ISSN: 0090-3973
Page Count: 15

A New Stable Nickel-Base Thermocouple

Starr, CD
Vice-President, Engineering and Research, and research directorMembers of ASTM,Wilbur B. Driver Co.,N.J.,

Wang, TP
Vice-President, Engineering and Research, and research directorMembers of ASTM,Wilbur B. Driver Co.,N.J.,

Abstract

The change in magnitude and direction of electromotive force (EMF) and the accompanying structural changes of nickel-chromium (Ni-Cr), nickel-silicon (Ni-Si), and nickel-silicon-magnesium (Ni-Si-Mg) alloy thermoelements at 1800°F (982°C) in air were studied in comparison with a standard type K thermocouple of the same size. The Ni-Cr versus Ni-Si-Mg couple changes only +200 µV versus +600 µV for the Type K couple after 21 200 hours at that temperature. The EMF changes of K+ and the Ni-Cr alloy are attributed to the simultaneous oxidation of Cr and Si and to the increase in oxygen. The EMF change of K- is caused by the preferential oxidation of silicon, aluminum, manganese, and iron and to the increase in oxygen. The EMF changes of Ni-Si and Ni-Si-Mg are caused by the preferential oxidation of Si and to the increase in oxygen. The presence of magnesium increases the stability of the Ni-Si system by blocking the reaction of silicon and oxygen.

Keywords:
electric potential, thermal conductivity, stability, drift (instrumentation), thermocouples, thermoelements, type K, nickel-based alloys, nickel-chromium, nickel-silicon, nickel-silicon-magnesium, oxygen, oxides, oxidation, preferential oxidation, internal oxidation, air atmosphere, surface morphology, microprobe analysis

Paper ID: JTE10507J
DOI: 10.1520/JTE10507J
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