Experimental Measurement of Pre-Melting and Melting of Thorium Dioxide

Thorium dioxide samples have been submitted to homogeneous four-beam laser pulses in an autoclave filled with different gases. Phase transitions were analysed through the produced thermal arrest stages, which were detection by a precise multichannel pyrometer. The spectral emissivity of the samples, measured on line during heating up to 4000 K, is also reported in the paper. Thoria exhibits a melting point at 3651 K, and a pre-melting transition at 3090 K. These temperatures are weakly influenced by chemical reduction of the sample. Whilst virtually stoichiometric thoria melts congruently, in the hypostoichiometric oxide, the liquidus and solidus temperatures could be clearly observed at O/Th = 1.98. The analysis of the measured data and, in particular, of the heat capacity shows that the features of the pre-melting transition can be described by a Debye-Hueckel oxygen defect model, where co-operative effects lead, in the oxygen sub-lattice, to a critical oder-disorder transition, whose thermodynamic nature depends on the parameters of the defect mean-field coulomb interaction.

RONCHI Claudio;  HIERNAUT Jean Pol; 

1996-08-09

JRC13201