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Vapor Phase Transport Synthesis of Zeolites from Sol-Gel Precursors

Description: A study of zeolite crystallization from sol-gel precursors using the vapor phase transport synthesis method has been performed. Zeolites (ZSM-5, ZSM-48, Zeolite P, and Sodalite) were crystallized by contacting vapor phase organic or organic-water mixtures with dried sodium silicate and dried sodium alumino-silicate gels. For each precursor gel, a ternary phase system of vapor phase organic reactant molecules was explored. The vapor phase reactant mixtures ranged from pure ethylene diamene, triethylamine, or water, to an equimolar mixture of each. In addition, a series of gels with varied physical and chemical properties were crystallized using the same vapor phase solvent mixture for each gel. The precursor gels and the crystalline products were analyzed via Scanning Electron Microscopy, Electron Dispersive Spectroscopy, X-ray mapping, X-ray powder diffraction, nitrogen surface area, Fourier Transform Infrared Spectroscopy, and thermal analyses. The product phase and purity as a function of the solvent mixture, precursor gel structure, and precursor gel chemistry is discussed.
Date: July 14, 2000
Partner: UNT Libraries Government Documents Department

Effects of solution precursor nature on sol-gel derived PZT thin film crystallization behavior and properties

Description: In fabricating lead zirconate titanate (PZT) films for nonvolatile memories and decoupling capacitors, various deposition methods have been investigated. Each can produce films with acceptable dielectric and ferroelectric properties, but sol-gel methods offer excellent control of film stoichiometry and coating uniformity. The sol-gel approaches for PZT film fabrication fall into two categories: processes that use 2-methoxyethanol as a solvent, and processes that use chelating agents, such as acetic acid, for reducing the hydrolysis sensitivity of the alkoxide compounds. Due to concerns about the toxicity of 2-methoxyethanol, we have concentrated on the second category. It was found that, in addition to reducing the hydrolysis sensitivity, the chelating agents serves to define the processing behavior of the films: film consolidation after deposition and densification and crystallization during heat treatment. This paper discusses the relations between precursor structure (reactions between chelating agents and the metal alkoxide starting reagents) and film consolidation, densification, and crystallization.
Date: September 1995
Creator: Schwartz, R. W. & DaSalla, R. S.
Partner: UNT Libraries Government Documents Department

Synthesis of novel precursors for PMN powders and the thin films obtained from them

Description: Sol-gel processing has been widely used in the preparation of lead zirconate titanate (PZT) thin films. The authors have applied this methodology to the formation of lead magnesium niobate (PMN) spin-cast deposited thin films. Since there is a limited number of soluble, commercially available compounds, the authors have recently synthesized a series of novel metal alkoxides for use as precursors for generation of PMN thin films and powders. The process for generation of the perovskite phase of these PMN powders and films are reported.
Date: March 1, 1995
Creator: Boyle, T.J.; Dimos, D.B. & Moore, G.J.
Partner: UNT Libraries Government Documents Department

Arylene-bridged 2,2,5,5-tetramethyl2,5-disila-1-oxacyclopentanes as precursors to non-shrinking polysiloxanes. A new route to sol-gel type polymers

Description: Sol-gel chemistry has been the focus of much attention in the design and preparation of highly crosslinked polysiloxane gels. Preparation of sol-gel processed silica or polysilesquioxane gels is carried out by the hydrolysis and condensation of alkoxysilyl monomers, usually in the presence of catalytic acid or base and an excess of water. Removal of the alcohol and water byproducts of the condensation reactions, in addition to the alcohol needed to co-dissolve the hydrophobic monomers with water, leads to substantial shrinkage during drying of the resulting gels. This limits the utility of sol-gel processing for applications requiring net-shape casting of artifacts, crack free coatings, or low vaporous organic contaminants (VOCs). It would be advantageous to have a sol-gel process based on an organosilicon monomer that would not require water as a reactant or produce water and alcohol condensation products and still result in siloxane network polymers capable of forming gels. Here, the authors show the synthesis and preparation of a novel sol-gel monomer which can easily be polymerized by ring opening polymerization to give highly crosslinked polysiloxane gels with no condensation byproducts.
Date: November 1, 1998
Creator: Rahimian, K. & Loy, D.A.
Partner: UNT Libraries Government Documents Department

