INFLUENCE OF TRANSITION-METAL OXIDE ADDITIONS ON THE CRYSTALLIZATION KINETICS, MICROSTRUCTURES AND THERMAL-EXPANSION CHARACTERISTICS OF A LITHIUM ZINC SILICATE GLASS

Lithium zinc silicate glasses can be used to prepare moderately high thermal expansion glass-ceramics, and these materials are ideally suited for the manufacture of hermetic seals to both nickel-based superalloys and stainless steel. On the basis of earlier work by the present authors, one particular composition from the lithium zinc silicate system was chosen for detailed investigation. This composition contains Na2O and B2O3 fluxing agents, together with P2O5 as the primary nucleating agent. The crystallization kinetics and resultant microstructures of this composition have been studied as a function of the heat-treatment parameters using differential thermal analysis, dynamic mechanical thermal analysis, scanning and transmission electron microscopy, ambient and high-temperature X-ray diffraction, and small-angle neutron scattering. Indirect evidence from the dynamic mechanical thermal analysis and small-angle neutron scattering suggests that the nucleated glass is phase separated on a very fine scale, of the order of 16 nm. A number of crystalline phases have been positively identified in the heat-treated glasses, including cristobalite, quartz, tridymite, beta-1-Li2ZnSiO4 and gamma-0-Li2ZnSiO4, the precise phases that are formed depending strongly on the heat-treatment parameters. The influence of a number of transition metal oxide additions on the resultant properties of the lithium zinc silicate composition has also been investigated, and it has been shown that the crystallization kinetics, microstructures and thermal expansion characteristics are all strongly affected by these additions. In particular, the activation energy for crystallization (which is related to the nucleating efficiency) is dependent on the ionic field strength of the transition metal ion species employed, with crystallization being favoured by solutes of high field strength.