Si poisoning on Al-5Ti-B master alloys has been restraining the effectiveness of grain refinement of hypoeutectic Al-Si casting alloys for over 60 years, and yet the underlying mechanism of this ...phenomenon remains unclear. In this work, Si poisoning in Al-Si/Al-5Ti-B system was systematically investigated by combining state-of-the-art electron microscopy, first-principles calculations and thermodynamic calculations. Different from the common belief that silicides coat and therefore poison TiB2, this study demonstrates that the segregation of Si atoms at the TiB2/α-Al interface is likely the cause of Si poisoning. Silicide was found to be thermodynamically unfavorable to form even in an alloy with 10 wt.%Si. On the other hand, an appreciable amount of Si (5–20 at.%) was found to segregate in the TiAl3 two-dimensional compound (2DC) which is critical for triggering the nucleation of α-Al on TiB2. The formation of Ti-Si covalent bond within TiAl3 2DC disturbs its lattice and reduces its chemical interaction with α-Al, which both obstruct the epitaxial nucleation of α-Al and hence leads to Si poisoning. This study suggests that composition engineering of TiAl3 2DC and TiB2 with elements less attractive to Si could be a viable way to mitigate Si poisoning.
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Synthesis of a continuous and intergrown metal-organic framework (MOF) membrane remains a challenging issue due to the grain boundary defects and intercrystalline cracks in MOF thin film. In this ...study, a ZIF-8 membrane was fabricated by using a plant polyphenol tannic acid (TA) layer as an intermediate layer to overcome this issue. Specifically, a TA layer was constructed on a polyethersulfone substrate via a facile and environmentally friendly aqueous coating process. Thereafter, the above modified membrane was immersed into a Zn(Ac)2 and Hmim mixture solution for in-situ growing a ZIF-8 thin film on membrane surface. It was found that the abundant pyrogallol groups in TA layer could coordinate with Zn2+ ions, promote heterogeneous nucleation and contribute to a continuous and intergrown ZIF-8 thin film. The as-prepared ZIF-8 membrane showed a pure water permeance of 3.6 L m−2 h−1 bar−1 with NaCl and Na2SO4 rejections of 64.7% and 92.2% respectively, overwhelming the controlled ZIF-8 membrane without TA as intermediate layer in both permeability and selectivity. This membrane is one of few MOF membranes that have a divalent salt rejection higher than 90%, demonstrating its great potential application in efficient desalination.
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•A tannic acid layer was used as intermediate layer in a ZIF-8 membrane.•The tannic acid layer can increase ZIF-8 heterogeneous nucleation.•A uniform and intergrown ZIF-8 membrane was synthesized.•Both permeance and salt rejection were enhanced after tannic acid modification.•It is one of few MOFs membranes that have divalent ion rejection higher than 90%.
This paper investigates the correlations between interfacial reaction, crystallographic orientation relationship on the interface and the required undercooling for nucleation on different ...crystallographic planes of MgO. Thermal analysis and high resolution transmission electron microscopy were used to study the nucleation behavior of liquid, high-purity Al droplet on single crystal MgO substrates using a DSC with an integrated image capture system and a sessile drop apparatus. The results showed that the original substrate MgO would be completely replaced by the reaction product MgAl2O4 at the interface owing to the chemical reaction between liquid Al and the MgO substrates. In addition, the same crystal structure with the original MgO substrate is achieved in the new MgAl2O4 layer. The orientation relationship between MgAl2O4 and Al is consistent with the theoretical prediction according to the Bramfitt's lattice misfit theory and Edge-to-Edge model. Consequently, the generated MgAl2O4 significantly influences the detected undercooling.
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•Only MgAl2O4 layer is produced at the interface of Al/MgO system due to chemical reaction occurred from 750 °C to 660 °C.•The well-defined orientation relationship is achieved at the interface of Al/MgAl2O4/MgO.•MgAl2O4 layer has a significant influence on the heterogeneous nucleation of Al.
•A frost model that considered the crystal growth period was constructed.•The approach was verified by comparison with five experimental results.•The model can reduce the maximum thickness and ...density errors by 17.4 % and 10.2 %.•The factors affecting the period of nucleation were analyzed.
In engineering applications, the formation and growth of frost layers can significantly affect the normal operation of equipment, leading to undesirable influences. Therefore, it is imperative to understand the frost formation mechanisms as a basis for developing effective anti-frosting and defrosting methodologies. This study proposes a model for the growth of horizontal cold surface frost layers using the classical nucleation theory and the Eulerian multiphase flow model. Compared to numerous models that investigate the frost formation issues using the computational fluid dynamics (CFD) method, our proposed model takes into account the crystal growth period (the heterogeneous nucleation process of ice). The predicted results of the proposed model were found to be in good agreement with the data collected from five related experimental studies. In this regard, the maximum frost layer thickness error was 23.9 % with a mean error of 5.1 %, and the maximum density error was 22.8 % with a mean error of 8.1 %. Compared to existing models in the literature, the proposed model can reduce the maximum thickness and density uncertainties by 17.4 % and 10.2 %, respectively. Additionally, the effects of the surface contact angle and environmental parameters on the nucleation process were analyzed using the established model. It was found that a larger contact angle, higher cold surface temperature, and lower air temperature, humidity, and velocity were unfavorable conditions for supporting the formation and growth of ice crystals. Moreover, the nucleation completion time was shortened with the increase in the moist air supersaturation degree, varying as an exponential function. The findings of this study provide an important theoretical basis for the development of optimizing strategies for anti-frosting and defrosting.