Fundamentals of sol-gel film deposition

Description: Results appear to confirm the concept of surfactant-templating of thin film mesostructures. Final film pore structure depends on starting surfactant and water concentrations and process time scale (governed by evaporation rate). Surfactant ordering at substrate-film and film-vapor interfaces orients the porosity of adjoining films, leading to graded structures. SAW experiments show that depending on processing conditions, the porosity may be open or closed (restricted). Open porosity is monosized. Upon pyrolysis, lamellar structures collapse, while the hexagonal structures persist; when both hexagonal and lamellar structures are present, the hexagonal may serve to pillar the lamellar, avoiding its complete collapse. Thick lamellar films can be prepared because the surfactant mechanically decouples stress development in adjoining layers. Upon drying and heating, each individual layer can shrink due to continuing condensation reactions without accumulating stress. During surfactant pyrolysis, the layers coalesce to form a thick crack-free layer. Formation of closed porosity films is discussed.
Date: December 31, 1996
Creator: Brinker, C.J.; Anderson, M.T.; Bohuszewicz, T.; Ganguli, R.; Lu, Y. & Lu, M.
Partner: UNT Libraries Government Documents Department

Thin-film silica sol-gels doped with ion responsive fluorescent lipid bilayers

Description: A metal ion sensitive, fluorescent lipid-b i layer material (5oA PSIDA/DSPC) was successfully immobilized in a silica matrix using a tetramethoxysilane (TMOS) sol-gel procedure. The sol-gel immobilization method was quantitative in the entrapment of seif-assembled Iipid-bilayers and yielded thin films for facile configuration to optical fiber piatforms. The silica matrix was compatible with the solvent sensitive lipid bilayers and provided physical stabilization as well as biological protection. Immobilization in the silica sol-gel produced an added benefit of improving the bilayer's metal ion sensitivity by up to two orders of magnitude. This enhanced performance was attributed to a preconcentrator effect from the anionic surface of the silica matrix. Thin gels (193 micron thickness) were coupled to a bifurcated fiber optic bundle to produce a metal ion sensor probe. Response times of 10 - 15 minutes to 0.1 M CUCIZ were realized with complete regeneration of the sensor using an ethylenediarninetetraacetic acid (EDTA) solution.
Date: January 12, 1999
Creator: Sasaki, D.Y.; Shea, L.E. & Sinclair, M.B.
Partner: UNT Libraries Government Documents Department

Preparation of microporous films with sub nanometer pores and their characterization using stress and FTIR measurements

Description: The authors have used a novel technique, measurement of stress isotherms in microporous thin films, as a means of characterizing porosity. The stress measurement was carried out by applying sol-gel thin films on a thin silicon substrate and monitoring the curvature of the substrate under a controlled atmosphere of various vapors. The magnitude of macroscopic bending stress developed in microporous films depends on the relative pressure and molar volume of the adsorbate and reaches a value of 180 MPa for a relative vapor pressure, P/Po = 0.001, of methanol. By using a series of molecules, and observing both the magnitude and the kinetics of stress development while changing the relative pressure, they have determined the pore size of microporous thin films. FTIR measurements were used to acquire adsorption isotherms and to compare pore emptying to stress development, about 80% of the change in stress takes place with no measurable change in the amount adsorbed. The authors show that for sol-gel films, pore diameters can be controlled in the range of 5--8 {angstrom} by ``solvent templating``.
Date: June 1, 1996
Creator: Samuel, J.; Hurd, A.J.; Swoll, F. van; Frink, L.J.D.; Contakes, S.C. & Brinker, C.J.
Partner: UNT Libraries Government Documents Department

