Diffusion and interdiffusion in Zn-disordered AlAs-GaAs superlattices

Several superlattices (SL's) with different layer thicknesses, grown by molecular beam epitaxy (MBE), were disordered via low temperature (550?600°C) Zn diffusions to investigate layer thickness effects on both the Zn diffusion process and the Al-Ga interdiffusion process. The Zn diffusion coefficients were measured using secondary ion mass spectroscopy (SIMS) and Auger electron spectroscopy (AES) and found to be ?10^?12 cm²/sec, increasing somewhat with decreasing layer thickness. The activation energy for the Zn diffusion process ranged from 3.1 eV for an 1100Å/period SL to 2.1 eV for a 320Å/period SL. The Al-Ga interdiffusion coefficient and the activation energy associated with the interdiffusion process were calculated from AES depth profiles. The coefficient is on the order of 10^?16 cm²/sec and the activation energy is approximately 1 eV, independent of the SL layer thickness.     

Correlation of the transport properties of simple fluids at low temperatures and high pressures based on the generalized Eucken relation and the molecular dynamics of hard sphere fluid

A generalized correlation is developed for the viscosity and thermal conductivity of isotropic fluids under high pressures (up to 200 MPa) and low temperatures (down to 85 K). Two known observations have been taken into consideration in the development of the correlation. First, the Alder correction factors for the Enskog theory values of transport coefficients obtained from molecular dynamics simulations for hard sphere fluids are incorporated. The inclusion of these corrections in the theory makes it possible to describe correctly the density dependence of the hard sphere viscosity and thermal conductivity at high pressures. The hydrodynamic cage effect, which is manifested in the molecular motions of dense fluid systems, is thus correctly accounted for. Second, the generalized Eucken relation, which relates the thermal conductivity to the viscosity, is incorporated. As a consequence, an internally consistent correlation is obtained, which can adequately predict the behavior of the thermal conductivity from given values of viscosity. Tests on simple fluids, such as argon, krypton, etc., show that the correlation is valid within a few percent for the entire fluid range where experimental data are available for comparison, and also along the vapor-liquid saturation line, with the exclusion of the critical region. Furthermore, since the variables appearing in the theory are in reduced form, a corresponding states correlation is established for isotropic fluids.     

An analysis for estimating the probability of particle coalescence in liquid phase sintered systems

During liquid phase sintering solid particles move about within the liquid and make contact with each other. Microstructural examination of liquid phase sintered alloys clearly indicates that some of these contacts lead to particle coalescence even in systems in which wetting of the solid by the liquid is presumed to occur. Under these circumstances, the grain boundary formed by the particles must have a rather low energy and hence, misorientation. In this study, attention is directed at the problem of calculating the probability of a collision between particles that lead to the formation of a low angle grain boundary and hence, coalescence. By making simple but physically plausible assumptions about the nature of low angle grain boundaries, the coalescence probabilityp _c can be determined as a function of a parameterθ where θ = withγ _SL =solid-liquid surface energy,γ _ss = average solid-solid grain boundary energy and У is the angular extent of the energy cusp associated with a low angle grain boundary. A simple analytical expression ofp _c ,p _c = 0.1θ (1-cosθ) is derived and found to be in excellent agreement with other techniques for calculatingp _c for values ofθ ≲ 10 deg, the range most probable for liquid phase sintered alloys. Some discussion of the accuracy and assumptions of the present model is also given.     

Symbolic gray code as a multikey hashing function

In this paper, we extend the binary Gray code to symbolic Gray code. We then show that this symbolic Gray code can be used as a multikey hashing function for storing symbolic records. The record stored at location k and the record stored at location k + 1 will be nearest neighbors if this hashing function is used. Thus, this symbolic Gray code hashing function exhibits some kind of clustering property which will group similar records together. This property is a desirable property for designing nearest neighbor searching (also called best match searching) systems. There are many other interesting properties of this hashing function. For instance, there exists an address-to-key transformation which can be used to determine the record stored at certain location k if this hashing function is used. Besides, if there are totally M records, we only have to reserve exactly M locations; there are no collisions and wasting of memory storage. At the end of this paper, it is shown that the resulting file exhibits the multiple-attribute tree structure.     

Mechanisms of cyclic softening in precipitation-hardening alloys—A ball model approach and tests at 78 K

Tests of random, alternating cuts on a ball-model of in Al–Cu alloy have been run to check the kinetics at which disordering occurs in an initially ordered precipitate subject to cyclic strain. In addition, fatigue tests at low temperature (78 K) and various microscopical observations have been made on Al–Ag alloy containing GP zones to check whether a structure containing an initially disordered precipitate will cyclically soften or not. It was thus found that the ball model disorders as a function of accumulated plastic strain consistently with the kinetics of cyclic softening in actual material. Further, Al–Ag alloy was found not to soften at 78 K. Both of these results supoort the disordering hypothesis of cyclic softening over the dissolution or shearing-off mechanisms, although other systems may be subject to these mechanisms. In spite of the low temperature at which the Al–Ag alloy was tested, small / precipitates were found to have formed in the longest lived test; however, this result did not interfere with the validity of the experiment as it did previously at room temperature. The fatigue lives at 78 K were much longer than those previously measured at room temperature although the failure mechanisms were not affected at high strain and only somewhat affected at low strain (there was increased incidence of transgranular crack nucleation and propagation).

