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Turbulence in the intracluster medium: Simulations, observables, and thermodynamics
No full textWe conduct two kinds of homogeneous isotropic turbulence simulations relevant for the intracluster medium (ICM): (i) pure turbulence runs without radiative cooling and (ii) turbulent heating + radiative cooling runs with global thermal balance. For pure turbulence runs in the subsonic regime, the rms density and surface brightness (SB) fluctuations vary as the square of the rms Mach number (mathcal M-textrms). However, with thermal balance, the density and SB fluctuations (SB/SB) are much larger. These scalings have implications for translating SB fluctuations into a turbulent velocity, particularly for cool cores. For thermal balance runs with large (cluster core) scale driving, both the hot and cold phases of the gas are supersonic. For small-scale (one order of magnitude smaller than the cluster core) driving, multiphase gas forms on a much longer time-scale but mathcal M-textrms is smaller. Both small- and large-scale driving runs have velocities larger than the Hitomi results from the Perseus cluster. Thus, turbulent heating as the dominant heating source in cool cluster cores is ruled out if multiphase gas is assumed to condense out from the ICM. Next we perform thermal balance runs in which we partition the input energy into thermal and turbulent parts and tune their relative magnitudes. The contribution of turbulent heating has to be lesssim 10 rm per cent in order for turbulence velocities to match Hitomi observations. If the dominant source of multiphase gas is not cooling from the ICM (but say uplift from the central galaxy), the importance of turbulent heating cannot be excluded. © 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
Effect of substrate transfer on performance of vertically stacked ultrathin MOS devices
No full textThis paper presents a low-temperature process to transfer devices on ultrathin silicon layers from a parent substrate to a foreign substrate or stack. MOS devices were fabricated on silicon-on-insulator(SOI) wafer. The device wafer was then temporarily bonded on a carrier wafer. The handle layer was etched and the remaining ultrathin silicon device layer of �1.4 μm was transferred to a foreign substrate using permanent bonding. Here, we explored two different bonding approaches, namely, 1) the gold-indium (Au-In) transient liquid phase (TLP) bonding and 2) the epoxy bonding. We demonstrate the advantages of epoxy bonding method over the TLP method. The unique characteristic of this epoxy bonding approach is its capability to vertically stack multiple thin silicon layers. Furthermore, we demonstrate three-layer stacking of the ultrathin silicon layers with functional metal-oxide-semiconductor field-effect transistors in each layer. Electrical characterization results of nMOS/pMOS devices in each layer is presented and compared for before and after transfer. Changes in measured device performance before and after stacking are studied using simulations. The maximum process temperature in this approach is 150 °C, which is considerably lower than those reported in the literature. This result demonstrates the feasibility of multilayer low-temperature stacking. © 2019 IEEE
Effects of superelasticity and plasticity on the spherical indentation response of shape memory alloys: A finite element analysis
No full textInstrumented indentation is particularly useful for characterizing the mechanical behavior of shape memory alloys (SMAs), which are often used as 'small volume' elements such as thin films or wires. Deciphering the measured indentation response, which is as such difficult for elastic-plastic materials due to the inhomogeneous state of stress underneath the indenter, becomes more complex for SMAs owing to the simultaneous occurrence of stress induced martensite transformation (SIMT) in conjunction with plastic deformation. In this work, a constitutive model that is able to capture the coupled nature of phase transformation and plastic deformation is employed to study, through finite element analyses, the spherical indentation behavior of SMAs at a temperature above the austenite finish temperature, A f . It is found that the concurrent development of plastic yielding and SIMT leads to slower evolution of martensite volume and a smaller transformed zone size. Also, in the absence of plastic yielding, the proportion of depth recovered by superelasticity is fairly constant. It is also observed, from a systematic comparison with a conventional elastic-plastic material, that the presence of the transformed zone significantly alters the stress distribution beneath the indenter. © 2019 IOP Publishing Ltd
