Phase Transformation Kinetics Research Articles

Black TiO2 and Oxygen Vacancies: Unraveling the Role in the Thermal Anatase-to-Rutile Transformation

Understanding the role of oxygen vacancies in the phase transformation of metal oxide nanomaterials is fundamental to design more efficient opto-electronic devices for a variety of applications, including sensing, spintronics, photocatalysis, and photo-electrochemistry. However, the structural mechanisms behind the phase transformation in reducible oxides remain poorly described. Here, we compare P25 and black TiO2 during the thermal anatase-to-rutile transformation using in situ synchrotron powder diffraction. The precise measurement of the phase fractions, unit cell parameters, and Ti-O bond sheds light on the phase transformation dynamics. Notably, we observe distinct temperature-dependent shifts in the relative phase fractions of anatase and rutile in both materials highlighting the role of the oxygen vacancy in promoting the phase transformation. We employ bond valence concepts for structural modeling, revealing unique trends in temperature evolution of Ti-O distances of black rutile, confirming that this TiO2 phase is preferentially reduced over anatase. These findings not only enhance our understanding of phase transitions in TiO2 but also open new ways for the design of advanced photocatalytic materials through targeted phase control.

Ingenious approach for retrieving valuable metals from gypsum via dehydration–rehydration two-step phase transition

Retrieving valuable metals (VMs) from industrial waste is an attractive approach to reduce the continuous consumption of metallic resources and prevent the environmental contamination by heavy metals. However, metal extraction efficiencies are extremely low due to their entrapment in crystal lattices. Herein, a universal strategy for extracting VMs from gypsum (CaSO4·2H2O), a common industrial waste, was reported using a two-step phase transition of dehydration–rehydration; metal extraction was achieved by designing an appropriate phase transition route and controlling the phase transformation kinetics. As the phase transition route to CaSO4·0.5H2O (path-0.5) proceeded faster than that to CaSO4 (path-0), higher extraction efficiencies were realized for Cr (99.9%) and Cd (98.8%) from gypsum. Mechanistic investigations indicated that during the selected path-0.5 dehydration, water molecule loss from gypsum initiated a direct solid-state transformation, triggering the first rearrangement of local atoms and excluding partially incorporated metals toward the edge of the CaSO4·0.5H2O lattice. The subsequent rehydration from CaSO4·0.5H2O to CaSO4·2H2O, with a phase transition rate faster than that of path-0, triggered the secondary atom rearrangement. The synergism between the two atomic rearrangements finally led to the complete release of trapped metals. Additionally, this strategy was used for retrieving other metals (e.g., Pb, Zn, Hg, As, and Sr) from gypsum, thereby potentially offering a new approach for designing an appropriate phase transition route for extracting VMs from other hydrous minerals.

Enhanced electrochemical oxidation of 5-hydroxymethylfurfural over tailored nickel nanoparticle assembly

Nickel-based materials are promising electrocatalysts for anodic oxidation of 5-hydroxymethylfurfural (HMF) to value-added 2, 5-furandicarboxylic acid (FDCA). However, their catalytic efficiency is impeded by the sluggish phase transformation of Ni(II) hydroxide to the active Ni(III) oxyhydroxide. Herein, we demonstrate for the first time that the phase transformation kinetics and the HMF oxidation activity of nickel nanoparticles can be modulated by creating self-assemblies with different particle aggregation structures: ordered nanoarrays, disordered nanoarrays, and random aggregates. Notably, the nanoparticle assembly featuring an ordered nanoarray structure exhibits the highest activity, achieving 99.8 % HMF conversion and 99.2 % FDCA yield at 1.36 V. In situ Raman spectroscopy and electrochemical analysis reveal that the ordered nanoarray effectively accelerates the transformation kinetics, attributed to the reduced dehydrogenation barrier of Ni(II) hydroxide as confirmed by density functional theory calculations. This work contributes new insights into the structure-performance relationship of Ni-based catalysts, offering valuable guidance for designing high-performing electrocatalysts.

