bluuebunny/crossref_metadata_2025 huggingface dataset:
Charge and Size Polydispersity Effects on the Scattering Properties and the High-Frequency Elasticity of Colloids
Authors: D'Aguanno Bruno , Klein Rudolf , Wagner Norman J.
Published: March 2011

Polydispersity in charge and in size of colloidal particles strongly influences the static light scattering < I(k) >. These effects are calculated by solving the Ornstein-Zernike integral equation together with the thermodynamically consistent Rogers-Young closure for a multicomponent mixture of up to 10 components. The results for < I(k) > for polydisperse systems show significant differences from results obtained by treating the system as an effective monodisperse one. For  a large increase is found at small k as well as a shift in the main peak. Finally, using the above results, high-frequency elastic moduli are predicted for a binary mixture.
Microscopic Processes in Microemulsion Polymerizations
Authors: Candau Francoise
Published: March 2011

The polymerization of acrylamide inside watcr-swollen micellas of Aerosol OT dispersed in toluene has been investigated. Incorporation of acrylamide enhances the interparticular attractive forces as shown by the intensity curves in small angle neutron scattering. It also induces sharp rises in conductivity which were attributed to a percolative process. The percolation threshold is lowered upon increasing the monomer content, reflecting the enhancement of the attractive interactions. In percolating systems, the structure affects the formation of polymer particles. Microscopy experiments show that particle nucleation occurs continuously throughout the polymerization.
Formation of Metal Colloids in Inverse Micelles and Microemulsions
Authors: Wilcoxon Jess P. , Williamson Rod L.
Published: March 2011

Gold colloidal suspensions are produced from both inverse micelle solutions and microemulsions through both chemical and photolytic reduction of the appropriate salt. A variety of surfactant/solvent combinations yield varying results with respect to metalloid growth kinetics and stability as revealed through dynamic light scattering, small-angle x-ray and neutron scattering, and transmission electron microscopy.
X-Ray Diffraction Studies of Zwitterionic Associating Polymers
Authors: Shen Y. , Safinya C. R. , Fetters L. , Adam M. , Witten T. , Liang K. S. , Chance R. , Stokes J.
Published: March 2011

We have carried out x-ray scattering studies in melts of a series of linear polyisoprenes with highly polar sulfo-zwitterion groups at one end. The zwitterion end groups cause aggregation in good solvents for polyisoprene. The aggregation number depends strongly on the molecular weight of the polyisoprene tails and on the polarity of the solvent. Molecular mechanics simulations of the interactions between the zwitterion head groups suggest tubelike or diskline structures for the aggregates. Spherical structures are not energetically favorable. High resolution synchrotron x-ray scattering studies were carried out for six different (chain) molecular weights between 2000 and 23,000. For low molecular weights (between 2000 and 4000) a tubular structure is found with the tubes organized on a well-defined, two-dimensional triangular lattice with very large domain sizes >2000Å. A structural phase transition to a cubic (bcc) phase with long range order is observed to occur for molecular weights >10,000. The lattice spacing increase over the molecular range was between 95Å (for MW-2000) and 206Å (for MW-23,000). For the high molecular weight melts, annealing transforms the structure from (bcc) (with long range order) to an (fcc) disordered structure with short range order.
Structure and Properties of Resorcinol-Formaldehyde Gels
Authors: Letts Stephan A. , Buckley Steven R. , Kong Fung-Ming , Lindsay Edward F. , Sattler Margaret L
Published: March 2011

The condensation polymerization of resorcinol and formaldehyde catalized by sodium carbonate produces a sol that aggregates to form a gel. Using viscometry the effect of temperature and catalyst concentration on the sol-gel transition was investigated. At a solids concentration of 3%, gelation occurred in 1500 minutes. The rate of increase in viscosity was a function of both catalyst concentration and temperature. The structure of the gels was studied by freeze-fracture TEM. In the sol phase the RF solutions contain particles of 5 to 20 nm diameter. After gelation the particles form a crosslinked network with a pore structure of 100 nm.
Thermal Property Evolution Toward Effectively Two-Dimensional Substrate-Free Systems
Authors: Geer R. , Stoebe T. , Huang C. C. , Pindak R. , Goodby J.
Published: March 2011

A high-resolution differential quasi-adiabatic calorimeter has been developed to investigate the physical properties of freestanding liquid-crystal films. Our recent heat-capacity studies near the smectic-A-hexatic-B transition of both 65OBC and 46OBC freestanding films clearly demonstrate the evolution towards twodimensional limiting behavior in four-layer films. As the film thickness decreases towards four molecular layers, the heatcapacity anomalies first display separate peaks for the surface and interior transitions and then evolve to one dominated by the surface transition.
Electrohydrodynamic Flow in Thick Liquid Crystal Cells
Authors: Van Winkle David H. , Gurung Jit , Biggers Rand
Published: March 2011

