A thin and defect-free UiO-66 membrane was fabricated by a heterogeneous nucleation assisted growth strategy, which showed a high CO2 permeance and excellent long-term stability in humid environment.
...Display omitted
•UiO-66 membranes were synthesized by a heterogeneous nucleation assisted growth method.•A small amount water promoted the heterogeneous crystal nucleation process.•Smooth PDMS interlayer guaranteed the defect-free growth of thin MOF membrane.•The optimized UiO-66 membrane showed a high CO2 permeance and moderate CO2/N2 selectivity.•UiO-66 membrane exhibited long-term stability under humid conditions.
Metal-organic frameworks (MOFs) have driven the development of polycrystalline membranes in the field of gas separation owing to the strong adsorption-desorption behavior and stable framework structure. However, the poor heterogeneous bonding ability between crystals and the substrate leads to the unsatisfactory gas separation as well as low stability of MOF membranes, especially in the humid environment. Herein, we prepared a thin and defect-free UiO-66 membrane by a crystal heterogeneous nucleation assisted growth strategy for efficient CO2/N2 separation. The addition of small amount of water in the precursor solution promoted the crystal nucleation process on the substrate surface. Meanwhile, the smooth PDMS interlayer guaranteed the heterogeneous well-intergrown of crystals into thin UiO-66 membranes, and provided a hydrophobic environment. Therefore, the dense UiO-66 membrane with a small thickness exhibited a high CO2 permeance (177.4 × 10−9 mol·m−2·s−1·Pa−1) with moderate CO2/N2 selectivity (24.3) under humid conditions. Furthermore, the prepared membrane demonstrated excellent renewable performance after dehumidification at high temperature, and maintained good hydrothermal stability during long-term operation under the humid environment. This work provides a new insight for the design of high-quality MOF membranes and promotes the development of MOF membranes in practical gas separation process.
Utilization of high-filler coal gangue (CG) to produce cost-effective composites is a promising strategy. To clarify the impact of high-filled CG on composite preparation and performance, in this ...research, the composite was prepared from CG and polyethylene (PE) through physical mixing and then hot-pressing. The role of CG in the properties of the composites and the influence of the interface formation mechanism are obtained by establishing the relationship between CG content and the composites' feature. The results show that: high-filler CG is helpful to the increment of the composite's mechanical and thermostable properties. When the CG addition is up to 80%, the density and hardness from 0.93 g/cm3 and 50.5 HD increased to 1.60 g/cm3 and 66.5HD, which was increased by 72.0% and 31.6%, respectively. Meanwhile, the bending strength and modulus reach 17.5 MPa and 3400.6 MPa, which is respectively exceeded by 56.3% and 628.6% than composite from pure PE (11.2 MP and 466.7 MPa). In addition, high-filler CG is beneficial for the enhancement of entanglement between CG and PE during the composite preparation, and has the effect of heterogeneous nucleation, which facilitated the crystallization and thermal stability of the composite. The properties of the composites prepared by this simple technique meet national standards and are more economical and environmentally sustainable than composites prepared with other fillers. This new material holds great promise for the development and realisation of carbon-neutral decorative materials such as assembly buildings, vehicles and coloured panels.
Display omitted
•A novel process for the high utilization of coal gangue was proposed in the polymer.•A low-cost and green composite was prepared with coal gangue and polyethylene.•The mechanism of CG-based composite formation was revealed.•The economic and environmental effects of composites were evaluated.
•Issues related to processes involving solid clathrate hydrates are discussed.•Current knowledge on the mechanism of hydrate formation is discussed.•Homo and heterogeneous nucleation of hydrates are ...both considered.•Proposed evidence for memory effects in hydrate reformation is discussed.•Directions to take in the future to understand these issues are outlined.
Among outstanding issues still to be understood regarding the clathrate hydrates are the mechanism of the processes involved in the formation and decomposition of clathrates: nucleation, decomposition, and the memory effect during reformation. The latter involves the shorter induction times required for solutions of decomposed hydrate to nucleate as compared to those for freshly prepared solutions. The formation of the clathrate hydrate phases of insoluble gases in water is accompanied by a ∼6000 fold concentration of the gas content in the solid phase compared to the aqueous phase from which it forms. The nucleation mechanism for the solid hydrate which allows the delivery of such high concentration of gas and water in one location has been the subject of much experimental and computational study. While these studies have improved our understanding of the nucleation process, many unknown aspects remain. These developments are described in this Opinion.
The formation of ikaite (CaCO3x6H2O) was studied in the presence and absence of quartz and mica surfaces using desupersaturation curves from cryo-mixed-batch-reactor experiments. Upon nucleation and ...growth within the reactor, the solution approached solubility of the precipitating carbonate phase. For ikaite, a solubility constant of log Ksp ikaite = −7.3 ± 0.1 was found (T = 0 °C). At supersaturations Ωikaite < 15, the nucleation of ikaite was significantly promoted by the presence of quartz or mica. This promotion prevented a competing nucleation of anhydrous calcium carbonates. In the presence of quartz or mica, therefore, ikaite forms over a much broader supersaturation range than in the absence. Similarly strong promotors of ikaite nucleation rather than anhydrous carbonate nucleation were previously attributed to calcite-inhibiting substances only.
