In this article, the novel octopus-tentacle-like Cu nanowire-Ag nanocrystals heterostructures have been fabricated in solution phase via heterogeneous nucleation and growth of Ag nanocrystals on ...presynthesized Cu nanowires. The growth environment and dynamic factors of Ag nanocrystals play an important role for formation of such heterostructures. Combined the physical constants of Cu and Ag with a series of control experiments, the epitaxial growth means of Ag nanocrystals on Cu nanowire is found to abide by “layer-plus-island” (Stranski-Krastanow) mode. Because of the presence of multiple junctions and strong synergistic effect of their constituents, the obtained heterostructures exhibit greatly enhanced electrocatalytic performance toward oxygen reduction reaction compared with that of pure Ag nanocrystals, Cu nanowires, and mechanically mixed dual components as well as recently reported some non-Pt materials, which can be served as an alternative cathodic electrocatalyst to apply in alkaline fuel cells. Moreover, our method can be extended to fabricate octopus-tentacle-like Cu nanowire-Au nanocrystals and Cu nanowire-Pd nanocrystals heterostructures.
The behavior for injection of magnesium vapor into molten steel and the potential of magnesium vapor for miniaturization of TiN size were evaluated by the experiment in tammann furnace and the ...experiment in vacuum furnace. From the results of these experiments, the partial pressure of magnesium reacting with molten steel could be increased, and the both high concentration in molten steel and high additive yield of magnesium into molten steel could be done together. It was confirmed that spinel MgO·Al2O3 was formed as a heterogeneous nucleus of TiN. The dissolved magnesium concentration governed the heterogeneous nucleus for TiN. The injection of magnesium vapor into molten steel could control the generation of heterogeneous nucleus during solidification.
Titanium-alloyed ferritic chromium steels are a competitive option to classical austenitic stainless steels owing to their similar corrosion resistance. The addition of titanium significantly ...influences their final steel cleanliness.
The present contribution focuses on the detailed metallographic characterization of titanium nitrides, titanium carbides and titanium carbonitrides with regard to their size, morphology and composition. The methods used are manual and automated Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy as well as optical microscopy. Additional thermodynamic calculations are performed to explain the precipitation procedure of the analyzed titanium nitrides.
The analyses showed that homogeneous nucleation is decisive at an early process stage after the addition of titanium. Heterogeneous nucleation gets crucial with ongoing process time and essentially influences the final inclusion size of titanium nitrides. A detailed investigation of the nuclei for heterogeneous nucleation with automated Scanning Electron Microscopy proved to be difficult due to their small size. Manual Scanning Electron Microscopy and optical microscopy have to be applied. Furthermore, it was found that during solidification an additional layer around an existing titanium nitride can be formed which changes the final inclusion morphology significantly. These layers are also characterized in detail.
Based on these different inclusion morphologies, in combination with thermodynamic results, tendencies regarding the formation and modification time of titanium containing inclusions in ferritic chromium steels are derived.
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•The formation and modification of TiN in the steel 1.4520 was examined.•Heterogeneous nucleation essentially influences the final steel cleanliness.•In most cases heterogeneous nuclei in TiN inclusions are magnesium based.•Particle morphology provides important information on inclusion formation.
Alumina ( - and γ-Al2O3) particles are formed in liquid Al-Mg alloys during the liquid dealing and cast processes. These native oxide particles have non-trivial influences on the microstructures and ...properties of the solidified parts, and may act as potential heterogenous nucleation sites during solidification. At present there is still a lack of understanding about the interaction and atomic arrangements at the interfaces between liquid-Al and γ-Al2O3 substrates. Here we investigate the liquid-Al/γ-Al2O3{1 1 1} interfaces by means of ab initio molecular dynamics simulations and electronic structure calculations. We found that the interfacial interaction at the interfaces leads to formation of an ordered terminating Al layer. This newly formed terminating Al layer is positively charged and chemically bonded to the substrate and thus, becomes part of the substrate. Analysis showed that the terminating Al layer contains vacancies and displacements, being atomically rough. The newly-formed Al layer is also structurally coupled with the substrates. These γ-Al2O3 particles are weak templates for nearby liquid to nucleate. The present study sheds some light on the role of alumina particles in grain refinement of Al-based alloys during solidification processing.
A considerable amount of B2 phase with a cellular morphology is retained in a 4Zr–4Nb-containing TiAl-based alloy. Heterogeneous precipitation of ordered ω from B2 is found to occur readily after ...HIPping: B2
→
ω with the B8
2-structure in cell regions and B2
→
ω with the D8
8-structure in cell-wall regions. Congregated D8
8-ω domains and particles form as a network surrounding the well-developed B8
2-ω cells. The heterogeneous formation of different ω variants is caused by a heterogeneous distribution of Zr
+
Nb elements across B2, which plays an important role in stabilizing vacancies and promotes the formation of D8
8-ω. Fine D8
8-ω particles are also observed to precipitate from the B8
2-ω cell matrix after ageing at 700
°C for 1000
h, showing a transformation path of β
→
B2
→
B8
2-ω
→
D8
8-ω for the aged cells. The heterogeneous formation of a D8
8-ω network and B8
2-ω cells is found to be detrimental to ductility and fatigue strength. A very brittle fine-grained TiAl alloy is produced as a result.
