THERMAL-PROPERTIES AND DEVITRIFICATION BEHAVIOR OF (1-CENTER-DOT(1-X)MGO-CENTER-DOT-2SIO(2) GLASSES(X)CAO)

The thermal properties (glass transformation, T-g, and softening, T-s, temperatures), the crystalline phases formed during heating in a diff erential thermal analysis (DTA) apparatus, the kinetic parameters and the mechanism of the devitrification process, of glasses of the system diopside-wollastonite were investigated. The substitution of CaO by M gO induces an increase in T-g and the crystal growth activation energy , E(c); this is probably linked to the greater coordination number of Ca2+ ions with respect to the Mg2+ ions. The substitution of CaO by Mg O lowers the nucleation rates of the diopside phase; wollastonite soli d solution nuclei form whose growth appears to leave a glassy matrix i n which diopside formation is inhibited. Only surface nucleation was o bserved, but, in finely powdered samples, which soften and efficiently sinter before devitrifying, surface nuclei behave as bulk nuclei. Whe n bulk crystallization occurs, the Avrami para meter m was found to be 2 for all glasses, except the diopside one, for which m=3.