P. Sujatha Devi

Ferromagnetism in nanoscale BiFeO3

A remarkably high saturation magnetization of ∼0.4μB∕Fe along with room temperature ferromagnetic hysteresis loop has been observed in nanoscale (4–40nm) multiferroic BiFeO3 which in bulk form exhibits weak magnetization (∼0.02μB∕Fe) and an antiferromagnetic order. The magnetic hysteresis loops exhibit exchange bias and vertical asymmetry which could be because of spin pinning at the boundaries between ferromagnetic and antiferromagnetic domains. Interestingly, both the calorimetric and dielectric permittivity data in nanoscale BiFeO3 exhibit characteristic features at the magnetic transition point. These features establish the formation of a true ferromagnetic-ferroelectric system with a coupling between the respective order parameters in nanoscale BiFeO3.

Indium tin oxide nano-particles through an emulsion technique

Abstract Spheroidal indium tin oxide (ITO) nano-particles with In/Sn=85:15 (at.%) were prepared by an emulsion technique under mechanical agitation without the aid of any surface-active agent. X-ray powder diffraction (XRD) indicated the stabilization of cubic ITO as the only phase at 250 °C. Particle size analysis showed a narrow distribution of 20–58 nm range particles with a mean size of 38 nm, thus, confirming their nano-structured nature. The pellets sintered at 1350 °C for 6 h were found to have 95–97% of theoretical density. Thus, well sinterable nano-particles of ITO powders were prepared at a significantly low temperature compared to that in the reported methods.

Synthesis and properties of nanocrystalline ceria powders

Nanocrystalline CeO2 powder was synthesized by a citrate-nitrate autoignition process and characterized by thermal analysis, x-ray diffraction, and impedance spectroscopy measurements. Nanocrystalline (20-40 nm) ceria powder with fluorite structure had formed in situ during the citrate-nitrate autoignition process. The powder prepared could be sintered to density more than 98% of theoretical density at 1450 degreesC. The nanocrystalline CeO2 exhibited an increase in conductivity in Ar and H-2 than air above 600 degreesC, suggesting a possible electronic contribution to the conductivity at low oxygen partial pressures. Impedance measurements on the sintered samples unequivocally established the potential of this process in developing phase pure ceria compositions

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.

A potential low-temperature oxide-ion conductor: La2-xBaxMo2O9

An oxide ion conducting material, La1.94Ba0.06Mo2O9, with ionic conductivity of the order of 0.084S∕cm at 800°C in air was prepared by a citrate-nitrate auto-ignition process. A 3% Ba doping has suppressed the resistive transition of unsubstituted La2Mo2O9, which in turn stabilized the high-temperature cubic phase at room temperature as confirmed from x-ray diffraction, differential thermal analysis, and dilatometric studies. Impedance measurements on sintered La1.94Ba0.06Mo2O9 further lend strong support that a small amount of Ba doping has increased the overall conductivity of the parent compound La2Mo2O9 to a notable extent both at low and high temperatures.

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.

Nb-Doped La2Mo2O9: A New Material with High Ionic Conductivity

A new series of La2Mo2O9-based oxides of the general formula La2Mo2-xNbxO9-delta, where x <= 0.4, was prepared by a citrate-nitrate combustion process, and the electrical and thermal properties were evaluated. Electrical conductivity measurements on the sintered samples confirmed that all the compositions underwent a phase transition below 570 degrees C with a concomitant increase in the oxide-ion conductivity. Although Nb doping did not suppress the order-disorder transition exhibited by pure La2Mo2O9 completely, it has significantly lowered both the transition temperature and the enthalpy change associated with the transition. The oxide-ion conductivity was found to decrease with increasing Nb concentration, and at 700 degrees C, La2Mo2-xNbxO9-delta compositions with x values of 0.06 and 0.1 exhibited oxide-ion conductivities of 0.069 and 0.062 S/cm, respectively. The composition with the lowest Nb doping, viz., La2Mo1.94Nb0.06O9-delta, exhibited an oxide-ion conductivity of 0.113 S/cm at 800 degrees C, which is nearly double that of the undoped material, and this is the highest value reported so far in this family of oxides. (c) 2005 The Electrochemical Society.

Defects in Chemically Synthesized and Thermally Processed ZnO Nanorods: Implications for Active Layer Properties in Dye-Sensitized Solar Cells

We have carried out the effect of post annealing temperatures on the performance of solution-grown ZnO rods as photoanodes in dye-sensitized solar cells. Keeping our basic objective of exploring the effect of native defects on the performance of DSSC, we have synthesized ZnO rods having length in the range of 2-5 μm by a modified sonication-induced precipitation technique. We performed extensive characterization on the samples annealed at various temperatures and confirmed that annealing at 300 °C results in ZnO rods with minimum native defects that have been identified as doubly ionized oxygen vacancies. The electron paramagnetic resonance measurements on the samples, on the other hand, confirmed the presence of shallow donors in the low temperature annealed samples. We also carried out electrochemical impedance measurements to understand the transport properties at different interfaces in the solar cell assembly. We could conclude that solution-processed ZnO rods annealed at 300 °C are better suited for fabricating DSSC with improved efficiency (1.57%), current density (5.11 mA/cm(2)), and fill factor (45.29%). On the basis of our results, we were able to establish a close connection between the defects in the metal oxide electron transporting nano system and the DSSC performance.

Comparison of citrate-nitrate gel combustion and precursor plasma spray processes for the synthesis of yttrium aluminum garnet

The influence of synthesis conditions on the formation of yttrium aluminum garnet (YAG) powders starting from the same solution precursors was investigated by employing a citrate–nitrate gel combustion process and a precursor plasma spraying technique. YAG powders were formed at ≥500 °C, through the citrate–nitrate gel combustion process, without any intermediate phase formation. Time-resolved x-ray experiments were performed for the first time on these citrate–nitrate precursor materials to understand their mode of decomposition. The in situ data confirmed a single-step conversion to YAG from the precursor powder without any intermediate phase formation. Ex situ experiments also produced similar results. However, the use of the same citrate–nitrate precursor solution as a liquid feedstock material in the precursor plasma spraying technique revealed an entirely different transformation mechanism to YAG through intermediate phases like H–YalO 3 and O–YalO 3 .