Yogesh K. Vohra

Los Alamos National Laboratory, Лос-Аламос, California, United States

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Publications (348)769.35 Total impact

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    ABSTRACT: Blends of poliglecaprone (PGC) and polycaprolactone (PCL) of varying compositions were electrospun into tubular conduits and their mechanical, morphological, thermal and in vitro degradation properties were evaluated under simulated physiological conditions. Generally, mechanical strength, modulus and hydrophilic nature were enhanced by the addition of PGC to PCL. In vitro degradation study in PBS (pH of 7.3) was carried out for up to one month to understand the hydrolytic degradation effect on the mechanical properties in both longitudinal and circumferential directions. Pure PCL and 4:1PCL/PGC blend scaffolds exhibited considerable elastic stiffening after a one-month long in vitro degradation. FT-IR and DSC techniques were used to understand the degradation behavior and the changes in structure and crystallinity of the polymeric blends. 3:1 PCL/PGC blend was concluded to be a judicious blend composition for tubular graft based on overall results from mechanical properties and performance after one month in vitro degradation study.
    Polymer International 11/2014; 64(4). DOI:10.1002/pi.4834 · 2.13 Impact Factor
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    ABSTRACT: High pressure structural transition studies have been carried out on rare earth metal gadolinium in a diamond anvil cell at room temperature to 169 GPa. Gadolinium has been compressed to 38% of its initial volume at this pressure. With increasing pressure, a crystal structure sequence of hcp → Sm-type → dhcp → fcc → dfcc → monoclinic has been observed in our studies on gadolinium. The measured equation of state of gadolinium is presented to 169 GPa at ambient temperature. Magnetic ordering temperature of gadolinium has been studied using designer diamond anvils to a pressure of 25 GPa and a temperature of 10 K. The magnetic ordering temperature has been determined from the four-point electrical resistivity measurements carried out on gadolinium. Our experiments show that the magnetic transition temperature decreases with increasing pressure to 19 GPa and then increases when gadolinium is subjected to higher pressures.
    High Pressure Research 10/2014; 34(4). DOI:10.1080/08957959.2014.977277 · 0.93 Impact Factor
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    ABSTRACT: Nanocrystalline titanium (nc-Ti) metal was investigated up to 161 GPa at room temperature using a diamond anvil cell. X-ray diffraction and electrical resistance techniques were used to investigate the compressibility and structural phase stability. nc-Ti is observed to undergo three structural phase transitions at high pressures, starting with at 10 GPa and followed by at 127 GPa and at 140 GPa. The observed structural phase transitions, as well as compressibility, are consistent with previously reported values for coarse grained Ti (c-Ti). The high pressure experiments on nc-Ti samples do no show any significant variation of the transition pressure under varying non-hydrostatic conditions. This is in sharp contrast to c-Ti, where a significant decrease in the transition pressure is observed under increasing non-hydrostatic conditions. This would indicate that the decrease in grain size in nano grained titanium makes the phase transition less sensitive to shear stresses as compared to bulk or c-Ti.
    09/2014; 1(3):035044. DOI:10.1088/2053-1591/1/3/035044
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    ABSTRACT: The structural phase stability of N-(4-hydroxyphenyl) acetamide (paracetamol) has been studied at ambient temperature up to 23 GPa using Raman spectroscopy. Spectral changes have provided further evidence for a highly kinetically driven Form I  II transition that occurs as a mixed phase from 4.8 - 6.5 GPa, and might complete as early as 7 GPa. Upon further compression to 8.1 GPa, a drastic shift in spectral signature was observed providing the first evidence for a previously undiscovered Form IV of paracetamol. Additional shifts in mode intensities were observed near 11 GPa indicating a potential restructuring of the hydrogen bonding network and/or structural modification to a potentially new Form V. Phase boundaries at 7 and 8 GPa were confirmed under hydrostatic conditions using Raman spectroscopy. Spectral changes indicate that the transition Form IV  V occurs near 11 GPa. Multiple ab initio harmonic frequency calculations at different levels of theory were performed with a B3LYP/6-31G** being used to provide a more robust mode assignment to our experimentally obtained Raman modes. High pressure X-ray diffraction (XRD) was performed up to 21 GPa which provided further evidence for a highly kinetically driven Form I  II transition in agreement with our Raman measurements. In addition, the XRD provided further evidence for the existence of Form IV near 8 GPa and Form V near 11 GPa with Form V persisting up to 21 GPa.
