Regions Of Different Composition Research Articles

Crystallization of Ge-Rich GeSbTe Alloys: The Riddle Is Solved

Among the phase-change materials, Ge-rich GeSbTe (GST) alloys are of considerable interest as they offer a much higher thermal stability than their congruent contenders, a desirable characteristic for embedded digital memories and neuromorphic devices. Up to now, the mechanisms by which such alloys crystallize and progressively switch from one resistivity state to the other remain unclear and very controversial. Using in situ synchrotron X-ray diffraction during isothermal annealing and advanced transmission electron microscopy techniques, we solve this riddle and unveil the mechanisms leading to the overall crystallization of such alloys. During annealing at 310 °C, the initially homogeneous and amorphous material undergoes a progressive phase separation, leading to the formation of Ge-rich regions of different compositions. During this decomposition, the first formed GeTe embryos crystallize and trigger the heterogeneous crystallization of the Ge cubic phase. As the phase separation proceeds, these embryos dissolve and the Ge phase gradually builds up through the nucleation of small grains. Only when this Ge cubic phase is largely formed, the remaining amorphous matrix may locally reach the Ge2Sb2Te5 composition at which it can crystallize as large grains. Our density functional theory calculations confirm that the quite exotic Pnma GeTe structure we have experimentally identified is more stable than the regular R3m structure at nanometric sizes.

Spatial characterization of heterogeneous nanopore surfaces from XCT scans of Niobrara shale

The multiphase fluid flow through natural and geo-architected nanopore structures is fundamentally controlled by the geometry of such nanopore structures, pore wettability and interfacial tension. Furthermore, multiphase fluid flow through the nanopore structures is compromised when the pore surfaces are heterogeneous, having surface regions of different composition, polarity, and wetting characteristics. In this regard, compositional analysis of the pore channel surface has been a big challenge in the past, since 3D analysis is required.Now, due to advances in nano X-ray CT, with a voxel resolution of 20 nm, it is possible to identify nanopore network structures and quantify their surface composition in 3D. In this paper, the development of procedures for detailed analysis to quantify the composition of the pore channel surface regions, including the corresponding area, is described using nanoCT images. Image analysis procedures, including preprocessing of the 3D tomographic images, correction for the partial volume effect (PVE), and the use of a new algorithm for determination of pore channel surface composition and area are discussed. Videos of the nanopore structure have been provided as supplementary information to further describe the heterogeneity of the nanopore surfaces.

Elemental Analysis of Beryllium Samples Using a Microzond-EGP-10 Unit

Results concerning the structural and elemental analysis of beryllium samples obtained via different technologies using a Microzond-EGP-10 unit with the help of the PIXE and RBS methods are presented. As a result, the overall chemical composition and the nature of inclusions were determined. The mapping method made it possible to reveal the structural features of beryllium samples: to select the grains of the main substance having different size and chemical composition, to visualize the interfaces between the regions of different composition, and to describe the features of the distribution of impurities in the samples.

Multiple Glass Transitions of Microphase Separed Binary Liquids Confined in MCM-41

The confinement of fluids in channels of nanometer size presents unprecedented opportunities to reveal emergent physicochemical properties that have no equivalent in the corresponding bulk system. We present an experimental study of fully miscible binary low-molecular weight liquids confined in nanopores. Under these conditions, the mixtures form a microphase-separated state with two regions of different compositions forming the core and the shell of a concentric tubular nanostructure. We combine neutron diffraction and DSC measurements to assess the thermal behavior of this unusual phase. These results show that crystallization is suppressed, leading to glassy behaviors characterized by anomalies in the density and the heat capacity. We reveal the coexistence of two different glass transitions, providing the direct proof for two different enthalpic relaxation times. It leads to the conclusion that the subcomponents of the microphase-separated mixture each obey distinct dynamical behaviors. This phenomeno...

Evolution of the amorphous-phase structure in metal–metal type metallic glasses

The structural changes in Fe90Zr10 metallic glass induced by low-temperature annealing are studied by transmission electron microscopy and largeand small-angle X-ray diffraction. It is shown that the crystallization of metallic glass is preceded by amorphous phase decomposition giving rise to amorphous regions of different compositions, which form various crystalline phases later. The properties of the structural changes point to the spinodal mechanism of the amorphous-to-crystalline transformation.

