Alternation of the flows of liquid components under asymmetric temperature oscillations

The mass transfer in a cylindrical inclusion of the melt with a rectangular cross-section in an anisotropic crystal in a nonstationary thermal field has been analyzed with due regard for the nonlinear solid-liquid interfacial kinetics. The possibility of the existence of a new effect is substantiated—alternation of the direction of the flows of liquid components depending on the rate of temperature variation. It is shown that the inclusion shape can be controlled by changing the shape of the temperature oscillations.     

Dendritic growth with interfacial energy anisotropy

A theory for dendritic growth of needle shaped crystals growing in supercooled pure melts and dilute alloys is proposed. This theory predicts the growth velocity and tip radii as functions of supercooling and alloy concentrations without introduction of additional conditions such as marginal stability. A key ingredient of the theory is introducing the effect of the interfacial energy anisotropy. Deviation of the shape of the dendrite from a paraboloid of revolution is permitted, consistent with a small interfacial energy anisotropy. The radius of curvature and the growth rate as functions of melt supercooling and alloy concentrations are determined in terms of the anisotropy, and are compared with experimental results. The predictions are in agreement with the experimental results, especially at large supercoolings. The deviation at lower supercoolings can be attributed to the neglect of natural convection in the present theory.     

Solubility data of trisodium citrate hydrates in aqueous solution and crystal‐solution interfacial energy of the pentahydrate

In this paper, the solubility of trisodium citrate dihydrate and trisodium citrate pentahydrate in water was experimentally determined. From solubility data, it was found that the relationship between trisodium citrate dihydrate and trisodium citrate pentahydrate is enantiotropic with a transition temperature at 315.4±1.0 K. Different hydrates can be isolated safely by controlling the crystallization temperature. The induction periods of trisodium citrate pentahydrate in aqueous solution were measured at different temperatures. The crystal–solution interfacial energy was calculated by using classical nucleation (CL) theory, mononuclear (MN) and polynuclear (PL) mechanisms through the relationship between induction period and supersaturation. It was found that the interfacial energy values calculated by using the CL theory and the MN model are nearly the same while interfacial energy calculated by PN model are about 40% higher. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the growth mechanism of sodalite single crystals grown by the hydrothermal method on single crystal seeds coated with informative informative interfacial layers

The growth mechanism of the early formation stages of sodalite single crystals grown by the method of hydrothermal synthesis on single crystal seeds coated with interfacial layers of polycrystalline silver has been studied at an electronmicroscopic scale. Coating with interfacial layers leads to a very weak adhesion between the overgrown single crystal and the surface of the interfacial layer on top of the seed, thus providing a unique possibility of detaching the overgrown single crystals from seeds and investigating the very early crystallization stages by the morphology of the growth surface. In local microregions of seed surfaces coated with interfacial layers discrete particles arise differing from one another in morphology, this being primarily associated with the electrical heterogeneity of seed surfaces. During crystallization, the space between the discrete particles was filled with the hydrothermal solution which represented a liquid interfacial layer exhibiting informative properties occurred under the influence of electrically active elements of the seed surface. At the boundary separating the liquid interfacial layers with particular informative properties from the rest of the solution volume, at early crystallization stages, together with the formation of discrete particles directly on the coated seed surface, growth of a continuous sodalite single crystal took place. The informative properties of seed surfaces, which are regularly modified due to coating with interfacial silver layers, determine the occurrence on local regions of seed surfaces (under appropriate crystallization conditions) of one or the other polymorphous modification: either hexagonal – cancrinite, or cubic – sodalite.     

Surface and interfacial structure of epitaxial SrTiO3 thin films on (0 0 1) Si grown by molecular beam epitaxy

Effects of relaxation of interfacial misfit strain and non-stoichiometry on surface morphology and surface and interfacial structures of epitaxial SrTiO₃ (STO) thin films on (0 0 1) Si during initial growth by molecular beam epitaxy (MBE) were investigated. In situ reflection high-energy electron diffraction (RHEED) in combination with X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectrometry (XPS) and transmission electron microscopy (TEM) techniques were employed. Relaxation of the interfacial misfit strain between STO and Si as measured by in situ RHEED indicates initial growth is not pseudomorphic, and the interfacial misfit strain is relaxed during and immediately after the first monolayer (ML) deposition. The interfacial strain up to 15 ML results from thermal mismatch strain rather than lattice mismatch strain. Stoichiometry of STO affects not only surface morphology but interfacial structure. We have identified a nanoscale Sr₄Ti₃O₁₀ second phase at the STO/Si interface in a Sr-rich film.     