    The Journal of Physical Chemistry A 07/2014; 118(31). DOI:10.1021/jp411810y · 2.78 Impact Factor
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    ABSTRACT: Iridium-based 5d transition-metal oxides are attractive candidates for the study of correlated electronic states due to the interplay of enhanced crystal-field, Coulomb and spin-orbit interaction energies. At ambient pressure, these conditions promote a novel Jeff = 1/2 Mott-insulating state, characterized by a gap of the order of ~0.1 eV. We present high-pressure electrical resistivity measurements of single crystals of Sr2IrO4 and Sr3Ir2O7. While no indications of a pressure-induced metallic state up to 55 GPa were found in Sr2IrO4, a strong decrease of the gap energy and of the resistance of Sr3Ir2O7 between ambient pressure and 104 GPa confirm that this compound is in the proximity of a metal-insulator transition.
    Journal of Physics Condensed Matter 06/2014; 26(25):255603. DOI:10.1088/0953-8984/26/25/255603 · 2.22 Impact Factor
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    ABSTRACT: Magnetic ordering temperatures in heavy rare earth metal dysprosium (Dy) have been studied using an ultrasensitive electrical transport measurement technique in a designer diamond anvil cell to a pressure of 69 GPa and a temperature of 10 K. Previous studies using magnetic susceptibility measurements at high pressures were able to track magnetic ordering temperature only till 7 GPa in the hexagonal close packed (hcp) phase of Dy. Our studies indicate that the magnetic ordering temperature shows an abrupt drop of 80 K at the hcp-Sm phase transition followed by a gradual decrease that continues till 17 GPa. This is followed by a rapid increase in the magnetic ordering temperatures in the double hcp phase and finally leveling off in the distorted face centered cubic phase of Dy. Our studies reaffirm that 4f-shell remains localized in Dy and there is no loss of magnetic moment or 4f-shell delocalization for pressures up to 69 GPa.
    High Pressure Research 05/2014; 34(2). DOI:10.1080/08957959.2014.903946 · 0.93 Impact Factor
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    ABSTRACT: 1,1-diamino-2,2-dinitroethylene (FOX-7) is a low sensitivity energetic material with performance comparable to commonly used secondary explosives such as RDX and HMX. At ambient pressure, FOX-7 exhibits complex polymorphism with at least three structurally distinct phases (α, β, and γ). In this study, we have investigated the high pressure-temperature stability of FOX-7 polymorphs using synchrotron mid-infrared (MIR) spectroscopy. At ambient pressure, our MIR spectra and corresponding differential scanning calorimetry (DSC) measurements confirmed the known α → β (~110 °C) and α → β (~160 °C) structural phase transitions; as well as, indicated an additional transition γ → (~210 °C), with the δ phase being stable up to ~251 °C prior to decomposition. In situ MIR spectra obtained during isobaric heating at 0.9 GPa, revealed a potential α → β transition that could occur as early as 180 °C, while β → β+δ phase transition shifted to ~300 °C with suppression of γ phase. Decomposition was observed slightly above 325 °C at 0.9 GPa.
    Journal of Physics Conference Series 05/2014; 500(5):052005. DOI:10.1088/1742-6596/500/5/052005
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    ABSTRACT: We have designed a portable pressure controller module to tune compression rates and maximum pressures attainable in a standard gas-membrane diamond anvil cell (DAC). During preliminary experiments, performed on zirconium (Zr) metal sample, pressure jumps of up to 80 GPa were systematically obtained in less than 0.2s (resulting in compression rate of few GPa/s up to more than 400 GPa/s). In-situ x-ray diffraction and electrical resistance measurements were performed simultaneously during this rapid pressure increase to provide the first time resolved data on α → ω → β structural evolution in Zr at high pressures. Direct control of compression rates and peak pressures, which can be held for prolonged time, allows for investigation of structural evolution and kinetics of structural phase transitions of materials under previously unexplored compression rate-pressure conditions that bridge traditional static and shock/dynamic experimental platforms.
