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The ‘blackest material yet’ could improve the performance of satellite based instrumentation

Carbon nanotubes have been seen as having a wide range of potential applications for many years, but their use in the ‘real world’ has been a long time coming. The first research into such structures was reported in 1952 by a Russian team, but it wasn’t until the 1990s that the technology began to gain some traction. Even today, their use – particularly in industrial applications – remains the exception

Despite the extended gestation period, applications in the electronics world continue to remain elusive. Whilst some see huge potential for carbon nanotubes (CNTs) to be used in supercapacitors – even ultracapacitors as a car battery replacement – research continues into their potential use in transistors in a post Moore’s Law industry. IBM, for example, is working in this field and expects to launch devices featuring CNTs within the next five years. But it remains a challenge to make CNTs with the correct orientation for use in such devices and, hence, the required properties.

Nevertheless, CNTs are finding application in what might be seen as complementary areas, including the space industry. And one application developed by nanomaterial specialist Surrey NanoSystems is what’s claimed as the blackest material yet made. Called Vantablack, the material is composed of vertically aligned nanotube arrays applied to a substrate using a manufacturing approach called photothermal chemical vapour deposition (PT-CVD). According to the company, this enables CNTs to be synthesised at temperatures compatible with widely used engineering alloys. In other words, it says, CNTs can be applied to ‘real’ products.

“Vantablack is a major breakthrough in the application of nanotechnology to optical instrumentation”, said Ben Jensen, the company’s chief technology officer. “For example, it reduces stray light, improving the ability of sensitive telescopes to see the faintest stars, and allows the use of smaller, lighter sources in space borne black body calibration systems.”

Because of the way in which Vantablack is engineered, it acts as an effective trap for incident radiation; the CNTs ensure that light is bounced around a number of times before the energy is absorbed. And, by defining the density of CNTs and their length, the reflectance of the material can be tuned for use with a particular part of the spectrum.

Jensen said: “Vantablack was originally intended for use in the infrared (IR) spectrum, where it exhibits a total hemispherical reflectance (THR) of less than 0.15% across wavelengths ranging from 1 to 15µm. It is therefore used as the functional coating in IR systems such as thermal cameras, calibration targets, analytical instruments and large scale scientific experiments, where absorption of incident radiation defines performance limits. This is equally true in defence related applications, such as target acquisition, night vision sights and signature reduction, where Vantablack’s ultra low reflectance and ultra high emissivity across a range of frequencies provides a tactical advantage.”

One particular application which Surrey NanoSystems has in mind is satellites, where the material is suitable for coating internal components, as well as for use in MEMS based optical sensors and Earth observation instruments. In fact, Vantablack will be used in an optical instrument aboard a satellite being launched in January 2016.

These instruments operate in the IR spectrum as the Earth’s atmosphere is largely transparent at these wavelengths. However, they need to be calibrated at a known low temperature. Jensen noted: “Excellent front to back thermal conductivity of the source is required, together with maximum uniform emissivity across wavelengths of interest to ensure maximum sensitivity and signal to noise ratio of the instrument.” He added that such devices need to be light, compact and able to withstand space flight.

In the past, this function has been performed by aluminium alloys which have been anodised using the Martin Marietta Black (MMB) treatment, but Jensen pointed to some downsides. “This treatment results in a THR of around 1.5% at the benchmark 5µm wavelength and there is a spectral feature near this wavelength. MMB coatings also exhibit some mass loss when exposed to space vacuum, giving the potential for contamination of sensitive optics.”

The company says that no other organisation – including NASA – has been able to achieve in situ deposition of aligned CNT arrays at temperatures of less than the melting point of space qualified aluminium alloys. As part of the process, different catalysts were developed to achieve the required density of aligned nanotubes, whilst ensuring strong adhesion to the underlying substrate.

A further problem surrounds MMB – it is subject to the US’ ITAR regulations and is, therefore, generally unavailable. While other approaches are available, all are seen to be less acceptable.

Jensen said that, for a solution to be better than MMB, it not only needs to have a better THR, it also needs a flat spectral response between 2 and 12µm. “The coating is also required to retain its optical properties after space qualification to ESA standards and to be thermally stable and chemically inert.”

Jensen pointed out that Vantablack also improves instrument reliability. “This is of paramount importance where subsequent intervention, such as the decontamination of optics, is impractical. Further, Vantablack’s flat spectral response ameliorates the need for compensating circuitry that otherwise adds complexity and weight.”

Vantablack is said to be the most effective light suppression material available. “Its blackness offers a range of aesthetic design possibilities,” Jensen concluded, “and it is the closest anybody will come to looking into a black hole.”

Flexoelectric MEMS device is just 70nm thick

Researchers from the Catalan Institute of Nanoscience and Nanotechnology, Cornell University and the University of Twente have developed the first integrated flexoelectric microelectromechanical system on silicon and claim the 70nm thick device could enable new applications.
Professor Guus Rijnders from the University of Twente believes it will be possible to create flexoelectric materials with a thickness of just a few atomic layers. “You could make sensors that can detect a single molecule, for example,” he said. “A molecule would land on a vibrating sensor, making it just fractionally heavier, slowing the vibration just slightly. The reduction in frequency could then easily be measured using the flexo-electric effect.” In addition to ultra sensitive sensors, flexoelectric materials could also be useful in applications that require a limited amount of power, such as pacemakers and cochlear implants.
According to the researchers, the desirable attributes of flexoelectricity are maintained at the nanoscale, while the figure of merit – bending curvature divided by the applied electric field – of the first prototype is comparable to state of the art piezoelectric bimorph cantilevers. Like piezoelectric materials, flexoelectric devices can either generate electricity when deformed or change their shape when a voltage is applied. However, while the piezoelectric effect decreases with thickness, the team claims the thinner the material, the stronger the flexoelectric effect becomes.
While piezoelectricity is hard to demonstrate in silicon, the flexoelectric effect can be exhibited by any dielectric material. The team believes that all high-k dielectric materials used currently in transistor technology should be flexoelectric, thus providing a route to integrating ‘intelligent’ electromechanical functionalities within current transistor technology.


Author
Graham Pitcher

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