Unsteady free convection flow under the influence of a magnetic field

Summary The unsteady free convection boundary layer at the stagnation point of a two-dimensional body and an axisymmetric body with prescribed surface heat flux or temperature has been studied. The magnetic field is applied parallel to the surface and the effect of induced magnetic field has been considered. It is found that for certain powerlaw distribution of surface heat flux or temperature and magnetic field with time, the governing boundary layer equations admit a self-similar solution locally. The resulting nonlinear ordinary differential equations have been solved using a finite element method and a shooting method with Newton's corrections for missing initial conditions. The results show that the skin friction and heat transfer coefficients, andx-component of the induced magnetic field on the surface increase with the applied magnetic field. In general, the skin friction, heat transfer andx-component of the induced magnetic field for axisymmetric case are more than those of the two-dimensional case. Also they change more when the surface heat flux or temperature decreases with time than when it increases with time. The skin friction, heat transfer andx-component of the induced magnetic field are significantly affected by the magnetic Prandtl number and they increase as the magnetic Prandtl number decreases. The skin friction andx-component of the magnetic field increase with the dissipation parameter, but heat transfer decreases.

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Instationäre freie Konvektionsströmung unter dem Einfluß eines magnetischen Feldes

Übersicht Untersucht wurde die instationäre freie Konvektionsgrenzschicht am Ruhepunkt eines zweidimensionalen und achsensymmetrischen umströmten Körpers bei vorgegebenem Wärmefluß bzw. bei vorgegebener Temperatur an der Oberfläche. Das magnetische Feld wird parallel zur Oberfläche angelegt, und der Einfluß des induzierten magnetischen Feldes wurde berücksichtigt. Es stellt sich heraus, daß bei bestimmter, zeitlicher Potenzgesetzverteilung des Wärmeflusses bzw. der Temperatur und des magnetischen Feldes an der Oberfläche die geltenden Grenzschichtgleichungen örtlich eine selbstähnliche Lösung erlauben. Die sich ergebenden nichtlinearen gewöhnlichen Differentialgleichungen wurden mittels einer Finite-Element-Methode und einer Shooting-Methode mit Newtonschen Korrekturen für fehlende Anfangsbedingungen gelöst. Die Ergebnisse zeigen, daß die Oberflächenreibung und die Wärmeübergangskoeffizienten sowie diex-Komponente des induzierten magnetischen Feldes an der Oberfläche mit dem angelegten magnetischen Feld zunehmen. Im allgemeinen sind die Oberflächenreibung, der Wärmeübergang und diex-Komponente des induzierten magnetischen Feldes im achsensymmetrischen Fall größer als die entsprechenden Werte im zweidimensionalen Fall. Außerdem verändern sich diese Werte beim zeitlichen Abfallen des Wärmeflusses an der Oberfläche bzw. der Temperatur in höherem Maße als bei der zeitlichen Zunahme dieser Werte. Die Oberflächenreibung, der Wärmeübergang und diex-Komponente des induzierten magnetischen Feldes werden durch die magnetische Prandtl-Zahl erheblich beeinflußt; sie nehmen mit abfallender magnetischer Prandtl-Zahl zu. Die Oberflächenreibung und diex-Komponente des magnetischen Feldes nehmen mit dem Wärmeableitungsparameter zu, der Wärmeübergang jedoch fällt ab.

     

Variable operator technique and the min-max theorem

We investigate a variation method where the trial function is generated from the application of a variable operator on a reference function. Two conditions are identified, one for obtaining a maximum and another for a minimum. Although the conditions are easy to understand, the overall formulation is somewhat unusual as each condition gives rise to a two-step variation process. As an example, projection operators are used to form the variable operator, and by this tactics one obtains the new interpretation that the pseudopotential formalism is in fact equivalent to a minimax procedure. The two-step variational process is nevertheless more flexible than the pseudopotential formalism, for it can also be used when the variable operator is not manifestly expressed in terms of projectors. This is illustrated by a comparison of the two-step method with the variational solution of Dirac’s relativistic electron equation. The same comparison leads to an alternative proof that the process of maximizing energy by varying the u–l coupling operator eliminates all negative-energy contributions from a trial spinor. The latter observation is crucial for the derivation of the min-max theorem in relativistic quantum mechanics.     

Tunable dielectric and conductivity properties of two 4-n alkoxy benzoic acid

We have presented dielectric and conductivity studies of two liquid crystal (LC) compounds- p-octyloxybenzoic acid (8OBA) and p-decyloxybenzoic acid (10OBA). Dielectric permittivity study of those compounds gives the evidence of space charge polarization and ionic conductance in the samples. Dielectric permittivity is found to be the highest for 8OBA than 10OBA. Both compounds found to exhibit positive dielectric anisotropy. Splay elastic constant as a function of temperature has also been investigated. Frequency and temperature dependent electrical conductivity of these two LC compounds have been studied in detail. Activation energy has been estimated from both dc and ac conduction process.     

