Silicon carbide nanowires (SiCNWs) exhibit some unique mechanical, electrical and optical properties. Potential applications include nanosensors. blue or UV-light emitting diodes or electron emitters. They can be used as reinforcing phase in composites. While for composite reinforcement the material with no specific parameters is usually needed, electronic and optical applications require precisely purified, debundled and fractionated SiCNWs.

We investigated SiCNWs obtained by Self-propagating High-temperature Synthesis (SHS) from micrometric Si and PTFE powders. The products of such reaction include not just nanowires but also bulk SiC crystals as well as unreacted carbon and silicon. Therefore some additional preparation steps are required to obtain pure and shape-controlled SiCNWs.

SiC is known to possess excellent thermal and mechanical resistance. It was expected that mechanical treatment of SiCNWs would assist in debundling of the nanowires to allow their fractioning while annealing in the presence of oxygen would cause a controlled surface oxidation and allow for decreasing of the SiCNWs lateral size. Surprisingly SiCNWs were found to be extremely sensitive to thermal energy and mechanical stresses. Here we report on some unexpected and specific transformations that SiCNWs undergo during thermal and mechanical treatment which was applied to purify and debundle the material.

Initial thermal treatment of SiCNWs was condicted at 750ºC in air in order to burn-out free carbon and oxidise free silicon. It was followed by boiling NaOH water solution in order to remove silica. The treatment produced pure SiC material consisting of long entangled nanowires and a considerable fraction of bulky crystallites.

Rubbing and milling of SiCNWs was though to be an effective way of nanowires debundling and shortening. The material was rubbed in a mortar and / or milled in a planetary ball mill. Depending on rubbing / milling conditions, the material recrystallized partially or totally into non-fibrous bulk of irregular SiC particles. The most destructive conditions for SiC nanowires is set up when the material is shortly milled in a planetary ball mill without any liquid media – all the wires transform into irregular SiC particles. Rubbing and milling are therefore not safe for SiC nanowires.

Annealing in the presence of oxygen was expected to cause limited surface oxidation of SiCNWs and possibly breaking them into shorter fragments. Annealing was conducted at 1000, 1100 and 1200ºC for 10 minutes in air. It was found that above 1000ºC the nanowires get converted into micrometric spherical entities composed of multiple SiC nanocrystals. Most probably the nanowires get dissolved in droplets of molten silica and recrystallize in a form of bulk nanocrystals.

The the present study indicates that silicon carbide nanowires are metastable objects which tend to change their morphology under conditions at which SiC is usually believed to be stable. Any harsh post-synthesis treatment appears to be destructive for them and the key to obtaining the of the SiCNWs of desired morphology lies in the synthesis process itself.

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