A controlled growth of two‐dimensional (2D) π‐conjugated metal‐organic frameworks (MOFs) on solid substrates can open exciting opportunities for the application of 2D MOFs as optoelectronic devices. ...Some factors like solvent composition and type of substrates are known to influence the properties of solution‐processed 2D MOF crystals; however, a mechanistic understanding of how interactions between solvent, substrate, and precursors affect heterogeneous nucleation has been limited. Here, it is reported that the structure of Ni‐catecholate (Ni‐CAT‐1) MOFs at a solid–liquid interface is controlled by solvent–substrate and solvent–MOF precursor interactions. Specifically, the structure of the MOF film can be controlled by varying the affinity of the solvent to the substrate. As a fraction of N,N‐dimethylformamide (DMF) in a binary solvent mixture of water and DMF increases, the arrangement of Ni‐CAT‐1 crystals varies from vertically aligned nanorods to the graphite substrate to less ordered nanorods with the lower initial nucleation number density of Ni‐CAT‐1 crystals on the surface.
The arrangement of two‐dimensional metal‐organic frameworks varies from well‐aligned nanorods orthogonal to substrate to less ordered nanorods by modulating solvent composition which affects interactions of solvent‐substrate and solvent‐precursor molecule. The results provide insights into the role of solvents in directing heterogeneous nucleation of crystals on substrates, which is essential for solution‐processed crystal growth for optoelectronic application.
High angle annular dark field scanning transmission electron microscope imaging and electron energy loss spectroscopy were used to elucidate the heterogeneous nucleation interface of AlB2 in an ...Al-7Si alloy. Si particles were observed in the vicinity of a AlB2 particle, but no significant Si-B-rich layer (such as SiB6) was observed at the interface between AlB2 and Al, strongly indicating that the proposed creation of a layer of SiB6 at the interface between AlB2 and Al may only be metastable and further transform to other stable phase (e.g. Si) during solidification. The solidification path of Al-Si-B alloy was also discussed based on the binary phase diagrams and a ternary Thermo-Calc simulation using the Scheil module in conjunction with the TCAL3 database. These results provide new insight into the heterogeneous nucleation behavior of Al on AlB2 influenced by solute Si.
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•Si particles were observed in the vicinity of a AlB2 particle in an Al-7Si alloy.•No significant Si-B-rich layer (such as SiB6) was observed at the interface between AlB2 and Al.•The solidification path of Al-Si-B alloy was discussed based on binary phase diagrams and a ternary Thermo-Calc simulation.
The valence electron structure, bond energy, and cohesive energy of Mg, Zr, and α-Mg containing Zr, and α-Zr containing Mg crystals were calculated using the empirical electron theory of solids and ...molecules (EET). The calculation results show that the bond and cohesive energies of Zr were much greater than those of Mg, so Zr particles could precipitate ahead of α-Mg in general magnesium alloy melts or insoluble Zr particles exist when the magnesium melt temperature is relatively low. The bond energy of α-Zr decreases with the increase in Mg content; therefore, at the end of the growth of Zr particles, the remaining Zr atoms in the melt exist in the form of Mg-Zr clusters. In order to reduce the surface energy of Zr particles, the outer surface of Zr particles tends to terminate with a Zr-Mg atomic layer, that is, a stable two-dimensional Zr-Mg atomic layer is formed first on the (0001) crystal surface of the outermost surface of Zr particles. Furthermore, on the basis of the calculated results, a complementary criterion to the edge-to-edge model of heterogeneous nucleation is also proposed. {ure and single Zr particles cannot become the heterogeneous nucleus of α-Mg, but when there is an atomic layer of two-dimensional Zr-Mg on its surface, the nucleation of particles can be activated. Mg atoms in the liquid phase preferentially attach to the Zr-Mg/Mg-Zr atomic layer on the surface of Zr particles to grow and form a stable ordered structure, which lastly transforms Zr particles into efficient heterogeneous cores.
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•Solid-state nucleation is investigated through molecular dynamics simulations.•A geometric model is demonstrated to evaluate the nucleus shape and free energy.•The results agreed ...with theory prediction in the size and shape of the nucleus.•A grain boundary pucker event is captured, which influents the nucleation process.
The study of solid-state nucleation through experiment is often limited by its tiny length range and short time scale. In this study, molecular dynamics (MD) simulations in a quasi 2D geometry are used to study the process of a BCC ferrite phase in pure Fe nucleating at a grain boundary (GB) in an FCC austenite polycrystalline system. In the MD simulations the critical nucleus can be identified, the bulk free energy difference between FCC and BCC is known for the Fe interatomic potential used and all relevant interface and GB energies are computed using a Gibbs–Cahn formulation. For nucleation events that exhibited low energy facets completely contained within the parent FCC phase, the results agreed well with predictions from classical nucleation theory (CNT) in terms of both the size and shape of the critical nucleus. For systems where the emerging nucleus contains facets that cross the GB plane the agreement with classical theory is less convincing and the observed nucleus does not exhibit parallel facets as predicted from the Winterbottom construction. The latter nucleation case involves a so-called pucker mechanism of the FCC grain boundary to accommodate the emerging nucleus and the effect of GB puckering on the incubation time is discussed.
The variation in unfrozen water content with temperature substantially affects coupled heat and water transport in frozen soil, causing frost heave and thaw settlement owing to the ice and water ...phase change and influencing soil stability in cold regions. Thus, analyzing the mechanism of water freezing and building a predictive model for the unfrozen water content of soils is paramount. In this study, an analytical model based on equivalent contact angle was developed to predict the unfrozen water content. The relationship between the equivalent contact angle and temperature was obtained based on the assumption that the heterogeneous nucleation rate nonlinearly decreased with temperature. The proposed analytical model was validated using existing unfrozen water content data at various temperatures for a silty clay soil material from the Qinghai–Tibet Plateau, and compared to several existing numerical models which predict unfrozen water content in soil materials. The results revealed a close relationship between the unfrozen water content and equivalent contact angle, and the equivalent contact angle increased as the temperature decreased. Meanwhile, the pore water in the soil first froze when the contact angle was smaller. Moreover, the values predicted by the analytical model for the unfrozen water content agreed well with the experimental results, especially under low‐temperature conditions and during the early stage of water freezing.
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