Showing papers in "Journal De Physique Iv in 2000"
TL;DR: In this article, a new analysis of data on the reduced glass transitions observed in thin freely-standing films is presented, which is highly suggestive of the existence of a mechanism of mobility in thin free standing films.
Abstract: In this paper we present a new analysis of data on the reduced glass transitions observed in thin freely-standing films. The empirical analysis presented is highly suggestive of the existence of a mechanism of mobility in thin freely-standing films that is inhibited in the bulk and distinct from the usual cooperative motion near the glass transition temperature T g . A tentative mechanism for T g reductions in thin freely-standing films is discussed.
50 citations
TL;DR: In this paper, a phenomenological continuum model for computational use has been developed to describe large amplitude transient wave propagation in heterogeneous multi-component materials, which is a physics-based treatment of the continuum response of microstructural components with markedly dissimilar elasticity and strength properties.
Abstract: A phenomenological continuum model for computational use has been developed to describe large amplitude transient wave propagation in heterogeneous multi-component materials. A key feature of the model is a physics-based treatment of the continuum response of microstructural components with markedly dissimilar elasticity and strength properties. A fundamental premise of the modeling effort is reliance solely on widely available dynamic material property data including Hugoniot equation-of-state and Hopkinson pressure bar strength data through either direct application or physically plausible theories. Average nonlinear iso-pressure and iso-strain solutions provide bounding responses of the multi-component material. Compressive deformation under pressure and concomitant dissipation is treated through methods of irreversible phase transformation. The model has been incorporated into a multidimensional Eulerian finite-difference shock physics code and used to examine the response of selected materials to dynamic loads.
44 citations
TL;DR: In this paper, the microstructure of proton conducting polymers (NAFION and a sulfonated polyetherketone) has been investigated by SAXS and the results are related to coefficients of transport in thermodynamic equilibrium (water-self diffusion and proton mobility) as well as hydrodynamic transport (water permeation and electroosmotic drag) as obtained from NMR, impedance spectroscopic and permeation experiments.
Abstract: The microstructure of proton conducting polymers (NAFION and a sulfonated polyetherketone) has been investigated by SAXS and the results are related to coefficients of transport in thermodynamic equilibrium (water-self diffusion and proton mobility) as well as hydrodynamic transport (water permeation and electroosmotic drag) as obtained from NMR, impedance spectroscopic and permeation experiments. A model comprising (i) simple geometric effects, (ii) effects of chemical interactions and (iii) the formation of inner space charges provides a semiquantitative scheme which relates the microstructure to the transport properties of such types of polymers.
38 citations
35 citations
TL;DR: In this article, the charge transfer process of 4-dimethylamino-4'-nitrostilbene in solution has been studied by pump-probe and transient CARS experiments with a 1 ps time resolution.
Abstract: Charge Transfert (CT) process of 4-dimethylamino-4'-nitrostilbene in solution has been studied by pump-probe and transient CARS experiments with a 1 ps time resolution. While pump-probe experiments give good evidence for formation of CT state in polar solvent, CARS experiments reveal structural changes associated to CT process. In acetonitrile, a 37 cm -1 downshift of the NO 2 stretching mode is assigned to conformational relaxation of the Internal Charge Transfer state. In toluene such behavior is not observed correlated to the absence of CT stabilization in non-polar solvent.
32 citations
TL;DR: In this article, the authors have carried out impact experiments on the mercury by a modified conventional split Hopkinson pressure bar apparatus, and the average axial pressure and volume strain histories in the mercury were obtained as assuming the uniaxial strain condition and Kolsky formulas.
