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Titolo: Sistemi e dispositivi meccanici applicati a processi fisici e chimici
Data di pubblicazione: 9-mag-2014
Abstract: The present Ph.D. thesis focuses on the development of innovative mechanochemical reactors and the improvement of existing ones. Mechanochemistry is the branch of Chemistry that relates to the effects of nonhydrostatic stresses and plastic strain on the chemical reactivity of molecules, crystals, and other aggregates of matter. Mechanochemical transformations occur on a local basis in the presence of unbalanced mechanical stresses, which result in non-Equilibrium thermodynamic conditions. Transformations are typically driven by ball milling, a powder metallurgy technique based on the occurrence of collisions between milling tools inside a reactor. At each collision, a fraction of the powder charge inside the reactor is trapped between colliding surfaces and submitted to a mechanical load. This induces cold-welding, fracturing and plastic deformation processes mediated by the generation, migration, and interaction of lattice defects. Depending on the nature of the powder and of its surroundings, the microstructural refinement can be accompanied by unusual physical and chemical transformations with no thermal counterpart. In the light of this peculiarity, mechanical activation processes show great promise in different fields of science and technology. However, their considerable potential did not find yet a definite industrial exploitation due to the unfortunate limitations affecting the performance of mechanical activation devices in terms of energy efficiency and production rates. Meeting the demand for improved mechanochemical reactors enabling innovative fundamental research and large-scale applications necessarily implies a deep revision of the reactor design. On the one hand, it is highly desirable to enhance the collision frequency and increase the amount of powder processed per unit time while saving energy and costs. On the other, fundamental experimental observables in frictional and collisional regimens must be rendered accessible to direct investigation. These contrasting requests definitely invoke atypical lines of approach in Mechanical Engineering, which makes any research in the field challenging. In this work, attention has been focused on a few main research issues, namely the ad hoc equipment of the reactor of a commercial ball mill following a previous investigation of the milling tools dynamics, and the development of three reactor prototypes addressing specific research needs. Complementary activities have been also carried out within the framework of collaborative research programmes.
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