William E. Mayo

Structural, optical, and surface acoustic wave properties of epitaxial ZnO films grown on (011̄2) sapphire by metalorganic chemical vapor deposition

High-quality ZnO films are receiving increased interest for use in low-loss high-frequency surface acoustic wave (SAW) devices, acousto-optic and optical modulators, as buffer layers for III-nitride growth, and as the active material in ultraviolet solid state lasers. In this work, high quality epitaxial ZnO films were grown on R-plane sapphire substrates by metalorganic chemical vapor deposition. The structural, piezoelectric, and optical properties of the ZnO films on R sapphire have been investigated. The epitaxial relationship between ZnO and R-Al2O3 was found to be (1120) ZnO∥(0112) Al2O3, and [0001] ZnO∥[0111] Al2O3. The interface between as-grown ZnO and R sapphire was atomically sharp and semicoherent, as evaluated by transmission electron microscopy. On annealing the films at temperatures above 850 °C, a solid state reaction occurred between ZnO and Al2O3, resulting in the formation of ZnAl2O4 (spinel) at the interface. A 15–20 nm spinel layer formed when the ZnO film was annealed at 850 °C fo...

High pressure/low temperature sintering of nanocrystalline alumina

Abstract Bulknanocrystalline α-Al2O3samples with a relative density >98% and a grain size

Theory of high pressure/low temperature sintering of bulk nanocrystalline TiO2

A model to predict the densification and grain growth rates ofn-TiO2 during high pressure/low temperature sintering has been developed and validated by experiments. In this model, densification during intermediate stage sintering is based on a modified grain boundary creep process. For late stage sintering, a modified grain boundary diffusion model that governs atom migration from interparticle boundaries to pores is used. The rate equations for densification during both stages are affected by the increased driving force and decreased diffusivity induced by high pressure. During the sintering process, the concurrent grain growth is modeled through a pore-controlled pinning model, which indicates that grains grow little during the intermediate sintering stage, but experience rapid growth during the final stage.

On the processing of nanocrystalline and nanocomposite ceramics

Abstract Two methods are being developed for the production of nanostructured bulk ceramics. The first method utilizes hot pressing to consolidate a flame-synthesized metastable nanopowder; taking advantage of a pressure-induced phase transformation to suppress grain growth during consolidation. This process; called Transformation Assisted Consolidation (TAC), is being used to prepare test samples of single phase nanoceramics- nanocrystalline ceramics. The second method utilizes plasma spraying of an aggregated powder feed to generate a splat-quenched metastable phase with extended solid solubility. In this case; TAC processing promotes co-nucleation of two or more nanophases from the rapidly quenched material; which provides an additional mechanism to limit grain growth during sintering. This variant on TAC processing is being used to prepare test samples of multiphase nanoceramics- nanocomposite ceramics.

Plasma-sprayed nanostructured Al2O3/TiO2 powders and coatings

Air plasma spray has been used to produce metastable oxide-ceramic powders and coatings, starting with commercially available Al2O3/13TiO2 powder feed. The feed material undergoes rapid melting and homogenization in the high-temperature zone of the plasma jet. A metastablex-Al2O3·TiO2 phase is formed when the molten droplets are quenched on a chilled substrate. The metastable phase has a defect spinel structure and a nanocrystalline grain size. When heated, it decomposes into an equilibrium two-phase structure, consisting ofα-Al2O3 andβ-Al2O3·TiO2. Both types of ceramic materials have potential as hard, wear-resistant coatings.

Structure and interface-controlled growth kinetics of ZnAl2O4 formed at the (112̄0) ZnO/(011̄2) Al2O3 interface

The solid state reaction between metalorganic chemical vapor deposition grown epitaxial ZnO films and the R-plane sapphire substrate after annealing at 1000 °C for various times in an O2/N2 atmosphere was studied in detail. Multiple epitaxial relationships between the reaction product (ZnAl2O4) and the reactants were observed, as determined by cross-sectional transmission electron microscopy. In the dominant epitaxial relationship (A1), the (220) plane of ZnAl2O4 was parallel to the (1101) plane of Al2O3. A twin (A2) of orientation A1, i.e. (220) ZnAl2O4//(1011) Al2O3, and a closely related orientation (B) wherein the (220) ZnAl2O4 plane is parallel to the (1210) ZnO plane (which is equivalent to a 5° clockwise rotation about the [112] ZnAl2O4 or [0001] ZnO zone axis relative to A2), were also observed. Enhanced growth was observed at grain boundaries. It was necessary to measure the spinel growth rate from grains with the same orientation far away from grain boundaries because the growth rate wa...

