Suppression Of Phase Separation Research Articles

Spherulitic crystallization behavior of a metallic glass at high heating rates

Understanding crystallization in metallic glasses is an important consideration in the development of processing and manufacturing techniques which will utilize these unique alloys to full advantage. In the present work, the crystalline morphology and crystallization mechanisms of a Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 metallic glass are examined at heating rates spanning 4 orders of magnitude. In contrast to the multi-phase nanocrystalline structure formed at low heating rates, high heating rates (>2.5 K/s) result in the formation of a non-equilibrium crystalline phase with spherulitic microstructure and uniform composition. Heating rates as fast as 103 K/s are found to be insufficient to avoid the formation of spherulites. The microstructure suggests that rapid heating suppresses phase separation and nucleation processes at the initial stage of crystallization, resulting in a growth-dominated crystallization behavior. The activation energy for spherulite formation is found to be less than that for nanocrystallization, which is attributed to the lack of phase separation and nucleation steps. Calculation of the Avrami exponent also indicates that spherulites form via the growth of quenched-in nuclei. The number density of quenched-in nuclei is estimated to be 1014 m−3.

Suppression of phase separation through blending of electron transporting materials in polymer electrophosphorescent devices

Herein, we report the concentration effect of an electron transporting material, 1,3-bis ((4-tert-butyl-phenyl)-1,3,4-oxidiazolyl) phenylene (OXD-7), on the performance of polymer electrophosphorescent devices. From the phase mode images of atomic force microscopy, we observed that phase separation between the host material and the triplet dopants was suppressed in the photoactive film while the concentration of OXD-7 was increased. Correspondingly, the single-layer blue light-emitting device with the best external quantum efficiency of 7.9% was achieved. We attributed the device enhancement to the improved compatibility between the host and the phosphorescent molecules after the addition of OXD-7 molecules.

Evolution of phase separation in In-rich InGaN alloys

Evolution of phase separation in InxGa1−xN alloys (x∼0.65) grown on AlN/sapphire templates by metal organic chemical vapor deposition has been probed. It was found that growth rate, GR, is a key parameter and must be high enough (>0.5 μm/h) in order to grow homogeneous and single phase InGaN alloys. Our results implied that conditions far from thermodynamic equilibrium are needed to suppress phase separation. Both structural and electrical properties were found to improve significantly with increasing GR. The improvement in material quality is attributed to the suppression of phase separation with higher GR. The maximum thickness of the single phase epilayer tmax (i.e., maximum thickness that can be grown without phase separation) was determined via in situ interference pattern monitoring and found to be a function of GR. As GR increases, tmax also increases. The maximum value of tmax for In0.65Ga0.35N alloy was found to be ∼1.1 μm at GR>1.8 μm/h.

Composition-temperature phase diagram of Be xZn 1− xO from first principles

Density-functional calculations have been performed to obtain the composition-temperature phase diagram for the Be x Zn 1− x O alloys. Consistent density-functional theory within local density approximation has been employed for relaxing the atomic structures, calculating the enthalpy of formation, and determining the lattice vibrational contributions to the free energy. The results show that lattice vibrations significantly reduce the critical temperature. The predicted phase diagram is consistent with the experimental observation that alloying Mg with Be x Zn 1− x O suppresses phase separation.

Nematic ordering drives the phase separation of mixed monolayers containing phospholipids modified with poly(ethylene glycol) at aqueous–liquid crystal interfaces

We report that homogeneous Langmuir monolayers formed from mixtures of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylethanolamine conjugated to poly(ethylene glycol) (Mw = 2000 g mol−1, DPPE-PEG2000), when transferred onto interfaces formed between aqueous phases and nematic liquid crystals (LCs), phase separate in a manner that leads to patterned orientations of the LCs. Quantitative fluorescence microscopy establishes that the patterned orientations of the LCs correspond to coexisting interfacial phases that differ in total lipid density as well as lipid composition. Subsequent investigation determined that the interfacial phase separation of lipid is driven by the combined influence of the nematic ordering of the LC and the interactions between the PEG chains of DPPE-PEG2000. The latter effects impact the packing of the lipid monolayer and thus its interaction with the LC. In contrast to the phase separation of PEG-lipids within Langmuir films, where the repulsive interactions between PEG chains suppress phase separation at high interfacial densities of lipid, the results of our study indicate that repulsive interactions between PEG chains promote the phase separation of lipid at the aqueous–LC interface. Our results reveal a fundamentally new mechanism leading to control of the interfacial phase behavior of PEG-lipids, and suggest new principles for manipulation of the organization of multi-component lipid assemblies at interfaces.

