Heterogeneous Nucleation Process Research Articles

Perfectly Ordered Patterns Formed by a Heterogeneous Nucleation Process of Block Copolymer Self-Assembly Under Rectangular Confinement.

The heterogeneous nucleation process during the phase separation of binary blends of the AB diblock and the C homopolymer induced by rectangular confinement is studied by cell dynamics simulation based on the time-dependent Ginzburg-Landau theory. The main goal is to yield large-scale ordered hexagonal patterns by tailoring the surface potentials of the sidewalls. Our study reveals a crucial condition to induce the desired heterogeneous nucleation process in which the nucleated domain grains grow and merge into a defect-free pattern. Specifically, nucleations are induced simultaneously by two parallel sidewalls with a strong surface potential, whereas the spontaneous nucleation and the heterogeneous nucleation at the other two walls with a weak surface potential are suppressed. Moreover, the confinement effect of the other two walls can ensure that the two rows of nucleated domains have correlated positions. Importantly, we find that the ordering process under the crucial condition exhibits a high tolerance to the rectangular sizes. Only a few defects in thousands of domains are occasionally caused that are observed to be annihilated in a short-annealing time via various mechanisms. This study may provide a facile route to prepare large-scale ordered patterns via a simple rectangular confinement.

Interannual variations of early winter Antarctic polar stratospheric cloud formation and nitric acid observed by CALIOP and MLS

Abstract. We use satellite-borne measurements collected over the last decade (2006–2015) from the Aura Microwave Limb Sounder (MLS) and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) to investigate the nitric acid distribution and the properties of polar stratospheric clouds (PSCs) in the early winter Antarctic vortex. Frequently, at the very start of the winter, we find that synoptic-scale depletion of HNO3 can be detected in the inner vortex before the first lidar detection of geophysically associated PSCs. The generation of "sub-visible" PSCs can be explained as arising from the development of a solid particle population with low number densities and large particle sizes. Assumed to be composed of nitric acid trihydrate (NAT), the sub-visible PSCs form at ambient temperatures well above the ice frost point, but also above the temperature at which supercooled ternary solution (STS) grows out of the background supercooled binary solution (SBS) distribution. The temperature regime of their formation, inferred from the simultaneous uptake of ambient HNO3 into NAT and their Lagrangian temperature histories, is at a depression of a few kelvin with respect to the NAT existence threshold, TNAT. Therefore, their nucleation requires a considerable supersaturation of HNO3 over NAT, and is consistent with a recently described heterogeneous nucleation process on solid foreign nuclei immersed in liquid aerosol. We make a detailed investigation of the comparative limits of detection of PSCs and the resulting sequestration of HNO3 imposed by lidar, mid-infrared, and microwave techniques. We find that the temperature history of air parcels, in addition to the local ambient temperature, is an important factor in the relative frequency of formation of liquid/solid PSCs. We conclude that the initiation of NAT nucleation and the subsequent development of large NAT particles capable of sedimentation and denitrification in the early winter do not emanate from an ice-seeding process. Finally, we investigate the patterns of interannual variability and compare the relative formation frequency of liquid and solid PSCs in the Antarctic lower polar stratosphere using the results of a cluster analysis to synthesize the combined CALIOP and MLS measurements into a relatively small number of interrelated categories.

Direct Growth of Crystalline Tungsten Oxide Nanorod Arrays by a Hydrothermal Process and Their Electrochromic Properties

Transparent crystalline tungsten oxide nanorod arrays for use as an electrochromic layer have been directly prepared on fluorine-doped tin oxide-coated glass via a facile tungsten film-assisted hydrothermal process using aqueous tungsten hexachloride solution. X-ray diffraction analysis and field-emission scanning electron microscopy were used to characterize the phase and morphology of the grown nanostructures. Arrays of tungsten oxide nanorods with diameter of ∼22 nm and length of ∼240 nm were obtained at 200°C after 8 h of hydrothermal reaction. We propose a growth mechanism for the deposition of the monoclinic tungsten oxide phase in the hydrothermal environment. The tungsten film was first oxidized to tungsten oxide to provide seed sites for crystal growth and address the poor connection between the growing tungsten oxide and substrate. Aligned tungsten oxide nanorod arrays can be grown by a W thin film-assisted heterogeneous nucleation process with NaCl as a structure-directing agent. The fabricated electrochromic device demonstrated optical modulation (coloration/bleaching) at 632.8 nm of ∼41.2% after applying a low voltage of 0.1 V for 10 s, indicating the potential of such nanorod array films for use in energy-saving smart windows.

