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Thermodynamic properties of solid solutions at high temperatures

Description: The research carried out under this cortract was concerned with the determination of thermodynamic properties of inorganic matenials at high temperatures. Experimental methods were developed which permit accurate and reliable determination of activity-composition relations in solid solutions, and stabilities of end-member compounds. The data provide insight irto the general laws governing the behavior of materials in high-temperature envirorments. The phases selected for the experimental studies include onthosilicates, metasilicates, oxides of periclase structure, spinels, and titarates. It has been shown that directions of conjugation lines between coexisting solid-solution phases provide a powerful tool for determnining relative stabilities of endmember compounds and activity-composition relations of solid solutions. In general, oxide phases (including silicates and titanates) forming complete solid-solution series at high temperatures show less deviations from ideality in their activity- composition relations than do alloy systems. Deviations from ideality are particularly small for many solid solutions of relatively simple structures (e.g. oxides of peniclase structure), whereas those of greater structural complexities (e.g. spinels) show larger deviations from ideality. (auth)
Date: January 1, 1973
Creator: Muan, A.
Partner: UNT Libraries Government Documents Department

Tape casting of magnesium oxide.

Description: A tape casting procedure for fabricating ceramic magnesium oxide tapes has been developed as a method to produce flat sheets of sintered MgO that are thin and porous. Thickness of single layer tapes is in the range of 200-400 {micro}m with corresponding surface roughness values in the range of 10-20 {micro}m as measured by laser profilometry. Development of the tape casting technique required optimization of pretreatment for the starting magnesium oxide (MgO) powder as well as a detailed study of the casting slurry preparation and subsequent heat treatments for sintering and final tape flattening. Milling time of the ceramic powder, plasticizer, and binder mixture was identified as a primary factor affecting surface morphology of the tapes. In general, longer milling times resulted in green tapes with a noticeably smoother surface. This work demonstrates that meticulous control of the entire tape casting operation is necessary to obtain high-quality MgO tapes.
Date: February 1, 2008
Creator: Ayala, Alicia; Corral, Erica L.; Loehman, Ronald E.; Bencoe, Denise Nora; Reiterer, Markus & Shah, Raja A.
Partner: UNT Libraries Government Documents Department

Adaptive Localization Regions for O(N) Density Functional Theory

Description: A linear scaling approach for general and accurate pseudopotential Density Functional Theory calculations is presented. It is based on a Finite Difference discretization. Effective O(N) scaling is achieved by confining the orbitals in spherical localization regions. To improve accuracy and flexibility while computing the smallest possible number of orbitals, we propose an algorithm to adapt localization regions during computation. Numerical results for a polyacethylene chain and a magnesium oxide ring are presented.
Date: January 15, 2008
Creator: Fattebert, J
Partner: UNT Libraries Government Documents Department

Development of Biaxially Textured YBa{sub 2}Cu{sub 3}O{sub 7} Coated Conductors in the U.S.

Description: Two new processes have been under development since 1991 that promise a new, cost-effective way to manufacture flexible, high current density wires made from YBa{sub 2}Cu{sub 3}O{sub 7} (YBCO). The key is to prepare a textured substrate, or ''template,'' on which the YBCO may be deposited as a biaxially aligned thick film. Ion beam assisted deposition (IBAD) of yttria stabilized zirconia or magnesium oxide on alloy tapes enables a final superconducting layer with grain-to-grain, in-plane alignment to within 3-5 degrees. Similar results are achieved on rolling-assisted, biaxially textured substrates (RABiTS) using a variety of oxide layers on textured nickel tapes. The performance of research lengths of prototype wires in strong magnetic fields at 65 K already exceeds that of NbTi and Nb{sub 3}Sn in liquid helium. A scalable, ex-situ process for the YBCO coating has been demonstrated on both types of substrates. Consistent values of critical current density (J{sub c }) greater than 1 x 10{sup 6} A/cm{sup 2} are now obtained on RABiTS, and J{sub c}'s in excess of 2 x 10{sup 6} A/cm{sup 2} have been obtained on both substrates. A nonmagnetic variation of RABiTS (Ni-13% Cr) has also been shown to yield Jc greater than 1.5 x 10{sup 6} A/cm{sup 2} . Six private companies in the U.S. are scaling up YBCO coated conductors for power and physics applications.
Date: November 13, 1999
Creator: Christen, D.K.; Hawsey, R.A. & Kroeger, D.M.
Partner: UNT Libraries Government Documents Department

