Liquidus temperatures of bioactive glasses

Melts in the composition range of bioactive glasses have their liquidus temperatures within or close to the working range. Additionally, bioactive glasses show a strong tendency to crystallization due to their composition. Thus, many bioactive glass compositions are unsuitable for demanding working procedures such as fiber drawing. In this work we discuss the suitability of different methods for measuring liquidus temperature of bioactive glasses. Thermal analysis was used, and complemented by sintering curves as given by hot stage microscopy. Hot stage microscopy could be used to measure liquidus also when the thermal effect associated with liquidus was low and consequently the DTA was not suitable. Liquidus of some glasses was measured with optical microcopy for samples heat treated in a gradient furnace. The values indicating the crystallization of the samples during viscosity measurement with rotational viscosimeter were compared with the other measurements. The crystalline phases formed at liquidus were identified with XRD and SEM-EDXA. The measured liquidus temperatures were also tested by drawing fibers directly from molten glass and from preforms heated to typical fiber drawing viscosity values. For all compositions the fiber drawing viscosity values were below liquidus. Continuous fibers could be manufactured of compositions only with low rates of crystal nucleation and growth.