Volume 20, Issue 5
August 1997, pages 623-735
pp 623-635 August 1997
In 1973, Usikov and Zilbershtein proposed that theα(hcp) →ω (a three atom hexagonal) transformation in Zr and Ti proceeds via theβ(bcc, a high temperature phase) intermediate. Based on this they derived two non-equivalent orientation relationships (OR) betweenα andω phases. Their transmission electron microscopy (TEM) study carried out on these elements, that wereα →ω-transformed under static high pressure, revealed only one of the two proposed ORs. Various TEM studies done thereafter on these elements and their alloys (ω transformed under static pressures) conform to either one of these ORs. In a recent TEM study by Song and Gray on Zr,ω-transformed under shock compression, a new OR has been observed which according to them is different than those given by UZ and they put forth the directα →ω transformation mechanism. In the present study, we have generated additional TEM data on shock compressed Zr samples and have reconciled the above conflicting results. We find all our ORs (which contain the OR of SG also) to be described by the OR reported by UZ. The latter OR (i.e. of SG) is shown to be a subset of the former. These observations show that the same type of mechanism of transformation is operative both, under static and shock compression. Mechanism of the transition is discussed in terms of the required strains.
pp 637-642 August 1997
Linear thermal expansion coefficient, dielectric data and transition temperature of KH2PO4-H3BO3 binary are reported. The transition temperature obtained from differential scanning calorimetry (DSC) is in concurrence with the dielectric and thermal expansion data. Further, this transition is classified as first order. Thermal expansion coefficients and enthalpy values of the binary are reported and compared with the data of pure KH2PO4.
pp 643-650 August 1997
An analytical theory is proposed to study the dynamic nucleation of crystals from melt at very high cooling rates (10−6 to 10−12 °K/sec). The mathematical framework is found to be isomorphic with the function space theory, wave and matrix mechanics, which enables application of various approximate methods of the latter disciplines. In principle, the mathematical apparatus and concepts of function space and wave mechanics can be utilized to study the time varying nucleation process. The Arrhenius law has been used to extrapolate the self-diffusion coefficient as a function of temperature above the melting point than those below. Since, applicability of Arrhenius equation at very high degrees of supercooling is not known and has to be substituted with appropriate constitutive relationship based on free volume theory of transport, the conclusion derived from the present analysis will not be unique with respect to the certainty of crystallization during the solidification process.
pp 651-665 August 1997
Many organic semiconductors with conjugate bond structure possess photoconductivity. Conduction mechanism of organic materials exhibits ‘dualism’ since both intramolecular as well as inter-molecular aspects are involved in the excitation, absorption and transport of charge carriers. Modulation spectroscopy promises to be the most accurate method for analysis of organic photoconductors, especially of thin films. In this technique a periodic perturbation is applied to the material under study and the effect of the perturbation is separated from reflection or absorption while scanning through a given wavelength range by use of lock-in phase sensitive detection method. In electromodulation, particularly in electrolyte electromodulation, the applied field on the material produces changes in the dielectric function which corresponds to the change in reflectance. When the applied field is low the line-shape of spectrum is third-derivative like in comparison with the unmodulated reflectance spectrum. Using Aspnes three-point method the transitions corresponding to critical points can be determined. When the field is intermediate Franz-Keldysh oscillations, which are dc bias dependent, appear on the higher energy side of the transition energy from which the role of intra-molecular as well as intermolecular aspects in conduction mechanism can be understood and the carrier concentration could be determined. Though the electroreflectance method has been developed for inorganic semiconductors, it could be effectively applied for organic/molecular semiconductors as well if the constituent molecules are assumed to be the lattices. The study of organic photoconductors is very important since they are more and more promising especially in photocopying, photovoltaic and solar cells.
pp 667-675 August 1997
The elastic moduli (Y, η, σ andH) and some thermodynamical parameters, such as Debye temperatureθD, diffusion constantDi and latent heat of melting ΔHm, of ZnF2-PbO-TeO2 glasses doped with some rare earth (Pr3+, Nd3+, Eu3+ and Tb3+) ions are determined as functions of temperature using piezo electric composite oscillator technique. All the parameters are found to decrease with increase in the atomic number Z of the rare earth ions and with increase in the temperature of measurement. The results are explained on the basis of the density of localized bonding defect states within the band gap of the material.
pp 677-683 August 1997
The subject of the study was the influence of oxides PbO, ZrO2 and Cr2O3 immobilized in cement matrix. The obtained results show the positive effect of the present oxides in hydrating cement on compressive strength development and the quality of the formating pore structure of the resulting material.
