Interfacial Nucleation Research Articles

The alloying processes in solid–solid and liquid–solid Li–Pb interfaces with atomistic simulations

The alloying processes of solid–solid (at 400K) and liquid–solid (at 500K) Li–Pb interfaces are investigated by molecular dynamics simulations with embedded-atom method (EAM) potentials. As a comparison, a critical Li–Pb interface at 450K near the melting point of Li is also studied. Three-stage feature, including the interface disordering, nucleation and growth of an intermetallic phase (B2-LiPb), has been clearly observed in these three cases. It is found that the alloying products are the same, however, the nucleation of the intermetallic phase at the solid–solid Li–Pb interface is earlier than that in the other two cases. The block Li in the solid–solid interface sample keeps the body-centered cubic structure during the course of simulation, while in the critical Li–Pb interface sample, it collapses from the excessive lattice distortion. As simulation time increasing, the interface widths increase gradually with decreasing growth rates, and the growth rate is the largest for the liquid–solid interface and the smallest for the solid–solid interface. The interface width of the solid–solid Li–Pb interface saturates at about 0.3ns for the diffusion barrier—B2-LiPb almost traversed across the entire interface plane at that time.

Precipitation Behavior of Cu-1.9Be-0.3Ni-0.15Co Alloy During Aging

In this paper, the evolutions of microstructure and mechanical properties of Cu-1.9Be-0.3Ni-0.15Co alloy were studied. The alloys in the condition of the solution treated (soft state) and 37% cold rolled (hard state) were aged at 320 °C for different time, respectively. The mechanical properties, electrical conductivity and microstructure of the alloy aged for different time were analyzed. Additionally, the precipitation kinetics of Cu-1.9Be-0.3Ni-0.15Co alloys was investigated. X-ray diffraction and transmission electron microscopy results reveal that both continuous precipitation and discontinuous precipitation existed in the hard-state Cu-1.9Be-0.3Ni-0.15Co alloy during the whole aging process; the sequence of continuous precipitation is G.P. zone → γ″ → γ′ → γ. Furthermore, the precipitation transformation mechanism of soft-state alloy is homogeneous nucleation, while hard-state alloy shows the heterogeneous nucleation (interface nucleation) with the nucleation rate of both states decaying rapidly to zero during aging at 320 °C.

Controlling the mechanism of phase transformation of colloidal In2O3 nanocrystals.

Controlling the crystal structure of transparent metal oxides is essential for tailoring the properties of these polymorphic materials to specific applications. The structural control is usually done via solid state phase transformation at high temperature or pressure. Here, we report the kinetic study of in situ phase transformation of In2O3 nanocrystals from metastable rhombohedral phase to stable cubic phase during their colloidal synthesis. By examining the phase content as a function of time using the model fitting approach, we identified two distinct coexisting mechanisms, surface and interface nucleation. It is shown that the mechanism of phase transformation can be controlled systematically through modulation of temperature and precursor to solvent ratio. The increase in both of these parameters leads to gradual change from surface to interface nucleation, which is associated with the increased probability of nanocrystal contact formation in the solution phase. The activation energy for surface nucleation is found to be 144 ± 30 kJ/mol, very similar to that for interface nucleation. Despite the comparable activation energy, interface nucleation dominates at higher temperatures due to increased nanocrystal interactions. The results of this work demonstrate enhanced control over polymorphic nanocrystal systems and contribute to further understanding of the kinetic processes at the nanoscale, including nucleation, crystallization, and biomineralization.

Symmetry breaking polymerization: one-pot synthesis of plasmonic hybrid Janus nanoparticles.

Asymmetric hybrid nanoparticles have many important applications in catalysis, nanomotion, sensing, and diagnosis, however ways to generate the asymmetric hybrid nanoparticles are quite limited and inefficient. Most current methods rely on interfacial adhesion and modification of already formed particles. In this article we report a one-pot, facile and scalable synthesis of anisotropic Au-polymer hybrid nanoparticles via interfacial oxidative dispersion polymerization. The interfacial nucleation and polymerization lead to spontaneous symmetry breaking and formation of the Janus particles. The reaction is initiated by monomer radicals generated by the strong oxidant HAuCl4, which is itself later reduced by the electron-rich monomers to self-nucleate and form Au nanoparticles (NPs). The competition between divinylbenzene adsorption and the PVP capping agent results in effective partial surface wetting, forming asymmetric Au-PDVB hybrid nanoparticles, by confining growth of each material to its own phase. Such spontaneous symmetry breaking, important in morphogenesis, with control over the subsequent growth processes should lead to significant advances in the synthesis of asymmetric nanostructures.

