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Vertical Current Transport in AlGaN/GaN HEMTs on Silicon: Experimental Investigation and Analytical Model
No full textWe investigate the vertical leakage mechanism in metal-organic chemical vapor deposition-grown carbon (C)-doped AlGaN/GaN High Electron Mobility Transistors (HEMTs) on 6-in silicon wafer. Substrate bias polarity-dependentI-V-s, temperature-dependentfitting, and band diagram analysis pointed to the Poole-Frenkel (P-F) type of conduction mechanism for vertical transport in the devices with breakdown as high as 580 V for a buffer of 4 mu m. Trap activation energy of 0.61 eV was estimated from the P-F fitting which matches well with values reported in the literature. We propose that higher dislocation density leads to shallower traps in the buffer and build an analytical model of dislocation-mediated vertical leakage around this. The variation in leakage as a function of dislocation density at a given field is predicted and is found to be the most abrupt in the range from similar to 10(7) to similar to 10(9) cm(-2) of dislocation density. This can be attributed to a sharp decrease in trap activation energy in the above range of dislocation density, possibly due to complex formation between point defects and dislocations
3D global simulations of RIAFs: convergence, effects of azimuthal extent, and dynamo
No full textWe study the long-term evolution of non-radiative geometrically thick (H/R approximate to 0.5) accretion flows using 3D global ideal magnetohydrodynamic simulations and a pseudo-Newtonian gravity. We find that resolving the scale height with 42 grid points is adequate to obtain convergence with the product of quality factors similar to << Q(theta)>><< Q(phi)>> >= 300 and magnetic tilt angle theta(B) similar to 13 degrees-14 degrees. Like previous global isothermal thin disc simulations, we find stronger mean magnetic fields for the restricted azimuthal domains. Imposing periodic boundary conditions with the azimuthal extent smaller than 2 pi makes the turbulent field at low m appear as a mean field in the runs with smaller azimuthal extent. But unlike previous works, we do not find a monotonic trend in turbulence with the azimuthal extent. We conclude that the minimum azimuthal extent should be >= pi/2 to capture the flow structure, but a full 2 pi extent is necessary to study the dynamo. We find an intermittent dynamo cycle, with alpha-quenching playing an important role in the non-linear saturated state. Unlike previous local studies, we find almost similar values of kinetic and magnetic alpha-s, giving rise to an irregular distribution of dynamo-alpha. The effects of dynamical quenching are shown explicitly for the first time in global simulations of accretion flows
Collapse loads for rectangular foundations by three-dimensional upper bound limit analysis using radial point interpolation method
No full textA three-dimensional kinematic limit analysis approach based on the radial point interpolation method (RPIM) has been used to compute collapse loads for rectangular foundations. The analysis is based on the Mohr-Coulomb yield criterion and the associated flow rule. It is understood that the internal plastic power dissipation function and flow rule constraints can be expressed entirely in terms of plastic strain rates without involving stresses. The optimization problem has been solved on basis of the semidefinite programming (SDP) by using highly efficient primal-dual interior point solver MOSEK in MATLAB. The results have been presented in terms of the variation of the shape factors with changes in the aspect ratio (L/B) of the footing for different values of soil internal friction angle (phi). Computations have revealed that the shape factors, s(c) and s(q), due to effects of cohesion and surcharge increase continuously with (1) decrease in L/B and (2) increase in phi. On the other hand, the shape factor s(gamma), due to the effect of soil unit weight, increases very marginally with an increase in L/B up to (1) phi = 25 degrees for a rough footing and (2) phi = 35 degrees for a smooth footing. Thereafter, for greater values of phi, the variation of s(gamma) with L/B has been found to be quite similar to that of the factors s(c) and s(q). The variations of (1) nodal velocity patterns, (2) plastic power dissipation, and (3) maximum plastic shear strain rates have also been examined to interpret the associated failure mechanism
Post-spreading behavior of impacting fuel drops on stainless steel surface