Self emulsifying drug delivery system: Optimization and its prototype for various compositions of oils, surfactants and co-surfactants

AimThe aim of the present study is to identify and optimize the self emulsifying regions of different compositions of various oils, surfactants and co-surfactants and its prototype. MethodsThe ternary phase diagram was constructed using series concentrations of oils, surfactants and co-surfactants to recognize the self emulsifying region. The selected self emulsifying compositions were evaluated and optimized using self emulsification time, robustness to dilution and globule size analysis. ResultsThe percentage of compositions of surfactants and co-surfactants with the oil phase plays a significant role for the formation of nano sized self emulsion. ConclusionThe prepared self emulsified prototype was ready to incorporate many poorly soluble drugs to improve its solubility and bioavailability as well.

Nanoscale Interactions of Lipids and Proteins in Live Cell Membranes Revealed by STED Nanoscopy

The plasma membrane seems to be dynamically structured in regions of different composition and function. However, the spatial and temporal scale of the respective membrane structure is not directly accessible by the diffraction limited resolution of conventional far-field optical microscopes. We report the detection of the membrane heterogeneities in nanosized areas in the plasma membrane of living cells using the superior spatial resolution of stimulated emission depletion (STED) far-field nanoscopy. By combining a (tunable) resolution of down to 30 nm with tools such as fluorescence correlation spectroscopy (FCS), we obtain new details of molecular membrane dynamics. Sphingolipids are transiently (∼ 10 ms) trapped on the nanoscale in cholesterol-mediated molecular complexes, while glycero-phospho-lipids diffuse freely. The results are compared to STED experiments on model membranes, which highlight that the nanoscale trapping in cells is not correlated with liquid order partitioning in model systems. The novel observations shine new light on the distribution and interaction of lipids and proteins in the plasma membrane with respect to the lipid raft hypothesis.

Influence of superheat before quench on the structure and stability of NiP metallic glasses studied by neutron scattering techniques

The temperature dependent structure of Ni 81P 19 metallic glasses, rapidly quenched from melts which have undergone different heat treatments, has been investigated by small angle neutron scattering and neutron diffraction. All samples contain in the as-quenched state particles or clusters of a very wide range of sizes. These are amorphous regions of different compositions in a large composition range and/or crystallites of another Ni composition than the matrix, such as Ni 3P, Ni 5P 2 or Ni 12P 5, or even “voids”. The results can be interpreted within the concept of a heterogeneous structure existing in eutectic melts in a certain temperature range. Comparison is made with results from the literature on samples from the same ribbons investigated by calorimetry as well as by fractography and Weibull analysis of mechanical properties.

Localized measurement of atherosclerotic plaque inflammatory burden with dynamic contrast–enhanced MRI

Inflammation plays an important role in progression and rupture of atherosclerotic plaque. Dynamic contrast-enhanced MRI has been proposed as a tool to evaluate inflammation in vivo by measuring the transfer constant and partial plasma volume, which are influenced by inflammation. This study sought to demonstrate the ability of dynamic contrast-enhanced MRI to provide localized measurements of transfer constant and partial plasma volume within plaque regions of different compositions. In order to do that, a highly automatic procedure for localized measurement of dynamic contrast-enhanced MRI parameters was developed. In 47 subjects, the average transfer constant and partial plasma volume were highest in loose matrix and fibrous tissue and substantially lower in intraplaque hemorrhage, lipid rich/necrotic core, and calcification. In addition, except for hemorrhage and calcification, statistically significant differences of transfer constant and partial plasma volume were observed for any pair of these components. This suggests that transfer constant and partial plasma volume could be helpful to differentiate different plaque components and that dynamic contrast-enhanced MRI has the potential to assess inflammatory burden in specific regions.

Cationic Disorder and Phase Segregation inLaAlO3/SrTiO3Heterointerfaces Evidenced by Medium-Energy Ion Spectroscopy

Medium-energy ion spectroscopy (MEIS) has been used to study the depth profile and deduce the distribution of possible cationic substitutions in LaAlO3/SrTiO3 (LAO/STO) heterointerfaces. Analysis of La and Sr peaks in aligned and random MEIS spectra indicates that the surface layers of LAO on an STO substrate are not homogeneous and stoichiometric if the film thickness is less than 4 unit cell layers. This is possibly caused by a redistribution of La and Sr at the interface. Kelvin probe force microscopy reveals an inhomogeneous distribution of the surface potential in a 4 unit cell LAO film, indicating micrometer-sized regions of different compositions. Our findings provide a novel view on the microstructural origin of the electrically conductive interfaces.

