A survey of predominantly industrial silicon carbide has
been carried out using Magic Angle Spinning nuclear magnetic
resonance (MAS nmr); a solid state technique.
Three silicon carbide polytypes were studied; 3C, 6H,
and 15R. The 13C and 29 Si MAS nmr spectra of the bulk SiC sample
was identified on the basis of silicon (carbon) site type in the
d iff ere n t pol Y t Y pes • Out to 5.00 A fro mac en t r a lsi 1 i con (0 r
carbon) atom four types of sites were characterized using
symmetry based calculations. This method of polytype analysis
was also considered, in the prelminary stages, for applications
with other polytypic material; CdBr 2 , CdI 2 , and PbI 2 "
In an attempt to understand the minor components of
silicon carbide, such as its surface, some samples were
hydrofluoric acid washed and heated to extreme temperatures.
Basically, an HF removable species which absorbs at -110 ppm
(Si0 2 ) in the 29 Si MAS nmr spectrum is found in silicon carbide
after heating. Other unidentified peaks observed at short
recycle delays in some 29 Si MAS nmr spectra are considered to be
impurities that may be within the lattice. These components
comprise less than 5% of the observable silicon.
A Tl study was carried out for 29 Si nuclei in a 3C
ii
polytype sample, using the Driven Equilibrium Single-Pulse
Observation of T1 (DESPOT) technique. It appears as though there
are a number of nuclei that have the same chemical shift but
different T1 relaxation times. The T1 values range from 30
seconds to 11 minutes. Caution has to be kept when interpreting
these results because this is the first time that DESPOT has been
used for solid samples and it is not likely in full working
order.
MAS nmr indicates that the 13C and 29 Si ~sotropic chemical
shifts of silicon carbide appear to have a reciprocal type of
relationship_ Single crystal nmr analysis of a 6H sample is
accordance with this finding when only the resultant isotropic
shift is considered. However, single crystal nmr also shows that
the actual response of the silicon and carbon nuclear environment
to the applied magnetic field at various angles is not at all
reciprocal. Such results show that much more single crystal nmr
work is required to determine the actual behavior of the local
magnetic environment of the SiC nuclei