K. G. K. Warrier

Anatase to rutile transformation in sol-gel titania by modification of precursor

Abstract The effect of pH on the formation of high temperature phases in titanium oxide, derived by hydrolysis of modified titanium isopropoxide has been reported. The anatase to rutile phase transformation is delayed in the case of acetic acid modified gel precursor at pH 3 and 4, showing the presence of anatase phase even at 1000°C. On the other hand, in the sample precursor prepared at pH 6, anatase to rutile transformation is complete at 800°C, under identical conditions of heat treatment. The transformation has been followed by thermal analysis and X-ray powder diffraction data. Impedance spectroscopy has been attempted as a useful and effective tool in identifying the anatase and rutile phases as well as transformation temperatures.

Sol–gel synthesis of nanosized anatase from titanyl sulfate

Nanosized titanium oxide with anatase structure holding high surface area has been synthesized by the sol–gel route from an aqueous solution of titanyl sulfate. A stable titania sol with extremely narrow range of particles having an average size of 30 nm has been obtained. The crystalline anatase particles have average particle size of 5–10 nm and specific surface area of 375–104 m2 g−1 between the calcination temperature 200–600 °C. The powders were characterized by XRD, FTIR, TGA/DTA and N2 adsorption analysis.

Phase transformation in sol–gel titania containing silica

Abstract Titanium dioxide containing 5–10% silica has been prepared by the co-hydrolysis of titanium isopropoxide at pH 4 in presence of TEOS in different solvents such as methanol, ethanol and water. The resultant gels have been analyzed by BET surface area measurements and thermal analysis. Addition of 5% silica to TiO2 increases initial surface area of 89 to 188 m2/g. As the amount of silica is further increased to 10%, the surface area is further increased to 203 m2/g. The composition containing TiO2–10% SiO2 in ethanol medium achieved the highest specific surface area 232 m2/g while the corresponding sample in methanol had 217 m2/g. Addition of silica has further resulted in considerable increase of anatase–rutile transformation temperature >800°C. Addition of methanol and ethanol to titania–silica systems enhances the formation of rutile phase. This phase transformation has been followed by X-ray powder diffraction data and further, through impedance measurements on the precursor gel pellets heated to various temperatures and the latter has been presented as a very simple and convenient tool to study phase transformation in titania ceramics.

Effect of nanoparticulate boehmite sol as a dispersant for slurry compaction of alumina ceramics

Abstract This work reports the use of monohydroxy aluminium oxide particulate sol [boehmite, AlOOH], as a dispersing medium for slurry compaction of submicron-sized alumina powders. Nanoparticulate boehmite sol, at controlled pH conditions, resulted in homogeneously dispersed alumina suspension which could be compressed into a definite shape by applying simple pressure. The partially flocculated boehmite sol at pH 6.2 acts as good dispersant and resulted in alumina–boehmite solids loading >80 wt.%. The alumina dispersed in boehmite sol containing 17.24 vol.% boehmite concentration showed maximum green and sintered density. The maximum theoretical green density was 63% and the maximum linear drying shrinkage was

Monohydroxy aluminium oxide (Boehmite, AlOOH) as a reactive binder for extrusion of alumina ceramics

Monohydroxy aluminium oxide (Boehmite, AlOOH) has been used as an extrusion aid along with α-alumina and γ-alumina particulates. Extended ball milling of the mixture using alumina grinding media imparts nucleating seeds to boehmite for an early conversion to α-alumina. The boehmite-γ-alumina mixture appears to result in a denser alumina ceramic accompanied by an enhanced grain growth indicating the higher reactivity at higher temperatures compared to the boehmite-α-alumina system. The reason for this novel behaviour is probably the coating of nanosize boehmite containing nucleating sites on the porous γ-alumina powders resulting in enhanced diffusion reactions. The present study therefore provides scope for low temperature alumina ceramics using boehmite binder along with partly dehydroxylated aluminium hydroxide.

