Abstract
Gel-precipitated (U, Th) spheres produced at AERE Harwell, were investigated to study the development of the structure of the gel spheres at various stages of production. Various parameters including surface area, true (matrix) and geometric densities, porosity, crystallite size, compliance and related properties were investigated. A careful examination of whole and cleaved spheres by Scanning Electron Microscope was also carried out. The xerogel spheres were debonded in a tubular furnace in an atmosphere of CO[2] to remove or decompose the gelling agent (polymer) and other volatiles which might be present. The isothermal and constant-rate-of-heating (CRH) sintering behaviour of partially and fully debonded spheres were studied. In the latter work the shrinkage data of a batch of spheres were obtained from geometric (Hg) density measurements instead of the conventional dilatometric method used for pellets. The study has shown that ageing and heavy metal composition have a substantial influence on the structure of the debonded spheres, the rate of sintering, and the microstructure of the sintered products. Ageing and increasing amount of thorium generally retards sintering. Partially debonded (to 750°C) spheres sinter faster than fully debonded spheres and the former also develop larger grains on sintering. Gel-precipitation is a versatile technique which can be used to produce spheres which sinter to > 98% of the theoretical density at a comparatively low temperature (≈ 1400 C). Grain-boundary diffusion was found to be the probable dominant material transport mechanism in the sintering of (U, 30% Th)O[2].