bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ultrasonic Propagation at Cylindrical Metal-Ceramic Interfaces in Composites
Authors: Wadley H. N. G. , Simmons J. A. , Drescher-Krasicka E. 
Published: March 2011

AbstractThe nature of ultrasonic propagation at cylindrical interfaces is being explored as a potential method for inferring the modulus of the interface in metal matrix composites. In many metal matrix composite systems leaky interface waves have been found to propagate along the interface. The velocity of these waves depends sensitively upon the local moduli and provides a potential basis for measuring these quantities. The opportunity exists to exploit the leaky character of these waves for microscopic imaging at the interface.
High Temperature Intermetallic Composites
Authors: Anton D. L. 
Published: March 2011

AbstractMany intermetallic compounds possess properties which make them excellent candidates for high temperature use in advanced gas turbine and aerospace applications. One method proposed for increasing damage tolerance in these brittle materials is to artificially composite them with high temperature fibers as utilized in both ceramic and glass composites. Fabrication of these composites is a formidable problem. One method of fabricating these structures, termed here Transient Liquid Phase Consolidation, TLPC, is demonstrated for a number of intermetallic/reinforcing fiber combinations. Thermal stability of the fibers in the intermetallic matrices was observed with FP Alumina being the most stable. Ambient temperature tensile property evaluations were made on monolithic, chopped and aligned FP fiber reinforced Al3 Ta with the aligned structure having the highest ultimate strength and the chopped fiber composite the greatest pseudo plastic response.
Molecular Growth Pathways in Silica Sol-Gel Polymerization
Authors: Klemperer W. G. , Ramamurthi S. D. 
Published: March 2011

ABSTRACTCapillary gas chromatography has been used to monitor the stoichi-ometries, structures, and abundances of low molecular weight polysilicate esters formed by partial hydrolysis of methanolic tetramethylorthosili-cate and redistribution of methanolie hexamethyoxydisiloxane under acidic (HC1) and basic (KOH) conditions. The polysilicate molecular size distributions, expressed in terms of mole percent of total silicon present as a function of degree of polymerization, show maxima near the number average degree of polymerization under acidic conditions. Under basic conditions, the distribution has its maximum at the monomer percent and extends to very high molecular weights, yielding a distribution far broader than the one observed under acidic conditions. These features are in agreement with covalent network theories of silica gelation, as opposed to particle aggregation theories. Weight fraction distributions of the type observed under acidic conditions are characteristic of linear polymers with a low degree of crosslinking and weight fraction distributions of the type observed under basic conditions are characteristic of branched polymers with high degree of crosslinking. The low degree of crosslinking obtained under acidic conditions is observed to arise from steric crowding that discourages branching in polysilicate structures.
Raman and NMR Studies of Hydrous Sodium Titanates
Authors: Bunker B. C. , Peden C.H.F. , Tallant D. R. , Martinez S. L. , Turner G. L. 
Published: March 2011

ABSTRACTRaman and solid state 17O NMR spectroscopies have been used to determine the molecular structure of amorphous hydrous sodium titanate catalyst support materials prepared by hydrolysis of titanium isopropoxide. Three types of oxygens have been identified: oxygens bridging three titanium atoms, oxygens bridging two titaniums, and non-bridging oxygens. Changes in the relative populations of the different oxygen types have been used to help understand the ion-exchange and polymerization properties of hydrous titanates.
Mathematical Modeling of the Structures and Bulk Moduli of TX2 Quartz and Cristobalite Structure-Types, T = C,Si,Ge and X = O,S
Authors: Gibbs G. V. , Boisen M. B. , Downs R. T. , Lasaga A. C. 
Published: March 2011

ABSTRACTModels of the oxide and sulfide structure-types of quartz and cristobalite have been made using potential energy surfaces derived from MO calculations on small molecules. The bond length and angle and the volume compressibility data calculated for quartz match those observed for pressures up to 40 kbars. An analysis of the force constants that define the potential energy surface indicates that the bulk modulus of the mineral is governed primarily by the bending force constant of the bridging angle. Similar calculations were completed for the GeO2 form of quartz, but the agreement with the observed data is somewhat poorer. A modeling of the CO2 form of quartz predicts that it would be significantly harder, more incompressible and show less expansibility than the SiO2 form.
Syneresis in Silica Gel
Authors: Scherer George W. 
Published: March 2011