Multiwall carbon nanotube-hyperbranched polymaleimide core–shell nanowires with hierarchical porous and polar structure as sulfur host for sustainable lithium-sulfur batteries

Lithium–sulfur (Li–S) batteries have gained considerable attention as a promising alternative to current advanced batteries because of their low cost, natural abundance, high energy density, and environmental benignity of sulfur. The commercial application of Li–S batteries is being seriously impeded by the shuttle effect of lithium polysulfides (LiPSs) and the sluggish reaction kinetics of solid phase transformation. Herein, a novel composite material (PPI@MWCNT-Y) with a hierarchical porous skeleton and polar channel is prepared as a sulfur host using a convenient in-situ one-pot polymerization. PPI@MWCNT-Y is composed of conductive mesoporous multi-wall carbon nanotubes (MWCNTs) core and a microporous hyperbranched polymaleimide (PPI) shell formed by the self-polymerization of N,N′-1,4-phenylenedimaleimide (PDM). The mesoporous and conductive channels in PPI@MWCNT-Y improve the redox kinetics of the active materials by serving as a fast electron and Li-ions transportation pathway. The microporous structure and abundant adsorption sites (N and O atoms) within the PPI@MWCNT-Y served as physical and chemical barriers, preventing the loss of LiPSs, and suppressing the shuttle effect. Consequently, the S/PPI@MWCNT-38 composite exhibited a high discharge capacity of 1338 mAh g−1 at 0.2 C and a low-capacity decay rate of 0.088% for 400 cycles at 1 C.

Navigating phase diagram complexity to guide robotic inorganic materials synthesis

AbstractEfficient synthesis recipes are needed to streamline the manufacturing of complex materials and to accelerate the realization of theoretically predicted materials. Often, the solid-state synthesis of multicomponent oxides is impeded by undesired by-product phases, which can kinetically trap reactions in an incomplete non-equilibrium state. Here we report a thermodynamic strategy to navigate high-dimensional phase diagrams in search of precursors that circumvent low-energy, competing by-products, while maximizing the reaction energy to drive fast phase transformation kinetics. Using a robotic inorganic materials synthesis laboratory, we perform a large-scale experimental validation of our precursor selection principles. For a set of 35 target quaternary oxides, with chemistries representative of intercalation battery cathodes and solid-state electrolytes, our robot performs 224 reactions spanning 27 elements with 28 unique precursors, operated by 1 human experimentalist. Our predicted precursors frequently yield target materials with higher phase purity than traditional precursors. Robotic laboratories offer an exciting platform for data-driven experimental synthesis science, from which we can develop fundamental insights to guide both human and robotic chemists.

WO3/ZnWO4 microcomposites with potential application in photocatalysis

The paper presents the results of a comprehensive study of the structural, optical and strength features, changes in the morphology of grains of (1-x)WO3 – xZnO ceramics depending on the concentration of the initial components when they vary. Using the X-ray diffraction method, it was found that mechanochemical grinding of samples of (1-x)WO3 – xZnO ceramics, regardless of the concentration of the components, does not lead to the initiation of phase transformation processes associated with the formation of the monoclinic ZnWO4 phase, the formation of which is observed during thermal annealing of samples at a temperature of 1000 °C. Based on data obtained using scanning electron microscopy and X-ray diffraction methods, the dynamics of phase transformations in ceramics of the type WO3/ZnWO4 → ZnWO4 → ZnO/ZnWO4 was established. It was found that with a given ratio of WO3: ZnO components equal to 0.7 : 0.3, under selected conditions of mechanochemical grinding and an annealing temperature of 1000 °C, highly ordered single-phase ZnWO4 ceramics with a wolframite structure can be obtained. Analysis of the optical characteristics and changes in the band gap depending on the ratio of the components showed a direct dependence of changes in the band gap on the phase composition of the ceramics. An assessment of the strength characteristics made it possible to establish that the formation of two-phase WO3/ZnWO4 ceramics leads to strengthening of the surface layer, while for two-phase ZnO/ZnWO4 ceramics a decrease in hardness is observed, the decrease of which directly correlates with changes in the content of the ZnO phase in the composition of the ceramics.

