Spinodal Mechanism Research Articles

Superconducting Properties and Structural Features of Solid Solutions on the Base of Barium-Samarium Cuprate

Superconducting properties, structural and microstructural features of single-phase polycrystalline samples of bariumsamarium cuprate and solid solution on the base of Sm1+xBa2–xCu3O6+y (123SmSS) with х = 0.0–0.7 prepared in air were studied. It was shown that the solid solution decomposes at 0.12 < х < 0.3 by the spinodal mechanism to form layered microstructures with a variable Sm/Ba ratio. The data obtained are useful for understanding the mechanisms of the formation of possible vortex pinning centers in Ln1+xBa2–xCu3O6+у phases with «light» Ln.

Stability of Ti–B–N solid solutions and the formation of nc-TiN/a-BN nanocomposites studied by combined ab initio and thermodynamic calculations

The ab initio density functional theory (DFT) has been used to calculate the properties of binary face-centered cubic (fcc)(NaCl)– and fcc (ZnS)–TiN and BN, hexagonal-close-packed (hcp)–TiB 2, and ternary Ti 1− x B x N and TiB x N 1− x solution phases. In order to study the stability of the ternary fcc(NaCl)–Ti 1− x B x N and of the nitrogen-deficient fcc(NaCl)–TiB x N 1− x solution, their mixing energies and the phase stability diagrams were constructed over the entire range of compositions. The results show that the fcc(NaCl)–Ti 1− x B x N should decompose by spinodal mechanism, whereas the substoichiometric fcc(NaCl)–TiB x N 1− x should decompose via nucleation and growth. The relatively large lattice mismatch between the fcc(NaCl)–TiN and fcc(NaCl)–BN, and the high lattice instability of the fcc(NaCl)–BN with respect to fcc(ZnS)–BN, suggests that the spinodal decomposition will, in the later stages, be accompanied by transformation of the fcc(NaCl)–BN to a more stable phase.

Effect of low-temperature decomposition on the resistive properties of Ba2YCu3O7 − δ ceramics

The effect of low-temperature decomposition of the nonstoichiometric (in oxygen) Ba2YCu3O7−δ phase on the critical characteristics of ceramic has been studied. The decomposition taking place in this system was found to occur via a spinodal mechanism. As a result of the decomposition, oxygen-depleted particles were shown to be formed in samples that are relatively rich in oxygen. These particles can play the role of pinning centers, which manifests itself in improving the dependence of the critical current density on the magnetic field.

Morphology change of asymmetric diblock copolymer micellar films during solvent annealing

The morphology change of an asymmetric polystyrene-block-poly(2-vinyl pyridine) (PS-b-PVP) diblock copolymer micellar film was investigated during solvent vapor annealing in chloroform. Initially, smaller islands in nanometer-length scale form at the film surface. Further annealing results in the growth of the islands composed of the PS-b-PVP cylinders above the bottom brush layer. For comparison, a film of the block copolymer prepared from THF solution (without micellar structure) was also studied. The surface morphology of the film from THF evolves via spinodal dewetting mechanism during solvent vapor annealing. At a long time solvent vapor annealing, the two kinds of the films display the same surface morphologies, which are determined by the interplay between the surface field and autodewetting.

Freezing of a Lennard-Jones Fluid: From Nucleation to Spinodal Regime

Using molecular dynamics, we investigate the crystal nucleation in a Lennard-Jones fluid as a function of the degree of supercooling. At moderate supercooling, a nucleation picture applies, while for deeper quenches, the phenomenon progressively acquires a spinodal character. We show that in the nucleation regime, the freezing is a two-step process. The formation of the critical nucleus is indeed preceded by the abrupt formation of a precritical crystallite from a density fluctuation in the fluid. In contrast, as the degree of supercooling is increased, crystallization proceeds in a more continuous and collective fashion and becomes more spatially diffuse, indicating that the liquid is unstable and crystallizes by a spinodal mechanism.

