Growth of GaN films by reactive sputtering of GaAs

GaN films were deposited by the reactive sputtering of GaAs target using 100% nitrogen as the sputtering and reactive gas at substrate temperatures ranging from 673 to 873 K. The films were studied using XRD and XPS techniques. All films exhibited peaks corresponding to hexagonal GaN with strong dependence of preferred orientation on the substrate temperature. The optical constants of the films were determined from the reflection and transmission spectra. The bandgaps of all films were found to be 3.4 eV which is the bandgap of hexagonal GaN. (C) 1998 Elsevier Science S.A. All rights reserved

Nanocrystalline gallium nitride thin films

Nanocrystalline gallium nitride (GaN) thin films were deposited on quartz substrates by reactive rf sputtering of GaAs target with nitrogen as the reactive cum sputtering gas. X-ray diffraction and transmission electron microscopy confirmed the presence of GaN crystallites with particle size increasing from 3 to 16 nm, as the substrate temperature was increased from 400 to 550 degrees C. The particle size in films grown at temperatures below 550 degrees C were less than the exciton Bohr radius of GaN. The band gap of these films obtained from absorption and photoluminescence measurements showed a blueshift with respect to bulk GaN. (C) 2000 [S0003-6951(00)03138-7]

Study of Layered Silicate Clays as Synergistic Nucleating Agent for Polypropylene

Effect of very small quantities of organically modified layered silicate clay on the nucleation of polypropylene (PP), as an additive at ppm levels dosage was investigated, in combination with two of the most commercially exploited organic nucleating agents, one of which is a cyclic aromatic phosphinate salt and the other is bis(3,4-dimethylbenzylidene) sorbitol, each representing a separate class of nucleating molecules by itself. Substitution of a considerable fraction of either of these organic nucleating agents with organically modified inorganic nanoclay was seen to result in a unique synergy between the two in nucleating PP. Polarized light microscopy studies of these synergistic formulations with organoclay to nucleating agent ratios of 1:1 and 1:3 totaling 0.2 weight percent in PP showed significant reduction in spherulite size from that of non-nucleated PP, and compared with the samples containing exclusive organic nucleating agent at similar loading. Differential scanning calorimetric studies provided evidence and insight into such synergistic behavior. Crystallization and supercooling temperatures for the synergistic formulations were comparable for those formulations containing only organic nucleating agents, indicating comparable nucleation efficiency, whereas organoclay alone, although showing some extent of nucleation, was clearly poorer in efficiency. Wide and small angle X-ray scattering studies further explained these observations. An increase in the gamma polytype fraction was seen in samples that contained both organoclay and nucleating agent, pointing to the role of organoclay as a gamma nucleator. Organoclay was found to be completely exfoliated in these synergistic formulations and was seen as well-dispersed, single platelets in the PP matrix. A hybrid network consisting of exfoliated organoclay platelets and organic nucleating agent molecules was proposed, which is more stable and stiffer than the network formed by nucleating agent alone. (C) 2010 . J Polym Sci Part B: Polym Phys 48: 1786-1794, 201

SAXS Analysis of Polypropylene-Layered Silicate Nanocomposites: An Integrated Correlations Functions Approach Using an Exfoliation Factor

Use of correlation function and interface distribution function to obtain the morphological parameters from Small Angle X-ray Scattering data makes it as an important quantitative method to evaluate lamellar morphology. Analysis using correlation function assumes lamellar stack morphology with variation of electron density along one dimension where lateral width of lamellae is much larger than the long period normal to the lamellae and the electron density varies with a rectangular profile for alternating crystalline and amorphous layers. In this work, a modified Porod law approach is used for the deviation from ideal two phase model and the thickness of transition zone with variation in electron densities is calculated. Morphological parameters of various grades of organically modified Polypropylene clay nanocomposites, such as long period, linear crystallinity, lamellar thickness and amorphous thickness are estimated using a combination of correlation and interface distribution function. Presence of transition zones does not influence the values for amorphous layer thickness, crystalline layer thickness and long period. Variations in values of long period calculated from correlation and interface function suggest a distribution of lamellar sizes in the polymer and nanocomposites. A new model consisting of the integrated correlation function of polymer and organoclay was developed to quantify the extent of exfoliation of organoclay in the nanocomposites by introducing a parameter, the exfoliation factor, P. For a highly non-exfoliated system, the large number of clay tactoids is highly correlated and interacting, the correlation function of nanocomposites will be similar to that of nanoclay samples with low values of exfoliation factor. The correlation function of well exfoliated nanocomposites is similar to neat polymer, and such systems have high values of the exfoliation factor. Moreover, the exfoliation factor, which is easily determined by a scattering experiment, correlated well to the important engineering properties of the nanocomposites

