Volume 19, Issue 5
October 1996, pages 717-835
pp 717-723 October 1996
Arcs of diffuse intensity appear in various shapes and positions in the diffraction patterns from the icosahedral phase, violating the parity rule for simple icosahedral (SI) symmetry. In the process of annealing treatment, the diffuse spots also evolve in the centre of the arcs and become sharp. These extra diffuse spots change the symmetry of the quasilattice fromP-type toF-type. The ordered and disordered structures in quasicrystal have been linked to the ordered and disordered structures present in the crystalline α (Al-Mn-Si) and α (Al-Fe-Si) alloys.
pp 725-729 October 1996
Porous silicon (PS) was formed on both polished and texturized single crystal silicon (100) by anodic etching. Photoluminescences (PL) from both of these silicon surfaces were measured and compared. A two-fold enhancement of PL from textured silicon surface was obtained. This enhancement could be ascribed to the geometry of the textured surface.
pp 731-736 October 1996
Dielectric constant and dissipation factor of lead phthalocyanine have been experimentally studied here as functions of frequency and temperature. Dielectric constant increases by one order of magnitude with temperature showing pyroelectric behaviour. It is explained that molecular distortion leads to increased dielectric constant.
pp 737-747 October 1996
The nature of pairing mechanism as well as transition temperature of yttrium cuprates is discussed using the strong coupling theory. An interaction potential has been developed for the layered structure with two conducting CuO2(a–b) layers in a unit cell. The interaction potential properly takes care of electron-electron, electron-phonon and electron-plasmon interactions. Furthermore, the electron-phonon coupling parameter (λ), the modified Coulomb repulsive parameter (μ*) and the 2D acoustic phonon (plasmon) energy as a function of oxygen deficiency is worked out. Finally, the superconducting transition temperature (Tc) is then evaluated by using these coupling parameters and obtainedTc = 95(92)K for Y(Yb)Ba2Cu3O7−δ superconductors withδ = 0·0. The model parameters estimated from the layered structure approach are consistent with the strong coupling theory. The result deduced on the variation ofTc withδ are in fair agreement with the earlier reported data on yttrium cuprates. The analysis of the above results are discussed.
pp 749-758 October 1996
Impact tests were carried out on five glassceramics tiles. The ballistic resistance was evaluated by the depth of penetration of the projectile into the backing. The fragmentation behaviour of the glassceramics showed that the glassceramics compared with with alumina and displayed some advantages over it.
pp 759-765 October 1996
Fracture mechanics studies on glass by indentation has become predominant in recent times. Interest in it has become more prominent due to a large application of such materials in areas of engineering applications encountering fluctuating stresses induced thermally, mechanically or physically. However, glass subjected to repeated indentation at a point prior to crack initiation with subcritical loads phenomenological to metal fatigue has not been systematically investigated. Repeated indentation at a single point with different subcritical loads (0·1N, 0·15N, 0·25N, 0·50N, 1·0N) was performed till radial cracks occurred. The length of the diagonal was measured after each indentation, which was found to increase with indentation cycle eventually leading to crack initiation. This observation was analysed considering the elastic plastic component and the residual stress developed during each cycle. A mathematical model has been postulated to correlate the contribution of cumulative residual stress for crack initiation.
pp 767-774 October 1996
Studies on lattice parameters, magnetization and ac susceptibility measurements for the ferrite system CdxMg1−xGdyFe2−yO4 (withx = 0·2, 0·3, 0·4 andy = 0·1, 0·2, 0·3, 0·4) are presented. The ac susceptibility was measured from room temperature to 800K at a constant magnetic field of 7 oersted.Xac vsT variations show that all the samples contain predominantly multidomain (MD) particles. Magnetization measurements of the system indicate that as the Cd2+ content increases magnetization increases while the addition of Gd3+ reduces the magnetization. The observations further indicate the existence of Y-K type of magnetic ordering in the system.
