Influence of Viscous Deformation at the Contact Point of Primary Particles on Compaction of Alkoxide‐Derived Fine SiO2 Granules under Ultrahigh Isostatic Pressure

Abstract

Viscous deformation and the adhesion force at the contact point between amorphous silica particles under ultrahigh isostatic pressure (up to 1 GPa) are important in the densification of powder compacts. The amount of viscous deformation and the strength of adhesion force have been changed in the present study by altering the calcination temperature and particle diameter, and the new values have been determined successfully using a diametral compression test. The diameter of spherical and monosized alkoxide‐derived silica powders has been controlled within the range of 10–400 nm. Close‐packed granules of these powders have been produced by spray drying. Because of viscous deformation, as‐spray‐dried ultrafine silica powders without calcination could be consolidated into highly dense compacts (>74% of theoretical density) by applying ultrahigh isostatic pressure (1 GPa). Relatively high temperature in the calcined particles (400°C) causes viscous deformation at the contact point to disappear almost completely and clearly increases the adhesion force, because of neck growth that has resulted from viscous sintering. At temperatures >200°C, the green density of the calcined powders decreases to 65% of theoretical density, even under 1 Gpa pressure. The relationship between green density and viscous deformation in silica particles at the point of contact has been analyzed quantitatively by the Hertz and Rumpf model. On the other hand, if relatively low isostatic pressure (Pc < 100 MPa) is applied, the green density and intergranular pore volume depend on the strength of the spray‐dried granules. The relationship between granule strength and neck growth at the contact point with calcination has been estimated quantitatively.