Thermodynamics and structural aspects of grain boundary segregation

Physical and chemical properties of solid materials are strongly. influenced by the chemical composition of internal interfaces, One of the crucial parameters affecting interfacial chemistry is the atomic structure of the interface. Due to its importance. a considerable amount of work was done to elucidate the relationship between structure and chemical composition of interfaces. This article reviews the present understanding of an important and fundamental part of this relationship, namely, the structural aspects of grain boundary segregation. After a brief outline of grain boundary structure and geometry. thermodynamic approaches to describe grain boundary segregation are summarized and their application to materials is discussed. covering particular sites at a single grain boundary as well as the role of interfaces in polycrystals. Both the experimental evidence of grain boundary segregation anisotropy and the theoretical results of computer simulations of grain boundary segregation are summarized. Useful methods of predicting grain boundary segregation are presented. Finally, segregation behavior of solutes at grain boundaries is compared with that at free surfaces, and examples of chemical composition of intexphase boundaries are given.     

A fixture for interfacial dilatational rheometry using a rotational rheometer

In characterizing complex fluid-fluid interfaces, interfacial rheometry has become an important tool to indirectly probe the interfacial microstructure and molecular interactions. It can also be useful to obtain the constitutive properties of an interface for calculating the interfacial flows of complex fluid-fluid interfaces. A number of devices for measuring interfacial shear rheology have been designed and have been thoroughly validated. However, although a range of devices for measuring interfacial dilatational rheology exist, they do not always allow for a proper separation of the effects of dynamic surface tension, curvature elasticity, Marangoni stresses, bulk flow effects and the desired dilatational rheological material functions. In the present work it is investigated if a fixture for a standard rotational rheometer can be designed which probes the dilatational viscoelastic properties of a planar complex fluid-fluid interface. A modification of the double wall ring geometry for shear rheometry is proposed, which creates a mixed but analyzable flow field. The use of a mixed flow field inherently limits the sensitivity for the dilatational properties, but some advantages over existing techniques are presented, in particular for insoluble monolayers. More importantly, the analysis illustrate some generic aspects on the use of mixed interfacial flow fields for measuring the surface rheological properties.     

Determination of interfacial parameters in fiber-polymer systems from pull-out test data using a bilinear bond law

We propose a new model for characterization of strength properties of fiber-polymer interfaces by means of a single fiber pull-out test. Our model is based on shear-lag analysis using a bilinear bond law (stress–slip relationship) which, in turn, is a simplified representation of the true stress behavior as a function of strain for cold-drawing polymers. According to this law, the fiber-polymer interface is subjected to the following successive processes: (1) linear loading within the elastic region; (2) yielding and subsequent bond strengthening with increasing strain; (3) local debonding and interfacial crack propagation along the interface; (4) post-debonding friction. Both crack propagation and extension of the yielded zone can be stable and unstable, depending on the values of interfacial parameters and the load applied to the free fiber end. The procedure of construction of theoretical force–displacement curves for a pull-out test is described in detail. Theoretical curves exhibit such features as multiple kinks and non-linear regions, whose positions and shape are related to interfacial parameters. By fitting experimental curves with theoretical ones, these parameters can be determined for each separate pull-out specimen. Practical examples are provided for basalt fiber–polypropylene and glass fiber–polypropylene specimens.     

An appraisal of composite interface mechanics models and some challenging problems

A critical review of previous mechanics models proposed for the evaluation of interfacial properties from single fibre tests is presented with regard to their applicability and limitations. New results which include the effects of some important factors, such as pre-existing fibre flaws. thermal residual stresses and matrix cracks. are provided for a single fibre fragmentation test. By comparing the stress distributions of single fibre fragment and multi-fibre fragment, a basic method to study the multi-fibre composite is introduced in order to relate the interfacial parameters to the mechanical properties of the bulk composite. Some challenging problems on fibre-matrix interfaces are discussed for future research work.     

