Nature Of Phase Separation Research Articles

Magnetic, dielectric and transport characteristics of Ln2CoMnO6 (Ln=Nd and Sm) double perovskite ceramics

Magnetic and dielectric properties of Nd2CoMnO6 and Sm2CoMnO6 double perovskite ceramics have been investigated by comparing with those of La2CoMnO6 and Ln2NiMnO6 (Ln=La, Nd and Sm) ceramics. Differing from the single magnetic state of Ln2NiMnO6 (Ln=La, Nd and Sm) ceramics, both Nd2CoMnO6 and Sm2CoMnO6 are multiphase magnetic state with the monoclinic symmetry (space group P21/n). Ferromagnetic Curie point TC generally decreases with decreasing 〈Ni/Co–O–Mn〉 bond angles which depends on lanthanide ionic radius (RLn). This result indicates that the 〈Ni/Co–O–Mn〉 bond angle plays an important role in magnetic properties of Ln2MMnO6 (Ln=La, Nd and Sm, M=Ni and Co) ceramics. Meanwhile, only one relaxor-like dielectric peak with strong frequency dispersion is observed in Nd2CoMnO6 and Sm2CoMnO6 ceramics, which is similar with that of La2CoMnO6. The dielectric constant ε′ also decreases with decreasing RLn. Both magnetic and dielectric properties of Ln2CoMnO6 (Ln=La, Nd and Sm) ceramics have similar variation tendency with decreasing RLn, and have been correlated with cos2〈Co–O–Mn〉, demonstrating some coupling between magnetic and dielectric response. In addition, transport behaviors of the present ceramics conform to the three-dimensional variable-range hopping mechanism, which may be attributed to the phase separation nature.

Historical Perspective of Advances in the Science and Technology of Polymer Blends

This paper will review the important developments in the field of polymer blends. The subject of polymer blends has been one of the most prolific areas in polymer science and technology in the past five decades judging from publications and patents on the subject. Although a continuing important subject, the peak intensity occurred in the 1970s and 1980s. The author has been active in this area for five decades and this paper is a recollection of some of the important milestones/breakthroughs in the field. The discussion will cover the development of the theory relevant to polymer blends, experimental methods, approaches to achieve compatibility in immiscible/incompatible blends, the nature of phase separation and commercial activity.

Characterization of a resorbable poly(ester urethane) with biodegradable hard segments

The rapid growth of regenerative medicine and drug delivery fields has generated a strong need for improved polymeric materials that degrade at a controlled rate into safe, non-cytotoxic by-products. Polyurethane thermoplastic elastomers offer several advantages over other polymeric materials including tunable mechanical properties, excellent fatigue strength, and versatile processing. The variable segmental chemistry in developing resorbable polyurethanes also enables fine control over the degradation profile as well as the mechanical properties. Linear aliphatic isocyanates are most commonly used in biodegradable polyurethane formulations; however, these aliphatic polyurethanes do not match the mechanical properties of their aromatic counterparts. In this study, a novel poly(ester urethane) (PEsU) synthesized with biodegradable aromatic isocyanates based on glycolic acid was characterized for potential use as a new resorbable material in medical devices. Infrared spectral analysis confirmed the aromatic and phase-separated nature of the PEsU. Uniaxial tensile testing displayed stress–strain behavior typical of a semi-crystalline polymer above its Tg, in agreement with calorimetric findings. PEsU outperformed aliphatic PCL-based polyurethanes likely due to the enhanced cohesion of the aromatic hard domains. Accelerated degradation of the PEsU using 0.1 M sodium hydroxide resulted in hydrolysis of the polyester soft segment on the surface, reduced molecular weight, surface cracking, and a 30% mass loss after four weeks. Calorimetric studies indicated a disruption of the soft segment crystallinity after incubation which corresponded with a drop in initial modulus of the PEsU. Finally, cytocompatibility testing with 3T3 mouse fibroblasts exhibited cell viability on PEsU films comparable to a commercial poly(ether urethane urea) after 24 h followed by 85% cell viability at 72 h. Overall, this new resorbable polyurethane shows strong potential for use in wide range of biomedical applications.

