Further study on sintering behavior of sol-gel ThO2 microspheres

By using two Th(NO3)4 sources with and without impurity sulphate, the sintering behavior of sol-gel ThO2 microspheres was further studied since the source used in the previous study had been found to contain a considerable amount of impurity sulphate. The study examined the effect of sulphate on the sintering behavior as well as that of sol Th-concentration, washing temperature of gel microspheres or sintering atmosphere. It was found that the previously reported dependence of sintered density on steam concentration in air-steam mixture of sintering atmosphere was valid only in the limited case of sulphate coexistence, low Th-concentration in sol and high temperature for washing gel microspheres. In the sulphate-absent case fully dense (≥99% TD) products were easily obtained, regardless of the steam concentration. In the sulphate-present case, higher Th-concentration of sol or lower washing temperature was found to make the sintered density higher. Such difference in the effect of sulphate on density was explained by the difference in microstructure during heating-up in air and in steam. (orig.)