Crystalline nucleation in undercooled liquid nickel

We investigate crystalline nucleation in undercooled liquid Ni by means of x-ray absorption temperature scans in powder samples dispersed in Al2O3. We show that the crystallization process is initially dominated by a heterogeneous nucleation process that induces a purity/size selection effect. The nucleation rate displayed by the residual mass fraction of about 30% (determined down to 364 K below the melting point) is in agreement with empirical data from previous single droplet experiments extrapolated to the deeper undercooling range presently achieved. We investigate the possibility that this ultimate nucleation process is actually homogeneous by computing the rate in the framework of the classical nucleation theory. We adopted a recently implemented kinetic Monte Carlo simulation approach using extrapolated values for the self-diffusion coefficient and empirical or simulated models for the excess free energy. The comparison between measured and calculated rates indicates that the homogeneous nucleation scenario is likely.

Misfit paradox on nucleation potency of MgO and MgAl2O4 for Al

MgO and MgAl2O4 are believed to be effective heterogeneous nuclei for Al based alloys due to their small lattice misfits with Al. However, there is a strong evidence to suggest that liquid Al reacts with MgO and MgAl2O4 phases but the heterogeneous nucleation behavior of such phases is rarely discussed. In order to identify the nucleation mechanism of Al, under the interference of the chemical reaction, the heterogeneous nucleation process is systematically investigated through thermal analysis and high resolution transmission electron microscopy (HRTEM). The observed multi-nucleation interfaces (Al/MgO, Al/MgAl2O4 and Al/Al2O3) and scattered experimental undercooling data indicate an independent multi-phase nucleation process in these systems.

Mechanical characteristics and thermal behaviours of polylactide blend films: influence of nucleating agent and poly(butylenes adipate-co-terephthalate)

Polylactide (PLA) blend films with poly(butylenes adipate-co-terephthalate) (PBAT) and a nucleating agent were prepared by the melt compounding technique. Thermal stability of the PLA decreased with added nucleating agent; in contrast, the decomposition temperature increased with the presence of PBAT. In addition, the differential scanning calorimetry thermograms demonstrated that the heterogeneous nucleation and cold crystallisation processes of the PLA blend films were accelerated.The influence of the type and level of the nucleating agent and the presence of PBAT on the tensile properties, impact resistance, thermal stability and non-isothermal crystallisation behaviours of the PLA blend films were investigated. Both the PLA/nucleating agents and the PLA/PBAT/nucleating agent blends showed significant effects from the changes in the nucleation process on their tensile properties, impact toughness and thermal behaviour. Furthermore, the impact energy that the PLA blends absorbed during the entire impact tension test was obviously enhanced by the increased content of the nucleating agent.

Heterogeneous Nucleation on Concave Rough Surfaces: Thermodynamic Analysis and Implications for Nucleation Design

Addressing heterogeneous nucleation process on materials with complex (superficial) structures is an important issue to understand the thermodynamic principles and forward the engineering applications. Based on classic nucleation theory, we developed a thermodynamic model that is comprised of both the geometry and the surface-structure effects of substrate to represent the process of forming a crystal or drop embryo on a rough concave surface. The model recovers many models developed previously. We extracted two featured factors: ratio factor that indicates the ratio of the critical radius of the embryos forming on the rough concave surface to that on the smooth surface, and shape factor that represents the relative free energy barrier of heterogeneous nucleation of the critical size to that of homogeneous nucleation. Anomalies of both the ratio factor and shape factor were observed for heterogeneous nucleation on the rough concave surface. Our study provides an essential route toward design (or manipulat...

