Activation Energy For Nucleation Research Articles

Study on Kinetics of Transformation in Medium Carbon Steel Bainite at Different Isothermal Temperatures.

Ultra-fine carbide-free bainitic (UCFB) steel, also known as nano-bainite (NB) steel, is composed of bainitic ferrite laths with nanoscale thickness and carbon-rich film-like retained austenite located between laths. The bainite transformation kinetic model can accurately describe the bainite transformation kinetics in conventional austempering (CA) processes based on the shear mechanism combined with the dilatometer test. UCFB steels with medium and high carbon composition are designed in this work to systematically study the transformation kinetics of bainite, and the evolution of its microstructure and properties, and reveal the influence of heat treatment processes on the microstructure and properties the UCFB steels. The results show that the activation energy for BF nucleation decreases during the CA process and isothermal transformation temperature decreases. The bainite transformation is first nucleated at the grain boundaries, and then nucleated at the newly formed bainitic ferrite/austenite interface.

Enhanced luminescence of self-crystallized Cs4PbBr6 quantum dots via regulating glass ceramic network structure

The most used method to obtain Cs4PbBr6 quantum dots (QDs) in glass is heat treatment, but the more energy-efficient method of self-crystallization is seldom explored. In this work, Cs4PbBr6 QDs are obtained in glass matrix through self-crystallization method. The results elucidate that with the increase of Na2O, glass network structure transforms from three-dimensional (3D) framework structure to two dimensional (2D) layered structure and phase separation process of glass is aggravated. The looser network structure facilitates atoms rearrangement, and the presence of phase interface reduces activation energy of nucleation. Both of the two aspects can promote the self-crystallization of Cs4PbBr6 QDs. Importantly, the PLQY of PG3 is 81.24 %, which is much higher than the heat treated one (CG3:11.89 %). Finally, a green backlit light-emitting diode (LED) with high color purity of 96.1 % and white light-emitting diode (WLED) with color coordinate of (0.3212, 0.3315) are fabricated. Our work puts forward a new perspective to investigate QDs glass ceramic and brings QDs glass ceramic significant prospect in the optoelectronic applications.

Reaction kinetics of shearing-enhanced goethite process for iron removal from zinc solution

A shearing-enhanced goethite process for iron removal is proposed to overcome the difficulties in process reaction control and non-uniform mass transfer. The reaction kinetics of the oxidation of Fe2+ to α-FeOOH and crystal growth of α-FeOOH were studied in a ZnSO4-FeSO4 system. Oxidation and crystal growth experiments were performed under the following conditions: shearing speed = 1000–4000 rpm, flow rate of oxygen = 0.3–1.5 L min−1, and reaction temperature = 70–90 °C. The results show that the oxidation rate increases with increasing shearing speed and reaction temperature, owing to the increase in oxygen mass transfer. The degree of crystallization of goethite improves and the needle-like structure is further refined as the shearing speed increases and the temperature decreases. The activation energy of nucleation is about 54 kJ mol−1. The rate equations are derived for the oxidation and oxygen mass transfer processes. The oxidation kinetics parameters are calculated by data fitting. The rate equations of oxygen mass transfer and Fe2+ oxidation are mO2=54.01kLO∙PO2HO2−CO2∙MO2∙vg∙n0.4 and r=47.08CFe2+2∙CO2CFe2++0.001865, respectively.

Effect of introduced vanadium carbide at the bay region on bainite transformation, microstructure and mechanical properties of high-carbon and high-silicon steel

