Two-phase Domain Research Articles

Structure and bonding in amorphous iron carbide thin films

We investigate the amorphous structure, chemical bonding, and electrical properties of magnetron sputtered Fe1−xCx (0.21 ⩽ x ⩽ 0.72) thin films. X-ray, electron diffraction and transmission electron microscopy show that the Fe1−xCx films are amorphous nanocomposites, consisting of a two-phase domain structure with Fe-rich carbidic FeCy, and a carbon-rich matrix. Pair distribution function analysis indicates a close-range order similar to those of crystalline Fe3C carbides in all films with additional graphene-like structures at high carbon content (71.8 at% C). From x-ray photoelectron spectroscopy measurements, we find that the amorphous carbidic phase has a composition of 15–25 at% carbon that slightly increases with total carbon content. X-ray absorption spectra exhibit an increasing number of unoccupied 3d states and a decreasing number of C 2p states as a function of carbon content. These changes signify a systematic redistribution in orbital occupation due to charge-transfer effects at the domain-size-dependent carbide/matrix interfaces. The four-point probe resistivity of the Fe1−xCx films increases exponentially with carbon content from ∼200 μΩ cm (x = 0.21) to ∼1200 μΩ cm (x = 0.72), and is found to depend on the total carbon content rather than the composition of the carbide. Our findings open new possibilities for modifying the resistivity of amorphous thin film coatings based on transition metal carbides through the control of amorphous domain structures.

Micromechanical modelling of reversible and irreversible thermo-mechanical deformation of oriented polyethylene terephthalate

In this article, the reversible and irreversible thermo-mechanical time-dependent deformation of oriented polyethylene terephthalate film is studied. A mean-field model is used to simulate these effects along with the long-term creep behaviour, taking into account the underlying material microstructure and differences in constitutive behaviour of the phases. The material is modelled as an aggregate of layered two-phase domains. Irreversible deformation, or partial shape recovery, results from the presence of an internal stress, which is characterised and incorporated into the constitutive behaviour of the non-crystalline phase. Using the micromechanical approach, the deformation mechanisms at the local scale are analysed.

A coupled Eulerian–Lagrangian extended finite element formulation for simulating large deformations in hyperelastic media with moving free boundaries

We present a coupled Eulerian–Lagrangian (CEL) formulation aimed at modeling the moving interface of hyperelastic materials undergoing large to extreme deformations. This formulation is based on an Eulerian description of kinematics of deformable bodies together with an updated Lagrangian formulation for the transport of the deformation gradient tensor. The extended finite element method (XFEM) is used to discretize the mechanical equilibrium and deformation gradient transport equations in a two-phase domain. A mixed interpolation scheme (biquadratic for the velocity and bilinear for the deformation gradient) is adopted to improve the accuracy of the numerical formulation. The interface describing the deformed shape of the body is represented by the level set function and is evolved using the grid based particle method. The performance of the scheme is explored in two-dimensions in the compressible regime. For an adequate spatial and temporal discretization, our numerical results are in good agreement with theory and with numerical results from the traditional Lagrangian formulation (in Abaqus). The advantage of the proposed formulation is that material motion is not coupled with that of the mesh; this eliminates the issues of mesh distortion and the need for remeshing associated with Lagrangian formulations when bodies undergo very large distortions. It is therefore well adapted to describe the motion of complex fluids and soft matter whose physical properties are intermediate between conventional liquids and solids.

Relationships between Mn3+ Content, Structural Ordering, Phase Transformation, and Kinetic Properties in LiNixMn2–xO4 Cathode Materials

Micrometer-sized LiNixMn2–xO4 (0.3 ≤ x ≤ 0.5) single crystals with (111) surface facets were synthesized and characterized by 6Li magic angle spinning nuclear magnetic resonance, Fourier transform infrared spectroscopy, and electrochemical studies. All three techniques were sensitive to cation disorder and the corroborated results showed that structural ordering improves with x. The transition from the ordered to the disordered spinel was triggered by an increase in Mn3+ content, which was accomplished either by a change in chemical composition or postsynthesis thermal treatment. Disordering led to increased solid solution behavior, reduced two-phase transformation domains, and improved transport properties during Li extraction and insertion. Further increasing Mn3+ content in already disordered structure extends the solid solution domain and eliminates the presence of phase II; however, this has limited effect on rate capability. The study demonstrates the dominant role of structural ordering in morpholo...

