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Doctor of Philosophy (PhD)
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The use of microelectronic sensors and actuators in harsh, high temperature environments, such as power plants, turbine engines, and industrial manufacturing, could greatly improve the safety, reliability, and energy efficiency of these processes. The primary challenge in implementing this technology is the breakdown and degradation of thin films used in fabricating these devices when exposed to high temperatures >800 °C and oxidizing atmospheres. Zirconium diboride, hexagonal boron nitride, and amorphous alumina are candidate materials for use as thin film sensor components due to their high melting temperatures and stable phases. Zirconium diboride thin films have metallic-like electrical conductivity and remain structurally stable for prolonged periods of annealing above 800 °C in vacuum, but oxidize rapidly in air. This oxidation leads to the crystallization of a zirconium oxide phase which causes the films to become electrically insulating and morphologically unstable.
In order to hinder the oxidation, protective capping layers of hexagonal boron nitride and amorphous alumina were deposited onto the zirconium diboride films, forming a compound, multilayer configuration. The oxidation resistance of hexagonal boron nitride is limited to temperatures below 700 °C, above which the boron nitride oxidizes and evaporates. An amorphous alumina layer, grown by atomic layer deposition, proved to be a more robust capping layer, but was still limited to temperatures below 800 °C. At higher temperatures, the slow oxidation of the zirconium diboride and film stress from thermal expansion caused the alumina layer to crack and expose the underlying zirconium diboride to rapid oxidation.
The growth of highly crystalline hexagonal boron nitride films by reactive magnetron sputtering, as shown in this work, is of great interest not only for oxidation resistant layers, but also for novel electronic devices constructed with 2D materials such as graphene. Furthermore, this work demonstrates the remarkable high temperature stability of zirconium diboride thin films in vacuum, and their instability in air. The use of oxidation resistant capping layers to provide protection from harsh atmospheres allows zirconium diboride to operate at higher temperatures. Further refinement of these capping materials will be required, however, in order to reliably extend film operation to temperatures above 800 °C.
Stewart, David Murdock, "Zirconium Diboride, Hexagonal Boron Nitride, and Amorphous Alumina Thin Films for High Temperature Applications" (2016). Electronic Theses and Dissertations. 2534.
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