Mechanism Of Phase Transition Research Articles

A thermoresponsive hydrogel with a wide‐range tunable phase transition temperature triggered by small molecules

AbstractThermoresponsive polymer hydrogels have great potential in the fields of smart window and information encryption due to the drastic changes in transmittance caused by the phase transition at specific temperatures. The synthesis of thermoresponsive hydrogels with a wide range and sensitive temperature response is current research goal. Here, we demonstrate a polymethacrylamide (PMAAm) hydrogel with the upper critical solution temperature (UCST) ranging from 62 to −1°C by simply polymerizing the monomer in different concentrations of acetic acid (Ac) solutions. The hydrogel exhibits sensitive temperature response with a temperature hysteresis within 2°C and can maintain stable transmittance changes after 200 cycles of testing. The phase transition mechanism is revealed by combining ultraviolet visible spectra, differential scanning calorimetry, Fourier transform infrared spectra, dynamic light scattering and scanning electron microscope measurements. This thermoresponsive hydrogel can be applied in smart window and information encryption. The smart window prepared with the hydrogel had a very high ΔTlum (The luminous transmittance modulation, 94.4%), ensuring daytime illumination and privacy protection at night. Meanwhile, the smart window exhibits an excellent thermal insulation capability for energy‐saving buildings. The hydrogels with different UCST are flexibly combined to display designed patterns at different temperatures, demonstrating the ability of the hydrogels to be used for information encryption.

Study on the Corrosion Behavior and Numerical Simulation of TC18 Titanium Alloy under Tensile Stress

Abstract Both understanding and simulation of the process of corrosion damage are crucial for the prediction of remaining service life of engineering structures, sound reliability analysis, and design for the purpose of enhancing overall resistance of the material to corrosion damage. A coupled mechano-electrochemical PD corrosion model was established by using the peridynamic (PD) corrosion theory and the mechano-chemical effect theory.The model is capable of simulating the occurrence of degradation caused by the conjoint and mutually interactive influences of mechano-electrochemical phenomena. Corrosion behavior of TC18 titanium alloy in EXCO solution under stress loads of 31% σ0.2, 47% σ0.2 and 62% σ0.2 was studied. The effect of tensile loads on the corrosion behavior of TC18 titanium alloy was examined by combining the micromorphology and electrochemical parameters to verify the dependence of reaction rate occurring at the anode on tensile stress. Results of this study shed light as the stress level increases, the corrosion potential of TC18 titanium alloy shifts negatively, the corrosion current density increases and the corrosion intensifies. When the phase transition mechanism is satisfied, boundary movement occurs spontaneously.This model can safely be employed to complex geometric shapes and as a basis for studying crack propagation in environments that are favorable or conducive for inducing corrosion.

Understanding the sulphur-oxygen exchange process of metal sulphides prior to oxygen evolution reaction

Dynamic reconstruction of metal sulphides during electrocatalytic oxygen evolution reaction (OER) has hampered the acquisition of legible evidence for comprehensively understanding the phase-transition mechanism and electrocatalytic activity origin. Herein, modelling on a series of cobalt-nickel bimetallic sulphides, we for the first time establish an explicit and comprehensive picture of their dynamic phase evaluation pathway at the pre-catalytic stage before OER process. By utilizing the in-situ electrochemical transmission electron microscopy and electron energy loss spectroscopy, the lattice sulphur atoms of (NiCo)S1.33 particles are revealed to be partially substituted by oxygen from electrolyte to form a lattice oxygen-sulphur coexisting shell surface before the generation of reconstituted active species. Such S-O exchange process is benefitted from the subtle modulation of metal-sulphur coordination form caused by the specific Ni and Co occupation. This unique oxygen-substitution behaviour produces an (NiCo)OxS1.33-x surface to reduce the energy barrier of surface reconstruction for converting sulphides into active oxy/hydroxide derivative, therefore significantly increasing the proportion of lattice oxygen-mediated mechanism compared to the pure sulphide surface. We anticipate this direct observation can provide an explicit picture of catalysts’ structural and compositional evolution during the electrocatalytic process.

