Date of Graduation

2018

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Industrial and Managements Systems Engineering

Committee Chair

Edward M Sabolsky

Committee Co-Chair

Xingbo Liu

Committee Member

Konstantinos A Sierros

Committee Member

Charter D Stinespring

Committee Member

Nianqiang Wu

Abstract

Many common electrical materials used in the current sensing applications suffer from structural and functional issues related to high operating temperatures (750°-1600°C), alternating reducing/oxidizing atmospheres, high pressures and corrosive environments. Therefore, there is a need for development of new materials, which are capable to operate under high-temperature and harsh-environments to provide real-time, accurate sensing during industrial processes such as coal gasification and power generation. In this study, electroconductive ceramic composites were fabricated by incorporating the 20-90 vol% of transition metal silicides (MoSi2, WSi2, NbSi2, TaSi 2, CrSi2) within refractory oxides (Al2O 3, ZrO2, Cr2O3), followed by sintering at 1370°-1600°C in argon. The densification, microstructural evolution, phase development and thermal stability of the composites were studied after sintering and further annealing by Archimedes, Scanning electron microscopy (SEM) and X-ray diffraction/Rietveld (XRD) techniques. Their non-isothermal and isothermal oxidation behavior was studied under ambient air using a thermogravimetric analyzer (TGA) at 50°-870°C to understand the key parameters influencing their high-temperature oxidation. The selected composites were also preoxidized at 1000°-1200°C for 10-120 minutes to better understand their high-temperature oxidation kinetics and surface layer formation, and hence, to enhance their oxidation resistance by forming a protective surface layer. XRD, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used for structural and surface characterization. In addition, their 4-point DC electrical conductivities were measured up to 800°-1000°C to study their electrical performance as a function of the composition and processing. A microstructural image analysis method was also developed for quantitatively characterizing the degree of distribution (homogeneity) within the composites. Lastly, thick-film embedded thermocouples (e.g. MoSi2-Al2O3//WSi 2-Al2O3) were fabricated using selected composites by a screen printing technique. Their thermoelectric properties were measured in argon by a typical hot-cold junction temperature measurements, while their structural and microstructural analyses were performed by XPS, XRD and SEM.

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