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1.
Thermal stability of Al-Mn-Be melt-spun ribbons
Gorazd Lojen, Tonica Bončina, Franc Zupanič, 2012, original scientific article

Abstract: As with other kinds of finely dispersed, small particles, icosahedral quasicrystals (IQCs) also have a distinct strengthening effect, which can be utilised to enhance the mechanical properties of aluminium alloys. In Al-Mn-Bealloys, IQCs already form at moderate cooling rates, which can be utilised when using some conventional casting processes, like mould or injection casting. In this case, however, crystalline intermetallic phases are also present and the mechanical properties are inferior to those of two-phase aAl-IQC alloys. Two-phase microstructures are feasible using rapid solidification techniques, e.g., melt spinning. Further processing often involves technologies (consolidation, extrusion etc.), which include the influence of heat. The alloy must not be overheated in order to preserve the strengthening effect of the metastable IQC-particles. In this investigation the Al-Mn-Be alloy was melt-spun using a free-jet melt spinner. Subsequently, the thermal stability of the IQCs was explored by annealing the ribbons for 24h at different temperatures. The samples were examined in the as-cast and heat-treated conditions using a dual-beam, scanning electron microscope (SEM-FIB), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). It was discovered that in the as-cast condition, the ribbons had a two-phase microstructure, consisting of an aAl matrix and finely dispersed IQCs. During annealing at temperatures up to 400 °C, the IQCs did not decompose and the phase composition remained unchanged. Annealing at 500 °C and at higher temperatures caused a decomposition of the IQCs, and only the crystalline intermetallic phases Al6Mn and Be4AlMn could be found in the aAl matrix.
Keywords: quasicrystal, Al-Mn-Be alloy, thermal stability
Published: 10.07.2015; Views: 940; Downloads: 95
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2.
Stability of proteinase from Carica papaya latex in dense gases
Maja Leitgeb, Mateja Primožič, Željko Knez, 2005, original scientific article

Abstract: Proteinase from Carica papaya latex was tested on its thermal stability at atmospheric pressure and in supercritical carbon dioxide, near-critical propane and dimethyl-ether. In supercritical carbon dioxide at 300 bar thermalactivation of the examined proteinase was improved in the comparison toatmospheric pressure. In propane and dimethyl-ether (300 bar) activity of the examined proteinase decreased. Influence of compressionžexpansion cycles on residual activity of the same proteinase in supercritical carbon dioxide (300 bar and 50 °C) was studied, as well. Different ways of transition from supercritical to low-pressure-state were used which affected residual activityof the proteinase.Addition of water in the system increased activity of proteinase from C. papaya, which was incubated in supercritical carbon dioxide for 24 h. Optimum amount of water was found to be between 0.5 and 0.7 g/L.
Keywords: chemical processing, high pressure technology, supercritical CO2, proteinase, thermal stability, pressure stability, enzyme activity, water content, dense gases
Published: 01.06.2012; Views: 1239; Downloads: 21
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3.
Particle formation using supercritical fluids
Željko Knez, 2006, review article

Abstract: Particle formation and the design of solid particles and powdery composites with unique properties is at the moment one of major the developments of supercritical fluid (synonyms: dense gases, dense fluids, high pressure) applications. Conventional well-known processes for the particle-size redistribution of solid materials are crushing and grinding (which for some compounds are carried out at cryogenic temperatures), air micronization, sublimation, and recrystallization from solution. There are several problems associated with the above-mentioned processes. Some substances are unstable under conventional milling conditions, in recrysfallization processes the product is contaminated with solvent and waste solvent streams are produced. The application of supercritical fluids may overcome the drawbacks of conventional processes, and powders and composites with special characteristics can be produced. Several processes for the formation and design of solid particles using dense gases have been studied intensively. The unique thermodynamic and fluid-dynamic properties of supercritical fluids can also be used for the impregnation of solid particles, for the formation of solid powderous emulsions, particle coatings, e.g. for the formation of solids with unique properties for use in different applications. This review will focus on the fundamentals and on recent advances of particle formation and design processes using supercritical fluids on their applications and the technological advantages and disadvantages of various processes.
Keywords: chemical processing, high pressure technology, supercritical CO2, proteinase, thermal stability, pressure stability, enzyme activity, water content, dense gases
Published: 31.05.2012; Views: 1705; Downloads: 66
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