bluuebunny/crossref_metadata_2025_split huggingface dataset:
Synchrotron X-Ray Diffraction Study of the Surface Layer in Poled Ceramic BaTiO3#
Authors: Balzar D. , Stephens P. W. , Ledbetter H. , Li J. , Dunn M. L. 
Published: April 2011

AbstractBoth unpoled and poled BaTiO3 were studied by laboratory and synchrotron x-ray sources. The shorter-wavelength synchrotron radiation was used to probe deeper below the specimen surface. Diffraction patterns revealed a distinctive surface layer of the same tetragonal structure as the main BaTiO3 fraction. A sin2 Ψ analysis of peak shift confirmed that no measurable change of elastic strain occurs upon poling. This indicates that poling may induce excessive strain that was relieved by microcracking or that a majority of domains reverse orientation. However, domain switching and possible microcracking may induce inhomogeneous strain (microstrain) and alter domain-size distribution. Line-broadening analysis showed large anisotropy of both coherently diffracting domain size and microstrain. The poled specimen shows a larger microstrain and smaller average domain size, which indicates possible effects of microcracking and additional defects created during poling.
CVD-Growth of Thin-Film Layered Se-Carbon Compounds
Authors: Grigorian L. , Fang S. , Sumanasekera G. , Rao A. M. , Schraderi L. , Eklund P. C. 
Published: April 2011

AbstractOriented films of layered Se-carbon compounds are grown on Ni substrates in evacuated sealed quartz tubes. Results of X-ray diffraction, Raman scattering and the c-axis electrical transport studies are reported. A discussion of possible models for the carbon -Se interaction is presented.
How does the Support Influence the Catalytic Activity of the Keggin Ion?
Authors: Smith Zakiyyah , Palmieri Michael , Buecheler Nancy , Jansen Susan A. 
Published: April 2011

AbstractHeteropoly acids, HPA are well known solid acid and oxidation catalysts that find application in hetergeneous and homogeneous reactions. In the former, surface area and stability problems are diminshed by supporting the HPA. Typical supports include oxide substrates and porous carbon materials. The HPA's show some instability on these supports however. In this work, we demonstrate that HPA encapsulated in sol-gel silica matrices show enhanced catalytic performance without compromising the catalytic activity of the HPA. In addition, the specific role of the support in the catalytic process is described as well.
Enhanced Oxidation of Air Contaminants on an Ultra-Low Density UV-Accessible Aerogel Photocatalyst
Authors: Dreyer M. , Newman G. K. , Lobban L. , Kersey S. J. , Wang R. , Harwell J. H. 
Published: April 2011

ABSTRACTThis research developed new forms of photocatalysts that could potentially move photocatalytic degradation of air contaminants into the main stream of industrially used remediation technologies. Tests of the photocatalytic activity of the TiO2 aerogel catalysts have been carried out using both acetone and methane as the air contaminant. For comparison, the same tests were carried out on a standard (non-aerogel) anatase powder. Despite having very low crystallinity, the aerogel decontaminates the air far more effectively than an equal volume of the anatase powder which indicates that a much larger fraction of the aerogel is activated by the UV light. Experimental data were used to determine adsorption equilibrium constants for acetone, and to determine reaction rate constants assuming a Langmuir-Hinshelwood type rate expression.
Aerogel Derived Catalysts
Authors: Reynolds John G. , Hair Lucy M. , Coronado Paul R. , Droege Michael W. , Wong Joe 
Published: April 2011

ABSTRACTAerogels are a class of colloidal materials which have high surface areas and abundant mesoporous structure. SiO2 aerogels show unique physical, optical and structural properties. When catalytic metals are incorporated in the aerogel framework, the potential exists for new and very effective catalysts for industrial processes. Three applications of these metal-containing SiO2 aerogels as catalysts are briefly reviewed in this paper — NOx reduction, volatile organic compound destruction, and partial oxidation of methane.
Characterization Studies of Copper Supported on a Variety of Carbonaceous Substrates
Authors: Ma J. , Rodriguez N. M. , Vannice M. A. , Baker R. T. K. 
Published: April 2011