No full textHydrocarbon fuel drops impacting on metallic solid surfaces kept at room temperature exhibit no receding after the completion of early inertia-driven primary spreading. The experimental data of impacting drops of different fuels on a smooth stainless steel surface reveal a sluggish spreading, referred to as post-spreading, after reaching the maximum spreading diameter at the end of primary spreading. A systematic analysis involving maximum spreading factor, final spreading factor and temporal variation of post-spreading diameter of impacting fuel drops of varying Weber number is reported to describe the behavior of the post-spreading process. It is found that the dynamics of post-spreading of impacting fuel drops is different from Tanner's spreading as the exponent n in the power law describing the temporal spreading behavior (beta proportional to tau(n)) varies with Weber number and is significantly less than 0.1. The extent of post-spreading is found to be as high as 20% of the maximum spread factor for the impact of low Weber number fuel drops. Due to the presence of post-spreading, the final spread factor of impacting fuel drops is always higher than the maximum spread factor
Long-Range Transport of Mineral Dust to the Northeast Indian Ocean: Regional versus Remote Sources and the Implications
No full textSynergizing satellite remote sensing data with vertical profiles of atmospheric thermodynamics and regional climate model simulations, we investigate the relative importance, transport pathways, and seasonality of contribution of dust from regional (Thar Desert and adjoining arid regions) and remote (southwest Asia and northeast Africa) sources over the northeast Indian Ocean i.e., the Bay of Bengal (BOB)]. We show that while over the northern BOB dust from the regional sources contribute more than 50% to the total dust load during the southwest monsoon period (June-September), interestingly; the remote dust sources dominate rest of the year. On the other hand, over the southern BOB, dust transported from the remote-source regions dominate throughout the year. During June, the dry elevated layer (at altitudes between 850 and 700 hPa) of dust, transported across the Indo-Gangetic Plain to the northern BOB, arises primarily from the Thar Desert. Dust from remote sources in the far west reaches the southern BOB after traversing over and around the southern Indian Peninsula. Since dust from these distinct source regions have different mineral composition (hence optical properties) and undergo distinct changes during atmospheric transport, it is important to understand source-specific dust contribution and transport pathways to address dust-climate feedback
Gate-Controlled Large Resistance Switching Driven by Charge-Density Wave in 1T-TaS2/2H-MoS2 Heterojunctions
No full text1T-TaS2 is a layered material that exhibits charge density wave (CDW) -induced distinct electrical resistivity phases and has attracted a lot of attention for interesting device applications. However, such resistivity switching effects are often weak, and cannot be modulated by an external gate voltage - limiting their widespread usage. Using a back-gated 1T-TaS2/2H-MoS2 heterojunction, we show that the usual resistivity switching in TaS2 due to different phase transitions is accompanied with a surprisingly strong modulation in the Schottky barrier height (SBH) at the TaS2/MoS2 interface - providing an additional knob to control the degree of the phase-transition-driven resistivity switching by an external gate voltage. In particular, the commensurate (C) to triclinic (T) phase transition results in an increase in the SBH owing to a collapse of the Mott gap in TaS2. The change in SBH allows us to estimate an electrical Mott-gap opening of approximately 71 +/- 7 meV in the C phase of TaS2. On the other hand, the nearly commensurate (NC) to incommensurate (IC) phase transition results in a suppression in the SBH, and the heterojunction shows a gate-controlled resistivity switching ratio up to 17.3, which is approximately 14.5 times higher than that of stand-alone TaS2. The findings mark an important step forward showing a promising pathway to externally control as well as amplify the CDW-induced resistivity switching. This will boost device applications that exploit these phase transitions, such as ultra-broadband photodetection, negative differential conductance, fast oscillator and threshold switching in neuromorphic circuits
A smart nanosensor for the detection of human immunodeficiency virus and associated cardiovascular and arthritis diseases using functionalized graphene-based transistors
