Minati Chatterjee

Indium tin oxide nano-particles through an emulsion technique

Abstract Spheroidal indium tin oxide (ITO) nano-particles with In/Sn=85:15 (at.%) were prepared by an emulsion technique under mechanical agitation without the aid of any surface-active agent. X-ray powder diffraction (XRD) indicated the stabilization of cubic ITO as the only phase at 250 °C. Particle size analysis showed a narrow distribution of 20–58 nm range particles with a mean size of 38 nm, thus, confirming their nano-structured nature. The pellets sintered at 1350 °C for 6 h were found to have 95–97% of theoretical density. Thus, well sinterable nano-particles of ITO powders were prepared at a significantly low temperature compared to that in the reported methods.

Hollow alumina microspheres from boehmite sols

Hollow alumina microspheres were obtained from emulsified boehmite sols by an ion extraction method. The viscosity and the equivalent alumina content of the sols were found to affect the characteristics of the derived microspheres. High-viscosity sols produced broken microspheres. A temperature of about 400–500 °C for boehmite to γ-Al2O3 transformation in the gel microspheres was observed by differential thermal analysis and X-ray diffraction. Complete crystallization of the gel microspheres to α-Al2O3 occurred at 1200 °C. A tentative mechanism for the formation of the hollow microspheres is presented.

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.

Thermal barrier coatings from sol gel-derived spray-grade Y2O3-ZrO2 microspheres

For the development of ceramic thermal barrier coatings, spray-grade yttria-stabilized zirconia microspheres were prepared by the sol-gel technique. Oxide microspheres were obtained by calcination of the corresponding gel spheres at 1000 °C. Scanning electron microscopic and optical microscopic observations revealed the material thus obtained to have a predominantly spherical morphology and the requisite size distribution (5–50 μm). The dense, calcined microspheres showed good flowability. X-ray diffraction studies indicated the presence of the tetragonal polymorph of ZrO2 as the major phase, in addition to about 14% monoclinic ZrO2. The plasma-sprayed YSZ coatings made from the sol-gel-derived microspheres showed a further decrease in the monoclinic ZrO2 content (6%). The coatings survived 40–50 thermal cycles (30 min at 1200 °C followed by a water quench), indicating good thermal shock resistance.

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.

Chromia microspheres by the sol-gel technique

Abstract Chromia microspheres were synthesized by the sol-emulsion-gel method. The solvent extraction technique was followed for the preparation of the sols. Cr 2 O 3 crystallized as single phase in the temperature range 400 to 1300 °C. Complete crystallization occurred at 1300 °C. Spherical morphology of the powder was confirmed by scanning electron microscopy. A size range of 3–30 μm was observed in the powder calcined at 1300 °C.

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 ).

Preparation of ZrO2CaO and ZrO2MgO fibres by alkoxide Sol-Gel processing

Abstract Extensive studies were made on the spinnability of Zr(O n C 3 H 7 ) 4 -derived sols for the preparation of ZrO 2 CaO and ZrO 2 MgO fibres. An improved preparative route was proposed in which acetylacetone controlled the hydrolyzability of the alkoxide and 2-methoxyethanol served as a slow-drying solvent. The advantages of the method of sol preparation over the known methods were highlighted. Crystallization behaviour of the gel fibres was studied mostly up to 1300°C for ZrO 2 CaO fibres (up to 20% mol CaO) and up to 1000°C for ZrO 2 MgO fibres (up to 12% mol MgO). ZrO 2 CaO fibres (7–11% mol CaO) were found to yield only the tetragonal and cubic polymorphs of ZrO 2 after heating at 1300°C/4h and furnace cooling. ZrO 2 MgO fibres, on the other hand, showed exsolution of MgO above 800°C and development of monoclinic ZrO 2 .

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.