On the dependence of interfacial tension on the size of two-dimensional nucleation

Among the assumptions in the derivation of the classical nucleation equations has been important that of the independence of the interfacial tension on the critical size of the two-dimensional nucleus. The one layer adsorption model based on Bragg-Williams approximation with empty sites has been used to investigate the dependence of interfacial tension on the number of molecules in the square shaped critical nucleus. Using the Gibbs adsorption at constant temperature for a two-component system, the number of molecules in the critical nucleus is exactly calculated in terms of the supersaturation, the interfacial tension and the interaction energies between the adsorbed molecules. The correction for the free energy of formation of critical two-dimensional nucleus due to the new term associated with the dependence of the interfacial tension on cluster size is derived. Assuming the correction term for the free energy is negligibly small, the expression for the variation of interfacial tension of an embryo from that of a plane surface is established. It is found that the deviation is significant for small clusters and decreases with increase in the size of thejnucleus.     

Surface Order Parameter of 4-n-Heptyl-4′-Cyanobiphenyl

We measured the order parameter near the interface between a liquid crystal and the solid substrate treated by several methods. The interfacial order parameter is different according to the surface treatment, and is larger than the bulk order parameter. Moreover, in the temperature above the clearing point, the interfacial order still has a finite value.     

Surface modification and grafting of carbon fibers: A route to better interface

This review is an audit of various Carbon fibers (CF) surface modification techniques that have been attempted and which produced results with an enhancement in the interfacial characteristics of CFRP systems. An introduction to the CF surface morphology, various techniques of modifications, their results and challenges are discussed here. CFs are emerging as the most promising materials for designing many technologically significant materials for current and future generations. In order to extract all the physic-mechanical properties of CF, it is essential to modulate a suitable environment through which good interfacial relation is achieved between the CF and the matrix. The interface has the utmost significance in composites and hybrid materials since tension at the interface can result in a deterioration of the fundamental properties. This review is aimed to provide a detailed understanding of the CF structure, its possible ways of modification, and the influence of interfacial compatibility in physic-mechanical and tribological properties. Both physical and chemical modifications are illustrated with specific examples, in addition to the characterization methods. Overall, this article provides key information about the CF based composite fabrication and their many applications in aerospace and electronics- where light weight and excellent mechanical strength are required.     

In situ observations in the melt growth of Pb5Ge3O11 crystals

The interface motion during the growth process of Pb₅Ge₃O₁₁ from the melt is visualized in an in situ observation system. The formation of the striation aggregation is observed, and the rates of the interface motion R_im are measured. It turns out that there are sharp changes in the growth rate during the formation of the striation aggregation. It has been found that the typical fluctuation in the rates can be associated with the kinetics and transport mixed control during the crystal growth. The non-uniform composition normal to the interface gives rise to the striation aggregation. Of particular importance in the investigation are the typical interfacial melt flows from the corners to the center of the interface observed in the experiments. The relation between the rates of this interfacial melt flow R_if and the time t is also provided showing that the change of supersaturation adjacent to the interface gives rise to the interfacial melt flow.     

Oxidation of Some Aliphatic Aldehydes in a Liquid-Liquid Interfacial System

In the present study the oxidation by chromic acid of some long-chain aliphatic aldehydes is investigated in a liquid-liquid interfacial system. For comparison the same oxidation was performed with the aldehydes dispersed in water. Higher yields were obtained in interfacial experiments as compared to the ones obtained when the aldehydes were dispersed in water. These results are rationalized by the fact that in the interfacial experiments reactants and products are exposed for short periods of time to the oxidative action of chromic acid, being in a way protected in the organic phase. Concerning the crucial problem of the reaction site i.e. where actually the oxidation occurs in interfacial experiments, it seems, based on kinetics and activation parameter determination as well as by modification of the interface with surfactants, that the oxidation occurs at the liquid-liquid interface and to a lesser extent in the aqueous phase. However, futher work is under way for the complete elucidation of the interfacial process.     

