Nov_Dec_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 7 9 Researchers found that certain optical properties in stable glasses are due to the layer-by-lay- er nature of the deposition that allows molecules to pack more tightly in the direction normal to the surface during deposition. that the structure is a new metastable form of crystal aluminum, and that— to their surprise—it has a density of just 0.61 g/cm 3 . This falls well below conventional aluminum’s density of 2.7 g/cm 3 , and even below water’s density of 1 g/cm 3 , allowing the ma- terial to float. For more informa- tion: Alexander Boldyrev, 435.797.1630, a.i.boldyrev@usu.edu, www.usu.edu , www.sfedu.ru/www/site.english. THROUGH THE LOOKING GLASS Researchers at the University of Pennsylvania, Philadelphia, develop- ed a unique form of amorphous stable glass—a type of closely packed glass produced by vapor deposition onto a cold substrate—and determined that its optical properties can be engineered without inducing order in its structure. To create the glass, the researchers designed and synthesized a mole- cule that can never perfectly align due to its spherical shape. Expecting the glass to be amorphous and isotropic, the team was surprised to discov- er its light refraction index differed in directions parallel and normal to the substrate. This birefringence typi- cally results from molecule align- ment, and the new glass exhibited a level analogous to having nearly 30% of its molecules ordered. However, subsequent photoluminescence ex- periments, crystal structure simula- tions, and refraction index calcula- tions confirm there is zero order in the material. The researchers learned that the deposition layering process allows mol- ecules to pack more tightly in the direc- tion normal to the surface, causing the birefringence. The denser the glass, the more extreme the effect, with the de- gree of densification controlled by sub- strate temperature. The work, which demonstrates that high-density amor- phous phases can exist, could allow en- gineers to manipulate the layering of a glass to tailor its mechanical properties, making it suitable for applications such as anti-scratch coatings. upenn.edu .