5 May 2011 Thin film electrodes and passivation coatings for harsh environment microwave acoustic sensors
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Abstract
°Stable nanostructured ultra-thin electrodes and protective passivation coatings have been developed for langasite (La3Ga5SiO14) based surface acoustic wave (SAW) sensors that can successfully operate in harsh high temperature environments up to 1000°C. Ultrathin (<100nm) nanocomposite Pt-Rh/ZrO2 electrode structures were fabricated by electron beam co-evaporation and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), 4-point resistivity, and scanning electron microscopy (SEM) studies. It was found that the incorporation of ZrO2 into the Pt-Rh electrode films retards recrystallization and de-wetting, thereby maintaining film continuity and low resistivity up to at least 1000°C. XPS results show that with heating at 800°C, the stoichiometry of the bare langasite SAW sensor surface becomes depleted of Ga in a reducing (vacuum) environment, but remains close to the bulk composition when heated in an oxidizing (air) environment. The incorporation of a thin oxynitride (SiAlON or SiZrON) coating over the entire sensor diminishes high temperature roughening and degradation of both the electrode and bare langasite surfaces. The viability and performance of these sensors was validated by experiments in which the SAW devices were tested in a controlled atmosphere laboratory furnace and also attached to rotating turbine blades within a small turbine engine operating with centripetal acceleration loads and temperatures in excess of 52,000g and 650°C, and under cyclical temperature shock conditions.
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Scott C. Moulzolf, David J. Frankel, George P. Bernhardt, Bryn Nugent, Robert J. Lad, "Thin film electrodes and passivation coatings for harsh environment microwave acoustic sensors", Proc. SPIE 8066, Smart Sensors, Actuators, and MEMS V, 806606 (5 May 2011); doi: 10.1117/12.886602; https://doi.org/10.1117/12.886602
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