ABSTRACTThe driving force for syneresis is generally attributed to the same chemical reactions that produce gelation, but it has also been proposed that shrinkage could be driven by interracial energy. The latter possibility is explored and discounted. The kinetics of syneresis depend on the driving force, the mobility of the gel network, and the rate of fluid flow through the contracting gel. A model that allows for viscoelastic behavior of the gel and fluid flow according to Darcy's law is shown to provide a quantitatively accurate representation of the shape of the shrinkage curves and the dependence of the shrinkage rate on sample size.
Adhesion of Hydrated Silicate Films
Authors: Michalské Terry A. , Keefer Keith D. 
Published: March 2011

ABSTRACTWe used fracture mechanics test techniques to measure the adhesive bond energy formed between hydrated silica glass surfaces and silicate species deposited from solution. In the case of silicate surfaces hydrated in room temperature water vapor, intermolecular bonding between hydrated surfaces can be attributed to either hydrogen bonding interactions between adsorbed water molecules (0.15 J/m2) or electrostatic bonds formed between adsorbed cations and anionie-nonbridging surface oxygen groups (2.0 J/m2). The bonding interaction observed at room temperature depends upon the glass surface composition and the degree of surface hydration. When hydrated silicate solution species are added to the interface and heat treated, adhesion energies as large as the cohesive energy of silica glass can be obtained with heat treatments as low as 200 C. The adhesion of the silicate interfacial film produced by the addition of solution species is greatest for silicate precursors showing a low degree of molecular crosslinking. In addition, the presence of alkali in the silicate solution greatly enhances interface adhesion for heat treatments below 200 C.
Comparison Between the Sol-Gel Processes of Zirconia and those of Alumina
Authors: Pierre Alain C. , Uhlmann Donald R. 
Published: March 2011

ABSTRACTZirconia gels have been synthesized from zirconium butoxide in excess water with HNO3. The structure and characteristics of the gels have been studied as a function of the acid content, and the transformation during heat treatment has been followed up to 1150°C.From the results of a previous similar study on aluminum hydroxide gels, a comparison is made between the sol-gel processes of zirconia and those of alumina. A discussion of the differences of the aqueous media chemistry of Al and Zr, and gelation mechanisms, is presented.
Alumina Powders from Aluminum Alkoxide.
Authors: Ayral A. , Phalippou J. , Droguet J. C. 
Published: March 2011

ABSTRACTA new way of producing alumina powder from the alkoxide is reported. Aluminum basic acetate is first synthesized by reactions which allow controlled precipitation. The chemical reactions are followed using IR spectroscopy. Alumina powder is obtained by subsequent thermal decomposition.
Preparation of BaTiO3 by Sol-Gel-Processing
Authors: Tomandl G. G. , Rösch H. , Stiegelschmitt A. 
Published: March 2011

ABSTRACTBarium acetate, titanium ethoxide, and the acetates of calcium and magnesium and zirconium-n-propoxide as additives, were dissolved in an acetic acid methanol mixture. By adding water, this sol was converted to a gel.Extended investigations of the calcination result in a crystalline barium tita-nate without other titanate phases, with a very small amount of carbonate and a negligible contents of free carbon. Aggregates could not be destroyed totally by the milling process. Therefore, the green density of the green bodies reaches only 60 to 65 %. Thus, a minimum sintering temperature of 1200° C was necessary to obtain a density of at least 95 %. In future work, a special treatment to destroy the aggregates must be developed to lower the sintering temperature.
Characterisation of Colloids and Gels
Authors: Ramsay John D.F. 
Published: March 2011

ABSTRACTSol-gel processes can provide a route to oxide ceramics with controlled microstructure and homogeneity. This feature can be exploited from an understanding of the inter-relationship between the characteristics of concentrated sols and the microstructure of the resultant gels produced on dehydration. Among several characterisation techniques employed (light scattering, incoherent neutron scattering, viscoelastic behaviour) neutron small angle scattering (SANS) has proved particularly powerful in the study of the mechanism of the sol to gel conversion process. Applications of SANS in the characterisation of sols containing (a) discrete particles (charge stabilised), (b) mixed oxide systems, and (c) particle aggregates, with fractal properties are illustrated.
Characterization of the Surface Area and Porosity of Sol-Gel Films Using Saw Devices
Authors: Frye Gregory C. , Ricco Antonio J. , Martin Stephen J. , Jeffrey Brinker C. 
Published: March 2011