Abstract: JAERI is carrying out R&D for constructing the facility of high intensity neutron spallation source which may bring us innovative science fields. A high power proton beam will be injected into the liquid mercury target for the neutron spallation. The mercury container will be subjected to the pressure waves generated by rapid thermal expansions because heating phenomena occurs in the mercury. In order to evaluate the pressure wave propagation in the mercury, we have carried out impact experiments on the mercury by a modified conventional split Hopkinson pressure bar apparatus. The chamber consisting of the input and out put bars and a collar was set between the two bars. The mercury was carefully inserted into the chamber without any air bubbles. Such a stiff collar may realize the constraint condition as a uniaxial strain condition in the mercury. The stress propagation on each bar and the liquid mercury was calculated by using an explicit FEM-code LS-DYNA. It is found that except very early time period just after stress wave injection into the mercury, a distinguished discrepancy is present between experimental and analytical results. While, in the early stage of impact, the analyses are appropriately representable to the experimental results. The average axial pressure and volume strain histories in the mercury were obtained as assuming the uniaxial strain condition and Kolsky formulas. It was recognized that the stiffness of mercury under high pressure is almost independent of the impact velocity in the experimental range. Many pits were found on the end surface of input bar being in contact with mercury, which may be associated with the cavitation in mercury.
32 citations
TL;DR: In this article, a rotational mechanism is proposed and presented in terms of dislocation energetics, which necessitates the stages of high dislocation generation and their organization into elongated cells.
Abstract: The microstructural evolution inside shear bands was investigated experimentally and analytically. A fine recrystallized structure (grains with 0.05-0.3 mu m) is observed in Ti, Cu, 304 stainless steel, Al-Li, and Ta, and it is becoming clear that a recrystallization mechanism is operating. The fast deformation and short cooling times inhibit grain-boundary migration; it is shown that the time is not sufficient for migrational recrystallization. A rotational mechanism is proposed and presented in terms of dislocation energetics. This mechanism necessitates the stages of high dislocation generation and their organization into elongated cells. Upon continued deformation. the cells become sub-grains with significant misorientations. These elongated sub-grains break up into equiaxed grains with size of approximately 0.05-0.3 mu m. It is shown that grain-boundary reorientation can operate within the time of the deformation process.
29 citations
TL;DR: In this paper, the influence of microstructure on strain hardening is studied through Kocks-Mecking plots in a number of systems showing precipitation hardening: Al Zn-Mg, Al Mg-Si-Cu, and Fe-Cu.
Abstract: The influence of microstructure on strain hardening is studied through Kocks-Mecking plots in a number of systems showing precipitation hardening: Al-Zn-Mg, Al-Mg-Si-Cu, and Fe-Cu. The presence of a supersaturated solid solution is shown to result in an extremely high work hardening rate, due to dynamic precipitation during the straining. When precipitation occurs, a drastic change in the work hardening capability is observed, which can be related to the type of precipitate-dislocations interactions and to the residual solute content. Shearable precipitates do not seem to influence greatly the work hardening behavior, which is then mostly controlled by the solute content. Non-shearable precipitates induce a high initial hardening rate. However this high initial value cannot be sustained to high strains due to extensive dynamic recovery in the solute-depleted matrix. From the analysis of the work hardening rate, it seems that precipitates remain shearable up to very large sizes and to very overaged states in the Al-Mg-Si-Cu and the Fe-Cu alloys, which has important consequences on the modeling of the hardening curve of these alloys.
28 citations
TL;DR: In this article, the ID wave propagation lagrangian code has been used to determine phase diagram of tin and the equation of state of tin developped as part of this study.
Abstract: To investigate shock behavior of tin, particularly β-bct polymorphic transition and melting, shock compression measurements have been carried out up to 53 GPa peak stress. Interface velocity measurements between tin target and LiF window have been recorded using VISAR measurement technique. Interface velocity profiles show not only direct and reverse polymorphic β-bct transition but also melting of tin on release for shock pressure above 34 GPa. In order to determine phase diagram of tin an analytic method of the experimental results has been developped. The equation of state of tin developped as part of this study has been implemented in a ID wave propagation lagrangian code. Experimental results and calculations obtained with this model of tin are in good agreement.
27 citations
TL;DR: In this article, a characterisation using X-rays of variations in texture and microstructure of copper billets obtained by the ECAE (Equal Channel Angular Extrusion) process is presented.
