pp 395-401 October 2004 Single Crystals
Polycrystalline spherulitic crystals of pure Gd-heptamolybdate and single and twinned crystals of substituted Gd–Ba-molybdate were grown by using gel encapsulation technique. The thermal behaviour of these crystals was studied using the thermoanalytical techniques, which included TG, DTA and DSC. Thermal analysis suggests decomposition of the materials in one or more than one stages. Results obtained on application of TG based models viz. Horowitz–Metzger, Coats–Redfern and Piloyan–Novikova, are reported. According to the results of the kinetics of thermal decomposition, the random nucleation model is shown to be the one that is relevant to the decomposition of single rare earth (Gd) containing material and contracting sphere to the decomposition of the substituted (Gd–Ba) one. The kinetic parameters viz. the order of reaction, frequency factor and energy of activation using above-mentioned models, are computed and shown to bear reasonably good agreement.
pp 403-407 October 2004 Phase Transitions
Phase transition in L-alaninium oxalate is studied by using TG, DTA and photoacoustic spectroscopy. A sharp transition at 378 K by photoacoustics is observed whereas at the same temperature the endothermic energy change observed by TG and DTA is not very sharp. This is discussed in detail with reference to the other known data for the organic crystals.
pp 409-415 October 2004 Composites
An experimental study was conducted to observe the effects of parallel-superposed flow condition on viscoelastic properties of LLDPE, Kevlar fibre reinforced LLDPE and hybrid of short glass fibre and Kevlar fibre reinforced LLDPE. Parallel-plate rheometer was employed for these tests. Rheological parameters such as loss modulus (𝐺″) and dynamic viscosity (𝜂′) do not vary significantly on superposing steady state shear with oscillatory shear in the studied range of experiment at 185°C in un-reinforced LLDPE. Kevlar fibre reinforced LLDPE and Kevlar/glass fibre reinforced LLDPE showed significant changes in the flow behaviour under various sets of superposed conditions. Storage modulus (𝐺′), and 𝐺″ become highly sensitive to low oscillatory angular frequencies (𝜔) under superposed conditions. These curves show two different regions with increased 𝜔 value. At low 𝜔 values, parameters 𝐺′ and 𝐺″ change sharply reaching a certain value, thereafter, changes are moderate with increased 𝜔. In case of 𝜂′ a maxima is observed, position of which, depends upon the value of steady shear rate. Maxima shifts towards higher frequencies with the increased steady shear rate.
pp 417-420 October 2004 Nuclear Related Materials
Ion irradiation of Si8+ ion beam of 100 MeV was scattered by a gold foil on a Mylar membrane of 25 𝜇m thickness in the form of film roll (width, 12.5 cm and length, 400 cm) at the Nuclear Science Centre, New Delhi. The characterization of etched nuclear tracks was carried out by gas permeation measurements. The samples cut from the film roll of required size for permeability measurements were etched in a controlled manner in a constant temperature bath of 6N NaOH solution. The opening of the conical etched tracks was characterized by hydrogen gas permeation.
pp 421-427 October 2004 Dielectrics
Investigation of solid solution of barium–strontium orthotitanates of the type, Ba2–𝑥Sr𝑥TiO4 (0 ≤ 𝑥 ≤ 2), show that pure phases exist only for the end members, Ba2TiO4 and Sr2TiO4, crystallizing in the 𝛽-K2SO4 and K2NiF4 structures, respectively. The intermediate compositions (till 𝑥 ≤ 1) lead to a biphasic mixture of two Ba2TiO4-type phases (probably through a spinodal decomposition) with decreasing lattice parameters, indicating Sr-substitution in both the phases. For 𝑥 > 1, Sr2TiO4 along with a secondary phase is obtained. The dielectric constant and dielectric loss were found to decrease with Sr substitution till the nominal composition of 𝑥 = 1. However, pure Sr2TiO4 shows higher dielectric constant compared to the solid solution composition. Sr2TiO4 shows very high temperature stability of the dielectric constant.
pp 429-432 October 2004 Superconductors
MgB2 superconductor has been synthesized using a simple technique at ambient pressure. The synthesis was carried out in helium atmosphere over a wide range of temperatures. Magnesium was employed in excess to the stoichiometry to prevent the decomposition of MgB2. Samples of MgB2 thus prepared have been almost free from MgO as compared to other methods. Resistivities of the samples are quite low with residual resistivity ratio (RRR) of around 3. 𝑇c (𝑅 = 0) is 38.2–38.5 K with 𝛥𝑇c of 0.6–1.0 K. Comparative studies of various methods of low pressure synthesis have been presented.
pp 433-439 October 2004 Thin Films
The fabrication method, technology route and structure performances of (Sr, Ba) TiO3 (SBT) ferroelectric thin film have been summarized in this paper. The tunability of dielectric constant, dielectric loss and leakage current are the basic parameters of tunable microwave devices. The thin films of SBT with high properties could be fabricated by means of RF magnetron sputtering and sol–gel processing. The electrical performances of thin film material can be improved largely by dopants. Some problems are put forward to pay attention to this material research process.
pp 441-444 October 2004 Thin Films
Temperature dependence of d.c. conductivity is studied in 𝑎-Se75In25–𝑥Pb𝑥 thin films where 𝑥 is varied from 0–10. From these measurements, the values of the pre-exponential factor (𝜎0) and activation energy (𝛥𝐸) are calculated for each glassy alloy. An approximate linear dependence of ln 𝜎0 on 𝛥𝐸 is observed in this glassy system with good agreement between the expected and calculated 𝜎0 values using Meyer–Neldel rule. Linear dependence of ln𝜎0 on 𝛥𝐸 in case of amorphous materials indicates that the conduction band tails a finite energy distance towards the valence band and Fermi level is controlled by fixed dominant hole levels deeper in the gap.