The thermal transport across a thick (0.66 cm) liquid crystal cell has been measured versus applied ac voltage and frequency. These measurements are correlated with the optically observed onset of flow and turbulence in cells as identical as practicable to those used for the thermal transport measurements. In addition, the measurements are compared with reported observations in thin cells. The thermal transport across the liquid crystal is characterized by an effective thermal conductivity Kf. It was found that Kf increases with increasing frequency, at constant voltage, to a maximum enhancement at about 40 Hz at room temperature. Optical observations on thick cells indicate that dynamic columnar domains of flowing liquid crystal are the primary mode of heat transport, as determined by correlating the structure and characteristic lifetime of such domains as a function of voltage and frequency. Optical observations at low voltages suggest that Williams Domains do not exist in these thick cells, and that all observed responses are functions of electric field strength, not applied voltage (as in thin Williams Domain cells).
Simple Theory for Persistent-Flexible Liquid Crystal Polymers Beyond the Second Virial Approximation
Authors: Hentschke Reinhard , Herzfeld Judith
Published: March 2011

Khokhlov and Semenov (KS) have proposed a theory for long persistent chains that quite accurately describes the liquid crystalline behavior of dilute solutions of long homogeneous bend-elastic macromolecules. By replacing their second virial description of excluded volume effects with Lee's generalization of the Carnahan-Starling equation, results are obtained which are valid over a wider range of polymer concentrations. Comparison of the calculated order parameter and equation of state with experimental data for Poly(γ-benzyl-L-glutamate) yields good agreement for large polymer axial ratios, where rigid particle theories deviate strongly from experimental results. In addition, we show how this approach to persistent flexibility can be connected to a recent model for translational order in lyotropic liquid crystals.
Weighted-Density Theory of Phase Transitions in Fluids Composed of Anisotropic Particles
Authors: Marko J. F. , Curtin W. A.
Published: March 2011

A nonperturbative approach to the density-functional description of phase transitions in fluids composed of anisotropic particles is presented. The theory is exact to second order in functional perturbation theory, and at higher orders satisfies all sum rules derived from density derivatives of the two-point direct correlations. We have applied this theory to orientational and translational freezing of hard ellipsoids, and we find that the description of the structure of crystalline phases is improved in comparison to the commonly used second-order theory. For the case of orientational freezing, it is found that the higher-order contributions do not modify the predictions of the second-order theory, and that accurate liquid structure appears to be the key factor leading to improved description of the nematic phase.
Modulated Phases in Amphiphilic Monolayers at the Water/Air Interface
Authors: Andelman David
Published: March 2011

Recently, modulated phases of insoluble monolayers of fatty acids and phospholipids spread on the water/air interface have been observed by fluorescence microscopy experiments. We propose a theoretical explanation of this observation by including electrostatic (dipolar) interactions in the total free energy calculation for the monolayer. Dipoles can originate from two sources: neutral amphiphiles have a permanent dipole and charged amphiphiles have an induced one. Modulated phases are found to be stable in two different limits: close to the liquid-gas transition and at low temperatures. Several phases with stripe and hexagonal symmetry are predicted and the phase transitions between them are calculated.
Surface Adsorption Studied by Optical SHG: Soluble Monolayers at the Air/Water Interface
Authors: Vogel Viola , Mullin C. S. , Shen Y. R. , Kim M. W.
Published: March 2011

Optical second harmonic generation (SHG) is applied to investigate the structure of the interfacial layer of soluble surfactants adsorbed to the air/water interface.
Nuclear Reaction Analysis Studies on the Interface Formation in Polymer Mixtures
Authors: Steiner Ullrich , Chaturvedi Kumar , Zak Omer , Krausch Georg , Schatz Günther , Klein Jacob
Published: March 2011

We have measured the composition profile between two polymer phases above and below their critical temperature for phase separation Tc, using the 2H(3He,4He)1H nuclear reaction in a direct profiling technique. Above Tc, we verify the “thtrmodynamic slowing down” effects on interdiffusion close to the critical point. Pelow Tc, we find that the width of the interface grows with time to a finite limiting value. The variation of this limiting value, as Tc, is approached from below is in good agreement with the predictions of mean-field theories. The time dependence of the initial interfacial broadening below Tc has been found to be significantly slower than t1/2.
Influence of Cationic Interactions on Fatty Acid Monolayers Studied by Ellipsometry
Authors: Kim Mahn Won , Sauer Bryan B. , Yu Hyuk , Yazdanian Mehran , Zografi George
Published: March 2011

Optical properties of fatty acid monolayers were measured as a function of surface density and under the influence of cations in the aqueous substrate. The monolayer behavior of pentadecanoic acid (C15) on an aqueous solution of HCl at pH=2.0 was studied. The change in the ellipsometric phase angle δΔ was mildly affected by increases in surface density at constant orientation but strongly influenced by the liquid-expanded/solid phase transition presumably due to a change in conformation.

The increase in the ellipsometric phase angle 6Δ with chain length C15, C16, C18, and C22 was attributed to an increase in thickness of fatty acids at the air/water interface. Separating the effect of optical density of head groups, the refractive indices calculated from the chain length data were independent of ionic environment. The magnitude of δΔ for fatty acids on aqueous substrates increased in the order HCl, CdCl2, and PbCl2. This was due to an increase in the refractive index of fatty acid head groups caused by association of divalent cations. Ellipsometry was also found to be a useful technique to monitor in-situ the degree of divalent cation association.
Molecular Dynamics Simulations of Contact Line Motion
Authors: Robbins Mark O. , Thompson Peter A.
Published: March 2011

