Electron paramagnetic resonance and Mössbauer spectroscopy of transition metal ion doped mullite

Appreciable difference in the properties of undoped and oxide-doped mullite are observed. The oxidation state of cation, its concentration and the position of the mullite lattice occupied by it appear to be the responsible factors. Mullite has, therefore, been doped with four transition metal ions, Mn, Fe, Cr and Ti. With the help of EPR and Mössbauer spectroscopy (supplimented by X-ray diffractometry) the oxidation states of these ions and the mullite lattice sites where they enter has been investigated. It was observed that Mn ion was present in Mn²⁺ and Mn³⁺ states, the former remained as clusters and the latter occupied the octahedral sites in the mullite lattice. Only Fe³⁺ ion was detected and conclusive evidence was obtained for the entry of Fe³⁺ in the octahedral lattice position of mullite from the analysis of Mössbauer spectra with the help of a specially written computer programme. The Cr ion entered the mullite structure only in the Cr³⁺ state. The change in lattice parameters of Cr doped mullite were measured by the XRD technique. The results showed that the expansion of b-axis was more than that of the a-axis which supported the presence of Cr³⁺ ion in the octahedral site of mullite lattice. The absence of signal in the EPR spectra of Ti doped mullite suggested the presence of only Ti⁴⁺ (3d⁰) ion. Very low electrical resistivity of Ti doped mullite and close similarity between mullite and Al₂TiO₅ structures stood as evidence for incorporation of Ti⁴⁺ ion in the octahedral site of mullite lattice by replacing Al³⁺ ion.     

Analysis of photo-excited thin superconducting films for optoelectronic device applications

This paper introduces a steady state photoresponse analysis in superconducting thin films. A model is developed for investigating the capability of superconductors for potential optoelectronic applications. First, we consider the thermodynamical variation under the application of continuous optical radiation in superconducting state. Then, the photo-induced changes in electrical parameters are evaluated from thermodynamical variations using the electrodynamics based on the BCS theory. All variations are considered as a function of optical characteristics along with the material properties. The numerical simulation gives us the optimum range of applied optical frequency and absorbed radiation intensity required for sensitive control of superconducting properties for optoelectronic and microwave applications.     

On the compositional dependence of the optical gap and structural properties of amorphous In x Se1−x thin films

The optical gap of the amorphous In _x Se_1–x thin film has been observed to decrease with increasing content of indium in the film (x). An attempt is made to interpret the compositional dependence of the optical gap in amorphous In _x Se_1–x thin film in an alloy-like approach. It has been found that introduction of higher concentrations of indium imparts greater stability to the structure of the indium selenium alloy. From the study of the radial distribution function (RDF) by X-rays it is observed that there is a correlation between the coordination number and the optical gap of indium selenide thin films.     

Oxide glass filaments and their superconducting phase by annealing

Bi₄Ca₃Sr₃Cu _x O _y (4334) oxide glasses with different concentrations of the transition metal copper have been prepared by rapid quenching technique from respective melts. These glasses have been characterized from electrical, magnetic, dielectric and other properties. The semiconducting glasses are not superconducting even down to 4·2 K. However, by properly annealing the glasses in air or in oxygen atmosphere they can be converted into the corresponding superconducting phases with superconducting transition temperatures between 70 and 110 K depending on the Cu ion concentrations. The dc conductivity of the glasses could be explained with small polaron adiabatic hopping mechanism. Analysis shows that the correlated barrier hopping (CBH) mechanism is the most appropriate model for explaining the a.c. conductivities of these glasses. Addition of a small amount (about 2 wt%) of B₂O₃ makes it possible to draw glass filaments which could also be converted into the superconducting phase withT _c values lower than those obtained from the respective pure glasses.     

Polymeric encapsulation of 1-2-3 ceramic superconductors

YBa₂Cu₃O_7−x pellets have been found to degrade and lose superconducting properties when exposed to the normal atmosphere for long durations. Hence, coating the superconducting pellets with a polymer to protect it from moisture and CO₂ of the ambient atmosphere is highly desirable. In this paper the criteria of selection of a polymer and some preliminary studies on polymeric encapsulation of 1-2-3 pellets are reported. Fevitite epoxy and stycast have been found to show encouraging results.     

Growth and microstructure of Cd1−xZnxS films

The growth and microstructure of Cd_1–x Zn _x S films were studied by depositing the films on to freshly cleaved NaCl, KCl, and LiF substrates at various x values (0相似文献   

Variational coupled-mode theory of optical couplers

A scalar coupled-mode theory is developed from the variational principle. The formulation is variational in the sense that the mode parameters are adjustable and can be optimized subject to the coupling between the guides. The theory is applied to slab and fiber couplers. In comparison with the conventional scalar coupled-mode theory in which only the amplitudes of the modes in the individual guides are adjustable, the variational scalar coupled-mode theory predicts more accurate propagation constants and field patterns of the normal modes of the couplers     

Design and Analysis of Ultra-Miniaturized Meandering Photonic Crystals Delay Lines

In this paper, we study the characteristics of a novel miniaturized optical delay line, which delays light in a meandering photonic crystal waveguide, and describe the design steps. We show how lattice parameters and refractive index difference of the photonic crystal affect the bandgap width and suggest a criterion to select these parameters. Next, we focus on the parallel waveguide channels in photonic crystal, and analyze the impact of the channel length and the interchannel spacing on crosstalk. We suggest a method for mitering the sharp corners in meandering lines which reduces the undesired reflections by 8 dB. Considering all these guidelines, we examine the propagation of light in the proposed delay line through calculating time-delay and insertion loss. To achieve longer delays in a small device area, we concentrate on coupled cavities in photonic crystals and propose an approximate method for calculating the group velocity of light in the coupled defects. We show how by replacing waveguide channels of a meandering delay line with coupled defects we achieve time-delays more than 9 ps within a device size around 27 m, which corresponds to a miniaturization factor of 100.     

Enhancement of over expression and chaperone assisted yield of folded recombinant aconitase in Escherichia coli in bioreactor cultures

A major portion of the over expressed yeast mitochondrial aconitase, a large 82 kDa monomeric TCA cycle enzyme, in Escherichia coli led to the formation of inclusion bodies. Bacterial chaperonin GroEL mediated the correct folding of aconitase with the assistance of its co-chaperonin GroES in an ATP dependent manner. Till date the chaperonin assisted folding of aconitase was limited to the shake flask studies with relatively low yields of folded aconitase. No attempt had yet been made to enhance the yield of chaperone mediated folding of aconitase using a bioreactor. The current report deals with the effect of co-expression of GroEL/GroES in the production of soluble, biologically active recombinant aconitase in E. coli by cultivation in a bioreactor at different temperatures under optimized conditions. It revealed that the yield of functional aconitase was enhanced, either in presence of co-expressed GroEL/ES or at low temperature cultivation. However, the outcome from the chaperone assisted folding of aconitase was more pronounced at lower temperature. A 3-fold enhancement in the yield of functional aconitase from the bioreactor based chaperone assisted folding was obtained as compared to the shake flask study. Hence, the present study provides optimized conditions for increasing the yield of functional aconitase by batch cultivation in a bioreactor.