US 2802894 A
Description (OCR text may contain errors)
Aug- 13, l957 G. SCHNEIDER ETAL THERMOCOUPLE Filed April l250, 1956 l l l l l l l l ATTORNES United States Patent iOtlice Patented` Aug. 13, 1957 l 2,802,894 THERMOCOUPLE Gnther Schneider, Frankfurt am Main Eschersheim, and YAlfred Boettcher, Hanau am Main Hohe Tanne, Germany, assignors to Deutsche Goldund Silber-Scheideanstalt vormals Roessler, Frankfurt am Main, Germany Application April 30, 1956, Serial No. 58i,479 .Claims priority, application Germany April 30, 1955 9 Claims. (Cl. 13G-S) The invention relates to apparatus for measurement of high temperatures and especially between 1500 and 2000 C. with the aid of thermocouples formed of high melting metals and their alloys.
It is known to employ thermoelements in the form of the so-called thermocouples for the measurement of temperatures. The best known thermocouple elements are those of nickel/ chromium-nickel and platinum/platinum-rhodium.
The simplicity of the use of thermocouples, as well as their accuracy, renders it desirable to provide thermocouples for use at temperatures above l500 C. However, up until now, the previous attempts employing combinations of the known high melting metals have led only to limited solutions. The thermocouples produced either did not meet the technological requirements or were only of limited application. The reason for this was that the wire employed for the thermocouples either still had too low a melting point or the sensitivity thereof especially against thermal influences, which can easilyA lead to undesirable embrittlement and therefore early destruction of the elements or at least to a diminished accuracy. In a similar manner, a change in alloy composition through evaporation of one of the alloy components during use of thermocouples at high temperatures can lead to diiiiculties. Also, in platinum metals, the sensitivity to the so-called platinum poisons is disadvantageous. Furthermore, it is, of course, also necessary to select such combinations for thermocouples that a suiiciently high thermoelectric E. M. F. is provided. For example, thermocouples of Ta/W, Mo/Ta and Mo/W only provide a relatively low thermoelectric E. M. F. and also the change in thermoelectric E. M. F. with change in temperature is too low for practical purposes.
According to the invention, it was found that excellent thermocouples could be produced for the measurement of high temperatures from combinations of iridium paired with rhenium-iridium alloys or of iridium-rhenium alloys paired with rhenium-iridium alloys of different composition. Preferably, the thermocouples according to the invention are sealed in suitable gas tight ceramic armatures in a known manner to protect them fully from the influence of the atmosphere or at least to such an extent that the life of the thermocouples is not deleteriously affected. Preferably, such armatures are tubes of high melting oxides, such as aluminum oxide, beryllium oxide, thorium oxide or zirconium oxide.
The thermocouples according to the invention, as a1- ready indicated above, essentially comprise an iridium oriridium-rhenium alloy (rich in iridium) element joined (paired) with a rhenium-iridium alloy (rich in rhenium). The rhenium-iridium alloy paired with a pure iridium element preferably contains up to 40% of iridium and alloys of 60%-.80% of `rhenium and 40%-20% of iridium have provedparticularly suitable, as within this range the thermoelectric E. M. F. of the thermocouple is rather insensitive to small changes in the composition of the alloy. It was also found that when rhenium-iridium alloys containing 70 to 90% of rhenium were paired with pure iridium the thermoelectric E. M. F. of thethermo- .couples was to a great extent independent of the temperature of the cold junction of the thermocouple so that in use it is not necessary to maintain au exact control over the temperature of the cold junction of such thermocouples. In the iridium-rhenium alloy/rhenium-iridium alloy thermocouples according to the invention, the iridium-rhenium alloy contains` up to 10% of rhenium, and preferably about 5 to 10% of rhenium, and the rhenium-iridium alloy contains 5090% of rhenium, and preferably 60-80% of rhenium. In the latter range it was again found that the thermoelectric E. M. F. of the iridium-rhenium/rhenium-iridium thermocouples is rather insensitive to small changes in the rhenium-iridium alloy composition.
