Aerogels are paradoxical materials. So ephemeral as to seem barely there, they have extraordinary properties. The MIT team, a collaboration between engineers from different specialities, has developed a material claimed to be capable of transmitting solar light while efficiently trapping solar heat.
In a paper in the journal ACS Nano, Evelyn Wang, head of the Department of Mechanical Engineering, graduate student Lin Zhao, professor of power engineering Gang Chen and colleagues describe how they produced the aerogel (which they refer to as a greenhouse medium) by controlling the hydrolysis and condensation rates of tetramethyl orthosilicate (TMOS). They discovered a novel process to do this, which, they explain, not only improves the optical transparency of the material but also reduces the fabrication time from weeks to days.
The key to making it work was in the precise ratios of the different materials from which the aerogel was synthesised, the paper explains. The process consists of mixing a catalyst with grains of a silica-containing compound in a liquid solution, and then drying the resulting gel to leave a matrix which is mostly air but which retains the mechanical strength of the original mixture. Producing a mixture which dries out much faster than conventional hydrogel precursors produced a gel with much smaller pore size between its grains, resulting in less scattering of light.
According to Wang, conventional hydrogels, although highly effective insulators, transmit a maximum of 70 per cent of incident light. The new hydrogel, however, lets through 95 per cent of incident sunlight but retains the highly insulating properties, meaning that the heat of the sunlight does not escape.
The team carried out tests on the material on a sunny day in the middle of winter, which in Cambridge, Massachusetts tends to be well below 0°C. They constructed a passive device consisting of a heat-absorbing dark material covered with a layer of the new aerogel. The dark material achieved and maintained a temperature of 220°C, they report. Such temperatures were previously only attained from sunlight by using a concentrating system of mirrors and lenses that focus the sunlight onto a central line or point. Such systems are more complex and expensive than the passive aerogel-collector combination.
The temperature reached is classified as intermediate in industrial terms, but would be suitable for generating domestic hot water on a rooftop system, for powering air-conditioning systems, or for a wide variety of applications in chemical or food manufacture offer many manufacturing processes, the team suggests. "The material we use to increase the temperature acts like the Earth’s atmosphere does to provide insulation, but this is an extreme example of it,” commented Zhao.
One aspect that still requires investigation is how to scale up the manufacture of the material, said Wang. This might involve working out a continuous rather than batch method of producing it. However, even the current method, which requires the use of a specialised piece of equipment called a critical point dryer, could be cost-effective for some applications, he added.
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