Volume 9, Issue 2
June 1987, pages 75-158
pp 75-80 June 1987
Copper chloride on alumina catalyst used for oxychlorination of ethylene represents a typical system where copper is in a dynamic equilibrium between Cu+ and Cu++ oxidation states. Surface studies on redox, structural and compositional properties have been undertaken to monitor the changes that occur on the catalyst during use and pinpoint causes for its deactivation. For this, a fresh and a spent catalyst were been chosen for characterization. A core consisting of copper in Cu+ state and present as CuAlO2 is formed at the centre of the catalyst sphere. Formation of such a nonreversible phase under reaction conditions upsets the redox equilibrium. XPS also shows the absence of potassium in the core and its precipitation as a new phase on use, probably KCl, formed by the decomposition of active component KCuCl3. These factors facilitate deactivation.
pp 81-87 June 1987
An all-glass open photoacoustic cell through which gases can be passed without affecting the signal has been described. The cell has been characterized for various chopping frequencies and temperatures. Several heterogeneous catalytic reactions have been studied under conditions of gas flow at elevated temperatures and the potential application of this method in the field of heterogeneous catalysis has been pointed out.
pp 89-96 June 1987
Ammonium hydrogen tartrate single crystals have been prepared by the reaction of NH4Cl and tartaric acid in silica gel. The morphology of some as-grown symmetrical crystals has been studied by optical microscopy. The growth morphology studies indicate that the grown crystals are needle-shaped, orthorhombic disphenoidal and tetrahedral disphenoidal. The growth mechanism has been assessed.
pp 97-101 June 1987
Shrinkage measurement is important in the study of sintering to ascertain the operative mechanism. Surface area reduction is related to shrinkage with the help of which operative mechanisms at different temperatures are inferred. Densification during sintering is mainly related to volume diffusion, grain boundary diffusion and plastic flow.
pp 103-110 June 1987
Radial distribution analysis of x-ray intensities diffracted by chrysotile samples untreated and treated at different temperatures upto 900°C has been carried out. Interatomic distances, coordination numbers, mean square displacements and the interatomic coupling constants for different pairs of atoms have been calculated from the radial distribution curves. The interatomic distances and octahedral co-ordination number is found to decrease marginally upto 640°C and thereafter decrease steadily upto 800°C. The hydroxyl water is completely expelled from the structure and the original chrysotile structure breaks down. The entire process of dehydration has been interpreted in terms of RDF data.
pp 111-115 June 1987
Second harmonic generation and temperature autostabilized nonlinear dielectric element (TANDEL) effect have been studied for ferroelectric solid solutions (NaxK1−x)VO3 and (NaxLi1−x)VO3 in the vicinity of the Curie temperature. The generated second harmonic is linear for low biasing d.c. fields with zero off-set. The deviations from linear behaviour and a sharp decrease in the amplitude of second harmonic is observed at higher d.c. bias voltages. The TANDEL elements, in the autostabilized state, adjust their impedance against the variation of applied a.c. voltage. The observed zero off-set might be due to the presence of defects which give internal bias and generate second harmonic.
pp 117-121 June 1987
The a.c. conductivity of ionic materials shows two regions of frequency-dependent conductivity over a wide range of frequencies. Jonscher’s law of dielectric response for ionic conductors enables us to characterize the conductivities. The region of low frequency dispersion approximates to a frequency-independent plateau enabling us to obtain the d.c. conductivity. In some other conductors, the presence of low-frequency dispersion cannot be neglected while determining the effective d.c. conductivity. We have used this method to extract the d.c. conductivity and hopping rate as well as to estimate concentrations of the mobile ions (carriers) in some NASICON analogues.
pp 123-130 June 1987
Measurements of DC electrical resistivity and Seebeck coefficient on the perovskite system La1−xPbx−δ Co1−xTixO3−δ for 0·2≤x≤0·9 have been made in the temperature range 300–800 K. AC conductivity,σa.c, of all the samples were measured as a function of temperature (300–573 K) and frequency (1 kHz-1 MHz). DC resistivity behaviour of all the samples is similar. However, the resistivity value, which varies over 5–6 orders of magnitude, depends on both the compositionx and the structure of the samples. All the samples exhibitp-type electronic conduction. The value of Seebeck coefficient,α, for samples withx≤0·5 initially increases with temperature up to a particular temperature. Above this temperature, the behaviour ofα for samples withx≤0·5 and for samples withx>0·5 over the entire temperature range is similar to that of La CoO3. All the samples exhibit frequency-dependent a.c. conductivity at low temperatures.
pp 131-136 June 1987
Resistivity and thermoelectric power studies have been carried out on two semiconductor alloy systems viz Pb0·8Sn0·2Te and Pb0·6Sn0·4Te up to 35 kbar pressure. Thermoelectric power and resistivity data on Pb0·8Sn0·2Te indicate that the energy gapEg=EL−6−EL6+ decreases with pressure resulting in a zero gap state near 35 kbar pressure. TEP studies on the alloy system Pb0·6Sn0·4Te provide direct evidence for a pressure induced L6−→L6+ cross over transition.
pp 137-147 June 1987
Three types of Vishnukundin coins belonging to Andhra region were subjected to chemical and microstructural studies. The source of the ores used, the nature of the alloy and the techniques employed have been inferred from the data obtained. Trace elemental data have shown that the coins were made of unrefined metal which was also confirmed by electron microprobe results.
pp 149-158 June 1987
The phase boundaries of the Cr-Mn-O system have been investigated by alloy-oxide equilibria at 1173 and 1273 K and by isopiestic technique at 1323 K. The oxide phases which coexist in equilibrium with the Cr-Mn alloys are determined by x-ray diffraction studies. The results of the experiments indicate the presence of MnO in equilibrium with Mn-rich alloy whereas MnCr2O4 and Cr2O3 phases coexist with almost pure Cr. A three-phase equilibrium consisting of MnCr2O4 and MnO phases has been detected at the alloy composition XMn=0·252 at 1323 K. The composition of the alloy delineates the phase boundaries in the isothermal sections of the system. The results are interpreted by thermodynamic analysis of the Cr-Mn-O system using the data from the isopiestic measurements and those available in the literature.
Volume 42 | Issue 6
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