Joseph A. Pask

Surfaces and interfaces in ceramic and ceramic-metal systems

Separate abstracts were prepared for each of the 53 included papers. Abstracts for the remaining papers appeared previously in ERA. (MOW)

Importance of starting materials on reactions and phase equilibria in the Al2O3SiO2 system

Abstract Disagreements in the high Al 2 O 3 side of the phase equilibrium diagram for the Al 2 O 3 SiO 2 system have been reported consistently. Some of the disagreements are significantly different and have been reported many times. It is thus necessary to eliminate experimental errors as being primarily responsible. An examination and analysis of the starting materials and the development of a fundamental understanding of the chemical reactions that take place provide an explanation for the reported differences. The objective of this report is to briefly review the disagreements and correlate them with the starting materials and the associated solid-state reactions that occur in reaching stable or metastable equilibrium.

The mechanism of creep in polycrystalline magnesium oxide

Polycrystalline magnesium oxide, nominally fully-dense (>99.8% theoretical), was tested in compressive creep at a temperature of 1200°C and under initial applied stresses of 3.44 × 108−13.76 × 108 dynes/cm2. The observed steady-state creep rates were independent of grain size in the range ~12–52μm, and proportional to σ3.3, where σ is the applied stress. The activation energy was determined as 51 ± 5 kcal/mole. The results show that the creep behavior of polycrystalline MgO is similar to that observed in many metals. An analysis of the data in terms of various creep theories suggests that a dislocation mechanism is rate-controlling, such as the climb of dislocations without accompanying glide.

Sintering of α-Al2O3—Amorphous silica compacts

Sintering of α-Al2O3—fused SiO2 compacts at temperatures of 1350–1500 °C is affected by metastable and stable phase reactions. Shrinkage maxima in the range 10 to 30 mole % Al2O3 are the result of cristobalite in the fused SiO2 and its reaction with α-Al2O3 to form the eutectic of the metastable phase equilibrium diagram for SiO2 (cristobalite)—α-Al2O3 without mullite. The retardation of densification of α-Al2O3 compacts with additions of SiO2 up to about 50 mole % at temperatures about 1400°C and above is associated with the appearance of mullite.

Wetting and spreading in the CuAg system

Abstract Sessile drop experiments were made in the copper-silver system at the eutectic temperature and 900°C. Wetting, or acute contact angles, were observed for specimens in chemical thermodynamic equilibrium. Spreading occurred when the substrate was not in chemical equilibrium with the liquid. The interface does not remain aligned with the free surface of the substrate only in the presence of reactions.

The Silica-Alumina System: Stable and Metastable Equilibria at 1.0 Atmosphere

Diffusion couples, consisting of sapphire and fused silica, which were annealed in the temperature range from 1678� to 2003�C and analyzed by electron beam microprobe, have provided data on the stable phase equilibrium of the silica-alumina system. Under stable equilibrium conditions, the intermediate compound of this system, mullite (3Al2O3 � 2SiO2), melts incongruently at 1828� � 10�C and its solid solution field extends from 70.5 to 74.0 percent (by weight) alumina. The stable phase diagram is a composite of the two binary eutectic diagrams: silica-mullite in the absence of alumina and silica-alumina in the absence of mullite. Under metastable conditions, mullite melts congruently at ≈ 1890� � 10�C and its solid solution field extends to ≈ 83.2 percent (by weight) alumina.

Sintering of α-Al2O3/quartz, and α-Al2O3/cristobalite related to mullite formation

Abstract Cristobalite and quartz react differently in mixtures with α-Al2O3 at 1415°C. With cristobalite, an eutectic liquid forms in accordance with the metastable phase equilibrium diagram for α-Al2O3-SiO2 (cristobalite) in the absence of mullite. With quartz, a liquid first forms on the surfaces of quartz because of the occurence of an intermediate liquid phase on transformation of quartz to cristobalite. These liquids act as precursors to the formation of mullite by reacting with α-Al2O3. Mullite was detected earlier in the cristobalite-containing mixtures under similar firing conditions because the growth of mullite becomes significant with the formation of the eutectic liquid at the α-Al2O3-cristobalite interface since it is already saturated with Al2O3. The kinetics of sintering are affected by the rates of the step reactions.

Reactions and wetting behaviour in the aluminium-fused silica system

Wetting of fused silica by molten aluminium at temperature of 800 to 1000° C and 3×10−5 Torr was dependent on the formation of a reaction zone by redox reactions. The reaction zone consisted of three layers. It was postulated that reactions proceeded by interdiffusion of Si2+, Al+, Al2+ and Al3+: the layer adjacent to the metal drop was primarily AlO stabilized by a solid solution of SiO; and adjacent to fused silica, a spinel of AlO and Al2O3 stabilized by SiO. On cooling these dissociated into Al, Si and θ-Al2O3 and/orα-Al2O3.

Wetting and reactions in the lead borosilicate glass-precious metal systems

Sessile drop experiments of lead borosilicate glass (liquid) on silver, gold and platinum were performed in air, vacuum and helium at 700° C. Wetting occurred in all cases. Strong adherence or bonding with the glass occurred for all three metals in air due to reactions with a thin oxide film present on the metal surfaces, and for silver in vacuum due to a redox reaction. Experiments in helium lead to poor adherence with all three metals.

Thermodynamic and geometric considerations of solid state sintering

Abstract Thermodynamic analysis of model sintering systems indicates that the minimum free energy configuration is represented geometrically by interpenetrating spheres of increasing size with no neck formation. The dihedral angle formed at the grain boundary increases as densification proceeds; the limit is the equilibrium angle determined by the ratio of the solid/solid and solid/vapor interfacial energies for the system. An intermediate free energy configuration favored by kinetic factors is the formation of a neck at grain/grain contacts with an equilibrium dihedral angle; the resulting reverse curvature in the surfaces of the particles provides the driving force for densification in this case. Conditions and mechanisms under which both configurations develop are discussed and are illustrated by experiments on MgO powder compacts that density considerably more rapidly in flowing water vapor than in static air.