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Grain growth kinetics in CoCrFeNi and CoCrFeMnNi high entropy alloys processed by spark plasma sintering
No full textNanocrystalline CoCrFeNi and CoCrFeMnNi high entropy alloys have been processed by mechanical alloying followed by spark plasma sintering. Grain growth kinetics has been estimated for both the alloys by subjecting them to heat treatment in the temperature range 1073-1373 K. These alloys possess a thermally stable single phase FCC structure along with Cr7C3 contamination. Electron back scattered diffraction (EBSD) has been used to determine grain size of all the heat treated samples. Both CoCrFeNi and CoCrFeMnNi alloys exhibit a grain growth exponent, n=3, suggesting long-range diffusion controlled grain growth in these alloys. Activation energies for grain growth are 134 and 197 kJ/mol for CoCrFeNi and CoCrFeMnNi, respectively, which are significantly lower than the activation energy of lattice diffusion in these alloys. Hardness is measured for CoCrFeMnNi alloy as function of grain size and is found to follow the Hall-Petch type relation. The strength coefficient (slope of Hall-Petch relation) is calculated as 1.92 GPa, which is nearly three times that of the value reported in literature for coarse grained CoCrFeMnNi. Presence of carbides enhances the hardness of these HEAs. The maximum contribution to strengthening comes from the FCC-carbide phase boundaries
De-symmetrizing periodically grafted amphiphilic copolymers: design, synthesis and generation of Janus folded chains
No full textPeriodically grafted amphiphilic copolymers (PGACs) were earlier shown to fold in a zigzag fashion to enable segregation of the two immiscible segments, namely the backbone and the pendant segments; crystallization of one or both segments led to the formation of a lamellar structure with the two segments located in alternate layers. In the present study two different pendant segments, namely PEG and fluorocarbon, were installed at periodic intervals in an alternating fashion along a polyethylene-like polyester backbone; the mutual immiscibility between the three different segments, namely backbone alkylene (HC), PEG and fluorocarbon (FC), causes the polymer to fold as earlier, but now the zigzag folded chain would carry fluoroalkyl segments on one side and PEG on the other, thereby generating de-symmetrized Janus-type folded chains. Using DSC, SAXS, WAXS, AFM and FT-IR measurements, we demonstrate the self-segregation and independent crystallization of all three segments, and the formation of a lamellar morphology bearing three different layers. Such layering of three different types of segments in a graft copolymer is unprecedented and could pave the way for interesting applications that require separation of two different functional entities at sub-10 nanometre length scales
Bootstrap and collider physics of parity violating conformal field theories in d=3
No full textWe study the crossing equations in d = 3 for the four point function of two U(1) currents and two scalars including the presence of a parity violating term for the s-channel stress tensor exchange. We show the existence of a new tower of double trace operators in the t-channel whose presence is necessary for the crossing equation to be satisfied and determine the corresponding large spin parity violating OPE coefficients. Contrary to the parity even situation, we find that the parity odd s-channel light cone stress tensor block do not have logarithmic singularities. This implies that the parity odd term does not contribute to anomalous dimensions in the crossed channel at this order light cone expansion. We then study the constraints imposed by reflection positivity and crossing symmetry on such a four point function. We reproduce the previously known parity odd collider bounds through this analysis. The contribution of the parity violating term in the collider bound results from a square root branch cut present in the light cone block as opposed to a logarithmic cut in the parity even case, together with the application of the Cauchy-Schwarz inequality
Enhancement of the fluorescence properties of double stranded DNA templated copper nanoparticles
No full textThe weak fluorescence emission from dsDNA templated copper nanoparticles necessitates the use of high-end detectors like photomultiplier tubes for their detection. This sets limitations on their applicability to in-situ analyte detection and point-of-care applications which utilize comparatively low cost and less sensitive detectors. In this article, a technique to improve the fluorescence properties of copper nanoparticles templated on dsDNA is reported. The fluorescence enhancement is achieved by introducing a modification in the conventional synthesis technique by using a combination of sodium ascorbate and Taq buffer. When compared to the existing methods, the proposed method achieves 11 times higher fluorescence signal intensity from the dsDNA templated copper nanoparticles and 4 times faster attainment of maximum fluorescence signal. The effect of the ionic strength of the individual constituent components of Taq buffer on the fluorescence emission from the copper nanoparticles is also studied here. The utility of this enhancement strategy for analyte measurement is demonstrated with the example of melamine detection from milk samples. A linear relationship was observed between the fluorescence intensity from the copper nanoparticles and the concentration of melamine in the range from 0.5 ppm to 100 ppm (R-2 = 0.9919), with a limit of detection of 0.1 ppm. The reported fluorescence enhancement technique also results in 2.95 times improved sensitivity of detection when compared to the conventional technique
