Effect of metal ion concentration on synthesis and properties of La0.84Sr0.16MnO3 cathode material

A reactive powder of La0.84Sr0.16MnO3 is synthesized as a cathode material for solid oxide fuel cells by a citrate–nitrate auto-ignition process and characterized by thermal analysis, X-ray diffraction and electrical conductivity measurements. The effect of starting metal ion concentration in the precursor solution on the properties of the final oxide is studied and correlated through particle size analyses, sintering studies and microstructural examination. Sintered La0.84Sr0.16MnO3 ceramics of relative density around 93% can be fabricated by preferably keeping the metal ion concentration in the precursor to less than 0.8 M, whereas to make porous ceramics (relative densities of 75–80%) a higher metal ion concentration is preferred. At 1000 °C, the 93% dense ceramics exhibit electrical conductivities of around 168–169 S cm−1 and the porous ceramics of around 136–146 S cm−1

Effect of boron addition on the dielectric properties of giant dielectric CaCu3Ti4O12

The recently discovered giant dielectric CaCu3Ti4O12(CCTO) has been reported to show dielectric constant value as high as 80,000 for single crystals and around 10,000 for ceramics. However the dielectric constant is also associated with high dissipation factor. In the present study, it has been observed that the loss factor of CCTO can be reduced by B2O3 addition. The low frequency dispersion of CCTO ceramics, which indicates Maxwell-Wagner type relaxation, is reduced by boron addition. Also the temperature dependence of dielectric constant is minimized by boron addition. From the present work it can be surmised that B2O3 addition can favourably modify the dielectric properties of CCTO ceramic for its practical applications as a capacitor material

Fast firing of lead zirconate titanate ceramics at low temperature

A fast firing technique for densification of PZT (up to 97% of theoretical density) at a low temperature (950 degrees C) has been found out. A small amount of excess PbO (3-5 wt.%) and a fast firing schedule are required to achieve the desired sintering. The final composition of the sintered ceramics can be kept close to the morphorropic phase boundary by modifying the firing time and post-sintering annealing treatment at 800 degrees C. The g(33) values of fast fired samples are comparable to those of conventionally sintered samples, though the d(33) values are somewhat lower than those reported for conventionally sintered (at 1250'C or above) sample

Structure and optical absorption of combustion-synthesized nanocrystalline LiCoO2

Nanocrystalline LiCoO2 powders (10-50 nm) were synthesized by a citrate-nitrate combustion process followed by calcination at different temperatures (300-800 degrees C) in air. Thermogravimetric analyses indicated a sharp combustion at a low temperature of 225 degrees C, producing fine crystallites. Quantitative phase analyses from the x-ray diffractograms showed that while annealing at 500 degrees C produced mixed phases of cubic and rhombohedral LiCoO2, annealing at 800 degrees C resulted in single-phase rhombohedral LiCoO2. Electronic transitions related to the Co 3d bands were investigated by ultraviolet-visible reflectance spectra in absorbance mode and were ascribed to the Co 3d intra-band transition involving t(2g) and e(g) orbitals. The d-d transitions underwent a blue shift of about 0.3 eV as the cubic LiCoO2 transformed into the rhombohedral structure with band gap values of about 1.4 and 1.7 eV

Role of Sb doping in governing the thermal conductivity of Bi-Pb-Sr-Ca-Cu-O superconductors between 10-K and 150-K

In this paper, we report on the role of Sb doping in causing differences in the nature of the thermal conductivity (k) versus temperature (T) plot between 10 and 150 K for Bi1.6Pb0.4Sr2Ca2Cu3Ox superconductors. The peak in the thermal conductivity plot shifts towards lower temperature and also becomes sharper with Sb doping. The increase in the static imperfections as a result of Sb doping is quantitatively estimated following the Tewordt and Wolkhausen [TW] model. The role of static imperfections in governing the nature of the k-T plot is discussed. We conclude that the sharp rise in k below the transition temperature (T(c)) for Sb-doped samples can be better accounted for by invoking the concept of suppression of quasiparticle scattering, rather than by the commonly believed concept of increased phonon mean free path due to condensation of carriers

Mechanism of silver-induced enhanced synthesis of YiBa2Cu3O7-x

It has been shown that silver addition enhances the formation of the Y1Ba2Cu3O7-x(123) phase during calcination of BaCO3, Y2O3, and CuO. The phase formation in nitrogen and air atmosphere in the presence of silver has been studied. A mechanism has been proposed to explain the role of silver in enhancing the formation of the 123 phase. The proposed mechanism can also explain enhanced sintering, grain growth and the promotion of high-temperature degradation of 123 to Y2BaCuO5(211) and other phases due to the presence of silver

Ion Dynamics Study of Nb(+5)-Substituted La(2)Mo(2)O(9) by AC Impedance Spectroscopy

To identify the ionic conductivity mechanism of Nb(+5)-substituted La(2)Mo(2)O(9), an ion dynamic study on these substituted materials has been performed. Although the ionic conductivity and mechanism of conduction has already been reported, the ion dynamic study of these materials has scarcely been carried out to establish the conduction mechanism. The frequency dependence of ac conductivity of Nb(+5)-doped La(2)Mo(2)O(9) has been explained by means of frequency dispersion of conductivity as well as in terms of modulus formalism. The values of the exponential parameter s of the universal dielectric response and the stretching exponent beta of the Kohlrausch-Williams-Watts relation are evaluated from curve fitting. Temperature dependence of such parameters is checked and is interpreted for the plausible mechanism of ionic motion in the material at low temperature. The scaling behavior of these materials are also studied to evaluate the composition and temperature dependence of the conduction mechanism

Formation of pore structure in tape-cast alumina membranes - effects of binder content and firing temperature

Disc type ceramic aluminium oxide membrane has been prepared by tape casting technique. Thickness of this single layer membrane is in the range 200-300 mu m. Porosity and pore size distribution have been determined by mercury porosimetry. Polymeric binder content of the green tape and the firing temperature are found to have strong influence on the average pore size, pore size distribution and overall porosity. Higher binder content promotes agglomeration of the ceramic particles, which on firing leads to wider pore size distribution and formation of closed pores. Pore coarsening is observed with increasing firing temperature. (C) 1998 Elsevier Science B.V

A wet-chemical process to form palladium oxide sensitiser layer on thin film zinc oxide based LPG sensor

The liquid petroleum gas (LPG) sensitivity characteristics of zinc oxide (ZnO) films have been studied for an optimised level of Pd loading. The sensor element comprises of a layer of chemically deposited ZnO on which an overlayer of palladium (Pd) sensitiser was formed by a chemical dipping technique. The room temperature resistance of the film was found to be a sensitive function of the quantity of palladium loading, which could be optimised for stable and reproducible sensor properties. The sensor characteristics that are dependent also on the operating temperature could be optimised at around 250degreesC. A sensitivity of 88% was observed in presence of 1.6 vol.% LPG in air at this optimum temperature with reasonably fast response and recovery times. (C) 2003 Elsevier B.V. All rights reserved