Probing the seeded protocol for high-concentration preparation of silver nanowires

Mass production of high-quality silver nanowires (Ag NWs) is of significant importance because of its potential applications in flexible transparent conductive devices. Halogen ions have been widely used for the synthesis of Ag NWs; however, owing to the lack of a deep insight into heterogeneous nucleation processes, usually a trace feeding amount (e.g. [Cl–] < 0.25 mM) is used, which in turn lowers the concentration of precursor ([Ag+]). Here we systematically investigated the nucleation and growth behavior of Ag NWs and concluded that the number of heterogeneous nucleation sites was determined by the total surface area of AgCl seeds, which indicated a linear relationship between the concentrations of Ag+ and Cl– during precipitation. Based on this mechanism, we successfully produced high-quality Ag NWs with Ag+ concentrations which were 20 times higher for a polyol system and 5 times higher for an aqueous system as compared to that in the previously reported strategies. Besides, by tailoring the heterogeneous nucleation sites by controlling the size of the AgCl seeds, the diameters of the final Ag NWs could be well controlled even at high Ag+ concentration. Based on the mechanistic understandings, this synthetic strategy could be extended to other AgX-seeds (X = Br–, I– and SO 4 2– ) and the basic principles can be applied to help rational synthesis of other high-yield metal NWs with tunable sizes.

Phase change behavior of latent heat storage media based on calcium chloride hexahydrate composites containing strontium chloride hexahydrate and oxidation expandable graphite

Calcium chloride hexahydrate (CaCl2·6H2O) composites PCMs containing strontium chloride hexahydrate (SrCl2·6H2O) and oxidation expandable graphite (EGO) were prepared and phase change behavior of CaCl2·6H2O/EGO/SrCl2·6H2O composite PCMs, including supercooling degree, phase change temperature, latent heat, density, thermal conductivity and thermal stability were systematically studied. Results demonstrate that thermal conductivity, heat transfer and the latent capacities of the CaCl2·6H2O/SrCl2·6H2O/EGO composite PCMs are significantly enhanced, supercooling of CaCl2·6H2O/SrCl2·6H2O/EGO composite PCMs are suppressed, mainly ascribe to that the EGO are homogenously dispersed in PCMs due to the existence of oxygen-containing functional groups in EGO, meanwhile, the well-dispersed EGO serving as nuclei to promote the heterogeneous nucleation and crystallization process of CaCl2·6H2O. Moreover, a fifty-run-cycling test verifies that the CaCl2·6H2O/SrCl2·6H2O/EGO composites PCMs contained with 3wt% SrCl2·6H2O, and 1.0wt% EGO possesses enhanced thermal behavior with no phase segregation observed; the latent heat was calculated to be 172.26J/g.

Crystallization and assembling of FAU nanozeolites on porous ceramic supports for zeolite membrane synthesis

In this study, a novel route for preparing nanocrystalline porous FAU layers s stably supported on porous α-Al2O3 tubular substrates is reported. A highly reactive sodium hydroxide rich hydrogel was used to promote the nucleation process on the support surface. After 24 h of treatment at near ambient condition, a heavily nucleated quasi-solid gel layer containing nanozeolite precursor species was first formed on the support surface. This matrix was used as starting material for the synthesis of nanozeolite-based membranes. The support coated with the extensively nucleated gel phase was isolated and cured at higher temperature in different conditions. A rapid and diffuse crystallization was observed by treating the gel matrix with a synthesis solution of lower alkalinity, leading to the formation of a dense nanocrystalline FAU layer firmly adhered to the substrate surface. The FAU layers (Si/Al ratio of ca. 2) have a thickness of ca. 2 μm and are constituted by closely packed, well intergrown nanocrystals (crystallite dimension of ca. 20–30 nm). The results of this study indicate that supersaturation as well as the elevate amount of sodium ions synergistically contribute to induce the assembling of soluble aluminosilicate species on the support surface and to effectively promote the heterogeneous nucleation process.

