Plasma-assisted ignition and deflagration-to-detonation transition

Non-equilibrium plasma demonstrates great potential to control ultra-lean, ultra-fast, low-temperature flames and to become an extremely promising technology for a wide range of applications, including aviation gas turbine engines, piston engines, RAMjets, SCRAMjets and detonation initiation for pulsed detonation engines. The analysis of discharge processes shows that the discharge energy can be deposited into the desired internal degrees of freedom of molecules when varying the reduced electric field, E/n, at which the discharge is maintained. The amount of deposited energy is controlled by other discharge and gas parameters, including electric pulse duration, discharge current, gas number density, gas temperature, etc. As a rule, the dominant mechanism of the effect of non-equilibrium plasma on ignition and combustion is associated with the generation of active particles in the discharge plasma. For plasma-assisted ignition and combustion in mixtures containing air, the most promising active species are O atoms and, to a smaller extent, some other neutral atoms and radicals. These active particles are efficiently produced in high-voltage, nanosecond, pulse discharges owing to electron-impact dissociation of molecules and electron-impact excitation of N(2) electronic states, followed by collisional quenching of these states to dissociate the molecules. Mechanisms of deflagration-to-detonation transition (DDT) initiation by non-equilibrium plasma were analysed. For longitudinal discharges with a high power density in a plasma channel, two fast DDT mechanisms have been observed. When initiated by a spark or a transient discharge, the mixture ignited simultaneously over the volume of the discharge channel, producing a shock wave with a Mach number greater than 2 and a flame. A gradient mechanism of DDT similar to that proposed by Zeldovich has been observed experimentally under streamer initiation.     

Influence of the methods of energy action on deflagration-to-detonation transition

The results of experimental gas-detonation investigations carried out in the past decade at the Department of Physical Gasdynamics of the Joint Institute of High Temperatures of the Russian Academy of Sciences (JIHT RAS) in connection with the problems of creation of a pulse detonation engine have been presented. Consideration has been given to the influence of the shape of injectors on the formation of detonation in a gas flow with separate feed of the fuel and the oxidant, the influence of acoustic action on the ignition region, reduction in the predetonation length in the flow of a mixture capable of detonation, and improvement in the efficiency of electric-discharge initiation of detonation by the self-magnetic and induced external magnetic field.     

Optimization of the switch-on and switch-off transition in a commercial laser

The response of a Class B laser to a rapid change in one of its parameters is known to be accompanied by delay and ringing. It has been theoretically and numerically shown that the transition can be modified by using adequate functional shapes for the control parameter (e.g., the laser pump) in order to steer the laser from one point of operation to another. Here we experimentally show the implementation of these ideas in a commercial device: a semiconductor laser. We establish a procedure for optimizing a controlled switch-on and switch-off and obtain a clean, fast, and reliable square pulse, either in a single shot or in a repetitive sequence. The generality of this procedure promises a wide field of application for a variety of laser systems.     

Brittle-to-ductile transition in steels and the critical transition temperature

Specimens of different thickness made of a low-alloy steel were tested to determine fracture toughness under static and impact loading at room and low temperatures. It was found that for the specimen of each thickness there was a specific brittle-to-ductile transition temperature T_psdefined as the upper temperature boundary up to which plane-strain conditions were valid at fracture. The temperature T_ps, as well as the brittleness transition temperature T ₅₀ (determined via the 50% ductile component of the fracture surface), rose with the increase in the thickness B of a specimen. The value of T_pswas found to be in a linear dependence on lgB for both static and impact loading.

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Résumé On a procédé à l'essai d'éprouvettes de différentes épaisseurs réalisées en acier faiblement allié en vue de déterminer la ténacité à la rupture sous charge statique et d'impact à température ambiante et à basse température. On a trouvé que pour les éprouvettes correspondantes à chaque épaisseur, il existait une température de transition spécifique ductile-fragile T_ps, définie comme la limite supérieure de température sous laquelle les conditions d'état plan de déformation sont atteintes à la rupture. La température T_psainsi que la température de transition de fragilité T₅₀, correspondant à une surface de rupture présentant 50% de ductilité, se relèvent avec un accroissement de l'épaisseur B de l'éprouvette. La valeur de T_ps a été trouvée en dépendance linéaire du lgB à la fois en contrainte statique et en contrainte d'impact.

