Currentunderstanding of the complex interrelationships among growth kinetics, microstructure, and adhesion of protective Cr2O3 and Al2O3 scales is critically reviewed. Similarities and differences in the behavior of these two systems are highlighted. The morphology of the alloy-scale interface appears to be a critical factor. Recent ideas are advanced to interpret the effect of oxygen-active elements on the development of a tortuous interface conducive to improved scale tenacity.

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Microstructure, adhesion and growth kinetics of protective scales on metals and alloys

Discovery-based design of transparent conducting oxide films

Towards hierarchically ordered functional porous polymeric surfaces prepared by the breath figures approach

Microstructural control in the processing of electronic ceramics

Controllable porous polymer particles generated by electrospraying

The effect of oxygen activity on the sintering of high-purity Cr2O3 is shown. Theoretical density was approached at the equilibrium O2 partial pressure needed to maintain the Cr2O3 phase (Po2=2×10−12 atm). The presence of N2 in the atmosphere during sintering did not prevent final sintering. The addition of 0.1 wt% MgO at this equilibrium pressure effectively controlled the grain growth and further increased the sintered density to very near the theoretical value. The solute segregation of MgO at the grain boundaries, followed by nucleation of spherulites of magnesium chromite spinel on the boundaries, accounted for the grain-growth control. It is speculated that these isolated spherulites locked the grain boundaries together, changing the fracture mode of the sintered oxide from inter-to intragranular and also that larger MgO additions produced a more continuous spinel formation at the boundaries, resulting in decreased sintered density. Weight loss, which was also monitored as a function of O2 activity, correlated with the changing predominant volatile species in the Cr-O system.

Final Sintering of Cr2O3

The effect of various solvents on the morphology of polymethyl methacrylate (PMMA) particles synthesized by spray drying is examined. It is concluded that the product PMMA particles, derived from the PMMA-acetone dilute solution, have a smaller particle size than those from the PMMA-THF dilute solution. This is due to the stronger PMMA-acetone interaction, since acetone is a good solvent for PMMA, while THF is a poor solvent for PMMA. By controlling the temperature of each section of the tube furnace, the heating rate was adjusted so that both solid and hollow particles could be obtained. When water was added to these dilute solutions, porous or honeycomb particles were produced due to the different evaporation rates of solvent and water. This was a result of a large difference in the solubility parameter values between PMMA and solvent. The strong interaction between PMMA and acetone results in the formation of porous particles while the weak interaction between THF and PMMA produced honeycomb structure particles.

Effect of the solvent on the particle morphology of spray dried PMMA

Titanium diboride is widely accepted to be completely wet by liquid aluminum, yet few published wetting studies demonstrate this behavior, and reported contact angles vary widely. Sessile drop substrates from four different sources were selected and their microstructures and chemistries characterized. The results of sessile drop experiments using four techniques to modify oxide film behavior were compared. The Al-TiB2 interfaces were examined in metallographic sections or after chemical removal of the Al drop. Al wets a material containing 5.5 wt% Ni in vacuum experiments before the hold temperature of 1025∘ C is reached. The other TiB2 substrates are completely wet by Al at 1025∘ C, but only after prolonged holds under vacuum. Elimination of boron oxide from the TiB2 surface leads to a spreading condition. The role of the substrate microstructure (porosity, grain size, roughness, and carbon content) in altering the wetting kinetics is discussed.

The Wettability of Titanium Diboride by Molten Aluminum Drops

In the production of silicon articles at an elevated temperature, a stream comprising a controlled mixture of an oxygen-containing first gas and a second gas is admitted to the processing chamber. The first gas is one which partially dissociates under the conditions in the chamber to form both oxygen and the second gas. The second gas is one which is not harmful to silicon at the conditions in the chamber. Substantially equilibrium conditions are established in the chamber so that the dissociation of the first gas to oxygen occurs reversibly. The partial pressure of oxygen (PO.sbsb.2) is sensed in the chamber during processing of the article. In response to the PO.sbsb.2 level, the ratio of the rates of flow of the oxygen-containing gas and the second gas is adjusted so as to maintain the PO.sbsb.2 at a level less than about 10-6 atmosphere, and usually no greater than about 10-10 atmosphere, at which the density of oxygen-related defects in the processed silicon article is acceptably low. Oxygen-related defects in the silicon are thereby reduced. If graphite structures are present in the hot zone of the processing chamber, they are preferably coated with an impervious coating which will stand the high temperature and will prevent the gas stream from coming into contact with the hot graphite. Carbon-related defects in the silicon are thereby also reduced.

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Control of oxygen- and carbon-related crystal defects in silicon processing

Al2O3-B4C whisker composites have been successfully densified by pressureless sintering. Low oxygen partial pressures, below 10
 
$$^{ - 10} $$

 atmospheres, were determined to be suitable for the composite densification, and the maximum densities were achieved at 1800 °C for 60 minutes. The B4C whiskers were screened classified into five size fractions and the effect of size and size distribution on the densification were studied. The small whiskers with a narrow size distribution (−325 + 400 mesh) yielded the highest density results. The maximum value of relative densities was 98%, 97%, 94% and 89% at 10, 20, 30 and 40 vol  % B4C whiskers, respectively. Addition of B4C between 5 and 15 vol % proved to be a sintering aid to the alumina densification via mechanisms other than being a grain growth inhibitor. An increased fracture toughness up to 6.2 MPa·m
 
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 was achieved in the composites containing 10–20 vol % B4C whiskers.

P. D. Ownby

Papers

Final Sintering of Cr2O3

Effect of the solvent on the particle morphology of spray dried PMMA

The Wettability of Titanium Diboride by Molten Aluminum Drops

Control of oxygen- and carbon-related crystal defects in silicon processing

Pressureless sintering of B4C whisker reinforced Al2O3 matrix composites