Kim Y-G, Lee Y, Kim J, 1998, "Composition change of type S thermocouples after high-temperature annealing at 1400 °C" High Temperatures - High Pressures 30(4) 465 – 470
Composition change of type S thermocouples after high-temperature annealing at 1400 °C
Yong-Gyoo Kim, Young Lee, Jae Kim
Received 23 July 1997, in revised form 09 September 1997
Abstract. The changes of thermoelectric properties and wire composition of a type S thermocouple after high-temperature exposure were investigated for wires from five different manufacturers. After 1440 h at 1400 °C, the thermal emfs decreased by about 3 ~ 4 mV and the immersion characteristics became two or three times worse. In the hot-junction area, electron probe microanalysis and scanning electron microscopy showed that large amounts of rhodium moved to the pure platinum side through air, and vaporisation of rhodium was preferential at the grain boundary area. About 4 wt% rhodium was detected in the pure platinum wire surface and only 5 wt% rhodium remained in the surface of the platinum - rhodium alloy leg. Inside the alumina insulator, rhodium vaporisation did not occur and only the grain coarsening phenomenon was observed. x-Ray photoelectron spectroscopy results on the hot-junction area indicated that rhodium had moved and been deposited on the pure platinum leg in its elemental state.
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