Phase Transformation Rate Research Articles

Simulation of effect of interfacial lithium flux on miscibility gap in non-equilibrium phase transformation of LiFePO4 particles

Non-equilibrium phase transformation and effect of interfacial Li flux on miscibility gap in two-phase transformation of LiFePO4 have been explored in this study. Our previously developed “Mushy-Zone” (MZ) model, accounting for sluggish Li diffusion across the two-phase interface, has been employed to study the non-equilibrium phase transformation in LiFePO4 materials of Li-ion batteries. Phase transformation rate, variation of two-phase miscibility gap, and interfacial Li composition profiles have been studied for different particle shapes at varying discharge rates. Sluggish Li flux across the two-phase interface, which is believed to be the origin of the kinetically-induced non-equilibrium phase transformation, has been calculated to explain the obtained results. It is found that small particle sizes (radii of 20 nm) and slow discharge rates tend to create homogenous phase transformation (i.e., shrunk or no miscibility gap). Two-phase transformation is remarkably delayed for spherical particles at low discharge rates, leading to a lower capacity compared to that of plated-shaped particles. However, at higher discharge rates spherical particles show better capacity.

Microwave-assisted hydrothermal annealing of binary Ni–Co oxy-hydroxides for asymmetric supercapacitors

Binary nickel–cobalt oxy-hydroxides (denoted as NCOH) with tunable crystal phases are prepared from an atomic mixture of Ni(OH)2 + Co(OH)2 in the 1:2 ratio (denoted as (Ni/2Co)(OH)2) through a microwave-assisted hydrothermal annealing (MAHA) method. The (Ni/2Co)(OH)2 powders are synthesized through a sol–gel process from an aqueous solution containing NiCl2·6H2O and CoCl2·6H2O. The MAHA process is demonstrated to be a novel and promising method for transforming/tuning the crystal phase and size of NCOH at relatively low annealing temperatures in short time. The MAHA temperature and time are varied to transform the crystalline phases from a hydroxide mixture to NiCo2O4 nanocrystals, which can be quantified by a crystalline index (denoted as CI) value obtained from X-ray diffraction (XRD) patterns. The higher content and larger size of NiCo2O4 nanocrystals are, the longer electrochemical activation time is needed. The rate of phase transformation can be enhanced by adding microwave adsorbents, graphene oxide (GO), to obtain a NiCo2O4/RGO composite with reduced crystal size (RGO: reduced GO). The variation in the crystalline index and the effect of the potential window on the specific capacitance of NCOH powders are systematically compared and discussed in this work.

Thermal and hydrothermal stability of flame hydrolytically synthesized SiO2/TiO2 mixed oxides

The phase stability of the two TiO2 modifications (anatase and rutile) in fumed SiO2/TiO2 nano-composites (0–24.8 wt-% silica) under thermal and hydrothermal conditions was investigated by X-ray powder diffraction, transmission electron microscopy (TEM) and gas adsorption methods (BET). The results show that the phase transformation from anatase to rutile type of structure and the growth of anatase crystallites are significantly retarded by mixing small amounts of SiO2 into TiO2, while the specific surface area is maintained. The SiO2/TiO2-composites reveal a remarkable shift in the anatase to rutile transformation temperature from approx. 500 °C (pure TiO2) to approx. 1000 °C (samples with SiO2 contents of more than 10%). The rate of phase transformation from anatase to rutile is enhanced under hydrothermal conditions compared to conventional thermal treatment, e.g. pure titania (AEROXIDE® TiO2 P25) annealed under hydrothermal conditions (100 g/m3 absolute humidity, 4 h at 600 °C) had a rutile content of 85%, while the same specimens annealed in absence of humidity contained only 46% rutile. However, the difference in rate of phase transformation became less pronounced when the silica content in SiO2/TiO2-composites was further increased.TEM results showed that the surface of the anatase crystallites was covered with silica. This averts coalescence of anatase crystallites and keeps them under a critical size during the annealing process. When the crystal domains grew larger, a rapid conversion to rutile took place. The critical size of anatase crystallites for the phase transformation was estimated to be 15–20 nm.

