Spray-On Technique Could Bring Carbon Nanotubes to Retailers’ Shelves

Carbon nanotubes appear to be getting back some of their glory—after seemingly being eclipsed by graphene—with the news yesterday that an entire computer could be made from the material.  Now researchers at Technische Universität München (TUM) in Germany are continuing the carbon nanotube comeback with a new, inexpensive process that promises to enable their use in a wide range of applications including electronic skin and sensors integrated into food packaging.

The process, which involves simply spraying the carbon nanotubes onto a flexible, plastic substrate, is described in the journal Carbon (“Fabrication of carbon nanotube thin films on flexible substrates by spray deposition and transfer printing”)

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Putting PENCIL to paper to create gas sensors

Scientists have made a carbon nanotube pencil that can draw gas sensors straight on to paper. This cheap and extremely quick prototyping method could spur huge advances in gas sensors, both for public health and in something as simple as toilet ventilation. The stability of the solid pencil composite also means it could be suitable for 3D printing, making gas sensors on demand and to the customer’s specification.

Volatile organic compounds (VOCs) are a huge health concern in public spaces and in the workplace: some VOCs contribute to photochemical smog and factory operators must be extremely careful that employees are not exposed to health-damaging vapours. For this reason gas sensors are increasingly in demand to monitor such molecules. According to lead author Tim Swager at the Massachusetts Institute of Technology, US, the gas sensor market is already worth $3 billion (£1.9 billion) a year and he expects that figure to grow significantly over the next few years.

gas_sensorBy milling the nanostructured carbon with different molecules gas sensors can be created that can detect many molecules © NAS

Swager’s gas sensors are based on the concept of a chemiresistor – a resistor that registers a change in conductivity as it interacts with a chemical. To create the chemiresistor PENCIL (process enhanced nanocarbon for integrated logic) the team mixed nanostructured carbon – graphite or carbon nanotubes – with a selector molecule, in this case naphthalene with a hexafluoroisopropyl moiety, which reacts selectively with different chemical vapours. They then ball milled the two solids together and compressed the PENCIL mixture into either a pellet or into the shape of a conventional pencil’s lead. Finally, the team drew the gas sensor onto cellulose paper that had gold electrodes at either end. The sensor was able to detect and distinguish between acetone, tetrahydrofuran and dimethyl-methylphosphonat vapour by changes in resistance when they interacted with the selector molecule. And because the production process is so simple a gas sensor with new selector molecules can be created in just 15 minutes.

According to Swager, the speed and ease of this method allows gas sensor prototypes to be tested very quickly, meaning scientists can look at combinations they never had the time to before. ‘We are reporting a discovery tool as much as a fabrication tool and because it’s such a rapid method we can start working with three compound mixtures.’

Such advanced sensors could have a huge impact. They could be used to test whether food has gone off – fish release amines when they go bad, for example. Another example of their potential importance is dynamic ventilation – these low-power, wireless sensors could be placed around a building and linked to the ventilation system. Air conditioning could then be turned on, for example, when carbon dioxide levels started rising in a lecture theatre, rather than being left on all the time.

Radislav Potyrailo, principal scientist at the GE Global Research Center in Niskayuna, US, is impressed by the work, and agrees that the applications of such gas sensors could be huge. ‘Solvent-free blending of chemically active ingredients with transducing ingredients (such as carbon nanotubes and graphite) is really innovative and far-reaching.’ He adds that the sensors could be used in areas ‘ranging from chemical leak detection, to food safety, homeland security [and] environmental testing’.

Ref: http://www.rsc.org/chemistryworld/2013/08/putting-pencil-paper-make-gas-sensors

Carbon Nanotubes Increase Light Absorption in Thin-Film Solar Cells

UW-Madison_CNT_CellMaterials engineers at the University of Wisconsin-Madison (US) have developed the first thin-film solar cell that takes advantage of a previously unexploited property of carbon nanotubes — their ability to absorb light. Using nanotubes as the principle light absorbing material, the proof-of-concept device converts more than 75% of the light it absorbs into electricity.

Carbon nanotubes could be highly attractive candidates for the primary light-absorbing component in a solar cell because they are strong light absorbers in addition to having high charge transport mobility and solution-processability. Nanotubes are also more air stable than most other semiconducting conjugated carbon materials.

Other research groups have previously used carbon nanotubes in photovoltaic devices, but mostly as secondary materials such as electrode materials or charge-collection or charge-transport aids rather than for their ability to absorb light. “We estimate more than 60% of the current in our cell comes from the nanotube component,” says Michael Arnold, an assistant professor of materials science and engineering at UW-Madison. “We achieve these results using highly pure semiconducting carbon nanotubes with band gap in the near-infrared and very few metallic nanotube impurities and tuning the optical interference effects within the device.” The cell has a bilayer structure with a planar heterojunction between the carbon nanotubes and fullerene-C60. The carbon nanotubes absorb light, generating bound electron-hole pairs — or excitons — in the nanotube film. The electrons are extracted by the fullerene-C60 layer and transported to the silver top electrode, where they are extracted from the device.

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Nanotube Supply Glut Claims First Victim from IEEE Spectrum

By Dexter Johnson

Just three years after announcing a huge capacity increase to its multi-walled carbon nanotube (MWNT) production, Bayer Material Science has announced that it will completely close down its MWNT production to focus on its core business.

This is no surprise since there was a huge glut of product resulting in industry utilization rates that must have been in the single digits. This oversupplied market was the result of a MWNT capacity arms race that started in the mid-2000s. While this steep ramping up of production capacity reduced pricing from $700/kg in 2006 to below $100/kg in 2009—with some estimates putting the price at $50/kg as of last year—the problem seemed to be that no matter how cheap you made the stuff nobody was buying it because there were no applications for it. This resulted in stories, at once humorous and worrisome, of big chemical companies that had gotten themselves caught up in this arm race making desperate phone calls to laboratory researchers pitching application ideas for the material.

While some observers believed that this price cut would result in the applications being developed, most people recognized that this was a case of putting the cart before the horse, or “technology push” ahead of the preferable “market pull.”

This is not to say strategically it was wrong for a company like Bayer Material Science to build out capacity for a product that nobody seemed to want at that moment but may in the future. A company like Bayer can ramp up production with relatively little capital cost and manage to price everyone else out of the market. It was worth the risk.

However, hindsight makes it pretty clear that MWNTs applications were never really going to materialize as had been hoped. This became painfully clear when after a few years into production one of the target applications being touted for the material was the blades of large wind turbines. That announcement smacked of desperation.

Despite this, the story of MWNT capacity growth has been very instructive for how the so-called “nanotechnology industry” will shake out.

First, it’s clear that small operations that have found a way to produce a nanomaterial cheaply will have a difficult time competing with large chemical companies. This is not because they can’t produce the material more cheaply or at a better quality, but because they do not have the supply chain that well-established chemical companies have.

Second, you don’t want to be in the business of producing a nanomaterial that serves just to make some other product. You want to be making the final product. Many small start-ups no longer exist because they figured that they could just license their technology to a company that would make a product from their nanomaterial.

Bayer Material Science is in the position where it can just mothball its production without too much pain, but there may be some other companies that are less diversified for which that may not be an option. Sometimes when one domino falls the rest go in quick succession. So this should be an area to watch in the near future.

Image: Martin McCarthy/iStockphoto

Ref : http://spectrum.ieee.org/nanoclast/semiconductors/nanotechnology/carbon-nanotube-supply-glut-claims-its-first-victim