The nature of phase separation in Ir–Sn–O ternary oxide electrocatalyst

A phase stability diagram of ruthenium–zirconium oxide (Ru–Sn–O) was constructed by a combination of ab initio density functional theory and thermodynamic calculations. Results suggest that the phase separation/segregation that has been reported in the literature for the RuO2–SnO2 system is through a typical spinodal decomposition mechanism. Ru0.45Sn0.55O2 films were prepared by thermal co-decomposition of precursors at 500 °C for varied duration. Quantitative phase analyses of the prepared films based on X-ray diffraction and high-resolution transmission electron microscopy confirmed the spinodal nature of the phase separation. The present fundamental study provides a theoretical guideline for the phase and microstructure design of Ru–Sn–O based mixed oxides for electrocatalysis applications.

Influence of thermal history on the photostructural changes in glassy As15S85 studied by Raman scattering and ab initio calculations

Photostructural changes—the hallmark of non-crystalline chalcogenides—are in essence the basis of a number of photoinduced effects, i.e., changes of their physical properties, which are exploited in a variety of applications, especially in photonics and optoelectronics. Despite the vast number of investigations of photostructural changes, there is currently lack of systematic studies on how the thermal history, which affects glass structure, modifies the extent of photostructural changes. In this article, we study the role of thermal history on photostructural changes in glassy As15S85. This particular sulfur-rich composition has been chosen based on the colossal photostructural response it exhibits under near-band gap light irradiation, which inherently originates from its nanoscale phase-separated nature. To control the thermal history, the glass was quenched to various temperatures and each of these quenched products was annealed under four different conditions. Off-resonant Raman scattering was used to study the equilibrium study of each product. Structural changes of interest involve changes of the sulfur atoms participating into S8 rings and Sn chains. Their ratio was found to depend on quenching/annealing conditions. Near-band gap light was used to perturb the rings-to-chain ratio and at the same time to record these changes through Raman scattering, revealing an intricate behavior of photostructural changes. Ab initio calculations were employed to determine the stability of various sulfur clusters/molecules thus aiding the correlation of the particular photo-response of glassy As15S85 with its structural constituents.

Visualization of Phase Evolution in Model Organic Photovoltaic Structures via Energy-Filtered Transmission Electron Microscopy

The morphology of the active layer in an organic photovoltaic bulk-heterojunction device is controlled by the extent and nature of phase separation during processing. We have studied the effects of fullerene crystallinity during heat treatment in model structures consisting of a layer of poly(3-hexylthiophene) (P3HT) sandwiched between two layers of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Utilizing a combination of focused ion-beam milling and energy-filtered transmission electron microscopy, we monitored the local changes in phase distribution as a function of annealing time at 140 °C. In both cases, dissolution of PCBM within the surrounding P3HT was directly visualized and quantitatively described. In the absence of crystalline PCBM, the overall phase distribution remained stable after intermediate annealing times up to 60 s, whereas microscale PCBM aggregates were observed after annealing for 300 s. Aggregate growth proceeded vertically from the substrate interface via uptake of PCBM from the surrounding region, resulting in a large PCBM-depleted region in their vicinity. When precrystallized PCBM was present, amorphous PCBM was observed to segregate from the intermediate P3HT layer and ripen the crystalline PCBM underneath, owing to the far lower solubility of crystalline PCBM within P3HT. This process occurred rapidly, with segregation already evident after annealing for 10 s and with uptake of nearly all of the amorphous PCBM by the crystalline layer after 60 s. No microscale aggregates were observed in the precrystallized system, even after annealing for 300 s.

