Activation Energy For Nucleation Research Articles

Crystallization kinetics of Al2O3-26mol%Y2O3 glass and full crystallized transparent Y3Al5O12-based nanoceramic

Transparent Y3Al5O12-based polycrystalline ceramics with excessive Al2O3 composition (Al2O3-26 mol%Y2O3, AY26) have been demonstrated as potential host materials for phosphor applications. However, the crystallization mechanism of AY26 glass has not been thoroughly investigated so far. In this work, the non-isothermal crystallization kinetics of AY26 glass was experimentally analyzed. The AY26 glass depicts high activation energy and crystallization mechanism of volume nucleation followed by three-dimensional crystal growth mode. Based on the analysis, a novel highly transparent YAG-Al2O3 nanoceramic material was elaborated at lower temperature of 963 ℃ by using pressureless glass crystallization. The biphase nanoceramic is characterized by the structure of YAG nanocrystals surrounded with homogeneous thin Al2O3 layers. It is extremely transparent from visible to mid-infrared region, particularly the transmittance can reach the theoretical limit of YAG transparent ceramic in NIR and MIR regions. Besides, it has almost same hardness of 21 GPa with YAG single crystal and YAG transparent ceramic.

Optimization of crystallization behavior and soft magnetic properties of the Si-free Fe-B-P-C-Cu nanocrystalline alloys by Cu content tuning

The effect of Cu content on amorphous forming ability (AFA), crystallization behavior and soft magnetic properties of the new Si-free Fe84-xB9P3C4Cux (x = 0.7, 0.9, 1.1, 1.3 and 1.5) alloys were systematically investigated. The phase structure identified by XRD reveals that the alloys for Cu content less than 1.3 at. % have good AFA. The dependence of activation energies of nucleation and growth on Cu content shows that the growth of α-Fe grains is more difficult than the nucleation for the alloy ribbons with Cu ≤ 1.1 at. %, which is beneficial to obtain finer nanocrystalline α-Fe grains during crystallization annealing, while for those with Cu > 1.1 at. %, the growth of α-Fe grains is easier than the nucleation. The relationship between coercivity (Hc) and the annealing temperature (Ta) indicates that appropriate Cu content is beneficial to improve the stability of soft magnetic properties, especially, when annealed near the temperature of the first crystallization peak. The nanocrystalline Fe82.9B9P3C4Cu1.1 alloy exhibits a high saturation magnetic flux density of 1.84 T ~ 1.85 T and a low Hc of 13.0 A/m ~ 8.9 A/m annealed in the range of 703 K ~ 763 K.

Seed-assisted synthesis of ZSM-48 zeolite with low SiO2/Al2O3 ratio for n-hexadecane hydroisomerization

Synthesis of highly-crystallized ZSM-48 zeolite with low SiO2/Al2O3 ratio was successfully performed via seed-assisted route. The influence of various factors on the seed-assisted synthesis was investigated systematically, including the seeds dosages, different seeds species, adding order of seeds, hydrothermal temperature and time, silica and aluminum sources, and gel compositions. The crystallinity, morphology, textural and acidic properties of ZSM-48 were characterized by various techniques, such as XRD, SEM, FTIR, Py-IR, N2 physisorption, 29Si and 27Al MAS NMR. Aluminum sources posed a critical influence on the crystallinity with NaAlO2 more beneficial to obtain well-crystallized ZSM-48, whereas the crystallinity seemed slightly susceptible to silica sources. SEM results revealed that the crystal size was significantly reduced from ca. 20 μm to around 2 μm after introduced the seeds. Kinetic analysis suggested that the added seeds can significantly reduce the apparent activation energy of nucleation and crystallization, encourage the formation of crystal nuclei and promote crystallization. The Pt-supported seed-assisted samples exhibited considerably higher density of acid sites than that loaded on seed-free sample. The hydroisomerization of n-hexadecane was investigated over various Pt-supported catalysts, and the Pt-supported seed-assisted samples exhibited the highest i-C16 yield of ca. 75 wt% at conversion over 87 wt%.

