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Author

Joseph A. Pask

Bio: Joseph A. Pask is an academic researcher from University of California, Berkeley. The author has contributed to research in topics: Wetting & Mullite. The author has an hindex of 43, co-authored 137 publications receiving 6340 citations. Previous affiliations of Joseph A. Pask include University of California & Lawrence Berkeley National Laboratory.
Topics: Wetting, Mullite, Grain boundary, Sintering, Ceramic


Papers
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Journal ArticleDOI
TL;DR: In this paper, a sigmoid curve of transformation vs. time was interpreted in terms of a nucleation-growth process, which led to activation energies for nucleation of 109, 136, and 148 kcal/mole.
Abstract: Transformation of three commercial anatase powders resulted in sigmoid curves of transformation vs. time, which were interpreted in terms of a nucleation-growth process. Analysis of these curves by several rate laws led to activation energies for nucleation of 109, 136, and 148 kcal/mole and for growth of 100, 115, and 190 kcal/mole, respectively. The rate of transformation and activation energy are governed by the nature and amount of impurities which determine the defect structure of the TiO2, i.e. the concentration of oxygen vacancies or interstitials. It is suggested that, in general, oxygen vacancies accelerate and the interstitials inhibit the transformation. CuO additions and hydrogen atmospheres accelerated the transformation by the introduction of oxygen vacancies and/or by reduction to a second TinO2n−1 Magneli phase which could act as a nucleating agent. The transformation is retarded in vacuum; this retardation is attributed to the introduction of titanium interstitials.

617 citations

Journal ArticleDOI
TL;DR: In this paper, concentration profiles of Al/sub 2/O/sub 3/ in diffusion couples made from sapphire and fused silica were used to determine the stable equilibrium phase diagram of the system.
Abstract: Concentration profiles of Al/sub 2/O/sub 3/ in diffusion couples made from sapphire and fused silica were used to determine the stable equilibrium phase diagram of the system SiO/sub 2/--Al/sub 2/O/sub 3/. The intermediate compound mullite, 3Al/sub 2/O/sub 3/.2SiO/sub 2/, melts incongruently at 1828 +- 10/sup 0/C; its stable solid-solution region ranges from 70.5 to 74.0 wt percent Al/sub 2/O/sub 3/ below 1753/sup 0/C and from 71.6 to 74.0 wt percent at 1813/sup 0/C. The microstructures of diffusion zones and heat-treated specimens also indicate the incongruency of mullite. Additional information is given for 3 metastable systems: SiO/sub 2/-Al/sub 2/O/sub 3/ in the absence of mullite, SiO/sub 2/-ordered-mullite in the absence of alumina, and SiO/sub 2/-disordered mullite in the absence of alumina. Under metastable conditions, ordered mullite melts congruently at approximately 1880/sup 0/C and its solid-solution range extends up to approximately 77 wt percent Al/sub 2/O/sub 3/. The solid-solution range of disordered mullite extends to approximately equal to 83 wt percent Al/sub 2/O/sub 3/ with an estimated congruent melting temperature of approximately 1900/sup 0/C. The existence of metastable systems is associated with superheating of mullite above the incongruent melting temperature and with nucleation of alumina and mullite in supercooled aluminum-silicate liquids. (auth)

410 citations

Journal ArticleDOI
TL;DR: In this paper, the thermodynamics of a solid-liquid-vapor system both under chemical equilibrium and nonequilibrium conditions, based on the model of Gibbs, are discussed, and the degree of wetting or nonwetting of a flat and nondeformable solid by the liquid is defined by Young's equation in terms of the static interfacial tensions.
Abstract: The thermodynamics of a solid-liquid-vapor system both under chemical equilibrium and nonequilibrium conditions, based on the model of Gibbs, is discussed. Under chemical equilibrium conditions, the degree of wetting or nonwetting of a flat and nondeformable solid by the liquid is defined by Young's equation in terms of the static interfacial tensions. Under chemical nonequilibrium conditions, mass transfer across an interface results in a transient decrease in the corresponding specific interfacial free energy and the interfacial tension by an amount equal to the free energy of the effective chemical reaction per area at that interface. When the reaction is between the solid and the liquid, this transient lowering of the interfacial tension can cause the liquid drop to spread on the solid substrate if the interfacial energy reduction is large enough and also if the diffusion rates of the reacting components in the solid phase are slow enough relative to the flow rate of the liquid to cause the liquid at the periphery of the drop to be in dynamic contact with unreacted solid.

