Vibrational properties of single-wall carbon nanotubes: a first-principles study

We present first-principles pseudopotential-based density functional theory (DFT) calculation of structures, full phonon dispersions and thermal properties of armchair single wall armchair carbon nanotubes (SWCNTs) in the isolated and bundle forms. Comparison between the properties of isolated and bundled nanotubes is used to estimate the intertube interaction. We determine correlation between vibrational modes of a graphene sheet and of the nanotube to understand how rolling of the sheet results in mixing between modes and changes in vibrational spectrum. The radial breathing mode hardens with increasing diameter (or decreasing curvature). We estimate thermal expansion coefficient of nanotubes within a quasiharmonic approximation and identify the modes that dominate thermal expansion of some of these SWCNTs both at low and high temperatures.     

Simulation of 3D gas–solid fluidized bed reactor hydrodynamics

In this article, an attempt is made to develop a 3D gas–solid fluidized bed reactor (FBR). Basically, it deals with simulation of a FBR in computational fluid dynamics (CFD) using the software, Ansys Fluent v14. The simulation of gas–solid flow is carried out using Eulerian multifluid model which is integrated with the solid particle kinetic theory. The coefficients of exchange momentum are estimated using the Syamlal & O'Brien, Gidaspow, Wen-Yu, and Huilin–Gidaspow drag functions. The results of the simulation have been validated with the experimental data available in literature and had proven that the model is capable to predict the hydrodynamics of FBR. The variation in kinetic energy of the solid phase is calculated by varying the restitution coefficient (RC) from 0.90 to 0.99. The predictions of pressure drop compare excellently with the experimental data. Finally, the effect of particle diameter on the expanded bed height has been studied for FBR.     

Removal of Cr(VI) from aqueous solutions using pre-consumer processing agricultural waste: a case study of rice husk

This paper reports the feasibility of using pre-consumer processing agricultural waste to remove Cr(VI) from synthetic wastewater under different experimental conditions. For this, rice husk, has been used after pre-treatments (boiling and formaldehyde treatment). Effect of various process parameters, namely, pH, adsorbent dose, initial chromium concentration and contact time has been studied in batch systems. The removal of chromium was dependent on the physico-chemical characteristics of the adsorbent, adsorbate concentration and other studied process parameters. Maximum metal removal was observed at pH 2.0. The efficiencies of boiled and formaldehyde treated rice husk for Cr(VI) removal were 71.0% and 76.5% respectively for dilute solutions at 20gl(-1) adsorbent dose. The experimental data were analyzed using Freundlich, Langmuir and Dubinin-Radushkevich (D-R) isotherm models. It was found that Freundlich and D-R models fitted well. The results revealed that the hexavalent chromium is considerably adsorbed on rice husk and it could be an economical method for the removal of hexavalent chromium from aqueous systems. FTIR and SEM were recorded, before and after adsorption, to explore number and position of the functional groups available for Cr(VI) binding on to studied adsorbents and changes in adsorbent surface morphology.     

Fiber-optic miniature sensor for in situ temperature monitoring of curing composite material

This study proposes a fiber-optic temperature sensor with a single-mode fiber tip covered with a thermo-sensitive polymer resin. The temperature is sensed by measuring the Fresnel reflection from the optical fiber/polymer interface. Because the thermo-optic coefficients differ between the optical fiber and the polymer, the in situ temperature can be measured even in curing composite materials. In initial experiments, the proposed sensor successfully measured and recovered the temperature information. The measured sensor data were linearly correlated, with an R² exceeding 0.99. The standard deviation in the long-term measurements of constant temperature was 2.6%. The durability and stability of the sensor head material in long-term operation was validated by Fourier transform infrared spectroscopy and X-ray diffraction analysis. In further experiments, the suggested miniature temperature sensor obtained the internal temperatures of curing composite material over a wide range (30–110 °C).     

One dimensional nonlocal integro-differential model & gradient elasticity model : Approximate solutions and size effects

In the present work, the close similarity that exists between Mindlin’s strain gradient elasticity and Eringens nonlocal integro-differential model is explored. A relation between length scales of nonlocal-differential model and gradient elasticity model has been arrived. Further, a relation has also been arrived between the standard and nonstandard boundary conditions in both the cases. C⁰-based finite element methods (FEMs) are extensively used for the implementation of integro-differential equations. This results in standard diagonally dominant global stiffness matrix with off diagonal elements occupied largely by the kernel values evaluated at various locations. The global stiffness matrix is enriched in this process by nonzero off diagonal terms and helps in incorporation of the nonlocal effect, there by accounting the long-range interactions. In this case, the diagonally dominant stiffness matrix has a band width equal to influence domain of basis function. In such cases, a very fine discretization with larger number of degrees of freedom is required to predict nonlocal effect, thereby making it computationally expensive. In the numerical examples, both nonlocal-differential and gradient elasticity model are considered to predict the size effect of tensile bar example. The solutions to integro-differential equations obtained by using various higher-order approximations are compared. Lagrangian, Bèzier and B-Spline approximations are considered for the analysis. It has been shown that such higher-order approximations have higher inter-element continuity there by increasing the band width and the nonlocal character of the stiffness matrix. The effect of considering the higher-order and higher-continuous approximation on computational effort is made. In conclusion, both the models predict size effect for one-dimensional example. Further, the higher-continuous approximation results in less computational effort for nonlocal-differential model.     

Effects of H2O Pretreatment on the Capacitance–Voltage Characteristics of Atomic-Layer-Deposited Al2O3 on Ga-Face GaN Metal–Oxide–Semiconductor Capacitors

Atomic layer deposition (ALD) of Al₂O₃ on Ga-face GaN is studied with respect to the effects of growth saturation, precursor injection sequence, and H₂O pretreatment. A metal–oxide–semiconductor capacitor (MOSCAP) structure is fabricated to measure the capacitance–voltage (C–V) characteristics. The origin of C–V hysteresis is explained by a model considering the different trapping behaviors of interface states and oxide border traps. The interface state density (D _it) is extracted as a function of band bending using an ultraviolet (UV)-assisted method. It is found that H₂O pretreatment followed by saturated ALD growth produces the best interface quality, with a reduced D _it compared with growth without H₂O pretreatment.