Ca-α SiAlON hollow spheres prepared by carbothermal reduction-nitridation from different SiO2 powders

The formation process of hollow spheres composed of nanosized Ca-α SiAlON particles was investigated using SiO2 starting powders with different characteristics in particle size, shape and crystalline state. TEM observations showed Ca-α SiAlON hollow spheres composed of a large number of nanosized particles in the products prepared at 1450 °C for 120 min in nitrogen. In all systems, the Ca-α SiAlON hollow spheres were always produced through an intermediate Si-Al-Ca-O liquid phase in the same mechanism, regardless of the characteristics of SiO2 starting powders used. Spherical solid particles consisted of amorphous phase containing Si, Al, Ca, O and a small amount of N were generated at the initial stage of carbothermal reduction-nitridation. These spherical solid particles changed into hollow particles with the progression of the reaction from the liquid phase to the crystalline Ca-α SiAlON with increasing temperature.