Superplastic behavior of fine-grained Ti-10V-2Fe-3Al alloy fabricated by friction stir processing

Ti-10V-2Fe-3Al alloy with fine-grained β phases was fabricated by friction stir processing with optimized processing parameters. The superplastic behavior of the specimens was investigated by tensile deformation at different strain rates and temperatures, and an optimal superplastic elongation of 634% was achieved at 700°C and 3 × 10–4/s. An annealing treatment at 650°C for 60 min showed a microstructure with α precipitates distributed in the β matrix in the friction stir specimen. Such pre-heat treatment improves the superplasticity of the specimen, achieving an elongation of up to 807% at 750°C and 3 × 10–4/s. The influences of tensile temperatures and strain rates on the microstructural evolution, such as grain size variation, grain morphology, and phase transformations, were discussed. The superplastic deformation behavior of fine-grained Ti-10V-2Fe-3Al alloy is controlled by grain boundary sliding and accompanied by dynamic phase transformation and recrystallization.

On the role of interfacial coherency and carbon in niobium segregation at ferrite/austenite interface: An atomistic study

Co-segregation of carbon (C) and substitutional atoms at the interface can remarkably influence the kinetics of phase transformation in steels, while decoupling their contributions by experiments remains challenging. Here, we employ Molecular Statics and hybrid Molecular Dynamics/Monte Carlo simulation approaches to comparatively investigate the niobium (Nb) segregation behaviors at both the incoherent and semi-coherent ferrite/austenite interfaces in Fe-C-Nb systems. The calculations reveal that the atomic Virial stress perpendicular to the interface is crucial for the binding energy of Nb to the interface, and the C presence at the interface has both physical and chemical interactions with Nb. It is conventionally believed that C addition can promote Nb segregation due to their attractive interaction, while the current study indicates that C and Nb atoms tend to form clusters in ferrite, thereby weakening the Nb segregation. The present study provides a promising computational approach for investigating the elemental co-segregation in steels.

Thermo-metallo-mechanical based phase transformation modeling for high-speed milling of Ti–6Al–4V through stress-strain and temperature effects

The optimization of machining parameters, tool longevity, and surface quality in High-Speed Milling (HSM) of Ti–6Al–4V relies immensely on understanding the local phase transformation. This study endeavors to build a Finite Element (FE) model capable of forecasting phase alterations during the rapid thermal fluctuations intrinsic to Ti–6Al–4V machining. Dynamic phase transformation models were initially introduced to capture rapid heating and cooling phenomena. Using a user-defined subroutine, the phase transitions predictive models were integrated into the HSM simulation within Abaqus/Explicit. Simulation outcomes unveiled phase transitions primarily occurring within the serrated chip and at the tool-workpiece interface. Notably, during rapid heating, when the cutting speed increased to 350 m/min, the β-phase volume fraction surged from 7.5 to 96.38%. A similar trend was observed with feed rate adjustments (i.e., 0.15–0.25 mm/tooth), where β-phase increased from 7.5 to 67.84%. Rapid cooling facilitated the reversion of the transformed β-phase back into the α'-phase. Finally, some advanced characterization techniques were employed to validate the developed thermo-metallo-mechanical coupled FE model for phase transformation. The simulation results verified by the experimental data promotes a better understanding of phase alteration mechanisms and microstructural evolution in HSM of Ti–6Al–4V. The current research is also beneficial for crucial insights into optimizing the machining conditions and their impact on tool-material interactions and surface integrity.

Stimulation of the Bainite Transformation Scenario by an External Magnetic Field

Introduction. It makes practical sense to change the properties of steels with a bainite structure, as with bainite transformation under the influence of a magnetic field, it is possible to improve the ductility of the steel while maintaining or even increasing its strength. Scientific research in this area has focused on the influence of the magnetic field on thermodynamics and on the change in the phase transformation scenario. However, there is no detailed description in open sources of the effect of a magnetic field on the structure and properties of the products of intermediate bainite transformation. The aim of the work is to study the peculiarities of the influence of an external magnetic field on the scenario and kinetics of phase transformation of steel.Materials and Methods. The study was conducted using samples made of 65G steel. Their chemical composition was monitored using a Magellan Q8 optical emission spectrometer. Heat treatment (resistive heating) was carried out in an IMASH 20–75 installation for high-temperature research. The heating temperature was approximately 1000 degrees 1000°C, and the holding time was 10 minutes. The sample was cooled down using water-cooled electrical contacts. An external magnetic field with a strength of 400 kA/m and 800 kA/m was created by an electromagnet integrated into the vacuum chamber of the installation.Results. The experiments confirmed the potential for altering the transformation pathway from pearlite into bainite in the presence of an external magnetic field of up to 1 MA/m. Images of the microstructure and surface relief of samples after cooling in a magnetic field were obtained. Kinetic changes and dependencies of the volumetric transformation rates on the duration of isothermal exposure were analyzed. It has been found that exposure to a constant magnetic field of 1.6 MA/m increased the volumetric transformation rate by 1.808 times (for 65G steel) and by 1.687 times (for 45H steel).Discussion and Conclusion. The results of observations of changes in the surface relief during cooling in the absence of a magnetic field, and in magnetic fields of various strengths, were recorded. This has allowed us to draw the conclusion that the external magnetic field stimulates the bainitic transformation instead of the original pearlitic one. Microstructural changes can be explained by the influence of the magnetic field on the initial phase magnetic state.