No full textHuman immunodeficiency virus (HIV), which isa worldwide public health issue, is commonly associated with cardiovascular disorders (CVDs) and rheumatoid arthritis (RA). A smart nanosensor was developed for the detection of HIV and its related diseases (CVDs and RA) using graphene-based field-effect transistors (FETs). In this study, amine-functionalized graphene (afG) was conjugated with antibodies anti-p24 for HIV, anti-cardiac troponin 1 (anti-cTn1) for CVDs, and anti-cyclic citrullinated peptide (anti-CCP) for RA] to detect various biomarkers. The antibodies were covalently conjugated to afG via carbodiimide activation. The bioconjugate (graphene-antibody) was characterized by various biophysical techniques such as UV-Vis, Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). The electrochemical performance of the sensor was evaluated with respect to changes in the resistance of the electrode surface due to the interaction of the antigen with its specific antibody. The developed sensor was highly sensitive and showed a linear response to p24, cTn1, and, CCP from 1 fg/mL to 1 mu g/mL. The limit of detection (LOD) was 100 fg/mL for p24 and 10 fg/mL for cTn1 and CCP under standard optimized conditions. The graphene-based smart nanodevice demonstrated excellent performance; thus, it could be used for the on-site detection of HIV, CVD, and RA biomarkers in real samples
CFD modelling and performance analysis of a twin screw hydrogen extruder
No full textAn intermeshing counter-rotating twin-screw extruder is reported to be more reliable and stable among various methods of extrusion techniques in pellet production systems for the plasma reactors. In the present study, CFD modelling has been successfully carried out using POLYFLOW module of ANSYS, which employs the Mesh Superposition Technique. The shear rate dependent shear stress of solid hydrogen modelled using Herschel-Bulkley equation has been used in the present analysis. A three-dimensional computation neglecting inertia and gravity effects has been carried out to simulate the flow through the extruder which involves leakage flows through the calendar, tetrahedron, flight and side gaps. The flow characteristics of the die was simulated independently and superimposed with the extruder pumping characteristics to arrive at the operating point. The CFD results have been compared with those obtained from an analytical model adopted from the literature on polymer extrusion research. It has been found that the deviation between them becomes narrow when the mechanical clearances of the extruder are made smaller. A systematic parametric analysis was carried out to arrive at an optimum design to produce a 3 mm diameter filament at about 400 mm(3)/s with a minimum viscous dissipation rate. The analysis revealed that an increase of pitch length or decrease of mechanical clearance gaps in association with either decrease of the number of `C' chambers or the speed will result in smaller viscous dissipation rate
A comparison of two biholomorphic invariants
No full textThe Fridman invariant, which is a biholomorphic invariant on Kobayashi hyperbolic manifolds, can be seen as the dual of the much studied squeezing function. We compare this pair of invariants by showing that they are both equally capable of determining the boundary geometry of a bounded domain if their boundary behavior is a priori known
Smartphone-based kanamycin sensing with ratiometric FRET
No full textSmartphone-based fluorescence detection is a promising avenue for biosensing that can aid on-site analysis. However, quantitative detection with fluorescence in the field has been limited due to challenges with robust excitation and calibration requirements. Here, we show that ratiometric analysis with Förster resonance energy transfer (FRET) between dye pairs on DNA aptamers can enable rapid and sensitive kanamycin detection. Since our detection scheme relies on ligand binding-induced changes in the aptamer tertiary structure, it is limited only by the kinetics of ligand binding to the aptamer. Our FRET-based kanamycin binding aptamer (KBA) sensor displays two linear ranges of 0.05-5 nM (detection limit of 0.18 nM) and 50-900 nM of kanamycin. The aptamer displays high specificity even in the presence of the 'natural' background from milk. By immobilizing the aptamer in the flow cell, our KBA sensor design is also suitable for repeated kanamycin detection. Finally, we show that the ratiometric FRET-based analysis can be implemented on a cheap custom-built smartphone setup. This smartphone-based FRET aptamer scheme detects kanamycin in a linear range of 50-500 nM with a limit of detection (LOD) of 28 nM. © 2019 The Royal Society of Chemistry