ON THE VISCOSITY OF GLASS MELTS AND POROUS SINTERED GLASSES

Suspensions of solid phase particles in molten grasses modify their flow behavior and therefore their viscosity. This work deals with the dependence of the effective viscosity (eta(eff)) of isotropic suspensions on the concentration of the solid phase particles. Independently of the shape of the inclusions the values of the effective viscosities lie between upper [eta(eff) = eta(L)(1 - c(D))(-14)] and lower bounds [eta(eff) = eta(L)(1 - c(D))(-3)],where eta(L) is the viscosity of the molten glass and c(D) is the volume fraction of solid inclusions. The lower bound is also valid for the effective viscosity of suspensions containing spherical inclusions. Pores present in a glass matrix affect its flow behavior and consequently its creep and sintering behavior. The effective viscosity of porous glasses (eta(P)) as a function of the volume fraction of pores, or porosity (P), also varies between an upper [eta(P) = eta(M) (1- P)(1.04)] and a lower bound [eta(P) = eta(M)(1- P)(14)], where eta(M) is the viscosity of the nonporous glass matrix. For spherical porosity the equation is: eta(P) = eta(M)(1 - P)(1.5) Measured values for the viscosity of different suspensions as reported in the literature as well as own experimental data on two glass-solid systems are compared with the theoretical values predicted by these equations. In a similar way literature data for the viscosity of porous sintered glasses are compared with the respective equations. In all cases a fair agreement between experiment and theory was found, in some cases the agreement was excellent. Therefore the presented equations constitute a reliable approach for the prediction of the viscosity of suspensions and porous sintered glasses and since they do not contain fitting parameters, they are of substantial practical relevance (for a comprehensive english treatment of the matter compare Glastechnische Berichte, Proc. Otto-Schott-Colloquium 1994).