Microscopic Phase Transitions Research Articles

Plastic evolution and phase transition mechanisms in γ-TiAl nano polycrystal by a molecular dynamics simulation

γ-TiAl based alloys are widely regarded as one important high-temperature structural material, however the deformation behaviors are still not clearly clarified. The microscopic plastic deformation and phase transition of γ-TiAl nano polycrystals under compression are investigated by molecular dynamics simulations. The results show that the initial plasticity is dominated by slip of partial dislocations whose continuous formation and secondary slip on the same slip plane result in the generation and disappearance of intrinsic stacking faults (ISFs). At the strain of about 0.2, a continuous structure transformation of “ISF → ESF → TB” is observed, during which the interaction between different ISFs causes the atomic misalignment corresponding to the change in atomic stacking order near ISF. As the compressive strain exceeds 0.2, the FCC-to-BCC phase transition occurs within polycrystals due to the high Mises stress, the high stress level within grains facilitates the expansion of BCC phase from initial FCC phase, and the newly formed BCC phase maintains the atomic-arrangement orderliness of original phase. The interface between FCC and BCC phases obeys the low-energy Kurdjumov-Sachs orientation relationship. This work is helpful for better understanding the atomic-scale deformation behaviors and provides a theoretical guidance for designing high-performance alloy materials.

A Two-Step Intermolecular Interaction Of Molecular Macroions With Multivalent Counterions.

Macroion-counterion interaction is essential for regulating the solution behaviors of hydrophilic macroions, as simple models for polyelectrolytes. Here, we explore the interaction between uranyl peroxide molecular cluster Li68K12(OH)20[UO2(O2)OH]60 (U60) and multivalent counterions. Different from interaction with monovalent counterions that shows a simple one-step process, isothermal titration calorimetry, combined with light/X-ray scattering measurements and electron microscopy, confirm a two-step process for their interaction with multivalent counterions: an ion-pairing between U60 and the counterion with partial breakage of hydration shells followed by strong U60-U60 attraction, leading to the formation of large nanosheets with severe breakage and reconstruction of hydration shells. The detailed studies on macroion-counterion interaction can be nicely correlated to the microscopic (self-assembly) and macroscopic (gelation or phase separation) phase transitions in the dilute U60 aqueous solutions induced by multivalent counterions.

Enhanced comprehension of the cross-scale forming mechanism in CF/PEEK resistance welding: Analysis of temperature gradients and meso–microscopic phase transitions

Resistance welding technology is an effective solution for connecting thermoplastic composites. However, limited research on non-uniform distribution of interfacial temperature and complex evolution mechanism of phase-change molding hinders its application. This reliance on traditional experimental and trial-and-error methods severely impedes its development. Therefore, this study employed a novel cross-scale numerical simulation method, being applied for the first time in the field of resistance welding for elucidating the underlying mechanism of joint formation through multi-component phase transformation. The constructed model accurately represented the transient heat transfer of a realistic wire mesh structure in three dimensions, while incorporating complex gradient effects arising from both spatial and temporal variations in heating element temperature due to air interference. Further, melting, flow, and consolidation behavior of the resin matrix in resistance welding of carbon fiber/polyetheretherketone (CF/PEEK) thermoplastic composite was investigated herein at a mesoscopic scale. To accomplish this objective, a three-phase flow spatial forming and evolution model for resistance welding was developed at the mesoscopic scale by integrating principles from fluid dynamics. Interestingly, introduction of an air phase reinstated the spatial voids formation and flow in resistance welding, providing robust evidence for the initiation mechanism of internal defects. Moreover, this study revealed variations in the melting mode of PEEK resin across different regions within the weld and highlighted distinct distributions of voids resulting from uneven heat transfer. The interfacial phenomenon and flow diffusion mechanism between components in resistance welding tests were further verified and discussed through analyzing scanning electron microscopy and energy dispersive spectroscopy results. Temporal and spatial correlation in void distribution within the welding zone was observed, which was found to be consistent with simulation results. Specifically, with the progress of the welding process, air near the wire mesh gradually dispersed into the laminate, with significantly more voids at weld seam edges than in central regions. This research methodology based on stepwise scale reduction offers a promising avenue for investigating resistance welding and other connection forming applications, in particular, within the context of engineering underlying logical behavior.

