with a lower lead content, it goes down to 0.40 percent by weight when the lead content is 72.45%. when the antimony limits ranging from 1.5 weight percent at 73.50 ¹ Vo lead to 0.20 weight percent of 72.45% lead. For thallium the limits of 1.5 weight percent at 73.50% of lead to 0.72 percent by weight were found at 72.45% lead. For arsenic, the values are 0.30% by weight for 73.50% lead and 0.10% by weight for 72.45% lead. The concentration range at which the polarity is reversed is that with thallium offset base alloy at 0.04 to 0.08 percent by weight, corresponding to the variation in the lead component of the lead-selenium base alloy between 72.45 and 73.50 "/ O. For arsenic, the corresponding values are 0.02 to 0.06 percent by weight and 72.45 to 73.50 percent lead. This behavior is likely to be due to the formation of a bismuth-lead-selenium, an antimony-lead-selenium, a thallium-lead-selenium or an arsenic-lead-selenium complex within the intermediate phase. All other positive and negative additives form complexes with the secondary or intermediate phase to a much lesser extent, so that their effect is of no importance for the invention.

In Tables VII and VIII refer to the information on the thermoelectric power and the specific resistance in each case on an alloy with 72.45% lead, the remainder essentially selenium. from Bismuth, antimony, thallium and arsenic were added to the minimum amount.

In Figs. 15 and 16 are the values for the specific resistance and the thermoelectric performance indicated at room temperature for positive additions.

The weight percentages and the specific resistance are logarithmic, the thermoelectric power given linearly.

For example, it is seen from Figs. 13B and 14B that bromine is the resistivity of the lead-selenium base alloy to more than about a twentieth and the thermoelectric power to about one Can reduce tenths.

The alloys are made by melting together the alloy components in the desired Quantities made and fused together. The lead-selenium alloys are allowed with exception of tellurium do not contain more than 0.001 percent by weight of impurities. Commercially available selenium is usually contain up to 1% tellurium.

The electrical properties of the alloys containing additives are to a lesser extent Depending on the heat treatment of the alloy, the fluctuations in the electrical properties are significant are lower than with the lead-selenium base alloys without additives. In general it can be said that the degree of stabilization with the increase of the additives up to the above-mentioned maximum limit elevated. For alloys with positive additives and when using temperatures close to 570 ° C To maintain the positive polarity, add the positive additives approximately in the maximum effective Limit exist. In addition, in the case of an electrically positive conductor, the positive additions be present in the lead-selenium base alloy in an amount which corresponds to the values in column 3 of the Table VIII does not fall below, d. H. in a concentration at which the polarity of the thermoelectric Performance becomes positive (point "α" of FIGS. 12 and 12A). As shown graphically in Figs is, there are practically usable Seebeck stresses and a power output in the alloys with positive additives only if the positive addition in the following minimum quantities, based on the lead-selenium base alloy, present: sodium 0.006 percent by weight, potassium 0.014 percent by weight. There are none for the thallium, lithium and arsenic additives Minimum concentrations given because, as can easily be seen from Table VIII, lead-selenium alloys with these additives have such a high specific resistance that the thermoelectric Efficiency is little better than that of metals. When checking the last-mentioned additions however, it has been shown that these do not have any noteworthy advantages.

One can use any of the negative electrical conductors described here along with any of those described here Use positive electrical conductor to produce a thermocouple, its output is the sum of the Seebeck voltages and the power output of the individual conductors.

In FIGS. 17, 17A and 18, the Seebeck voltages are shown for a temperature gradient of 555 ° C. (10 to 565 ° C) that can be added to lead-selenium alloys with different amounts in the there specified areas. In Figures 19, 19A, 19B and 20 the additives are in percent by weight plotted against the values of the short-circuit power in watts, which are increased by a cubic centimeter cube. ^ caustic lead-selenium alloys with a uniform temperature gradient between the surfaces to be developed. The surfaces are kept at 10 or 565 ° C and electrodes are applied to them. To calculate the maximum power output at a corresponding load, you have to use the Divide the power values from FIGS. 19, 19A, 19B and 20 by 4, since the load resistance is then is half the resistance of the entire circuit.

It can be seen from Fig. 17A that, for. B. a lead-selenium alloy with a content of up to 0.60 percent by weight bromine, a negative Seebeck voltage from 162 to 35 millivolts against stainless steel at a temperature gradient of 555 ° C. Thermocouples made from the same base alloy with a content of 0.006 to 0.08 percent by weight sodium show positive Seebeck voltages from 50 to 140 millivolts (see Fig. 18). If you have these two When conductors are connected in series as components of a thermocouple, they develop voltages from 85 to 302 millivolts with a temperature gradient of only 555 ° C. Such Seebeck voltages are around one several times larger than the known metallic thermocouples even with a temperature gradient, that's only half the size.