Obtaining a fine bainitic ferrite for high carbon steel at low temperature requires a long period. Thus, creating strategies to accelerate bainite transformation is important. This investigation aims to explore the introduction of vanadium carbide at the bay region on bainitic transformation kinetics and mechanical properties by comparing isothermal bainitic transformation treatment directly and in combination with bay treatment prior to bainite transformation. Results show that introduced fine VC precipitation during bay treatment notably accelerates the initial bainite transformation with the incubation period being reduced by 91%. Decrease in activation energy for bainite nucleation at vanadium carbide/austenite interfaces and generation of preferred nucleation sites lead to a favorable acceleration of bainitic formation kinetics. However, indirect refinement of grain size for intragranular bainitic ferrite nucleation and consumption of available vanadium carbide/austenite interfaces result in overall rate of the subsequent bainite transformation. For the sample with prior bay treatment, the lath directions of bainitic ferrite are more complex and the size of block RA is smaller than that of the conventional sample. Moreover, the introduction of vanadium carbide at the bay region can simultaneously enhance the tensile strength and impact toughness of the steel.

Analytical approach for pop-in and post-pop-in deformation behavior during nanoindentation: effect of solute Si in interstitial free steel

Effect of solute atom on dislocation nucleation and subsequent elasto-plastic deformation observed in nanoindentation tests was investigated using Si-alloyed interstitial-free steel (IF). The onset shear stresses for plastic deformation τp at the first pop-in where the dislocation nucleation occur were greatly affected by the presence of solute Si irrespective of the Si content tested. The analysis of the cumulative probability of τp indicate that the dislocation nucleation occurred through heterogeneous nucleation where the nucleation sites are solute Si atoms. It has suggested that Si has ability to increase the activation energy for dislocation nucleation. The variation in the magnitude of displacement burst at the first pop-in showed that more kinetic energy of moving dislocations is dissipated by the presence of solute Si resulting in decrease of travel distance of dislocations. This trend can be explained by classical solid solution theory. The staircase yielding observed only in Si-alloyed sample indicate that the re-activation of dislocations formed at the pop-in became difficult by the solute Si.

Effect of pre-existing nuclei on microstructure and magnetic properties of high Bs FINEMET-like nanocrystalline alloys

The effect of conventional annealing and rapid annealing on the magnetic properties and microstructure of Fe78+2xSi7.2-xB13-xCu0.8Nb1 (x = 0, 1, 2) alloys is discussed systematically. The study was found that the existence of a large number of pre-existing nuclei in the amorphous phase of Fe82Si5.2B11Cu0.8Nb1 alloy can produce tiny nanocrystals and thus result in excellent soft magnetic properties. When the average nanocrystal size is 16 nm using rapid annealing, the best soft magnetic performance can be obtained, in which Bs reaches 1.80 T, Hc is 5 A/m, and μ is 20,000 at 1 kHz. The crystallization kinetics shows that as the Fe content increases, the incubation time of nucleation of the alloys decreases, and the nucleation is faster and easier. In addition, a large number of pre-existing crystal nuclei not only greatly reduces the nucleation activation energy of the alloy but also achieves grain refinement and excellent magnetic properties through grain competition growth.

Manipulating the Crystallization Kinetics by Additive Engineering toward High‐Efficient Photovoltaic Performance

AbstractAdditive processing is proven to be an effective method to improve the efficiency and stability of perovskite solar cells; however, its intrinsic role in directing the crystallization pathway and thus morphology formation remains unknown. In situ grazing‐incidence wide‐angle x‐ray scattering (GIWAXS) is applied to study the function of a 1,8‐diiodooctane (DIO) additive in manipulating the crystallization behavior of perovskite thin films. It is seen that the DIO additive could induce multi‐stage intermediate crystallization phases and increases the activation energy for nucleation and growth, which postpones the perovskite phase transformation time and broadens the transition zone. The elongated crystallization process affords improved perovskite thin film crystallinity and reduces defect density, which enables a longer carrier diffusion length. As a result, improved device efficiency, moisture, and thermal stability can be achieved. The current study provides a new prospective in understanding the additive function in perovskite thin film morphology control from fundamental parameters, indicating the importance of minor processing conditions in global property management toward high device performance.

