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The X-Ray Diffraction Method for Study of Growth Defects in CVD Diamond Single Crystals :: Science Publishing Group

X-ray diffraction from Chemical Vapor Deposition (CVD) gem-quality colorless diamond single crystals, grown with nitrogen addition in methane –hydrogen plasma mixture, was studied by imaging plate area detector (IPD) giving pixel pattern. Growth defects are responsible for rising x-ray diffuse scattering beyond reciprocal lattice points (rlp), described in the framework of kinematical theory of X-ray diffraction. In the particular case of CVD diamond crystals grown epitaxially on (001) substrate, the additional scattering in the close vicinity of rpl’s was registered. This observation was possible because of the improved angular resolution of x-ray intensity measurements using imaging plate detector (IPD). The pronounced differences in scattering around 111 rlp’s for natural and CVD growth sector were demonstrated. Oscillation and stationary crystal methods allowed registration of diffraction spots that are different from natural diamond crystals. The diffraction patterns include features of short- and long-range atomic order. There are satellite reflections at the positions corresponding to the interatomic distances at 1.51 Å and 1.57 Å in the vicinity of 111 Bragg reflections, which are characteristic to 1.54 Å of cubic diamond. The displacement disorder of atoms, understood as disturbance of lattice periodicity can be explained by hypothesis about linear defects running in and directions. Along twin line tetrahedra share edges. Hydrogen atoms are presumably incorporated along this linear twin to protect chemical bonding stability. Bragg reflections exhibit anisotropy and considerable broadening compared to the diffraction standards. The ratio of peak intensity of forbidden by the diamond space group symmetry 222 reflection to 111 reflection is larger than for natural crystal. Raman spectra from (001) CVD crystals fit well to the spectrum from nearly perfect natural diamond crystal. The X-ray scattering around Bragg reflection is characteristic for a given crystal and can be applied as a gem quality criterion for distinguishing among crystals of different origin, or different growth sectors or grown by different methods. The scattering around 111 CVD diamond reflection is the strongest among the rlp’s.

Electrical Performance and Stability of ZnO Thin-Film Transistors Incorporating Gadolinium Oxide High-k Dielectrics :: Science Publishing Group

This work investigates the performance and gate bias stress instability of ZnO-based thin film transistors (ZnO-TFTs) incorporating amorphous gadolinium oxide, a high-k dielectric material. ZnO thin films produced via radio frequency (RF) reactive magnetron sputtering were used as channel layers. The source/drain electrodes were achieved by the thermal evaporation of aluminium on a bottom gate inverted staggered ZnO TFT structure. Gadolinium oxide (Gd2O3) deposited by metal-organic chemical vapour deposition (MOCVD) served as the gate dielectric. The electrical characterisation of the ZnO-TFTs produced showed improvement in performance and stability in comparison to thermally-grown SiO2-based ZnO TFTs fabricated under the same conditions. The effective channel mobility, on-off current ratio and subthreshold swing of the TFTs incorporating Gd2O3 dielectric were found to be 33.5 cm2 V-1s-1, 107, and 2.4 V/dec respectively when produced. The electrical characterisation of the same devices produced with SiO2 dielectrics exhibited effective mobility, on-off current ratio and subthreshold swing of 7.0 cm2 V-1s-1, 106 and 1.4 V/dec respectively. It is worth noting that, the ZnO active layer was sputtered under room temperature with no intentional heating and post-deposition annealing treatment. On application of gate bias stressing on these thin film transistors, it was observed that threshold voltage instability increased with stress period in all device types. Transistors incorporating Gd2O3 however, were found to exhibit lesser threshold voltage related instability with regards to gate bias stressing in comparison to similar devices incorporating SiO2 as gate dielectric. It was also observed that the effective mobility in both devices tend to stabilize with prolonged gate bias application. In this work, it is demonstrated that Gd2O3 dielectric is a potential alternative to SiO2 for the fabrication of ZnO TFTs with improved performance and electrical stability under prolonged use.

