bluuebunny/crossref_metadata_2025_split huggingface dataset:
Properties and Structural Characterization of the Laser Synthesized Nano-Composite Si-N-C Powders
Authors: Li Ya-li , Liang Yong , Hu Zhuang-qi 
Published: April 2011

AbstractThe properties and structure characteristics of the ultrafine Si-N-C powders synthesized from laser induced hexamethyldisilazane and ammonia gas phase reactions were characterized by multiple techniques. Result shows that the Si-N-C particles are uniform in diameter with narrow size distributions and less agglomerated. The powders exhibited a short distance ordered structure which was distorted at high temperature annealing. HTEM observation shows that the particles are amorphous or have nano-composite structure with SiC particles dispersed in the amorphous Si-N-C matrix. The nucleation and growth of the composite particles occurred by the polymerization of the ligand formed from the decomposition of Si-CH3and N-H bonds in hexamethyldisilazane accompanied by the release of small hydrocarbon gases.
In-Situ Formation and Characterization of α-Si3N4 Whiskers from Nano Amorphous Si-N-C Powders
Authors: Li Ya-Li , Liang Yong , Hu Zhuang-Qi 
Published: April 2011

Abstractα-Si3N4 whiskers were formed from laser-synthesized nanoscale amorphous Si-N-C powders at 1873K under 1 atm N2. The as-formed whiskers were characterized by TEM, STEM, XRD techniques and the process conditions for the whisker growth were studied. The whiskers exhibit various morphologies such as the long thick straight, the prismatic, the ribbon-like, and knuckled whiskers. The gas phase reaction among N2, SiO, and CO gases leads to Si3N4 whisker growth on the pre-crystallized α-Si3N4 grains by the Vapor-Solid (VS) mechanism along specific crystal planes such as {1101}., which ensures an in-situ formation. No addition of other catalyst and the atomic combination of the elements in the Si-N-C powders ensure a high purity of the whiskers.
Modeling the GAS-Phase Chemistry of Silicon Carbide Formation
Authors: Allendorf Mark D. , Osterheld Thomas H. 
Published: April 2011

AbstractMethyltrichlorosilane (MTS) is a precursor commonly used in the production of silicon carbide for composites, protective coatings, and structural ceramics. Experiments suggest that MTS decomposes in the gas phase to produce a large number of species whose surface reactivity is expected to vary widely. In this work, we discuss the development of a gas-phase mechanism that describes the decomposition of MTS. Rate coefficients for the reactions were obtained from literature sources, theoretical methods, and comparisons with analogous reaction chemistry. Several unknown reaction rates were fit to experimental measurements of MTS decomposition. The results are used to predict the major reaction paths, identify reactions to which product concentrations are sensitive, and determine the rate-limiting step for MTS decomposition.
MOCVD of Yttria Stabilized Zirconia on Co-Cr
Authors: Greenwald Anton C. , Loges Peter G. , Kaloyeros Alain E. 
Published: April 2011

AbstractThin films of yttria stabilized zirconia (YSZ) were deposited by metal organic chemical vapor deposition (MOCVD) on cobalt-chromium (Co-Cr) alloys used for orthopaedic implants. TMHD derivatives were preferred for the metalorganic sources over the less expensive fluorinated beta-diketonates as the latter left up to 1% fluorine in the film, as determined by Auger spectroscopy. Crystalline cubic structure of these YSZ films was verified by X-ray diffraction. Columnar grain growth gave a rough surface on thicker YSZ films, which increased wear of the mating polyethylene component during simulation tests. YSZ films under one micron thick were mirror smooth and reduced wear of the polymer component by 70% compared to uncoated Co-Cr. Film adhesion was excellent.
The Application of the X-Ray Surface Forces Apparatus (XSFA) to Studies of Confined Complex Fluid Systems
Authors: Koltover I. , Idziak S. H. J. , Safinya C. R. , Steinberg S. , Israelachvili J. N. , Liang K. S. 
Published: April 2011

