Mechanical properties and thermodynamics of nanomaterials
1) Hydrogen absorption in Pd nanoparticles
In order to analyze the influence of shape on the hydrogen absorption properties of Pd at the nano-scale we simulated pressure-composition (P-C) 300K isotherms at the atomistic level for nanoparticles of similar sizes. The simulations were performed by the Monte Carlo (MC) technique in the TPchemicalpotentialN ensemble and embedded atom type potentials (EAM) to model the interaction among atoms. We generated nanoparticles of different shapes (sphere, icosahedron, octahedron, cubooctrahedron) and compared the resulting P-C isotherms with the bulk and a few available experimental results. Instead of the plateau corresponding to the alpha-beta PdH equilibrium in the bulk, the nanoparticles isotherms show at least a two-plateaux behavior, corresponding to a subsurface hydride formation and a subsequent filling of the core of the nano-particle with H atoms. We also studied the evolution of the atomic pressure profiles inside nanoparticles. We used different criteria to define the atomic volumes at the surfaces and calculated the atomic stresses.
2) Statistical Physics and Thermodynamic of nanosystems
The two levels system and the Einstein model of a crystalline solid are taught in every course of Statistical Mechanics and they are solved in the microcanonical formalism because the number of accessible microstates can be easily evaluated. However, their solutions are usually presented using the Stirling approximation to deal with factorials. In this paper those two models are solved without any approximation, using the gamma function and its derivatives. Exact values are calculated for the entropy, temperature and specific heat, and the relative error between our exact solution and the approximate one using the Stirling approximation. This error is significant for small systems, with a number of particles N ∼ 100, as in studies of atomic clusters or nanoscale structures.
Integrantes:
Publicaciones:
1- Exact solution of two level system and the Einstein solid in the microcanonical formalism. Dalía S Bertoldi, Eduardo M Bringa, Enrique N Miranda. Eur. J. Phys. 32 (2011) 1–9
2- Hydrogen absorption in Pd nanoparticles of different shapes, Eduardo A Crespo, Margarita M Ruda, Susana Ramos de Debiaggi, Eduardo M Bringa, Fabian U Braschi, Graciela Bertolino. International Journal of Hydrogen Energy, in press(2012).