The Stabilization of Metal Oxide Nanocrystals by the Addition of Alumina

A problem in the study of nanoparticles is that they will tend to grow at moderate temperatures. For example, most oxides (e.g. SnO₂, ZrO₂, MgO) will show significant grain growth at 400°C. This severely limits experimental studies that require measurements over an extensive temperature range. In this contribution we demonstrate that the incorporation of A1₂O₃, can significantly restrict grain growth in MgO and ZrO₂ even at high temperatures.     

The existence of pure surface modes in elastic materials with orthorhombic symmetry

A pure surface mode in a semi-infinite elastic body with a traction-free boundary is a progressive harmonic surface wave in which the displacement is everywhere co-planar with the wave and surface normals. When the elastic material has orthorhombic symmetry, with one of the 2-fold symmetry axes normal to the bounding plane, there are generally only two directions in which a pure surface mode may propagate, but additional directions appear when the transmitting material has higher symmetry. It is shown that for all the possibilities arising in this way the positive definiteness of the strain energy is a sufficient condition for the existence of a unique pure surface mode. The properties of pure modes in elastic materials belonging to the orthorhombic, tetragonal, hexagonal and cubic systems are discussed with particular reference to conditions under which the motion has the same features as a Rayleigh wave.     

Visualization of flameholding mechanisms in a supersonic combustor using PLIF

Laser-induced fluorescence of OH and CH₂O was imaged to investigate the flame stabilization mechanism in a flameholder with a Mach 2.4 free stream. Ethylene was burned in a rectangular cavity with two points of injection: the aft wall and the cavity floor. When injected from the aft wall, the fuel came into immediate contact with hot combustion products from the reaction zone under the shear layer. Primary combustion occurred under the shear layer and in the aft region of the cavity volume. In contrast, when fuel was injected from the floor, a jet-driven recirculation zone of hot products near the upstream wall of the cavity served as a flameholder. The reaction then occurred on the underside of the shear layer. In conditions near lean blowout, significant changes in the flameholding mechanisms were observed. Improved CH₂O fluorescence signal was obtained by taking advantage of the long fluorescence lifetime at low pressures and delaying the camera gate to reduce the background signal.