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Nanoscale Mass Transport and Carbon Nanotube Based Membranes
 

Carbon nanotube based membranes known as buckypaper may be used as filter media for analytical mission instruments or implantable device support for astronaut health monitoring.

Benefit

Fluid flows on the nanoscale are studied through experimental permeability measurements and validated with a continuum model. These have applications ranging for MEMs sensors to purification technology. Future sensors developed for explorations will likely rely on MEMs and NEMs based fluidic systems where sample transport issues may be of import. As an advanced membrane material, the buckypaper will have potential applications in a variety of areas for manned space missions, ranging from fuel cells to waste purification and air and water filtration.

Research Overview

We have constructed membranes out of CNTs as a material known as "buckypaper" (Fig. 1) and have begun characterization of ion and gas permeabilities of the buckypaper. The buckypaper displays a high surface area (~450 m2/g) and an average pore size of 4-10 nm. Gas transport through buckypaper displays unusual dependencies, (Fig. 2) with throughput dramatically increasing as pressure is increased, in contradiction with common gas transport models. Further characterization is necessary to ascertain whether the buckypaper will behave as an effective gas separation media. Ion permeability studies have shown the buckypaper to be more conducive as an ionic transport media than it is for gas transport.

carbon nanotube buckypaperFigure 1. A SEM image of carbon nanotube based “buckypaper.” Bundles of single-walled nanotubes may be seen on the surface of the buckypaper.

We have also studied transport through regularly ordered 200 nm Alumina pores. The permeability of the porous alumina was then measured using a pressure/flow apparatus. A finite element code with adjustable slip boundary conditions developed by collaborators at Kettering University was used to model transport through the alumina. Transport was well described by a no slip condition. A uniform 20-30 nm thick carbonaceous coating was then formed over the pores by chemical vapor deposition, forming an amorphous carbon nanotubule. With this coating present, the flux through the pore increased by a factor of 2. This demonstrates the ability to modify micro- and nano- channels with surface treatments to enhance gas transport. Other coatings may conceivably give greater enhancements.

For example, our studies on a 10 nm diameter pore in polycarbonate has shown an order of magnitude increase in throughput over what is expected based on ordinary Knudsen diffusion models.

Background

Nanoporous media may have practical applications in separations and catalysis, allowing for size-selective molecular separations or high surface area catalysis. Applications in this general vein include air purification for manned space missions. Micro- and nano- electromechanical systems are being developed for a variety of applications including gas sensing and materials analysis. It is expected that the properties of transport in nanometer sized media may differ significantly from classical media for fluid mechanics, therefore detailed investigation of flow properties through these media are necessary to understand and implement these materials. As the channels through which gas flows becomes smaller, the channel surface plays a large role in impacting the gas flow properties. Particularly, surfaces can contribute dray, where gas molecules are slowed down by wall interactions. It is possible to modify the surfaces involved to control this slip and thereby enhance gas throughput over traditional expectations. This may have application anywhere micro- or nano- flows are required, such as MEMs sensors, air purification, supported catalysis or gas (e.g. hydrogen) storage.

Pressure decay rate through buckypaper as a function of pressure.Figure 2. Pressure decay rate through buckypaper as a function of pressure. The buckypaper displays a strong dependence of gas throughput on applied pressure.

At the limit of small diameter pore materials are carbon nanotubes, with diameters on the order of 1 nm.

Molar flux through 200nm anodized alumina pores as a function of applied pressure.Figure 3. Molar flux through 200nm anodized alumina pores as a function of applied pressure. By coating the pore with carbon, it is possible to increase the throughput by a factor of 2 or more.

The small diameters of CNTs allow for selective transport of different species, making them into potentially useful materials in separation technologies. Additionally, the large surface area available has led to proposals for use in gas storage, purification, or as a support material for catalysis. Simulations with CNTs have shown that wall slip is significant, with gas transport through the CNTs exceeding the Knudsen diffusion limit by as much as two orders of magnitude.