Structure and reactivity of environmental interfaces: Application of grazing angle X-ray spectroscopy and long-period X-ray standing waves

Chemical processes occurring at environmental interfaces (e.g. mineral–fluid, mineral–organic matter and mineral–biofilm interfaces) have a profound impact on the environmental fate and bioavailability of aqueous metals and other contaminant species. However, the direct analysis of molecular scale structure and properties of environmental interfaces, particularly under “high-pressure” or “wet” conditions is highly challenging. Synchrotron based X-ray scattering and spectroscopic approaches offer numerous advantages, such as the high penetrating power and molecular scale information inherent to X-ray techniques. Yet, the ability to localize information content to environmental interfaces requires challenging experimental configurations. Here, the application of grazing angle X-ray fluorescence techniques is reviewed, including the presentation of a model formalism that allows for quantitative analysis of fluorescent yield profiles and discussion of the experimental setup. Illustrative examples are discussed, particularly in the context of combining results of GI measurements with the results of other complementary interface probes such as crystal truncation rod diffraction and X-ray microprobe spectroscopic studies.     

Spectroscopic investigation of buried interfaces and liquids with soft X-rays

When, in general, two entities interact, they do it by forming an interface. The properties of such interfaces are determined not only by the properties of the two interface partners, but also to a large degree by the peculiarities of the interface-formation process itself. This is of particular importance in solid-state devices composed of two or more different materials. Unfortunately, the investigation of such interfaces is very difficult for two reasons. First they are, by their nature, buried. Secondly, interfaces generally form a thin layer within a larger ensemble and thus give very weak signals. Nevertheless, a few experimental techniques are available to study such buried interfaces. This report demonstrates that a combination of soft-X-ray spectroscopies (X-ray emission, photoemission, and X-ray absorption) is extremely well suited for this task. As examples, the electronic and chemical properties of several material systems are discussed, including II–VI semiconductors, Cu(In,Ga)(S,Se)₂ thin-film solar cells, organic thin films, and liquids. PACS 82.80.Ej; 73.20.-r; 68.35.Fx     

Force-field dependence on the interfacial structure of oil–water interfaces

We investigate the performance of different force-fields for alkanes, united (TraPPE) and all atom (OPLS-AA) models, and water (SPC/E and TIP4P-2005), in the prediction of the interfacial structure of alkane (n-octane, and n-dodecane)–water interfaces. We report an extensive comparison of the interfacial thermodynamic properties as well as the interfacial structure (translational and orientational). We use the recently introduced intrinsic sampling method, which removes the averaging effect of the interfacial capillary waves and provides a clear view of the interface structure. The alkane interfacial structure is sensitive to the environment, i.e. alkane–vapour or alkane–water interfaces, showing a stronger structure when it is in contact with the water phase. We find that this structure is fairly independent of the level of detail, full or united atom, employed to describe the alkane phase. The water surface properties show a small dependence on the water model. The dipole moment of the SPC/E model shows asymmetric fluctuations, with a tendency to point both towards the alkane and water phases. On the other hand the dipole moment of the TIP4P-2005 model shows a tendency to point towards the water phase only. Analysis of the intrinsic electrostatic field indicates that the surface water potential is confined to an interfacial region of about 8 Å. Overall we find that the intrinsic structure of alkane–water interfaces is a robust interfacial property, which is independent of the details of the force-field employed. Hence, it should provide a good reference to interpret experimental data.     

Reactive blending for improving interfacial behavior

Multicomponent polymer blends, for improving both process and product related properties, often require the optimization of interfaces/ interphases. This is particularly true in the case of blends based on recycled polymers. Besides a review of existing approaches, two examples are presented in more detail. The former is related to the toughening of recycled PET (poly(ethylene terephthalate)) by reactive ethylene-ethyl acrylate^_glycidyl methacrylate elastomers. The effect of catalysts of the reaction between elastomer epoxide functions and PET carboxyl and hydroxyl end-groups and of impurities introduced during recycling has been assessed. Variations of rheological behavior, morphology of chips and molded parts, and mechanical properties have been examined and related to interfacial chemistry. The formulation with glass fibers has also been experimented with, gaining interesting information on the compromise between resilience and rigidity. The second example is related to the so called 'light fraction' of recycled polymers, concerning mainly polyethylene (PE) and polypropylene (PP). Mixing has been performed in various relative amounts, as such and in the presence of initiators inducing both bulk and interfacial radical reactions. Effects on rheology, morphology and mechanical properties have been assessed, observing that the initiation of reactions is an important tool for improving the interfacial behavior.     

