Suited to defence - nanotech protection against chemical warfare

Materials World magazine
2 Mar 2010
Showing (top) a vehicle in the visible spectrum, conventional optical camouflage and (bottom) the same vehicle in the IR spectrum. The warm motor, transmission and exhaust pipe are clearly visible. © Saab Barracuda

Warfare is moving to the nanoscale to defend soldiers against modern chemical and biological weapons. Steven Savage, Research Director from the Swedish Defence Research Agency in Linköping, illustrates the benefits of these small yet significant solutions.

There are already hundreds of nanotechnology consumer products on the market, such as odour-preventing additives for socks, tennis racquets and high performance automotive lubricants. Scientists and engineers are actively engaged at the nanoscale in R&D of materials and surface coatings for defence applications. These include protection against chemical and biological (CB) agents, camouflage and inhibiting the impact on sensors from laser radiation.

Undercover agent

Uniforms and shelters, vehicles, weapons and other equipment must be decontaminated if exposed to CB agents. The contaminants are preferably neutralised in situ rather than washed off, as this may dilute and spread the contaminant. The nature of the service places demands on the techniques used.

Neutralisation involves washing with strongly oxidising, acidic or alkaline solutions and foams, which themselves can be harmful to humans and the environment. The decontaminating fluids, equipment, transport and personnel required present a major logistics burden. Progress in nanomaterials science has been made to create surface coatings for easy cleaning and nanoparticles to neutralise and decompose toxic agents. This crosses into civilian applications in terms of materials and coating techniques, as well as applications in air and water filtration, remediation and hygiene.

By designing a surface to be non-wettable by CB agents, the ability of the agent to penetrate the surface is also reduced. Hydrophobic surface coatings have been available for some time and can effectively reject liquid droplets. However, by patterning the surface, as well as creating a texture or structure with micro- or nanometre dimensions, the contact area between the surface and a liquid drop is further decreased, thereby reducing adhesion.

An alternative self-cleaning mechanism is where the surface is made superhydrophilic, using a titanium dioxide (TiO2) coating with decontamination based on film flow. This type of coating, which has been commercialised, can be made transparent and incorporated in anti-reflective coatings. There are also applications in anti-fog coatings.

Titanium dioxide is a wide band gap semiconductor exhibiting photocatalytic activity, when illuminated by ultraviolet light, contaminants are oxidised to harmless salts, water and CO2. A controlled surface texture is not needed, giving a technological advantage, although available TiO2 coatings as yet are only effective in sunlight.

Out of sight

When human eyes are the only sensors available, operating in the visible spectrum, even with binoculars conventional camouflage technology using paints and abstract colour patterns provides adequate protection. However, infrared (IR) sensors (thermal detectors) are widely available and frequently used in cameras and weapons such as heat seeking missiles. This makes conventional camouflage inadequate.

The situation is more difficult when radar sensors are also considered. A multispectral camouflage must be effective from visible wavelengths of 400nm through IR with wavelengths of >10µm to radar with wavelengths of tens of centimetres. The IR ‘signature’ emission and reflectivity of an object is determined by factors such as the temperature of the object and the background, the materials used and weather conditions. In The IR photograph above shows a jagged dark pattern at the rear of the vehicle behind a warm exhaust pipe, due to the lower emissivity of the blue camouflage paint. However, the temperature is the same as the rest of the vehicle panel.

Advanced materials can be used for spectral design of surface reflectance and emittance. Both the composition and structure are important, in all wavelengths. For example, the IR signature of normal green vegetation results from cellulose fibre and cell structure, which is independent of the green colour of the chlorophyll. Normal green vegetation’s reflectance spectrum is shown above, top left. By incorporating nanoporous beads that reproduce the structure of cellulose, and by doping them with a pigment, the optical colour can be chosen independently of the IR.

Another example is photonic crystals, which theoretically have 100% reflectance in a well-defined region of the spectrum. Butterfly wings, beetle carapaces and peacock plumages do not get colour from pigments, but from photonic crystals with dimensions of the wavelengths involved. Mimicking these structures gives more freedom when designing a camouflage surface coating. Again, the optical can be de-coupled from the IR signature.

Optical power

Lasers are widely used in military operations for aiming, range-finding, target illumination and identification. The more frequent use in sensor systems and consequently in electronic warfare enhances the demand for laser protection, both to defend the sensor system and to protect the human operator.

A self-activating optical limiter material should be highly transparent during normal conditions, but react upon laser illumination and limit the transmission to a safe level. The ability of modern lasers to switch wavelengths means that a protective device needs to act as a broadband optical limiter to counter the threat from different wavelengths.

Laser activated non-linear optical limiting materials fulfill these requirements. They respond rapidly (self-activation within nanoseconds) to intense laser illumination, have low transmittance upon irradiation but high transmission for normal harmless light, and the possibility to give broadband protection. When non-linear optical limiting materials are exposed to an intense laser light, absorption occurs bya non-linear mechanism. As the illumination intensity increases, a greater relative absorption occurs and, as the incident optical power increases, the transmitted energy is clamped.

The non-linear optical compounds can be dissolved in an appropriate solvent and transferred to a cuvette, forming a liquid-based nonlinear optical limiting filter, or doped into a glass material, creating a solid-state non-linear optical limiting filter. Solid-state filters can more easily be incorporated into optical systems and have several obvious advantages over liquids, including flexible shape processability for optical device construction, enhanced chemical, physical and mechanical long-term stability, and ease of handling.

An obstacle to preparing high-performance solid-state optical limiting materials is the difficulty in obtaining high levels of doping in the glass materials. A sol-gel method, which allows preparation of nanocomposite glass materials with high dopant concentrations, has been developed by FOI. With this new technique, glasses with optical limiter loadings of 30-40wt% have been obtained. The materials can be cut and polished to high quality optical filters, as shown in the image below, right.

The Swedish Defence Research Agency in Linköping has investigated multi-doped nanocomposite glasses to protect optical sensors working at visible wavelengths, and glasses with a photopic (normal) transmission >70% have been developed and tested. The glasses are effective as broadband filters for visible wavelengths and the best are able to limit the transmitted energy to below one microjoule. The agency has also explored how the self-activating filters might protect a charged coupled device camera from laser damage. The protected device experienced no damage on illumination for several different laser wavelengths in the visible spectrum.

Further information: Swedish Defence Research Agency FOI