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Search Results: 1 - 10 of 10520 matches for " Jacob Philip "
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Nanostructured Ceramics of Potassium Sodium Bismuth Titanate: Hydrothermal Synthesis and Piezoelectric Response at Morphotropic Phase Boundary  [PDF]
Nissamuddeen Kunnath, Jacob Philip
New Journal of Glass and Ceramics (NJGC) , 2019, DOI: 10.4236/njgc.2019.91001
Abstract: Potassium Sodium Bismuth Titanate (KNBT) ceramics, with the general formula (1 - x)K0.5Bi0.5TiO3 -xNa0.5Bi0.5TiO3, have been synthesized following hydrothermal route, starting with solid solutions of pure perovskite nanoceramics of KBT and NBT in desired stoichiometric weight ratios, followed by sintering between 850°C and 1000°C for few hours. Pure KNBT nanoceramics with perovskite structure, having mean particle size around 30 nm, could be obtained. Morphology of the samples is found to depend strongly on composition. A change of composition results in a phase change, as evident from X-ray structure analysis. This phase change is a result of rhombohedral to tetragonal morphotropic phase boundary (MPB) in the sample with x around 0.80. Composition dependent occurrence of MPB leads to formation of needle like structures with micrometer length scales. These are typical of tetragonal lamellar structures, suggesting partial induction of tetragonal polar order from rhombohedral structure at MPB. Dielectric and piezoelectric properties, such as dielectric constant and loss, piezoelectric coefficients and figures of merit, exhibit threshold maxima in their values at the composition corresponding to MPB. These values reported for
Hyperosmolar Hyperglycemic Non Ketotic Coma and Rhabdomyolysis; An Uncommon Clinical Encounter
C Philip, J Jacob, S Gupta
Libyan Journal of Medicine , 2009,
Abstract: Rhabdomyolysis is usually attributed to trauma. However there is an association of rhabdomyolysis with hyperosmolar states. Recognition of this association will enable better management of the patient and reduce the burden on the care taker by preventing the onset of complications that can prove fatal. It is also important to realise that hyperosmolar coma can be the presenting complaint of a diabetic seeking medical attention for the first time.
Efficiency of Particle Trapping in the Outer Regions of Protoplanetary Disks
Jacob B. Simon,Philip J. Armitage
Physics , 2014, DOI: 10.1088/0004-637X/784/1/15
Abstract: We investigate the strength of axisymmetric local pressure maxima (zonal flows) in the outer regions of protoplanetary disks, where ambipolar diffusion reduces turbulent stresses driven by the magnetorotational instability. Using local numerical simulations we show that in the absence of net vertical magnetic fields, the strength of turbulence in the ambipolar dominated region of the disk is low and any zonal flows that are present are weak. For net fields strong enough to yield observed protostellar accretion rates, however, zonal flows with a density amplitude of 10-20% are formed. These strengths are comparable to those seen in simulations of ideal MHD disk turbulence. We investigate whether these zonal flows are able to reverse the inward radial drift of solids, leading to prolonged and enhanced concentration as a prelude to planetesimal formation. For commonly assumed mean surface density profiles (surface density proportional to radius to the -1/2 power or steeper) we find that the predicted perturbations to the background disk profile do not correspond to local pressure maxima. This is a consequence of radial width of the simulated zonal flows, which is larger than was assumed in prior analytic models of particle trapping. These larger scale flows would only trap particles for higher amplitude fluctuations than observed. We conclude that zonal flows are likely to be present in the outer regions of protoplanetary disks and are potentially large enough to be observable, but are unlikely to lead to strong particle trapping.
Yield Implications of Site Preparation Treatments for Lodgepole Pine and White Spruce in Northern British Columbia
Francesco Cortini,Philip G. Comeau,Jacob O. Boateng,Lorne Bedford
Forests , 2010, DOI: 10.3390/f1010025
Abstract: We evaluated the effects of site preparation treatments on growth of lodgepole pine and white spruce in north-eastern British Columbia, Canada. These treatments can provide yield gains of up to 10 percent for lodgepole pine and white spruce at 60 and 80 years, respectively (estimated using TASS). Stands of these two species are showing a Type 1 response. Using growth multipliers, based on measurements collected at ages 10 to 20 results in inflated estimates of potential yield responses while the age-shift method provides the most appropriate estimates of yield gains when measured during the first 20 years of growth.