ABSTRACTCopper catalysts supported on different allotropie forms of carbon, such as activated carbon, graphite, pitch-based carbon fibers and diamond have been prepared and characterized using TPR, XRD, TPO, TEM and CAEM techniques. Preparation procedures were established for each support system and were dependent upon the oxidation characteristics of the particular carbonaceous solid. The ease of the reduction of copper oxide to metallic copper was found to be dependent upon the nature of the carbonaceous supports as well as the possible interactions between these two components. It was postulated that the epitaxial relationship that might exist between metallic copper and the diamond would make it easy for copper oxide to be reduced to its metallic state on this support. Attack of the graphite surface occurs in the presence of copper species in hydrogen at low temperatures. This behavior is rationalized according to the notion that dissociation of molecular hydrogen takes place on Cu2O and the active atomic species “spill-over” onto the graphite and undergo reaction with the π-electrons of the basal plane resulting in the creation of pits in the surface. In-situ electron diffraction highlighted the changes in the chemical state of copper species. Cu2O was found to be stabilized at certain stages of the reaction possibly due to its interaction with the carbonaceous supports. It is suggested that this type of copper species might exhibit some unique catalytic behavioral patterns.
Dynamical heterogeneities in glass-forming materials
Authors: Heuer A. , Kuebler S. C. , Tracht U. , Spiess H. W. , Okún K. 
Published: April 2011

ABSTRACTCooperative dynamics around the glass transition leads to complex motional behavior of the individual molecules, resulting in non-exponential relaxation. The nature of this non-exponentiality is clarified theoretically as well as experimentally. The non-exponentiality may be due to heterogeneous relaxation (superposition of exponential processes with different rate constants) or homogeneous relaxation (identical intrinsically non-exponential processes). A precise definition of both limits is given. It is shown that the type of relaxation, i.e. to which degree heterogeneous and homogeneous contributions are present, reflects geometrical properties of the dynamics. The heterogeneous contribution can be further classified according to the timescale of fluctuations within the underlying distribution of relaxation rates, thereby introducing the concept of the rate memory. Determination of the type of relaxation and the rate memory essentially boils down to analysing the properties of multi-time correlation functions. Experimentally they are accessible by multidimensional NMR methods. Alternatively, they can be directly obtained from computer simulations. In order to clarify these concepts, the dynamics of polymers above the glass transition is analysed experimentally as well as numerically. The rotational dynamics of polymer segments turns out to be mainly heterogeneous with only small homogeneous contributions. Interestingly, the fluctuations within the heterogeneous distribution occur on the same timescale as the reorientation process itself.
Slow Dielectric Relaxation of Supercooled Liqutos Investigated by Nonresonant Spectral Hole Burning
Authors: Chamberlin R. V. , Schiener B. , Böhmer R. 
Published: April 2011

ABSTRACTWhen supercooled propylene carbonate and glycerol are subjected to a large-amplitude, low-frequency electric field, a spectral hole develops in their dielectric relaxation that is significantly narrower than their bulk response. This observation of nonresonant spectral hole burning establishes that the non-Debye response is due to a distribution of relaxation times. Refilling of the spectral hole occurs abruptly, indicative of a single recovery rate that corresponds to the peak in the distribution. The general shape of the spectral hole is preserved during recovery, indicating negligible interaction between the degrees of freedom that responded to the field. All relevant features in the behavior can be characterized by a model for independently relaxing domains that are selectively heated by the large oscillation, and which recover via connection to a common thermal bath, with no direct coupling between the domains.
Fluctuation Model for Structural Relaxation and the Glass Transition
Authors: Moynihan C. T. , Whang J.-H. 
Published: April 2011

ABSTRACTThe fluctuation or independently relaxing nanoregion model attributes the distribution of structural relaxation times in a glassforming melt to a physical distribution of nanoregions which vary in their properties. A quantitative test of this model is described, in which parameters derived from relaxational data on B2O3 glass are shown to be capable of predicting the anomalous light scattering in the glass transition region. It is also shown that the local inhomogenieties which lead to the distribution of structural relaxation times make only a very minor contribution to the distribution of electrical relaxation times in ionically conducting glasses and melts.
Modeling DSC Annealing Peaks for Polyetherimide: Incorporation of Temperature Gradients
Authors: Simon Sindee L. 
Published: April 2011

