Department Seminar with Marcio Luis Ferreira Nascimento:”Formation and Migration Enthalpy from Elemental and Cooperative Diffusion in Lead Silicate Supercooled Liquid and Glass”

Title: Formation and Migration Enthalpy from Elemental and Cooperative Diffusion in Lead Silicate Supercooled Liquid and Glass

Abstract: Diffusivity, conductivity, and viscosity data of the PbO×SiO₂ were collected in the liquid, supercooled liquid, and glassy states. The difference in the dependence of diffusivity, viscous flow and ionic conductivity on temperature below and above the glass transition temperature (T_g) is interpreted as a discontinuity in the charge carrier’s mobility mechanisms. In particular, charge carrier displacement occurs by an activated mechanism below T_g and through a cooperative mechanism above this temperature. Fitting diffusivity and conductivity data with the proposed model allows one to determine the enthalpies of charge carrier formation and migration separately. In particular, we present experimental results of lead and silicon diffusion ions (D_Pb and D_Si) at deep and low undercoolings in PbSiO₃ – considering 16 orders of magnitude and compare the effective diffusivity for viscous flow, D_h, and its activation energy. The enthalpy of charge carrier formation is 0.596 ± 0.011 eV, while the migration enthalpy is 0.946 ± 0.012 eV. Based on these values, the charge carrier mobility and concentration in the glassy state can then be calculated. The activation energy E_A^h for the isostructural viscosity was 1.253 ± 0.026 eV and is similar to the diffusion and conduction activation energies for lead below T_g of E_A^D = 1.070 ± 0.026 eV and E_A^s =1.245 ± 0.006 eV, respectively. A decoupling temperature T_d between the cationic diffusivity and the diffusivity calculated from viscosity, i.e. D_h < (D_Si » D_Pb) was noted. In fact, a noticeable change in the lead self-diffusion coefficients around T_d = 1.19T_g also contributed to this analysis. Thus, above T_d, silicon and lead control the transport mechanism involved in viscous flow, while below this temperature some more complex structure must control the transport process. Such results suggest that viscous flow requires a cooperative motion of some “structural units” rather than just jumps of one or a few isolated atoms, as it occurs in conductivity. Also, the cooperatively rearranging regions or the size of the structural units are quite similar for both processes above T_d.

Bio: Marcio Luis Ferreira Nascimento is a Brazilian physicist graduated from the University of São Paulo (USP) in 1997 and received a Ph.D. in Materials Science and Engineering from the Federal University of São Carlos (2004). Now he is a professor at the Federal University of Bahia (UFBA) in the Department of Chemical Engineering. He teaches math, applied statistics, chemical engineering and materials science to undergraduate and graduate students. Currently he has strong interest in topics such as glasses, innovation, history of science, multivariate analysis, machine learning, Big Data and especially mathematics for laymen.