Effect of interface nucleation time of the GaN nucleation layer on the crystal quality of GaN film

In this paper, the influences of the growth time of low-temperature (LT) GaN nucleation layer on the crystal quality and optical properties of GaN film are investigated. It is found that the optimal LT nucleation layer growth time can effectively reduce the crystal defects and is favorable to forming the annihilation of dislocations. GaN films are grown on c-plane sapphire substrates by metal-organic chemical vapor deposition. Crystal quality and optical properties are characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), high-resolution X-ray diffraction (HRXRD), and photoluminescence spectra, respectively. In the AFM images, the island density decreases as growth time increases, while the size of island becomes larger and the uniformity of island size deteriorates as growth time increases, leading to the phenomenon that the number of interfaces formed during the nucleation island coalescence, first decrease and then increase as detected by SEM, which also induces the screw dislocation density and edge dislocation density to first decrease and then increase as measured by HRXRD. This first-decrease-and-then-increase variation trend is consistent with the first-increase-and-then-decrease variation trend of the ratio of the band edge emission peak intensity to the yellow luminescence peak intensity tested by photoluminescence, which is confirmed by HRXRD. It is shown that GaN islands with different sizes and densities could lead to different mechanisms of dislocation evolution, thereby forming GaN epitaxial layers with different dislocation densities and optical properties. Through controlling the nucleation time, GaN films with the smallest dislocation density could be obtained.

Modeling Precipitation Kinetics During Heat Treatment with Calphad-Based Tools

Sophisticated precipitation reaction models combined with well-developed CALPHAD databases provide an efficient way to tailor precipitate microstructures that maximize strengthening via the optimization of alloy chemistries and heat treatment schedules. The success of the CALPHAD approach relies on the capability to provide fundamental phase equilibrium and phase transformation information in materials of industrial relevance taking into consideration composition and temperature variation. The newly developed TC-PRISMA program is described. The effect of growth modes, alloy chemistries, and cooling profiles on the formation of multimodal microstructures has been examined in order to understand the underlying thermodynamics and kinetics. Practical issues that are critical to the accuracy and applicability of the current simulations, such as modifications that overcome mean field approximations, compatibility between CALPHAD databases, and selections of key parameters (particularly interfacial energy and nucleation site densities), are also addressed.

Unique interfacial and confined porous morphology of PLA/PS blends in supercritical carbon dioxide

In this study, a typical immiscible system, a poly(lactic acid) (PLA)/polystyrene (PS) bioblend is used to investigate the effect of interface and phase structure on bubble nucleation and porous morphologies using supercritical carbon dioxide as a physical foaming agent. A unique microcellular skin-core structure with a porous core and surface, and a nonporous skin layer which is embedded in a solid PLA phase are observed. The involved possible mechanism of the interfacial nucleation and confined foaming behavior in multi-phase systems has been discussed. Because of the higher gas concentration and lower activation energy barrier, bubble nucleation preferentially occurs at the interface of the PS and PLA phases. Due to the constrained effect of the crystalline PLA phase, smaller space could be provided for the expansion of the PS phase during the foaming process, hence gas bubbles in the interior are restrained from further growth. The porous structures of the PS phases are similar when the blends have comparable phase morphology. The results indicated that the confinement effect on the foam behavior is not only related to the crystalline PLA phase but also the phase structure of the blend.

Understanding the true shape of Au-catalyzed GaAs nanowires.

With increasing interest in nanowire-based devices, a thorough understanding of the nanowire shape is required to gain tight control of the quality of nanowire heterostructures and improve the performance of related devices. We present a systematic study of the sidewalls of Au-catalyzed GaAs nanowires by investigating the faceting process from the beginning with vapor-liquid-solid (VLS) nucleation, followed by the simultaneous radial growth on the sidewalls, and to the end with sidewall transformation during annealing. The VLS nucleation interface of our GaAs nanowires is revealed by examining cross sections of the nanowire, where the nanowire exhibits a Reuleaux triangular shape with three curved surfaces along {112}A. These curved surfaces are not thermodynamically stable and adopt {112}A facets during radial growth. We observe clear differences in radial growth rate between the ⟨112⟩A and ⟨112⟩B directions with {112}B facets forming due to the slower radial growth rate along ⟨112⟩B directions. These sidewalls transform to {110} facets after high temperature (>500 °C) annealing. A nucleation model is proposed to explain the origin of the Reuleaux triangular shape of the nanowires, and the sidewall evolution is explained by surface kinetic and thermodynamic limitations.

Hierarchically porous titania networks with tunable anatase:rutile ratios and their enhanced photocatalytic activities.