Electrical Switching and Optical Bandgap Studies on Quaternary Ag-Doped Ge-Te-In Thin Films
No full textAmorphous Ge15Te80-xIn5Agx (6 <= x <= 24) thin films prepared in sandwich geometry exhibit memory switching behavior unlike the bulk sample that has shown both threshold switching for less current (1-2mA) andmemory switching for current greater than 2 mA. As anticipated, the threshold voltage of Ge(15)Te(80-x)In5Ag(x)(6 <= x <= 24) thin films is found to be lower as contrast to those of bulk counterparts. The compositional dependence of amorphous system shows an extensive plateau in the range of 6 <= x <= 12 which literally stands for the intermediate phase afterwhich there is a drastic increase in the threshold field. Shifted rigidity percolation threshold has also been confirmed fromthe compositional dependence of threshold field of amorphousGe(15)Te(80-x)In(5)Ag(x) thin films. In addition, the optical bandgap of a-Ge15Te80-xIn5Agx thin-film sample has been reckoned considering absorption spectra, and the compositional dependence has been described based on average bond energy of the system
Investigating thermoacoustic instability mitigation dynamics with a Kuramoto model for flamelet oscillators
No full textIn this paper, we present experimental observations and phenomenological modeling of the intermittent dynamics that emerge while mitigating thermoacoustic instability by rotating the otherwise static swirler in a lean premixed, laboratory-scale combustor. Starting with a self-excited thermoacoustically unstable combustor, here we find that a progressive increase in swirler rotation rate does not uniformly decrease amplitudes of coherent, sinusoidal pressure or heat-release-rate oscillations. Instead, these oscillations emerge as high-amplitude bursts separated by low-amplitude noise in the signal. At increased rotational speeds, the high-amplitude coherent oscillations become scarce and their duration in the signal reduces. The velocity field from high-speed particle image velocimetry and simultaneous pressure and chemiluminescence data support these observations. Such an intermittent route to instability mitigation is reminiscent of the opposite transition implemented by changing the Reynolds number from a fully chaotic state to a fully unstable state. To model such dynamics phenomenologically, we discretize the swirling turbulent premixed flame into an ensemble of flamelet oscillators arranged circumferentially around the center body of the swirler. The Kuramoto model is proposed for these flamelet oscillators which is subsequently used to analyze their synchronization dynamics. The order parameter r, which is a measure of the synchronization between the oscillator phases, provides critical insights on the transition from the thermoacoustically unstable to stable states via intermittency. Finally, it is shown that the Kuramoto model for flamelet oscillator can qualitatively reproduce the time-averaged and intermittent dynamics while transitioning from the state of thermoacoustic instability to a state of incoherent noisy oscillations
Occurrence of Phoma Sacc. in the phyllosphere of Neogene Siwalik forest of Arunachal sub-Himalaya and its palaeoecological implications
No full textThe present study reports in situ occurrence of two new epiphyllous fungal species of Phomites (comparable to modern genus Phoma Sacc.) on angiospermic leaf remains recovered from the Siwalik sediments (middle Miocene to early Pleistocene) of Arunachal Pradesh, eastern Himalaya. We describe two new species i.e. Phomites siwalicus Vishnu, Khan et Bera S, sp. nov. and Phomites neogenicus Vishnu, Khan et Bera S, sp. nov. on the basis of structural details of pycnidia. The pycnidium is a globose or slightly lens shaped, ostiolate with a collar layer consisting of thick walled cells, sunken in leaf cuticle, with one-celled conidiospores and short-ampulliform conidiogenous cells. Host leaves resemble to those of extant Dipterocarpus C. F. Gaertn., Shorea Roxb. ex C. F. Gaertn. (Dipterocarpaceae), Dysoxylum Blume (Meliaceae), and Poaceae Barnhart. In situ occurrence of two Phomites morphotypes on the said leaf remains suggests a possible host-parasite interaction in the moist evergreen forest of Arunachal sub-Himalaya during Mio-Pleistocene period. The occurrence of Phomites in appreciable numbers indicates a humid climate favored by high rate of precipitation during Siwalik sedimentation, which is also consistent with our previously published climatic data obtained from the study of the macroscopic plant remains. (C) 2018 British Mycological Society. Published by Elsevier Ltd. All rights reserved
