Phase-separated Material Research Articles

Local matrix-cluster interactions in a phase separated perovskite

Magnetoelectronic phase separation plays an integral part in many recent advances in the understanding of correlated electron systems. We have studied the magnetically phase separated material ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Co}{\mathrm{O}}_{3}$ and observe unambiguous evidence of magnetic interactions between hole-rich ferromagnetic clusters and the neighboring hole poor matrix. We suggest that this mechanism detected here plays an important role in the rich electrical and magnetic properties of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Co}{\mathrm{O}}_{3}$.

The role of alkali content in the devitrification mechanisms of SiO2–Fe2O3–Na2O–PbO SYSTEM

Three batch compositions of pure oxides (SiO2, Fe2O3, PbO, Na2O) with equivalent SiO2, Fe2O3 and PbO contents and a gradually increased Na2O content were vitrified through heating in a high temperature electric furnace and subsequent quenching. The resulting vitreous products were thermally treated in order to study the devitrification behaviour, under conditions designated from differential thermal analysis experiments. Depending on the Na2O content, crystal phase separation gave rise to the growth of acmite and hematite or maghemite. A uniformly phase separated glass-ceramic material, with crystallites of similar size and population density, was produced from devitrification of the vitreous product with the higher Na2O content.

Multi-Scale Characterisation of Hybrid Organic-Inorganic Materials Using New Solid-State NMR Methods

Recent developments of high-resolution NMR techniques for proton in the solid state open the way to obtaining a more accurate description of the structure of proton containing materials at different length scales (from 0.1 up to 200 nm). This can be achieved through the combined use of Lee-Golburg homonuclear proton decoupling and proton spin-diffusion. In that scope we describe a multi-scale characterisation of a layered and phase-separated zinc phosphonate material.

Superconducting properties of mixed phases of nominal stoichiometry Rb 2NaC 60

When attempting to prepare Rb 2NaC 60, one obtains a phase-separated material, consisting of the energetically more stable superconducting Rb 3C 60 and nonsuperconducting Na x C 60. This sample exhibits a very sharp superconducting transition temperature at 26.3 K. The decrease in T c with respect to that of pure-phase Rb 3C 60 is explained as the result of the decrease in density of states in Josephson-like junctions formed between the two phases. This composition displays a large volume fraction of superconductivity, and all measurements of superconducting parameters are extremely well defined, implying high phase purity of the Rb 3C 60. The zero temperature values of the upper and lower critical magnetic fields are H c2(0)=165 kOe and H c1(0)=62 Oe, calculated from measurements of the field dependence of the magnetization as a function of temperature. The coherence length and penetration depth are 44 Å and 3400 Å, significantly higher than earlier literature estimates.

Morphology and elastomeric properties of isotactic polypropylene/ hydrogenated poly(styrene- co-butadiene) blends: a potential for a new thermoplastic elastomer

A single-phase melt of the isotactic polypropylene (iPP)/hydrogenated poly(styrene-co-butadiene) (hSBR) blend, which has a virtual upper critical solution temperature (UCST)-type phase boundary below the melting point of iPP (T.), was prepared at 50/50 wt. ratio and quenched below UCST. By transmission electron microscopy and dynamic mechanical analysis, the blend was shown to be a phase-separated material in which hSBR-rich domains with uniform diameter of ca. 20 rim are dispersed in an iPP-rich matrix. The regular structure is believed to be formed by spinodal decomposition which is pinned at an early stage by crystallization in the iPP-rich region. The blend showed good strain recovery after large deformation, suggesting a potential for a new class of thermoplastic elastomer. Wide-angle X-ray diffraction (WAXD) studies showed that iPP crystallites in the blend are smaller than those in neat iPP and there exists an optimum size of ca. 10 nm (crystal size by the Scherrer equation) for this strain recovery. Such crystallites suffered from less plastic deformation and were hardly oriented under bulk deformation. That is, the crystallites that developed in the presence of polymer impurity (hSBR) seem to be different from those in neat iPP, and play the role of tie points to provide the elastomeric character of the matrix itself.

Superconducting Properties of Alkali Doped C60 Prepared by Precipitation from Liquid Methylamine or Ammonia

Abstract An extremely convenient method of preparing alkali doped face-centered cubic phases of C60 consists of the partial solubilization of the constituent materials in liquid ammonia or monomethylamine followed by precipitation and annealing. The preparation and properties of Rb3C60, K3C60 and Rb2CsC60 prepared in this manner from monomethylamine are decribed. When attempting to prepare Rb2NaC60 from liquid ammonia, one obtains a phase separated material, consisting of the energetically more stable superconducting Rb3C60 and nonsuperconducting NaxC60. This sample exhibits a very sharp superconducting transition temperature at 26.3 K. The decrease in Tc with respect to that of pure-phase Rb3C60 is explained as the result of the decrease in density of states in Josephson-like junctions formed between the two phases. This composition displays a large volume fraction of superconductivity, and all measurements of superconducting parameters are extremely well defined, implying high phase purity of the Rb3C60...