We report on molecular dynamics simulations of incompressible Lennard-Jones fluids sheared between two solid walls. Boundary conditions and flow fields were studied for both single fluid and immiscible two-fluid systems. For single fluid systems, equal wall and fluid densities or strong wall-fluid couplings crystallized the first layers of fluid atoms. Different wall densities and weaker coupling led to normal Couette flow with a no-slip boundary condition. Our simulations of two fluid systems indicate that there is always slip within ∼ 2 atomic spacings from the contact line. This slip appears to be associated with the breakdown of hydrodynamics at atomic scales. Changes with capillary number in the interface shape and dynamic contact angle were consistent with previous analytic results.
A Computer Model for the Average “Cluster” Size in Polymer Aggregates
Authors: Balazs Anna C. , Hu Jenny C. , Lentvorski Andrew P. , Lewandowski Steven , Lantman Christopher W.
Published: March 2011

A reversible diffusion-limited aggregation model for self-assembling polymers has been developed with the aid of computer simulations. The calculations have been performed in three dimensions and apply to linear chains bearing an associating site at each terminus. The results reveal that the associating sites form finite-sized clusters that, in turn, lead to the assemblage of a supermolecular network. The average cluster size is found to vary with the degree of reversibility of site binding.
What can we Learn from Molecular Dynamics Simulations of Macromolecular Liquids?
Authors: Grest Gary S. , Kremer Kurt , Murat Michael
Published: March 2011

We describe how molecular dynamics simulations for a relatively simple coarse grained model can be very useful for investigating the static and dynamic properties of polymers and other macromolecular liquids. We show that it is important to use a simplified coarse grained model instead of a detailed microscopic model if one is interested in studying on modern supercomputers large systems which also relax slowly. As examples we present results for isolated star polymers with f-arms and diluted gelation/percolation clusters. We find in agreement with recent neutron scattering experiments that diluted percolation clusters swell and that their fractal dimension is reduced from 2.5 to 2. We also discuss our results for a dense melt of entangled linear polymers to show that the method is effective at high density. Our results for the entangled melt cover the crossover from Rouse to reptation and strongly support the concept of reptation.
Interface Effects with Ga0.47In0.53As Layers on InP Substrates Prepared by Organometallic Chemical Vapor Deposition
Authors: Whiteley J. S. , Ghandhi S. K.
Published: March 2011

Lattice-matched Ga0.47In0.53As was epitaxially grown on InP substrates by the reaction of triethylgallium, triethylindium and arsine. The mobility and carrier concentration in these layers were determined by sequential etch and Hall effect measurements made on the grown layers. These measurements show a considerable fall–off in mobility in the vicinity of the interface, accompanied by a rapid increase in electron concentration. In situ chloride etching of the substrate, prior to Ga–In–As growth, is shown to reduce significantly but not eliminate these interface effects. In this paper we outline possible reasons for these effects, based on measurements made on films grown with and without substrate etching and also on measurements of the effect of etching on the substrate itself.
Structural Studies of Metal-Semiconductor Interfaces by Means of Surface Extended X-Ray Absorption Edge Fine Structure
Authors: Stohr J. , Jaeger R. , Kendelewicz T. , Rossi G. , Lindau I.
Published: March 2011

We report surface extended X-ray absorption edge fine structure (SEXAFS) studies of the early stages of Schottky barrier formation. Results will be presented for aluminum, silver and palladium on Si(111) 7 × 7 in the range from ⅓ monolayer to five monolayers. The SEXAFS signal reveals the local structure around the metal atoms in terms of bond lengths and coordination numbers. Complex ordered and disordered surface structures are observed ranging from weak reaction between the components (e.g. silver clusters on silicon) to strong reaction (e.g. Pd2Si formation). At elevated temperatures the Ag–Si interface is particularly interesting and consists of a well-defined ordered interface layer (√3 × √3 Ag on Si(111)) with silver clusters growing on top. While most other surface structural techniques fail to yield complete information the power of SEXAFS as a local structural technique becomes most apparent for such complex systems. Structure determinations for some metalsemiconductor interfaces will be presented.
Ion-Beam-Induced Silicide Formation in Nickel Thin Films on Silicon
Authors: Chen L. J. , Hou C. Y.
Published: March 2011

As+-ion-induced silicide formation in nickel thin films on silicon was investigated by Rutherford backscattering spectrometry and transmission electron microscopy. The emphasis was on the study of ion-beam-induced microstructural changes.

For 160 keV As+ implantation, amorphization of the interface occurred at a dose of 5 × 1014 cm−2. Ni2Si was found together with an amorphous layer after a 1 × 1015 cm−2 bombardment. For Ni/Si(100) the surface layer became completely amorphous after implantation to 5×1015 cm−2. Silicides were found after a 1×1016 cm−2 irradiation. The amorphous layer was not stable enough to withstand the enormous chemical driving force causing the formation of crystalline silicides as the composition ratio Nsi/NNi reached a critical value. A similar trend for ion-beam-induced reactions was found for 190 keV As+ implantation on Ni/Si(111) as for 160 keV implantation.

The results of post-implantation annealing showed major differences from those obtained for directly annealed samples.
Effects of Two-Step Annealing on the Epitaxialgrowth of CoSi2 on Silicon
Authors: Chen L. J. , Chang T. T.
Published: March 2011

The formation of epitaxial CoSi2 on silicon by both conventional and two-step annealing of cobalt thin films on silicon was studied by transmission electron microscopy.

