The Experts below are selected from a list of 23724 Experts worldwide ranked by ideXlab platform
Jian Luo - One of the best experts on this subject based on the ideXlab platform.
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vanadia based Equilibrium Thickness amorphous films on anatase 101 surfaces
Applied Physics Letters, 2007Co-Authors: Haijun Qian, Jian LuoAbstract:Nanometer-thick, surficial amorphous films are found to form in a model “monolayer” catalyst system: vanadia on TiO2 anatase (101) surfaces. These films exhibit a self-selecting or “Equilibrium” Thickness; once a thermodynamic Equilibrium is reached, the film Thickness, which corresponds to the Gibbsian surface excess of vanadia adsorbates, is independent of synthesis methods, the fraction of secondary vanadia phase, and the heat treatment history. These (multilayer) adsorbate films are largely amorphous (quasiliquid) at subeutectic temperatures, where analogies to premelting and prewetting phenomena are made. Reversible film Thickness versus temperature (with a hysteresis loop) is observed and explained from a force-balance model.
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thin intergranular films and solid state activated sintering in nickel doped tungsten
Acta Materialia, 2007Co-Authors: Vivek Gupta, Danghyok Yoon, Harry M Meyer, Jian LuoAbstract:Nickel-doped tungsten specimens were prepared with high purity chemicals and sintered. Although activated sintering starts more than 400 °C below the bulk eutectic temperature, the nickel-rich crystalline secondary phase does not wet the tungsten grain boundaries in the solid state. These results contrast with the classical activated sintering model whereby the secondary crystalline phase was presumed to wet grain boundaries completely. High resolution transmission electron microscopy and Auger electron spectroscopy revealed the presence of nanometer-thick, nickel-enriched, disordered films at grain boundaries well below the bulk eutectic temperature. These interfacial films can be regarded as metallic counterparts to widely observed Equilibrium-Thickness intergranular films in ceramics. Assuming they form at a true thermodynamic Equilibrium, these films can alternatively be understood as a class of combined grain boundary disordering and adsorption structures resulting from coupled premelting and prewetting transitions. It is concluded that enhanced diffusion in these thin intergranular films is responsible for solid-state activated sintering.
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Equilibrium Thickness amorphous films on surfaces of bi2o3 doped zno
Journal of The European Ceramic Society, 1999Co-Authors: Jian Luo, Yetming ChiangAbstract:Abstract Model experiments and thermodynamic calculations revealed that a nanometer-thick liquid/amorphous film is the Equilibrium configuration of the {1 1 2 0} surfaces of Bi 2 O 3 -doped ZnO. In samples equilibrated both above and below the eutectic temperature, bismuth enriched amorphous films were observed. The film Thickness is fairly uniform. The average Thickness of ninety-seven films formed at 780°C (above the eutectic temperature, 740°C) is 1·54 nm, with a narrow standard deviation of 0·28 nm. In addition, the Thickness was found to be independent of firing time or second phase fraction. Therefore, we conclude that the surface film has an ‘Equilibrium-Thickness.’ A thermodynamic model is presented. This new observation may have relevance to the understanding and control of surface coatings in ceramics and related properties.
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origin of solid state activated sintering in bi2o3 doped zno
Journal of the American Ceramic Society, 1999Co-Authors: Jian Luo, Haifeng Wang, Yetming ChiangAbstract:Activated sintering in Bi2O3-doped ZnO has been studied with emphasis on the mechanistic role of intergranular amorphous films. The atomic-level microstructures and bismuth solute distributions in doped powders have been investigated using high-resolution electron microscopy and scanning transmission electron microscopy. Densification is observed to be significant below the bulk eutectic temperature in the presence of Bi2O3 concentrations as low as 0.58 mol%. Transmission electron microscopy of as-calcined and sintered powders shows that significant neck growth and particle coarsening occur in the solid state. Intergranular amorphous films of ∼1 nm Thickness, terminating in wetting menisci at sinter-necks, are observed to form concurrently with the onset of activated sintering. In a few instances, amorphous films are also observed at surfaces of the ZnO particles. These films appear to be the free-surface counterpart to Equilibrium-Thickness intergranular films. Activated sintering in this binary system is attributed to rapid mass transport through subeutectic, Equilibrium-Thickness intergranular films, with the amorphous phase also providing capillary pressure.
Yetming Chiang - One of the best experts on this subject based on the ideXlab platform.
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Equilibrium Thickness amorphous films on surfaces of bi2o3 doped zno
Journal of The European Ceramic Society, 1999Co-Authors: Jian Luo, Yetming ChiangAbstract:Abstract Model experiments and thermodynamic calculations revealed that a nanometer-thick liquid/amorphous film is the Equilibrium configuration of the {1 1 2 0} surfaces of Bi 2 O 3 -doped ZnO. In samples equilibrated both above and below the eutectic temperature, bismuth enriched amorphous films were observed. The film Thickness is fairly uniform. The average Thickness of ninety-seven films formed at 780°C (above the eutectic temperature, 740°C) is 1·54 nm, with a narrow standard deviation of 0·28 nm. In addition, the Thickness was found to be independent of firing time or second phase fraction. Therefore, we conclude that the surface film has an ‘Equilibrium-Thickness.’ A thermodynamic model is presented. This new observation may have relevance to the understanding and control of surface coatings in ceramics and related properties.