Phase separation and superconductivity in La 2CuO 4+δ: Effects of oxygen diffusion

In oxygen-annealed La 2CuO 4+δ (δ ∼0.03), bulk magnetization measurements show an increase of ∼4K in the superconducting T c when the sample is cooled slowly through a narrow temperature range near 195K. At this temperature, 139La NQR spectra exhibit a concomitant appearance of an anomalous feature associated with the metallic phase whose spectral weight increases with cooling rate. The data suggests a distribution of local structures and annealing-dependent volume fraction in this part of the sample. Interpretation of these results in terms of oxygen diffusion in a phase separated material is considered and a picture consistent with the observed changes in T c is obtained. The activation energy of oxygen diffusion derived from the model, Q ∼ 0.25 eV, is in the same order of magnitude as the results obtained in La 2−xSr xCuO 4− y .

Characterization studies of low pressure chemical vapour deposition SICARB layers for wide band gap emitters

The microstructure of low pressure chemical vapour deposition SICARB layers with carbon concentrations [C] up to 50% has been determined by spectroscopic ellipsometry (SE), IR transmission, X-ray diffraction and cross-sectional transmission electron microscopy. For [C] ≲ 15% a phase-separated material is obtained, including polycrystalline silicon, while at higher concentrations the material consists of SiC microcrystallites in an amorphous matrix. Good correlation of the techniques has been obtained and compositions and layer thicknesses have been determined by detailed fitting of the SE spectra.

Conductivity in polymer ionics. Dynamic disorder and correlation

Theoretical constructs are developed for discussing diffusivity and conductivity in polymer ionic materials. Such materials are characterized by extensive disorder, either static (lack of long-range order) or static and dynamic (lack of long-range order with short-range order evolving with time). Beginning with a dynamic percolation model, we show that, in general, so long as the mean-square displacement of the charged particle obeys a certain growth law, the observed charged-particle motion will be diffusive, both in the ballistic regime, corresponding to electronic motion with strong scattering, and in the ionic-hopping regime, corresponding to dynamic disorder renewal of the hopping situation. Some general behaviour for transport under these conditions is predicted, including definite statements about the frequency dependence of the conduction, the relationship between the growth law in a single interval and the growth law for observation times long compared to scattering or renewal times, and the behaviour in the neighbourhood of the percolation threshold for the static problem. Interpretations are suggested both for ion and electron-hopping situations.A statistical thermodynamic model is developed for analysis of contact ion pair formation and its effect on conductivity in ion-conducting polymer systems. In this model, the energy (due to solvation and polarization) favouring formation of a homogeneous complex in which the cations are solvated by the polymer host, competes with an entropic term favouring the separated structures (free polymer and contact ion pairs). We derive general conditions for this phase separation, and an expression for the number of polymer-bound, homogeneously solvated ions. We show that this number will, in general, decrease monotonically with increase in temperature, due to entropic favouring of the phase-separated material, this is reminiscent of the lower consolute temperature phenomenon in liquid mixtures.

Analytical Electron Microscopy of a Polymer Blend

A polymer blend which is composed of poly-p-phenylene benzobisthiazole (PBT) and poly-2,5(6)benzimidazole (ABPBI) has been processed into both a phase-separated material and a “molecular composite”. In the molecular composite, the PBT and ABPBI components are dispersed at a scale finer than 3 nm. This results in high mechanical properties as the rod-like, high strength PBT reinforces the flexible-coil ABPBI matrix. In the phase- separated blend, micron-sized aggregates form within a more ductile matrix. This study qualitatively examines the structure and composition of the phase- separated 20% PBT / 80% ABPBI blend using the analytical electron microscopy (AEM) techniques of energy dispersive x-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS), and microdiffraction. Beam damage of the components is also considered.

The structure of azoester-modified polybutadiene elastomers

It is demonstrated that the improved tack and green strength accompanying the IAD modification of polybutadiene arise strictly from the introduction of bulky, polar groups. The fact that addition of 1,4-PB occurs in a blocked arrangement is not a factor in the development of high green strength, as can be seen by the large increase in strength of modified 1,2-PB, wherein the IAD groups are randomly attached. The tack is also elevated through random IAD addition; however, the relevance of blocked vs. random structure in this regard cannot be completely judged since it depends as well on the absolute IAD level and the polymer's Tg, as well as the test temperature. At high levels of IAD (about 38 mol %), the modified PB becomes a phase-separated material, exhibiting high stiffness and brittleness. This heterogeneous system is quite unlike the PB modified with only 15 mol % IAD. These latter elastomeric materials are homogenous, and remain so during deformation. The attainment of high tack and green strength relies upon maximal addition of the IAD groups, provided the molecules retain their flexibility. In blending the modified PB with other rubbers, optimum behavior in the blends appears to be achieved when the IAD–PB is present as a continuous phase.