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Crystal growth and roughening of solid D{sub 2}

Description: Near the triple point, growth shapes of vapor deposited hexagonal close packed D{sub 2} crystals reveal two crystal orientations contain facets which persist up to the melt. This observation is in contrast with previous experiments on rare gas solids and H{sub 2} where the highest T{sub r} measured is 0.8 T{sub tp}.
Date: March 26, 1997
Creator: Kozioziemski, B.J.; Collins, G.W. & Bernat, T.P.
Partner: UNT Libraries Government Documents Department


Description: Due to their aesthetic value and high compressive strength, dentists have recently employed ceramics for restoration materials. Among the ceramic materials, zirconia provides high toughness and crack resistant characteristics. Residual stresses develop in processing due to factors including grain anisotropy and thermal coefficient mismatch. In the present study, polychromatic X-ray (Laue) micro-diffraction provided grain orientation and residual stresses on a clinically relevant zirconia model ceramic disk. A 0.5 mm x 0.024 mm region on zirconia was examined on a 500 nm scale for residual stresses using a focused poly-chromatic synchrotron X-ray beam. Large stresses ranging from - to + 1GPa were observed at some grains. On average, the method suggests a relatively small compressive stress at the surface between 47 and 75 MPa depending on direction.
Date: January 1, 2009
Creator: Bale, H. A.; Tamura, N.; Coelho, P.G. & Hanan, J. C.
Partner: UNT Libraries Government Documents Department

Characterizing the nano and micro structure of concrete to improve its durability

Description: New and advanced methodologies have been developed to characterize the nano and microstructure of cement paste and concrete exposed to aggressive environments. High resolution full-field soft X-ray imaging in the water window is providing new insight on the nano scale of the cement hydration process, which leads to a nano-optimization of cement-based systems. Hard X-ray microtomography images on ice inside cement paste and cracking caused by the alkali-silica reaction (ASR) enables three-dimensional structural identification. The potential of neutron diffraction to determine reactive aggregates by measuring their residual strains and preferred orientation is studied. Results of experiments using these tools will be shown on this paper.
Date: October 22, 2008
Creator: Monteiro, P. J. M.; Kirchheim, A. P.; Chae, S.; Fischer, P.; MacDowell, A. A.; Schaible, E. et al.
Partner: UNT Libraries Government Documents Department

Influence of orientation pinning on the Goss-texture in Fe-3%Si electrical steel

Description: Despite a large number of investigations on the formation of the Goss-texture in Fe-3%Si electrical steels, the exact mechanisms leading to the preference of this particular orientation are not completely understood so far. As an alternative to the standard explanation of a favored growth of Goss-oriented grains during secondary recrystallization, recently the concept of orientation pinning has been proposed, which considers that the growth of grains with special orientation relationships corresponding to low-angle and twin grain boundaries is disfavored. The present paper present preliminary EBSD-results on the growth of Goss-grains during secondary recrystallization in high-permeability (HiB) transformer steel sheets. A semi-quantitative model to simulate the effect of orientation pinning on the evolution of the Goss-texture is introduced.
Date: December 1, 1998
Creator: Engler, O. & Friedel, F.
Partner: UNT Libraries Government Documents Department

Grain boundary dissociation by the emission of stacking faults

Description: A range of <110> symmetric tilt grain boundaries (GBs) are investigated in several fcc metals with simulations and high resolution electron microscopy. Boundaries with tilt angles between 50.5{degree} and 109.5{degree} dissociate into two boundaries 0.6 to 1.1 nm apart. The dissociation takes place by the emission of stacking faults from one boundary that are terminated by Shockley partials at a second boundary. This is a general mode of GB relaxation for low stacking fault energy metals. The reasons for the occurrence of this relaxation mode are discussed using the theory of GB dislocations.
Date: May 1995
Creator: Rittner, J. D.; Seidman, D. N. & Merkle, K. L.
Partner: UNT Libraries Government Documents Department

Growth and Structure of Metallic Barrier Layer and Interconnect Films I: Experiments