Aerial oxidation of tetraethyl silicate and effect on ammonia catalyzed hydrolysis

Description: Colloidal suspensions of Si0{sub 2} in ethanol prepared by the ammonia catalyzed hydrolysis of tetraethyl silicate (TEOS) in ethanol have been routinely used for over 10 years to prepare antireflective (AR) coatings on the fused silica transmissive optical components of high power fusion lasers. Very high purity coatings are required to avoid laser damage and these are obtained when the TEOS is fractionally distilled under N{sub 2} prior to use. Recently we found that products from aerial oxidation of distilled TEOS, had a significant effect on the particle size of our coating suspensions to the detriment of the optical performance. We require particle sizes less than 20 nm to avoid light loss due to scatter and contaminated TEOS gave suspensions with much higher particle sizes. Oxidation products were identified by GC mass spectroscopy and included acetaldehyde, acetic acid, silicon acetates and reaction products of these compounds with ethanol. Acetic acid and silicon acetates were found to be the major cause of large particle formation. These could be removed by careful redistillation preferably in the presence of a small quantity of magnesium ethoxide. Storage in sealed containers over N{sub 2} avoided further problems.
Date: June 25, 1997
Creator: Thomas, I. M.
Partner: UNT Libraries Government Documents Department

Epitaxial Growth of Yb<sub>2</sub>O<sub>3</sub> Buffer Layers on Biaxially Textured-Ni (100) Substrates by Sol-Gel Process

Description: In order to develop an alternative buffer layer architecture using the sol-gel process to produce YBCO (YBa2Cu307+) coated conductors, Yb203 has been chosen as the candidate material. Buffer layers of fi03 were epitaxkdly grown on biaxially textured-Ni (100) substrates by the sol gel process for the first time. The ~03 precursor solution was prepared from an alkoxide sol-gel route in 2-xnetho~ethanol and was deposited on textured-Ni (100) substrates by either spin coating or dip coating methods. The amorphous film was then processed at 1160oC under flowing (96%)MH2(4%) gas mixture for one hour. The fi03 iihn exhibited a strong c-axis orientation on the Ni (100) substrates. The phi and omega scans indicated good in plane and out of plane orientations. The X-ray (222) pde figure showed a cube-on-cube epitaxy. High current YBCO films were grown on the Y&03 sol-gel buffered-Ni substrates.
Date: April 5, 1999
Creator: Beach, D.B.; Chirayil, T.G.; Christen, D.K.; Feenstra, R.; Goyal, A.; Kroeger, D.M. et al.
Partner: UNT Libraries Government Documents Department

Practical superconductor development for electrical power applications: Quarterly report for the period ending December 31, 1999

Description: This is a multiyear experimental research program focused on improving relevant material properties of high-T{sub c} superconductors (HTSS) and on development of fabrication methods that can be transferred to industry for production of commercial conductors. The development of teaming relationships through agreements with industrial partners is a key element of the Argonne (ANL) program. Recent results on substrate deposition for coated conductors, vortex studies, development of hardened Ag-alloy sheaths for powder-in-tube conductors, and sol-gel processing of NdBa{sub 2}Cu{sub 3}O{sub x} (Nd-123) are presented.
Date: February 2, 2000
Partner: UNT Libraries Government Documents Department

Polysilsesquioxanes through base-catalyzed redistribution of oligohydridosiloxanes

