NAMS 2002 Workshop - ICOM 2008

Inorganic Membranes I – 6
Tuesday July 15, 5:00 PM-5:30 PM, Moloka’i
High-Density, Vertically-Aligned Carbon Nanotube Membranes with High
Flux
M. Yu (Speaker), University of Colorado, Boulder, Colorado, USA - miao.yu@colorado.edu
H. Funke, University of Colorado
J. Falconer, University of Colorado
R. Noble, University of Colorado
Several studies have reported carbon nanotube (CNT) membranes that consist
of aligned nanotubes sealed in a polymer or inorganic matrix 1-3 . These
membranes had single gas selectivities that were approximately Knudsen, and
they had high permeation fluxes for liquid and gas feeds in nanotubes. Because
the aligned CNTs grew with a low density (~ 1011 CNTs per cm 2 of surface
area), only a few percent (0.08 ~ 2.7) of the membrane consisted of CNTs; most
of it was the sealing material. Thus, although the fluxes per cm 2 of CNT area
were orders of magnitude higher than other types of membranes, the fluxes per
actual membrane area (CNTs plus polymer or inorganic sealant) were much
lower.
We have prepared vertically-aligned CNT membranes with a CNT density of 2.9
x 1012 CNTs/cm 2 , which is approximately 20 times higher than these previous
studies by eliminating the need for a polymer or inorganic filler. Aligned CNTs
were grown on a silicon wafer with catalyst thin films 1-nm Fe/10-nm Al2O3 that
were formed by e-beam evaporation, the nanotubes were removed from the
silicon surface by in-situ water etching, and the nanotubes were collapsed to
about 5% of their original area by solvent evaporation. The tops of the CNT
membrane are expected to be open due to water etching 4, 5 , and the bottoms are
also expected to be open because the silicon wafers can be reused for several
times for CNT growth after detaching the CNT arrays. This preparation is much
simpler than that used for composite membranes, and the membranes have
much higher fluxes because of the much higher CNT density and additional
interstitial transport pathway between nanotubes. These membranes exhibit light
gas selectivities that are equal to or greater than Knudsen selectivities, but their
permeances are not independent of pressure. Instead, for most gases the
permeances decrease with increasing pressure. The permeance at 1 bar
pressure drop for N2 through a membrane that was 750 mm thick was 1.2 x 10 -4
mol/m 2 -s-Pa. This corresponds to a permeability of 27 cc (STP)/ m 2 -s-atm. Thus,
these permeabilities are one to four orders of magnitude higher than those
reported for composite membranes.