Amorphous and nanostructured silica and aluminosilicate spray-dried microspheres

► The study is focused on development of nanocrystals on amorphous microspheres. ► The nanostructure is significant to biomedical applications. ► The nucleation induced by heat treatment starts on particle surface. ► The microspheres break down when they become highly crystalline. ► First NMR studies on spray dried aluminosilicate microspheres. Amorphous silica and aluminosilicate microspheres with diameters in the 0.1–20 μm range were produced by spray drying method. SEM, TEM and AFM images showed the spherical shape of the obtained particles. Based on thermal analysis data, several heat treatments have been applied on the as-prepared samples in order to check the amorphous state stability of the microspheres and to develop nanosized crystalline phases. As-prepared microspheres remain amorphous up to 1400 °C. By calcination at 1400 °C, cristobalite type nanocrystals are developed on silica sample, while in aluminosilicate sample first are developed mullite type nanocrystals and only after prolonged treatment are developed also cristobalite type nanocrystals. 29Si and 27Al MAS NMR results show that the local order around aluminum and silicon atoms strongly depend on the thermal history of the microspheres.