The densification of Si3N4 with nano-sized sintering aids that were in situ incorporated by a combustion process was studied in comparison with that of sintering aids mixed by ball milling. The combustion process directly produces amorphous and nano-sized Y–Al oxides within the Si3N4 powder. X-ray diffraction results indicate that amorphous Y–Al oxides begin to crystallize into Y3Al5O12 at about 600 °C. Additionally the nano-sized sintering aids are more homogeneously distributed and thereby promote the formation of eutectic melts at lower temperatures during liquid-phase sintering. Therefore, the densification process of Si3N4 during liquid-phase sintering is strongly accelerated. The microstructure of as-sintered parts from combusted powder seems more dense and homogeneous.

Microreactors as a novel concept in chemical technology enable the introduction of new reaction procedures in chemistry, pharmaceutical industry, and molecular biology. These miniaturized reaction systems offer many exceptional technical advantages for a large number of applications. One major application is in the bulk synthesis of nanoparticles. Despite the availability of a plethora of nanoparticle synthesis processes, there exist many difficulties in controlling the shape, size, and purity of nanoparticles in large quantities in a safe and cost-effective manner. These difficulties have been the principal factors adversely limiting the applications of ceramic nanoparticles. Recent experiments have shown that to study the process of growth and formation of nanoparticles, a reactor having much smaller dimensions, namely a microreactor is more appropriate. These studies have also shown that a microchannel reactor provides control over the mean residence time and hence over the nanoparticle size and shape. This paper deals with the design, fabrication, and testing issues related to a high temperature, ceramic microreactor by investigating the use of reactive gas streams in arrays of microchannel reactors. These innovations offer the potential to overcome the barriers associated with synthesis of ceramic nanoparticles in large quantities.

This work highlights the recovery of water from sewage effluents using alumina ceramic membranes with pore sizes of 0.2 and 0.45m 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 Darcy's and Poiseuille's laws of filtration. The results show that alumina ceramic membrane with disc geometry having a pore size of 0.2m is more effective in filtering the total suspended solids, turbidity and microbes of the sewage effluents as compare to that of 0.45m 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.

Dead-end microfiltration of live Saccharomyces cerevisiae (yeast) suspensions of low concentration was carried out in a stirred membrane cell and corresponding permeate flux as a function of trans-membrane pressure (TMP) and membrane pore size were estimated. The deposition mechanism on the membrane surface and/or into its porous structure was analyzed in terms of various kinetic models, viz., cake filtration, standard blocking and complete pore-blocking models. It was seen that an initial intense flux decline due to external blockage followed by an internal deposition (partially retained cells) or the formation of a cake. Steady state permeate fluxes increased with the increase in the pore size of the membranes and decreased with the increase in yeast concentrations. In the present study, linear regression analysis of the filtration data showed that β values (filtration characteristic constant) were in the same order of magnitude as those of theoretical values. It was found that specific cake resistance increased with the increase in the TMP. The β values were found to vary in the range of –0.5 to 1.4. Compressibility factor (n) was found to be 0.853. According to the values, it has been found that all the above filtration mechanisms were simultaneously occurring rather than contributing individually.

The work presents an on-line filtration treatment for biomass-based power plant effluents using microporous ceramic symmetric membranes with an emphasis on establishment of fouling mechanisms and regeneration options in the process. The work also establishes the relationships among the membrane filterability, quality of permeate and flux to that of the pore size of ceramic membranes. For these purposes, two types of ceramic membrane configurations, i.e., seven-channeled and single-channeled (hollow type) tubes with identical pore size of 1.2 μm were selected for the experiments. On the other hand, raw effluents generated by the power plant were first pre-filtered using a 0.5 mm sieve and the resultant filtrate was further used for the present microfiltration trials by varying transmembrane pressure (TMP). Differences in the filterability of the effluents by the two membrane configurations at variable levels of TMP were explained using various membrane-fouling models. The filtration resistances that are acting in series were calculated using Darcy's and Poiseuille's law. Establishment of fouling mechanism/s of the membranes and its regeneration by adopting chemical cleaning and backflushing techniques were also an integral part of the study. The results suggest that microfiltration of effluents in a continuous crossflow mode with backflushing in regular intervals offer great advantage over efficiency and effectiveness in monitoring the contaminants in the raw effluents by physical methods. The study hence finds an on-line filtration solution in monitoring biomass-based power plant effluents in a benign way using microporous ceramic membranes.

