Interfacial Transitions Research Articles

High-temperature tribological properties of a novel AlFeCoNiCr–Cr3C2 cermet coatings

Microstructural instability of NiCr–Cr3C2 coating greatly limits its high-temperature tribological applications. In this work, we present a design concept for NiCr alloys based on an enthalpy-guided approach. A novel AlFeCoNiCr–Cr3C2 coating was fabricated using a supersonic plasma spraying followed by annealing. The high-temperature tribological results suggested that, due to the precipitation strengthening of Cr23C6 nano-grains in the interfacial transitions and the high plasticity of the disordered BCC (A2) and FCC (A1) phases in the AlFeCoNiCr alloy, the wear rate of the coating after annealing at 600 °C decreased by 47.6 % compared to the NiCr–Cr3C2 coating. The high density of mesoscopic defects further contributed to the strengthening and plasticity of the coating. After annealing at 900 °C, a net-shaped agglomeration of Cr23C6 and A2 phases accelerated oxidation diffusion and abrasive wear, leading to an increase of wear rate of coating. The produced wear debris mainly came from NiCr2O4, Cr2O3, NiO, Al2O3 and Fe2O3, where the NiCr2O4 was the product from the reaction between the Cr2O3 and NiO phases.

Modeling and simulation of multi-component immiscible flows based on a modified Cahn–Hilliard equation

In this study, an efficient method will be developed for the phase-field model of multi-component immiscible phases. The formulation of surface tension requires the interfaces to satisfy the hyperbolic tangent property. However, the interfacial transitions between different phases are not hyperbolic tangent profiles. The enclosed area is not preserved although the total mass is conserved by the original Cahn–Hilliard equation. This study is an extended research based on our previous study (Li et al., 2016). This work aims to apply the modified Cahn–Hilliard model into the multi-phase system. The interface is forced to be hyperbolic tangent by the modified Cahn–Hilliard system. The computational accuracy of the surface tension is improved under our multiphase framework. The mass loss of each phase can be reduced and the enclosed area can be preserved by the proposed method. We show various numerical results to demonstrate the robustness of the proposed modified model.

Preparation of Graphene Liquid Cells for the Observation of Lithium-ion Battery Material.

In this work, we introduce the preparation of graphene liquid cells (GLCs), encapsulating both electrode materials and organic liquid electrolytes between two graphene sheets, and the facile synthesis of one-dimensional nanostructures using electrospinning. The GLC enables in situ transmission electron microscopy (TEM) for the lithiation dynamics of electrode materials. The in situ GLC-TEM using an electron beam for both imaging and lithiation can utilize not only realistic battery electrolytes, but also the high-resolution imaging of various morphological, phase, and interfacial transitions.

Probing charge transfer states at organic and hybrid internal interfaces by photothermal deflection spectroscopy

In organic and hybrid photovoltaic devices, the asymmetry required for charge separation necessitates the use of a donor and an acceptor material, resulting in the formation of internal interfaces in the device active layer. While the core objective of these interfaces is to facilitate charge separation, bound states between electrons and holes may form across them, resulting in a loss mechanism that diminishes the performance of the solar cells. These interfacial transitions appear in organic systems as charge transfer (CT) states and as bound charge pairs (BCP) in hybrid systems. Despite being similar, the latter are far less investigated. Herein, we employ photothermal deflection spectroscopy and pump-push-probe experiments in order to determine the characteristics and dynamics of interfacial states in two model systems: an organic P3HT:PCBM and hybrid P3HT:ZnO photovoltaic layer. By controlling the area of the internal interface, we identify CT states between 1.4 eV and 1.8 eV in the organic bulk-heterojunction (BHJ) and BCP between 1.1 eV and 1.4 eV in the hybrid BHJ. The energetic distribution of these states suggests that they not only contribute to losses in photocurrent, but also significantly limit the possible maximum open circuit voltage obtainable from these devices.

Sodium storage in a promising MoS2-carbon anode: elucidating structural and interfacial transitions in the intercalation process and conversion reactions.

Sodium-ion batteries and capacitors are considered as low-cost energy storage devices, compared to Li-ion counterparts. However, most anodes for sodium-ion devices show sluggish kinetics and poor structural stability caused by the large radius (1.02 Å) of Na+. One candidate anode is MoS2, a 2D atomic layered material with a large interlayer spacing of 6.2 Å, that can take up and release Na+via two working principles: a two-electron intercalation process and a four-electron conversion reaction. Herein, we report a facile method to synthesize a MoS2-amorphous carbon (MoS2-AC) nanocomposite and further study the effect of the two working principles on the structure, interphase, and charge storage properties of MoS2-AC. The two-electron intercalation reaction enables the MoS2-AC electrode to have a higher rate capability and superior stability than that via the four-electron Na+ conversion reaction. This favorable Na+ charge storage performance of MoS2-AC via the two-electron intercalation process is attributed to its pseudocapacitive behavior, a stable solid electrolyte interphase and robust stability of the structure, which enables us to fabricate a sodium-ion capacitor that can deliver high energy density at a high rate. This work underscores the potential importance of realizing fast Na+ charge storage via an intercalation process as a strategy for the fabrication of high-performance sodium-ion capacitors and batteries.