Phase behaviour and interfacial properties of ternary system CO2 + n-butane + n-decane: coarse-grained theoretical modelling and molecular simulations

ABSTRACT

This work illustrates the application of a three-party approach based on theoretical modelling, molecular dynamic (MD) simulations and available experimental data for describing the phase equilibrium and interfacial properties for the ternary system: carbon dioxide + n-butane + n-decane and its corresponding binary sub-systems at 344.3 K. Specifically, a coarse-grained force field is employed for both theoretical predictions and MD. The interfacial region is described by the square gradient theory where the homogenous Helmholtz energy density contribution is provided by the Statistical Associated Fluid Theory equation of state for potentials of variable range for molecules conformed of segments interacting through the Mie potential (SAFT-VR Mie) and MD simulations in the canonical ensemble where the molecules are represented by a coarse-grained Mie force field. The novelty here is that both the theory and the simulations uniquely share the same underlying intermolecular potentials; hence, the experimental data are employed to verify both the theory and simulations. In this schema, the ternary mixture is full predictive as its parameters are only based on pure fluids parameters and binary interactions. It is observed that the phase equilibria and the interfacial properties are equally well represented by the used approach.     

Modelling of mass-transfer induced instabilities at liquid–liquid interfaces based on molecular simulations

Interfaces and especially mass transfer across interfaces are of great importance in many fields of chemical engineering. Interfacial convection, which is generally called the Marangoni effect, may improve mass transfer across interfaces quite drastically and has not been investigated adequately in detail. In order to investigate the influence of mass transfer on a liquid–liquid interface molecular computer simulations have been performed. Since many molecules have to be considered for a significant modelling of the interface, cubic lattice systems have been chosen for the simulation which proceeds according to the Monte-Carlo scheme. The parameters that describe the thermodynamic and transport properties resemble those of realistic standard EFCE test systems for extraction. Results of various Monte-Carlo simulations show that under certain conditions mass transfer across interfaces induces the formation of nano droplets in the close vicinity of the interface. The different combinations of the nano droplet behaviour due to attractive or repulsive long-range forces together with the characteristics of coalescence may lead to different macroscopic interfacial instabilities such as spontaneous emulsification or eruptions. Based on diffusive and thermodynamic properties of the chosen lattice system a first stability criterion which allows the prediction of the onset of nano droplet formation is developed. The theoretical results compare well with experimental observations at a single drop and in a two-phase cell where the instabilities are investigated optically via Schlieren optics.     

The electronic structure of the interface between thin conjugated oligomer films and inorganic substrates with different work function

In this work, a comparison of the interfacial electronic properties between a semiconducting oligomer and a variety of substrates with different properties—metal, semiconductor and oxide layers—is reported. The interface formation was studied by X-ray and Ultraviolet photoelectron spectroscopies (XPS, UPS). High purity oligomer films with thickness up to 10 nm were prepared by stepwise evaporation on the clean substrates under ultrahigh vacuum (UHV) conditions. Analysis of the oligomer and substrate related XPS spectra clarified the interfacial chemistry and band bending in the semiconducting materials. The valence band structure and the interfacial dipoles were determined by UPS. The barriers for hole injection were measured at the interfaces of the organic film with all substrates. The interfacial energy band diagrams were deduced in all cases from the combination of XPS and UPS results. Emphasis was given on the influence of the substrate work function (eΦ) on the electronic properties of these interfaces.     

Self-organization processes at active interfaces

Four examples for active processes at interfaces are studied and reveal complex pattern formation phenomena including complex defect dynamics, standing waves and turbulence, bistability and domain formation and Turing patterns. The examples studied range from active Langmuir monolayers to thin films with floating molecular machines and biomembranes with active proteins. It is shown that linear stability analysis and numerical simulations of the resulting continuum model equations allow to qualitatively reproduce previous experimental observation in some cases and offer intriguing predictions for future investigations in the laboratory.     

