On the sinterability of crystallizing glass powders

We define an adimensional parameter that is capable of predicting if a given 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)=g/[NS.r.U(T).eta(T)], where g is the glass/vapor surface energy, NS the density of nucleation sites on the glass surface, U(T) the crystal growth rate, eta(T) the viscosity, and r the average particle size radius. For high temperatures, T>0.8Tm, where Tm is the melting point, another expression can be used: ShT(T)=2PI.g-R.Tm^2.Vm^3/2/[NS.r.kB.DHm.DT^2], where Vm is the molar volume, R is the gas constant, kB is Boltzmann s constant, DHm is the melting enthalpy of the crystal phase, and DT is the undercooling. This expression avoids the (time consuming) measurement of U(T) and eta(T). Predictions can be made by S or ShT avoiding the need to perform 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-ceramic