The micro lost in the nano

21 May 2012

A team from the University of California, San Diego and the Samsung Advanced Institute of Technology report the production of thin-film carbon nanotube materials that are effective absorbers of X-band radiation, which can be ‘tuned’ to target specific narrowband frequencies.

Hide and shield

Materials that absorb specific frequencies of EM waves to protect vulnerable systems or conceal objects from EM wave-based detection systems are of interest for a wide range of reasons. From military aircraft ‘stealth’ technologies to concerns about long-term effects of human exposure to mobile communication transmissions; the demand for such absorbers is increasing.

Polymer composites have been used successfully as EM absorbers in land, sea and air vehicle applications for concealment and for protection of sensitive electronics. Polymer composites have also shown good characteristics in weight, flexibility and corrosion resistance, and the advent of the carbon nanotube (CNT) has created opportunities to further enhance these materials.

The spread of CNTs

The US–Korea team considered CNTs ideal for microwave absorption applications owing to their physical dimensions – very high aspect ratio (length/diameter) and large interfacial area (more than 1300 m2g) – and the ease with which their surfaces can be functionalised for suitable interactions with polymer matrix groups through the use of coupling agents.

Previous attempts to produce single-walled carbon nanotube (SWCNT)/polymer composite microwave absorber materials have suffered from aggregation and clumping of the CNTs producing uneven distributions within the polymer matrix.  This non-uniformity of course leads to non-uniform performance of the materials as absorbers and makes it hard to repeatedly produce materials with consistent absorption properties, which is a serious obstacle to large-scale production.

The team have addressed this problem by using coupling agents to functionalise the CNT surfaces for enhanced interactions with the polymer matrix. Specifically, they have used acid treatment to add carboxylic acid (COOH) groups to the CNT surfaces. The COOH groups facilitate epoxide ring rupture in the polymer matrix to allow the CNTs to disperse uniformly.  

Uniform CNT dispersion within the polymer maximises the permittivity increase for the composite that can be achieved for a given proportion of CNTs. Conversely, this means that fewer CNTs are required to achieve a desired change in electrical properties. More uniform results also mean greater repeatability. The result is composites that are cheaper and more structurally robust owing to the use of fewer CNTs, and that can be produced consistently – a prerequisite for use in commercial products.

Tube tuning

Another major contribution of the work is the introduction of a method to design SWCNT/polymer composites to selectively absorb a specific frequency range of EM energy. Team member Dr Paul Theilmann explained “Creating material that absorbs radiation over a very wide bandwidth is extremely challenging, most successful materials are very thick and high in carbon content, for example the carbon foam insulation used in anechoic chambers. Such materials cannot easily be integrated into small wireless devices or applied to the surfaces of aircraft.”

So the team concentrated on creating very thin, low-cost materials with good absorption at a specific frequency band.  Theilmann went on to say, “The design problem then reduces to an impedance matching issue. By adjusting the thickness and CNT percentage-by-weight of the composite on a metallic plane, its impedance can be made equivalent to that of freespace at a specific frequency. This is a well-known matching method in transmission line theory, which we simply apply to this freespace absorption problem.”

Combined with the ability to reliably produce uniform composite material, these readily controlled factors, which can be used to select the peak absorption frequency, provide powerful tools for producing absorber materials targeted at specific applications.

A carbon-based future

There is still much to explore in the relationships involved: both thickness and CNT content affect the frequencies of the absorption peaks, and there are other factors to consider; for example the effect of CNT content on the depth of the peaks. These need to be investigated to discover the best combination of CNT content and thickness to maximise absorption at a given frequency.

Beyond that question, the team aim to develop a better understanding of the physical processes at work that produce these effects and to construct a model of them with a view to better understanding the trade-offs.

In addition to the wide range of electronics applications of these materials the team also plan to explore their application in other fields, both for lightweight structural reinforcement and taking advantage of the composites’ hybrid properties. As team member Dr Sunghoon Park explained, “We are also considering how the unique electrical and mechanical properties of CNTs can be used within the biomedical field. The miniscule size of CNTs allows them to reach and interact with humans cells on a microscopic level.”

The Letter presenting the results on which this article is based can be found on the IET Digital Library

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Cover of Electronics Letters, volume 50, issue 19

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