The Growth of Zn–Sb Nanowires by Heat Treatment of Zn–Sb Nanoparticles Obtained by Electrodeposition

Zn–Sb nanowires are prepared by heat treatment of Zn–Sb nanoparticles at 640 °C under an Ar atmosphere. The precursor, Zn–Sb nanoparticles, was obtained by electrodeposition. The length of the nanowires is more than 30 μm and the typical diameter is about 90 nm (see figure). A spinodal decomposition liquid solid mechanism is proposed to explain the Zn–Sb nanowire growth.

Diffusive phase transformations: Example of Fe–M alloys

A correlation has been performed between the existing theoretical notions of the diffusive phase transformations and the experimental data obtained by us earlier when studying Fe–M alloys by means of structural methods. The conclusions following from this correlation suggest that the phase transformations occurring at the level of changes in the electronic structure determine the possibility of further “secondary” phase transformations at the level of changes in the microstructure of alloys and, that the solid solution regions in the phase diagrams are in actuality regions in which a tendency towards phase separation takes place. Under suitable conditions, a microstructure of phase separation is formed. The transitions occurred at the level of a microstructure, no matter how the tendency (towards ordering or phase separation) manifests itself in an alloy, begin always with cluster formation and are followed by a spinodal mechanism.

Microstructure and properties of a Cu–Ti–Co alloy

It is now well established that dilute Cu–Ti alloys with titanium in the range of 2.5–5 wt.% decompose by a spinodal mechanism and, also, that the decomposition begins during the process of quenching itself from the temperature at which the alloy is solution treated. To study the initial stages of the decomposition reaction, the necessary suppression of the decomposition was attempted and achieved either by addition of a small quantity of a third element, namely, cobalt to the binary Cu–Ti alloy or by melt-spinning the binary alloy. Both the approaches were found to be successful. The results obtained through the second approach were recently published. The present work reports the results obtained through the former approach. In this work, the decomposition reaction in the ternary alloy was studied at temperatures of 673 and 723 K. The important findings relate to the effect of cobalt on the precipitation process and the properties of the alloy. During aging, the metastable Cu 4Ti (D1 a) was seen to evolve in this ternary alloy without the initial 1 1/2 0 ordering in the titanium enriched regions and the formation of the special point N 3M phase that were seen to precede the formation of the metastable Cu 4Ti (D1 a) in the melt spun alloy. Also, the ternary alloy was found, in the aged condition, to be extremely strong and ductile. Moreover, unlike the Cu–Ti binary alloy, the ternary alloy was found to be resistant to over-aging.

Spinodal-like dewetting of thermodynamically-stable thin polymer films.

Energetic considerations indicate that long-range Van der Waals forces stabilize thin polystyrene (PS) films against height fluctuations on silicon substrates. Nevertheless, we report here on the amplification of capillary waves of specific wavelengths for 15 nm thick PS films on silicon, ultimately leading to dewetting in a "spinodal-like" process. However, the temporal dependence of the wavelength of the growing instability does not agree with the "classical" spinodal dewetting mechanism. Therefore, this phenomenon is ascribed to the existence of "structural" forces resulting either from the restructuring of the films or from density variations within the films during annealing, in accordance with recent theoretical treatments. The process is shown not to be limited to polystyrene films, which indicates the generality of our findings.

On the sequence of clustering and ordering in a meltspun Cu–Ti alloy

Although it is well established that dilute Cu–Ti alloys with titanium in the range of 2.5–5 wt.% decompose by a spinodal mechanism, the sequence of ordering and clustering processes in the early stages has been a matter of controversy. An attempt has been made in this work to resolve some of the issues by carrying out transmission electron microscopy (TEM) investigations on a dilute meltspun Cu–Ti alloy. Decomposition was studied as a function of ageing over a temperature range of 573–723 K. The results seem to suggest that clustering precedes the formation of long range ordered (LRO) phases in this alloy. Formation of a transitory special point N3M phase was observed for the first time in this work. This disappeared on prolonged ageing giving rise to the metastable Cu4Ti (D1a).

