Volume 20, Issue 1
February 1997, pages 1-140
pp 1-7 February 1997
Carbon onions are found along with carbon nanotubes and other carbon nanoparticles in the cathodic deposit in the arc-vaporization of graphite. Atomic force microscopy has been used to characterize these particles on the basis of their sizes and shapes. Onion-like particles have three-dimensional, near spherical structure and are distinct from two-dimensional graphitic particles. The spherical shape and height to diameter ratios obtained using atomic force microscope, afford a distinction between onion-like structures and other carbon nanoparticles.
pp 9-22 February 1997
Interruption of growth and H-plasma exposure on stacking layers of Si:H film resulted in a remarkable change in material properties. Widening of optical gap and increase in dark conductivity were simultaneous with the reduction in photoconductivity, bonded hydrogen content and optical absorption. An associated change in the network structure from amorphous towards crystalline was observed. Enhanced dose of plasma exposure resulted in the gradual lowering in the size of nanograins and increase in their number density. Systematic widening in optical gap during dehydrogenation of the network appears to be a unique feature related to amorphous semiconductors, which suggests nanocrystallization and quantum size effect in hydrogenated binary alloy.
pp 23-25 February 1997
A new phase of Ba(Mg1/3Ta2/3)O3 has been obtained by calcination at 1400°C. It belongs to the cubicPm3m witha = 4·08840(4)Å and the crystal structure has been found to be the ideal perovskite by the Rietveld method from powder X-ray diffraction data.
pp 27-35 February 1997
α-Fe2O3 sols were prepared by forced hydrolysis method. The effect of time of aging, concentrations of ferric chloride and hydrochloric acid on the size, shape and percent yield were studied. Static light scattering was employed to determine the spherical shape, further confirmed by cryo-SEM technique.
pp 37-48 February 1997
The mixed crystals of neodymium praseodymium oxalate are grown by the diffusion of a mixture of aqueous solutions of neodymium nitrate and praseodymium nitrate (as an upper reactant) into the set gel embedded with oxalic acid. By varying the concentration (by volume) of rare earth nitrates in the upper reactant, the incorporation of Nd and Pr in the mixed crystals has been studied. Tabular crystals with the well defined hexagonal basal planes are observed in the mixed crystals of varying concentrations. X-ray diffraction patterns of these powdered samples reveal that these mixed crystals are ‘isostructural’, while IR and FTIR spectra establish the presence of oxalate groups. TGA and DSC analyses show the correctness of the chemical formula for the mixed crystals, by the release of water molecules (endothermic) and of CO and CO2 (exothermic), with the rare earth oxides as the stable residue. X-ray fluorescence (XRF) and energy dispersive X-ray analyses (EDAX) establish the presence of heavy rare earth elements qualitatively and to a good extent quantitatively. X-ray photoelectron spectroscopic (XPS) studies confirm the presence of rare earth elements (Nd and Pr) as their respective oxides. The findings of these techniques of characterization are in excellent agreement with the proposed empirical structure for the mixed rare earth oxalates. The implications are discussed.
pp 49-66 February 1997
Optically transparent alumina sonogels with optical transmittance of over 90% have been prepared by chemically modifying a partially hydrolysed aluminium sec-butoxide precursor with ethyl acetoacetate. Chelating the alkoxide in a 1:2 molar ratio has permitted an enhanced control over microstructural changes during the sol to sonogel synthesis. Particle size and morphology changes during controlled sonogelling indicate that the sol to sonogel evolution in chelated alkoxides occurs by a dissolution-nucleation process in which the chelated species dissolves and aluminium hydroxide nucleates. The chelated agglomerates break down with increasing amount of water, and finally dissolve into the alcohol rich medium. The breakdown of chelated agglomerates is accompanied by the formation of nanosized aluminium hydroxide particles which grow with water addition into aggregated micron sized, spherical species. The chelated agglomerates evolve from a spherical geometry to a cylindrical morphology to a tapelike structure with a preferred orientation, and finally to a wavy sheet-like matrix. The nano-aluminium hydroxide particles increase in size at every water addition and grow to about 1 μm at the sonogel point.