ABSTRACTA novel technique for accurately obtaining nitrogen adsorption isotherms on thin porous films has been developed. These isotherms are useful for characterizing the surface area and pore size distribution of porous samples. The sensitivity to adsorbed nitrogen is increased by several orders of magnitude over conventional techniques by forming the test film on the substrate of a surface acoustic wave (SAW) device. This device functions as a microbalance able to detect less than 100 pg/cm2 of film. Surface areas and pore size distributions calculated from adsorption isotherms obtained with this technique on silicate sol-gel films are compared to those for bulk samples prepared from similar sol-gel solutions.
Structural Evolution During the Sol to Gel Transition of Silicon-Alkoxide Based Sols Observed by Cryogenic Transmission Electron Microscopy (CRYO-TEM)
Authors: Bailey Joseph K. , Mecartney Martha L. 
Published: March 2011

ABSTRACTThe gelation reaction for tetraethoxysilane in both acid and base catalyzed environments was studied using cryogenic transmission electron microscopy. The evolution of the gel structure was observed in wet samples by the technique of fast-freeze vitrification, in which the three dimensional gel structure is preserved in vitreous solvent. The sample is then observed in the electron microscope using a cold stage. Samples were prepared and allowed to react until a specific point of gelation was reached, then were vitrified at that point. Results from this technique show a coarse network structure for base catalyzed gels. This network is composed of silica clusters ≈40Å in size. The acid catalyzed gels show an extremely fine texture (<10Å) at end point of gelation. The structures at intermediate stages of gelation were also determined.
Conventional and Chemical Processing of High Tc Superconductors
Authors: Yan M. F. , Ling H. C. , O'Bryan H. M. , Gallagher P. K. , Rhodes W. W. 
Published: March 2011

ABSTRACTConventional and chemical processing of the superconducting YBa2Cu3Ox ceramic powders are reviewed. Conditions for calcination, sintering and microstructural development are shown to be important considerations for the superconducting properties of YBa2Cu3Ox ceramics. We examine different forming techniques, e.g. dry pressing, hot pressing, tape casting and screen printing, to prepare superconducting components with different sizes, shapes and configurations. We also identify and discuss problem areas related to incomplete decomposition of BaCo3 precursor; oxidation of YBa2Cu3Ox ceramics; sensitivity of the YBa2Cu3Ox phase to water; reactivity of the YBa2Cu3Ox phase with substrates and co-firing materials; low critical current density in bulk ceramics of YBa2Cu3Ox and low mechanical strength of YBa2Cu3Ox ceramics.
Synthesis of Superconducting Powders by Freeze-Drying
Authors: Johnson Sylvia M. , Gusman Michael I. , Hildenbrand Donald L. , Eom C. B. 
Published: March 2011

ABSTRACTThe freeze-drying method of synthesizing powders of the superconducting oxide YBa2Cu3O7-δ is described. This process produces homogeneous, submicron powders of high purity. The effects of salt selection, solution concentration and pH on the process are described. Some evaluation of the sintering behavior and the effects on critical current density are included.
Tailored Organometallics as Low-Temperature CVD Precursors to Thin Films
Authors: Girolami Gregory S. , Jensen James A. , Gozum John E. , Pollina Deborah M. 
Published: March 2011

