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Parameter N (parameter + n)

Distribution by Scientific Domains

Chemistry	44%

Selected Abstracts

Modelling of diffraction from fibre texture gradients in thin polycrystalline films

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2007
M. Birkholz
Crystallographic textures in thin polycrystalline films typically exhibit a rotational symmetry, i.e. they occur as a fibre texture with the texture pole being orientated in the direction of the substrate normal. As a further characteristic of thin-film textures, it was often observed that the degree of preferred orientation increases with increasing thickness. It is shown in this work how a fibre texture gradient may be modelled in kinematical X-ray diffraction and which effects it has on the intensity mapping of the IHKL reflection, when the HKL pole is the fibre axis. A general expression for IHKL is derived for a depth-dependent fibre texture that is based on the finite Laplace transform of the texture distribution. The concept is outlined for the cosn, function to model the tilt-angle dependence of intensity, with the parameter n denoting the degree of texture. It is found that the measured intensity distribution sensitively depends on the ratio of texture gradient over X-ray attenuation coefficient. For particular cases, it is found that the maximum intensity may occur for non-zero tilt angles and thus arise at a different tilt angle from the pole of the fibre texture. [source]

Subcritical crack growth behavior of Al2O3 -glass dental composites

JOURNAL OF BIOMEDICAL MATERIALS RESEARCH, Issue 2 2003
Qingshan Zhu
Abstract The purpose of this study is to investigate the subcritical crack growth (SCG) behavior of alumina-glass dental composites. Alumina-glass composites were fabricated by infiltrating molten glass to porous alumina preforms. Rectangular bars of the composite were subject to dynamic loading in air, with stressing rates ranging from 0.01 MPa/s to 2 MPa/s. The SCG parameter n was determined to be 22.1 for the composite, which is substantially lower than those of high-purity dense alumina. Investigations showed that glass phases are responsible for the low n value as cracks propagate preferentially within glass phases or along the interface between glass phases and alumina phases, due to the fact that glasses are more vulnerable to chemical attacks by water molecules under stress corrosion conditions. The SCG behavior of the infiltration glass was also investigated and the SCG parameter n was determined to be 18.7. © 2003 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 65B: 233,238, 2003 [source]

Phorbol esters and adenosine affect the readily releasable neurotransmitter pool by different mechanisms at amphibian motor nerve endings

THE JOURNAL OF PHYSIOLOGY, Issue 2 2003
T. J. Searl
Phorbol esters and adenosine have been proposed to interact at common sites downstream of calcium entry at amphibian motor nerve endings. We thus studied the actions and interactions of phorbol esters and adenosine using electrophysiological recording techniques in conjunction with both binomial statistical analysis and high-frequency stimulation at the amphibian neuromuscular junction. To begin this study, we confirmed previous observations that synchronous evoked acetylcholine (ACh) release (reflected as endplate potentials, EPPs) is well described by a simple binomial distribution. We then used binomial analysis to study the effects of the phorbol ester phorbol dibutyrate (PDBu, 100 nm) and adenosine (50 µm) on the binomial parameters n (the number of calcium charged ACh quanta available for release) and p (the average probability of release), where the mean level of evoked ACh release (m) =np. We found that PDBu increased m by increasing the parameter n whilst adenosine reduced m by reducing n; neither agent affected the parameter p. PDBu had no effect on either the potency or efficacy of the inhibition produced by adenosine. Subtle differences between these two agents were revealed by the patterns of EPPs evoked by high-frequency trains of stimuli. Phorbol esters increased ACh release during the early phase of stimulation but not during the subsequent plateau phase. The inhibitory effect of adenosine was maximal at the beginning of the train and was still present with reduced efficacy during the plateau phase. When taken together with previous findings, these present results suggest that phorbol esters increase the immediately available store of synaptic vesicles by increasing the number of primed vesicles whilst adenosine acts at a later stage of the secretory process to decrease the number of calcium-charged primed vesicles. [source]

Explicit Calculation of the Friction Factor for Non-Newtonian Fluids Using Artificial Neural Networks

ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 1-2 2005
W. H. Shayya
An explicit procedure based on artificial neural networks (ANN) was developed for calculating the friction factor (f) for Herschel-Bulkley fluids under laminar and turbulent flow conditions in closed pipes. The Regula-Falsi method was used as an iterative procedure to estimate the f values for a range of flow behavior indexes (n), Reynolds numbers (Re), and Hedstrom numbers (He). In developing the ANN model, the input parameters Re and He and the output parameter f were transformed using a logarithmic scale to the base 10, while the input parameter n was taken on a linear scale. An ANN configuration with 16 neurons in each of two hidden layers was found to be optimal. However, a simpler ANN model with eight neurons in one hidden layer also produced reasonably good predictions. These values were in close agreement with those obtained using the numerical technique. The developed ANN model may offer significant advantages when dealing with flow problems that involve repetitive calculations of the friction factor such as those encountered in the hydraulic analysis of viscous non-Newtonian fluids in pipe networks. [source]