Room- and elevated-temperature nano-scaled mechanical properties of low-density Ti-based medium entropy alloy

The incipient plasticity and creep behavior of body-centered cubic (BCC) Ti65(AlCrNb)35 medium entropy alloy (MEA) were investigated via nanoindentation. The elastic modulus and hardness obtained from different methods were compared. The activation volume for plastic deformation was calculated to be 11 b3, indicating a larger-scaled Peierls deformation mechanism typical for BCC metals. The orientation effect on incipient plasticity was studied on (100), (110) and (111) orientations at room temperature. The orientation difference is attributed to the different Schmid factors for different planes. The activation volume and activation energy of dislocation nucleation on Ti65(AlCrNb)35 was estimated as 0.8–0.9 b3 and 0.43 eV, which suggests that the mechanism of incipient plasticity is heterogeneous dislocation nucleation assisted by pre-exist defects, with a very localized activated volume. Also, the activation volume and energy of Ti65(AlCrNb)35 are higher than those of conventional metals (Ni, Pt, Cr and Mg) and lower than those of high entropy alloys (HEAs), which suggests that the dislocation nucleation in Ti65(AlCrNb)35 is more difficult than that in metals but easier than that in HEAs. Creep responses of the (100), (110) and (111) grains at 500–600 °C were found to be all dominated by the power-law dislocation creep mechanism (stress exponent n ~ 5) with the activation energy ~130 kJ/mol. The high stress applied by the indenter tip led to the stress-induced pipe diffusion, assisting the dislocation climb and lowering the activation energy. The activation volume was measured as 3–6 b3, related to dislocation climb and atom diffusion at elevated temperatures. The creep displacements and creep rates for the (100), (110) and (111) planes are quite similar, probably resulting from (i) complex slip systems in BCC structure underneath the indenter tip, (ii) a complex strain field produced by concentrated solute atoms, and (iii) the stress-induced pipe diffusion.

Isothermal crystallization kinetics of (Cu60Zr25Ti15)99.3Nb0.7 bulk metallic glass

This paper reports the crystallization kinetics of (Cu60Zr25Ti15)99.3Nb0.7 bulk metallic glass under isothermal conditions. Differential scanning calorimetry (DSC) has been employed for isothermal annealing at ten different temperatures prior to the onset of crystallization (To) temperature. X-ray diffraction and transmission electron microscopy have been used to confirm the amorphous structure of the as cast sample. Crystallized volume fractions (x) are calculated from the exothermic peaks of DSC scans. Crystallized volume fractions (x) against time show sigmoidal type of curves as well as the curves become steeper at higher annealing temperatures. Continuous heating transformation diagram has been simulated to understand the stability of the bulk metallic glass. Crystallization kinetics parameters are calculated using Arrhenius and Johnson–Mehl–Avrami equations. Activation energy (Ea) and Avrami exponential factor (n) have exhibited strong correlation with crystallized volume fraction (x). The average activation energy for isothermal crystallization is found to be 330 ± 30 kJ/mol by Arrhenius equation. Nucleation activation energy (Enucleation) is found to be higher than that of growth activation energy (Egrowth). The Avrami exponential factor (n) indicates about the diffusion controlled mechanism of the nucleation and three-dimensional growth.

An atomistic gateway into capturing twin nucleation in crystal plasticity

ABSTRACT Capturing twin nucleation in crystal plasticity is a long-standing problem due to its localisation and site sensitivity. Quantifying local energy to induce a stable twin is a unique challenge and profoundly informs the microstructural evolution. We performed nudged elastic band atomistic calculations identifying minimum energy path and activation energy for twin nucleation. This enables gauging the transformations from an initial state without twins to a final state with twins under various boundary conditions. The role of stress and atomic structure in twinning can be understood by the minimum energy path, energy barrier and relaxed energy.

