A paradigm shift from hard to flexible, organic-based optoelectronics requires fast and reversible mechanical response from actuating materials that are used for conversion of heat or light into mechanical motion. As the limits in the response times of polymer-based actuating materials are reached, which are inherent to the less-than-optimal coupling between the light/heat and mechanical energy in them, a conceptually new approach to mechanical actuation is required to leapfrog the performance of organic actuators. Herein, we explore single crystals of 1,2,4,5-tetrabromobenzene (TBB) as actuating elements and establish relations between their kinematic profile and mechanical properties. Centimeter-size acicular crystals of TBB are the only naturally twinned crystals out of about a dozen known materials that exhibit the thermosalient effect-an extremely rare and visually impressive crystal locomotion. When taken over a phase transition, crystals of this material store mechanical strain and are rapidly self-actuated to sudden jumps to release the internal strain, leaping up to several centimeters. To establish the structural basis for this colossal crystal motility, we investigated the mechanical profile of the crystals from macroscale, in response to externally induced deformation under microscope, to nanoscale, by using nanoindentation. Kinematic analysis based on high-speed recordings of over 200 twinned TBB crystals exposed to directional or nondirectional heating unraveled that the crystal locomotion is a kinematically complex phenomenon that includes at least six kinematic effects. The nanoscale tests confirm the highly elastic nature, with an elastic deformation recovery (60%) that is far superior to those of molecular crystals reported earlier. This property appears to be critical for accumulation of stress required for crystal jumping. Twinned crystals of TBB exposed to moderate directional heating behave as all-organic analogue of a bimetallic strip, where the lattice misfit between the two crystal components drives reversible deformation of the crystal.
Polycrystalline transparent magnesium aluminate ''spinel'' ceramics were fabricated by hot-pressing and hot isostatic pressing (HIPing) using commercially available MgO and Al 2 O 3 powders. Al 2 O 3 content of spinel was systematically changed that can be expressed as MgOÁnAl 2 O 3 with n ¼ 1:0, 1.5 and 2.0. UV/visible and near-IR wavelength region light reflection and transmission behaviors of the spinel ceramics were quantitatively correlated to their microstructure to account for the optical quality of the fabricated materials. The stoichiometric spinel ceramic with n ¼ 1:0 revealed a relatively poor optical transparency due to pronounced light scattering at the microcracked grain boundaries with a specular light transmission of $20{40% in the visible wavelength range. On the other hand, Al 2 O 3 rich compositions revealed a specular transmission of $40{60% in the same wavelength range with a high degree of transparency. Additionally, effect of chemical composition on the fracture toughness of spinel ceramics was investigated applying indentation and chevron notched specimen fracture toughness measurement techniques. The spinel ceramic with n ¼ 2:0 revealed the highest fracture toughness with a mean value of $2:02 MPaÁm 1=2 . Based on their optical and mechanical properties, potential of Al 2 O 3 rich non-stoichiometric polycrystalline spinel ceramics for engineering applications requiring high optical transparency and improved fracture toughness was addressed.
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