144 research outputs found
Extracción en fase sólida de β-sitosterol y α-tocoferol de destilados de aceite de girasol desodorizado utilizando zeolita desilicada
In this study, the efficiency of using zeolite-based adsorbents in a solid phase extraction (SPE) procedure of α-tocopherol and β-sitosterol isolation from Sunflower Oil Deodorizer Distillate (SuDOD) without pre-treatment was investigated. The results showed that 99.2% α-tocopherol and 97.3% β-sitosterol were successfully isolated as pure fractions from SuDOD, when desilicated ZSM-5-type zeolite (DSiZSM-5) was used as adsorbent on a SPE. A simple and rapid HPLC method for simultaneous α-tocopherol and β-sitosterol analysis was developed and validated according to AOAC guidelines. It was found that the inclusion of a DSiZSM-5 SPE step increased the precision of the α-tocopherol and β-sitosterol analysis. In conclusion, DSiZSM-5 zeolite was proven to be an efficient adsorbent which can be used not only for the recovery of α-tocopherol and β-sitosterol from SuDOD in industrial scale, but also in a laboratory scale clean-up method prior to the analysis of α-tocopherol and β-sitosterol.En este estudio, se investigó la eficacia del uso de adsorbentes a base de zeolita en el procedimiento de extracción en fase sólida (EFS) para el aislamiento de α-tocoferol y β-sitosterol a partir de destilados de aceites de girasol desodorizados (SuDOD) sin ningún tratamiento previo. Los resultados mostraron que el 99,2% de α-tocoferol y el 97,3% de β-sitosterol se aislaron con éxito como fracciones puras de SuDOD, cuando se usó zeolita de tipo ZSM-5 desilicado (DSiZSM-5) como adsorbente en una EFS. Se desarrolló y validó un método HPLC simple y rápido para el análisis simultáneo de α-tocoferol y β-sitosterol de acuerdo con las directrices de la AOAC. Se encontró que la inclusión del paso DSiZSM-5 EFS aumentó la precisión del análisis de α-tocoferol y β-sitosterol. En conclusión, se demostró que la zeolita DSiZSM-5 es un adsorbente eficiente que puede usarse, no solo para la recuperación de α-tocoferol y β-sitosterol de SuDOD a escala industrial, sino también en un método de limpieza a escala de laboratorio antes del análisis de α-tocoferol y β-sitosterol
Melting of Flux Lines in an Alternating Parallel Current
We use a Langevin equation to examine the dynamics and fluctuations of a flux
line (FL) in the presence of an {\it alternating longitudinal current}
. The magnus and dissipative forces are equated to those
resulting from line tension, confinement in a harmonic cage by neighboring FLs,
parallel current, and noise. The resulting mean-square FL fluctuations are
calculated {\it exactly}, and a Lindemann criterion is then used to obtain a
nonequilibrium `phase diagram' as a function of the magnitude and frequency of
. For zero frequency, the melting temperature of the
mixed phase (a lattice, or the putative "Bose" or "Bragg Glass") vanishes at a
limiting current. However, for any finite frequency, there is a non-zero
melting temperature.Comment: 5 pages, 1 figur
Force fluctuation in a driven elastic chain
We study the dynamics of an elastic chain driven on a disordered substrate
and analyze numerically the statistics of force fluctuations at the depinning
transition. The probability distribution function of the amplitude of the slip
events for small velocities is a power law with an exponent
depending on the driving velocity. This result is in qualitative agreement with
experimental measurements performed on sliding elastic surfaces with
macroscopic asperities. We explore the properties of the depinning transition
as a function of the driving mode (i.e. constant force or constant velocity)
and compute the force-velocity diagram using finite size scaling methods. The
scaling exponents are in excellent agreement with the values expected in
interface models and, contrary to previous studies, we found no difference in
the exponents for periodic and disordered chains.Comment: 8 page
Steric repulsion and van der Waals attraction between flux lines in disordered high Tc superconductors
We show that in anisotropic or layered superconductors impurities induce a
van der Waals attraction between flux lines. This attraction together with the
disorder induced repulsion may change the low B - low T phase diagram
significantly from that of the pure thermal case considered recently by Blatter
and Geshkenbein [Phys. Rev. Lett. 77, 4958 (1996)].Comment: Latex, 4 pages, 1 figure (Phys. Rev. Lett. 79, 139 (1997)
Abrupt Change of Josephson Plasma Frequency at the Phase Boundary of the Bragg Glass in Bi_2Sr_2CaCu_2O_{8+\delta}
We report the first detailed and quantitative study of the Josephson coupling
energy in the vortex liquid, Bragg glass and vortex glass phases of
Bi_2Sr_2CaCu_2O_{8+\delta} by the Josephson plasma resonance. The measurements
revealed distinct features in the T- and H-dependencies of the plasma frequency
for each of these three vortex phases. When going across either
the Bragg-to-vortex glass or the Bragg-to-liquid transition line,
shows a dramatic change. We provide a quantitative discussion on the properties
