Date of Award

Spring 5-5-2023

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Advisor

Robert J. Lad

Second Committee Member

Robert W. Meulenberg

Third Committee Member

James P. McClymer

Additional Committee Members

MacKenzie R. Stetzer

Carl P. Tripp

Abstract

Yttria-stabilized zirconia (YSZ) is a widely used ceramic material in solid oxide fuel cells, oxygen sensors, and sensing applications due to its high ionic conductivity, chemical inertness, and thermal stability. YSZ is promising active coating for use in miniaturized harsh environment wireless surface acoustic sensors to monitor gases such as H2. Adding catalytic Pt nanoparticles can enhance gas reactivity and lead to associated film conductivity changes.

In this work, thin films with an (8% Y2O3 - 92% ZrO2) composition were deposited onto piezoelectric langasite substrates using RF magnetron sputtering in Ar:O2 - 95:5 gas mixture. Films were grown using growth temperatures (30 - 7000C), deposition rates (0.03 - 0.07 nm/s), and substrate bias (-300 - +300 V). Platinum was deposited in-situ via e-beam evaporation at either 30oC or 400oC and then subsequently annealed to cause nanoparticle formation. YSZ and Pt/YSZ films ionic conductivities were measured and characterized with electrochemical impedance spectroscopy (EIS) in pure N2, or in a 4% H2 - 96% N2 mixture.

X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were also used to analyze the surface composition, crystal structure and nanoparticles morphology, respectively.

By manipulating the deposition parameters, either (111) or mixed (111)/(200) YSZ film crystallographic texture can be achieved. Post-deposition annealing up to 1000oC in air causes grain growth, strain relief and yttria segregation. EIS measurements from YSZ films over the range 400oC - 600oC indicate that ionic conductivities are strongly dependent on yttria segregation and film nanostructure.

For YSZ films decorated with Pt nanoparticles, the surface becomes reactive towards hydrogen. Pt nanoparticles form (111) oriented crystallites, and the amount of yttria segregation is less than that for Pt-free films. Ionic conductivities and sensitivities towards hydrogen depend on the nanoparticle size and film nanostructure. Pt nanoparticles lower the H2 adsorption energy and facilitate the interaction. The conductivity changes that occur corresponding to pure N2 versus exposure to 4% H2 - 96% N2 were found to be reversible. These results indicate that Pt/YSZ films hold promise as hydrogen sensing films that can be incorporated onto a variety of sensing platforms for H2 gas detection and management.

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