278 research outputs found
Relaxation Time of Proton Adsorption from Solution onto Magnetite and Anatase: Classical and New Theoretical Approach*
The pH dependence of relaxation time of proton adsorption was analyzed by the classical Theory of Activated Adsorption and Desorption (TAAD) and the recently developed Statistical Rate Theory of Interfacial Transport (SRT). It was found that both models predicted different behaviour of relaxation time as a function of pH. Since SRT and TAAD give an identical result for relaxation time of adsorption of uncharged species, it is suggested that the surface potential is a key factor in ion adsorption kinetics. Additionally, only the order of magnitude of the ion adsorption rate constant can be estimated from the analysis of experimental data because the surface potential and the total concentration of adsorption sites are not known exactly. Keywords: magnetite anatase proton adsorption kinetics relaxation time statistical rate theory CROATICA CHEMICA ACTA CCACAA 80 (3-4) 345¿349 (2007
Thermodynamic parameters of bonds in glassy materials from viscosity-temperature relationships
Doremus's model of viscosity assumes that viscous flow in amorphous materials is mediated by broken bonds (configurons). The resulting equation contains four coefficients, which are directly related to the entropies and enthalpies of formation and motion of the configurons. Thus by fitting this viscosity equation to experimental viscosity data these enthalpy and entropy terms can be obtained. The non-linear nature of the equation obtained means that the fitting process is non-trivial. A genetic algorithm based approach has been developed to fit the equation to experimental viscosity data for a number of glassy materials, including SiO2, GeO2, B2O3, anorthite, diopside, xNa2O–(1-x)SiO2, xPbO–(1-x)SiO2, soda-lime-silica glasses, salol, and α-phenyl-o-cresol. Excellent fits of the equation to the viscosity data were obtained over the entire temperature range. The fitting parameters were used to quantitatively determine the enthalpies and entropies of formation and motion of configurons in the analysed systems and the activation energies for flow at high and low temperatures as well as fragility ratios using the Doremus criterion for fragility. A direct anti-correlation between fragility ratio and configuron percolation threshold, which determines the glass transition temperature in the analysed materials, was found
Validity of temperature and time equivalence in metallic glasses during shear deformation
The ecological diversity of vegetation within Urban Parks in the Dabrowski Basin (southern Poland)
The aim of this work is to present the diversity of flora in terms of ecological requirements. The research was
conducted in the area of two urban parks in the area of two cities in southern Poland: Bedzin and Czeladz. These parks were
established in different historical periods, and were planned (and are managed) differently. The results of the investigation
have shown that the occurrence of 192 vascular species has been observed in the Gora Zamkowa (Castle Hill) Park, while in
the Grabek park, 334 such species are known to exist. Such disparity is the result of the occurrence of micro-habitats and of
the differences between the ways the two parks are managed. It is also due to these parks’ different functions. In the first case,
the park area is protected by law. In the latter case, human activity has created a new ecological niche for organisms with a
high degree of ecological tolerance. Based on the ecological values, the following groups of plants were distinguished: saxifrages
grasslands, xerothermic grasslands, beech forests, alder forests and artificial planted trees. Analysis has shown that urban
parks are potential places for growth various type of vegetation and also for increasing biodiversity, and can constitute
particularly important hotspots for biodiversity in the cityscape, even if their primary role is recreational. As the study shows,
the environment of a highly urbanized and industrialized region can also have a positive influence on ecological and floristic
diversity
Topologically disordered systems at the glass transition
The thermodynamic approach to the viscosity and fragility of amorphous oxides was used to determine the topological characteristics of the disordered network-forming systems. Instead of the disordered system of atoms we considered the congruent disordered system of interconnecting bonds. The Gibbs free energy of network-breaking defects (configurons) was found based on available viscosity data. Amorphous silica and germania were used as reference disordered systems for which we found an excellent agreement of calculated and measured glass transition temperatures. We reveal that the Hausdorff dimension of the system of bonds changes from Euclidian three-dimensional below to fractal 2.55 ± 0.05-dimensional geometry above the glass transition temperature
Frequency dependent specific heat of viscous silica
We apply the Mori-Zwanzig projection operator formalism to obtain an
expression for the frequency dependent specific heat c(z) of a liquid. By using
an exact transformation formula due to Lebowitz et al., we derive a relation
between c(z) and K(t), the autocorrelation function of temperature fluctuations
in the microcanonical ensemble. This connection thus allows to determine c(z)
from computer simulations in equilibrium, i.e. without an external
perturbation. By considering the generalization of K(t) to finite wave-vectors,
we derive an expression to determine the thermal conductivity \lambda from such
simulations. We present the results of extensive computer simulations in which
we use the derived relations to determine c(z) over eight decades in frequency,
as well as \lambda. The system investigated is a simple but realistic model for
amorphous silica. We find that at high frequencies the real part of c(z) has
the value of an ideal gas. c'(\omega) increases quickly at those frequencies
which correspond to the vibrational excitations of the system. At low
temperatures c'(\omega) shows a second step. The frequency at which this step
is observed is comparable to the one at which the \alpha-relaxation peak is
observed in the intermediate scattering function. Also the temperature
dependence of the location of this second step is the same as the one of the
peak, thus showing that these quantities are intimately connected to
each other. From c'(\omega) we estimate the temperature dependence of the
vibrational and configurational part of the specific heat. We find that the
static value of c(z) as well as \lambda are in good agreement with experimental
data.Comment: 27 pages of Latex, 8 figure
Computer Simulations of Supercooled Liquids and Glasses
After a brief introduction to the dynamics of supercooled liquids, we discuss
some of the advantages and drawbacks of computer simulations of such systems.