of these phase transitions, including the first order nature of the
Bragg-to-vortex glass transition.Comment: 5pages, 4figure
Partially fluidized shear granular flows: Continuum theory and MD simulations
The continuum theory of partially fluidized shear granular flows is tested
and calibrated using two dimensional soft particle molecular dynamics
simulations. The theory is based on the relaxational dynamics of the order
parameter that describes the transition between static and flowing regimes of
granular material. We define the order parameter as a fraction of static
contacts among all contacts between particles. We also propose and verify by
direct simulations the constitutive relation based on the splitting of the
shear stress tensor into a``fluid part'' proportional to the strain rate
tensor, and a remaining ``solid part''. The ratio of these two parts is a
function of the order parameter. The rheology of the fluid component agrees
well with the kinetic theory of granular fluids even in the dense regime. Based
on the hysteretic bifurcation diagram for a thin shear granular layer obtained
in simulations, we construct the ``free energy'' for the order parameter. The
theory calibrated using numerical experiments with the thin granular layer is
applied to the surface-driven stationary two dimensional granular flows in a
thick granular layer under gravity.Comment: 20 pages, 19 figures, submitted to Phys. Rev.
Phase Transitions of the Flux Line Lattice in High-Temperature Superconductors with Weak Columnar and Point Disorder
We study the effects of weak columnar and point disorder on the
vortex-lattice phase transitions in high temperature superconductors. The
combined effect of thermal fluctuations and of quenched disorder is
investigated using a simplified cage model. For columnar disorder the problem
maps into a quantum particle in a harmonic + random potential. We use the
variational approximation to show that columnar and point disorder have
opposite effect on the position of the melting line as observed experimentally.
Replica symmetry breaking plays a role at the transition into a vortex glass at
low temperatures.Comment: 4 pages in 2 columns format + 2 eps figs included, uses RevTeX and
multicol.st
Vortex Matter Transition in BiSrCaCuO under Tilted Fields
Vortex phase diagram under tilted fields from the axis in
BiSrCaCuO is studied by local magnetization
hysteresis measurements using Hall probes. When the field is applied at large
angles from the axis, an anomaly () other than the well-known
peak effect () are found at fields below . The angular dependence of
the field is nonmonotonic and clearly different from that of
and depends on the oxygen content of the crystal. The results suggest existence
of a vortex matter transition under tilted fields. Possible mechanisms of the
transition are discussed.Comment: Revtex, 4 pages, some corrections are adde
Disordered periodic systems at the upper critical dimension
The effects of weak point-like disorder on periodic systems at their upper
critical dimension D_c for disorder are studied. The systems studied range from
simple elastic systems with D_c=4 to systems with long range interactions with
D_c=2 and systems with D_c=3 such as the vortex lattice with dispersive elastic
constants. These problems are studied using the Gaussian Variational method and
the Functional Renormalisation Group. In all the cases studied we find a
typical ultra-slow loglog(x) growth of the asymptotic displacement correlation
function, resulting in nearly perfect translational order. Consequences for the
Bragg glass phase of vortex lattices are discussed.Comment: 12 RevTex pages, uses epsfig, 2 figure
A Bragg glass phase in the vortex lattice of a type II superconductor
Although crystals are usually quite stable, they are sensitive to a
disordered environment: even an infinitesimal amount of impurities can lead to
the destruction of the crystalline order. The resulting state of matter has
been a longstanding puzzle. Until recently it was believed to be an amorphous
state in which the crystal would break into crystallites. But a different
theory predicts the existence of a novel phase of matter: the so-called Bragg
glass, which is a glass and yet nearly as ordered as a perfect crystal. The
lattice of vortices that can contain magnetic flux in type II superconductors
provide a good system to investigate these ideas. Here we show that neutron
diffraction data of the vortex lattice in type II superconductors provides
unambiguous evidence for a weak, power-law decay of the crystalline order
characteristic of a Bragg glass. The theory also predicts accurately the
electrical transport properties of superconductors; it naturally explains the
observed phase transition and the dramatic jumps in the critical current
associated with the melting of the Bragg glass. Moreover the model explains
experiments as diverse as X-ray scattering in disordered liquid crystals and
conductivity of electronic crystals.Comment: 9 pages, 4 figure
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