MoSi2- and WSi2-based embedded ceramic composite thermocouples for high-temperature and harsh-environment sensing

MoSi2- and WSi2-based ceramic composite thick-film thermocouples were deposited on alumina substrates by a screen printing technique, and subsequently thermally processed and embedded within an alumina preform. The one leg of the thermocouple was formed with a MoSi2-Al2O3 composite, while the other leg was composed of a WSi2-Al2O3 composite. Four compositions were prepared by adjusting the volume percentages of MoSi2 and WSi2 from 50 to 90 vol%. The phase formation and microstructure of the sintered thermocouples were studied by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The thermoelectric response of the thermocouples was measured by a typical hot-cold junction temperature measurements using secondary thermocouples. The thermoelectric voltage and Seebeck coefficients of thermocouples generally increased with increasing metal silicide content, as well as, temperature difference. The highest thermoelectric performance was achieved with [90-10] vol% MoSi2-Al2O3//[90-10] vol% WSi2-Al2O3 composite thermocouples, which may be related to the increased amount of the 5-3 metal silicide phases (Mo5Si3, W5Si3) that were present in the composite systems after sintering. The 5-3 metal silicide content was shown to increase with metal silicide content after sintering. The measured effective Seebeck coefficients of the MoSi2-Al2O3//WSi2-Al2O3 composite thermocouples were higher than their estimated Seebeck coefficients. A long thermocouple (22.9 cm length, 0.6 cm width) with the legs composed of [90-10] vol% MoSi2-Al2O3 and [90-10] vol% WSi2-Al2O3 displayed stable thermoelectric performance at 1350 °C with a peak thermoelectric voltage of 19.3 mV.