Modeling the effects of material chemistry on water flow enhancement in nanotube membranes
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n One of the main applications of nanofluidics is the possibility to build efficient filtration membranes.1–4 After the first experiments5–6 on fluid flow inside nanotubes, interest grew because channels with diameters of a few nanometers would mean the ability to filter by mechanical separation, starting from ultrafiltration (diameters of 10–100 nm), through nanofiltration (1–10 nm) up to reverse osmosis ranges (0.5–1 nm). Tubular carbon structures with an open central pore were the first nanotubes considered that have low-friction transport thanks to graphitic surfaces. We distinguish carbon nanotubes (CNTs) from carbon nanopipes (CNPs) produced by chemical vapor deposition (CVD) of amorphous carbon in alumina templates.7 More recently, boron nitride nanotubes (BNNTs) and silicon carbide nanotubes (SiCNTs) have also been used.8 The tubes are infiltrated in a dense material, referred to as the filling material, which can be a polymer or inorganic material. Ideally, the tubes are vertically aligned (VA) and the number of tubes is high. In this way, both tortuosity—defined as the ratio between the mean length of the flow path and the thickness of the
membrane—and porosity, which depends strongly on the ratio between pore volume and filling material volume, are favorable.2,9 It is a great challenge to produce VA membranes with these features. We will discuss the modeling of liquid flow so that the effects of solid–liquid interactions,8,10,11 both with the tube wall and with the filling material, can be better understood.
Definitions and main assumptions The first experimental evidence of high water flow in carbon nanotubes was obtained using vertically aligned carbon nanotube (VA-CNT) membranes.1–6 These were produced by infiltrating a nonpermeable matrix material into the gaps of a nanotube array grown by CVD on silicon wafers or similar substrates.12–16 The filling material (a polymer or inorganic material) is dense and, therefore, the porosity of VA-CNT membranes consists solely of the inner channel of the nanotubes, allowing the construction of membranes capable of separating molecules based on the inner diameter of the nanotubes, which is controlled during the synthesis process. CNT membranes with sizes in the ultrafiltration, nanofiltration,
Francesco Calabrò, Department of Electrical and Information Engineering, University of Cassino and Southern Lazio, Italy; [email protected] doi:10.1557/mrs.2017.58
• VOLUME Core • www.mrs.org/bulletin 2017 Materials Research Society MRS BULLETIN 42 • terms APRIL 2017 Downloaded© from https:/www.cambridge.org/core. Queen Mary, University of London, on 19 Apr 2017 at 06:49:26, subject to the Cambridge of use, available at https:/www.cambridge.org/core/terms. https://doi.org/10.1557/mrs.2017.58
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MODELING THE EFFECTS OF MATERIAL CHEMISTRY ON WATER FLOW ENHANCEMENT IN NANOTUBE MEMBRANES
and reverse osmosis range have all been produced.9 The higher the density of the initial nanotube arrays, the higher the overall membrane permeability, provided the filling material can still be infiltrated arou
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