pp 435-440 October 2001 Crystal Growth
Experiments on the growth of mixed rare earth (didymium—a combination of La, Nd, Pr and Sm) molybdates in silica gel medium are reported. The optimum conditions conducive for the growth of these crystals are described and discussed. Concentration programming is reported to enhance the size of crystals by two-fold; the maximum size obtained being about 1 mm3. EDAX results suggest the crystals to be heptamolybdates of type R2Mo7O24, bearing composition La1.23Nd0.43Pr0.29 Sm0.05Mo7O24. The didymium molybdate crystals assume morphologies corresponding to those of spherulites, platelets, cuboids and coalesced crystals. Twinned structure in didymium molybdate crystals are also reported. It is explained that spherulitic morphologies result from aggregates of crystals joining in a spherical envelope. It is suggested that the crystals of didymium molybdates grow by two-dimensional spreading and piling up of layers.
pp 441-444 October 2001 Crystal Growth
Ni𝑥Mg1–𝑥SO4.7H2O single crystals were grown by the slow evaporation method from aqueous solutions. Density was measured by the floatation method. X-ray diffraction data were collected for powder samples and used for the estimation of lattice variation and thermal parameters like Debye–Waller factor, mean-square amplitude of vibration and Debye temperature. Lattice volumes approximately obey a relation similar to Retger’s rule. Values of thermal parameters do not follow any particular order with composition. The results obtained are reported.
pp 445-453 October 2001 Crystal Growth
Indium antimonide and gallium antimonide were synthesized from the respective component elements using an indigenously fabricated synthesis unit. Bulk crystals of indium antimonide and gallium antimonide were grown using both the vertical and horizontal Bridgman techniques. Effect of ampoule shapes and diameters on the crystallinity and homogeneity was studied. The grown crystals were characterized using X-ray analysis, EDAX, chemical etching, Hall effect and conductivity measurements. In the case of gallium antimonide, effect of dopants (Te and In) on transport and photoluminescence properties was investigated.
pp 455-459 October 2001 Crystal Growth
The rapid crystallization of KH2PO4 (KDP) from solution is demonstrated at a rate up to ≈7.5 mm/day along  and 22 mm/day along  in a crystallizer of 5 l capacity, using accelerated crucible rotation technique (ACRT) and simulated platform geometry for controlling the hydrodynamic conditions. On an experimental basis we have grown the crystals up to 40 × 43 × 66 mm3 size in about 3 days. Comparative analysis of the main structural and optical properties of crystals grown by conventional and rapid crystallization technique, is discussed.
pp 461-468 October 2001 Crystal Growth
Single crystals of calcium iodate and barium iodate were grown by simple gel technique by single diffusion method. The optimum conditions were established by varying various parameters such as pH of gel solution, gel concentration, gel setting time, concentration of the reactants etc. Crystals having different morphologies and habits were obtained. Prismatic, dendritic crystals of barium iodate and prismatic, needle shaped, hopper crystals of calcium iodate were obtained. Some of them were transparent, some transluscent, and few others were opaque. Both the crystals were studied using XRD, FT-IR, and thermal analysis. The crystals were doped by iron impurity. The effect of doping was studied using IR spectroscopy and thermal analysis.
pp 469-473 October 2001 Crystal Growth
Microhardness measurements were undertaken on twelve rare earth garnet crystals. In yttrium aluminium garnet and gadolinium gallium garnet, there was no measurable difference in the hardness values of pure and nominally Nd-doped crystals. The hardness values were correlated with the lattice and elastic constants. An analysis of hardness data in terms of the interatomic binding indicated a high degree of covalency.
pp 475-482 October 2001 Crystal Growth
The growth of Sb-based crystals (InSb, GaSb etc) was undertaken using resistive heater furnace by vertical directional solidification (VDS) technique. Crystal–melt interface shape during the growth was shown to convert from concave to convex along the crystal axis of the ingots. Many antimonide (Sb) crystals of 8 mm to 18 mm diameter were grown by optimized growth parameters. The forced convection and absence of conducting support to ampoule showed improvement in crystal quality of as grown ingots. Crystals showed preferred orientation and self-seeding. Results on interface shape and crystallinity of ingots were found to be in good agreement with the experiments.
pp 483-485 October 2001 Electrical Properties
The solid solutions of Sn1–𝑥Ti𝑥O2 (0.0 < 𝑥 < 0.25) were prepared by the usual high temperature ceramic processing techniques with small amounts of dopants such as Nb, Co, Al. The electrical measurements (log 𝐼–log 𝑉) on the ohmically metal electroded sintered pellets showed the non-linear behaviour. The nonlinear coefficient was found to decrease from 12 to 3 with increase in 𝑥. On the other hand, the breakdown voltage (𝐸b) showed the increase with the increase in 𝑥. This complex electrical behaviour is explained on the effects of the three dopants (as varistor former, performance enhancer and highlighter) on the microstructural features of the dense pellet.