Suppression of phase separation in InGaN layers grown on lattice-matched ZnO substrates

Sapphire and SiC are typical substrates used for GaN growth. However, they are non-native substrates and result in highly defective materials. The use of ZnO substrates can result in perfect lattice-matched conditions for 22% indium InGaN layers, which have been found to suppress phase separation compared to the same growths on sapphire. InGaN layers were grown on standard (0 0 0 2) GaN template/sapphire and (0 0 0 1) ZnO substrates by metalorganic chemical vapor deposition. These two substrates exhibited two distinct states of strain relaxation, which have direct effects on phase separation. InGaN with 32% indium exhibited phase separation when grown on sapphire. Sapphire samples were compared with corresponding growths on ZnO, which showed no evidence of phase separation with indium content as high as 43%. Additional studies in Si-doping of InGaN films also strongly induced phase separation in the films on sapphire compared with those on ZnO. High-resolution transmission electron microscopy results showed perfectly matched crystals at the GaN buffer/ZnO interface. This implied that InGaN with high indium content may stay completely strained on a thin GaN buffer. This method of lattice matching InGaN on ZnO offers a new approach to grow efficient emitters.

Hexagonal phase induced by a reversible photo-cross-link reaction in a polymer mixture

A hexagonal phase was found during the synthesis of interpenetrating polymer networks composed of polystyrene (PS) and poly(methyl methacrylate) (PMMA). By using confocal microscopy, it was found that the regularity of this hexagonal phase further increases upon de-cross-linking of the PS networks in the matrix phase by irradiation with shorter uv wavelengths. We conclude that the cooperation between the cross-link-induced suppression of phase separation and the elastic repulsion between the dispersed PMMA-rich domains is responsible for the emergence of this hexagonal phase.

Metalorganic Vapor Phase Epitaxial Growth Parameter Dependence of Phase Separation in Miscibility Gap of InGaAsP

The generation mechanism of the immiscibility of InGaAsP grown by metalorganic vapor phase epitaxy (MOVPE) is investigated. The growth condition dependence of the immiscibility is examined in detail using particular photoluminescence (PL) characteristics observed in the boundary composition between stable and unstable compositional regions. A lower growth rate or a larger V/III ratio is effective for suppressing phase separation. To explain this growth condition dependence, some factors related to surface energy should be taken into consideration.

Model for reversible nanoparticle assembly in a polymer matrix

The clustering of nanoparticles (NPs) in solutions and polymer melts depends sensitively on the strength and directionality of the NP interactions involved, as well as the molecular geometry and interactions of the dispersing fluids. Since clustering can strongly influence the properties of polymer-NP materials, we aim to better elucidate the mechanism of reversible self-assembly of highly symmetric NPs into clusters under equilibrium conditions. Our results are based on molecular dynamics simulations of icosahedral NP with a long-ranged interaction intended to mimic the polymer-mediated interactions of a polymer-melt matrix. To distinguish effects of polymer-mediated interactions from bare NP interactions, we compare the NP assembly in our coarse-grained model to the case where the NP interactions are purely short ranged. For the "control" case of NPs with short-ranged interactions and no polymer matrix, we find that the particles exhibit ordinary phase separation. By incorporating physically plausible long-ranged interactions, we suppress phase separation and qualitatively reproduce the thermally reversible cluster formation found previously in computations for NPs with short-ranged interactions in an explicit polymer-melt matrix. We further characterize the assembly process by evaluating the cluster properties and the location of the self-assembly transition. Our findings are consistent with a theoretical model for equilibrium clustering when the particle association is subject to a constraint. In particular, the density dependence of the average cluster mass exhibits a linear concentration dependence, in contrast to the square root dependence found in freely associating systems. The coarse-grained model we use to simulate NP in a polymer matrix shares many features of potentials used to model colloidal systems. The model should be practically valuable for exploring factors that control the dispersion of NP in polymer matrices where explicit simulation of the polymer matrix is too time consuming.

Annealing effects on the nanoscale indium and nitrogen distribution in Ga(NAs) and (GaIn)(NAs) quantum wells

III∕V semiconductors containing dilute amounts of nitrogen are metastable and need to be thermally treated after growth to optimize optoelectronic properties. The influence of thermal annealing on the nitrogen depth profile in metal organic vapor phase epitaxygrown Ga(NAs)∕GaAs as well as (GaIn)(NAs)∕GaAs heterostructures is examined on a nanometer scale by combining several high resolution transmission electron microscopy techniques, also with Rutherford backscattering spectrometry. Annealing conditions, which are optimized for quaternary alloys with respect to photoluminescence intensity, do not result in element redistribution for the In containing material. Contrary to the quaternary material, the result of annealing the ternary Ga(NAs) is a pronounced pileup of the nitrogen profile without any out diffusion of nitrogen. These findings have important influence on device structures, which often contain Ga(NAs) barriers for strain-compensation purposes together with (GaIn)(NAs) active regions. In the light of metastability considerations for the ternary and quaternary alloy, one can conclude that the In contained in the quaternary material stabilizes the material and suppresses phase separation. Consequently (GaIn)(NAs) is more stable than its ternary counterpart Ga(NAs).

