Diffusion Barrier Coatings to Reduce Oxidation Effects in Platinum-Silicon Carbide Thermocouples

A platinum (Pt): silcon carbide (SiC) thermocouple system has been developed for high resolution surface temperature measurements on ceramic matrix composites or CMC’s. High purity platinum was deposited by rf sputtering onto a SiC-SiC CMC substrate coated with a dielectric, which allows the SiC-SiC CMC itself to be utilized as one of thermoelements of the thermocouple. The purpose of the dielectric was to reduce oxidation effects at the Pt:SiC junctions. The thermoelectric output, hysteresis and drift were measured at temperatures up to 1000C. This technology replaces the platinum-palladium thin film thermocouples, which exhibited low thermoelectric output and thus, prompted the current research for a higher resolution sensor approach. The thermoelectric power generated by the Pt:SiC thermocouple is an order in magnitude greater than conventional Pt:Pd or Type S thermocouples, with outputs as large at large as 180μV/K , in comparison to 10 μV/K generated by the Pt:Pd and Type S thermocouples. The results show that the Pt:SiC thermocouples exhibit excellent stability at high temperatures, very low rates of drift on upon thermal cycling , and little diffusivity with the use of a diffusion barrier coating, enabling a very robust temperature sensor technology