Polymer derived ceramics, high temperature sensor, temperature dependence conductivity, raman, solid state nmr, epr
Polymer-derived ceramic (PDC) is the name for a class of materials synthesized by thermal decomposition of polymeric precursors which excellent thermomechanical properties, such as high thermal stability, high oxidation/corrosion resistance and high temperature multifunctionalities. Direct polymer-to-ceramic processing routes of PDCs allow easier fabrication into various components/devices with complex shapes/structures. Due to these unique properties, PDCs are considered as promising candidates for making high-temperature sensors for harsh environment applications, including high temperatures, high stress, corrosive species and/or radiation. The SiAlCN ceramics were synthesized using the liquid precursor of polysilazane (HTT1800) and aluminum-sec-tri-butoxide (ASB) as starting materials and dicumyl peroxide (DP) as thermal initiator. The as-received SiAlCN ceramics have very good thermal-mechanical properties and no detectable weight loss and large scale crystallization. Solid-state NMR indicates that SiAlCN ceramics have the SiN4, SiO4, SiCN3, and AlN5/AlN6 units. Raman spectra reveals that SiAlCN ceramics contain “free carbon” phase with two specific Raman peaks of “D” band and “G” band at 1350 cm1 and 1600 cm1 , respectively. The “free carbon” becomes more and more ordered with increasing the pyrolysis temperature. EPR results show that the defects in SiAlCN ceramics are carbon-related with a g-factor of 2.0016±0.0006. Meanwhile, the defect concentration decreases with increasing sintered temperature, which is consistent with the results obtained from Raman spectra. iv Electric and dielectric properties of SiAlCN ceramics were characterized. The D.C. conductivity of SiAlCN ceramics increases with increasing sintered temperature and the activation energy is about 5.1 eV which higher than that of SiCN ceramics due to the presence of oxygen. The temperature dependent conductivity indicates that the conducting mechanism is a semiconducting band-gap model and follows the Arrhenius equation with two different sections of activation energy of 0.57 eVand 0.23 eV, respectively. The temperature dependent conductivity makes SiAlCN ceramics suit able for high temperature sensor applications. The dielectric properties were carried out by the Agilent 4298A LRC meter. The results reveal an increase in both dielectric constant and loss with increasing temperature (both pyrolysis and tested). Dielectric loss is dominated by the increasing of conductivity of SiAlCN ceramics at high sintered temperatures. SiAlCN ceramic sensors were fabricated by using the micro-machining method. High temperature wire bonding issues were solved by the integrity embedded method (IEM). It’s found that the micro-machining method is a promising and cost-effective way to fabricate PDC high temperature sensors. Moreover IEM is a good method to solve the high temperature wire bonding problems with clear bonding interface between the SiAlCN sensor head and Pt wires. The Wheatstone bridge circuit is well designed by considering the resistance relationship between the matching resistor and the SiAlCN sensor resistor. It was found that the maximum sensitivity can be achieved when the resistance of matching resistor is equal to that of the SiAlCN v sensor. The as-received SiAlCN ceramic sensor was tested up to 600 C with the relative output voltage changing from -3.932 V to 1.153 V. The results indicate that the relationship between output voltage and test temperature is nonlinear. The tested sensor output voltage agrees well with the simulated results. The durability test was carried out at 510 C for more than two hours. It was found that the output voltage remained constant for the first 30 min and then decreased gradually afterward by 0.02, 0.04 and 0.07 V for 1, 1.5 and 2 hours.
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Doctor of Philosophy (Ph.D.)
College of Engineering and Computer Science
Materials Science Engineering
Materials Science and Engineering
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Dissertations, Academic -- Engineering and Computer Science, Engineering and Computer Science -- Dissertations, Academic
Shao, Gang, "Development Of Polymer Derived Sialcn Ceramic And Its Applications For High-temperature Sensors" (2013). Electronic Theses and Dissertations, 2004-2019. 2690.