Some 300 years ago, Robert Hooke studied the microstructure of cork with his new optical microscope and drew what he could see, both along its growth axis and in a radial section. A modern electron microscope is capable of far greater magnification, since the effective wavelength of an electron is about 0.01 nm, compared to the 500 nm or so of visible light. But the electron micrographs of cork that Jin-Chong displayed showed recognisably the same structure.
The engineered porous materials which were to be the subject of the talk could be divided by the scale of their pores, e.g. macroporous, mesoporous, microporous, nanporous, the scale ranging from cm to nm. The macroporous materials at the larger end of the scale included lightweight metallic foams, which could be of two types, closed-cell ones which were not really porous in the usual sense, and open-cell ones, in which the cavities were linked to each other. Applications could be structural, but they were also used for heat exchangers, sound absorption and in fuel cells.
There were also fibre-network materials, made of stainless fibres, 10-100 µm in diameter, sintered together at 1200°C. Micrographs showed them as networks of randomly oriented rods, obviously highly porous. One application was for acoustic damping in aircraft gas-turbines.
Microporous materials include zeolites. Inorganic zeolites are hydrated alumino-silicates, which have a linear structure with pores about 1 nm in diameter, and an internal surface area of hundreds of square metres per gram of the material. They may be naturally-occurring or synthetic, and are used in e.g. petrochemical cracking, as molecular sieves for absorption or separation (some molecules can get through, others not), and ion exchange (water softening). The global demand for such materials is of the order of millions of tonnes per annum.
By linking metals with organic units on a molecular scale, we can create hybrid inorganic-organic materials, in crystal form or as thin-films about 2 µm thick. A single gram of such a material can have an internal surface area of 7000 m2, about the same as football pitch! There are numerous applications, but one is to separate CO2 from other gases such as nitrogen or methane. A film of such material can let the one gas through almost at once, but the CO2 gets stored in the lattice, and only starts to come out the other side after a minute or two. Another version of such a film can pass CO2 but not methane. Other composite materials can be used for controlled drug delivery, absorbing the drug and then letting it out over an extended period.
A quite different application for these very versatile materials is photoluminescence, for instance emission of visible light when excited by ultraviolet photons.
Members of the audience raised several questions: about whether there were dangers of toxicity in the drug-delivery application (one did need to consider it); noise reduction in aircraft (Jin-Chong showed slides of some tests on reducing the noise from low-flying small aircraft); and the extent to which gas separation and storage could be scaled up for practical applications.
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