Description: We present experimented results directed at understanding the growth and structure of metallic barrier layer and interconnect films. Numerical simulation results associated with this experimental work are presented in an accompanying paper in these proceedings. Here, thin films of Al, Ti, Cu and Ta have been grown by magnetron sputtering onto oxidized Si substrates. Using a specially-constructed substrate holder, the orientation of the substrate with respect to the growth direction was varied from horizontal to vertical. Films were grown at both low and high argon pressure; in the case of Ta, the cathode power was varied as well. The film structure and in particular the surface roughness was measured by X-ray reflectance and also by atomic force microscopy. We find that the surface roughness increases markedly with orientation angle in the case of Ta and Cu films, and in Ti films grown at high argon pressure. At low pressure, however, the Ti film surface roughness remains constant for all substrate orientations. No variation in roughness with either orientation angle or argon pressure was observed in the Al films. These results suggest that, under certain circumstances, shadowing effects and/or grain orientation (i.e., texture) competition during growth can give rise to lower density, more porous (and thus more rough) films, particularly at large orientation angles, as on sidewalls in sub-micron trenches.
Date: April 5, 1999
Creator: Baumann, F.H.; Gilmer, G.H.; O'Sullivan, P.L.; Sapjeta, J.; Torre, J.D. & Windt, D.L.
Partner: UNT Libraries Government Documents Department

HRLEED and STM study of misoriented Si(100) with and without a Te overlayer

Description: The growth of high quality Te on misoriented Si(100) is important as an intermediate phase for epitaxial growth of CdTe. The misorientation angle plays a key role in the growth quality of CdTe/Si(100); this incited the curiosity to investigate the effect of the misorientation angle on the topography of the surface structure of Si(100). The main goal is to show the relation between the misorientation angle, the terrace width and the step height distributions. HRLEED (High Resolution Low Energy Electron Diffraction) provides information in reciprocal space while STM gives real space topographic images of the surface structure. STM and HRLEED measurements were performed on Si(100) with misorientation angle {var_theta} = 0.5{degree}, 1.5{degree} and 8{degree} towards the [110] direction and {var_theta} = 4{degree} towards the [130] direction. Except for the 8{degree} misorientation in which case a regular step array with diatomic step height was observed, for the other misorientations the terrace width was variable. The average terrace width decreased with increasing misorientation angle. A mixture of diatomic and monatomic step heights was observed on the 0.5{degree} and 1.5{degree} misoriented Si(100) samples. It proves that one can not assume purely monatomic step height for low misorientation angles. The results do not agree with the belief that at low miscut angle A and B terraces are equal and that as the misorientation angle increases the B terrace tends to be wider than the A terrace. In fact, pairing of terraces was not observed at all. Te was deposited at a substrate temperature of 200 C. The authors observed a significant reduction in the terrace widths for all miscut angles.
Date: December 1, 1996
Creator: Yala, S. & Montano, P.A.
Partner: UNT Libraries Government Documents Department

Biomimetic processing of oriented crystalline ceramic layers

Description: The aim of this project was to develop the capabilities for Sandia to fabricate self assembled Langmuir-Blodgett (LB) films of various materials and to exploit their two-dimensional crystalline structure to promote the growth of oriented thin films of inorganic materials at room temperature. This includes the design and synthesis of Langmuir-active (amphiphilic) organic molecules with end groups offering high nucleation potential for various ceramics. A longer range goal is that of understanding the underlying principles, making it feasible to use the techniques presented in this report to fabricate unique oriented films of various materials for electronic, sensor, and membrane applications. Therefore, whenever possible, work completed in this report was completed with the intention of addressing the fundamental phenomena underlying the growth of crystalline, inorganic films on template layers of highly organized organic molecules. This problem was inspired by biological processes, which often produce exquisitely engineered structures via templated growth on polymeric layers. Seashells, for example, exhibit great toughness owing to their fine brick-and-mortar structure that results from templated growth of calcium carbonate on top of layers of ordered organic proteins. A key goal in this work, therefore, is to demonstrate a positive correlation between the order and orientation of the template layer and that of the crystalline ceramic material grown upon it. The work completed was comprised of several parallel efforts that encompassed the entire spectrum of biomimetic growth from solution. Studies were completed on seashells and the mechanisms of growth for calcium carbonate. Studies were completed on the characterization of LB films and the capability developed for the in-house fabrication of these films. Standard films of fatty acids were studied as well as novel polypeptides and porphyrins that were synthesized.
Date: October 1, 1997
Creator: Cesarano, J. & Shelnutt, J.A.
Partner: UNT Libraries Government Documents Department

