Volume 21, Issue 5
October 1998, pages 363-438
pp 363-374 October 1998 Materials Preparation
Various criteria of formation of perovskites and K2NiF4-type compounds, which are the end members of homologous Ruddlesden-Popper series of phases, have been discussed. RP-phases show some general trends in their physical properties. Literature survey of the formation of these phases, their properties, and effect of oxygen stoichiometry and doping is presented. Some conditions for their synthesis are also described.
pp 375-380 October 1998 Materials Preparation
Single crystals of lanthanum neodymium oxalate (LNO) are grown in sodium meta silicate gels, by the diffusion of a mixture of aqueous solutions of lanthanum nitrate and neodymium nitrate into the test tube having the set gel containing oxalic acid. The bluish pink coloured tabular crystals of LNO having well defined hexagonal basal planes appear either as ‘foggy’ or ‘clear’, the latter at the greater depths inside the gel. The colouration of LNO visually observed is evidenced in UV-visible spectrum, by the revelation of well pronounced characteristic peaks in the visible region (500–900 nm). X-ray diffraction (XRD) of powdered LNO is ordered, meaning crystalline in nature, besides its ‘isostructurality’ with similarly grown lanthanum samarium oxalate crystals. The single crystallinity of LNO is established by its oscillation XRD pattern. Thermogravimetric analysis (TGA) and differential scanning colorimetry (DSC) support that LNO loses water of crystallization around 120°C and CO and CO2 around 350–450°C, while the infrared absorption (IR) spectrum of LNO establishes the presence of oxalate (C2O4)2− ions. Energy dispersive X-ray analysis (EDAX) confirms the presence of La and Nd in the sample. X-ray photoelectron spectroscopic (XPS) studies of LNO establish the presence of La and Nd in their respective oxide states. An empirical structure for LNO has been proposed on the basis of these findings. The ‘smokiness’ in the foggy LNO crystal has been attributed due to the ‘gel inclusion’ during the growth process.
pp 381-385 October 1998 Materials Preparation
α-Fe2O3 films were prepared by sol-spinning process using ferric nitrate as a precursor and 2-methoxy ethanol as the solvent. The films were grown on various substrates by spin coating and were subjected to different annealing temperatures. These were characterized using X-ray and fourier transform infrared spectroscopy (FTIR). The films showed crystallinity at about 500°C. The surface morphology of these films was studied using scanning electron microscopy (SEM) which revealed cracks for films having thickness of the order of 2 µm. The band gap of these films was observed to be 2·1 eV from UV-vis spectroscopy.
pp 387-391 October 1998 Materials Preparation
Single crystals of cerium oxalate are grown by the gel method. Ce(NO3)3 is allowed to diffuse into a gel in which oxalic acid is incorporated. Cerium oxalate crystals are formed by chemical reaction and the growth process is observed. Crystals are yellow-white in colour. Morphology and size of the crystals are highly influenced by the acidity of the feed solution. With 30% HNO3 long needle shaped, with 50% HNO3 thin hexagonal and with 100% HNO3, well developed crystals are obtained. Natural etch pits are observed on the grown crystals due to the presence of HNO3. Appearance of a single, large interfacial crystal is observed for 0·5 M cerium nitrate. IR spectrum confirms the presence of water molecules and carboxylic group. X-ray diffractogram gives well defined peaks. Peaks are indexed. Unit cell dimensions are determined. Thermal analysis of the samples done using TGA and DTA show the release of water molecules (endothermic) and of CO and CO2 (exothermic) with the rare earth oxide as stable residue.
pp 393-398 October 1998 Materials Preparation
Properties of substituted La-manganites are known to be very sensitive to La site substitution. The critical parameters appear to be (a) the charge and concentration of the substituted ions, which determine the Mn3+/Mn4+ ratio, (b) the average ionic radius at theA site and (c) the tolerance factor,t. The latter two parameters determine the distortion and structural tuning of the perovskite structure. Here we report the influence of minor variations in the above parameters on the magnetotransport of compounds around the basic composition La0·6M0·07Ca0·33MnO3 (M=Er, Yb and Bi). The influence of the method of synthesis on different properties is also discussed. The ionic radii of Er3+, Yb3+ and Bi3+ ions are 1·00, 0·985 and 1·11 Å, respectively, compared to 1·32 Å for La3+ and this provides a systematic variation for the investigation of the effects ofA site substitution. The zero field resistivity data are essentially similar above the maxima which corresponds, at least approximately, to the Curie temperature as determined by magnetization measurements. However, there are variations below the transition temperature, where there is magnetic ordering, and large variations are often seen in the magnetic data between samples which are very similar in composition. Substantial similarities between field dependent magnetization and resistivity measurements appear to be related to domain wall displacement and rotation. These results will be discussed in the framework of structural tuning which has a strong influence on exchange parameters such as the Mn-O-Mn bond length and angle.