Analysis and Optimization of Thermophysical Properties and Phase Change Behavior of Expanded Vermiculite-Based Organic Composite Phase Change Materials

In this study, the optimized expanded vermiculite (EVM) was used as the matrix and a series of novel form-stable composite phase change materials (fs-CPCMs) were prepared by the physical impregnation method. The results showed that acid leaching and organic intercalation EVM had a better package effect (≥74.9 wt %) and heat storage capacity (≥139.2 J/g) compared with EVM, which showed a good optimization effect, revealing that proper modification treatment was beneficial to the improvement of the thermophysical properties of composite PCMs. Nonisothermal crystallization kinetics demonstrated that the EVM matrix was beneficial to reduce the nucleation activation energy (ΔEa) of PEG, and the optimized EVM could further reduce the ΔEa, which effectively improved the crystal nucleation process of PEG. However, the matrix would restrict the crystal growth process of PEG, which had certain adverse effects. Moreover, XRD, TGA, and cycle tests showed that the prepared novel fs-CPCMs had excellent chemical stability, thermal stability, and cycle reliability. This research provided a certain theoretical reference and technical support for the preparation of mineral-based organic composite phase change materials with high heat storage capacity and excellent phase change behavior.

Microstructures of enstatite in fine-grained CAIs from CV3 chondrites: Implications for mechanisms and conditions of formation

Enstatite is a ubiquitous phase in chondritic meteorites, interplanetary dust particles, and cometary samples. In equilibrium condensation models, enstatite is predicted to condense via a reaction between pre-condensed forsterite and gaseous SiO. However, previous studies have shown that some enstatites in chondrite matrices and AOAs do not have a genetic relationship with forsterite, arguing against formation by the predicted forsterite-gas reaction. Here we report the occurrence of enstatite in a unique, fine-grained, spinel-rich inclusion (FGI) Ef1014-01 in the Efremovka CV3 chondrite. Enstatite in this FGI is present as an outer layer on spinel-anorthite-diopside nodules and separates the FGI from an amoeboid olivine aggregate (AOA) -like material. Enstatite shows elevated CaO and Al2O3 contents (up to a few weight percent). Four FIB sections were extracted from this FGI to investigate the microstructures of enstatite and its relationship with other phases using TEM techniques. The TEM observations show that the enstatite is dominantly low-temperature clinoenstatite (LCLEN), which displays abundant twinning, and is sometimes associated with thin orthoenstatite (OREN) lamellae. Clinoenstatite grains commonly have a crystallographic orientation relationship with adjacent diopside, but do not exhibit any replacement relationship with forsterite in the AOA-like material surrounding the FGI. Investigations of several other fine-grained CAIs from the Efremovka and Leoville CV3 chondrites show that enstatite is more common in these inclusions than previously thought and typically forms discontinuous layers or islands on the diopside layers.Based on SEM and TEM observations, we suggest that the LCLEN-OREN intergrowths in Ef1014-01 formed by transformation from a protoenstatite (PEN) precursor, which may be a product of direct condensation or reheating in the solar nebula. The crystallographic orientation relationship between enstatite and diopside suggests that epitaxial growth of enstatite occurred, lowering the activation energy for nucleation and facilitating direction condensation of enstatite from the gas phase, rather than by reaction of the gas with forsteritic olivine. The microstructures of enstatite are indicative of an extremely rapid cooling rate (∼104 K/h) that is within the range of chondrule cooling rates. Such a rapid cooling rate may imply that the cooling rates of FGIs are indeed much higher than other types of refractory inclusions. Alternatively, the rapid cooling rate may not reflect the primary cooling of the FGIs, but is the result of rapid cooling after a short-lived secondary reheating event in the solar nebula.A fractionated gas with a lower Mg/Si ratio than the solar value is required to condense enstatite. Such a gas could be produced by isolation of pre-condensed forsterite or repeated evaporation-recondensation processes. The presence of both enstatite-bearing and enstatite-free CAIs in CV3 chondrites suggests that at least two gaseous reservoirs with different Mg/Si ratios were present in the CAI-forming regions.