Texture evolution in upset-forged P/M and wrought tantalum: Experimentation and modeling

Description: Preferred orientations in polycrystalline materials can significantly affect their physical and mechanical response through the retention of anisotropic properties inherent to the single crystal. In this study the texture evolution in upset-forged PIM and wrought tantalum was measured as a function of initial texture, compressive strain, and relative position in the pressing. A <001>/<111> duplex fiber texture parallel to the compression axis was generally observed, with varying degrees of a radial component evident in the wrought material. The development of deformation textures derives from restricted crystallographic slip conditions that generate lattice rotations, and these grain reorientations can be modeled as a function of the prescribed deformation gradient. Texture development was simulated for equivalent deformations using both a modified Taylor approach and a viscoplastic self-consistent (VPSC) model. A comparison between the predicted evolution and experimental results shows a good correlation with the texture components, but an overly sharp prediction at large strains from both the Taylor and VPSC models.
Date: November 1, 1997
Creator: Bingert, J.F.; Desch, P.B.; Bingert, S.R.; Maudlin, P.J. & Tome, C.N.
Partner: UNT Libraries Government Documents Department


Description: The electrochemical responses of AlO{sub x}/Al thin films have been investigated as a function of film growth conditions which produce films with different grain orientation, size and morphology. Films with smooth, 150 nm diameter, randomly oriented grains show a higher pitting potential and lower passive current than those films with large grain-boundary grooving from a mixture of smooth micron-sized, (200)-oriented grains and 300--500 nm diameter, (220)-oriented grains. These results suggest that surface roughness from grain-boundary grooving affects the pitting resistance more strongly than does the grain boundary density.
Date: November 1, 1998
Partner: UNT Libraries Government Documents Department

Experimental issues in in-situ synchrotron x-ray diffraction at high pressure and temperature by using a laser-heated diamond-anvil cell

Description: An integrated technique of diamond-anvil cell, laser-heating and synchrotron x-ray diffraction technologies is capable of structural investigation of condensed matter in an extended region of high pressures and temperatures above 100 GPa and 3000 K. The feasibility of this technique to obtain reliable data, however, strongly depends on several experimental issues, including optical and x-ray setups, thermal gradients, pressure homogeneity, preferred orientation, and chemical reaction. In this paper, we discuss about these experimental issues together with future perspectives of this technique for obtaining accurate data.
Date: December 1, 1997
Creator: Yoo, C.S.
Partner: UNT Libraries Government Documents Department

The development of cube and non-cube recrystallization textures

Description: The development of recrystallization textures in cold rolled copper has been characterized using an electron backscatter pattern (EBSP) technique. Cube oriented grains exist in materials that have been annealed after a series of rolling strains, between 1.0 and 4.5 (von Mises strain). The strength and sharpness of these cube textures increases with increasing strain thus replacing the random texture produced by recrystallization of moderately deformed copper. The preferential formation of the cube, texture is attributed to the homogenization of stored energy gradients adjacent to randomly oriented nucleation sites. This process, coupled with the development of the microstructure adjacent to deformed cube sites, favors nucleation and growth of cube grains over randomly oriented grains.
Date: July 1, 1995
Creator: Necker, C.T.; Rollett, A.D. & Doherty, R.D.
Partner: UNT Libraries Government Documents Department

Orientation-dependent shock response of explosive crystals

Description: Some orientations of PETN crystals have anomalously high shock initiation sensitivity around 4 to 5 GPa. Results of a series of laser interferometry experiments at 4.2 GPa show that this is associated with an elastic-plastic, two-wave structure with large elastic precursors. Implications for the initiation mechanism in single crystals is discussed. Initial work on beta phase, monoclinic HMX is also described.
Date: September 1, 1995
Creator: Dick, J.J.
Partner: UNT Libraries Government Documents Department

In situ biaxial texture analysis of MGO films during growth on amorphous substrates by ion beam-assisted deposition