Polymorphic phase transition in CoCrNi medium-entropy alloy under impact loadings

Polymorphic phase transition in metallic materials under high pressure is a critical aspect of dynamic properties and has been attracting a great interest. Despite the extensive researches have been made on understanding of this phase transition in traditional single-principal element alloys, little is known about the phase transition in recently emergent multi-principal medium and high entropy alloys, especially compressed under high strain rates. In this work, based on molecular dynamic simulations, three impact loading strategies with distinct loading paths, such as single-shock, double-shock and ramp-wave loading are carried out on the single crystalline CoCrNi medium-entropy alloy (MEA) to investigate the phase transition under high strain-rate compression. Careful characterizations show that the phase transition of CoCrNi MEA is loading-path dependent, as evidenced by the significant differences in macroscopic pressure evolution and microscopic structural phase transition among the samples under various thermodynamic paths. An intriguing pressure “overshoot” is found and demonstrated as the characteristic of the critical structural phase transition from face-centered cubic (FCC) structure to hexagonal-close-packed (HCP) structure mediated by body-centered cubic (BCC) like clusters. We show that such loading-path dependence is attributed to the strain rate and temperature rise in the loading process, which control the evolution of microstructure and deformation field. The inherent correlation between the atomistic process of phase transition and loading strategies results in polymorphic phase transition under high strain rates. These findings shed new light on the nature of impact phase transition of multi-principal alloys.

Nanoscale insight into the thermal properties of lauric acid and CuO based phase change material used for thermal energy storage

Lauric acid (LA) based phase change material (PCM) has been broadly used for thermal energy storage, whereas its poor thermal performance limits its practical application. In this paper, the microscale structure of LA as the PCM and CuO nanoparticles based composite PCM was established, the thermal properties including density, thermal diffusion, thermal conductivity, melting enthalpy were predicted by means of molecular dynamics (MD) simulations. The microscopic structure evolution and phase transition characteristic of the CPCM system were observed and investigated by structure snapshots and thermal diffusion behaviors. The addition of CuO nanoparticle makes the self-diffusion coefficient of LA based PCM decreased by 15.13 %, and addition of CuO nanoparticle is an effective way to enhance the thermal conductivity of LA based PCM significantly, and it can lead to a 33.765 % increase on average in thermal conductivity with an addition fraction of 6.86 wt%, the melting enthalpy was decreased by 17.57 %, and possible mechanism was revealed at nanoscale. This study is expected to be helpful for the preparation, performance enhancement and practical application of LA based PCM.

Probing the Effects of Ultrasound-Generated Nanobubbles on Hydrate Nucleation: Implications for the Memory Effect

Understanding the microscopic hydrate phase transition process is of crucial importance to the application of hydrate-based techniques. This process is nevertheless significantly limited by the insufficient enrichment of hydrophobic gas molecules in the solution. Here, in this work, ultrasonic technology was applied to generate stable and large amount of nanobubbles in pure water to promote hydrate formation kinetics. The particle size distribution of the nanobubbles was following the log-normal distribution, with a concentration of ∼109 mL–1 and a diameter concentrated in 100–120 nm. The concentration of the nanobubbles increased with the ultrasonic duration and power. Notably, the concentrations of nanobubbles produced by hydrate decomposition were slightly higher with the bubble size concentrated around 200 nm. The effect of nanobubbles on the induction time of hydrate formation was further analyzed by forming from the nanobubble solution with different concentrations and particle size distributions. Hydrate formation induction time was reduced by up to 61.13%. We further analyzed the mechanism of the nanobubble dominant hydrate memory effect. The results proved ultrasound as an approach to enhance the solubility of gases in water and thus promote hydrate formation kinetics, providing evidence for the dominant role of nanobubbles in the memory effect.

Evolution of insulating layers during Heat–Treatment and their effects on magnetic behavior of Fe–Si–Al based soft magnetic composites

As a symbolic ceramic oxide, the reaction of SiO2 as an insulating layer with the Fe–Si–Al/SiO2 soft magnetic composites (SMCs), substrate during heat treatment, as well as the resulting microscopic phase transition of the core–shell structure and its influence on the magnetic properties, are worth investigating. In this study, based on the successful synthesis of Fe–Si–Al based SMCs using fluidised chemical vapour deposition combined with an electric hot–press sintering method, the evolution of the insulating layers was investigated under different heat–treatment temperatures and closely linked to the magnetic performance of the composites. During heat treatment at high temperatures, the formation of high–resistivity oxides (Al2O3) after heat treatment results in a low inter–particle eddy current loss, while excessive temperature causes disintegration of the core–shell heterostructure. This leads to a discernible increase in eddy current and hysteresis losses and a decrease in permeability and resistivity. The SMCs had an Ms of 115.8 emu/g, a resistivity of 6.86 mΩ cm, and a hysteresis loss of 56.3 kW/m3 at 10 mT and 100 kHz when heat-treated at 1373 K for 120 min, which was the best among all samples. This research provides a novel approach to building SMCs with inorganic ceramic insulating layers and a comprehensive understanding of the relationship between the heat–treatment process parameters, microstructure, and magnetic properties which is highly significant for industrial production.