ABSTRACTEnthalpy recovery of polyetherimide is measured during heating with differential scanning calorimetry (DSC) after cooling at various rates. The resulting annealing peaks are fit using the Moynihan-Tool-Narayanaswamy model of structural recovery. A self-consistent phenomenological equation is used to describe the experimentally observed structure and temperature dependence of the relaxation time in both glass and equilibrium regimes. Temperature gradients in the DSC sample are incorporated into the model calculations. When no thermal gradients are assumed, model parameters are found to vary with thermal history despite the use of the self-consistent equation for the relaxation time. Accounting for the presence of thermal gradients in the DSC sample is found to affect the values of the model parameters needed to fit the data. However, thermal gradients are unable to account for the thermal history dependence of the model parameters or for the discrepancy between the observed and calculated shapes of the DSC annealing peaks.
Reorientational Dynamics and Intermolecular Cooperativity of Reactive Systems
Authors: Mijovic Jovan 
Published: April 2011

ABSTRACTAn investigation was conducted of the effects of the advancement of chemical reactions and the prepolymer molecular weight on the reorientational dynamics and intermolecular cooperativity in reactive epoxy-amine systems. Experimental results were obtained by dielectric spectroscopy over a wide range of frequency (1 mHz to 1 GHz) and temperature (−170 to +200 °C). A strong effect of the progress of reaction on reorientational dynamics was noted and, for the first time, an explanation was put forward within the framework of the coupling theory. It was proposed that the molecular-level characteristics of a reactive system that affect its intermolecular cooperativity can be classified into two categories: 1) molecular architecture - determined by molecular symmetry, rigidity and steric hindrance; and 2) dielectric architecture - determined by the type and concentration of all dielectrically active species.
Temperature Dependence of Spatial and Dynamic Heterogeneities above the Ising Spin Glass Transition
Authors: Glotzer S. C. , Poole P. H. , Coniglio A. , Jan N. 
Published: April 2011

ABSTRACTThe temperature dependence of the microstructure and local dynamics in the paramagnetic phase of the d = 2 and d = 3 ± J Ising spin glass model is examined by comparing the equilibrium distributions of local flip-rates and local energies calculated in large-scale Monte Carlo simulations. The emergence in this model of fast processes as the glass transition is approached corresponds with recent experimental results.
Slow Dynamics and Glass-Like Behavior of Liquid Crystals Dispersed in Nanoporous Media
Authors: Aliev F. M. , Sinha G. P. 
Published: April 2011

ABSTRACTWe have investigated the dynamic behavior of liquid crystals (LC), which are not glass formers when in bulk form, confined in porous matrices with randomly oriented, interconnected pores as well as in parallel cylindrical pores with different pore sizes by photon correlation (time range: 20 ns-103s) and dielectric spectroscopies (frequencies: 0.1 Hz-1.5GHz). We observed that in random pores (pore size is 10 nm and 100 nm) LC does not crystallize at temperatures about 25° C below bulk crystallization temperature and the non-Debye relaxational processes studied by both photon correlation and dielectric experiments were found not to be frozen. Slow relaxational process which does not exist in bulk LC and a broad spectrum of relaxation times (10−8− 10)s appear not only for LC in random pores but in cylindrical pores as well. However in matrices with random pores of 100 Å, glass-like behavior of slow mode (τ> 1ms) was observed. The relaxation time (determined in photon correlation experiment) of this slow process strongly increases when temperature decreases from 300 K up to 270 K varying from 0.2ms to 14 s and it's temperature dependence is described by the Vogel-Fulcher law.
Small Atom Diffusion and Breakdown on Stokes-Einstein Relation in the Supercooled Liquid State of Zr-Ti-Cu-Ni-Be Alloys
Authors: Geyer U. , Schneider S. , Qiu Y. , Macht M.-P. , Tombrello T. A. , Johnson W. L. 
Published: April 2011

