Metal Silicide-Refractory Oxide Ceramic Composites for High-Temperature and Harsh-Environment Sensing: Processing, Stability and Thermoelectric Properties
Authors: Yakaboylu Gunes Alp , Chockalingam Pillai Rajalekshmi , Sabolsky Katarzyna , Haynes Daniel J. , Sabolsky Edward M. 
Published: February 2020

Extreme harsh-environments are emphasized as one of the major challenges in many industries such as coal gasification, power generation, oil refineries and steel/glass manufacturing. These harsh conditions are due to the very high operating temperatures (1200°-1650°C), oxidizing/corrosive environments and high pressures. Technologies are sought to monitor the process conditions such as temperature and pressure, and structural health/degradation of the ceramic refractories used in the high-temperature reactors. This is crucial for achieving better process control, improved efficiency, reduced environmental impact and increased lifetime of the process units. However, materials commonly used for real-time sensing in harsh-environments, like noble metals (Pt, Rh etc.) and their alloys, suffer from serious structural and functional issues such as selective oxidation via formation of different volatile oxides, detrimental microstructural and compositional changes, and mechanical degradation. Therefore, there is a great demand for advanced sensing materials capable of operating under such extreme conditions reliably and precisely for long-term without high-temperature degradations.
The objective of this work was to develop transition metal silicide-based (MoSi2, WSi2, TaSi2 etc.) ceramic composites reinforced by various refractory oxide (Al2O3, ZrO2 etc.) particles for fabricating embedded ceramic composite thermocouples and their use in high-temperature and harsh-environments. Prior to the fabrication of thermocouples, the metal silicide-oxide composite compositions having 20-90 metal silicide volume fraction were synthesized via a mixed-oxide route, and sintered up to 1600°C under argon atmosphere to investigate densification, thermal and chemical stability, microstructural evolution, grain growth kinetics, and high-temperature electrical properties as a function of the composite composition (type of metal silicide/oxide and their fractions) and processing. The four-point DC electrical conductivity measurements were performed up to 1000°C. The effect of homogeneity/distribution and secondary phase formation (Mo5Si3, W5Si3 etc.) on the composite properties was studied. The studied composite materials exhibited high chemical/thermal stability, reduced grain growth rates and sufficient electrical properties at high-temperatures. Based on these results, the physical and electrical properties were specifically manipulated by altering the level of percolation of the conductive species (metal silicide) within the refractory constituent (refractory oxide).
Thick-film ceramic composite thermocouples (MoSi2-Al2O3//WSi2-Al2O3, MoSi2-Al2O3//TaSi2-Al2O3 etc.) were fabricated by screen-printing the developed metal silicide-refractory oxide composite materials on alumina substrates, and by incorporating that design within the alumina microstructure via tape lamination processing. These metal silicide-oxide composite thermocouples embedded within alumina substrates were sintered at 1500°C under argon atmosphere to study the phase development/stability, microstructure and thermoelectric performance. The thermoelectric response of these thermocouples was measured up to 1000°C by a typical hot-cold junction temperature measurements using secondary thermocouples. Additionally, various ceramic composite//platinum thermocouples were fabricated, and their thermoelectric response was recorded to estimate the intrinsic Seebeck coefficients (S) of the composite materials and effective S of the ceramic composite thermocouples for a better understanding of their thermoelectric performance. The thermoelectric voltage and Seebeck coefficients of thermocouples generally increased with increasing silicide fraction, as well as, temperature difference. The embedded composite thermocouples successfully generated thermoelectric voltage above 14.1 mV at 1000°C with a sensitivity up to 31.3 µV/K. For reference, a Pt-Rh-based B-type thermocouple produces a thermoelectric voltage of 4.83 mV at the same temperature.

Acknowledgements:

This research was funded by the U.S. Department of Energy, National Energy Technology Laboratory under contract no. DE-FE0012383. The authors would like to thank the project monitor, Maria Reidpath, for her insightful discussions and guidance. The authors would also like to acknowledge West Virginia University Shared Research Facilities for support through materials characterization.