Simulation of recrystallization and martensite revision in 304L austenitic stainless steel after multi-pass rolling processes

In this work, non-isothermal annealing of cold-rolled 304L austenitic stainless steel was studied at temperatures ranging between 400 and 800 °C. In the first place, a dual-phase structure containing work-hardened austenite and strain-induced martensite was produced by means of multi-pass plate rolling, and then, the deformed steel was subjected to annealing heat treatments. To determine the distribution of stored strain energy in the rolling operations, an elastic–plastic finite-element modeling was conducted. Afterwards, an artificial neural network technique was coupled with the thermal-cellular automata model for prediction of martensite reversion, temperature history, and recrystallization progress during subsequent non-isothermal heat treatments. Experimental methods including X-ray diffraction, optical metallography, and Feritscope measurements were carried out to assess the material constants as well as to verify the predictions. Comparison between the predicted results and experimental data showed a good agreement indicating the capability of the model under practical conditions. It was found that that martensite revision started at about 550 °C, while the onset of recrystallization occurred around 670 °C. The activation energies for nucleation and growth during static recrystallization were determined as 180 kJ/mole and 240 kJ/mole, respectively. In addition, the homogenous nucleation mechanism was found to be operative in the rolled steel subjected to total reduction of 35% or higher that resulted in a uniform fine-grained structure.

Solvent-free synthesis of MFI-type zeolites and their degradation properties of gas-phase styrene

In this study, a series of MFI-type zeolites with various compositions were synthesized by a solvent-free synthesis method at a comparatively mild condition. The results of investigations showed that the as-synthesized samples displayed a good crystallinity grade and regular morphology of layered structure. The crystallization process of TS-1 was systematic investigated. It was revealed that the transition stage of crystallization process was the formation of nucleation intermediates (Na2SiF6), which was the key factor for dropping Ti atoms into MFI framework. Meanwhile, it showed a high apparent nucleation activation energy (64.8 kJ mol−1), and low growth activation energy (25.3 kJ mol−1) in a spontaneous nucleation system, and the nucleation activation energy could be reduced to 17.2 kJ mol−1 in non-spontaneous nucleation system. The as-synthesized zeolites were evaluated for their catalytic activity for the degradation of gas-phase styrene. The titanium silicalite-1 sample doped with iron exhibited excellent catalytic activity with 100% degradation of styrene. The method employed in this work not only decreases the production cost and energy-consumption of MFI-type zeolites, but also significantly enhances their production yield. Moreover, it is an efficient pathway to solve the pollution of volatile organic compounds using solid waste.

Crystallization kinetics of Cu60Zr25Ti15 and (Cu60Zr25Ti15)95Ni5 bulk metallic glasses by differential scanning calorimetry (DSC)

Amorphous structures of the as-cast bulk metallic glass samples (BMGs) of Cu60Zr25Ti15 and (Cu60Zr25Ti15)95Ni5 alloys are confirmed by X-ray diffraction. Isothermal crystallization kinetics of these BMGs are investigated using differential scanning calorimetry. For both the alloys, the apparent activation energy (Ea) shows a strong dependency on crystallization volume fraction (x). Isothermal crystallization kinetic parameters are calculated by Arrhenius and Johnson–Mehl–Avrami equations. Avrami exponential factor (n) is also found to show clear correlation with the crystallization volume fraction (x). It is found that the crystallization process is easier in Cu60Zr25Ti15 BMG than in (Cu60Zr25Ti15)95Ni5 BMG as the Ea decreases constantly for the former alloy though in the case of second alloy, the Ea is almost unchanged up to x = 60%. For both the alloys, nucleation activation energy (Enucleation) is found to be higher than that of growth activation energy (Egrowth). This indicates that the growth of crystals during isothermal crystallization in these glasses is easier than nucleation, and the minor addition of Ni causes the higher thermal stability against crystallization in isothermal condition.

The role of grain-boundary cementite in bainite formation in high-carbon steels

Low temperature bainite formation in high-carbon steels leads to highly refined microstructures. However, the rate of bainite formation becomes impractically low as the bainite formation temperature decreases. Thus, it is important to create strategies to accelerate the low-temperature bainite formation kinetics in high-carbon steels. In this work, it is observed that decorating the parent austenite grain boundaries with grain-boundary cementite prior to bainite formation leads to an appreciable acceleration of subsequent bainite formation kinetics due to decrease in activation energy for bainite nucleation at cemenite/austenite interfaces.