Polymer Spherulitic Growth Kinetics Mediated by Nanoparticle Assemblies

We systematically examine the role of the self-assembly state of polymer-grafted silica nanoparticles (GNPs) on the spherulitic growth kinetics of a poly(ethylene oxide) (PEO) matrix. The nanoparticles (NPs) are functionalized with either unimodal or bimodal polymer brushes to systematically control their self-assembly within the semicrystalline polymer matrix. In the former case, we employ a poly(methyl methacrylate) (PMMA) brush (miscible with PEO), while the latter has a short dense polystyrene (PS carpet) brush, which is immiscible with the PEO matrix, and a long, sparse PMMA brush. The unimodal GNPs always yield well-dispersed NPs possibly because both the silica NP surface and the PMMA interact favorably with the PEO. The addition of the short PS carpet makes the interaction between the surface and the matrix PEO unfavorable. However, since the NPs also have grafted PMMA chains, they act akin to surfactants and provide access to a range of self-assembled structures. In all cases, the addition of NPs decreases the PEO spherulitic growth rates. Surprisingly, we find that the spatial dispersion of NPs does not change the secondary nucleation activation energy barrier of the PEO crystallization, such that the temperature dependence of spherulite growth kinetics is relatively unaffected by the NPs. Instead, the main effect of the NPs on spherulitic growth kinetics arises from variations of the melt’s viscosity. Two apparently “universal” trends are found—bare NPs and large assemblies of GNPs appear to approximately follow the same dependence for the role of additives on polymer viscosity. On the other hand, all self-assembled NP structures which have at least one nanoscale dimension (well-dispersed NPs, one-dimensional strings or two-dimensional sheets) follow another general behavior, but one where the viscosity is much larger. Such unusual viscosity increases have been previously observed in the pioneering experiments of Composto and Winey, but there is currently no available theoretical description that can capture these changes and their effects on polymer crystallization.

A Non-Isokinetic Approach for Modeling Solid-State Transformations: Application to Crystallization of a Fe-B Amorphous Alloy.

Solid-state phase transformations like crystallization of amorphous alloys can be described by an analytical model incorporating a nucleation index a. However, this model cannot be used to examine isochronal transformations with abrupt changing of enthalpy differences performed with differential scanning calorimetry. Based on the model, a non-isokinetic approach is proposed and applied to analyze the isochronal crystallization kinetics of Fe85B15 amorphous alloy. The approach enabled us to obtain the kinetic parameters and activation energies for nucleation and growth.

Thermodynamic Effect of Pressure on Nucleation Activation Energy

The first stage of the solidification of metal alloys is nucleation, during this stage is defined the number of grains present in the final structure, just as the conditions in which the same will grow. The application of pressure in the solidification is mostly studied as an operational parameter, and its effects are described and commented extensively in literature. The objective of this paper is to study the effect of pressure on nucleation, still not to describe but to explain it, for this purpose the pressure is treated as a thermodynamic variable. From this analysis it was possible to determine the real influence of pressure on the activation energy, being clear that its effect is equivalent and co-dependent to that of the undercooling, and can thus cause considerable variations in the activation energy, much larger than what is proposed in previously published papers that approach the subject, causing a considerable stimulus on nucleation. It was also observed that the curve that represents the effect of pressure on the activation energy proposed in this paper, has a similar form to the curves that describe how the pressure refines structures, resulting from the process of solidification under pressure reported in literature.

Nucleation behaviour of racemic and enantiopure histidine

Small non-centrosymmetric domains are observed in the racemic crystal, due to the low nucleation activation energy of the conglomerate.

A Mechanistic Investigation of Sustainable Solvent-Free, Seed-Directed Synthesis of ZSM-5 Zeolites in the Absence of an Organic Structure-Directing Agent.