Cosolvent effects on the growth of protein aggregates formed by a single domain globular protein and an intrinsically disordered protein
No full textCosolvents modulate the stability of protein conformations and exhibit contrasting effects on the kinetics of aggregation by globular proteins and intrinsically disordered proteins (IDPs). The growth of ordered protein aggregates after the initial nucleation step is believed to proceed through a dock-lock mechanism. We have studied the effect of two denaturants guanidinium chloride (GdmCl) and urea and four protective osmolytes (trimethylamine N-oxide (TMAO), sucrose, sarcosine, and sorbitol) on the free energy surface (FES) of the dock-lock growth step of protein aggregation using a coarse-grained protein model and metadynamics simulations. We have used the proteins cSrc-SH3 and Aβ 9-40 as model systems representing globular proteins and IDPs, respectively. The effect of cosolvents on protein conformations is taken into account using the molecular transfer model (MTM). The computed FES shows that protective osmolytes stabilize the compact aggregates, while denaturants destabilize them for both cSrc-SH3 and Aβ 9-40 . However, protective osmolytes increase the effective energy barrier for the multistep domain-swapped dimerization of cSrc-SH3, which is critical to the growth of protein aggregates by globular proteins, thus slowing down the overall aggregation rate. Contrastingly, denaturants decrease the effective barrier height for cSrc-SH3 dimerization and hence enhance the aggregation rate in globular proteins. The simulations further show that cSrc-SH3 monomers unfold before dimerization and the barrier to monomer unfolding regulates the effective rate of aggregation. In the case of IDP, Aβ 9-40 , protective osmolytes decrease and denaturants increase the effective barriers in the dock-lock mechanism of fibril growth, leading to faster and slower growth kinetics, respectively. © 2019 American Chemical Society
Overlap of Radial Dangling Orbitals Controls the Relative Stabilities of Polyhedral B n H n-X Isomers (n = 5-12, x = 0 to n-1)
No full textThe removal of H atoms from polyhedral boranes results in the formation of dangling radial orbitals with one electron each. If there is a requirement of electrons for skeletal bonding to meet the Wade's rule, these are provided from the exohedral orbitals. Additional electrons occupy a linear combination of the dangling orbitals. Stabilization of these molecular orbitals depends on their overlap. The lateral (sideways) overlap of dangling orbitals decreases with the decreasing cluster size from 12 to 5 boron atoms as the orbitals become more and more splayed out. Thus, as the number of dangling orbitals increases, the destabilization of their combinations increases at a higher rate for smaller polyhedral boranes, leading to flat structures with the removal of a fewer number of hydrogens. Though exohedral orbitals form better overlap in larger polyhedral clusters, the increase of electrons with the removal of H atoms results in occupancy of antibonding skeletal orbitals (beyond Wade's rules) and leads to flat structures. The reverse happens when hydrogens are added to a flat cluster. Substitution of BH by Si does not change structural patterns. © Copyright © 2019 American Chemical Society
Temperature-Dependent Photoluminescence and Energy-Transfer Dynamics in Mn 2+ -Doped (C 4 H 9 NH 3 ) 2 PbBr 4 Two-Dimensional (2D) Layered Perovskite
No full textReported here are the low-temperature photoluminescence (PL), energy-transfer mechanism, and exciton dynamics of Mn 2+ -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 330 ps, which is much faster than the band edge exciton lifetime (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 Mn 2+ sensitization that ultimately dictates the observed temperature dependence of the dopant emission (intensity, lifetime). © 2019 American Chemical Society
On the Use of Dropouts in Neural Networks for System Identification and Control
No full textAs universal function approximators, neural networks have been successfully used for nonlinear dynamical system identification and control. Recent developments in neural network regularisation methods include dropouts. Here, the outputs of hidden units are dropped randomly with a probability which is tuned as a hyperparameter. In this paper, we first present a scheme to conduct a rigorous analysis on the effect of changing hyperparameters in neural network learning by comparing their ability to generalise and achieve convergence. Second, we conduct an analysis with dropouts using the proposed scheme through a grid search over the dropout probabilities in each hidden layer. In the context of system identification and control, our results show that dropout is at best as good as standard back propagation in terms of the amount of data required for generalisation without adapting. This is achieved by the proposed scheme which efficiently summarises hundreds of simulations with typical examples. © 2018 IEEE
