HOT PRESSING OF BERYLLIUM OXIDE WITH ADDITIVES.

The effect of variations in temperature, pressure and pressure cycle on the density vs grain size relationship was determined for hot pressed, sulphate-derived UOX beryllium oxide; the effects of selected additives on the final grain size using constant hot pressing conditions were also determined. A single-ended pressing technique, incorporating an incremental build-up in the pressing pressure, was developed to produce hot pressed specimens of near theoretical density with a uniform grain size of 25u. Smaller grain sizes were not obtained by this technique owing to the onset of discontinuous grain growth at a bulk density of 97 per cent theoretical. However, by introducing a 'dwell' or 'equilibration' time at temperature before the application of the load, a theoretically dense specimen (3.010 g/cm3) was produced with a uniform 8u grain size. Carbon was the only additive which consistently refined the grain size at all levels of addition. A hot pressed specimen of 99 per cent theoretical density and 2u grain size was obtained using a standard condition of 1500°C, 250 kg/cm2 for 25 minutes, 5 volume per cent of colloidal graphite additive.