Fast electronic switching of ultrathin films of phase-change materials render nonvolatile color changes
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t electronic switching of ultrathin films of phase-change materials render nonvolatile color changes
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hase-change materials (PCMs) switch between amorphous and crystalline solid states. The process can occur at ultrahigh speeds (on the order of MHz), and may be driven thermally, optically, or electrically. Controlling the associated changes in the PCM’s optoelectronic properties is a great motivator to investigate and characterize such systems. As reported in the July 10 issue of Nature (DOI: 10.1038/ nature13487; p. 206), Peiman Hosseini and Harish Bhaskaran at the University of Oxford and C. David Wright at the University of Exeter induced phase switching within sputtered Ge2Sb2Te5 (GST) films. These phase changes take place within low-dimensional
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MRS BULLETIN
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VOLUME 39 • SEPTEMBER 2014
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by layer. The pattern is generated slice by slice from a 3D CAD model, and projected through a liquid-crystal-on-silicon chip, which acts as a reconfigurable digital photomask, onto the surface of a bath of UV-curable resin. After a layer hardens, the sample is lowered in the polymer bath, new resin coats the surface, and the next layer of the 3D structure is projected and polymerized onto the layer beneath. The result is an extended microlattice of octet truss unit cells made of solid polymer struts. By coating these struts with a nickel-phosphorus alloy through electroless nickel plating, and removing the polymer struts through thermal decomposition, a hollow-tube metallic Ni-P microlattice can be formed. Similarly, by depositing the ceramic Al2O3 by atomic layer deposition onto the polymer struts and removing the polymer, a hollow ceramic Al2O3 microlattice is formed. For yet another configuration, Al2O3 nanopowder is mixed in with the polymer before the process begins, resulting in a structure consisting of a polymer–ceramic hybrid core. Subsequent heat treatment decomposes
the polymer and sinters the Al2O3 to form a solid ceramic microlattice. In each case (polymer, metallic, hollow ceramic, solid ceramic), uniaxial compression studies yielded plots of relative compressive stiffness and relative compressive strength versus relative density. These plots showed the stiffness and strength to be linear functions of the relative density of the material for each of these stretch-dominated lattices. In contrast, a bend-dominated, solid polymer Kelvin foam made by the same process for comparison decreased in strength and stiffness by a power of two with decreasing density. “We fabricated an ultrastiff, ultrastrong material that is primarily void space, which makes it very light weight,” Spadaccini said. “Then we took that a step further by combining additive micromanufacturing processes with nanoscale coating processes to give us a material that is about as light as an aerogel. The mechanical properties, relative to the material’s density, go through the roof.” Tim Palucka
nanoscale-sized regions, and the optical properties (e.g., color) in these regions change in a reversible, stable manner. The researchers started b
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