Microstructural properties of K0.8 Fe1.6 S2, K0.8 Fe1.75 Se2-y Sy(0 ≤ y ≤ 2) and K0.8 Fe1.5+x S2(0 < x ≤ 0.5) single crystals

The structural features of the antiferromagnetic K0.8Fe1.6S2 have been studied in the temperature range from 300 K up to 700 K by means of in situ transmission electron microscopy (TEM). The superstructure with a wave vector originating from a Fe-vacancy order has been clearly observed; moreover, the structural analysis shows that K0.8Fe1.6S2 undergoes a transition from the Fe-vacancy order to disorder at about 585 K. The S substitution effect on the phase separation and superconductivity in the K0.8Fe1.75Se2−ySy materials has been systematically investigated by SEM and TEM structural analyses, as well as by electrical resistivity measurements. Our experimental results reveal that the S element adopts a homogeneous distribution in all investigated materials, and the essential phase-separation nature is very similar to what was observed in the K0.8Fe1.75Se2 superconductor. A phase-separated state formed by the coexistence of two Fe-vacancy orders with wave vectors and in K0.8Fe1.5+xS2 (0 < x < 0.1) has been briefly discussed.

Coordination nature of phase separation in oxide melts

The behavior of uranium oxide-silica, zirconia-alumina, zirconia-iron oxide, uranium oxideiron oxide, zirconia-alumina-iron oxide, and uranium oxide-iron oxide-zirconia melts was experimentally investigated in the air. The existence of two-phase fluids in these systems was confirmed. It is proposed that the reason for the phase separation is the formation of complexes with the general formula x[M3+O8/2] [Me4+O8/2]. The influence of a complex concentration on the density and surface tension of melts in the ZrO2-Al2O3 system was demonstrated.

Microstructural phase separation related in-plane fourfold symmetric superconductivity in K0.8Fe1.65Se2 crystals

Through angle-resolved out-of-plane electrical transport measurements on K0.8Fe1.65Se2 crystals, it is found that the upper critical field Hc2, the out-of-plane resistivity ρc, and the activation energy U for flux motion all show a distinct fourfold symmetry which is probably related to the fourfold symmetric superconducting stripes arising from the special phase separation nature. A scaling approach was proposed to analyze the fourfold symmetry of Hc2, which fits the experimental data very well, and the fourfold symmetric behaviors of ρc in flux flow region were described by a modified Bardeen-Stephen equation after considering the phase separated stripes.

The nature of phase separation in a Ru–Sn–O ternary oxide electrocatalyst

A phase stability diagram of ruthenium-zirconium oxide (Ru-Sn-O) was constructed by a combination of ab initio density functional theory and thermodynamic calculations. Results suggest that the phase separation/segregation that has been reported in the literature for the RuO(2)-SnO(2) system is through a typical spinodal decomposition mechanism. Ru(0.45)Sn(0.55)O(2) films were prepared by thermal co-decomposition of precursors at 500 °C for varied duration. Quantitative phase analyses of the prepared films based on X-ray diffraction and high-resolution transmission electron microscopy confirmed the spinodal nature of the phase separation. The present fundamental study provides a theoretical guideline for the phase and microstructure design of Ru-Sn-O based mixed oxides for electrocatalysis applications.

Stability of Solid-Solution Phase and the Nature of Phase Separation in Ru–Zr–O Ternary Oxide

Equilibrium phase diagram of ruthenium–zirconium oxide (Ru–Zr–O) was calculated by a combination of ab initio density functional theory and thermodynamic calculations. The phase diagrams suggest that the solubility between ruthenium oxide and zirconium oxide is very low under normal experimental conditions, in good agreement with the prior reports in literature. Also provided is the theoretical support for the reported phenomenon that doping or amorphization of Ru–Zr–O might suppress phase separation of the oxide. To study the spinodal decomposition of Ru–Zr–O, we successfully prepared an amorphous Ru0.48Zr0.52O2 film of a solid solution phase by a thermal decomposition at a low temperature (563 K). In situ transmission electron microscopy was utilized to witness the spinodal decomposition of the amorphous Ru0.48Zr0.52O2 solid solution by electron–beam annealing. The present fundamental study of the phase behavior of Ru–Zr–O provides a guideline for the phase and microstructure design of Ru-based mixed ox...

