• Sangeeta

      Articles written in Bulletin of Materials Science

    • Characterization of ferroelectric and superconducting ceramics prepared from precursor carbonates

      D Pandey V S Tiwari A K Singh Sangeeta Chaudhry

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      The advantages of a new semi-wet method for the synthesis of (Ba, Ca)TiO3 and YBa2Cu3O7 − y powders from precursor carbonates are described. The precursor carbonates are prepared using chemical coprecipitation technique under controlled pH conditions to ensure uniform distribution of Ba+2/Ca+2 and Y+3/Ba+2 ions. It is shown that the powders synthesized by this route possess better chemical homogeneity as compared to those obtained by the conventional ceramic technique involving calcination of a mixture of BaCO3, CaCO3, TiO2 and Y2O3, BaCO3, CuO powders. The behaviour of the ferroelectric (Ba, Ca)TiO3 and the superconducting YBa2Cu3O7 − y ceramics prepared from powders obtained by our method are found to be markedly different from those known for the conventionally prepared ceramics.

    • Growth of highTc Bi-Sr-Ca-Cu-O thick films

      Sangeeta Chaudhry Neeraj Khare A K Gupta K C Nagpal V N Ojha G S N Reddy V S Tomar

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      Thick films of Bi-Sr-Ca-Cu-O were deposited on (100) MgO substrates by screen-printing technique with the starting composition 1112. To attain the superconducting state, the films were subjected to two-step heat-treatment. R-T and XRD have been studied for films annealed at different durations of the second step. InitiallyTc (R=0) increased from 77 to 103 K as the annealing duration was increased after whichTc decreased. Kinetics of the growth of highTc phase is discussed in the light of our results.

    • Study of galvanomagnetic effects in Bi-Sr-Ca-Cu-O films

      Ramesh Chandra Neeraj Khare A K Gupta D K Walia Sangeeta Chaudhry V N Ojha V S Tomar

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      Galvanomagnetic effect has been studied at 77 K on Bi-Sr-Ca-Cu-O films as a function of d.c. bias current. These films were prepared by spray pyrolysis and screen-printing techniques, withTc (R=0) 80 K and 100 K respectively. Magnetic field dependence ofIc of sprayed-film showed very small hysteresis whereas screen-printed film showed greater hysteresis. Application of small magnetic field on these films destroyed the zero-resistance state and a finite resistance was developed. The slope of the resistance vs magnetic field curveΔR/ΔH depended on the current flowing through the sample. To increaseΔR/ΔH, meander-shape pattern was prepared on the sprayed film. In generalΔR/ΔH increased by a factor of 103 after sample patterning. The possibility of using these films as a sensitive magnetic field sensor is discussed.

    • Mössbauer, xrd and positron annihilation studies on natural magnetite and hematite ore from Ari Dongri, central India

      D Das M B Chakraborti K Choudhury P M G Nambissan B R S Babu P Sen Sangeeta C K Majumdar

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      Natural magnetite and hematite samples taken from iron ore deposits associated with Precambrian banded iron-formation (BIF) at Ari Dongri (20°23′N:81°3°E), Bastar district in Central India have been studied by Mossbauer, XRD and positron annihilation techniques. Three magnetite samples show a genetic association with α-Fe2O3 with a wide range of variations in Fe3O4:α-Fe2O3 ratio. The fourth sample, a typical specular hematite, shows α-Fe2O3 content of the order of 90%, the rest being magnetite. The magnetite present in the samples was found to be stoichiometric. None of the samples contains maghemite (γ-Fe2O3). Some geological implications of the observed variation in the oxidation states of the samples are considered.