Effects of temperature,pH, and ionic strength on the adsorption of nanoparticles at liquid–liquid interfaces

The effects of temperature, pH and sodium chloride (NaCl) concentration on the equilibrium and dynamic interfacial tension (IFT) of 4.4-nm gold nanoparticles capped with n-dodecanethiol at hydrocarbon–water interfaces was studied. The pendant drop technique was used to study the adsorption properties of these nanoparticles at the hexane–water and nonane–water interfaces. The physical size of the gold nanoparticles was determined by TEM image analysis. The interfacial properties of mixtures of these nanoparticles, having different sizes and capping agents, were then studied. The addition of NaCl was found to cause a decrease of the equilibrium and dynamic IFT greater than that which accompanies the adsorption of nanoparticles at the interface in the absence of NaCl. Although IFT values for acidic and neutral conditions were found to be similar, a noticeable decrease in the IFT was found for more basic conditions. Increasing the temperature of the system was found to cause an increase in both dynamic and equilibrium IFT values. These findings have implications for the self-assembly of functionalized gold nanoparticles at liquid–liquid interfaces.     

Theory of defects and defect processes

Most studies of imperfect solids concentrate on the properties of individual isolated defects. These include electronic structure, formation energies and diffusion parameters of point defects and point defect aggregates. Many physical phenomena are determined by defect processes, in which defects interact or the defect state of the lattice evolves. Such phenomena include the radiation damage process and subsequent development of microstructure, the matter and charge transport in the growth of oxide films, the recombination—or ionization—assisted diffusion in many systems, and a range of degradation mechanisms. These and other examples are discussed. The emphasis is on those cases where quantitative theory can unravel phenomena which experiment alone cannot easily do, notably those situations where the time scales are inconvenient or the system too complex for easy experimental analysis.     

Chemical and physical interactions at metal/self-assembled organic monolayer interfaces

The purpose of research on metals (M) deposited onto self-assembled monolayers (SAMs) is to understand the interactions between metal (M) and eventually metal oxide overlayers on well-ordered organic substrates. Application of M/SAM and inorganic/SAM research results to the understanding of real inorganic/ organic interfaces in vacuum and under environmental conditions can potentially play a key role in the development of advanced devices with stable interfacial properties. The M/SAM approach to interface research is delineated as a new subfield in surface science in the context of other approaches to inorganic/organic interface research. Current issues in M/SAM research are outlined, including chemical compound formation, the morphology (spreading, clustering, or penetration) of the metal species, the kinetics of the metal morphology, the effect of the metal on the degree of order in the SAM, and the rate of metal penetration into the SAM. Probes are recommended that are suitable for M/SAM research. The results of M/SAM studies to date are reviewed, and M/SAM combinations are ranked according to reactivity and penetration. Key probes for addressing gaps in the research results are identified. The effects of defects, disordering, air exposure, and X-ray and electron beam exposure on the experimental results to date are evaluated. Thus far, the results have successfully revealed qualitative relationships of M/SAM chemistry, temperature, and penetration. The chemical interactions that have been found are applicable to real M/polymer interfaces as formed in vacuum. It has yet to be shown that M/SAM research will yield quantitative understanding of interface formation or that M/SAM interfaces are entirely analogous to M/polymer interfaces in the details of interface formation. The future of this subfield of surface science lies in its expansion from M/SAM interfaces in vacuum to other inorganic/SAM interfaces in vacuum and, eventually, under environmental conditions.     

Numerical simulation of micro-scratch tests for coating/substrate composites

In this paper the micro-scratch test is simulated by ANSYS finite element code for thin hard coating on substrate composite material system. Coulomb friction between indenter and material surface is considered. The material elastic-plastic properties are taken into account. Contact elements are used to simulate the frictional contact between indenter and material surfaces, as well as the frictional contact after the detachment of coating/substrate interfaces has taken place. In the case of coating/substrate interfaces being perfectly bonded, the distributions of interfacial normal stress and shear stress are obtained for the material system subjected to normal and tangential loading. In the case of considering the detachment of interfaces, the length of interfacial detachment and the redistribution of stresses because of interfacial detachments are obtained. The influences of different frictional coefficients and different indenter moving distances on the distributions of stresses and displacements are studied. In the simulation, the interfacial adhesion shear strength is considered as a main adhesion parameter of coating/substrate interfaces. The critical normal loading from scratch tests are directly related to interfacial adhesion shear strengths. Using the critical normal loading known from experiments, the interfacial adhesion shear strength is obtained from the calculation. When the interfacial adhesion shear strength is known, the critical normal loading is obtained for different coating thicknesses. The numerical results are compared with the experimental values for composite materials of thin TiN coating on stainless steel substrate.     