For two-step annealing, samples were first annealed at 350°C for 1 h, followed by high temperature annealing at 650–950°C for 1 h. The scheme was found to bevery effective in promoting the epitaxial growth of CoSi2 on silicon as well aseliminating faceted structures on Si(111). The results are discussed in terms of the driving away of impurities from the interfaces.
Atomic Motion of Dopant During Interfacial Silicide Formation
Authors: Wittmer M. , Ting C.-Y. , Tu K. N.
Published: March 2011

The redistribution of implanted dopant atoms during silicide formation has attracted much interest recently because of its important implications for shallow junction device technology. Ion channeling and electrical measurements have shown that dopant atoms are pushed ahead in front of the moving silicide-silicon interface during the growth of near-noble metal silicides. However, dopant redistribution has not been observed with refractory metal silicides. This unique feature of near-noble metal silicides is discussed in conjunction with the growth kinetics of these silicides.
Oxidation Behavior of Pd-Si Compounds
Authors: Cros A. , Pollak R. A. , Tu K. N.
Published: March 2011

The room temperature oxidation of PdSi, Pd2Si and Pd4Si has been studied using X-ray photoelectron spectroscopy (X-ray photoemission spectroscopy or electron spectroscopy for chemical analysis). We find that only silicon atoms in these silicides are oxidized and the oxidation of Pd4Si surfaces is enhanced compared with that of Pd2Si and PdSi, as is evidenced by both a higher silicon oxidation state and thicker oxide films. This behavior is discussed in terms of silicide stability and a spillover effect where palladium atoms catalyze molecular oxygen dissociation.
Materials Research Frontiers In Semiconductor Heterojunctions
Authors: Bauer Robert S.
Published: March 2011

New materials preparation techniques such as molecular beam epitaxy (MBE) provide dramatically improved possibilities for interface control and novel device structures. However, compared with our knowledge of Schottky barriers or semiconductor-insulator interfaces, our microscopic information on the basic science underlying semiconductor heterojunctions is very limited. Insight into the growth, extent and composition of such interfaces is needed to illuminate the origins of the electronic and optoelectronic behavior. Most recent experimental studies have not properly monitored the detailed structural characteristics of the heterojunctions being measured.

In this paper we discuss the scientific exploration of the interrelationship between growth and electronic properties of hetero-interfaces prepared by MBE. In situ low energy electron diffraction and synchrotron radiation photoemission were performed on germanium interfaces with GaAs and AlAs polar and non-polar surfaces. The individual surface and subsurface bonding interactions (rather than bulk thermodynamic properties of the semiconductors) are shown to control the formation and electrical properties of hetero-interfaces of either germanium on the compound or the inverse deposition sequence. Simple arguments based on bulk semiconductor heats of formation incorrectly predict the relative stability of interfaces in the lattice-matched Ge/GaAs/AlAs system. Rather the formation of a GeAsx surface layer provides a lower energy free surface during MBE growth and causes arsenic diffusion from the interface layer to lower the free energy when a pure germanium surface begins to form. Significant variations in the band edge offsets of heterostructures are observed when the semiconductor-semiconductor interface microstructure changes with substrate orientation and overlayer crystallinity. Neither the common electron affinity difference “rule” nor recent tight-binding models for determining heterojunction barriers allows for effects of interface microstructure. Measurement and analysis of such “materials-related” aspects of semiconductor interfaces are central to the understanding and control of device characteristics.

These studies are partially funded by the U.S. Office of Naval Research (L. R. Cooper), Contract N00014-81-C-0696. Part of this work was performed at SSRL and Stanford Linear Accelerator Center, supported by National Science Foundation Grant DMR 77–27489 and the U.S. Department of Energy.
Electronic Characterization of Compound Semiconductor Surfaces And Interfaces
Authors: MÖnch Winfred
Published: March 2011

Metal-semiconductor contacts and semiconductor heterojunctions are wellestablished concepts in semiconductor device technology. The key parameters characterizing such junctions are the barrier height and valence band discontinuity at the interface and the electronic interface states. Clean cleaved GaAs(110) surfaces exhibit no intrinsic surface states in the bulk band gap but do exhibit extrinsic cleavage-induced states. Furthermore, local segregates of arsenic were detected. The chemisorption of metals, semiconductors, hydrogen and oxygen causes depletion layers to form on both n-and p-type crystals. The surface states responsible for these band bendings, which persist even under thick layers of metals and semiconductors, are thought to be related to chemisorption-induced defects. Possible candidates are discussed. Chemical trends are also considered.
Rutherford Scattering-Channeling Analysis of Semiconductor Interfaces
Authors: Feldman L. C.
Published: March 2011

The main concepts in ion scattering analysis of interfaces are reviewed and relevant literature is cited.
The Origin of Schottky Barriers on the Cleavage P1Lane of III–V Semiconductors: Review of Some Recent Theoretical Work
Authors: Zunger Alex
Published: March 2011

Recent theoretical ideas on the origin of the Schottky barrier on the cleavage plane of III–V semiconductors are reviewed, using the Al/GaAs(l10) system as a benchmark.
Lattice Structure and Electrical Properties of Epitaxial Aluminum on GaAs
Authors: Fernando  , Ponce A. , Eglash Stephen J.
Published: March 2011

The crystal structure of aluminum layers grown by molecular beam epitaxy (MBE) on GaAs(100) was studied using high resolution transmission electron microscopy. We obtained cross-sectional images with nearly atomic resolution of the lattice structure at Al-GaAs interfaces. GaAs epitaxial layers were grown at 585 °C and cooled in the ultrahigh vacuum MBE growth chamber prior to aluminum deposition. After the desired substrate temperature had been achieved, aluminum was immediately deposited onto arsenic-rich surfaces at a rate of 1000 Å h−1.