    • Deposition of polymer bilayer configuration by pulsed laser ablation and its use for study of polymer-polymer interface

      Sangeeta Kale J P Jog V M Nadkarni

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      Thin films of polyphenylene sulphide (PPS) and polyethylene (PE) polymers have been deposited in a bilayer configuration using pulsed excimer laser ablation. Such bilayer specimens have been annealed at different temperatures, up to a maximum of 120°C, and for different time intervals, up to a maximum of 110 min, to investigate the evolution of the interface. By employing the technique of spectroscopic ellipsometry, the nature and the degree of thermally induced polymeric transport across the interface are brought out.

    • Sintering of ultra high molecular weight polyethylene

      Sangeeta Hambir J P Jog

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      Ultra high molecular weight polyethylene (UHMWPE) is a high performance polymer having low coefficient of friction, good abrasion resistance, good chemical resistance etc. It is used in shipbuilding, textile industries and also in biomedical applications. UHMWPE is processed by powder processing technique because of its high melt viscosity at the processing temperature. Powder processing technique involves compaction of polymeric powder under pressure and sintering of the preforms at temperature above its melting point. In this study, we report our results on compaction and sintering behaviour of two grades of UHMWPE with reference to the powder morphology, sintering temperatures and strength development.

    • Preparation and characterization of magnesium-aluminium-silicate glass ceramics

      Madhumita Goswami T Mirza A Sarkar Shobha Manikandan Sangeeta S L Verma K R Gurumurthy V K Shrikhande G P Kothiyal

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      Synthesis of machinable quality magnesium aluminium silicate (MgO-Al2O3-SiO2) for fabrication of insulators/spacers usable in high voltage applications under high vacuum conditions has been carried out following two different routes i.e. (i) sintering route, and (ii) glass route. A three-stage heating schedule involving calcination, nucleation and crystallization, has been evolved for the preparation of magnesium aluminium silicate (MAS) glass ceramic with MgF2 as a nucleating agent. The effect of sintering temperature on the density of compacted material was studied. Microstructure and machinability of samples obtained from both routes were investigated. They were also characterized for microhardness. Initial studies on material obtained by glass route reveal that these samples are superior to those obtained from sintered route in respect of their high voltage breakdown strength and outgassing behaviour. Outgassing rate of 10−9 Torr l·s−1 cm−2 and breakdown strength of 160 kV/cm were obtained. Different types of spacers, lugs, nuts and bolts have been prepared by direct machining of the indigenously developed glass ceramic.

    • Electroless deposition, post annealing and characterization of nickel films on silicon

      Subir Sabharwal Siddharth Palit R B Tokas A K Poswal Sangeeta

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      Electroless deposition of nickel (EN) films on 𝑛-type silicon has been investigated under different process conditions. The interface between the film and substrate has been characterized for electrical properties by probing the contact resistances. X-ray diffraction and atomic force microscopy have been performed to obtain information about the structural and morphological details of the films. As a comparative study, nickel films have also been sputter deposited on silicon substrates. An as-deposited electroless film is observed to form non-ohmic contact while in a sputtered film prepared without the application of substrate heating, the formation of metal–insulating–semiconductor type junction is seen.

    • X-ray absorption spectroscopy of PbMoO4 single crystals

      D Bhattacharyya A K Poswal S N Jha Sangeeta S C Sabharwal

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      X-ray absorption spectra of PbMoO4 (LMO) crystals have been investigated for the first time in literature. The measurements have been carried out at Mo absorption edge at the dispersive EXAFS beamline (BL-8) of INDUS-2 Synchrotron facility at Indore, India. The optics of the beamline was set to obtain a band of 2000 eV at 20,000 eV and the channels of the CCD detector were calibrated by recording the absorption edges of standard Mo and Nb foils in the same setting. The absorption spectra have been measured for three LMO samples prepared under different conditions viz.

      grown in air from stoichiometric starting charge,

      grown in argon from stoichiometric starting charge and

      grown in air from PbO-rich starting charge.

      The results have been explained on the basis of the defect structure analysed in LMO crystals prepared under different conditions. The Mo absorption edge is significantly influenced by the deviations in crystal stoichiometry.

