1,452 research outputs found
Insights into the room temperature magnetism of ZnO/Co3O4 mixtures
The origin of room temperature (RT) ferromagneticlike behavior in ZnO-based
diluted magnetic semiconductors is still an unclear topic. The present work
concentrates on the appearance of RT magnetic moments in just mixed ZnO/Co3O4
mixtures without thermal treatment. In this study, it is shown that the
magnetism seems to be related to surface reduction of the Co3O4 nanoparticles,
in which, an antiferromagnetic Co3O4 nanoparticle (core) is surrounded by a
CoO-like shell. This singular superficial magnetism has also been found in
other mixtures with semiconductors such as TiO2 and insulators such as Al2O3
Reforming of Diesel Fuel for Hydrogen Production over Catalysts Derived from LaCo0.8M0.2O3 (M = Ni, Ru, Fe, Ga)
Morphological and structural features of Co/TiO<sub>2</sub> catalysts prepared by different methods and their performance in the liquid phase hydrogenation of α, β unsaturated aldehydes
The effect of the preparation method of Co/TiO2 catalysts on the performance in the
liquid phase hydrogenation of cinnamaldehyde and crotonaldehyde has been studied. The Co/TiO2 catalysts were prepared by impregnation on titania (TiO2, P-25), sol-gel, and precipitation procedures. Different reduction-oxidation-reduction cycles were carried out in order to detect possible changes in the nature of the phases and their reduction degrees. The catalysts were characterized by specific area measurements, TEM, ED, XPS and TPR/TPO studies. The catalysts prepared by the sol-gel and precipitation method exhibit higher resistance to sintering when they are treated under the same reduction-oxidation-reduction cycles, than the impregnated catalyst. Morphological changes observed by TEM were used to explain the catalytic behavior showed by the catalysts. The presence of Coº, CoO and Co3O4 phases in the reduced samples were detected by different methods. XPS results show the existence of small particles of CoO, which are interacting with the support, which are very difficult to reduce. TEM and electron diffraction structural studies show an effect of metalsupport interaction by the appearance of channels and pill-box form in the particles. The highest yield in both catalytic hydrogenation reactions were obtained with the Co/TiO2 catalysts prepared by precipitation and sol gel methods. The selectivity was closed to 50 and 30 % for cinnamyl alcohol and crotyl alcohol, respectively.Centro de Investigación y Desarrollo en Ciencias Aplicada
On random graphs and the statistical mechanics of granular matter
The dynamics of spins on a random graph with ferromagnetic three-spin
interactions is used to model the compaction of granular matter under a series
of taps. Taps are modelled as the random flipping of a small fraction of the
spins followed by a quench at zero temperature. We find that the density
approached during a logarithmically slow compaction
- the random-close-packing density - corresponds to a dynamical phase
transition. We discuss the the role of cascades of successive spin-flips in
this model and link them with density-noise power fluctuations observed in
recent experiments.Comment: minor changes, to appear in EP
Structure and photoactivity for hydrogen production of CdS nanorods modified with In, Ga, Ag-In and Ag-Ga and prepared by solvothermal method
This work studies the variation in the photocatalytic properties of CdS derived from the insertion of In,
Ga, Ag-In and Ag-Ga in the CdS lattice through solvothermal methodology. Solvothermal synthesis of
CdS-M photocatalysts has been succesful for the insertion of Ga3þ, In3þ, Ga3þ/Agþ and In3þ/Agþ into the
hexagonal crystal lattice of one-dimensional CdS. The insertion of In, Ga, Ag-In and Ag-Ga modifies the
band gap and the relative position of EVB. CdS modified with In3þ or Ga3þ shows an increase in the band
gap and upshift in the relative position of the valence band energy which leads to a low efficiency
hydrogen production. The co-addition of Agþ-In3þ or Agþ-Ga3þ favours the insertion of Agþ ions into the
CdS lattice with narrower band gap. Of all the co-substituted photocatalysts, the CdS-AgGa was the only
one that showed a higher photoactivity with respect to the CdS. The increase in the photoactivity of the
CdS-AgGa photocatalyst is related to the band gap narrowing and downshift in the relative position of
the valence band energy which enhance their visible light absorption and potential for oxidation. The
CdS-AgGa photocatalyst shows small segregation of metallic Ag nanoparticles at the surface which also
assist in the photoactivity of the sample.publishe
An invariant distribution in static granular media
We have discovered an invariant distribution for local packing configurations
in static granular media. This distribution holds in experiments for packing
fractions covering most of the range from random loose packed to random close
packed, for beads packed both in air and in water. Assuming only that there
exist elementary cells in which the system volume is subdivided, we derive from
statistical mechanics a distribution that is in accord with the observations.