A novel fiber Bragg grating system for eye tracking
No full textEye movement evaluation is vital for diagnosis of various ophthalmological and neurological disorders. The present study proposes a novel, noninvasive, wearable device to acquire the eye movement based on a Fiber Bragg Grating (FBG) Sensor. The proposed Fiber Bragg Grating Eye Tracker (FBGET) can capture the displacement of the eyeball during its movements in the form of strain variations on a cantilever. The muscular displacement generated by the eyeball over the lower eyelid, by its swiveling action while moving the gaze on a target object, is converted into strain variations on a cantilever. The developed FBGET is investigated for dynamic tracking of the eye-gaze movement for various actions of the eye such as fixations, saccades and main sequence. This approach was validated by recording the eye movement using the developed FBGET as well as conventional camera-based eye tracker methodology simultaneously. The experimental results demonstrate the feasibility and the real-time applicability of the proposed FBGET as an eye tracking device. In conclusion, the present study illustrates a novel methodology involving displacement of lower eyelid for eye tracking application along with the employment of FBG sensors to carry out the same. The proposed FBGET can be utilized in both clinical and hospital environment for diagnostic purposes owing to its advantages of wear-ability and ease of implementation making it a point of care device. (C) 2019 The Authors. Published by Elsevier B.V. on behalf of Cairo University
Reliability analysis of jointed rock slope considering uncertainty in peak and residual strength parameters
No full textStability analysis of rock slopes is a complex problem because of uncertainties involved in the rock mass properties. The probabilistic approach is a rational way to deal with these uncertainties. This article investigates the stability of a rock slope by deterministic and probabilistic approaches by considering uncertainty in peak and residual strength parameters and strength-drop in stress-strain behavior of the rock mass. A Geological Strength Index (GSI) based on a quantitative approach was used to estimate the statistical parameters of peak and residual strength parameters. Reliability index of the slope was then estimated using Hong's Point Estimate Method coupled with the finite element method. The approach is demonstrated using an important case study of a Himalayan rock slope supporting the piers of the world's highest railway bridge. It was observed that the factor of safety and reliability index for the slope was highly sensitive to residual strength parameters, and; hence, ignoring strength-drop from the rock mass behavior and uncertainty in residual strength parameters can overestimate the factor of safety and the reliability index of rock slopes. A parametric study is carried out to evaluate the influence of the coefficient of variation of uniaxial compressive strength of intact rock, the Hoek-Brown strength parameter of intact rock, roughness and alteration parameters of the joints on the probability of failure and the reliability index. The approach is verified by comparing the estimated displacements along the slope with in-situ measured displacements observed during field monitoring over the years. The approach used can be extended to the rock slopes or rock slides where high displacements in the slopes are expected due to triggering forces like seismic forces, excavation or structural loads
Temperature-Dependent Photoluminescence and Energy-Transfer Dynamics in Mn2+-Doped (C4H9NH3)(2)PbBr4 Two-Dimensional (2D) Layered Perovskite
No full textReported here are the low-temperature photoluminescence (PL), energy-transfer mechanism, and exciton dynamics of Mn2+-doped two-dimensional (2D) perovskites that show interesting differences from their three-dimensionally doped counterpart. Dopant emission in 2D system shows increased PL intensity and shortened lifetime with increase of temperature and strong dopant emission even at low temperatures. Transient absorption (TA) spectroscopy reveals the dominant role of ``hot'' excitons in dictating the fast energy-transfer timescale. The operative dynamics of the generated hot excitons include filling up of existing trap states (shallow and deep) and energy-transfer channel from hot excitons to dopant states. Global analysis and target modeling of TA data provide an estimate of excitons (hot and band edge) to a dopant energy-transfer timescale of similar to 330 ps, which is much faster than the band edge exciton lifetime (similar to 2 ns). Such fast energy-transfer timescale arises due to enhanced carrier exchange interaction resulting from higher exciton confinement, increased covalency, and involvement of hot excitons in the 2D perovskites. In stark contrast to three-dimensional systems, the high energy-transfer rate in 2D system results in high dopant emission intensity even at low temperatures. Increased intrinsic vibronic coupling at higher temperatures further supports efficient Mn2+ sensitization that ultimately dictates the observed temperature dependence of the dopant emission (intensity, lifetime)