Design and Analysis of Thermoacoustic Refrigerators Using Air as Working Substance
No full textIn this paper, the design of 50 W thermoacoustic refrigerators operating with air as working substance at 10 bar pressure and 3 drive ratio for a temperature difference of 28 K is described. The design strategies discussed in this paper help in design and development of low cost thermoacoustic coolers compared to helium as the working substance. The design and optimization of spiral stack and heat exchangers, and the promising 0.2λ and 0.15λ resonator design with taper and divergent section with hemispherical end are discussed. The surface area, volume, length and power density of the hemispherical end design with air as working substance is found better compared to the published 10 and 50 W coolers using helium as the working substance. The theoretical design results are validated using DeltaEC software simulation results. The DeltaEC predicts 51.4 improvement in COP (1.273) at the cold heat exchanger temperature of -2.7°C with air as working substance for the 50W 0.15λTDH resonator design compared to the published 50W 0.25λTDH resonator design with helium as working substance. © 2019 World Scientific Publishing Company
Partially-disordered to frozen-state crossover induced magnetocaloric properties of the antiferromagnetic one-dimensional spin-chain Sr 3 CoIrO 6
No full textLow-dimensional and geometrically frustrated spin-chain Sr 3 (Co/Ni)IrO 6 compounds exhibit interesting physical phenomena due to the exotic anisotropy associated with the spin-orbit entangled Ir 4+ state. Temperature dependent magnetic susceptibility of polycrystalline Sr 3 CoIrO 6 characterizes the partially-disordered antiferromagnetic with T N1 � 90 K and the frozen state of this antiferromagnetic order below T N2 � 30 K. The fitting of �T versus T (K) data to the Ising model reveal that the intra-chain exchange interactions are ferromagnetic, whereas the inter-chain exchange correlations are antiferromagnetic. Field-induced alteration of the partially-disordered antiferromagnetic state in the temperature regime of T N1 � T � T N2 is responsible for the anomalous magnetic hysteresis loop. Further, a large value of isothermal magnetic-entropy change (�S M � 22 J kg -1 K -1 for 7 T), absence of thermal hysteresis in magnetization and highly insulating nature consents Sr 3 CoIrO 6 to be classified as a good magnetocaloric material for the lowerature refrigeration. © 2019 IOP Publishing Ltd
Failure analysis of BMC insulators used for third rail traction system
No full textBulk molding compound (BMC) or Glass Reinforced plastic (GRP) composite insulators are extensively used for the third rail electric traction system. The performance of these insulators primarily depends on the local environmental conditions that cause degradation over a period of time. Degradation leads to partial arcing which grow more severe and can generate combustion that ignites flammable debris, smoke including fires resulting in service interruptions and safety risks of the third rail traction system. In the present investigations, exhaustive analysis is conducted to understand the root cause of degradation and arcing on the field aged insulators rated for 750Vdc. Leakage current and surface resistance measurements are conducted under dry, wet and contaminated conditions. Further to better interpret Physico-chemical analysis are conducted using Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray (EDAX) and Fourier Transform Infra-Red (FTIR) spectroscopy to investigate surface morphology and chemical changes in the samples, also Thermo-Gravimetric Analysis (TGA) is performed to find the thermal characteristics of the insulator. Interesting results obtained from the investigations clearly present evidence to loss of epoxy material from the bulk eventually leading to the exposure of glass fibers causing degradation, arcing and failure of the insulators. The study further highlights the essential procedures to be edopted to locate, isolate, and better anticipate failures of BMC insulators used in third rail traction system. © 2019 Elsevier Lt