Variation of the viscosity of molten potassium niobate with annealing time

Using the double-wire torsion pendulum method, we have measured the viscosity of a potassium niobate molten system with excess K2O from 50 to 56 mol% at temperatures up to 1190 degrees C in air. Both the viscosity and its activation energy show strong time dependence. While the values for viscosity increase for melts with 50 and 51 mol% K2O, which are annealed at 1150 degrees C in air, the viscosity, however, of the melt with more than 2 mol% excess K2O shows a completely different time dependence, i.e. drastic decrease with annealing time. A similar striking reversed variation of temperature-dependent viscosity with annealing time is observed when the K2O content in the melt is up to 52 mol%. These anomalous variations in viscosity give clear evidence why the KNbO3 single crystal should be grown from mother melts with excess K2O above 51 mol%, and after being annealed for a long time. This result also confirms that the double-wire torsion pendulum method is a useful tool to study the time-dependent viscosity of melts at high temperature in air.