A Homogeneous Phase Change Model for Two-Phase Mechanical Seals With Three-Dimensional Face Structures

A homogenous phase change model (HPCM) based on the mass conservation law is proposed to analyze the flow field of a two-phase mechanical seal with 3D face structures. The two-phase flow domain is governed by the simultaneous partial differential equation set containing a mass transfer governing equation for each phase with a source term derived from the Rayleigh–Plesset model and a Reynolds equation for the mixture, where the pressure and the liquid fraction are unknowns. A numerical solution is developed based on finite element method (FEM). The results from the present model are in good agreement with those from the previous two-phase mechanical seal models. A two-phase mechanical seal with wavy-tilt-dam face structure is calculated. The results indicate that the 3D face structure affects the phase distribution by altering the film pressure field. The present model is especially useful to analyze the two-phase film flow field bounded by the complex solid surfaces.

Performance of thermal lattice Boltzmann and finite volume methods for the solution of heat conduction equation in 2D and 3D composite media with inclined and curved interfaces

In the present study, the accuracy of predicted effective thermal conductivity of composite materials, determined by the finite volume method (FVM) and the thermal lattice Boltzmann method (TLBM) using single relaxation time (SRT), is investigated. In addition, computational efforts required by both solution methods are also compared. Two- and three-dimensional heat conduction problems in two-phase domains with a high thermal conductivity ratio are considered, where oblique and curved interfaces are represented by the staircase approximation. The discretisation is carried out on a uniform mesh. For TLBM, different stencils are employed and a wide range of relaxation frequencies, corresponding to relative magnitudes of thermal conductivities of different phases, is investigated. For comparison, wherever available, the analytical expression for the effective thermal conductivity is considered. Otherwise, the reference effective thermal conductivity is determined using a commercial software employing an appropriate body-fitted mesh. The present results show that the accuracy of TLBM depends on the employed stencil, relaxation frequency and arrangement of two materials in the computational domain. It is also observed that TLBM can be even more accurate than FVM, although the accuracy of FVM is independent of individual values of thermal conductivities. For the same convergence criterion, however, TLBM requires a higher number of iterations than FVM, although TLBM shows a potential for speed-up.

Characterisation and modelling of anisotropic thermo-mechanical behaviour of oriented polyethylene terephthalate

The long-term and short-term anisotropic mechanical behaviour of a biaxially stretched polyethylene terephthalate foil is measured. The orientation of the crystalline phase is characterized and the representative foil microstructure is discussed. Using the obtained information, a mean-field model is used to simulate the elasto-viscoplastic behaviour of the oriented polymer foil, taking into account the different constitutive behaviour of the phases. The material is modelled as an aggregate of connected two-phase domains. The parameters of the constitutive behaviour of the crystalline and non-crystalline phases have been determined, and the ability to simulate the large-strain anisotropic behaviour of polyethylene terephthalate in the strain-rate-controlled regime and the long-term creep has been demonstrated. The model is extended to include pre-orientation of the non-crystalline phase. In addition, deformation at the microscopic level is analysed using the model results.

Occurrence of Dynamic Alpha-Phase Nucleation in Ti-5553 during Hot Deformation

This paper discusses the microstructural changes during hot deformation, mainly dynamic nucleation and growth of a precipitates, occurring in the b metastable titanium alloy Ti-5553 (Ti–5Al–5Mo–5V–3Cr–0.5Fe). The effect of process variables on flow response and microstructure evolution during hot working of the alloy with initial bimodal microstructure was established using isothermal hot-compression tests. Testing was conducted at 4 strain rates between 0.001 and 1 s−1 and 5 temperatures between 720 °C and 850 °C, on material with prior b grain size of 450 μm, a nodular size of 3 µm and a known primary a-phase fraction. All flow curves exhibited a peak stress followed by moderate flow softening in the two-phase domain. Flow softening was interpreted in terms of deformation heating and substructure or texture evolutions. The dependence on strain rate and temperature of the kinetics of dynamic a-phase nucleation during straining is complex and appears to be of second-order importance compared to the effects of strain. This suggests that the nucleation and growth of a phase in the temperature range between 720 °C and 990 °C results from a mixed-mode displacive-diffusional transformation, similar to the austenite / ferrite transformation above the Ae3 temperature reported by some authors.

Boiling phenomena in near-critical SF6 observed in weightlessness

Boiling phenomena in the two-phase region of SF6 close to its critical point have been observed using the high-quality thermal and optical environment of the CNES dedicated facility ALI-DECLIC on board the International Space Station (ISS). The weightlessness environment of the fluid, which cancels buoyancy forces and favorites the three-dimensional spherical shape of the gas bubble, is proven to be an irreplaceable powerful tool for boiling studies. To identify each key mechanism of the boiling phenomena, the ALI-DECLIC experiments have benefited from (i) the well-adapted design of the test cells, (ii) the high-fidelity of the ALI insert teleoperation when long-duration experiment in stable thermal and microgravity environment are required and (iii) the high repeatability of the controlled thermal disturbances. These key mechanisms were observed by light transmission and interferometry technique independently with two sample cells filled with pure SF6 at a near-critical density. The fluid samples are driven away from thermal equilibrium by using a heater directly implemented in the fluid, or a surface heater on a sapphire optical window. In the interferometry cell, the bulk massive heater distinguishes two symmetrical two-phase domains. The modification of the gas bubble shape is observed during heating. In the direct observation cell, the gas bubble is separated by a liquid film from the thin layered transparent heater deposited on the sapphire window. The liquid film drying and the triple contact line motion during heating are observed using light transmission. The experiments have been performed in a temperature range of 10K below the critical temperature Tc, with special attention to the range 0.1mK≤Tc−T≤3mK very close to the critical temperature. The unique advantage of this investigation is to provide opportunities to observe the boiling phenomena at very low heat fluxes, thanks to the fine adjustment of the liquid–vapor properties, (e.g. surface tension), by precise control of the distance to the critical point. We present the new observations of the gas bubble spreading over the heating surface which characterizes the regime where vapor bubbles nucleate separately and grow, as well as liquid drying, vapor film formation, triple contact line motion, which are the key mechanisms at the origin of the boiling crisis when the formed vapor film reduces the heat transfer drastically at the heater wall.

A metallurgical approach to individually assess the rheology of alpha and beta phases of Ti–6Al–4V in the two-phase domain

An original hot torsion procedure is presented using Ti–6Al–4V in the two-phase domain. Kinetics of phase transformation are used to obtain stress–strain curves in off-equilibrium states. The individual influences of phase fraction and of temperature are quantitatively assessed in the two-phase domain. Between the two forging temperatures 1143K (870°C) and 1228K (955°C), a 95MPa stress difference is observed. The paper shows that this flow stress drop is mainly due to thermal activation, by 80MPa, while 15MPa is due to the change of phase fraction. The stresses in each phase are individually assessed and stress–strain curves for each phase are obtained within the two-phase domain.

Small-angle neutron scattering investigation of polyurethane aged in dry and wet air

The microstructures of Estane 5703 aged at 70°C in dry and wet air have been studied by small-angle neutron scattering. The samples were swollen in deuterated toluene for enhancing the contrast. The scattering data show the charac- teristic domain structure of polyurethanes consisting of soft and hard segments. Debye-Anderson-Brumberger function used with hard sphere structure factor, and the Teubner-Strey model are used to analyze the two-phase domain structure of the polymer. The combined effects of temperature and humidity have a strong disruption effect on the microstructures of Estane. For the sample aged at 70°C in wet air for 1 month, the domain size, described by the correlation length, increases from 2.3 to 3.8 nm and their distance, expressed by hard-sphere interaction radius, increases from 8.4 to 10.6 nm. The struc- ture development is attributed to degradation of polymer chains as revealed by gel permeation chromatography. The hydrol- ysis of ester links on polymer backbone at 70°C in the presence of water humidity is the main reason for the changes of the microstructure. These findings can contribute to developing predictive models for the safety, performance, and lifetime of polyurethanes.

Characterization of bubble mobility in channel flow with fibrous porous media walls

In composites processing, resin is introduced into a fibrous domain to cover all the empty spaces between the fibers. It is important to extract air bubbles from the domain before the resin solidifies. Failure to do so will entrap these voids in the final part, which is detrimental to its performance. Hence, there is a need to understand bubble motion in a fibrous porous domain in which the bubbles move with the resin in channels surrounded by fibrous walls. A rising bubble model is presented that consists of a single spherical void in a cylindrical axisymmetric two-phase domain of resin and air surrounded by porous media boundaries. The motion of a bubble in a channel flow with porous boundaries is modeled by replacing the walls with a slip velocity. Focus is on how the porous media permeability influences the bubble motion. A parameter called bubble mobility is defined as the ratio of bubble rise velocity to the resin free surface velocity. Results suggest that fabric permeability and fluid properties can be optimized to increase bubble mobility and ultimately lead to reduction in void content during composites processing.

Crystal Structures and Photoluminescence across the La2Si2O7–Ho2Si2O7 System

It is well-known that when an RE2Si2O7 matrix is doped with active lanthanide ions, it displays promising luminescent responses for optical applications. The crystalline structure adopted by the silicate matrix as well as the distribution of the dopants among the available RE crystallographic sites have important effects on the luminescent yields of these compounds. The present study is aimed at analyzing the structural behavior as well as the luminescent properties of Ho(3+)-substituted La2Si2O7. Several compositions across the La2Si2O7-Ho2Si2O7 system were synthesized using the sol-gel method followed by calcination at 1600 °C. The resulting powders were analyzed by means of X-ray and neutron diffraction to determine the phase stabilities across the system. The results indicated a solid solubility region of G-(La,Ho)2Si2O7 which extends to the La0.6Ho1.4Si2O7 composition. Compositions richer in Ho(3+) show a two-phase domain (G+δ), while δ-(La,Ho)2Si2O7 is the stable phase for Ho(3+) contents higher than 90% (La0.2Ho1.8Si2O7). Anomalous diffraction data interestingly indicated that the La(3+) for Ho(3+) substitution mechanism in the G-(La,Ho)2Si2O7 polymorph is not homogeneous, but a preferential occupation of Ho(3+) for the RE2 site is observed. The Ho(3+)-doped G-La2Si2O7 phosphors exhibited a strong green luminescence after excitation at 446 nm. Lifetime measurements indicated that the optimum phosphor was that with a Ho(3+) content of 10%.

Micromechanical modelling of short-term and long-term large-strain behaviour of polyethylene terephthalate

A micromechanically based model is used to describe the mechanical behaviour of polyethylene terephthalate (PET) under uniaxial compression up to large strains and at different temperatures. The creep behaviour of isotropic PET is simulated and compared to experimental data to demonstrate the applicability of the model to describe the long-term response. The material is modelled as an aggregate of two-phase layered domains, where different constitutive laws are used for the phases. A hybrid interaction law between the domains is adopted. The crystalline phase is modelled with crystal plasticity and the amorphous phase with the Eindhoven Glassy Polymer model, taking into account material ageing effects. Model parameters for the selected constitutive laws of the phases are identified from uniaxial compression tests for fully amorphous material and semicrystalline material. Texture evolution during the deformation predicted by the model adequately matches previously observed texture evolution.

H electro-insertion into Pd/Pt(1 1 1) nanofilms: an original method for isotherm measurement coupled to in situ surface X-ray diffraction structural study

In order to get a thorough comprehension of the mechanisms governing hydrogen insertion into nanometric metallic films, we have studied ultra-thin Pd/Pt(111) layers. In this paper we propose an original method allowing the measurement of hydrogen insertion electrochemical isotherms. The use of a hanging meniscus rotating disc electrode and a new calculation approach permit to remove the contributions to the insertion charge of both hydrogen evolution and hydrogen oxidation reactions. Indeed, compared to hydrogen insertion such terms become non-negligible in the case of nanometric deposits, due to their large surface/bulk atom ratio.We have measured hydrogen insertion isotherms for Pd/Pt(111) films from 14ML down to 4ML. Independently from the film thickness, the maximum hydrogen insertion rate (H/Pd)max is smaller than that of bulk Pd. The so-called two-phase region is still present, but contrarily to bulk Pd it is characterized by a slope. Both hydrogen solubility and the two-phase domain width diminish with the decrease of the film thickness.In the present work the behaviour of hydrogen electrochemical insertion isotherms is interpreted in the light of the Pd nanofilms structure obtained with in situ surface X-ray diffraction. The lattice constraints induced by the substrate result in a lower insertion rate in the Pd deposit close to the Pt–Pd interface. Only the outermost region of the film is relaxed and behaves like bulk Pd. This description quantitatively accounts for the experimental behaviour of (H/Pd)max as a function of the film thickness. The obtained Pd/Pt(111) films structure also corresponds to the presence of non-equivalent hydrogen insertion sites, surely contributing to the slope observed in the two-phase domain.

Two-dimensional pseudo-phase-equilibrium approach on the cathode gas diffusion layer of proton exchange membrane fuel cell under varying degrees of humidification

We propose a novel 2D water management for cathodes of proton exchange membrane fuel cells (PEMFCs). A pseudo-phase-equilibrium function is incorporated to the model to relate the vapor fraction to the liquid water within the cathode gas diffusion layer, so that explicit tracking of the liquid water front between the single phase and two-phase domains becomes completely unnecessary. In the proposed model, liquid water transport in the porous medium is governed by the capillary pressure gradient, whereas the multi-component gas transports of oxygen, nitrogen, and water vapor are described using the Stefan–Maxwell equations. The effects of the gas diffusion layer thickness, porosity, temperature, pressure, and varying degrees of feed humidification are investigated. In addition to the steady-state performance, the dynamic response is computed for a detailed exploration of both the oxygen diffusion and the transient water imbibition and drainage. It concludes that the proposed method can be successfully applied to the two-phase fuel cell models in 2D geometry for both the steady-state and dynamic analyses.

Influence of Zinc Oxide on the Conductivity of Ceria

ZnO was studied as dopant or additive for CeO2 in the concentration range x = 0.00 to x = 0.95 referred to (CeO2)1-x(ZnO)x. From XRD, the solubility limit of ZnO in the bulk of CeO2was estimated as 3 ± 0.7 mol% at room temperature. The total conductivity was measured for all compositions in air by impedance spectroscopy. The contributions from bulk and grain boundaries were evaluated separately. Additionally, the electronic conductivities were measured using a Hebb-Wagner technique with microelectrodes. Up to the solubility limit, the total conductivity of the bulk in air increases almost linearly with the ZnO content from 300°C to 800°C, the same holds for the p-type electronic conductivity (in the range 600°C–800°C). Hence, zinc ions act as acceptors on cerium sites, zinc doped ceria is a moderate mixed conductor. In the two-phase domain from 3 mol% up to 20 mol%, the total conductivity increases slightly and the partial electronic conductivity dependence remains nearly constant. Starting at 30 mol% ZnO, the grain boundary conduction rises steeply and the electronic conductivity predominates the total conductivity for this and higher ZnO contents reaching 0.03 S/cm at 800°C in air for a sample with the composition x = 0.95 mol%.

Kinetics of hydrogen sorption by palladium nanoparticles

The purpose of this communication is to compare the hydriding kinetics of bulk (250 μm) and nano-sized (3.6 nm) palladium particles. Sorption/desorption isotherms have been measured at 25 °C from volumetric experiments. It is shown that nano-particles retain only some of the characteristics of bulk systems. Several major differences such as different hydrogen solubility, different magnitude of hysteresis and different plateau pressure slopes have been observed. Hydrogen sorption/desorption kinetics has been analyzed in the frequency domain. A model has been used to fit experimental impedance diagrams and determine microscopic rate-parameters in the solid solution, two-phase domain and hydride phase. It is shown that microscopic rate parameters of micro- and nano-particles do not significantly differ.

Revealing Structural Detail in the High Temperature La2Si2O7–Y2Si2O7 Phase Diagram by Synchrotron Powder Diffraction and Nuclear Magnetic Resonance Spectroscopy

High resolution synchrotron powder XRD, 89Y CPMG NMR, and 139La MAS NMR spectroscopy have been applied to eventually draw the phase diagram of the La2Si2O7–Y2Si2O7 system. The diagram presents a solid solubility region of G-(La,Y)2Si2O7, which extends to the La0.9Y1.1Si2O7 composition at any temperature of this study. Compositions richer in Y show two-phase domains, with G + α at T < 1450 °C and G + δ at T > 1450 °C. The Y-rich extreme is more complex, showing two solid solution regions of δ- and γ-(La,Y)2Si2O7 polymorphs which appear with increasing Y content, respectively. It is interesting to note that the La for Y substitution mechanism in the G-(La,Y)2Si2O7 polymorph is not homogeneous, but a preferential occupation of Y for the RE2 site is observed. Finally, the 89Y and 139La isotropic chemical shift values in G-(La,Y)2Si2O7 have been described here for the first time and assigned to the different RE crystallographic sites of the unit cell.

Impact response and dynamic strength of partially melted aluminum alloy

Solid solution treated aluminum alloy 6061 was tested in a series of planar impact experiments in which the initial sample temperature was varied between 296 and 902 K with special attention paid to the temperatures of the solid-liquid two-phase region. In the experiments, the free surface velocity histories were recorded using velocity interferometer system for any reflector (VISAR). It has been found that both the dynamic yield and the dynamic tensile strengths of the alloy decline monotonously through the two-phase domain and vanish completely as the initial sample temperature approaches 902 K and as the content of the liquid phase in the alloy approaches 20%. Through the same temperature interval the plastic wave rise-time and the time interval between elastic and plastic waves behave non-monotonously and display a sharp maximum at 876 K, some 20 K above the solidus temperature of the alloy. We presently assume that the non-monotonous behavior is caused by a compressive-induced solidification of a thin liquid layer between the grains of the solid phase.