VISCOSITY AND DENSITY OF BAB2O4, BAB2O4-NAF AND BAB2O4-NA2O MELTS

Using the double-wire torsion pendulum method, we have measured the vi scosity of BaB2O4, BaB2O4-NaF and BaB2O4-Na2O melts, respectively, at high temperature of circa 910 degrees C up to 1150 degrees C in air am bient atmosphere. It was found that at the reported growth temperature (1050 degrees C), the viscosity of BaB2O4 was as high as 197.2 Pa s, and the addition rate of the logarithmic viscosity becomes bigger in t he supercooling region than that of the normal melt. Both the activati on energy and the volume of how unit are different around the growth t emperature of beta-BaB2O4 crystal. With the addition of NaF or Na2O as flux to the BaB2O4 melt, both the viscosity and the activation energy of the melt decrease severely. Density of the melts with the fluxes w as also determined, showing that similar obvious variations of density and its temperature dependence were observed. In terms of the Stokes- Einstein equation and the estimated volumes of the flow units in the m elt, the role of NaF and Na2O fluxes on the BaB2O4 crystal growth was discussed. (C) 1998 Elsevier Science B.V. All rights reserved.