Thermal conductivity and phase separation of the crust of accreting neutron stars

Recently, crust cooling times have been measured for neutron stars after extended outbursts. These observations are very sensitive to the thermal conductivity kappa of the crust and strongly suggest that kappa is large. We perform molecular dynamics simulations of the structure of the crust of an accreting neutron star using a complex composition that includes many impurities. The composition comes from simulations of rapid proton capture nucleosynthesis followed by electron captures. We find that the thermal conductivity is reduced by impurity scattering. In addition, we find phase separation. Some impurities with low atomic number Z are concentrated in a subregion of the simulation volume. For our composition, the solid crust must separate into regions of different compositions. This could lead to an asymmetric star with a quadrupole deformation. Observations of crust cooling can constrain impurity concentrations.

Point-spread functions for backscattered imaging in the scanning electron microscope

One knows the imaging system's properties are central to the correct interpretation of any image. In a scanning electron microscope regions of different composition generally interact in a highly nonlinear way during signal generation. Using Monte Carlo simulations we found that in resin-embedded, heavy metal-stained biological specimens staining is sufficiently dilute to allow an approximately linear treatment. We then mapped point-spread functions for backscattered-electron contrast, for primary energies of 3 and 7 keV and for different detector specifications. The point-spread functions are surprisingly well confined (both laterally and in depth) compared even to the distribution of only those scattered electrons that leave the sample again.

Line Tension and Coalescence in Heterogeneous Membranes

Membrane inhomogeneity gives rise to a perturbation energy that may be manifested as line tension between regions of different compositions. Here we calculate the perturbation energy of a phase-separated fluid membrane composed of domains embedded in a continuous phase and relate it to the line tension in the system. We find that the effective line tension due to the thickness mismatch between the phases varies nonmonotonically as a function of the domain size and spacing when those decrease below about 20 nm. Although we find that the characteristic time scale for domain coalescence increases with domain size, the coalescence time for domains on the order of 25 to 50 nm may be as long as several days. The time scale for domain coalescence is found, quite surprisingly, to be maximal when the bending and area moduli of the domain and continuous phase are similar and to decrease when one phase is stiffer than the other. Domain stability is also found to vary as a function of spacing; as a result, small domains on the order of 5 nm become unstable and tend to coalesce when the domain-phase area fraction increases above 0.5, whereas larger domains on the order of 50 nm become unstable only when the area fraction increases above 0.8.

Application of the parametric bootstrap method to determine statistical errors in quantitative X‐ray microanalysis of thin films

We applied the parametric bootstrap to the X-ray microanalysis of Si-Ge binary alloys, in order to assess the dependence of the Ge concentrations and the local film thickness, obtained by using previously described Monte Carlo methods, on the precision of the measured intensities. We show how it is possible by this method to determine the statistical errors associated with the quantitative analysis performed in sample regions of different composition and thickness, but by conducting only one measurement. We recommend the use of the bootstrap for a broad range of applications for quantitative microanalysis to estimate the precision of the final results and to compare the performances of different methods to each other. Finally, we exploited a test based on bootstrap confidence intervals to ascertain if, for given X-ray intensities, different values of the estimated composition in two points of the sample are indicative of an actual lack of homogeneity.

Numerical investigations on methods to control the rate of heat release of HCCI combustion using reduced mechanism of n-heptane with a multidimensional CFD code

In this study a semi-reduced reaction scheme developed previously was used to derive a 26 step reduced mechanism, using the sensitivity approach and the steady state approximation (QSS) with Chemkin code. This 26 step model has been implemented in a CFD combustion code (Star-CD/Kinetics) to study combustion process in homogeneous charge compression ignition (HCCI) engines. The first results obtained have confirmed the very rapid combustion phase and fast heat release with completely homogeneous mixtures, for a wide range of operating conditions. This numerical approach has been used first to study the effects of natural thermal stratification when the mixture is initially homogeneous. In a second step, the different possible methods to control the heat release rate have been studied. The stratification with several homogeneous regions of different composition is shown to be very efficient; the limits of this process are discussed.

Nuclear quadrupole resonance study of local bonding in glassyAsxSe1−x

Nuclear quadrupole resonance (NQR) experiments were performed on glassy ${\mathrm{As}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ to study the local structural order. The bonding in ${\mathrm{As}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ is governed by preferential bonding (chemical ordering) between arsenic and selenium at arsenic concentrations $x\ensuremath{\leqslant}0.40$; however, the bonding for higher arsenic concentrations is governed mostly by statistical considerations. At concentrations $x\ensuremath{\geqslant}0.45$ the glasses are inhomogeneous with the presence of local regions of different composition. Measurements of the NQR spin echo intensity identify regions formed by arsenic atoms bonded to zero, one, two, or three selenium atoms. The NQR spectral line shapes and the longitudinal relaxation times suggest that these regions have a more ordered structure as compared to the homogeneous samples with low arsenic content. The existence of a small concentration of crystalline inclusions in amorphous ${\mathrm{As}}_{0.60}{\mathrm{Se}}_{0.40}$ is also suggested.

