• Deepak

      Articles written in Bulletin of Materials Science

    • Critical properties of random magnetic systems

      Deepak Kumar

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      A review of the recent applications of renormalisation group techniques to the calculations of the critical properties of random magnetic systems is presented.

    • A low cost laser-raman spectrometer

      A K Bandyopadhyay Nita Dilawar Arun Vijayakumar Deepak Varandani Dharambir Singh

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      A Jobin Yvon-Spex (HR640) monochromator with a notch filter (514·5 nm) and Ar+ ion laser has been used to set up a low cost laser Raman spectrometer. The selection and setup of the various optical components used in the present work has been solely carried out in our laboratory. The calibration of the monochromator was established from the studies of various standard mercury lines and the obtained data fitted with the reported data. Raman signals have been recorded for a number of samples e.g. diamond, ruby, carbon tetrachloride (CCI4), benzene (C6H6) and ethanol (C2H5OH). The obtained results are found to be in excellent agreement with the reported values for these materials in the literature.

    • A detailed model for defect concentration and dopant activation in GaAs

      Deepak N Lakshminarayana

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      Defects in semi-insulating (SI) GaAs are especially critical in determining the properties of devices in which dopants are introduced by ion-implantation. The defects in GaAs are native to the material and their concentrations are subsequently modified after ion-implantation and annealing. In this work, we have extended the existing models in the literature by incorporating a large set of defects and using the most recent values for formation energies of these defects. The model includes eight types of point defects, the vacancy of Ga and As, their antisites and interstitials of Ga and As on both sub-lattices, along with carbon related defects always present in SI–GaAs. We have also included Si and related defects when this element is implanted as an 𝑛-type dopant. All these defects are considered in several charge states allowed by their stability conditions. The model assumes thermodynamic equilibrium between the point defects at an anneal temperature. Then the GaAs wafer is quenched so that the number of defects remain the same as those at the anneal temperature, but redistribution of charges occurs in various charge states. We find that the defect concentrations are extremely sensitive to the crystal stoichiometry, and good agreement with experimental data is shown. However, when we calculate the dopant activation in implanted GaAs, the quantitative agreement with experiments is not adequate. This discrepancy is explained on the basis of available formation energies for the defects.

    • Variations in first principles calculated defect energies in GaAs and their effect on practical predictions

      Deepak D Balamurugan K Nandi

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      There is an abundant literature on calculations of formation and ionization energies of point defects in GaAs. Since most of these energies, especially the formation energies, are difficult to measure, the calculations are primary means of obtaining their values. However, based on the assumptions of the calculations, the reported values differ greatly among the various calculations. In this paper we discuss the sources of errors and their impact on practical predictions valuable in GaAs device fabrication. In particular, we have compared a large set of computed energies and selected the most appropriate values. Then, in the context of GaAs material quality, we investigated the impact of errors in calculation of formation energies on the performance of the GaAs substrate for device fabrication. We find that in spite of the errors inherent in ab initio calculations, it is possible to correctly predict the behaviour of GaAs substrate.

    • Evaluation of a.c. conductivity behaviour of graphite filled polysulphide modified epoxy composites

      Navin Chand Deepak Jain

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      Composites of epoxy resin having different amounts of graphite particles have been prepared by solution casting method. Temperature dependence of dielectric constant, tan 𝛿 and a.c. conductivity was measured in the frequency range, 1–20 kHz, temperature range, 40–180°C for 0.99, 1.96 and 2.91 wt% graphite filled and unfilled epoxy composites. It was observed that the dielectric constant, tan 𝛿 and a.c. conductivity increase with increasing temperature. Near the transition temperature the materials show anomalous behaviour for the observed properties. Peaks of dielectric constant, tan 𝛿 and a.c. conductivity were observed to shift towards lower temperature with increasing frequency. Clear relaxation (tan 𝛿) peaks around 169°C were observed in epoxy resin, which shifted to lower temperature side on increasing the frequency. Addition of 2.91 wt% graphite shifted the tan 𝛿 peaks towards higher temperature side by creating hindrances to the rotation of polymer dipoles. Addition of 2.91 wt% graphite leads to an increased relaxation time 𝜏 of dipoles in polysulphide epoxy from 1.44 × 10-5-3.92 × 10-5 (s) at 90°C by creating the hindrance to the rotation of dipoles.