Molecular weight dependence of the growth rate of spherulite of cyclic poly(ε-caprolactone) polymerized by ring expansion reaction

To elucidate the molecular weight (Mw) dependence of the growth rate of spherulite (G) of cyclic polymers polymerized by the ring expansion reaction, we studied the G of cyclic poly(ε-caprolactone) (C-PCL) growing from the melt as a function of the crystallization temperature (Tc) and degree of supercooling (ΔT) using polarizing optical microscopy. We prepared C-PCLs comprising an exclusively repeating unit, excluding other structural units with Mw = 17000, 33000, 41000, 74000 and 85000 and linear PCLs (L-PCL) with Mw = 16000, 35000 and 60000, where Mw was determined by GPC using linear polystyrene standards.G obeyed the equation, G = G0exp(−B/TΔT), for all the samples. The activation energies of secondary nucleation (B) of C-PCL were almost identical except for the C-PCL with the lowest Mw. Since B is proportional to the end surface free energy of the secondary nucleus, it implies that the regularity of the folding surface of C-PCL with high Mw is independent of Mw.Compared to C-PCL and L-PCL having almost the same Mw, G of L-PCL is higher than that of C-PCL at the same TΔT. This suggests that G of C-PCL and L-PCL is not solely controlled by the entanglement because the entanglement of C-PCL is lower than that of L-PCL. We found the following associations: G0∝Mw−1.2 for C-PCL and G0∝Mw−0.67 for L-PCL. The difference in the power of the G0 dependence on Mw reflects the difficulty of performing sliding diffusion in the secondary nucleus. This strongly supports the hypothesis that the absorption behavior of C-PCL chains in the secondary nucleus on the growth front is significantly different from that of L-PCL.

Simulation of bicrystal deformation including grain boundary effects: Atomistic computations and crystal plasticity finite element analysis

Grain boundaries play a pivotal role in dictating the deformation behavior of crystalline materials. Modeling their effects within the framework of physically based crystal plasticity approaches demands a multiscale description of the underlying phenomena. This work puts-forth a two-scale atomistic to crystal plasticity approach for determining the deformation behavior of a ductile face-centered cubic material. The approach uses atomistic computations to quantify the activation energies for nucleation of partial dislocations from a grain boundary under tensile loading. To this end, embedded-atom method based atomistic simulations involving nudged elastic band method are utilized to compute stress-dependent activation parameters of the grain boundary. The extracted parameters are then used as input to the flow rule of crystal plasticity at higher length scale, which is based on transition state theory. At this scale, the grain boundaries are explicitly accounted for by assigning a finite thickness and are differentiated from their bulk counterparts by prescribing distinct flow parameters extracted from atomistic simulations. The predictive capabilities of the proposed methodology are then assessed by performing numerical simulations on uniaxial tensile behavior of Ni bicrystal and validating them from the published experiments and computations available in the literature. This research paradigm can be pushed forward by incorporating more complex grain boundary behaviors at higher length scales while preserving the richness provided by atomic scales.

Improved magnetic performance of Cobalt-based ribbons by nanocrystallization through magnetic annealing

Phase transformation driven soft magnetic properties have been correlated through different stages of nano-crystallization of Co-based amorphous alloys, via transverse magnetic annealing, by combining structural, magnetothermal, domain imaging, and AC/DC magnetometry techniques. The nano-crystallization starts by nucleation and growth process of soft magnetic meta-stable, thermodynamically favored, Co23B6 phase with less nucleation activation energy compared to other stable phases. In the second crystallisation stage, Co2B and Co3B, as a semi-hard magnetic phase, are identified in the alloys, magnetically annealed at 525 and 550 °C, respectively. Field-induced anisotropy dominates over the residual contributions of magneto-crystalline anisotropy of the phases, precipitated after field annealing at 510, 515, and 525 °C. The anomalous loss is significantly reduced as by annealing in a transverse magnetic field due to the reorientation of the preferred magnetisation axis, and consequently, change in dominant magnetization reversal mechanism from domain wall motion to magnetization rotation. In addition, magnetic annealing causes a measurable decrease in the domain width, which, in turn, promotes pinning and inhibits domain wall motion, thus further favours coherent domain rotation as the primary mechanism of magnetization. The combined mechanism of nanocrystallisation and coherent magnetisation rotation accounts for a 70% decrement in the anomalous loss in the so-processed ribbons at 525 °C, which renders them attractive for applications in mid- and high-frequency power supplies and inverters.

