SUPERCOOLING, INDUCTION TIMES, AND CLUSTERING IN THE FLUX GROWTH OF W03

Differential thermal analysis (DTA) and slow-cooling crystal growth experiment~ were carried out on W03 crystallising from four systems chosen to have closely related compositions: Molar composition 1 o 2. 100 W03 + 15PbO + 7Na 20 3. 100 W0 3 + 44NaF 4. 100 W03 + 22PbF 2 The supercooling and the crystals grown in these systems are compared and related to the probable clusters in the melts. In W03-NaF the supercooling was extensive enough for the induction time (the time for the melt to nucleate when held at a constant temperature) to be measured at various temperatures, and after different heat treatments. The results are interpreted a~ indicating that clusters of (W03)X exist above a~swell as below the liquidus. They quickly reach an equilibrium state at each soak temperature. After a soak period above the liquidus, a second soak below the liquidus resulted in shorter induction times at a fixed induction temperature. This suggest an increase in size of the clusters; however a stable nucleus did not form until at lea~t 35°C below the liquidus. This large critical supercooling is similar to that observed in the system Pb2V207-V20s, apparently the only other case where induction times have been measured in flux system