pp 445-451 October 2004 Thin Films
Cubic silicon carbide (3C-SiC) thin films were grown on (100) and (111) Si substrates by CVD technique using hexamethyldisilane (HMDS) as the source material in a resistance heated furnace. HMDS was used as the single source for both Si and C though propane was available for the preliminary carbonization. For selective epitaxial growth, patterned Si (100) substrates were used. The effect of different growth parameters such as substrate orientation, growth temperature, precursor concentration, etc on growth was examined to improve the film quality. The surface morphology, microstructure and crystallinity of grown films were studied using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis and X-ray photoelectron spectroscopy (XPS).
pp 453-457 October 2004 Thin Films
Thermal spray coatings of surfaces with metal, alloy and ceramic materials for protection against corrosion, erosion and wear is an intense field of research. The technique involves injection of the powder into a plasma flame, melting, acceleration of the powder particles, impact and bonding with the substrate. Feedstock powders of metals, alloys and ceramics for thermal spray applications have to meet several requirements. Particle shape, size and its distribution, powder flow characteristics and density are the important factors to be considered in order to ensure high spray efficiency and better coating properties. For smooth and uniform feeding of powders into plasma jet, the powder particles have to be spherical in shape. High temperatures and steep temperatures present in thermal plasma is exploited to spheroidize particles in the present investigation. Nickel powder particles in the size range from 40–100 𝜇m were spheroidized using plasma processing. SEM and optical micrographs showed spherical shape of processed particles.
pp 459-466 October 2004 Ionic Conductors
The study of ionic conductivity vs reciprocal temperature of pure KCl and KCl crystal doped with 0.1, 0.3 and 0.5 mole% gadolinium has been carried out in as grown, quenched from elevated temperatures (100, 350 and 500°C) and annealed at various timings i.e. 2–3 h and deformed by different percentages. The plots exhibit three well-known regions, II, III and IV (extrinsic regions). The intrinsic region I was not observed in the plots as the conductivity measurements were taken up to 575°C. From the analysis of these plots, activation energies for the migration of cation vacancy and the association of gadolinium ion with cation vacancy in the lattice of KCl crystals are calculated. These values are compared with previously reported values. Further, an attempt is made to explain the existence of oxidation state of gadolinium ion in + 3 state rather than in + 2 state as reported earlier. The variation in conductivity with effect of concentration of impurity ion, quenching and annealing and deformation with various percentages are explained on the basis of formation of impurity vacancy dipoles, vacancy – vacancy pairs (which appear in the form of precipitation), storage of cation vacancies in the form of defects, introduction of fresh dislocations, etc.
pp 467-470 October 2004 Ionic Conductors
A novel biomimetic approach in designing and fabricating engineering ceramic materials has gained much interest in recent times. Following this approach, synthesis has been made of dense Si–SiC duplex ceramic composites and highly porous SiC ceramics in the image of the morphological features inherent in the caudex stem of a local monocotyledonous plant. The process route involves making of a carbonaceous biopreform and its subsequent reaction with an infiltrating silicon melt to yield the biomorphic Si–SiC ceramic composites with flexural strength and Young’s modulus of 264 MPa and 247 Gpa, respectively and loss in weight of only ∼ 9% during oxidative heating up to 1200°C in flowing air.
The Si–SiC composites were transformed into porous (49 vol.%) SiC ceramics with complete preservation of microcellular anatomy of the parent plant, by depleting residual silicon phase in channel pores through reaction with carbon. SiC based materials so derived can be used in structural applications and in designing high temperature filters and catalyst supports.
pp 471-482 October 2004 Instrumentation
Convergent beam electron diffraction (CBED) is a powerful technique to estimate lattice distortion and lattice strain in crystals. The positions of the higher-order Laue-zone (HOLZ) lines in the transmitted disc of CBED patterns are very sensitive to the lattice parameter, and can therefore be used to estimate changes in the lattice parameter. This offers the possibility to calculate lattice misfit and lattice strain. The positions of the HOLZ lines depend not only on the lattice parameter, but also on the operating voltage of the microscope. It is essential to know the actual voltage of the microscope. In the present work, (1 0 0) GaAs crystal has been used as a standard. Cross-sectional TEM specimens were prepared by argon ion beam thinning technique using a liquid nitrogen cold stage. 〈0 1 2〉 on-zone CBED technique has been used to estimate the actual voltage of the transmission electron microscope (Philips EM430T TEM), when the voltage was set at 250 kV. CBED–HOLZ simulation and analyses have been done, using JEMS software, to correlate with the experimental data. The methodologies adopted for estimating the actual voltage of TEM are discussed in this paper. The studies have also been cross-checked using 〈0 1 2〉 and 〈2 3 3〉 zone axes using (1 0 0) silicon standard. The techniques established are found to be suitable for TEMs operating at a setting voltage of about 250 kV. For the TEM studies, a regular double-tilt specimen holder is required in order to be able to get to the desired zone axes. When the experiments were repeated using a cryogenic double-tilt holder, an improvement in the sharpness of HOLZ lines was observed. Wherever possible, the use of the cryogenic double-tilt holder is recommended. Care must, however, be taken to ensure that effects such as lattice parameter changes (due to temperature changes), phase transformations etc can be properly accounted for.
Volume 42 | Issue 5
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