Wide Range Of External Parameters Research Articles

Equilibrium structure of a dense trimesic acid monolayer on a homogeneous solid surface: from atomistic simulation to thermodynamics.

A general methodology for determining the thermodynamic characteristics of rigid organic crystals on the atomistic level is presented. The proposed approach is based on a combination of grid interpolation of the precalculated intermolecular potential and kinetic Monte Carlo simulation of the gas-crystal system with an explicit interphase. The two-phase system is stabilized in a wide range of external parameters with an imposed external potential and damping field. The damping field reduces the intermolecular potential at the edges of the crystals and turns it off in the gas phase. To determine the thermodynamic characteristics of a crystal the conditions of equality of chemical potentials in coexisting phases are used. The intermolecular pairwise potential can be calculated on the atomistic or quantum level. In the kinetic Monte Carlo simulations, a grid interpolation of the precalculated potential is performed on each iteration of the algorithm. We have applied the approach to the thermodynamic analysis of a dense monolayer of trimesic acid on a homogeneous surface. The calculated free energy and entropy for the dense "superflower" and filled chicken-wire phases obey the Gibbs-Duhem equation, which confirms the thermodynamic consistency of our approach. Using the proposed approach, we have revealed that the dense "superflower" phase becomes metastable at zero pressure and 470-500 K. Under these conditions, the filled chicken-wire structure with partially released hexagonal cages is thermodynamically favourable. The proposed approach is a potentially universal tool for the thermodynamic analysis of crystals formed by "rigid" organic molecules of any complexity on the atomistic level.

Nanoelectromechanical rotary current rectifier

Nanoelectromechanical systems (NEMS) are devices integrating electrical and mechanical functionality on the nanoscale. Because of individual electron tunneling, such systems can show rich self-induced, highly non-linear dynamics. We show theoretically that rotor shuttles, fundamental NEMS without intrinsic frequencies, are able to rectify an oscillatory bias voltage over a wide range of external parameters in a highly controlled manner, even if subject to the stochastic nature of electron tunneling and thermal noise. Supplemented by a simple analytic model, we identify different operational modes of charge rectification. Intriguingly, the direction of the current depends sensitively on the external parameters.

Bromate-Anion Electroreduction at Rotating Disc Electrode under Steady-State Conditions: Comparison of Numerical and Analytical Solutions for Convective Diffusion Equations in Excess of Protons

The article contains results of numerical analysis of convective-diffusion transport equations for the components of the bromate anion electrochemical reduction process at rotating disk electrode via the redox-mediator autocatalysis (EC″) mechanism. The problem is solved taking into account the difference in the diffusion coefficients of the components. It is assumed that the concentration of protons inside the solution is constant, including the diffusion layer, due to its high value compared to the concentration of bromate-anions. Comparison of the obtained results with the predictions of an approximate analytical study of the same system (Vorotyntsev, M.A., Antipov, A.E., Electrochim. Acta, 2017, vol. 246, p. 1217) confirms the adequacy of the developed analytical approach to the calculating of both the concentration profiles of the system’s components (with the exception of the case of very thick diffusion layer) and the current density for a wide range of external parameters: the solution composition, rate of the comproportionation reaction, the convection intensity (the electrode rotation velocity) and the passing current.

The formation of new quasi-stationary vortex patterns from the interaction of two identical vortices in a rotating fluid

Within the framework of the quasi-geostrophic approximation, the interactions of two identical initially circular vortex patches are studied using the contour dynamics/surgery method. The cases of barotropic vortices and of vortices in the upper layer of a two-layer fluid are considered. Diagrams showing the end states of vortex interactions and, in particular, the new regime of vortex triplet formation are constructed for a wide range of external parameters. This paper shows that, in the nonlinear evolution of two such (like-signed) vortices, the filaments and vorticity fragments surrounding the merged vortex often collapse into satellite vortices. Therefore, the conditions for the formation and the quasi-steady motions of a new type of triplet-shaped vortex structure are obtained.

Demkov–Kunike transition dynamics in a nonlinear two-level system

We investigate the Demkov–Kunike transition in a nonlinear two-level system. We find that nonlinearity can dramatically affect the transition dynamics. We show the asymmetry of transition probability in different initial modes. For the weak interaction, the transition probability could be quickly stabilized at 100% in the initial state $$\psi _{1}$$ and could be optionally obtained from zero to 100% over a very wide range of external parameters in the initial state $$\psi _{2}$$ . For the strong interaction, the quantum transition of the two states could be completely blocked. In the adiabatic case, for different initial modes we analyze the asymmetry of transition probability by the eigenenergy level structure of the system. We also show the influence of static detuning on transition probability. Possible applications of our theory to actual physical systems are discussed.