Subsequently we present the results of computer simulations in which the
dynamics of a fragile glass former, a binary Lennard-Jones system, is compared
to the one of a strong glass former, SiO_2. This comparison gives evidence that
the reason for the different temperature dependence of these two types of glass
formers lies in the transport mechanism for the particles in the vicinity of
T_c, the critical temperature of mode-coupling theory. Whereas the one of the
fragile glass former is described very well by the ideal version of
mode-coupling theory, the one for the strong glass former is dominated by
activated processes. In the last part of the article we review some simulations
of glass formers in which the dynamics below the glass transition temperature
was investigated. We show that such simulations might help to establish a
connection between systems with self generated disorder (e.g. structural
glasses) and quenched disorder (e.g. spin glasses).Comment: 37 pages of Latex, 11 figures, to appear as a Topical Review article
in J. Phys.: Condens. Matte
An extension of the momentum transfer model to time-dependent pipe turbulence
We analyze a possible extension of Gioia and Chakraborty's momentum transfer
model of friction in steady turbulent pipe flows (Phys. Rev. Lett. 96, 044502
(2006)) to the case of time and/or space dependent turbulent flows. The end
result is an expression for the stress at the wall as the sum of an steady and
a dynamic component. The steady part is obtained by using the instantaneous
velocity in the expression for the stress at the wall of a stationary flow. The
unsteady part is a weighted average over the history of the flow acceleration,
with a weighting function similar to that proposed by Vardy and Brown (Journal
of Sound and Vibration 259, 1011 (2003); ibid. 270, 233 (2004)), but naturally
including the effect of spatial derivatives of the mean flow, as in the Brunone
model (B. Brunone et al., J. of Water Resources Planning and Management 236
(2000)).Comment: 15 pages. 2 figures (included) arXiv admin note: text overlap with
arXiv:1005.040
Phase transitions in TbMnO₃ manganites
X-ray diffraction and magnetic measurements of polycrystalline and nanosize TbMnO₃ manganites have been
performed. All the compounds studied crystallize in the orthorhombic crystal structure (space group Pnma) at
room temperature. The nanosize manganites were synthesized with a sol-gel method at different (800 and
900°C) temperatures. The average size of synthesized nanoparticles (from 45 to 70 nm) was estimated by using
the x-ray diffraction and low-temperature adsorption of argon methods. An information on the evolution of
properties of TbMnO₃ with changing grain size, temperature and magnetic field was obtained. The crystal structure
parameters of nanospecimens change slightly with changing the nanoparticle size. The peculiarities of magnetic
ordering in polycrystalline and nanosize TbMnO₃ were compared. Magnetization and the Nèel temperature
corresponding to antiferromagnetic ordering of the Tb³⁺ sublattice decrease as the particle size is reduced. The
inverse magnetic susceptibility of the nanoparticle samples deviates from the Curie–Weiss law below 50 K, that
is connected with the magnetic ordering of the Mn³⁺ moments. Specific heat of the nanosize samples exhibits
anomalies related to the magnetic ordering of the Tb³⁺ and Mn³⁺ sublattices
Low-dimensional network formation in molten sodium carbonate
Molten carbonates are highly inviscid liquids characterized by low melting points and high solubility of rare earth elements and volatile molecules. An understanding of the structure and related properties of these intriguing liquids has been limited to date. We report the results of a study of molten sodium carbonate (Na2CO3) which combines high energy X-ray diffraction, containerless techniques and computer simulation to provide insight into the liquid structure. Total structure factors (Fx(Q)) are collected on the laser-heated carbonate spheres suspended in flowing gases of varying composition in an aerodynamic levitation furnace. The respective partial structure factor contributions to Fx(Q) are obtained by performing molecular dynamics simulations treating the carbonate anions as flexible entities. The carbonate liquid structure is found to be heavily temperature-dependent. At low temperatures a low-dimensional carbonate chain network forms, at T = 1100 K for example ~55% of the C atoms form part of a chain. The mean chain lengths decrease as temperature is increased and as the chains become shorter the rotation of the carbonate anions becomes more rapid enhancing the diffusion of Na+ ions
- …