•FC occurs when a polymer is divided in a large number of microdomains (MDs).•When the number of heterogeneities in bulk equals the number of MDs, FC occurs.•The injection of nucleating agents or ...self-nuclei in every MD eliminates FC•Nucleation goes from heterogeneous to homogeneous by increasing confinement.•Crystallization kinetics can change from sigmoidal to first order by confinement.
The crystallization of heterogeneously nucleated bulk polymers typically occurs in a single exothermic process, within a narrow temperature range, i.e., a single exothermic peak is detected by differential scanning calorimetry when the material is cooled from the melt. However, when a bulk semicrystalline polymer is subdivided or dispersed into a multitude of totally (or partially) isolated microdomains (e.g., droplets or cylinders), in number comparable to that of commonly available nucleating heterogeneities, several separated crystallization events are typically observed, i.e., fractionated crystallization. This situation is often found for the minor crystallizable component in immiscible blends.
When the bulk polymer is dispersed into a number of microdomains that is several orders of magnitude higher than the available number of heterogeneities within it, most microdomains will be heterogeneity-free. In these clean microdomains the nucleation can occur by contact with the interfaces (i.e., surface nucleation) or by homogeneous nucleation inside the microdomain volume. These cases can be easily encountered in cylinders or spheres within strongly segregated block copolymers, or in infiltrated polymers within nanopores of alumina templates.
In this work, a comprehensive review of the known cases of fractionated crystallization is provided. The changes upon decreasing microdomain sizes from a dominant single heterogeneous nucleation, through fractionated crystallization, to surface or homogeneous nucleation are critically reviewed. Emphasis is placed on the common features of the phenomenon across the different systems, and thus on the general conclusions that can be drawn from the analysis of representative semicrystalline polymers. The origin of the fractionated crystallization effects and their dramatic consequences on the nucleation and crystallization kinetics of semicrystalline polymers are also discussed.
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Coupling high-resolution characterization and phase-field microelasticity modeling, the non-conventional transformation pathway, mediated by ω precipitates, that leads to super-refined intragranular ...α precipitates in a metastable β Ti-alloy (Ti–5Al–5Mo–5V–3Cr, all in wt%) has been rationalized for the first time. Samples are initially solution treated above the β transus, rapidly cooled to room temperature, and then slowly heated (in the range 20°C–1 °C/min) to 600 °C. Interrupted tests, coupled with characterization involving scanning electron microscopy (SEM), transmission electron microscopy (TEM), high angle annular dark field-high resolution scanning transmission electron microscopy (HAADF-HRSTEM) and atom probe tomography (APT), have revealed the role of the ω phase on the copious nucleation of α leading to super-refined precipitation. Thus, both stress and compositional variations associated with the development of isothermal ω during heating contribute to an extra driving force for nucleation. It is also found that the interfaces between the super-refined α and the β matrix are relatively coherent, unlike the case of such interfaces in α/β Ti alloys. The experimental results have been combined with CALPHAD and phase field microelasticity modeling to assist in developing a robust notion of the factors influencing copious nucleation in these samples.
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Al-Ti-B based master alloys have been widely used for grain refining of Al-alloys in industry for many decades. However, the effectiveness of such grain refiners is severely compromised when a few ...hundred ppm of Zr is present in the Al melt, and this phenomenon is referred to as Zr poisoning in the literature. So far the exact mechanisms for Zr poisoning are not clear albeit significant research effort on the subject in the last few decades. In this work we investigated the mechanism for Zr poisoning through extensive examinations of the Al/TiB2 interface using the state-of-the-art electron microscopy and ab initio molecular dynamics simulations. We found that the presence of Zr in Al melts leads to (i) the dissolution of the Al3Ti 2-dimensional compound (2DC) formed on the (0 0 0 1) TiB2 surface during the grain refiner production process; and (ii) the formation of an atomic monolayer of Ti2Zr 2DC on the (0 0 0 1) TiB2 surface, which replaces the original Ti-terminated TiB2 basal surface. This monolayer of Ti2Zr not only has large lattice misfit (4.2%) with α-Al, but also is atomically rough, rendering the TiB2 particles impotent for heterogeneous nucleation of α-Al. This work, in combination of our previous work, demonstrates that heterogeneous nucleation can be effectively manipulated, either enhanced or impeded, by chemical segregation of selected alloying/impurity elements at the liquid/substrate interface.
