Qualitative Evolution of Asymmetric Raman Line-Shape for NanoStructures

A qualitative evolution of an asymmetric Raman line-shape function from a Lorentzian line-shape is discussed here for application in low dimensional semiconductors. The step-by-step evolution reported here is based on the phonon confinement model which is successfully used in literature to explain the asymmetric Raman line-shape from semiconductor nanostructures. Physical significance of different terms in the theoretical asymmetric Raman line-shape has been explained here. Better understanding of theoretical reasoning behind each term allows one to use the theoretical Raman line-shape without going into the details of theory from first principle. This will enable one to empirically derive a theoretical Raman line-shape function for any material if information about its phonon dispersion relation, size dependence, etc., is known.     

Photoluminescence properties and energy transfer in γ-irradiated Dy3+, Eu3+-codoped fluoroaluminoborate glasses

The photoluminescence (PL) properties of singly doped (Dy³⁺) and codoped (Dy³⁺, Eu³⁺) fluoroaluminoborate glasses, with an emphasis on the white light generation, are studied. The γ-irradiation led to the formation of defects in Dy³⁺-doped glasses and photoreduction of Eu³⁺ to Eu²⁺ in codoped (Dy³⁺, Eu³⁺) glasses. The electron paramagnetic resonance spectra confirm the presence of divalent europium ions and defects in Dy³⁺, Dy³⁺–Eu³⁺-doped glasses. The FTIR spectra mainly establish the compaction of glass network due to γ-irradiation. From the PL spectra, the intensity ratio of Dy³⁺ emission bands yellow to blue (⁴F_9/2–⁶H_13/2/⁴F_9/2–⁶H_15/2) defines the site symmetry, covalency, and feasibility of extracting white light. The existence of an energy transfer (ET) from Dy³⁺ to Eu³⁺ ions are established due to the decrease in intensity of Dy³⁺ peaks with an increase of Eu₂O₃ content. Moreover, the non-exponential nature of decay curves was well fitted with the generalization of Yokota–Tanimoto model for electric dipole-quadrupole (S = 8) interaction that is responsible for ET process from sensitizer (Dy³⁺) to activator (Eu³⁺).     

3-Methyltrimethylammonium poly(2,6-dimethyl-1,4-phenylene oxide) based anion exchange membrane for alkaline polymer electrolyte fuel cells

Hydroxyl ion (OH^?) conducting anion exchange membranes based on modified poly (phenylene oxide) are fabricated for their application in alkaline polymer electrolyte fuel cells (APEFCs). In the present study, chloromethylation of poly(phenylene oxide) (PPO) is performed by aryl substitution rather than benzyl substitution and homogeneously quaternized to form an anion exchange membrane (AEM). ¹H NMR and FT-IR studies reveal successful incorporation of the above groups in the polymer backbone. The membrane is characterized for its ion exchange capacity and water uptake. The membrane formed by these processes show good ionic conductivity and when used in fuel cell exhibited an enhanced performance in comparison with the state-of-the-art commercial AHA membrane. A peak power density of 111 mW/cm² at a load current density of 250 mA/cm² is obtained for PPO based membrane in APEFCs at 30 °C.     

Role of lead in the growth of the highT c phase in the Bi2−x Pb x Sr2Ca2Cu3O y systems

Bi_2−x Pb _x Sr₂Ca₂Cu₃O _y superconducting samples with 0<x<0·3 have been synthesized and characterized using X-ray powder diffraction.T _c and superconducting volume fraction have been measured using a.c. magnetic susceptibility, d.c. electrical resistivity as well as X-band microwave surface resistance in the normal state. The data indicate the growth of the highT _c (2223) phase with corresponding reduction of the lowT _c (2122) phase with increasingx, up to 0·25. Beyond this value ofx there is a slight deterioration of the superconducting behaviour.     

Mechanistic Modelling of the Catalytic Pyrolysis of Methylcyclohexane

A detailed chemical kinetic mechanism describing the thermal and catalytic cracking of methylcyclohexane over a 12CaO.7Al₂O₃ catalyst is presented. The model is based on balanced equations for both molecular and radical species describing an experimental fixed‐bed reactor system. The mechanistic model is based on overall first‐order decomposition kinetics, which satisfactorily describes experimental data. The simulated product distributions show a reasonably good agreement with the experimental results and confirm the hypothesis that the catalyst does not affect the pyrolysis mechanism and only increases the rate of the initiation steps.     

Dynamic analysis of flexible multi-body systems using mixed modal and tangent coordinates

This paper presents a mixed modal and tangent coordinate technique for computer aided analysis of flexible mechanical systems whose components undergo large translations and large rotations. In this model the configuration of a flexible component is identified by using two sets of generalized coordinates, namely rigid body and elastic coordinates. The rigid body coordinates define the location and orientation of a body axis, whereas the elastic coordinates define the displacement field of a component with respect to its body axis. The elastic coordinates are introduced by using finite element discretization to model flexible components with complex geometries. A modal analysis technique is used to identify the elastic mode shapes and to eliminate insignificant higher frequency modes. An orthonormalization of constraint Jacobian matrix associated with rigid body coordinates is used to identify the rigid body tangent coordinates. The resulting modal and tangent coordinates are used to develop an automated numerical integration scheme to solve the system differential and algebraic equations. Two numerical examples are considered to show the feasibility of dynamic analysis of flexible mechanical systems using this scheme.     

Microstructure and Wear Characteristics of Al-4.5Cu-5Pb Alloy

Al-4.5Cu-5Pb alloy was prepared by sand and chill casting. The same alloy was also spray deposited at a gas pressure of 1.6 MPa. The microstructural features exhibit a coarse to fine dendritic morphology for sand and chill cast alloys. Equiaxed grains were observed for spray fOrmed alloys. Wear testing employing a pin-on-disc type set-up, reveaIs considerably lower wear of spray deposited alloy compared to that of chill and sand cast alloys. The morphological features of wear track on specimen and debris indicated a mixed oxidative-cum-adhesive wear mechanisms for these alloys tested in the present investigation     

Rare earth cobaltite catalysts: Relation of activity to spin and valence states of cobalt

Catalytic activities of rare earth cobaltites and related compounds are shown to be related to the spin and valence states of cobalt.