Cooperative effects at the interface of nanocrystalline metal–organic frameworks

Controlling the chemistry at the interface of nanocrystalline solids has been a challenge and an important goal to realize desired properties. Integrating two different types of materials has the potential to yield new functions resulting from cooperative effects between the two constituents. Metal–organic frameworks (MOFs) are unique in that they are constructed by linking inorganic units with organic linkers where the building units can be varied nearly at will. This flexibility has made MOFs ideal materials for the design of functional entities at interfaces and hence allowing control of properties. This review highlights the strategies employed to access synergistic functionality at the interface of nanocrystalline MOFs (nMOFs) and inorganic nanocrystals (NCs).

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Penetration behaviour of individual hydrophilic particle at a gas–liquid interface

The penetration behaviour of a hydrophilic particle impacting on a gas–liquid interface was studied both experimentally and mathematically. The aim of this study was to determine the critical impact velocity below which a falling hydrophilic particle would remain on a horizontal liquid surface. A model to predict the critical velocity has been developed based on energy balance of both the particle and liquid volume in the vicinity of the impact zone. The model also includes the effect of hydrophobicitiy (contact angle) of the particle as well as the change in potential energy of the impacted liquid. Experiments were performed using spherical glass beads of diameter 0.97–1.66 mm, and using liquids with varying density (1000–1182 kg/m³), viscosity (1.002–4.796 mPa s) and surface tension (50.31–87.42 mN/m). High speed video camera was used to obtain the particle impact velocity, cavity profile and velocity of the three-phase contact line (TPCL) at the critical conditions. The TPCL line velocity and cavity profile were used as inputs for the model. The fitted advancing contact angle was employed in the model. It was found that the model was in good agreement with the experimental observations, and the fitted advancing contact angle agreed with the combined molecular-hydrodynamic model well.     

The radiation-induced galvanic effect at a metal–dielectric interface

The effect observed upon interaction between the electromagnetic radiation with quantum energy of 25–1000 eV and a dielectric with metal coating is investigated. The radiation source was a megampere Z-pinch. Measurements performed on optical glass samples showed that radiation with a power of ~10⁶ W/cm² in the electric circuit switching on the metalized dielectric induces the current. It is shown that the observed galvanic effect originates from the generation of hot electrons in the dielectric.     

Zener–Stroh crack at the interface of multi-layered structures

A Zener–Stroh (Z–S) crack can be nucleated on the interface of a multi-layered structure when a dislocation pileup is stopped by the interface which works as an obstacle. During the entire fracture of a crack, Z–S crack mechanism controls the initial stage, or the first phase of crack initiation and propagation. In our current research, investigation on a Z–S crack at the interface of a multi-layered structure is carried out. The problem is formulated into a set of singular integral equations by applying the distributed dislocation based fracture mechanics. The obtained integral equations are then solved with numerical method after the singularities at crack tips are carefully checked. In the solution procedure, the contact zone model is adopted to cease the oscillation behavior. The contact zone length, the stress field near the crack tips and the stress intensity factors (SIFs) of the crack are discussed based on the numerical results of two typical structures. It was found that the contact zone length could be very large and was determined by the properties of all the three materials and loading conditions. Our analysis also shows that the thickness of the middle thin layer plays a critical role for the fracture behavior of the crack when it is comparable to the crack length.     

Special features of the interface between glass–metal coatings of superhard materials powders and metal matrices

The main results of the investigation of special features of the formation of the interface between the glass coating on the diamond and cBN powders with metals, that are bonds of grinding tools are described and generalized. It has been shown that the mutual diffusion occurring in the contact zone results in the increase of the adhesion at the interphase boundary, which ensures a strong fastening of a metal coating on a glass aggregate and the glass aggregate itself in a metal bond.     

Modeling of the solid–liquid bubble interface in partial nucleate boiling

Several models of heat transfer in partial nucleate boiling are identified in order to determine the relationship between the dominant physical parameters. The correlations are different for different models, so the main goal of this analysis is to determine the validity of each model and to identify the most dominant physical phenomenon in the nucleate boiling heat transfer. This is done by comparing the results of different models with a vast range of reliable experimental data. The comparison shows that the Sakashita and Kumada model gives the best results in the nucleate boiling heat transfer. It is also shown that the most dominating phenomenon in isolated partial bubbles zones is the transient conduction taking place mainly under the bubbles. This is in contradiction with a majority of the models that consider convection as the most important mode in the nucleate boiling heat transfer. The selected model can also be extrapolated and used in the case of fully developed bubbles zones.     

