EXPERIMENTAL-VERIFICATION OF THE FORMAL NUCLEATION AND GROWTH-RATE EQUATIONS - INITIAL UH3 DEVELOPMENT ON URANIUM SURFACE

A Hot Stage Microscopy (HSM) technique was utilized for continuous obs ervations of UH3 nucleation and growth on hydrogen exposed uranium (U- 0.1 wt% Cr) surface. Such observations enabled the quantitative evalua tion 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, nam ely, the relation between the extended and the experimental reacted fr actions, is very accurate over a wide range of surface coverages. Howe ver, the time dependence of the nucleation rate function was not monot onous, as commonly applied in the conventional treatments. Instead, th e nucleation rate first accelerated, passed through a maximum and fina lly decayed again. At about 7-10% of the reaction, the nucleation rate decayed to zero, and saturation in the number of growing nuclei was a ttained. This nucleation behavior lends to overall kinetics which do n ot fit the simplified power law time dependence that is usually applie d in the kinetic analysis of nucleation and growth processes. (C) 1998 Elsevier Science S.A.