The derived values of log D, were then plotted against 1/T. Since D , = D~ exp ( g ) the preexponential, Do, and activation energy, Q, can be deter-Morphological stages in the internal crystallization of a 3Ba0-., rate and morphology of crystalkation were studied. The peak nucleation temperature was between 700" and 725"C, and all
X-ray diffraction patterns have been made of two lime-phosphate glasses with compositions of 23% and 28% of CaO by weight, and of one lime-borate glass with 25% o f CaO. Fourier analyses of these patterns were carried out, and the radial distribution curves were plotted. In the lime-phosphate glasses, each phosphorus is tetrahedrally bonded to 4 oxygens at a distance of 1.57 A, and each oxygen is bonded to either 1 or 2 phosphorus atoms. The calcium ions, Ca++, are situated in the holes in the phosphorusoxygen network and have about 7 oxygen neighbors. In the lime-borate glass, some of the borons are triangularly bonded to 3 oxygens, and the others are tetrahedrally bonded to 4 oxygens.
An attempt is made to apply the ideas of crystal chemistry to the problem of immiscibility in glass systems. Miscibility is favored by the tendency of the networkforming cations, Si, B, and P, to bond with all available oxygens in the melt. Immiscibility is favored in compositions which do not allow the other cations, such as Na+ and Ca++, to be properly surrounded by unsaturated oxygens. The importance of this effect may be related to the valence and size of the cation. A qualitative discussion is given for a number of binary and ternary systems.
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