Growth mechanisms and properties of coiled whiskers of silicon nitride and carbon

This paper reviews recent studies of regularly coiled whiskers of Si3N4 prepared by the chemical vapor deposition (CVD) method as well as those of carbon by the catalytic pyrolysis method. Coiled Si3N4 whiskers have been obtained from a gas mixture of Si2Cl6 and NH3 at 1200°C on substrates on which metal impurity was painted. The most effective impurity for the growth of the whiskers was Ni for the quartz substrate and Fe for the graphite. A vapor liquid solid (VLS) growth mechanism was suggested from morphology of the whiskers. Coiled carbon whiskers have been grown by the catalytic pyrolysis of acetylene at 300-750°C using Ni powder as a catalyst. A small amount of H2S was indispensable for the growth of the coiled carbon whiskers. A Ni compound seed observed on the tip of the pair-coiled carbon whisker is a single crystal. It is suggested that each crystal plane of the Ni compound seed has a different catalytic ability for the growth of the coiled carbon whiskers. The growth mechanism for the coiled carbon whiskers involves the surface diffusion of carbon atoms on the Ni compound seed. Structure of the coiled whiskers of Si3N4 and carbon was investigated by a scanning electron microscope (SEM) and a transmission electron microscope (TEM). Furthermore, extension characteristics of these whiskers were examined.