ABSTRACTWe have been investigating the use of “tailored” organo-metallic precursors for the chemical vapor deposition (CVD) of thin films in an effort to open up new approaches to the low-temperature synthesis of ceramic, electronic, and optical materials that contain transition metals. Following our approach to the successful deposition of high-quality titanium carbide thin films at 150–250°C from tetraneopentyltitanium, we have extended this technique to the preparation of other carbides, such as ZrC, HfC, TaC, and WC, at temperatures some 1000°C lower than previous CVD methods from the metal halides and methane. The details of the thermolysis of the organometálico precursors are most consistent with a mechanism involving homolysis of M-C bonds to give alkyl radicals followed by C-C and C-H bond activation steps. Metal diboride thin films such as TiB2, ZrB2, and HfB2 can be prepared at ca. 250 °C by CVD of suitable tetrahydroborate precursors: Ti(BH4)3-(MeOCH2CH2OMe), Zr(BH4)4, and Hf(BH4)4. In these cases, the principal byproducts are hydrogen and diborane. Mirror-bright, adhesive thin films up to 2 μm thick can be grown on a variety of substrates including glass, steel, aluminum, copper, silicon, and graphite. Despite the amorphous nature of the films, their electrical resistivities are essentially identical with those of the bulk polycrystalline material.
An Investigation Into the Preparation, Properties, and Processing of SiC/AIN and Si3N4/AIN Solid Solutions from Organometallic Precursors.
Authors: Interrante Leonard V. , Czekaj Corinna L. , Hackney Micheal L.J. , Sigel Gary A. , Schields Paul J. , Slack Glen A. 
Published: March 2011

ABSTRACTSolid solutions of SiC and AIN have been prepared by the co-pyrolysis of polysilanes and organoaluminum amides. In the presence of ammonia. Si3N4and AIN are formed. These materials are fully characterized by TEM, SAD, SEM, XRPD, and elemental analyses. The influence of reaction conditions, as well as precursor properties and reactivity, on the composition and morphology of these materials is addressed.
A Novel Route to Aluminum Nitride Ceramics Using a Polyaminoalane Precursor
Authors: Seibold M. , Rüssel C. 
Published: March 2011

ABSTRACTMetallic Aluminum is dissolved anodically in an electrolyte, containing a primary amine, an aprotic solvent and a tetraalkylammonium salt. Removal of the solvent and subsequent heating lead to a polymer network. Initial drying causes a more viscous liquid, further drying up to 150°C accelerates the polymerization process and a gel-like consistency is obtained. Pyrolysis is carried out under NH3 or inert gas atmosphere. X-ray diffraction data show amorphous products up to 800°C, treatment above 800°C leads to the desired ceramic product aluminum nitride. Peak broadening indicates extremely small crystallite sizes. Pressureless sintering without additives gives a compact product, having densities up to nearly 100% of the theoretical density.
Molecular Building Blocks in Low PH Silicon Sol-Gels: A Silicon-29 NMR Study
Authors: Kelts Larry W. , Armstrong Nancy J. 
Published: March 2011

ABSTRACTHigh field Silicon-29 NMR is used to study the structural intermediates in tetramethyl and tetraethyl orthosilicate (TMOS and TEOS) low pH sol-gel reactions. Linear oligomers as well as ring components of various sizes are identified and their evolution in the sols is followed. Differences in the number of compact ring structures are related to differences in gel times. Reactions are followed for various silicon alkoxide:water:acid molar ratios.
Preparation of Titanium Nitride (TiN1-x-yCxOy) from Ti(OPri)4-Triethanolamine Condensation Product by Pyrolysis
Authors: Kuroda Kazuyuki , Tanaka Yuji , Sugahara Yoshiyuki , Kato Chuzo 
Published: March 2011

ABSTRACTIn order to develop new precursors for the preparation of titanium nitride, a Ti alkoxide-nitrogen containing polyol condensation product was prepared and heat-treated. A stoichiometric mixture of titanium tetra-isopropoxide (Ti(OC3H7)4) and triethanolamine (N(CHM2H4OH)3) was heated at ca. 200°C under reduced pressure. The distilled liquid was isopropyl alcohol and its amount indicated that the transesterification reaction proceeded almost quantitatively.The condensation product was a transparent glassy solid on cooling to room temperature. Thermal treatment of the condensation product at 1600°C in a nitrogen flow yielded titanium nitride (TiN1-x-yCxOy) as the main phase with a small amount of Ti3O5. The thermal treatment for a longer time reduced the amount of the oxide. The heat treatment of the product in an Ar flow also yielded nitrogen containing ceramics, which indicated the effectiveness of nitrogen in the condensation product during the conversion.
The Use of Organometallic Precursors to Silicon Nitride as Binders
Authors: Schwab Stuart T. , Blanchard-Ardid Cheryl R. 
Published: March 2011

