Flickering of fusion pores during exocytotic release of hormones and neurotransmitters is well documented, but without assays that use biochemically defined components and measure single-pore dynamics, the mechanisms remain poorly understood. We used total internal reflection fluorescence microscopy to quantify fusion-pore dynamics in vitro and to separate the roles of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins and lipid bilayer properties. When small unilamellar vesicles bearing neuronal v-SNAREs fused with planar bilayers reconstituted with cognate t-SNARES, lipid and soluble cargo transfer rates were severely reduced, suggesting that pores flickered. From the lipid release times we computed pore openness, the fraction of time the pore is open, which increased dramatically with cholesterol. For most lipid compositions tested, SNARE-mediated and nonspecifically nucleated pores had similar openness, suggesting that pore flickering was controlled by lipid bilayer properties. However, with physiological cholesterol levels, SNAREs substantially increased the fraction of fully open pores and fusion was so accelerated that there was insufficient time to recruit t-SNAREs to the fusion site, consistent with t-SNAREs being preclustered by cholesterol into functional docking and fusion platforms. Our results suggest that cholesterol opens pores directly by reducing the fusion-pore bending energy, and indirectly by concentrating several SNAREs into individual fusion events.

Transparent ceramic materials with optical qualities comparable to single crystals of similar compositions have been developed in recent years, as a result of the improved understanding of powder-processing-fabrication- sintering-property inter-relationships. These high-temperature materials with a range of thermal and mechanical properties are candidate envelopes for focused-beam, short-arc lamps containing various fills operating at temperatures higher than quartz. This paper reviews the composition, structure and properties of transparent ceramic lamp envelope materials including sapphire, small-grained polycrystalline alumina, aluminium oxynitride, yttrium aluminate garnet, magnesium aluminate spinel and yttria–lanthana. A satisfactory thermal shock resistance is required for the ceramic tube to withstand the rapid heating and cooling cycles encountered in lamps. Thermophysical properties, along with the geometry, size and thickness of a transparent ceramic tube, are important parameters in the assessment of its resistance to fracture arising from thermal stresses in lamps during service. The corrosive nature of lamp-fill liquid and vapour at high temperatures requires that all lamp components be carefully chosen to meet the target life. The wide range of new transparent ceramics represents flexibility in pushing the limit of envelope materials for improved beamer lamps.

Silicon carbide ceramics containing up to 24.6 vol% dispersed TiC particles yielded fully dense composites by hot‐pressing at 2000°C with 1 wt% Al and 1 wt% C added. The micro‐structure consists of fine TiC particles in a fine‐grained SiC matrix. Addition of TiC particles increases the critical fracture toughness of SiC (to ∼6 MPa·m1/2 at 24.6 vol% TiC) and yields high flexure strength (≥680 MPa), with both properties increasing with increasing volume fraction of TiC. The strengths at high temperatures are also improved by the TiC additions. Observations of the fracture path indicate that the improved toughness and strength are a result of crack deflection by the TiC particles.

The development of hot-pressed Sic-whisker-reinforced alumina has resulted in composites with fracture toughness values -8.7 MPa.m"2 at 20 vol% S i c . Whisker orientation during processing leads to anisotropy in both fracture toughness and fracture strength. Fracture strengths are also limited by the ability to disperse the S i c whiskers; however, use of bothfine alumina powders and ultrasonic dispersion techniques yields strengths =800 MPa. R, Prewo and Brennan' demonstrated that the fracture toughness of glass-ceramics can be increased substantially by reinforcing them with continuous high-strength S i c fibers. In that study, notched-beam KIc values of up to =20 MPa-m"' (where the notch is perpendicular to the fiber axes in uniaxially aligned fiber compositions) and flexure strengths of 4 5 0 MPa were obtained for a borosilicate-matrix Sic-fiber composite. As pointed out by Rice' and S h e t t~,~ load transfer to long, high-strength fibers requires fiber-to-matrix Young's modulus (E) ratios of greater than two, which is the case in the Prewo and Brennan' study. Crack deflection4 as well as fiber or whisker pullout3 toughening processes can occur in composites containing short, strong whiskers where the whisker-to-matrix modulus ratios are somewhat lower. The potential for toughening by fiber reinforcement processes becomes very attractive if short, strong whiskers could be incorporated into various polycrystalline ceramics by conventional powder processing techniques. Clearly, the degree of toughening that can be achieved in ceramics by incorporating short, strong whiskers needs to be assessed. With this in mind, the fracture behavior was examined for the case of polycrystalline alumina containing highstrength S i c whiskers, where the fiber-tomatrix modulus ratio is ==I .5.* RESULTSAND DISCUSSION Fully dense, fine-grained ( s 4 p m ) alumina matrix composites were obtained by hot-pressing a mixture of alumina pow-CONTRIBUT[NC Eo[ToR-R. J. STOKES