21 research outputs found
Anisotropic cosmological models with two fluids
In this paper, aniostropic dark energy cosmological models have been
constructed in a Bianchi-V space-time with the energy momentum tensor
consisting of two non-interacting fluids namely bulk viscous fluid and dark
energy fluid. Two different models are constructed based on the power law
cosmology and de Sitter universe. The constructed model also embedded with
different pressure gradients along different spatial directions. The variable
equation of state (EoS) parameter, skewness parameters for both the models are
obtained and analyzed. The physical properties of the models obtained with the
use of scale factors of power law and de Sitter law are also presented.Comment: 10 pages, 12 figure
Axially magnetized Dark Energy cosmological model
We investigate the behaviour of the skewness parameters for an anisotropic
universe in the framework of General Relativity. Non interacting dark energy is
considered in presence of electromagnetic field. A time varying deceleration
parameter simulated by a hybrid scale factor is considered. The dynamics of the
universe is investigated in presence and absence of magnetic field. The
equation of state parameter of dark energy evolves within the range predicted
by the observations. Magnetic field is observed to have a substantial effect on
the cosmic dynamics and the skewness parameters. The models discussed here end
in a big rip and become isotropic at finite time.Comment: 9 pages, 13 figures, Version accepted for publication in Mod. Phys.
Lett.
Effect of bulk viscosity in cosmic acceleration
In this paper, we have investigated the late time cosmic acceleration issue
in the context of gravity. The matter field is considered to be that
of viscous fluid. The model has been framed as a mathematical formalism and the
effect of viscous fluid on the cosmic expansion has been shown. The equation of
state parameter indicates the quintessence behaviour of the Universe at late
time. The theoretical results obtained here shows its alignment with the
cosmological observations result.Comment: 11 pages, 8 Figures. Accepted version IJGMM
Cosmological models with Big rip and Pseudo rip Scenarios in extended theory of gravity
In this paper, we have presented the big rip and pseudo rip cosmological
models in an extended theory of gravity. The matter field is considered to be
that of perfect fluid. The geometrical parameters are adjusted in such a manner
that it matches the prescriptions given by cosmological observations, to be
specific to the range of Hubble tension . The models favor phantom
behavior. The violation of strong energy conditions are shown in both the
models, as it has become essential in an extended gravity. The representative
values of the coupling parameter are significant on the evolution of the
universe.Comment: 14 pages, 18 figure
Effect of W substitutions on the phase stability and oxidation behaviour of Mo-Si-B alloys
Mo-Si-B alloys are potential candidate materials for extreme environments, especially at temperature regimes beyond the operating limits of superalloys. Metal rich compositions show excellent creep resistance and fracture toughness, but the oxidation resistance is poor due to low Si and B content. Intermetallic rich compositions show excellent oxidation resistance, but poor fracture toughness. Therefore, the inherent challenge in this system is designing an alloy with excellent prime reliance on its intrinsic oxide scale, with excellent creep and adequate fracture toughness. We address this conundrum by attempting to destabilize the brittle phase A15 Mo3Si in the intermetallic phase field. In the current work, ab-initio calculations were used to evaluate the thermodynamic phase stability of the A15 phase. Experiments indicated that this phase becomes unstable beyond a critical W content (~ 10 atom%). Single crystal studies revealed the site occupancies with W addition to be in accordance with the thermodynamic models. Following studies on phase stability, a series of oxidation experiments were carried out at different temperatures and time intervals with sintered as well as cast alloys. The microstructural length scales and morphologies changed significantly with processing conditions. Transient oxidation studies reveal a strong microstructure dependence of oxidation in this alloy. In addition to transient oxidation studies at 1100 and 1400°C, we shall also present isothermal oxidation behavior of this alloy in the 1100 – 1500°C range. Tungsten additions modify the pesting range of this material, due to the higher volatilization temperature of (WO3)3. The lower vapor pressure of (WO3)3, in comparison to (MoO3)3 also results in a lower initial metal recession, especially in alloys with finer microstructures. The oxidized cross-sections revealed the formation of a continuous borosilicate scale that covers the alloy surface completely resulting in excellent high temperature oxidation resistance
