°°M
Figure 4.21: 2,S i chemical shifts (from the tetramelhylsilane reference peak) for (a) a SiC-Si,N4 composite (with > 8 0 wt% SiC) fabricated at ultra-high pressures and (b) the 'as received' SiC platelets. Both spectra were acquired using a 30 minute relaxation delay time.
explained by the corresponding SiC lattice sites having particularly long spin-lattice relaxation times, as is discussed by Hartman et al. [151].
Comparison of the post-UHP spectrum for the platelets with that for the original (i.e. 'as received') platelets, in figure 4.21 (b), reveals that the individual peaks in the NSi chemical shift spectrum have been markedly broadened as a result of the UHP processing, suggesting an increase in crystalline disorder, consistent with the observations made from the XRD spectra (section 4.7). The broadening of the spectrum after UHP treatment partially obscures the original peak at -14.5 ppm and also, may be preventing resolution of the two closely spaced peaks at -20.1 and -20.4 ppm in the
original spectrum. A particularly striking difference between these spectra is the
apparent growth of the peak at -16 ppm, in figure 4.21 (a), relative to the very small peak at this position in the original spectrum (figure 4.21 b). Other differences between
figures (a) and (b) can be seen in the relative intensities of the peaks between -2 2 ppm
and -25 ppm.
4.8.2 SiC Polytype Identification Using 29Si Chemical Shift Data
Using the SiC phase identification obtained by XRD and the results of the published research which examined the characteristic chemical shifts in the more common SiC phases (summarised in appendix C), it was possible to identify the various chemical shift positions of the unprocessed platelets with those of the different SiC polytypes. In table 4.3, the chemical shifts have been analysed for the spectrum shown in figure 4,21 (b), in which the best peak resolution was obtained.
Table 4.3: Analysis o f chemical shifts for the 'as received' SiC platelets. Si chemical shift (in ppm) Relative peak intensity SiC Polytype -20.1 very strong 4H -22.9 strong -14.5 medium 6H -20.5 very strong -25.0 medium shoulder, between -14.3 and -13.5 weak 15R -23.9 weak -15.9 very weak 3C
The particularly strong intensity of the peaks between -20.1 and -20.5 ppm is
believed to arise from some overlapping of peaks for the 4H and 6H phases and
probably also from the 15R peak expected to lie in this region. The remaining two
peaks ascribed to the 6H phase are of fairly similar intensity, in accordance with the 6H
lattice sites being equally populated and the greater intensity of the 4H peak at -22.9 ppm is consistent with there being a greater proportion of the 4H phase [65]. Having
identified the most intense peaks with the 4H and 6H phases, the presence of other
polytypes, such as 15R SiC and trace amounts of 3C SiC is suggested by the weaker
peaks. An analysis of the phases present in the post-UHP processed platelets is
presented in table 4.4, for data taken from the spectrum in figure 4.21 (a).
Table 4.4: Analysis o f chemical shifts for SiC platelets processed at ultra-high pressures. Si chemical shift (in ppm) Relative peak intensity SiC Polytype -15.9 very strong 3C -20.2 very strong 4 H /6 H / 15R -22.8 strong 4H -14.5 medium 6H -24.1 medium 15R -25.0 medium 6H
4.8.3 The Structural Stability o f Silicon Carbide Platelets During Ceramic Fabrication
Additional MAS-NMR experiments were conducted to establish whether these observed structural changes were caused by the extreme pressures or the high sintering
temperatures. Such experiments involved SiC platelet-Si3N4 ceramics that had been
fabricated by hot pressing at 20 MPa and 1700°C and by hot isostatic pressing (HIP) at 160 MPa and 1725°C. (The HIPed ceramic was prepared by Dr. S. M. Ketchion, University of Warwick, using a separate batch of SiC platelets.) In figure 4.22, the spectrum for the UHP-processed SiC is reproduced from figure 4.20 for ease of comparison. The 29Si chemical shift peaks at -47 and -48 ppm in figure 4.22 (b) arise
from a mixture of the a and 0-Si3N4 phases [155] which were also present in the hot-
pressed ceramic. The peaks associated with the SiC lattice sites in the hot-pressed ceramic show little difference from those in the spectrum for the unprocessed platelets.
~ l i f fw,
The Effect of
In figure 4.23 (a) the peak at -48 ppm is associated with the 0-Si3N4 phase and the remaining peaks, associated with SiC lattice sites, show a strong resemblance to the spectrum for the unprocessed platelets. Figure 4.23 (b) was obtained with a shorter relaxation delay of one minute and this may explain the more prominent diffuse background, as the structurally disordered features would be expected to have much shorter relaxation times [150].
Both the hot-pressed and HIPed ceramics were fabricated at temperatures believed to be comparable with those attained during the UHP ceramic fabrication and yet no significant SiC structural changes were revealed by the MAS-NMR spectra taken from these ceramics. Therefore, the experiments infer that the observed differences between the MSi chemical shifts for the SiC platelets before and after UHP processing were not caused solely by the high sintering temperatures, but by the application of extreme pressures.
However, it was found that the structural changes could not be effected by the extreme pressures alone. A final experiment was carried out to subject SiC platelets to the same UHP conditions as were used in ceramic fabrication, but at ambient temperature. No change in atomic structure was indicated by the MAS-NMR spectrum obtained from the platelets after this experiment (figure 4.24), from which it was inferred that the combination of both high temperature and extreme pressure was necessary to induce structural changes in the platelets.
Another observation, revealed by comparison of figures 4.22 and 4.23, is a
variation between the different SiC batches. In particular, for the HIPed ceramic
(fabricated from an 'earlier' batch of SiC platelets), the peak at -16 ppm is of a much greater intensity than that for the hot-pressed material. A chemical shift of -16 ppm (from tetramethylsilane) has been identified as being the characteristic, single resonance peak for the 3C polytype [151-153]. Therefore, the stronger intensity of this peak in figure 4.23 suggests there to be a higher proportion of 3C SiC (also known as (3-SiC) in the platelet component of the HIPed ceramic.
The dramatic growth of a peak at -16 ppm in the chemical shift spectra for all the
SÍC-SÍ3N4 composites sintered at UHP conditions is indicative of a systematic