pp 775-789 October 1996
The photoluminescence spectra at atmospheric pressure and high pressure photoluminescence spectra of the tetracyanoplatinate salts (MCP) KLiCP·2H2O, KNaCP·3H2O, Cs2Ca(CP)2·nH2O and Cs2CP·H2O encapsulated sol-gel glasses are reported. In all these sol-gel glass systems we have found large pressure dependent shift of the photoluminescence maxima under hydrostatic high pressure condition (up to 13 nm/kbar) and therefore, prove to be as most sensitive probes to enable us to calculate pressure at different stages of gelation, aging and drying of the gel glass. The pressure dependent shift, along with the position of the peak maxima and band width at half height, are measured for the photoluminescence spectra at atmospheric pressure as well as at high pressure. Concentration effect has also been studied in the present system to arrive at more conclusive picture of the dopant matrix and dopant-dopant interaction. The fluorescence life time, as well as integrated emission intensity as a function of concentration, have also been studied in all the four glass systems which allows us to identify the dimer and cluster spectra.
pp 791-798 October 1996
Large single crystals of an organic nonlinear optical (NLO) material, benzil, have been grown by employing an indigenously built Bridgman-Stockbarger crystal grwoth system. The dielectric constant (εr), dielectric loss tangent (tanδ) and impedance (Z) of these crystals, monitored along the crystallographica-axis, as a function of frequency in the 100kHz-10MHz range, exhibit sharp resonance effects in the 100–300kHz region. These strong resonance effects are attributed to the piezoelectric nature of the crystal. The origin of the appearance of the sharp peaks in the frequency variation ofεr and tanδ is in response to the elastic resonances induced via the externally applied electric field. The resonance peak positions shift towards lower frequencies with increase in temperature as a consequence of the decrease in the stiffness coefficient (C11) of the crystal.
pp 799-806 October 1996
A single crystal of copper of size 80 mm dia. and 50 mm length was grown by the Bridgman method. Neutron diffraction investigations were carried out to determine the mosaic spread and reflectivity of this as grown crystal. The crystal was found to be of good quality having structure free and symmetric rocking curves with mosaic spread of 14 min.arc and reflectivity of ∼ 50%.
pp 807-821 October 1996
The interaction of different metal oxides such as Co3O4, NiO, Al2O3, Cr2O3, Fe2O3 and SiO2 with Na2SO4 at a temperature of 1100 and 1200 K in flowing oxygen has been studied. The thermogravimetric studies for each system were carried out as a function of Na2SO4 in the mixture. The presence of different constituents in the reaction products were identified by X-ray diffraction analysis and the morphologies of the reaction products were characterized using metallography and scanning electron microscopy (SEM). The formation of products was also investigated by thermodynamic computation of free energies of the reactions and the study of relevant equilibrium phase diagrams. The soluble species in the aqueous solutions of the reaction products were determined quantitatively using atomic absorption spectrophotometry.
The high temperature interaction products usually contain a 3-phase structure namely, Na2O·M2Ox, M2Ox and metal sulphide and/or metal sulphate. The formation of Na2O·M2Ox depends upon the solid state solubility of metal oxide in the molten salt at high temperatures. Under limited solubility conditions Na2O·M2Ox is invariably formed, but as soon as this condition is relaxed the oxide. M2Ox, precipitates and forms a separate phase.
pp 823-829 October 1996
The paper gives a method of model XLPE medium voltage cable insulation testing for partial discharges and electrical treeing with a point to plane test geometry. Based on this method, a comparative estimate of insulation resistance to treeing and partial discharges have been made. XLPE crosslinked by different methods (steam medium and hot nitrogen medium) and with varying contents of inorganic filler kaolin were tested. The characteristics of partial discharges were measured by means of a pulse height analyser characterized with a microscope. The data have been analysed statistically.
pp 831-835 October 1996
The potentiodynamic polarization curves of Fe3Al and Fe3Al + Cr intermetallics obtained in aerated pH 4 H2SO4 acidic solution have been theoretically analyzed. The role of chromium in minimizing the hydrogen embrittlement (HE) of the intermetallic Fe3Al (resulting in its poor ductility) has been addressed based on the analysis. In the case of the chromium-alloyed iron aluminide, calculations indicate that hydrogen liberation does not occur on the surface due to the shift of the corrosion mixed potential to a value nobler than the electrode potential for the hydrogen evolution reaction. This shift occurs due to the induction of passivity on alloying with Cr resulting in the formation of a passive film. The minimization of HE of iron aluminides on alloying with Cr can thus be understood.
Volume 42 | Issue 5
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