Sukumar Roy

Low temperature synthesis of ultrafine Pb(Zr, Ti)O 3 powder by sol-gel combustion

Lead zirconate titanate powders are derived from a novel aqueous-based citrate-nitrate/oxynitrate sol-gel combustion process. Aqueous solutions of metal nitrates or oxynitrates are transformed into gels with citric acid under heating. The received gels undergo a self-propagating combustion reaction on heating to 180{degree}C and subsequently yield voluminous ashes. These ashes form single phase perovskite Pb(Zr{sub 0.53}Ti{sub 0.47})O{sub 3} powder with a specific surface area of 8m{sup 2}/g upon calcination at 550{degree}C. The ashes show a homogeneous distribution of lead, zirconium, and titanium ions which guarantees short diffusion paths in solid state formation of PZT perovskite. The redox behavior of the gels was studied with the help of DTA experiments. Powders are characterized in terms of XRD, SEM, and EDX analysis. {copyright} {ital 1997 Materials Research Society.}

Synthesis of YAG phase by a citrate-nitrate combustion technique

Abstract A white precipitate is yielded on heating a solution of Y(NO3)3 (equivalent to 8 wt.% Y2O3), Al(NO3)3 (equivalent to 7 wt.% Al2O3 and molar ratio of 1:1.9378 for Y2O3 to Al2O3) and citric acid in isopropanol with a citrate–nitrate molar ratio of 0.098 to 60°C. The dried precipitate is then combusted at 200°C and produces a solid product (ash). X-ray powder diffraction patterns of the ash and its calcined forms show that the ash is amorphous and remains amorphous up to 600°C. The ash starts crystallizing to form a YAG phase at 800°C and completely transforms into YAG below 900°C. A certain amount of YAM phase co-exists with the YAG phase between 850°C and 900°C. Finally at 900°C, only the YAG phase exists. This combustion technique for synthesis of YAG phase can be used for incorporation of sintering additives in non-oxide ceramics such as Si3N4.

NANOSTRUCTURED YTTRIA POWDERS VIA GEL COMBUSTION

Nanostructured yttria powders were prepared by a gel combustion technique. The technique involves exothermic decomposition of an aqueous citrate-nitrate gel. The decomposition is based on a thermally induced anionic redox reaction. A variety of yttria powders with different agglomerate structures can be made by altering the citrate-nitrate ratio γ. The gel with γ = 0.098 in situ yields nanostructured yttria powder at 258 °C that is porous and agglomerated with an average of 25 nm primary particles. Its specific surface area is 55 m 2 /g. The decomposition of the gels was investigated by simultaneous thermogravimetry analysis (TGA) and differential thermal analysis (DTA) experiments. The produced ashes and calcined powders are characterized by x-ray diffraction (XRD), ir spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Brunauer, Emmett, and Teller (BET) analysis.

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.

Polyaniline Membranes for Separation and Purification of Gases, Liquids, and Electrolyte Solutions

Abstract Conjugated polymers are excellent barriers for membrane separations because their porosity can be controlled at the molecular level through chemical doping. Polyaniline (PANI) is particularly attractive because simple acid/base doping/undoping enables a controllable level of doping that can be readily achieved using dopants of different sizes and shapes. PANI, which belongs to an important member of the family of electrically conducting polymers, has been studied extensively as a membrane due to its distinct electrochemical properties and environmental stability. Adding dopants to PANI leads to a decrease in gas permeability, while removal of these dopants would produce extremely high permeability. This review provides an overview of the use of PANI membrane in gas separation (GS), pervaporation (PV) and electrodialysis (ED) applications. Our discussion will be concerned with the utility of PANI as a homopolymer, blend and composite membrane, discussing a considerable amount of background information on their developments and applications. Various modifications of PANI as efficient membranes and their future prospects in membrane separation and purification technology are discussed. This article was the CEPS Communication # 106. The authors dedicate this review article in honor of Professor Alan G. MacDiarmid, the University of Texas at Dallas, USA (Nobel Laureate in Chemistry, 2000), who visited CEPS in December 2004. His untiring energy to visit and inaugurate the Center of Excellence in Polymer Science at Karnatak University, Dharwad has been a great inspiration to our younger students and scientists.

