not Available.

The mechanism of material transport in sintering can be elucidated in some cases by direct observation of the rate of interface growth and approach of centers between spherical particles. Measurements with glass, sodium chloride, and copper indicate that with these materials viscous flow, evaporation-condensation, and self-diffusion are the rate-determining mechanisms. Values of viscosity, vapor pressure, and diffusion constants have been determined, but calculations of diffusion constants from these data are subject to uncertainties of interpretation. A model is presented for the behavior of copper during the initial stages of sintering, which is in agreement with available experimental data, and which requires vacancy elimination at dislocations or grain boundaries. Data for refractory oxides indicate the importance of purity and fabrication pressure, but the sintering mechanism for these materials is not determined by the present data.

The driving force leading to densification during sintering in the presence of a liquid phase and the material transport phenomena have been analyzed and relationships for the densification rate during the rearrangement process, the solution-precipitation process, and the final coalescence process have been determined. These relationships allow an experimental determination of the mechanism of sintering in the presence of a liquid phase on the basis of the time, particle size and temperature dependence of the densification rate. In addition, they allow direct calculations of densification rates to be made for certain simple systems for which property data are available.

Porous structures having a continuous solid phase with isolated pores were prepared by the addition of different amounts of crushed naphthalene to an alumina casting slip. Samples of from 5 to 500/, porosity were fired together for comparable grain development, eliminating structural variables except porosity. Eff ects of porosity and temperature on strength, elastic modulus, modulus of rigidity, and coefficient of thermal expansion were investigated. Effects of porosity on thermal stress resistance and torsional creep properties were studied at constant temperature.

The sources and calculation of thermal stresses are considered, together with the factors involved in thermal stress resistance factors. Properties affecting thermal stress resistance of ceramics are reviewed, and testing methods are considered.