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10,783 results on '"KUMAR, PANKAJ"'

27. Article in Hindi

Author

Pandey, Ambeshvari Kumar and Kumar, Pankaj

Published
2022

45. First-principles determination of refractory binary alloy mechanical properties for the microscopic design of complex structural materials

Author

Bijjala, Surya T., Atlas, Susan R., and Kumar, Pankaj

Subjects
Condensed Matter - Materials Science
Abstract

The elastic tensor provides valuable insight into the mechanical behavior of a material with lattice strain, such as disordered binary alloys. Density functional theory (DFT)-based methods provide a powerful mechanism for computing and probing the microscopic features of elastic tensor-related properties. Here we present results for the rigid-ion and relaxed-ion elastic tensors computed using density functional perturbation theory (DFPT), for a comprehensive set of structural refractory body-centered cubic (BCC) binary alloys of molybdenum (Mo), niobium (Nb), tantalum (Ta), and tungsten (W). Intermediate quantities (force-response internal-strain tensor, $\Lambda$, and displacement-response internal-strain tensor, $\Gamma$) needed to compute the relaxed-ion elastic tensor are used here to map heterogeneity in elastic constants at each lattice site in a given alloy and associated relaxation fields. Derived polycrystalline aggregate properties -- the bulk modulus ($B$), shear modulus ($G$), Young's modulus ($E$), and Poisson's ratio ($\nu$) and elastic anisotropy -- are reported as a function of composition for all binaries. Pugh's ratio ($B/G$) and the Cauchy pressure ($C'$) derived from computed elastic moduli are analyzed in order to evaluate the effect of alloying on the mechanical properties of the refractory BCC binary alloys. The computed mechanical properties data for the parent unary materials and binary alloys at systematically-varied Mo, Nb, Ta, and W compositions are in excellent agreement with available experimental data. These results, together with computed microscopic field data, establish a foundation for the principled design of compositionally-complex, high-temperature-structural refractory alloys with desired elastic properties., Comment: 26 pages 16 figures

Published
2024

46. X-ray/Radio Quasi-periodic Pulsations Associated with Plasmoids in Solar Flare Current Sheets

Author

Kumar, Pankaj, Karpen, Judith T., and Dahlin, Joel T.

Subjects
Astrophysics - Solar and Stellar Astrophysics, Astrophysics - High Energy Astrophysical Phenomena
Abstract

Plasmoids (or magnetic islands) are believed to play an important role in the onset of fast magnetic reconnection and particle acceleration during solar flares and eruptions. Direct imaging of flare current sheets and formation/ejection of multiple plasmoids in extreme ultraviolet (EUV) images, along with simultaneous X-ray and radio observations, offers significant insights into the mechanisms driving particle acceleration in solar flares. Here we present direct imaging of the formation and ejection of multiple plasmoids in flare plasma/current sheets and associated quasi-periodic pulsations (QPPs) observed in X-ray and radio wavelengths, using observations from SDO/AIA, RHESSI, and Fermi GBM. These plasmoids propagate bidirectionally upward and downward along the flare current sheet beneath the erupting flux rope during two successive flares associated with confined/failed eruptions. The flux rope exhibits evidence of helical kink instability with formation and ejection of multiple plasmoids in the flare current sheet, as predicted in an MHD simulation of a kink-unstable flux rope. RHESSI X-ray images show double coronal sources (``loop-top" and higher coronal sources) located at both ends of the flare current/plasma sheet. Moreover, we detected an additional transient faint X-ray source (6-12 keV) located between the double coronal sources, which was co-spatial with multiple plasmoids in the flare current sheet. X-ray (soft and hard) and radio (decimetric) observations unveil QPPs (periods$\approx$10-s and 100-s) associated with the ejection and coalescence of plasmoids. These observations suggest that energetic electrons are accelerated during the ejection and coalescence of multiple plasmoids in the flare current sheet., Comment: ApJ (in press), 27 pages, 17 figures

Published
2024

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