Diffusion, Viscosity and Ionic Transport Properties in Lead Metasilicate Glass

Reference	Presenter	Authors (Institution)	Abstract
10-025	Marcio Luis Ferreira Nascimento	Nascimento, M.L.(Federal University of Bahia); Cassar, D.R.(Federal University of São Carlos); Rodrigues, A.C.(Federal University of São Carlos);	Diffusivity, conductivity and viscosity data of lead metasilicate glass ($\mathrm{PbO}·{\mathrm{SiO}}_2$) were collected in the glassy and liquid states. The difference in the dependence of diffusivity ($D$), viscous flow ($\mathrm{\eta }$) and ionic conductivity ($\mathrm{\sigma }$) on temperature below and above their glass transition temperatures ($T_g$) are interpreted by a discontinuity in the charge carrier’s mobility mechanisms. In particular, charge carrier displacement occurs through an activated mechanism below $T_g$ and through a Vogel-Fulcher-Tammann-Hesse (VFTH) mechanism above this temperature. Fitting diffusivity and conductivity data with the proposed model allows one to determine separately the enthalpies of charge carrier formation and migration. In particular, we present measurement of lead and silicon diffusion ions ($D_{\mathrm{Pb}}$ and $D_{\mathrm{Si}}$) at deep and low undercoolings – these are the slowest diffusivity species, considering 16 orders of magnitude and compared the effective diffusivity for viscous flow, $D_{\eta }$, and its activation energy. The enthalpy of charge carrier formation is 0.513 $\pm$ 0.010 eV, while the migration enthalpy is 0.861 $\pm$ 0.006 eV. Based on these values, the charge carrier mobility ($\mu$) and concentration ($n$) in the glassy state can then be calculated. The activation energy $E_A^{\eta }$ for isostructural viscosity was 1.306 eV, and is similar to diffusion and conduction activation energies for lead below $T_g$, of $E_A^D=$ 1.010 $\pm$ 0.033 eV and $E_A^{\mathrm{\sigma }}=$ 1.115 $\pm$ 0.035 eV, respectively. Therefore, viscous flow and cationic diffusion are coupled near melting point, but there is a decoupling temperature $T_d$ between the cationic diffusivity and the diffusivity calculated from viscosity, i.e. $D_{\eta }<D_{\mathrm{Si}}\approx D_{\mathrm{Pb}}$.
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