Description: We used a previously reported kinematical electron scattering model to develop a RHEED based method for performing quantitative analysis of mosaic polycrystalline thin film in-plane and out-of-plain grain orientation distributions. RHEED based biaxial texture measurements are compared to X-Ray and transmission electron microscopy measurements to establish the validity of the RHEED analysis method. In situ RHEED analysis reveals that the out of plane orientation distribution starts out very broad, and then decreases during IBAD MgO growth. Other results included evidence that the in-plane orientation distribution narrows, the grain size increases, and the film roughens as film thickness increases during IBAD MgO growth. Homoepitaxy of MgO improves the biaxial texture of the IBAD layer, making X-ray measurements of IBAD films with an additional homoepitaxial layer not quantitatively representative of the IBAD layer. Systematic offsets between RHEED analysis and X-ray measurements of biaxial texture, coupled with evidence that biaxial texture improves with increasing film thickness, indicate that RHEED is a superior technique for probing surface biaxial texture.
Date: January 1, 2001
Creator: Brewer, R. T. (Rhett T.); Arendt, P. N. (Paul N.); Atwater, H. A. (Harry A.) & Groves, J. R. (James R.)
Partner: UNT Libraries Government Documents Department

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

Description: TEM, while capable of determining misorientation of adjacent grains, can practicably provide information only for a small number of grain boundaries. A more complete description of the structure of a polycrystal can be obtained using a new technique OIM, which uses crystallographic orientation data obtained from Backscattered Electron Kikuchi patterns (BEKP), collected using SEM. This paper describes general OIM results for 99.7 and 99.99% Al{sub 2}O{sub 3} samples with grain sizes 4-27 {mu}m. The results include image quality maps, grain boundary maps, pole figures, and lattice misorientations depicted on MacKenzie plots and in Rodrigues space. High quality BEKPs were obtained from all specimens. Images and data readily reveal the grain morphology, texture, and grain boundary structure. Subtle differences in texture and grain boundary structure (crystallite lattice misorientations) are observed for the different alumina specimens. Distributions of misorientations for cracked boundaries in alumina are compared to the bulk distribution of boundaries and generally larger misorientations are observed.
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

Comparison of the structure of grain boundaries in silicon and diamond by molecular-dynamics simulations

Description: Molecular-dynamics simulations were used to synthesize nanocrystalline silicon with a grain size of up to 75 {angstrom} by crystallization of randomly misoriented crystalline seeds from the melt. The structures of the highly-constrained interfaces in the nanocrystal were found to be essentially indistinguishable from those of high-energy bicrystalline grain boundaries (GBs) and similar to the structure of amorphous silicon. Despite disorder, these GBs exhibit predominantly four-coordinated (sp{sup 3}-like) atoms and therefore have very few dangling bonds. By contrast, the majority of the atoms in high-energy bicrystalline GBs in diamond are three-coordinated (sp{sup 2}-like). Despite the large fraction of three-coordinated GB carbon atoms, they are rather poorly connected amongst themselves, thus likely preventing any type of graphite-like electrical conduction through the GBs.
Date: June 1, 1997
Creator: Keblinski, P.; Phillpot, S.R.; Wolf, D. & Gleiter, H.
Partner: UNT Libraries Government Documents Department

{open_quotes}Local texture, current flow, and superconductive transport properties of Tl1223 deposits on practical substrates{close_quotes}

Description: Quantitative investigations of the crystal grain orientations and electrical transport properties of high temperature superconducting (HTS)TiBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub 8+x} (Tl1223) deposits on polycrystalline substrates show that current flow comprises percolative networks of strongly-coupled material. Superconductive transport properties on different samples, on the same samples at different widths, and on samples with artificially-induced strong flux pinning defects confirm the nature of current flow, and suggest that these materials may be useful as a new class of HTS conductors.
Date: May 1, 1996
Creator: Christen, D.K.; Specht, E.D. & Goyal, A.
Partner: UNT Libraries Government Documents Department

Molecular dynamics simulations of grain boundaries in thin nanocrystalline silicon films