Study of phase transformation behaviour of alumina through precipitation method

Alumina hydrates were prepared by the wet chemical method under different pH values and subsequently calcined through both conventional and microwave heating approaches. After calcination at 500 °C for 2 h, amorphous precursor was partially transformed to α-alumina. NH4NO3 remaining in the dried precursor without further washing after precipitation reaction was found to increase the transformation temperature, which is different from previous published results. Microwave calcination was also employed and can dramatically enhance the phase transformation rate in comparison with the conventional method. A combination of microwave heating and amorphous precursor may be a viable strategy to fabricate α-alumina at low temperatures rapidly.

Phase Stability and Mechanical Properties of Zirconia and Zirconia Composites

Monolithic zirconia materials (3Y‐TZP, 10Ce‐TZP, and 12Ce‐TZP) and their composites with 30 vol% alumina were produced. Low‐temperature aging degradation (LTAD) and mechanical properties of materials were investigated. For assessment of phase stability in the materials, aging experiments were performed in water at 90°C for 32, 64, and 128 days. The aging phenomenon was characterized and monitored using X‐Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Four‐point bending was used to determine the flexural strength of materials before and after aging treatment in water at 90°C for 2, 4, and 6 months. The aging experiments resulted in different phase transformation rates for the materials studied. The 12Ce‐TZP containing materials showed the highest resistance to low‐temperature aging and 3Y‐TZP containing materials showed the highest bending strength. When compared, no change in flexural strength was observed between the materials not exposed to aging and the aged materials.

The study on crystal-crystal transformation of isotactic poly(1-butene)/poly(1-butene-co-propylene) in-reactor alloy by FTIR

The isotactic poly(1-butene)/ poly(1-butene-co-propylene) in-reactor alloy was prepared by heterogeneous titanium catalyst system. The differences of crystal phase transformation time, phase transformation rate and phase transformation time between Form I and Form II at a given transformation degree at 25°C by fourier transform infrared spectroscopy(FT-IR) were discussed. It can be seen that the transformation from Form II to I of the alloy was significantly accelerated by the increase of propylene structure unit. With the further increase of the content of the propylene unit in the alloy, the transformation may be ignored and Form I can be formed immediately at 25°C. The change of the spherulite of pure poly(1-butene) and alloy also be observed and discussed.

Controlled Synthesis of Manganese Dioxide Nanostructures via a Facile Hydrothermal Route

Manganese dioxide nanostructures with controllable morphological structures and crystalline phases were synthesized via a facile hydrothermal route at low temperatures without using any templates or surfactants. Both the aging duration and aging temperatures were the main synthesis parameters used to influence and control the rate of morphological and structural evolution of MnO2 nanostructures. MnO2 nanostructures comprise of spherical nanoparticulate agglomerates and highly amorphous in nature were formed at lower temperature and/or short aging duration. In contrast, MnO2 nanostructures of sea‐urchin‐like and nanorods‐like morphologies and nanocrystalline in nature were prepared at the combined higher aging temperatures and longer aging durations. These nanostructures underwent notable phase transformation from δ‐MnO2 to α‐MnO2 upon prolonged hydrothermal aging duration and exhibited accelerated rate of phase transformation at higher aging temperature.

Effect of low-level impurities on low-temperature performance properties of biodiesel

Biodiesel, a renewable fuel consisting of fatty acid methyl esters and made from lipid feedstocks, has presented persistent cold weather operability problems that are not predicted using the standards tests common in the petroleum refining industry. These problems have been referred to as “precipitate formation above cloud point” and are known to be caused by minor impurities. This study investigates the fundamental causes of this issue. To this end, water, steryl glucosides (SG), and saturated monoglycerides (SMGs) were spiked into 100% biodiesel (B100) from various feedstock sources. Only SMGs were found to have a significant effect on the cloud point (CP) and final melting temperature (FMT) in four B100 samples with a range of CP. A large difference between FMT and CP indicates that a metastable phase of SMG forms initially and can transform into a more stable, less soluble polymorph over time or upon heating. This occurred for SMG content above approximately 0.2 to 0.3 wt%. For more saturated B100, a large FMT–CP difference was only observed at slower heating rates, suggesting a slower rate of phase transformation. β-monostearin solubility (the most stable phase) in B100 was measured as a function of temperature. CP measurements suggest the metastable phase is as much as 10 times more soluble than the β phase. Differential scanning calorimetry experiments suggest that the metastable phase is a hydrated α-gel. SMGs at concentrations above 0.24 wt% caused failure of the cold soak filtration test (ASTM D7501); however, at higher water concentrations (∼1200 ppm), the effect of SMGs was significantly reduced. Addition of SGs had no effect on cold soak filterability.

