Effect of matrix composition in the kinetic of grow of Pb nanodroplets in lead-borate glass

_Full text: We studied the effect of matrix composition on the formation and growth of Pb nanodroplets embedded in (x-2)PbO-(100-x)B₂O₃-2SnO₂-2Pb glasses during isothermal treatments at several temperatures. Glasses with different concentrations of PbO (x = 40, 45, 50, 55 and 60 in mol %) were produced using the melt-quenching method. The mixture of fine grained PbO, B₂O₃ and SnO powders was melted in vacuum at 780 °C during 1 hour to improve homogeneity. SnO was used to partially reduce the PbO leading to the formation of Pb and SnO₂ homogeneously diluted in the glasses. After melting the material was fast quenched down to room temperature using the splat-cooling technique. As a result thin glass platelets transparent to visible light and having thickness ranging between 60 and 110 micrometers were obtained. Samples of the different glasses were studied by small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Studies by DSC showed that the glass transition temperature Tg decreases with increasing concentration of PbO. TEM images of glasses previously treated at temperatures favorable to the formation and growth of Pb nanodroplets showed that nanoparticles have nearly spherical shape and some dispersion in their size. The analysis of SAXS intensity curves, measured in situ at several times during the thermal treatments, allowed the determination of the dependence on time of the average radius, size dispersion, number density and total volume occupied by the nanodroplets. The results showed that a substantial number of small nanoparticles are already formed during the production of the glass platelets. Three different successive stages were observed during the growth of Pb droplets: 1) an initial period in which occurs the dissolution of nanodroplets with subcritical size, 2) a classical nucleating and growth stage, in which droplets grow at the expense of the Pb atoms initially dissolved in the glass, and 3) a final stage where the formed Pb droplets continue to grow at a progressively smaller rate. The results showed that during the stage of nucleation and growth the diffusion of Pb atoms in glasses follows an Arrhenius-type law. This behavior allowed to determine the activation energy of growth of Pb nanodroplets for the here studied compositions of lead-borate glasses. The results of SAXS also allowed to determine the conditions (composition, temperature and time of thermal treatment) that minimize the dispersion in size of Pb nanoparticles. (author)