Carbothermal synthesis of β-sialon from mechanochemically activated precursors

Reaction mixtures of halloysite clay and fine carbon for carbothermal reduction and nitridation (CRN) synthesis of β-sialon were ground in a planetary ball mill under flowing nitrogen for varying periods before being converted to sialon by heating in nitrogen at 1200–1400 °C. After 4 h grinding the XRD reflections of the halloysite were destroyed and some of the octahedrally-coordinated Al was converted to four- and five-fold coordination. 27Al and 29Si MAS NMR gave no evidence of the formation of AlN or SiN bonds upon grinding. Upon subsequent heating in nitrogen, the ground samples show significant differences from the unground control, the intermediate compound mullite being replaced by β-sialon (z ≈ 2) a temperature at least 100 °C lower, but the formation of corundum (α-Al2O3) also occurs at a lower temperature and is more persistent than in the unground control. MAS NMR spectroscopy shows that the products from the ground mixtures contain relatively less AlON units and that the formation of SiC (a transient reaction intermediate) is also facilitated by grinding. The optimum grinding time for this system was found to be 12 h.