Thermal sprayed nanostructured WC/Co hardcoatings

Hardcoatings of WC/Co, produced by high-velocity oxy-fuel (HVOF) and air plasma spray (APS) deposition, have been studied. During HVOF deposition, nanostructured powder experiences more decarburization than conventional powder, whereas in APS deposition, just the opposite effect occurs This is explained in terms of the influence of the highly porous, spherical-shell morphology of the nano-WC/Co particles on their melting characteristics and reaction kinetics. In particular, heterogeneous melting and localized superheating of the high-surface-area powder is considered to be a controlling factor in decarburization. The situation is further complicated in APS deposition by high-temperature vaporization of Co and C.

High pressure and low temperature sintering of bulk nanocrystalline TiO2

Abstract Bulk n-TiO2 samples with a relative density as high as 95% and a grain size less than 50 nm were fabricated by hot-pressing at temperatures as low as 400 °C and at pressures up to 1.5 GPa. During hot-pressing, the anatase phase transformed to the rutile phase and the amount of transformation increased with sintering pressure. The grain size in both the anatase and the rutile phase increased with sintering pressure at a constant temperature but the grain size of the transformed phase is always smaller than that of the starting material. We believe that the smaller grain size of the rutile phase is related to multiple nucleation events in the anatase phase during sintering at very high pressure. The average grain size increased from 27 nm in the original powder to only 45 nm in the compact after hot-pressing. Analysis of the grain size and closed porosity by transmission electron microscopy suggested that closed pores at grain boundary triple junctions might also retard the grain boundary migration and thus prevent grain growth. A competing mechanism is also proposed in which the rate of grain growth is controlled by the pressure effect on the bulk diffusion rate and interface energy.

Transformation-assisted consolidation of bulk nanocrystalline TiO2

Abstract Bulk nanocrystalline TiO2 samples have been produced by high pressure/low temperature sintering. Nanophase TiO2 powder with metastable anatase structure and particle size ∼ 38 nm was used as the starting material. During sintering, the anatase phase transformed to either rutile or srilankite phase, depending on the pressure/temperature combination. Grain growth was limited by the low sintering temperature and multiple nucleation events in the parent phase. The transformed rutile grain size was found to decrease with sintering pressure, due to increasing nucleation rate and decreasing growth rate with high pressure. In contrast to previous researchers who found exaggerated grain growth during sintering of n-ceramics, we have demonstrated that it is possible to produce a dense sintered compact with grain size even smaller than that of the starting powder by proper selection of sintering parameters.

Predicting IGSCC/IGA susceptibility of Ni-Cr-Fe alloys by modeling of grain boundary chromium depletion

Abstract Computer simulations based in part on a semi-empirical model of chromium diffusion in Alloy 600 (Ni-16 Cr-9 Fe) have been performed to predict heat treatment regimes that produce high susceptibility to intergranular stress corrosion cracking (IGSCC) or intergranular attack (IGA). The model is based in part on empirical data to establish boundary conditions for the one dimensional discretized diffusion equation that describes chromium depletion in the presence of semi-continuous grain boundary carbides. The simulations correctly predict the minimum Cr concentration (C min ) at the grain boundary and also correctly estimate the depletion zone halfwidth for the thermal treatments for which experimental data is available. Once the model was verified by the limited available experimental data, calculations were then extended to much wider temperature and time ranges. Based on these calculations, a time-temperature contour plot is generated for estimating C min that predicts regions of high IGSCC/IGA susceptibility. Agreement between the calculated contour plot of C min and the limited experimental observations of IGSCC and IGA is good.