Compatibility of Materials for Use at High Temperatures with W–Re Thermocouples

Abstract

Material changes caused by exposure to harsh environments are well known to be the major source of decalibration (drift) in thermocouples and arise from changes in the Seebeck coefficient in the region of the temperature gradient along the thermoelements. Methods of self-validation, which consist of a miniature temperature fixed-point cell mounted over the tip of the thermocouple in situ, have been developed to determine and counter the impact of the drift. Putting this additional component into the hot region introduces complications due to inter-diffusion among the different materials. In order to protect the thermocouple from additional influences due to the presence of the self-validating cell, NPL has performed controlled tests to characterize the material changes which occur when typical high temperature W–Re thermocouples are exposed to temperatures up to $$2300\,^{\circ }$$ C in the presence of other materials. Combinations of the cell crucible material (carbon), thermocouple sheath material (tantalum), and thermoelement insulator materials (boron nitride, hafnia, silicon carbide, and yttria–stabilized zirconia) have been tested with controlled exposure at selected temperatures between $$1700\,^{\circ }$$ C and 2300 $$^{\circ }$$ C, for a period of 10 h to 24 h. Detailed visual results are presented and supported with scanning electron microscope surface characterization, allowing conclusions on the compatibility of these individual materials to be drawn. Importantly, the tantalum reaction with carbon is found to be such that a typical sheath thickness of 0.5 mm reached a saturation composition within 10 h at 2300 $$^{\circ }$$ C, when in direct contact with carbon.