• Ashok K Ganguli

      Articles written in Bulletin of Materials Science

    • Comparative study of dielectric properties of MgNb2O6 prepared by molten salt and ceramic method

      Vishnu Shanker Ashok K Ganguli

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      Magnesium niobate (MgNb2O6) powder was synthesized by the conventional ceramic route as well as by the molten salt route using a eutectic mixture of NaCl–KCl as the salt and Mg(NO3)$_2\cdot$6H2O and TiO2 as the starting materials. Pure phase of MgNb2O6 could be obtained by the molten salt method at 1100°C. However, in ceramic method the pure phase of MgNb2O6 was obtained by heating at 1025°C for 20 h. On sintering at 1100°C the dielectric constant and dielectric loss of MgNb2O6 obtained by the molten salt method was found to be 19.5 and 0.004 at 100 kHz at room temperature. Lower values were obtained for these oxides prepared by the ceramic route, 16.6 and 0.000518, respectively. In both cases the dielectric constant was quite stable with frequency.

    • Investigation of Ba2–𝑥Sr𝑥TiO4: Structural aspects and dielectric properties

      Vishnu Shanker Tokeer Ahmad Ashok K Ganguli

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      Investigation of solid solution of barium–strontium orthotitanates of the type, Ba2–𝑥Sr𝑥TiO4 (0 ≤ 𝑥 ≤ 2), show that pure phases exist only for the end members, Ba2TiO4 and Sr2TiO4, crystallizing in the 𝛽-K2SO4 and K2NiF4 structures, respectively. The intermediate compositions (till 𝑥 ≤ 1) lead to a biphasic mixture of two Ba2TiO4-type phases (probably through a spinodal decomposition) with decreasing lattice parameters, indicating Sr-substitution in both the phases. For 𝑥 > 1, Sr2TiO4 along with a secondary phase is obtained. The dielectric constant and dielectric loss were found to decrease with Sr substitution till the nominal composition of 𝑥 = 1. However, pure Sr2TiO4 shows higher dielectric constant compared to the solid solution composition. Sr2TiO4 shows very high temperature stability of the dielectric constant.

    • (La2/5Ba2/5Ca1/5)(Mn(2/5)–𝑥Ni𝑥Ti3/5)O3: Rietveld studies, dielectric and magnetic properties of new perovskite-related oxides

      Pika Jha Saroj L Samal Kandalam V Ramanujachary Samuel E Lofland Ashok K Ganguli

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      Oxides of the type (La2/5Ba2/5Ca1/5)(Mn(2/5)–𝑥Ni𝑥Ti3/5)O3 (0 ≤ 𝑥 ≤ 0.4) have been synthesized by the ceramic route. All the above oxides have been found to crystallize in the cubic perovskite structure. Rietveld refinement of the Ni-based oxide, (La2/5Ba2/5Ca1/5)(Ni2/5Ti3/5)O3 gave rise to a composition (La0.44Ba0.38Ca0.18) (Ni0.42Ti0.58)O2.85(6) and the refined lattice parameter obtained was 3.9411(2) Å (space group 𝑃𝑚$\bar{3}$𝑚; 𝑅(𝐹2) = 0.026, 𝑅p = 0.074, 𝑤𝑅p = 0.087). A shift from antiferromagnetic to paramagnetic behaviour is observed with increase in nickel concentration, the Mn-rich phases showing antiferromagnetism around 5 K. There is a systematic decrease in the dielectric constant, 𝜀 and loss tangent with increase in Ni concentration (from 𝜀 = 592 for 𝑥 = 0 to 𝜀 = 78 for 𝑥 = 0.4).