Isobutanol-methanol mixtures from synthesis gas. Quarterly technical progress report, October 1, 1996--December 30, 1996

Description: A series of MgO-based Cu catalysts have been prepared by coprecipitating the corresponding metal nitrates with a mixed solution of potassium carbonate and potassium hydroxide. The bulk composition of the catalyst has been measured by atomic absorption (AA) analysis and the Cu dispersion has been determined by N{sub 2}O titration at 363 K. Kinetic studies of ethanol coupling reactions on Cu{sub 0.5}Mg{sub 5}CeO{sub x} and 1.0 wt % K-Cu{sub 0.5}Mg{sub 5}CeO{sub x} catalyst indicates that at similar steady-state acetaldehyde concentrations, the presence of K increases the rates of base-catalyzed aldol coupling reactions to acetone and butyraldehyde. Aldol coupling chain growth reaction rates on 1.2 wt % K-Cu{sub 7.5}Mg{sub 5}CeO{sub x} are higher than on 1.0 wt % K-Cu{sub 0.5}Mg{sub 5}CeO{sub x} even though basic site densities are similar on both samples, suggesting that Cu metal sites are also involved in rate-determining steps required for condensation reactions. Cu appears to enhance the desorption of H{sub 2} via the migration of H species from basic to Cu sites and makes the basic sites available for subsequent C-H bond activation steps. Addition of CO{sub 2} decreases the rate of base-catalyzed chain growth reaction to acetone, but does not affect the rate of ethanol dehydrogenation reaction on Cu metal sites
Date: March 1, 1997
Partner: UNT Libraries Government Documents Department

Atomic structure of a symmetric 27{degrees}[001] tilt grain boundary in MgO

Description: The atomic structure of a symmetric 27 degree[001] tilt grain boundary in magnesium oxide has been determined by high-resolution Z-contrast imaging using a 300kV VG HB603U scanning transmission electron microscope with a 1.3 A probe. The atomic configuration in the grain boundary core is found to be considerably less open than the structures proposed for similar materials.
Date: April 1, 1997
Creator: Yan, Yanfa; Chisholm, M.F. & Pennycook, S.J.
Partner: UNT Libraries Government Documents Department

Growth of large single crystals of MgO

Description: The progressive identification of new high-technology applications and requirements for MgO single crystals in the commercial realm, as well as in DOE and other government-agency project areas, has resulted in an increased demand and international market for this material. Specifically, the demand for MgO crystals in large sizes and quantities is presently increasing due to existing and developing applications that include: (a) MgO substrates for the formation of electro-optic thin films and devices, (b) epitaxial substrates for high-temperature thin-film superconducting devices MgO optical components - including high-temperature windows, lenses, and prisms, and (d) specialty MgO crucibles and evaporation sources for thin-film production. In the course of CRADA ORNL92-0091, carried out with Commercial Crystal Laboratories of Naples, Florida as the commercial participant, we have made major progress in increasing the size of single crystals of MgO produced by means of the submerged-arc-fusion technique-thereby increasing the commercial utility of this material. Prior to the accomplishments realized in the course of this CRADA, the only commercially available single crystals of MgO were produced in Japan, Israel, and Russia. The results achieved in the course of CRADA ORNL92-0091 have now led to the establishment of a domestic commercial source of MgO single-crystal substrates and components, and the U.S. is no longer totally dependent on foreign sources of this increasingly important material.
Date: June 12, 1997
Creator: Boatner, L.A. & Urbanik, M.
Partner: UNT Libraries Government Documents Department

Atomic structure of polar ceramic/metal interface: {l_brace}222{r_brace} MgO/Cu

Description: {l_brace}222{r_brace}MgO/Cu is one of the most extensively characterized ceramic/metal interfaces, with atom-probe field-ion-microscopy, Z-contrast scanning-transmission-electron-microscopy (STEM), and spatially-resolved electron-energy-loss-spectroscopy (EELS) measurements recently performed by the present authors, as well as high-resolution electron microscopy (HREM) of this system by others. Atomistic simulations with local density functional theory (LDFT) and molecular dynamics (MD) have been performed to gain additional insight into the structure of this interface. This presentation describes an interface interatomic potential for {l_brace}222{r_brace}MgO/Cu derived from LDFT total energy calculations, and its application to structural properties, including the terminating species, the absence of dislocation standoff, and the symmetry of the interfacial dislocation network.
Date: December 1, 1997
Creator: Benedek, R.; Shashkov, D.A.; Seidman, D.N.; Muller, D.A.; Silcox, J.; Chisholm, M.F. et al.
Partner: UNT Libraries Government Documents Department