Thermodynamic and viscoelastic constitutive model of TPI/HDPE hybrid shape memory polymer

In recent years, shape memory smart materials have developed rapidly, and more and more novel shape memory polymers have been developed and applied. In sharp contrast to the rapid development of shape memory smart materials, its theoretical research is obviously lagging behind. Therefore, it is necessary to clearly understand and master the thermodynamic dynamic response mechanism of shape memory polymers. In view of the fact that the influence of thermodynamic properties of hybrid shape memory polymer under the coupling effect of multiple factors is still very vague, a novel hybrid shape memory polymer composites (SMPC) thermodynamic viscoelastic constitutive model is proposed for the first time by combining the macroscopic phenomenological viscoelastic theory with the microscopic phenomenological phase transition model, and a microstructure composed of two parts of reversible phase and stationary phase in parallel is constructed. At the same time, a three-dimensional parameter equation with temperature effect and capable of describing the coordinated changes of various parameters in the phase transition process is established. Through the validation of the model, it can be found that the variation trend of the simulation curve is in good agreement with the experimental curve during the whole static loading process, and the error of the fracture stress is less than 2%. Through the dynamic evolution of multi-content and wide-temperature range, the consistency between the simulation results and the experimental results is successfully proved, and the microscopic mechanism of the macroscopic static mechanical behavior of the hybrid SMPC is revealed. The model established in this paper can accurately describe the internal phase change of the mixed matrix, and realize the effective prediction of the stress-strain response under the coupling effect of the matrix composition and the external environment. The proposal of this model provides a new idea for the exploration of hybrid SMPC thermodynamic viscoelastic theory, and also provides an important theoretical reference for the development and application of novel hybrid shape memory materials in the future.

The Hofmeister effect on protein hydrogels with stranded and particulate microstructures

The infiltration of ions into hydrogel matrix could significantly affect the microstructure and macroscopic mechanics of the gels. Here, the Hofmeister effect of various salts on the whey protein isolate hydrogels with fine-stranded and particulate microstructures is investigated by soaking the preformed hydrogels in the sodium salts of different anions. The infiltration of kosmotropic anions yield stiffer hydrogels, whereas the chaotropic anions soften the hydrogels. The hydrogels with fine-stranded microstructures are more sensitive to the salts comparing to the particulate ones due to the microscopic phase transitions and enhanced hydrophobic interactions between polymer chains occurred in fine-stranded hydrogels. Besides, despite the significant difference in water binding ability of different salts, the water holding capacity of the salt-treated hydrogels was mainly determined by the gel stiffness instead of the salt types. Similar mechanical responses of BSA and egg white protein hydrogels to the Hofmeister series was also demonstrated, suggesting that the results shown here could potentially be generalized for other globular protein hydrogels.

Interplay between Macroscopic Stretching and Microscopic Phase Transition Revealed in Butene-1/1,5-Hexadiene Random Copolymers

The intrinsic coupling effect between the multiscale microstructure and macroscopic performance is a fundamental issue in polymer engineering and polymer physics. Combing the tensile testing and in...

Developing a self-consistent AGB wind model – II. Non-classical, non-equilibrium polymer nucleation in a chemical mixture

ABSTRACTUnravelling the composition and characteristics of gas and dust lost by asymptotic giant branch (AGB) stars is important as these stars play a vital role in the chemical life cycle of galaxies. The general hypothesis of their mass-loss mechanism is a combination of stellar pulsations and radiative pressure on dust grains. However, current models simplify dust formation, which starts as a microscopic phase transition called nucleation. Various nucleation theories exist, yet all assume chemical equilibrium, growth restricted by monomers, and commonly use macroscopic properties for a microscopic process. Such simplifications for initial dust formation can have large repercussions on the type, amount, and formation time of dust. By abandoning equilibrium assumptions, discarding growth restrictions, and using quantum mechanical properties, we have constructed and investigated an improved nucleation theory in AGB wind conditions for four dust candidates, TiO2, MgO, SiO, and Al2O3. This paper reports the viability of these candidates as first dust precursors and reveals implications of simplified nucleation theories. Monomer restricted growth underpredicts large clusters at low temperatures and overpredicts formation times. Assuming the candidates are present, Al2O3 is the favoured precursor due to its rapid growth at the highest considered temperatures. However, when considering an initially atomic chemical mixture, only TiO2-clusters form. Still, we believe Al2O3 to be the prime candidate due to substantial physical evidence in presolar grains, observations of dust around AGB stars at high temperatures, and its ability to form at high temperatures and expect the missing link to be insufficient quantitative data of Al-reactions.

