Vertically aligned carbon nanotube membranes on macroporous alumina supports

Vertically aligned multi-walled carbon nanotube (CNT) membranes are grown on the porous α-alumina support by a multi-step method consisting of growth of vertical CNTs by chemical vapor deposition, filling of inter-CNT gaps with polystyrene and removal of the polystyrene over-layer and CNT tips by polishing and acid treatment. The membranes are defect free and exhibit gas permeance independent of mean transmembrane pressure. CNT membranes grown on the porous alumina support have lower areal tube density of 1.87 × 109 CNT/cm2, lower than the CNT membranes on dense silicon and quartz support reported in the literature. The CNT layer consists of fairly straight carbon nanotubes of 6.3 nm in pore diameter running parallel to each other with a tortuosity factor of about 1.3. Gas permeance through the porous alumina-supported CNT membranes is inversely proportional to the squared root of the gas molecules suggesting a Knudsen diffusion mechanism. However, the diffusivity values measured are about four times larger than the values predicted from the pore size, molecular weight and temperature using the Knudsen diffusion model.