Microstructural Changes During Static Recrystallization of Austenitic Stainless Steel 304L: Cellular Automata Simulation

Static recrystallization and microstructural changes in austenitic stainless steel 304L were studied. The rolling experiments at 200 °C were carried out, and then, annealing treatment was made at temperatures ranging between 500 and 830 °C. A model was also developed to simulate the kinetics of non-isothermal recrystallization within the rolled steel. The distribution of plastic strains during rolling was predicted utilizing an elastic–plastic finite element formulation performed in ABAQUS/Explicit, while the predicted results were used to generate the as-rolled microstructure and to estimate the stored energy. Finally, microstructural–thermal model based on cellular automata was developed to evaluate the rate of static recrystallization within the rolled steel. The comparison between experimental and simulations showed a good consistency. The predictions illustrated that inhomogeneous distribution of plastic strain was produced during multi-pass rolling leading to different rates of recrystallization in the center and the surface regions of the rolled plate. The onset temperature of recrystallization was found about 700 °C, and the activation energies for nucleation and growth for recrystallization were determined as 180 kJ/mol and 240 kJ/mol, respectively. It was found that homogenous nucleation mechanism can be operative in recrystallization of multi-pass rolled steel, i.e., for reduction of 40% or higher.

Synthesis of hierarchical zeolite T nanocrystals with the assistance of zeolite seed solution

Hierarchical zeolite T aggregates with the size range of 200–400 ​nm were successfully synthesized by zeolite seed solution-assisted method. The addition of zeolite seed solution can provide active precursors such as secondary building units and organic template to promote crystallization rate and enhance the purity and crystallinity. Compared to zeolite T samples synthesized under the similar synthesis conditions in the previous literatures, zeolite crystals obtained in the presence of zeolite seed solution have 1/10–1/5 crystal size and 1/5–1/2 synthesis time. The synthesized samples typically display the elliptic-like zeolite T aggregates composed of tightly packed nanosized fiber-like shaped crystals, resulting in the additional intercrystalline mesopores of samples between these nanocrystals. Crystal growth of zeolite samples synthesized using zeolite seed solution was investigated at 353–393 ​K. Both of nucleation and crystal growth rate increase when the crystallization temperature increase from 353 ​K to 393 ​K, and the nucleation is rate-limiting step since the value of nucleation rate is less than that of crystal growth rate at each temperature. Moreover, the magnitudes of nucleation and crystal growth activation energies obtained from crystallization data are 35.0 and 28.9 ​kJ ​mol−1, respectively, implying that crystal growth process is controlled by surface kinetics.

Генерация неравновесных зародышей намагниченности в быстро нарастающем магнитном поле в синтетических ферримагнетиках Pt/Co/Ir/Co/Pt

In a Pt/Co/Ir/Co/Pt synthetic ferrimagnet with perpendicular magnetic anisotropy, stochastically occurring peak of magnetic moment was detected in the rapidly increasing magnetic field of 16 kOe / s sweep rate at T = 300 K. The peak arises at the coinciding switching fields between three equilibrium magnetization states. The reason of the peak is nucleation of the nonequilibrium magnetic domains in the vicinity of structural defects of the Co film, where diminished activation energy of magnetic nucleation takes place.

N-Methyl-2-pyrrolidone-promoted crystallization of MEL zeolite and its acceleration mechanism

NMP can break the hydration layer around TBA+ and aluminosilicate and reduce the apparent nucleation activation energy, thus accelerating nucleation.

Primary Nucleation of Benzoic Acid in Aqueous Ethanol Solution

The knowledge of crystallization kinetics, especially crystal nucleation, is crucial for crystallization process design and control. In the present paper, the primary nucleation kinetics of benzoic acid in water-ethanol solution were estimated from induction time measurements, where focused beam reflectance measurement (FBRM) was utilized to detect the onset point of nucleation. As a result, the induction times have been found to decrease with increasing mass fraction of water under the same supersaturation. These data were then used to determine the interfacial energy together with critical size and activation energy for nucleation based on classical nucleation theory. The resulting interfacial energy differed from 1.83 to 6.46 mJ/m2 at various solvent composition. Also, it has been demonstrated that the interfacial energy increased with increasing water mass fraction, which finally caused the increase of the critical size and activation energy for nucleation, suggesting significant effects of water mass...

