Influence of the size and shape of free nanoparticles on the local changes in the lattice parameter and on the structural stability of body-centered cubic zirconium and iron

The influence of the shape and size of nanocrystals on the lattice relaxation of body-centered cubic metals (zirconium, iron) at a constant temperature has been investigated using the molecular dynamics method with the many-body interatomic interaction potential obtained in terms of the embedded-atom model. The calculations have been performed for isolated clusters with sizes ranging from 2.5 to 17 nm for zirconium and from 2 to 14 nm for iron. It has been demonstrated that, in free zirconium and iron particles, the relaxation of the lattice constant along the [100], [010], and [001] directions has an oscillatory character. Irrespective of the size of the zirconium and iron particles, the equilibrium distances between atoms at the center of cubic clusters are minimum compared to those observed in near-surface layers and the equilibrium value of the lattice parameter for the bulk sample. In spherical clusters, the region of a maximum contraction corresponds to a depth approximately equal to 0.2 particle diameter from the surface. An increase in the size of both cubic and spherical clusters leads to a decrease in the deviation of the local lattice parameter from the equilibrium value for the bulk sample. It has been established that the size and shape of the cluster substantially affect the temperature and mechanism of the structural transformation from the body-centered cubic phase into the hexagonal close-packed phase.     

Interfacial dipole orientation—A thermodynamic approach

Summary A thermodynamic relation between the affinity and the rate of orientation is shown for rigid dipoles at the interface of two immiscible media. The influence of the temperature and of the dielectric constant is analysed. A relaxation process towards equilibrium for the interfacial tension is discussed.     

Relaxation of in-plane and out-of-plane oH2 pairs

Previous measurements of the nuclear spin-lattice relaxation time for nearest neighbor oH₂ pairs have been analyzed. The analysis has yielded the relaxation time for both in-plane and out-of-plane pair configurations. The relaxation time for the in-plane pairs was found to be 97ms and that for out-of-plane pairs was 12 ms. An analysis of the pair signal amplitude versus time indicated that the equilibrium time constant for the formation of in-plane pairs was over an order of magnitude longer than that for out-of-plane pairs.     

On the thermodynamic equilibrium in the <Superscript>3</Superscript>He-aerogel system at low temperatures

A new method for studying the processes of the establishment of the thermodynamic equilibrium in the adsorbed ³He layers in highly porous media has been proposed. Using this method, the thermalization of adsorbed ³He on silica aerogel at a temperature of 1.5 K has been studied. The process of the establishment of the thermodynamic equilibrium has been controlled by measuring the pressure in an experimental cell, the amplitude of the NMR signal, and the nuclear spin-spin and spin-lattice relaxation times of adsorbed ³He. It has been shown that the establishment of the thermodynamic equilibrium in the adsorbed ³He-aerogel system is characterized by a time of 26 min.     

Remagnetization of synthetic antiferromagnetic cells by a magnetic field pulse

The theory of the dynamic remagnetization of a synthetic antiferromagnetic system and magnetic points located on a magnetic substrate in an external magnetic field has been considered. The energies of the equilibrium states of the system have been calculated. The conditions of switching between equilibrium states have been described. The conditions of applicability of this theory have been formulated. It has been shown that the process of remagnetization can be implemented in an inertialess regime, escaping the long-term relaxation of the system to a new equilibrium position with the use of a special shape of the field signal. The possibility of the reduction of the switching field amplitude by varying the pulse duration has been demonstrated.     

Statistical mechanics of a nonequilibrium steady-state classical particle system driven by a constant external force

A classical particle system coupled with a thermostat driven by an external constant force reaches its steady state when the ensemble-averaged drift velocity does not vary with time. In this work, the statistical mechanics of such a system is derived solely based on the equiprobability and ergodicity principles, free from any conclusions drawn on equilibrium statistical mechanics or local equilibrium hypothesis. The momentum space distribution is determined by a random walk argument, and the position space distribution is determined by employing the equiprobability and ergodicity principles. The expressions for energy, entropy, free energy, and pressures are then deduced, and the relation among external force, drift velocity, and temperature is also established. Moreover, the relaxation towards its equilibrium is found to be an exponentially decaying process obeying the minimum entropy production theorem.     

Evolution of the cosmological constant in effective gravity

In contrast to the phenomenon of nullification of the cosmological constant in equilibrium vacuum, which is the general property of any quantum vacuum, there are many options in modifying the Einstein equation to allow the cosmological constant to evolve in a nonequilibrium vacuum. An attempt is made to extend the Einstein equation in the direction suggested by the condensed matter analogy of the quantum vacuum. Different scenarios are found depending on the behavior of and the relation between the relaxation parameters involved, some of these scenarios having been discussed in the literature. One of them reproduces the scenario in which the effective cosmological constant emerges as a constant of integration. The second one describes the situation when, after the cosmological phase transition, the cosmological constant drops from zero to a negative value; this scenario describes the relaxation from this big negative value back to zero and then to a small positive value. In the third example, the relaxation time is not a constant but depends on matter; this scenario demonstrates that vacuum energy (or its fraction) can play the role of cold dark matter.     

