Precipitate particle size distribution in rigid and floppy networks

It is a usual assumption that oxide glasses build up continuous random networks. A further and very important development of this approach is the rigidity networks idea. According to it, the network switches from floppy to rigid if the number of bridges between the network formers exceeds a certain threshold value. In oxide glasses, only properties which require a motion over a sufficiently large distance will be subject to angular constraints at oxygen bonds thus there can be a switch from floppy to rigid modes. The kinetics of phase transition (and most nucleation processes) are very sensitive to the rigidity of the network. Slightly above the threshold concentration of rigid bonds some tiny floppy regions still exist inside the rigid network. They can serve as active centers for the nucleation process. Much later; nucleation can appear in the rigid part of the network. Therefore, we predict a certain interval of concentration of rigid bonds in which a bimodal size distribution function of precipitated particle will be observed. This prediction is confirmed by experimental data. (C) 2000 Elsevier Science B.V. All rights reserved.