Controlling the Temperature and Speed of the Phase Transition of VO2 Microcrystals

We investigated the control of two important parameters of vanadium dioxide (VO2) microcrystals, the phase transition temperature and speed, by varying microcrystal width. By using the reflectivity change between insulating and metallic phases, phase transition temperature is measured by optical microscopy. As the width of square cylinder-shaped microcrystals decreases from ∼70 to ∼1 μm, the phase transition temperature (67 °C for bulk) varied as much as 26.1 °C (19.7 °C) during heating (cooling). In addition, the propagation speed of phase boundary in the microcrystal, i.e., phase transition speed, is monitored at the onset of phase transition by using the high-speed resistance measurement. The phase transition speed increases from 4.6 × 10(2) to 1.7 × 10(4) μm/s as the width decreases from ∼50 to ∼2 μm. While the statistical description for a heterogeneous nucleation process explains the size dependence on phase transition temperature of VO2, the increase of effective thermal exchange process is responsible for the enhancement of phase transition speed of small VO2 microcrystals. Our findings not only enhance the understanding of VO2 intrinsic properties but also contribute to the development of innovative electronic devices.

Experimental research of heterogeneous nuclei in superheated steam

A mobile steam expansion chamber has been developed to investigate experimentally homogeneous and heterogeneous nucleation processes in steam, both in the laboratory and at power plants using the steam withdrawn from the steam turbine. The purpose of the device is to provide new insight into the physics of nonequilibrium wet steam formation, which is one of the factors limiting the efficiency and reliability of steam turbines. The expanded steam or a mixture of steam with a non-condensable gas rapidly expands in the expansion chamber. Due to adiabatic cooling, the temperature drops below the dew point of the steam at a given pressure. When reaching a sufficiently high supersaturation, droplets are nucleated. By tuning the supersaturation in the so-called nucleation pulse, particles of various size ranges can be activated. This fact is used in the present study to measure the aerosol particles present in the air. Homogeneous nucleation was negligible in this case. The experiment demonstrates the functionality of the device, data acquisition system and data evaluation methods.

Selective heterogeneous nucleation of gold nanoparticles on one-dimensional cadmium silicate for enhanced nonlinear optical responses

A facile route to uniformly assemble Au nanoparticles (NPs) on the surface of cadmium metasilicate (CdSiO3) nanowires (NWs) via a heterogeneous nucleation process was demonstrated.

Synthesis of α-MoO3nanobelts with preferred orientation and good photochromic performance

Stable phase α-MoO3 nanobelts with preferred orientation were successfully evolved on transforming from metastable phase h-MoO3via an aqueous approach. H+ ions play a key role in the nuclei formation and crystal growth processes. Variation of the Mo : H+ molar ratio led to MoO3 crystals with different polymorphs. Oriented α-MoO3 nanobelts along the [001] direction were achieved under a Mo : H+ ratio of 1 : 9, which is relative to a heterogeneous nucleation process on the surface of metastable phase h-MoO3. The α-MoO3 nanobelts along the [100] direction were obtained under a Mo : H+ ratio of 1 : 10; homogeneous nucleation dominated the synthesis without the presentation of h-MoO3 microrods. The α-MoO3 nanobelts with distinguishing crystalline growth orientations exhibited different photochromic behaviors.

Randall’s plaque and kidney stones: Recent advances and future challenges

Alexander Randall described eight decades ago a heterogeneous nucleation process at the tip of renal papillae giving birth to calcium oxalate stones. Kidney stones were for the first time described to originate from calcium phosphate plaques growing in the interstitial tissue, breaking urothelium and then promoting calcium oxalate crystals aggregation at their contact. During the next decades, few studies were dedicated to these Randall's plaques but the increasing incidence of calcium oxalate kidney stones, the development of endoscopic procedures allowing plaques visualization and series evidencing the high proportion of calcium oxalate stones generated on these plaques renewed interest in Randall's plaque. Although some progress has been carried during the past two decades, the origin of Randall' plaques, their composition, their role in kidney stones epidemic, their specific affinity for some crystalline phases, and their potential deleterious effects in kidney tissues are among the challenges that lie ahead.