History effects in harmonic generation in sintered pellets of YBaCuO

Abstract We present measurements of harmonic generation in the magnetization of a sintered YBa2Cu3O7 pellet subject to HDC + HAC × cos cot. The magnitude of the harmonics depend strongly on whether the pellet was cooled to 77K in HDC, or whether HDC was applied after zero-field-cooling. In the latter case, we also observe a large hysteresis between the HDC increasing and HDC decreasing cases. The results are explained qualitatively.

Microstructure dependent hardness and fracture behavior in liquid-phase-sintered Al2O3

Abstract The liquid-phase-sintered Al 2 O 3 (LPS) derived from commercial powders of different particle size, e.g. coarse (70–100 μm), medium (3.6–7.0 μm) and reactive ( 2 O 3 into the glassy phase. A high flexural strength was achieved with the LPS of medium powder. A high K ic-short always resulted either due to (i) the MgO/(CaO+BaO+KNaO) ratio of nearly 1 in the chemical composition of LPS, or (ii) higher modulus of elasticity to hardness ratio, or (iii) reinforcement of coarse grains (>12 μm) in the fine-grained (∼2 μm) microstructure. The crack path was predominantly intergranular at lower MgO/(CaO+BaO+KNaO) ratio ( K ic-short was observed due to precipitation of anorthite phase in the LPS with a high MgO/(CaO+BaO+KNaO) ratio. Finally the sintered density of 91–94 wt% LPS materials comprising of all powders produced a linear relationship with both the hardness and the modulus of elasticity.

Magnetic field dependence of the harmonic generation in sintered pellets of YBaCuO: The history effects

Abstract Results are presented for a detailed study of harmonic generation in the magnetization of a sintered YBa 2 Cu 3 O 7 pellet subjected to various combinations of AC and DC magnetic field. On cooling the sample below T c in zero field, a large hysteresis is observed between H DC increasing and decreasing cases. General features of this observation can be understood in terms of the hysteretic properties of the weak-linked intergrain region mediated by trapped flux inside the grains. However, there remain some finer aspects which do not have a simple explanation within this model.

Second harmonic generation as a sensitive technique for estimating Hc1 of high Tc materials

A simple and interesting technique for measuring the Hc1 of the superconducting grains in bulk high Tc superconductor (HTSC) samples is presented herein. This technique takes advantage of interesting history effects observed in the nonlinear magnetic response of HTSC samples. The hysteretic second harmonic magnetization shows sharp structure at low applied fields due to the magnetic flux trapped inside the grains. This feature is used to track the penetration of magnetic field inside the superconducting granular regions.

Recovery of Water from Sewage Effluents using Alumina Ceramic Microfiltration Membranes

Abstract This work highlights the recovery of water from sewage effluents using alumina ceramic membranes with pore sizes of 0.2 and 0.45 µm respectively in dead‐end filtration mode. The work demonstrates the ability and advantages of alumina‐based microfiltration (MF) membranes in filtering microbes and other harmful pollutants normally present in sewage effluents in dead‐end filtration mode. The fouling behavior of the membranes in the filtration cycle is identified, which in turn helped to regenerate the fouled membranes for subsequent usage. Regeneration studies of fouled membranes also suggest that though chemical cleaning was effective in recovering membrane performance, the fouling had still been progressed slowly and the membranes showed the ability to perform at least five filtration cycles of highly‐contaminated sewage effluents. As expected, the filtration efficiency and flux characteristics at various transmembrane pressure (TMP) of the membranes varies with the pore size of the membrane and is explained in light of Darcys and Poiseuilles laws of filtration. The results show that alumina ceramic membrane with disc geometry having a pore size of 0.2 µm is more effective in filtering the total suspended solids, turbidity and microbes of the sewage effluents as compare to that of 0.45 µm membrane to a level in which the permeate water appears to be benign for discharging into the surface thereby offering the possibility of recycling or reusing the recovered water from the sewage effluents for suitable purposes.