Biomedical subjects

Francesco Delogu

Publications and source records attributed to Francesco Delogu.

6 recordsLinked to original sources

PubMed2006

Mechanistic aspects of homogeneous and heterogeneous melting processes.

Molecular dynamics simulations have been employed to explore the response of crystalline Ar systems with and without a free surface to a gradual temperature rise. The surface-free crystalline bulk undergoes a homogeneous melting process at the limit of superheating, whereas the semicrystal terminating with a free plane surface melts with a heterogeneous mechanism at a temperature corresponding to the equilibrium melting point. Numerical findings suggest that the gradual disordering of the crystalline lattice as well as the homogeneous and heterogeneous melting processes are mediated by atoms with defective coordination. Their concentration in the regions close to the semicrystal surface at the equilibrium melting point is found to be approximately the same as in the surface-free bulk at the limit of superheating.

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PubMed2006

Cooperative atomic displacements and melting at the limit of superheating.

This paper shows how the melting of superheated crystals originates from the localization of thermal disorder in excited regions of the crystalline structure. Within such regions, disordered thermal motion is found to induce the formation of bulk topological defects. These consist of atoms with a number of nearest neighbors different from the equilibrium one. Such defectively coordinated atoms arrange according to pseudolinear clusters, the number and size of which depend on temperature. Characterized by high mobility, defective atoms and their nearest neighbors are seen to undergo a cooperative dynamics that can result in net atom displacements between equilibrium lattice sites.

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PubMed2005

Thermodynamics on the nanoscale.

Classical thermodynamics is applied to the melting of nanometer-sized Sn particles with radii in the range 5-50 nm. Such particles display a depression of both the melting point and the latent heat of fusion depending on the particle size. The size dependence can be explained with the formation of a structurally perturbed layer at the particle surface. The experimental measurement of both melting temperatures and latent heats of fusion allowed for estimation of the thickness of the perturbed layer. This permitted in turn the evaluation of the excess Gibbs free energy associated with the perturbed layer at melting and the determination of its variation with particle size and temperature.

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PubMed2005

Defect-mediated melting in superheated noble gas crystals.

Molecular dynamics simulations have been used to investigate the mechanisms governing the homogeneous melting of pure noble gases at the limit of superheating. For each chemical species considered, the heterogeneous melting point was estimated by monitoring the thermal behavior of crystalline systems containing a high-angle grain boundary. To determine the limit to superheating, calculations were instead carried out on a perfect crystalline bulk. The temperature was gradually increased to bring the systems within the metastable region above the equilibrium melting point. The static order parameter was employed to monitor the structural disordering during the slow temperature increase and to determine the temperature at which the crystalline lattice collapses to a liquid. Structural disorder was further characterized by studying the appearance of atoms with defective coordination. Their relative number and spatial correlation appeared to play a fundamental role in destabilizing the crystalline lattice bulk and triggering the homogeneous melting. The fraction of atoms with defective coordination and the total length of the stringlike clusters they form in the vicinity of the homogeneous melting point were found to be approximately the same for all of the chemical species considered. These findings have been compared with theoretical predictions.

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PubMed2005

Cooperative dynamics and self-diffusion in superheated crystals.

Molecular dynamics simulations have been used to study the atomistic scale dynamics of superheated crystals under different temperature and pressure conditions. The limit of superheating was determined by monitoring a suitable order parameter. The occurrence of homogeneous melting was related to the generation of structural defects characterized by the presence of pairs of particles having defective coordination. At temperatures close to the homogeneous melting point such particles formed extended stringlike clusters. Particles involved in clusters change continuously as a result of local structural rearrangements. These can result in the displacement of particles from one lattice site to another, thus providing a mechanism for self-diffusion.

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PubMedDate not supplied

Mechanochemical degradation of aromatic sulfonic acids.

Sulfonic acids and sulfonates are widely used as surfactants in various industrial processes. Although partial biodegradation occurs, their release in the environment has determined a widespread pollution problem due to their remarkable tendency towards accumulation. For this reason, various methodologies to effectively degrade sulfonic acids and sulfonates in real matrices are currently under investigation. In this study, we focused our attention on the mechanochemical degradation of the 1,5-naphthalene disulfonic acid using strongly reducing agents such as pure Mg and Ca metals. The mechanochemical processing induced either a gradual degradation of the organic compound or a combustion-like reaction depending upon the experimental conditions. A partial degradation of the sulfonic acid was observed as a result of gradual reactions. An almost complete removal of the pollutant was instead observed after combustion-like reactions.

Calcium↗