pp 487-495 October 2001 Electrical Properties
The compound, LaBi5Fe2Ti3O18, is a five-layered material belonging to the family of bismuth layered structure ferroelectromagnetics. D.c. and a.c. conductivity measurements were performed on the samples. Dielectric measurements were also performed on these samples. Combined impedance and modulus plots were used as tools to analyse the sample behaviour as a function of frequency. Cole–Cole plots showed non-Debye relaxation.
pp 497-504 October 2001 Electrical Properties
The effect of lanthanum doping is studied on ferroelectric properties of Ba1–𝑥La𝑥TiO3 with 𝑥 = 0.0005, 0.001, 0.003 prepared through solid state sintering route. Dielectric and impedance spectroscopic studies have been carried out. The tetragonal distortion of the unit cell decreased and ferroelectric transition temperature, 𝑇c increased with the increase of lanthanum content.
Combined impedance and admittance spectroscopy was used to analyse impedance data. The electromechanical parameters were calculated from the resonant and anti-resonant frequencies from vector admittance plots. The electromechanical coefficients for Ba1–𝑥La𝑥TiO3 with 𝑥 = 0.003 were found to be much larger than that of pure barium titanate.
pp 505-509 October 2001 Electrical Properties
Dielectric properties such as dielectric constant (𝜀') and dielectric loss tangent (tan ⋹ 𝛿) of mixed Mn–Zn–Er ferrites having the compositional formula Mn0.58Zn0.37Fe2.05–𝑥Er𝑥O4 (where 𝑥 = 0.2, 0.4, 0.6, 0.8 and 1.0) were measured at room temperature in the frequency range 1–13 MHz using a HP 4192A impedance analyser. Plots of dielectric constant (𝜀') vs frequency show a normal dielectric behaviour of spinel ferrites. The frequency dependence of dielectric loss tangent (tan 𝛿) was found to be abnormal, giving a peak at certain frequency for all mixed Mn–Zn–Er ferrites. A qualitative explanation is given for the composition and frequency dependence of the dielectric constant and dielectric loss tangent. Plots of dielectric constant vs temperature have shown a transition near the Curie temperature for all the samples of Mn–Zn–Er ferrites. However, Mn0.58Zn0.37Er1.0Fe1.05O4 does not show a transition. On the basis of these results an explanation for the dielectric mechanism in Mn–Zn–Er ferrites is suggested.
pp 511-514 October 2001 Electrical Properties
Pure and impurity added (with urea and thiourea) KDP single crystals were grown by the gel method using silica gels. Electrical conductivity measurements were carried out along both the unique axis and perpendicular directions at various temperatures ranging from 30 to 140°C by the conventional two-probe method. The present study shows that the conductivity in KDP crystals, for both the impurities considered, increases with the increase in impurity concentration and temperature. Activation energies were also determined and reported.
pp 515-521 October 2001 Magnetic Materials
Magnetic sub-micron sized particles (with diameters in the range 100–600 nm) of Co, Ni and Co–Ni alloys, protected with polyvinylpyrrolidone have been prepared in gram quantities using the polyol process. Experiments carried out with different metal precursors and starting compositions have yielded reliable routes to produce particles of the desired diameters in the 100–600 nm range. The particles were characterized with X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, thermogravimetric analysis and magnetic measurements. The particles are found to be stable under ambient conditions indefinitely. The coercivity values of the Co and Ni particles are ∼ 50% higher compared to the corresponding bulk values. The alloy particles follow a trend similar to the bulk alloys.
pp 523-528 October 2001 Superconductors
In this paper we have reported investigations on the effect of simultaneous substitution of Bi and Tl at the H𝑔 site in the oxygen deficient H𝑔O𝛿 layer of H𝑔Ba2Ca2Cu3O8+𝛿 cuprate superconductor. Bulk polycrystalline samples have been prepared by the two-step solid state reaction process (precursor route). It has been observed that the as grown H𝑔Bi0.2–𝑥Tl𝑥Ba2Ca2Cu3O8+𝛿 (with 𝑥 = 0.00, 0.05, 0.10, 0.15, 0.20) corresponds to the 1223 phase. It has been found that the 𝑇c varies with the average cationic size $\langle R_d \rangle$ of the dopantcations. The optimum 𝑇c of ∼ 131 K has been found for the composition H𝑔Bi0.15Tl0.05Ba2Ca2Cu3O8+𝛿. This composition leads to the average dopant cation size of ∼ 1.108 Å which is very close to the size of H𝑔2+ (∼ 1.11 Å). The microstructure for H𝑔Bi0.15Tl0.05Ba2Ca2Cu3O8+𝛿 has been found to be most dense and this phase exhibits the highest stability. The 𝐽c of the optimum material H𝑔Bi0.15Tl0.05Ba2Ca2Cu3O8+𝛿 is found to be ∼ 1.29 × 103 A/cm2 at 77 K.
