Minimization Of Interfacial Energy Research Articles

Phase Field Simulation of the Lamellar Microstructure Formation in TiAl Alloys

The details of the lamellar microstructure in TiAl intermetallic alloys, such as the lath thickness and interfaces type governs the strength, ductility, creep properties and the long term microstructure stability of the alloy. The lamellar microstructure coarsening may induce property degradation of materials when the working temperature is high especially for the aero-engine turbine blades. At the same time, the reliability of the structure will decreases dramatically during long term working. In order to customize highly stable microstructure in high temperature, the phenomenon of lamellar formation during the solid-solid α→α2+γ phase transformation in fully lamellar TiAl alloys was investigated by phase field simulations. The lamellar structure morphology obtained with simulation is coincides with the experimental results. It is found that the independent nuclei and twin-related nuclei co-exist in the nucleation stage for the random noise nucleation. During growth stage, the independent nuclei grow slowly or disappear for the interfacial energy and elastic energy minimization. While most twin-related nuclei survived. During the following coarsening stage, big nuclei swallow small nuclei for interfacial energy minimization. The statistical character of twin area fraction changes complicated during these processes and it will be analyzed in detail. These findings could shed light on the understanding of the lamellar formation and coarsening mechanisms during phase transformation in TiAl alloys.

Disentangling Epitaxial Growth Mechanisms of Solution Derived Functional Oxide Thin Films

This study investigates the mechanisms of epitaxial development and functional properties of oxide thin films (Ce0.9Zr0.1O2−y, LaNiO3, and Ba0.8Sr0.2TiO3) grown on single crystal substrates (Y2O3:ZrO2, LaAlO3, and SrTiO3) by the chemical solution deposition approach. Rapid thermal annealing furnaces are very powerful tools in this study providing valuable information of the early stages of nucleation, the kinetics of epitaxial film growth, and the coarsening of nanocrystalline phases. Advanced transmission electron microscopies, X‐ray diffraction, and atomic force microscopy are employed to investigate the film microstructure and morphology, microstrain relaxation, and epitaxial crystallization. This study demonstrates that the isothermal evolution toward epitaxial film growth follows a self‐limited process driven by atomic diffusion, and surface and interface energy minimization. All investigated oxides experience a transformation from the polycrystalline to the epitaxial phase. This study unequivocally evidences that the film thickness highly influences the epitaxial crystallization rate due to the competition between heterogeneous and homogeneous nucleation barriers and the fast coarsening of polycrystalline grains as compared to epitaxial growth. The investigated films possess good functional properties, and this study successfully confirms an improvement at long annealing times that can be correlated with grain boundary healing processes. Thick epitaxial films can be crystallized by growing sequential individual epitaxial layers.

How a Surface Nanodroplet Sits on the Rim of a Microcap.

The location and morphology of femtoliter nanodroplets that nucleate and grow on a microcap-decorated substrate in contact with a liquid phase were investigated. We experimentally examined four different wetting combinations of the flat area and the microcaps. The results show that depending on the relative wettability, the droplets sit either on the plain surface or on the top of the microcap or on the rim of the microcap. The contact angle and, for the last case, the radial positions of the nanodroplets relative to the microcap center were characterized, in reasonable agreement with our theoretical analysis, which is based on an interfacial energy minimization argument. However, the experimental data show considerable scatter around the theoretical equilibrium curves, reflecting pinning and thus nonequilibrium effects. We also provide the theoretical phase diagram in parameter space of the contact angles, revealing under which conditions the nanodroplet will nucleate on the rim of the microcap.