Equilibrium ensembles of quantum dots in atomically inhomogeneous pentagonal nanowires

A theoretical model has been proposed for describing the relaxation of elastic stresses in an atomically inhomogeneous pentagonal nanowire due to the formation of quantum dots in the form of precipitates of the second phase. Quantum dots have been considered as finite-height coaxial cylindrical inclusions subjected to intrinsic axial dilatation and located along the axis of the nanowire. The optimum shape and sizes of the quantum dots have been calculated for specified nanowire parameters. It has been shown that such quantum dots can form different equilibrium periodic structures in the pentagonal nanowires.     

Phenomenological theory of structural relaxation based on a thermorheologically complex relaxation time distribution

The aim of this work is to explore the consequences on the kinetics of structural relaxation of considering a glass-forming system to consist of a series of small but macroscopic relaxing regions that evolve independently from each other towards equilibrium in the glassy state. The result of this assumption is a thermorheologically complex model. In this approach each relaxing zone has been assumed to follow the Scherer-Hodge model for structural relaxation (with the small modification of taking a linear dependence of configurational heat capacity with temperature). The model thus developed contains four fitting parameters. A least-squares search routine has been used to find the set of model parameters that fit simultaneously four DSC thermograms in PVAc after different thermal histories. The computersimulated curves are compared with those obtained with Scherer-Hodge model and the model proposed by Gómez and Monleón. The evolution of the relaxation times during cooling or heating scans and also during isothermal annealing below the glass transition has been analysed. It has been shown that the relaxation times distribution narrows in the glassy state with respect to equilibrium. Isothermal annealing causes this distribution to broaden during the process to finally attain in equilibrium the shape defined at temperatures above T _g .     

Determination of the relaxation function in a quartz single crystal

A method is presented for determining the relaxation function from experimental frequency-temperature dependences of the dielectric constant on the basis of a general dispersion relation. Three relaxation processes have been detected in a quartz single crystal with the aid of this method. The relaxation times of these processes and their contribution to the relaxation function at elevated temperatures have been determined.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 76–81, October, 1986.We thank S. V. Kolodieva and her co-workers for supplying the samples of single-crystal quartz, the arrangement for measuring the dielectric parameters, and for discussion of the results of our work.     

An Ising model for the stress relaxation of solids

A kinetic theory of stress relaxation of solids as a cooperative process is proposed. The theory is based on a two-state model for the relaxation. It is shown that the conventional mean field approximation leads to an exponential dependence of the rate of stress relaxation on the stress while the multiplicative approximation of Vol'kenstein et al. leads to a power law. It is argued that the exponential law should be valid initially in the relaxation process while the power law is appropriate when the system is nearer equilibrium, which is in qualitative agreement with recent experimental findings.     

Absorption and Luminescence Spectra of Tetra (3-Pyridyl)Porphyrazine: A Convergent Spectroscopic Method for the Elucidation of Association Reactions in Solution

Tetra(3-pyridyl)porphyrazine is soluble in trifluoroethanol yielding intense blue solutions. The electronic spectra exhibit the characteristic Soret and Q bands around 330 and 610 nm, respectively; however, the absorption profile is strongly dependent on the concentration, reflecting the occurrence of an association process. The system has been elucidated by solving the equilibrium and the absorbance-concentration equations for all the wavelengths in the spectral range, according to a variational procedure. In this way the consistent spectra of the individual species have been generated. The study has also been extended to the luminescence properties of the porphyrazine molecule, and discussed in terms of the ground state association and the exciplex formation involving the monomeric species.     

Critical behaviour of the penetration depth into a metastable commensurate structure

A penetration of the equilibrium configuration into the metastable one is considered within the Frenkel-Kontorowa model with a free end. Using an accelerated relaxation method we calculate the critical variation of the penetration depth on approaching the unpinned structure. A relation to the scaling of the Lyapunov exponent near the analyticity breaking transition is proposed.     