Theoretical study of the auto-catalyzed hydrolysis reaction of sulfur dioxide

The hydrolysis reaction of sulfur dioxide (SO2) to form sulfurous acid involving additional sulfurous acid (H2SO3) was investigated using high-level computational methods. With H2SO3, the reaction takes place via a double proton transfer process with a cage-like structure, which is different from the planar ring structure involved in a corresponding process with an additional water molecule (served as a catalyst). Our results show that H2SO3 is a better catalyst than water, as the barrier height for the H2SO3-catalyzed reaction is only 5.5kcal/mol, compared to over 25.0 and 15.0kcal/mol for the reaction without a catalyst and the H2O-catalyzed reaction, respectively. In addition, the sulfurous acid dimer from the H2SO3-catalyzed reaction is more stable than hydrated H2SO3 from the H2O-catalyzed reaction. Considering the existence of sulfurous acid in the aqueous phase and acidic aerosols, as well as the importance of SO2 and H2O in the atmosphere, our results will have potentially significant implications on the homogeneous and heterogeneous nucleation processes.

Thermodynamics and Characteristics of Heterogeneous Nucleation on Fractal Surfaces

Clarifying heterogeneous nucleation process on fractal surfaces is an important issue to understand its thermodynamic principles and forward the engineering aspects. We developed a thermodynamic model to capture the process of forming a crystal or drop embryo on fractal surfaces and the corresponding characteristic parameters. We showed that the differences between the critical size of the embryos on the fractal surfaces and those on the flat surfaces are negligible for the hydrophobic nuclei, but become significant for the hydrophilic nuclei. We reported the available domains of forming the critical embryos and obtained the shape function for the first time. The results also recover the data reported in the literature. Finally, we present the possible mechanisms of vanishing of the energy barriers against forming the critical embryos on the fractal surfaces as the apparent contact angle is relatively small. The obtained results may help to engineer the nucleation process by designing substrate surfaces i...

Surface inorganic scale formation in oil and gas industry: As adhesion and deposition processes

Scale formation on surfaces can normally be divided into two distinct processes: a “deposition process” which refers to the process of heterogeneous nucleation and growth at the asperities of the surface and an “adhesion process” which refers to the sticking of pre-existing crystals, which have nucleated in the bulk solution, and which build up as a layer on the surface. It has been presented in this paper that the surface scale formation rate is more dominantly controlled by the “deposition process” rather than the “adhesion process”. However, the level of agitation could have inverse effects on one process to another. Only a small amount of research has been done to understand the differences of the kinetics of each of these processes. The presented work represents an experimental study of scaling tests to assess the effect of hydrodynamic conditions, using Rotating Cylinder Electrode (RCE), in a complex scaling environment, particularly supersaturated with barium/strontium sulphate and calcium carbonate, on the stainless steel substrate coated with a wide range of different industrial coatings. In addition, the effect of the surface energy and surface roughness on both processes has been studied. The paper provides data that will assist in the understanding of the controlling parameters in scale formation in different conditions, and also describes what characteristics of the surface can make it a good anti-scale surface for inorganic scale; however, the results have showed that merely one parameter cannot assure a surface as a good antifouling surface.

Nucleation: The Birth of a New Protein Phase

Nucleation is a process that initiates phase transitions. Since the classical work of Gibbs on nucleation thermodynamics at the end of the nineteenth century, research into nucleation processes has been spread into a huge variety of scientific fields. To date, nucleation has been studied in disciplines ranging from biophysics to cosmology in systems spanning atomic to planetary scales and beyond (1). Proteins in solution are also known historically to form a variety of states and structures through nucleation (2,3).

Crystallization Behavior of Poly(lactic acid)/Titanium Dioxide Nanocomposites

In this study, a poly(lactic acid) (PLA) with various titanium dioxide (TiO 2 ) nanoparticles loading were prepared by a manual laboratory mixing method. The effect of TiO 2 nanoparticles on the non-isothermal and the isothermal crystallization behavior of PLA were investigated by polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The presence of TiO 2 nanoparticles decreased the spherulite growth rate of PLA, whereas it initiated faster crystallization through the heterogeneous nucleation process as observed by optical microscopy. The results of DSC analyses confirmed that the TiO 2 nanoparticles act as an efficient nucleating agent for PLA crystallization. The cold crystallization temperature and crystallization half-time of PLA decreased, while degree of crystallinity of PLA increased in relation to increases of TiO 2 nanoparticles.