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Generation of Chloride Active Defects at the Aluminum Oxide Surface for the Study of Localized Corrosion Initiation

Description: The generation of surface defects on electron cyclotron resonance (ECR) plasma derived aluminum oxide films has been studied. We find that Cl active O vacancies can be generated using electron and ion irradiation yielding surface concentrations of 3 xl 013 to 1X1014 sites"cm-2. These values correspond to surface defect concentrations of 3 to 10% when compared to ordered, crystalline u-alumina. The vacancies appear to be responsible for increased surface O concentrations when immersed in water. Anodic polarization of irradiated films yields a decrease in the stable pitting potential which correlates with electron dose.
Date: December 7, 1998
Creator: Barbour, J.C.; Missert, N.; Son, K.-A; Wall, F.D. & Zavadil, K.R.
Partner: UNT Libraries Government Documents Department

Variations in the Nature of Metal Adsorption on Ultrathin Al(2)O(3) Films

Description: First-principles density-functional calculations are used to study metal adsorption (Li, K, Y, Nb, Ru, Pd, Pt, Cu, Ag, Au, and Al at 1/3-4 monolayer coverages) atop 5 ~ A1203 films on Al(Ill). The oxide-metal bond is ionic at Iow coverages but, with interesting exceptions, caused by polari@i ,~-cE!vED at high coverages where the overlayer is metallic. Binding trends are explained in terms of s'imp e concepts. Increasing overlayer thickness can cause the adsorbate-oxide interface structure to than . %lEc o ~ 1998 and while some metals wet, most do not.
Date: November 24, 1998
Creator: Bogicevic, A. & Jennison, D.R.
Partner: UNT Libraries Government Documents Department

Characterization of microstructure and crack propagation in alumina using orientation imaging microscopy (OIM). December 1996

Description: A more complete description requires the lattice orientations of a statistically significant number of grains, coupled with morphology such as grain size and shape; this can be obtained using orientation imaging microscopy (OIM), which uses crystallographic orientation data from Backscattered Electron Kikuchi patterns (BEKP) collected using a SEM. This report describes the OIM results for alumina; these include image quality maps, grain boundary maps, pole figures, and lattice misorientations depicted on MacKenzie plot and in Rodrigues space. High quality BEKP were obtained and the images and data readily reveal the grain morphology, texture, and grain boundary misorientations, including those for cracked boundaries. A larger number of grains should be measured to make statistical comparisons between materials with different processing histories.
Date: December 1, 1996
Creator: Glass, S.J.; Michael, J.R.; Readey, M.J.; Wright, S.I. & Field, D.P.
Partner: UNT Libraries Government Documents Department

Lightweight alumina refractory aggregate. Phase 2, Pilot scale development

Description: Kilogram quantities of refractory aggregate were prepared from both a paste and a pelletized form of extruder feed material in both bench and pilot-scale equipment. The 99{sup +} % alumina aggregate exhibited a bulk density approaching 2.5 g/cm{sup 3} and a fired strength slightly lower than fused alumina. Based on initial evaluation by two refractory manufacturers in brick or castable applications, the new aggregate offered adequate strength with thermal conductivity reductions up to 34%, depending on the temperature and application of the new aggregate in these initial trials. The new aggregate was simply substituted for Tabular{trademark} in the refractory formulation. Thus, there is room for improvement through formulation optimization with the lightweight aggregate. The new aggregate offers a unique combination of density, strength, and thermal properties not available in current aggregate. To this point in time, technical development has led to a pelletized formulation with borderline physical form leaving the Eirich mixer. The formulation requires further development to provide more latitude for the production of pelletized material without forming paste, while still reducing the bulk density slightly to reach the 2.5 g/cm{sup 3} target. The preferred, pelletized process flowsheet was outlined and a preliminary economic feasibility study performed based on a process retrofit into Alcoa`s Arkansas tabular production facilities. Based on an assumed market demand of 20,000 mt/year and an assumed selling price of $0.65/lb (25% more than the current selling price of Tabular{trademark}, on a volume basis), economics were favorable. Decision on whether to proceed into Phase 3 (full- scale demonstration) will be based on a formal market survey in 1994 October.
Date: November 1, 1994
Creator: Swansiger, T.G. & Pearson, A.
Partner: UNT Libraries Government Documents Department

