Fabrication and properties of dense beryllium oxide

Abstract

Nuclear grade beryllium oxide powders from commercial suppliers may be pressed at temperatures of 1500° to 1900° C and under pressures from 2000 to 6000 psi in graphite dies into a variety of shapes having densities in the range 98 to 100 % of the theoretical value. Addition of 1 % MgO facilitates the process and improves the rate and uniformity of densification. The compaction of BeO as a function of pressure, temperature and time may be described by a flow relation involving a viscosity and a critical shear stress for grain boundary movement. Thermal expansion is isotropic to about 1700° C and is well described to 2000° C by a quadratic function of the temperature. The specific heat of BeO at 1500° C is 0.52 cal/g-°C, slightly above the Dulong and Petit value. From 500° to about 1800° C the thermal conductivity decreases as 1 T , from a high value at room temperature of about 0.50 cal/cm-°C for dense material. A decrease in density of 5 % diminished the thermal conductivity by approximately 13 %. Information given earlier on the creep, elastic and anelastic properties of dense material has been extended, and relations have been employed showing that significant fractions of thermal stresses encountered in practice may be relaxed if given time. Anelasticity due to grain boundaries also appears to confirm the view on flow during densiflcation in hot pressing. A preliminary estimate of rupture strength for completely dense material with grain size of approximately 35 μ is 36 000 psi at room temperature. Lowering the density 1 % reduces the strength about 4100 psi. The status of knowledge of other properties is reviewed briefly.