Valproate induced thrombocytopenia complicating acute febrile illness
Elizabeth K,Gopakumar H,Zachariah Philip,Jacob Roy
Annals of Indian Academy of Neurology , 2006,
Abstract: Sodium valproate is a commonly used anticonvulsant. Thrombocytopenia is one of the lesser known adverse effects of valproate. Fever with thrombocytopenia is a frequent clinical presentation especially in tropical countries like India with emerging and reemerging infections like dengue fever. We present here two cases of resistant thrombocytopenia in febrile children which responded dramatically to withdrawing valproate. Both the children had valproate drug level in the normal therapeutic range. Based on the Naranjo adverse drug reaction probability scale a probable association was found between valproate and the thrombocytopenia.
Two timescale dispersal of magnetized protoplanetary disks
Philip J. Armitage,Jacob B. Simon,Rebecca G. Martin
Physics , 2013, DOI: 10.1088/2041-8205/778/1/L14
Abstract: Protoplanetary disks are likely to be threaded by a weak net flux of vertical magnetic field that is a remnant of the much larger fluxes present in molecular cloud cores. If this flux is approximately conserved its dynamical importance will increase as mass is accreted, initially by stimulating magnetorotational disk turbulence and subsequently by enabling wind angular momentum loss. We use fits to numerical simulations of ambipolar dominated disk turbulence to construct simplified one dimensional evolution models for weakly magnetized protoplanetary disks. We show that the late onset of significant angular momentum loss in a wind can give rise to "two timescale" disk evolution in which a long phase of viscous evolution precedes rapid dispersal as the wind becomes dominant. The wide dispersion in disk lifetimes could therefore be due to varying initial levels of net flux. Magnetohydrodynamic (MHD) wind triggered dispersal differs from photoevaporative dispersal in predicting mass loss from small (less that 1 AU) scales, where thermal winds are suppressed. Our specific models are based on a limited set of simulations that remain uncertain, but qualitatively similar evolution appears likely if mass is lost from disks more quickly than flux, and if MHD winds become important as the plasma beta decreases.
Turbulent Linewidths in Protoplanetary Disks: Predictions from Numerical Simulations
Jacob B. Simon,Philip J. Armitage,Kris Beckwith
Physics , 2011, DOI: 10.1088/0004-637X/743/1/17
Abstract: Sub-mm observations of protoplanetary disks now approach the acuity needed to measure the turbulent broadening of molecular lines. These measurements constrain disk angular momentum transport, and furnish evidence of the turbulent environment within which planetesimal formation takes place. We use local magnetohydrodynamic (MHD) simulations of the magnetorotational instability (MRI) to predict the distribution of turbulent velocities in low mass protoplanetary disks, as a function of radius and height above the mid-plane. We model both ideal MHD disks, and disks in which Ohmic dissipation results in a dead zone of suppressed turbulence near the mid-plane. Under ideal conditions, the disk mid-plane is characterized by a velocity distribution that peaks near v \simeq 0.1cs (where cs is the local sound speed), while supersonic velocities are reached at z > 3H (where H is the pressure scale height). Residual velocities of v \approx 0.01cs persist near the mid-plane in dead zones, while the surface layers remain active. Anisotropic variation of the linewidth with disk inclination is modest. We compare our MHD results to hydrodynamic simulations in which large-scale forcing is used to initiate similar turbulent velocities. We show that the qualitative trend of increasing v with height, seen in the MHD case, persists for forced turbulence and is likely a generic property of disk turbulence. Percent level determinations of v at different heights within the disk, or spatially resolved observations that probe the inner disk containing the dead zone region, are therefore needed to test whether the MRI is responsible for protoplanetary disk turbulence.