The solvent-free, seed-directed synthesis using natural precursors has emerged as a sustainable route for the synthesis of zeolite. Albeit the significant progress in the synthesis techniques, the crystallization behaviors of zeolites are somewhat elusive. Herein, we performed a detailed investigation of the crystallization behaviors of ZSM-5 zeolites synthesized through the solvent-free, seed-directed route using rice husk silica as starting materials. The crystallization at 180 °C is completed rapidly in 10 h, with an ultrahigh zeolite yield of at least 95%. Moreover, we evaluated the crystallization kinetics at different temperatures using the nonlinear Avrami equation and found instantaneous nucleation with three-dimensional growth in the studied temperature range, with activation energies for nucleation, transition, and crystal growth of 137, 51, and 51 kJ mol–1, respectively, indicating that nucleation is the rate-determining step. Further investigation of the structural and morphological evolution revealed a preference for secondary nucleation over the seed-growth mechanism. Crystallization proceeds via structural rearrangement within the solid system. We anticipate that our work will provide extensive insights that increase the understanding of zeolite crystallization and expand the highly sustainable production of zeolites.

Effect of cobalt content on non­isothermal crystallization kinetics of Fe­based amorphous alloys

In the present study, FeSiBP and FeCoSiBP ribbons with a fully amorphous structure were made by melt spinning technique. A detailed analysis of the isochronal crystallization behavior is presented in this paper. The influence of cobalt on the crystallization kinetics of the alloys was studied under isochronal conditions using differential scanning calorimetry (DSC). Apparent and local activation energy values were determined by Kissinger, Ozawa and Kissinger-Akahira-Sunose (KAS) methods. The results indicate that appropriate amounts of cobalt can significantly enhance the thermal stability of Fe-based alloys, through an increase in nucleation activation energy from 538kJ/mol to 701kJ/mol, obtained by Kissinger method. Furthermore, with the method proposed by Matusita, it was possible to obtain global values for the Avrami exponent, noting that from a general perspective, Co changes the mechanism from diffusion controlled to interface controlled. This leads to the conclusion that the crystallization process is complex and takes place in more than one stage. Therefore, the determination of nucleation mechanisms and dimensional growth is difficult due to the inapplicability of the Johnson-Melh-Avrami (JMA) model. As such, a study under isothermal conditions is suggested, in order to achieve a full understanding of the mechanisms involved.

Investigating the Kinetic Parameters of SiO2-Al2O3-CaO-CaF2-K2O Oxyfluoride Glass

Oxyfluoride glass-ceramics containing CaF2 nanocrystals are kind of attractive materials for the optical applications due to their low phonon energy and high transparency. Moreover, the crystallization control and consequently, the kinetic properties are important for oxyfluoride glasses. Therefore, in the present research, the crystallization kinetics of isochronal transformation of the 37.26SiO2-28.11Al2O3-7.73CaO-26.89CaF2-4.5 K2O (wt%) glass have been determined upon the basis of maximum transformation rate using Differential Thermal Analysis (DTA) technique. Hence, it is concluded that the crystallization of the mentioned glass is a process controlled by Avrami nucleation, three-dimensional diffusion-controlled growth, and anisotropic growth impingement mode. The effective activation energy Qp =181 kJ.mol-1, growth exponent n=2.272, nucleation activation energy QN= 123, and growth activation energy QG=211 have been determined.

Kinetics and in situ observation of nonisothermal crystallization in Bayan Obo tailing-based nanocrystalline glass-ceramic

In recent years, the preparation of CMAS nanocrystalline glass-ceramics has shown potential as an application of secondary resourcing technology in utilizing Bayan Obo iron ore tailings containing rare earth elements. The crystallization mechanism for nanodiopside-type glass-ceramics was studied via an investigation of the nonisothermal crystallization kinetics of the glass system, combined with the in situ observation of softening and crystallization of the basic glass using a high-temperature laser confocal microscope. The results show that the activation energy of nucleation in the glass system is higher than that of crystal growth by using the Ozawa model. The crystallization mechanism changes as the crystallization fraction increases, that is, from the three-dimensional growth in which the nucleation rate increases with time in an interface-controlled manner (a > 1, b = 1, m = 3) at the initial stage of crystallization to a decreased nucleation rate in a diffusion-controlled growth (a = 0.5, b = 0.5, m = 3) at the middle and later stages. This process involves both surface crystallization and volume crystallization. The crystallization was observed in situ, and it was further confirmed that there exists a critical nucleation temperature between Tg and Tx, which is related to the interface free energy and critical Gibbs free energy difference. When the temperature exceeds the critical value of Tg + 55 K, the system begins to exhibit visible crystallization. With an increase in temperature, the basic glass softened considerably, while the crystal grew significantly. In addition, the surface roughness can be used to characterize the crystallization process, providing a new research method for crystal growth.