Mixed-phase hierarchically porous titania networks (PTNs) with 3D interconnected porous frameworks and tunable rutile contents have been synthesized via a facile sol-gel templating and calcination process. The products were characterized using scanning electron microscopy, powder X-ray diffraction, and nitrogen gas sorption analysis, and their photocatalytic activities were evaluated by measuring the photocatalytic degradation of methylene blue, a typical effluent from the textile industry, under UV light illumination. The hierarchically macro-/mesoporous titania structure formed after templating followed by calcination in air. The reduced interfaces between titania nanocrystals in these PTN materials can significantly decrease interface nucleation of the rutile phase and effectively retard the anatase to rutile phase transformation, therefore giving rise to porous titania photocatalysts featuring tunable rutile ratios (from 0 to 100 wt %), reduced crystal sizes, hierarchically porous structure, and relatively high specific surface areas (up to 71.0 m(2) g(-1)). The photocatalytic performance of the materials was correlated to the anatase:rutile ratio and specific surface area of the materials, with the mixed-phase (rutile content of 15.4%) nanocrystalline titania calcined at 600 °C for 6 h showing the highest photocatalytic activity. This study demonstrates that a substantial improvement in photocatalytic activity of the titania can be achieved by controlling morphology and carefully tuning phase composition via a feasible solid-state phase transformation at a relatively low temperature (600 °C). This concept for the rational design and development of high-performance photocatalysts using an industrially simple process would be capable of mass production.

Effects of hyperthermia induced crystalline aggregation on properties of TiO2 thin films

TiO2 thin films were prepared on fused quartz by using the sol–gel dip coating method followed by annealing. The effects of annealing temperature on the phase transition, crystalline agglomeration, surface topography and optical properties of the TiO2 films were studied. Activation energy of the phase transition from the anatase to rutile phase was estimated about 64 kJ mol−1. Crystallite size did not vary significantly with the annealing temperature, but higher temperature led to the aggregation of nanocrystallines into large particles with wide size distribution due to the surface nucleation (SN) mode gradually replaced the interface nucleation (IN) mode. Both the optical transmittance and band gap of the TiO2 thin films decrease as the annealing temperature increase. The highest optical transmittance over the visible wavelength region was obtained by the film annealed at 800 K, which was composed entirely of anatase phase.

Melting and resolidification of gold film irradiated by laser pulses less than 100 fs

The rapid melting and resolidification of gold films irradiated by laser pulses less than 100 fs are investigated using the dual-hyperbolic two-step model. The solid–liquid interfacial velocity in the ultrafast phase change process is obtained by coupling a hyperbolic interfacial energy balance equation and nucleation dynamics. The results are compared with the experimental data for the 28-fs laser. The effects of laser pulse widths and fluences on melting process are investigated. A phase chart of the variations of pulse widths and fluences is established. The relationship between the melting threshold and ablation threshold is also presented.

First-principles study of Al/A13Ti heterogeneous nucleation interface

Abstract The interfacial adhesion, stability, wetting, and bonding nature of Al(1 1 1)/Al 3 Ti(1 1 2) and Al(0 0 1)/Al 3 Ti(0 0 1) are calculated and compared by using first-principles density functional method. Three Al(1 1 1)/Al 3 Ti(1 1 2) models with different stacking sites (top-, bridge-, and center-sites), and two center-sited Al(0 0 1)/Al 3 Ti(0 0 1) models with different terminations (Al- and Al + Ti-terminations) are investigated. With the largest work of adhesion and smallest interface energy, the center-sited Al(1 1 1)/Al 3 Ti(1 1 2) is the most stable among the five models. The epitaxial stacking style is confirmed, and its adhesion work is larger than Al/Al(1 1 1). The heterogeneous nucleating mechanism of α-Al on Al 3 Ti(1 1 2) substrate can be interpreted as the perfect wetting, and stronger adhesion. Valence electron density and partial density of states (PDOS) are also analyzed. The interfacial bonding mainly comes from Al–Al metallic and Al–Ti covalent interactions.

