The stuff of pottery and piggy banks can be transformed into a nanocomposite that is stronger than steel, light as plastic and cheap as mud
JULIAN EVANS leans back on his chair in the chemistry department of University College London. For the past hour or so we have been discussing a new wonder material, and now it’s time for me to see it for myself. He shows me a close-up picture of a sample. It looks like the plastic wrapping on a pack of supermarket fruit. “It’s see-through,” I observe, surprised.
Lots of materials are transparent, of course, but they are rarely noted for their strength. Yet this stuff is stronger than steel, and perhaps even a match for Kevlar, the material used in bulletproof vests. It’s also as light as plastic yet as stiff as carbon fibre.
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“This stuff is perhaps even a match for Kevlar, which is used in bulletproof vests”
Perhaps the most surprising thing is that it is made from clay – the same stuff we use to make bricks and crockery and which sticks to your spade as you dig your garden. But treated in the right way, clay’s properties can be transformed. It is also dirt cheap. “All you need is to pull it out of the ground and wash it,” Evans says.
This is music to the ears of aircraft and car manufacturers, who are crying out for alternatives to steel and aluminium. At the moment their best option is carbon fibre composite. This is a great material – Volkswagen, for instance, has used it to achieve astonishing levels of fuel efficiency in its . But producing carbon fibre requires temperatures of 1500 °C, which makes carbon-fibre composites far too costly to use in the average family car and gives them an environmental impact to match.
Making composites based on clay is much less energy-intensive, perhaps no surprise given that they draw inspiration from one of nature’s wonder materials, mother-of-pearl. The unusual strength of this material has been recognised since the 1970s ().
The basic ingredient of mother-of-pearl is aragonite, a form of calcium carbonate that in pure form is brittle and soft. Yet when sheets of aragonite are alternated with layers of protein and other biopolymers around 10 to 50 nanometres thick in a “bricks and mortar” structure, the result is a material with exceptional mechanical properties that surpass those of any of its ingredients (see diagram). When the shell is struck – by the beak of a hungry seagull, say – any crack that begins to form has to zigzag back and forth through the biopolymer layers. This rapidly dissipates the energy, halting the crack in its tracks.
If an unremarkable substance like aragonite could be transformed in this way by adding polymers, it occurred to materials scientists that perhaps clay could do the same. There are numerous different types of clay but all share the same basic structure: flat mineral sheets called platelets which are around a nanometre thick and remarkably strong. It is the water between the platelets that makes freshly dug clay soft and pliable.
Researchers working for the car maker Toyota were the first to give clay composites a spin. In 1985 they blended small amounts of clay with nylon to create a “nanocomposite”, in which nylon molecules coating the platelet surfaces were also linked to each other by strong bonds. Within four years, Toyota had begun using its clay-based material to make cam-belt covers for its engines, and since then it has found a host of other applications.
Such clay nanocomposites are only a fraction of the strength of mother-of-pearl, however. Whereas mother-of-pearl is 99 per cent aragonite by volume, with just a smidgen of protein binding it together, clay accounted for only 5 per cent of Toyota’s material. Simply adding more clay to the mix didn’t solve the problem, as beyond this proportion the platelets clump together, destroying the structure essential for strength. In fact, increasing the proportion of platelets beyond 5 per cent produced a weaker material.
What the material was lacking was the highly ordered bricks-and-mortar structure of mother-of-pearl. Molluscs achieve this by growing their shells slowly over several months. Simple mixing is not enough. Researchers elsewhere have since tried just about everything to encourage clay platelets to mimic mother-of-pearl, from letting them settle in suspension to spinning them in a centrifuge. For years, the resulting materials were all disappointingly weak.
The breakthrough came in 2007, when Nicholas Kotov of the University of Michigan, Ann Arbor, and his team discovered how to achieve a structure remarkably similar to mother-of-pearl by growing it layer by layer. They dipped a glass slide alternately into suspensions of polyvinyl acetate and clay to build up an incredibly strong and light material. Kotov calls it “plastic steel” (). If Kotov’s clay nanocomposites could be mass-produced, they would be a cheap and environmentally friendly alternative to carbon fibre.
Right now, that’s a big if. It took Kotov’s group two days of continual dipping to make a piece smaller than a playing card and a fraction of the thickness of a human hair. Although the team has since speeded up the process more than 100-fold, it is not clear whether the layer-by-layer technique will ever be fast enough for mass production.
However, the potential for clay composites is alluring enough for others to try to solve the problem of making them. Evans has been working on it for four years and now believes the trick might be as simple as filtering a suspension of clay platelets and suffusing the reside with polymers (). The filtering step helps the platelets line up in a well-ordered structure, Evans thinks.
Peng Jiang and colleagues at the University of Florida, Gainesville, use an electric field to pull charged clay platelets and polymer molecules in solution towards an electrode. Within 5 minutes they can get a nanocomposite film more than 0.1 millimetres thick – roughly a hundred times thicker than Kotov’s – whose area is limited only by that of the electrodes (). It is about half as strong as Kotov’s, but Jiang reckons that’s mainly because Kotov’s group use a more complex polymer with superior binding properties. Jiang’s team is working on an improved polymer mixture.
We’re still a long way from a composite that could be used to build cars or aeroplanes. But with the basics now worked out, a new wonder material inspired by molluscs and made from clay looks increasingly ready to fly.
