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Silica
7-20 usd/kg
Silica is an abundant mineral – sand is a form of silica, although it often contains many impurities. It is an important industrial material and a key ingredient in building materials (concrete and bricks), manufacturing (food and pharmaceutical processing) and high-performance glass (fused silica and quartz). The difference between silica and quartz, is that quartz is made up of naturally occurring crystalline mineral, which is primarily silica, but with the addition of a very small amount of impurity.
Silica is inert to most chemicals (except hydrofluoric acid and phosphoric acid), an electrical insulator, incredibly resistant to thermal shock and very stable at high temperatures, and has extensive optical transmission properties, from ultra violet through (80%) visible (93%) and infra red spectrums. These attributes are utilised in laboratory equipment (lenses and crucibles for example), manufacturing (food and pharmaceutical, for example), lenses and lamps for sensing and imaging, and parts for radiation environment equipment.
Producing shaped parts from silica and quartz is challenging, due to the high melting point (it begins to soften at 1,630 degC), and is limited to glass blowing and machine cutting. Lenses are cut and ground from quartz blocks to a precise finish using diamond tools, for example.
Fine silica dust (such as produced by abrading and machining) is a known health risk and too much exposure can be hazardous. Regulations are in place in many countries and regions to reduce the risk of exposure.
Sustainability concerns
Non-renewable ingredients
Raw material generates polluting by-products
Low circularity potential
Silica aerogel is a rigid (and very fragile) foam and one of the lightest materials to have been discovered. The first useful application came at CERN (the European Laboratory for Particle Physics) in Geneva during the 1980s to detect fast subatomic particles. Commercial spin-offs continually emerge, inspired by the use of aerogel in the most extreme applications including NASA’s Mars Path under Rover and Stardust mission.
Aerogels are fabricated using the sol-gel process. The operation begins by mixing the necessary liquid ingredients. The molecules contained within combine to form nanometer-sized particles. Held in suspension (a stage known as ‘sol’), the particles are encouraged to form a gel (semi-solid) network, such as with the use of a catalyst. The three-dimensional structure of the gel and very low density are maintained by removing the liquid by supercritical drying (conversion of the liquid to gas without evaporation). Conventional drying would cause the structure to collapse because the surface tension created during drying – as it passes the liquid-gas boundary – would cause failure.