Thermodynamics of Defect Formation and Hydration of Y<sub>2</sub>O<sub>3</sub>

Defect formation in yttria with a small content of acceptor impurities in equilibrium with a hydrogen-containing gas phase is studied theoretically. A statistical-thermodynamic description of the yttriagas equilibrium is based on the approach developed for compounds with a complex electronic structure [Phys. Stat. Sol. B (1991) Vol. 168, p. 233]. The considered model of electronic structure for Y2O3 includes, besides valence and conduction bands, acceptor and F-center states. The energy of F-centers was calculated in the framework of the variational quantum-mechanical approach combined with the molecular statics method. It is shown that acceptor states appreciably affect the thermodynamics of defect formation, while the F-centers contribution in a wide range of external parameters is small. The concentrations of defects (protons, oxygen vacancies, electronic defects) and the Fermi level position are determined as functions of temperature and gas phase parameters.

Diagnostics and modeling of CH4–CO2 plasmas for nanosmooth diamond deposition: Comparison to experimental data

Microwave plasma-assisted chemical vapor deposition of very smooth diamond coatings is an important process for various applications including mechanical and micromechanical systems and acoustic wave devices. Nanosmooth coatings have been deposited from CH4–CO2 gas mixtures at moderate temperature, the order of 600 °C. In order to increase the knowledge of the process and the control of the final characteristics of the films, a modeling of these plasmas is necessary. This has been carried out here from the prior determination of the plasma parameters. Optical emission spectroscopy was used in order to determine the gas kinetic temperature. Microwave interferometry and Langmuir double probe were used to determine the electron density and the electron temperature, respectively. All these experimental data have been obtained for a wide range of external parameters, such as the inlet composition, the pressure, the gas flow rate, and the power injected in the plasma. Then modeling of CH4–CO2 plasmas was developed by coupling chemical kinetics with a two-dimensional description of hydrodynamics and a surface-wall recombination of main radicals. The kinetic description of the CH4–CO2 plasmas was done by combining a specific mechanism of dissociation by electrons to a slightly modified version of a combustion mechanism for neutral-neutral interactions. This model has been validated by comparing the calculated species concentrations and the experimental results obtained by molecular beam mass spectrometry as a function of various external parameters. The influence of the inlet composition at three microwave power densities has been especially emphasized here. The calculations are in good agreement with the experimental results. It is shown that among the various parameters that influence the diamond growth from CH4–CO2 plasmas, the power density injected in the plasma is very important as it changes strongly the degree of completion of the chemical system and then the deposition conditions.

Rosen-Zener transition in a nonlinear two-level system

We study the Rosen-Zener transition (RZT) in a nonlinear two-level system in which the level energies depend on the occupation of the levels, representing a mean-field type of interaction between the particles. We find that the nonlinearity could affect the quantum transition dramatically. At certain nonlinearity the 100% population transfer between two levels is observed and found to be robust over a very wide range of external parameters. On the other hand, the quantum transition could be completely blocked by a strong nonlinearity. In the sudden and adiabatic limits we have derived analytical expressions for the transition probability. Numerical explorations are made for a wide range of parameters of the general case. Possible applications of our theory to Bose-Einstein condensates (BECs) and solid-state physics are discussed.

Electron energy distribution function measurements and plasma parameters in inductively coupled argon plasma

Electron energy distribution functions (EEDFs) have been measured in a cylindrical inductively coupled plasma (ICP) with a planar coil over a wide range of external parameters (argon pressure, discharge power and driving frequency). The measurements were performed under well-defined discharge conditions (discharge geometry, rf power absorbed by plasma, external electrical characteristics and electromagnetic field and rf current density profiles). Problems found in many probe measurements in ICPs were analysed and a rationale for designing probe diagnostics that addresses these problems is presented in this paper. A variety of plasma parameters, such as, plasma density, effective and screen electron temperatures, electron–atom transport collision frequency, effective rf frequency and rates of inelastic processes, have been found as appropriate integrals of the measured EEDFs. The dependence of these ICP parameters over a wide range of argon pressure, rf power and frequency results in experimental scaling laws that are suitable for comparison with ICP models and helpful in ICP design for many applications.

A critical nonequilibrium phase transition and [formula omitted] noise in a current-carrying thin HTSC film-boiling nitrogen system

Critical phenomena at a nonequilibrium phase transition in a system of a current-carrying thin film of a high-temperature superconductor (HTSC) cooled by boiling nitrogen have been studied experimentally. It is seen that there arises a 1 f noise with a large scope of amplitudes in a wide range of external parameters, that is explained by the interaction of nonlinear thermal processes in a current-carrying superconductor and boiling coolant.

Novel optical fibres for sensing applications

Optical fibres have some degree of sensitivity to a wide range of external parameters. On the other hand the effects are usually small and are minimised as far as possible in fibres designed for telecommunications. Furthermore it is often difficult to differentiate between a number of small responses that may occur simultaneously. Considerable advantages can be gained by appropriate selection of core and cladding materials and by novel fibre structures and designs.