    Journal of Physics Conference Series 05/2014; 500(3):032020. DOI:10.1088/1742-6596/500/3/032020
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    ABSTRACT: Nanostructured diamond (NSD) films were grown on silicon and Ti-6Al-4V alloy substrates by microwave plasma chemical vapor deposition (MPCVD). NSD Growth rates of 5 mu m/h on silicon, and 4 mu m/h on Ti-6Al-4V were achieved. In a chemistry of H-2/CH4/N-2, varying ratios of CH4/H-2 and N-2/CH4 were employed in this research and their effect on the resulting diamond films were studied by X-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, and atomic force microscopy. As a result of modifying the stock cooling stage of CVD system, we were able to utilize plasma with high power densities in our NSD growth experiments, enabling us to achieve high growth rates. Substrate temperature and N-2/CH4 ratio have been found to be key factors in determining the diamond film quality. NSD films grown as part of this study were shown to contain 85% to 90% sp(3) bonded carbon.
    Materials 12/2013; 7(1):365-374. DOI:10.3390/ma7010365 · 1.88 Impact Factor
  • Sunil Karna, Yogesh Vohra
    10/2013; 3(1). DOI:10.5539/jmsr.v3n1p43
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    ABSTRACT: High-pressure superconductivity in a rare-earth doped Ca0.86Pr0.14Fe2As2 single crystalline sample has been studied up to 12 GPa and temperatures down to 11 K using designer diamond anvil cell under a quasi-hydrostatic pressure medium. The electrical resistance measurements were complemented by high pressure and low temperature x-ray diffraction studies at a synchrotron source. The electrical resistance measurements show an intriguing observation of superconductivity under pressure, with Tc as high as ~51 K at 1.9 GPa, presenting the highest Tc reported in the intermetallic class of 1-2-2 iron-based superconductors. The resistive transition observed suggests a possible existence of two superconducting phases at low pressures of 0.5 GPa: one phase starting at Tc1 ~48 K, and the other starting at Tc2~16 K. The two superconducting transitions show distinct variations with increasing pressure. High pressure low temperature structural studies indicate that the superconducting phase is a collapsed tetragonal ThCr2Si2-type (122) crystal structure. Our high pressure studies indicate that high Tc state attributed to non-bulk superconductivity in rare-earth doped 1-2-2 iron-based superconductors is stable under compression over a broad pressure range.
    High Pressure Research 10/2013; 34(1). DOI:10.1080/08957959.2013.879867 · 0.93 Impact Factor
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    ABSTRACT: Simultaneous low-temperature electrical resistivity and Hall effect measurements were performed on single-crystalline Bi_{2}Se_{3} under applied pressures up to 50 GPa. As a function of pressure, superconductivity is observed to onset above 11 GPa with a transition temperature T_{c} and upper critical field H_{c2} that both increase with pressure up to 30 GPa, where they reach maximum values of 7 K and 4 T, respectively. Upon further pressure increase, T_{c} remains anomalously constant up to the highest achieved pressure. Conversely, the carrier concentration increases continuously with pressure, including a tenfold increase over the pressure range where T_{c} remains constant. Together with a quasilinear temperature dependence of H_{c2} that exceeds the orbital and Pauli limits, the anomalously stagnant pressure dependence of T_{c} points to an unconventional pressure-induced pairing state in Bi_{2}Se_{3} that is unique among the superconducting topological insulators.