Methane coupling by membrane reactor. Quarterly technical progress report, September 25, 1995--December 24, 1995

Description: The performance of the third type of catalytic membrane reactor configuration, with catalyst deposited in the membrane and no catalyst or inert materials in the tube side, was evaluated. The C{sub 2} selectivity obtained was about 10% due to the gas phase reaction in the empty tube side of the reactor. The membrane reactor with an oxygen-permeable dense membrane has been built. The use of a dense membrane will eliminate the loss of hydrocarbon from the tube side to the shell side, as observed in the Vycor glass membrane reactor. Also, air can be used as the oxygen source without contaminating the product. La/MgO was synthesized and will be used as the catalyst for the dense membrane reactor. This catalyst was reported in the literature to show significant improvement of C{sub 2} selectivity and yield for oxidative coupling of methane in a packed-bed reactor by using the operation mode of staged-feed of oxygen. A reactor mode for methane oxidative coupling in reactors with both distributed oxygen feed and C{sub 2} product removal was developed based on the general model of cross-flow reactors reported in the last quarterly report. A distributed oxygen feed could give rise to much higher C{sub 2} yield than the co-feed reactor as long as the space time is long enough. In the case of a two-membrane reactor, where oxygen is supplied by one membrane and products are removed through the other membrane, a high separation factor of C{sub 2} product to methane for the product-removal membrane is critical to achieve high C{sub 2} yield.
Date: March 15, 1996
Partner: UNT Libraries Government Documents Department

Neutron scattering investigations of the dynamics of thin films adsorbed on solid surfaces

Description: While not commonly thought of as surface sensitive probes, high-resolution inelastic and quasielastic neutron scattering (INS) techniques can be used to investigate the dynamical properties of films adsorbed on solid surfaces. These studies are possible because the adsorption takes place on samples which exhibit a large surface-to-volume ratio. By combining dynamical measurements with data from structural, thermodynamic and computer simulation studies it is then possible to develop a comprehensive picture of the microscopic properties of an adsorbed film system. In order to illustrate the power of employing INS techniques we will discuss the continuous melting of monolayer ethylene and the layer-by-layer melting of multilayer argon films on the basal plane of graphite and the rotational tunneling of methane films on the MgO(100) surface.
Date: November 1, 1997
Creator: Larese, J.Z.
Partner: UNT Libraries Government Documents Department

Atomic structure of a polar ceramic/metal interface: {l_brace}222{r_brace}MgO/Cu

Description: {l_brace}222{r_brace}MgO/Cu is one of the most extensively characterized ceramic/metal interfaces, in view of the atom probe field ion microscopy, Z-contrast Scanning Transmission Electron Microscopy (STEM), and spatially resolved Electron energy loss spectroscopy (EELS) measurements performed by the present authors, as well as the high resolution electron microscopy (HREM) of this system by others. Atomistic simulations with local density functional theory (LDFT) and Molecular Dynamics (MD) have been performed to gain additional insight into the structure of this interface. This presentation describes an interface interatomic potential for {l_brace}222{r_brace}MgO/Cu derived from LDFT total energy calculations, and its application to structural properties, including the terminating species, the absence of dislocation standoff, and the symmetry of the interfacial dislocation network.
Date: February 1, 1998
Creator: Benedek, R.; Shashkov, D.A.; Seidman, D.N.; Muller, D.A.; Silcox, J.; Chisholm, M.F. et al.
Partner: UNT Libraries Government Documents Department

Metal-ceramic interfaces: Overlayer-induced reconstruction and magnetism of 4d transition-metal monolayers

Description: Structural, electronic, and magnetic properties of metal-ceramic interfaces, M/MgO(001) (M=Pd, Rh, and Ru), have been investigated using the full potential linearized augmented-plane-wave method. Ru and Rh monolayers are found to be able to retain large spin magnetic moments on MgO(001) (1.95 {mu}{sub B} and 1.21 {mu}{sub B} for Ru and Ph; respectively) -- indicating, in principle, the potential application of MgO(001) as a benign substrate for 4d monolayer magnetism. Significantly, according to our atomic-force determinations, the metal overlayers induce a sizable buckling reconstruction in the interfacial MgO layer, which enhances the M-MgO binding energy by 0.1 eV. The weak M-0 interaction is mainly via tail effects; however, it affects the density of states at the Fermi level for Pd/Mg0(001) significantly and completely eliminates the small magnetic moment of the free Pd monolaver (0.34{mu}{sub B}).
Date: February 15, 1995
Creator: Wu, R. & Freeman, A.J.
Partner: UNT Libraries Government Documents Department