ABSTRACTBe diffusivity data in the bulk metallic glass forming alloys Zr41.2Ti13.8Cu12.5Ni10Be22.5 and Zr46.7Ti8.3Cu7.5Ni10Be27.5 are reported for temperatures between 530K and 710K, extending up to 80K into the supercooled liquid states of the alloys. At the glass transition temperature, Tg, a change in temperature dependence of the data is observed in both alloys, and above Tg the diffusivity increases faster with temperature than below. The data in the supercooled liquid can be described by a modified Arrhenius expression containing the communal entropy of the supercooled liquid and based on a diffusion mechanism suggested earlier. The comparison with viscosity data in the supercooled liquid state of Zr46.7Ti8.3Cu7.5Ni10Be27.5 reveals a breakdown of the Stokes- Einstein relation, whereas D(T) and η(T) follow a relation close to van den Beu-kel's. The breakdown of the Stokes- Einstein relation indicates a cooperative diffusion mechanism in the supercooled liquid state of the ZrTiCuNiBe alloys.
Spac-Ttme-Correlations for Cation Motion in Alkali Silicates
Authors: Kieffer John 
Published: April 2011

ABSTRACTThe motion of structural components in alkali silicates has been studied using molecular dynamic simulations. The relevant time correlation functions have been recorded over periods of up to 0.5 nanoseconds. The van Hove correlation functions reveal that alkali cation diffusion occurs by hopping between rather well defined sites, even at temperatures far above the melting point. The accessibility of such sites is controlled by the relaxation of the surrounding network, and constitutes the mechanism of coupling between the motion of network and modifier elements. The intermediate scattering functions exhibit a stretched exponential behavior, with a wavevector dependent stretching exponent. This is the result of a distribution of residence times at individual cation sites. When the diffusion process is examined on a coarser scale, at which Brownian motion is applicable, this distribution remains undetected. As an alternative to the Kohlrausch function, several mechanistic approaches for describing the relaxation behavior of these structures are discussed.
A Crystallographic Guide to the Structure of Borate Glasses
Authors: Wright Adrian C. , Vedishcheva Natalia M. , Shakhmatkin Boris A. 
Published: April 2011

ABSTRACTBorate glasses are an enigma in that there is now increasing evidence that their structures are dominated by superstructural units, which comprise well defined arrangements of the basic BO3 and BO4 structural units, with no internal degrees of freedom in the form of variable bond or torsion angles. In the present paper, it is shown that considerable insight into the structure of borate glasses can be gained from a study of the corresponding crystalline polymorphs. A simple, model is proposed to predict the fraction, x4, of 4-fold co-ordinated boron atoms in vitreous borate networks and the topological criteria for the formation of such networks are discussed, taking into account the degrees of freedom necessary for conventional glass formation.
Development of the Glassy State of Benzophenone and Effect of Heating Rate from the Glassy State on Solidification
Authors: Thoma Paul E. , Boehm John J. 
Published: April 2011

ABSTRACTBenzophenone supercools to a glass when cooled to −100°C. In fact, it is difficult to freeze benzophenone on cooling. In this investigation, the effect of cooling rate and the minimum cooling rate to obtain benzophenone as a glass is determined. From the glassy state, the influence of heating rate on the solidification temperature of benzophenone is determined. When heated at 3°C/min., solidification starts at about −29°C. Upon additional heating, melting usually starts at about +24°C, which is 23°C lower than the solid equilibrium structure melting temperature of 47°C. Occasionally the solid that forms at about −29°C undergoes a solid state phase transformation at about +22°C, when heated at 3°C/min. If this solid state phase transformation occurs, then the solid benzophenone starts to melt at 47°C. When solid benzophenone with the equilibrium structure is cooled to −100°C, no solid state phase transformation occurs. It appears that the structure that solidifies at −29°C is metastable.
The Effect of Iron and Oxygen Additions on the Properties of Zr-Al-Cu-Ni Bulk Metallic Glass Forming Alloys
Authors: Eckert J. , Mattern N. , Seidel M. , Schultz L. 
Published: April 2011

ABSTRACTThe effect of iron and oxygen additions on the thermal stability of rapidly quenched amorphous Zr65Al7.5Cu17.5Ni10) was studied by x-ray diffraction and differential scanning calorimetry. With increasing Fe content the glass transition temperature Tg and the crystallization temperature Tx shift to higher temperatures. The increase is more significant for Tg than for Tx, resulting in a decrease of the supercooled liquid region with increasing Fe content. For oxygen additions Tx decreases with increasing oxygen content whereas Tg increases slightly, causing a decrease of the supercooled liquid region with increasing oxygen content. The results reveal that even minor iron or oxygen contaminations lead to drastic changes in the glass transition behavior and the crystallization mode. Large iron or oxygen contents lead to the formation of nanocrystalline microstructures instead of coarse-grained material.
Synthesis and Properties of Bulk Metallic Glasses in Pd-Ni-P and Pd-Cu-P Alloys
Authors: He Y. , Schwarz R. B. 
Published: April 2011

