Effect of Cr implantation on structural and optical properties of AlN thin films

Molecular beam epitaxy (MBE) grown AlN thin layer on sapphire substrates have been implanted with Cr⁺ ions for various dose from 10¹³ to 10¹⁵ cm⁻². The analyses were carried out by an X-ray diffractometer (XRD), Raman spectroscopy, a spectrophotometer and spectroscopic ellipsometry (SE) for structural and optical analyses. E₂(high) and A₁(LO) Raman modes of AlN layer have been observed and analyzed. The behavior of Raman shift and the variation in intensity and in peak width of Raman modes as a function of ions flux are explained on the basis of chromium substituting aluminum atom and implantation-induced lattice damage. Both Raman and X-ray analyses reveal that the incorporation of chromium atoms increases in the host lattice with the increasing of Cr ions fluence. The band gap energy was determined by using transmission spectra. It was found that the band gap energy decreases as the ion dose increases. The band gap of the unimplanted AlN is 6.02 eV and it decreases down to 5.92 eV for the Cr⁺-implanted AlN with a ion dose of 1×10¹⁵ cm⁻². Optical properties such as optical constants of the samples were examined by using a spectroscopic ellipsometer. It was observed that the refractive index (n) decreases with the increasing of ion dose.     

Effects of exponential volume fraction law on the natural frequencies of FGM cylindrical shells under various boundary conditions

In the present work, vibration characteristics of thin functionally graded cylindrical shells are studied under the influence of various boundary conditions. Fabrication of FGM cylindrical shell is carried out by using exponential volume fraction law. Strain- and curvature-displacements relationships are taken from Love’s thin shell theory. The frequency equation in the form of eigenvalue problem is obtained by adapting the Rayleigh-Ritz method. Characteristic beam functions are assumed to approximate the axial modal dependence. Effects of exponential volume fraction law on the natural frequencies of the FGM cylindrical shells for various boundary conditions are studied against circumferential wave number, length to radius ratio and thickness to radius ratio for different values of power law exponents. Results evaluated show good agreement with those available in the literature.     

Velocity correlations in dense granular flows observed with internal imaging

We show that the velocity correlations in uniform dense granular flows inside a silo are similar to the hydrodynamic response of an elastic hard-sphere liquid. The measurements are made using a fluorescent refractive-index-matched interstitial fluid in a regime where the flow is dominated by grains in enduring contact and fluctuations scale with the distance traveled, independent of flow rate. The velocity autocorrelation function of the grains in the bulk shows a negative correlation at short time and slow oscillatory decay to zero similar to simple liquids. Weak spatial velocity correlations are observed over several grain diameters. The mean square displacements show an inflection point indicative of caging dynamics. The observed correlations are qualitatively different at the boundaries.     

Synthesis of Starch-Grafted Polymethyl Methacrylate via Free Radical Polymerization Reaction and Its Application for the Uptake of Methylene Blue

In this research, a new biodegradable and eco-friendly adsorbent, starch-grafted polymethyl methacrylate (St-g-PMMA) was synthesized. The St-g-PMMA was synthesized by a free radical polymerization reaction in which methyl methacrylate (MMA) was grafted onto a starch polymer chain. The reaction was performed in water in the presence of a potassium persulfate (KPS) initiator. The structure and different properties of the St-g-PMMA was explored by FT-IR, 1H NMR, TGA, SEM and XRD. After characterization, the St-g-PMMA was used for the removal of MB dye. Different adsorption parameters, such as effect of adsorbent dose, effect of pH, effect of initial concentration of dye solution, effect of contact time and comparative adsorption study were investigated. The St-g-PMMA showed a maximum removal percentage (R%) of 97% towards MB. The other parameters, such as the isothermal and kinetic models, were fitted to the experimental data. The results showed that the Langmuir adsorption and pseudo second order kinetic models were best fitted to experimental data with a regression coefficient of R² = 0.93 and 0.99, respectively.     

Adiabatic small polaron-hopping conduction in Ln0.85Ca0.15MnO3 (Ln=Nd, Pr and Sm) perovskites

Polycrystalline bulk ferromagnetic insulating (FMI) Ln_0.85Ca_0.15MnO₃ (Ln=Nd, Pr and Sm) samples are prepared by standard solid-state reaction route and characterized. Powder X-ray diffraction (XRD) data of the manganites show single-phase character. Existing theoretical models predict that the high temperature (T>θ_D/2, θ_D being the Debye temperature) dc conductivity (σ_dc) of these samples is due to adiabatic small polaron-hopping conduction. Greaves’ and Mott's variable range hopping (VRH) conduction mechanisms are not suitable to explain the σ_dc data at low temperature (T<θ_D/2).     

Vibration characteristics of FGM circular cylindrical shells filled with fluid using wave propagation approach

The vibration characteristics of a functionally graded material circular cylindrical shell filled with fluid are examined with a wave propagation approach. The shell is filled with an incompressible non-viscous fluid. Axial modal dependence is approximated by exponential functions. A theoretical study of shell vibration frequencies is analyzed for simply supported-simply supported, clamped-simply supported, and clamped-clamped boundary conditions with the fluid effect. The validity and the accuracy of the present method are confirmed by comparing the present results with those available in the literature. Good agreement is observed between the two sets of results.     

Optimization study of polyethylene glycol and solvent system for gas permeation membranes

Cellulose acetate (CA) membranes blended with Polyethylene glycol (PEG) in acetone–water solvent system were synthesized by using solution-casting method that resulted in the formation of flexible, white membranes. Different molecular weight (MW) grades of PEG (including MW 1000, 10,000 and 20,000?g/mol) were used. Cast membranes were tested for tensile strength and permeability at different loading of PEG MW 10,000 and 20,000?g/mol. Excellent flexible membranes were produced in acetone–water solvent system in the presence of PEG, which were otherwise brittle. Surface structure and morphology were analysed using scanning electron microscopy. The presence of different functional groups was confirmed using Fourier transform infra-red spectroscopy and the mechanical characteristics were studied by tensile testing. The introduction of PEG caused an increase in permeability of the membranes. The increase in permeability is due to the opening up of pores as the membrane becomes more flexible, when the plasticizer is added. The permeability continues to increase with the addition of PEG. Moreover, the resulting membranes are not only more flexible, but also have largely improved tensile strength as compared to the CA membranes without PEG. This improved tensile strength can also be attributed to the improved flexibility of the membrane. A trade-off is reached between tensile strength and permeability as increasing amount of PEG improves tensile strength but the resulting membrane becomes too permeable to be used for gas separation. Moreover, using PEG of higher MW resulted in porous membranes, even at low amounts of PEG. Therefore, we concluded that CA membrane with less amount of low-MW PEG (i.e. 5% PEG of MW 1000?g/mol) must be used to optimize both permeability and tensile strength of the membrane.