The following Tables I and II tabulate the thermoelectric E. M. F. of various Ir/Relr and IrRe/Relr thermocouples at temperatures from 1500 C. to 2100 C. in m. volts, as well as change in thermoelectric E. M. F. for 1 C. change in temperature at 2000 C. in p. volts.
Table I Re 90%, I1' 10% 5. 36 7. 87 10. 37 13.32 14.8 Re Ir 20%-- 6. 45 8.63 11.26 14. 37 15.6 Re 70%, Ir 30". 6. 92 9. 25 11. 99 15.22 16. 2 Re 60%, Ir 40%.- 7. 46 9. 66 12. 40 15. 70 16. 5 Re 40%, Ir 60%.. 5. 81 6.83 8.37 10. 24 9. 4
Table II IrRe (Ir 95% Re 5%)lReIr THERMOGOUPLES E in m. volts at- DE/dt Composition of Relr Alloy in ,t
1,500 1,700 1,900 2,100 Volts C. C. C
Re Ir 10% 3.36 5. 37 7.07 8.07 5. 0 Re 70%, It 30?". 4. 92 6.75 8. 69 10.22 7. 7 Re 60%, Ir 40% 6. 46 7.16 9.10 10.70 8. 0 Re 50%, Ir 509'. 5.17 6, 42 7. T2 8.72 5.0
As indicated above, it is preferable to protect the thermocouples according to the invention against inuence of the atmosphere by providing a non-metallic gas tight casing for the thermocouples themselves. It is known that high melting oxides, such as aluminum oxide, zirconium oxide, thorium oxide and others, when suitably pure, can besintered to produce practically gas tight shaped bodies. Such shaped bodies have already been employed as protective tubes for thermoelements. However, for the purpose of the invention, it is important that every inuenee of the atmosphere, especially of oxygen containing gases, be excluded with certainty. This, for example, can be accomplished by iilling the casings containing the thermocouples according to the invention with inert gases or to continuously flush the casings with gases which have no deleterious eifect upon the materials of the thermocouples at the temperatures to be measured. When the casing is sealed absolutely gas tight, it is generally not necessary to employ a protective gas. However, in accordance with an advantageous modification according to the invention, it has been found desirable to introduce a getter metal into the casing to remove any last traces of air or oxygen. The getters can be introduced into the casing either in powder form or as shaped bodies. Getter metals, such as titanium, tantaluln, thorium or zirconium, are capable of binding gases such as oxygen, nitrogen 'or vaporsof metalloids with the formation of solid compounds at elevated temperatures, such as v such as given in Table III.
non-metallic protective casings for the thermocouples can be provided with a glaze-like coating of mixtures of higher melting oxides `and appropriate -quantities otflux-V ,ing oxides in order to ensurea gas tight seal. Such -oxide mixtures are especially desirable for bondingand sealing the joints in the casing when it` is not .made of one piece. Preferably, the composition of the glazes is selected so thatV itsy melting or softeningpoint can be selected `with regard to the temperatures to be measured betweenvlOOO" C. and l700 C.l These glazes wet the non-,metallic material of the'casings upon-fusion and form a `uniform tirmly adhering coating.` The bond of such it possible to use such oxide mixtures to seal the casing and especially the joints thereof for thermocouples to be used in measuring temperatures of up to 21007 C., even though such temperatures substantially exceed the actual melting point of such oxide mixtures, without danger of failure of the seal. Oxide mixtures which canbe used either to form a glazed `coating on the casing or to bond the joints between separate parts of the casings are given by way of example in the following table:
v Table III aluminum oxide and sealing masses 27 and 28 of an oxide mixture such asy given' inV Table III. A tube 29 of a getter metal, such as titanium, is provided within protective tube 21. An outerfglazecoating 30 is also provided on tube 21. Y
`We claim: Y f g l. A thermocouple comprising an iridium4 rich fele' ment selected from the group consisting of aV pure iridium element and iridium rich iridium-rhenium alloy'` elements l containing at least 90% of iridium joined with a rheniumiridium alloy element containing up to 60% of iridium.
