Cracking a problem
Another use for a fashionable, new material
LIKE all other human activities, science is subject to fads. One of the latest is for graphene. This wonder material—a form of carbon that comes in films a single atomic layer thick—won Andre Geim and Konstantin Novoselov the Nobel prize for physics last year. Because of its unusual electrical properties it has been touted as a way of making everything from touch screens to solar cells. Now a humbler use is being proposed. Another of graphene's qualities is that it is strong. That, suggests Erica Corral of the University of Arizona, makes it ideal for reinforcing ceramics.
Engineers like ceramics because they can be heated without melting. Unlike metals and plastics, though, they are brittle. Because they crack easily, using them in places that are exposed to a lot of physical punishment is difficult. But a paper just published in the American Chemical Society's journal, Nano, by Dr Corral and her colleagues, suggests a sprinkling of graphene may deal with that.
The ceramic they experimented with was silicon nitride, a material much admired in the aerospace industry. Components are made by sintering it in powder form in a mould, at 1,000ºC, for several hours. Unfortunately, graphene disintegrates above 600ºC, so the team had to think of a clever way of mixing the two materials. Their solution was to take advantage of graphene's electrical conductivity, by running a current through the mixture. This generated a temperature of 1,650ºC—more than enough to sinter the silicon nitride. The graphene, however, did not break down. Why, is not entirely clear. But rapid electrical heating did not seem to affect it in the way that slower oven heating would. The result was a composite that was able to withstand twice as much pressure as unalloyed silicon nitride.
When the team examined what they had done under a microscope, they found that the graphene had wrapped itself around the silicon-nitride grains, forming continuous walls. When they looked at samples that they had whacked hard enough to come close to cracking, they found that these walls were encouraging the cracks to spread in three dimensions—in contrast with the two-dimensional pattern usually seen in silicon nitride. Dr Corral suspects that extending cracking into the third dimension dissipates the energy faster and stops the fault spreading. A cracking idea, as it were.