Volume 22, Issue 6
October 1999, pages 937-1012
pp 937-945 October 1999 Quasicrystals
In recent decades some complex crystalline phases, as also many rational approximants to quasicrystalline phases with rather large unit cells, have been reported with orthorhombic symmetry in aluminium-transition metal (Al-TM) alloys. Furthermore, quite a few quasicrystalline phases, icosahedral as well as decagonal, forming in Al-TM alloys on normal or rapid solidification have been interpreted during the last decade as multiply-twinned orthorhombic crystals growing as superstructures of an orthorhombic cell that forms through welding in three perpendicular directions in the liquid state of 13-atom icosahedral clusters.
Following exemplification of this new approach to quasicrystals based on the analysis of the Debye-Scherrer diffraction data from the comparatively defect-free Al-Cu-Fe icosahedral phase, the three types of orthorhombic phases in aluminium-rich Al-TM alloys, numbering 36 in all, have been examined as icosahedral cluster compounds nucleating from icosahedral atomic clusters present in the molten alloys. A detailed analysis of their lattice parameters supports the postulate that all such phases can be viewed as complex and, occasionally, as very large superstructures of a small basic orthorhombic cell.
pp 947-951 October 1999 Quantum Well Studies
A series of InxGa1−xAs (x=0·47) quantum wells with InP barrier layers have been grown on InP substrates by metalorganic vapour phase epitaxy (MOVPE) at 625°C. The nominal well widths were defined during growth at (i) 25 Å, 39 Å, 78 Å and 150 Å for one sample and (ii) 78 Å for all 4 wells in another sample. The InP barrier widths have been kept constant at 150 Å. These layers have been characterized by X-ray diffraction (XRD) which from simulation gave the nominally 78 Å well width as 84 Å and the nominally 150 Å barrier width as 150·5 Å. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) have been carried out on etched and ion-milled samples for direct measurement of well and barrier widths. The well widths found from TEM are 25 Å, 40 Å, 75 Å and 150 Å. TEM micrographs revealed that, while the InP barrier layer is of good quality and the growth is confirmed to be epitaxial, dipoles are detected at the interface and the quantum well has some small disordered regions. These thickness measurements are in good agreement with earlier photoluminescence (PL) and secondary ion mass spectrometry (SIMS) studies.
pp 953-957 October 1999 Spray Pyrolysis
A large number of thin films of cadmium oxide have been prepared on glass substrates by spray pyrolysis method. The prepared films have uniform thickness varying from 200–600 nm and good adherence to the glass substrate. A systematic study has been made on the influence of thickness on resistivity, sheet resistance, carrier concentration and mobility of the films. The resistivity, sheet resistance, carrier concentration and mobility values varied from 1·56–5·72×10−3 Ω-cm, 128–189 Ω/□, 1·6–3·9×1021 cm−3 and 0·3–3 cm2/Vs, respectively for varying film thicknesses. A systematic increase in mobility with grain size clearly indicates the reduction of overall scattering of charge carriers at the grain boundaries. The large concentration of charge carriers and low mobility values have been attributed to the presence of Cd as an impurity in CdO microcrystallites. Using the optical transmission data, the band gap was estimated and found to vary from 2·20–2·42 eV. These films have transmittance around 77% and average reflectance is below 2·6% in the spectral range 350–850 nm. The films aren-type and polycrystalline in nature. SEM micrographs of the CdO films were taken and the films exhibit clear grains and grain boundary formation at a substrate temperature as low as 523 K.
pp 959-963 October 1999 Spray Pyrolysis
Liquid phase co-spray forming (LPCSF) technique was employed to produce Al-Pb and Al-Si-Pb alloys to show that it is possible, using this technique, to distribute lead into very fine-sized particles in Al/Al alloy matrix at low melt temperatures. Microstructural studies were carried out to explore the mechanisms governing lead distribution in the matrix of the alloys during processing. Results showed that, regardless of the alloy compositions and experimental conditions, the microstructures of the preforms exhibited great similarity, i.e. less uniform distribution of Pb particles in the base region, and uniform distribution of fine Pb particles in the equiaxed region. During LPCSF process, the behaviour of Pb droplets was similar to that of ceramic particles, except that the shape and size of liquid Pb phase varied corresponding to local solidification condition.
pp 965-969 October 1999 Synthesis And Crystal Growth
BaGd0·5(PO4)2: Eu0·53+ and ABaGd0·5(PO4)2: Eu0·53+ (where A+=Li, Na or K) double phosphates were synthesized by solid state diffusion method. These powders were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. XRD results showed many distinct lines in the XRD spectra. SEM measurement showed grains of different sizes in double phosphates. The Eu3+-ion emission lines for5D0→7Fj (j=1, 2, 3, and 4) transitions showed a splitting into 3, 2, 1, and 2 components, respectively. Strength of these lines was enhanced by the addition of alkali ion to BaGd0·5(PO4)2: Eu0·53+ double phosphate. However, as the alkali ion varied from Li+ to K+, the intensity of5D0→7F2 line (617 nm) in the red region increased at the cost of5D0→7F1 line (599 nm) in the orange region. Suitable explanation has been proposed for this phenomena.