Charged End-Group Terminated Poly(N-isopropylacrylamide)-b-poly(carboxylic azo) with Unusual Thermoresponsive Behaviors

We report unusual thermoresponsive behaviors of one poly(N-isopropylacrylamide) (PNIPAm)-based diblock copolymer containing carboxylic azo moieties due to the introduction of one charged end-group. On heating, the ionized end-group prevented macroscopic phase separation of the block copolymer aqueous solution under basic conditions, leading to the absence of cloud point. The micelle solution showed a quick micelle-to-vesicle morphological transition via the end-group stabilized large micelles as a transition state when being stored at 50 °C for 2 h. Such a quick micelle-to-vesicle transition can be attributed to the joint effect of the charged end-group stabilization and the changed hydrophilic–hydrophobic balance of PNIPAm. When adding a tiny amount of Ca2+, macroscopic phase transition reoccurred by breaking the end-group protection layer. This study represents a unique separation of microscopic dehydration and macroscopic phase transition, providing a new way for design of stimuli-responsive polymers.

Absorbing phase transitions in deterministic fixed-energy sandpile models.

We investigate the origin of the difference, which was noticed by Fey etal. [Phys. Rev. Lett. 104, 145703 (2010)PRLTAO0031-900710.1103/PhysRevLett.104.145703], between the steady state density of an Abelian sandpile model (ASM) and the transition point of its corresponding deterministic fixed-energy sandpile model (DFES). Being deterministic, the configuration space of a DFES can be divided into two disjoint classes such that every configuration in one class should evolve into one of absorbing states, whereas no configurations in the other class can reach an absorbing state. Since the two classes are separated in terms of toppling dynamics, the system can be made to exhibit an absorbing phase transition (APT) at various points that depend on the initial probability distribution of the configurations. Furthermore, we show that in general the transition point also depends on whether an infinite-size limit is taken before or after the infinite-time limit. To demonstrate, we numerically study the two-dimensional DFES with Bak-Tang-Wiesenfeld toppling rule (BTW-FES). We confirm that there are indeed many thresholds. Nonetheless, the critical phenomena at various transition points are found to be universal. We furthermore discuss a microscopic absorbing phase transition, or a so-called spreading dynamics, of the BTW-FES, to find that the phase transition in this setting is related to the dynamical isotropic percolation process rather than self-organized criticality. In particular, we argue that choosing recurrent configurations of the corresponding ASM as an initial configuration does not allow for a nontrivial APT in the DFES.

In Situ STM and Vibrational Study of Nanometer-Scale Reorganization of a Phospholipid Monolayer Accompanied by Potential-Driven Headgroup Digestion.

In situ dynamic observation of model biological cell membranes, formed on a water/gold substrate interface, has been performed by the combination of electrochemical scanning tunneling microscopy and reflection infrared absorption vibrational spectroscopy. Monolayers of 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) were formed on alkanethiol-modified gold surfaces in a buffer solution, and the microscopic phase transitions driven by electrochemical potential control were observed more in detail than our previous study on the same system [Electrochem. Commun. 2007, 9, 645-650]. This time the transitions were associated with the chemistry of DHPC by the aid of vibrational spectroscopy and the utilization of deuterium-labeled DHPC molecules. A negative potential shift solidifies the fluidic lipid layers into static striped or grainy features without notable chemical reactions. The first positive potential shift over the virginal DHPC monolayer breaks DHPC into choline and the corresponding phosphatidic acid (DHPA). This is the first case of a phospholipid electrochemical reaction microscopically detected at the solid surface.

On the stress calculation within phase-field approaches: a model for finite deformations

Numerical simulations based on phase-field methods are indispensable in order to investigate interesting and important phenomena in the evolution of microstructures. Microscopic phase transitions are highly affected by mechanical driving forces and therefore the accurate calculation of the stresses in the transition region is essential. We present a method for stress calculations within the phase-field framework, which satisfies the mechanical jump conditions corresponding to sharp interfaces, although the sharp interface is represented as a volumetric region using the phase-field approach. This model is formulated for finite deformations, is independent of constitutive laws, and allows using any type of phase inherent inelastic strains.

