Date

2017

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Materials Science and Engineering

First Adviser

Helen M. Chan

Abstract

Metal-ceramic composites are of interest for a range of applications, including heat exchangers, electronic packaging, piezoelectric actuators and transducers, gas turbine generators and engine components. These composites will derive their properties from the properties of the constituent materials, and from the arrangement of the micro- and nano-structures. Microstructural features are often limited by the available processing methods, so new methods are needed to further refine and control composite microstructures. Partial reduction heat treatment of mixed oxide ceramics is one technique that has been investigated as a way to produce metal-ceramic composites in near-net shapes. As part of this dissertation, novel, hierarchical metal-ceramic microstructures using partial-reduction reactions have been produced. The crystal structure of the starting oxide, delafossite-structured CuAlO2, was shown to determine the structure of these hierarchical nanocomposites, with metallic copper lamella growing fastest on the CuAlO2 (0003) basal plane. An orientation relationship was identified in the nanocomposites as Cu[110]//θ-Al2O3[001] and Cu(111)//θ-Al2O3(-401). It was found that the structure of the nanocomposites can be further controlled and tailored by changing process variables to alter the rates of decomposition and diffusion in the system. SEM and S/TEM characterization techniques were used to investigate the crystal structure before, during, and after the partial reduction process. Atomic resolution images of the reduction front elucidated the change in crystal structure and identified orientation relationships between the CuAlO2 and resulting Cu/Al2O3 nanocomposites. A few regions of the composite microstructures showed evidence of discontinuous coarsening of the nanocomposite. This discontinuous coarsening reaction was shown to induce a phase transformation in the alumina phase from the θ-Al2O3 variant to the δ-Al2O3, while retaining the orientation of the copper phase across the coarsening interface. The decomposition behavior of CuAlO2 was applied to grain boundary interfaces and complexions. Films of CuAlO2 were produced in α-Al2O3 by annealing in an oxidizing atmosphere. The CuAlO2 films were decomposed by a reduction heat treatment and examined for evidence of copper at the α-Al2O3 grain boundaries. No evidence of copper segregation to the α-Al2O3 grain boundaries was found during HAADF-STEM imaging. The formation of thick films of CuAlO2 after the oxidation anneal was shown to increase the surface conductivity of alumina by roughly 3 orders of magnitude. 1000ppm titanium was added to improve the wettability of copper via a reactive wetting mechanism. After oxidizing heat treatments, Ti/Cu segregation at the atomic level was observed in the α-Al2O3 grain boundaries. Similar structures were observed after a subsequent heat treatment in a reducing atmosphere. Electron Energy-Loss Spectrometry (EELS) was used to investigate the valence state of the segregant elements. These experiments have demonstrated the use of a mixed copper-aluminum oxide to produce unique, tailored metal-ceramic composites using a near-net shape process. These findings allow the production of these composites, and sets a foundation for future studies of decomposition of similarly structured mixed oxides with different compositions.

Available for download on Saturday, September 01, 2018

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