The Effects of Metal Oxides on the Synthesis of Si<sub>3</sub>N<sub>4</sub> Whiskers from Diatomaceous Earth

Abstract

Two types of Si3N4 whisker and a Si3N4 powder were obtained by the reduction and nitridation of diatomaceous earth in a flowing mixture of nitrogen and ammonia at 1350°C. It is revealed that the addition of 2wt% various metal oxides brought about remarkable effects on the shape and the yield of Si3N4. Addition of either vanadium, manganese, yttrium or cobalt oxide increased the yield of inside whiskers which have a needle-like shape and form on the Si3N4 powder. Addition of manganese oxide promoted the formation of outside whiskers significantly, which have a wool-like shape and form on the wall at the lower part of the reactor tube. Most of the outside whiskers produced in the presence of metal oxide had droplets on their top, which were mainly composed of Si and Fe. These results indicate that the outside whiskers have grown by the vapor-liquid-solid (VLS) mechanism and that Fe is necessary for the growth of the outside whiskers. During the reaction, most of the added metal oxides seem to be reduced to the corresponding metals. The melting point of metallic manganese is the lowest among the added metals. Moreover, manganese has a strong affinity to nitrogen. Therefore, it is estimated that manganese and iron oxides in diatomaceous earth are reduced to metallic Mn and Fe. Then the metallic Mn and Fe evaporate and migrate to the position where the outside whiskers are produced, and whereupon the vapor condenses to droplets of Fe-Mn alloy. Incorporation of nitrogen into the droplets will be augmented by the Mn contained in the droplets. This may account for the enhanced formation of the outside whiskers.