A Volume Averaged Approach to the Numerical Modeling of Phase-Transition Intercalation Electrodes Presented for LixC6

An approach for the volume averaged numerical modeling of phase-transition intercalation electrodes is presented for lithiated graphite, LixC6, in lithium-ion batteries. The proposed method directly treats phase formation and growth through a modified form of the Avrami equation enabling the physics-based mathematical model to capture the additional time constant observed in the two phase regions of lithiated graphite as well as a portion of the hysteresis commonly observed between charge and discharge voltage. The graphite phase diagram was taken to be composed of three stages, or phases, each with a Nernstian or ideal solution based equilibrium potential function. The behavior of the potential rise from a current pulse in the two-phase region is matched well with this methodology resulting in higher valued diffusion coefficients than found when only a single-phase approach is used. Simulated results for mesocarbon microbeads show the co-existence of all three phases within the electrode during higher rate discharges. Concentration dependent diffusion coefficients are found to be necessary to match experimental results at rates significantly higher than 1C. The model is shown to be capable of exhibiting core-shell behavior when fitted phase-transformation rate constants are sufficiently high in value, although not observed for MCMB.

Effects of current intensity and cumulative exposure time on the localized current-activated sintering of titanium nickelides

This article discusses the processing and properties of titanium nickelides locally sintered via Current-Activated Tip-based Sintering (CATS), a new localized sintering process. One of the advantages of CATS is the ability to apply orders of magnitude higher current densities than conventionally possible, which can promote rapid sintering and phase transformation rates. Mechanically alloyed equi-atomic Ni–Ti powder was for the first time tip sintered at varying current intensities and cumulative current exposure time. The effect of current-control processing conditions on the evolution of the locally sintered Ni–Ti microstructure and properties are discussed. The size of the locally sintered process zone was found to increase with cumulative current exposure time. The degree of sintering, phase transformations, and properties were found to depend on the current intensity, cumulative current exposure time and distance away from the tip/compact interface. Fully/near fully dense material was achieved rapidly at locations exposed to the highest current densities.

α-Chalcocite Nanoparticle Synthesis and Stability

While intensively investigated as a solar light absorber in photovoltaic applications, the use of α-chalcocite (α-Cu2S) as such has been hindered by its conversion to a copper-deficient, nonstoichiometric phase, djurleite. Here we study this transformation in copper sulfide nanoparticles. We discuss the relative stabilities of α-chalcocite and djurleite as synthesized here, how this influences the conditions for obtaining these phases, and the affect of the reaction conditions on the phase transformation rate. Nanoparticle phases are characterized by powder X-ray diffraction (PXRD) and differential scanning calorimetry (DSC). Size and morphology was characterized by transmission electron microscopy (TEM). Additionally, the α-chalcocite particles were characterized by UV/visible/NIR absorption spectroscopy (UV/vis/NIR) and energy dispersive X-ray spectroscopy (EDS).

Stress-induced phase transformation and strain rate effect in polycrystalline Mo nanowires

Using molecular dynamics and modified analytic embedded atom methods, the stress-induced phase transformation and strain rate effect on deformation characteristics of polycrystalline molybdenum nanowires are studied at room temperature under uniaxial tensile strain. The results show the tensile ductility of the nanowires increases with increasing strain rate. The surface rupture occurs at one position as the strain is bigger than 6% at a strain rate of 0.01% ps −1, but it does at multiple positions in the range of larger strain at higher strain rate. Moreover, an increased strain rate delays the appearance of “face-centered cubic (fcc)” atoms, but increases the peak fraction of “fcc” atoms. During the deformation process, two kinds of structure transformation are observed: (1) the “body-centered cubic (bcc)” configuration transforms into “other” configurations, and subsequently the “other” configurations transform into the “fcc” or “hexagonal close-packed (hcp)” configuration and (2) the “fcc” configuration converts into “other” configurations, and then into “bcc” or “hcp” configuration. In addition, the mechanical properties of the polycrystalline nanowires are compared with those of single-crystalline nanowires.