Aluminum layers deposited onto substrates at 80 °C exhibit an Al(110)/GaAs(100) epitaxial relationship along an abrupt interface. In some regions of the interface we observed isolated arsenic crystallites which were epitaxial with the GaAs substrate. The current-voltage (I-V) and capacitance-voltage (C–V) characteristics of this material are well behaved. Barrier heights of 0.79 eV and ideality factors of 1.04 were determined from room temperature I–V measurements.

For aluminum deposited onto GaAs substrates at 400 °C, evidence of twodimensional nucleation and growth was observed. The resulting pyramid-shaped aluminum crystallites had facets along crystallographic planes. Two epitaxial relationships were observed: Al(100)/GaAs(100) and Al(110)/GaAs(100). We observed 12 Å steps along the Al(100)–GaAs(100) interface. C–V and scanning electron microscopy measurements indicate that the individual aluminum crystallites are electrically and physically separated.
GaAs Microcircuit Contact and Interconnect Technology
Authors: Welch Bryant .M
Published: March 2011

The purpose of this paper is to review the current status of and to discuss key technological issues relative to GaAs integrated circuit contact and interconnect technology. Rapid progress and performance advantages exhibited by GaAs MSI circuits in both ultrahigh speed and low power dissipation has recently provided the motivation for the development of GaAs large-and very-large-scale integration (LSI and VLSI). The complexity, density and small feature size requirements of high speed LSI and VLSI places severe demands on GaAs contact and interconnect technology with respect to yield, reliability and performance.

The key device used today in GaAs integrated circuits is the depletion-mode Schottky barrier field effect transistor. Results on the optimization of GaAs field effect transistor Schottky barriers and ohmic contacts using X-ray photoelectron spectroscopy and Auger electron spectroscopy surface analysis techniques in conjunction with thermal reliability studies of various metal systems on GaAs will be presented. The established reliability for GaAs integrated circuits with a goldbased metallization system has been shown to have a mean time to failure of more than 109 h at room temperature.

The microcircuit lithography requirements of GaAs LSI and VLSI rely heavily on lift-off and dry etch replication techniques. Several high yield techniques, which have been specifically developed for GaAs, will be described. Finally, recent trends in GaAs device research relative to new non-alloyed contacts and high temperature Schottky barriers will be presented.
Contact Resistivity and Dopant Activation in Pulsedlaser-Annealed Au-Ge/GaAs Contacts
Authors: Aina O. , Katz W. , Rose K.
Published: March 2011

The electrical activation of dopants in the heavily doped GaAs layers of pulsedlaser-annealed Au–Ge/GaAs contacts were deduced and shown to be higher than that in bulk GaAs. The electrical activation is shown to decrease towards the GaAs surface and is used to explain the previously observed minimum in contact resistivity profiles in terms of arsenic loss during the laser annealing. These results will be shown to be consistent with observations of concentration–dependent activation in bulk GaAs.
Tin as a Diffusion Barrier Between CoSi2 or PtSi and Aluminum
Authors: Schutz R. J.
Published: March 2011

The effectiveness of a thin (360Å) layer of reactively sputtered TiN as a diffusion barrier between aluminum and two silicides (PtSi and CoSi2) was evaluated. The chemical composition, structural phases and electrical properties of silicide/Al and silicide/TiN/Al contacts to n-type silicon were studied by Rutherford backscattering spectroscopy, glancing angle X-ray diffraction and Schottky barrier height measurements respectively. The results show that TiN is an effective barrier in these two systems up to at least 450°C, the typical temperature at which aluminum contacts are sintered.
Rapid Thermal Processing Of High Temperature Superconducting Fiber
Authors: Halloran J.W. , Neal M.J. , Ginley D.S. , Venturini E.L. , Kwak J.F. , Baughman R.J. , Mitchell M.A. , Morosin B. , Basu S.N. , Mitchell T.E.
Published: March 2011

We report on the rapid thermal processing (RTP) of YBa2Cu3O7 (Y-123) fibers. Unsintered fibers are densified by RTP. We show that fibers which were originally semiconducting and tetragonal before RTP form normal twinned orthorhombic material after processing for 2–4 seconds above 1000°C with a 3 min. cool down in oxygen. They subsequently show Tc to 90K and magnetization indicative of substantially shielding. We present the effects of varying the RTP parameters on the morphology, phase, and superconducting properties of a number of tetragonal and orthorhombic Y-123 fibers.
Preparation of Porous Oxide Beads Using Polymeric Beads to Control Bead Size and Shape
Authors: Hardy Anne B. , Rhine Wendell E. , Bowen H. Kent
Published: March 2011