Modulated photophysics of 2-anthracene sulphonate in micellar assembly

The association of a non-ionic surfactant of polyoxyethylene-p-(1,1,3,3-tetramethylbutyl)phenyl ether (Triton X) series with 2-AS in aqueous solution has been studied by means of steady-state, time-resolved fluorescence and fluorescence anisotropy techniques. The effect of the hydrophobic chain length on the structural dynamism of the fluorophore has been reported. Experimental results demonstrate that the equilibrium of this dynamism is sensitive to the environment. The association constant of the probe molecule with the non-ionic micelles of Triton X (TX), location of the probe in the micellar environment, have been determined from the change in emission characteristics of the probe as a function of surfactant concentration. The rate constant of quenching and mode of quenching of probe in micellar media have been ascertained. Quantitative estimates of the micropolarity at the binding sites of the probe molecule have been determined. Some of the environment-dependent relevant fluorescence parameters, fluorescence anisotropy (r), have been monitored for exploring the imposed motional restriction of the microenvironment around the probe. An attempt has been made to correlate the steady-state results with time resolved study.     

Near-oscillatory relaxation behavior of levitation force in infiltration and growth processed bulk YBCO/Ag superconducting composites

Time relaxation behavior of levitation force has been studied in IGP bulk YBCO/Ag superconductor using levitation force measurements as these measurements throw light on the magnetic relaxation in superconductors and the underlying vortex dynamics, pinning mechanisms and the nature of pinning forces. The measurements have revealed a hitherto unknown near-oscillatory relaxation of the levitation force with varying magnetic field. This kind of behavior is found to be more pronounced at smaller gap distances between the permanent magnet and the superconductor. A switch-type polarity bistable equilibrium model for the supercurrent structure has been proposed based on the understanding that even the permanent magnet gets magnetized in the presence of the superconductor, which has also been verified and reported here. This model satisfactorily explains the observed oscillatory behavior of relaxation rates.     

Molecular relaxation of some rigid polar molecules and their mixtures

The dielectric behavior of benzene solutions of four rigid polar molecules, benzophenone, benzotrichloride, ortho- and meta- dichlorobenzenes and their mixtures has been studied in the microwave region over a range of temperatures. The dielectric data has been used to determine the relaxation times and the thermodynamic parameters for the activated state. The relaxation times for single component solutions agree well at temperatures at which literature values are available. The relaxation times for mixtures are consistent with those of other rigid molecules studied previously. The results have been also compared with the computed values obtained from a relation proposed by us.     

Relaxation processes in non-Debye dielectrics

The specific features of the relaxation processes in non-Debye dielectrics have been investigated. The nature of the difference between the relaxation frequencies of the dielectric constant and dielectric loss (conductivity) has been explained. It has been shown that the average relaxation frequency of the conductivity is considerably (in some cases, by several orders of magnitude) higher than the relaxation frequency of the dielectric constant owing to an increase in the conductivity spectra of the statistical weight of the relaxation processes with short relaxation times.     

Theoretical investigations of the vapour-liquid equilibrium and dielectric properties of dipolar Yukawa fluids in an external field

In a low field approximation, using the dipolar Yukawa fluid model (in mean spherical approximation as a reference system) a consistent field-dependent free energy expression is proposed for the calculation of the vapour-liquid equilibrium of polar fluids in an applied electric field. A perturbation theory high field approximation expression of the free energy is also proposed to study the field-dependent properties of fluids. In the high field approximation, equations for the field-dependent polarization and for the nonlinear dielectric constant (or Piekara constant) are also predicted. It has been discussed that our approximations are appropriate to describe the vapour-liquid-like phase equilibria and the magnetization curves of magnetic fluids.     

Relaxation modes in glassy polymers: A temporal analysis by the simplex method of isothermal depolarisation current measurements

Molecular dynamics associated to relaxation phenomena in the glass transition temperature domain is often investigated by means of thermostimulated depolarisation current TSDC technique. It is a very sensitive method and the data are traditionally obtained according to two protocols leading to the well known complex spectra and elementary spectra. The aim of this work is to use a new TSDC protocol which analyses the relaxation current kinetics obtained after submitting the sample to an electrical field pulse at a constant temperature. A new temporal analysis of the return equilibrium isothermal transient current I(T) is proposed. The signal fitting is obtained by a simplex optimisation method. The entire signal recorded for all the temperatures can be fitted with a sum of two exponentials allowing the definition of two different relaxation times called τ₁ and τ₂. This new protocol has been used to analyse the glass transition domain of amorphous PET.     

Self-consistent pseudopotential calculations of the equilibrium properties of bulk and surface Si

A momentum-space formalism is used study the equilibrium properties of bulk and surface Si based on the self-consistent pseudopotential method. The calculated equilibrium lattice constant and the crystal energy are in agreement with experiment. By minimizing the total energy of the system, the equilibrium relaxation for the Si (111) surface was found to be 0.15 Å, in excellent agreement with measurements on the impurity-stabilized relaxed surface.