Abstract: This study gives a characterisation using X-rays of variations in texture and microstructure of copper billets obtained by the ECAE (Equal Channel Angular Extrusion) process. Billets were produced using the so-called route C with an extrusion angle of 120° It is shown that a significant texture gradient is observed through the width of the ECAE processed billet. In addition, we demonstrate that the amount of micro-strain and the size of the coherently diffracting domains depend on which peak of a given family of planes is investigated as well as on the exact location in the sample.
24 citations
TL;DR: In this article, the concept of laser-propelled ram accelerator (L-RAMAC) was proposed, which is capable of achieving a higher launch speed than that by a chemical ram accelerator because a higher specific energy can be input to the propellant gas.
Abstract: The concept of laser-propelled ram accelerator (L-RAMAC)' is proposed. Theoretically it is capable of achieving a higher launch speed than that by a chemical ram accelerator because a higher specific energy can be input to the propellant gas. The laser beam is supplied through the muzzle, focused as an annulus behind the base of the projectile. The performance of L-RAMAC is analized based on generalized Rankine-Hugoniot relations, suggesting that a superorbital muzzle speed is achievable out of this device.
TL;DR: Experimental and computational studies of the properties of water confined in different environments are reviewed in this article, with particular emphasis on thermodynamics and microscopic structure; the diffusional dynamics is also considered.
Abstract: Experimental and computational studies of the properties of water confined in different environments are reviewed. Particular emphasis is given to thermodynamics and microscopic structure; the diffusional dynamics is also considered. The need for studies of the vibrational dynamics is pointed out.
TL;DR: In this article, an integrated modelling approach of Chemical Vapour Infiltration of a fibrous preform by methyltrichlorosilane, featuring coupled reactor fluid mechanics and porous medium gas transport has been developed.
Abstract: An integrated modelling approach of Chemical Vapour Infiltration of a fibrous preform by methyltrichlorosilane, featuring coupled reactor fluid mechanics and porous medium gas transport has been developed. The coupling between transport/reaction phenomena inside and outside the preform is rather strong, both physically and numerically. An important counter-effect of the reactive preform on the surrounding gas-phase composition is evidenced; also, an appreciable dissymmetry of the densification rate inside the preform, even with negligible convective transport inside it . Such an approach brings new informations for understanding the physico-chemical phenomena of CVI and new guidelines for preform densification optimisation.
TL;DR: In this article, a concise derivation of the quantum-diffraction term at the order n 5/2 in the expansion with respect to density n of Helmholtz' free energy F for Coulomb plasma within the effective potential approach was given.
Abstract: This paper gives a concise derivation of the quantum-diffraction term at the order n 5/2 in the expansion with respect to density n of Helmholtz' free energy F for Coulomb plasma within the effective-potential approach. Starting with the many-body cluster expansion of F, it is shown that this particular term comes from the irreducible part of the screened three-particle Slater sum in the third cluster integral.
TL;DR: In this article, the results of a molecular dynamics computer simulation of a binary Lennard-Jones liquid confined in a narrow pore are presented, where the surface of the pore has an amorphous structure similar to that of the confined liquid.
Abstract: We present the results of a Molecular Dynamics computer simulation of a binary Lennard-Jones liquid confined in a narrow pore. The surface of the pore has an amorphous structure similar to that of the confined liquid. We find that the static properties of the liquid are not affected by the confinement, while the dynamics changes dramatically. By investigating the time and temperature dependence of the intermediate scattering function we show that the dynamics of the particles close to the center of the tube is similar to the one in the bulk, whereas the characteristic relaxation time τ q (T, p) of the intermediate scattering function at wavevector q and distance p from the axis of the pore increases continuously when approaching the wall, leading to an apparent divergence in the vicinity of the wall. This effect is seen for intermediate temperatures down to temperatures close to the glass transition. The p-dependence of τ q (T,ρ) can be described by an empirical law of the form τ q (T,ρ) = f q (T) exp[Δ q /(ρ ρ - ρ)], where Δ q and ρ q are constants, and f q (T) is the only parameter which shows a significant temperature dependence.
