Amitava Chakraborty

Low temperature sintering of La(Ca) CrO3 prepared by an autoignition process

Abstract Ultrafine powders of calcium substituted lanthanum chromite with average particle size of 0.57 μm, synthesized by a simple process of autoignition of citrate–nitrate gels, can be sintered at relatively low temperatures (1200–1250°C) compared to those normally reported. The sintered samples show adequate electrical conductivity and thermal expansion behavior at 1000°C for using it as an interconnect material for solid oxide fuel cell.

Low temperature synthesis and some physical properties of barium-substituted lanthanum manganite (La 1− x Ba x MnO 3 )

Barium-substituted lanthanum manganite (La 1− x Ba x MnO 3 ) powders have been synthesized by a novel autoignition technique, and the effect of barium content on the autoignition characteristics, stability of the compound, and the powder characteristics have been investigated. X-ray examinations show that the material exists as a single phase having perovskite structure up to 40 at. % substitution of Ba for La, beyond which mixed phases of LaMnO 3 and BaMnO 3 are formed at least up to the highest limit of substitution (50 at. %) and calcination temperature (1350 °C) investigated. Electrical conductivity and thermal expansion behavior of the material have been studied for plausible use as cathode material in solid oxide fuel cells.

Low-temperature synthesis of ultrafine La0.84Sr0.16MnO3 powder by an autoignition process

A simple and convenient method for low-temperature synthesis of La0.84Sr0.16MnO3 powder is described. The technique involves autoignition of a carboxylate (citrate + acetate)-nitrate gel resulting from a thermally induced anionic oxidation-reduction reaction to yield an ash, which upon calcination produces the desired powder. The resulting powder is pure, homogeneous, and possesses ultrafine particle size of the order of 0.3 to 0.5 mum. The autoignition is restricted to a particular range of carboxylate to nitrate ratio in the gel. Attempts have been made to understand the ignition process with the help of Thermogravimetry (TG) and Differential Thermal Analysis (DTA) of the samples. The process appears to have a higher degree of reproducibility and a good material yield (more than 96%) suitable for large-scale production.

Preparation of La1−xSrxMnO3 (0 ⩽ x ⩽ 0.6) powder by autoignition of carboxylate-nitrate gels

Abstract Fine powders of Sr-substituted LaMnO3 have been prepared at relatively low temperatures by autoignition of citrate-acetatenitrate gels. The ignition temperature and the enthalpy change during the combustion vary with strontium content leading to a variation of average particle size in the range 0.3 to 3 μm. As-ignited powders, particularly with less strontium substitution, are amorphous in nature. However, the crystallographic phase becomes either pseudocubic or rhombohedral or orthorhombic depending on the calcination temperature as well as the extent of substitution.

Electrical resistivity of porcelain in relation to constitution

The electrical resistivity of porcelain samples has been measured from room temperature to 1200 degrees C and it was found to depend on the concentration of the phases as well as the size and size-distribution of mullite crystals. The resistivity of porcelain was found to decrease with increase in amount, size and asymmetry of size-distribution of mullite crystals but increase with increase in quartz, cristobalite and glass content. Irrespective of constitution, resistivity of porcelain decreased with rise in temperature from about 10(13) ohm-cm at room temperature to about 10(4) ohm-cm at 1200 degrees C. The change in electrical resistivity of porcelain was primarily controlled by the defect structure of mullite and the composition and viscosity of the glassy phase

Bi2O3 as an effective sintering aid for La(Sr)MnO3 powder prepared by autoignition route

Sintering characteristics of the Sr-doped lanthanum manganite [La(Sr)MnO3] powder, prepared by autoignition of citrate–nitrate gel, have been studied by measuring density and evaluation of microstructural information. Bi2O3 is found to be an effective sintering aid for this material. Increased concentration of point defects arising from substitution of Bi in La site is the plausible cause of enhanced sintering. Measurement of electrical conductivity and thermal expansion coefficient indicate that Bi2O3 addition does not have any significant effect on these properties. ©

Electrodeposition and characterisation of cuprous oxide films

Abstract Semiconducting cuprous oxide films were prepared by electrodeposition onto copper substrates from an alkaline CuSO4 bath at temperatures between 40 and 60°C. The Cu2O films, which were deposited using a potentiostatic method, were found to exhibit exponential growth kinetics. X-ray diffraction studies revealed the formation of only Cu2O films with (200) preferred orientation. The observed current-voltage characteristics of the Cu 2 O Cu device structures were found to be similar to that of a metal-insulator-semiconductor (MIS) tunnel diode, indicating the presence of a thin unidentified interfacial insulating layer between the copper substrate and the cuprous oxide film.

Evidence of relaxation of Jahn-Teller polarons above TC in La1-xSrxMnO3 (0.1<xleq0.5)

We report the resistivity ()-temperature (T) patterns in La1-xSrxMnO3 (0.1<x0.5) systems over a temperature regime of 20-1273 K. Apart from a general trend of metallic -T pattern below TC (Curie point) and insulating above it, we have observed a re-entrant metallic -T pattern beyond a characteristic temperature T* above TC for the compositions 0.25 x0.50. Such behaviour possibly reflects the relaxation of Jahn-Teller (JT) polarons (which form as a result of splitting of Mn3+ 3d outer levels and associated distortion in MnO6 octahedra) at a higher temperature. The polaron formation energy EJT has been calculated from fitting a simple two-channel conduction model with the resistivity data. It appears to vary significantly with the Sr2+ ion concentration (x) or the concentration of JT-active Mn3+ ions which reflects the variation of the extent of Jahn-Teller coupling. The relaxation of polarons and the change in the behaviour of -T pattern above T* are not associated with any commensurate magnetic transition which seems to be supporting the notion that Jahn-Teller polarons influence the properties like TC, conductivity, magnetoresistivity etc significantly.