Extrusion characteristics of alumina-aluminium titanate composite using boehmite as a reactive binder

Abstract Alumina–aluminium titanate composite was fabricated through paste extrusion using boehmite gel as a binder. The rheological characteristics of alumina paste containing various amounts of boehmite binder were studied at different shear rates. The results of investigations on the development of torque, yield stress, viscosity and fluidity are presented. In general, Bingham type plastic behavior was observed. Alumina pastes containing 18.5 vol.% of boehmite exhibits adequate plasticity and fluidity for extrusion. The torque value for this composition is 68 kPa at shear rate 28.5 s−1. The viscosity curves of alumina pastes showed “shear thinning” flow behavior. The mixture of 54.15 vol.% alumina and 18.5 vol.% boehmite resulted in 61% of theoretical green density on extrusion. These conditions were extended to synthesis of alumina–aluminium titanate composites, and the extrusion was carried out using 18.5 vol.% boehmite. Aluminium titanate was synthesized through sol–gel method and dispersed uniformly in alumina matrix through colloidal process. The composites could be sintered at 1400°C to 98% of its theoretical value. The SEM fractograph of sintered composite showed reasonably dense, uniformly distributed grains in the order of 2–3 μm. The present study shows that boehmite binder phase yields good workability for extrusion and contributes high green and sintered densities to the composites.

Synthesis of high surface area silica by solvent exchange in alkoxy derived silica gels

Abstract Silica gels have been prepared by the hydrolysis of tetraethyl orthosilicate at pH 9.5 with a high water: TEOS ratio. The gels were exchanged with different solvents such as ethanol, methanol, isopropanol and acetone. The resultant gels were calcined at 500°C for 3 h at heating rate of 3°C min−1. The surface area and total pore volume of the calcined gels measured using nitrogen adsorption show much dependence with the nature of the liquid used for the exchange. The gel washed with ethanol under the conditions of present investigation shows maximum surface area. The adsorption isotherm of this gel also shows the characteristics of Type IV behaviour. Other gels usually show Type II behaviour. Solvents of low vapour pressure and high molecular volume favour the formation of gels with high pore volume. The method of exchange of solvents also influences the characteristics of the final gel precursor in terms of surface area and pore volume. Repeated washing of the gels with solvents and subsequent removal result in high surface area, while evaporating the gels with solvents results in relatively less porous gels.

Effect of seeds on the formation of sol-gel mullite

Abstract Mullite precursor gels have been prepared from a mixture of particulate boehmite sol and tetraethoxysilane at a pH of 4. The sol has been seeded with submicron size α-Al 2 O 3 , γ-Al 2 O 3 and mullite limited to two percent by weight of the mullite phase. These gels have been invesitgated with respect to phase formation, densification and microstructural development. Dilatometric studies on the seeded precursor gels indicate marked shrinkage in their profiles. Of the different seed nuclei, fine mullite particles have shown excellent influence in the early ceramic phase formation as well as densification. Further they induce complete transformation of the precursor gel to the high temperature phase as well as a uniform fine-grained microstructure.

Thermal decomposition characteristics of boehmite gels under microwave heating and associated microstructural features

Abstract High temperature densification and associated microstructural features of alumina derived through sintering of microwave dehydroxylated boehmite are presented. α-alumina has been obtained through a series of transitional alumina phases within 60 min of exposure of boehmite gel to microwaves of 2.45 GHz and 600 W power. The phase formation has been identified by XRD. The surface area measurements on various gel samples exposed to microwaves at intermediate time intervals are also reported.The densification characteristics of microwave dehydroxylated gel precursors when sintered by conventional route resulted 95% of theoretical density at 1400°C and an average grain size of 2 μm. On the other hand, part of the same gel precursor conventionally dehydroxylated and sintered at 1400°C resulted 97% theoretical density with very fine microstructure. Thus, the present study indicates the possibility of using low power microwaves for dehydroxylation of boehmite precursor gels to active phases which further can be sintered under conventional heating to dense, fine grained alumina.

Delamination through sonication for hydroxy metal oxide sol intercalation of montmorillonite

Abstract Boehmite intercalated alumina pillared montmorillonite has been prepared from a 10% aqueous suspension of the sodium exchanged montmorillonite in as short a period as 20 min through ultrasonication at 32 °C. This technique has resulted in Al-PILC having specific surface area 165 m 2 /g and d -spacing 17 A, stable up to 700 °C. The delamination of the alumino silicate layers through the sonication technique enhances the extent of intercalation as well as the stability, in addition to reducing considerably the time of reaction practiced at present for the synthesis of pillared montmorillonites. Further, the delamination is effective in suspensions of high solid content. The samples were characterised by chemical analysis, XRD, surface area (BET), TGA, DTA and SEM. The properties were compared to a sample prepared after 30 h of intercalation at 32 °C using boehmite species as intercalant.