On the sinterability of crystallizing glass powders

We define an adimensional parameter, S, that is capable of predicting if a glass powder compact can be fully densified by viscous flow sintering or if concurrent surface crystallization will hinder densification. The proposed sinterability parameter is S(T) = gamma/left perpendicular root N-s.U(T).n(T).rright perpendicular, where gamma is the glass-vapor surface energy, N-s the density of nucleation sites on the glass surface, U(T) the crystal growth rate, n(T) the viscosity, and r the average particle radius. For high temperatures, T >= 0.85T(m), where T-m is the melting point of the crystal phase, an approximate expression can be used: S-ht(T) = 2 pi.gamma.N-A.T-m(2).(3)root V-m(2)/[10 root N-s.r.Delta H-m.Delta T-2], where V, is the molar volume, N-A is Avogadro's number, Delta H-m is the melting enthalpy of the crystal phase, and Delta T = T-m - T is the undercooling. This expression avoids the (time consuming) measurement of U(T) and n(T). Predictions can be made by S or S-hT thus avoiding the need of any sintering experiment. For a given glass-forming composition the physical properties are fixed, but higher temperatures and smaller particle sizes increase S and privilege sintering over surface crystallization. We demonstrate that the condition to successfully densify any glass powder at a given temperature is S > 50. This new parameter is a very useful aid for the development of sintered glasses and glass-ceramics. (c) 2008 Elsevier B.V. All rights reserved