Crystal phases and lattice dynamics of slip-cast {beta}{prime}-sialons

Description: The crystal structures and phonon densities of states (DOS) of {beta}{prime}-Sialon ceramics, Si{sub 6-z}Al{sub z}O{sub z}N{sub 8-z} (O {le} z {le} 6), prepared by a novel slip-cast method were studied by neutron scattering techniques. A Rietveld analysis of the diffraction patterns shows that samples of z < 4 form a single-phase solid solution of Si-Al-O-N isostructural to {beta}-Si{sub 3}N{sub 4} (space group P6{sub 3}/m). Within this structure there is a consistent preferred occupation of O on the 2c sites and N on the 6h sites. For z > 4 the materials exhibit multiple-phase structure. The observed phonon DOS of the O {le} z {le} 4 ceramics displays phonon bands at about 50 and 115 meV. These features are considerably broader than the corresponding ones in {beta}-Si{sub 3}N{sub 4} powder. As z increases, effects due to atomic disorder lead to an overlap of the two phonon bands and a complete fill up of the phonon gap at {approximately} 100 meV observed in Si{sub 3}N{sub 4}.
Date: December 31, 1995
Creator: Loong, C.K.; Richardson, J.W. Jr.; Suzuki, S. & Ozawa, M.
Partner: UNT Libraries Government Documents Department

Low-energy deposition of high-strength Al(0) alloys from an ECR plasma

Description: Low-energy deposition of Al(O) alloys from an electron cyclotron resonance (ECR) plasma offers a scaleable method for the synthesis of thick, high-strength Al layers. This work compares alloy layers formed by an ECR-0{sub 2} plasma in conjunction with Al evaporation to 0-implanted Al (ion energies 25-200 keV); and it examines the effects of volume fraction of A1{sub 2}0{sub 3} phase and deposition temperature on the yield stress of the material. TEM showed the Al(O) alloys contain a dense dispersion of small {gamma}-Al{sub 2}0{sub 3} precipitates ({approximately}l nm) in a fine-grain (10-100 nm) fcc Al matrix when deposited at a temperature of {approximately}100C, similar to the microstructure for gigapascal-strength 0-implanted Al. Nanoindentation gave hardnesses for ECR films from 1.1 to 3.2 GPa, and finite-element modeling gave yield stresses up to 1.3 {plus_minus} 0.2 GPa with an elastic modulus of 66 GPa {plus_minus} 6 GPa (similar to pure bulk Al). The yield stress of a polycrystalline pure Al layer was only 0.19 {plus_minus} 0.02 GPa, which was increased to 0.87 {plus_minus} 0.15 GPa by implantation with 5 at. % 0.
Date: December 31, 1995
Creator: Barbour, J.C.; Follstaedt, D.M.; Knapp, J.A.; Myers, S.M.; Marshall, D.A. & Lad, R.J.
Partner: UNT Libraries Government Documents Department

Mechanical behavior of robocast alumina

Description: Direct fabrication of alumina parts by robocasting was completed. This method is based on three-dimensional deposition of binderless aqueous alumina slurries. Parts were made with different deposition paths and mechanical testing performed to determine the effects of bead alignment. Properties were also compared to alumina processed more traditionally.
Date: December 1, 1998
Creator: Denham, H.B.; Cesarano, J. III; King, B.H. & Calvert, P.
Partner: UNT Libraries Government Documents Department

Advanced process development for high reflector coatings on solar concentrator panels. Final letter report

