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 reduce the absolute value of the Gibbs energy of crystallization, leading to cancellation of the driving force for homogeneous and heterogeneous nucleation of compounds in the amorphous interlayer. A thermodynamic critical concentration gradient is defined, above which there is no available driving force for nucleation. As the concentration gradient diminishes during growth of the amorphous layer, the driving force for nucleation is restored. This yields a thermodynamic critical maximum amorphous layer thickness, which has been observed experimentally. The cases of formation of a compositionally different compound embryo and of polymorphous (or partitionless) nucleation are both treated and discussed. The heterogeneous nucleation of compounds under a concentration gradient, which has been proven to be at the origin of the cessation of amorphization, is treated in more detail. The nucleus shape anisotropy and non-stoichiometry of the nucleating compound are taken into account. The results are applied to the NiZr system.