     

Probing the localized-to-delocalized transition

Detailed understanding of the transition between localized and delocalized behaviour in mixed valence compounds has been elusive as evidenced by many interpretations of the Creutz-Taube ion, [(NH3)5Ru(pz)Ru(NH3)5]5+. In a review in 2001, experimental protocols and a systematic model to probe this region were proposed and applied to examples in the literature. The model included: (i) multiple orbital interactions in ligand-bridged transition metal complexes, (ii) inclusion of spin-orbit coupling which, for dpi5-dpi6 complexes, leads to five low-energy bands, two from interconfigurational (dpi-->dpi) transitions at the dpi5 site and three from intervalence transfer transitions, (iii) differences in time scale between coupled vibrations and solvent modes which can result in solvent averaging with continued electronic asymmetry defining 'class II-III', an addition to the Robin-Day classification scheme, and (iv) delineation of coupled vibrations into barrier vibrations and 'spectator' vibrations. The latter provide direct insight into localization or delocalization and time scales for electron transfer. In this paper, the earlier model is applied to a series of mixed-valence molecules.     

Numerical modeling of the ductile-brittle transition

Numerical studies of the ductile-brittle transition are described that are based on incorporating physically based models of the competing fracture mechanisms into the material's constitutive relation. An elastic-viscoplastic constitutive relation for a porous plastic solid is used to model ductile fracture by the nucleation and subsequent growth of voids to coalescence. Cleavage is modeled in terms of attaining a temperature and strain rate independent critical value of the maximum principal stress over a specified material region of the order of one or two grain sizes. Various analyses of ductile-brittle transitions carried out within this framework are discussed. The specimens considered include the Charpy V-notch test and cracked specimens under mode I or mode II loading conditions. The fracture mode transition emerges as a natural outcome of the initial-boundary value problem solution. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cluster model of the glass transition

A cluster model of the glass transition has been developed, treating the relative size of the cluster as an order parameter. The model accounts for some of the features of the glass transition.     

About the Coulomb phase transition

High Temperature - The possibility of detecting a gas-liquid Coulomb phase transition in nondegenerate nonideal two-component low-temperature plasma is discussed using the results of calculating...     

Diffusion of the superconducting transition in HTSC

In the present study it is shown that the transition to the superconducting state can be described within the framework of the phenomenological theory of diffused phase transitions, due to the structural material heterogeneity. The paraconductivity due to the appearance of fluctuating Cooper pairs at temperatures above T_c, demonstrates 2D–3D crossover, which also occurs in a certain temperature range, located above the superconducting transition interval.     

Connotation of transition spaces outside the dwelling

A study of the ways in which residents of two different UK Local Authority house types have appropriated space in front of their dwellings suggest that this practice is related to the meaning of the transition zone between the public and private realms of the house. It has become apparent that spatial boundaries around the dwelling unit serve either as borders representing the synbolic and judicial differentiation of space, or as physical barriers which regulate visibility and permeability. Thus, the relationship between the dwelling and its immediate surroundings is more complex than traditional design criteria (such as setbacks from the street, plot ratios and site density) suggest. In this study, the meaning and the use of the space around the dwelling is considered from a sociological and a psychological perspective. This is a different approach to that adopted by the UK Department of the Environment for example, and suggests that the findings of this well known study are limited. A diversification of the concept of site layout for houses is presented, which has implications for architects and planners involved in the design of residential areas.     