Correlation Between Shear Strengths of Disturbed and Undisturbed Soils at Three Sites in Burkina Faso :: Science Publishing Group

In the field of construction, performing laboratory tests of soil mechanics and geotechnics is a key step. Tests performed on undisturbed soil samples are sometimes required. The major difficulty in the success of these tests is in the prevention of any change in the state of these materials which may be due to coring, transport and sampling in the laboratory conditions. Physical factors, such as rainfall, temperature, are ones of the most which can change samples natural state. To facilitate obtaining the parameters of the mechanical behavior of undisturbed samples, this paper proposes to establish correlations between shear strengths of disturbed samples and those of undisturbed samples of tropical soils of Burkina Faso. Shear tests were performed using the Casagrande cell on disturbed and undisturbed samples from Kamboinsé; Loumbila and Saba sites. The tests carried out were Unconsolidated-Undrained; Consolidated-Undrained and Consolidated-Drained. For each test, linear correlation between shear strengths of disturbed and undisturbed samples was etablished under similar moisture content conditions. Results displayed a good correlation between shear strengths of undisturbed samples and those of disturbed samples. All correlation coefficients are greater than 0.9. Samples particles sizes distributions and their clay content did not have significant impacts on shear strengths.

Formulation of Aqueous Dispersions of PEKK by a Quantitative Structure Property Relationship Approach and Application to Thermoplastic Sizing on Carbon Fibers :: Science Publishing Group

The development of formulations for thermoplastic sizing on carbon fibers requires water dispersions of small polymer particles (< 20 µm). PolyEtherKetoneKetone (PEKK) is a high-performance polymer used as a matrix in Carbon Fiber Reinforced Polymers (CFRP) or as a sizing agent. To limit the formulation steps and the use of organic solvents, the sonofragmentation process can be used to deagglomerate polymers, directly in the final aqueous formulation. The sonofragmentation process is controlled by multiple parameters and, in order to identify the key parameters, a quantitative structure property relationship (QSPR) study was performed using artificial neural networks (ANN). The 40 formulations of this study were characterized with the aim of quantifying the sonofragmentation effect. Various physicochemical techniques were used: Photon Correlation Spectroscopy (PCS), destabilization velocity of the dispersions by ytical centrifugation, and scanning electron microscopy. The results obtained showed that only two parameters (m concentration of surfactant and duration of sonication) had a notable effect on the sonofragmentation process. By controlling these two parameters, it was possible to define a design space in the stability domain of the formulations and to calculate a sonofragmentation efficiency (ϕ) for four singular zones. Image ysis showed that the sonofragmentation process was accompanied by an increase in the number of particles with Particle size (Ps) < 20 µm. In optimized aqueous formulations, the majority of particles should have Ps < 20 µm.

Role of [Cu]/[In] Molar Ratio in Controlling Structural, Morphological and Optical Properties of Sprayed CuInS

CuInS2 thin films were deposited by chemical spray pyrolysis from aqueous solutions containing CuCl2, InCl. and thiourea at substrate temperature of 300°C and annealed at 500°C in air. [Cu]/[In] molar ratio was varied from 0.8 to 1.4 in precursor solution. The influence of [Cu]/[In] molar ratio on structural, morphological and optical properties of CuInS2 thin films was investigated. X-ray diffraction ysis shows that all the films have chalcopyrite structure with the preferential orientation along (112) plane. CuInS2 films with indium excess have poor crystallinity and consist of large quantity of small particles while copper-rich films exhibit good crystallinity with large grains. The best film crystallinity is obtained for [Cu]/[In] = 1.2. The scanning electron microscopy and atomic force microscopy images indicated that [Cu]/[In] molar ratio has a strong influence on the microstructure and surface morphology of the films. It was observed that films obtained with [Cu]/[In] ratio of 0.8 are porous. But films surface became gradually dense with increase in [Cu]/[In] molar ratio. As consequence films roughness decreased from 236.12 nm to 110.30 nm. Optical ysis shows that all the films have good absorbance in the visible. The optical absorbance of films is found to increase with increase in [Cu]/[In] molar ratio.