Adsorption and dissociation of molecular oxygen on the (0001) surface of double hexagonal close packed americium
In our continuing attempts to understand theoretically various surface
properties such as corrosion and potential catalytic activity of actinide
surfaces in the presence of environmental gases, we report here the first ab
initio study of molecular adsorption on the double hexagonal packed (dhcp)
americium (0001) surface. Dissociative adsorption is found to be energetically
more favorable compared to molecular adsorption. The most stable configuration
corresponds to a horizontal approach molecular dissociation with the oxygen
atoms occupying neighboring h3 sites, with chemisorption energies at the NSOC
and SOC theoretical levels being 9.395 eV and 9.886 eV, respectively. The
corresponding distances of the oxygen molecule from the surface and
oxygen-oxygen distance were found to be 0.953 Ang. and 3.731 Ang.,
respectively. Overall our calculations indicate that chemisorption energies in
cases with SOC are slightly more stable than the cases with NSOC in the
0.089-0.493 eV range. The work functions and net magnetic moments respectively
increased and decreased in all cases compared with the corresponding quantities
of the bare dhcp Am (0001) surface. The adsorbate-substrate interactions have
been analyzed in detail using the partial charges inside the muffin-tin
spheres, difference charge density distributions, and the local density of
states. The effects, if any, of chemisorption on the Am 5f electron
localization-delocalization characteristics in the vicinity of the Fermi level
are also discussed.Comment: 6 tables, 10 figure
A first-principles based description of the Hf-Ni system supported by high-temperature synchrotron experiments
Hf-Ni is an important binary system for high temperature alloys and shape memory alloys which has been investigated several times in the literature but often using samples of Hf contaminated by Zr. The thermodynamics of this system are remodeled in this work based on first-principles calculations and additional experiments using Hf with relatively low Zr contamination (0.25 wt. %). Diffusion couples in the Ni-rich portion of the Hf-Ni system heat treated at 1173, 1273 and 1373 K are used to measure phase stability and Hf solubility in the fcc phase. The solubility observed in fcc Ni from Ni/Ni50Hf50 (at.%) diffusion couples is larger than that observed in previous experiments. These results are the only source fit to during modeling of the fcc solubility to mitigate effects from Zr contamination. Data in the literature suggests that the high temperature crystal structure of the B33 NiHf phase is, in fact, the B2 structure. High temperature synchrotron measurements provide confirmation of this crystal structure. Modeling of the B2 phase was aided by first-principles calculations using special quasi-random structures (SQS). The present CALPHAD model will prove useful when designing shape memory alloys containing Hf and when modeling the Hf activity in Ni-base high temperature alloys
Observation of gravitational waves from the coalescence of a 2.5−4.5 M⊙ compact object and a neutron star
Ultralight vector dark matter search using data from the KAGRA O3GK run
Among the various candidates for dark matter (DM), ultralight vector DM can be probed by laser interferometric gravitational wave detectors through the measurement of oscillating length changes in the arm cavities. In this context, KAGRA has a unique feature due to differing compositions of its mirrors, enhancing the signal of vector DM in the length change in the auxiliary channels. Here we present the result of a search for U(1)B−L gauge boson DM using the KAGRA data from auxiliary length channels during the first joint observation run together with GEO600. By applying our search pipeline, which takes into account the stochastic nature of ultralight DM, upper bounds on the coupling strength between the U(1)B−L gauge boson and ordinary matter are obtained for a range of DM masses. While our constraints are less stringent than those derived from previous experiments, this study demonstrates the applicability of our method to the lower-mass vector DM search, which is made difficult in this measurement by the short observation time compared to the auto-correlation time scale of DM