ABSTRACTWe report here on the application of the new technique of the X-Ray Surface Forces Apparatus (XSFA) to the study of the smectic liquid crystal 8CB (4-cyano-4′-octylbiphenyl) and a zwitterionic polyisoprene melt. The XSFA allows one to study the structure of fluid films under confinement and flow using intense synchrotron x-ray radiation. The above systems were investigated with the distances between the confining surfaces ranging from 0.4μm to a few tens of microns. Two different kinds of confining surfaces were used leading to different structural behavior of the samples as a function of the confining gap.
Nanorheology of Polymers, Block Copolymers, and Complex Fluids
Authors: Demirel A. Levent , Cai Lenore , Dhinojwala Ali , Granick Steve , Drake J. M. 
Published: April 2011

ABSTRACTThe shear rheology of molecularly-thin films of fluids has been studied experimentally as it depends on sinusoidal frequency (linear response) or on sliding velocity (nonlinear response). Building upon previous identification of a solidlike state that is induced by confinement, we find the shearinduced transition to a sliding state in which the viscous dissipation is essentially velocity-independent. The mechanism appears to involve wall slip but Fourier transforms of the response reveal fluctuations, intrinsic to the sliding state, over all accessible frequencies. Other ongoing studies involve shear-induced changes in the fluorescence of confined fluorescent probes, shear dilatancy, and the contrast between the shear of simple nonpolar fluids, and block copolymers.
Energetically and Pressure Driven Liquid-Solid Transitions in Molecularly Thin Hydrocarbon Films
Authors: Ballamudi Ravi K. , Bitsanis Ioannis A. 
Published: April 2011

ABSTRACTIn this study we used molecular dynamics simulations to investigate the structural changes in nanoscopically thin n-octane films confined between smooth solid surfaces as a result of a) increasing solid-methylene unit affinity and b) increasing pressure. Increasing solid-methylene unit energetic affinity resulted in the solidification of the film at a critical value. A similar transition is observed at a critical value of pressure. Bulk n-octane was a liquid in both cases. The transition was signaled by an abrupt increase in the intermolecular order and was facilitated by a precipitous extension of the octane molecules, which adopted almost fully extended configurations. A discontinuous jump in the film density at the critical values of solid affinity and pressure was evident. The characteristics of the transitions showed that it was a mild first order transition from a highly ordered liquid to a poorly organized solid.
Probing Surface Roughness and Porosity through Adsorption of Wetting Layers
Authors: Krim J. , Panella V. 
Published: April 2011

ABSTRACTWe have employed a quartz microbalance technique to record adsorption isotherms on silver and gold surfaces whose roughness has also been characterized by x-ray reflectivity or scanning tunneling microscopy. We observe strikingly different behavior for two different liquid adsorbates (oxygen and nitrogen), and attribute this to a difference in their surface tension. Our results demonstrate the impact that capillary condensation phenomena can have on the interpretation of adsorption data, particularly with regards to the fractal nature of the substrate. Valuable information on surface morphology can nonetheless be obtained from adsorption isotherms, if combined with alternate experimental techniques.
Hysteresis in a Capillary Condensation System
Authors: Lilly M. P. , Hallock R. B. 
Published: April 2011

ABSTRACTWe report measurements of the hysteretic capillary condensation of liquid 4He into the porous material Nuclepore. Results are presented for global and hysteretic behavior for 300 Å diameter pores. These observations deviate from the predictions of the Preisach model of independent hysteresis domains. By modifying the Preisach model so as to restrict the ability of some of the pores to drain normally we are able to model many features of the experimental data.
Quenched Molecular Reorientation in Confinement
Authors: Korb J.-P. , Malier L. , Cros F. , Xu Shu , Jonas J. 
Published: April 2011

ABSTRACTThe effect of steric hindrance on reorientational molecular dynamics has been studied both theoretically and experimentally in well-defined geometries. We show that, even for a nonwetting fluid, confinement quenches reorientation. This is unambiguously probed by the nuclear relaxation of 33S and 13C in a set of calibrated porous glasses saturated with CS2.
Freezing/Melting Transition of Physically Restricted n-Decane
Authors: Hoffmann P. M. , Malhotra V. M. 
Published: April 2011

