The increasing importance of powder materials fabrication by use of the HIP has led to recent emphasis on analytical techniques for describing and understanding the process. Modeling of particle adhesion and consolida...
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The increasing importance of powder materials fabrication by use of the HIP has led to recent emphasis on analytical techniques for describing and understanding the process. Modeling of particle adhesion and consolidation during the HIP process has been achieved through the characterization of the particle system densiflcation behavior by the deformation of a representative particle due to forces transmitted through its interparticle contacts. Instead of assuming that all of these forces are equal, a model is developed based on forces which vary to produce a uniform rate of contraction (densiflcation) across a compact. Initial parameters of the systems are obtained from radial distribution data available for monosized and bimodal-sized particle systems. Deformation mechanism maps are predicted for superalloys under temperature conditions of 873–1373 K and pressure conditions of 1–1000 MPa by coupling the equations for athermal yield and power-law creep densification mechanisms. The deformation of large vs small particles in a bimodal-sized particle system is also analyzed and assumptions important in these models are discussed. L'importance croissante de la fabrication de matériaux à partir de poudres en utilisant la pression isostatique à chaud (PIC) a conduit à un intérêt récent pour les techniques analytiques décrivant et expliquant ce procédé. La modélisation de la consolidation et de l'adhésion de particules pendant le procédé PIC à été réalisée grâce à la caractérisation du comportement du système de densification des particules par a déformation d'une particule représentative, due aux forces transmises par contacts interparticulaires. Au lieu de considérer toutes ces forces égales, on développe un modèle basé sur des forces qui varient de façon à générez un taux uniforme de contraction (densification) sur le système compact. Les paramètres initiaux des systèmes sont obtenus à partir des distributions radiales des systèmes de particules à une seule taille et b
The photoelastic stress-freezing technique is applied to observe the stress distribution inside two spheres of different sizes compressed together elastically. After the stress is frozen in, thin slices of the materia...
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The photoelastic stress-freezing technique is applied to observe the stress distribution inside two spheres of different sizes compressed together elastically. After the stress is frozen in, thin slices of the material containing the symmetry axis are prepared for observation through a polariscope. The stress distribution is compared with both the finite element numerical analysis and the Hertz analytical theory which is limited to small deformations. Among the three, the agreement between the experimental results and the finite element analysis is the best. The deviation from the Hertz theory is less in the larger sphere contacting a smaller one than in the smaller sphere contacting a larger one.
References1. H.D. Keith and F.J. Padden. J. Appl. Phys. 34 (1963), pp. 2409–2421. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (212)H.D. Keith and F.J. Padden. J. Appl. Phys. 35 (1964), pp. 127...
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References
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Blade coating is the most common method of coating paper, and it is applied in coating magnetic suspensions and adhesives. Typically, the blade's upstream edge is clamped into a rigid holder so that the blade and ...
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Blade coating is the most common method of coating paper, and it is applied in coating magnetic suspensions and adhesives. Typically, the blade's upstream edge is clamped into a rigid holder so that the blade and substrate form a converging wedge. The translating substrate drags liquid into the wedge, where liquid forces develop and deflect the elastic blade, but it is loaded externally to counteract the deflection. Liquid dragged past the blade and thus the coating thickness depend on the elastohydrodynamic interaction among blade, liquid, and loading. Shell theory and lubrication flow theory describe the interaction by means of differential equations that are shown to be equivalent to Saita and Scriven's (1985) earlier formulation but that lead to a computationally more efficient analysis. Predictions computed agree with those of the earlier formulation and with experiments. Moreover, the new predictions illustrate effects of normal force loading and broaden the theory of blade coating.
Small-Angle X-ray Scattering (SAXS) has been used as an analytical tool to study the development of microstructure with increasing Heat Treatment Temperature (HTT) of a carbonized phenolic resin, which is often used a...
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Small-Angle X-ray Scattering (SAXS) has been used as an analytical tool to study the development of microstructure with increasing Heat Treatment Temperature (HTT) of a carbonized phenolic resin, which is often used as matrix in commercial carbon-carbon composites. Processing parameters-heating rate and maximum heat treatment temperature-were the primary variables of interest. Results from SAXS have led to the consideration of a model for microstructure development in carbonized phenolic resins. Local density variations on the 10 angstrom-100 angstrom scale seem to be the predominant feature of these carbonized resins in the temperature range studied (450-900-degrees-C). Heat treatment results in a gradual destruction of crosslinks, which leads to clustering of the aromatic units. These clusters enclose pores, the high concentrations of which give rise to scattering maxima in SAXS data. Concentration and dimensions of pores are sensitive both to the heating rate used and the maximum carbonization temperature.
The stress field of an edge dislocation near a finite length crack depends on where the dislocation is originated. If the dislocation is emitted from the crack which is originally stress-free, the image force increase...
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The stress field of an edge dislocation near a finite length crack depends on where the dislocation is originated. If the dislocation is emitted from the crack which is originally stress-free, the image force increases with decreasing crack length. But the opposite is true if the dislocation is originated from elsewhere. At the same distance from the crack tip, the image force for the former is always larger than that for the latter. They become equal only if the crack is semi-infinitely long. If the dislocation is emitted from one of the crack tips, it generally will shield this crack tip but antishield the other tip. However the dislocation will shield or antishield both crack tips if it is originated from elsewhere.
