Search results for "grain"
showing 10 items of 752 documents
SHORT-TERM SYNAPTIC PLASTICITY IN MIGRAINE MOTOR CORTEX: EVIDENCE BY PRECONDITIONING OF HIGH-FREQUENCY REPETITIVE TRANSCRANIAL MAGNETIC STIMULATION (…
2011
Background: Brief 5Hz repetitive transcranial magnetic stimulation (rTMS) trains at 120% of the resting- motorthreshold (RMT) intensity determine in healthy subjects a potentiation of motor evoked potentials (MEPs), likely due to short-term presynaptic plasticity facilitation. We recently showed paradoxical intensity-dependent MEP changes to 5Hz rTMS in migraine with MEP facilitation at 110% and inhibition at 130% RMT. This provides evidence of both hyper-responsivity and self-limiting hyperexcitability capacity in migraine, likely due to earlier activation of inhibitory homeostatic plasticity mechanisms. To explore this, we applied in migraineurs cathodal transcranial Direct Current Stimul…
Determination of acrylamide in foods by pressurized fluid extraction and liquid chromatography-tandem mass spectrometry used for a survey of Spanish …
2006
An automated and rapid method for the determination of acrylamide in different food products is presented. The method involves pressurized fluid extraction (PFE) of foods with acetonitrile and precipitation with Carrez reagents. The final extract is analysed by liquid chromatography coupled to electrospray ionization tandem mass spectrometry (ESI-MS-MS). The main parameters affecting the performance of ESI-MS-MS and PFE were optimized using a design of experiments approach. The limit of quantification of the method was 5 microg kg(-1), and recoveries from incurred samples ranged between 93 and 101%. The accuracy was evaluated using the reference test materials FAPAS T3002, T3005 and T3011. …
Evaluation of enniatins A, A1, B, B1 and beauvericin in Portuguese cereal-based foods.
2012
Sixty-one samples of Portuguese cereal-based foods were analysed for the occurrence of emerging mycotoxins called enniatins (A, A1, B and B1) and beauvericin. Samples were extracted with a mixture of acetonitrile/water (85/15, v/v) using an Ultra-Turrax homogeniser, and mycotoxins were detected with liquid chromatography coupled to a mass spectrometer. This method was validated and adequate values of recovery (70-103%) and relative standard deviation (15%) were obtained. Signal suppression/enhancement was studied and matrix-matched calibration used to minimise this effect, but no additional clean-up step was necessary. The mass spectrometer was operated in selected reaction monitoring (SRM)…
Phase transformation kinetics in d-dimensional grains-containing systems: diffusion-type model
1998
Abstract An analytical approach to the phase transformation in d-dimensional grains-containing complex systems is offered. It is based on considering the mechanism of surface material exchange among neighbouring grains as the so-called state-dependent diffusion process, where the diffusion function is related to the magnitude of the grain boundary. The approach proposed deals with the kinetics of that ensemble under circumstances of a volume increase of the new phase or microstructure. Probabilistic characteristics of the process are derived and analyzed. A comparison with 2D modelling of similar kind is presented for the 3D case, and some possible practical realizations of the situation un…
Hard-Core Thinnings of Germ‒Grain Models with Power-Law Grain Sizes
2013
Random sets with long-range dependence can be generated using a Boolean model with power-law grain sizes. We study thinnings of such Boolean models which have the hard-core property that no grains overlap in the resulting germ‒grain model. A fundamental question is whether long-range dependence is preserved under such thinnings. To answer this question, we study four natural thinnings of a Poisson germ‒grain model where the grains are spheres with a regularly varying size distribution. We show that a thinning which favors large grains preserves the slow correlation decay of the original model, whereas a thinning which favors small grains does not. Our most interesting finding concerns the c…
Stress induced grain boundary migration in very soluble brittle salt
1999
Abstract Grain boundary migration (GBM) was studied in-situ at room temperature, atmospheric pressure and an applied diffmfwerential stress of ∼9.5 MPa under the optical microscope, in a wet aggregate of an elastic-brittle salt (sodium chlorate). The aggregate was previously deformed predominantly by a combination of grain boundary sliding, pressure solution and cataclastic solution creep. After deformation, but when the sample was still under differential stress, undeformed, fracture-free grains were observed to grow at the cost of deformed, intensely fractured grains. GMB rates typically fell in the range 2--10 μm/day. GBM took place only as long as the sample was under stress. Boundaries…
The structure of reactive grain-boundaries under stress containing confined fluids
2006
We present numerical experiments on structure development in grain-boundaries during dissolution–precipitation creep. Two solids that are represented by an elastic spring configuration are pressed together with a compressible fluid in the grain-boundary. The solid can dissolve or precipitate depending on elastic and surface energy as well as fluid pressure and concentration of dissolved material in the fluid. We perform a number of numerical experiments with different starting configurations that represent a large-scale island-channel interface with solid–solid contacts across the islands, a rough grain-boundary interface with a fluid along the whole interface and a smooth thin-film interfa…
Effect of microcracking on pressure-solution strain rate: The Gratz grain-boundary model
1998
Different, but reasonable and well-accepted assumptions made about grain-boundary structure during pressure-solution (PS) creep may easily have an effect of more than 10 orders of magnitude on the calculated PS deformation rate. Understanding of grain-boundary structure during PS creep is therefore extremely important. Experimental evidence is presented in support of a grain-boundary model previously proposed by A. J. Gratz on the basis of observations on naturally deformed rocks. In this model, boundaries are assumed to have a static island-channel network structure. Channels are located where microcracks intersect the boundary. The rate of material transport is governed by thin-film diffu…
The effect of elastic strain on the microstructure of free surfaces of stressed minerals in contact with an aqueous solution
2001
The influence of gradients in bulk elastic strain energy on the dissolution and growth behaviour of minerals in rocks is commonly considered negligible. We experimentally observed, however, that regular arrays of macroscopically visible etch grooves may develop on the originally smooth free surfaces of soluble crystals held in an undersaturated aqueous solution if the crystals are only elastically stressed. These grooves are oriented perpendicular to the compressive stress. They disappear soon after the stress is taken off. The formation of the grooves is well explained by recent theories on the instability of the surface of stressed solids. Development of such instabilities could significa…
STRONTIUM CONCENTRATION DEPENDENCE OF SELECTED STRUCTURAL AND MECHANICAL PROPERTIES OF POLYCRYSTALLINE Ba1−XSrXTiO3
2009
ABSTRACT Microstructure and material constants such as Young modulus E, shear modulus G, Poisson ratioν of polycrystalline ferroelectric materials Ba1-xSrxTiO3 for 0≤x≤0.4 were investigated to determine their dependence on strontium concentration. The performed investigations showed that the material is chemically homogeneous. It was found that the strontium concentration had no significant influence on the grain size. The highest value of Young's modulus is observed for Ba0.6Sr0.4TiO3 sample. The performed investigations of mechanical properties made it possible to evaluate the usability of this material for manufacturing transducers and sensors which could work under load.