From Figures 17-20 inclusive, it can be seen that lower concentrations of negative additions higher Seebeck stresses result, but do not produce a greater output, and indeed as a result the resistivity of the alloy at such a concentration. On the other hand, have higher Concentrations of negative additives ensure maximum power output at lower Seebeck voltages result. The concentration of negative additives can therefore be varied to meet the actual application needs of the negative component, e.g. B. the resistance of the load circuit to meet.

The Seebeck voltage and the short-circuit power dt-r alloys with positive additions among the The measurement conditions given above also depend on the concentrations of the additives concerned. in the In contrast to the negative alloys, there is an increase in the concentration of positive additives in the Lead-selenium alloys cause an increase in the Seebeck voltage and the short circuit power result. This is explained in FIGS. 17 to 20 inclusive, in which the specification refers to a unit cube of lead-selenium alloys among the specified Relate conditions; the specified Seebeck voltage is the one that you get when using of stainless steel as the second component in a thermocouple.

The values of the Seebeck voltages and the short-circuit power for alloys with low concentrations positive additions (FIGS. 18 and 20) are shown as a dashed line in the vicinity of the concentrations at which the Seebeck voltage reverses its polarity. The actual properties in this area depends largely on the type of residual contamination in the lead-selenium alloy, which is on the order of 0.001 percent by weight and less, and to a lesser extent on the heat treatment of the alloys.

Table IX shows the power output and thermal efficiency of a known metallic Thermocouple with the corresponding values of a thermocouple according to the invention a negative and a positive electrical conductor made of lead-selenium alloys containing additives. The numbers refer to a cubic centimeter cube with a uniform Thermal gradient, with the surfaces being kept at 565 or 10 ° C. The one for thermal efficiency numbers given are due to the total heat flow through the cube and the developed thermoelectric current.

Table IX

Thermocouple tension
mV Performance
delivery
at ent
speaking
load
"Watt Thermal
Effect
Degree
»/ 0 Iron constantan ..
Pb-Se alloy
with 0.03% Br ..
Pb-Se alloy
with 0.1% Na ... 30.8
• 252.0 2.7
0.87 0.5 to 0.6
2.2

Thermocouple components made from lead-selenium alloys containing additives can be classified as non-oxidizing Use atmosphere at a temperature of 565 ° C at the hot joint. they are Stable even at 815 ° C, but the lead-selenium alloy sublimes to a small extent at temperatures above about 570 ° C.

Final alloys consisting essentially of lead and sulfur, with various concentrations of the negative and positive additions, are shown graphically in Figures 21 and 21A. In both Fig. 21 and Fig. 21A, the central vertical line shows the electrical properties of the lead-sulfur base alloy at room temperature. The two drawings, FIGS. 21 and 21A, have no scales since the concentrations for each additive change as a result of the change in the atomic weights and the concentration limits. It should be noted that for a given alloy containing additives, the maximum resistivity and polarity reversal of the thermoelectric power occur at the same concentration. This concentration is labeled "a" in Figures 21 and 21A.

In the first column of Table X, the elements are listed that are used in lead-sulfur alloys are effective as negative additives. In the second column of Table X are the amounts required specified, which are to be regarded as a maximum at which the electrical properties of the base alloys can still be effectively changed. Have concentrations in excess of the specified amounts no noticeable effect on the electrical properties. In the third and fourth columns of the table X are the electrical properties of the lead-sulfur alloys after annealing at high Temperature indicated.

Table X

Negatives
additions border
the most
effective
concentration
Weight percent Thermo
electrical
power
Microvolt
each ° C More specific
"Resistance
Ohm-cm Zirconium ..
Indium
bromine
chlorine
titanium
iodine
Tantalum
bismuth
Antimony ...
Gallium ....
niobium
uranium 0.40
0.50
0.35
0.15
0.20
0.55
0.70
1.0 to 3.0 *
0.50 to 3.0 *
0.30
0.40
1.0 -36
-54
-54
-54
-68
-99
-99
-118
-126
-117
-108
-99 0.00022
0.00024
0.00024
0.00025
0.00029
0.00042
0.00042
0.00061
0.00064
0.00058
0.00023
0.00024

The specified area is discussed below.
22, 22 A, 23 and 23 A show the effect on the specific resistance and the thermoelectric performance at room temperature, which is achieved by the addition of the negative additives to the lead-sulfur base alloys.
The positive additions for the lead-sulfur alloys are given in column 1 of Table XI. In the second column the concentration limits are given in percent by weight. Concentrations in excess of the specified amounts do not exert any

noticeable effect on the electrical properties.