The combination of fluctuation-assisted crystallization and interface-assisted crystallization in a crystalline/crystalline blend of poly(ethylene oxide) and poly(ε-caprolactone)

The nucleation and crystallization of poly(ethylene oxide) (PEO) and poly(e-caprolactone) (PCL) in the PEO/PCL blends have been investigated by means of optical microscopy (OM) and differential scanning calorimetry (DSC). During the isothermal or nonisothermal crystallization process, when the adjacent PEO is in the molten state, PCL nucleation preferentially occurs at the PEO and PCL interface; after the crystallization of the adjacent PEO, much more PCL nuclei form on the surface of the PEO crystal. However, PEO crystallizes normally and no interfacial nucleation occurs in the blend. The concentration fluctuation caused by liquid–liquid phase separation (LLPS) induces the motion of PEO and PCL chains through interdiffusion and possible orientation of chain segments. The oriented PEO chain segments can assist PCL nucleation, and the heterogeneous nucleation ability of PEO increases with the orientation of PEO chains. Oriented PCL chain segments have no heterogeneous nucleation ability on PEO. It is postulated that the interfacial nucleation of PCL in the PEO/PCL blend follows the combination of “fluctuation-assisted crystallization” and “interface-assisted crystallization” mechanisms.

Two-step phase transformation of anatase to rutile in aqueous suspension

Kinetic data tracking the anatase to rutile phase transformation under acidic, hydrothermal conditions (200 °C, pH 1) are consistent with a two-step transformation mechanism. Fitting the experimental data using phase transformation models, interface–nucleation dominates the early stages of transformation and dissolution–precipitation dominates at later stages. During the first stage, the rate of transformation by interface–nucleation is consistent with a substantially higher number of nucleation sites, possibly generated via crystal growth by oriented aggregation. Characterization of the hydrothermally treated samples using HRTEM revealed that anatase crystal morphologies indicative of crystal growth by oriented aggregation are common. Furthermore, twinned rutile crystals, which are consistent with transformation of anatase twinned across the {112} to rutile twins by interface–nucleation, were also observed. At later stages, the kinetics of the anatase to rutile transformation are consistent with dissolution–precipitation, and the number concentration of rutile crystallites reaches a plateau. These results are consistent with a decrease in crystal growth by oriented aggregation as the anatase particle size increases and the dissolution of anatase and reprecipitation onto already existing rutile crystals due to the larger bulk energy and comparatively small size of anatase.

Effects of lithium perchlorate on the nucleation and crystallization of poly(ethylene oxide) and poly(ε-caprolactone) in the poly(ethylene oxide)–poly(ε-caprolactone)–lithium perchlorate ternary blend

The effects of lithium perchlorate (LiClO4) on the nucleation and crystallization of poly(ethylene oxide) (PEO) and poly(ε-caprolactone) (PCL) in the PEO–PCL–LiClO4 ternary blend were studied by using a polarizing optical microscope (POM), differential scanning calorimetry (DSC) and Fourier transformation infrared spectroscopy (FTIR). LiClO4 has a heterogeneous nucleation effect on PEO in the blend, irrespective of the presence of PCL. The coordination interaction between PEO and LiClO4 decreases with the increase of the PCL content, so the heterogeneous nucleation ability of LiClO4 on PEO decreases with the increase of the PCL content in the ternary blends. LiClO4 has no heterogeneous nucleation effect on PCL. In the ternary blend, the heterogeneous nucleation effect of PEO on PCL is remarkably promoted with the addition of LiClO4. POM observations show that the miscibility of PEO and PCL improves after the addition of LiClO4, leading to an increase in the interface volume of PEO and PCL phase domains. The increased concentration-fluctuation at the interface promotes the interfacial nucleation of PCL in the ternary blend. The combination of “fluctuation-assisted crystallization” and “interface-assisted crystallization” mechanisms of the interfacial nucleation of PCL in the PEO–PCL binary blend is also suitable for the interfacial nucleation of PCL in the PEO–PCL–LiClO4 ternary blend.

First-principles calculations on LaAlO3 as the heterogeneous nucleus of austenite

In this work, the interface atomic structure, bonding character, adhesion work and interfacial energy of LaAlO3 (100)/austenite (100) interface were studied and the effectiveness of the LaAlO3 as the heterogeneous nucleus of austenite was analyzed by first-principles calculations. The results indicate that, in LaAlO3 bulk, the charge transfer and sp2 orbital hybridization not only appear between La atom and O atom, but also between Al atom and O atom, which indicate that the chemical bond in LaAlO3 bulk is a mixed one with both covalent and ionic characteristics. For LaAlO3 (100) surface, when La chemical potential (ΔμLa) is low, AlO2 terminated structure is stable than LaO terminated one. However, when La chemical potential (ΔμLa) is high, LaO terminated structure is stable than AlO2 terminated one. For austenite (100) surface, doped C terminated structure is stable than lack C terminated one all along. In the four LaAlO3 (100)/austenite (100) interfaces, the LaO-doped C terminated structure has the largest adhesion work and the smallest interface distance, which indicate that it is the most stable. The effectiveness of LaAlO3 as the heterogeneous nucleus of austenite is related with ΔμLa and ΔμC. When ΔμLa is low, the LaAlO3 (100)/austenite (100) interface with AlO2 terminated structure meets the energy requirements as the heterogeneous nucleation interface. With the increase of ΔμLa, the LaAlO3 (100)/austenite (100) interface with AlO2 terminated structure cannot be the LaAlO3/austenite heterogeneous nucleation interface any longer, while that with LaO terminated structure, especially with LaO-doped C terminated structure, meets the energy requirements as the heterogeneous nucleation interface greatly.