    • Investigations on gradient a.c. conductivity characteristics of bamboo (Dendrocalamus strictus)

      Navin Chand Deepak Jain Archana Nigrawal

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      Effect of temperature and frequency variation on a.c. conductivity of bamboo was determined by using a 4274 A Multi-Frequencies LCR meter. Electrical measurements were carried out in the temperature range 24–120°C and in the frequency range 4–100 kHz. It was observed that the a.c. conductivity increased initially and then decreased with increase of temperature and frequencies. The increase of distance from outer surface to the inner surface side increased the a.c. conductivity values and showed the grading in a.c. conductivity behaviour. Two phases of a.c. conductivity behaviour with temperature exist in bamboo. At 10 mm distance a.c. conductivity suddenly increases which is the critical depth from skin for this bamboo. Increase of temperature, at all the frequencies increases the a.c. conductivity initially and then decreases. Downward peaks in a.c. conductivities are observed at all the frequencies due to the presence of moisture in bamboo, which liberated on heating. Sharp peak is observed in case of sample 4, which is inner most strip. Maximum sharp peak is observed at lowest 4 kHz frequency.

    • X-ray study of weak interactions in two flavonoids

      Deepak Sharma Vivek K Gupta Goutam Brahmachari Sadhan Mondal Arindam Gangopadhyay

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      X-ray diffraction studies were carried out on single crystals of two flavonoids, viz. 5-hydroxy-6,7,4′-trimethoxyflavone, C18H16O6, (I) and 5-hydroxy-3,7,4′-trimethoxyflavone, C18H16O6, (II). Crystal structures of both the flavonoids were solved by direct methods and refined by full-matrix least-squares procedures. In both the molecules, the benzopyran moiety is planar. The dihedral angle between the phenyl ring and the benzopyran portion is 5.50(4)° in (I) and 29.11(5)° in (II). In (I), the crystal packing is influenced by O–H…O hydrogen bonds, and weak C–H…O and $\pi \ldots \pi$ interactions whereas in (II) the crystal structure is stabilized by the presence of four intermolecular short contacts of the type C–H…O. There is also one C–H$\ldots \pi$ hydrogen bond with H… centroid distance of < 2.7 Å. The molecules are further stabilized by 𝜋–𝜋 interactions.

    • Formation of InN nanoparticle and nanorod structures by nitrogen plasma annealing method

      Ajay Kumar Mann Deepak Varandani Bodh Raj Mehta Lalit Kumar Malhotra G Mangamma A K Tyagi

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      In the present study, a novel method involving nitrogen plasma annealing has been reported for preparing InN nanoparticle/nanorod structures and for improving the properties of InN nanoparticle layers. Plasma annealed structures have been characterized by X-ray diffraction, atomic force microscopy and photoluminescence spectroscopy techniques. InN nanoparticle layers have been prepared using activated reactive evaporation set up. It has been observed that there is a remarkable improvement in the conductivity and crystallinity of InN nanoparticle layers on annealing in nitrogen plasma. This has been attributed to the increase in the nitrogen content of the samples. Experiments involving plasma annealing of In nanorods deposited oxide template has also been carried out. It was found that on plasma treatment In nanorods get converted to mixed phase InN nanorods with hexagonal and cubic fractions.

    • Studies on blends of cycloaliphatic epoxy resin with varying concentrations of carboxyl terminated butadiene acrylonitrile copolymer I: Thermal and morphological properties

      Garima Tripathi Deepak Srivastava

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      Differential scanning calorimetric (DSC), thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) of the blends of cycloaliphatic epoxy (CAE) resin toughened with liquid elastomer such as carboxyl terminated butadiene acrylonitrile copolymer (CTBN) have been carried out. Exothermal heat of reaction due to cross linking of the resin in the presence of diamino diphenyl sulphone (DDS, an amine hardener) showed a decreasing trend with increasing rubber concentration. Enhancement of thermal stability as well as lower mass loss of the epoxy–rubber blends with increasing rubber concentration have been observed in thermogravimetric analysis (TGA). Dynamic mechanical properties reflected a monotonic decrease in the storage modulus (𝐸′) with increasing rubber concentration. The loss modulus (𝐸″) and the loss tangent (tan 𝛿) values, however, showed an increasing trend with rise of temperature up to a maximum (peak) followed by a gradual fall in both cases.