Dual Thermoresponsive and pH-Responsive Poly(vinyl alcohol) Derivatives: Synthesis, Phase Transition Study, and Functional Applications

Novel poly(vinyl alcohols) (PVA) functionalized with pendant thermo- and pH-responsive groups were prepared by carbonyldiimidazole (CDI)-mediated couplings of N1,N1-diethylethane-1,2-diamine (DEEDA) with controllable modification degree. Nuclear magnetic response (NMR) and IR have verified the successful modification of PVA. The macro- and microscopic phase transition behavior of the obtained PVA-DEEDA-t (t = 10, 30, 70, and 90 h) was thoroughly characterized using various techniques, including turbidity measurement, NMR, and dynamic light scattering (DLS). PVA-DEEDA-t is demonstrated to possess tunable lower critical solution temperature (LCST) between 58 and 24 °C. LCST is dependent on solution pH and degree of PVA modification (14.1–20.9%). By combining DLS and DOSY characterizations, it can be concluded that both coil-to-globule transition and aggregation occurred to PVA-DEEDA-t during phase transition, while only coil-to-globule transition can be detected for the pristine PVA. 2D NOESY proved that th...

A shrinkable dendritic glutamic acid based supromolecular gel: From microscopic volume phase transition to functional applications

Supramolecular gel is an important class of soft materials formed through the self-assembly of low molecular weight molecules in water or organic solvents via various non-covalent interactions to immobilize the solvents. Supramolecular gels have attracted widespread attentions due to their structural and functional diversity. One of the important features of the gel materials is their responsiveness, which include the reversible gel-to-sol transition, macroscopic volume phase transition (shrinkage-swelling) and so on depending upon the external stimulus such as temperature, light, metal ions, electrical or magnetic field. Among these, volume phse transition is important since it can be used to develop various functions in drug release, biosensing, controlled microvalves, self-healing and so on. Although there are large amount of reports on the volume phase transition in polymer gels, there is fewer reproted on the supramolecualr gels due to the the inherent weak character of non-covalent interactions. Supramolecular hydrogels suffer the reality that the brittle 3D structures have difficulty in maintaining physical change such as folding and curling of the supramolecular chains as in polymer hydrogels to accordingly exhibit phase transition in volume (shrinkage-swelling) and shape (deformation- recovery). Among several shrinkable supramolecular gels, the dendritic glutamic acid amphiphile OGAc containing multi-carboxylic acid as the head group and a long alkyl chain appeared to be one of the good examples of such gels. This minireview focuses on the formation of the supramolecular hydrogel OGAc, the shrinkage-swelling transition, the stimuli-responsive together with such volume phase transition and the potential applications by utilizing the shrinkage/ swelling properties. The paper firstly introduces the unique reversible shrinking and swelling supramolecular gel triggered by divalent metal ions. Then the function of the various metal ions as the inducer for the gel shrinkage was discussed and a mechanism was proposed. Based on these discussions, the design concept for the shrinkable supramolecular gels was suggested. Secondly, the pH responsiveness of the OGAc-Mg2+ is discussed. Thirdly, in order to functionalize the supramolecular gels, charged organic compounds were introduced into the shrinkable gels. It was found that OGAc-AZOC2Py complex system also exhibited reversible shrinkage/swelling transition. Simultaneously, the gel showed a change in supramolecular chirality. Fourthly, the potential applications of the supramolecular assemblies by using the shrinkable gel are discussed, which mainly highlights the application in the efficient separation of the mixtures composed of positively and negatively charged dyes, visualized recognition of chiral amines based on OGAc- Mg2+-PIC system, the construction of dual reversible thermal and light-responsive chiroptical switches for OGAc-AZOC2Py hydrogel and controlled step-wise release of drug or biological substances in OGAc-Mg2+ gels. Finally, the future development and application of supramolecular shrinkable gel is outlooked. It is anticipated to design more sophisticated multi-functional materials based on reversible macroscopic volume phase transition and also to combine the advantage of the shrinkable polymer gels to develop new soft materials. It is expected that the shrinkable gels will find more applications in biological system and biotechnology.

Entropic screening preserves non-equilibrium nature of nematic phase while enthalpic screening destroys it.