Direct and controllable preparation of uniform PLGA particles with various shapes and surface morphologies

Abstract Biodegradable polymeric particles have been widely used in the biomedical fields such as drug carriers, tissue engineering, and medical imaging. It has been recognized that the physical properties of particles, such as size and shape, have a profound influence on their functions. However, it is not easy to prepare non-spherical particles by the conventional emulsion-based method because the minimization of interfacial energy leads to the spontaneous formation of spherical emulsions and it is difficult to control the uniformity and size. In order to solve this problem, we developed a direct method to fabricate non-spherical polylactic-co-glycolic acid (PLGA) particles with uniform size. This method employed a common agent, phosphate buffer saline (PBS), as the deformation initiator and combined the premix membrane emulsification technique with the solvent evaporation method to prepare uniform non-spherical particles of various shapes with aspect ratios from 2 to 40 and various sizes ranging from 800 nm to 60 μm (long axis). We found that the external driving force of stirring and the interfacial tension of the droplet dictated by the constitution of PBS and polyvinyl alcohol (PVA) have synergistic effects on the deformation. Through confocal laser microscopy (CLSM), the deforming process was observed visually in the solvent and gel state during the solidification. Moreover, octreotide acetate as a model peptide was successfully encapsulated into the non-spherical particles. The results indicated that the deformation process did not influence the encapsulation efficiency and bioactivity of the peptide, proving the feasibility of using non-spherical particles as potential drug delivery systems.

A Microfluidic Platform to Produce and Manipulate Liposomes - Towards Synthetic Cells on Chip

Liposomes are versatile supramolecular assemblies that are widely used in basic as well as applied sciences. Importantly, they mimic the semi-permeable lipid-bilayer boundary of living cells, and thus lie at the very heart of creating synthetic cells. Using liposomes as the basic architectural element and a controlled microfluidic environment, we wish to establish a continuous growth-division cycle of liposomes.We have developed a new and robust microfluidic technique to form unilamellar, cell-sized (5-20 μm), and monodisperse liposomes with an excellent encapsulation efficiency. Named octanol-assisted liposome assembly (OLA), our method involves a bubble-blowing process where an inner aqueous phase and a biocompatible 1-octanol phase containing lipids are simultaneously focused and pinched-off by an outer aqueous phase to form a double-emulsion droplet. This double-emulsion droplet quickly develops into an intermediate complex of a liposome connected to a 1-octanol pocket, and owing to interfacial energy minimization, the pocket spontaneously separates within a few minutes, leaving behind a fully functional liposome. We show the unilamellarity of the OLA-based liposomes by inserting functional α-hemolysin pores in the membranes. OLA is robust to physiological salt concentrations and variations in the bilayer composition, rendering it a very versatile high-throughput technique. It also circumvents the common problems such as remnants of unwanted organic solvents in the lipid bilayer and the time-consuming solvent-extraction associated with existing methods.In order to gain further control and increase the stability of the liposomes, we incorporate pneumatic valves, decoupling the production phase from the manipulation phase, and use dielectrophoresis to separate the liposomes from 1-octanol droplets. Future plans are aimed at exploring the division of liposomes using a combination of physical and biological approaches. Establishing on-chip growth-division cycle of liposomes would present a breakthrough towards synthetic cells.

Octanol-assisted liposome assembly on chip.

Liposomes are versatile supramolecular assemblies widely used in basic and applied sciences. Here we present a novel microfluidics-based method, octanol-assisted liposome assembly (OLA), to form monodisperse, cell-sized (5–20 μm), unilamellar liposomes with excellent encapsulation efficiency. Akin to bubble blowing, an inner aqueous phase and a surrounding lipid-carrying 1-octanol phase is pinched off by outer fluid streams. Such hydrodynamic flow focusing results in double-emulsion droplets that spontaneously develop a side-connected 1-octanol pocket. Owing to interfacial energy minimization, the pocket splits off to yield fully assembled solvent-free liposomes within minutes. This solves the long-standing fundamental problem of prolonged presence of residual oil in the liposome bilayer. We demonstrate the unilamellarity of liposomes with functional α-haemolysin protein pores in the membrane and validate the biocompatibility by inner leaflet localization of bacterial divisome proteins (FtsZ and ZipA). OLA offers a versatile platform for future analytical tools, delivery systems, nanoreactors and synthetic cells.

