10 December 2013
Carbon nanotubes find real world applications
No one disputes that carbon nanotubes have the potential to be a wonder technology: their properties include a thermal conductivity higher than diamond, greater mechanical strength than steel – orders of magnitude by weight – and better electrical conductivity than copper.
But, like other 'great technologies of the the future', are we over hyping nanotubes? Are they near passing the real test – that of widespread practical use? The answer is a qualified yes. Qualified, because there are two distinct kinds of nanotube – single wall and multiwall (SWNT and MWNT). A carbon nanotube is a seamless cylinder of either one or many layers of graphene, hence the term single or multiwall. Typically, MWNTs are being used in practical applications. SWNTs are mostly much more expensive, although they hold out huge potential for the future.
The success of MWNTs is proved by a surprising statistic: worldwide commercial production capacity presently exceeds several thousand tons per year, according to Dr Michael De Volder, previously with imec, but now a lecturer in nanomanufacturing and engineering design at Cambridge University's Institute or Manufacturing. But it's a level of production that has taken around 20 years to achieve.
"The beginning of widespread carbon nanotube research was preceded in the 1990s by the first scientific report of MWNTs, although hollow carbon was reported as early as the 1950s," Dr De Volder says. "However, carbon nanotube related commercial activity has grown most substantially during the past decade. Since 2006, worldwide carbon nanotube production capacity has increased at least tenfold."
A summary of some carbon nanotube applications now available commercially gives a flavour of just how widespread a real impact the technology is starting to make. Take water and oil purifiers, for example. the size, surface area (500m2/g) and adsorption properties of carbon nanotubes make them an ideal membrane for filtering toxic chemicals, dissolved salts and biological contaminants from water. That makes them a potential technology for producing clean water and drinking water from the sea.
US company Seldon Technologies has developed the MineralWater System using its Nanomesh Purification Technology – a carbon nanotube filtration system – to do just that. It says its system delivers drinking water without the use of chemicals, heat, or power: vital for use in developing countries where it is most needed.
Nanomesh removes pathogens and contaminants such as viruses, bacteria, cysts and spores, delivering water that meets or exceeds the USEPA Drinking Water Standard. It is suitable for use in homes, offices, schools, clinics, and other commercial environments, Seldon says.
The huge surface area of carbon nanotubes is also being exploited when they are used as the electrodes in capacitors to provide more current and better electrical and mechanical stability than other materials. Their large surface area means energy is stored all along them, not just at the ends as in conventional capacitors. Research labs at Stanford and MIT have been working to create carbon nanotube based ultracapacitors that would rival batteries in cars. Bringing them to market is FastCAP Systems of Boston, using carbon nanotube to create ultracapacitors that it claims offer long life spans, durability, and recharge times and power levels beyond the traditional batteries and other capacitors. They also contain no lithium and carry no risk of thermal explosions.
The properties of carbon nanotubes make them ideal for supporting different kinds of structures – for example, sports equipment, body armour, vehicles, rockets and building materials, where they are being widely used. The nanotubes create networks within the composite material to bear the load of the weight and strain placed upon it. This can apply to medical areas as well: the University of Delaware's Center for Composite Materials is researching carbon nanotube as a 'smart skin' to sense changes in a structure's integrity.
There are many other medical uses for carbon nanotubes, including: bone scaffolding; cell therapy – achieved by delivering drugs or silencing genes, with modified carbon nanotubes recently used to control the damage caused by a stroke; synthetic muscles; biosensors; and dental implants.
"Microelectronics is one area where carbon nanotubes have been studied for some time and where work is being done towards using carbon nanotubes for flexible electronics," Dr De Volder says. "Companies like IBM are looking to make the smallest possible transistors consisting of only one nanotube, but they are also aiming to create slightly larger transistors containing many nanotubes for use in applications like flexible mobile phones and for integration in textiles.
"You could call such applications traditional, but I have been happily surprised to see other very interesting and promising applications of carbon nanotubes, for example the portable water filtration devices developed by Seldon Technologies for use in developing countries."
The growing use of carbon nanotubes is coming as a result of improvements in the production of nanotube materials – prices have come down significantly as volumes increase, vital for applications like water filtration, which have to be very affordable.
Another use of carbon nanotubes that is already quite well established is their addition to polymer composites to enhance stiffness and improve damping. Sports manufacturers use them in tennis and badminton rackets, and bicycle frames as with BMC Switzerland.