ABSTRACTBulk amorphous Pd-Ni-P and Pd-Cu-P alloy rods with diameters ranging from 7 to 25 mm have been synthesized over a wide composition range using a fluxing technique. For most bulk amorphous Pd-Ni-P alloys, the difference ΔT = Tx - Tg between the crystallization temperature Tx and the glass transition temperature Tg is larger than 90 K, while for bulk amorphous Pd-Cu-P alloys, ΔT varies from 27 to 73 K. Pd40Ni40P20 has the highest glass formability, and 300-gram bulk amorphous cylinders, 25 mm in diameter and 50 mm in length, can be easily produced. This size, however, is not an upper limit. The paper presents the glass formation ranges for both ternary alloy systems and data on the thermal stability of the amorphous alloys, as well as their specific heat, density, and elastic properties.
Fast Dynamics in Glass Forming Systems: Vibrations vs Relaxation
Authors: Sokolov A. P. 
Published: April 2011

ABSTRACTTwo contributions specific for the spectra of the fast dynamics in glass forming systems, a broad quasielastic scattering and the boson peak, are analyzed. It is shown that the vibrational contribution (the boson peak) decreases strongly in fragile systems. Some speculations about dependence of the degree of fragility (a la Angeli) on peculiarity of the spectrum of fast dynamics are presented. The existence of some intrinsic relation between the broad quasielastic contribution and the boson peak is demonstrated from analysis of the recent neutron and Raman scattering data. It is shown that this relation can be explained in framework of the model of damped oscillator. The model ascribes the quasielastic contribution to the scattering of light or neutrons on the vibrations around the boson peak, which are damped by some relaxation channel and have a quasielastic part in their response function. It is demonstrated that the model can explain many peculiar properties of the fast dynamics in the Raman, neutron and far-infrared absorption spectra.
Interfacial Composition Measurements in Complex Ni-Base Superalloys
Authors: Hall E. L. , Bentley J. 
Published: April 2011

ABSTRACTIn this study, three different Ni-base superalloy / heat treatment combinations are studied in an attempt to assess the role of grain boundary morphology, composition, and phase distribution on mechanical properties, particularly time-dependent fatigue crack growth. The alloys chosen include one in which crack growth can be slowed by slow-cooling, and one in which crack growth is slow in the fast-cooled state. Both x-ray spectroscopy and energy-filtered imaging in the analytical electron microscope were used to measure grain boundary composition. The x-ray spectroscopy showed some enhancement of Cr, Mo, and W in the γ matrix at grain boundaries in the fast-cooled state, which was not present after slow cooling. Energy-filtered imaging showed no significant enhancement in alloying elements at interfaces in any of the samples studied. The results did show the tendency for the γ matrix to quickly equilibrate by second-phase precipitation, and a preference to avoid γ ‘- γ’ interfaces. The conclusions of this study are that time-dependent fatigue crack growth behavior in these alloys cannot be completely explained on the basis of grain boundary composition of major alloying elements.
Microstructure and Microchemistry of Interfaces in Structural Ceramic Composites
Authors: Alexander K. B. , Becher P. F. , Rice P. M. , Braski D. , Sun E. Y. 
Published: April 2011

ABSTRACTThe performance of reinforced ceramics, particularly the toughness and creep resistance, is often determined by the nature of the interface between the reinforcement and the ceramic matrix. Specially-designed experiments to investigate the role of the interfacial characteristics on toughening mechanisms and crack propagation in reinforced (silicon carbide whisker reinforced alumina) and self-reinforced (silicon nitride) ceramic composites will be described. In the whisker-reinforced composites, the interfacial topography and chemistry were of primary importance, whereas in the silicon nitride materials the formation of interfacial phases and glassy-phase chemistry influenced the interfacial debonding process. The composite interfaces were characterized by high resolution electron microscopy and high spatial resolution microchemical analysis, including energy-dispersive X-ray and electron energy loss spectroscopy. Results from energy-filtered images from ceramic interfaces will also be shown.