Influence of temperature on different optoelectronic characteristics of InGaN light emitting diodes

We have measured the electroluminescence (EL) and carrier lifetime characteristics in InGaN/Sapphire purple light emitting diode (LED), namely, UV3TZ-405-30 in a temperature range from 350 to 120 K and have compared them with those of a similar LED (UV3TZ-395-15) but with different Indium concentration, measured earlier. While it is found that for the present device the EL intensity decreases drastically with lowering of temperature after reaching a maximum (99%) at 228 K, this is markedly different from the previous device where intensity continues to increase monotonically till lowest temperature. This qualitatively distinct temperature dependence indicates difference in nature of localisation of carriers in the multiple quantum wells for varying Indium content in the two devices. The light–current–temperature data have been analysed in terms of the semiconductor rate equations to determine different optoelectronic properties. Next, estimating the ideality factor from the current–voltage (I–V) measurements, the effective carrier lifetime has been evaluated from the open circuit voltage decay process. Using the above measurements, the temperature dependence of the internal quantum efficiency of the device has been calculated and it is found to attain a maximum value of 99.88% at 228 K. Unlike all previous calculations, a unique feature of the present approach has been to include the effect of temperature dependence of the radiative recombination coefficient (B) in the rate equation analysis. Finally, a comparative study of the temperature dependence of the different optoelectronic properties of both devices is presented with and without this effect.     

Perspectives on Higgs boson and supersymmetry

We review the recent discovery of the Higgs like particle at ～ 125 GeV and its implications for particle physics models. Specifically the implications of the relatively high Higgs mass for the discovery of supersymmetry are discussed. Several related topics such as naturalness and supersymmetry, dark matter and unification are also discussed.     

Mössbauer spectral studies of di- and triorganotin(IV) histidinylleucinates

Some new di- and triorganotin(IV) derivatives of the formulae, R₂SnL, where R =Me, Ph, and n-Oct, and L is the dianion of histidinylleucine (H₂L abbreviated as H₂(His-Leu)) and R′₃ SnHL, where R^′?= Me and Ph, HL = monoanion of histidinylleucine, have been synthesized. The bonding and coordination behaviour in these derivatives are discussed on the basis of FT-IR and ¹¹⁹Sn Mössbauer spectroscopic studies. The ¹¹⁹Sn Mössbauer studies, together with the IR data suggest that the ligand in R₂Sn(His-Leu) acts as dianionic tridentate coordinating through C(O)O^???, –NH₂ at axial positions and N_peptide at equatorial position while in case of R′₃Sn(HHis ? Leu) the ligand acts as a monoanionic bidentate coordinating through C(O)O^??? and –NH₂ giving a distorted trigonal–bipyramidal environment around tin.     

Why does the open-circuit voltage in a micro-crystalline silicon PIN solar cell decrease with increasing crystalline volume fraction?

In hydrogenated micro-crystalline silicon (μc-Si:H) thin film solar cells, the open-circuit voltage (V_OC) shows a decline when the crystalline volume fraction (F_c) of the intrinsic μc-Si:H layer increases from 60% to over 90%. In this article we have simulated the experimental characteristics of solar cells, having intrinsic layers of different degrees of crystallinity to understand the reasons why. In order to model all aspects of the characteristics, we had to assume (a) wider band tails, (b) a higher mid-gap defect density and (c) a lower band gap for the more crystallized material. Modeling reveals that all three factors lower the field in the volume of the device and hence V_OC, due to higher photo-generated hole-trapping close to the P/I interface. The third factor brings the quasi-Fermi levels closer to the band-edges, resulting in higher free and trapped carrier densities throughout the device, with the trapped hole population particularly high at the P/I interface. We further show that V_OC is higher in a crystalline silicon PN cell, in spite of a sharply reduced band gap, because the lower effective density of states at the band-edges and sharply reduced band gap defect density overcome the effect of the smaller band gap.     

Synthesis and characterization of active cuprous oxide particles and their catalytic application in 1,2,3‐triazole synthesis via alkyne‐azide cycloaddition reaction in water

Active cuprous oxide materials are synthesized from CuSO₄.5H₂O using sodium stannite as reducing agent in the presence of various stabilizers, viz., cetyl trimethyl ammonium bromide, sodium dodecyl sulphate, and polyvinyl pyrrolidone. The synthesized cuprous oxide materials are well characterized by powder X‐ray diffraction and Fourier transform infrared spectroscopy to ascertain their identity, while field emission scanning electron microscopy and energy‐dispersive spectroscopy analysis were used to study their morphology and composition, respectively. We have compared the catalytic prowess of the various cuprous oxide materials in the cycloaddition reaction of alkynes and azides to synthesize 1,4‐disubstituted‐1,2,3‐triazoles. A wide variety of substitutions can nicely be tolerated in our optimized reaction conditions to produce very good to excellent yields of the corresponding triazoles in water at 55 °C. The reactions are carried out in water without any assistance of organic cosolvent or other additives, which renders the catalytic method as economical and environment friendly.