A method was developed for preparing silica gels by silicon alkoxide hydrolysis using
water-swellable polymers to control the shape. A variety of polymer beads with different water absorbencies and different bead sizes were used. Silica beads in a wide range of sizes were prepared from polymer beads. The final bead size was dependent on the size and water absorbency of the beads, on the extent of reaction, and on the final density. It was shown that porosity could be formed in two regions. The silica beads contained pores of 3–4 nm that were attributed to the intrinsic gel structure. In addition, when large amounts of polymer were removed, additional porosity on the scale of 20–50 nm was formed. It was also shown that surface areas varied dramatically with calcining. For beads containing only small pores, surface areas varied from 400 m2/g to less than 0.2 m2/g as the calcining temperature was increased from 600°C to 1000°C. Beads containing porosity formed by removal of the polymer remained porous even after calcining at 1000°C.
Boron Nitride and Composite Aerogels from Borazine Based Polymers
Authors: Lindquist David A. , Smith Douglas M. , Datye Abhaya K. , Johnston Gregory P. , Borek Theodore T. , Schaeffer Riley , Paine Robert T.
Published: March 2011

Poly(borazinylamines) have been processed in THF and toluene solvents and aerogel forms produced by supercritical drying methods. The polymer aerogels have been pyrolyzed and porous amorphous and crystalline BN monoliths have been produced. These have been characterized and effects of processing factors on surface area and pore structure are described.
Small Angle X-ray Scattering Studies of Polymeric Zirconium Species in Aqueous Solution
Authors: Jutson J.A. , Richardson R.M. , Jones S.L. , Norman C.
Published: March 2011

Small angle X-ray scattering techniques have been used to investigate the shape and size distribution of Zr(IV) species in aqueous solution. This study has shown that when zirconyl chloride solutions, containing the zirconyl tetramer, are subjected to various treatments polymerisation occurs. While ageing and addition of base produces an increase in particle size the shape remains globular. Refluxing the solutions produces “rod” like particles of varying length but constant cross sectional radius.
High Pressure Raman Study of the Hydrolysis Reaction of Tmos and Teos
Authors: Hoang G. , Watson J. , Zerda T. W.
Published: March 2011

High pressure Raman spectroscopy is used to monitor the hydrolysis reaction of TMOS in solutions with methanol, acetonitrile, acetone, dioxane and formamide, and of TEOS as a function of pH and the catalyst used. The rate constants for various solvents, temperatures and pressures are experimentally determined from the time dependence of Raman band intensities. It is shown that the reaction is slow in dioxane and fast in methanol or formamide. The volume of activation is found from the pressure dependence of the rate constant. The volume of activation, the dielectric constant, dipole moments and hydrogen bonding properties and their role in the hydrolysis reaction are discussed. It is shown that solvents which can form hydrogen bonding with Si-OR groups can increase the rate of the reaction.
Rheology and Microstructure of Concentrated Suspensions
Authors: Mondy Lisa A. , Graham Alan L.
Published: March 2011

We give an overview of some of the experiments currently underway to study the coupling of the microstructure and rheology of concentrated suspensions. Nuclear magnetic resonance imaging, real-time x-ray radiography, and refractive index matching allow the viewing of particles in concentrated suspensions. Both shear flow experiments and falling ball rheometry are reviewed. In the slow flow of these suspensions of large, hard, particles in a viscous Newtonian fluid, colloidal forces are negligible and hydrodynamic forces dominate.

Large local concentration changes are shown to occur rapidly in suspensions of uniform spheres subjected to flow between concentric rotating cylinders. Suspensions of spheres with a bimodal size distribution not only show similar phenomena, but also exhibit particle separation according to size. In addition, the large particles in the bimodal suspension migrate into ordered, concentric, cylindrical sheets, parallel to the axis of the cylinders. These sheets of particles rotate relative to each other. The particle migration and structure formation induced by this inhomogeneous shear flow is believed to be responsible for torque reductions and other anomalous behavior witnessed during the rheological testing of concentrated suspensions reported in the literature. Thus, suspensions may not always be characterized by a viscosity that is a scalar material property.

Suspensions of fibers also show markedly different rheological properties when the particles are aligned by flow. Falling ball rheometry is shown to be an effective tool to determine the bulk viscosity of a suspension while only slightly influencing the microstructure. This is illustrated by showing that falling ball rheometry can isolate the effect of orientation on the viscosity of a suspension of fibers.
Molecular Architecture and its role in Silica Sol-Gel Polymerization
Authors: Cagle P. C. , Klemperer W. G. , Simmons C. A.
Published: March 2011

Sol-gel polymerization of [Si8O12](OCH3)8 in CH3CN under neutral conditions yields very high surface area (SBET > 900 m2/g) xerogels. This property is seen to result from the structure of the gel on the molecular level. According to N2 adsorption studies, model studies, and TEM studies, the large size and rigidity of the cubic [Si8O12] core structure leads to polymers whose rigidity inhibits extensive crosslinking of the type observed in orthosilicate derived xerogels.
S.A.N.S. Study Of Fractal Alumino-Silicate Aerogels
Authors: Chaput F. , Boilot J.P. , Dauger A. , Devreux F. , De Geyer A.
Published: March 2011

Small angle scattering clearly shows that aluminosilicate aerogels, prepared from Si-Al double precursor, form mutually self-similar volume fractals at least in the range of densities 40-160kg/m3. The structure, insensitive to the pH of the added water, consists of primary homogeneous rough units attached into volume fractal clusters (D=2.1) .Heating at 800°C oxidizes residual organic groups and decreases the roughness of the unit particles. Densification above 900°C both implies increasing of the size of units and decreasing of the fractal domain.
Characterization Of Partially Condensed Silica Formed From The Partial Hydrolysis Of Tmos
Authors: Tillotson T.M. , Hrubesh L.W.
Published: March 2011

