Material Problems in Using High-Temperature Thermocouples

The material compatibility and thermal stability of ceramic-composite coatings of different oxide ceramics deposited on alumina tubes to prevent the reduction of the alumina were investigated in the high-temperature range between 1750 A degrees C and 1850 A degrees C. It turned out that the coatings were thermally unstable and did not provide adequate protection against the reduction of the alumina tubes. The oxide ceramics formed eutectic compositions with low melting temperatures and were also prone to reduction to elementary metals by carbon. A new type of high-temperature thermocouple on the basis of refractory and noble metals was tested in the temperature range between 1325 A degrees C and 1800 A degrees C. Two metal-sheathed prototypes were constructed. The thermoelectric behavior of the tungsten5%rhenium/iridium thermocouples (W5%Re/Ir) was investigated by different high-temperature exposures, and the thermoelectric stability was checked by repeated measurements at the ice point.