2. A thermoelectric element comprising a thermocougroup consisting of a pure iridium element and iridium l5 rich iridium-rhenium alloy` elements containing atleast 90% of iridium joinedV with a rhenium.-iridium alloy element containing'up to 60% of iridiun'rsealed` in gas tight casing of non-metallic thermostable'imaterial.
3. A thermoelectric elementY comprisingga thermocouple comprisingan iridium rich element selected from'the group consisting of a pure iridium element and iridium.
rich iridium-rhenium alloy elements containing; atleast 90% of iridium joined with a rhenium-iridiumalloy ele.-
rnent containing up to 60% of iridium sealed'V with a getter in a gas tight casing of non-metallick thermosta'ble material. i
4. A thermoelectric element lcomprising a thermocou:
ple comprising an iridium rich element selected from the group consisting of a pure iridium element and iridium,
rich iridium-rhenium alloy elements containing atleast 90% of iridium joined with a rhenium-,iridium alloyele-V Melting Point A1203 B20; BeO CaO FezOs MgO The accompanying drawings illustrate several modications of thermocouple elements according to theinvention.
In such drawings:
Fig. l shows a longitudinal section of one form of such an element; and
. Fig. 2 shows a longitudinal section of another form of such an element. j
In the modification shown in Fig. 1, the thermocouple 13 comprises an iridium wire or an iridium-rhenium alloy wire joined with a rhenium-iridium alloy wire at terminal or point 14 to form the hot junction` of the thermocouple. A protective tube 11 of a high melting oxide, for ex-r ample, sintered aluminum oxide, surrounds the hot junction and the wires of the thermocouple are supported in the two capillaries provided in support 12 which `preferably is formed of the same material as tube 11. Sealing mass 15 seals the joint between tube 11 and support 12 and sealing mass 16 provides a seal at the end of support 12 where the wires of the thermocouple 13 leave such support. The sealing mass is a mixture of oxides An outer glaze coating 17 of ythesarne material is provided on tube 11.
In the modication shown in Fig. 2, the thermocouple 24 comprises an iridium wire joined with a rheniumiridium alloy Wire at terminal or point 25 to form the hot junction of the thermocouple. One of the wires of suchV thermocouple is provided with a sintered aluminum oxide insulating tube 22 and the other with va plurality of short insulating tubes 23 of the same material. `A protective tube 21 of aluminum sintered oxide surrounds the hot junction of the thermocouple and the open end thereof is sealed oi with the aid of stopper 26 of sintered ment'containing up to 60% of iridium sealed in` a gas tight casing of thermostable sintered oxide. l
5. A thermoelectric element comprising a thermocouple comprising an iridium rich element selectedjfrornS the group consisting of a pure iridium element and iridiuml rich iridium-rhenium alloy elements containingat'leastv 90% of iridium joined with a rhenium-iridium alloy ele-` ment containing up to 60% of iridium sealed in algas` tight casing of thermostable sintered oxide provid'edrwith a glaze having a softening point betweenQlOOO C. and;` l700 C. composed of a mixture of oxides and a'ux."'
joined with a rhenium-iridium alloy element ,containing up to 40% of iridium.
7. A thermocouple comprising a pure iridiumV eleliije Y with a rhenium-iridium alloy element containingrO,to'`
% of rhenium.
References Cited in the tile of this patent Y UNITED STATES PATENTS 2,012,465 Godecke v Aug. V27, 1935 i i, j FOREIGN PATENTs' ple comprising an iridium rich element selected from the 6. A thermocouple comprising a pure iridium element