EFFECT OF SHARP CONCENTRATION GRADIENTS ON CRYSTAL NUCLEATION IN AMORPHOUS LAYERS OBTAINED BY SOLID-STATE REACTION

Using thermodynamic arguments it is shown that the sharp concentration gradient, occurring during diffusive mixing in multilayers, may reduc e the absolute value of the Gibbs energy of crystallization, leading t o cancellation of the driving force for homogeneous and heterogeneous nucleation of compounds in the amorphous interlayer. A thermodynamic c ritical concentration gradient is defined, above which there is no ava ilable driving force for nucleation. As the concentration gradient dim inishes during growth of the amorphous layer, the driving force for nu cleation is restored. This yields a thermodynamic critical maximum amo rphous layer thickness, which has been observed experimentally. The ca ses of formation of a compositionally different compound embryo and of polymorphous (or partitionless) nucleation are both treated and discu ssed. The heterogeneous nucleation of compounds under a concentration gradient, which has been proven to be at the origin of the cessation o f amorphization, is treated in more detail. The nucleus shape anisotro py and non-stoichiometry of the nucleating compound are taken into acc ount. The results are applied to the NiZr system.