Enthalpy landscapes and the glass transition

A fundamental understanding of the glass transition is essential for enabling future breakthroughs in glass science and technology. In this paper, we review recent advances in the modeling of glass transition range behavior based on the enthalpy landscape approach. We also give an overview of new simulation techniques for implementation of enthalpy landscape models, including techniques for mapping the landscape and computing the long-time dynamics of the system. When combined with these new computational techniques, the enthalpy landscape approach can provide for the predictive modeling of glass transition and relaxation behavior on a laboratory time scale. We also discuss new insights from the enthalpy landscape approach into the nature of the supercooled liquid and glassy states. In particular, the enthalpy landscape approach provides for natural resolutions of both the Kauzmann paradox and the question of residual entropy of glass at absolute zero. We further show that the glassy state cannot be described in terms of a mixture of equilibrium liquid states, indicating that there is no microscopic basis for the concept of a fictive temperature distribution and that the glass and liquid are two fundamentally different states. We also discuss the connection between supercooled liquid fragility and the ideal glass transition.     

Spin-flip transition in the chromium-tungsten system

The magnetic susceptibility χ of polycrystalline Cr and Cr-0.3 at.% W samples has been studied in the neighborhood of the spin-flip transitionT _sf with a relative precision of one part in 10⁴. In pure Cr, χ exhibits a step-type anomaly atT _sf=123.0±0.5 K. The χ behavior of the Cr-0.3 at.% W alloy is different from that of pure Cr: No step anomaly exists, but atT _sf=112.5±0.5 Kdχ/dT shows a sharp minimum.     

Fracture toughness in the transition region

In order to study the origin of the scatter in the fracture toughness in the transition region and to specify the lower bound of the scatter, approximately 100 pieces 0.5T-CT NiCrMoV steel specimens were tested in the transition region, and their fracture surfaces were investigated. Major portion of the scatter was caused by the scatter in the length of preceding dimple crack which was generated at the fatigue precrack before conversion to final cleavage fracture. A method to predict the lower bound of the scatter in the small-specimen fracture toughness was proposed. It employs Weibull plot of new parameter J₁ which represents the scatter in the cleavage strength of material. The cleavage fracture origin may be associated with micro-stress-concentration in the microstructure which may not be caused by the micromechanism in terms of grain size unit.     

Thermoelectric power above the superconducting transition

The thermoelectric power in the fluctuation region above the superconducting transition temperatureT _c is studied theoretically. Use is made of the time-dependent Ginzburg-Landau (TDGL) equation due to Ebisawa and Fukuyama, which includes the correction term of the order ofT/μ as well, whereT is the temperature and μ is the chemical potential. It is shown that the thermoelectric power tends to zero as (T?T _c ) ln [T/(T?T _c )] and as (T?T _c ) as the temperature approaches the transition temperatureT _c for the two-dimensional and the one-dimensional systems, respectively.     

Fullerenes catalyze the graphite-diamond phase transition

The effect of fullerenes on the graphite-diamond phase transition was experimentally studied. Adding a catalytic amount of fullerenes to graphite during the diamond synthesis at relatively low pressures (4.5–5.5 GPa) and temperatures (about 1200°C) increases the percentage graphite to diamond conversion by a factor of 1.8 as compared to the case of an initial charge containing only graphite or fullerene. Adding fullerene to graphite allows the pressure and temperature of the synthesis to be decreased.     

Fluctuation phenomena below the superconducting transition

The fluctuation propagator for the superconducting order parameter belowT _c is derived for a gapless superconductor. The fluctuation of the order parameter consists of two distinct modes: the fluctuations of the phase and the modulus of the order parameter. The former fluctuation obeys a phononlike dispersion, while the latter fluctuation is described by a damping diffusion type equation. The present theory gives an account of the fluctuation-induced Josephson effect belowT _c observed recently by Carlson and Goldman.     

Editorial transition

Foreword

Editorial transition     

Fluctuation conductivity in the vicinity of the superconducting transition

The conductivity of the metal in the vicinity of the superconductivity transition point has been obtained. The nonlinear fluctuation effects change the temperature dependence of the conductivity in a wide range of temperatures, in which fluctuation corrections to thermodynamic quantities are still small. At comparatively strong pair breaking these effects decrease the fluctuation correction, which may exceed the conductivity of normal metal far from the transition temperature.