Transformation of carrier recombination mechanism as increasing the Germanium content in Cu2ZnGexSn1-xS4 single crystal prepared by molten salt method

Return to the physics origin of Cu2ZnSnS4 type (CZTS-type, include CZGS, CZGTS, CZTSSe, etc.) material system, studying the influence of replacement elements on the optoelectronic properties by preparing high-quality single crystal materials to clarify the relationship between the key defect states of materials and carrier dynamics is the only way which must be passed to find efficient paths to improve the existing performance of CZTS-type optoelectronic devices. Herein, the optical behavior of high-quality Cu2ZnGexSn1-xS4 (CZGTS) single crystal prepared by the molten salt method and its relationship with Ge content were systematically studied using a photoluminescence (PL) and absorption tests at room temperature. Combined with the simple electrical tests, the transformation of carrier recombination mechanism as increasing the Germanium content in CZGTS single crystal before and after phase transition were reported for the first time. The transmission electron microscopy images, X-ray diffraction spectra and Raman scattering spectra of CZGTS were adopted to reveal the evolution mechanism of single crystal growth and phase transition caused by the change of Ge composition. The results show that the tunable blue-shift of PL peak emerges with the increase of Ge content and the excitation power dependence has a saturation phenomenon in the CZGTS single crystal particles. In consideration of the optical bandgap, PL peak position, PL intensity, carrier lifetime and the macroscopic resistivity, it is confirmed that the intrinsic PL behavior could be effectively tailored by different Ge amounts. Compared with the recombination PL of interband carrier (Ep > Eg) in Cu2ZnSnS4, the band tailed carriers (Ep < Eg) dominated by defect states would play a powerful role in CZGTS single crystal. Our work provides a strategy for tailoring the bandgap of CZGTS materials and a new perspective for understanding the influence of defect states on carrier dynamics.

Understanding the phase transition mechanism in the lead halide perovskite CsPbBr3 via theoretical and experimental GIWAXS and Raman spectroscopy

Metal-halide perovskites (MHPs) exhibit excellent properties for application in optoelectronic devices. The bottleneck for their incorporation is the lack of long-term stability such as degradation due to external conditions (heat, light, oxygen, moisture, and mechanical stress), but the occurrence of phase transitions also affects their performance. Structural phase transitions are often influenced by phonon modes. Hence, an insight into both the structure and lattice dynamics is vital to assess the potential of MHPs. In this study, GIWAXS and Raman spectroscopy are applied, supported by density functional theory calculations, to investigate the apparent manifestation of structural phase transitions in the MHP CsPbBr3. Macroscopically, CsPbBr3 undergoes phase transitions between a cubic (α), tetragonal (β), and orthorhombic (γ) phase with decreasing temperature. However, microscopically, it has been argued that only the γ phase exists, while the other phases exist as averages over length and time scales within distinct temperature ranges. Here, direct proof is provided for this conjecture by analyzing both theoretical diffraction patterns and the evolution of the tilting angle of the PbBr6 octahedra from molecular dynamics simulations. Moreover, sound agreement between experimental and theoretical Raman spectra allowed to identify the Raman active phonon modes and to investigate their frequency as a function of temperature. As such, this work increases the understanding of the structure and lattice dynamics of CsPbBr3 and similar MHPs.

Structure defects engineering in Prussian blue cathode materials for high-performance sodium-ion batteries