ABSTRACTCurrent techniques of advanced ceramic component fabrication are based on pressureless green body consolidation technology in which voids often remain in the final microstructure. Much of this residual porosity is created when the organic binder used to consolidate the ceramic powder burns off on sintering. A new concept in the field of ceramic processing is the use of an organometallic binder material which will pyrolyze on sintering and convert into a predetermined ceramic. Silicon nitride (Si3N4) is a ceramic material in great demand for elevated temperature applications because of its excellent high temperature properties. At the present time, silicon nitride cannot be successfully processed without the use of costly pressure sintering (e.g., hot pressing), or the addition of a glassy phase, which degrades the high temperature properties.Various preceramic polymers to be used as binders for cold pressing operations have been synthesized and studied. It has been demonstrated that these polymers may be dissolved in an appropriate organic solvent and mixed with a powder. Removal of the solvent yields homogeneous mixtures which may be consolidated into highly dense (> 66% of theoretical) green bodies. The ceramic yields of these polymers have also been determined.
Synthesis and Properties of YBaSrCu3O6.9 Prepared from Soluble Precursors
Authors: Nigrey Paul J. , Kwak J. F. , Venturini E. L. , Eatough M. O. , Baughman R. J. 
Published: March 2011

ABSTRACTSolution chemistry routes have been used to prepare single-phased superconducting ceramics of YBaSrCu3O6.9 (1113). Resistivity measurements on air annealed samples showed a superconducting onset temperature of 85 K with zero resistance at 78 K which improves to 81 K upon oxygen annealing. Magnetization data show ca. 78% flux exclusion at 5 K for 1113. X-ray diffraction experiments revealed that an orthorhombic to tetragonal transition occurs near 590°C when 1113 was heated in air. This work has demonstrated that such techniques can be utilized to prepare solid solution material with properties similar to those observed in YBa2Cu3Ox (123).
Partially Hydrolyzed Alkoxysilanes as Precursors for Silica Aerogels
Authors: Tillotson T. M. , Hrubesh L. W. , Thomas I. M. 
Published: March 2011

ABSTRACTThe classical sol-gel process for synthesizing SiO2 aerogels involves the hydrolysis/condensation of tetraethyoxysilane (TEOS) and/or teramethyoxysilane (TMOS) to produce a gel which can then be super-critically extracted to a low density, highly porous aerogel glass. Controlled hydrolysis of TEOS and TMOS leads to partially hydrolyzed compounds that can be subsequently water processed to form silica aerogels in the density range from .020 to .500 gms/cc. The partially hydrolyzed compounds are stable when sealed from moist air and can be stored for future use.We discuss the controlled conditions used to obtain these compounds and present data that characterize their structure. We detail the procedures for preparing the wide range of aerogel densities. We also report on their use as an adhesive.
Precursor Chemistry and the Structure of Silica Aerogels
Authors: Schaefer D. W. , Brinker C. J. , Wilcoxon J. P. , Wu D.-Q. , Phillips J. C. , Chu B. 
Published: March 2011

ABSTRACTSmall-angle X-ray scattering is used to characterize the structure of aerogels prepared by two-stage polymerization processes. Second-stage catalysis controls the resistance to collapse during drying with the base-catalyzed system being most resistant. Base catalysis in the second stage leads to compaction of the polymer network on short length scales. This short-scale rigidity makes the networks sufficiently robust to withstand the surface tension forces present during solvent extraction and re-exposure to the atmosphere. Aging in solution also improves aerogel quality. In this case, a dissolution-repolymerization process leads to short length scale circuits and improved rigidity.
Supercritical Fluid Applications in Advanced Materials Processing
Authors: Wagner Richard A. , Krukonis Val J. , Coffey Michael P. 
Published: March 2011

ABSTRACTThe feasibility of using supercritical fluids to impregnate carbon/carbon composites with a ceramic precursor was successfully demonstrated. Improvements in mechanical properties and oxidation resistance were correlated with the distribution of silicon within the microstructure of the composites. In addition, supercritical fluids were also used to fractionate silicon carbide precursors and thereby control the physical properties and char yield.