Silver has a number of properties which make it desirable as a coating for microparticulates. It is highly electrically and thermally conductive and has excellent reflectivity across the entire visible spectrum and into the infrared region. In addition, the surface of silver is slow to oxidise, which is an issue for other electrical conductors, such as copper or nickel, as it reduces the efficiency of charge transfer between surfaces. The highly conductive nature of silver also means that it can be used to shield sensitive electronic components from electromagnetic interference from other current carrying devices.

At present, solid silver particles or flakes are used in specialist paints, polymers and adhesives to coat or encase structures and components where efficient dissipation of electrostatic charge build-up is required. This property is of obvious importance in the aerospace industry, where protection of sensitive components and dissipation of lightning strikes is clearly desirable. However, solid silver is a particularly dense material (d = 10.4 g/cm3) and so the level of usage has to be balanced against the increase in weight afforded by this filler in aerospace applications.

A further issue with the use of solid silver particles for these types of application is the rising price of silver itself. The current price is five times higher than in 2005, and was recently more than 7 times higher. Stock market predictions suggest that the price of precious metals will continue to increase as investors seek refuge from the ongoing global financial instability.

Using particle coating technology, these density and cost issues can be addressed simultaneously. The use of a low density core material, whether hollow glass or polymer, as support for a nanometer thin shell of metal, reduces both the density of the final composite and the amount of metal required per unit of final product. This will be true of all metal coatings, but the opportunity to offer significant cost reductions is most pronounced with precious metals. Despite the low levels of conductive metal present on the coated particles, it is still enough to provide useful functionality. A silver coating thickness of 50 to 100 nanometers on a 20 micron sphere is sufficient to permit passage of an electrical current through a coat of paint into which the particles have been formulated at 30% by volume.

With this in mind, if the density of the coated filler is only 1/10th that of solid silver and each particle contains only 30% silver, it follows that there is a 30 fold lower materials cost associated with silver coated cores than solid particles.

Known potential application areas –

• As an additive in paints, polymers and adhesives to effect electrical conductivity in these materials (Conductospheres). This conductive property also results in the material providing shielding from electromagnetic interference.
• Flow visualisation seeding material (Isospheres).
• Platform for diagnostics through surface enhanced raman spectroscopy (SERS).
• Micromirrors for use in academic projects.

Several diverse application areas have been addressed by MTL’s silver coated hollow glass spheres. The company currently offers a range of conductive, metal coated hollow glass spheres termed Conductospheres. The ‘standard’ materials offered allow new customers to choose from a number of sizes and densities for their initial development work. However, the option to tailor the product to best suit the client’s requirements is also offered should these standard products not be of the required specification.

MTL also offers a silver coated hollow glass microsphere with sufficient silver loading to bring the average particle density up to that of water. This low volume, high value product is termed Isospheres and is marketed to the field of flow visualisation/marine architecture as a reflective, neutrally buoyant seeding particle for analysing fluid turbulence effects.

In addition to these proven applications, researchers at the Robert Gordon University, Aberdeen, UK, are currently assessing a range of MTL’s silver coated hollow glass spheres for their potential as a diagnostics platform for marine pollutant detection. The technique, SERS, requires a particular surface morphology to enable sensitive detection of analytes and MTL’s materials may offer a novel vehicle through which this technology can be applied.

MTL can readily adjust the size, density, coating thickness and core material to produce conductive materials with specifications not previously available, thus allowing the customer to achieve the best material for their application. This has certainly been the attraction for academic groups working in the area flow visualisation and diagnostics, where achieving a specific density and surface morphology are the key interest drivers.