Formation and annihilation of multi-antiskyrmion defects during skyrmion nucleation

Magnetic skyrmions and antiskyrmions are intriguing topological defects in spin textures. A single-antiskyrmion defect would form and collapse to straightforwardly complete the topological transition for the nucleation of a skyrmion phase from a ferromagnetic background. Here, a novel multi-antiskyrmion defects-driven skyrmion nucleation mechanism is investigated in both the ferromagnetic single layer and synthetic antiferromagnetic trilayers with the spin-polarized current stimuli. The multi-antiskyrmion defects mechanism needs lower nucleation activation energy than the single-antiskyrmion defect mechanism and brings about intriguing fluctuations in topological charge. The multi-antiskyrmion defects mechanism can survive even at room temperature and is robust against simulation parameters. Our finding not only unravels an unprecedented skyrmion nucleation process but also provides a platform for investigating antiskyrmions as topological defects during topological transition.

Influence of cooling rate on the precipitation kinetics of nanoscale isothermal ω-phase in metastable β-Ti alloy, Ti–5Al–5Mo–5V–3Cr

In metastable β-Ti alloys, nanoscale isothermal ω-phase (ωiso) precipitates are regarded as the nucleation sites for the α strengthening phase. Here we investigate the precipitation kinetics of the ωiso precipitates as a function of cooling rate (air cooling and water quenching) after β-solutionising. A combined in situ small-angle neutron scattering (SANS) and electrical resistivity measurement approach was used during ageing of Ti–5Al–5Mo–5V–3Cr wt% (Ti-5553) alloy at 300 °C and 325 °C up to 8 h. The SANS modelling was consistent with ellipsoid shaped particles for the ωiso precipitates, for both air-cooled and water-quenched samples. The precipitates attained a maximum size (equatorial diameter) of ∼21 nm and ∼17 nm after 2 h and 4 h of ageing the water-quenched and air-cooled samples respectively. Although the air-cooled samples showed delayed nucleation in comparison to water-quenched sample, the volume fraction became approximately the same (∼11%) after ageing for 8 h. The average value of the activation energy for ωiso nucleation from the β-phase matrix was determined as 122 kJ mol−1 from electrical resistivity data using a modified Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The hardness increased with ageing time, with water quenching leading to a higher final value of hardness than air cooling.

Molecular models for creep in oriented polyethylene fibers

Highly oriented and crystalline polyetheylene (PE) fibers have a large failure stress under rapid tensile loading but exhibit significant creep at much smaller stresses that limits applications. A possible mechanism is slip of chains due to stress-enhanced, thermally activated nucleation of dislocations at chain ends in crystalline regions. Molecular dynamics simulations are used to parameterize a Frenkel-Kontorova model that provides analytic expressions for the limiting stress and activation energy for dislocation nucleation as a function of stress. Results from four commonly used hydrocarbon potentials are compared to show that the qualitative behavior is robust and estimate quantitative uncertainties. In all cases, the results can be described by an Eyring model with values of the zero-stress activation energy Ea 0≈1.5 eV and activation volume V* ≈ 45 Å3 that are consistent with the experimental results for increasingly crystalline materials. The limiting yield stress is ∼8 GPa. These results suggest that activated dislocation nucleation at chain ends is an important mechanism for creep in highly oriented PE fibers.