Shelley R. Gilliss

The monitoring of grain-boundary grooves in alumina

The geometry of grain-boundary grooves in polycrystalline alumina has been studied using a combination of visible-light microscopy (VLM) and atomic force microscopy (AFM). Samples were heat treated in air to induce thermal grooving. A VLM map was made from the central region of each sample. Profiles of grooves located within the VLM maps were measured using AFM. Using again VLM maps, the same regions were re-examined using AFM after an additional heat treatment. This approach allowed the geometries of grooves which formed at migrating grain boundaries to be directly compared with those at stationary boundaries. The groove profiles measured were asymmetric at migrating grain boundaries and more nearly symmetric at stationary grain boundaries.

Nanoscale EELS analysis of oxides: composition mapping, valence determination and beam damage

Beam damage to ceria abrasive particles during EELS measurements with ~1 nm probes of ~1nA was negligible for typical analysis times (1-5 s). Ce3+ and tri-valent impurities reduce near-surface regions. Ce valence was measured from Ce M5/M4 white line ratios. By defocusing a 1 nA probe to ~5 nm, beam damage to nanoscale MgAl2O4 spinel was sufficiently slowed to allow spectrum imaging measurements of composition variations. Recording spectrum lines in TEM mode can be attractive when dose-rate rather than dose is the limiting factor in beam damage. Multivariate statistical analysis of data from a CoO-Co3O4 interface has revealed an additional interface-related component.

Glass and metals on crystalline oxides

Wetting and dewetting of liquid metals and glasses on ceramic substrates has been investigated using a combination of microscopy techniques. The influence of surface structure on dewetting behavior and the influence of the dewet droplets on the morphology of surface steps has been shown with the aid of experimental observations from many systems. The role of chemistry and kinetics on the wetting behavior has also been demonstrated. Finally, the possibility of exploiting reconstructed ceramic surfaces for nanopatterning of metal particles has been illustrated.

Iron oxide on (001) MgO

Abstract Transmission and scanning electron microscopy have been used to characterize thin films of Fe oxide grown on (001)-oriented MgO using pulsed-laser deposition. Several films, each with a longer deposition time, and hence increasing thickness, were grown under similar conditions of oxygen pressure, deposition rate and substrate temperature. For shorter deposition times, i.e. thinner films, a spinel-structured Fe oxide film occurred, while for longer deposition times a corundum-structured Fe oxide film formed. In some instances it was found that the film had a mixture of the two structures of Fe oxide. The analysis of these films determined that the corundum-structured Fe oxide was being formed via a transformation from the spinel structure. This transformation process led to interesting thin-film morphologies and epitactic alignments.

Nanochemistry of Ceria Abrasive Particles

Surfaces of ceria (CeO2) particles have been studied by electron energy-loss spectroscopy in a field-emission gun scanning transmission electron microscope. All the ceria particles analyzed contained Ce3+ at the surface. Rare-earth impurities such as La were enriched at the surface and were observed for particles ranging from tens to hundreds of nanometers in size. The oxidation state of the cerium ion is measured from the Ce M5/M4white-line intensity ratio.

Monitoring Faceting on Ceramic Surfaces

Faceting is the transformation of a planar surface into two or more surfaces of lower energy. Metal, semiconductor and ceramic surfaces can all undergo faceting. The evolution of facets formed on the m-plane (10 1 0) of alumina has been monitored using atomic-force microscopy (AFM). When heat-treated, the (10 1 0) surface reconstructs into a hill-and-valley morphology. The present study investigates the manner in which facets originally form and grow to cover a surface. A gravity-loaded indenter (load of 25 grams) was used to mark a 25 μm × 25 μm square area on as-received, polished alumina specimens. An initial heat-treatment of 1400°C for 10 minutes is carried out to initiate faceting. With the indents as guides the same area can be identified and imaged after each subsequent heat-treatment. The morphology of the facets can be described as being comprised of a “simple” and “complex” surface. The simple surface corresponds to the (1 1 02) plane which is stable over the course of heat treatments, whereas the complex surface gradually transforms to a lower energy surface after several heat treatments and acts as a nucleation site for new facet growth.

Dewetting of liquids on ceramic surfaces at high temperatures.

The influence of surface structure and chemistry on high-temperature dewetting of silicate liquids on ceramic surfaces has been investigated. Model systems based on well-defined crystallography and known chemistry have been used to illustrate the effect of surface roughness and chemistry on the dewetting process. Reconstructed ceramic surfaces provide ideal substrates to study effects of surface roughness. It has been shown that the morphology of dewet droplets depend on the length scale and the crystallography of the facets on the surface. Complex pattern formation due to solute redistribution during dewetting is illustrated in the case of SiO2 dewetting on (001) rutile substrates. The role of kinetics on the dewetting process has also been clarified.

Investigation of Surface Grooves from Migrating Grain Boundaries

Visible-light microscopy (VLM) and atomic-force microscopy (AFM) were used to study the progression of grain-boundary grooving and migration in high-purity alumina (Lucalox ™ ). Groove profiles from the same grain boundaries were revisited using AFM following successive heat-treatments. The grooves measured from migrating grain boundaries were found to have asymmetric partial-angles compared to those measured from boundaries that did not migrate during the experiment. For a moving boundary, the grain with the larger partial-angle was consistently found to grow into the grain with the smaller partial-angle. Migrating boundaries were observed to leave behind remnant thermal grooves. The observations indicate that the boundary may be bowing out during the migration process.

Studying alumina boundary migration using combined microscopy techniques

Thermal grooving and migration of grain boundaries in alumina have been investigated using a variety of microscopy techniques. Using two different methods, polycrystalline alumina was used to investigate wet, (implying the presence of a glassy phase), and dry grain boundaries. In the first, single-crystal Al2O3 was hot-pressed via liquid phase sintering (LPS) to polycrystalline alumina with an anorthite glass film at the interface. Pulsed laser deposition was used to deposit approximately 100-nm thick glass films. Specimens were annealed in air at 1650°C for 20 h to induce boundary migration. Boundary characterization was carried out using visible light (VLM) and scanning electron (SEM) microscopies. Effects on migration due to surface orientation of grains were investigated using electron backscatter diffraction (EBSD). The second method dealt with heat treating dry boundaries in polycrystalline alumina to monitor boundary migration behavior via remnant thermal grooves. Heat treatments were conducted at 1650°C for 30 min. The same region of the sample was mapped using VLM and atomic force microscopy (AFM) and followed over a series of 30 min heat treatments. Boundary migration through a pore trapped inside the grain matrix was of particular interest.

Technique for Monitoring the Etching Rate of Alumina

The effect of chemical and thermal treatments on the grains and grain boundaries of polycrystalline μ-Al 2 O 3 has been examined using a combination of microscopy techniques. Commercially available alumina samples (Lucalox™) were chemically etched in phosphoric acid at 200°C in increments of 15 min. Thermal treatments were carried out at 1650°C before chemical treatments. Using maps obtained by visible-light microscopy (VLM) as a guide, the same regions were re-examined using atomic force microscopy (AFM) after subsequent treatments. Variations in the dissolution rates of different grains and grain boundaries could then be studied using AFM. The geometry of the grain-boundary grooves was compared after thermal and chemical treatments. Electron backscattered diffraction (EBSD) patterns recorded in the scanning electron microscope (SEM) were used to obtain crystallographic orientations of the grains which enabled variations in dissolution rates between grains to be correlated to orientation.