Description: Using molecular dynamics simulations, the grain boundaries in thin polycrystalline silicon films (considered as promising material for future nanoelectronic devices) are investigated. It is shown that in polysilicon film with randomly oriented grains the majority of grain boundaries are disordered. However, some grains with small mutual orientation differences can form extended crystalline patterns. The structure of the grain boundaries satisfies the thermodynamical criterion. The majority of atoms in the grain boundaries are tetrahedrally coordinated with the nearest neighbors, even though the grain boundaries are disordered. The grain boundary matter is characterized as an amorphous phase with a characteristic tetragonality value.
Date: October 1, 1997
Creator: Berman, G.P.; Doolen, G.D.; Mainieri, R.; Campbell, D.K. & Luchnikov, V.A.
Partner: UNT Libraries Government Documents Department

Interfacial Microstructure Formed by Reactive Metal Penetration of Al into Mullite

Description: Microstructures in the reaction interface between molten Al and dense mullite have been studied by transmission electron microscopy to provide insight into mechanisms for forming ceramic-metal composites by reactive metal penetration. The reactions, which have the overall stoichiometry, 3Al{sub 6}Si{sub 2}O{sub 13} + (8 + x)Al {r_arrow} 13Al{sub 2}O{sub 3} + xAl + 6Si, were carried out at temperatures of 900, 1100, and 1200 C for 5 minutes and 60 minutes, and 1400 C for 15 minutes. Observed phases generally were those given in the above reaction, although their proportions and interfacial microstructure differed strongly with reaction temperature. After reaction at 900 C, a thin Al layer separated unreacted mullite from the {alpha}-Al{sub 2}O{sub 3} and Al reaction products. No Si phase was found near the reaction front. After 5 minutes at 1100 C, the reaction front contained Si, {alpha}-Al{sub 2}O{sub 3}, and an aluminum oxide phase with a high concentration of Si. After 60 minutes at 1100 C many of the {alpha}-Al{sub 2}O{sub 3} particles were needle-shaped with a preferred orientation. After reaction at 1200 C, the reaction front contained a high density of Si particles that formed a continuous layer over many of the mullite grains. The sample reacted at 1400 C for 15 minutes had a dense {alpha}-Al{sub 2}O{sub 3} reaction layer less than 2 {micro}m thick. Some isolated Si particles were present between the {alpha}-Al{sub 2}O{sub 3} layer and the unreacted mullite. Using previously measured reaction kinetics data the observed temperature dependence of the interfacial microstructure have been modeled as three sequential steps, each one of which is rate-limiting in a different temperature range.
Date: March 3, 1999
Creator: Du, T.B.; Ewsuk, K.G.; Fahrenholtz, W.G.; Loehman, R.E. & Lu, P.
Partner: UNT Libraries Government Documents Department

Modeling of nucleation during recrystallization

Description: The number density and spatial distribution of nuclei determine the kinetics, and the evolution of microstructure and texture during recrystallization. The potential nucleation sites are the high angle boundaries that exist in the deformed microstructure. These could be either the original grain boundaries prior to deformation or boundaries created due to non-uniform plastic deformation on the microscopic scale. Discontinuous subgrain growth with or without the formation of high angle boundaries may also occur during recrystallization in the presence of long range orientation gradients existing in the deformation substructure. The paper presents Monte Carlo simulation results on the evolution of substructures for each of the above nucleation mechanisms. The evolution of recrystallized grain structure from a realistic substructure obtained by modeling the deformation of fcc polycrystals at the microstructural level is presented, and shows the collective contribution of the above nucleation mechanisms during recrystallization and grain growth. The simulations capture not only the kinetics and microstructural evolution but also the evolution of texture during recrystallization.
Date: November 1, 1998
Creator: Radhakrishnan, B.; Sarma, G. & Zacharia, T.
Partner: UNT Libraries Government Documents Department

Segregation of copper to (100) and (111) silicon surfaces from internal Cu{sub 3}Si precipitates