pp 529-534 October 2001 Polymers
Physical and chemical responses of 70 MeV carbon ion irradiated Kapton-H polymer were studied by using UV-visible, FTIR and XRD techniques. The ion fluences ranging from 9.3 × 1011–9 × 1013 ions cm–2 were used. Recorded UV-visible spectra clearly showed a decrease in absorption initially with fluence, but for the higher fluences it showed a recovery characteristic. A decrease in band-gap energy of 0.07 eV was observed. The FTIR analysis indicated the high resistance to radiation induced degradation of polymer. The diffraction pattern of Kapton-H indicates that this polymer is semi-crystalline in its nature. In case of irradiated one, there was an average increase of crystallite size by 20%, but diffuse pattern indicates that there was a decrease in crystallinity, which may be attributed to the formation of complex structure induced by the cross-linking of the polymeric chains.
pp 535-538 October 2001 Polymers
Interpenetrating polymer networks (IPNs) of glycerol modified castor oil polyurethane (GC–PU) and poly[2-hydroxyethylmethacrylate] (PHEMA) were synthesized using benzoyl peroxide as initiator and N,N-methylene bis acrylamide as crosslinker. GC–PU/PHEMA interpenetrating polymer networks were obtained by transfer moulding. These were characterized with respect to their resistance to chemical reagents and mechanical properties such as tensile strength, per cent elongation and shore A hardness. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were undertaken for thermal characterization. The changes in NCO/OH ratio and GC–PU/PHEMA composition on the properties of the IPNs were studied.
pp 539-545 October 2001 Glasses
Optical properties of chlorophosphate glasses of the type 50P2O5–30Na2HPO4–20RCl (R = potassium and lead) activated by 0.1 and 0.5 mol% of Pr3+ and Nd3+ have been investigated. Optical band gaps (𝐸opt) have been reported for 0.1 and 0.5 mol% concentrations of Pr3+ and Nd3+ doped potassium and lead chlorophosphate glasses. Energy levels and optical transitions of Pr3+ and Nd3+ are assigned. Spectroscopic parameters (𝐸1, 𝐸2, 𝐸3, 𝜉4f and 𝛼), spectral intensities (𝑓expt), Judd–Ofelt intensity parameters (𝛺2, 𝛺4 and 𝛺6) and radiative lifetimes (𝜏R) are correlated for 0.1 and 0.5 mol% concentrations of these two ions in potassium and lead chlorophosphate glasses
pp 547-554 October 2001 Rapid Solidifications
Thermal history of droplets associated with gas atomization of melt has been investigated. A mathematical model, based on classical theory of heterogeneous nucleation and volume separation of nucleants among droplets size distribution, is described to predict undercooling of droplets. Newtonian heat flow condition coupled with velocity dependent heat transfer coefficient is used to obtain cooling rate before and after nucleation of droplets. The results indicate that temperature profile of droplets in the spray during recalescence, segregated and eutectic solidification regimes is dependent on their size and related undercooling. The interface temperature during solidification of undercooled droplets rapidly approaches the liquidus temperature of the alloy with a subsequent decrease in solid–liquid interface velocity. A comparison in cooling rates of atomized powder particles estimated from secondary dendrite arm spacing measurements are observed to be closer to those predicted from the model during segregated solidification regime of large size droplets.
pp 555-558 October 2001 Corrosion Studies
Quantitative data on corroding steel reinforcement in reinforced concrete structures are undoubtedly very useful for evaluation of their service life and timely repairs. The method of electrode potential measurement is a convenient and simple test for this purpose, but it provides no quantitative data on corrosion rate and only information regarding active or passive state of steel reinforcement can be obtained. We show here the possibility of obtaining quantitative data on degree of corrosion of steel reinforcement by a potentiodynamic method. The developed method is based on experimentally estimated mathematical relation between the results of potentiodynamic method and degree of corrosion of steel reinforcement. It is possible to calculate the degree of corrosion of steel reinforcement using this mathematical relation and the measured values of current density by the potentiodynamic method.
pp 559-562 October 2001 Hydrogen Embrittlement
The critical hydrogen concentration for hydrogen embrittlement in iron aluminide, Fe3Al has been estimated (0.42 wppm). The estimated critical hydrogen content has been correlated to structural aspects of the decohesion mechanism of hydrogen embrittlement.
Volume 42 | Issue 5
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