Positron spectroscopy of solids
Research focus
The Peer review has evaluated this group as Average
The ‘free volume’, that is the presence of cavities with atomic or slightly larger sizes, strongly influences mechanical, thermal and electrical properties of metallic, semiconductor and insulator systems, as well as the structural transformations these materials can undergo. Research in this field has strong practical interest, as determined by the technological applications of the chosen materials (metallic light alloys, semiconductors, composites, polymers), but it can also contribute to fundamental problems such as the nature of the glassy state, the role of secondary transitions in macromolecules, the importance of vacancies in the transport of matter in solid solutions and in the energetic stability of aggregates. The group is formed by a team of specialists of Positron Annihilation Spectroscopy (PAS), which is the ideal tool for studying free volumes of atomic or nanometric size. The potential of PAS is reinforced by synergy with the other experimental techniques adopted by the group (SAXS, DSC) and by an extended national and international net of active collaborations with specialists of complementary techniques and with leading theoretical groups. Several research projects have been developed during the reporting period. The main focus was on: · Study of age hardening in light alloys. The formation of vacancy-defect clusters is followed since the early stages of decomposition of supersaturated solid solutions with the aim of isolating the effects of the individual solute species in different thermal regimes. This study, which is relevant to technological applications of age hardening in the metallurgy of aluminium and magnesium alloys, has been carried out on several laboratory or commercial Al-based alloys of the series AA2xxx and 7xxx as well as on Mg-based alloys WE43 and EV31. Collaboration with Italian and foreign laboratories has made possible to combine PAS and SAXS 67 measurements performed by the group with other standard and nonstandard observation techniques (calorimetry, TEM, 3D-atom probe). · Development of the Coincidence Doppler Broadening (CDB) method as a tool for quantitative chemical analysis of nanoclusters. The method is based on the measurement, via spectrometry of the electron-positron annihilation radiation, of the momentum distribution of electrons in contact with open volume defects. The measured spectrum depends of the chemistry of the local environment at the positron annihilation site. A combined experimental-theoretical effort has been made in order to solve the non-trivial problem of the analysis of the measured momentum spectrum in terms of elemental concentrations. · Depth-resolved analysis of defects in overlayers and sub-surface layers of semiconductors. While putting in operation a new energy-controlled positron beam in the Como campus of the Politecnico di Milano, the group has been collaborating with the University of Trento for the study of defects produced by ion bombardment in non-stoichiometric overlayers of silicon oxide on silicon. The ion bombardment was optimized for producing optically active clusters of Si in the oxide matrix. · Study of polymer degradation by weathering and by gamma irradiation. PAS and thermal analysis were combined for experiments on polypropylene and on various polyesters. Chain break induced by irradiation is found to be the main process at low doses (up to 100 kGy) and it is influenced both by the length of aliphatic chain and by the presence of aromatic rings. Crosslinking prevails at intermediate doses while the importance of oxidative degradation, present at doses higher than 300 kGy, increases with the amorphous fraction of the polymer. · Dependence of physical properties of styrene-butadiene copolymers on the crosslinking degree. The glass transition temperature was found to increase with the crosslinking degree; opposite behaviour is shown by free volume and by the thermal expansion coefficient. Nanoholes density does not depend on the temperature and decreases with the content of sulphur. · Evolution of nanoporosity in Portland cement paste. Cements display complex time dynamics, in particular in the first few weeks from their formation. The results show that gel pore concentration increases with ageing as well as by increasing water concentration; on the other hand, typical sizes of the pores do not show significant variations. PAS measurements were aimed at estimating the shape parameters the size distribution of the finest gel nanopores, which cannot be probed by traditional porosimetry. · Advances in sub-nanometric porosimetry by PAS. The commonly adopted spherical model for the estimate of the sub-nanometric sizes of polymer cavities was the object of a critical evaluation and revision.
Dipartimento di afferenza
Docenti afferenti
Full Professors
Alfredo Dupasquier
Associate Professors
Giovanni Consolati
Fiorenza Quasso
Assistant Professors<7em>
Paola Folegati
Rafael Ferragut