Phase identification and interfacial transitions in ternary polymer blends by ToF-SIMS

Phase identification and the study of the interphase region in multi-component polymer blends with a chemically similar structure using conventional techniques is a challenge. In this work, the detailed morphological analysis of such systems is examined. A ternary blend comprised of poly butylene succinate (PBS); poly lactic acid (PLA); and polycaprolactone (PCL) with a partial wetting morphology is carefully selected since all three components are polyesters with low interfacial tensions. It will be shown that a novel technique by applying multivariate analysis (MVA) on time-of-flight secondary ion mass spectrometry (ToF-SIMS) data can effectively identify the complex phase structure, especially in blends with chemically similar components. Furthermore, for the first time for such systems, this technique provides detailed information about interfacial thicknesses and transitions. By employing the principal component analysis (PCA) method on the ToF-SIMS data of pure polymers, specific peaks with a certain molecular ion mass related to each polymer are determined. Using overlaid mappings on the surface of the blend by ToF-SIMS and selected ion masses to identify each polymer results in the differentiation of the various phases represented as a morphological image. In a second step, the multivariate curve resolution (MCR) method is used as a “self modeling curve resolution” for the recovery of pure components from a multi-component mixture when little or no prior information is available. Total pseudo-color RGB images of PBS/PLA/PCL show that PLA droplets unambiguously partially wet the PBS and PCL phases. Since each pixel from the analysis in the high lateral resolution image represents a 200 nm diameter, the interfacial transitions can also be studied for both PLA/PBS and PLA/PCL interfaces. The results show the concentration variation of phases across the interfaces. A complete trace line across the two interfaces (PLA/PBS and PLA/PCL) allows for the quantitative determination of interfacial thickness for the first time for such systems.

High-temperature interfacial electroluminescence in type-II broken-gap heterostructures based on InSb quantum dashes in n-InAs matrix

Room-temperature electroluminescence is observed for the first time in type-II heterostructures based on InSb quantum dashes embedded in a narrow-gap n-InAs matrix. The heterostructures exhibit positive luminescence at wavelengths in the range 3–4 μm. This is due to the interfacial radiative transitions of electrons from self-consistent quantum wells on the side of InAs matrix layers across the broken-gap type-II InSb/InAs heterointerface to quantum-well hole levels in the InSb quantum dashes, situated in the energy gap of the matrix near the InAs conduction-band bottom.

Temperature effect on the surface phase transitions of monolayer films of C12E1 at air/water interface

The temperature-dependent conformational states of a monolayer film of ethylene glycol monododecyl ether (C12E1) at the air/water interface have been investigated using ellipsometry, surface tension, external reflection–absorption FTIR spectroscopy and two-dimensional infrared (2DIR) correlation analysis. The ellipticity coefficients and the entropy associated with C12E1 adsorption changed almost discontinuously at certain temperatures, which manifested the interfacial phase transitions. The phase transition and coexistence of two phases were further clarified using 2DIR correlation analysis with temperature perturbation. The asynchronous correlation maps revealed that both bands of asymmetric and symmetric C–H stretching vibration in one-dimensional IR were split into two components, which confirmed the coexistence of two phases at the interface.

Interfacial rheology and conformations of triblock copolymers adsorbed onto the water–oil interface

The conformation and the dilatational properties of three non-ionic triblock PEO–PPO–PEO (where PEO is polyethyleneoxide and PPO is polypropyleneoxide) copolymers of different hydrophobicity and molecular weight were investigated at the water–hexane interface.The interfacial behavior of the copolymers was studied by combining dilatational rheology using the oscillating drop method and ellipsometry.From the dilatational rheology measurements the limiting elasticity values, E0, of the Pluronics as function of surface pressure, Π, and adsorption time were obtained, i.e. E0(t) and E0(Π). Here, it is shown that E0(t) depends on the number of PEO units and on the bulk concentration, showing maximum and minimum surface elasticity values which indicate conformational changes in the interfacial layer.Furthermore, in the framework of the polymer scaling law theory, conformational transitions were discussed in E0 vs. Π plots. In a dilute regime (Π<14mNm−1) at the water–hexane interface, E0=2Π fits well all the data, which indicates a two-dimensional “stretched chain” conformation. Increasing Π, two other interfacial transitions could take place. The different behavior of Pluronic copolymers could be also described by the local minima of E0, which depends on the hydrophobicity of the copolymers.Conformational transitions observed by interfacial rheology were compared to ellipsometric data. Experimental results were discussed and explained on the basis of two- and three-dimensional copolymer structure taking into account that PPO chains could be partially immersed in hexane and water.