pp 67-77 February 1997
Glasses in the system (65 −x) [SrO·TiO2] − (35) [2SiO2·B2O3] − (x) [Bi2O3] wherex = 1, 5, 10 (wt%) prepared by melting in alumina crucible (1375–1575 K), were subjected to different heat treatment schedules followed by DTA studies. Crystallization study showed the formation of Sr2B2O5 as major phase at low temperature (≈950°C) heat treatment. At high temperatures, TiO2 and SrTiO3 with or without Sr2B2O5 crystallize out depending on heat treatment. In this paper, the influence of variation in composition, thermal treatment on the nature of crystallizing phases as well as on the resulting microstructures are investigated through XRD, IR and SEM. Uniform crystallization was achieved by suitable addition of Bi2O3 and proper heat treatment.
pp 79-92 February 1997
Results of dielectric and conduction properties of vacuum evaporated tellurium (Te) thin film capacitors (Al-Te-Al) have been reported in the frequency range 1–100 kHz at various temperatures (303–423 K). Loss factor (tanδ) which shows a maximum with frequency increases with rise of temperature and tanδmax shift towards high frequency region. The large values of capacitance and dielectric constant (ɛ′) in the low frequency region indicate the possibility of an interfacial polarization mechanism.I-V characteristics show ohmic, space charge limited (SCLC) and thermionic emission conduction mechanisms to operate at low, intermediate and high voltages respectively. Various transport parameters have been calculated. It has been observed that the Schottky type of conduction is predominant in the high field region and the Schottky barrier height has been determined. The Hall coefficient, Hall mobility and carrier concentration are also discussed.
pp 93-101 February 1997
The alternating current (a.c.) low field susceptibility vs temperature, magnetization and57Fe Mössbauer effect measurements are reported for the spinel solid solution series MgxCo1−xFe2O4 synthesized by a wet-chemical method before and after high temperature annealing. The observed features for the wet samples, such as the coexistence of paramagnetic doublet and magnetic sextets in Mössbauer spectra and lower saturation magnetization values confirm small particle ferrite behaviour. Especially, Mössbauer spectra of wet samples reveal the presence of superparamagnetic particles which exist simultaneously with ferrimagnetic regions in the materials well supported by a.c. susceptibility data. The high temperature annealing changes the wet-prepared ferrites into the ordered magnetic structure of ceramic ferrites.
pp 103-109 February 1997
Step-wise thermal dehydration of WO3·2H2O has been investigated and the various hydrated tungsten trioxide phases characterized. A cubic phase of tungsten trioxide with 0·36 moles of water is obtained by heating WO3·2H2O to 498 K.
pp 111-123 February 1997
The paper presents a ballistic performance index for metallic armour materials in terms of the commonly determined mechanical properties such as strength and modulus. The index is derived using an energy-balance approach, where the kinetic energy of the projectile is assumed to be absorbed by the elastic and the plastic deformation involved in the penetration process as well as the kinetic energy imparted to the target material during deformation. The derivation assumes two distinct stages to exist during the penetration of the projectile. At the striking face of the armour, the material is assumed to flow radially in a constrained deformation region but longitudinally at the rear surface leading to typically observed bulging of the armour without constraint. The index is validated using the available experimental and empirical data obtained in the case of small arm projectiles for an impact velocity of about 800 m/sec. This index is expected to facilitate the development of metallic armour, since the number of the ballistic experiments can be reduced significantly and only the promising materials need to be considered.
pp 125-133 February 1997
The effect of electrolyte temperature on the polarization characteristics of an Al-Li alloy of composition Al-1·90 Li-1·80 Cu-1·00Mg-0·09Zr in 0·1 M NaOH solution was studied by the potentiodynamic polarization method. The electrolyte temperature was the equilibrium temperature established due to the prevailing weather conditions at different times in the year. The alloys exhibited active-passive behaviour at all the temperatures. It was found that the temperature of electrolyte affected the polarization behaviour of the alloys. The zero current potential and the open circuit potential were nobler with decreasing temperature. The corrosion current density, critical current density and passive current density increased with temperature, indicating the activated nature of the electrochemical process. The passive range was higher at 35°C compared to other temperatures. The temperature effect on polarization behaviour has also been explained by considering the dissolved oxygen concentration in the electrolyte at different temperatures. The ennoblement of Cu on the Al-Li alloy surface has also been addressed.
pp 135-140 February 1997 Rapid Communication
A biological study is reported where implants ofβ-TCP/t-ZrO2 has been examined in rabbit mandibles. The composite is found to be fully biocompatible and evidence of good osteoconduction and tissue ingress has been noticed.
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