TL;DR: In this paper, the authors report on the special sub-Doppler features of high-resolution spectroscopy in a thin cell, in conditions when the atomic free path is anisotropic as due to the special cell geometry.
Abstract: We report on the special sub-Doppler features of high-resolution spectroscopy in a thin cell, in conditions when the atomic free path is anisotropic as due to the special cell geometry. The most recent developments include linear spectroscopy under normal incidence, an extension to transition between excited states, and the observation of atoms flying under grazing incidence in a pump-probe scheme with spatially separated beams.
TL;DR: In this paper, a modified SHPB set-up is presented, which extends the scope of this technique to the tensile loading of sheet metals, and a special method of wave analysis is then used to determine the forces and displacements occurring at the specimen ends from the strain signals recorded in the bars.
Abstract: The testing of sheet metal at high strain rates requires the use of the SHPB set-up. This technique provides satisfactory results as far as the compression and shear are concerned. A modified SHPB set-up is presented here, which extends the scope of this technique to the tensile loading of sheet metals. In this case, many measurement problems arise due to the way in which the specimen is connected to the bars, which never completely prevents unexpected slipping and is liable to induce interference waves due to an impedance mismatch. The technique described here was based on the hat specimen technique proposed by Lindholm [1]. A two-dimensional hat shaped specimen is subjected to compression loading. A tensile state is induced in a symmetrically necked area, where higher strains are concentrated. Connecting supports of an appropriate shape are attached to the bars. They have the same impedance as that of the bars to avoid the occurrence of any interference waves. The hat shape prevents sliding, and to ensure perfect initial contact conditions, the complete system is elastically pre-stressed. A special method of wave analysis is then used to determine the forces and displacements occurring at the specimen ends from the strain signals recorded in the bars.
TL;DR: In this paper, a tensile and compression tests were performed on steels 30CrNiMo8 and 25CrMo4 at strain rates from quasi-static to high strain rates up to ∼5x10 3 s -1.
Abstract: Tensile and compression tests have been performed on specimens of steels 30CrNiMo8 and 25CrMo4 at strain rates from quasi-static to high strain rates up to ∼5x10 3 s -1 . The flow stress under tensile loading shows a different behaviour than that under compression. Constitutive equations are developed to describe the behaviour in both cases. The strength-differential values are calculated from experimental data and from constitutive equations, and are presented as a function of strain and strain rate.
TL;DR: In this article, the authors propose a modele based on dynamique moleculaire and quantique quantique for developper a mecanisme catalytique in a complexe enzyme-inhibiteur.
Abstract: L'aldose reductase est un enzyme implique dans des complications diabetiques. Il fait l'objet d'etudes approfondies dans le but de developper des inhibiteurs. La structure d'un complexe enzyme-inhibiteur, determinee a resolution sub-atomique, a ete utilisee pour developper un modele du mecanisme catalytique. Ce modele a ete affine en utilisant une combinaison de dynamique moleculaire et de calculs quantiques. Il montre que la donation du proton, objet de nombreuses controverses, est due a l'effet combine de trois residus: Lys 77, Tyr 48 et His 110. Lys 77 polarise le groupement OH de Tyr 48, dont le proton est donne a His 110, qui est alors doublement protonee. His 110 est alors deplacee et donne le proton au substrat. L'information cle fournie par la structure a resolution sub-atomique est l'orientation du cycle et la protonation simple de His 110 dans le complexe enzyme-inhibiteur. Ce modele est en complet accord avec toutes les donnees experimentales disponibles.
TL;DR: In this paper, a path-dependent model for gilding copper is presented, which is an extension of a successful pathdependent model developed by Goldthorpe for the deformation of fcc materials.
