A new crop of high-performance bio-minerals is being created by a research group at Leeds University using a fraction of the energy required in current ceramics manufacture.
Inspiration for the materials comes from biological minerals of the sort found in seashells, which can rival ceramics in terms of hardness and mechanical properties but are created in aqueous environments at ambient temperatures.
‘With calcium carbonate, if it’s a pure geological sample you simply have to tap it with a hammer and it will fall apart along the cleavage planes,’ said Prof Fiona Meldrum of Leeds University. ‘But if you do this to a biogenic calcite crystal it fractures with extreme difficulty.’
The key feature of bio-minerals is that they are composites, made from an inorganic mineral such as calcium carbonate, with a small amount (often only 0.1 per cent by weight) of organic material, usually a protein.
‘You still have a perfect single crystal but inside this crystal you find these proteins. Quite how they are included in the structure isn’t well understood but it makes an enormous difference to the properties.’
The group set about creating similar minerals in the lab, but rather than using proteins it designed polymer nanoparticles that are incorporated into the architecture of the crystal as it grows.
‘Usually when you crystallise something, that crystal wants to be perfect and every atom wants to be in the right place so there’s a tendency to push out impurities. But a number of systems can actually retain these impurities within the crystal and if we can understand how to do that then we can really start to tailor the properties.’
The team has a raft of techniques at its disposal to characterise the minerals it produces, including high-resolution X-ray diffraction, infrared spectroscopy, transmission electron microscopy (TEM) and atomic force microscopy — the latter allowing it to actually watch as the organics get incorporated into the crystal.
This wealth of data is then related to mechanical tests using a nano-indenter — a small chisel-like tool that can probe a material and record its response to a force.
Crucially, this allows a far greater level of design and control than is possible in the high-temperature and pressure environments used to manufacture current high-performance ceramics. Meldrum and her team will continue to hone their minerals and will then focus on transferring techniques to industry.
‘I absolutely don’t see a problem — it’s a fully scalable, one-pot method. It’s as simple as it suggests — you just grow crystal with the appropriate additives.’