Prussian blue analogues (PBAs) have been considered as promising cathode materials for sodium-ion batteries (SIBs) due to high theoretical energy density and stable three-dimensional framework structure, while the existence of anionic vacancies and coordinated water have become the key scientific issues affecting electrochemical performances. Herein, PBA materials with different structural defects, constructed via regulating electronic state of Fe-ion, are regarded as cathode materials for SIBs. It is confirmed that anionic vacancies and coordinated water molecules gradually decrease with the increase of FeⅢ-ion with the weaker metal activity than FeⅡ. As a result, FeⅢ[FeⅢ(CN)6] electrode with the lowest structural defects displays the highest reversible specific capacity of 121.4 mA h g−1 with an energy density of 356.9 Wh kg−1 at 10 mA g−1 and the best cyclic stability compared with NaFeⅢ[FeⅡ(CN)6] and Na2FeⅡ[FeⅡ(CN)6]. Na-ion intercalation/deintercalation proceeds by a reversible phase transition mechanism (cubic↔cubic/rhombohedral↔rhombohedral) relied on 2-electron transfer per formula using FeⅡ/FeⅢ as redox sites, which is verified by ex situ XRD and XPS tests. Besides, high anionic vacancies and specific surface area boost the NaFeⅢ[FeⅡ(CN)6] electrode to exhibit fast Na-ion transfer ability, thus contributing superior rate property. This work elucidates a new technique strategy to regulate the structural defects and improve Na-ion storage properties of PBAs.

Signatures of Quantum Criticality in the Complex Inverse Temperature Plane

Concepts of the complex partition functions and the Fisher zeros provide intrinsic statistical mechanisms for finite temperature and real time dynamical phase transitions. We extend the utility of these complexifications to quantum phase transitions. We exactly identify different Fisher zeros on lines or closed curves and elucidate their correspondence with domain-wall excitations or confined mesons for the one-dimensional transverse field Ising model. The crossover behavior of the Fisher zeros provides a fascinating picture for criticality near the quantum phase transition, where the excitation energy scales are quantitatively determined. We further confirm our results by tensor network calculations and demonstrate a clear signal of deconfined meson excitations from the disruption of the closed zero curves. Our results unambiguously show significant features of Fisher zeros for a quantum phase transition and open up a new route to explore quantum criticality.

Mechano-/seeding-triggered crystal-to-crystal phase transition in luminescent switching cocrystals

Currently, there are only a few cases of both mechanical stimulation-induced and seeding-triggered fluorescence conversion in which the mechanism of phase transition remains less well investigated. Herein, the first report of a luminescent switch based on mechano-/seeding-triggered crystal-crystal phase transition in a cocrystal polymorphic system is present. Based on the structure analysis and theoretical calculations, the luminescent switching can be attributed to the orbital overlap of the chromophores and exciton coupling. Furthermore, a mechanism of the phase transition was proposed, in which the synthons undergo movements including slipping and twisting during the phase transition. The effects of heating, local mechanical stimulus, and seed contact on the transition were discussed. It was proposed that contacting seeds can lower the energy barrier of the phase transition by inducing the formation of intermediate transition states of synthons via molecular recognition. The energy barrier was found to be the key factor in such mechano-/seeding-triggered phase transition.

Na+ Migration Mediated Phase Transitions Induced by Electric Field in the Framework Structured Tungsten Bronze

Framework structured tungsten bronzes serve as promising candidates for electrode materials in sodium-ion batteries (SIBs). However, the tungsten bronze framework structure changes drastically as mediated by the sodium ion concentration at high temperatures. While the three-dimensional ion channels facilitate fast ion storage and transport capabilities, the structural instability induced by Na+ migration is a big concern regarding the battery performance and safety, which unfortunately remains elusive. Here, we show the real-time experimental evidence of the phase transitions in framework structured Na0.36WO3.14 (triclinic phase) by applying different external voltages. The Na+-rich (Na0.48WO3, tetragonal phase) or -deficient (NaxWO3, x < 0.36, hexagonal phase) phase nucleates under the positive or negative bias, respectively. Combined with the theoretical calculations, the atomistic phase transition mechanisms associated with the Na+ migration are directly uncovered. Our work sheds light on the phase instability in sodium tungsten bronzes and paves the way for designing advanced SIBs with high-stability.

