Ibram Ganesh

A new sintering aid for magnesium aluminate spinel

Abstract A dense MgAl 2 O 4 sintered spinel has been prepared following a conventional double stage firing process. A new type of sintering aid, AlCl 3 , can improve the bulk density, apparent porosity and water absorption of MgAl 2 O 4 spinel sintered at 1550°C devoid of any contamination. In order to see the effect of AlCl 3 on densification and formation of a MgAl 2 O 4 spinel, 0.01–0.03 mol% AlCl 3 was added separately prior to calcination, subsequent to calcination as well as simultaneously at both stages. AlCl 3 is found to be equally effective in enhancing spinel formation during calcination as well as in increasing sintered density. The efficacy of AlCl 3 as a “spinelizer” or as a sintering aid is also compared with the conventional mineralizer, AlF 3 , which is generally added only prior to calcination to enhance the spinel formation. AlCl 3 is found to be active as a sintering aid provided the powder contains some free Al 2 O 3 and MgO and probably the hygroscopic nature of AlCl 3 is responsible for its effectiveness as a sintering aid.

Surface passivation of MgAl2O4 spinel powder by chemisorbing H3PO4 for easy aqueous processing.

A stoichiometric MgAl 2O 4 spinel (MAS) powder was synthesized by heat treating at 1400 degrees C for 2 h a compacted mixture of alpha-Al 2O 3 and calcined caustic MgO, followed by crushing and milling. The surface of this powder was then passivated against hydrolysis with H 3PO 4 and Al(H 2PO 4) 3 in an ethanol solution. The as-passivated powder could then be dispersed in water using tetramethylammonium hydroxide (TMAH) and an ammonium salt of poly(acrylic acid) (Duramax D-3005) as dispersing agents and gelcast to form green consolidates with relatively high strength (>15 MPa). The good dispersing behavior of the passivated powder in water was confirmed by the low viscosity of its suspension containing 41-45 vol % solids, demonstrating the viability of replacing organic solvents by water in colloidal processing of MAS-based ceramics. The Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and energy dispersive X-ray (EDAX) studies revealed that only negligible amounts of phosphate ions at the surface are required to effectively protect the powder from reacting with water.

Thermal anisotropy in sintered cordierite monoliths

Cordierite honeycombs with oxide compositions of 49.5-51 wt.% SiO 2 , 35.5-36 wt.% Al 2 O 3 and 14-14.5 wt.% MgO were prepared from clay, talc, and alumina using conventional extrusion process. Effects of sintering parameters such as heating rate, soaking time, cooling rate as well as amount of calcined clay in the raw material compositions on the overall coefficient of thermal expansion (CTE) as well as thermal anisotropy in terms of CTE difference between the extrusion direction and across the extrusion direction of the cordierite honeycombs were studied. For this purpose, extruded honeycombs were sintered with various heating and cooling rates varying from 80 to 180°C h -1 up to a peak temperature of 1420°C for various soaking times ranging from 4 to 8 h. All the sintered cordierite honeycombs were characterized with respect to phase composition (XRD), morphology (SEM), bulk density, water absorption, apparent porosity, and CTE (dilatometric studies). XRD studies reveal that all the investigated sintered honeycombs contain >90% cordierite phase with smaller quantities of mullite, spinel and α-Al 2 O 3 . Raw materials compositions containing 60% calcined clay of total clay yielded crack free honeycombs with lower thermal anisotropy. Among the various parameters studied, honeycombs sintered with heating rate of 80°C h -1 , soaking time of 8 h and cooling rate of 180°C h -1 showed minimum thermal anisotropy.

Surface Passivation of MgAl 2 O 4 Spinel Powder by Chemisorbing H 3 PO 4 for Easy Aqueous Processing

A stoichiometric MgAl 2O 4 spinel (MAS) powder was synthesized by heat treating at 1400 degrees C for 2 h a compacted mixture of alpha-Al 2O 3 and calcined caustic MgO, followed by crushing and milling. The surface of this powder was then passivated against hydrolysis with H 3PO 4 and Al(H 2PO 4) 3 in an ethanol solution. The as-passivated powder could then be dispersed in water using tetramethylammonium hydroxide (TMAH) and an ammonium salt of poly(acrylic acid) (Duramax D-3005) as dispersing agents and gelcast to form green consolidates with relatively high strength (>15 MPa). The good dispersing behavior of the passivated powder in water was confirmed by the low viscosity of its suspension containing 41-45 vol % solids, demonstrating the viability of replacing organic solvents by water in colloidal processing of MAS-based ceramics. The Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and energy dispersive X-ray (EDAX) studies revealed that only negligible amounts of phosphate ions at the surface are required to effectively protect the powder from reacting with water.