At supersaturations Ωikaite ≥ 8, application of classical nucleation theory on induction periods of ikaite formation yielded an effective interfacial energy of 15 ± 3 mJ/m2. Compared to data of anhydrous CaCO3 phases, this interfacial energy is low and expresses the highly hydrated character of ikaite. At supersaturations Ωikaite ≥ 18, a transient amorphous phase appeared besides ikaite.
Our results show that a comprehensive understanding of ikaite formation in natural settings requires consideration not only of supersaturation and presence of calcite-inhibitors but also of the presence or absence of mineral surfaces capable of promoting heterogeneous nucleation of ikaite.
Display omitted
•Mineral surfaces promote the formation of ikaite significantly.•The presence of mineral surfaces is as important for ikaite nucleation as calcite inhibiting substances.•The low interfacial energy for ikaite nuclei implies a nucleation mechanism different to anhydrous CaCO3 minerals.•An amorphous phase occurs at increased supersaturations.
The solidification path and precipitation mechanism of the C-HRA-3 Ni-Cr-Co-Mo-based heat-resistant alloy are investigated by methods such as differential scanning calorimetry analysis, in-situ ...observations, quenching experiments, and scanning transmission electron microscopy. The results confirm that the primary solidification precipitates in the center of the C-HRA-3 alloy VAR ingot consist of Ti(C,N), M6C-M23C6 symbiotic phases, Ti(C,N)-M6C-M23C6 symbiotic phases and surrounding dispersed M6C and M23C6 carbides. The precipitation temperatures for Ti(C,N), M6C, and M23C6 are 1310 °C, 1280 °C, and 1230 °C, respectively. The C-HRA-3 alloy exhibits a wide solidification temperature range of 176 °C (1390 °C ∼ 1214 °C). Within 1390 °C ∼ 1355 °C, the γ phase grows vigorously, accompanied by a significant reduction in the fraction of liquid phase. At 1355 °C ∼ 1214 °C, the decrease in liquid phase fraction slows down, accompanied by significant enrichment of Mo, Cr, C and Ti elements. There is a clear functional relationship between the liquid phase percentage and temperature. M6C carbides undergo two heterogeneous nucleation mechanisms in the liquid phase. Due to the degradation of M6C carbides discharging Cr and C atoms, M23C6 carbides form at their edge. Dispersed M6C and M23C6 carbides nucleate at misfit dislocations around the symbiotic phases. Appropriately reducing the cooling rate during the stage above the precipitation temperature and increasing the cooling rate below the precipitation temperature can significantly reduce the volume percentage and size of the symbiotic phases.
•The solidification path and precipitation behavior of the C-HRA-3 alloy was objectively clarified.•The formation mechanisms of precipitates of C-HRA-3 alloy was identified.•A new idea to control the dendritic segregation and the size and volume fraction of precipitates was proposed.
Antimony (Sb) has been identified as a promising candidate for replacing toxic lead (Pb) in perovskite materials because Sb-based perovskite-like halides exhibit not only intrinsic thermodynamic ...stability but also a unique set of intriguing optoelectronic characteristics. However, Sb-based perovskite-like halides still suffer from poor film morphology and uncontrollable halide constituents, which result from the disorder of the growth process. Herein, we propose a simple strategy to facilitate heterogeneous nucleation and control the dimension transformation by introducing bis(trifluoromethane)sulfonimide lithium (LiTFSI), which produces high-quality two-dimensional MA3Sb2I9–x Cl x films. As the spacer molecule among Sb-based pyramidal clusters, LiTFSI plays a role in forming a zero-dimensional intermediate phase and retarding crystallization. The slower dimension transformation well stabilizes the band gap of perovskite-like films with a fixed Cl/I ratio (∼7:2) and avoids random “x” values in MA3Sb2I9–x Cl x films prepared from the conventional method. Based on this method, Sb-based perovskite-like solar cells (PLSCs) achieve the highest recorded power conversion efficiency (PCE) of 3.34% and retain 90% of the initial PCE after being stored under ambient conditions for over 1400 h. More importantly, semitransparent Sb-based PLSCs with PCEs from 2.62 to 3.06% and average visible transparencies from 42 to 23% are successfully obtained, which indicates the great potential of the emerging Pb-free halide semiconductor for broad photovoltaic applications.
Display omitted
The smallest nanodrop tractable with macroscopic notions such as the interfacial energy could be determined by comparing heterogeneous nucleation observations and capillary theory ...predictions at decreasing drop diameters dp.