Nanoscale ceramic powders offer attractive prospects as building blocks for microscale and mesoscale 3-D sintered structures for various high performance applications due to their excellent mechanical, thermal, dielectric and corrosion properties. Despite the availability of a plethora of nanoparticle synthesis processes the difficulties in controlling the shape, size, and obtaining highly pure and stable nanoparticles in large quantities in a safe and costeffective manner, have been the factors adversely limiting the applications of ceramic nanoparticles. Recent experiments have shown that to study the process of growth and formation of nanoparticles, a reactor having much smaller dimensions, namely a microreactor is more appropriate. Prior work has shown that the shape, size, and yield of nanoparticles are strongly influenced by the mean residence time required to produce the nanoparticles. A microreactor provides control over the mean residence time and hence over the nanoparticle size and shape. This paper deals with the design, fabrication, and testing issues related to a high temperature, ceramic microreactor and investigate the use of reactive gas streams in arrays of microchannel reactors to overcome the barriers associated with synthesis of ceramic nanoparticles in large quantities.

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.

A solution combustion technique for the synthesis of different beta-alumina compositions in the Na2O center dot xAl(2)O(3) system (where x = 5, 6, or 7) is described along with the structural characterization of the materials prepared. The amorphous powder obtained after a combustion reaction between the nitrate salts of the cations and aminoacetic acid was calcined in air at different temperatures in the range from 600 degrees C up to 1300 degrees C. The phases were investigated by powder X-ray diffraction (XRD) and infrared spectroscopic measurements. A metastable mullite-like alumina phase was found to form as an intermediate at a minimum calcination temperature of 750 degrees C and stable up to 1000 degrees C, which transformed completely into beta/beta"-alumina phases beyond a temperature of 1100 degrees C. The crystal structure of the mullite-like alumina phase was deduced by rietveld refinement of slow scan powder XRD data. A better understanding of the crystal structure of the mullite-like alumina was possible using other supplementary experimental evidences.

Dielectric properties of liquid-phase-sintered alumina (LPS) ceramics prepared using commercial powders of different particle size distribution and impurities (Na2O) content were studied. So far as the particle size distribution of the commercial powder is concerned, LPS ceramics, those derived from powders of both medium (3.1–8.4 μm) and coarse (70–100 μm) grades, showed similar dielectric loss, whereas it was higher in the case of LPS derived from the reactive powders (<1 μm). While considering the impurity levels of the powders, higher Na2O content (0.57 wt.%) in the powder showed significantly higher dielectric loss compared to that of the lower Na2O content (<0.2 wt.%). Furthermore, the MgO/(CaO+BaO+KNaO) ratio in the chemical composition of the LPS within the range of 0.4–1.8 was found to influence the dielectric properties in the frequency range of 102–107 Hz. The dielectric loss in the frequency band of 102–103 Hz was found to be higher in the case of the LPS with a MgO/(CaO+BaO+KNaO) ratio of 1.6. Besides the starting powder and the chemical composition, the orientation of alumina platelets arising out of fabrication route (e.g. tape casting) was also found to have a profound influence on the dielectric properties. A higher loss was observed in the tape cast specimen.

The alumina grains in liquid‐phase‐sintered (LPS) materials prepared from different commercial sources have a predominantly platelet morphology. Generally, the MgO:(CaO + BaO + Na2O + K2O) ratio in the chemical composition controls the morphology in LPS alumina that is 91–94 wt% pure. Within a given range of SiO2 content (i.e., 4.3–5.2 wt% in the chemical composition), a low MgO:(CaO + BaO + Na2O + K2O) ratio (i.e., <1.0) in the LPS compositions favors the formation of elongated grains, whereas ratios of >1.0 result in equiaxed grains. SiO2 contents outside the 4.3–5.2 wt% range favor the formation of elongated grains. A tendency to form platelike grains is observed for LPS alumina with a purity of 91–94 wt% when both the MgO:(CaO + BaO + Na2O + K2O) ratio and the SiO2 content are relatively low. The sintered density generally increases as the SiO2 content in the chemical composition decreases.

The liquid-phase-sintered Al2O3 (LPS) derived from commercial powders of different particle size, e.g. coarse (70-100 mu m), medium (3.6-7.0 mu m) and reactive (< 2.0 mu m) showed a substantial differences in Vickers indentation fracture behavior depending upon their grain size distribution and thermal expansion coefficient mismatch between the matrix grain and the intergranular phases. A high true hardness and a low indentation crack length was observed in the case of the LPS with reactive powder which was attributed due to enhanced dissolution of Al2O3 into the glassy phase. A high flexural strength was achieved with the LPS of medium powder. A high Kic-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 Irm) in the fine-grained (similar to 2 mu m) microstructure. The crack path was predominantly intergranular at lower MgO/(CaO + BaO + KNaO) ratio (<1.0) for the indentation load in between 9.81 and 49.03 N, whereas it was transgranular at a higher ratio (similar to 1.6) A low Kic-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. (C) 2000 Elsevier Science Ltd and Techna S.r.l. All rights reserved.