Dissipative particle dynamics simulation study on the binary mixture phase separation coupled with polymerization

The influence of polymerization on the phase separation of binary immiscible mixtures has been investigated by the dissipative particle dynamics simulations in two dimensions. During polymerization, the bulk viscosity increases, which consequently slows down the spinodal decomposition process. The domain size growth is monitored in the simulations. The absence of 23 exponent for inertial hydrodynamic mechanism clearly reflects the suppressing effect of polymerization on the phase separation. Due to the increasing viscosity, the individual phase may be trapped in a metastable stage instead of the lamellar morphology identified for symmetric mixtures. Moreover, the polymerization induced phase separation in the binary miscible mixture has been studied. The domain growth is strongly dependent on the polymerization probability, which is naturally related to the activation energy for polymerization. The observed complex phase separation behavior is attributed to the interplay between the increasing thermodynamic driving force for phase separation and the increasing viscosity that suppresses phase separation as the polymerization proceeds.

The Study of Hafnium Silicate by NO Gas Annealing Treatment

The physical and electrical properties of nitrided Hf-silicate films, incorporated by NO gas annealing, were investigated by XPS, NEXAFS, TEM and C-V measurement. We confirmed the nitrogen incorporation during NO gas annealing treatment effectively enhances the thermal stability of Hf-silicate. The suppression of phase separation was observed in Hf-silicate films with high nitrogen contents. The negative shift of threshold voltage is caused by the incorporation of nitrogen in the hafnium silicate films.

Suppression of Phase Separation in PC/PMMA Blend Film by Thermoset Oligomer

Phase separation of bisphenol A polycarbonate (PC) and poly(methyl methacrylate) (PMMA) thin blend film is suppressed by addition of solid epoxy oligomer. Epoxy has strong intermolecular interactions with both PC and PMMA, while PC and PMMA are quite incompatible with each other. Consequently, phase separation in the PC/PMMA blend film pushes epoxy to the interface; at the same time, PC and epoxy react readily at the interface to form a cross-linking structure, binding PMMA chains together. Therefore, the interface between PC and PMMA is effectively reinforced, and the PC/PMMA thin blend film is stabilized against phase separation. On the other hand, only an optimal content of epoxy (i.e., 10 wt %) can serve as an efficient interfacial agent. In contrast to the traditional reactive compatibilization, here we observed that the cross-linking structure along the interface is much more stable than block or graft copolymers. Atomic force microscopy (AFM) is used to characterize the morphological changes of the blend films as a function of annealing time. Two-dimensional fast Fourier transform (2D-FFT) of AFM data allows quantitative investigation of the scaling behavior of phase separation kinetics.

Single-particle versus pair condensation of hard-core bosons with correlated hopping

We investigate the consequences of correlated hopping on the ground state properties of hard-core bosons on a square lattice as revealed by extensive exact diagonalizations and quantum Monte Carlo simulations. While for noninteracting hard-core bosons the effective attraction induced by the correlated hopping leads to phase separation at low density, we show that a modest nearest-neighbor repulsion suppresses phase separation, leading to a remarkable low-density pairing phase with no single particle Bose-Einstein condensation but long-range two-particle correlations, signaling a condensation of pairs. We also explain why the unusual properties of the pairing phase are a real challenge for standard one-worm quantum Monte Carlo simulations.

Change in phase separation and electronic structure of nitrided Hf-silicate films as a function of composition and post-nitridation anneal

The thermal stability and electronic structure of nitrided xHfO2∙(100−x)SiO2 (HfSiO) (x=25%, 50%, and 75%), prepared using an NH3 annealing treatment, were investigated. The quantity of N incorporated into the Hf-silicate film was dependent on the mole fraction of SiO2 in the film: i.e., a silicate film containing a high mole fraction of SiO2 contained a higher quantity of N, resulting in the suppression of phase separation. In particular, the incorporated N easily diffuses out through a silicate film that contains a small quantity of SiO2 during the post-nitridation anneal, while in a film with a high quantity of SiO2, it is relatively stable. The phase separation effect in the nitrided film with a low SiO2 mole fraction was significantly influenced by the stability of N in the film and interface.

