Milan Kanti Naskar

Zirconia fibre mats prepared by a sol-gel spinning technique

Abstract Fine grained, polycrystalline, stabilized zirconia (with 10 wt.% Y 2 O 3 ) fibre mats having a web-like structure were fabricated directly during synthesis by a sol-gel spinning technique using a multi-orifice spinneret. Points of fusion in gel fibre mats helped in the formation of a web-like fibrous body having reasonable strength and very little dust formation after calcination. Strong and resilient fibres with diameters in the range 3–10 micron were obtained by a single-step sol-gel method from spinnable acetate sols devoid of organics as the binder aid. The Y 2 O 3 additive in the fibre mats helped to retain the tetragonal phase even at 1600°C by inhibiting grain growth. The health hazard involved with airborne fibres was significantly minimized in the developed zirconia mats by applying sol-gel coatings on their surfaces followed by calcination. The resulting samples exhibited reasonably coherent coatings by anchoring loose surface fibres in a network of zirconia.

Effect of anion type on the synthesis of mesoporous nanostructured MgO, and its excellent adsorption capacity for the removal of toxic heavy metal ions from water

Mesoporous MgO nanostructures of different morphologies like nanoplates, nanosheets and nanoparticle assembled microspheres were prepared by a simple hydrothermal method at 180 °C/5 h in the absence of any organic templates. The products were characterized by X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetry (TG), Fourier transform infrared spectroscopy (FTIR), N2 adsorption–desorption, field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The role of anions of different magnesium precursors on the morphology, textural properties, and heavy metal ion (Pb(II) and Cd(II)) adsorption capacity of the products was studied. The adsorption data were interpreted with Langmuir and Freundlich models. The effect of contact time, adsorbate concentration, pH and temperature on the adsorption capacity of the products was investigated. Nanosheet-like MgO obtained from chloride and nitrate salts of magnesium having higher pore size rendered better adsorption capacity than that prepared from sulphate and carbonate sources of magnesium. The prepared MgO nanostructures showed maximum adsorption capacities up to 3900 mg g−1 and 2980 mg g−1 for Pb(II) and Cd(II) ions, respectively.

Sol-gel alumina fibre mats for high-temperature applications

Abstract Polycrystalline, high-alumina fibre mats (with 4 wt.% SiO 2 ), for application as high-temperature thermal insulation materials having a web-like structure, were fabricated in situ by a single-step sol-gel spinning technique using a multi-orifice spinneret. Strong and resilient fibres with diameters in the range 3–10 μm were obtained from spinnable alumina–silica sols devoid of organics as the binder aid. Incorporation of 4 wt.% SiO 2 as an additive increased the stability of the metastable transition alumina phases, which helped to increase the strength of the fibre mats. The mats exhibited a small linear shrinkage of about 2.75% at 1600 °C. Disintegration of the mats was also found to be negligible (about 2.25 wt.%) at 1600 °C. The health hazard involved with airborne fibres was drastically minimised in the developed fibre mats by applying sol-gel coatings on their surfaces and subsequent calcination at 1200 °C. The oxide-coated fibres exhibited coherent coatings by anchoring loose surface fibres in a network of alumina.

Sol-emulsion-gel synthesis of hollow mullite microspheres

Hollow mullite microspheres were obtained from emulsified diphasic sols by an ion extraction method. The surfactant concentration and viscosity of the sols were found to affect the characteristics of the derived microspheres. The gel and calcined microspheres were investigated by using thermogravimetry analysis (TGA), differential thermal analysis (DTA), Fourier transformed infrared spectroscopy (FTIR), X-ray diffraction (XRD), optical and scanning electron microscopy (SEM) and particle size analysis. TGA indicated the removal of most of the volatiles, i.e. 30.77 wt% up to about 500°C. Crystallization of the Si-Al spinel at 900°–970°C in gel microspheres was confirmed by DTA and XRD. XRD results also showed the formation of orthorhombic mullite at 1200°C. FTIR indicated the sequence of transformations taking place during heat-treatment of gel microspheres at different temperatures. The optical and scanning electron microscopy confirmed the spherical morphology of the gel and calcined particles. Formation of hollow microspheres with a single cavity was identified by SEM. The particle size distribution of the mullite microspheres calcined at 1300°C/1h exhibited a size range of 6–100 μm with an average particle size (d50) of 22.5 μm.

Role of organic solvents and surface-active agents in the sol-emulsion-gel synthesis of spherical alumina powders

Spherical alumina particles were prepared following the sol-emulsion-gel method by systematic variation of (i) the concentration of the surfactant Span 80 above or below the critical micelle concentration (CMC) in different organic solvents and (ii) hydrophilic–lipophilic balance (HLB) of different surfactants in cyclohexane. The experimentally determined CMC of Span 80 was found to increase with increasing dielectric constant of the organic solvent, influencing the sol droplet and alumina particle size. With an increase in the HLB value of the surfactants, the tendency of monodispersed sphere formation among the particles increased with a decrease in the size distribution and average particle size ( d 50 ).

