Investigation of the paramagnetic centres and electronic properties of silicon carbide nanomaterials

Ultrafine powders of silicon carbide (SIC) exhibit a substantial electric c onductivity (sigma(dc) similar to 0.05 S cm(-1)) after annealing at tempera tures between 1500 degrees C and 1800 degrees C. The origin of this phenome na is discussed with respect to structural modifications, sample compositio n and the delocalized paramagnetic centres involved in these materials. The structural evolution of SiC nanomaterials with annealing was monitored by Si-29 and C-13 Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). A consistent analysis of the electron paramagnetic resonance (EPR) spectra w as performed and showed the coexistence of two paramagnetic centres (DI, DI I) localized in the crystalline sites of the SiC polytypes. Delocalized DII I paramagnetic species were partially created by thermal conversions of DII defects with energies not exceeding 13 meV. The spin susceptibility, deduc ed from EPR measurements, was found to be marked by delocalized paramagneti c species above 150 K and by strongly correlated (DIII) unpaired spins arou nd 30 K. An attempt is made to correlate the delocalized unpaired spins wit h the electric properties of these materials.