Ronald J. Kerans

Structural ceramic composites

Abstract Major recent advances: Near-stoichiometric, small-diameter SiC fibers with excellent properties are now available. A new polycrystalline oxide fiber offers significantly improved temperature limits. Oxide fiber coatings to substitute for C and BN for deflecting cracks and promoting distributed damage appear feasible. Two show substantial low-temperature plasticity in constrained loading. Properties of SiC–BN–SiC and coatingless oxide composites have been improved markedly, but combustion atmospheres have proven more problematic than thought for the former.

A Model for Transitions in Oxidation Regimes of ZrB2

A mechanistic model that interprets the transition in oxidation behavior of zirconium diboride as the temperature is varied from 600°C to 2500°C is presented. Available thermodynamic data and literature data for vapor pressures, oxygen permeability in boria, and viscosity of boria were used to evaluate the model. Three regimes and the temperatures of transition between them were identified. In the intermediate temperature regime, viz., 1000°C to 1800°C, good correspondence was obtained between theory and experiments for weight gain, recession, and scale thickness as functions of temperature and oxygen partial pressure. In this regime, the rate-limiting step is the diffusion of dissolved oxygen through a film of liquid boria in capillaries at the base of the oxidation product. At lower temperatures, an external boria scale forms, but it was not found to contribute significantly to oxidation resistance. Comparison with literature data on recession is very good, but weight gain is predicted to be higher than experimentally observed unless flow of viscous boria is included. At higher temperatures, the boria is lost by evaporation, and the oxidation rate is limited by diffusion of molecular oxygen through the capillaries between nearly columnar blocks of the oxide MO2.; this regime is soon followed by a rapid acceleration of recession due to vaporization of the oxide MO2 itself.

Processing of 3D interconnected porous aluminum nitride composites for electronic packaging

Abstract A simple process was used to fabricate 3D interconnected porous aluminum nitride composites. For this purpose aluminum powders were cold pressed and sintered under controlled conditions of temperature and time to initiate particle necking and coarsening, but with minimum shrinkage. The resultant compacts were characterized for their microstructure and density using optical microscopy and pycnometry measurements. The sintered compacts contain ≈ 28% porosity comprising of 5–75 μm pores which are well connected and continuous to the surface. The pores were then infiltrated with borosilicate glass to a depth of ≈ 30–80 μm. Such composites would potentially exhibit optimum dielectric and thermal conductivity values making them viable substrate materials for electronic packaging.