Description: Segregation of transition metal impurities to surfaces or interfaces can have detrimental or beneficial effects in silicon-based microelectronic devices. Controlled segregation of impurities to regions remote from device structures, i.e. gettering is routinely used to prevent uncontrolled segregation to critical regions which may cause failure. Internal gettering is a widely used process in which oxide precipitates and associated lattice defects provide sites for precipitation of metal-silicide phases. Segregation of impurities onto surfaces of internal microcavities has also been examined as a potential gettering process. It was observed that gettering to cavities can dissolve pre-existing internal metal silicide precipitates of Cu, Au and Ni. The energetics of copper segregation to silicon surfaces were examined by measuring the Cu coverage after equilibration between Cu on the surface and internal Cu{sub 3}Si, for which the Cu chemical potential is known. For oxide-free surfaces the Cu coverage was close to one monolayer on (111) surfaces but was much smaller on (100) surfaces. The Cu coverage was greatly reduced by oxide passivation of the surface. LEED showed the 7 x 7 structure of the clean (111) silicon surface converted to a quasiperiodic 5 x 5 structure after equilibrating with Cu{sub 3}Si. The 2 x 1 LEED patterns for (100) surfaces indicated no change in surface structure due to the Cu{sub 3}Si. These results show that the free energy of copper in Cu{sub 3}Si is higher than that of copper on (111) surfaces but lower than that of copper on (100) surfaces.
Date: December 1, 1996
Creator: Wampler, W.R.
Partner: UNT Libraries Government Documents Department

Pit initiation in AlO{sub x}/Al thin films

Description: The electrochemical responses of AlO{sub x}/Al thin films have been investigated as a function of film growth conditions which produce films with different grain orientation, size and morphology. Films with smooth, 150 nm diameter, randomly oriented grains show a higher pitting potential and lower passive current than those films with large grain-boundary grooving from a mixture of smooth micron-sized, (200)-oriented grains and 300--500 nm diameter, (220)-oriented grains. These results suggest that surface roughness from grain-boundary grooving affects the pitting resistance more strongly than does the grain boundary density.
Date: December 1998
Creator: Son, K. A.; Barbour, J. C.; Missert, N.; Wall, F. D.; Copeland, R. G.; Martinez, M. A. et al.
Partner: UNT Libraries Government Documents Department

A comparison of different texture analysis techniques

Description: With the advent of automated techniques for measuring individual crystallographic orientations using electron diffraction, there has been an increase in the use of local orientation measurements for measuring textures in polycrystalline materials. Several studies have focused on the number of single orientation measurements necessary to achieve the statistics of more conventional texture measurement, techniques such as pole figure measurement using x-ray and neutron diffraction. This investigation considers this question but also is extended to consider the nature of the differences between textures measured using individual orientation measurements and those measured using x-ray diffraction.
Date: August 1, 1996
Creator: Wright, S.I. & Kocks, U.F.
Partner: UNT Libraries Government Documents Department

Low-temperature growth and orientational control in RuO{sub 2} thin films by metal-organic chemical vapor deposition

Description: For growth temperatures in the range of 275 C to 425 C, highly conductive RuO{sub 2} thin films with either (110)- or (101)-textured orientations have been grown by metal-organic chemical vapor deposition (MOCVD) on both SiO{sub 2}/Si(001) and Pt/Ti/SiO{sub 2}/Si(001) substrates. Both the growth temperature and growth rate were used to control the type and degree of orientational texture of the RuO{sub 2} films. In the upper part of this growth temperature range ({approximately} 350 C) and at a low growth rate (< 30 {angstrom}/min.), the RuO{sub 2} films favored a (110)-textured. In contrast, at the lower part of this growth temperature range ({approximately} 300 C) and at a high growth rate (> 30 {angstrom}/min.), the RuO{sub 2} films favored a (101)-textured. In contrast, a higher growth temperatures (> 425 C) always produced randomly-oriented polycrystalline films. For either of these low-temperature growth processes, the films produced were crack-free, well-adhered to the substrates, and had smooth, specular surfaces. Atomic force microscopy showed that the films had a dense microstructure with an average grain size of 50--80 nm and a rms. surface roughness of {approximately} 3--10 nm. Four-probe electrical transport measurements showed that the films were highly conductive with resistivities of 34--40 {micro}{Omega}-cm ({at} 25 C).
Date: August 1, 1996
Creator: Bai, G.R.; Wang, A.; Foster, C.M.; Vetrone, J.; Patel, J. & Wu, X.
Partner: UNT Libraries Government Documents Department