Description: Objectives were to develop and demonstrate the manufacturing process for vacuum deposition of low-cost thin-film high reflectance coatings onto large solar concentrator panels; demonstrate thin-film deposition processes for commercialization of this technology by United Solar Technologies (UST); apply reflective coatings to solar concentrator panels for prototype application by UST.
Date: October 1, 1996
Creator: Martin, P.M.; Stewart, C.D.; Bennett, W.D. & Johnston, J.W.
Partner: UNT Libraries Government Documents Department

Microstructural Characterization of Water-Rich Boehmite (AlO(OH)): TEM Correlation of Apparently Divergent XRD and TGA Results

Description: An understanding of the solid-phase thermodynamics and aqueous speciation of aluminum is critical to our ability to understand and predict processes in a wide variety of geologic and industrial settings. Boehmite (AIO(OH)) is an important phase in the system Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O that has been the subject of a number of structural and thermodynamic studies since its initial synthesis [l] and discovery in nature [2]. Unfortunately, it has long been recognized that thermogravimetric analysis (TGA) of both synthetic and natural boehmite samples (that appear well crystallized by powder XRD methods) yields significant excess water - typically losing 16-16.5 wt. % on heating as compared with a nominal expected weight loss of 15.0 wt. % [3,4]. The boehmite used in our experiments was synthesized hydrothermally from acid-washed gibbsite (Al(OH)<suv>3</sub>) at 200°C. Powder XRD and SEM examination showed no evidence of the presence a contaminant phase. The TGA patterns do not suggest that this is due to adsorbed water, so a structural source is likely. We therefore undertook to examine this material by TEM to clarify this phenomenon.
Date: August 1999
Creator: Allard, L. F.; Anovitz, L. M.; Benezeth, P.; Coffey, D. W.; Palmer, D. A.; Porter, W. D. et al.
Partner: UNT Libraries Government Documents Department

Atomic structure of the {sigma}5 (210)/[001] symmetric tilt grain boundary in yttrium aluminum garnet

Description: The {Sigma}5(210)/[100] symmetric tilt grain boundary in YAG was produced by UHV diffusion bonding precisely oriented single crystals. The boundary has been characterized by HREM along two different directions, parallel and perpendicular to the tilt axis. Models of the atomic structure of the boundary were formed following the Coincident Site Lattice scheme. The resulting models are equivalent to twins formed at the atomic scale. The high resolution images show no rigid crystal translations away from the perfect mirror reflection relation. Comparison of the simulated images using the atomic model as input with the experimental images identifies the plane of mirror symmetry. The atomic model is shown to be in good agreement with the experimental images when viewed parallel to tilt axis, but disagrees with the images perpendicular to tilt axis. Agreement between simulated and experimental images can be improved by changing the composition of the grain boundary with respect to the bulk. To reach a more certain conclusion on the structure of the grain boundary will require additional theoretical calculations.
Date: June 24, 1996
Creator: Campbell, G.H. & King, W.E.
Partner: UNT Libraries Government Documents Department

Residual stress distribution in an Al{sub 2}O{sub 3}-Ni joint bonded with a composite layer

Description: Neutron diffraction was used to study the residual stress distribution in an axisymmetric Al{sub 2}O{sub 3}-Ni joint bonded with a 40 vol% Al{sub 2}O{sub 3}-60 vol% Ni composite layer. A series of measurements was taken along the axis of symmetry through the Al{sub 2}O{sub 3} and composite layers. It is shown that after taking into account the finite neutron diffraction sampling volume, both the trends and peak values of the experimental strain distribution were in excellent agreement with calculations of a simple finite element model, where the rule-of-mixtures approach was used to describe the constitutive behavior of the composite interlayer. In particular, the predicted steep strain gradient near the interface was confirmed by the experimental data.
Date: July 1, 1996
Creator: Wang, X.L.; Watkins, T.R.; Rabin, B.H.; Williamson, R.L. & Bruck, H.A.
Partner: UNT Libraries Government Documents Department