    Physical Review Letters 08/2013; 111(8):087001. · 7.73 Impact Factor
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    ABSTRACT: Neutron diffraction and electrical transport measurements have been carried out on the heavy rare-earth metal terbium at high pressures and low temperatures in order to elucidate the onset of ferromagnetic (FM) order as a function of pressure. The electrical resistance measurements show a change in slope as the temperature is lowered through the FM Curie temperature. The temperature of this FM transition decreases at a rate of−16.7 K/GPa up to a pressure of 3.6 GPa, at which point the onset of FM order is suppressed. The neutron diffraction measurements as a function of pressure at temperatures ranging from 90 to 290 K confirm that the change of slope in the resistance is associated with the FM ordering, since this occurs at pressures similar to those determined from the resistance results at these temperatures. A disappearance of FM ordering was observed as the pressure is increased above 3.6 GPa and is correlated with the phase transition from the ambient hexagonal close packed structure to an α-Sm-type structure at high pressures.
    High Pressure Research 08/2013; 33(3):555-562. DOI:10.1080/08957959.2013.806503 · 0.93 Impact Factor
  • Journal of Neurotrauma 08/2013; 30(15):A61-A61. · 3.97 Impact Factor
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    ABSTRACT: We have used X-ray diffraction to study the structural phase of CeCoIn5 in external pressure. Using high-pressure X-ray diffraction, we find that the crystalline phase is stable in the P4/mmm phase for pressures ≤51.2 GPa. From our measured equation of state, we find a bulk modulus given by B 0 = 72.8 ± 2.9 GPa and a first pressure derivative of B ′ = 5.1 ± 0.3. Measurement of the electrical resistivity of CeCoIn5 to pressures as high as 34.4 GPa shows the existence of a peak in resistivity at p ∗ = 8.2 ± 0.2 GPa.
    Physics of Condensed Matter 07/2013; 86(7). DOI:10.1140/epjb/e2013-40563-7 · 1.46 Impact Factor
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    ABSTRACT: Uranium sulfide belongs to a class of uranium monochalcogenides that crystallize in the rocksalt structure and exhibit ferromagnetism at low temperature. The magnetism is believed to play a role in the low-temperature rhombohedral distortion, possibly due to its large magnetic anisotropy. We have performed electrical and structural characterization along with density-functional theory calculations as functions of pressure to help understand the interplay between structure and magnetism in US. Theoretical calculations suggest that ferromagnetic order is responsible for the small distortion at ambient pressure and low temperature. Under pressure, the Curie temperature is reduced monotonically until it discontinuously disappears near a pressure-induced deformation of the crystal structure. This high-pressure distortion is identical to the one correlated with the onset of magnetic order, but with a larger change in the cell angle. Calculations imply a reduction in the electronic band energy as the driving force for the pressure-induced structure, but the loss of magnetic order associated with this distortion remains a mystery. The high-pressure electronic phase diagram may shed light on the magnetostructural free energy landscape of US.
    Physical review. B, Condensed matter 06/2013; 87(21). DOI:10.1103/PhysRevB.87.214104 · 3.66 Impact Factor
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    ABSTRACT: High pressure electrical resistance and x-ray diffraction measurements have been performed on ruthenium-doped Ba(Fe0.9Ru0.1)2As2, up to pressures of 32 GPa and down to temperatures of 10 K, using designer diamond anvils under quasi-hydrostatic conditions. At 3.9 GPa, there is an evidence of pressure-induced superconductivity with Tc onset of 24 K and zero resistance at Tc zero of ~14.5 K. The superconducting transition temperature reaches maximum at ~5.5 GPa and then decreases gradually with increase in pressure before completely disappearing above 11.5 GPa. Upon increasing pressure at 200 K, an isostructural phase transition from a tetragonal (I4/mmm) phase to a collapsed tetragonal phase is observed at 14 GPa and the collapsed phase persists up to at least 30 GPa. The changes in the unit cell dimensions are highly anisotropic across the phase transition and are qualitatively similar to those observed in undoped BaFe2As2 parent.