Morphology of phase separated blends of poly(cyclohexyl methacrylate) with poly(vinyl acetate)

AbstractBlends of poly(vinyl acetate) (PVAc) and poly(cyclohexyl methacrylate) (PCHMA) labeled by copolymerization with 4‐methacryloylamine‐4′‐nitrostilbene (Sb), with (1‐pyrenylmethyl)methacrylate (Py) or with 3‐(methacryloylamine)propyl‐N‐carbazole (Cbz), were prepared by casting dilute solutions in tetrahydrofurane (THF) or chloroform onto silanized glass plates. The resulting films were studied by epifluorescence microscopy, microfluorescence spectroscopy, DSC and optical microscopy. Epifluorescence micrography probes the chemical composition of the different regions in phase separated blends, with black areas corresponding to PVAc rich regions and colored areas corresponding to labeled PCHMA rich regions. The technique also visualizes primary and secondary morphologies, which depend on the composition of the polymer blend and on the casting solvent. Mixtures containing 80 wt % PCHMA show, in general, a bicontinuous primary morphology suggesting a spinodal demixing mechanism. Solvent effects are particularly relevant for 50% and 20% PCHMA samples showing morphologies composed of PCHMA rich domains, in a matrix of solvent‐dependent compositions. Samples cast from chloroform are more homogeneous and the matrix is always highly fluorescent. In contrast, the domains of samples cast from THF are heterogeneous in size and shape and the matrix is non‐fluorescent, being thus formed by nearly pure PVAc. Small voids are formed in the polymer‐air interface. They are submicrometric for THF cast films and disappear with annealing at 122°C. For chloroform cast samples they are much less frequent and appear well ordered, forming a mostly hexatic two dimensional network. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1284–1290, 2003

Filler effect on polymerization kinetics and phase separation in polymer blends formed in situ

AbstractThe kinetics of formation of a mixture of two polymers [poly(methyl methacrylate) and polyurethane] in situ in the course of two simultaneously proceeding reactions was studied in the presence of various amount of a filler (fumed silica). It was shown that the filler affects the rates of both reactions. In addition, the filler exerts an influence on the phase separation induced by the chemical reaction increasing the amount of a filler increases the time for the onset of phase separation. The effects observed may be explained both by the increase in the viscosity of the reaction system due to introduction of a filler and by selective adsorption of reaction system components at the interface with filler particles. In all cases, phase separation in the early stages of reaction proceeds in a four‐component system (two polymers formed and two initial compounds) and obeys the spinodal mechanism. It is also shown that the final morphology is determined far from the end of the reaction and before establishing the equilibrium state.© 2003 Society of Chemical Industry

Effect of the morphology of two phase polymer blends on glass transition temperature

Blends of poly(cyclohexylmethacrylate), PChMA, with poly(vinylacetate), PVAc, were prepared by casting THF and chloroform solutions. A calorimetric and morphologic study was performed. Differential scanning calorimetry investigations of the blends show two glass transition temperature that give evidence of inmiscibility. For polymer less flexible (PChMA) was observed an enhancement of T g by 20 °C approximately in blends of PVAc-rich composition. Epifluorescence microscopy, using PChMA labelled by copolymerization pirenyl-methylmethacrylate, Py, shows also phase separation providing imaging of the distribution of PChMA in the different domains and in the matrix. Blends containing 80% w/w PChMA show a bicontinuous primary morphology suggesting a spinodal phase separation mechanism. The 50 and 20% PChMA samples show morphologies composed of PChMA-rich domains in a matrix composed by PVAc mainly. Blends with domain–matrix morphology present a higher T g than pure homopolymer more rigid (PChMA) due to packing in microphases in matrix of more flexible polymer (PVAc).

Phase separation in blends of linear polymers formed in situ according to different mechanisms

AbstractThe in situ formation of blends of two incompatible polymers was studied. The kinetics of simultaneously proceeding reactions of radical polymerization of methyl methacrylate and polyaddition reaction of polyurethane formation was investigated for various ratios of monomeric compounds in the reaction mixture. It was established that the kinetics of formation of each polymer in the mixture depends on the presence of the other and on the kinetics of its formation. These parallel reactions are accompanied by phase separation at a definite conversion degree of each monomer. The onset of phase separation does not affect the course of the kinetic curves. The regularities of phase separation in connection with reaction kinetics were investigated by light scattering. It was found that because of the difference in the rates of the two reactions, the ratio of the polymers formed in the reaction mixture continually changes in time. In spite of this, initial stages of the phase separation may be considered as proceeding according to the spinodal mechanism. Values of amplification factors were calculated which depend on the component ratio and reaction rates. It was established that reaction‐induced phase separation determines the formation of the interconnected morphology immediately after the onset of phase separation, when the composition of the polymer blends has not yet reached its equilibrium value.© 2002 Society of Chemical Industry