A stable mixture of partially condensed polysilicates is formed from the substoichiometric, acid catalysed hydrolysis of tetramethoxysilane [Si(OCH3)4] and the removal of the reaction- generated methanol by distillation. This mixture has been used as a precursor to make silica aerogels that show microstructural differences from base catalysed TMOS aerogels. In this paper, we report the analysis of this mixture for a determination of the type and distribution of the polysilicate species formed, and we include measurements of some physical properties of this fluid.
Crystallization and Phase Transformation of ZrO2-SiO2Gels
Authors: Li Xiaoming , Johnson P.F.
Published: March 2011

Pure SiO2, ZrO2and SiO2−ZrO2gels have been prepared by hydrolysis and gelation of tetraethylorthosilicate (TEOS) and zirconium acetate solution. The crystallization and phase transformation of SiO2and ZrO2in these gels have been followed up to 1450°C using differential thermal analysis (DTA), thermal gravimetric analysis (TGA) and X-ray diffraction analysis (XRD). The crystallization and phase transformation behavior of SiO2or ZrO2is quite different in different gel systems. It was found that in pure SiO2gel, the cristobalite phase was precipitated out at about 1000°C. In the pure ZrO2system, tetragonal (t−) phase zirconia was crystallized at about 430°C, t-ZrO2crystals grew three dimensionally and the activation energy for growth was calculated as about 365 kJmol−1. But in the SiO2-ZrO2system, the presence of ZrO2suppressed the crystallization of SiO2, and SiO2increased the initial crystallization temperature and the stability of t-ZrO2.
Studies in the Ba-Ca-Al-O System Chemical Modification of Aluminum Alkoxide by Propionic Acid
Authors: Hebert V. , Vilminot S. , Brion D.
Published: March 2011

Barium and calcium aluminate powders have been elaborated using chemical methods like coprecipitation, alkoxide polymerization and metalloorganic decomposition. These powders have been successfully used for impregnation of porous tungsten cathodes for use in electronic tubes. Spectroscopic studies on the modification of aluminum s-butoxide reveal that carboxylic acid substitutes OR groups forming preferentially bidentate chelating complexes.
Optical Interference Filters By Sol-Gel Processing
Authors: Keddie J.L. , Giannelis E.P.
Published: March 2011

Optical interference filters have been synthesized by sol-gel. The selected filter is a multilayer of alternating TiO2 and SiO2 films. In transmission, the filter edge depends on the angle of incident light, which can be tailored through control of thickness and refractive index of the individual films. Theoretical modeling of the filter with film thicknesses obtained by Rutherford backscattering spectrometry is in good agreement with the experimental optical response.
Microwave Processing of Sol-Gel Derived Potassium Niobate
Authors: Fathi Z. , Ahmad I. , Clark D.E.
Published: March 2011

Microwave energy has been used in a novel processing method. The relationship between processing, structure and properties of sol-gel-derived potassium niobate sol as well as thin films has been investigated. A comparison has been established between microwave and conventional heating for drying, calcining and crystallizing sol-gel-derived samples. Structural and phase changes were monitored by thermogravimetric analysis (TGA), differential thermal analysis (DTA) and x-ray diffraction (XRD).
Dielectric Characterization of Polyceram Films
Authors: Teowee G. , Boulton J.M. , Fox H.H. , Koussa A. , Gudgel T. , Uhlmann D.R.
Published: March 2011

Polycerams are an emergent class of hybrid, multifunctional materials which combine the properties of organic and inorganic materials. Films have been prepared from silicon alkoxides and reactive, functionalized polymers such as triethoxysilyl modified polybutadiene (MPBD), (N-triethoxysilylpropyl)O polyethylene oxide urethane (MPEOU) and trimethoxysilylpropyl substituted polyethyleneimine (MPEI). Characterization of dielectric constant and tan δ of the films has been carried out over a range of frequency from 500 Hz to 100 kHz; and the results are used to consider the potential of Polycerams as dielectric materials.
Sol-Gel Synthesis of Lead Titanate Fibres for Use in the Fabrication of Piezoceramic-Polymer Composites
Authors: Janusson H. , Millar C.E. , Milne S.J.
Published: March 2011

Lead titanate fibres for use in piezoceramic-polymer composites have been prepared by extrusion of polymeric gels. The gels are formed from solutions of lead acetate, titanium isopropoxide and 2-methoxyethanol. The concentration of the solution, and amount of water and acid added, proved to have a strong influence on the suitability of the gel for extrusion purposes. The drying and firing conditions have been altered to obtain dense, fully crystalline PbTiO3 fibres of 100 μm diameter.
Electron Diffraction Analysis of the Structure of SiO2 Gel-Film
Authors: Ohsaki Hisashi , Aegerter Michel A. , Shichiri Takaki
Published: March 2011

The structure of self-supported SiO2 gel-films prepared from acid and basic TEOS solutions is analysed by high energy transmission electron diffraction method. The reduced radial distribution function (RDF) curves show that all the films are already well dense despite the low drying temperature (≤50°C) and short drying time (≤30 s). The Si-O bond length of the gelfilms prepared from highly acid and basic solutions is about 1.58 Å; it is smaller than that of bulk vitreous silica (1.61 Å) but similar to that of BO Å thick evaporated a-SiO2 film.
Thin Anisotropic Coatings Based on Sol-Gel Technology.
Authors: Randall Holmes-Farley S. , Yanyo Lynn C.
Published: March 2011

Using sol-gel technology, thin organic/ceramic (ceramer) coatings have been applied to metal surfaces in order to enhance such surface properties as adhesion promotion and corrosion prevention. Isotropic coatings have been found to be effective in certain applications such as corrosion prevention, but the formation of anisotropic coatings permits greater flexibility over the resulting properties. Isotropic coatings derived from tetraethoxysilane, for example, effectively inhibit corrosion while being only 100 to 1000 Å thick. These coatings do not, however, promote adhesion. Thin coatings made from traditional silane adhesion promoters alone are unable to prevent corrosion of metallic substrates.