ABSTRACTWe undertook differential scanning calorimetry (DSC) measurements at 170 K < T < 300 K on n-decane, physically confined in 8 nm (= diameter D) porous silica derivatized with various functional groups, to understand how surface structure of the confining media affects the freezing or melting transition of the n-decane. Though we observed a typical depression (ΔT) in the freezing or melting transition temperature of the physically confined decane, our results failed to manifest usual linear dependence of ΔT on D−1 when the expected contraction in D, due to the presence of aminopropyl-, hexyl-, phenyl- or trimethyl-groups on the silica surface, was taken into account. However, it is worth noting that a linear behavior was observed between ΔT and D−1 if only alkane-chain derivatized hosts were considered. Our results also indicate that a large fraction of physically confined n-decane (35 to 70 %), depending on the host silica, does not participate either in the melting or freezing transition. The most interesting behavior observed in the present study is the occurrence of the unusual two peaks associated with the freezing transition of physically confined decane. This bimodal behavior is strongly dependent on the chemistry of the confining silica host's surface.
Study of an A+2B--->C Reaction-Diffusion System with Initially Separated Components
Authors: Yen Andrew , Kopelman Raoul 
Published: April 2011

ABSTRACTThe presence of a reaction front is a characteristic feature of a variety of physical, chemical and biological processes. A chemical reaction exhibits a front (spatially localized region where concentration of product is non zero), provided the diffusing reactants are separated in space. We study the reaction front dynamics of a termolecular A+2B--->C reaction with initially separated components in a capillary. The reaction tetra+2Ni2+--->1:2 complex is used, where ‘tetra’ is disodium ethyl bis(5-tetrazolylazo) acetate trihydrate. We measure and compare with theory the dynamic quantities that characterize the kinetic behavior of the system: the global reaction rate R(t), the location of the reaction center xf(t), the front's width w(t), and the local production rate R(xf,t). The non-classical nature of this dynamical system is confirmed.
Adhesion Force Between High Energy Surfaces in Vapor Atmosphere
Authors: Crassous Jerome , Loubet Jean-Luc , Charlaix Elisabeth 
Published: April 2011

ABSTRACTWe report experimental measurements of the adhesion force between metallic substrates in undersaturated heptane vapor atmosphere, with a surface force apparatus. The attractive force between the substrates is strongly dependant of the condensation of a liquid bridge connecting the surfaces. The results show the importance of wetting phenomena for the maximum attractive force: we find that this maximum attraction varies as the power two-third of the curvature of the meniscus connecting the surfaces, in good agreement with the theory of Van der Waals wetting.
Molecular Motion through a Fluctuating Bottleneck
Authors: Eizenberg N. , Klafter J. 
Published: April 2011

ABSTRACTMolecular motion in a series of cavities dominated by time dependent bottlenecks is studied as a model for molecular pathways in biomolecules. The problem is formulated by coupled rate and Langevin equations and is shown to be equivalent to n-dimensional reaction-diffusion equation where n is the number of cavities visited by the molecules. Results are presented for two cavities and a comparison is made between steady state and non steady state results.
The Escape of Particles from a Quantum Well
Authors: Lavine James P. 
Published: April 2011

ABSTRACTThe escape rate is calculated for an electron in a one-dimensional potential well. First-order time-dependent perturbation theory is used with solutions of Schrödinger's equation and a set of coupled rate equations is numerically solved. The time evolution of an ensemble of one-electron systems is followed and the fraction of systems that remain in a bound state is found to decay exponentially as time passes. The characteristic time constant for the decay grows exponentially with an increase in the well depth. This is analogous to Kramers' result for the classical escape problem.
Diffusion-Controlled Reactions in Micellar Systems
Authors: Tachiya Masanori , Barzykin Alexander V. 
Published: April 2011

ABSTRACTReaction kinetics in micellar solutions are studied theoretically with an emphasis on diffusioncontrolled luminescence quenching. Different spatial arrangements of reactants within individual micelles are analyzed and a general method for treating diffusion-controlled reactions in a finite volume employing an effective potential approximation is developed. Several models are considered for the exchange of reactants between micelles including migration mediated by the bulk phase and successive multiparticle hopping through transient channels connecting micelles during their sticky collisions. These results are combined in a general stochastic theory of reaction kinetics in micellar solutions with exchange. The theory is further extended to reactions in clusters of micelles using a continuous time random walk approach. Once the principal features of micellar kinetics are understood, one can extract important structural and dynamic information on the aggregates and their guest molecules by analyzing suitably designed experiments.
Global Reaction Rates from Local Cluster Distributions
Authors: Argyrakis Panos , Kopelman Raoul , Lindenberg Katja 
Published: April 2011