Synchrotron SAXS study on the micro-phase separation kinetics of segmented block copolymer

The phase transition behavior and isothermal micro-phase separation kinetics of polyester-based thermoplastic elastomer were studied using the synchrotron X-ray scattering (SAXS) method. The structural changes occurring during heating period were investigated by determining the changes of the one-dimensional correlation function, interfacial thickness and Porod constant. Based on the abrupt increases of the domain spacing and interfacial thickness, a major structural change occurring well below the melting transition temperature is suggested. Those changes are explained in terms of melting of the thermodynamically unstable hard domains or/and the interdiffusion of the hard and soft segments in the interfacial regions. SAXS profile changes during the micro-phase separation process were also clearly observed at various temperatures and the separation rate was found to be sensitively affected by the temperature. The peak position of maximum scattering intensity stayed constant during the entire course of the phase separation process. The scattering data during the isothermal phase separation process was interpreted with the Cahn-Hilliard diffusion equation. The experimental data obtained during the early stage of the phase separation seems to satisfy the Cahn-Hilliard spinodal mechanism. The transition temperature obtained from the extrapolation of the diffusion coefficient to zero value turned out to be about 147±2°C, which is close to the order-disorder transition temperature obtained from the Porod analysis. The transition temperature was also estimated from the invariant growth rate. By extrapolating the invariant growth rate to zero, a transition temperature of about 145±2°C was obtained.

Self-organized structures in thin liquid films on chemically heterogeneous substrates: Effect of antagonistic short and long range interactions

Surface instability, dynamics, and morphology in spontaneous dewetting of a thin liquid film on a chemically heterogeneous substrate are studied based on nonlinear simulations for a system subjected to a long range van der Waals attraction and soft short-range repulsion. Characteristics of dewetting by a heterogeneity are clearly contrasted with the spinodal dewetting on a homogeneous surface. In the presence of a chemical heterogeneity, the instability is engendered by the gradient of intermolecular interactions that lead to a microscale wettability contrast. The time scale of instability can be substantially less than the spinodal time scale, especially for thinner films close to the critical thickness, and it varies inversely with the potential difference induced by the heterogeneity. Heterogeneity, on a very small length scale, can even destabilize a spinodally stable film. A local ordering of the structure (droplets and holes) around the heterogeneity produces “castle-moat,” “ripples,” and “flower” like morphologies, which are not predicted by the spinodal mechanism. A local enhancement of the long-range attraction on the patch leads to a longer ranged ordering of holes around the heterogeneous patch for relatively thicker films. In contrast, a decrease in the short-range repulsion on the heterogeneous patch encourages an even longer-ranged ordering of droplets for the thinner films.

Spontaneous Dewetting and Ordered Patterns in Evaporating Thin Liquid Films on Homogeneous and Heterogeneous Substrates

The growth of instabilities and the initial stages of dewetting of volatile thin aqueous films on partially wettable solid substrates are investigated based on 2D nonlinear simulations. Dewetting by the formation of holes occurs by a spinodal mechanism due to the hydrophobic attraction on chemically homogeneous surfaces. The number density of holes and, consequently, the rate of dewetting can be enhanced by as much as an order of magnitude by evaporation on a homogeneous surface. At moderate to high rates of evaporation, all the holes do not form at the same time uniformly over the surface but form gradually in a rather ordered way around the earliest holes which act as “seeds”. On a chemically heterogeneous substrate, spatial gradients of the interaction potential and the rate of evaporation engender the surface instability. A chemical heterogeneity can induce faster rupture at a higher mean thickness and, thus, control the hole size distribution and the pattern of drying very significantly. A locally ordered, complex pattern often forms that consists of a central giant “nucleated” hole surrounded by a few concentric rings of smaller spinodally created satellite holes. An increase in the rate of evaporation encourages the formation of a larger number of ringlike structures containing the satellite holes but reduces the size difference between the spinodal satellite holes and the heterogeneously nucleated holes. The results obtained are in accord with recent experimental observations.