Liquid-liquid phase separation and static light scattering of concentrated ternary mixtures of bovine α and γB crystallins

We have used light scattering, turbidimetry, and thermodynamic analysis to study the phase diagram of concentrated aqueous mixtures of the bovine lens proteins, gammaB crystallin, and alpha crystallin. We find that dilute alpha crystallin raises the phase separation temperature of concentrated gammaB crystallin, while more concentrated alpha crystallin suppresses phase separation. Very concentrated alpha/gammaB mixtures can reversibly cloud above 37 degrees C, even though gammaB alone phase separates only below temperatures near 0 degrees C, and alpha does not phase separate. At the scattering vector magnitude used, high-concentration alpha/gammaB mixtures scatter less light than the weighted average of their component alpha and gammaB solutions, while low-concentration alpha/gammaB mixtures scatter more than such a weighted average. We use a mean-field thermodynamic analysis of such ternary mixtures to show that the observed light scattering and phase boundaries of alpha and gammaB crystallin mixtures give evidence for prominent local fluctuations of relative protein composition. In the single phase, these fluctuations scatter comparatively little light, but are associated with enhanced thermodynamic instability. By applying this analysis to the experimental tie lines we estimate the magnitude of the saddlelike component of the free energy near the aqueous-gammaB critical point.

Suppression of phase separation in Hf-silicate films using NH3 annealing treatment

The structural characteristics of Hf-silicate films and nitrogen incorporated Hf-silicate films, prepared using a NH3 annealing treatment, were investigated by various measurements. Hf-silicate films annealed in a N2 ambient at 900°C show the evidence of crystallization in local regions, resulting in the phase separation of HfO2 and SiO2. In addition, a SiO2 overlayer is formed on the Hf-silicate films, due to the diffusion of Si by postannealing in an ambient of N2 at 900°C. However, in nitrogen incorporated Hf-silicate films, prepared using a NH3 annealing treatment, phase separation is effectively suppressed and no SiO2 overlayer is present. The incorporated N is distributed into the film and interfacial layer, and obstructs the diffusion of Si from the substrate as well as the film. Structural changes in films affect electrical characteristics such as the dielectric constant and flatband voltage.

Simulation and theory of self-assembly and network formation in reversibly cross-linked equilibrium polymers

A simulation model of hard spheres capable of reversible assembly into chains, which then may reversibly cross-link into networks, has been studied through grand canonical Monte Carlo simulation. Effects of varying intra- and interchain bond strengths, chain flexibilities, and restrictions on cross-linking angle were investigated. Observations including chain-length distributions and phase separation could be captured in most cases using a simple model theory. The coupling of chain growth to cross-linking was shown to be highly sensitive to the treatment of cross-linking by chain ends. In some systems, ladderlike domains of several cross-links joining two chains were common, resulting from cooperativity in the cross-linking. Extended to account for this phenomenon, the model theory predicts that such cooperativity will suppress phase separation in weakly polymerizing chains and at high cross-link concentration. In the present model, cross-linking stabilizes the isotropic phase with respect to the nematic phase, causing a shift in the isotropic-nematic transition to higher monomer concentration than in simple equilibrium polymers.

Simulation and theory of fluid demixing and interfacial tension of mixtures of colloids and nonideal polymers

An extension of the Asakura-Oosawa-Vrij model of hard sphere colloids and nonadsorbing polymers is studied with grand canonical Monte Carlo simulations and density functional theory. Polymer nonideality is taken into account through a repulsive step-function pair potential between polymers. Simulation results validate previous theoretical findings for the shift of the bulk fluid demixing binodal upon increasing strength of polymer-polymer repulsion, indicating suppression of phase separation. For increasing strength of the polymer-polymer repulsion, simulation and theory consistently predict the interfacial tension of the free interface between the colloidal liquid and the colloidal gas phase to decrease significantly for fixed colloid density difference in the coexisting phases, and to increase for fixed polymer reservoir packing fraction.

Anisotropic suppression of phase separation in polymer solutions by oscillatory shear

We consider the effects of subjecting a polymer solution to a simultaneous oscillatory shear flow and temperature jump into the two-phase region of the phase diagram. We predict that if the oscillatory shear stresses are significant enough then the flow suppresses phase separation in the flow direction, leading to the possibility of creating strongly aligned structures. We construct a quantitative dynamic phase diagram in the amplitude-frequency plane showing the conditions for the growth or decay of concentration fluctuations. Further, we discuss the time dependence of periodic structure factors in the flow-vorticity and flow-gradient planes. It is also shown that significantly enhanced scattering occurs at large scattering vectors regardless of whether the unbounded growth of fluctuations at smaller scattering vectors is suppressed.