Journal of the American Ceramic Society, Vol.102, No.12, 7116-7124, 2019
The mechanochemical route of vaterite synthesis using sodium hexametaphosphate as an inorganic additive
Vaterite is a metastable phase of CaCO3 and was prepared mechanochemically for the first time with the assistance of sodium hexametaphosphate (SHMP). First, CaCO3 was prepared without SHMP and was characterized using X-ray diffraction (XRD) to study the effect of milling times and speeds on the polymorphs of product. The results indicate that the reaction is complete at 60 minutes producing only calcite. Additionally, amorphous CaCO3 (ACC) was obtained at a milling speed of 300 rpm, while calcite was obtained at 600 and 1000 rpm. Then, the effect of SHMP concentration on the fraction of vaterite was investigated, and the vaterite fraction increased with increasing SHMP amount. Subsequently, the effect of milling speed in the presence of 0.8 g of SHMP was studied, and the vaterite fraction increased with decreasing milling speed. Finally, gentler manual milling was employed, and the effect of the amount of added SHMP on vaterite formation was evaluated. The results confirmed that vaterite increased with increasing amounts of SHMP, and that vaterite formed more readily via manual milling than via mechanical milling. Observations with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that calcite and vaterite particles formed by mechanical milling were irregular agglomerates composed of primary nanoparticles while calcite particles formed by manual milling were irregular microparticles. Moreover, vaterite readily aggregated into spherical particles as the amount of SHMP increased. To investigate the reaction process and mechanism, the ethanol-washed product was characterized using XRD, SEM/EDS and TEM/SAED. The results demonstrate that ACC and calcite were concomitant during the milling process, and ACC transformed into vaterite during subsequent water washing.