Quantitative Thin-Film X-ray Microanalysis by STEM/HAADF: Statistical Analysis for Precision and Accuracy Determination

Silicon-germanium thin films have been analyzed by EDS microanalysis in a field emission gun scanning transmission electron microscope (FEG-STEM) equipped with a high angular dark-field detector (STEM/HAADF). Several spectra have been acquired in the same homogeneous area of the cross-sectioned sample by drift-corrected linescan acquisitions. The Ge concentrations and the local film thickness have been obtained by using a previously described Monte Carlo based "two tilt angles" method. Although the concentrations are in excellent agreement with the known values, the resulting confidence intervals are not as good as expected from the precision in beam positioning and tilt angle position and readout offered by our state-of-the-art microscope. The Gaussian shape of the SiKalpha and GeKalpha X-ray intensities allows one to use the parametric bootstrap method of statistics, whereby it becomes possible to perform the same quantitative analysis in sample regions of different compositions and thicknesses, but by doing only one measurement at the two angles.

Behavior of a Co-Cr-Mo biomaterial in simulated body fluid solutions studied by electrochemical and surface analysis techniques

The behavior of a Co-Cr-Mo biomaterial used as dental prosthesis was studied by electrochemical (Open Circuit Potential (OCP) and potentiodynamic polarization) and surface analysis techniques (Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)) in artificial saliva, artificial saliva with modified temperature (7 °C - 47 °C) and pH (1.4 - 13.4), and containing fluoride (1500 ppm). The results revealed that the surface morphology after potentiodynamic polarization is highly heterogeneous exhibiting three regions of different composition. Soluble metal species such as CrO4(2-) were released to the solution during the potentiodynamic polarization. The electrochemical processes were markedly influenced by solution temperature and pH, whereas the presence of fluoride did not produce changes in the system.

Mapping organic coatings on atmospheric particles

To date there is no direct evidence of the distribution or characteristics of organic compounds in individual particles because techniques for chemically identifying organic compounds are not sufficiently sensitive to detect molecules or functional groups with masses below 10−15 g [Husar and Shu, 1975]. Here we present detailed maps of organic groups (aromatic, alkyl, ketonic carbonyl and carboxylic carbonyl groups) and inorganic ions (potassium, carbonate, calcium) in individual dry particles with diameters ranging from 0.2 to 10 μm using a new technique for aerosol characterization by soft X‐ray spectromicroscopy at atmospheric pressure. The maps show both the compounds present in individual particles and regions of different compositions within particles. The particle compositions on surfaces are enhanced in shorter chain or more oxygenated groups, providing the first observations of surface active carboxylic acids in organic coatings in atmospheric particles.

Isochore chromosome maps of eukaryotic genomes

Analytical DNA ultracentrifugation revealed that eukaryotic genomes are mosaics of isochores: long DNA segments (≫300 kb on average) relatively homogeneous in G+C. Important genome features are dependent on this isochore structure, e.g. genes are found predominantly in the GC-richest isochore classes. However, no reliable method is available to rigorously partition the genome sequence into relatively homogeneous regions of different composition, thereby revealing the isochore structure of chromosomes at the sequence level. Homogeneous regions are currently ascertained by plain statistics on moving windows of arbitrary length, or simply by eye on G+C plots. On the contrary, the entropic segmentation method is able to divide a DNA sequence into relatively homogeneous, statistically significant domains. An early version of this algorithm only produced domains having an average length far below the typical isochore size. Here we show that an improved segmentation method, specifically intended to determine the most statistically significant partition of the sequence at each scale, is able to identify the boundaries between long homogeneous genome regions displaying the typical features of isochores. The algorithm precisely locates classes II and III of the human major histocompatibility complex region, two well-characterized isochores at the sequence level, the boundary between them being the first isochore boundary experimentally characterized at the sequence level. The analysis is then extended to a collection of human large contigs. The relatively homogeneous regions we find show many of the features (G+C range, relative proportion of isochore classes, size distribution, and relationship with gene density) of the isochores identified through DNA centrifugation. Isochore chromosome maps, with many potential applications in genomics, are then drawn for all the completely sequenced eukaryotic genomes available.