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origin of solid state activated sintering in bi2o3 doped zno
Journal of the American Ceramic Society, 1999Co-Authors: Jian Luo, Haifeng Wang, Yetming ChiangAbstract:Activated sintering in Bi2O3-doped ZnO has been studied with emphasis on the mechanistic role of intergranular amorphous films. The atomic-level microstructures and bismuth solute distributions in doped powders have been investigated using high-resolution electron microscopy and scanning transmission electron microscopy. Densification is observed to be significant below the bulk eutectic temperature in the presence of Bi2O3 concentrations as low as 0.58 mol%. Transmission electron microscopy of as-calcined and sintered powders shows that significant neck growth and particle coarsening occur in the solid state. Intergranular amorphous films of ∼1 nm Thickness, terminating in wetting menisci at sinter-necks, are observed to form concurrently with the onset of activated sintering. In a few instances, amorphous films are also observed at surfaces of the ZnO particles. These films appear to be the free-surface counterpart to Equilibrium-Thickness intergranular films. Activated sintering in this binary system is attributed to rapid mass transport through subeutectic, Equilibrium-Thickness intergranular films, with the amorphous phase also providing capillary pressure.
Roger G Horn - One of the best experts on this subject based on the ideXlab platform.
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possible electrical double layer contribution to the Equilibrium Thickness of intergranular glass films in polycrystalline ceramics
Journal of the American Ceramic Society, 1993Co-Authors: David R Clarke, Thomas M Shaw, Albert P Philipse, Roger G HornAbstract:The plausibility of the entropic repulsion of electrical double layers acting to stabilize an Equilibrium Thickness of intergranular glass films in polycrystalline ceramics is explored. Estimates of the screening length, surface potential, and surface charge required to provide a repulsive force sufficiently large to balance the attractive van der Waals and capillary forces for observable Thicknesses of intergranular film are calculated and do not appear to be beyond possibility. However, it has yet to be established whether crystalline particles in a liquid-phase sintering medium possess an electrical double layer at high temperatures. If they do, such a surface charge layer may well have important consequences not only for liquid-phase sintering but also for high-frequency electrical properties and microwave sintering of ceramics containing a liquid phase.
Erik Nies - One of the best experts on this subject based on the ideXlab platform.
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a polyethylene chain investigated with replica exchange molecular dynamics simulation Equilibrium lamellar Thickness and melting point ordering and free energy
Polymer, 2010Co-Authors: Zhouting Jiang, Erik NiesAbstract:Abstract In the present work, a polyethylene chain with N = 200 CH 2 units was simulated using replica exchange molecular dynamics (REMD). Simulations were performed in a broad temperature range and for intra-chain interactions varying from the fully interacting to the ideal spring chain. Our work demonstrates that REMD is a very efficient method to obtain Equilibrium data. It is found that the coil-to-globule transition is dominated by the vdW energy, whereas the globule-to-folded chain transition is accompanied by transitional behavior in the torsion and vdW energies. Our data clearly show that for the chain length considered here, the chain folded crystal to globule transition is a continuous transition. Nevertheless, we can establish with good accuracy the Equilibrium transition temperature for the chain folded crystal to globule transition. A set of orientational order parameters was used to investigate the order in the polymer chain. At the globule-to-folded chain transition an abrupt change in the value of the order parameter is observed, whereas there is no or almost no change in the value of the order parameter at the coil-to-globule transition temperature. The (apparent) order in the disordered globular and coiled states indicated by some studied order parameters is related to the definition of the order parameter and depends on the chain length of the polymer. Below the Equilibrium melting temperature the (largest principal component of the) radius of gyration and the Equilibrium lamellar Thickness of the folded chain crystal decrease with increasing temperature, which gives support to the theory of Muthukumar but is opposite to the prediction of classical crystallization theories. The agreement between simulations and theory may hint to universal behavior of the relative Equilibrium Thickness versus the relative super cooling.
D T Wasan - One of the best experts on this subject based on the ideXlab platform.
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novel approach for calculating the Equilibrium foam nanofilm meniscus contact angle and the film free energy
Journal of Colloid and Interface Science, 2019Co-Authors: Alex Nikolov, Pingkeng Wu, D T WasanAbstract:Abstract The film meniscus is a capillary system that is part of everyday observed phenomena, such as in foams, emulsions, liquid suspensions of nanoparticles (nanofluids), and liquid-wetting solids. The capillarity of a microscopic free foam lamella with a meniscus is important for a fundamental understanding of the role of the surface forces vs. Thickness and stability of dispersed systems. The film-meniscus transition region, known as the Gibbs-Plateau border, and macroscopic contact angle, defined by the extrapolated meniscus Laplace surfaces, are the characteristics of capillary systems that reveal how the surface forces contribute to the stability of the dispersed systems. The foam nanofilm formed from a nanofluid due to nanoparticle self-layering under the film surface confinement thins in a multiple regular stepwise manner (not like soap films) above the CMC. The Equilibrium Thickness of the nanofilm is governed by the film area rather than the capillary pressure, as was reported for common and Newtonian films. Our video clip shows that the nanofilm thins layer by layer as the film area decreases. Our observation reveals that the nanofilm with a small film area remains at the Equilibrium Thickness with several layers. An iterative method is proposed to locate the film meniscus contact line. The film-meniscus profile of the transition region is examined using the reflected light interferometry and by applying the two radii of curvature. The micro- and macroscopic contact angles between nanofilm and meniscuses are calculated. The foam nanofilm’s structural free energy is calculated vs. the number of layers. The knowledge gained from this research will help to improve our understanding of the dispersion stability of foams, emulsions, and liquid suspensions of nanoparticles.