The Formation of Porous Membranes by Filtration of Aerosol Nano-particles

Flame-generated aerosol particles of Al2O3 were deposited by gas filtration on two types of porous and ceramic tubes of α-Al2O3 with mean pore diameters of 450 and 2700 nm, respectively. The particles were aggregates with average mobility diameters in the range of 30¨¢100 nm and primary particle diameters of 4¨¢8 nm. The particles are characterized by differential mobility analysis, transmission electron microscopy, and by their specific surface area. The deposited membranes are characterized by gas permeability measurements, scanning electron microscopy, and by their pore size distribution from nitrogen capillary condensation. The particles form a distinct, homogeneous membrane layer with a porosity of ∼90% on top of the substrate surface and only penetrate slightly into the substrate structure. The mean pore sizes of the deposited membranes determined by nitrogen condensation agree approximately with those determined by gas permeation and the specific surface area. The mean pore diameter varies in the range of 30¨¢70 nm. The gas permeability of the deposited membranes is related to the specific surface area but influenced by the high porosity. The mean pore size and the permeability of the membranes are almost independent of the substrate structure. The development of a membrane with uniform properties is preceded by a short initial period in which the deposited particles, with an equivalent membrane thickness of roughly 2 m, have a significantly lower permeability than the ultimately developed uniform membrane layer. This effect is particularly significant for the aerosol particles with the lowest mean size, probably due to particles deposited in the pore mouths of the substrate. The particles and the deposited membranes are X-ray amorphous but retain their specific surface area on heating to even high temperatures. When the membranes are heated to 1473K for 10 h, X-ray diffraction shows a mixture of θ- and α-alumina, accompanied by a partial disintegration of the membrane and a considerable loss of surface area.