Molecular size pores in beryllium oxide powders

Abstract

BeO prepared by thermal decomposition of β-Be(OH)₂in vacuo consists of porous aggregates of crystallites ∼30 Å. The aggregates are relics of individual hydroxide crystals which undergo a volume shrinkage on decomposition of only ∼30 % of that theoretically possible, and have a density ∼50 % that of dense BeO. Of the intra-aggregate pore volume, only ∼20 % is available to CCl₄ and ∼50 % available to N₂ and hence there is a substantial volume of pores <∼6 Å and <∼3 Å. Such small pores render “surface area” measurement from gas adsorption meaningless, cause a small irreversibility in N₂ adsorption at low pressures and exhibit measurable He adsorption even at 100°C. Although of comparable molecular size to N₂, H₂O is able to penetrate most or all of the pores (due to the higher interaction energy to H₂O with the oxide surface) and after desorption some of these become accessible to N₂. Similarly, H₂O adsorption and desorption increases the pore volume accessible to CCl₄, and various H₂O treatments (including decomposition of the hydroxide in the presence of H₂O vapour) lead to a microstructure in which pores <∼6 Å are virtually absent.