    Physics of Condensed Matter 04/2013; 87(3). DOI:10.1140/epjb/e2014-40265-8 · 1.46 Impact Factor
  • Sunil Karna, Yogesh Vohra
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    ABSTRACT: The growing demand of wide band semiconductors entice researcher to investigate electronic properties in diamond. The chemical vapor deposition (CVD) method has shown that various level of doping can be possible in diamond films. The purpose of this study was to investigate the growth morphology and quality of boron doped diamond film with deposition parameters. Various level of boron doped diamond films were synthesized epitaxially on synthetic (100) ib type diamond substrate using microwave plasma assisted CVD. The structural, optical and electrical characterizations were made to study effect of deposition parameters and pretreatment of substrates on surface morphology and growth quality. Raman spectra showed shape modification of the zone center optical phonon line and its downshift with the increasing boron content in the film. Additional bands were also observed in lower wavelength region below optical phonon line. Surface modification of films with increasing boron content has been observed in atomic force microscopy. High growth rate and high quality films were obtained with the addition of a few ppm of nitrogen in feed gas during deposition with little compromise on conductivity. Electrical measurement showed carriers have been transported via two different conduction mechanisms.
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    ABSTRACT: We have investigated the insulator-to-metal transition in thin film vanadium dioxide as a function of pressure at ambient temperature using a designer diamond anvil cell (DAC). Four-point probe resistance measurements show a monotonic decrease over the entire pressure range studied with no significant discontinuity. High-pressure X-ray diffraction measurements observe an M1 (P21/c) phase at 0 GPa, an M2 (C2/m) phase from 0.8 GPa to 1.1 GPa, and a reentrant M1 phase from 1.1 GPa to 13.5 GPa. Crystal refinement above 1.1 GPa shows a monotonically decreasing a, b and c lattice constants and a minimum in the monoclinic angle, β, near 8.5±0.5 GPa. The atomic positions show that the first V-V nearest neighbor distance (d) decreases over the entire pressure range, the second nearest neighbor distance (s) increases until 5 GPa after which it is constant with s f 3.2 å. The next most closely spaced V-V distance (f), which corresponds to V atoms in different unit cells, is approximately constant across the entire pressure range measured.
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    ABSTRACT: Simultaneous low-temperature electrical resistivity and Hall effect measurements were performed on single-crystalline Bi2Se3 under applied pressures up to 50 GPa. As a function of pressure, superconductivity is observed to onset above 11 GPa with a transition temperature Tc and upper critical field Hc2 that both increase with pressure up to 30 GPa, where they reach maximum values of 7 K and 4 T, respectively. Upon further pressure increase, Tc remains anomalously constant up to the highest achieved pressure. Conversely, the carrier concentration increases continuously with pressure, including a tenfold increase over the pressure range where Tc remains constant. Together with a quasi-linear temperature dependence of Hc2 that exceeds the orbital and Pauli limits, the anomalously stagnant pressure dependence of Tc points to an unconventional pressure-induced pairing state in Bi2Se3 that is unique among the superconducting topological insulators.
    Physical Review Letters 02/2013; 111(8). DOI:10.1103/PhysRevLett.111.087001 · 7.73 Impact Factor

Publication Stats

5k Citations
769.35 Total Impact Points

Institutions

  • 2014
    • Los Alamos National Laboratory
      • Shock and Detonation Physics Group (DE-9)
      Лос-Аламос, California, United States
  • 1993–2014
    • University of Alabama at Birmingham
      • • Department of Physics
      • • Department of Materials Science and Engineering
      Birmingham, Alabama, United States
  • 2013
    • University of Maryland, College Park
      • Department of Physics
      College Park, MD, United States
  • 1991–2012
    • Lawrence Livermore National Laboratory
      • Condensed Matter and Materials Division
      Livermore, California, United States
  • 1985–2011
    • Cornell University
      • Department of Materials Science and Engineering
      Ithaca, New York, United States
  • 1979–2001
    • Bhabha Atomic Research Centre
      • Chemistry Division
      Mumbai, State of Maharashtra, India
  • 1982–2000
    • Universität Paderborn
      Paderborn, North Rhine-Westphalia, Germany
  • 1995
    • University of Alabama
      Tuscaloosa, Alabama, United States