Volume 20, Issue 2
April 1997, pages 141-286
pp 141-230 April 1997
Fullerenes which were thought to be ‘superaromatics’ are actually ‘superalkenes’. Reactions in fullerenes are varied, ranging from the addition of simple molecules like H2 to large molecules like dendrimers. The synthesis, structure, characterization, along with simple reactions like halogenation, oxygenation, metalation, cycloaddition, polymerization and dendrimer addition are discussed.
pp 231-238 April 1997
Thermodynamical feasibility study and the growth of layer structured transition metal dichalcogenide single crystals of WSe2, using iodine as transporting agent, has been reported in this paper. The characterization of the grown samples have been done by X-ray analysis.
pp 239-245 April 1997
A series of phosphorous-containing nadicimide end-capped resins, having different backbone, were prepared by reacting endo-5-norbornene- 2,3-dicarboxylic anhydride (nadic anhydride), pyromellitic dianhydride (PMDA)/3,3′,4,4′-benzophenone tetracarboxylic acid dianhydride (BTDA) andtris [3-azo(4-aminophenyl)] phosphine oxide (TAZP) in dimethyl acetamide. Yields were essentially quantitative. Structural characterization of resins was done by elemental analysis, IR and1H-NMR. Thermal behaviour of the resins was investigated by differential scanning calorimetry and thermogravimetric analysis, which revealed good high temperature performance. Isothermal ageing studies done at 340°C for various time periods showed good thermo-oxidative stability.
pp 247-257 April 1997
Pure spodumene and spodumene-zirconia (5, 10, 15 mol%) composite powders were prepared using aqueous sol-gel method employing lithium formate, aluminium formate, zirconium formate and tetraethoxy silane (TEOS) as starting materials in aqueous medium. The gels prepared by this method were dried at 100°C for 24h and then calcined for 2h at different temperatures ranging from 500°C to 800°C. X-ray powder diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis (TGA) and infra-red spectroscopy analysis (IR) were utilized to characterize the gel powders and calcined powders. Transmission electron microscope (TEM) was used to measure the average particle size of the calcined powders.
pp 259-264 April 1997
We have investigated the effects of oxygen deficiency (δ) on the transition temperature (Tc) of (Sm, Er)Ba2Cu3O7−δ superconductors by incorporating the effects of the two dimensional (2D) acoustic phonons and plasmons in the framework of strong coupling theory. The proposed approach for yttrium cuprates properly takes care of the double CuO2 plane in a unit cell and has been found earlier to be successful in describing the pairing mechanism as well as the variation ofTc withδ in Y Ba2Cu3O7−δ system. The coupling strength (λ), the screening parameter (μ*) and the two dimensional acoustic phonon (plasmon) energyħω−(ω+) as a function of oxygen deficiency is worked out. Finally, the transition temperature is evaluated and is found to be consistent with the earlier experimental data on yttrium cuprates. Thus, coupled phonon-plasmon mechanism is adequate to understand the nature of pairing mechanism and oxygen deficiency dependence of transition temperature in 90 K (Sm, Er)Ba2Cu3O7−δ superconductors.
pp 265-277 April 1997
Lead magnesium niobate (PMN) based binary with lead titanate (PT), lead iron niobate (PFN) and lead zinc niobate (PZN) and ternary with PZN-PT and PFN-PT solid solutions have been synthesized. In depth characterization using X-ray diffraction and SEM techniques have revealed the major perovskite phase formation. Amongst all the solid solutions, PMN-PZN-PT has given the highest values of permittivity as 19,740 and 23,700 withTc as 34°C when sintered at 1080°C and 1180°C respectively on the one hand and on the other, PFN containing solid solutions exhibited distinct deviation from the relaxor behaviour.
pp 279-282 April 1997
Heat treatment of poly nickel phthalocyanine leads to a charge transfer and hence there is an improvement in conductivity by 5 orders of magnitude. DC electrical conductivity measurements have been carried out in the heat-treated samples in the temperature range 270 K-10 K. Mott’s variable range hopping conduction in one dimension is observed in the temperature range 270K-77K and the conductivity remains constant in the temperature range 77K-10K.
pp 283-286 April 1997 Rapid Communication
Mullite (3Al2O3-2SiO2) powder was prepared by using an aqueous sol-gel method where aluminium formate and tetraethoxy silane (TEOS) and precipitated silica were used as the precursor compounds and water was used as reaction medium instead of the conventionally used solvent alcohol. The gels were calcined at 1000°C, 1200°C and 1300°C for 1 h and characterized by X-ray powder diffraction, infrared spectroscopy, differential thermal analysis, thermogravimetry analysis and transmission electron microscopy. This method is shown to provide homogeneous, reactive amorphous powder at comparatively low temperatures.
Volume 42 | Issue 5
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