Bibliothek

Notizen: 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.

Notizen: 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.

Notizen: 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.

Notizen: The phenolic composition was determined in the leaves, petioles and bark tissues of male and female plants of two papaya cultivars. The same kind of phenolics were isolated from the male and female plants. However, a marked difference was observed between the plant organs of different cultivars. The important free and bound phenolics extracted after acidic and alkaline hydrolysis were caffeic acid, gentisic acid, m-coumaric acid, p-coumaric acid, salicylic acid and quercetin. Four phenolic compounds were not identified. The amounts of free, acid-hydrolysable and alkali-hydrolysable phenolic compounds were considerably higher in male plants.

Notizen: [Auszug] In the auroral zone electron penetration is almost vertically downwards. Electrons penetrating the dense atmosphere give rise to linearly polarised bremsstrahlung X-ray photons. The degree of linear polarisation is a function of the kinetic energy of scattering electrons, the energy of the emitted ...

Notizen: [Auszug] It is well known that the global thunderstrom activity is at a maximum at the temperate latitude and decreases with increasing latitudes7. The parameters or mechanisms controlling the guidance of thunderstorm-generated low-frequency waves are less effective at low latitude, however, which seems to ...

Notizen: 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.