Replacement reaction induced by ball milling of silica in nitrogen

Abstract

Metal oxides are considered more thermally stable than metal nitrides. Thus for conventional nitriding processes that involve heating at high temperature in N[sub 2]/NH[sub 3] under pressure, it is impossible to produce nitrides in an atmosphere of oxygen. However, mechanical alloying has been well recognized as a process that produces non-equilibrium phases. In the case of gas-solid reactions, a supersaturated nitrogen solid solution and a high-temperature phase have been produced at room temperature. Further, TiN and ZrN, which are metastable compared with their corresponding oxides, have been formed by ball milling of pure elemental powders in a leaking mill into which air penetrated. It has also been reported that ball milling of titanium powder in air induced the formation of TiNO[sub 0.5]. These results suggest that oxidation is not always a dominant reaction and that nitridation can be more favorable under mechanical attrition. To study the possibility of nitridation reaction of an oxide, a crystalline silica SiO[sub 2] was ball milled in a pure nitrogen atmosphere. The crystalline silica structure is more reactive than an amorphous silica during milling. [alpha]-Si[sub 3]N[sub 4] is stable to oxidation reactions below 1,400 C.