Also known as nacre, mother of pearl is the hard iridescent coating found on the outside of pearls, and the inside of certain mollusc's shells. It's one of nature's hardest, stiffest, most stable materials – and Swiss scientists have now figured out how to make an artificial version of it, which can be "tuned" for different applications.
Natural nacre has a microstructure much like that of a brick wall, in that it's composed of stacked brick-like calcium carbonate plates. These "bricks" are joined together via a biopolymer "mortar," along with tiny interconnecting bridges made of minerals.
Led by Prof. André R. Studart, a team at Switzerland's ETH Zurich research institute set out to develop synthetic nacre which could be mass-produced, and tweaked for specific uses.
The researchers started with commercially-available aluminum oxide plates, which stood in for the calcium carbonate. Measuring a few dozen micrometers in length, these plates were suspended in a liquid that was subjected to a rotating magnetic field. This caused all the plates to align themselves in one direction, like the bricks in a wall.
An epoxy resin along with titanium oxide particles were then added, and the whole mixture was placed under high pressure while being heated to approximately 1,000 ºC (1,832 ºF). This caused the resin to harden into a mortar between the bricks, with the titanium dioxide melting at 800 ºC (1,472 ºF) to form the bridges. The aluminum oxide plates themselves didn't melt.
Once the material had cooled, it formed a very nacre-like product – according to ETH, it has set a new world's record for combining stiffness, strength and toughness. It's possible to control which of these characteristics are the most pronounced, by varying the pressure and temperature at which the material is made.
It is hoped that the technology could ultimately find uses in fields such as construction, aviation or spacecraft design. A paper on the research was recently published in the journal PNAS.
Source: ETH Zurich
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