Polyacrylamide-Based Block Copolymer Bearing Pyridine Groups Shows Unexpected Salt-Induced LCST Behavior

Thermal-responsive block copolymers are a special type of macromolecule that exhibit a wide range of applications in various fields. In this contribution, we report a new type of polyacrylamide-based block copolymer bearing pyridine groups of polyethylene glycol-block-poly(N-(2-methylpyridine)-acrylamide; Px) that display distinct salt-induced lower critical solution temperature (LCST) behavior. Unexpectedly, the phase-transition mechanism of the salt-induced LCST behavior of Px block copolymers is different from that of the reported LCST-featured analogues. Moreover, their thermo-responsive behavior can be significantly regulated by several parameters such as salt species and concentration, urea, polymerization degree, polymer concentration and pH values. This unique thermal behavior of pyridine-containing block copolymers provides a new avenue for the fabrication of smart polymer materials with potential applications in biomedicine.

Phase Transition Kinetics of MAPbI3 for Tetragonal-to-Orthorhombic Evolution.

Despite the commonly observed phase-instability-induced photovoltaic degradation of MAPbI3, the phase transition kinetics at the atomic level remains elusive. Herein, by developing a stepwise NEB method, we clarify a nonsynergistic minimum-energy pathway for the tetragonal-to-orthorhombic phase transition. It is kinetically driven by the tilting of PbI6 4- that induces a spontaneous small rotation of adjoining MA+ and ends with stepwise ∼110° reorientations of two nonadjacent MA+ enabled by the cavity expansion. Compared to the common concerted mechanism, this process gives a low barrier of 0.08 eV/unit, demonstrating the easiness of the transition at extremely low temperatures and the importance of rotational entropies in regulating transition at elevated temperatures. With an explicit phase transition mechanism, we explore the structure-induced property response and reveal that introducing even low content of large-sized organic cations could help maintain the quasi-stable low-temperature performance of MAPbI3 solar cells.

Mechanism of dynamical phase transitions: The complex-time survival amplitude

Dynamical phase transitions are defined through nonanalyticities of the survival probability of an out-of-equilibrium time-evolving state at certain critical times. They ensue from zeros of the corresponding survival amplitude. By extending the time variable onto the complex domain, we formulate the complex-time survival amplitude. The complex zeros of this quantity near the time axis correspond, in the infinite-size limit, to nonanalytical points where the survival probability abruptly vanishes. Our results are numerically exemplified in the fully connected transverse-field Ising model, which displays a symmetry-broken phase delimited by an excited-state quantum phase transition. A detailed study of the behavior of the complex-time survival amplitude when the characteristics of the out-of-equilibrium protocol change is presented. The influence of the excited-state quantum phase transition is also put into context.

Full-scale simulation and experimental verification of the phase-transition temperature of a VO2 nanofilm as smart window materials

Vanadium dioxide (VO2), a highly important thermotropic phase-transition material, undergoes a reversible metal-insulator transition near temperatures of 68 °C and has potential applications in the energy-efficient smart window field. Recently, it has been found that the phase-transition temperature of VO2 films with nanometer thickness approached the ideal of room temperature (30 °C); however, deciphering this nanoscale phase-transition mechanism is still difficult in experiments and computer simulations. Here, a full-scale computational model was carefully constructed using molecular dynamics and first principles based on the artificial neural net algorithm and fully validated via accurate atomic layer deposition experiments. Overall, the machine-learning training results indicated that experimental observations and numerical simulation were in good agreement, demonstrating the feasibility and accuracy of our full-scale nanothermodynamics model. The computational simulations showed that the surface 3-layer atomic occupancy of the VO2 nanomaterials was a critical regulator of the phase transition temperature, implying as an important strategy for phase transition behavior in large-scale VO2 nanofilms designed on first principles. Moreover, the precise atomic layer deposition results agree with the simulation calculations by 98%, revealing that the computational model effectively and accurately combines with the experiments, and thus can accurately regulate the phase transition temperature of the VO2 nanofilm. This work was of crucial importance for potential applications of VO2 film in smart windows.

