• Tokeer Ahmad

      Articles written in Bulletin of Materials Science

    • 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.

    • 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.

    • Fabrication of nano-sized solid solution of Zn1−𝑥Mn𝑥O (𝑥 = 0.05, 0.10, 0.15) in reverse microemulsions: Structural characterization and properties

      Sarvari Khatoon Aparna Ganguly Tokeer Ahmad

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      Mn-doped ZnO nanoparticles were synthesized by reverse micellar method using Tergitol NP9 as a surfactant for the first time. These nanoparticles were characterized using powder X-ray diffraction, transmission electron microscopy and selected area electron diffraction analysis. Structural analysis and optical studies revealed that manganese is incorporated into the ZnO host lattice forming a solid solution. Transmission electron microscopic studies show that the particle size increases from 20–50 nm on increasing the dopant concentration from 0.05–0.15. The specific surface area of Zn1−𝑥Mn𝑥O (𝑥 = 0.05, 0.10 and 0.15) as calculated using BET method was found to be 202.62, 145.78 and 75.66 m2g-1, respectively which are higher than the reported values so far.

    • Optical, magnetic and structural characterization of Zn1−𝑥Co𝑥O nanoparticles synthesized by solvothermal method

      Tokeer Ahmad Sarvari Khatoon Kelsey Coolahan

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      Nanoparticles of Co-doped ZnO with 3.8, 7.2 and 11.5 wt% were synthesized by solvothermal method through oxalate precursor route. X-ray diffraction studies showed the formation of hexagonal ZnO structure for 𝑥 = 0.038, however, secondary phase of Co3O4 arises on increasing the Co content up to 11.5%. Transmission electron microscopic studies showed that particles are in the nano-metric regime and the grain size decreases on increasing the Co concentration. Optical reflectance measurements showed an energy bandgap, which decreases on increasing Co concentration. Specific surface area of these nanoparticles was found to be very high and comes out to be 97.6, 112.1 and 603.8 m2g-1, respectively. All the solid solutions showed paramagnetism with weak antiferromagnetic interactions. It is seen that the antiferromagnetic interaction increases on increasing Co concentration.

    • Structural characterization and properties of YCrO$_3$ nanoparticles prepared by reverse micellar method


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      YCrO$_3$ nanoparticles were prepared by reverse micellar method by the use of surfactant tergitol after heating theprecursor sample at 800$^{\circ}$C. As-prepared YCrO$_3$ nanoparticles were characterized by various sophisticated techniques like X-ray diffraction (XRD), transmission electron microscope, Brunauer–Emmett–Teller surface area analyzer, high frequency LCR-meter, superconducting quantum interface device magnetometer and P–E loop tracer. Powder XRD study revealsthe formation of highly crystalline orthorhombic monophasic YCrO$_3$ nanoparticles. The average grain size of as-preparednanoparticles was found to be 35 nm with the surface area of 348 m$^2$ g$^{−1}$.Wedge-shaped hysteresis for ferromagnetism andthe room temperature ferroelectricity confirm the multiferroic characteristics in the nanoparticles.

    • Synthesis, characterization and dielectric properties of TiO$_2$–CeO$_2$ ceramic nanocomposites at low titania concentration


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      TiO$_{2(x)}$–CeO$_{2(1−x)}$ nanocomposites were prepared at low TiO$_2$ composition of 5, 10, 15 and 20%, by using TiO$_2$ and CeO$_2$ nanoparticles obtained by polymeric citrate precursor method. These nanocomposites were characterized by using powder X-ray diffraction, transmission electron microscopy, scanning electron microscopy, energy dispersive analysis of X-rays and BET surface area studies. BET studies showed the specific surface area of as-prepared nanocomposites in the range of 239–288 m$^2$ g$^{−1}$. Twenty percent of TiO$_2$-based titania–ceria nanocomposites have smallest average particle size of 30 nm and highest surface area of 288 m$_2$ g$^{−1}$ among all the as-prepared nanocomposites. The dielectric characteristics were measured as a function of frequency and temperature. The dielectric constant of TiO$_{2(x)}$–CeO$_{2(1−x)}$ at room temperature was 35.6 (maximum) at 500 kHz for $x = 0.20$.

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      Posted on October 12, 2020

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      Chemical Sciences 2020

      Prof. Surajit Dhara — School of Physics, University of Hyderabad, Hyderabad
      Physical Sciences 2020

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