    • An investigation in InGaO3(ZnO)m pellets as cause of variability in thin film transistor characteristics

      Sonachand Adhikari Rajeev Gupta Ashish Garg Deepak

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      Indium–gallium–zinc oxide (IGZO) is a novel amorphous oxide semiconductor, which recently has received much attention for thin film transistors (TFTs) in flat panel displays. Published literature reports significant variations in the properties of thin films and TFTs prepared from IGZO even though the reported process conditions are similar.We demonstrate that these differences could arise from the method for preparation of targets from which the films are made. Accordingly, we also propose simple and appropriate conditions, specifically using much lower sintering temperatures and thus avoiding use of sealed Pt tubes for preparation of IGZO targets in composition range, InGaO3(ZnO)m, with 1 ≤ m ≤ 5. These target materials are suitable in physical vapour deposition processes such as pulsed laser deposition and sputtering. In developing the process for sintering, the phase analysis of the target pellets was carried out using X-ray diffraction (XRD). The chemical compositions of the phases are also confirmed with inductively coupled plasma optical emission spectrometry (ICP-OES) and energy dispersive X-ray (EDX) techniques. We also demonstrate successful deposition of amorphous IGZO thin films by pulse laser deposition using the targets prepared by the proposed sintering process. Finally, we demonstrate that unmonitored method of making pellets for films deposition is a cause of variability associated in published literature on IGZO TFTs.

    • A 1 V supercapacitor device with nanostructured graphene oxide/polyaniline composite materials

      Deepak Kumar Anjan Banerjee Satish Patil Ashok K Shukla

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      Polyaniline and graphene oxide composite on activated carbon cum reduced graphene oxide-supported supercapacitor electrodes are fabricated and electrochemically characterized in a three-electrode cell assembly. Attractive supercapacitor performance, namely high-power capability and cycling stability for graphene oxide/polyaniline composite, is observed owing to the layered and porous-polymeric-structured electrodes. Based on the materials characterization data in a three-electrode cell assembly, 1 V supercapacitor devices are developed and performance tested. A comparative study has also been conducted for polyaniline and graphene oxide/polyaniline composite-based 1 V supercapacitors for comprehending the synergic effect of graphene oxide and polyaniline. Graphene oxide/polyaniline composite-based capacitor that exhibits about 100 F g−1 specific capacitance with faradaic efficiency in excess of 90% has its energy and power density values of 14 Wh kg−1 and 72 kW kg−1, respectively. Cycle-life data for over 1000 cycles reflect 10% capacitance degradation for graphene oxide/polyaniline composite supercapacitor.

    • Assessment of mechanical and three-body abrasive wear peculiarity of TiO$_2$- and ZnO-filled bi-directional E-glass fibre-based polyester composites

      AKANT KUMAR SINGH SIDDHARTHA DEEPAK

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      This paper is about the development of bi-directional E-glass fibre-based polyester composites filled with zinc oxide (ZnO) and titanium dioxide (TiO$_2$) fillers, respectively. The mechanical characterization of these composites is performed. The three-body abrasive wear characteristic of fabricated composites has been assessed under different operating conditions. For this, the three-body abrasion test is done on dry abrasion test rig (TR-50)and analysed using Taguchi’s experimental design scheme and analysis of variance. The results obtained from these experiments are also validated against existing microscopic models of Ratner–Lancaster and Wang. A good linear relationship is obtained between specific wear rate and the reciprocal of ultimate strength and strain at tensile fracture of these composites. It indicates that the experimentally obtained results are in good agreement with theseexisting models. It is found that the tensile strength decreases with filler loading, while hardness, flexural strength, inter-laminar shear strength and impact strength are increased. TiO$_2$-filled composites were observed to performbetter than ZnO-filled composites under abrasive wear situations. The wear mechanism is studied in correlation with the SEM micrograph of the worn-out surface of composites. Performance optimization of composites is doneby using VIKOR method.

