Silica Aerogels possess the lowest density, highest thermal insulation and highest surface area per unit volume of any known solid. However, methods of making silica gels are expensive and time consuming which prohibits their use in mainstream applications such as domestic insulation. Aerogels are formed from “wet gels” by replacing the liquid with gas (air) to leave a nano-structured, porous, silica foam. The drying of the wet gel to aerogel is a difficult step and can be achieved by either:
• Super critical drying (SCD) exchanges the liquid inside wet-gels for super critical CO2 at high temperature and pressure. The CO2 is purged from the gel as a gas as the system is allowed to return to ambient temperature and pressure to give the dried aerogel.
• Ambient pressure drying (APD) is a multistep process which exchanges the liquid inside wet-gels for an organic drying solvent. The organic solvents have to be chosen with care as those with high surface tension effectively “pull down” the aerogel structure from within, due to the capillary force during drying.
The specialist equipment, high pressure and temperature required to dry the gel make the SCD process energy intensive and therefore produces materials that are too expensive except for highly specialised use. APD of gels provides an alternative, less energy intensive route but relies on the replacement of costly organic solvents.
Researchers at Newcastle University have developed a process of drying silica aerogels at ambient pressure inspired by the mechanism by which dragonflies form their wings, which are aerogels. The new method does not require the use of expensive, low surface tension organic solvents. Instead it uses an inorganic solution which is ~80 times cheaper. In the new process, inorganic solution and reagent produce gas within the pores of the gel.
When the gas is formed the gel is incompressible and therefore the pore diameter is increased, reducing capillarity, driving out solvent and enlarging the radius of the pore. The overall result is to dry the aerogel from the inside out.
Newcastle University is currently seeking industrial partners to develop and scale up the process to provide sustainable, greener, low cost manufacturing of aerogels. A single breakthrough in large-scale production method can fuel tremendous growth in aerogel enabled products such as insulation, smart windows and replacements for catalytic converters.
Current research is concerned with salt doped aerogels as the process generates the salt as a by-product. The salt can be easily removed from the aerogel but little is known about its effect on the material’s properties.
A GB patent application has been filed for this technology. The patent has entered the PCT phase.
Application no: GB1502613.1
Filing date: 17/02/2015
Dr Tim Blackburn, Science, Agriculture & Engineering Enterprise Team, Research and Enterprise Services, Devonshire Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK