Phase Coexistence Properties Research Articles

Composition-driven phase coexistence and functional properties of the (1-x)BZT-(x)BCT ceramics near the phase convergence region

The concept of composition-induced phase transformation in Lead Zirconate Titanate (PZT) at the Morphotropic Phase Boundary (MPB) has been employed to improve functional properties of the (1-x)BZT-(x)BCT ceramic. However, it was observed that the phase diagram of the (1-x)BZT-(x)BCT ceramic is different to the PZT. As a result, the nature of the superior functional properties found in (1-x)BZT-(x)BCT ceramic is unlike PZT and still unclear so far. In this work, functional properties; dielectric, ferroelectric, energy storage, and piezoelectric properties, of the (1-x)BZT-(x)BCT ceramics where x = 0.3 mol% to 0.6 mol% were evaluated at room temperature in comparison to the identification of phase coexistence using synchrotron x-ray powder diffraction (SXPD). This work found that changes of BCT content had a strong impact on the observed coexisting phases and functional properties. Moreover, the composition that showed the highest piezoelectric properties did not present the largest of saturation polarization. This implies that the functional properties of the (1-x)BZT-(x)BCT ceramics are not dependent on the presence of polarizations under the application of electric field. The contribution of non-180° domain switching also plays a vital role, especially in the piezoelectric properties. These findings would help to extend our knowledge of the nature of the (1-x)BZT-(x)BCT ceramic.

Investigations on phase coexistence and functional properties of BCZT lead-free piezoceramic

Large piezoelectric properties was observed in (1-x)BZT-(x)BCT where x=0.5 or Ba0.85Ca0.15Ti0.9Zr0.1O3 (denoted as BCZT), leading to a promising candidate for lead-free piezoelectric materials. However, phase formation of the BCZT is controversial and still unclear since various phase coexistences were identified in the literatures, for instances, the mixed phases of rhombohedral-tetragonal (R-T), ortho-rhombic-tetragonal (O-T) or rhombohedral-orthorhombic-tetragonal (R-O-T). Additionally, it is well known that the crystal structure plays a crucial role on the occurrence of polarization in the piezoceramics. Therefore, this work aims to investigate the coexistence of phase formation at room temperature for the BCZT powder and ceramic. Moreover, the electrical properties as a function of temperature, frequency and electric field were observed in order to evaluate the extrinsic contribution of piezoelectric response. It was found that, according to the results from temperature-dependent dielectric properties as well as Rietveld refinement of XRD profiles, the coexistence of O-T phase was observed in the BCZT powder and ceramic. Furthermore, the enhancement of Ca2+ substitution into Ba2+ site in BCZT ceramic caused the shrinkage of unit cell, leading to the shift of XRD profile and Raman spectra. In addition, it was found that the applications of frequency and electric field can influence on changes of domain-wall motion and micro-polar cluster in the BCZT piezoceramic.

Promoting Metamagnetic Transition by Interphase Magnetic Coupling

AbstractMagnetic mixed‐phase materials with competing ground states are important from both scientific and technical points of view due to their magnetic phase transition and phase coexistence properties. Unlike artificially fabricated multi‐phase composites, it should be possible to modify their physical properties more efficiently by making use of the interphase coupling effects in mixed‐phase materials. In this study, the magneto‐transport properties are investigated to provide evidence of the existence of interphase coupling in a magnetic mixed‐phase system, specifically, Ni‐doped FeRh thin films. The rotation of the external magnetic field induces an emergent promotion effect of the antiferromagnetic (AFM)–ferromagnetic (FM) phase transition, which can be attributed to the modification of the magnetic free energy by interphase magnetic coupling across the AFM–FM phase boundary. The results shed light on the availability of an extra degree of freedom for manipulating the physical properties of material systems with mixed magnetic phases and help to further exploit their potential applications.

Magnetic phase separation in polycrystalline Pr0.5−xBixSr0.5MnO3 (x ≤ 0.15)

In this paper, magnetic phase coexistence and magnetotransport properties of Pr0.5–xBixSr0.5MnO3 (x ≤ 0.15) samples have been studied. Our specimens demonstrate complicated magnetic properties through the presence of several transitions as a function of temperature. With increase in bismuth content, a spin–glass–like–state appears at low temperatures and long–range charge ordering can be observed for x = 0.15. Magnetization and resistivity seems to be sensitive to magnetic field cycling, testifying the presence of training effect. Both, magnetic phase separation phenomenon and 6s2 lone pair electrons of Bi3+ ions, play a key role in the governance of the physical response of studied specimens.

Theory and computer simulation of hard-core Yukawa mixtures: thermodynamical, structural and phase coexistence properties

We report extensive calculations, based on the modified hypernetted chain (MHNC) theory, on the hierarchical reference theory (HRT), and on Monte Carlo simulations, of thermodynamical, structural and phase coexistence properties of symmetric binary hard-core Yukawa mixtures (HCYM) with attractive interactions at equal species concentration. The obtained results are throughout compared with those available in the literature for the same systems. It turns out that the MHNC predictions for thermodynamic and structural quantities are quite accurate in comparison with the MC data. The HRT is equally accurate for thermodynamics, and slightly less accurate for structure. Liquid-vapor (LV) and liquid–liquid (LL) consolute coexistence conditions as emerging from simulations, are also highly satisfactorily reproduced by both the MHNC and HRT for relatively long ranged potentials. When the potential range reduces, the MHNC faces problems in determining the LV binodal line; however, the LL consolute line and the critical end point (CEP) temperature and density turn out to be still satisfactorily predicted within this theory. The HRT also predicts with good accuracy the CEP position.The possibility of employing liquid state theories HCYM for the purpose of reliably determining phase equilibria in multicomponent colloidal fluids of current technological interest, is discussed.

Extending the parQ transition matrix method to grand canonical ensembles.

Phase coexistence properties as well as other thermodynamic features of fluids can be effectively determined from the grand canonical density of states (DOS). We present an extension of the parQ transition matrix method in combination with the efasTM method as a very fast approach for determining the grand canonical DOS from the transition matrix. The efasTM method minimizes the deviation from detailed balance in the transition matrix using a fast Krylov-based equation solver. The method allows a very effective use of state space transition data obtained by different exploration schemes. An application to a Lennard-Jones system produces phase coexistence properties of the same quality as reference data.

Effect of surface-screening parameter of the Yukawa potential model on vapour–liquid phase coexistence and critical-point properties of confined Yukawa fluid

We report the effect of surface-screening parameter of Yukawa potential model on vapour–liquid phase coexistence and critical-point properties of slit–pore-confined Yukawa fluid, using grand canonical transition-matrix Monte Carlo along with the histogram reweighting method. The effect of surface-screening parameter on the vapour–phase coexistence density is insignificant for the studied system. On the other hand, significant effect of surface-screening parameter is observed on liquid phase coexistence density. With increasing surface-screening parameter, liquid phase coexistence density decreases. Critical-point properties have shown monotonic decreasing trends with increase in surface-screening parameter. Moreover, the effect of change of surface-screening parameter is least on critical temperature changes as compared to critical density and critical pressure changes for the studied Yukawa system in this work.

A general treatment of polar-polarizable systems for an equation of state

In this work, a general theory to account for any kind of polarization arising from polar as well as ions induced interactions in fluid mixtures is proposed. The general treatment is based on the self-consistent mean field theory (SCMF) that was originally proposed and applied for pure components using integral equation theories and molecular simulation studies. The extended SCMF is consistent with theory-based equations of state applied to hard chain mixtures. The theory is extended to mixtures and compared to molecular simulation data. The comparison to molecular simulation data shows good to excellent results for phase co-existence properties, energy and effective dipole moment. The validity of the theory is demonstrated by studying VLE of highly non-ideal mixtures of aldehyde and ketones using statistical association fluid theory.

Rhombohedral–orthorhombic phase coexistence and electrical properties of Ta and BaZrO3 co-modified (K, Na)NbO3 lead-free ceramics

The BaZrO3 and Ta have been used to improve piezoelectric properties of (K, Na)NbO3 ceramics by the construction of the phase boundary, and (1 − x)K0.48Na0.52(Nb0.95Ta0.05)O3–xBaZrO3 [(1 − x)KNNT–xBZ] ceramics were prepared by the conventional solid-state method. The effect of BZ content on their phase structure, microstructure, and electrical properties has been investigated. A rhombohedral and orthorhombic phase coexistence has been observed in the compositional range of 0.05 ≤ x ≤ 0.07. With increasing BZ content, their Tc and To–t values decrease gradually, and the dielectric constant increases linearly. The ceramic with x = 0.06 exhibits an enhanced piezoelectric behavior (d33 ∼ 193 pC/N and kp ∼ 32.6%) because of the coexistence of two phases together with a dense microstructure. As a result, the construction of a rhombohedral and orthorhombic phase boundary is an effective way to improve the piezoelectric properties of KNN-based ceramics.

Monte Carlo Simulation Methods for Computing Liquid-Vapor Saturation Properties of Model Systems.

We discuss molecular simulation methods for computing the phase coexistence properties of complex molecules. The strategies that we pursue are histogram-based approaches in which thermodynamic properties are related to relevant probability distributions. We first outline grand canonical and isothermal-isobaric methods for directly locating a saturation point at a given temperature. In the former case, we show how reservoir and growth expanded ensemble techniques can be used to facilitate the creation and insertion of complex molecules within a grand canonical simulation. We next focus on grand canonical and isothermal-isobaric temperature expanded ensemble techniques that provide a means to trace saturation lines over a wide range of temperatures. To demonstrate the utility of the strategies introduced here, we present phase coexistence data for a series of molecules, including n-octane, cyclohexane, water, 1-propanol, squalane, and pyrene. Overall, we find the direct grand canonical approach to be the most effective means to directly locate a coexistence point at a given temperature and the isothermal-isobaric temperature expanded ensemble scheme to provide the most effective means to follow a saturation curve to low temperature.

Vapor-liquid phase coexistence and transport properties of two-dimensional oligomers

Grand-canonical transition-matrix Monte Carlo and histogram reweighting techniques are used herein to study the vapor-liquid coexistence properties of two-dimensional (2D) flexible oligomers with varying chain lengths (m = 1-8). The phase diagrams of the various 2D oligomers follow the correspondence state (CS) principle, akin to the behavior observed for bulk oligomers. The 2D critical density is not influenced by the oligomer chain length, which contrasts with the observation for the bulk oligomers. Line tension, calculated using Binder's formalism, in the reduced plot is found to be independent of chain length in contrast to the 3D behavior. The dynamical properties of 2D fluids are evaluated using molecular dynamics simulations, and the velocity and pressure autocorrelation functions are investigated using Green-Kubo (GK) relations to yield the diffusion and viscosity. The viscosity determined from 2D non-equilibrium molecular dynamics simulation is compared with the viscosity estimated from the GK relations. The GK relations prove to be reliable and efficient for the calculation of 2D transport properties. Normal diffusive regions are identified in dense oligomeric fluid systems. The influence of molecular size on the diffusivity and viscosity is found to be diminished at specific CS points for the 2D oligomers considered herein. In contrast, the viscosity and diffusion of the 3D bulk fluid, at a reduced temperature and density, are strongly dependent on the molecular size at the same CS points. Furthermore, the viscosity increases and the diffusion decreases multifold in the 2D system relative to those in the 3D system, at the CS points.

First principles predictions of thermophysical properties of refrigerant mixtures

We present pair potentials for fluorinated methanes and their dimers with CO(2) based on ab initio potential energy surfaces. These potentials reproduce the experimental second virial coefficients of the pure fluorinated methanes and their mixtures with CO(2) without adjustment. Ab initio calculations on trimers are used to model the effects of nonadditive dispersion and induction. Simulations using these potentials reproduce the experimental phase-coexistence properties of CH(3)F within 10% over a wide range of temperatures. The phase coexistence curve of the mixture of CH(2)F(2) and CO(2) is reproduced with an error in the mole fractions of both phases of less than 0.1. The potentials described here are based entirely on ab initio calculations, with no empirical fits to improve the agreement with experiment.

Methodology for the Development of Thermodynamic Models Describing Substances That Exhibit Complex Association Interactions

This work describes a methodology for the development of a thermodynamic model describing the substances that show strong self- and cross-association interactions. The methodology is fundamentally based on the chemical theory of association interactions. The system used as a case study in this work is a binary mixture containing hydrogen fluoride and water (HF + H2O). Earlier studies have failed to provide a reasonable description of this binary mixture because of the complex association interactions between these compounds, which were not adequately modeled. In this work, the phase behavior of this mixture is understood by exploring these complex association interactions. Pure HF was modeled using 14 different association schemes that allow the formations of different physically meaningful oligomers with different distribution schemes (1−2, 1−6, 1−2−6, etc.), where the 1−2 scheme allows the formation of monomers and dimers and likewise. The parameters for these pure component schemes were obtained by correlating the phase coexistence properties of pure HF and were also used to predict several other pure component properties (ΔHvap, CP, Cv, Z, etc.) The dominance of these association patterns and their distribution were understood on the basis of their predictive ability. The pure component association schemes that were developed for HF and water were extended to the binary mixture. The phase coexistence properties were correlated using different association patterns for the pure components with and without considerations for the strong association between them. The significance of these self- and cross-association patterns are studied and understood on the basis of the correlative and the predictive ability of the association schemes. The effect of including the cross-associates that are most likely to be formed in this mixture, from a molecular level hybrid meta-density functional theory study, is also discussed. The methodology described in this work can be utilized to understand and predict the bulk-phase thermodynamic properties of substances that show complex association interactions at a molecular level.

Rhombohedral-Tetragonal Phase Coexistence and Piezoelectric Properties of (NaK)(NbSb)O3-LiTaO3-BaZrO3 Lead-Free Ceramics

The phase transitional behavior and composition-dependent piezoelectric properties of (NaK)(NbSb)O3–LiTaO3–BaZrO3 (NKNS–LT–BZ) pseudo-ternary system were investigated. A composition–temperature phase diagram was generalized within a certain range of BZ content in which a rhombohedral–tetragonal (R–T ) ferroelectric phase boundary connecting orthorhombic and cubic phase zones is formed near room temperature between two trifurcate points. Piezoelectric and electromechanical properties of NKNS–LT–BZ ceramics exhibit optimum values of d33=365 pC/N and kp=45% in the vicinity of the R–T phase coexistence zone. The dielectric and ferroelectric properties and the phase transition of NKNS–LT–BZ ceramics were discussed from a crystallographic point of view.

Vapour–liquid phase equilibria of simple fluids confined in patterned slit pores

We present the influence of surface heterogeneity on the vapour–liquid phase behaviour of square-well fluids in slit pores using grand-canonical transition-matrix Monte Carlo simulations along with the histogram-reweighting method. Properties such as phase coexistence envelopes, critical properties and local density profiles of the confined SW fluid are reported for chemically and physically patterned slit surfaces. It is observed that in the chemically patterned pores, fluid–fluid and surface attraction parameters along with the width of attractive and inert stripes play fundamentally different roles in the phase coexistence and critical properties. On the other hand, pillar gap and height significantly affect the vapour–liquid equilibria in the physically patterned slit pores. We also present the effect of chemically and physically patterned slit surfaces on the spreading pressure.

Modeling the phase behavior in binary mixtures involving blowing agents and thermoplastic resins

AbstractThe thermophysical properties of mixtures of thermoplastic resins and blowing agents, together with the knowledge of the solubilities of these components, are the basis for the manufacturing of plastic foams. In this work, the solubilities of blowing agents trichlorofluoromethane, dichlorodifluoromethane, chlorodifluoromehane, and 1,2‐dichloro‐1,1,2,2‐tetrafluoroethane in thermoplastic resins poly(styrene), high density poly(ethylene), low density poly(ethylene), poly(propylene), poly(vinyl chloride), poly(carbonate) and poly(propylene oxide) were modeled by using the Perturbed Chain‐Statistical Associating Fluid Theory (PC‐SAFT) and the Sánchez‐Lacombe equations of state (EoS), fitting a single temperature‐dependent binary interaction parameter. PC‐SAFT is a theoretically based equation of state with three pure component parameters that describe efficiently the thermodynamics of complex systems. Earlier works with this EoS have already predicted the phase coexistence properties of various refrigerants and higher order alkane series compounds, along with their mixtures. The pure component parameters for the blowing agents were obtained by regression of vapor pressure and liquid density data, while the pure component parameters for the thermoplastic resins were obtained by regression of pure liquid PVT data. The parameter estimation was performed by using a modified maximum likelihood method. The solubility results obtained with both EoS have been compared; the results from PC‐SAFT showed a higher accuracy in terms of solubility pressure deviations. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

The effects of interaction range, porosity and molecular association on the phase equilibrium of a fluid confined in a disordered porous media

Grand-canonical transition-matrix Monte Carlo (GC-TMMC) is employed to analyse the effects of range of interaction, packing fraction and molecular association on phase coexistence properties of square-well (SW) based fluids in disordered pores. The nature of the phase equilibria were studied inside a repulsive disordered porous media with packing fractions, η m = 0.05 and 0.10. Three values of the SW attractive well range parameter were studied: λ = 1.5, 1.75, and 2.0. Coexistence number probability distribution reflects the signature of the disordered structure of the porous matrix. Yet, no multiple fluid–fluid transition was observed. The effect of strength of molecular association on coexistence densities, density profile, saturation pressure, and monomer fraction for the SW based dimerizing fluids inside a repulsive disordered media is reported. Association is found to increase as the packing fraction of the matrix increase. Critical properties of these confined fluids are calculated via a rectilinear diameter approach. Fractional shift in the critical temperature linearly decreases with the increase in the attractive well width for non-associating fluids. The rate of decrease in the critical temperature shift increases with the increase in packing fraction. Associating sites are found to suppress the shift in the critical temperature.

Comparing the Use of Gibbs Ensemble and Grand-Canonical Transition-Matrix Monte Carlo Methods to Determine Phase Equilibria

We present results from a computational study investigating the use of Gibbs ensemble and grand-canonical transition-matrix Monte Carlo (GC-TMMC) methods to determine the liquid−vapor phase coexistence properties of pure molecular fluids of varying degrees of complexity. The molecules used in this study were ethane, n-octane, cyclohexane, 2,5-dimethylhexane, 1-propanol, and water. We first show that the GC-TMMC method can reproduce Gibbs ensemble results found in the literature. Given the excellent agreement for each molecule, we then compare directly the performance of Gibbs ensemble and GC-TMMC simulations at both low and high reduced temperatures by monitoring the relative uncertainties in the saturation properties as a function of computational time. In general, we found that the GC-TMMC method yielded limiting uncertainties in the saturated vapor density and pressure that were significantly smaller, by an order of magnitude in some instances, than those of the Gibbs ensemble method. Limiting Gibbs ensemble uncertainties for these properties were generally in the 0.8−5.0% range. However, both methods yielded comparable limiting uncertainties in the saturated liquid density, which fell within the range of 0.1−1.0%. In the case of water at 300 K, we found that the Gibbs ensemble outperformed GC-TMMC. The relatively poor performance of the GC-TMMC method in this situation was tied to the slow convergence of the density probability distribution at this low temperature. We also discuss strategies for improving the convergence rate under these conditions.

Simulation of Chemical Potentials and Phase Equilibria in Two- and Three-Dimensional Square-Well Fluids: Finite Size Effects

We study the simulation cell size dependence of chemical potential isotherms in subcritical square-well fluids by means of series of canonical ensemble Monte Carlo simulations with increasing numbers of particles, for both three-dimensional bulk systems and two-dimensional planar layers, using Widom-like particle insertion methods. By estimating the corresponding vapor/liquid coexistence densities using a Maxwell-like equal area rule for the subcritical chemical potential isotherms, we are able to study the influence of system size not only on chemical potentials but also on the coexistence properties. The chemical potential versus density isotherms show van der Waals-like loops in the subcritical vapor/liquid coexistence range that exhibit distinct finite size effects for both two- and three-dimensional fluids. Generally, in agreement with recent findings for related studies of Lennard-Jones fluids, the loops shrink with increasing number of particles. In contrast to the subcritical isotherms themselves, the equilibrium vapor/liquid densities show only a weak system size dependence and agree quantitatively with the best-known literature values for three-dimensional fluids. This allows our approach to be used to accurately predict the phase coexistence properties. Our resulting phase equilibrium results for two-dimensional square-well fluids are new. Knowledge concerning finite size effects of square-well systems is important not only for the simulation of thermodynamic properties of simple fluids, but also for the simulation of models of more complex fluids (such as aqueous or polymer fluids) involving square-well interactions.

Phase diagrams of Ising fluids with Yukawa-Lennard-Jones interactions from an integral equation approach

An integral equation approach is developed to investigate phase coexistence properties of Ising spin fluids with Yukawa ferromagnetic and Lennard-Jones nonmagnetic interactions in the presence of an external field. The calculations are carried out on the basis of the Duh and Henderson closure with a specific Duh-like partitioning of the total potential. The coupled set of the Ornstein-Zernike equation, the closure relation and the external field constraint are solved using an efficient numerical algorithm. The phase diagrams are evaluated in a wide range of varying the external field and the ratio of strengths of Yukawa to Lennard-Jones interactions. Different types of the phase diagram topology as well as various external field dependencies of critical temperatures and densities are identified. The complexity with respect to simple Lennard-Jones fluids is explained by coupling between spatial and spin degrees of freedom in the system. A comparison of the obtained theoretical results with simulation data is made and a good agreement is observed.