Stability of eutectic interface during directional solidification

Description: Directional solidification of eutectic alloys shows different types of eutectic morphologies. These include lamellar, rod, oscillating and tilting modes. The growth of these morphologies occurs with a macroscopically planar interface. However, under certain conditions, the planar eutectic front becomes unstable and gives rise to a cellular or a dendritic structure. This instability leads to the cellular/dendritic structure of either a primary phase or a two-phase structure. The objective of this work is to develop a fundamental understanding of the instability of eutectic structure into cellular/dendritic structures of a single phase and of two-phases. Experimental studies have been carried out to examine the transition from a planar to two-phase cellular and dendritic structures in a ceramic system of Alumina-Zirconia (Al{sub 2}O{sub 3}-ZrO{sub 2}) and in a transparent organic system of carbon tetrabromide and hexachloroethane (CBr{sub 4}-C{sub 2}Cl{sub 6}). Several aspects of eutectic interface stability have been examined.
Date: April 23, 1996
Creator: Han, S.H.
Partner: UNT Libraries Government Documents Department

Development of high performance refractory fibers with enhanced insulating properties and longer service lifetimes: Phase 2, Improved refractory fiber and industrial benefit development. Final report

Description: This is Phase II of a three-phase study for the development of high performance refractory fibers with enhanced insulating properties and longer service lifetimes, for use in the aluminum, glass, cement, and iron and steel industries. Fiberization of 24 out of 25 compositions in the Al{sub 2}O{sub 3}-Si0{sub 2}-Zr0{sub 2} system were achieved. These 24 and three existing fiber compositions were evaluated: The shrinkage and the crystalline and vitreous phases were determined vs heat treatment time and temperature. Four theoretical models were developed: Shrinkage, devitrification kinetics, density change, and fiberization. Although some of the fibers formed during Phase II had properties as good as the reference ASZ fiber, no fiber had a significantly improved performance. This work, although not entirely successful, did produce significant benefits to refractory insulating fiber manufacturers and users: Mechanisms of both linear and thickness shrinkage for vitreous refractory fibers were determined, devitrification kinetics were quantified and used in models to predict shrinkage during service, and the mechanism of fiber formation in the melt spinning process was studied.
Date: May 1, 1995
Creator: Cai, Yifang; Curtis, J.M.; DePoorter, G.L.; Martin, P.C. & Munoz, D.R.
Partner: UNT Libraries Government Documents Department

Strain measurements in thermally grown alumina scales using ruby fluorescence

Description: We have measured strains in alumina scales thermally grown on Fe-Cr- Al alloys by exploiting the strain dependence of the ruby luminescence line. Measurements were done on Fe-5Cr-28Al and Fe-18Cr-10Al (at.%, bal. Fe) oxidized between 300-1300 C with periodic cycling to room temperature. Significantly different levels of strain buildup were observed in scales on these alloys. Results on similar alloys containing a dilute reactive element (Zr or Hf) are also presented. We observe that scales on alloys containing a reactive element (RE) can support higher strains than scales on RE-free alloys. With the luminescence technique, strain relief associated with spallation thresholds is readily observed. In early stage oxidation, the evolution of transition phases is monitored using Raman and fluorescence spectroscopies. The fluorescence technique also provides a sensitive probe of early stage formation of {alpha}-Al{sub 2}O{sub 3}. It appears that, in presence of Cr{sub 2}O{sub 3} or Fe{sub 2}O{sub 3}, the {alpha}-alumina phase can form at anomalously low temperatures.
Date: December 31, 1996
Creator: Veal, B. W.; Natesan, K.; Koshelev, I.; Grimsditch, M.; Renusch, D. & Hou, P. Y.
Partner: UNT Libraries Government Documents Department