Supersaturation Dependence of Induction Period for Three-Dimensional Nucleation of Substances from Aqueous Solutions Revisited

The available experimental data on the supersaturation dependence of induction period for the nucleation of some compounds crystallizing from solutions are analysed from the standpoint of the classical nucleation theory and the mononuclear and polynuclear models in order to obtain values of interfacial tension and pre-exponential factor for homogeneous and heterogeneous nucleation. It was found that the contributions of homogeneous and heterogeneous nucleations to the experimental data can be separated by assuming that the nucleation rates due to the two nucleations are additive and that the polynuclear model in combination with the separated contribution of homogeneous nucleation to the induction period gives a satisfactory agreement between theoretical estimates and experimental values of the interfacial tension and the pre-exponential factor.     

Selective epitaxy and lateral overgrowth of 3C-SiC on Si – A review

This review article attempts to present a comprehensive picture of the progress in selective epitaxial growth (SEG) of cubic silicon carbide (3C-SiC) to make it a cheap and practical material for high temperature and high power, high frequency and MEMS (Micro Electromechanical Systems) applications. Selective epitaxial growth followed by epitaxial lateral overgrowth (ELO) is a suitable approach to minimize the interfacial defects and other planar defects in case of thin film growth. Different techniques of SEG and its application to Si, GaAs and III–V nitrides are reviewed briefly in the first section of this article. Various SEG techniques like epitaxial lateral overgrowth, pyramidal growth and pendeo epitaxial growth, etc. have been discussed extensively for growing 3C-SiC on Si, together with the characterization of the grown films. The influence of various experimental parameters such as temperature of growth, choice of mask material, influence of an etchant, pattern shape and size, etc. is also discussed. On the basis of these data, it is believed that SEG and related techniques are a promising approach for heteroepitaxial growth of 3C-SiC films useful for devices and MEMS applications.     

Nucleation of struvite (MgNH4PO4 · 6 H2O) single crystals and aggregates

The conditions (concentration and pH) for preparation of single crystals of struvite at 25 °C from ammoniacal solutions equimolar with respect to magnesium and phosphate ions are given. The limits are found above which crystal aggregates (twins or dendrites) precipitate as dominant morphology. The mechanisms of nucleation are discussed and the interfacial tension between crystals and solution is evaluated.     

Dielectric Spectroscopy of Ferronematics Based on 6CHBT Liquid Crystal

In our study, the dielectric behaviour of the rod-like liquid crystal (6CHBT) doped with magnetic nanoparticles of spherical shape was investigated by means of dielectric spectroscopy in the frequency range from 20 Hz to 2 MHz. A low frequency dielectric dispersion in the nematic and isotropic phases of the pure liquid crystal (LC) has been assigned to the space charge polarization. After doping the host LC with the magnetic nano particles, a nearly Debye-like relaxation process was observed with the temperature dependence obeying the Arrhenius law. Considering a possible electric double layer formation on the particle surfaces, the detected relaxation process in the doped LC can be associated with the electric double layer polarization. The experimental results point out that in the measured frequency range the space charge and interfacial effects constitute the main dielectric response. Any anchoring effects were not observed and are therefore expected to appear in higher frequencies.     

Effects of surface tension anisotropy on interfacial instability in directional solidification

Effects of surface tension anisotropy on the planar interfacial stability are studied with asymptotic analysis method in both the solidification from undercooled pure melts and the unidirectional solidification of binary alloys. The asymptotic approach developed by Xu is adopted to study the interfacial stability here, which is different from that used by other investigators previously in their works. A simple linear analysis result is obtained, i.e., the surface tension anisotropy may compete to determine interfacial stability near some critical conditions in unidirectional solidification of binary alloys. The exsitstence of the surface tension anisotropy enlarges the instability region of disturbed wave number. And the threshold of instability is strongly affected by surface tension anisotropy, especially at high pulling velocity or high temperature gradient. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of textured c-Si surface morphology on the interfacial properties of heterojunction silicon solar cells