Mechanochemical Polymorphic Transformation of Photocatalyst Material TiO<sub>2</sub> and its Dependence on the Milling Operation

Some dry and wet grinding experiments have been respectively conducted on titanium dioxide which is a noble photocatalyst material in a mortar, a tumbling mill and a planetary mill. Anatase is apt to transform to rutile via a metastable phase brookite in every kind of mills in the case of dry grinding. And it hardly takes place for phase transformation from rutile to other forms. It is shown that the kind of mill has not decisive effect on the mechanochemical polymorphic transformation of titanium dioxide, which merely influences the rate of phase transformation. On the other hand, the addition of other liquid media, such as water and acetone, is helpless for phase transformation of anatase. Only anatase can transform to metastable phase brookite by wet grinding. When ground titanium dioxide is heated, the amorphous phase is easier to transform to rutile than metastable phase brookite at lower temperature.

Thermodynamics approach to the hydrogen diffusion and phase transformation in titanium particles

The hydrogen diffusion and phase transformation in a titanium particle were studied based on thermodynamic calculation. The mechanisms of hydrogen diffusion in different phases (α-Ti, β-Ti and TiH x ) were analyzed. A mobility database was developed for titanium–hydrogen system based on the experimental works on hydrogen diffusion coefficient reported in literature and the fundamental of diffusion. To implement the calculation, a commercial software package for the simulation of diffusion-controlled phase transformation was used. The hydrogen diffusion process, hydrogen distribution, phase transformation and phase growth rate during hydrogenization of a titanium particle at temperatures of 560 K, 800 K and 1000 K were discussed. The thermodynamics and kinetics analysis provided quantitative insight into the diffusion process and improved the understanding of diffusion mechanism and phase transformation.

Influence of Temperature on Solvent-Mediated Anhydrate-to-Hydrate Transformation Kinetics

To achieve an in-depth understanding of the underlying mechanism of the acceleration or deceleration effect of temperature on solvent-mediated anhydrate-to-hydrate phase transformation. The effect of temperature on the phase transformation rate and onset time of two model compounds was investigated using in situ Raman spectroscopy. The thermodynamic driving force of the phase transformation (e.g. supersaturation) at different temperatures was determined by measuring the solubility of the anhydrate and the hydrate. Both acceleration and deceleration effects of temperature on the phase transformation were observed. The mechanism of these temperature effects was studied by exploring the influence of temperature on supersaturation level and crystallization kinetics. Increasing temperature usually leads to accelerated phase transformation kinetics, but it simultaneously decreases supersaturation, which has the opposite effect on the kinetics of the phase transformation. The overall effect of temperature on the phase transformation is therefore determined by the combined effects of supersaturation and temperature on the nucleation and crystal growth kinetics of the hydrate. By differentiating and comparing the effects of temperature and supersaturation on the anhydrate-to-hydrate phase transformation, a deeper understanding of the underlying principle of the acceleration and deceleration effects of temperature on the phase transformation has been achieved.

Reaction Synthesis and Oxidation Behaviours of Ludwigite

Using ludwigite as raw material, the phase transformation and mass loss rate of ludwigite in the process of oxidizing roasting are investigated by DTA, isothermal TG, scanning electron microscopy (SEM), X-ray diffraction (XRD) techniques. The results showed that magnetite is transformed into hematite, serpentine is decomposed into forsterite at lower temperature (T＜700°C). The weight of ludwigite has a small loss below 600°C. The decomposed of szaibelyite dehydrated and formed into suanite about 700°Cis the main reason of causing ludwigite mass losses. By comparing the curves of ludwigite at different temperature from 700 to 900°C, the process of oxidizing roasting can be divided into three phases: characterized by a period of fast weight loss, and then followed by a mass gain. Finally, weight of sample is no longer change with prolongation of time. The final weight loss is 6.062%, 6.658% and 7.442% respectively for test temperature. Suanite can not be decomposed to form B2O3 and volatilized when the temperature of oxidizing roasting is below 1142 °C. It is found by XRD that paigeite and magnoferrite are the most stable composition without deterioration on oxidizing roasting. The experiment results can provide theoretical references for agglomeration and blast furnace smelting of ludwigite.

