Experimental verification of the formal nucleation and growth rate equations – initial UH 3 development on uranium surface

Abstract

A Hot Stage Microscopy (HSM) technique was utilized for continuous observations of UH 3 nucleation and growth on hydrogen exposed uranium (U–0.1 wt% Cr) surface. Such observations enabled the quantitative evaluation of both, nucleation rate function, N ⋅ (t) , and the average lateral growth velocity, u, of the individually growing nuclei. Knowing these kinetic parameters, the validity of the different assumptions made in the derivation of the formal nucleation and growth rate equations (i.e. the Avrami-Erofeev or Johnson-Mehl equations) can be checked. It was found that the basic assumption underlying the Avrami's approach, namely, the relation between the extended and the experimental reacted fractions, is very accurate over a wide range of surface coverages. However, the time dependence of the nucleation rate function was not monotonous, as commonly applied in the conventional treatments. Instead, the nucleation rate first accelerated, passed through a maximum and finally decayed again. At about 7–10% of the reaction, the nucleation rate decayed to zero, and saturation in the number of growing nuclei was attained. This nucleation behavior leads to overall kinetics which do not fit the simplified power law time dependence that is usually applied in the kinetic analysis of nucleation and growth processes.