Electro-hydrodynamic programming reshapes liquid crystal dynamics in free-form director fields

This study unveils a groundbreaking technique leveraging the superposition of electric field vectors to manipulate liquid crystals (LCs). Demonstrated through a simple configuration of four independent electrodes at the corners of a rectangular enclosure, notably, this configuration can be further simplified or modified as needed, showcasing the versatility of the approach. Significantly, the design showcased in the paper eliminates the need for an alignment layer, highlighting the versatility of the method. Through nuanced adjustments in waveforms, amplitudes, frequencies, and phases in AC or DC from these electrodes, precise control over LC shape deformation and dynamic phase transformation is achieved in both temporal and spatial dimensions. In contrast to traditional methods, the approach presented here abolishes alignment layers and intricate electrode-array systems, opting for a streamlined configuration with varying AC frequencies and DC electric signals. This innovative methodology, founded on simplified governing equations from Q-tensor hydrodynamics theory, demonstrates true 3D control over LCs, displaying efficiency in electrode usage beyond current arrays. The study's contributions extend to temporal control emphasis, superposition techniques, and the elimination of fixed electrodes, promising unprecedented possibilities for programming LC materials and advancing the field of programmable LC devices.

Characterization on thermal hysteresis of shape memory alloys via macroscopic interface propagation

A specimen of Shape Memory Alloy (SMA) starts martensitic phase transformation normally by the nucleation of a macroscopic austenite-martensite interface at the specimen's boundary (i.e., the nucleation is sensitive to boundary conditions). By contrast, the interface propagation only needs to overcome the energy barrier of the incompatibility between the austenite and martensite phases (i.e., the interface-propagation driving force reflects material's intrinsic properties). In this paper, we observe the thermally induced forward and reverse quasi-static propagation of the macroscopic austenite-martensite interface in a Ni-Mn-Ga single-crystal bar. It is found that the temperature difference between the forward and reverse propagation is only 2.8 °C, which is obviously less than the thermal hysteresis evaluated from Differential Scanning Calorimetry measurement (6.3 °C) on the same material. This result not only gives a better characterization of the thermal hysteresis of the phase transformation, but also helps deeply understand the relation between the phase-transformation kinetics and the microstructures of the macroscopic austenite-martensite interface.

Rheology of Ti-6Al-4V alloy under non equilibrium conditions during β forging

During the “β-forging” of Ti-6Al-4V structural aircraft components that follows pre-heating above the β-transus, the skin zone of the blank actually cools down to below the β-transus whilst being deformed, as a result of heat losses towards the atmosphere or the dies. The core areas of the bulkiest sections of the blank, however, can be deformed above the β-transus during the whole processing time. This paper focuses on the development of an innovative rheological model to support the numerical modelling of the forging process, suitable for both the equilibrium conditions above the β-transus and the non-equilibrium conditions below. For that purpose, small-scale hot compression tests were performed with the thermomechanical simulator Gleeble 3500, after pre-heating in the β-field of Ti-6Al-4V at 1100°C and rapid cooling to the compression temperature under the β-transus, which was varied between 750 °C and 950 °C. In some cases, a lag time -or stabilization time- at constant temperature was introduced between the cooling step and the deformation step. Effects of lag time and deformation temperature on microstructural evolutions and rheological properties of Ti-6Al-4V were investigated. Results show that the lack of stabilization time leads to a significant reduction of the flow stress at low strain. Such a behavior is associated with non-equilibrium thermodynamic conditions at the beginning of deformation and the progress of phase transformation during deformation. Based on that observation, an analytical formulation for the targeted rheological model is proposed. It includes a sub-model for the kinetics of phase transformation derived from the literature.