ABSTRACTThe well-known model reactions A+A⇢0 and A+B⇢0 are examined in terms of the microscopic geometric arrangements of the reactants that may possibly lead to reaction. The reaction takes place only upon physical short-ranged collision of the reactants. Different dimensionalities are examined, and for each one of these arrangements all possible arrangements of the reactants are enumarated. Then, the frequency (probability) is taken into account, and an overall rate is formed. This rate is monitored in the course of the reaction in the simulation calculation and compared to the overall rate derived the traditional way from the densities. These two rates agree very well. Finally, the pair correlation functions are calculated as a function of time from the simulation data, and they are also in good agreement with previous studies.
Reaction Kinetics at a Fluctuating Surface: Proton Exchange in Proteins
Authors: Dewey T. Gregory 
Published: April 2011

ABSTRACTExperimental data for the exchange of protons between proteins and the aqueous solvent is re-examined using a fractal model. The fraction of protons unexchanged from the protein, f is seen to follow a stretched exponential, at long times. For the protein, lysozyme, data over a range of temperatures were considered and accurate fits were obtained with a single, unadjusted scaling exponent, α. The time constant, τ, followed an Arrhenius law and gave an activation energy comparable to that obtained for free peptide exchange. A model is proposed where proton exchange occurs as a result of solvent reacting at the fractal surface of the protein. The protein itself is not treated as static but has units that diffuse to the surface. The diffusion of these units in the long time domain is assumed to be classical. In this model the scaling exponent, α, is related to the spectral dimension of the system. Treating the problem as a reaction of the type A+B→C in a confined region, the exponent is given by: α = (3 − ds)/4 where dS/4 is the fractal surface dimension of the protein. Using the value of 2.17 previously established for the surface dimension of lysozyme, data at 6 different temperatures could be fit with the corresponding α of 0.21. These results show that the fractal structure of a protein can influence difflusional processes of small molecules associating with the protein.
Dynamics of Ferroelectric and Nematic Liquid Crystals Confined in Porous Matrices
Authors: Aliev Fouad M. 
Published: April 2011

ABSTRACTWe performed dielectric spectroscopy measurements to study dynamics of collective modes of ferroelectric (FLC) and molecular motion of nematic (NLC) liquid crystals with polar molecules confined in silica macroporous and microporous glasses with average pore sizes of 1000 Å (volume fraction of pores 40%) and 100 Å (27%) respectively. For FLC the Goldstone and the soft modes are found in macropores. The rotational viscosity associated with the soft mode is about 10 times higher in pores than in the bulk. These modes are not detected in micropores although low frequency relaxation is present. The last one probably is not connected with the nature of liquid crystal but is associated with surface polarization effects typical for two component heterogeneous media. The difference between the dynamics of orientational motion of the polar molecules of NLC in confined geometries and in the bulk is qualitatively determined by the total energy Fs of the interaction between molecules and the surface of the pore wall, which is found Fs ≈ 102erg/cm2.
Critical Dynamics of Contact Line Depinning
Authors: Ertaş Deniz , Kardar Mehran 
Published: April 2011

ABSTRACTThe depinning of a contact line is studied as a dynamical critical phenomenon by a functional renormalization group technique. In D = 2 - ∈ “line” dimensions, the roughness exponent is ζ = ∈/3 to all orders in perturbation theory. Thus, ζ = 1/3 for the contact line, equal to the Imry-Ma estimate for equilibrium roughness. The dynamical exponent is z = 1 – 2∈/9 + O(∈2) < 1, resulting in unusual dynamical properties. In particular, a characteristic distortion length of the contact line depinning from a strong defect is predicted to initially increase faster than linearly in time. Some experiments are suggested to probe such dynamics.
Synthetic Design of Cobalt Fischer-Tropsch Catalysts
Authors: Soled Stuart L. , Baumgartner Joseph E. , Reyes Sebastian C. , Iglesia Enrique 
Published: April 2011

ABSTRACTWe describe synthetic strategies for designing cobalt Fischer-Tropsch catalysts in order to maximize synthesis rates and C5+ selectivity. The support cobalt-precursor interaction must be balanced to maximize dispersion yet allow facile low temperature reduction. The cobalt time yield is independent of support and varies directly as the cobalt dispersion for dispersions in the 0.02 to 0.12 range [1].
Encapsulated Silver Clusters as Oxidation Catalysts
Authors: Crane R. A. , Chao L. C. , Andres R. P. 
Published: April 2011