Nucleophilic Reactivities of Azulene and Fulvenes,

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 8 2009
Mariusz K, dziorek
Abstract The kinetics of the reactions of azulene (1), 6,6-dimethylfulvene (2), 6-[4-(dimethylamino)phenyl]fulvene (3) and 6-(julolidin-9-yl)fulvene (4) with a set of benzhydrylium ions (reference electrophiles) have been investigated in MeCN. The second-order rate constants for these reactions correlate linearly with the electrophilicity parameters (E) of the benzhydrylium ions. According to the linear free-enthalpy relationship log,k2(20 °C) = s(N + E), the nucleophilicity parameters N and s of the ,-nucleophiles 1,4 were determined and compared with those of other types of nucleophiles. Azulene (1, N = 6.66) is about 10 times more nucleophilic than N -methylpyrrole and comparable to 2-methylindole. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

Nucleophilic Reactivities of Pyrroles,

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2008
Tobias A. Nigst
Abstract The second-order rate constants of the reactions of alkyl-substituted pyrroles with a series of benzhydrylium ions were determined in acetonitrile, and the reaction products were fully characterized by NMR spectroscopy and mass spectrometry. The formation of the , adducts is the rate-limiting step of these reactions. Because the second-order rate constants correlate linearly with the electrophilicity parameters of the benzhydrylium ions, the determination of the nucleophilicity parameters N and s according to the linear free energy relationship log k2 (20 °C) = s(N + E) was achieved. With these findings, a direct comparison of the nucleophilic reactivities of these ,-excessive heterocycles with other nucleophiles became possible, and the pyrroles were integrated into the comprehensive scale of nucleophilicity, covering a range of 8,9 orders of magnitude from N -(triisopropylsilyl)pyrrole (N = 3.12), the weakest nucleophile of this series, to kryptopyrrole (3-ethyl-2,4-dimethylpyrrole, N = 11.63). Thus, highly reactive pyrroles show similar nucleophilic reactivities as enamines, whereas those of less-reactive pyrroles are comparable to allylsilanes or indoles. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

Lifetime prediction of CAD/CAM dental ceramics,

JOURNAL OF BIOMEDICAL MATERIALS RESEARCH, Issue 6 2002
Ulrich Lohbauer
Abstract The dynamic fatigue method was used to obtain subcritical crack growth parameters n and A for a commercial feldspathic dental porcelain and for a lanthanum-glass-infiltrated alumina glass ceramic. Five stress rates d,/dt ranging from 50 to 0.01 MPa s,1 were applied. The inert strength values were calculated with the use of Weibull statistics and maximum-likelihood approaches for the Weibull parameter m. Strength,probability,time (SPT) diagrams were derived for both materials. The alumina glass composite showed a high fracture strength ,0 (442 MPa) at a failure probability of PF = 63.2% and a high resistance against subcritical crack growth (n = 36.5). The development of strength under fatigue conditions was calculated for exemplary 1 year. The strength of the alumina glass material dropped to 228 MPa within this period. This fact is due to a low content of infiltrated lanthanum glass phase in the composite material (25 wt%). In contrast, for the high-silica-glass-containing porcelain a distinct decrease of strength ,0 from initial 133 to 47 MPa after 1 year was predicted. This, mainly because of a low crack growth resistance (n = 16.8) of the feldspathic porcelain. Much lower strength values were calculated, assuming a failure probability of PF = 5%. The decrease is mainly caused by the sensitivity of high,glass-containing ceramics against water corrosion. © 2002 Wiley Periodicals, Inc. J Biomed Mater Res (Appl Biomater) 63: 780,785, 2002 [source]

An exact sinusoidal beam finite element

PROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2008
Zdzislaw Pawlak
The purpose of the paper is to derive an efficient sinusoidal thick beam finite element for the static analysis of 2D structures. A two,node, 6,DOF curved, sine,shape element of a constant cross,section is considered. Effects of flexural, axial and shear deformations are taken into account. Contrary to commonly used curvilinear co,ordinates, a rectangular co,ordinates system is used in the present analysis. First, an auxiliary problem is solved: a symmetric clamped,clamped sinusoidal arch subjected to unit nodal displacements of both supports is considered using the flexibility method. The exact stiffness matrix for the shear,flexible and compressible element is derived. Introduction of two parameters "n" and "t" enables the identification of shear and membrane influences in the element stiffness matrix. Basing on the principle of virtual work a full set of 18 shape functions related to unit support displacements is derived (total rotations of cross,sections, tangential and normal displacements along the element). The functions are found analytically in the closed form. They are functions of one linear dimensionless coordinate of x,axis and depend on one geometrical parameter of sinusoidal arch, height/span ratio "c" and on physical and geometrical properties of the element cross,section. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]