Carbon Nanotube Manufacturing, Experiments & Simulations
Now we’re cooking with gas. This article in physorg.com entitled Breakthrough for carbon nanotube materials lays out how
NanoTech Institute of the University of Texas at Dallas (UTD) – CSIRO has achieved a major breakthrough in the development of a commercially-viable manufacturing process for a range of materials made from carbon nanotubes.
The article gives their bonafides:
As reported in today’s edition of the prestigious international scientific journal, Science – the UTD/CSIRO team recently demonstrated that synthetically made carbon nanotubes can be commercially manufactured into transparent sheets that are stronger than steel sheets of the same weight.
How is it done? More importantly, what’s their production rate?
Starting from chemically grown, self-assembled structures in which nanotubes are aligned like trees in a forest, the sheets are produced at up to seven meters per minute. Unlike previous sheet fabrication methods – using dispersions of nanotubes in liquids – this dry-state process produces materials made from the ultra-long nanotubes required to optimise their unique set of properties.
How long will it be before this process is available for commercialization?
“Rarely is a processing advance so elegantly simple that rapid commercialisation seems possible, and rarely does such an advance so quickly enable diverse application demonstrations”, says Dr Ray H. Baughman of the NanoTech Institute.
Please someone make sure that funding is available to synch this manufacturing work with the carbon nanotube work being done at LLNL. My wag is that we’re talking about funding $.2 million- $2 million to adapt this process for carbon nanotube membranes. One guy on the ball is all it takes.
A while back I asked a member of the LLNL team what the best investment of dollars would be for research in this field. He said that the best investment currently would be “in coming up with scalable (economical) processes for producing membranes that use nanotubes or other useful nanomaterials for desalination.”
Now that we have the “scalable (economical) processes” –the next job is to adapt it to desalination membranes.
In what looks like a first for the University of North Carolina at Chapel Hill–a team there has produced some experimental results for the way water behaves inside carbon nanotubes.
The team of scientists, led by Yue Wu, Ph.D., professor of physics in the UNC College of Arts and Sciences, examined carbon nanotubes measuring just 1.4 nanometers in diameter (one nanometer is a billionth of a meter). The seamless cylinders were made from rolled up graphene sheets, the exfoliated layer of graphite.
“Normally, graphene is hydrophobic, or ‘water hating’ – it repels water in the same way that drops of dew will roll off a lotus leaf,” said Wu. “But we found that in the extremely limited space inside these tubes, the structure of water changes, and that it’s possible to change the relationship between the graphene and the liquid to hydrophilic or ‘water-liking’.”
The UNC team did this by making the tubes colder. Using nuclear magnetic resonance – similar to the technology used in advanced medical MRI scanners – they found that at about room temperature (22 degrees centigrade), the interiors of carbon nanotubes take in water only reluctantly.
However, when the tubes were cooled to 8 degrees, water easily went inside. Wu said this shows that it is possible for water in nano-confined regions – either human-made or natural – to take on different structures and properties depending on the size of the confined region and the temperature.
How is this applicable to semipermiable membranes?
In terms of potential practical applications, Wu suggested further research along these lines could impact the design of high-tech devices (for example, nano-fluidic chips that act as microscopic laboratories), microporous sorbent materials such as activated carbon used in water filters, gas masks, and permeable membranes.
“It may be that by exploiting this hydrophobic-hydrophilic transition, it might be possible to use changes in temperature as a kind of ‘on-off’ switch, changing the stickiness of water through nano-channels, and controlling fluid flow.”
I would think too that the next experiment would be in which you varied the pressure on the carbon nanotube. Subsequently, you’d want to build a simulation that modeled for variations of temperature and pressure across a carbon nanotube membrane.
I posted on this story back in June about how reseachers at LLNL were working at the 1.6 nanometer level. Their work confirmed simulations that showed salt would be rejected at these levels–and that the primary rejection driver would be charge.
What to do next? Well do a simulation.
This time simulations were done at the 1.0 nanometer level:
Professor N.R. Aluru at the Beckman Institute for Advanced Science and Technology at the University of Illinois at Urbana-Champaign, and Sony Joseph, who defended his Ph.D. thesis recently, have used computer simulations to explore a method by which water transport through smaller carbon nanotubes could be further enhanced.
“Until now,” Sony Joseph tells PhysOrg.com, “previous simulations had shown that single file water movement in short carbon nanotubes have net transport in both directions. But if you could get the water to orient in one direction, in a long tube, you could have net transport along that direction.
A second press release from the University of Illinois on the subject dated September 16, 2008 elaborates:
“Extraordinarily fast transport of water in carbon nanotubes has generally been attributed to the smoothness of the nanotube walls and their hydrophobic, or water-hating surfaces,” said Narayana R. Aluru, a Willett Faculty Scholar and a professor of mechanical science and engineering at the U. of I.
“We can now show that the fast transport can be enhanced by orienting water molecules in a nanotube,” Aluru said. “Orientation can give rise to a coupling between the water molecules’ rotational and translational motions, resulting in a helical, screw-type motion through the nanotube,” Aluru said.
Using molecular dynamics simulations, Aluru and graduate student Sony Joseph examined the physical mechanism behind orientation-driven rapid transport. For the simulations, the system consisted of water molecules in a 9.83 nanometer long nanotube, connected to a bath at each end. Nanotubes of two diameters (0.78 nanometers and 1.25 nanometers) were used. Aluru and Joseph reported their findings in the journal Physical Review Letters.
For very small nanotubes, water molecules fill the nanotube in single-file fashion, and orient in one direction as a result of confinement effects. This orientation produces water transport in one direction. However, the water molecules can flip their orientations collectively at intervals, reversing the flow and resulting in no net transport.
In bigger nanotubes, water molecules are not oriented in any particular direction, again resulting in no transport.
Water is a polar molecule consisting of two hydrogen atoms and one oxygen atom. Although its net charge is zero, the molecule has a positive side (hydrogen) and a negative side (oxygen). This polarity causes the molecule to orient in a particular direction when in the presence of an electric field.
Creating and maintaining that orientation, either by directly applying an electric field or by attaching chemical functional groups at the ends of the nanotubes, produces rapid transport, the researchers report.
“The molecular mechanism governing the relationship between orientation and flow had not been known,” Aluru said. “The coupling occurs between the rotation of one molecule and the translation of its neighboring molecules. This coupling moves water through the nanotube in a helical, screw-like fashion.”
In addition to explaining recent experimental results obtained by other groups, the researchers’ findings also describe a physical mechanism that could be used to pump water through nanotube membranes in next-generation nanofluidic devices.
I would think that first generation carbon nanotube desalination membranes –in order to keep the flow in one direction–could obtain the charge by “directly applying an electric field”. Then later generation membranes membranes could obtain charge by “attaching chemical functional groups”.
Why is this important?
Joseph and Aluru, are especially interested in using this technology for water purification and nanofiltration. “We are trying to show how this would aid the process of reverse osmosis,” Aluru says.
Joseph and Aluru emphasize that, right now, this work is largely based on computer simulations with theoretical models. Joseph explains that right now water transport through nanotube membranes of two nanometers have been achieved, but that scientists are working toward pumping water through membranes that are less than one nanometer.
“We’ve shown that it is theoretically possible to get this sort of water transport,” Joseph points out. “The next step is getting to the point where this could be tested.”
This looks like it builds on the work of Jason Holt mentioned in my last post on LLNL work.
However, if manufacturers are already able to get commercial production volumes for the longer nanotubes–it may not be so important to do further work with the shorter nanotubes.
Anyhow, the simulation articles are here:membrane research, Water Desalination Research and Development