pp 971-973 October 1999 Synthesis And Crystal Growth
Large single crystals of CsB3O5 (CBO), free of cracks and inclusions, were successfully grown by the Kyropoulos technique from a slightly Cs2)-enriched melt. The growth conditions were experimentally established. Maximum dimensions obtained were 45×41×44 mm fora×b×c-axes. Optical uniformity of these as-grown crystals was verified by means of striation observation, and the optical transparency characteristic was measured in the region from vacuum ultraviolet (VUV) to infrared (IR) wavelengths.
pp 975-979 October 1999 Phase Transitions
Polycrystalline samples of Ba5RTi3Nb7O30 [R=Nd, Eu, Gd], were prepared using high-temperature solid-state reaction technique. Preliminary X-ray structural analysis of the compounds shows the formation of single phase compounds (orthorhombic crystal system) at room temperature. Detailed studies of dielectric properties (ɛ, tanδ,σ) as a function of frequency (400 Hz to 10 kHz) and temperature (30° to 380°C) show that these compounds exhibit diffuse ferroelectric phase transition.
pp 981-986 October 1999 Phase Transitions
Vanadium oxide as well as molybdenum oxide thin films have been found to show electrochromism and metal insulator transitions. Incorporation of vanadium ion into molybdenum oxide lattice not only changes the lattice parameters but also shows prominent spectral changes in X-ray emission spectra. Kβ1-3 emission spectra of vanadium in different alloy films of V2O5-MoO3 system have been studied here and these have been compared with the X-ray emission spectra of similar composition of V2O5-MoO3 powders. For Kβ1-3 emission, the difference in values of asymmetry indices of thin films and powders is marginal, although considerable tailing towards high energy is observed in thin films. By curve fitting procedure, the Kβ1-3 emission spectra have been resolved to several gaussian peaks, and these have been explained on the basis of vacancy-induced electronic states, crossover transitions advocated in molecular orbital theory and plasmon oscillation.
pp 987-990 October 1999 Magnetic Materials
The nitriding thermochemical treatment (NTT) is commonly used for steels. In this paper, the experimental conditions required for NTT, and the influence of such treatments on the structure and hysteresis loops of Co74Fe8B12Si6 and Co74Fe4Mn4B12Si6 ribbons are reported. The results have been compared with those obtained with ribbons treated according to conventional thermal treatment (CTT) as well.
pp 991-997 October 1999 Magnetic Materials
Synthesis of nanocomposites of mixed oxides of Fe and Cr in a copolymer matrix of aniline and formaldehyde at room temperature along with IR, XRD and57Fe Mössbauer studies on as synthesized as well as the samples on heating at different temperatures is described. The XRD and57Fe Mössbauer studies show the formation of nanosized iron oxide particles. These studies further show the formation of nanosized particles of metal oxides and formation of solid solution of iron and chromium oxide on heating the samples at 700°C. Further, the IR studies show that the polymeric backbone is strongly influenced by different reaction conditions and lead to variable magnetic character in the heated samples as shown by their Mössbauer studies.
pp 999-1001 October 1999 Sensors
The recovery characteristics of optical hydrogen sensor, using Pd film, have been investigated in order to clarify their distinctive behaviour showing three-stage curve. The obvious two peaks observed in differentiated reflectance as a function of elapsed time during hydrogen desorption were found to correspond to the phase boundaries of Pd hydride. The phase transformation determined by these peaks was consistent with that observed within situ X-ray diffraction analysis. Moreover, the change in resistivity with hydrogen desorption indicated distinctive recovery curve, which was also connected with the phase transformation. These results clarified that the hydrogen desorption obviously consisted of three-stage process, and the distinctive behaviour was due to the phase transformation of Pd hydride.
pp 1003-1008 October 1999 Electrical Properties
Current-voltage (I–V) characteristics of pure polyvinyl formal (PVF) were investigated at different fields, range 5–100 kV/cm, as a function of temperature, range 313–363 K. It was observed that while at low fields (up to 25 kV/cm), the conduction was Ohm’s law-dependent at high fields (beyond 25 kV/cm), the conduction was Poole-Frenkel (P-F) mechanism-dependent. An attempt was made to identify the nature of the current by comparing its observed dependence on temperature, electric field and electrode materials with their respective characteristic features of the existing theories of electrical conduction. The current showed a strong dependence on temperature. To identify the possible mechanism of conduction, current versus square root of field characteristics were drawn with aluminium, silver, copper and gold as upper electrodes and Al as the lower electrodes. The observed characteristic suggested that the charge carriers were generated by the field-assisted lowering of coulombic barriers at the traps, and were subsequently conducted through the bulk of the material by a hopping process between the localized states by a Jonscher-Ansari-modified P-F mechanism. The calculated value of the modified P-F barrier was ⋍ 1·94×10−19 J (1·21 eV).
pp 1009-1012 October 1999 Mechanical Properties
The primary objective of this work was to evaluate the influence of various reinforcements and matrix materials on the dynamic hardness of polymer matrix materials (PMC). Therefore, the impact of a WC ball, using a gravity drop system, was studied on PMCs. The two parameters, coefficient of restitution and dynamic hardness, of PMCs were determined. The mechanical properties of the PMCs were evaluated, and the correlation of the two parameters with the mechanical properties of PMCs was attempted.
Volume 42 | Issue 6
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