Emergent Mesoscale Phenomena in Magnetized Accretion Disc Turbulence
Jacob B. Simon,Kris Beckwith,Philip J. Armitage
Physics , 2012, DOI: 10.1111/j.1365-2966.2012.20835.x
Abstract: We study how the structure and variability of magnetohydrodynamic (MHD) turbulence in accretion discs converge with domain size. Our results are based on a series of vertically stratified local simulations, computed using the Athena code, that have fixed spatial resolution, but varying radial and azimuthal extent (from \Delta R = 0.5H to 16H, where H is the vertical scale height). We show that elementary local diagnostics of the turbulence, including the Shakura-Sunyaev {\alpha} parameter, the ratio of Maxwell stress to magnetic energy, and the ratio of magnetic to fluid stresses, converge to within the precision of our measurements for spatial domains of radial size Lx \geq 2H. We obtain {\alpha} = 0.02-0.03, consistent with recent results. Very small domains (Lx = 0.5H) return anomalous results, independent of spatial resolution. The convergence with domain size is only valid for a limited set of diagnostics: larger spatial domains admit the emergence of dynamically important mesoscale structures. In our largest simulations, the Maxwell stress shows a significant large scale non-local component, while the density develops long-lived axisymmetric perturbations (zonal flows) at the 20% level. Most strikingly, the variability of the disc in fixed-sized patches decreases strongly as the simulation volume increases. We find generally good agreement between our largest local simulations and global simulations with comparable spatial resolution. There is no direct evidence that the presence of curvature terms or radial gradients in global calculations materially affect the turbulence, except to perhaps introduce an outer radial scale for mesoscale structures. The demonstrated importance of mean magnetic fields, seen in both large local and global simulations implies that the growth and saturation of these fields is likely of critical importance for the evolution of accretion discs. (abridged)
Turbulence in Global Simulations of Magnetized Thin Accretion Disks
Kris Beckwith,Philip J. Armitage,Jacob B. Simon
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.19043.x
Abstract: We use a global magnetohydrodynamic simulation of a geometrically thin accretion disk to investigate the locality and detailed structure of turbulence driven by the magnetorotational instability (MRI). The model disk has an aspect ratio $H / R \simeq 0.07$, and is computed using a higher-order Godunov MHD scheme with accurate fluxes. We focus the analysis on late times after the system has lost direct memory of its initial magnetic flux state. The disk enters a saturated turbulent state in which the fastest growing modes of the MRI are well-resolved, with a relatively high efficiency of angular momentum transport $< < \alpha > > \approx 2.5 \times 10^{-2}$. The accretion stress peaks at the disk midplane, above and below which exists a moderately magnetized corona with patches of superthermal field. By analyzing the spatial and temporal correlations of the turbulent fields, we find that the spatial structure of the magnetic and kinetic energy is moderately well-localized (with correlation lengths along the major axis of $2.5H$ and $1.5H$ respectively), and generally consistent with that expected from homogenous incompressible turbulence. The density field, conversely, exhibits both a longer correlation length and a long correlation time, results which we ascribe to the importance of spiral density waves within the flow. Consistent with prior results, we show that the mean local stress displays a well-defined correlation with the local vertical flux, and that this relation is apparently causal (in the sense of the flux stimulating the stress) during portions of a global dynamo cycle. We argue that the observed flux-stress relation supports dynamo models in which the structure of coronal magnetic fields plays a central role in determining the dynamics of thin-disk accretion.
Turbulent Linewidths as a Diagnostic of Self-Gravity in Protostellar Discs
Duncan H. Forgan,Philip J. Armitage,Jacob B. Simon
Physics , 2012, DOI: 10.1111/j.1365-2966.2012.21962.x
Abstract: We use smoothed particle hydrodynamics simulations of massive protostellar discs to investigate the predicted broadening of molecular lines from discs in which self-gravity is the dominant source of angular momentum transport. The simulations include radiative transfer, and span a range of disc-to-star mass ratios between 0.25 and 1.5. Subtracting off the mean azimuthal flow velocity, we compute the distribution of the in-plane and perpendicular peculiar velocity due to large scale structure and turbulence induced by self-gravity. For the lower mass discs, we show that the characteristic peculiar velocities scale with the square root of the effective turbulent viscosity parameter, as expected from local turbulent-disc theory. The derived velocities are anisotropic, with substantially larger in-plane than perpendicular values. As the disc mass is increased, the validity of the locally determined turbulence approximation breaks down, and this is accompanied by anomalously large in-plane broadening. There is also a high variance due to the importance of low-m spiral modes. For low-mass discs, the magnitude of in-plane broadening is, to leading order, equal to the predictions from local disc theory and cannot constrain the source of turbulence. However, combining our results with prior evaluations of turbulent broadening expected in discs where the magnetorotational instability (MRI) is active, we argue that self-gravity may be distinguishable from the MRI in these systems if it is possible to measure the anisotropy of the peculiar velocity field with disc inclination. Furthermore, for large mass discs, the dominant contribution of large-scale modes is a distinguishing characteristic of self-gravitating turbulence versus MRI driven turbulence.
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