Exploring applications of quasicrystals at small scales
(Nanowerk Spotlight) The discovery of quasicrystals three decades ago unveiled a class of matter that exhibits long-range order but lacks translational periodicity (Dan Shechtman was awarded the Nobel Prize in Chemistry for the discovery of quasicrystals in 2011). Owing to their unique structures, quasicrystals possess many unusual and useful properties.
Bulk quasicrystals are very brittle materials at room temperature – even more brittle than metallic glasses – rendering them difficult to process and often unsuitable for usage. A problem that must be resolved towards to the applications of quasicrystals is the tendency for cracking due to the materials' extreme brittleness.
Whether it is possible to achieve high formability in quasicrystals and how quasicrystals are plastically deformed at room temperature have been long-standing questions since their discovery.
In new work, an international group of researchers led by scientists at ETH Zurich have found that a typically brittle quasicrystal exhibits superior ductility (ductility is a solid material's ability to deform under stress without fracture) at the sub-micrometer scales and at room temperature.
Furthermore, their experiments indicate that 'dislocation glide' could be the dominating deformation mechanism for quasicrystals under high-stress and low temperature conditions, which has been poorly understood before.
"Understanding materials behavior in large temperature and length-scale ranges is not only of academic interest but also essential for technological applications," Dr. Yu Zou, a former PhD student in the Laboratory for Nanometallurgy at ETH Zurich, currently a postdoctoral researcher at MIT, and first author of the paper, tells Nanowerk. "Our paper demonstrates an effective approach to work on this problem."
In situ SEM and TEM of i-Al–Pd–Mn pillars during bending tests. (a) SEM snapshots captured during the bending test of a pillar with the diameter of ∼300 nm. An initial crack occurs near the pillar base at the bending angle of ∼20–30° and eventual fracture happens at the bending angle of ∼40°. (b) TEM snapshots during bending tests on a pillar in the diameter of ∼100 nm, showing a homogenous deformation without any fracture, and the maximum tensile strain at the pillar centre estimated to be over 50%. Scale bars, 300nm (a) and 100 nm (b). (Image: Yu Zou, ETH Zurich) (click on image to enlarge)
The researchers found that typically brittle, small-sized quasicrystals can show over 50% compressive and tensile strains as well as a high yield strength of about 4.5 GPa. Such small-sized quasicrystals show the highest specific strength (strength divided by density or elastic energy density, ~1 MJ/kg) among all metallic micro/nano-pillars reported to date.
Early studies of the plastic deformation of quasicrystals focused on an easily to grow icosahedral quasicrystal (i-Al-Pd-Mn) in the high-temperature regime above ∼600°C (because it is too brittle at room temperature). Despite several investigations having sought to explore the plastic deformation of quasicrystals at or near room temperature using indentation or by confining gas or solid pressures, so far no common conclusion has been reached.
"Our report suggests the possibility of using quasicrystals as strong and ductile materials in micro- or nanodevices," Zou points out. "Together with their interesting physical properties – unusual thermal and electronic conductivity, light absorption and hydrogen storage – small-scale quasicrystals are potentially useful for the design of multifunctional devices."
As far as practical applications are concerned, these quasicrystals might be used to store energy. Small dimensional quasicrystals having superior strength and ductility, together with their interesting functional properties, may also permit access to being both structurally and functionally useful in micro- or nanoelectromechanical systems.
Exploring potential applications of quasicrystals at small scales, such as thin films and nano-pillar arrays, is now one of the areas the team is looking at. Furthermore, they plan to use these micro-compression techniques to explore a fundamental understanding of the deformation mechanisms of other brittle materials, such as complex metallic alloys and high-entropy alloys in a large temperature range.
"For fundamental research, it would be interesting to understand the deformation mechanisms of brittle materials in the low temperature regimes (0 K to ~100 K)," notes Zou. "Towards technological applications, the coupling mechanical properties with other functional properties in quasicrystals would be attractive for micro- or nano-electromechanical systems."
Of course, how to scale up these materials and devices for real applications is a significant challenge as well.