Abstract: A constitutive model for gilding copper is presented. The model is an extension of a successful path-dependent model developed by Goldthorpe for the deformation of fcc materials. Gilding copper is a face-centred cubic metal and its deformation behaviour is, therefore, fully dependent upon the load path taken. This path-dependent model is the first constitutive model that treats path-dependency in a physically realistic manner and gives a complete description of loading and unloading paths; it can be implemented into hydrocodes with no significant cost. The necessity for extending the model using gilding copper as an exemplar material is to include the effects of alloying and of prior processing into the model. It introduces a generic principle in which the underlying dislocation dynamics are governed by the copper. All aspects of the model are presented and explained and determination of the constants for the model from experimental data is shown. Validation of the model by comparison with mechanical tests performed under changing conditions is presented together with predictions for the same tests using the Armstrong-Zerilli model.
TL;DR: In this article, the authors present results of molecular dynamics simulations for a supercooled polymer melt confined between two smooth and purely repulsive walls, and show that structural relaxation times exhibit a power-law behavior in the vicinity of a critical temperature T c (D) 0.39.
Abstract: We present results of molecular dynamics simulations for a supercooled polymer melt confined between two smooth and purely repulsive walls. The thickness D of the film is about 7 times the bulk radius of gyration. For all temperatures studied, a significant increase of the monomer and chain mobilities with respect to the bulk is observed. Preliminary results suggest that structural relaxation times exhibit a power-law behavior in the vicinity of a critical temperature T c (D) 0.39 (in Lennard-Jones units). This estimate of T c (D) is about 14% smaller than the corresponding bulk value. Despite this significant decrease the time dependence of various mean-square displacements seems to be unaffected by confinement if data for the same reduced temperatures T - T c of the bulk and the film are compared.
TL;DR: In this paper, the spallation strength of single crystal copper foils 100 micrometers thick of the and orientations and of polycrystalline copper foam was determined using laser driving.
Abstract: The spallation strength of single crystal copper foils 100 micrometers thick of the and orientations and of polycrystalline copper foils has been determined. Laser driving was used to launch miniature plates. Also, the spallation strength of polycrystalline copper was measured using conventional gas gun techniques and plates for comparison. Experimental measurements include VISAR records of the free surface particle velocity and micrographs of recovered flyers. A 1D hydrocode was used to simulate the VISAR records to determine the dynamic pressure history of the spallation process. Judging by the calculated pressures, it was found that the foils were somewhat stronger than the foils, which were about twice as strong as the polycrystalline foils. These were, in turn, substantially stronger than the larger sample plates (1.5 mm thick) of the gas gun. In the polycrystalline foils, small voids are found in recovered samples impacted at flyer plate velocities almost producing spallation in the gas gun, indicating that this sparse damage in the foils is able to grow substantially more in the much larger gas gun plates and under much slower gas gun loading.
TL;DR: In this paper, Monte Carlo simulations of a simplified lattice model are used to study the dynamic behavior of thin polymer films, where the dynamics is accelerated with respect to the bulk, leading to a decrease of the extrapolated glass transition temperature with decreasing film thickness.
Abstract: The dynamic behavior of thin polymer films is studied by Monte Carlo simulations of a simplified lattice model. The film geometry is realized by two opposite hard walls whose distance is varied in the simulations. In the films the dynamics is accelerated with respect to the bulk, leading to a decrease of the extrapolated glass transition temperature with decreasing film thickness.
TL;DR: In this article, the authors examined the dependence of the liquid-liquid coexistence line as the degree of size-asymmetry between cations and anions is varied, measured by the ratio ω of the diameters of the cationic and anionic species.