    • Optical and X-ray photoelectron spectroscopy of PbGeO3 and Pb5Ge3O11 single crystals

      S C Sabharwal S N Jha Sangeeta

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      Pb5Ge3O11 crystals are found to exhibit pale yellow colouration while PbGeO3 are colourless. X-ray photoelectron spectroscopy (XPS) measurements show lead deficiency in both the crystals. The results also reveal a stronger ionic character for PbGeO3 as compared to Pb5Ge3O11 crystal. The binding energy of Ge3𝑑 core level in the case of Pb5Ge3O11 crystal is found to be smaller than the binding energy of germanium oxide, thereby indicating the incomplete oxidation of Ge ions in the crystal lattice. On gamma ray irradiation, the transmission of both the crystals is observed to deteriorate uniformly over the entire wavelength range, which has been attributed to the oxidation of some of the lattice Pb ions. On gamma irradiation the changes observed in O1𝑠 core level energies for both the crystals are seen to be consistent with the changes noted in the Pb4𝑓7/2 and Ge3𝑑 spectra. Interestingly, the results reveal oxidation of surface Ge atoms with atmospheric oxygen under gamma irradiation.

    • Dielectric behaviour of Pb-substituted BZT ceramics

      Parveen Kumar Sangeeta Singh J K Juneja K K Raina Chandra Prakash

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      Material series with compositional formula Ba1.0–𝑥Pb𝑥Ti0.90Zr0.10O3 (0 ≤ 𝑥 ≤ 0.20, in steps of 0.05) were prepared by conventional solid state method. All the samples were subjected to X-ray diffraction (XRD) studies and found to be single phase with perovskite structure. SEM measurements were done in order to collect micro-structural information. Different transition temperatures were found to depend on the Pb content. Tetragonality (𝑐/𝑎) and Curie temperature (𝑇c) increase with increase in lead content 𝑥. Dielectric properties were studied as a function of temperature and frequencies.

    • EXAFS investigations on PbMoO4 single crystals grown under different conditions

      A K Poswal D Bhattacharyya S N Jha Sangeeta S C Sabharwal

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      Extended X-ray absorption fine structure (EXAFS) measurements on PbMoO4 (LMO) crystals have been performed at the recently-commissioned dispersive EXAFS beamline (BL-8) of INDUS-2 Synchrotron facility at Indore, India. The LMO samples were prepared under three different conditions viz.

      grown from a stoichiometric starting charge in air ambient,

      grown from a stoichiometric starting charge in argon ambient and

      grown from PbO-rich starting charge in air ambient.

      The EXAFS data obtained at both Pb 𝐿3 and Mo K edges of LMO have been analysed to determine Pb–O, Pb–Mo and Mo–O bond lengths in the crystals. The information thus obtained has been used to examine the microscopic defect structures in crystals grown under different conditions.

    • Characterization and optical properties of Pr$_2$O$_3$-doped molybdenum lead-borate glasses

      SANGEETA B KOLAVEKAR N H AYACHIT R V ANAVEKAR

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      Pr$^{3+}$ doped molybdenum lead-borate glasses with the chemical composition 75PbO−[25–($x+y$)B$_2$O$_3$]–$y$MoO$_3$–$x$Pr$_2$O$_3$ (where $x = 0.5$ and 1.0 mol% and y = 0 and 5 mol%) were prepared by conventional melt-quenching technique. Thermal, optical and structural analyses are carried out using DSC, UV and FTIR spectra. The physical parameters,like glass transition ($T_g$), stability factor ($\Delta T$), optical energy band gap ($E_{gopt}$), of these glasses have been determined as a function of dopant concentration. The $T_g$ and optical energy gaps of these glasses were found to be in the range of 290–350$^{\circ}$C and 2.45–2.7 eV, respectively. Stability of the glass doped with Pr$^{3+}$ is found to be moderate ($\sim$40). The results are discussed using the structural model of Mo–lead-borate glass

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