844 research outputs found
Resolving complex mixtures by means of pulsed field gradient NMR experiments
No full textWe present a method to analyse pulsed gradient spin-echo (PGSE) NMR data from a mixture of compounds sharing the same NMR resonance (e.g. polymer solutions or mixtures of aliphatic compounds). If all the spin-bearing species undergo Brownian motion, their contribution to the experimental echo decay is exponential (i.e. e(-sD), with s a function of the parameters of the PGSE-NMR experiment and D the self-diffusion coefficient). For the case of more than one diffusing species at a given chemical shift, the (normalized) echo attenuation is the Laplace transform of the distribution function of the self-diffusion coefficients. The Laplace transform can be reduced to a Fredholm integral equation of the first kind in the variable z proportional to e(-sD) (in the interval [0,1]). Applying the algorithm previously developed by us (L. Ambrosone, A Ceglie, G. Colafemmina and G. Palazzo, J. Chem. Phys. 1999, 110, 797) we solve the integral equation, obtaining the distribution function of the diffusion coefficients. The method is tailored for small data sets (10-30 points) typical of PGSE-NMR measurements. Moreover, the relevant variable (z) being an exponential function of the self-diffusion coefficient, it allows insight on the ne structure of the diffusion spectrum. The method was successfully tested on a three-component solution and on an aqueous solution of seven PEG oligomers. In the latter case an estimate of the molecular mass distribution function was obtained. The reported results indicate that such an approach permits determination of self-diffusion coefficients differing by 15% with a high accuracy (6-3%)
Microcrystalline silicon-carbon films deposited by silane-methane mixture highly diluted in hydrogen
No full textA Formula For The Power In Partially Failed PV Fields
No full textIn this paper we present analytical formulae which give the curves of power as a function of failed cells, for some configurations. The configurations analysed are simple (maximum four connection levels) because the first levels are the more responsible for the power decrease
COMPARISON BETWEEN POWER AND ENERGY METHODS OF ANALYSES OF PHOTOVOLTAIC PLANTS
No full textDifferent methods of evaluating the performance of photovoltaic systems have been used to date; the main differences are in the details used in the system component descriptions and the temporal step used for the energy balance. In this paper, an approximate energy analysis model and a more complex power analysis model are analyzed and compared. By simulating the performance of several completely different photovoltaic systems, we show that the results become nearly coincident for increasing values of a parameter proportional to the storage capacity. Therefore, this parameter must be considered to decide a priori the reliability of the approximate energy analysis
Size distribution in emulsions
No full textEmulsions are generally opaque and scatter light widely. This characteristic is a considerable obstacle to the use of optical techniques such as dynamic light scattering and optical microscopy.
However, the optical appearance of a sample is totally irrelevant for nuclear magnetic resonance (NMR) measurements. In this paper we use general methods to calculate the size distribution of a collection of droplets in dilute emulsion systems using NMR pulse gradient spin-echo measurements. We show that such methods are also successful in describing a bimodal distribution of droplets
Growth-Etching Competitive Mechanism Governing The Structure And Chemical Composition Of Plasma Deposited Silicon-Based Materials
No full textDefect characterization of a-SiC:H and a-SiN:H alloys produced by ultra high vacuum plasma enhanced chemical vapor deposition in different plasma conditions
No full textHigh electronic quality a-SiC : H and a-SiN : H films with optical gap up to 2.3 eV have been deposited by ultrahigh vacuum plasma enhanced chemical vapour deposition in undiluted and hydrogen diluted reactive gas mixtures. Optical and photoelectrical characterizations have been performed. Successful progresses towards the deposition of a-SiC : H and a-SiN : H having high electronic properties and low defect densities have been obtained. Modulated photocurrent technique has been applied to study gap state energetic profiles
Optical and structural properties of siliconlike films prepared by plasma-enhanced chemical- vapor deposition
No full textAmorphous siliconlike thin films (Si:O-x:C-y:H-z), deposited by plasma-enhanced chemical-vapor deposition using hexamethyldisiloxane as monomer and Ar as feed gas, have been investigated for their optical and structural properties as a function of the deposition power, in the range of 100-400 W. The films have been analyzed by Fourier transform infrared spectroscopy (FTIRS), UV-VIS-NIR spectrophotometry, and atomic force Microscopy (AFM). From the analysis of FTIR spectra it results that films assume a marked inorganic character as the power increases. Indeed, at high power, Si-O-Si groups prevail over Si(CH3)(x) groups, so that the film chemistry approaches the SiO2 one. Reflectance/transmittance spectra, acquired in the range of 200-2500 nm, allow us to describe the film absorption edge for interband transitions. The relationship between the optical energy band gap, deduced from the absorption coefficient curve, and the deposition power has been investigated. The reduction of the optical energy gap from 3.86 to 3.61 eV and the broadening of the optical-absorption tail with the increase of power from 100 to 400 W are ascribed to the growth of thermal and structural disorders. Moreover, the refractive index has been evaluated and related to the film morphology. The AFM analysis confirms the amorphous character of the films and shows how the deposited layers become flatter and more compact when power increases. We consider the densification of the film responsible for the growth of the refractive index from 1.90 to 1.97 in the power range 100-400 W
Kinetics of charge recombination in bacterial reaction ceters incorporated into liposomes
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