pp 341-344 October 2000
The template carbonization of polyphenyl acetylene yields hollow, uniform cylindrical carbon nanotubes with outer diameter almost equal to pore diameter of the template used. High resolution transmission electron microscopic investigation reveals that Pt-Ru nanoparticles are highly dispersed inside the tube with an average particle size of 1.7 nm.
pp 345-348 October 2000
Solid solutions of compositions LiNiyCo1−yO2 (y = 0.0, 0.1 and 0.2) were prepared by solid-state fusion synthesis from carbonate precursors. Material characterization was carried out using XRD. Formation mechanisms of the products are discussed in the light of TG/DTA results. Nickel-containing compositions gave higher discharge capacities and smaller hystereses in their charge-discharge profiles which make them more attractive than pristine LiCoO2 as cathode materials in high-energy lithium cells. The lower loss in capacity per cycle for cells with unsubstituted LiCoO2, as determined from cycling studies up to 25 cycles, makes it more suitable than the substituted ones for long cycle-life cells with low capacity fade.
pp 349-354 October 2000
Fine powders of zirconia were prepared by employing combustion method with varying fuel to pre-cursor molar ratios. The zirconia powders contained more amount of monoclinic phase as the fuel content was increased. This aspect was studied using XRD, IR and UV-vis diffuse reflectance techniques. The surface acid-base properties of these samples were also investigated by indicator titration method. The catalytic activity was probed with transfer hydrogenation reaction in liquid phase. It was found that combustion synthesized zirconia did not provide required active sites for transfer hydrogenation reactions in liquid phase unlike hydrous zirconia.
pp 355-359 October 2000
The hydroxides of Mg, Ni, Cu and Zn transform into layered double hydroxide (LDH)-like phases on ageing in solutions of Al or Cr salts. This reaction is similar to acid leaching and proceeds by a dissolution-reprecipitation mechanism offering a simple method of LDH synthesis, with implications for the accepted theories of formation of LDH minerals in the earth’s crust.
pp 361-367 October 2000
The dielectric properties of nanophase Ag2HgI4 and Ag2HgI4-Al2O3 nanocomposites at different frequencies have been studied over a temperature range covering the stability range of β phase of Ag2HgI4 and beyond the β to a phase transition temperature. έ′, tan δ and σa.c. of nanophase Ag2HgI4 and Ag2HgI4-Al2O3 nanocomposites were found to be larger than the reported values for polycrystalline pellets of Ag2HgI4. The dielectric properties of the nanocomposites were found to be a function of the wt.% of nano alumina. The observed changes are attributed to the grain boundary properties of nanophase materials and to the microsize space charge effects.
pp 369-375 October 2000
Results of differential scanning calorimetry (DSC) studies of Se85Te15−xPbx (x = 4, 6, 8 and 10) glasses have been reported and discussed in this paper. The results have been analyzed on the basis of structural relaxation equation, Matusita’s equation and modified Kissinger’s equation. The activation energies of structural relaxation lie in between 226 and 593 kJ/mol. The crystallization growth is found to be onedimensional for all compositions. The activation energies of crystallization are found to be 100–136 kJ/mol by Matusita’s equation while 102–139 kJ/mol by modified Kissinger’s equation. The Hruby number (indicator of ease of glass forming and higher stability) is the highest for Se85Te9Pb6 glass while S factor (indicator of resistance to devitrification) is highest for Se85Te7Pb8 glass at all heating rates in our experiment. Further the highest resistance to devitrification has the highest value of structural activation energy and the activation energy of crystallization is maximum for the most stable glass by both Matusita’s equation and the modified Kissinger’s equation.
pp 377-382 October 2000
Synthesis of machinable quality magnesium aluminium silicate (MgO-Al2O3-SiO2) for fabrication of insulators/spacers usable in high voltage applications under high vacuum conditions has been carried out following two different routes i.e. (i) sintering route, and (ii) glass route. A three-stage heating schedule involving calcination, nucleation and crystallization, has been evolved for the preparation of magnesium aluminium silicate (MAS) glass ceramic with MgF2 as a nucleating agent. The effect of sintering temperature on the density of compacted material was studied. Microstructure and machinability of samples obtained from both routes were investigated. They were also characterized for microhardness. Initial studies on material obtained by glass route reveal that these samples are superior to those obtained from sintered route in respect of their high voltage breakdown strength and outgassing behaviour. Outgassing rate of 10−9 Torr l·s−1 cm−2 and breakdown strength of 160 kV/cm were obtained. Different types of spacers, lugs, nuts and bolts have been prepared by direct machining of the indigenously developed glass ceramic.