(a, b) High resolution STEM HAADF images showing an atomic monolayer of Ti2Zr 2-dimensional compound (2DC) on (0 0 0 1) surface of TiB2 being viewed along (a) 1 1 -2 0TiB2 and (b) 1 0 -1 0TiB2 direction respectively, and (c) 3D construction of the Ti2Zr 2DC on top of TiB2. Display omitted
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•Heavy metal hydroxide’s heterogeneous nucleation on struvite was detected.•Struvite has enhancement effect in heavy metal precipitation.•△Gi were intensively abated through ...heterogeneous nucleation on struvite.•Model of sorption, nucleation, and precipitation by struvite was established.
During the struvite (MgNH4PO4·6H2O) recovery from agricultural or industrial wastewater, the extensive existence of heavy metals would pose great threats to the planting and environment. This work revealed the association of kinds of heavy metals, as possible substances in the wastewater, with the struvite. The struvite has been synthesized in situ and employed to contact with both high and low concentration heavy metal (including Cu, Ni, Pb, Zn, Mn, Cr(III)) wastewater. The heavy metal precipitation rates under different pH values from 6.0 to 10.0, and under a series of struvite addition amount have been investigated. The precipitation of heavy metals without addition of struvite is functioned as control experiments. The struvite’s presence can enhance the heavy metal precipitation rate under all pH values. The Ni and Mn have relatively lower precipitation rate compared with other metals. For extremely low heavy metal concentration the precipitation rate is 99.1%, 97.9%, 99.9%, 98.9%, 96.9%, and 98.3%, which is much higher than that of control group (10.5%, 8.7%, 13.2%, 14.1%, 6.7%, and 10.5%). Through XPS and TEM & EDS analysis, the heavy metal hydroxides is found to be precipitated on the struvite surface. Based on TEM observations, it has been found the copper hydroxides had nucleation and growth on the struvite’s surface. The heterogeneous nucleation mechanism has been proposed, which provides relationship model of sorption, nucleation, and precipitation of heavy metals in the form of hydroxides on struvite surfaces. It has been calculated that the △Gi value that thermodynamic activation energy barrier constraints on the metal hydroxide formation can be intensively abated through heterogeneous nucleation on struvite.
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Superhydrophobic surfaces can effectively prevent the freezing of supercooled droplets in technological systems. Droplets on superhydrophobic surfaces commonly not only wet the top ...asperities (Cassie State), but also partially penetrate into microstructure due to surface properties, environment, and droplet impact occurring in real-world applications. Implications on ice nucleation can be expected and are little explored. It remains elusive how anti-icing surfaces can be designed to exploit intermediate wetting phenomena.
We utilized engineered micro-/nanostructures, specifically micropillars, to modulate the wetting fraction in the microstructure. The behavior of intermediate wetting with supercooling and resulting implications on ice nucleation delay when potential nucleation sites are formed in the microcavities were investigated using experimental, theoretical, and simulation components.
The temperature-dependent wetting fraction in the microstructure increased at supercooled temperatures, partly activated by condensation in the microcavities. At −10/−20 °C, a critical wetting fraction led to maximum ice nucleation delays, with experimental results consistent with theoretical predictions. This critical wetting fraction minimized the effective contact area solid-to-liquid along the partially wetted microstructure. The study establishes physical relations between ice nucleation delays, geometrical surface parameters and wettability properties in the intermediate wetting regime, providing guidance for the design of ice resistant microstructured surfaces.
•The multi-step spin coating method can form the layered and superimposed CsPbBr3 perovskite films.•The method can form CsPbBr3 phase with consistent particle size and vertical distribution.•The ...method can effectively eliminate the non-CsPbBr3 phase.•The power conversion efficiency of CsPbBr3 perovskite solar cells with annealing temperature of 100 °C is 10.02%•The efficiency is one of the highest reported efficiency of the CsPbBr3 perovskite solar cell so far.
We have demonstrated that the preparation technology of high-quality CsPbBr3 perovskite thin film based on multi-step spin coating method by using solvent engineering. We effectively avoided that both heterogeneous nucleation on the substrate surface and homogeneous nucleation on the surface could form a large number of grains during spin coating. The multi-step spin coating method can form the layered and superimposed CsPbBr3 perovskite films by spin coating for many times with different annealing temperatures, which can form CsPbBr3 phase with consistent particle size and vertical distribution. Can And Thus, it can greatly improve the efficiency of carrier extraction and collection, and comprehensively enhance the PCE of the CsPbBr3 solar cells. Moreover, the multi-step spin coating method can effectively fabricate the required thickness of CsPbBr3 perovskite films and eliminate the non-CsPbBr3 phase in the process of film formation. The results showed the power conversion efficiency of CsPbBr3 perovskite solar cells with annealing temperature of 100 °C is up to 10.02% with the open circuit voltage (Voc) of 1.47 V, the short circuit current density (Jsc) of 8.22 mA/cm2 and the filling factor (FF) of 83%, which is one of the highest reported efficiencies of the CsPbBr3 perovskite solar cells so far. The efficiency basically retained the original value when the optimal CsPbBr3 soalr cell was placed in the air at room temperature after 350 days. Meanwhile, the efficiency of CsPbBr3 soalr cells at 120° C was decreased only by 10%, showing excellent thermal stability.
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