Description: Organopolysilsesquioxanes have recently gained much interest as materials for low-K dielectrics, ceramic precursors and photoresists. Typical sol-gel synthesis of polysilsesquioxanes involves the hydrolysis of organotricholorosilanes and/or organotrialkoxysilanes in the presence of acid or base catalysts and organic solvents. However, under sol-gel conditions most organotrialkoxysilanes do not afford silsesquioxane gels. This limits the range of organic functionalities that can be introduced into these hybrid organic-inorganic materials. An alternative route to polysilsesquioxanes is through oligohydridosiloxanes. Catalytic disproportionation, by titanium complexes, of linear or cyclic oligomers of methylhydridosiloxanes can lead to polymethylsilsesquioxanes. The authors have shown that disproportionation of oligomethylhydridosiloxanes can also be catalyzed by tetrabutylammonium hydroxide to yield polymethylsilsesquioxanes (scheme 1). This replaces the step-growth sol-gel polymerization process of organotrialkoxysilanes, which requires solvent, stoichiometric water and produces alcohol and water condensation by-products. Tetraalkylammonium hydroxides, as catalysts, are also attractive because they readily decompose by heating above 150 C; thus, they can be easily removed from the final materials. In this paper the authors report on both the catalytic and stoichiometric redistribution of organohydridosiloxanes to produce polysilsesquioxane foams and gels of the formula (RSiO{sub 1.5}){sub n} which otherwise cannot be obtained through traditional sol-gel means.
Date: May 1, 2000
Partner: UNT Libraries Government Documents Department

Sol-gel chemistry by ring-opening polymerization

Description: Sol-gel processing of materials is plagued by shrinkage during polymerization of the alkoxide monomers and processing (aging and drying) of the resulting gels. The authors have developed a new class of hybrid organic-inorganic materials based on the solventless ring-opening polymerization (ROP) of monomers bearing the 2,2,5,5-tetramethyl-2,5-disilaoxacyclopentyl group, which permits them to drastically reduce shrinkage in sol-gel processed materials. Because the monomers are polymerized through a chain growth mechanism catalyzed by base rather than the step growth mechanism normally used in sol-gel systems, hydrolysis and condensation products are entirely eliminated. Furthermore, since water is not required for hydrolysis, an alcohol solvent is not necessary. Monomers with two disilaoxacyclopentyl groups, separated by a rigid phenylene group or a more flexible alkylene group, were prepared through disilylation of the corresponding diacetylenes, followed by ring closure and hydrogenation. Anionic polymerization of these materials, either neat or with 2,2,5,5-tetramethyl-2,5-disila-1-oxacyclopentane as a copolymer, affords thermally stable transparent gels with no visible shrinkage. These materials provide an easy route to the introduction of sol-gel type materials in encapsulation of microelectronics, which they have successfully demonstrated.
Date: February 7, 2000
Creator: Rahimian, Kamyar & Loy, Douglas A.
Partner: UNT Libraries Government Documents Department

The Physics of Evaporation-Induced Assembly of Sol-Gel Materials

Description: Remarkable materials ordered at the nanoscale emerge when a sol-gel solution becomes co-organized with a surfactant. At sufficiently high concentration, the surfactant forms crystalline or liquid-crystalline arrays of micelles in the presence of the sol-gel, and as gelation proceeds the arrays become locked into the gel. Recent experiments demonstrate that the degree of order in the resulting mesoporous ceramic phase can be enhanced and controlled by continuous dip coating in which the solution, initially dilute, evolves through the critical micelle concentration by steady-state evaporation. The long-range order and microstructural orientation in these films suggest that the propagation of a critical-micelle-concentration transition front, with large physico-chemical gradients, promotes oriented self assembly of surfactant aggregates. This steep-gradient view is supported by results from unsteady evaporation of aerosols of similar solutions, in which internally well-ordered but complex particles are formed.
Date: July 24, 2000
Creator: Hurd, Alan J. & Steinberg, Lev
Partner: UNT Libraries Government Documents Department

Polymerization of trialkoxysilanes. Effect of the organic substituent on the formation of gels