On the Metastable Ferrite Hot Rolling of Low‐Grade Electrical Steels: Its Microstructure, Texture and Low‐Temperature Annealed Structure

The hot‐rolled sheets of low‐grade electrical steels undergo complete austenite‐ferrite phase transformation, resulting in fine equiaxed grains in the sheets. These fine initial grains eventually lead to a strong detrimental {111} texture in the final product after cold‐rolling and annealing process. This work, based on the phase transformation delay of columnar grain structure in cast slabs, proposes a novel approach called metastable ferrite hot rolling, namely, a two‐stage heating at 900 °C and then at 1100 °C shortly and hot rolling, to avoid complete transformation and to retain original {100} oriented regions. In addition, low‐temperature annealing is employed to increase grain sizes of hot‐rolled sheets thereby improving the final texture and magnetic properties after cold‐rolling and annealing. The results indicate that metastable ferritic columnar grains can be maintained during the two‐stage heating process at 1100 °C and primarily undergoes subgrain growth. The transformation from ferrite to austenite at 1100 °C is very slow. During the hot rolling with a high 86% reduction by two passes, the notable grain refinement is mainly caused by dynamic recrystallization rather than dynamic phase transformation as evidenced by the observation from the rolling plane.

Phase transformation sequence and kinetics of ω formation in Ti-3573 titanium alloy during continuous heating

The work explores the phase transformation sequence and the kinetics of ω formation in a novel metastable β titanium alloy of Ti–3Al–5Mo–7V–3Cr (wt%) through dilatometer test and phase/microstructure characterizations. The phase transformation sequence has been identified as β → β + ω, β + ω → α + β, β → β + α, and α → β during the continuous heating. The formation of ω phase has underwent nucleation and growth stages revealed by the S-shaped kinetic curve. The diffusion controlled mechanism has been analyzed via Avrami exponent. The average activation energy for β → β + ω phase transformation has been evaluated 90.42 kJ/mol via Kissinger–Akahira–Sunose equation. The activation energy difference between early and late stages of ω formation has been compared among metastable β titanium alloys. The small value of Ti-3573 alloy may imply a thermo-kinetic inconvenience of ω assisted α formation.

Beyond Constant Current: Origin of Pulse-Induced Activation in Phase-Transforming Battery Electrodes.

Mechanistic understanding of phase transformation dynamics during battery charging and discharging is crucial toward rationally improving intercalation electrodes. Most studies focus on constant-current conditions. However, in real battery operation, such as in electric vehicles during discharge, the current is rarely constant. In this work we study current pulsing in LiXFePO4 (LFP), a model and technologically important phase-transforming electrode. A current-pulse activation effect has been observed in LFP, which decreases the overpotential by up to ∼70% after a short, high-rate pulse. This effect persists for hours or even days. Using scanning transmission X-ray microscopy and operando X-ray diffraction, we link this long-lived activation effect to a pulse-induced electrode homogenization on both the intra- and interparticle length scales, i.e., within and between particles. Many-particle phase-field simulations explain how such pulse-induced homogeneity contributes to the decreased electrode overpotential. Specifically, we correlate the extent and duration of this activation to lithium surface diffusivity and the magnitude of the current pulse. This work directly links the transient electrode-level electrochemistry to the underlying phase transformation and explains the critical effect of current pulses on phase separation, with significant implication on both battery round-trip efficiency and cycle life. More broadly, the mechanisms revealed here likely extend to other phase-separating electrodes, such as graphite.

X‐ray imaging for structural evolution and phase transformation dynamics of battery electrodes

AbstractThe impending energy crisis entails sustainable battery technologies with improved energy density, reliability, safety and lifetime. Hence, it is essential to gain detailed insights into the surface reactions, ionic diffusion, structural and morphological evolution, and degradation mechanisms of battery electrodes. Recently, X‐ray techniques emerged as revolutionary tools to reveal an in‐depth understanding of the battery during operations. This review provides an overview of the use of in situ/operando X‐ray techniques to understand the different functionalities of electrode materials inside the battery. It will focus on the phase transformation, structural evolution and dynamic properties of the battery electrodes, and discuss the relationship between battery failure and electro‐chemo‐mechanical failure in the electrode. Finally, the limitations of these methods are also discussed with the prospects for effective use of these techniques in the development of advanced battery technologies.