But while carbon nanotubes are being used in practical applications, it doesn't imply their more widespread use will not be problem free.
"There are a number of obstacles we have been working on which we haven't solved yet," Dr De Volder says. "Particularly in high end targets, like the search for better transistors, the exact morphology of the nanotube and the orientation of the graphene lattice with respect to the tube axis – referred to as its chirality – is really important. At this moment, we have little ability to synthesise carbon nanotubes with very specific types of chirality and it is this that determines the semiconducting versus conducting properties of the carbon nanotubes.
"One of the interesting things happening is the improvement in computer simulations of how carbon nanotubes are synthesised, which will hopefully enable us to tweak the fabrication process. And electron microscopy is making it possible to look at the carbon nanotubes while they are being formed, which is helping increase the deep understanding of the process."
Dr De Volder himself is working on the challenge of mass producing devices featuring hundreds of thousands of nanotubes.
"Unfortunately, when you bring them together in large numbers, the figures of merit for their properties are often disappointing compared with what you get from an individual carbon nanotube. I am trying to develop techniques for bringing particles together in more efficient ways, or looking at new emerging properties of the materials depending on how you bring the carbon nanotubes together."
Nevertheless, progress is now happening with SWNTs, with UK company Thomas Swan being a world leader in making SWNTs with its Elicarb material, now being used in areas like advanced composites, electronics, energy storage, print, paper and packaging and fuel cells.
Another recent development in SWNTs – announced in June by Linde Electronics – is the development of a carbon nanotube ink for use in displays, sensors and other electronic devices. Potential applications include smartphones with a roll up screen and a see through GPS device embedded in the windshield of a c
"Linde is now making its nanotube inks available to developers," says Dr Sian Fogden, market and technology development manager for Linde's nanomaterials unit. "These inks contain single walled carbon nanotubes and are produced without damaging or shortening the nanotubes and therefore they preserve the unique nanotube properties."
Linde claims this is a landmark development that drastically improves the performance of transparent conductive thin films made from the inks and opens the door for the development of carbon nanotube applications in not only consumer electronics, but also the healthcare and sensor manufacturing sectors.
Because nanotubes are long and thin, they have high van der Waals forces between them and they stick together. The standard way to separate them is by using high powered sound waves. But this can damage the nanotubes and affect their properties.
"With our inks, we use a process called Salt Enhanced Electrostatic Repulsion (SEER) that doesn't require sonication but which produces solutions of individual carbon nanotubes while maintaining the length of the nanotube," Dr Fogden says. "Only very recently have products such as touch screens begun to be produced which contain single walled carbon nanotubes and these devices have yet to be launched into the full consumer market. Only when the raw carbon nanotube material can be fully processed in a reliable and repeatable manner will they be used in consumer electronics on a large scale."
Another recent intriguing development in electronics and computing comes from US company Nantero, which says it is commercialising carbon nanotube based semiconductor devices, including memory, logic and others.
"We have developed NRAM, a high density nonvolatile RAM and the aim is for it to serve as a universal memory technology," says ceo Greg Schmergel. "NRAM can be manufactured for both standalone and embedded memory applications and samples have already been shipped to selected customers and are under development at several production cmos fabs by Nantero and its licensees. These samples are multimegabit arrays that demonstrate high yield, high speeds, reliability and low power consumption."
Nantero claims it is the first company to actively develop semiconductor products using carbon nanotubes suitable for production in a standard cmos fab.
"The main obstacle in the past has been the fact that carbon nanotubes have not been compatible with existing semiconductor fabs," Schmergel says. "At Nantero, we have solved that by developing a cmos compatible carbon nanotube material that can be accepted into any fab in the world and manufacturing processes compatible with existing semiconductor manufacturing equipment. So our memory and other carbon nanotube devices can be made in any cmos fab at high volumes.
Using existing processes means reliability and reproducibility is far higher." Nantero's microelectronic grade carbon nanotube material is now available commercially through licensee Brewer Science.
This could be a pointer to the longer term future, involving mainstream computing. At Stanford University recently, a team announced the first functioning computer built from carbon nanotubes. Despite featuring just 178 transistors and running at 1kHz, the computer is nevertheless 'Turing complete', meaning it can do anything today's machines can do, just much slower.
But, in a few years time, billions of carbon nanotubes may be on our desks and in our pockets.
University of Cambridge
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