Study of the effect of compaction on the surface area and porosity of six powders by measurement of nitrogen sorption isotherms

Abstract

The effect, on the surface area and porosity of six powders, of compacting at pressures up to 1480 GN m–2 has been studied by measurements of the sorption isotherms of nitrogen at 77 K. The powders covered a variety of particle shapes, viz. spherical (alumina and silica), platy (mica, illite), aggregates of plates (kaolin) and trough-like (halloysite). With each powder, as expected, compaction produced interstices between the particles falling within the mesopore range of size, increased compaction pressure leading to reduced pore volume and a shift of the size distribution towards the finer end.

Except with kaolin (which showed a slight increase, ∼ 2 %), the specific surface diminished on compaction, but the magnitude of the effect varied greatly from substance to substance. Silica and mica showed marked diminution (∼ 30 %), halloysite a diminution of a few per cent, and alumina only ∼ 2 %; important factors involved are relative softness (silica as compared with alumina), platy character and, probably, lattice regularity.

Examination of the f-plots [graphs of f against relative pressure, where f=n/n(ref), n and n(ref) being adsorption on experimental substance and reference substance respectively] indicates that in the compacts of silica, mica and illite capillary condensation is largely inhibited during adsorption probably because with mica and illite most of the pores formed by compaction are parallel-sided, and because with silica flat areas (which hinder the advance of a liquid meniscus) are formed by deformation under compaction.

Halloysite particles, despite their hollow character, appear to be remarkably resistant to crushing.