第一原理研究界面弛豫对InAs/GaSb超晶格界面结构、能带结构和光学性质的影响

通过广义梯度近似的第一原理全电子相对论计算, 研究了不同界面类型InAs/GaSb超晶格的界面结构、电子和光吸收特性. 由于四原子界面的复杂性和低对称性, 通过对InAs/GaSb超晶格进行电子总能量和应力最小化来确定弛豫界面的结构参数. 计算了InSb, GaAs型界面和非特殊界面(二者交替)超晶格的能带结构和光吸收谱, 考察了超晶格界面层原子发生弛豫的影响.为了证实能带结构的计算结果, 用局域密度近似和Hartree-Fock泛函的平面波方法进行了计算. 对不同界面类型InAs/GaSb超晶格的能带结构计算结果进行了比较, 发现界面Sb原子的化学键和离子性对InAs/GaSb超晶格的界面结构、 能带结构和光学特性起着至关重要的作用.     

Adsorption at liquid interfaces: A comparison of multiple experimental techniques

It has proven to be a challenging task to quantitatively resolve the interfacial profile at diffuse interfaces, such as, the adsorption profile near a bulk binary liquid mixture critical point. In this contribution we examine the advantages and disadvantages of a variety of experimental techniques for studying adsorption, including neutron reflectometry, X-ray reflectometry and ellipsometry. Short length scale interfacial features are best resolved using neutron/X-ray reflectometry, whereas, large length scale interfacial features are best resolved using ellipsometry, or in special circumstances, neutron reflectometry. The use of multiple techniques severely limits the shape of the adsorption profile that can describe all experimental data sets. Complex interfaces possessing surface features on many different length scales are therefore best studied using a combination of neutron/X-ray reflectometry and ellipsometry.     

Influence of nano-SiO2 on dilational viscoelasticity of liquid/air interface of cetyltrimethyl ammonium bromide

An investigation was reported on the interfacial rheology of nano-SiO₂ dispersions in the presence of cetyltrimethyl ammonium bromide (CTAB). The interfacial dilational viscoelasticity had been measured as a function of the nano-particle concentration. The properties of the interface were affected by different processes such as the surfactants adsorption at the liquid or at the particle interfaces. It was found that the influence of nano-SiO₂ particles on the interfacial properties was evident and complex. The property of SiO₂ particles would change from hydrophilic to hydrophobic when CTAB molecules absorbed at their surface. The reorganization of surfactants and the participation of hydrophobic SiO₂ at the surface were offered to explain the process. In particular, the interaction between surfactants and particles, and the transfer of particles from bulk to the surface played an important role in changing the properties of the interface.     

SH ultrasonic guided waves for the evaluation of interfacial adhesion

Shear-Horizontally (SH) polarized, ultrasonic, guided wave modes are considered in order to infer changes in the adhesive properties at several interfaces located within an adhesive bond joining two metallic plates. Specific aluminium lap-joint samples were produced, with different adhesive properties at up to four interfaces when a glass–epoxy film is inserted into the adhesive bond. EMAT transducers were used to generate and detect the fundamental SH₀ mode. This is launched from one plate and detected at the other plate, past the lap joint. Signals are picked up for different propagation paths along each sample, in order to check measurement reproducibility as well as the uniformity of the adhesively bonded zones. Signals measured for four samples are then compared, showing very good sensitivity of the SH₀ mode to changes in the interfacial adhesive properties. In addition, a Finite Element-based model is used to simulate the experimental measurements. The model includes adhesive viscoelasticity, as well as spatial distributions of shear springs (with shear stiffness K_T) at both metal–adhesive interfaces, and also at the adhesive–film interfaces when these are present. This model is solved in the frequency domain, but temporal excitation and inverse FFT procedure are implemented in order to simulate the measured time traces. Values of the interfacial adhesive parameters, K_T, are determined by an optimization process so that best fit is obtained between both sets of measured and numerically predicted waveforms. Such agreement was also possible by adjusting the shear modulus of the adhesive component. This work suggests a promising use of SH-like guided modes for quantifying shear properties at adhesive interfaces, and shows that such waves can be used for inferring adhesive and cohesive properties of bonds separately. Finally, the paper considers improvements that could be made to the process, and its potential for testing the interfacial adhesion of adhesively bonded composite components.