Learning from nature
A new process of toughening ceramics in ambient conditions has been discovered. Kathryn Allen reports.
Ophiocoma wendtii, a brittle starfish, which has hundreds of focal lenses on its arms, has inspired a new technique to toughen ceramics without the need for conventional toughening methods, such as heating and quenching.
A team of researchers, led by Technion – Israel Institute of Technology and the European Synchrotron Radiation Facility (ESRF), France, studied the starfish to see how it creates its toughened optical lenses in the sea. The method could strengthen ceramics used in lenses, automotive turbochargers and biomaterial implants.
The starfish inhabits coral reefs. The lenses, made from calcite, are created in ambient conditions and, therefore at lower temperatures than those needed to conventionally strengthen materials. Glass, for example, is tempered to increase its strength. This requires rapid heating, to around 620°C, and cooling, causing the outside of the material to cool quicker than the inside, compressing the material.
Boaz Pokroy, Professor in the Department of Materials Science and Engineering and the Russell Berrie Nanotechnology Institute (RBNI), Technion, commented, ‘What is impressive about biominerals is that they are made from materials available to the organism, for example, chalk. Engineers would never choose chalk as a durable building material, but nature does, and successfully, using different toughening and strengthening strategies. Moreover, the creatures produce these biominerals under available conditions, without furnaces and pressure-generating devices available in research laboratories.’
The crucial stage
The researchers concluded that the key stage in the lens formation is the change from the amorphous phase – between liquid and solid – to the crystalline phase, when magnesium-rich calcite nanoparticles separate from the rest of the material. According to the team, ‘during formation of a single crystalline structure from supersaturated amorphous precursor, the excess of magnesium depletes from the matrix via precipitation of high-magnesium nanoparticles. The thermodynamic solubility of magnesium in calcite is only a few percent.’
Hardness, density and pressure differ throughout the material because of the varied magnesium concentration. The magnesium-rich nanoparticles exert pressure on the inner lens as it crystallises, increasing its toughness.
The formation of the lenses was observed using powder X-ray diffraction, transmission electron microscopy and tomography, at facilities including the ESRF synchrotron and Technion’s Titan microscope.
Pokroy told Materials World, ‘Using ex situ and in situ X-ray powder diffraction (XRD) measurements we revealed that magnesium rich nanoprecipitates are present within the lenses and are coherent with the crystalline matrix. High-resolution XRD coupled with in situ heating equipment allowed us to observe the coherency loss and appearance of the magnesium-rich phase after heat treatment at 400°C.’
Dr Iryna Polishchuk, Research Scientist at Technion, added, ‘Sub-micron diffractometry on [ESRF microfocus beamline] was used to map the variation in d-spacings and small angle X-rays scattering signal of a single lens. These measurements enabled us to reveal curved lines parallel to the surface. These are in fact the regions of alternating concentration of magnesium-rich nanoprecipitates.’
The team also carried out nano-computed tomography, observing the layers of alternating density in the lenses. Combined with the results of the sub-micron diffractometry, this allowed them to determine that the alternating densities are likely to be a result of the different concentrations of magnesium-rich nanoprecipitates.
To read Coherently aligned nanoparticles within a biogenic single crystal: A biological prestressing strategy, visit bit.ly/2Dg3wzy