• Ravi Kumar

      Articles written in Bulletin of Materials Science

    • Characteristics of laser-induced plasma from highTc superconductor

      G Padmaja A V Ravi Kumar V Vidyalal P Radhakrishnan V P N Nampoori C P G Vallabhan

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      The spectroscopic analysis of the emission from the plasma produced by irradiating a highTc superconducting GdBa2Cu3O7 target with a high power Nd:YAG laser beam shows the existence of the bands from different oxides in addition to the lines from neutrals and ions of the constituent elements. The spectral emissions by oxide species in laser-induced plasma show considerable time delays as compared to those from neutral and ionic species. Recombination processes taking place during the cooling of the hot plasma, rather than the plasma expansion velocities, have been found to be responsible for the observed time delays in this case. The decays of emission intensities from various species are found to be non-exponential.

    • Elastic properties of ZnF2-PbO-TeO2 glasses doped with certain rare earth ions

      V Ravi Kumar N Veeraiah

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      The elastic moduli (Y, η, σ andH) and some thermodynamical parameters, such as Debye temperatureθD, diffusion constantDi and latent heat of melting ΔHm, of ZnF2-PbO-TeO2 glasses doped with some rare earth (Pr3+, Nd3+, Eu3+ and Tb3+) ions are determined as functions of temperature using piezo electric composite oscillator technique. All the parameters are found to decrease with increase in the atomic number Z of the rare earth ions and with increase in the temperature of measurement. The results are explained on the basis of the density of localized bonding defect states within the band gap of the material.

    • Dielectric properties of LiF-B2O3 glasses doped with certain rare earth ions

      A V Ravi Kumar B Apparao N Veeraiah

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      Dielectric constantɛ, loss tanδ and a.c. conductivityσ of LiF-B2O3: Ln3+ (where Ln=Ce, Pr, Nd and Tb) glasses are studied as functions of frequency (in the range 102–106 Hz) and temperature (range 30–200°C). The dielectric breakdown strength of these glasses was also determined at room temperature in an air medium. The rate of increase ofɛ and tanδ with temperature decreases with decrease in the ionic radius of RE3+ ion whereas the dielectric breakdown strength, the activation energy for a.c. conduction in the high temperature region decreases with increase in the ionic radius of RE3+ ion. An attempt has been made to explain the a.c. conduction in these glasses on the basis of quantum mechanical tunnelling (QMT) model.

    • 1/f Noise properties of swift heavy ion irradiated epitaxial thin films of YBCO

      S K Arora Ravi Kumar D Kanjilal G K Mehta S Khatua R Pinto Vijay Kumar A K Gupta

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      Effect of 250 MeV107Ag ion irradiation induced columnar defects on the noise properties of the YBCO superconductor in the normal and superconducting state have been investigated. Magnitude of the spectral density of the noise is found to scale inversely with the frequency and exhibit a quadratic dependence on the bias current confirming that the noise arises due to the resistance fluctuations. The magnitude ofSv has been found to decrease with decrease in temperature and shows a noise peak in the transition region. The noise performance of these materials in the vicinity of the superconducting transition as well as in the normal state is found to improve by an order of magnitude after irradiation with 250 MeV107Ag ions. The decrease in the magnitude of 1/f noise peak is due the irradiation induced enhanced flux pinning of the material which suppresses the flux motion induced noise in the vicinity ofTc.

    • Chitin and chitosan fibres: A review

      M N V Ravi Kumar

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      Chitin is the most abundant natural amino polysaccharide and estimated to be produced annually almost as much as cellulose. It has become of great interest not only as an underutilized resource, but also as a new functional material of high potential in various fields and the recent progress in chitin chemistry is quite noteworthy. The purpose of this review is to take a closer look at fibres made of chitin and its derivatives. Based on the current research and existing products, some new and futuristic approaches, in the development of novel fibres and their applications have been thoroughly discussed.

    • Thermoluminescence properties of 𝛾-irradiated Bi doped BaS nanostructures

      Surender Singh Ankush Vij S P Lochab Ravi Kumar Nafa Singh

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      Bismuth doped barium sulphide nanocrystallities were prepared and characterized by XRD. Thermoluminescence (TL) studies of these samples after exposure to gamma radiation were carried out. The TL glow curve of the phosphors have two peaks at 403 K and 658 K while in their bulk counterparts these peaks were reported at 486 K and 570 K (Rao 1986). We noted that TL intensity increases with gamma exposure time in the range 30 min – 41 h which may be explained on the basis of track interaction model (TIM) and a high surface to volume ratio for the nanostructures. The kinetic parameters at various heating rates namely activation energy (E), order of kinetics (b) and frequency factor (s) of BaS : Bi (0.4 mol%) sample was determined using Chen’s method. The deconvolution of curve was done using the GCD function suggested by Kitis. The effect of different heating rates and different amount of dose has also been discussed.

