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Notes: We have synthesized La0.83Na0.11MnO2.93 by heating La2O3 and MnCO3 in NaCl melt at 900 °C. The exact composition was arrived by analyzing each ion by an independent chemical method. The compound crystallized in a rhombohedral structure and showed an insulator-to-metal transition at 290 K. Epitaxial thin films were fabricated on LaAlO3 (100) using a pulsed laser deposition technique. The film also showed an insulator-to-metal transition at 290 K. Magnetoresistance [ΔR/R0=(RH−R0)/R0] was −71% near the insulator-to-metal transition temperature of 290 K at 6 T magnetic field. © 1997 American Institute of Physics.

Notes: Models of retrograde metamorphism in many orogenic belts are based on crustal thickening and erosion of continental lithosphere. The pressure and temperature evolutions in such models are generally obtained by solving the advection diffusion equation with the lithosphere-asthenosphere boundary (LAB) either fixed at a specified depth or moving downwards at the same rate as erosion from the surface. However, this boundary is defined as a solid-partial-melt boundary in many geophysical interpretations. In the present work, we discuss a solution of the above problem considering the LAB as a phase boundary that moves. This solution is obtained by the Fourier series approach for a general case of surface erosion and basal heat transport. The results obtained for different models of crustal thickening show a significant movement of the LAB in response to erosion from the surface. A corresponding variation in the lithospheric thickness is also a significant result of this analysis. The earlier notion of a fixed-base lithosphere seems to be a good approximation when analysing metamorphic data, as we obtain nearly the same temperature profile in the crust as after including the LAB motion effect. However, the erosion of thickened crust is found to affect the lithospheric growth. These results indicate that metamorphic data (sampling the thermal structure of the upper lithosphere) do not preserve the signatures of such boundary motion.

Notes: Abstract We have deduced the steady-state lithospheric geotherm at c. 1 Ga in the south Indian shield area using the available data on the concentration of radioactive elements, and the P-T conditions of Proterozoic mantle xenoliths in the south Indian kimberlites as constraints. The geotherm was adjusted back to 2.5 Ga by keeping the surface temperature constant and calculating the temperature change at the top of convecting upper mantle. The reduced or mantle heat flux, which was treated as an adjustable parameter, was 20.9–21.3 mW/m2 at 1–2.5 Ga. Comparison of the calculated steady-state geotherm with the available P-T data of the Archaean (c. 2.5 Ga) charnockites and granulites from southern India suggests that the granulite facies metamorphism in this region had resulted from a major thermal perturbation, which was c. 400° C at 25 km.Seismic tomographic and gravity data essentially preclude any significant magma underplating of the granulitic crust in southern India. Previous workers have suggested that the formation of charnockites in this region was associated with copious CO2 influx from a deep-seated source, possibly the mantle. In this work, we have evaluated both the transient and steady-state thermal effects of the heat convected by CO2 outgassing from upper mantle. It is shown that the thermobarometric array of charnockites and granulites can be produced by the convective perturbation of the steady-state geotherm, and that a flux of CO2 of ±90 mol/m2 yr (corresponding to Darcy velocity of ±0.30 cm/yr) for a period of ±30 Ma was needed to produce the required perturbation. This is c. 150 times the average CO2 flux through the tectonically active area of the Earth's crust at the present time. There is, however, an uncertainty of a factor of 3 in this value.Seismic tomographic and gravity data independently suggest thickening of the crust beneath the granulite terrane compared with the adjacent Dharwar craton. This suggests thermal perturbation due to overthrusting as a major potential cause for the granulite facies metamorphism in south India. Overthrusting of a 30–35-km-thick thrust block was needed to produce the required thermal effect. The estimated thickness of the original crust from geobarometric and seismic tomographic data south of the orthopyroxene isograd or ‘transition zone’is compatible with the emplacement of a thrust block of this magnitude. However, the latter fails to match the estimated pre-uplift crustal thickness at the transition zone, if it is assumed that the crust has not thinned by non-erosional processes since the Archaean. Thus, we propose a combination of overthrusting and CO2 fluxing from a deep-seated source as the cause for the formation of charnockites in this zone. The required focusing of CO2 in this case is c. 40% of that estimated in the model where CO2 fluxing was considered to be the sole reason for thermal perturbation. This combined thrusting—CO2 fluxing model also helps explain the development of patchy charnockites in the transition zone from amphibolite facies rocks.