The nature of the surface basicity of two series of highly pure, chemically and/or thermally pretreated carbons has been investigated by (a) HCl adsorption, (b) stepped temperature-programmed desorption, (c) electroph...
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The nature of the surface basicity of two series of highly pure, chemically and/or thermally pretreated carbons has been investigated by (a) HCl adsorption, (b) stepped temperature-programmed desorption, (c) electrophoresis, and (d) mass titration. Following a comprehensive review of the pertinent literature, it is concluded that two mechanisms are necessary and sufficient to account for the basic properties of carbons. For carbons having low oxygen contents, molar HCl/O adsorption ratios exceeding two indicate that an oxygen-free type of site is involved in the adsorption process. Evidence favoring electron donor-acceptor (EDA) interactions of the type C(pi) + H3O+ double-line arrow pointing left and right C(pi)-H3O+ is presented. The postulated C(pi) site and its interaction with adsorbed H3O+ ions is described in detail. As the oxygen content of the carbon increases, the molar HCl/O ratios decrease sharply and level off at nonzero values, which are small due to the presence of nonbasic surface oxides. When these are removed by heat treatment (at 1073 K), the molar HCl/O adsorption ratio is again found to exceed two at low oxygen coverages, but levels off at ca. 1 for higher oxygen coverages. Of all the basic surface oxides proposed in the literature, only the pyrone-type groups can account for this HCl/O ratio. Therefore, it is proposed that the basicity of carbon surfaces arises from a combination of EDA and pyrone-type interactions. The predominance of either will be dictated by the oxygen content of the carbon in question.
Experimental evidence of cooperative behavior during the ferroelectric phase transitions in granular and powder KNO3 at atmospheric pressure is presented. Three different experimental studies were performed in which p...
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Experimental evidence of cooperative behavior during the ferroelectric phase transitions in granular and powder KNO3 at atmospheric pressure is presented. Three different experimental studies were performed in which phase transitions were detected and characterized by heat flow calorimetry: (1) the distribution of SiC powder in granular KNO3 was varied;(2) the volume fraction of SiC in powdered KNO3 was varied;and (3) pure KNO3 powder was thermally cycled. All three studies provided evidence of cooperative behavior between the KNO3 particles during the III-II phase transition. The cooperative behavior reduced the temperature range of phase III stability from approximately 97-124-degrees-C to that characteristic of bulk material (approximately 110-124-degrees-C). Separate KNO3 particles behaved as individual ferroelectric domains, with each particle making the phase transition independently near the expected Curie temperature. Particles of KNO3 in intimate physical contact tended to behave cooperatively as a single large ferroelectric domain leading to sharper phase transitions more characteristic of single crystals. The degree of cooperative behavior was dependent upon the extent to which the individual particles were in physical contact. The absence of the III-II phase transition in KNO3 powder that has been reported in the literature can be understood from the results obtained using SiC powder to separate KNO3 particles during heat flow calorimetry measurements.
The gasification reactivity profiles of different carbons (chars) are analyzed from both a theoretical and an experimental point of view. The virtues of and/or problems with utilizing the concepts of total surface are...
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The gasification reactivity profiles of different carbons (chars) are analyzed from both a theoretical and an experimental point of view. The virtues of and/or problems with utilizing the concepts of total surface area (TSA), active surface area (ASA), and reactive surface area (RSA) to explain or predict gasification rate variations with conversion are discussed. An analysis of several models of char gasification kinetics which predict the evolution of TSA with conversion revealed that the experimentally observed reactivity variations with conversion in carbon dioxide may be explained using just the initial properties of the char. An experimental investigation of char gasification in carbon dioxide using the transient kinetics approach gave a direct measurement of RSA. Gasification rates normalized with respect to RSA were essentially constant over the entire conversion range. A temperature-programmed desorption technique was also used to determine the amount of reactive surface intermediate formed on these chars during gasification in carbon dioxide; the results were in agreement with those obtained from transient kinetics. The application of these two independent but complementary techniques provided the heretofore elusive quantitative experimental explanation of reactivity variations with conversion for char gasification in carbon dioxide.
An investigation of the influence of crystalline morphology on fatigue crack propagation (FCP) resistance in a slightly branched polyethylene is presented. Various thermal histories have been utilized to generate samp...
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An investigation of the influence of crystalline morphology on fatigue crack propagation (FCP) resistance in a slightly branched polyethylene is presented. Various thermal histories have been utilized to generate samples with different crystalline microstructures and the samples were characterized thoroughly using standard methods. Estimation of tie molecule densities was obtained from measurements of brittle fracture stress. Differences in FCP behaviour for the quenched and annealed samples were shown to be dictated by a competing effect between the degree of crystallinity and tie molecule density. Further, larger spherulite size and distribution appeared to have a deleterious effect on fatigue properties. In general, crystalline microstructure is shown to have a significant influence on fatigue crack propagation behaviour.
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