A SEARCH FOR ALLOYS SUITABLE FOR HIGH-TEMPERATURES THERMOCOUPLES (in Russian)
The prime requirement of a high-temperature thermocouple is stability of voltage reading at as high a temperature as possible. Thus, for temperatures from 1300 to 1600 tained C a Pt-PtRh couple can be used, but the readings are stable only up to 1300 tained C. The refractory metals tungsten and molybdenum are used to measure temperatures up to 2000 tained C, but these suffer from the disadvantage that they are readily oxidized in air. Alloys of the noble metals are the most stable to oxidizing conditions at high temperatures. The principal reasons for variations in voltage readings are oxidation of the alloy, volatility of the metal, diffusion at the hot junction, and chemical reaction with the surium form stable oxides which have a relatively high vapor pressure. The problem of diffusion can be minimized by using different contents of the metal in both junctions. The thermocouples (Pt + 30% Rh)-(Pt + 6% Rh) and Rh-(Pt + 20% Rh) show low voltage readings at the lower temperatures (0 to 600 tained C), but this characteristic is an advantage in high temperature work since the cold junction need not be kept at a constant temperature. A variation in temperature of the cold junction from 0 to 200 tained C will introduce an error of only about 1% in measuring a temperature of 1800 tained C. The calibration for the thermocouple Ir- (Ir + 60% Rh) is practically linear at high temperatures. The melting point of this thermocouple is about 2340 tained C. The stability of the voltuge reading of various thermocouples at high temperatures was investigated. A change in temperature reading of 1% at 1550 tained C was shown by the thermocouple Pt-(Pt + 10% Rh) after 75 hours operations, by (Pt + 30% Rh)-(Pt + 6% Rh) after 230 hours operation and by Rh-(Pt + 20% Rh) after 1500 hours. Tests at 1800 tained C showed that the thermocouple Rh-(Pt + 20% Rh) operated for 100 hours with a change in voltage of combination rat 0.5%, when special pains were taken to exclude silica from the insulator by substituting aluminum or beryllium oxides. It is evident that the diffusion of platinum into the rhodium is of no consequence. The thermocouple (Ir + 60% Rh)-Ir showed signs of recrystallization after operation at 1800 tained C for 25 hours that resulted in an increase of voltage of 0.6%. Subsequent voltage changes did not exceed combination rat 0.3%. The replacement of pure rhodium in the thermocouple Rh-(Pt + 20% Rh) by alloys of platinum and rhodium containing some iridium, palladium or ruthenium was unsatisfactory, since these combinations were less stable. The specific electrical resistances of Rh, Ir, Pt + 20% Rh and Pt + 20% Rh + 10% Ir were determined as a function of temperature. The prime advantage of the noble metal alloys is their resistance to oxidation. (TTT)
- Research Organization:
- Originating Research Org. not identified
- NSA Number:
- NSA-14-019187
- OSTI ID:
- 4185167
- Journal Information:
- Zhur. Neorg. Khim., Journal Name: Zhur. Neorg. Khim. Vol. Vol: 5
- Country of Publication:
- Country unknown/Code not available
- Language:
- Russian
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Related Subjects
ALUMINUM OXIDES
BERYLLIUM OXIDES
CHEMICAL REACTIONS
COATING
DIFFUSION
EFFICIENCY
ELECTRIC CONDUCTIVITY
ELECTRIC POTENTIAL
ENGINEERING AND EQUIPMENT
HIGH TEMPERATURE
IRIDIUM
IRIDIUM ALLOYS
IRIDIUM OXIDES
MEASURED VALUES
MELTING POINTS
METALS
MOLYBDENUM
OXIDATION
PALLADIUM
PLATINUM
PLATINUM ALLOYS
RECRYSTALLIZATION
REFRACTORIES
RHODIUM
RHODIUM ALLOYS
RUTHENIUM
RUTHENIUM OXIDES
SILICON OXIDES
STABILITY
TEMPERATURE
THERMOCOUPLES
TUNGSTEN
VARIATIONS
VOLATILITY