Discussion notes on “Universality in the dependence of the drop contact angle on liquid-solid interactions and temperature obtained by the density functional theory”, by G.O. Berim &amp; E. Ruckenstein

Discussion notes on “Universality in the dependence of the drop contact angle on liquid-solid interactions and temperature obtained by the density functional theory”, by G.O. Berim &amp; E. Ruckenstein

Simulation of boiling heat transfer in small heaters by a coupled cellular and geometrical automata

Automata are entities defined by mathematical states that change following iterative rules representing neighborhood interactions. A model of automata for pool boiling heat transfer simulation consisting in collections of virtual spheres that change their volumes and move around a certain environment is presented. The approach is an alternative technique to describe the turbulent features of boiling phenomena, such as interfacial topological transitions and fluid-wall interaction. The novel computer model presented here is able to capture the essential features underlying boiling heat transfer and crisis above a small heater, showing good agreement with experimental data reported in the open literature.

Tests of nonuniversality and finite-size scaling for two-dimensional wetting with long-ranged forces

Effective Hamiltonian models predict nonuniversal critical singularities for two-dimensional wetting transitions with marginal long-ranged forces. We test these predictions by studying interfacial delocalization transitions in an infinitely long Ising strip, of width L (lattice spacings), with external fields that are long ranged and have opposite signs at each surface. Finite-size scaling suggests that the shift of the delocalization temperature T(c)(L) below the (semi-infinite) wetting temperature T(w) scales as L(-1/beta(s)) with beta(s) the adsorption critical exponent. Density-matrix renormalization-group methods allow us to study the behavior of T(c)(L) for L up to several hundred lattice spacings. For short-ranged forces the method recovers the universal value of beta(s)=1 known from the exact solution. While marginal long-ranged forces strongly influence the finite-size scaling of T(c)(L) , the extrapolated asymptotic value for the exponent beta(s) does not appear to confirm the predicted nonuniversality, but instead approaches the same universal value representative of systems with short-ranged forces.

Direct regrowth of thin strained silicon films on planarized relaxed silicon–germanium virtual substrates

We discuss a method for fabricating strained Si layers via deposition directly onto planarized relaxed SiGe virtual substrates, a process termed direct regrowth (DRG). We show that a trade-off exists between surface roughness and cleanliness of the Si/SiGe interface. Using this knowledge, we discuss process requirements to ensure that strained Si wafers produced via direct regrowth are free of interfacial contamination, exhibit ultra-low surface roughness, and feature abrupt Si/SiGe interfacial transitions. Finally, we show that metal-oxide-semiconductor field-effect transistors produced on strained Si channels fabricated via direct regrowth exhibit excellent device performance, suggesting the viability of the DRG process for manufacture of high quality strained Si wafers.

Statistical mechanics of fluids adsorbed in wedges and at edges

Some of the history and modern significance of wetting phenomena involving wedge and edge geometries is discussed. The adsorption of fluids onto structured surfaces or colloidal objects yields rich behaviour, including new classes of interfacial phase transitions and alternative ways to consider classical capillarity. I describe how the equilibrium properties of these systems can be described with exact virial theorems obtained from statistical mechanics, that are of immediate relevance to computer simulation procedures, density functional theories and experiments capable of measuring inhomogeneous fluid density profiles. In wedge geometry the physics behind these sum rules can be illustrated with the use of the Derjaguin approximation, familiar to colloidal science. Three specific examples of the utility of statistical mechanics to this field are described: (i) a sum rule analysis is provided of capillarity phenomena involving two-phase coexistence within a linear triangular wedge, whose walls could be chemically distinct, (ii) an example emphasizing that experimental systems will typically involve both wedge and edge geometries that cannot be readily separated and (iii) an example concerning adsorption within an annular wedge, taken from colloidal science.

Interactions and phase transitions of colloidal dispersions in bulk and at interfaces

Recent progress in the theory and computer simulation of effective interactions and phase transitions of colloidal dispersions is reviewed. Particular emphasis is put on the role of the discrete solvent in determining the effective interaction between charged colloids, bulk fluid–fluid phase separation of star-polymer–colloid mixtures, and on interfacial freezing transitions of sterically stabilized colloids on patterned substrates.