    • Cation substitution induced blue-shift of optical band gap in nanocrystalline Zn$_{(1−x)}$Ca$_x$O thin films deposited by sol–gel dip coating technique

      NRIPASREE NARAYANAN N K DEEPAK

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      Transparent nanocrystalline Zn${(1−x)}$Ca$_{x}O ($0 ≤ x ≤ 0.20$) thin films were deposited on glass substrates by sol–gel dip coating method. The X-ray diffraction (XRD) pattern revealed the polycrystalline nature of the films withhexagonal wurtzite structure and confirmed the non-existence of the secondary phase corresponding to CaO indicating the monophasic nature of the deposited films. The crystallinity of the films deteriorated with higher dopant concentration due to the segregation or separation of dopant ions in grain boundaries. The lattice parameters and the unit cell volume increased to a higher Ca-dopant concentration. This was due to the successful incorporation of Ca2+ ions with larger ionic radius inthe host zinc oxide (ZnO) lattice. The optical transmittance spectra of the samples showed transmittances above 60% in thevisible spectral range and the absorption edge in the near ultra-violet region got blue-shifted with cation substitution. Theestimated optical energy gaps confirmed the band gap widening with increase in Ca-dopant concentration. The calculatedvalues increased from 3.30 eV for undoped ZnO to 3.73 eV for Zn0.8Ca0.2O thin films giving 13.03% enhancement in theenergy gap value due to the electronic perturbation caused by cation substitution as well as deterioration in crystallinity.

    • A first-principle investigation into effect of B- and BN-doped C$_{60}$ in lowering dehydrogenation of MXH$_4$ (where M $=$ Na, Li and X $=$ Al, B)

      MEENAKSHI DEEPAK AGNIHOTRI KIRAN JEET HITESH SHARMA

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      The present paper reports the effect of B- and BN-doped C$_{60}$ as catalysts for lowering the dehydrogenationenergy inMXH4 clusters (M $=$ Na and Li, X $=$ Al and B) using density functional calculations.MXH4 interacts strongly withB-doped C$_{60}$ and weakly with BN-doped C$_{60}$ in comparison with pure C$_{60}$ with binding energy 0.56–0.80 and 0.05–0.34 eV, respectively. The hydrogen release energy (EHRE) of MXH$_4$ decreases sharply in the range of 38–49% when adsorbed on B-doped C$_{60}$; however, with BN-doped C$_{60}$ the decrease in the EHRE varies in the range of 6–20% as compared with pure MXH$_4$ clusters. The hydrogen release energy of second hydrogen atom in MXH$_4$ decreases sharply in the range of 1.7–41% for BN-doped C$_{60}$ and decreases in the range of 0.2–11.3% for B-doped C$_{60}$ as compared with pure MXH$_4$ clusters. The results can be explained on the basis of charge transfer within MXH$_4$ cluster and with the doped C$_{60}$.

    • Improvement in light-extraction efficiency of light emitting diode using microlenses fabricated by a novel and cost-effective method

      RAHUL K SHARMA MONICA KATIYAR DEEPAK

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      A low cost, solution-based novel process was proposed, which avoids any photo-lithographically fabricated Si template and yet yields small dimension microlenses. The microlenses were fabricated using a well-known chemical technique called dewetting of polymer thin film. Polystyrene (PS) was used to make a polymer thin film with thickness ranging from 20 to 40 nm. Then, this PS film was kept in dewetting solution namely methylethylketone (MEK) to obtain the microlenses. Dimension of these microlenses was measured to be the order of 1–2 $\mu$m. The effect of various microlens parameters such as diameter and area fraction on light-extraction efficiency was systematically studied. Improvement of 4% in extraction efficiency was obtained by employing it on white light emitting diode. The area fraction of microlenses was increased up to 0.34 by reducing the spin speed. The light-extraction efficiency was further enhanced up to 7% uponincreasing the area fraction of microlenses.

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