Rare-earth doped zirconia fibres by sol-gel processing

Polycrystalline zirconia fibres, doped with 2–8 mol% of oxides of trivalent lanthanum, praseodymium, neodymium, samarium, gadolinium, and dysprosium (in decreasing cation size), were prepared by spinning of acetate-derived sols and baking the gel fibres thus obtained at 900–1300 °C for 1 h. The larger sized dopants lanthanum, praseodymium and neodymium (Group A) gave rise to tetragonal zirconia, with or without cubic zirconia, at 900 °C which converted partly or fully to monoclinic zirconia, in certain cases accompanied by a cubic zirconate phase at higher temperatures. The smaller sized dopants samarium, gadolinium and dysprosium (Group B) generated only tetragonal or cubic, or both polymorphs of zirconia, depending on the cation type, concentration and temperature. This “stabilization” of higher symmetry polymorphs with Group B dopants was associated with relatively large crystallite size (especially when calcined at 1300 °C). The maximum tensile strength values of usable fibres calcined at 1300 °C were found to decrease with increasing size in dopant dysprosium > gadolinium > samarium > neodymium, praseodymium, lanthanum=0). Although all the dopant cations were larger in size than Zr4+ (in the same oxygen coordination), the relative closeness in size of Group B cations with Zr4+ was considered to be the reason behind the obtained differences in properties.

Synthesis of mesoporous γ-alumina nanorods using a double surfactant system by reverse microemulsion process

Mesoporous γ-Al2O3 nanorods were synthesized by a reverse microemulsion (water in oil, w/o) technique using an aqueous-based alumina sol, the anionic surfactant AOT (sodium bis(2-ethylhexyl sulfosuccinate)), the non-ionic co-surfactant Span 80 (sorbitan monooleate) and cyclohexane as an organic solvent. The γ-Al2O3 phase was obtained at 500 °C with a slight transformation to θ-Al2O3 at 700 °C–900 °C, while a mixed phase of γ-, θ- and α-Al2O3 resulted at 1000 °C. The BET surface area, pore volume and average pore size of the 500 °C-treated sample was found to be 59.3 m2 g−1, 0.040 cm3 g−1 and 2.65 nm respectively, which changed slightly in the temperature range of 500 °C–900 °C. The mesoporous γ-alumina nanorods were formed through rod-like micelle formation in the presence of surfactant/co-surfactant in organic solvent, which was illustrated by a proposed mechanism.

Mullite Fibre Mats by a Sol-Gel Spinning Technique

Fine grained, microcrystalline mullite (Al4+2xSi2−2xO10−x) fibre mats with a web-like structure were fabricated by a sol-gel spinning technique using a multi-orifice spinneret. Points of contact in gel fibre mats helped in the formation of a web-like fibrous body having reasonable strength and very little dust formation after calcination. Strong and resilient fibres with diameters in the range 3−12 μm were obtained by a single-step sol-gel method from spinnable sols devoid of organics as the binder aid. Crystallization of γ-alumina and mullite at about 900°C and 1250°C, respectively was confirmed by differential thermal analysis (DTA) and X-ray diffraction (XRD). Thermogravimetry (TG) indicated the removal of most of the volatiles at about 500°C accompanied by a weight loss of about 48%. Scanning electron microscopy (SEM) shows the presence of small grains (80–150 nm in size) in the fibres calcined at 1250°C. Fourier transformed infrared spectroscopy (FTIR) indicated the sequence of transformations taking place during heat-treatment of gel fibres at different temperatures. The individual fibres in the mats calcined at 1250°C exhibited a tensile strength of 1300–1600 MPa.

A rapid one-pot synthesis of hierarchical hollow mesoporous CuO microspheres and their catalytic efficiency for the decomposition of H2O2

Hierarchical hollow mesoporous CuO microspheres were synthesized by a rapid one-pot hydrothermal method at 150 °C for 2 h, using aqueous based precursors like copper acetate, urea and water. The as-prepared (uncalcined) and calcined (300–500 °C) particles were characterized by differential thermal analysis (DTA), thermogravimetry (TG), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), multipoint BET (Brunauer–Emmett–Teller) surface area, field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The catalytic efficiency of the synthesized microspheres was examined by tracking the decomposition of H2O2. The size of the hollow microspheres was 3–8 μm comprised of small particles (100–300 nm) which were formed by the self-assembly of nanometer sized (5–20 nm) particles with a stone-wall-like structure. The formation mechanism of the hierarchical hollow CuO microspheres is discussed. The BET surface area and total pore volume of the 300 °C-treated sample were found to be 51 m2 g−1 and 0.25 cm3 g−1, respectively, and it exhibited the highest catalytic performance for the decomposition of H2O2 with a rate constant of 27.49 × 10−3 min−1, the highest value ever published. The catalytic activity was studied in terms of the textural properties and calcination temperatures of the microspheres. The hollow CuO microspheres were used as recyclable catalyst.

Effects of processing parameters on the fabrication of near-net-shape fibre reinforced oxide ceramic matrix composites via sol–gel route

The sol infiltration technique was found to be very effective for the fabrication of near-net-shape mullite fibre reinforced ceramic matrix composites (CMCs). The infiltrated sol, in single- and bi-component oxide systems with equivalent molar compositions of Al2O3 (A), 60 Al2O3:40 SiO2 (AS), 87 Al2O3: 13 ZrO2 (AZ), and 94 ZrO2:06 Y2O3 (ZY), after drying and calcination, formed the matrix. The discontinuous mullite fibres (as preforms with 15 vol.% fibre content) acted as the reinforcement agents. The characteristics of the CMCs were found to be strongly dependent on the type of the sols (infiltrates) and their viscosity, presence of non-reactive fillers in the sol, number of infiltrations, intermediate and final sintering temperatures and in-situ deposition of carbon in the fabricated materials. The CMCs were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and the three point bend test. SEM indicated fibre pull-out in the fracture surface of the CMCs. The pseudo-ductile character, developed in the CMCs, was evident from the load–elongation curve of the three-point bend test. The carbon-containing CMCs exhibited a modulus value of almost 51 GPa.