Modeling solid-state sintering with externally applied pressure: a geometric force approach

Solid-state sintering is one of the most widely used material processing technologies in modern manufacturing. The preponderance of previous computer simulations focused on free sintering (without externally applied pressures), in which the morphology evolution and densification in the system are driven by surface energy reduction. In this work, we develop an efficient algorithm to simulate solid-state sintering with hot pressing by explicitly considering interfacial diffusion as well as rigid body movement under external pressure. Particle movements including both translations and rotations are taken into account in the model, which are directed by the associated local stress state in the sintering system. A novel “geometric force” is also introduced to stochastically model geometrically necessary plastic deformations of the sintering particles to accommodate the initial fast densification due to the applied pressure. Subsequent evolution of the overall morphology and inter-particle connections are mainly controlled by interfacial energy minimization, while coarsening is considered in the later sintering stages. The utility of our method is illustrated by sintering 2D compacts of polydisperse circular particles and equal-sized elliptical particles. Significant coarsening occurs in the polydisperse particle system, while no significant grain growth is observed in the equal-sized ellipse system.

Interface and Strain Energy Revolution Texture Map To Predict Structure and Optical Properties of Sputtered PbSe Thin Films.

The preferred growth orientation of the sputtered lead selenide (PbSe) thin films on Si(100) substrates was thermodynamically simulated and calculated on the basis of the density functional theory. The results showed that the total free energy variation during the grain growth is dominated by the interface and strain energy minimization under certain conditions, indicating that the preferred growth orientation and related optical properties of the PbSe thin films can be effectively modified by these two energy variations. Thermodynamically, the PbSe[200] and PbSe[220] preferred orientations are obtained when the interface and strain energy minimization dominate the total free energy variation, respectively. A texture map related to the interface and strain energy revolution was obtained, which can be used to predict the structure and optical properties of the sputtered PbSe thin films, and its applicability was confirmed by the real X-ray diffraction and Fourier transform infrared spectroscopy experimental results of four midfrequency sputtered PbSe thin films with designed thickness and microstrain deposited on Si(100) substrates.

Bi2Te3 nanoflowers assembled of defective nanosheets with enhanced thermoelectric performance

Bi2Te3 assembled by disordered nanosheets with defects was prepared using a one-step easy synthesis strategy based on easy solvothermal synthesis routes. The high compatibile plane structure, minimization of nucleation and interfacial angle energies are the driving force of the crystal connection and the formation of the defects. To survey the impact of the defects on their thermoelectric properties, thermoelectric properties of bulk pellets have been investigated, and they exhibited preferable thermoelectric properties with a high ZT value of 0.68 due to its unique structural characteristics.

Spinel type twins of the new cubic Er6Zn23Ge compound

Abstract The crystal structure of the new Er6Zn23Ge intermetallic compound was established by X-ray diffraction analysis on a twinned crystal (space group Fm3̅m, Wyckoff sequence: f2edba, cF120–Zr6Zn23Si, a =12.7726(6) Å). The crystal is composed of two nearly equal size domains, whose mutual orientation is described by a 180° rotation around the cubic [111] axis, i.e. a spinel-type twinning law, not common for intermetallics. Applying the nanocluster approach, Er6Ge octahedra and centered two-shell Zn45 clusters were found as structural building blocks, filling the crystal space in a NaCl-like arrangement. This description was adopted to interpret the twinning in terms of stacking faults in the fcc cubic close packed arrangement. Moreover, the assembly of the nanocluster units is proposed as a possible mechanism for crystal growth and twin formation, in agreement with the principle of the interface energy minimization. Experimental conditions such as supersaturation and co-formation of other phases are also considered as favorable factors for Er6Zn23Ge twin formation.

Fabrication of Hierarchical Metallic Nanocomposite Core/Metal Shell Nanostructures by Self-Assembly.

Direct self-assembly of metals with inorganic nanoparticles into hierarchical nanostructures is highly demanded. Here we developed a simple methodology for direct self-assembly of metals (pure or alloy) and nanoparticles into hierarchical metallic nanocomposites core/metal shell nanostructures in immiscible molten salt, driven by the minimization of interfacial energy of the system. The core metals and nanoparticles firstly assemble into metal-nanoparticle nanocomposite microspheres. The formed nanocomposite microspheres could be utilized as new building blocks for a subsequent self-assembly procedure to be coated form a metal shell. This work provide a novel and simple method to fabricate hierarchical nanocomposites core/metal shell nanostructures for numerous applications.