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Résumé Des essais d'application de coups alternés et aléatoires sur un modèle de simulation par des billes de la phase dans les alliages de Cu–Al ont été effectués en vue de vérifier la cinétique suivant laquelle se produisent des désordres dans un précipité initialement ordonné et soumis à des déformations cycliques. En outre, des essais de fatigue à basse température (78°K) ainsi que diverses observations microscopiques ont été effectués sur un alliage Al–Ag comportant des zones GP en vue de vérifier si une structure comportant un précipité initialement en désordre subirait un adoucissement cyclique ou non. On a donc trouvé que le modèle de utilisant des billes fait état de désordres en fonction de l'accumulation de la déformation plastique et ce en rapport avec la cinétique d'adoucissement cyclique dans le matériau réel. En outre, l'alliage Al–Ag s'est révélé ne pas s'adoucir à 78°K. Ces deux résultats soutiennent l'hypothèse suivant laquelle un adoucissement cyclique s'accompagne de désordres plutôt que de mécanismes de disolution ou de séparation par cisaillement, bien que d'autres systèmes puissent être sujets à ces mécanismes. En dépit de la basse température à laquelle l'alliage Al–Ag a été essayé, on a trouvé de petits précipités / qui se sont formés au cours des essais, les plus longs. Cependant, ce résultat ne gène pas la validité de l'expérience comme c'était le cas précédemment à température ambiante. Les vies en fatigue à 78°K ont été trouvées bien plus longues que celles mesurées précédemment à température ambiante, en dépit du fait que le mécanisme de rupture n'ait pas été affecté sous les déformations élevées et ne l'a été que quelque peu sous les déformations faibles, auxquelles apparait une incidence croissante d'une nucléation et d'une propagation d'une fissure transgranulaire.

     

Assay of aflatoxin in peanuts and peanut products using acetone-hexane-water for extraction

A quantitative method is described for the assay of aflatoxin in peanut products. The procedure involves extraction of aflatoxin from the sample with a homogeneous acetone-hexane-water solvent mixture followed by purification of the extract by phasic extraction of the aflatoxin with aqueous sodium chloride and then with chloroform. The purified chloroform extract is analyzed by thin-layer chromatography by comparison of the intensity of fluorescence of any aflatoxin with the intensity of a known standard. The aflatoxin analyses of peanuts were found to be very variable due to sampling, and this variability has been greatly reduced by finely grinding and thoroughly mixing 2 kg of the sample before removal of an aliquot for assay. The method is sensitive to approximately 2 parts per billion. Honorable mention, Bond Award competition. Presented at AOCS Meeting, Chicago, Illinois, October, 1964. So. Reg. Res. Lab., New Orleans, Louisiana, one of the laboratories of the So. Utiliz. Res. and Dev. Div., ARS, USDA.     

Beta brass bicrystal and tricrystal stress strain behavior

Beta brass single crystal, bicrystal and tricrystal stress strain behavior were determined for a series of isoaxial specimens in which the bicrystals and tricrystals were incompatible in shear along the grain boundary. Volume fractions of grain boundary deformation were also determined to ascertain the effect of increased constraint on grain boundary strengthening. It was found that bicrystal flow stress was considerably higher than single crystal flow stress and that tricrystal strain hardening was only slightly larger than that of the bicrystal. For the tricrystals the grain boundary deformation zone is larger than that of corresponding bicrystals because of both configurational and constraint effects. This larger zone indicates that secondary stresses extend further from the boundary in tricrystals than in bicrystals. It was concluded that despite increased constraint, , the average grain boundary stress for the tricrystals, is smaller than that for corresponding bicrystals and that because of the increased extent of the secondary stress,σ _{b i}, the stress in the center component away from the grain boundary, is higher than that of the single crystal at corresponding strains. The relationship of these observations to corresponding stresses in polycrystalline material is examined. where Taek Dong Lee was formerly a Graduate Student. This paper is based on a thesis to be submitted by Taek Dong Lee in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Polytechnic Institute of New York.     

Deformation behavior of blended elemental ti-6ai-4v compacts

A blended elemental Ti-6A1-4V alloy powder, sintered to 99 pct theoretical density, was tested in tension and fatigue and characterized by optical, scanning, and transmission electron microscopy and by X-ray energy dispersive analysis. The microstructure consisted of a combination of equiaxed and low aspect ratio lamellar alpha phase. Two types of pores were observed: macropores at grain boundaries or triple points, and micropores within alpha regions. Chemical homogeneity was achieved within individual alpha or beta regions with slight compositional fluctuations between different grains. The deformation modes in alpha regions were investigated by transmission electron microscopy on thin foils prepared from tensile and cyclically deformed specimens. Tensile deformation occurred by homogeneous slip on both prismatic and pyramidal alpha slip planes. In cyclic deformation, on the other hand, little slip activity was found except for some heterogeneous slip on the basal plane associated with a relatively large micropore. Micropores of ?0.5 μm size were found to act as dislocation pinning sites.     

Measuring the economic benefits of the Youngsan River Restoration Project in Kwangju,Korea, using contingent valuation

This study examines the economic benefits of a river restoration project in the Youngsan River, Kwangju, Korea, using a contingent valuation method. It employs a spike model to address the high proportion of zero responses. The results indicate that a household's annual mean willingness to pay is KRW3410 in Kwangju and KRW1744 in other regions. The resulting benefit-cost ratio is 2.29, indicating that the project would be socially profitable, and that households appear willing to support it financially.