This is done here for the condensation of n-butanol vapors on polyethylene glycol nano-globules (3 nm ≤ dp ≤ 9 nm). We use published activation probability measurements P(w,dp), where w is the accurately controlled saturation ratio of n-butanol vapor in a gas stream exiting a saturator. The maximal saturation ratio achieved in the nucleation region by cooling this gas-vapor stream in the apparatus of Gallar et al. satisfies Smax = Cw. The key unknown constant C and the preexponential term K governing the nucleation rate are determined by assuming that classical theory applies to the largest particles used. This yields P(Smax,dp) data, directly comparable with capillary theory with perfect wetting.
Excellent agreement is found above 5 nm for the critical dependence Smax(dp) resulting from the constraint P(Smax,dp) = 0.5. The entire P(Smax,dp) curves also agree closely between 5 and 7 nm. Smaller particles depart only slightly from theory, even at dp = 3 nm. Capillary theory hence describes accurately the heterogeneous nucleation process above 3–5 nm, provides a reliable method to determine Smax, and yields experimentally the nucleation rate constant K.
Organic-inorganic hybrid perovskite nanomaterials hold immense potentials for photovoltaic and optoelectronic devices, while their structural instability under external stimuli has seriously impeded ...their practical applications. Meanwhile, the microscopic understanding of structural degradation mechanisms for these low-dimensional perovskite nanomaterials remains incomplete. Herein, we report the degradation mechanisms on surface of perovskite methylammonium lead iodide (MAPbI3) nanowires under electron-beam irradiation by in situ transmission electron microscopy. Our atomic-scale study has directly identified the decomposition of MAPbI3 and the subsequent growth of PbI2 nanocrystals are highly anisotropic. Furthermore, PbI2 nucleation follows the heterogeneous two-step mechanism with the growth kinetically controlled by the coherent MAPbI3/PbI2 interface.
Display omitted
•The surface degradation pathway of MAPbI3 nanowires is microscopically revealed.•The facet-dependent degradation of MAPbI3 induces anisotropic growth of PbI2.•A non-classical two-step nucleation mechanism is identified for PbI2 nanocrystals.•The coherent interface of MAPbI3/PbI2 is critical for the structural transition.
The effect of rare earth on the solidification structure of 3.2%Si-0.9%Al non-oriented silicon steel was investigated using industrial trials. The outputs demonstrated that increasing rare earth ...content leads to a decrease in the average size of equiaxed crystals in the casting billets. In order to further understand the rare earth on the grain refinement of δ-ferrite, the conventional inclusion detection technology, was used to investigate the distribution characteristics of inclusions, together with theoretical calculation of the equilibrium partition coefficients, pinning forces and mismatch degrees. The detection results of inclusions and the calculation results of pinning force showed that the effect of rare earth on the pinning force of inclusions was marginal. Thermodynamic calculation indicated that Ce addition had negligible effect on the equilibrium partition coefficient of Si, Al and Mn. Combined with the calculation results of GRF model, it is reasonable to consider that the contribution of rare earth element to the refinement of equiaxed crystals can be ignored. Further, the outcomes obtained from the E2EM model calculations revealed that the principal mechanism responsible for the refinement of equiaxed crystals through rare earth treatment can be attributed to the heterogeneous nucleation effect of (La, Ce)2O2S.
Entrained double oxide films, or bifilms, can seriously alter the structural integrity, microstructure, and in this way, the mechanical performance of aluminum alloys. Bifilms, which usually preexist ...in suspension in the liquid metals, are known to be potential heterogeneous nucleation sites for certain intermetallic phases during the solidification of the alloys. However, the investigations on the possibility of the nucleation of titanium-containing intermetallic phases on double oxide films, the possible effects of this phenomenon on the melt quality, as well as on the resulting microstructure, were absent to date. In this work, a novel melt treatment technique is proposed, which was used to induce the precipitation of (Al,Si)3Ti particles in a liquid multicomponent AlSi alloy. Differential thermal analysis (DTA), scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) were used to characterize the phases formed during the experiment. SEM-EDS investigations proved that the (Al,Si)3Ti intermetallic phase precipitated on MgAl2O4 double oxide films and the wetted side of the surface oxide layer of the melt. As revealed by glow discharge optical emission spectroscopy (GDOES) and optical microscopic investigations of the microstructure, the majority of the precipitated (Al,Si)3Ti particles sedimented to the bottom region of the melt. Based on the computed tomographic (CT) analysis of reduced pressure test (RPT) samples, this sedimentation extensively reduced the bifilm content in the upper regions of the melt. The theoretical basis of a new melt treatment technique is laid down.
Display omitted
•The heterogeneous nucleation of TiAlSi intermetallics on bifilms was investigated.•A novel, controlled precipitation-based melt treatment technique is proposed.•The sedimentation of TiAlSi compounds can significantly improve melt quality.•Nucleation on the surface oxide layer causes Ti-macrosegregation.
PDF