Hairpin Defects in Liquid-Crystalline Polymers

Conventionally, the term “defects” in liquid-crystalline systems refers to microscopic faults in the orientational order, which are usually visible optically. These are discussed in other articles in this issue. Our use of the term defect is entirely different. The defects we shall be considering, “hairpins,” occur on the scale of several angstroms and are abrupt reversals in the trajectory of a single liquid-crystalline-polymer (LCP) chain (Figure 1). In comparison to conventional defects, the direct observation of hairpin defects is much more difficult, yet their presence has important effects. Among the affected properties are the dimensions and the elasticity of the chains, the elastic behavior of the bulk nematic, and its dielectric response. Their presence should also give rise to a family of interfacial phase transitions in solutions of LCPs in nematic solvents. In turn, these are of interest in the design of liquid-crystalline displays. Hairpins in LCPs are superficially reminiscent of similar configurations in proteins and in homopolymers that undergo fold crystallization. These similarities are misleading because hairpins in proteins are permanent structures due to the chemical bonds. The folds in crystalline polymers are also fixed structures. In marked contrast, hairpin defects are mobile topological excitations that are created and annihilated continuously. Our emphasis in this article is on main-chain, semiflexible, nematic LCPs, consisting of mesogenic monomers joined by flexible spacer chains (Figure la). These polymers combine the orientational order of monomeric nematics with the flexibility and randomness inherent in polymers. The appearance of an oriented, nematic phase can be controlled either by temperature or by concentration.

Finite-size effects at wetting transitions.

A systematic analysis of finite-size effects at interfacial phase transitions is presented. Both static and (model A) dynamic phenomena are considered in a cubic ${L}^{d\mathrm{\ensuremath{-}}1}$ geometry with periodic boundary conditions in the directions parallel to the interfaces, but with no restriction on the interface separation z\ensuremath{\in}[0,\ensuremath{\infty}). In contrast to bulk critical phenomena, the finite-size behavior is shown to be qualitatively different at coexistence (with h=0) and for finite values of the symmetry-breaking field h. In particular, it is shown that the static finite-size scaling functions are singular in the finite-size limit when h\ensuremath{\ne}0. At coexistence, there are exponentially long relaxation times for L\ensuremath{\gg}${\ensuremath{\xi}}_{?}$, where ${\ensuremath{\xi}}_{?}$ is the interfacial correlation length, and a crossover to quasi-one-dimensional diffusive growth in the finite-size limit.

A series approach to wetting and layering transitions. III. The chiral clock model

For pt.II see ibid., vol.21, p.159-171, (1988). In the third of this triad of papers which study interfacial phase transitions using series expansions the authors treat the wetting transition of an interface in the three-state chiral clock model. Previous work has shown that on a simple cubic lattice at low temperatures the interface wets through a large, possibly infinite, number of layering transitions. They extend the low-temperature series results to an arbitrary number of nearest neighbours and show that, in the mean-field limit of infinite coordination number, only two layering transitions are seen. This is in agreement with numerical solutions of mean-field equations. Hence the mean-field approximation in this case does not provide a correct description of interface behaviour in three dimensions.

Dielectric spectroscopy of semi-insulating gallium arsenide

Dielectric spectroscopy of semiconductors (DSS) employs dielectric measuring techniques to study delayed electronic transitions in and out of localised energy levels in the forbidden gap. 'Horizontal' transitions between levels normally take place in the volume of the material and involve relatively small changes of energy. 'Vertical' transitions between deep levels and the free bands involve energy changes of the order of half the band gap and take place mainly in interfacial space-charge regions and at semiconductor-metal interfaces. DSS is uniquely able to resolve the spectra of these delayed transitions and measurements on semiinsulating (SI) GaAs in the frequency range 10-2-104 Hz and between 90 and 380K show that none of them conform to the expected exponential time dependence. The various transitions are found to have well recognisable spectral 'signatures' involving fractional power-law dependences on frequency and hence on time. The exponents are sensitive to the nature of the transitions and to the types of impurities, and may also depend on the crystalline order. The authors report for the first time in the context of single-crystal SI GaAs the phenomena of low-frequency dispersion both in the volume and in interfacial transitions, and also of negative capacitance, all of which involve some structural instabilities and the likelihood of electrochemical energy storage. They also show the existence of electronic phase transitions involving subtle reversible changes of electron-lattice interactions as well defined temperatures. They associate the power laws with many-body interactions affecting the transitions in the localised levels, which are not well understood probably on account hitherto of lack of experimental evidence. This shows the usefulness of DSS as a unique technique for studies of semiconductor transitions.

Interfacial properties and phase transitions of a system of anisotropic molecules

By employing an anisotropic scaling of the cartesian coordinates, the properties of a system of perfectly aligned ellipsoidal molecules can be exactly related to the analogous properties of a system of spherical molecules. This approach is used to determine for such a model system the structure of the liquid surface and the orientational dependence of the surface tension. It is found that the surface tension and surface stiffness are no longer equal as they are in an isotropic liquid. The smectic phase is absent, while a continuum of crystal phases exist, differing in the relative orientation between the molecular axis and the crystal axis.