Crystallisation and microstructure of lithium orthosilicate pebbles

Slightly overstoichiometric lithium orthosilicate (Li4SiO4) pebbles are fabricated by a melt-spraying process, where the droplets are rapidly quenched to room temperature. Depending on their size, the solidified pebbles exhibit different microstructures. While the crystallised pebbles always display some pores and cracks, the amorphous ones are mostly free of any defects. The crystallisation process of amorphous pebbles was therefore investigated to decrease the amount of cracks and pores with the aim of improving the mechanical properties of the final product. The development of phases during crystallisation was studied by thermoanalysis, X-ray diffraction and electron microscopy. The crystallisation takes place in three steps. At about 300-350 °C an unidentified, intermediate phase is nucleated. In a second step the formation of Li4SiO4 takes place at about 500 °C, while lithium metasilicate (Li2SiO3) is the last phase to appear at temperatures above 600 °C.