pp 383-391 October 2000
A hydrogen-storage alloy of the composition Zr0.5Ti0.5V0.6Cr0.2Ni1.2 has been investigated for corrosion resistance and hydrogen-evolution reaction (HER) in KOH electrolyte of varying concentrations. Activation of the electrode by absorption of hydrogen takes place after prolonged cathodic polarization in the potential range of HER. Prior to activation, the open-circuit potential is about −0.4 V vs Hg/HgO, OH−, at which the alloy electrode tends to undergo corrosion with oxygen-reduction reaction (ORR) as the conjugate reaction. The corrosion-current density measured from Tafel polarization of ORR is found to be independent of KOH concentration and has an average value of about 30 μA cm−2. Subsequent to activation, the open circuit potential of the electrode is shifted to about −0.93 V vs Hg/HgO, OH−, which is equal to the reversible potential of HER. The exchange current density values measured from Tafel polarization of HER are marginally higher in relation to the values obtained before the electrode is activated. Alternating-current impedance spectra in the Nyquist form contain two overlapped semicircles. The high-frequency semicircle is attributed to the electrode geometry, while the low-frequency semicircle is due to the charge-transfer reaction and double-layer capacitance. The impedance data are analyzed by a non-linear least square curve fitting technique and impedance parameters are evaluated.
pp 393-398 October 2000
Austenitization process of three SG irons with varying compositions and as cast matrix microstructure has been studied at three austenitization temperatures of 850, 900 and 950C for different time periods. Microstructure, hardness and X-ray diffraction have been used to reveal the nature of dependence of the process on austenitization temperature, time and as cast structure. The optimum austenitization time is maximum for ferritic and minimum for pearlitic matrix.
pp 399-403 October 2000
The corrosion behaviour of two carbon-alloyed intermetallics of composition Fe-28.1Al-2.1C and Fe-27.5Al-3.7C has been studied and compared with that of binary intermetallics. Potentiodynamic polarization studies indicated that the intermetallics exhibited active-passive behaviour in an acidic solution of pH = 1, whereas they exhibited stable passivity in a buffer solution of pH 8.4. Corrosion rates were also obtained by immersion testing. The variation of corrosion rate as a function of time was similar for both the intermetallics. The variation in corrosion rate as a function of time has been explained based on the observed potentiodynamic polarization behaviour. Scanning electron microscopy of corroded surfaces indicated that the carbon-alloyed intermetallics were susceptible to galvanic corrosion, due to the presence of carbides.
pp 405-411 October 2000
In commercial practice, two-step ageing is commonly used in Al-Zn-Mg alloys to produce a fine dispersion of η′ precipitates to accentuate the mechanical properties and resistance to stress corrosion cracking. While this is true in Al-Zn-Mg alloys, two-step ageing leads to inferior properties in Al-Mg-Si alloys. This controversial behaviour in different alloys can be explained by Pashley’s Kinetic model. Pashley’s model addresses the stability of clusters after two-step ageing. In the development of the model, the surface energy term between cluster and matrix is taken into account while the coherency strains between the cluster and matrix are not considered. In the present work, a model is developed which takes into account the coherency strains between cluster and matrix and defines a new stability criterion, inclusive of strain energy term. Experiments were done on AA 7010 aluminium alloy by carrying out a two-step ageing treatment and the results fit the new stability criterion. Thus it is found that the new model for two-step ageing is verified in the case of Al-Zn-Mg alloy.
pp 413-418 October 2000
A series of interpenetrating polymer networks (IPNs) of castor oil based polyurethane/polyacrylonitrile (PU/PAN: 80/20, 60/40, 50/50, 40/60 and 20/80) were synthesized by condensation reaction of castor oil with methylene diisocyanate and acrylonitrile, employing benzoyl peroxide (BPO) and ethylene glycol dimethylacrylate (EGDM) as initiator and crosslinkers respectively. The physical, chemical, optical and some of the mechanical properties of PU/PAN were studied. Phase stabilization in IPNs was investigated by wide angle X-ray (WAXS) profile analysis. Variation of crystal size distribution was studied in these polymer networks.
pp 419-423 October 2000
An appreciable electric current is observed in a system consisting of a polyvinyl formal (PVF) film in a sandwich configuration, in the temperature range 30–110°C. The maximum value of the current during first heating is found to be of the order of 10−10 A and its thermograms exhibit one transition (i.e. current peak) at around 60°C. The position of the current peak in thermal spectrum shifts with the heating rate. A temperature dependence of the open circuit voltage is also observed. The activation energy of the process responsible for the current is determined. The magnitude of the current is more in the case of dissimilar electrode systems. It is proposed that the electric current arising from unpolarized metal-polymer-metal system is a water activated phenomenon, which is influenced by the transitional changes of the polymer.
