Jow-Lay Huang

ZnO nanopencils: Efficient field emitters

ZnO nanopencils were synthesized on a silicon wafer without catalysts at a low temperature of 550 ° C through a simple two-step pressure controlled thermal evaporation. Penholders were well-hexagonal faceted and the diameter of pen tips on the nanopencils was in the range of 20–30 nm. High-resolution transmission electron microscopy shows that the nanopencils were single crystals growing along the [0001] direction and the pen tips subtend a small angle with multiple surface perturbations. Field-emission measurements on the nanopencils show a low turn-on field of 3.7V∕μm at a current density of 10μA∕cm2. The emission current density reached 1.3mA∕cm2 at an applied field of 4.6V∕μm. The emission at the low field is attributed to the sharp tip and surface perturbations on the nanopencils.

The effects of r.f. power and substrate temperature on the properties of ZnO films

ZnO thin films were prepared by r.f. magnetron sputtering from a zinc oxide target. The effects of r.f. power and substrate temperature on the properties of ZnO films were studied. The composition of ZnO films was analyzed by X-ray photoemission spectroscopy. The Zn atomic composition in the films was higher than the O atomic composition at any r.f. power and growth temperature conditions. This behavior might be explained by the preferential sputtering of zinc atoms rather than oxygen atoms in the ceramic target. In this study, ZnO films with epitaxial grain growth had a preferred (0 0 2) orientation. The crystallographic orientation and the piezoelectric properties of ZnO films were influenced by the sputtering parameters. A good condition exists for making the c-axis oriented films in the range of deposition rate and substrate temperature under the experimental conditions in this study. ZnO films showed different surface morphologies and surface roughness under different deposition conditions. ZnO films were transparent in the visible region, but had obvious absorption in the UV region. There were no obvious changes in the optical properties of ZnO films by varying the r.f. power and substrate temperature in this study. The phase velocity in this study was relatively higher at the optimal r.f. power and substrate temperature.

Single-crystalline AlZnO nanowires/nanotubes synthesized at low temperature

Single-crystalline AlZnO nanomaterials were synthesized through a proposed alloy-evaporation deposition method at the low temperature of 550°C by thermal chemical vapor deposition. Transmission electron microscopy images show that AlZnO nanowires, or nanowire/nanotube junction structures, can be synthesized where the Al∕(Al+Zn) atomic ratio is determined to be about 2.5 and 12at.%, respectively, by electron energy loss spectrometry. Room-temperature cathodoluminescence measurements show that the AlZnO nanowires exhibit a strong ultraviolet emission, which shifts to a higher energy from 3.29to3.34eV due to Al incorporation.

ZnO symmetric nanosheets integrated with nanowalls

Diverse ZnO integrated nanostructures, constructed by epitaxial nanowalls and symmetric single-crystalline nanosheets, were successfully synthesized via a strain-assisted self-catalyzed process at a low temperature of 500°C. The nanostructures started with the growth of ZnO nanowires, nucleated on a rugged ZnO single-crystalline film via a strain-assisted self-catalyzed growth mechanism. The nanowalls were then formed by the interconnection of the nanowires. Finally, the nanosheets were grown from the edges of the nanowalls. The growth mechanisms were supported by direct experimental evidence. Room-temperature cathodoluminance spectra show a relatively strong and sharp ultraviolet emission as well as a weak and broad green emission. The integrated nanostructure may be applied to develop self-inclusive nanoelectronics.

Effects of r.f. bias and nitrogen flow rates on the reactive sputtering of TiA1N films

Abstract The effects of r.f. bias and nitrogen flow on the microstructurc and mechanical properties of TiA1N films were investigated. The preferred orientations, grain size, density, adhesive strength and hardness were substantially affected by substrate bias and nitrogen flow rate. The optimized wear resistance occurred 6 ml min −1 higher than the critical N 2 flow rate at which a stoichiometric composition could be obtained. This could be related to different kinetics of nitridation of Ti and A1.

Effect of the electrical discharge machining on strength and reliability of TiN/Si3N4 composites

Conductive TiN/Si3N4 hot pressed composites were processed by electrical discharge machining (EDM) and their microstructure and conductivity investigated. The dependence of surface texture, surface roughness, and materials removal rate on electrical discharge machining conditions including working voltage and feed rate were also analyzed. The flexural strength and strength reliability in terms of Weibull modulus of TiN/ Si3N4 processed by EDM and conventional cutting and polishing are compared. Higher working voltage and current, as well as higher content of TiN result in greater material removal rate. Comparison of the flexural strength and Weibull modulus of the composites processed by EDM with conventional polished samples revealed that the surface with low roughness lead to an increase in strength and reliability for electrical discharge machining.

ZnO hexagonal arrays of nanowires grown on nanorods

ZnO single-crystalline nanowire-type nanostructures were synthesized on silicon by thermal chemical vapor deposition without catalysts through a two-step pressure-controlled vapor-reflected process at a low temperature of 550 °C where self-organized hexagonal crystalline or porous nanowire arrays were grown on nanorods. The nanowire diameter is around 20 nm and number of nanowires is selected by the nanorod size. Cathodoluminescence spectra exhibit strong green emissions, indicative of high oxygen-vacancy density, which sheds a light on further applications for multichannel nanoconductors in nanodevices.

Effect of Y2O3 and Yb2O3 on the microstructure and mechanical properties of silicon nitride

The effect of Y2O3 and Yb2O3 on the microstructure and mechanical properties of silicon nitride was investigated. The effect of heat-treatment on the crystallization of grain boundary phases and on the high-temperature strength was also studied. Sinterability and α to β phase transformation of silicon nitride with Y2O3 were more pronounced than that with Yb2O3, for the same additive content. A wider grain size distribution and a larger average grain size were observed in the specimens with Y2O3. The specimens with Yb2O3 showed higher flexure strength, hardness and fracture toughness owing to a more frequent pullout of grains. Strength retention of silicon nitride at elevated temperatures was attributed to the crystallization and refractoriness of the grain boundary phases.

Formation of BN films on carbon fibers by dip-coating

Turbostratic boron nitride (BN) films (amorphous with some microcrystallinity) were successfully deposited onto carbon fibers and graphite substrates by dip-coating in methanolic boric acid and urea solutions, followed by nitriding in an ammonia flow at 1000 °C. BN coatings exhibited an orange-peel or pebble-like structure. Surface morphology results indicated that the sizes of the grain-like particles increased with the concentration of the dipping solution. The thickness of the BN film exhibited parabolic relationships with the viscosity of the dipping solution and the withdrawal speed. With the homogeneous surface, the thickness of the BN film increased with the concentration of the reactant in the solution. However, stripping and cracking at the surface have been observed while the reactant concentration was higher than 0.9 M. The yield of BN increased with the nitriding temperature. The increasing trend was slowed down at 800 °C and attained a maximum at 1000 °C.

The effects of nitrogen flow on reactively sputtered TiAlN films

Abstract A thin coating of TiAlN was deposited on a high speed steel cutting tool material by a reactive sputtering process. The effects of N 2 flow rate on the microstructure, chemical composition and mechanical properties of such TiAlN films were investigated. A stoichiometric composition of TiAlN was obtained at a certain critical nitrogen flow rate ( fc N 2 ). A further increase in N 2 flow led to no detectable changes in chemical composition. A dense, isotropic, fine-grained microstructure was obtained; the optimized hardness, toughness and wear resistance occurred at an N 2 flow rate greater than fc N 2 . This was probably due to the different kinetics of nitridation of Ti and Al.