Vapour Phase Synthesis of Isobutyraldehyde From Methanol and Ethanol Over Mixed Oxide Supported Vanadium Oxide Catalysts

The vapour phase synthesis of isobutyraldehyde from methanol and ethanol in one step was investigated over titania-silica, titania-alumina, titania-zirconia, titania-silica-zirconia, and magnesia supported vanadium oxide catalysts at 623 K and under normal atmospheric pressure. Among various catalysts the titania-silica binary oxide supported vanadia provided higher yields than the other single or mixed oxide supported catalysts. The high conversion and product selectivity of V2O5/TiO2-SiO2 catalyst (20 wt% V2O5) was related to the better dispersion of vanadium oxide over titania-silica mixed oxide support in addition to other acid-base and redox characteristics. A reaction path for the formation of isobutyraldehyde from methanol and ethanol mixtures over these catalysts was described.

Effect of rubber encapsulation on the comparative mechanical behaviour of ceramic honeycomb and foam

Abstract Cordierite–mullite honeycombs with square cells were prepared by an extrusion technique. Mechanical behaviour of the honeycomb was compared with that of commercial ceramic foam with and without rubber encapsulation. While impact testing, honeycombs as well as foam had shown low energy absorption, which was increased substantially upon rubber encapsulation in both cases. It has been found that the honeycomb absorbs significant amounts of energy under compression when loaded in the direction parallel to channel walls in comparison with when it is loaded in the perpendicular direction. Upon rubber encapsulation, honeycomb had shown substantial decrease in the absorbed energy parallel to channels, whereas, there was modest increase in the perpendicular direction. However, in both the orientations the critical stress was decreased upon rubber encapsulation. The foam on other hand absorbed significantly lower energy under compression in comparison with that of the bare honeycombs. Interestingly, upon rubber encapsulation, the foam had shown increased amounts of energy absorption as well as enhancement in the critical stress, contrary to the behaviour observed in the case of honeycombs. Failure mechanisms for the different types of configurations with and without rubber encapsulation are proposed in this paper.

Synthesis of Nanosized Ceria-Zirconia Solid Solutions by a Rapid Microwave- Assisted Combustion Method

By adopting a simple cost effective microwave-assisted synthesis methodology, nanosized ceria-zirconia solid solution (MW) has been prepared. For comparison purpose, ceria-zirconia solid solution with the same composition was also synthesized by a coprecipitation method (CP) and calcined at 773 K. Both the samples were examined by different characterization techniques namely, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, transmis- sion electron microscopy, BET surface area and other methods, and evaluated for CO oxidation activity. XRD studies re- vealed formation of monophasic Ce0.5Zr0.5O2 solid solution in the MW sample and Ce0.75Zr0.25O2 solid solution in the CP sample. TEM studies showed the presence of nanometre sized particles with broad particle size distribution in the sample prepared by microwave method. Raman spectroscopy and oxygen storage capacity measurements suggested the presence of more oxygen vacancies and lattice defects in the MW sample. XPS studies indicated a high reducibility and surface en- richment of Ce 3+ ions in the MW sample. Higher CO oxidation activity and lower light off temperature was observed for

Chemisorption of phosphoric acid and surface characterization of As passivated AlN powder against hydrolysis.

By simply refluxing a commercial AlN powder in a mixture solution of ethanol, H(3)PO(4), and Al(H(2)PO(4))(3) for 24 h at 80 degrees C, the powder was successfully passivated against hydrolysis. The phosphate layer formed on the surface of AlN powder was found to be quite stable toward protecting the powder from hydrolysis. The efficacy of the coating was established by suspending the treated and the untreated powders in water for 72 h and subsequently characterizing them by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and Raman analysis. The good dispersing behavior of the treated AlN powder in water was confirmed by the low viscosity of an AlN suspension containing 50 vol % solids demonstrating the viability of replacing organic solvents by water in colloidal processing of AlN-based ceramics.

Vapour phase synthesis of quinoline from aniline and glycerol over mixed oxide catalysts

Abstract The vapour phase synthesis of quinoline from aniline and glycerol (1:2 mole ratio) in a single step was investigated over ZnO–Cr 2 O 3 , CuO–ZnO/Al 2 O 3 , MoO 3 –V 2 O 5 /Al 2 O 3 and NiO–MoO 3 /Al 2 O 3 catalysts in the presence of air at 623–723 K under normal atmospheric pressure. Among these catalysts investigated, the CuO–ZnO/Al 2 O 3 combination effectively performed this reaction with high activity and selectivity.

Phosphoric acid treated AlN powder for aqueous processing of net-shape dense AlN and β-SiAlON parts

Abstract Due to fast hydrolysis of AlN when in contact with water, AlN based ceramics are processed in organic solvents, which are volatile, expensive, harmful to the health and environment. The present work intends to protect AlN powder against hydrolysis in order to enable aqueous processing of AlN based ceramics. A commercial AlN powder was treated in an ethanol solution of H3PO4 and Al(H2PO4)3 kept at 80°C for 24 h to protect it from hydrolysis. The dispersing behaviour of the treated AlN powder in water and the ability to consolidate defect free parts of AlN and β-Si4Al2O2N6 ceramics from aqueous suspensions by slip casting and gelcasting techniques were studied. The consolidated parts were then sintered in a graphite furnace and the sintered bodies exhibited properties [bulk density (BD), apparent porosity (AP), crystalline phase composition and microstructural features] similar to those reported in literature for the same materials processed from organic media.