Highly Transparent Thermoplastic Elastomer From Isotactic Polypropylene and Styrene/Ethylene-Butylene/Styrene Triblock Copolymer: Structure-Property Correlations

Polypropylene (PP) is one of the most useful general purpose plastics. However, the poor transparency and brittleness of PP restricts its applications in the field of medical and personal care where silicone and polyvinyl chloride (PVC) are presently used. This work concentrates on developing highly transparent elastomeric PP blends and also thermoplastic elastomer by blending isotactic polypropylene (I-PP) with styrene/ethylene-butylene/styrene (SEBS) triblock copolymer. PP/SEBS blend derived from high melt flow index (MFI) PP and high MFI SEBS exhibit remarkable transparency (haze value as low as 6%) along with good percentage of elongation and processability. The reduction in difference of refractive index (RI) between PP and SEBS has been observed by blending SEBS with PP. The wide angle X-ray diffraction studies show that there is significant reduction in the percentage crystallinity of PP by the addition of SEBS block copolymer. Temperature-dependent polarized light microscopy studies reveal the reduction in spherulites size by the addition of SEBS block copolymer. Transmission electron micrographs show that the SEBS polymer forms a fine lamellar structure throughout the PP matrix with phase inversion at higher SEBS concentration. Development of phase morphology, crystalline morphology, and crystallinity in different blends has been analyzed and microstructure-haze correlations have been developed. POLYM. ENG. SCI., 50:331-341, 2010. (C) 2009 Society of Plastics Engineer

Quantification of organoclay dispersion and lamellar morphology in poly(propylene)–clay nanocomposites with small angle X-ray scattering

Intensity profiles of small angle X-ray scattering (SAXS) curves were analyzed to simultaneously gain quantitative information on nanoclay dispersion as well as lamellar ordering in polypropylene–clay nanocomposites. Different types of PP nanocomposites prepared with PP homopolymer (HPP), random propylene–ethylene copolymer (RCP) and a high impact polypropylene–ethylene propylene rubber (ICP) were analyzed. Various one-dimensional models for stacked structures were applied on Lorentz corrected SAXS spectra to derive long period, thicknesses of alternating high and low electron density layers and their distributions, and the number of stacks for both nanoclay and PP lamellae. We applied a mixed thickness distribution model comprising combined Gaussian and exponential for a simple stack of finite thickness, which was found to explain the experimental data better for both nanoclay tactoids and lamellar stacks, compared to simple Gaussian and exponential thickness distributions. Long period X and number of stacks N were derived as important parameters signifying changes in levels of nanoclay exfoliation in PP. Among the three types of polypropylenes studied, better nanoclay exfoliation was obtained for the high impact ICP grade compared to HPP and RCP. Complete exfoliation of nanoclay was achieved in ICP matrix, employing a masterbatch processing route. Moreover, role of nanoclay as a γ nucleating agent was evident from small and wide angle X-ray analyses, and was seen strongly in RCP. Changes in lamellar structure of PP as a result of nanoclay incorporation, double population consisting of both α and γ polytypes in the nanocomposites from that of a primarily α population in neat polymer matrices, were also analyzed in detail with the mixed thickness distribution model, thereby demonstrating its usefulness.© Elsevie