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Thermodynamics and mechanisms of sintering

Description: A phenomenological overview and exploration of the thermodynamic and geometric factors play a role in the process of densification of model compact systems consisting of crystalline spheres of uniform size in regular and irregular packing that form grain boundaries at every contact point. A further assumption is the presence of isotropic surface and grain boundary energies. Although such systems are unrealistic in comparison with normal powder compacts, their potential sintering behavior can be… more
Date: October 1, 1978
Creator: Pask, J.A.
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Partner: UNT Libraries Government Documents Department
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Fundamentals of wetting and bonding between ceramics and metals

Description: The conditions and the reactions that lead to the realization of chemical equilibrium at interfaces and thus to chemical bonding is discussed with specific attention to the platinum/glass and gold/glass systems. Wetting of the solid by the liquid and formation of an interface are part of the problem. Sessile drop experiments and their interpretation are included. 10 figs, 12 refs. (GHT)
Date: January 21, 1977
Creator: Pask, J. A.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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Influence of surface silica impurities on the sintering behavior of alumina powders

Description: The filtering, pressing and sintering behavior of an alumina powder with and without silica impurity on surface has been studied. When silica is removed from surface by HF-treatment and compactibility is considerably enhanced due to the different acidity/basicity nature of the OH of the hydroxyl layers. The silica impurity decreases the activation energy of the initial stage of sintering and grain boundary energy, improves the densification process and exerts a moderate inhibitor effect on the … more
Date: February 1, 1980
Creator: Moya, J.S. & Pask, J.A.
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Partner: UNT Libraries Government Documents Department
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Wetting, spreading and reactions at liquid/solid interfaces. [Glass/metal]

Description: Chemical (thermodynamic) stable equilibrium at glass-metal interfaces is established when the liquid glass becomes saturated with the low valence oxide of the metal. Stable or metastable equilibrium conditions at glass-metal interfaces result in a constant contact angle in a sessile drop experiment. If chemical equilibrium is absent, then a driving force exists for the occurrence of a reaction to attain equilibrium. All reactions that take place contribute to the maintenance of the necessary sa… more
Date: July 1, 1980
Creator: Pask, J.A. & Tomsia, A.P.
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Partner: UNT Libraries Government Documents Department
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Direct observation of densification and grain growth in a W--Ni alloy

Description: Densification and grain growth in a tungsten--nickel alloy containing 32 vol % of liquid at 1550/sup 0/C were studied by conventional methods aided by hot stage scanning electron microscopy and cinematography. This technique yields important additional qualitative information on the mechanisms. Two stages can be discerned. In stage 1, essentially complete pore elimination, rapid grain growth and adjustment of microstructural geometry take place. In the second stage, microstructure coarsening oc… more
Date: April 1, 1979
Creator: Riegger, H.; Pask, J.A. & Exner, H.E.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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Influence of grain boundary silica impurity on alumina toughness

Description: In a series of previous reports the effect of silica impurity on aggregation state and on electropheretic, pressing, filtering and sintering behavior on alumina powders was presented. The results obtained showed that the silica surface impurity plays an important role in the ceramic processing of powders by (a) decreasing the pH values of the isoelectric point (i.e.p.), which affects the aggregation state of the powder, and (b) decreasing the compactability and the activation energy for the ini… more
Date: August 1, 1980
Creator: Moya, J.S.; Kriven, W.M. & Pask, J.A.
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Partner: UNT Libraries Government Documents Department
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Theory of Positron Annihilation in Helium-Filled Bubbles in Plutonium

Description: Positron annihilation lifetime spectroscopy is a sensitive probe of vacancies and voids in materials. This non-destructive measurement technique can identify the presence of specific defects in materials at the part-per-million level. Recent experiments by Asoka-Kumar et al. have identified two lifetime components in aged plutonium samples--a dominant lifetime component of around 182 ps and a longer lifetime component of around 350-400ps. This second component appears to increase with the age o… more
Date: February 13, 2003
Creator: Sterne, P A & Pask, J E
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Partner: UNT Libraries Government Documents Department
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Real-space formulation of the electrostatic potential and total energy of solids

Description: We develop expressions for the electrostatic potential and total energy of crystalline solids which are amenable to direct evaluation in real space. Unlike conventional reciprocal space formulations, no Fourier transforms or reciprocal lattice summations are required, and the formulation is well suited for large-scale, parallel computations. The need for reciprocal space expressions is eliminated by replacing long-range potentials by equivalent localized charge distributions and incorporating l… more
Date: May 12, 2004
Creator: Pask, J E & Sterne, P A
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Partner: UNT Libraries Government Documents Department
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Large-scale quantum mechanical simulations of high-Z metals

Description: High-Z metals constitute a particular challenge for large-scale ab initio calculations, as they require high resolution due to the presence of strongly localized states and require many eigenstates to be computed due to the large number of electrons and need to accurately resolve the Fermi surface. Here, we report recent findings on high-Z materials, using an efficient massively parallel planewave implementation on some of the largest computational architectures currently available. We discuss … more
Date: January 3, 2007
Creator: Yang, L H; Hood, R; Pask, J & Klepeis, J
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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Partition-of-Unity Finite-Element Method for Large Scale Quantum Molecular Dynamics on Massively Parallel Computational Platforms