In column 3 of Table XI, the concentration of the positive additives is given in percent by weight, in which the polarity of the conductivity and the thermoelectric power of the additional alloy reverses. These values are for the respective additions by the point "α" in FIGS. 21 and 21A, respectively designated.

Columns 4 and 5 show the thermoelectric power and the specific resistance at room temperature indicated after annealing at high temperature and subsequent slow cooling.

Table XI

Positive additions Maximum effective
Concentration limit
Weight percent Concentration,
Weight percent,
at which the polarity
reverses
(Point "a"
of Figs. 21 and 21 A) Thermoelectric
power
Microvolt per ° C More specific
"Resistance
Ohm-cm sodium
potassium
Rubidium
silver 0.20
0.30
0.70
2.0 0.02
0.04
0.20
0.30 + 364
+446
+450
+270 0.24
0.32
0.20
0.02

The lead-sulfur alloy is a two-phase alloy in which the additives are between the two Distribute phases. The type of distribution has a negligible effect on the properties of the Alloy except for bismuth and antimony. 3 percent by weight bismuth is the maximum effective Concentration in lead-sulfur alloys with a lead content of 87.10 ° / », it decreases at Base alloys with a lower lead content up to 1.0 percent by weight if the lead content is 86.63%. For antimony, the corresponding values are 3.0 percent by weight for 87.10 percent lead and 0.50 percent by weight at 86.63% lead. This behavior of the two elements is likely a consequence of the Formation of a bismuth-lead-sulfur or an antimony-lead-sulfur complex within the intermediate phase, which takes up part of the addition.

In Tables X and XI, the details of the thermoelectric power and the specific relate Resistance to the alloy with 86.63% lead, the remainder sulfur. For bismuth and antimony is that lower effective value indicated.

FIGS. 24 and 25 explain the effect of the additives on the specific resistance and the thermoelectric Performance measured at room temperature.

In Figs. 22 to 25, weight percentages and resistivity are logarithmic and are thermoelectric power given linearly.

From FIGS. 21 to 25 it can be seen that by adding positive or negative additives, the electrical Properties can be influenced within wide limits. For example, by adding of indium the resistivity to more than about one twentieth and the thermoelectric Reduce the power to a fifth (see Figs. 22A and 23A).

The lead-sulfur alloys are made by melting them together of the ingredients prepared in the desired quantities, it should be noted that the Alloys must have a high degree of purity, d. that is, they must not be more than 0.001 percent by weight Contain impurities. In contrast, tellurium and selenium concentrations cause about 1%, as occurs in the pure sulfur of the trade, no substantial changes in the electrical properties Properties of the alloys.

The alloy containing additives is a two-phase alloy with better electrical properties than the corresponding lead-sulfur base alloy. For alloys with positive additives and if the application temperature approaches 570 ° C, the positive additives should be used in concentrations approaching the maximum effective limit in order to maintain the positive polarity to ensure the alloy. In addition, if the electrical conductor is positive, the positive Additives are present in the lead-sulfur base alloy in an amount that is not below the in Column 3 of Table IX is the values given, d. H. in a concentration in percent by weight which the polarity of the thermoelectric power becomes positive (point "α" in FIGS. 21 and 21 A). As As shown in FIGS. 27 and 29, useful Seebeck voltages and powers occur with the aforementioned Alloys with positive additives only if the positive additive is in the following minimum

109 607067

amounts available: silver 0.40 percent by weight of the lead-sulfur alloy. No minimum concentrations are given for the base alloys to which sodium, potassium or rubidium have been added, as they have such a high resistivity that their thermal conversion efficiency is only is little better than that of metals. When testing the last-mentioned additives, however, it has been shown that that these do not bring any noteworthy advantages.

26, 26 A and 27 show the Seebeck voltages for a temperature gradient of 555 ° C (10 to 565 ° C), which is given in lead-sulfur alloys with additives in various amounts within the areas shown in the figures can achieve. To get the maximum power output at To calculate a corresponding load, the power value from FIGS. 28, 28 A and 29 must be passed through 4 can be divided.