Instability and morphology of thin liquid films on chemically heterogeneous substrates

A new mechanism for the surface instability and dewetting of thin films on chemically heterogeneous substrates is identified and simulated. The time scale for instability varies inversely with the potential difference due to the heterogeneity. Heterogeneities can even destabilize spinodally stable films, reduce the time of rupture substantially for thicker films, and produce complex and locally ordered morphological features (e.g., ripples and castle-moat structures, lack of undulations before hole formation) that are not predicted by the spinodal mechanism.

Comparison of the phase behaviour of the liquid-crystalline polymer/poly(methyl methacrylate) and poly(vinyl acetate)/poly(methyl methacrylate) blends

The phase behaviour of blends of a liquid-crystalline polymer (LCP) and poly(methyl methacrylate) (PMMA), as well as the phase state of blends of PMMA and poly(vinyl acetate) (PVA) has been investigated using light scattering and phase-contrast optical microscopy. The blends of LCP and PMMA have been obtained by coagulation from ternary solutions. The cloud point curves were determined. It was established that both pairs demix upon heating, ie have an LCST. In the region of intermediate composition, the phase separation proceeds according to a spinodal mechanism; however for LCP/PMMA blends, the decomposition proceeds according to a non-linear regime from the very onset. In the region of small amounts of LCP, the phase separation follows a mechanism of nucleation and growth. For PMMA/PVA blends, the spinodal decomposition proceeds according to a linear regime, in spite of the molecular mobility that PVA chains develop at lower temperatures. Only after prolonged heat treatment does the process transit to a non-linear regime. The data show a similarity between the phase behaviour of blends of liquid-crystalline and of flexible amorphous polymers. The distinction consists of the absence of a linear regime of decomposition for LCP-PMMA blends. © 1999 Society of Chemical Industry

Dynamics of Rims and the Onset of Spinodal Dewetting at Liquid/Liquid Interfaces

Atomic force microscopy was used to investigate dewetting instabilities of liquid films from solid and liquid substrates in three limiting cases. We examined dewetting of a poly(styrene-co-acrylonitrile) (SAN) copolymer of viscosity ηT(SAN) from a solid substrate, SiOx, and from a liquid polystyrene (PS) substrate whose viscosity ηS(PS) ≫ ηT (SAN). The third case involved the dewetting of PS liquid films from liquid SAN substrates of varying thickness, HS. In this latter case, the viscosity of the top PS liquid layer ηT(PS) ≫ ηS(SAN), the viscosity of the SAN substrate layer. SAN films dewet SiOx substrates by a spinodal process when they were of thickness H < 300 Å. For H > 300 Å the SAN films were stable. For the situations in which the substrates were liquid PS films, SAN films of thickness HT(SAN) < 1000 Å dewet via a spinodal mechanism whereas SAN films of HT(SAN) > 1000 Å dewet the PS substrates by a mechanism involving the nucleation of holes with outer rims and the subsequent growth of these holes. Two regimes of growth were identified: an early stage regime where the hole initially nucleates and a rim is yet to form and a later stage regime where the rim exists. In this later stage, the hole radii RSAN ∼ t and the width of the rims lSAN ∼ t1/2. The height of the rims, qSAN, increased at a more rapid rate with time than the width of the rims and lSAN ≫ qSAN. In the final situation examined, where the PS films dewet the liquid SAN substrates (ηT(PS) ≫ ηS(SAN)) of varying thickness, we identified a transition from low-viscosity liquid substrate-like behavior to solid substrate-like behavior when the thickness of the SAN substrate layer became sufficiently thin. Finally, we discuss significant differences in the mechanisms by which holes grow when the substrate viscosity is liquidlike and when it is solidlike.