Determination of the thickness of the surface microlayer from which aerosols are formed by burst of gas bubbles at the liquid–gas interface

The surface diameters of gas bubbles at the liquid–gas interface whose burst leads to the formation of aerosol from a thin surface microlayer of thickness 1 μm and less have been determined experimentally. Precisely the anomalous concentrations of such a microlayer are responsible for the fractionation of substances in the process of ocean–atmosphere exchange.     

Numerical–experimental assessment of roughness-induced metal–polymer interface failure

A numerical–experimental method is presented to study the initiation and growth of interface damage in polymer–steel interfaces subjected to deformation-induced steel surface roughening. The experimentally determined displacement field of an evolving steel surface is applied to a numerical model consisting of a polymer coating and interface layer. The measured displacement field is obtained with a Finite Element based Digital Image Correlation method.The resulting simulations provide novel insights into the mechanical behaviour of the polymer–steel interface during interface roughening. The appearance of local hills and valleys on the evolving steel surface results in local bands of intensified stress in the polymer layer. These localized deformation bands trigger interface damage, which grows as the surface deformation increases. Polymer ageing initially delays the initiation of interface damage. However, for increased polymer ages the average interface damage increases. Likewise, the critical strain, at which the interface integrity is locally compromised, decreases.The presented method allows for a detailed study of the interface integrity during deformation-induced steel surface roughening. With properly identified material parameters, it becomes possible to tailor the polymer–steel material properties to minimize interface damage during production and storage of cans or canisters, e.g. for food and beverages.     

Heat transfer from Ni–W tapes in liquid nitrogen at different orientations in the field of gravity

Ni–W tapes of the micrometric thickness are considered as the basis for the cost-effective manufacturing of coated conductors – the 2nd generation of high-temperature superconductor (HTS). Many HTS applications involve widely-available and inexpensive liquid nitrogen. The transition from superconducting to normal state may however occurs due to unexpected temperature fluctuations. In this case Ni–W tape is significantly heated by electrical current propagating through it. The amount of heat transferred from the tape to coolant is defined by heat transfer from the surface of tape to liquid nitrogen. The heat transfer, in turn, is strongly dependent on the tape orientation in the field of gravity. The present paper reports the experimental results on the heat transfer from Ni–W tape to a pool of liquid nitrogen. The heat transfer coefficients are quantified for three subsequent heat transfer regimes: natural convection of liquid nitrogen, nucleate boiling regime and film boiling. The dependence of heat transfer coefficient on inclination angle of the tape from vertical are experimentally clarified for each regime. The expression for the heat transfer coefficient at different inclination angles is derived for the case of nucleate boiling.     

Evolution of the metal–oxide interface during the initial stage of the high temperature oxidation of ferritic stainless steels

Abstract

Two grades of ferritic stainless steel, a bi-stabilised Ti, Nb (AISI 441) and a stabilised Ti (AISI 439), were oxidised at 1060°C under the simulated process atmosphere for durations between 45 and 1800 s. Focused ion beam coupled with field emission gun and scanning electron microscopy was carried out to investigate the cross-section morphology of the oxide growing on ferritic stainless steels. Matrix protrusions localised at the metal – chromia interface through the silica layer are observed and the following mechanism for their formation is proposed. During the first step of oxidation, interface undulation, induced by growth stresses, in combination with silica precipitation at the metal – oxide interface lead to the formation of matrix protrusions in the chromia layer. For an increased oxidation time, due to the laterally silica growth matrix protrusions are trapped into the Cr₂O₃ layer as matrix inclusions.     

Charge transport and ammonia sensing properties of flexible polypyrrole nanosheets grown at air–liquid interface

Flexible polypyrrole nanosheets (thickness ∼150 nm) grown at the air–liquid interface have been investigated for charge transport and NH₃ sensing application. Polypyrrole nanosheets films exhibited a uniform and dense morphology. Temperature dependent charge transport measurements revealed that the PPy films obey Mott's 3-D variable range hopping mechanism. The mobility values calculated using temperature dependent current voltage characteristics indicated them to obey Arrhenius behaviour. These films exhibited a reversible response towards NH₃ at room temperature. The sensor exhibited a sensitivity of ∼12% with a typical response and recovery times of 240 s and 50 min, respectively towards 50 ppm of NH₃. Raman studies indicated that there is an increase in the antisymmetrical C–N stretching upon exposure to higher concentration of NH₃ (500 ppm) and could be assigned to the interaction of NH₃ with the carbon backbone of PPy film. Our results clearly emphasize that these flexible PPy films could be used to realize flexible sensors.     