pp 399-402 October 1998 Materials Preparation
Due to a lack of proper understanding about the formation mechanism of oxygen related donors during the transition temperature range (465–540°C) which exhibits the transition of TDs to NDs, an attempt has been made to study their behaviour in the present context. We have found the formation and diffusion of molecule like oxygen at low temperature annealing of silicon and observed that second order kinetics of oxygen diffusion holds good. A relatively low value of 0·6 eV has been estimated to be the activation energy for the diffusion of oxygen in silicon which is supposed to be due to the hydrogen passivation.
pp 403-407 October 1998 Transport Properties
Heterobinuclear manganese-molybdenum complex with the Schiff base prepared by the condensation of diethylenetriamine with salicylaldehyde yielded a polymeric mixed metal coordination complex with the composition of [Mn1·5MoO3L]. The infrared spectral data indicated the presence of azomethine (HC=N) and Mo=O groups.1H NMR further confirmed the coordination of azomethine nitrogen and absence of phenolic hydrogen. The EPR studies suggested the presence of Mn(II) in the complex. The solid state conductivity studies with the variation of potential and temperature indicated a specific conductivity of 1·6×10−5 ohm−1 cm−1. The activation energy was found to be 0·019 eV. Hall measurements indicate that the material isp-type which is further confirmed by iodine doping which increases the conductivity by two orders of magnitude.
pp 409-413 October 1998 Magnetic Properties
The structural and magnetic properties of the mixed spinel Co1+xSixFe2−2xO4 system for 0·1≤x≤0·6 have been studied by means of X-ray diffraction, magnetization, and Mössbauer spectroscopy measurements. X-ray intensity calculations indicate that Si4+ ions occupy only tetrahedral (A) sites replacing Fe3+ ions, and the added Co2+ ions substitute for (B) site Fe3+ ions. The Mössbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe3+ at the A and B sites, forx≤0·3. The Mössbauer intensity data shows that Si possesses a preference for the A site of the spinel. The variation of the saturation magnetic moment per formula unit measured at 300 K with the Si content, is explained on the basis of Neel’s collinear spin ordering model forx≤0·3 which is supported by Mössbauer, and X-ray data. The Curie temperature decreases nearly linearly with increase of the Si content, forx=0·1–0·6.
pp 415-419 October 1998 Glasses
Optical properties of Sm3+ ion in phosphate glassy matrices were studied. The absorption, fluorescence, excitation spectra and decay patterns were obtained at room temperature. The oscillator strengths of the transitions betweenJ manifolds were calculated using Judd-Ofelt theory and compared with the ones obtained experimentally. Various laser intensity parameters such as J-O parameters, radiative transition probabilities, lifetimes, branching ratio, and integrated absorption cross-section were evaluated. Radiative lifetimes of the excited states were determined and used to obtain nonradiative transition rates and quantum yields. Integrated intensity and dynamics of the fluorescence originating from the excited states were also studied as a function of Sm3+ concentration.
pp 421-425 October 1998 Glasses
A special vitreous enamel coating suitable for use in air preheater heating elements (waste heat recovery system) in boilers of thermal power plants has been developed. The preparation of coating materials, techniques of application, evaluation and characterization of the coating and prediction of life expectancy using a mathematical model based on Brandon’s method have been reported.
pp 427-431 October 1998 Positron Annihilation Studies
Time differential perturbed angular correlation (TDPAC) and positron life-time measurements on cold-worked and hydrogen chargedNbHf indicate a strong binding of hydrogen decorated vacancy clusters with Hf impurities. Both TDPAC and positron lifetime results are mutually consistent on this aspect. Transformation of H-vacancy clusters into H-bubbles is indicated by the positron lifetime behaviour at annealing temperatures beyond 900 K. Impurity effects like oxygen pickup and coating of voids cannot be ruled out at elevated temperatures.
pp 433-438 October 1998 Instrumentation
A Jobin Yvon-Spex (HR640) monochromator with a notch filter (514·5 nm) and Ar+ ion laser has been used to set up a low cost laser Raman spectrometer. The selection and setup of the various optical components used in the present work has been solely carried out in our laboratory. The calibration of the monochromator was established from the studies of various standard mercury lines and the obtained data fitted with the reported data. Raman signals have been recorded for a number of samples e.g. diamond, ruby, carbon tetrachloride (CCI4), benzene (C6H6) and ethanol (C2H5OH). The obtained results are found to be in excellent agreement with the reported values for these materials in the literature.
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