Effect of molecular weight on the polymorphic transformation of isotactic poly(1-butene)

The effect of molecular weight on the polymorphic transformation of isotactic poly(1-butene)(iPB) under room temperature had been investigated by fourier transform infrared spectroscopy(FT-IR). The phase transformation time, phase transformation rate and phase transformation time difference between phase I and phase II at a given transformation degree were used to analysis the phase transformation kinetics of iPB aging at room temperature. The results show that the reduction of phase II can occur quickly at room temperature and seem less dependent on the molecular weight. However, the molecular weight has great effect on the formation of phase I. When the phase transformation degree for phase I reach 90%, a distinct transformation time difference can be observed. In order to clearly explain the difference in the reduction of phase II and the growth of phase I, a phase transformation model from the chain conformation level for iPB with different molecular weight have been drawn. DSC analysis was used to support the proposed model.

Biomimetic transformations of amorphous calcium phosphate: kinetic and thermodynamic studies

The biomimetic synthesis and phase transformation of XRD amorphous calcium phosphate were studied by application of kinetic, chemical and spectral (XRD and IR) methods and thermodynamic simulations. Two SBFs (SBFc and SBFr), differing in their HCO(3)(-) and Cl(-) ion contents, were used in the maturation studies. It has been proven that the biomimetic maturation accelerated the phase transformation of less thermodynamically stable amorphous calcium phosphate to poorly crystalline hydroxyapatite. Several regularities have been found: (i) kinetic reasons determined the biomimetic precipitation of XRD-amorphous calcium deficient phosphate (ACP); (ii) the precipitated ACP always contained impurities due to co-precipitation, ion substitution and incorporation phenomena; (iii) the increased content of HCO(3)(-) ions in the surrounding microenvironments increased the rate of phase transformation and the concentration of MeHCO(3)(+) (Me = Ca, Mg) species in the solution, but the solubility of CaCO(3) has only been decreased and its precipitation accelerated, thus playing a crucial role in the process under study.

Dynamic Tensile Behaviour of TRIP and DP Steels at Different Temperatures

The stress-strain response of TRIP 700 and DP 600 steels was studied at a wide range of strain rates and temperatures using a special high/low temperature tensile Hopkinson Split Bar (THSB) device. The mechanical properties of the studied steels, especially of the TRIP steel, were found to be strongly affected by both temperature and strain rate. The beneficial TRIP effect in the studied steel reached its maximum at temperatures between 75-150 °C. The transformation behaviour of the retained austenite in the TRIP steel was studied by XRD, revealing that the phase transformation rate increases with decreasing temperature and decreases with increasing strain rate. A phenomenological numerical model was also presented to describe the behaviour of the TRIP and DP steels at different temperatures and strain rates.

Kinetics of reactions in FeAl synthesis studied by the DTA technique and JMA model

The phenomena of Fe–Al intermetallic phase formations preceding the SHS reaction were investigated. The XRD analysis and SEM observations have shown that not only the Fe 2Al 5 phase and the solid solution of aluminum in iron, but also the FeAl 3 and FeAl 2 phases are formed before the start of the SHS reaction. The kinetics of the sintering process of elemental Fe and Al powders was investigated by the DTA technique. The kinetics of the phase transformations was determined at different temperatures with heating rates of 0.5–10 °C/min for non-isothermal conditions using the Johnson–Mehl–Avrami model (JMA). The activation energy of specific phase formations was determined by the Kissinger method. Moreover, the Avrami coefficients, describing the rate and the character of specific phase transformations were calculated. The influence of the heating rate of the powder mixture on the rate of phase transformation was shown. It was found that a heating rate lower than 2.5 °C/min yields the full sequence of phase transformations: Fe + Al → FeAl 3 → Fe 2Al 5 → FeAl 2 → FeAl, compatible with the binary phase diagram of Fe–Al.