Solid-particle erosion of an Al{sub 2}O{sub 3}-SiC-TiC composite

Description: An electrodischarge-machinable Al{sub 2}O{sub 3}-SiC-TiC composite developed by Industrial Ceramic Technology, Inc., has a high fracture toughness, 9.6{+-}0.6 MPm{sup 1/2}, as measured by indentation, and a Vickers hardness of 20.3{+-}0.6 GPa. The composite`s resistance to solid-particle erosion was measured for 143-{mu}m dia SiC particles impacting at 20-90{degree} angles and 50-100 m/s velocities. Erosion rate exhibited a maximum for normal incidence, and the erosion resistance was better than that of commercial Al{sub 2}O{sub 3}. SEM indicated that material wastage was by a combination of brittle fracture and microplasticity.
Date: November 1, 1996
Creator: Jiang, M.; Goretta, K.C.; Singh, D.; Routbort, J.L. & Schuldies, J.J.
Partner: UNT Libraries Government Documents Department

Photovoltaic manufacturing technology monolithic amorphous silicon modules on continuous polymer substrates. Annual technical progress report, 5 July 1995--4 June 1996

Description: Iowa Thin Film Technologies` goal is to develop the most cost-effective photovoltaic manufacturing process possible. During the first year, they developed the capability of sputtering a high-quality (Zn(Al)O) successfully implemented increased deposition rates for the ZnO top contact deposition; improved registration and ink-line width to reduce area loss due to interconnects; developed a new alignment process and sensor to improve the speed and accuracy of registration for the patterning processes; developed a new Silver ink composition that allows finer print lines and lower series resistance; demonstrated an 8% overall improvement in area utilization; evaluated water-based insulator inks for compatibility with their processes; investigated and tested the use of roll-based lamination as a means to reduce the cost of assembly; developed straight roll lamination capability using pressure-sensitive adhesives and thermally activated bonding; and evaluated the use of the standard EVA/Tefzel encapsulant with a roll laminator.
Date: February 1, 1997
Creator: Jeffrey, F.
Partner: UNT Libraries Government Documents Department

Processing and Characterizing Alumina/Aluminum Composites with Tailored Microstructures Formed by Reactive Metal Penetration

Description: In industry, the need to maximize energy efficiency depends on the availability of suitable advanced materials. Ceramic composites are exemplary materials for many advanced engineering applications because they exhibit good thermal stability, oxidation resistance and enhanced toughness. Presently, ceramic composite fabrication processes are costly, often requiring high temperatures and pressures to achieve reasonable densities. Our research is focused on developing a processing technique, that will allow production of alumina/aluminum composites using relatively low temperatures and without the application of an external force, thus reducing the processing costs. Our composites were formed using Reactive Metal Penetration (RMP), which is a process involving the reaction of molten Al with a dense ceramic preform. The result is a near net shape ceramic/metal composite with interpenetrating phases. The volume fraction of metal in the composites was varied by doping an aluminosilicate ceramic preform with silica. For this study we fabricated composites using pure mullite and mullite doped with 23 and 42 weight percent silica, yielding 18, 25, and 30 volume percent metal in the composites, respectively. Optical and Scanning Electron Microscopy were used to characterize the homogeneity and scale of the microstructure. The scale of the microstructure varied with preform composition, the reaction temperature and with secondary heat treatments. Four-point bend testing was used to evaluate the influence of microstructure on strength and reliability. During these studies a gradient in the microstructure was observed, which we further characterized using microhardness testing. Alumina/aluminum composites formed by RMP show higher toughness then monolithic alumina and have the potential for improved reliability when compared to monolithic ceramics.
Date: January 28, 1999
Creator: Corral, E.; Ellerby, D.; Ewsuk, K.; Fahrenholtz, B. & Loehman, R.
Partner: UNT Libraries Government Documents Department