313 citations

Book
30 Jun 1981
TL;DR: In this article, separate abstracts were prepared for each of the 53 included papers, and abstracts for the remaining papers appeared previously in ERA and MOW (MOW).
Abstract: Separate abstracts were prepared for each of the 53 included papers. Abstracts for the remaining papers appeared previously in ERA. (MOW)

201 citations

Journal ArticleDOI
TL;DR: In this paper, the strength of single-phase polycrystalline mullite was investigated under compressive stress-strain and creep testing conditions at 1400° and 1500°C.
Abstract: The high strength potential of single-phase mullite was investigated under compressive stress-strain and creep testing conditions at 1400° and 1500°C. In single-crystal experiments, no plastic deformation and, hence, no dislocation glide was observed. Polycrystalline mullite was deformed via the Nabarro-Herring mechanism accompanied by grain-boundary sliding and some cavitation. In stress-strain tests, deformation was enhanced by intergranular separation.

193 citations


Cited by
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Journal ArticleDOI
TL;DR: In this paper, photo-induced superhydrophilicity was used on the surface of a wide-band gap semiconductor like titanium dioxide (TiO 2 ) for photocatalytic activity towards environmentally hazardous compounds.

4,241 citations

Journal ArticleDOI
TL;DR: In this article, the temperature at which the maximum deflection is observed varies with heating rate for certain types of reactions, and an expression can be derived relating this variation with the kinetics of the reaction.
Abstract: In differential thermal analysis, the temperature at which the maximum deflection is observed varies with heating rate for certain types of reactions. An expression can be derived relating this variation with the kinetics of the reaction. By making a number of differential thermal patterns at different heating rates, the kinetic constants can be obtained directly from the differential thermal data. Measurements of the variation of peak temperature with heating rate have been made for several minerals of the kaolin group, the values of the kinetic constants determined, and these values compared with corresponding values obtained for both the same samples and similar material by conventional isothermal techniques. Some factors affecting the results are discussed. The method of differential thermal analysis (DTA) has been universally accepted by mineralogical laboratories as a rapid and convenient means for recording the thermal effects that occur as a sample is heated. Changes in heat content of the active sample are indicated by deflections shown by a line representing the differential temperature. It is conventional to represent an endothermic effect by a negative deflection and an exothermic effect by a positive deflection. The deflections, whether positive or negative, are called peaks.

3,138 citations

Journal ArticleDOI
TL;DR: A comprehensive analysis of the reported effects of dopants on the anatase to rutile phase transformation and the mechanisms by which these effects are brought about is presented in this article, yielding a plot of the cationic radius versus the valence characterised by a distinct boundary between inhibitors and promoters.
Abstract: Titanium dioxide, TiO2, is an important photocatalytic material that exists as two main polymorphs, anatase and rutile. The presence of either or both of these phases impacts on the photocatalytic performance of the material. The present work reviews the anatase to rutile phase transformation. The synthesis and properties of anatase and rutile are examined, followed by a discussion of the thermodynamics of the phase transformation and the factors affecting its observation. A comprehensive analysis of the reported effects of dopants on the anatase to rutile phase transformation and the mechanisms by which these effects are brought about is presented in this review, yielding a plot of the cationic radius versus the valence characterised by a distinct boundary between inhibitors and promoters of the phase transformation. Further, the likely effects of dopant elements, including those for which experimental data are unavailable, on the phase transformation are deduced and presented on the basis of this analysis.

2,570 citations

Journal ArticleDOI
TL;DR: In this paper, the authors provide a quantitative framework for the description of weathering and alteration in sedimentary petrology, using the available experimental data on dissolution of silicates to verify quantitatively the usual mineral stability series.
Abstract: The recent literature on the kinetics of water-rock interactions is reviewed. The data are then extended to provide a quantitative framework for the description of weathering and alteration. The available experimental data on dissolution of silicates verifies quantitatively the usual mineral stability series in sedimentary petrology. The rate of hydration of carbonic acid is shown to be a possible limiting factor in water-rock interactions. The framework is developed to enable use of laboratory dissolution experimental results and thermodynamics to arrive at a rate law applicable up to equilibrium and therefore applicable to natural systems. The kinetic justification for the significance of a water-rock ratio is discussed. With a proper treatment of fluid flow, the equations are applied to the weathering profile leading to the development of bauxites from nepheline syenites.

1,105 citations

Journal ArticleDOI
TL;DR: Transparent polycrystalline YAG with nearly the same optical characteristics as those of a single crystal were fabricated by a solid-state reaction method using high-purity powders (>99.99 wt% purity) as mentioned in this paper.
Abstract: Transparent polycrystalline YAG with nearly the same optical characteristics as those of a single crystal were fabricated by a solid-state reaction method using high-purity powders (>99.99 wt% purity). The average grain size and relative density of the 1.1 at.% ND:YAG ceramics obtained were about 50 {micro}m and 99.98%, respectively. An oscillation experiment was performed on a cw laser by the diode laser excitation system using the fabricated ceramics. The experimental results indicated an oscillation threshold and a slope efficiency of 309 mW and 28%, respectively. These values were equivalent or superior to those of the 0.9 at.% ND:YAG single crystal fabricated by the Czochralski method.

1,098 citations