Liquid-phase-sintered (LPS) Al2O3 ceramics prepared using different powders of varying particle size, i.e, medium (3.6-7.0 mu m), coarse (70-100 mu m) and reactive (<2.0 mu m) showed a significant difference in wear resistance in different wear test environments, e.g. abrasion and erosion tests with dry silica sand, and wet-milling test with alumina grits. LPS materials derived from the reactive powders yielded a highs wear resistance that was possibly due to dissolution of a higher amount of Al2O3 into the intergranular glassy phase. The wear resistance of 88-94 wt.% LPS Al2O3 increased linearly upon increasing the MgO/(CaO+BaO+Na2O+K2O) ratio in the chemical composition (within the experimental limits) of the sintered material that was derived from the medium and coarse powders. The precipitation of anorthite on the Al2O3 grain boundary led to a lower wear resistance in 91-94 wt.% LPS Al2O3 of medium and coarse powders. Extensive plastic deformation was observed in the high-wear resistant LPS material whereas extensive microcracking was noticed in the low-wear resistant LPS material. (C) 2000 Published by Elsevier Science S.A.

A novel approach for in-situ incorporation of Al2O3 and Y2O3 additives into Si3N4 powder by a combustion technique is described. A suspension is made by mixing an alcoholic solution of Al/Y nitrates and citric acid with Si3N4 powder. The suspension forms a gray precipitate upon heating at 60°C. This precipitate undergoes a combustion reaction upon heating at 200°C and produces an amorphous phase of Al5Y3O12 (YAG) on the Si3N4 powder. The amorphous YAG phase shows a homogeneous distribution on the Si3N4 powder. Pellets of the composite powder are fabricated by cold isostatic pressing and sintered at 1750°C for 2h at 5 bar of nitrogen pressure. The microstructure of the sintered body prepared by this method reveals a high density, the fracture toughness (K1C) is increased by 13·4%, compared to that of a sintered Si3N4 body formed with identical amounts of alumina and yttria additives prepared by planetary milling.

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.

The magnetic critical-current density (${\mathit{J}}_{\mathit{c}\mathit{m}}$) in the low-field limit (where grains are essentially electromagnetically coupled) is found to increase with film thickness for granular ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ films, which is quite an opposite trend in comparison to the variation of the transport ${\mathit{J}}_{\mathit{c}}$ (${\mathit{J}}_{\mathit{c}\mathit{t}}$) with film thickness. To explain the thickness dependence of ${\mathit{J}}_{\mathit{c}\mathit{m}}$, theoretical estimation of the several microstructural parameters, e.g., grain-boundary width, thickness, etc., and their variation with the film thickness is attempted. Though such estimation does provide information regarding the sample characteristics, it cannot throw light on the observed discrepancy between ${\mathit{J}}_{\mathit{c}\mathit{t}}$ and ${\mathit{J}}_{\mathit{c}\mathit{m}}$ over the entire thickness regime. To accommodate the discrepancy it has been proposed that in addition to the microstructural parameters, the trapping of extra vortex chains---in the case of magnetic measurement---between two oppositely directed current loops is also governing the ${\mathit{J}}_{\mathit{c}\mathit{m}}$ significantly. The trapped extra flux varies with film thickness and is smaller in case of thinner films (25 \ensuremath{\mu}m). However, in the thinner regime the geometric factor plays a dominant role in controlling the ${\mathit{J}}_{\mathit{c}\mathit{t}}$ while it does not have any influence on ${\mathit{J}}_{\mathit{c}\mathit{m}}$.

An attempt has been made to explain the experimental observations regarding variation of critical current density with the film thickness between 3 and 65 μm in electrophoretically deposited YBa2Cu3O7−x films on silver (Ag) substrates. It is not possible to explain the phenomenon using the commonly believed concept of self-field degradation of critical current density of weak links between the grains. On the other hand, an increased grain-boundary resistance due to a lesser degree of silver penetration from the substrate with increasing film thickness is proposed to be the cause of the observed degradation of critical current density. The experimental data agree quite well with those theoretically calculated.

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 anionie 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 μm. 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 simple and convenient method for the synthesis of YBa2Cu3O7−x powder is described. The technique involves autoignition of a citrate-nitrate gel resulting from a thermally induced oxidation-reduction reaction to yield an ash that upon calcination produces the desired compound. The resulting powder is pure, homogeneous, and possesses a reasonably fine particle size. The autoignition is restricted to a particular range of citrate-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 potential for large-scale production.