ABSTRACTA novel supported metal catalyst has been prepared, that consists of Ag clusters a few nanometers in diameter which are encapsulated in an amorphous silica coating only a few Ångstroms in thickness and are deposited on an oxide support. The synthesis of this catalyst and its physical characterization are described. Its potential use as a selective oxidation catalyst is discussed.
Performance Characteristics of Low-Temperature Co Oxidation Catalysts
Authors: Hoflund Gar B. , Gardner Steven D. , Schryer David R. , Upchurch Billy T. , Schryer Jacqueline , Kielin Erik J. 
Published: April 2011

ABSTRACTA series of Au/MnOx catalysts with the Au content ranging from 0 to 10 at%, a 19.5 wt% Pt/SnOx catalyst and a commercially available 2 wt% Pt/SnOx catalyst have been examined for activity toward low-temperature CO oxidation. A 1 atm, 10 sccm stoichiometric mixture of CO (1 vol.%) and O2 in He were reacted at 55°C over 100 mg of each catalyst for time periods as long as 20,000 min. Under these conditions a 10 at% Au/MnOx catalyst exhibits the highest activity and the lowest decay. Outgassing experiments indicate that CO2 retention is primarily responsible for the activity decay of the Pt/SnOx catalysts and that the 10 at% Au/MnOx catalyst is not significantly affected by this process.
Preparation and Characterization of New Molybdenum Nitride or Oxynitride Phases
Authors: Marchand Roger J. , Gouin X. , Tessier F. , Laurent Y. 
Published: April 2011

ABSTRACTSeveral methods of synthesizing molybdenum nitride or oxynitride fine powders are presented.We have prepared a γ-Mo2N type oxynitride phase by reacting ammonia with MoO3. The surface area and morphology of the oxynitride powders depend on the synthesis conditions. Characterization of the solids by elemental analysis, X-ray and neutron diffraction, and thermogravimetric analysis shows dramatic modification of the stoichiometry of conventional Mo2N nitride. Aging at room temperature under air results in decreasing the material surface area. The initial surface area can be recovered be fine tuning of experimental conditions. MoCl5 and Ca3N2 are reacted in a molten CaCl2 medium leading to a new Mo2N structure type.The reaction between molybdenum sulfide and NH3 produces two different phases depending on the reaction conditions. They are structurally related to δ-MoN.
Novel Microfabricated Pd-Au/SiO2Model Catalysts for the Hydrogenation of 1,3-Butadiene
Authors: Krauth Anthony C. , Wolf Eduardo E. 
Published: April 2011

ABSTRACTModel Pd-Au/SiO2catalysts were prepared via a microfabrication methodology and their catalytic activity was evaluated using the hydrogenation of 1,3-butadiene as a probe reaction. The catalytic particles of the microfabricated catalyst were Pd-Au squares, ranging in composition, each approximately 4 µm in size and separated by 4 µm on a silica/silicon support. The reaction was carried out at atmospheric pressure with a hydrogen to hydrocarbon ratio of 125. The bimetallic catalysts exhibited lower apparent activation energies than the pure Pd catalyst with a minimum occurring at ˜65 at% Pd. Corresponding to the decrease in activation energy, there is an increase in the turnover frequency over the monometallic catalyst at low temperatures, i.e. room temperature. At high temperatures (above 200°C) the pure Pd catalyst has the highest conversion (0.61) and the largest pre-exponential factor (4.71×1016cc/g Pd/s), while the 73 at% Pd catalyst has the lowest conversion (0.45) and the smallest pre-exponential factor (5.59 × 106cc/g Pd/s). The bimetallic catalysts also showed a higher selectivity towards 1-butene than the monometallic catalyst; however, at lower conversions (less than 25%) the pure Pd catalyst was more selectivity towards 1-butene. AFM was used to characterize the surface morphology of each of the samples.
Fine Chemicals Synthesis by Heterogeneous Asymmetric Catalysis
Authors: Wan Kam T. , Davis Mark E. 
Published: April 2011

ABSTRACTThe application of heterogeneous asymmetric catalysis to fine chemicals synthesis is discussed. The asymmetric synthesis of naproxen, a non-steroidal antiinflammatory drug, using a novel, heterogeneous, chiral catalyst is used as an example to illustrate the feasibility to industrial application.