Progress of Seepage Law and Development Technologies for Shale Condensate Gas Reservoirs

With the continuous development of conventional oil and gas resources, the strategic transformation of energy structure is imminent. Shale condensate gas reservoir has high development value because of its abundant reserves. However, due to the multi-scale flow of shale gas, adsorption and desorption, the strong stress sensitivity of matrix and fractures, the abnormal condensation phase transition mechanism, high-speed non-Darcy seepage in artificial fractures, and heterogeneity of reservoir and multiphase flows, the multi-scale nonlinear seepage mechanisms are extremely complicated in shale condensate gas reservoirs. A certain theoretical basis for the engineering development can be provided by mastering the percolation law of shale condensate gas reservoirs, such as improvement of productivity prediction and recovery efficiency. The productivity evaluation method of shale condensate gas wells based on empirical method is simple in calculation but poor in reliability. The characteristic curve analysis method has strong reliability but a great dependence on the selection of the seepage model. The artificial intelligence method can deal with complex data and has a high prediction accuracy. Establishing an efficient shale condensate gas reservoir development simulation technology and accurately predicting the production performance of production wells will help to rationally formulate a stable and high-yield mining scheme, so as to obtain better economic benefits.

Study on the Influence of Cu Content on Phase Transition Characteristics of Fe75Ni(25-x)Cu x Elastocaloric Refrigeration Alloys

In this study, the mechanical alloying (AM) and powder metallurgy (PM) techniques were adopted to synthesize the Fe75 Ni(25-x) Cu x elastocaloric refrigeration alloys to research the effect of Cu contents on phase transition characteristics and microstructure of Fe75Ni(25-x) Cux elastocaloric refrigeration alloys. The conclusion was drawn that, with increasing Cu content, the phase transition characteristics changed and presented different changing trends. It was found that the phase enthalpy change value was 43.34J/g when the Cu content was 7at%, which was about 1.26 times of Fe15Ni (wt%) alloy. In addition, the FeNiCu alloy was a single-phase alloy with a crystal space group structure of Im-3m when the Cu content was 7at%, and the FeNiCu alloy was a two-phase alloy with crystal space group structures of Im-3m and Fm-3m when the Cu content was more than 7at%. This study provided an experimental basis for the theoretical investigation of the phase transition mechanism of FeNiCu elastocaloric refrigeration alloys.

Modulating Trinary‐Heterostructure of MoS2 via Controllably Carbon Doping for Enhanced Electrocatalytic Hydrogen Evolution Reaction

AbstractUnderstanding the phase transitions process of 2D transition metal dichalcogenides (2D‐TMDs) from semiconducting (2H) to metallic (1T, 1T′) phase provides directionality for the iteration of hydrogen evolution catalysis. So far, the phase engineering methods are intensively explored, serving as practical tools for discovering low‐cost novel nanomaterials for electronic and electrode devices in the realm of energy storage and catalysis. However, the heterostructures between 2H/1T, 2H/1T′, or 1T/1T′, functionalizing as critical active sites in the electrocatalytic process, are overlooked. Herein, a facile carbon doping paradigms, enabling augmentation of MoS2 phase transition, together with density functional theory calculations and rationales to explain the counterintuitive directionality of transitions is reported. The experiment and simulation results indicate that the existence of carbon as interstitial atoms is more favorable to the phase transition than the substitution atoms. The heterogeneous interfaces between 2H and 1T or 1T′ are more conducive to charge transfer. As expected, the trinary‐heterostructure nanofilm displays excellent electrocatalytic activities both in micro‐electrochemical measurements and conventional electrolytic cells. The results provide a fresh insight into the 2D‐TMDs phase transition mechanism and guide for trinary‐heterostructure electrocatalysts for scalable production.

Observation of distinct phase transitions in a nonlinear optical Ising machine

Optical Ising machines promise to solve complex optimization problems with an optical hardware acceleration advantage. Here we study the ground state properties of a nonlinear optical Ising machine realized by spatial light modulator, Fourier optics, and second-harmonic generation in a nonlinear crystal. By tuning the ratio of the light intensities at the fundamental and second-harmonic frequencies, we experimentally observe two distinct ferromagnetic-to-paramagnetic phase transitions: a second-order phase transition where the magnetization changes to zero continuously and a first-order phase transition where the magnetization drops to zero abruptly as the effective temperature increases. Our experimental results are corroborated by a numerical simulation based on the Monte Carlo Metropolis-Hastings algorithm, and the physical mechanism for the distinct phase transitions can be understood with a mean-field theory. Our results showcase the flexibility of the nonlinear optical Ising machine, which may find potential applications in solving combinatorial optimization problems.

