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

We report an experimental investigation on the sintering process induced in fumed silica powders byisochronal thermal treatments at T=1270 K. Three types of fumed silica are considered, consistingof amorphous SiO2 a-SiO2 particles with mean diameters 7, 14, and 40 nm. The study isperformed by atomic force microscopy AFM , to follow the morphological changes, and by Ramanscattering, to obtain information on the concomitant structural modifications. The former methodindicates 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 typeconsidered, the quantitative analysis of the AFM images shows that the grain growth process takesplace approximately at constant rate for thermal treatment durations up to 290 h. Nevertheless, thevalue of the grain growth rate is sensitive to the system properties. In fact, it is found to increasewith decreasing the particle mean diameter, giving a strong quantitative evidence of thesize-dependence of the sintering process. On the other hand, Raman measurements indicate that thestructure of the as-received fumed silica nanoparticles is significantly modified with respect to thatof ordinary bulk a-SiO2, in agreement with previous experimental evidences. However, it rapidlyrelaxes upon thermal treatment at T=1270 K and its characterizing features are almost completelylost after treatment for about 80 h. Finally, the comparison of AFM and Raman data shows that thenanoparticles structure are completely relaxed resembling that of bulk a-SiO2 when the grainsformed by thermal treatment reach diameters greater than about 43 nm, indicating that it representsthe characteristic size above which the effects of spatial confinement on the structure of the materialbecome almost negligible.