It can be seen from Fig. 26A that a lead-sulfur alloy is up to 0.50 weight percent Indium has a negative Seebeck voltage of 184 to 58 millivolts against stainless steel at one Has a temperature gradient of 555 ° C, depending on the concentration of indium. Thermoelectric components the same base alloy with a content of 0.40 to 2.0 percent by weight silver show among the positive Seebeck voltages of 50 to 152 millivolts under the same measurement conditions (see FIG. 27). From these two alloy conductors of a thermocouple develop when connected in series, at a temperature gradient of 555 ° C a voltage of 108 to 336 millivolts. This Seebeck tension are several times larger than those of known thermocouple components, even one Temperature gradient that is only half of that given above.

With regard to Figures 26 to 29 inclusive, it should be mentioned that low concentrations are more negative Additions high negative Seebeck voltages, but due to the specific resistance of the alloy do not give maximum power output at such concentrations. On the other hand generate high Concentrations of negative additives result in high power output at low Seebeck voltages. The concentration of the negative additives is therefore adapted to the resistance of the load circuit.

The Seebeck voltages and the short-circuit power of the alloys with positive additions, measured under the above conditions, also depend on the concentration of the additives in question the way that, in contrast to the negative alloys, an increase in the concentration of the positive Additions to the lead-sulfur alloys are increasing the Seebeck voltages and the short-circuit power causes. This is shown in FIG. 26 up to and including 29, the values referring to a unit cube of lead-sulfur alloys refer to the specified conditions and the Seebeck voltage is that voltage which is measured using stainless steel as the second component on a thermocouple.

The values of the Seebeck voltages and the short-circuit power for alloys with positive additives in low concentration (FIGS. 27 and 29) are at the points where the polarity of the Seebeck stress reverses, indicated by dotted lines. The actual properties in this The type of contamination remaining in the lead-sulfur alloy depends largely on the area should be no more than about 0.001 weight percent, and to a lesser extent from heat treatment the alloys.

Table XII shows the power output and thermal efficiency of a known Thermocouple with the corresponding values of a thermocouple from a negative and positive electrical conductors made of additional lead-sulfur alloys compared. The data relate to

ίο on thermocouples made up of two components, each with Component has the dimensions of a cubic centimeter with a uniform temperature gradient accordingly a surface temperature of 565 or 10 ° C. The information for the thermal efficiency are calculated based on the total heat flow through the cube.

Thermocouple Table XII Performance Thermal delivery at "Effect 20th correspond Degree Iron constantan .. tension the burden Pb-S alloy watt 0.5 to 0.6 with 0.05 ° / »In .. mV 2.7 25th 30.8 Pb-S alloy 1.4 with 1.0% Ag ... 0.54 ■ 280.0

In the case of thermocouple components made of lead-sulfur alloys containing additives, the hot connection point can be used Bring to 565 ° C when enclosed in a non-oxidizing atmosphere. Such Components are mechanically stable even at 815 ° C, but it has been observed that above about 570 ° C the lead-sulfur alloy sublimates somewhat. Components the alloy described above are stable when exposed to a temperature gradient; one Recrystallization and diffusion in the temperature gradient have no adverse effects had.

The alloys according to the invention made of lead and at least one of the elements tellurium, selenium and sulfur can be produced by the following process. The constituents, which should be free of metallic impurities and preferably in a reduced state, are mixed in the desired proportions and enclosed in a tube or a container, preferably made of quartz, which has been evacuated. The tube and its contents are then heated to the melting point of the latter. It is about 920 ° C. for the final alloys shown on the extreme left of FIG. 1, about 1085 ° C. for those shown on the extreme right of FIG. 1 and for the alloys shown on the extreme left of FIG and at about 1115 ⁰ C for the final alloys at the extreme right side of Fig. 2. In Figs. 30 and 3oA the dependence of the melting point is given of the composition of the alloys. The molten mass is preferably agitated during the heating.

After the alloy has solidified, the mass is removed from the pipe and z. B. in graphite forms poured under an inert atmosphere. It is particularly useful to shape the mold during casting with an inert gas, e.g. B. with argon or carbon dioxide to cover under pressure. This gas under-

suppresses the evaporation of alloy components and thereby reduces the porosity of the casting.

The melting and casting should be done in crucibles that are not containing the alloy react or contaminate them. Melting pots made of carbon, aluminum oxide and pre-annealed have proven to be useful Lavit or quartz.

After casting, the castings can be machined, if necessary. The molded castings are then preferably in a reducing atmosphere in the range from 540 to 815 ° C 10 to Annealed for 20 hours. This annealing ensures the homogeneous structure of the casting and promotes it its electrical and physical properties.