Phase behavior of polystyrene acrylonitril copolymer and polymethylmethacrylate blends under shear

Polymer blends undergo external stresses such as pressure and shear in course of processing cycles. The knowledge of their phase behavior at each step of these cycles is crucial for understanding their physical properties and eventually improves their performance in practical applications. The effects of shear on the phase diagram of binary polymer blends are considered. A theoretical formulism is used upon which the free energy is the sum of two terms. The first term is modeled with the Flory–Huggins free energy of mixing and describes the thermodynamic behavior of the system in the quiescent state. The second term represents the excess free energy stored during flow. In the presence of shear flow, the excess free energy is expressed in terms of the viscosity and the shear modulus. Both quantities depend on composition and shear rate. The curvature of the variation of viscosity versus composition has a tremendous impact upon the nature of phase separation. Phase diagrams are described by the spinodal curves and show for the case considered here miscibility enhancement with increasing shear rate. A good correlation is found with experimental data of the literature on blends of polystyrene acrylonitril copolymer and polymethylmethacrylate. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Uncharacteristic phase separation trends with the ionic size in cobaltites

Using elastic neutron scattering on single crystals of ${\text{La}}_{1\ensuremath{-}x}{A}_{x}{\text{CoO}}_{3}$ ($A={\text{Ca}}^{2+}$, ${\text{Sr}}^{2+}$, and ${\text{Ba}}^{2+}$), we found the development of magnetic superstructures below the global magnetic transition to be strongly dependent on the size of the $A$-site dopant, $⟨{r}_{A}⟩$, in an unusual way. Upon reducing the $⟨{r}_{A}⟩$ (i.e., as with Ca doping), only a commensurate ferromagnetic cluster phase is evident. On expanding the $⟨{r}_{A}⟩$, the tendency toward coexistence of competing ferromagnetic and antiferromagnetic orders increases giving rise to an inhomogeneous ground state. The antiferromagnetic ordered state, initially incommensurate, continuously strengthens and becomes commensurate with long-range order and a characteristic cubic wave vector of ${\stackrel{P\vec}{Q}}_{c}=(0.25,0.25,0.25)$ with $x$. The two competing order parameters become comparable in magnitude indicative of the phase-separated nature of the cobalt perovskite system.

Mechanical Properties of Ternary Blends of ABS + HIPS + PETG

The effect of a commercial styrene/butadiene/styrene-based compatibilizer (Styroflex) on the tensile and impact properties of ternary blends of poly(acrylonitrile-co-butadiene-co-styrene) (ABS), high impact poly(styrene) (HIPS) and poly(ethylene terephthalate-co-cyclohexanedimethanol terephthalate) (PETG) was investigated. The tensile yield strengths and the moduli of the blends were of similar magnitude as the parent polymers. However, notched Charpy impact properties showed significant deviations with high synergy in ABS/PETG blends and strong antagonism in HIPS/PETG blends. Addition of Styroflex improved the impact properties of all blends containing HIPS and ABS. Dynamic mechanical analysis studies confirm the phase separated nature of ABS/PETG binary blends.

Dissimilar Joining of Aluminum Alloy and Steel by Resistance Spot Welding

Recently, there are strong demands for light-weight, fuel efficient cars. An effective joining technique to join light metals, such as Al and Mg alloys, with steel is required. Mg alloy and steel are especially difficult pair of metals to join. Due to not only Mg alloy's surface oxide, but also their complete phase separation nature, Mg alloy and steel do not form an intermetallic at the joint interface. We aimed to join Mg alloy and steel by using Mg-Zn eutectic reaction, Zn layer was inserted between Mg alloy and steel as plating on steel. A conventional resistance spot welding machine was used, and joint strength was evaluated by tensile strength tests. It was found that Mg alloy surface oxide are removed along with eutectic melt at low temperature, and at the same time, thin and uniform intermetallic layer of Fe-Al and Al-Mg were formed at the joint interface.