Fabrication and evaluation of uniform and gradient density epoxies

Description: Filled epoxy materials which vary in density in a designed manner have been fabricated and their mechanical properties evaluated. Density variations were produced by incorporating different volume fractions of either glass microballoons (GMB) or alumina. Several different sample types were evaluated including uniform density (0.8 g/cm{sup 3} < {rho} < 2.0 g/cm{sup 3}) samples and gradient density samples (GMB only, 0.8 g/cm{sup 3} < {rho} < 1.2 g/cm{sup 3}). The uniform density specimens were evaluated for the effects of filler type and concentration on modulus and toughness. Results indicated that addition of alumina filler significantly increased the resulting modulus while addition of GMB had little measurable effect. These differences could be understood in terms of the differing moduli of the additives relative to that of the epoxy matrix. In the former case the alumina particulates had a modulus much greater than that of the epoxy while in the latter case, the modulus of the GMB additive was only slightly greater than that of the matrix. Addition of either filler significantly degraded the toughness of the composite specimens and precluded the use of gradients to enhance toughness performance. Discontinuous {open_quotes}block{close_quotes} gradients used for testing were fabricated by simple sequential pours of formulations with different GMB loadings and were evaluated for modulus, strength and ductility. Continuous gradients were fabricated in process studies by programmed shifts in the peristaltic pumping/mixing ratio of epoxies filled with either alumina or GMB. None of the continuous gradient materials were mechanically tested. These results suggest that applications utilizing gradient materials containing alumina and similar high modulus fillers to provide designed stiffness rather than improved toughness are the most appropriate targets for future investigation.
Date: November 1, 1997
Creator: Domeier, L.A.; Skala, D.M. & Goods, S.H.
Partner: UNT Libraries Government Documents Department

Design, performance and production of the Fermilab TESLA RF input couplers

Description: The TeV Energy Superconducting Linear Accelerator (TESLA) requires as one of its technical components a radiofrequency (rf) input coupler that transfers 1.3 GHz rf energy from the rf distribution system to a nine-cell superconducting accelerating cavity operating at a temperature of 1.8 K. The input coupler design is driven by numerous design criteria, which result in a rather complicated implementation. The production of twelve input couplers for the TESLA Test Facility (TTF) is underway at Fermilab, with the first two couplers having been delivered late in 1995. This paper discusses the Fermilab TESLA rf input coupler design, recent test results, and production issues.
Date: October 1, 1996
Creator: Champion, M.
Partner: UNT Libraries Government Documents Department

Alumina strength degradation in the elastic regime

Description: Measurements of Kanel et. al. [1991] have suggested that deviatoric stresses in glasses shocked to nearly the Hugoniot Elastic limit (HEL) relax over a time span of microseconds after initial loading. Failure (damage) waves have been inferred on the basis of these measurements using time-resolved manganin normal and transverse stress gauges. Additional experiments on glass by other researchers, using time-resolved gauges, high-speed photography and spall strength determinations have also lead to the same conclusions. In the present study the authors have conducted transmitted-wave experiments on high-quality Coors AD995 alumina shocked to roughly 5 and 7 GPa (just below or at the HEL). The material is subsequently reshocked to just above its elastic limit. Results of these experiments do show some evidence of strength degradation in the elastic regime.
Date: August 1, 1997
Creator: Furnish, M.D. & Chhabildas, L.C.
Partner: UNT Libraries Government Documents Department

Recent developments in freeform fabrication of dense ceramics from slurry deposition

Description: A freeform fabrication technique for dense ceramics and composites has been developed. The technique requires less than 2 volume percent of organic additives and relies on the principle of layerwise deposition of highly loaded colloidal slurries. Components can be manufactured into complex geometries with thick solid sections as well as with thin-walled sections with high aspect ratios. Process feasibility and quality is dependent on the processing parameters of solids loading, slurry rheology, deposition rate, and drying rate. These interrelated parameters must be controlled so that sintering defects are prevented and shape tolerance is maintained. A review of this freeform fabrication technique, called robocasting, will be discussed for fabrication of aluminum oxide parts. Recent developments for a finite element analysis technique for modeling the drying process will also be presented.
Date: November 1, 1997
Creator: Cesarano, J. III; Baer, T.A. & Calvert, P.
Partner: UNT Libraries Government Documents Department