Notes: Abstract LaNi1−x FexO3 (x = 0, 0.25, 0.5) has been synthesized by the hydroxide solid solution precursor method for electrochemical characterization as oxygen anode in strongly alkaline medium. Studies were made at the oxide film, which was obtained by the oxide-slurry painting technique. The cyclic voltammetric study showed the formation of a diffusion-controlled quasireversible redox couple, Ni(ii)/Ni(iii), (E 0 ∼- 430 ± 10 mV) at the oxide surface in 1 m KOH. The reaction was observed to follow approximately first-order kinetics in OH− concentration. Values of the Tafel slope ranged between 59 and 86 mV decade−1 with all the oxide film electrodes. The electrocatalytic activity was found to be greatest with the Ni/LaNi0.75Fe0.25O3 electrode. A comparison was made between the electrocatalytic activities of LaNiO3 prepared by the hydroxide solid solution precursor and by the hydroxide coprecipitation technique.

Notes: Abstract A kinetic study of the electroformation and electroreduction of oxygen in KOH solutions has been carried out on a LaNiO3 electrode obtained through a malic acid precursor route. The new electrocatalyst was found to show greatly enhanced activity for both oxygen evolution and reduction. The apparent electrocatalytic activity of this electrode for oxygen reduction was more than 10 times higher than those reported for similar electrodes obtained by other methods. It was, however, observed to be less active electrocatalytically for oxygen evolution as compared to the active LaNiO3 electrode obtained by the Bockris–Otagawa coprecipitation method. The electrochemical reaction order with respect to OH− concentration was found to be approximately 1 in the case of oxygen evolution while that for oxygen reduction, was approximately −1. The Tafel slope values for the reactions were ∼2.3RT/F and ∼2×2.3RT/3F, respectively.

Notes: Abstract The thermal shock behaviour of NicalonTM fibre-reinforced chemical vapour infiltrated SiC matrix composites with three different types of fibre architecture, unidirectional, 0°/90°, and 2-D woven, has been studied using the water quench technique. Thermal shock induced damage was characterized by the destructive four-point flexure technique and the nondestructive technique of Young's modulus measurement by the dynamic resonance method. It was shown that the unidirectional and 0°/90° composites did not possess satisfactory mechanical properties or resistance to thermal shock because these fibre architectures prevented the composites from attaining high density during infiltration. Excess carbon coating was also found in the unidirectional and 0°/90° composites. Oxidation of this carbon coating contributed to the property degradation at high quench temperature difference. By contrast, the composite with 2-D woven fibre architecture created using the 0°/30°/60° cloth lay-up showed superior mechanical properties and thermal shock resistance. The nondestructive technique of Young's modulus measurement by the dynamic resonance method was successfully used in detecting the thermal shock damage.

Notes: Abstract Tensile testing of a modified 9Cr–1Mo steel in two microstructural conditions (710T– normalized at 1100°C, tempered at 710°C, and 550T – normalized at 1100°C tempered at 550°C) in the temperature range 25–450°C, under strain rates of 2.3×10-5–2.3×10-3s-1, exhibited serrated flow curves, with serrations appearing almost at the onset of deformation and disappearing before ultimate strengths were attained. The serrated flow curves (characteristics of dynamic strain ageing) were accompanied by increased ultimate strengths, loss of ductility and negative strain-rate sensitivity, relative to the ambient temperature properties. However, the increase in ultimate strength and the reduction in ductility were much larger for 710T specimens, as compared to 550T ones. In the dynamic strain ageing regime, the work-hardenability of 710T specimens increased rapidly while that of 550T specimens remained practically unaffected. Based on the microstructural consideration and the observed activation energy of 45 kJ mol-1, it is proposed that serrations are initiated by a nitrogen atmosphere formation on the waiting dislocations by a pipe diffusion mechanism, and they disappear by diffusion to the precipitate sinks during deformation. Because the fine alloy carbide precipitates in 550T specimens are more effective sinks than those of 710T ones, they can cause much faster depletion of the atmosphere, resulting in a much smaller effect of dynamic strain ageing on the tensile properties of 550T specimens.