Another method of making the alloy involves melting the components into one open crucible and under an inert and / or reducing atmosphere. Because of the relatively The expected loss of these constituents must be the high vapor pressure of selenium and sulfur must be taken into account in the quantities used. Otherwise this procedure is the same as that described above similar.

If less pure components are used as the starting material, the impurities can get through Recrystallization can be reduced. This is best done by melting the impure alloy and slowly solidifying progressively from one end of the melt to the other. Collect thereby The impurities in the starting components are mainly at the point of the melt, which last stiffens. This piece can then be discarded and the process repeated if necessary. With this one Cleaning process requires a small amount of additional lead, i.e. that is, it must be about 0.1 to 2.5 percent by weight can be added to the finished alloy to bring them to the ranges given above to adjust.

Claims (8)

PATENT CLAIMS: 40 1. Tellurium and selenium or selenium and sulfur-containing ones suitable as legs of thermocouples Lead-based alloy, characterized in that the lead-tellurium-selenium alloys in addition to lead the remainder is the sum of the tellurium and selenium contents within the linear range which with traces of selenium 37.0 to 38.05 percent by weight tellurium and with traces of tellurium 26.5 to 27.55 percent by weight Selenium makes up and in the case of lead-selenium-sulfur alloys, in addition to lead, the remainder is the sum of the selenium and sulfur contents is within the linear range of 26.5 to 27.55 percent by weight for traces of sulfur Selenium and traces of selenium make up 12.9 to 13.37 percent by weight of sulfur, and another Part of a small addition to z. B. gallium, zirconium, titanium, tantalum, bismuth, chlorine, bromine or iodine is added, which influences the thermal voltage and the specific resistance. 2. Alloys according to claim 1, characterized in that lead-tellurium-selenium alloys with only traces of selenium, the addition of up to 0.06 percent by weight sodium, 0.10% potassium, 0.10% aluminum, 0.25% gallium, 0.25% zirconium, 0.15% titanium, 0.50% tantalum, 0.80% uranium, 0.25% manganese, 0.10% chlorine, 0.20% bromine, 0.25% iodine or from 0.25 to 1.0% thallium, 0.60 to 1.20% bismuth or 0.07 to 0.25% arsenic. 3. Alloys according to claim 1, characterized in that lead-selenium-tellurium alloys with only traces of tellurium, the addition of up to 0.08 percent by weight sodium, 0.15% potassium, 0.30% copper, 0.35% gold, 0.03% aluminum, 0.15% gallium, 0.20% indium, 0.10% silicon, 0.10% titanium, 0.60% zirconium, 0.35% niobium, 0.60% tantalum, 0.02% fluorine, 0.20% chlorine, 0.60% Bromine, 0.50% iodine or 0.40 to 2.5% bismuth or 0.20 to 1.5% antimony. 4. Alloys according to claim 1, characterized in that lead-selenium-sulfur alloys with only traces of selenium, the addition of up to 2.0 percent by weight silver, 0.30% gallium, 0.50% indium, 0.40% zirconium, 0.20% titanium, 0.70% tantalum, 0.40% niobium, 1.0% uranium, 0.15% Chlorine, 0.35% bromine, 0.55% iodine or 1.0 to 3.0% bismuth or 0.50 to 3.0% antimony. 5. A method for producing the alloy according to claim 1 to 4, characterized in that that the ingredients are melted under an inert or reducing atmosphere, the finished alloy heated to 540 to 815 ° C for a long time, preferably 10 to 20 hours, slowly cooled to not below 204 ° C and then quenched. 6. Thermocouple according to claim 1, 2 and 5, characterized in that one of the two electrical conductor as a negative addition aluminum, gallium, titanium, zirconium, bismuth, tantalum, Uranium, manganese, chlorine, bromine or iodine and the other as a positive addition sodium, potassium, Contains thallium or arsenic. 7. Thermocouple according to claim 1, 3 and 5, characterized in that one of the two electrical conductor as a negative addition copper, gold, gallium, aluminum, indium, silicon, zirconium, Titanium, bismuth, antimony, tantalum, niobium, fluorine, chlorine, bromine or iodine and the other than positive additive contains sodium or potassium. 8. Thermocouple according to claim 1, 4 and 5, characterized in that one of the two electrical conductor as a negative addition gallium, indium, titanium, zirconium, antimony, bismuth, Contains tantalum, niobium, uranium, bromine, chlorine or iodine and the other as a positive additive silver. Considered publications:
German Patent Nos. 872 210, 906 813;
U.S. Patent No. 2,602,095. In addition 8 sheets of drawings © 109 60-7 / 357 5.61