Palladium‐catalyzed phosphonation of SEBS block copolymer

AbstractPhosphonic acid‐bearing styrene–ethylene/butylene–styrene (SEBS) block copolymer was synthesized by bromination and subsequent palladium‐catalyzed phosphonation of SEBS. The phosphonated block copolymer was characterized by spectroscopic, thermal, and conductivity measurements. The new polymer shows good ion‐exchange capacity of ∼0.7 meq/g and proton conductivity of around 2–4 mS/cm (at room temperature and 100% relative humidity) which is in good agreement with literature value of other phosphonated materials. This value was obtained despite a relatively low degree of phosphonation, demonstrating the ability of the phase separated nature of block copolymers to promote proton conductivity. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5431–5441, 2008

Polysulfones Grafted with Poly(vinylphosphonic acid) for Highly Proton Conducting Fuel Cell Membranes in the Hydrated and Nominally Dry State

Mechanically strong and flexible membranes with very high local concentrations of immobilized proton-conducting phosphonic acid have been achieved by grafting poly(vinylphosphonic acid) side chains onto polysulfones. The graft copolymers were prepared by anionic polymerization of diethyl vinylphosphonate initiated from lithiated polysulfones, followed by quantitative cleavage of the ester functions. The resulting phosphonic acid units acted like monoprotic acids to indicate a high level of intramolecular interactions, and the phase-separated nature of the copolymers was shown by dual glass transitions. Fully polymeric membranes were conveniently cast from solution and showed high proton conductivities, e.g., 5 mS/cm under nominally dry conditions at 120 °C and up to 93 mS/cm under 100% relative humidity at the same temperature. The corresponding conductivities measured for Nafion 115 under the same conditions were 0.04 and 105 mS/cm, respectively. The former membranes furthermore showed high thermal stability with decomposition temperatures exceeding 300 °C under air. Additions of 2−5 wt % of a perfluorosulfonic acid polymer to the phosphonated membranes were found to reduce the water uptake significantly, thus improving the mechanical properties. The conductivity of these fully polymeric doped membranes was generally observed to be enhanced, or at least to remain at the same level, under both humidified and nominally dry conditions. The findings of this study demonstrate that phosphonated membranes with a proper macromolecular design may potentially show some important advantages in relation to the more commonly studied sulfonated membranes in fuel cell applications.

Aggregation Kinetics and the Nature of Phase Separation in Two-Dimensional Dipolar Fluids

The kinetics of aggregation in a monolayer of dipolar particles are studied using stochastic dynamics computer simulations. Transient concentrations of end defects (at low density) and Y-shaped defects (at high density) clearly exceed those at equilibrium. Although very large dipole moments are expected to disfavor such defects at equilibrium, it is found that the transient defect concentrations increase with increasing dipole moment. The results suggest that the conditions for defect-driven condensation--as proposed by Tlusty and Safran [T. Tlusty and S. A. Safran, Science 290, 1328 (2000)]--could be met by kinetic trapping, giving rise to a metastable phase transition between isotropic fluid phases.

Sr and Ba substitution in charge ordered Pr 0.5Ca 0.5MnO 3: Sharp steps in the magnetic and transport properties for a narrow composition range

The magnetic and transport properties of Pr 0.5Ca 0.5− x A x MnO 3 (A = Sr, Ba) have been investigated in this paper. The substitution of Ca with bigger cations such as Sr and Ba can favour the field-induced ferromagnetism and induce sharp steps in the magnetization versus field and resistivity versus field curves. These properties strongly depend on the thermal history of the samples. All the results have been interpreted by a martensitic-like mechanism based on phase separation induced by A-site size mismatch. The above model can also explain the result that the less efficient ability of Sr substitution than Ba substitution to induce ferromagnetism and sharp steps. Another interesting feature in the system is the existence of an optimum substitution range to induce magnetization steps and reach high field-induced magnetization values for both Sr ( x = 0.07–0.10) and Ba ( x = 0.01–0.08) substitution. We suggest that the disappearance of the steps beyond the optimum substitution range possibly results from the different nature of phase separation in the optimum substitution range and beyond this range.

Magnetoresistance in phase-separated La0.5Ca0.5MnO3 manganite

Abstract We report measurements of magnetoresistance and magnetization in a polycrystalline La 0.5 Ca 0.5 MnO 3 sample. The results show the way in which a magnetic field is applied may change the phase-separated nature of the compound, clearly distinguishing two different effects: domain alignment and field-induced ferromagnetic fraction enlargement. The latter is strongly enhanced in a field-cooled cooling experiment, giving rise to low resistivity values, and providing clear evidence for the percolative nature of the colossal magnetoresistance of manganites.