This universal distribution function for granular media is analogous to the
Maxwell-Boltzmann distribution for molecular gasses.Comment: 4 pages 3 figure
Work and heat fluctuations in two-state systems: a trajectory thermodynamics formalism
Two-state models provide phenomenological descriptions of many different
systems, ranging from physics to chemistry and biology. We investigate work
fluctuations in an ensemble of two-state systems driven out of equilibrium
under the action of an external perturbation. We calculate the probability
density P(W) that a work equal to W is exerted upon the system along a given
non-equilibrium trajectory and introduce a trajectory thermodynamics formalism
to quantify work fluctuations in the large-size limit. We then define a
trajectory entropy S(W) that counts the number of non-equilibrium trajectories
P(W)=exp(S(W)/kT) with work equal to W. A trajectory free-energy F(W) can also
be defined, which has a minimum at a value of the work that has to be
efficiently sampled to quantitatively test the Jarzynski equality. Within this
formalism a Lagrange multiplier is also introduced, the inverse of which plays
the role of a trajectory temperature. Our solution for P(W) exactly satisfies
the fluctuation theorem by Crooks and allows us to investigate
heat-fluctuations for a protocol that is invariant under time reversal. The
heat distribution is then characterized by a Gaussian component (describing
small and frequent heat exchange events) and exponential tails (describing the
statistics of large deviations and rare events). For the latter, the width of
the exponential tails is related to the aforementioned trajectory temperature.
Finite-size effects to the large-N theory and the recovery of work
distributions for finite N are also discussed. Finally, we pay particular
attention to the case of magnetic nanoparticle systems under the action of a
magnetic field H where work and heat fluctuations are predicted to be
observable in ramping experiments in micro-SQUIDs.Comment: 28 pages, 14 figures (Latex
Dynamic heterogeneities in attractive colloids
We study the formation of a colloidal gel by means of Molecular Dynamics
simulations of a model for colloidal suspensions. A slowing down with gel-like
features is observed at low temperatures and low volume fractions, due to the
formation of persistent structures. We show that at low volume fraction the
dynamic susceptibility, which describes dynamic heterogeneities, exhibits a
large plateau, dominated by clusters of long living bonds. At higher volume
fraction, where the effect of the crowding of the particles starts to be
present, it crosses over towards a regime characterized by a peak. We introduce
a suitable mean cluster size of clusters of monomers connected by "persistent"
bonds which well describes the dynamic susceptibility.Comment: 4 pages, 4 figure
Heterogeneities in systems with quenched disorder
We study the strong role played by structural (quenched) heterogeneities on
static and dynamic properties of the Frustrated Ising Lattice Gas in two
dimensions, already in the liquid phase. Differently from the dynamical
heterogeneities observed in other glass models in this case they may have
infinite lifetime and be spatially pinned by the quenched disorder. We consider
a measure of local frustration, show how it induces the appearance of spatial
heterogeneities and how this reflects in the observed behavior of equilibrium
density distributions and dynamic correlation functions.Comment: 8 page
Effective pair potentials for spherical nanoparticles
An effective description for spherical nanoparticles in a fluid of point
particles is presented. The points inside the nanoparticles and the point
particles are assumed to interact via spherically symmetric additive pair
potentials, while the distribution of points inside the nanoparticles is taken
to be spherically symmetric and smooth. The resulting effective pair
interactions between a nanoparticle and a point particle, as well as between
two nanoparticles, are then given by spherically symmetric potentials. If
overlap between particles is allowed, the effective potential generally has
non-analytic points, but for each effective potential the expressions for
different overlapping cases can be written in terms of one analytic auxiliary
potential. Effective potentials for hollow nanoparticles (appropriate e.g. for
buckyballs) are also considered, and shown to be related to those for solid
nanoparticles. Finally, explicit expressions are given for the effective
potentials derived from basic pair potentials of power law and exponential
form, as well as from the commonly used London-Van der Waals, Morse,
Buckingham, and Lennard-Jones potential. The applicability of the latter is
demonstrated by comparison with an atomic description of nanoparticles with an
internal face centered cubic structure.Comment: 27 pages, 12 figures. Unified description of overlapping and
nonoverlapping particles added, as well as a comparison with an idealized
atomic descriptio
- …