The present manuscript describes the role of entropic and enthalpic forces mediated by organic non-polar (hexane) and polar (methanol) solvents on the bulk and microscopic phase transition of a well known nematic liquid crystalline material MBBA (N-(4-methoxybenzylidene)-4-butylaniline) through Differential Scanning calorimetry (DSC), UV-Visible (UV-Vis), and Fourier Transform Infrared (FTIR) spectroscopy. DSC study indicates continuous linear decreases in both nematic-isotropic (N-I) phase transition temperature and enthalpy of MBBA in presence of hexane while both these parameters show a saturation after an initial decay in methanol. These distinct transitional behaviours were explained in terms of the "depletion force" model for entropic screening in hexane and "screening-self-screening" model for methanol. Heating rate dependent DSC studies find that non-Arrhenius behaviour, characteristic of pristine MBBA and a manifestation of non-equilibrium nature [Dan et al., J. Chem. Phys. 143, 094501 (2015)], is preserved in presence of entropic screening in the hexane solution, while it changes to Arrhenius behaviour (signifying equilibrium behaviour) in presence of enthalpic screening in methanol solution. FTIR spectra show similar dependence on the solvent induced screening in the intensities of the imine (-C = N) stretch and the out-of-plane distortion vibrations of the benzene rings of MBBA with hexane and methanol as in DSC, further establishing our entropic and enthalpic screening models. UV-Vis spectra of the electronic transitions in MBBA as a function of temperature also exhibit different dependences of intensities on the solvent induced screening, and an exponential decrease is observed in presence of hexane while methanol completely changes the nature of interaction to follow a linear dependence.

Surface Adsorption of Oppositely Charged SDS:C(12)TAB Mixtures and the Relation to Foam Film Formation and Stability.

The complexation, surface adsorption, and foam film stabiliztation of the oppositely charged surfactants, sodium dodecyl sulfate (SDS) and dodecyl trimethylammonium bromide (C12TAB), is analyzed. The SDS:C12TAB mixing ratio is systematically varied to investigate whether the adsorption of equimolar or irregular catanionic surfactant complexes, and thus a variation in surface charge (i.e., surface excess of either SDS or C12TAB), governs foam film properties. Surface tension measurements indicate that SDS and C12TAB interact electrostatically in order to form stoichometric catanionic surfactant complexes and enhance surface adsorption. On the other hand it can be demonstrated that the SDS:C12TAB mixing ratio and, thus, a change in surface charge and composition plays a decisive role in foam film stabilization. The present study demonstrates that varying the mixing ratio between SDS and C12TAB offers a tool for tailoring surface composition and foam film properties, which are therefore not exclusively mediated by the presence of equimolar catanionic surfactant complexes. The SDS:C12TAB net amount and mixing ratio determine the type, stability, and thinning behavior of the corresponding foam film. These observations indicate the formation of a mixed surface layer, composed of the catanionic surfactant species surrounded by either free SDS or C12TAB molecules in excess. Furthermore, a systematic variation in CBF-NBF transition kinetics is rationalized on the basis of a microscopic phase transition within the foam films. Fundamental knowlegde gained from this research gives insight into the surface adsorption and foam film formation of catanionic surfactant mixtures. The study helps researchers to understand basic mechanisms of foam film stabilization and to use resources more efficiently.

G0100203 モルフォトロピック相境界における多結晶PZTのマルチスケール解析

PZT is widely used for actuators and sensors of electromechanical devices and it has multiple crystal systems. Although PZT outputs large piezoelectricity, it shows a complicated hysteresis behavior caused by domain switching and structural phase transition at morphotropic phase boundary (MPB). It is important for development and improvement of lead-free piezoelectric materials to understand the mechanism of huge piezoelectricity in PZT. In this study, a multiscale nonlinear finite element method was developed to estimate ferroelectric material properties. The homogenization theory was employed for scale-bridging between macrostructure and microstructure. We utilized an incremental form of fundamental constitutive law in consideration with physical property change caused by domain switching and structural phase transition. Crystal morphologies, which are characterized as an inhomogeneous structure composed of many grains and domains with individual orientations, are modeled at a micro scale. Then the homogenized physical properties of macrostructure can be estimated with perfect correlation to microstructural changes such as domain switching and structural phase transition. The proposed multiscale finite element method was applied to a polycrystalline PZT, and the relation between the macroscopic properties and microscopic domain switching and structural phase transition were investigated. Especially the difference among tetragonal single phase, rhombohedral single phase and their dual phases were discussed.