Description: Hydrolysis and condensation of trialkoxysilanes, R-Si(OR{prime}){sub 3}, generally leads to the formation of silsesquioxane oligomers and polymers. These polymers are composed of a monomer repeat unit, [R-SiO{sub 1.5}]{sub n}, with a single silicon atom attached to other repeat units in the polymer through one to three siloxane bonds. The remaining substituent is an organic group attached to the silicon through a silicon-carbon single bond. Silsesquioxanes have been the subject of intensive study in the past and are becoming important again as a vehicle for introducing organic functionalities into hybrid organic-inorganic materials through sol-gel processing. Despite all of this interest, there has not been a systematic study of the ability of trialkoxysilanes to form gels through the sol-gel process. In fact, it has been noted that silsesquioxanes are generally isolated as soluble resins rather than the highly crosslinked network polymers (gels) one would expect from a tri-functional monomer. In this study, the authors have examined the sol-gel chemistry of a variety of trialkoxysilanes with different organic substituents (R = H, Me, Et, n-Pr, i-Pr, n-Bu, i-Bu, t-Bu, n-octadecyl, n-dodecyl, cyclohexyl, vinyl, phenyl, benzyl, phenethyl), with methoxide or ethoxide substituents on silicon, at varying monomer concentrations ranging up to neat monomer, and with different catalysts (HCl, NaOH, formic acid, fluoride). Gels were prepared from tetramethoxysilane and tetraethoxysilane at identical concentrations for purposes of comparison.
Date: September 1, 1998
Creator: Loy, D.A.; Baugher, B.M. & Schneider, D.A.
Partner: UNT Libraries Government Documents Department

Sol-gel processing of energetic materials

Description: As part of a new materials effort, we are exploring the use of sol- gel chemistry to manufacture energetic materials. Traditional manufacturing of energetic materials involves processing of granular solids. One application is the production of detonators where powders of energetic material and a binder are typically mixed and compacted at high pressure to make pellets. Performance properties are strongly dependent on particle size distribution, surface area of its constituents, homogeneity of the mix, and void volume. The goal is to produce detonators with fast energy release rate the are insensitive to unintended initiation. In this paper, we report results of our early work in this field of research, including the preparation of detonators from xerogel molding powders and aerogels, comparing the material properties with present state-of-the-art technology.
Date: August 18, 1997
Creator: Tillotson, T.M.; Hrubesh, L.H.; Fox, G.L.; Simpson, R.L.; Lee, R.W.; Swansiger, R.W. et al.
Partner: UNT Libraries Government Documents Department

Hydroxylapatite Otologic Implants

Description: A Cooperative Research and Development Agreement (CRADA) between Lockheed Martin Energy Research Corporation (LMER) and Smith and Nephew Richards Inc. of Bartlett, TN, was initiated in March 1997. The original completion date for the Agreement was March 25, 1998. The purpose of this work is to develop and commercialize net shape forming methods for directly creating dense hydroxylapatite (HA) ceramic otologic implants. The project includes three tasks: (1) modification of existing gelcasting formulations to accommodate HA slurries; (2) demonstration of gelcasting to fabricate green HA ceramic components of a size and shape appropriate to otologic implants: and (3) sintering and evaluation of the HA components.
Date: January 1, 2000
Creator: McMillan, A.D.; Lauf, R.J.; Beale, B. & Johnson, R.
Partner: UNT Libraries Government Documents Department

LDRD final report on nano-scale engineering of smart membranes

Description: A new approach to the fabrication of porous, amorphous inorganic membranes using organic pore templates was investigated. The pore templates were a new family of hybrid organic-inorganic monomers. As background for membrane work, the monomers were polymerized by sol-gel techniques to make crosslinked polymers. Molecular modeling was used to create computer simulations of the materials and provide insight into their composites, were then converted into porous silicas using low temperature oxygen plasma techniques. A select few of the monomers were copolymerized with silica monomers to form non-porous thin films on mesoporous substrates. The films were converted into porous silica thin films with thermal oxidations and the resulting membranes were tested for gas selectivities and flux.
Date: September 1, 1997
Creator: Loy, D.A.; Jamison, G.M. & Assink, R.A.
Partner: UNT Libraries Government Documents Department

Oriented inorganic thin film channel structures with uni-directional monosize micropores

