Influence of Microstructure and Phase Morphology on the Stability of High Temperature Irradiation Resistant Thermocouples

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Development of in-core instrumentation is driven by the pursuit of safer and more economic energy production from both existing nuclear reactors and Generation IV reactor designs. Idaho National Laboratory (INL) has developed high temperature irradiation resistant thermocouples (HTIR-TCs) for temperature sensing inside Generation IV nuclear reactors. These thermocouples are composed of phosphorus-doped niobium (Nb-P) and lanthana-doped molybdenum (Mo-LaO) thermoelements, an alumina (Al2O3) insulation, and a niobium sheath. HTIR-TCs require an initial heat treatment exceeding the maximum service temperature to stabilize the generated electromotive force (EMF) signal; however, the mechanism behind this stabilization is not well understood. This work evaluates the impact of the stabilization heat treatment on the thermoelements' microstructures, chemical stability, and electrical properties to determine the mechanisms by which the EMF signal stabilization occurs. Accordingly, during the preliminary heat treatment, a secondary Nb3P phase formed within the Nb-P, along with an interaction region at the Al2O3/niobium interface. The formation of secondary phases within the niobium leg of the thermocouple causes an increase in the Seebeck coefficient. Stabilization of the HTIR-TC EMF signal was found to be dependent upon both the equilibrium of a diffusion interaction region at the Nb-P/Al2O3 interface and the formation of Nb3P precipitates.