Search results for "fluid"
showing 10 items of 5513 documents
Coherent coupling between vortex bound states and magnetic impurities in 2D layered superconductors
2021
Bound states in superconductors are expected to exhibit a spatially resolved electron-hole asymmetry which is the hallmark of their quantum nature. This asymmetry manifests as oscillations at the Fermi wavelength, which is usually tiny and thus washed out by thermal broadening or by scattering at defects. Here we demonstrate theoretically and confirm experimentally that, when coupled to magnetic impurities, bound states in a vortex core exhibit an emergent axial electron-hole asymmetry on a much longer scale, set by the coherence length. We study vortices in 2H-NbSe2 and in 2H-NbSe1.8S0.2 with magnetic impurities, characterizing these with detailed Hubbard-corrected density functional calcu…
Structure and pair correlations of a simple coarse grained model for supercritical carbon dioxide
2009
A recently introduced coarse-grained pair potential for carbon dioxide molecules is used to compute structural properties in the supercritical region near the critical point, applying Monte Carlo simulations. In this model, molecules are described as point particles, interacting with Lennard-Jones (LJ) forces and a (isotropically averaged) quadrupole–quadrupole potential, the LJ parameters being chosen such that gratifying agreement with the experimental phase diagram near the critical point is obtained. It is shown that the model gives also a reasonable account of the pair correlation function, although in the nearest neighbour shell some systematic discrepancies between the model predicti…
Extraction of wheat germ oil by supercritical CO2 : Oil and defatted cake characterization
2003
In this paper the working conditions for the extraction of wheat germ oil in a supercritical CO2 pilot plant of 1-L extraction capacity were studied. The best conditions were: pressure, 38 MPa; temperature, 55degreesC; wheat germ particle size, about 0.35 mm; CO2 flow rate, 1.5 L min(-1). These conditions gave yields of about 92% of total oil after 3 h of processing. The obtained oils and the partially defatted cake were investigated with regard to their FA, tocol (tocopherol and tocotrienol), carotenoid, and sterol compositions and to their quality characteristics (FFA, PV, para-anisidine value, and color of the by-product). Moreover, the oil quality was evaluated in relation to the progre…
Supercritical CO2 extraction of essential oil from orange peel; effect of the height of the bed
2000
Abstract The influence of the height of the particle bed on the kinetics of supercritical fluid extraction (SFE) of essential oil from orange peel is analyzed in this article. Peel of dehydrated oranges of the satsuma and naveline cultivars was used. A series of experiments were designed wherein, for the same conditions, particle height varied widely. These experiments were also carried out on different scales with extraction volumes of 0.5 and 5 l. The results of the experiments were interpreted using Sovova's extended flow model as has been done in previous studies. The bibliography indicates that a number of phenomena may distort the process. One of these phenomena is the formation of ma…
Effective recovery of perfluoropolyether surfactants from PVDF and PTFE by supercritical carbon dioxide extraction
2007
Abstract The extraction of ammonium carboxylate perfluoropolyether surfactants from poly(vinylidenefluoride) (PVDF) and poly(tetrafluoroethylene) (PTFE) was performed with supercritical carbon dioxide. When proper conditions in terms of temperature, pressure and flow rate were used, extraction recoveries higher than 90% were obtained. For PVDF (with a concentration of surfactant of 5% w/w), at low pressure (12 MPa) and 40 °C, more than 100 min are necessary to have a quantitative extraction. On the other hand, at higher pressure (20 MPa) a quantitative recovery was obtained after few minutes. A strong influence of the temperature on the extraction recovery was also found. In the case of PTF…
Copolymerization of VDF and HFP in Supercritical Carbon Dioxide: A Robust Approach for Modeling Precipitation and Dispersion Kinetics
2011
A kinetic model is developed for the heterogeneous free-radical copolymerization of vinylidene fluoride and hexafluoropropylene in supercritical CO2. The model accounts for polymerization in both the dispersed (polymer-rich) phase and in the continuous (polymer-free) supercritical phase, for radical interphase transport, diffusion limitations, and chain-length-dependent termination in the polymer-rich phase. A parameter evaluation strategy is developed and detailed to estimate most of the kinetic parameters a priori while minimizing their evaluation by direct fitting. The resulting model predictions compare favorably with the experimental results of conversion and MWD at varying monomer fee…
Critical Micelle Density for the Self-Assembly of Block Copolymer Surfactants in Supercritical Carbon Dioxide
1999
The parameters which influence the self-assembly of molecules in solution include the temperature and solvent quality, and this study illustrates the use of these variables to regulate the degree of association of block copolymer amphiphiles in highly compressible supercritical carbon dioxide. Small-angle neutron scattering (SANS) has been used to examine the association behavior of a block copolymer containing a CO2-phobic moiety, poly(vinyl acetate), and a CO2-philic block, poly(1,1-dihydroperfluoro-octylacrylate). By adjustment of the density of the medium through pressure and temperature profiling, the self-assembly can be reversibly controlled from unimers to core−shell spherical micel…
A supercritical-fluid method for growing carbon nanotubes
2007
Large‐scale generation of multiwalled carbon nanotubes (MCNTs) is efficiently achieved through a supercritical fluid technique employing carbon dioxide as the carbon source. Nanotubes with diameters ranging from 10 to 20 nm and lengths of several tens of micrometers are synthesized (see figure). The supercritical‐fluid‐grown nanotubes also exhibit field‐emission characteristics similar to MCNTs grown by chemical‐vapor deposition.
Supercritical fluid crystallization of adipic acid using urea as habit modifier
2008
The crystal morphology of adipic acid mediated by the action of urea as additive has been investigated using the supercritical antisolvent precipitation (SAS). The process was performed using acetone and carbon dioxide as solvent and antisolvent, respectively. The effect of urea concentration in the liquid solution and of precipitation pressure on the crystal habit was investigated; the products were characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRPD), differential scanning calorimetry (DSC), and infrared spectrometry (FT-IR). Urea was found to be very effective in modifying the crystal habit of adipic acid from needle-like shape with a length of several hun…
Baeyer—Villiger Oxidation in Supercritical CO2 with Potassium Peroxomonosulfate Supported on Acidic Silica Gel.
2006
Supercritical carbon dioxide (scCO2) is an efficient reaction medium to perform the Baeyer-Villiger oxidation with hydrated silica-supported potassium peroxomonosulfate (h-SiO2.KHSO5) under flow-through conditions. Hydration modulates the reactivity of the active surface by softening the acidity of the KHSO4 present in the supported reagent. The reaction in scCO2 is much more efficient than in n-hexane under similar conditions, which is attributed to better transport and solvating properties of the supercritical medium with regard to n-hexane.