Papers by Keyword: Polycrystalline Alumina

Abstract: In this work we present the investigations on the determination of the mechanism of sodium loss in High Pressure Sodium (HPS) lamps. The transport through the ceramic arc tube consists of several steps (solution in the ceramics, diffusion through the ceramics, leaving the bulk phase, evaporation from the surface). Among the listed processes the mechanism of the diffusion was investigated by model-experiments. A ceramic arc tube was applied as a reactor chamber, in which polycrystalline ceramic plates were heated at 600-1100°C in controlled Na and Xe atmosphere. The sodium diffusion profiles developed by the heat-treatment were measured by XPS and SIMS and from the measurements we tried to draw a conclusion about the mechanism of sodium diffusion through the ceramic discharge tube. The ceramic discharge tube were measured by X- ray diffraction, too. We found traces of some Na-O-Al compound.

Abstract: In this work we present the investigations on the blackening of the outer jacket of the High Pressure Sodium (HPS) lamps. For this purpose ceramic arc tubes and their glass outer jackets taken from lamps burned for different times (0h-10000h) were examined by X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectroscopy (SIMS) in order to determine the composition of the deposit on the internal wall of glass outer jacket. In the case of the several thousand hours burned lamps the outer bulb was found to be covered by aluminium, which comes to a black deposit on the glass surface. XPS identified Al on the glass surface can be found in two different chemical state (elemental Al, Al2O3). The assumed evaporation of aluminium-oxide ceramic tube might be described by a dissociative mechanism of the alumina evaporation. This papers described the possible mechanism of the alumina evaporation. Na was also found in the black deposit, whose cause of this in the case of the several thousand hours burned lamps is probably related to the sodium transport through the walls of the arc tube

Abstract: Fracture behavior under multiaxial stress state of polycrystalline alumina was studied from the view point of an artificial crack propagation and fracture from a natural flaw. The former was studied by mixed-mode fracture toughness tests; asymmetric four-point bending and diametral compression techniques were carried out using precracked and notched specimens. The latter was studied by biaxial fracture tests in compression and torsion loading; multiaxial fracture statistics was applied to the measured fracture envelope. The ratio KIIC/KIC obtained from the biaxial tests was higher than that obtained by the mixed-mode fracture toughness tests. It revealed that the fracture from an artificial flaw does not simulate the fracture from a naturall flaw in polycrystalline ceramics.