Description: The goal of this project was to develop a novel technology that may be used to eventually manufacture a new generation of inorganic membranes and sensors with oriented, unidirectional monosized pores. The premise is that very thin membranes with oriented channels as pores will have a high flux in addition to being highly selective. Applications include: (1) gas separation membranes for oxygen enrichment, partial oxidation, dehydrogenation, and purification of natural gas; (2) refractory catalytic membrane reactors; and (3) molecular recognition sensors. The methodology for making such membranes was to combine Langmuir - Blodgett (LB) technology with sol-gel chemistry to engineer pore channels within the range 3 to 20 K The channel structure was fabricated of amorphous SiO{sub 2} because of its good thermal, chemical, and mechanical stability. Our approach was to use LB techniques to uniformly place organic molecular spacers throughout a thin silica precursor matrix and apply this film to a substrate. LB films of solid solutions of commercially available silane amphiphiles and organic amphiphiles were fabricated. The siloxane groups were then hydrolyzed to form silica and the organic portions of the amphiphiles removed by thermal decomposition. With the completely fugitive organic spacer amphiphiles removed, a thin silica film with micropores resulted. The pore size was in the range of 6 - 8 {angstrom} and in an ultra-thin configuration. With further development this technique may be useful for fabrication of inorganic membranes which satisfy all the criteria of the ideal membrane.
Date: October 1, 1997
Creator: Cesarano, J. III; Sasaki, D.Y.; Singh, S. & Brinker, C.J.
Partner: UNT Libraries Government Documents Department

A Mechanistic Investigation of Gelation. The Sol-Gel Polymerization of Bridged Silsesquioxane Monomers

Description: The study of a homologous series of silsesquioxane monomers has uncovered striking discontinuities in gelation behavior. An investigation of the chemistry during the early stages of the polymerization has provided a molecular basis for these observations. Monomers containing from one to four carbon atoms exhibit a pronounced tendency to undergo inter or intramolecular cyclization. The cyclic intermediates have been characterized by {sup 29}Si NMR, chemical ionization mass spectrometry and isolation from the reaction solution. These carbosiloxanes are local thermodynamic sinks that produce kinetic bottlenecks in the production of high molecular weight silsesquioxanes. The formation of cyclics results in slowing down or in some cases completely shutting down gelation. An additional finding is that the cyclic structures are incorporated intact into the final xerogel. Since cyclization alters the structure of the building block that eventually makes up the xerogel network, it is expected that this will contribute importantly to the bulk properties of the xerogel as well.
Date: July 14, 2000
Partner: UNT Libraries Government Documents Department

Microporous Silica Prepared by Organic Templating: Relationship Between the Molecular Template and Pore Structure

Description: Microporous silica materials with a controlled pore size and a narrow pore size distribution have been prepared by sol-gel processing using an organic-templating approach. Microporous networks were formed by pyrolytic removal of organic ligands (methacryloxypropyl groups) from organic/inorganic hybrid materials synthesized by copolymerization of 3-methacryloxypropylsilane (MPS) and tetraethoxysilane (TEOS). Molecular simulations and experimental measurements were conducted to examine the relationship between the microstructural characteristics of the porous silica (e.g., pore size, total pore volume, and pore connectivity) and the size and amount of organic template ligands added. Adsorption measurements suggest that the final porosity of the microporous silica is due to both primary pores (those present in the hybrid materials prior to pyrolysis) and secondary pores (those created by pyrolytic removal of organic templates). Primary pores were inaccessible to N{sub 2} at 77 K but accessible to CO{sub 2} at 195 K; secondary pores were accessible to both N{sub 2} (at 77 K) and CO{sub 2} (at 195 K) in adsorption measurements. Primary porosity decreases with the amount of organic ligands added because of the enhanced densification of MPS/TEOS hybrid materials as the mole fraction of trifunctional MPS moieties increases. pore volumes measured by nitrogen adsorption experiments at 77 K suggest that the secondary (template-derived) porosity exhibits a percolation behavior as the template concentration is increased. Gas permeation experiments indicate that the secondary pores are approximately 5 {angstrom} in diameter, consistent with predictions based on molecular simulations.
Date: September 3, 1999
Partner: UNT Libraries Government Documents Department