Structure Evolution of Iron (Hydr)oxides under Nanoconfinement and Its Implication for Water Treatment.

In the development of nanoenabled technologies for large-scale water treatment, immobilizing nanosized functional materials into the confined space of suitable substrates is one of the most effective strategies. However, the intrinsic effects of nanoconfinement on the decontamination performance of nanomaterials, particularly in terms of structural modulation, are rarely unveiled. Herein, we investigate the structure evolution and decontamination performance of iron (hydr)oxide nanoparticles, a widely used material for water treatment, when confined in track-etched (TE) membranes with channel sizes varying from 200 to 20 nm. Nanoconfinement drives phase transformation from ferrihydrite to goethite, rather than to hematite occurring in bulk systems, and the increase in the nanoconfinement degree from 200 to 20 nm leads to a significant drop in the fraction of the goethite phase within the aged products (from 41% to 0%). The nanoconfinement configuration is believed to greatly slow down the phase transformation kinetics, thereby preserving the specific adsorption of ferrihydrite toward As(V) even after 20-day aging at 343 K. This study unravels the structure evolution of confined iron hydroxide nanoparticles and provides new insights into the temporospatial effects of nanoconfinement on improving the water decontamination performance.

Examining the Impact of Intermediate Cooling on Mechanical Properties of 22MnB5 in a Tailored Tempering Process

Tailoring the properties of hot-stamped components offers the potential to enhance crash performance while simultaneously improving downstream joining processes. In recent years, an innovative technology suited for achieving tailored properties involving the utilization of a specialized furnace chamber, known as the TemperBox®, has been introduced. Within this chamber, a cooled aluminum mask shields specific areas of the blank from incoming heat radiation and concurrently absorbs the blank’s own radiation. The duration of the heat radiation exchange can influence the diffusion-dependent phase transformation and, consequently, the resulting mechanical properties. Hence, the intermediate cooling duration assumes a pivotal role as a parameter, as is investigated in this study. To examine the effects, specimens of the steel 22MnB5 AS150 are subjected to intermediate cooling of varying durations, followed by forming and partial quenching. The temperature profile of the blank during intermediate cooling prior to forming and quenching is analyzed. Subsequently, the tailored hot-stamped components are assessed for hardness, strength, ductility, and thickness strain. The study reveals that with increasing duration of partial intermediate cooling and targeted radiation exchange, a homogeneous ferritic–pearlitic structure is formed from an austenitic structure. This uniform structure of ferrite and pearlite is reflected in lower hardness and strength values, along with improved ductility. Additionally, this paper introduces a simulation methodology designed to calculate the dynamics of thermal radiation and the kinetics of phase transformation.

Effect of austenitizing temperature on isothermal quenching microstructure and mechanical properties of 52100 bearing steel

In general, isothermal quenching heat treatment in salt bath can improve the toughness but sacrifice the strength and hardness of 52100 bearing steel. Many previous studies focusing on the isothermal quenching process have been carried out to balance the strength and toughness. And the optimal mechanical properties are limited to approximately 60–61 HRC hardness and 100 J impact toughness. In this study, in order to further improve the mechanical properties, the influences of austenitizing temperature on microstructural evolution and strengthening-toughening mechanism in isothermal quenched bainite in 52100 bearing steel have been systematically investigated. The increase of austenitizing temperature decreases the volume fraction of undissolved carbides, and increases the carbon concentration in austenite matrix. These changes not only strongly influence the kinetics of bainite phase transformation and bainite microstructure, but also cause the conflict between the coarsening of prior austenite grains, as well as bainite sheaves, and the refining of substructure of lower bainite. Austenitizing at 870 °C can balance this conflict well and obtain a good combination of hardness and toughness. It is found that the carbon content and dislocation density in bainite ferrite reach the peak value at 870 °C austenitized specimens, which effectively strengthens the bearing steels. At the same time, the 870 °C austenitized specimens retain appropriate content of undissolved carbides, which avoids stress concentration and inhibits crack propagation, and finally improve the impact toughness. Therefore, the optimal austenitizing temperature (870 °C) achieves the improvement of mechanical properties with the hardness and toughness of 59.7 HRC and 273 J, respectively.