For the HIT solar cells, the properties of interface between intrinsic thin film and c-Si are critical for the resulting device. The interfacial properties mainly depend on the surface passivation quality of c-Si, which is found to be affected by the morphology of textured surfaces. In this study, four kinds of textured c-Si substrates are fabricated: large pyramids without chemical polished (CP), large pyramids with CP, small pyramids without CP and small pyramids with CP. We investigated the effects of textured-surface morphology on the passivation of c-Si, the thin layer coverage and the interfacial properties of heterojunction prepared by HWCVD. Minority carrier lifetime measurements show that the wafer with small pyramids leads to better surface passivation than the one with large pyramids. The good coverage and contact between the thin film and the substrate can be achieved and no epitaxial growth occurs on the wafer with small pyramids through the study of TEM. Dark I-V measurements reveal that the heterojunction on wafer with small pyramids and CP has low recombination at the a-Si:H/c-Si interface. Our results indicate that the surface with small pyramids and low surface roughness is beneficial to the performance of HIT solar cells.     

Dielectric relaxation and electro-optical switching behavior of nematic liquid crystal dispersed in poly(methyl methacrylate)

Polymer dispersed liquid crystal composite films were prepared from poly(methyl methacrylate) and nematic liquid crystal E44 by solvent induced phase separation method. In the present investigation we report effect of liquid crystal concentration on the electro-optical and dielectric properties of the composite films. The results were interpreted in terms of phase separation of liquid crystal and polymer, shape and size of liquid crystal droplet, interfacial charge layer effect, liquid crystal loading and miscibility of liquid crystal in the polymer matrix. The miscibility between two phases at interface was investigated by employing Fourier‐Transform Infrared Spectroscopy and differential scanning calorimetry. Morphological study showed that liquid crystal phase is embedded in a spongy poly(methyl methacrylate) matrix and homogeneous distribution increased with increasing E44 content. Electro optical behavior of these composite films under the condition of an externally applied AC electric field (0–200 Vp-p, 50–1000 Hz) and wide range of temperature was determined using He–Ne laser (wave length 632.8 nm) as a light source. It was found that Poly(methyl methacrylate)/E44 (30/70) wt.% composite has more significant properties than the other concentrations. The performance of all composites showed variations with respect to applied voltage as well as temperatures. Dielectric measurement of polymer dispersed liquid crystals has been carried out in the frequency range from 20 Hz to 20 MHz and over the temperature range from 24 °C to 100 °C. The Maxwell–Wagner effect due to interfacial charge accumulation between boundaries of liquid crystal droplets and surrounding of polymer matrix has been observed.     

Metastable zonewidth, induction period and interfacial energy of zinc tris(thiourea) sulfate

Metastable zonewidth and induction period measurements on ZTS and ZTSP crystals are presented. The effect of pH on the solubility, metastable zonewidth and induction period has been studied. The effect of substitution of phosphate on the nucleation parameters has been presented. The interfacial energy for pure and phosphate mixed ZTS solutions at 30°C has also been estimated. There is a significant variation in the induction period and interfacial energy due to decrease in solution pH and phosphate substitution.     

Homogeneous nucleation in cooling crystallization

Nielsen's equation has been applied for cooling crystallization of KCl from watery solutions. In the calculations activities are used instead of concentrations, the temperature and concentration dependence of both the activity and diffusion coefficients are considered. It is found that A) the interfacial tension between the crystal and its saturated solution is < 2.5 erg/cm², which is less by two orders of magnitude than that of between crystal and vacuum; B) homogeneous nucleation can take place even at very small supersaturations (ln S < < 0.01), which is unusual in precipitation.     

Nucleation Studies in Supersaturated Triglycine Sulphate Solutions

Rates of nucleation of supersaturated aqueous solutions of Triglycine sulphate are measured by the induction period over the temperature range 30–50 °C. The effects of temperature and supersaturation on the induction period are reported. The interfacial tension, energy of formation and radius of the critical nuclei of the crystal are calculated in accordance with classical nucleation theory. Nucleation rate increases with increase in supersaturation and temperature, while interfacial tension decreases with increase in temperature.