Advanced Materials Research Vols. 29-30

Abstract: We report the microstructure and optical properties of gallium nitride (GaN) epilayers grown on lens shape patterned sapphire substrate (PSS) using metalorganic chemical vapor deposition (MOCVD) for various growth times. A lens shaped pattern was used to reduce the threading dislocation density and to improve optical emission efficiency. A scanning electron microscope (SEM) image shows flat and smooth surface of GaN grown on PSS at 80 min which could be achieved by lateral growth from the trench region. From the DCXRD spectra, full width at half maximun (FWHM) value was decreased with increasing growth time. FWHM of the sample grown at 80 min was 473.5 arc sec. This indicates there is an improvement in crystalline quality of the GaN grown on PSS as the growth time increases. From photoluminescence (PL) spectra, an increase in band edge emission intensity and a decrease in defect related yellow luminescence was observed for GaN on PSS as the growth time increased. From the PL spectra, FWHM was 82.2 meV at peak position 363.9 nm for the sample grown for 80 min. It is clearly seen that the threading dislocations can be reduced by lateral growth improving the light emission efficiency by internal light reflection on the lens surface for GaN grown on PSS.

Abstract: GaN epilayers were grown on lens shaped patterned sapphire substrate (PSS) (0001) and unpatterned sapphire substrate (UPSS) (0001) by metal-organic chemical vapor deposition (MOCVD). The quality of the grown GaN epilayers on the PSS and UPSS were compared. Structural characteristics, surface morphology and optical properties of the GaN epilayers were investigated using double crystal X-ray diffraction (DCXRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and photoluminescence (PL). A lens shaped pattern was formed on the sapphire substrate to reduce threading dislocation (TD) density and also to improve the optical emission efficiency by internal reflection on the lens. Scanning electron microscopy images show the growth of GaN epilayers at various times. Full coalescence is observed at a growth time of 80 min. It is seen from the DCXRD rocking spectrum that full width at half maximum (FWHM) of the GaN grown on PSS was 438.7 arcsec which is less than UPSS value. The lower value of FWHM indicates that the crystalline quality of the GaN epilayers grown on PSS is improved compared to GaN grown on UPSS. It is clearly seen from the AFM images that the dislocation density is less for the GaN grown on PSS. A strong and sharp photoluminescence (PL) band edge emission was observed for the GaN grown on PSS compared to UPSS. Defect related yellow luminescence was observed for GaN grown on UPSS which did not appear for PSS. The FWHM at the 364.3 nm peak position was evaluated to be 50.7 meV from the PL spectra for GaN grown on PSS. The above result indicates GaN epilayers can be grown on PSS with low TD density and will be useful for optical emission.

Abstract: Changes in crystal phase and electrical properties of oxides composed of Mn, Co, and Ni with the molar ratios of 5:2:1 and 3:3:1 were investigated. Starting oxides were fired from 250°C to 800°C, then maintained at prescribed temperatures for 3 h in air. In the 5:2:1 specimen, a monophase consisting of a cubic spinel oxide that is important to electrical conductivity was obtained at firing temperature of 800°C. In the 3:3:1 specimen, the monophase was obtained at temperatures ranging from 600°C to 800°C. Electrical resistance decreased exponentially with increasing temperature for all specimens fired at temperatures ranging from 250°C to 800°C, indicating that the oxides have intrinsic thermistor characteristics with negative temperature coefficient (NTC). The temperature dependence of the thermistor constant (B value) necessary for practical application was considered to be related to the existence ratio of cubic spinel-type and ilmenite-type structures and the lattice constant of the cubic spinel-type structure. The electrical conduction was stabilized by annealing at prescribed temperatures for more than 720 minutes.

Abstract: Conducting polymer (CP) actuators undergo volumetric changes due to the movement of dopant ions into the film during the electrical oxidation process. In this work, PPy/SPE/PPy electroactive tri-layer actuator was prepared by the electrochemical polymerization of pyrrole and the actuation characteristics were studied. An all-solid actuator, consisting of two polypyrrole (PPy) films and a solid polymer electrolyte (SPE) based on poly(vinylidene fluoride) (PVDF), clearly showed a reversible displacement in an atmosphere when a voltage was applied.

Abstract: Water contaminated by heavy metals remains a serious environmental and public health problem. The toxic effects of heavy metals on the biosphere have been demonstrated by a number of studies. Since the main sources of heavy metals for humans are water and food, the monitoring of the heavy metals content in natural water is of paramount importance. Diverse technologies have been used to reduce the contents of heavy metals in water. Recently, adsorption methods have been widely used because of their low cost. The novel nano-water-purifying material used in our work is composed of AlO(OH) nanoparticles loaded onto glass fibre and supported by active carbon felt. The Al nano-powder starting material, was prepared using an electric explosive technique. The products were characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope, and BET techniques. It was found that the particles consisted of AlO(OH) nanofibres of pure boehmite structure. They exhibited a surface area of 431.7 m2 / g. The composite material was found to be effective in removing cadmium from dilute aqueous solutions and could find broad application in heavy metal removal.

Abstract: NixZn1-xFe2O4 ferrites with initial unit molar ratio of NiO/ZnO were prepared by self-propagating high temperature synthesis (SHS) at the oxygen partial pressures of 0.25 MPa. Neutron diffraction analysis shows that final product is Ni0.38Zn0.62Fe2O4 (χ2=1.68). The combustion temperature and propagating rate were about 1180 K and 4.2 mm/sec, respectively. The ferrite particle size was less than about 350 nm. The coercive force, maximum magnetization, residual magnetization, susceptibility and curie temperature of Ni0.38Zn0.62Fe2O4 powders prepared by SHS are 2878 A/m, 6.805 Wb/m2Kg, 1.27 Wb/m2Kg , 0.042 m3/kg and 42.20 °C, respectively.

Abstract: Microsilica 600 (MS60) is a highly reactive pozzolan processed from a natural white geosilica deposit found in New Zealand’s Rotorua district. Like silica fume it is a very fine amorphous silica and falls into the microsilica family of products. As part of a feasibility study of using MS600 in the manufacturing process of Golden Bay Cement, this work establishes the improved performance characteristics that can be achieved with MS600 When added with Portland cement, MS600 facilitates high performance concrete by achieving: increased compressive strength, improved sulphate resistance, reduced alkali silica reactity, very low chloride diffusion, reduced water permeability, improved abrasion resistance. improved resistance to chemical attack, improved stability in geothermal environments, superior plastic properties (moderate concrete bleed, lower heat generated, high quality finish) and low drying shrinkage characteristics. It has been shown that MS600 will satisfy performance requirements in applications requiring an advanced concrete material having long serviceability in harsh environments or where structural performance beyond current limitations are pre-requisites.