Date of Award

Spring 6-21-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Alan W. Weimer

Second Advisor

Charles B. Musgrave

Third Advisor

Sanghamitra Neogi

Fourth Advisor

Colin A. Wolden

Fifth Advisor

J Will. Medlin

Abstract

Many metal and ceramic materials fail due to environmentally triggered corrosion reactions. The development of advanced environmental barrier coatings (EBCs) to inhibit these types of reactions is imperative for materials with desirable characteristics to be utilized in hostile environments. Both materials discovery and advanced deposition methods are important avenues to the protection of materials from environmentally related side reactions.

One significant example of this type of failure is the use of silicon carbide (SiC) is oxidative environments such as water based nuclear reactors and high-temperature steam systems. In order to modify SiC to be used in high-temperature solar driven water splitting systems oxidation resistant, thin, pinhole-free, crystalline coatings were deposited using atomic layer deposition (ALD). Three different coating materials were compared experimentally. Mullite, alumina (Al2O3), and boron nitride (BN) coated SiC particles were exposed to high-temperature steam with the use of thermogravimetric analysis to assess the oxidation resistance of the films. The best performing film reduced the oxidation of silicon carbide by 62% relative to uncoated particles under these conditions.

Two film thicknesses of Al2O3 ALD coatings were used to experimentally probe the kinetic effects of ALD films on the oxidation of SiC in high-temperature steam. The kinetic triplet for each sample was determined by fitting the resulting data to the D4 kinetic model (representing 3-dimensional diffusion). Reaction rate data for the oxidation reaction of uncoated SiC and SiC coated with a 10 nm film indicate that the coating reduces the oxidation of a SiC component by up to a factor of 5 for steam oxidation at temperatures between 1050°C and 1150°C.

To study the performance of ALD EBCs, films were applied to SiC tubes that were used in a lab system to study long-term steam exposure and at the NREL high flux solar furnace (HFSF) to study the performance of the coating in a pilot scale solar hydrogen system. Hybrid mullite/alumina coatings were applied to tubes for both applications. One of the tubes was exposed to high-temperature steam in a laboratory environment where weight change and visible oxidation was monitored relative to an uncoated SiC sample. Visual examination of the tubes after these experiments clearly shows a difference in the oxide formation on the surface of the tubes. Additionally, mass change measurements show the uncoated tube oxidizing at three times the rate of the uncoated. The other tube was installed in a pilot scale solar reactor at NRELs HFSF to study the effects of the film during redox driven water splitting. The coating of the tube visibly reduced oxidation of the external containment surface during on-sun redox. Additionally, post testing elemental analysis determined that active materials did not chemically interact with either pure SiC nor the coating applied.

Another significant application of EBCs is the use of low-cycle alumina ALD films to enhance cycling stability of lithium ion battery cathodes. The true nature and deposition mechanism of these films is characterized with a comparison of eight film thicknesses. The films generated within the first 10 cycles of deposition are shown to improve the cycling stability of lithium ion battery cathodes due to the preferential deposition of trimethyl aluminum (TMA) onto transition metal sites in the lattice over intercalated lithium. This results in covering and stabilizing metals sensitive to dissolution in electrolyte without blocking lithium intercalation pathways.

Available for download on Thursday, January 27, 2022

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