pp 425-429 October 2000
A siliconized epoxy interpenetrating network (IPN) was synthesized from commercially available DGEBA epoxy resin GY250 (Ciba-Geigy, epoxy equivalent = 182–192, viscosity = 9000–12000 cP) and hydroxyl terminated polydimethylsiloxane (PDMS). PDMS and GY250 were thoroughly mixed at 30°C to get the prepolymer. Stoichiometric amounts of PDMS-epoxy prepolymer, γ-aminopropyltriethoxysilane, aliphatic amine curing agent (HY951), and dibutyltindilaurate catalyst, were thoroughly mixed and cast in a mould after evacuating the entrapped air. The cured material was then taken out and post cured at 70°C for 10 h. IPN was characterized by FTIR spectroscopy, SEM, DSC, TGA and viscosity measurements. Incorporation of PDMS in the epoxy matrix increased the viscosity and lowered the exotherm and pot-life. PDMS in IPN increasedTg, heat-distortion temperature and reduced the percentage weight loss with increase in temperature. Incorporation of PDMS drastically reduced the tensile and flexural strengths and hardness. By reducing the tensile and flexural modulus, the siloxane moiety effectively reduced the internal stress of IPN thereby improving its impact strength and percentage elongation. PDMS increased the electric potential gradient of IPN to withstand without breakdown. An increase in the tracking index and arc resistance of IPN were observed, because of the presence of Si-O-Si, which minimized the possibility of forming carbonized path. Volume and surface resistivities of IPN also increased with the incorporation of PDMS. The siliconized epoxy IPN, with better impact and thermal resistance, may be used in automobile and aerospace applications to withstand high temperature, and mechanical stress. The PDMS-epoxy IPN may be used for encapsulation, high temperature and high voltage application due to their low shrinkage and lesser internal stress. With the improved electrical characteristics, IPN may be used for high performance electrical insulation, insulator housings, and encapsulation to withstand high voltage, moisture, oxidation, chemical attack, biological attack, outdoor weathering, contamination, electrical, mechanical and thermal stress.
pp 431-437 October 2000
The system SrBi4Ti4−3xFe4xO15 belonging to bismuth layer structured ferroelectric (BLSF) materials withx = 0, 0.1 and 0.2 has been prepared through solid-state double sintering method. Increase of iron content in SrBi4Ti4O15 resulted in densification of the samples. The normal puckering observed in the sample has been found to decrease and the lattice distortion increased. Dielectric measurements indicate the ferroelectric transition temperature of the samples to be 530°C, 560°C and 606°C. The increase of transition temperature with increasing iron content is correlated with lattice distortion. Conductivity measurements show two slopes and the activation energies have been found to increase with the iron content. Impedance spectroscopy measurements confirm insulating behaviour at lower temperatures. With increase in temperature the samples become conducting and show relaxational effects. The Cole-Cole plots at higher temperatures are found to be semicircles. The relaxation times decrease with the increase of iron content.
pp 439-445 October 2000
Thermal conductivity of a set of (Bi0.8Pb0.2−yVy)2Sr2Ca2Cu3O10+δ (0 =y ≤ 0.05) pellets in the temperature range between 10 and 150 K is reported. Vanadium substitution influences strongly the magnitude of thermal conductivity (λ,) over the entire temperature range. But the nature of λ(T) dependence remains similar to that generally observed for HTSCs. The electronic contribution to the total λ in the normal state is estimated to be ∼ 25%. We have attempted to examine our data, assuming the role of both electrons and phonons in the origin of the λ(T) behaviour belowTc. Observed temperature variation of λ(T) for the present set of samples could be explained very well assuming this electron + approach. Some of the microscopic quantities estimated from the best-fit parameters give reasonable values.
pp 447-452 October 2000
The dielectric properties (dielectric constant and loss) for the system CuxFe3−xO4 with x = 1.0, 0.8, 0.6, 0.4 and 0.2, were studied in the temperature range 300 ∼ 800 K and also in the frequency range 1 kHz ∼ 1 MHz. A.c. conductivity was derived from dielectric constant and loss tangent data. The conduction in this system is interpreted as due to small polaron hopping. The dielectric relaxation was observed for the compositions with tetragonal structure whereas normal behaviour was observed for cubic structure.
Volume 42 | Issue 5