A surprising effect showing the paralysis of the positive effect of heavy metal oxide admixture has been shown by the increasing of the added quantity of PbO (from 1 to 4%) and the used w/c ratio (0·7 instead of 0·4). The increase of the quantity overcoming the optimum of the reaction product of the interaction between hydrating cement and PbO added for the cause of the paralysis is supposed. The explanation of the adverse effect and the identification of the reaction product in question needs more detailed study.
pp 685-691 August 1997
An improvement in fracture toughness and strength is observed in carbon atmosphere sintered alumina on reheating the sintered samples in air at 1200°C. SEM photographs show the presence of secondary precipitate particles at grain boundaries of the sintered samples, which disappear after reheating in air. Infrared spectroscopy indicates that the C-O bond intensity increases on reheating in air. An AES/XPS study reveals that carbon enters the alumina lattice on reheating. These results lead to the conclusion that the enhanced fracture toughness and strength are related to the dissolution of precipitates and consequent dissolution of carbon in alumina lattice.
pp 693-697 August 1997
Investigations were done at dip-forming, contirod and SCR copper wire rods. These continuously cast and hot rolled copper wire rods (8 mm in diameter) were cold drawn in two drawing programmes (soft and hard drawn) to various diameters without any intermediate annealing. The results of influence of deformation and oxygen content on the flow stress, tensile elongation and number of twist to failure are given. Strength and strain hardening rates of hard-drawn wires always were higher than the soft-drawn wires. Oxygen effect on the ductility of copper wires measured with the tensile elongation and the number of twist to failure is not clear at large drawing prestrains.
pp 699-705 August 1997
The electrical transport behaviour of ferrocene mixed poly (methyl methacrylate) (PMMA) films (≈ 20 µm in thickness) deposited by the isothermal immersion technique has been studied in the temperature range of 333–373 K and field from (2·0–4·0)×104 V/cm. It has been found that at higher fields and temperatures, the observed conduction behaviour could be consistently described by the Richardson-Schottky emission. The increase in current due to doping has been attributed to the formation of charge transfer complexes. The dopant molecules act as an additional trapping centre and provide a link between polymer molecules in amorphous region leading to the formation of charge transfer complex.
pp 707-711 August 1997
The simple method for the preparation of phosphors such as aluminates, Y2O3, YVO4 and LaPO4 by the combustion synthesis described earlier in the literature is reported. It is shown here that the phosphors prepared by such synthesis do not possess all the required characteristics. Suitable modifications by way of annealing at >1200°C in controlled atmospheres, however, can yield the phosphors with characteristics which may fulfil the demands of lamp industry.
pp 713-717 August 1997
The room temperature diffusivity of hydrogen in a fully B2 ordered iron aluminide of composition Fe-35·8 Al was estimated from the experimental hydrogen depth profile to be 2·38×10−15m2/s. The mathematical procedure utilized for data analysis has been described. The estimated diffusivity is a lower bound value due to surface trapping effects.
pp 719-725 August 1997
Graphite was irradiated by CO2 laser of energy 10J and pulse width 4 µs. SEM and XRD examinations were carried out before and after laser irradiation. It was observed that XRD pattern changed after irradiation and new entities were seen on surface. Amorphous graphite was also irradiated and it was found that it became partly crystalline. The changes were explained on the basis of phase changes induced by high pressures and temperatures during laser irradiation.
pp 727-735 August 1997
Stability of polyurethane potting compounds based on castor oil/SMDI and caprolactone polyol/SMDI in repeated gamma radiation sterilization was studied. Radiation-induced degradation and leaching of low molecular weight fragments are higher in castor oil based polyurethane than in caprolactone polyol based polyurethane. For castor oil and caprolactone polyol based polyurethanes degradation increases up to 5 Mrad dose of sterilization. Further increase of dose of sterilization decreases leaching in caprolactone polyol based polyurethane which has resulted from secondary reactions leading to crosslinking. In the case of castor oil based polyurethane such crosslinks undergo cleavage at 10 Mrad dose of sterilization.
Volume 42 | Issue 6
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