Cytosolic and mitochondrial ribosomal proteins mediate the locust phase transition via divergence of translational profiles

The phase transition from solitary to gregarious locusts is crucial in outbreaks of locust plague, which threaten agricultural yield and food security. Research on the regulatory mechanisms of phase transition in locusts has focused primarily on the transcriptional or posttranslational level. However, the translational regulation of phase transition is unexplored. Here, we show a phase-dependent pattern at the translation level, which exhibits different polysome profiles between gregarious and solitary locusts. The gregarious locusts exhibit significant increases in 60S and polyribosomes, while solitary locusts possess higher peaks of the monoribosome and a specific "halfmer." The polysome profiles, a molecular phenotype, respond to changes in population density. In gregarious locusts, ten genes involved in the cytosolic ribosome pathway exhibited increased translational efficiency (TE). In solitary locusts, five genes from the mitochondrial ribosome pathway displayed increased TE. The high expression of large ribosomal protein 7 at the translational level promotes accumulation of the free 60S ribosomal subunit in gregarious locusts, while solitary locusts employ mitochondrial small ribosomal protein 18cto induce the assembly of mitochondrial ribosomes, causing divergence of the translational profiles and behavioral transition. This study reveals the translational regulatory mechanism of locust phase transition, in which the locusts employ divergent ribosome pathways to cope with changes in population density.

Molecular Dynamics Study on the Reaction of RDX Molecule with Si Substrate.

RDX is widely used in various explosion situations, and there are many studies on its detonation performance, safety, preparation, etc. Research on preparation of β-RDX is mainly conducted by experiments. In recent years, part of the research points to the use of substrate as a medium to produce β-RDX faster. Based on this guidance, our work aims to theoretically solve the physical and chemical processes that RDX may experience in the production process through numerical simulation. In this work, molecular dynamics simulation is set up for the interaction between RDX and a Si clean surface and a Si hydroxyl saturated surface separately, and a higher precision simulation is set up to verify the reliability of the results. NCI analysis is also used to guess the possible phase transition mechanism in the simulation results. In the simulation process, a 7 × 7 Si clean surface, a 3 × 3 Si clean surface, and a 7 × 7 Si-OH surface are set, and each surface adsorbs one α-RDX. The semiempirical Gfn1-xtb method is used for the 7 × 7 surface, and the DFT method is used for the 3 × 3 surface. The calculation results confirmed by high-precision results show that RDX molecules will react with the dangling bonds on the Si surface. Three conformations of RDX were found on the hydroxyl saturated surface of Si. The isosurface generated by the NCI method is used to analyze the reasons for the formation of these conformations.

Interlayer Chemical Modulation of Phase Transitions in Two-Dimensional Metal Chalcogenides.

Two-dimensional metal chalcogenides (2D-MCs) with complex interactions are usually rich in phase transition behavior, such as superconductivity, charge density wave (CDW), and magnetic transitions, which hold great promise for the exploration of exciting physical properties and functional applications. Interlayer chemical modulation, as a renewed surface modification method, presents congenital advantages to regulate the phase transitions of 2D-MCs due to its confined space, strong guest-host interactions, and local and reversible modulation without destructing the host lattice, whereby new phenomena and functionalities can be produced. Herein, recent achievements in the interlayer chemical modulation of 2D-MCs are reviewed from the aspects of superconducting transition, CDW transition, semiconductor-to-metal transition, magnetic phase transition, and lattice transition. We systematically discuss the roles of charge transfer, spin coupling, and lattice strain on the modulation of phase transitions in the guest-host architectures of 2D-MCs established by electrochemical intercalation, solution-processed intercalation, and solid-state intercalation. New physical phenomena, new insight into the mechanism of phase transitions, and derived functional applications are presented. Finally, a prospectus of the challenges and opportunities of interlayer chemical modulation for future research is pointed out.