Abstract: For the class of size-asymmetric primitive models (SAPM) of binary electrolyte solutions (models comprising equal numbers of positively and negatively charged hard-spheres of different diameter) we examine the dependence of the liquid-liquid coexistence line as the degree of size-asymmetry between cations and anions is varied. The asymmetry is measured by the ratio ω of the diameters of the cationic and anionic species. The phase diagram for liquid-liquid equilibria is calculated under a modified Bjerrum theory, in which the electrolyte solution is represented as a mixture of free ions and (cation-anion) ion-pairs. More specifically, we have extended to the SAPM model the theory of Ebeling and Grigo originally formulated for the restricted primitive model (RPM). Several schemes for estimating the association constant for ion-pair formation are considered. We find that the features of the coexistence line are very sensitive with respect to ω, with the critical temperature, the critical density, and the width of the coexistence line increasing as the asymmetry of the SAPM model increases. We also report results for the critical parameters of the RPM calculated under the associative-mean spherical approximation; these values serve to gauge the limitations of the simpler Ebeling-Grigo theory.
TL;DR: In this paper, the authors validate a numerical model for monolithic polycarbonate or polymeric multilayer material in order to adapt them to different impact conditions (type of threat, striking velocity and test temperature).
Abstract: The aim of this work is to validate a numerical model for monolithic polycarbonate or polymeric multilayer material in order to adapt them to different impact conditions (type of threat, striking velocity and test temperature). Thus, this paper deals with the experimental and numerical investigations of PC plates impacted at velocities up to 400 m/s for a temperature range between -20°C and 50°C. In this context, impact tests on 6 and 9.7 mm thick polycarbonate plates have been performed. A high speed camera records the target profile during the interaction and the displacement history of the projectile. Photographs of impact on polycarbonate plates show that rupture occurs either by petalling or plugging depending on the projectile striking velocity and on the target thickness. Numerical simulations after fitting procedures agree well with the experimental results and show the temperature and polycarbonate thickness influence on the capacity to absorb the impact energy by plastic deformation.
TL;DR: A survey of the state of the field and insights into possible reasons for the apparent discrepancies that suggest paths for further investigation can be found in this article, with a focus on the effects of size and confinement on the glass transition of both small molecules and polymers.
Abstract: Since the first measurements of the reduction of the glass transition of small molecule liquids in nanometer pores, there has been an increasing number of studies of the effects of size and confinement on the glass transition of both small molecules and polymers. The measurements coming from different groups often give apparently conflicting results. This paper surveys the state of the field and provides insights into possible reasons for the apparent discrepancies that suggest paths for further investigation.
TL;DR: In this article, the molecular dynamics of the liquid crystals 8CB and 5CB confined to the mesopores of the molecular sieve Al-MCM-41 with pore diameter less than 2.5 nm was investigated by broadband dielectric spectroscopy.
Abstract: The molecular dynamics of the liquid crystals 8CB and 5CB confined to the mesopores of the molecular sieve Al-MCM-41 with pore diameter less than 2.5 nm was investigated by broadband dielectric spectroscopy. The phase transitions of the liquid crystal cannot be detected for the molecules confined to the pores. Moreover it has been found that even about 20 K below the melting temperature of the bulk material a relaxation process occurs for molecules confined to the mesoporous material which does not exist in the bulk liquid crystal state. The temperature dependence of the relaxation rate of that process obeys a Vogel-Fulcher-Tammann law.
TL;DR: In this article, quasi-phase matched conditions induced by two photon absorption in a thermally poled glass are reported, and simultaneous formation of a linear non-permanent index grating is also recorded.
Abstract: Quasi-phase matched conditions induced by two photon absorption in a thermally poled glass are reported. The simultaneous formation of a linear non-permanent index grating is also recorded.
TL;DR: In this paper, a detailed description of microscopic void nucleation and growth dynamics is presented, which demonstrates the competitive role of void nucleations and growth in the development of the spall.
Abstract: In order to achieve incipient and complete spallation, plane impact experiments were performed on three types of copper with different impurity levels. Dependence of the damage behavior on the material microstructure was found. The VISAR results of these experiments were used to examine the NAG model of Curran et al [Phys Rev. 147 (1987)], which includes a detailed description of microscopic void nucleation and growth dynamics. All measurements could be fitted by change of a single parameter in the model, the initial nucleation rate. The analysis demonstrates the competitive role of void nucleation and growth in the development of the spall.