NAMS 2002 Workshop - ICOM 2008

Ultra- and Microfiltration II - Processes – 7
Thursday July 17, 12:00 PM-12:30 PM, Moloka’i
Pressurized Porous Nanocrystalline Silicon Membranes Exhibit High
Permeability to Water and Gas
T. Gaborski (Speaker), University of Rochester, Rochester, New York, USA
D. Fang, University of Rochester, Rochester, New York, USA
C. Striemer, University of Rochester, Rochester, New York, USA
M. Kavalenka, University of Rochester, Rochester, New York, USA
J. Snyder, University of Rochester, Rochester, New York, USA
M. Hoffman, University of Rochester, Rochester, New York, USA
J. DesOrmeaux, SiMPore Inc., West Henrietta, New York, USA
P. Fauchet, University of Rochester, Rochester, New York, USA
J. McGrath, University of Rochester, Rochester, New York, USA - jmcgrath@bme.rochester.edu
We recently introduced porous nanocrystalline silicon (pnc-Si) as a molecularly
thin membrane material capable of size and charged based separation of
proteins and other nanometer-sized solutes (Striemer et al. Nature, 2007). The
membranes can be produced in massive arrays with membranes freely
suspended over millimeter support spacings. Mechanical tests indicate surprising
strength with failure at or above 15 psi with no fatigue prior to rupture. Average
membrane pore sizes can be tuned between 5 nm to 100 nm with porosities
between 0.1-10%.
The structure of pnc-Si membranes suggests that they should display
extraordinary permeability to water and gas under pressure. To test this
prediction, we formatted membranes for easy assembly into gas pressure cells
and centrifuge tube inserts. For membranes with mean pore sizes ~ 20 nm and
porosities ~ 2%, we measured hydraulic permeabilities of nearly 2 x 10 -8 m/(s-Pa)
and air permeability in excess of 5 x 10 -5 (m/s-Pa). These values are at least
tenfold higher than the permeability values for commercial ultrafiltration
membranes measured in side-by-side comparisons. The permeabilities to air and
water are also more than one order higher than literature values for carbon
nanotube/polymer composite membranes. Because pores can be directly imaged
in electron microscopy, we employ known pore sizes and distributions to test
existing theories for gas and water permeability of ultrathin membranes (Tong et.
al. Nano Letters, 2004). For both water and air, we find that the existing theories
are predictive of the flow rates we measure through specific membranes.
Interestingly, native pnc-Si membranes are impermeable to water if one side of
the membrane is left dry, highlighting the significance of surface tension and high
curvature for liquid flow through nanoporous membranes.