Monolayer studies of sugar- and nucleoside-terminated dendrimers at the air–water interface

Sugar- and adenosine-terminated dendrimers, [1,2-o-Isopropylideneribosyl-(G₁-12acid), -(G₂-36acid)] and [Adenosyl-(G₁-12acid), -(G₂-36acid)], were synthesized using Newkome's dendrimer synthetic method. Langmuir and Langmuir–Blodgett (LB) monolayers of these dendrimers have been constructed and characterized at the air–water interface and on solid substrates by measuring surface pressure–molecular area (Π–A) isotherms, atomic force microscopy (AFM), ellipsometry and contact angle measurement. Π–A isotherms and AFM images showed that these dendrimers formed stable and homogeneous monolayers without aggregation on pure water surface. The first and second generation of sugar-terminated dendrimers show molecular areas of 647 and 1359 Ų, respectively. Ellipsometry measurement indicates that the thickness of both the first and the second generation of sugar-terminated dendrimers were about 10 Å. This reflects a flat orientation of both molecules at the air–water interface. On the other hand, the first generation of adenosine-terminated dendrimer shows an area of 105.6 Ų per molecule with a thickness of 16 Å, and for the second generation, the area was 738.4 Ų with a thickness of 27 Å. These results suggested that adenosine-terminated dendrimers maintain a spherical form at the air–water interface. It was found that small difference in the structure of thymine and uracil in the subphase critically affects the interaction of the molecules and conformation of the dendrimers at the interface.     

Measurement of liquid tin heat transfer coefficients within the temperature range of 506–1170 K

The thermal conductivity and thermal diffusivity coefficients of liquid tin within the temperature range of 506–1170 K were measured by the laser flash technique. The measurement errors for the heat transfer coefficients were equal to ±(2.5–3.5)%. Approximation equations and the reference data tables were obtained for the temperature dependency of the properties. The measurement results were compared with the available literature data. The Lorentz number temperature dependence was calculated up to 1000 K.     

A facile room temperature synthesis of ZnO nanoflower thin films grown at a solid–liquid interface

Hierarchical ZnO films consisting of nanoflower particulates are successfully grown by a solid–liquid interface reaction technique at room temperature without additives like surfactants, capping agent, or complexing agent. The structural, morphological, and photocatalytic properties of these films are studied using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and UV–Vis spectroscopy. The nucleation, growth processes and hence the resulting morphology of the end product can be regulated by changing the concentration of LiOH and the time of reaction. SEM throws light on the chronology of the flower formation by studying the intermediate morphology. Electron microscopy results indicated that these ZnO nanostructures self-assembled to produce flower-like nanostructures. The highest photocatalytic efficiency was observed for the films prepared at the concentration of LiOH 0.5 mg/mL in ethanol at 24 h. On the basis of the results, a plausible growth mechanism for the formation of flower-like ZnO nanostructures is discussed.     

Refinement of the V-O phase diagram in the range 25–50 at % oxygen

The boundaries of the V₁₄O₆ + V _x O _z two-phase region in the V-O system at temperatures from ? 1050 to ? 1650 K have been determined experimentally. The V-O phase diagram has been refined in the range 25–50 at % oxygen using structural and microstructural data for vanadium oxides containing less than 50 at % oxygen in conjunction with earlier results. The possibility of ordering of cubic vanadium monoxide has been examined.     

Frictional stresses at the disc–jaw interface during the standardized execution of the Brazilian disc test

An attempt to more accurately describe the boundary conditions of the standardized Brazilian disc test is presented. Specifically addressed is the problem of quantitatively relating the radial pressure with the tangential (frictional) stresses generated at the disc–jaw interface according to a physically acceptable law. A novel approach is proposed based on the notion that friction is directly related to the mismatch between the tangential components of displacement of the disc and jaw along their common interface due to the different deformability of the two materials. The surface displacements in both jaw and disc are determined using the complex potentials method, and the difference between their tangential components along the common contact arc is calculated. This difference in combination with the radial contact pressure tends to generate relative lateral displacements between the disc and jaw that are counterbalanced by frictional forces. The distribution of friction stresses along the contact rim obtained from the present approach fulfils all physical and intuitive imposed conditions. In addition, it is strongly skewed, attaining its maximum value at two-thirds distance from the centre of the contact arc, in good agreement with the earlier results based on a completely different approach.