Using monomers with appropriate reactivities permits the single-step synthesis of anisotropic coatings that can both promote adhesion and prevent corrosion. These types of anisotropic coatings allow the physical and chemical properties of a coating to be varied as a function of distance from the substrate and confer properties to the substrate that would not be possible from a single isotropic coating. The principle behind the construction of these anisotropic coatings is general enough that it can be used in many applications where microengineering of surface structures is important.
Ceramic Fiber Coating by a Non-Aqueous Sol-Gel Process
Authors: Montgomery F. C. , Streckert H. H. , Harrington R. O. , Kaae J. L. , Paguio S. P. , Wall D. R.
Published: March 2011

Thin coatings of Al2O3, and Y2O3, have been prepared on desized Nicalon yarn. The layers were deposited by dip coating in homogeneous, alcohol solutions of partially hydrolyzed metal alkoxides. The coating thicknesses could be controlled by multiple dipping.

The uniformity of the coatings was affected by the alkoxide concentration. Solutions containing greater than 1 wt % equivalent oxide resulted in oxide bridging the void between the fibers.

Transmission Electron Microscopy (TEM) studies on alumina coatings formed by dip coating 300 mesh nickel grids, have shown that either eta or gamma alumina is formed after heating at temperatures as low as 700 °C. The rate of grain growth increases as the temperature increases. In addition, increased grain growth is observed for films produced from solutions with higher water:metal ratios.

The strength of coated fibers heat treated in air after coating has been determined. Alumina, and yttria coatings reduce the strength of the fiber after heating at temperatures above 900 °C. Scanning Auger analysis suggests that the coating is reacting with the fiber at these high temperatures.
Improvement of Building Materials Performance by Sol-Gel Derived Coatings
Authors: Beier Wolfram , Odler Ivan
Published: March 2011

Different multicomponent solutions have been developed by which building materials were coated. After drying and annealing, the coatings convert into silicate based glassy films. TEOS or fumed silica were used as silica carriers and by adding lead compounds also low melting compositions were prepared. In some cases it proved to be convenient to adjust the viscosity of the solutions by additions of suitable organic polymers. The heat treatment was done either in an electric furnace or by means of a special acetylene burner. The produced films can protect building materials against corrosion, may reduce the absorption and permeation of water or improve abrasion resistance.
Effect of Precursors on Lithium Infiltrated Silica Gels Studied by Xps
Authors: Ho S.F. , Klein L. C.
Published: March 2011

Lithium silicate gels of composition 15 mole % lithia - 85 mole % silica were prepared by soaking highly porous silica gels in aqueous LiNO3 or methanolic LiOH solutions. The fracture surfaces of these samples were studied by X-ray photoelectron spectroscopy (XPS). High resolution XPS core level spectra were collected such that both chemical shift and atomic concentration could be determined. While the average lithium content was slightly higher in LiNO3 gels, only some of the lithium was associated with the silica network. In contrast, most of the lithium in LiOH gels was associated with the network.
Sol-Gel Film Formation by Dip Coating
Authors: Hurd Alan J. , Jeffrey Brinker C.
Published: March 2011

The physical aspects of sol-gel film formation are discussed, including the steady state film profile during dip coating, evaporation, and capillary phenomena. It is argued that, since the evaporation rate increases singularly near a sharp boundary (analogous to an electric field singularity near a sharp conductor), the film profile near the drying line falls off precipitously, following the inverse form of the evaporation singularity. Finally, the large tensile pressures in the solvent during the final stage of drying of a porous film are discussed from the point of view of controlling the degree of capillary collapse.
Oxometalate-Glass Composites and Thin Films
Authors: Moller Karin , Bein Thomas , Jeffrey Brinker C.
Published: March 2011

New glass-composites with ion exchange properties have been developed. Ammonium 12-molybdophosphate (AMP) (NH4)3PMo12O40, and ammonium 12-tungstophosphate (AWP) (NH4)3PW12O40, known for their ion exchange capabilities, are included either in preformed aerogels with defined pore size, or are added to sol-gel mixtures during the process of gel formation. Characterization is carried out by FTIR, Raman and EXAFS spectroscopy. Ion exchange capacities for the oxometalate precursors are determined for silver and rubidium and are compared to those of the glass composites. Glass composites show high ion exchange capacity, but some portion of the metalate complexes leaches from the glass during the procedure. This is in contrast to thin composite films, which have almost no porosity and do not show loss of metalate. EXAFS spectroscopy demostrates that the oxometalate microstructure is maintained in glass composites and that rubidium ions after ion exchange in glasses occupy similar cation positions as in the precursor compounds.