Actuating Fluid–Fluid Interfaces for the Reconfiguration of Light

Advances in microfluidics and the associated ability to manipulate fluidic interfaces in the submillimeter scale have fueled the rapid development of a new class of highly reconfigurable optofluidic devices. In this type of devices, the optical interface is formed at the interface between two fluids. Actuation of the shape or the position of the fluidic interface allows dynamic reconfiguration of the optical properties of the device. Since a fluidic interface is intrinsically smooth as a result of minimization of interfacial energy, polishing is unnecessary in these optofluidic devices. It is also simple to obtain a graded profile of refractive index by using diffusion between miscible liquids possessing different refractive indices. This paper aims to review recent development of optofluidic devices based on the actuation of fluid-fluid interfaces, and also to illustrate their principles of operation.

Properties of three-dimensional structures prepared by Ge dewetting from Si(111) at high temperatures

The formation of three-dimensional (3D) structures during Ge deposition on Si(111) at about 800 °C is studied with scanning tunneling, Kelvin probe and electron microscopies, and scanning tunneling and Raman spectroscopies. The observed surface morphology is formed by dewetting of Ge from Si(111), since it occurs mainly by means of minimization of surface and interfacial energies. The dewetting proceeds through massive Si eroding around growing 3D structures, providing them to be composed of SiGe with about a 30% Ge content, and leads to the significant reduction of the SiGe/Si interface area. It is found that the SiGe top component of 3D structures forms sharp interfaces with the underlying Si. The minimization of interfacial and strain energies occurs on the way that the 3D structures appear to get the dendrite-like shape. The Ge distribution in the 3D SiGe structures is inhomogeneous in the lateral dimension with a higher Ge concentration in their central areas and Ge segregation on their surface.

A level set method for materials with texturally equilibrated pores

Textural equilibrium controls the distribution of the liquid phase in many naturally occurring porous materials such as partially molten rocks and alloys, salt–brine and ice–water systems. In these materials, pore geometry evolves to minimize the solid–liquid interfacial energy while maintaining a constant dihedral angle, θ, at solid–liquid contact lines. We present a level set method to compute an implicit representation of the liquid–solid interface in textural equilibrium with space-filling tessellations of multiple solid grains in three dimensions. Each grain is represented by a separate level set function and interfacial energy minimization is achieved by evolving the solid–liquid interface under surface diffusion to constant mean curvature surface. The liquid volume and dihedral angle constraints are added to the formulation using virtual convective and normal velocity terms. This results in an initial value problem for a system of non-linear coupled PDEs governing the evolution of the level sets for each grain, using the implicit representation of the solid grains as initial condition. A domain decomposition scheme is devised to restrict the computational domain of each grain to few grid points around the grain. The coupling between the interfaces is achieved in a higher level on the original computational domain. The spatial resolution of the discretization is improved through high-order spatial differentiation schemes and localization of computations through domain composition. Examples of three-dimensional solutions are also obtained for different grain distributions networks that illustrate the geometric flexibility of the method.

Fluidic assisted thin-film device heterogeneous integration: Surface tension as driving force and magnetic as guiding force

This paper demonstrates a fluidic assisted heterogeneous integration of optical thin-film device using surface tension as driving force and magnetic field as guiding force. Thin-film devices can be auto-aligned and integrated using surface tension onto their predesigned locations on a host substrate due to minimization of interfacial energy. By inserting a layer of nickel (Ni) into device metallization step, magnetic force was employed to increase mobility and contact probability of thin-film devices to their binding sites on the host substrate. A thin-film gallium arsenide (GaAs) inverted Metal–Semiconductor–Metal (MSM) photodetector (PD) has been successfully integrated onto a silicon host substrate with the proposed integration approach. The influence of the nickel layer to the PD performance was also investigated. Due to the self-assembly capability and thin-film format of the device, the proposed method has potential for wafer-scale implementation and is compatible with the matured silicon-based CMOS technology. This is a critical step towards a scalable manufacturing process to create advanced photonic/optoelectronic systems that are low-cost, compact, high performance, and complex in multi-material functionality.