    • High resolution X-ray diffraction studies on unirradiated and irradiated strontium hexaferrite crystals

      Balwinder Kaur Monita Bhat F Licci Ravi Kumar K K Bamzai P N Kotru

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      High-resolution X-ray diffraction technique, employing a three-crystal monochromator–collimator combination is used to study the irradiation induced defects in flux grown Sr-hexaferrite crystals irradiated with 50 MeV Li3+ ion beams at room temperature with a fluence value of 1 × 1014 ions/cm2. The diffraction curves of the irradiated crystals suggest the possibility of creation of low angle grain boundaries and other point/clusters of defects causing amorphization in the irradiated crystals. The perfection of the irradiated and unirradiated (0001) cleaved surfaces of the crystals is studied using the bulk method of X-ray topography. The topographs supplement the findings suggestive of modifications in the crystalline quality of SrFe12O19 on irradiation with SHI of Li3+. Etching of the (0001) cleaved surfaces in H3PO4 at 120°C suggests that the dissolution characteristics of the surfaces get affected on irradiation with SHI of Li3+, besides supporting the findings of HRXRD and X-ray topography regarding modifications in the perfection of SrFe12O19 on irradiation.

    • Elastic, thermoluminescence and dielectric characterization of lead antimony silicate glass composites doped by small concentrations of Cr₂O₃


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      A series of glass composites [(Sb₂O₃)0.05(SiO₂)0.65]–[(PbO)(0.3−x):(Cr₂O₃)x ](0 ≤ x ≤ 0.01 mol%) were synthesized. Elastic, thermoluminescence, direct current (dc) conductivity and dielectric characteristics of these glass com-posite materials were studied. The investigations on these glass composites have indicated that the chromium ions exhibit two different oxidation states, such as Cr³⁺ and Cr⁶⁺. The variation in poison ratio (σ ) with micro-hardness (H) plots suggests that the glass composites were prepared with a strongly and covalently connected internal structure. The observed thermoluminescence output increased with an increase in the dose of UV-irradiation. The observed peak positions of these thermoluminescent curves were shifted towards higher temperature regions. The enhanced thermoluminescence output results even recommended that both e− and h+ trap cantres were created at the deeper trap levels and contribute to thermolu-minescence emissions at higher temperatures. The dc conductivity (σ_{dc}) and activation energy evaluations are also observed. Dielectric parameters such as density of energy states $N$($E$_{f} ), temperature region of relaxation and loss tangent (tan δ)were calculated. The linear relationship between variation in alternating current conductivity (σ_{ac}) and activation energy suggests that good amounts of polaron hopping with an increase in Cr₂O₃ concentration in these materials were observed. The grades of loss tangent and the number of energy states near the Fermi level suggest that the materials prepared are highly useful in dielectrics.

    • Influence of Bi$_2$O$_3$, Sb$_2$O$_3$ and Y$_2$O$_3$ on optical properties of Er$_2$O$_3$-doped CaO–P$_2$O$_5$–B$_2$O$_3$ glasses


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      The glasses of the composition 25CaO–10(M$_2$O$_3$)–14P$_2$O$_5$–50B$_2$O$_3$:1Er$_2$O$_3$ (where M$=$Bi/Sb/Y) are prepared. The prepared samples are characterized by XRD, thermoluminescence (TL),UVand photoluminescence (PL) techniques. TL reports suggest that the sample of 10 mol%Y2O3 concentration (EY) exhibiting good shape symmetry factor ($u = 0.484$) and low AEs ($E_{\tau} = 1.189$, $E_{\delta} = 1.218$ and $E_{\omega} = 1.210$) under $\gamma$-irradiation might be a good TL asset. The optical absorptions of present glasses are studied well through the Judd–Ofelt theory.The evaluations such as Judd–Ofelt parameters are additionallyprojected. They are found to be the best ($\Omega_2 = 1.62 \times 10^{−21}$ cm$^2$, $\Omega_4 = 1.56 \times 10^{−21}$ cm$^2$ and $\Omega_6 = 0.59 \times 10^{−21}$ cm$^2$) for the sample of 10 mol% Y$_2$O$_3$ concentration (E$_{\rm Y}$), which may be a good optical asset to develop a novel class of laser resources. The radiative evolutions corresponding to the emissive transition ${}^4$S$_{3/2}$ $\to$ ${}^4$I$_{15/2}$ are assessed by PL technique. The evaluations are found to be the best ($A_t = 7089$ s$^{−1}$, $\tau_{\rm rad} = 140$ $\mu$s and $\beta = 96.1$%) for the sample of 10 mol% Y$_2$O$_3$concentration (E$_{\rm Y}$), which may be a useful luminescent resource.

    • Evaporation-driven self-assembly in the mixtures of micro and nanoparticles


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      We report experimental studies on the self-assembly of silica microspheres and Laponite nanoplatelets (NPs) in evaporating sessile droplets and in thin films, respectively. A ring-like stain of the silica microspheres with positionalorder is observed after the evaporation of sessile droplets due to the coffee-ring effect. This effect is suppressed in the binary mixtures of silica microspheres and Laponite NPs. A depletion zone has been observed in the mixtures during the sessile droplet evaporation, the width of which can be tuned by varying the compositions. We demonstrate a simple method for preparing core–shell particles by evaporating thin films of binary mixtures in which the Laponite NPs self-assemble to form a crystalline shell on the amorphous silica microspheres.We present a possible orientation of the Laponite NPs in the shell.

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    • Dr Shanti Swarup Bhatnagar for Science and Technology

      Posted on October 12, 2020

      Prof. Subi Jacob George — Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bengaluru
      Chemical Sciences 2020

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

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      Posted on July 25, 2019

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