Atomic force microscopy and Raman investigation on the sintering process of amorphous SiO2 nanoparticles

Abstract

We report an experimental investigation on the sintering process induced in fumed silica powders by isochronal thermal treatments at T=1270 K. Three types of fumed silica are considered, consisting of amorphous SiO2 a-SiO2 particles with mean diameters 7, 14, and 40 nm. The study is performed by atomic force microscopy AFM , to follow the morphological changes, and by Raman scattering, to obtain information on the concomitant structural modifications. The former method indicates that the sintering process proceeds by aggregation of single particles into larger grains, whose sizes increase with the thermal treatment duration. Furthermore, for each fumed silica type considered, the quantitative analysis of the AFM images shows that the grain growth process takes place approximately at constant rate for thermal treatment durations up to 290 h. Nevertheless, the value of the grain growth rate is sensitive to the system properties. In fact, it is found to increase with decreasing the particle mean diameter, giving a strong quantitative evidence of the size-dependence of the sintering process. On the other hand, Raman measurements indicate that the structure of the as-received fumed silica nanoparticles is significantly modified with respect to that of ordinary bulk a-SiO2, in agreement with previous experimental evidences. However, it rapidly relaxes upon thermal treatment at T=1270 K and its characterizing features are almost completely lost after treatment for about 80 h. Finally, the comparison of AFM and Raman data shows that the nanoparticles structure are completely relaxed resembling that of bulk a-SiO2 when the grains formed by thermal treatment reach diameters greater than about 43 nm, indicating that it represents the characteristic size above which the effects of spatial confinement on the structure of the material become almost negligible.