Epitaxial and non-epitaxial platinum, palladium and silver films on yttrium-stabilised zirconia

Platinum, palladium and silver films have been prepared on differently orientated YSZ (yttrium-stabilised zirconia) substrates by PLD (pulsed laser deposition). The deposition temperatures for platinum were 200 °C and 400 °C, whereas palladium and silver were deposited only at 200 °C. The microstructure of the films depends on the particular metal, on the orientation of the substrate and on the deposition temperature. Platinum – deposited at 400 °C – forms single crystalline, epitaxial (111), (311) and (110) orientated as well as (111) orientated polycrystalline films. Platinum, palladium and silver – deposited at 200 °C – always form (111) orientated and polycrystalline films, in some cases also with a fraction of epitaxial grown grains. The formed microstructures were discussed on the basis of interface and surface energy minimization and structure zone models.

Parameters for Inherently Homogeneous Sintering Processes

Thermodynamic criteria are introduced to calculate the driving force for the formation of heterogeneous metastable states during sintering. As already shown for one special case of liquid phase sintering, these metastable states significantly affect the reliability of sintered ceramics. Inherently safe sintering processes are proposed, providing a homogeneous evolution of microstructure. A desintering parameter is defined which can be used for a quantitative evaluation of different sintering processes in terms of their tendency for formation of heterogeneity. The desintering parameter has been calculated using sintering simulations in 3D representative volume elements. For that, a deterministic model has been developed. It considers interface energy minimization and grain growth for random arrangements of particles. Computational effort could be significantly reduced by periodic continuation of the microstructure at the sides of the RVE. Basic parameters controlling inherently homogeneous sintering are presented. Surface diffusion is positive at large dihedral angles. Unexpectedly, grain growth can be advantageous as well, provided that it acts continuously during the entire sintering cycle.

Non-equilibrium oxidation states of zirconium during early stages of metal oxidation

The chemical state of Zr during the initial, self-limiting stage of oxidation on single crystal zirconium (0001), with oxide thickness on the order of 1 nm, was probed by synchrotron x-ray photoelectron spectroscopy. Quantitative analysis of the Zr 3d spectrum by the spectrum reconstruction method demonstrated the formation of Zr1+, Zr2+, and Zr3+ as non-equilibrium oxidation states, in addition to Zr4+ in the stoichiometric ZrO2. This finding resolves the long-debated question of whether it is possible to form any valence states between Zr0 and Zr4+ at the metal-oxide interface. The presence of local strong electric fields and the minimization of interfacial energy are assessed and demonstrated as mechanisms that can drive the formation of these non-equilibrium valence states of Zr.

Regulating cell behaviors on micropillar topographies affected by interfacial energy

Micropillar topographies can greatly influence the individual hepatic stellate cell behaviors, being triggered by a minimum interfacial energy.

Thermal stability of platinum, palladium and silver films on yttrium-stabilised zirconia

Platinum, palladium and silver films with different microstructures have been prepared on differently orientated yttrium-stabilised zirconia (YSZ) substrates by pulsed laser deposition and then annealed at temperatures between 200°C and 850°C. Thereby, an influence of the type of metal, of the microstructure of the as-prepared film and of the orientation of the substrate on the annealing behaviour could be determined. The following annealing effects were observed for platinum, palladium and silver films: i) sharpening of the film boundary, ii) smoothing of the film surfaces, iii) sharpening of the texture [thereby: reduction of the fraction of small angle and twin grain boundaries], iv) grain growth and accordingly reduction of the fraction of grains as well as v) grooving at grain boundaries, vi) void formation at the metal|YSZ-interface, vii) hole formation within the films and viii) reduction of the fraction of droplets. In the case of palladium films also ix) oxidation [between 300°C≤T<750°C] and stronger de-wetting phenomena than for platinum [with x) waving of the film and xi) island formation at T≥750°C] have been found. Silver films are not oxidised, but show stronger de-wetting phenomena than platinum and palladium, with xi) island formation and xii) evaporation of the silver at T≥550°C. Interestingly, silver films on (111) orientated YSZ are thermally much more stable than silver films on the other orientated substrates up to 750°C. The annealing effects were described by interface, grain boundary and surface energy minimization.