Description: Single crystals of magnesium-aluminate spinel MgAl{sub 2}O{sub 4} were irradiated with 340 keV Xe{sup 2} ions at {minus}173 C ({approximately} 100 K). A fluence of 1 x 10{sup 20} Xe/m{sup 2} created an amorphous layer at the surface of the samples. The samples were annealed for 1 h at different temperatures ranging from 130 C to 880 C. Recrystallization took place in the temperature interval between 610 C and 855 C. Transmission electron microscopy (TEM) images show two distinct layers near the surface: (1) a polycrystalline layer with columnar grain structure; and (2) a buried damaged layer epitaxial with the substrate. After annealing at 1100 C for 52 days, the profile of implanted Xe ions did not change, which means that Xe ions are not mobile in the spinel structure up to 1100 C. The thickness of the buried damaged layer decreased significantly in the 1100 C annealed sample comparing to the sample annealed for 1 h at 855 C.
Date: April 1, 2000
Creator: AFANASYEV, I. & AL, ET
Partner: UNT Libraries Government Documents Department

Solvent Influences on the Molecular Aggregation of Magnesium Aryloxides

Description: Magnesium aryloxides were prepared in a variety of solvents through the reaction of dibutyl magnesium with sterically varied aryl alcohols: 2,6-dimethylphenol (H-DMP), 2,6-diisopropylphenol (H-DIP), and 2,4,6-trichlorophenol (H-TCP). Upon using a sufficiently strong Lewis-basic solvent, the monomeric species Mg(DMP){sub 2}(py){sub 3} (1, py = pyridine), Mg(DIP){sub 2}(THF){sub 3}, (2a, THF = tetrahydrofuran) Mg(TCP){sub 2}(THF){sub 3} (3) were isolated. Each of these complexes possesses a five-coordinate magnesium that adopts a trigonal bipyramidal geometry. In the absence of a Lewis base, the reaction with H-DIP yields a soluble trinuclear complex, [Mg(DIP){sub 2}]{sub 3} (2b). The Mg metal centers in 2b adopt a linear arrangement with a four-coordinate central metal while the outer metal centers are reduced to just three-coordinate. Solution spectroscopic methods suggest that while 2b remains intact, the monomeric species (1, 2a, and 3) are involved in equilibria, which facilitate intermolecular ligand transfer.
Date: July 14, 2000
Partner: UNT Libraries Government Documents Department

First-principles simulations and Z-contrast imaging of impurities at <001> tilt grain boundaries in MgO

Description: First-principles density-functional calculations were used to study the effects of CA impurities on the {Sigma} = 5 (310) <001> tilt grain boundaries in MgO. An equilibrium structure and two metastable structures of the grain boundaries in pure MgO have been established. The calculations further demonstrated that Ca impurities segregate at particular sites in the metastable grain boundary and induce a structural transformation. This result is consistent with atomic resolution Z-contrast imaging. The calculations also found that the impurities at the grain boundaries do not induce states in the band gap. The mechanism of the transformation is also discussed.
Date: December 1, 1997
Creator: Yan, Y.; Chisholm, M.F.; Pennycook, S.J.; Duscher, G.; Maiti, A. & Pantelides, S.T.
Partner: UNT Libraries Government Documents Department


Description: Neutron inelastic scattering measurements of the polar TO phonon mode in the cubic relaxor Pb(Mg{sub 1/3}Nb{sub 2/3})O{sub 3}, at room temperature, reveal anomalous behavior similar to that recently observed in Pb(Zn{sub 1/3}Nb{sub 2/3}){sub 0.92}Ti{sub 0.08}O{sub 3} in which the optic branch appears to drop precipitously into the acoustic branch at a finite value of the momentum transfer q = 0.2 {angstrom}{sub {minus}1}, measured from the zone center. By contrast, a recent neutron study indicates that PMN exhibits a normal TO phonon dispersion at 800 K. The authors speculate this behavior is common to all relaxor materials, and is the result of the presence of nanometer-scale polarized domains in the crystal that form below a temperature T{sub d}, which effectively prevent the propagation of long wavelength (q = 0) phonons.
Date: March 9, 2000
Partner: UNT Libraries Government Documents Department