Description: Over the course of the past two decades, quantum mechanical calculations have emerged as a key component of modern materials research. However, the solution of the required quantum mechanical equations is a formidable task and this has severely limited the range of materials systems which can be investigated by such accurate, quantum mechanical means. The current state of the art for large-scale quantum simulations is the planewave (PW) method, as implemented in now ubiquitous VASP, ABINIT, and… more
Date: February 28, 2011
Creator: Pask, J. E.; Sukumar, N.; Guney, M. & Hu, W.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department
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HIGH TEMPERATURE STRESS-STRAIN BEHAVIOR OF MgO IN COMPRESSION

Description: Compressive stress-strain curves for several types of polycrystalline MgO specimens were correlated with those for single crystals and analyzed as a function of grain size and grain-boundary character at 1200 and 1400 C for several strain rates. The results for fully dense specimens were explained in terms of grain-boundary sliding and intergranular separation in addition to slip. The modification of grain-boundary nature concurrent with heat treatment for grain growth, caused by residual LUF, … more
Date: January 1, 1979
Creator: Dokko, P.C & Pask, J.A.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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Calculation of positron observables using a finite-element-based approach

Description: We report the development of a new method for calculating positron observables using a finite-element approach for the solution of the Schrodinger equation. This method combines the advantages of both basis-set and real-space-grid approaches. The strict locality in real space of the finite element basis functions results in a method that is well suited for calculating large systems of a thousand or more atoms, as required for calculations of extended defects such as dislocations. In addition, t… more
Date: November 4, 1998
Creator: Klein, B. M.; Pask, J. E. & Sterne, P.
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Partner: UNT Libraries Government Documents Department
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Theoretical investigations of two Si-based spintronic materials

Description: Two Si-based spintronic materials, a Mn-Si digital ferromagnetic heterostructure ({delta}-layer of Mn doped in Si) with defects and dilutely doped Mn{sub x}Si{sub 1-x} alloy are investigated using a density-functional based approach. We model the heterostructure and alloy with a supercell of 64 atoms and examine several configurations of the Mn atoms. We find that 25% substitutional defects without vacancies in the {delta} layer diminishes half metallicity of the DFH substantially. For the allo… more
Date: August 3, 2007
Creator: Fong, C Y; Snow, R; Shaughnessy, M; Pask, J E & Yang, L H
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Partner: UNT Libraries Government Documents Department
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Electronic and magnetic properties of zincblende half-metal superlattices

Description: Zincblende half-metallic compounds such as CrAs, with large magnetic moments and high Curie temperatures, are promising materials for spintronic applications. They explore layered materials, consisting of alternating layers of zincblende half-metals, by first principles calculations, and find that superlattices of (CrAs){sub 1}(MnAs){sub 1} and (CrAs){sub 2}(MnAs){sub 2} are half-metallic with magnetic moments of 7.0{mu}{sub B} and 14.0{mu}{sub B} per unit cell, respectively. They discuss the n… more
Date: November 5, 2003
Creator: Fong, C Y; Qian, M C; Pask, J; Yang, L H & Dag, S
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Partner: UNT Libraries Government Documents Department
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Design of Spintronic Materials with Simple Structures

Description: A brief comparison of conventional electronics and spintronics is given. The key features of half metallic binary compounds with the zincblende structure are presented, using MnAs as an example. We discuss the interactions responsible for the half metallic properties. Special properties of superlattices and a digital ferromagnetic heterostructure incorporating zincblende half metals are also discussed.
Date: May 3, 2007
Creator: Fong, C Y; Qian, M C; Liu, K; Yang, L H & Pask, J E
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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Spin-polarized ballistic transport channel in a thin superlattice composed of zincblende half-metallic compounds

Description: The authors examine theoretically conduction processes near the Fermi energy of thin layers of zincblende structure half metals, using as an example a superlattice consisting of monolayers of GaAs and MnAs, a bilayer of CrAs, and a bilayer of GaAs. By artificially separating bilayers, they show that non-fourfold coordinated Cr states thwart half metallicity. However, capping the metal-As bilayers restores half metallicity and ballistic conduction of electrons around 0.3 eV above the Fermi level… more
Date: May 11, 2004
Creator: Qian, M C; Fong, C Y; Pickett, W E; Yang, L H; Pask, J E & Dag, S
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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Half Metallic Digital Ferromagnetic Heterostructure Composed of a (Delta)-doped Layer of Mn in Si

Description: The authors propose and investigate the properties of a digital ferromagnetic heterostructure (DFH) consisting of a {delta}-doped layer of Mn in Si, using ab initio electronic-structure methods. They find that (1) ferromagnetic order of the Mn layer is energetically favorable relative to antiferromagnetic, and (2) the heterostructure is a two-dimensional half metallic system. The metallic behavior is contributed by three majority-spin bands originating from hybridized Mn-d and nearest-neighbor … more
Date: May 30, 2006
Creator: Qian, M C; Fong, C Y; Liu, K; Pickett, W E; Pask, J E & Yang, L H
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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