Crystallization and hydrolytic degradation behaviors of poly(l-lactide) induced by carbon nanofibers with different surface modifications

Interfacial interaction exhibits great role in determining the microstructure evolution and performance of the polymer composites. In most cases, it is expected to enhance the degree of the interfacial interaction as much as possible. In this work, carbon nanofibers (CNFs) were modified through two different methods, i.e. carboxyl functionalization in compounded H2SO4/HNO3 (3:1, vol/vol) to obtain the f-CNFs and then further reaction with poly (ethylene glycol) (PEG) to obtain the PEG-grafted CNFs (g-CNFs). The results showed that the grafting ratio of functional groups on f-CNFs was about 13.09 wt%, while after being grafted by PEG, the grafting ratio was further enhanced up to 50.95 wt%. Largely enhanced dispersion stability in aqueous was achieved for f-CNFs and g-CNFs. Surface modification also improved the dispersion states of CNFs in the poly (l-lactic acid) (PLLA) composites, and stronger interfacial adhesion was achieved in the PLLA/f-CNF and PLLA/g-CNF composites. Studying on crystallization behaviors of the composites showed that raw CNFs (r-CNFs) and f-CNFs exhibited good nucleation effects on PLLA crystallization with nucleation activation energies of −12.49 and −11.81 kJ/mol, respectively, while the nucleation effect of the g-CNFs was very weak and the nucleation activation energy was −5.45 kJ/mol. However, compared with the PLLA/f-CNF composite, the PLLA/g-CNF composite exhibited largely enhanced growth rate of spherulites during the isothermal crystallization process. Researches about the hydrolytic degradation behaviors of the PLLA composites showed that incorporating f-CNFs accelerated the hydrolytic degradation while it was greatly suppressed by g-CNFs. For example, the PLLA/g–CNF–2 sample showed much lower hydrolytic degradation rate (0.0134%/h) compared with the PLLA/r–CNF–2 (0.0521%/h) and PLLA/f–CNF–2 (0.086%/h) samples. This work confirms that appropriate interfacial interaction is more favorable for the realization of the nucleation effect of CNFs and the acceleration of the hydrolytic degradation of the PLLA matrix, while strong interfacial interaction achieved through grafting polymer chains not only suppresses the nucleation effect but also enhances the hydrolytic degradation resistance of the PLLA matrix.

Scalable synthesis of multilayer h-BN on AlN by metalorganic vapor phase epitaxy: nucleation and growth mechanism

We studied the nucleation and growth of hexagonal BN (h-BN) on AlN template on c-plane sapphire by metalorganic vapor phase epitaxy as functions of growth temperature, deposition time, and triethylboron (TEB) partial pressure. A lateral growth rate of about 25 nm min−1 for h-BN nuclei was obtained by atomic force microscopy and a nucleation activation energy of 2.1 eV was extracted from the temperature dependence of the nucleation density. A large TEB flow rate strongly enhances the formation of h-BN nuclei. At a reduced TEB flow rate, we observed a significantly decreased nuclei density and a delay in nucleation due to TEB desorption. By fine tuning the growth parameters, single-crystalline multilayer h-BN was successfully formed on AlN surface, as confirmed by x-ray diffraction and transmission electron microscopy (TEM). The epitaxial relationship between h-BN and AlN was [0 0 0 1]h-BN || [0 0 0 1]AlN and [1 0 −1 0]h-BN || [1 1 −2 0]AlN from TEM and electron backscatter diffraction measurements. In addition, TEM showed that the initial h-BN layers are not parallel and tend to form half-domes. On those half-domes (cap-shaped-like) a 2D lateral growth sets on, resulting in a well-oriented 2D multilayer observed in TEM. Thus, the surface topography further develops to form a relatively flat surface without wrinkles and finally a typical hexagon-like wrinkled surface at thicker h-BN layers. Particularly, the small h-BN nuclei have dangling bonds at their periphery that can interact with the substrate, forming actual bonds with AlN. Hence the choice on the substrate is important, despite the basal planes of multilayer h-BN are bonded by a weak van der Waals force.