    • Synthesis of nanocrystalline materials through reverse micelles: A versatile methodology for synthesis of complex metal oxides

      Ashok K Ganguli Sonalika Vaidya Tokeer Ahmad

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      We have been successful in obtaining monophasic nanosized oxides with varying chemical compositions using the reverse micellar method. Here we describe our methodology to obtain important metal oxides like ceria, zirconia and zinc oxide. The oxalate of cerium, zirconium and zinc were synthesized using the reverse micellar route. While nanorods of zinc oxalate with dimension, 120 nm in diameter and 600 nm in length, could be obtained, whereas spherical particles of size, 4–6 nm, were obtained for cerium oxalate. These precursors were heated to form their respective oxides. Mixture of nanorods and nanoparticles of cerium oxide was obtained. ZrO2 nanoparticles of 3–4 nm size were obtained by the thermal decomposition of zirconium oxalate precursor. ZnO nanoparticles (55 nm) were obtained by the decomposition of zinc oxalate nanorods. Photoluminescence (PL) studies at 20 K shows the presence of three peaks corresponding to free excitonic emission, free to bound and donor–acceptor transitions. We also synthesized nanoparticles corresponding to Ba1–𝑥Pb𝑥ZrO3 using the reverse micellar route. The dielectric constant and loss were stable with frequency and temperature for the solid solution.

    • Magnetic and photocatalytic properties of nanocrystalline ZnMn2O4

      Menaka Mohammed Qamar Samuel E Lofland Kandalam V Ramanujachary Ashok K Ganguli

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      The present study describes the synthesis of ZnMn2O4 nanoparticles with the spinel structure. These oxide nanoparticles are obtained from the decomposition of metal oxalate precursors synthesized by

      the reverse micellar and

      the coprecipitation methods.

      Our studies reveal that the shape, size and morphology of precursors and oxides vary significantly with the method of synthesis. The oxalate precursors prepared from the reverse micellar synthesis method were in the form of rods (micron size), whereas the coprecipitation method led to spherical nanoparticles of size, 40–50 nm. Decomposition of oxalate precursors at low temperature (∼ 450°C) yielded phase pure ZnMn2O4 nanoparticles. The size of the nanoparticles of ZnMn2O4 obtained from reverse micellar method is relatively much smaller (20–30 nm) as compared to those made by the co-precipitation (40–50 nm) method. Magnetic studies of nanocrystalline ZnMn2O4 confirm antiferromagnetic ordering in the broad range of ∼ 150 K. The photocatalytic activity of ZnMn2O4 nanoparticles was evaluated using photo-oxidation of methyl orange dye under UV illumination and compared with nanocrystalline TiO2.

    • Anisotropic silica mesostructures for DNA encapsulation

      Aparna Ganguly Ashok K Ganguli

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      The encapsulation of biomolecules in inert meso or nanostructures is an important step towards controlling drug delivery agents. Mesoporous silica nanoparticles (MSN) are of immense importance owing to their high surface area, large pore size, uniform particle size and chemical inertness. Reverse micellar method with CTAB as the surfactant has been used to synthesize anisotropic mesoporous silica materials. We have used the anisotropic silica nanostructures for DNA encapsulation studies and observed a loading capacity of ∼8 𝜇g mg-1 of the sample. On functionalizing the pores of silica with amine group, the amount of DNA loaded on the rods decreases which is due to a reduction in the pore size upon grafting of amine groups.

    • Self-assembling behaviour of Pt nanoparticles onto surface of TiO2 and their resulting photocatalytic activity

      M Qamar Ashok K Ganguli

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      In the present study, self-assembling behaviour of guest nanoparticles (platinum) onto the surface of host support (titanium dioxide) during photodeposition process as a function of solution pH has been explored in detail by means of transmission electron microscope (TEM). The photocatalytic activity of the resulting bimetallic nanoassembly (Pt/TiO2) was evaluated by studying the degradation of two organic pollutants viz. triclopyr and methyl orange. Microscopic studies revealed that the deposition and/or distribution of Pt nanoparticles onto the surface of TiO2 were strongly guided by the ionization state of support which in turn was regulated by the solution pH of photodeposition process. A direct relationship between the solution pH of deposition process and the photocatalytic activity of resulting bimetallic catalyst has been observed. A mechanism based on the interparticle interaction between TiO2 and hydrolytic products of metal ions has been proposed for the differences in the photocatalytic activity of the resulting nanocomposite.

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