Oxygen defect dependent variation of band gap, Urbach energy and luminescence property of anatase, anatase–rutile mixed phase and of rutile phases of TiO2 nanoparticles

TiO2 nanoparticles are prepared by a sol–gel method and annealed both in air and vacuum at different temperatures to obtain anatase, anatase–rutile mixed phase and rutile TiO2 nanoparticles. The phase conversion from anatase to anatase–rutile mixed phase and to rutile phase takes place via interface nucleation between adjoint anatase nanocrystallites and annealing temperature and defects take the initiate in this phase transformation. The samples are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–vis and photoluminescence spectroscopy (PL). Anatase TiO2 exhibits a defect related absorption hump in the visible region, which is otherwise absent in the air annealed samples. The Urbach energy is very high in the vacuum annealed and in the anatase–rutile mixed phase TiO2. Vacuum annealed anatase TiO2 has the lowest emission intensity, whereas an intense emission is seen in its air annealed counterpart. The oxygen vacancies in the vacuum annealed samples act as non-radiative recombination centers and quench the emission intensity. Oxygen deficient anatase TiO2 has the longest carrier lifetime. Time resolved spectroscopy measurement shows that the oxygen vacancies act as efficient trap centers of electrons and reduce the recombination time of the charge carriers.

Effect of nucleation time on bending response of ionic polymer–metal composite actuators

An autocatalytic electro-less plating of nickel is attempted to replace an electroless impregnation-reduction (IR) method in ionic polymer–metal composite (IPMC) actuators to reduce cost and processing time. Because nucleation time of Pd–Sn colloids is the determining factor of overall processing time, we used the nucleation time as our control parameter. To optimize nucleation time and investigate its effect on the performance of IPMC actuators, we analyzed the relationship between the nucleation time, interface morphology and electrical properties. The optimized nucleation time was 10h. The trends of the performance and electrical properties as a function of nucleation time were attributed to the fact that the Ni penetration depth was determined by the minimum diffusion length of either Pd–Sn colloids or reducing agent ions. The Ni-IPMC actuators can be fabricated less than 14h processing time without deteriorating performance of the actuators, which is comparable to Pt-IPMC prepared by IR method.

Formation of solid shell nanoparticles with liquid ω-3 fatty acid core

A major challenge for food and pharmaceutical industries is the engineering of nanostructures that can efficiently encapsulate bioactive compounds with enhanced physical and chemical stability, and high load. The influence of surfactant properties on the physical and chemical stability of (i) nanostructured lipid carriers (NLC) containing tristearin and ω-3 fish oil, (ii) tristearin solid lipid nanoparticles (SLN), and (iii) ω-3 fish oil-in-water emulsions was investigated. As surfactants we used low (LM)- and high-melting (HM) lecithins. Results indicated that the presence of fish oil reduced the crystallisation temperature, melting temperature, and melting enthalpy of tristearin. NLC stabilized with HM-lecithin inhibited the oxidation of ω-3 fatty acids ⩾90% compared to those stabilized with LM-lecithin. This was attributed to the solidified surfactant layer of HM-lecithin inducing crystallisation of the shell by interfacial heterogeneous nucleation. The results showed that the saturated HM-lecithin was the key in controlling the crystallisation behaviour, and thereby enabled the formation of oxidatively and physically stable lipid nanoparticles.

Formation of Large-Grain Ge(111) Films on Insulator by Gold-Induced Layer-Exchange Crystallization at Low Temperature

A formation technique of orientation-controlled large-grain (< 10 μm) Ge crystals on insulator at low temperature ({less than or equal to} 350oC) has been investigated to realize advanced flexible electronics. Previously, we proposed a gold-induced crystallization technique, and demonstrated low temperature crystallization (250oC) of Ge on insulator. In the present study, we intentionally inserted thin Al2O3 layers between gold and a-Ge layer, to control crystal nucleation. Consequently, (111)-oriented large-grain (< 20 μm) Ge crystals are obtained at 350oC by optimizing interfacial oxide layer thickness. It is speculated that this phenomena is attributed to suppression of random bulk nucleation of Ge in Au films and resultant dominance of interfacial nucleation at Ge/SiO2 interfaces.