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)=γ/⌊NS·U(T)·η(T)·r⌋, where γ is the glass–vapor surface energy, NS the density of nucleation sites on the glass surface, U(T) the crystal growth rate, η(T) the viscosity, and r   the average particle radius. For high temperatures, T⩾0.85TmT⩾0.85Tm, where Tm is the melting point of the crystal phase, an approximate expression can be used: ShT(T)=2π·γ·NA·Tm2·Vm23/[10NS·r·ΔHm·ΔT2], where Vm is the molar volume, NA is Avogadro’s number, ΔHm is the melting enthalpy of the crystal phase, and ΔT=Tm-TΔT=Tm-T is the undercooling. This expression avoids the (time consuming) measurement of U(T) and η(T). Predictions can be made by S or ShT 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.