3350 G. Brauer et al. /Applied Surface Science 252(2006)3342-3351 4. Conclusions o SiC/SiC, 500 eV It has been experimentally demonstrated by XRD that a macroscopic SiC/SiC composite sintered from nano-crystalline 3C-Sic consists exclusively 3C-SiC containing still some graphite inclusions AFM measurements on different areas of the composite sample reveal that micrometer sized 2D plies of Sic fibres are embedded in a matrix of crystallites with diameters in the range between 30 and SPIS investigations underline the perfectness of the Stopping paene,a 4 -5 composite at an atomic size level due to the sintering process used, and demonstrate that depth profiling of Fig. 6. Re-emitted positron spectra from the Sic/Sic composite and defects is not hindered by a large surface roughness a crystalline 6H-SiC sample as a function of stopping potential. From positron affinity calculations it becomes clear Solid line fit to the data. the two arrows indicate zero and that graphite embedded in 3C-Sic is attractive to maximum positron energies, yielding a value for the positron work function=3.0±0.3 Positron lifetime measurements indicate also the to within statistical uncertainty. This supports the studied view that there may be low-level work-function re- Appreciable re-emission from the Sic/SiC sample emission from the sintered sample at 0.5 keV. is only observed for incident positron energies below However, the unavoidable experimental scatter I kev, characteristic of epithermal positron emission he data may obscure a small epithermal tail to higher energies(on the right of the plot). The work function suggested by these measurements is +=3.0 Acknowledgement +0.3 ev, close to the previously measured values The authors express their gratitude to Dr V. Heera SPis data of the Sic/Sic sample and a 6H-SiC(FZ Rossendorf) for valuable discussions of various sample, taken just for comparison at the Bath, UK aspects of this work positron beamline, give diffusion lengths of 186 and 45 nm for the composite and crystalline samples respectively, when analysed by VEPFIT.These diffusion lengths are consistent with the curves drawn References on Fig. 5 for work-function re-emission, although for [1] L.L. Snead, R.H. Jones, P Fenici, A Kohyama. J Nucl. Mater. the composite sample epithermal re-emission dom- 233-237(1996)26. inates at low energies. The bulk S value is found to be [2] Advanced SiC/SiC ceramic composites: developments and higher for the SiC/SiC sample, in agreement with plications in energy systems, in: A. Kohyama, M. Singh, results presented in Fig. 3 -T. Lin, Y. Katoh(Eds ), Ceramic Transactions, vol. 144. In summary, the positron re-emission measure- merican Ceramic Society, Westerville, OH, 2002. ments(Figs. 5 and 6)-in combination with co- [3] V. Heine, C. Cheng, R. Needs, J. Am. Ceram Soc. 74(1991) 2630 mparative SPIS studies- suggest that, although the [4] MJ. Rutter, V. Heine, J Phys. Condens. Matter 9(1997) diffusion length for thermalized positrons in 82 composite is rather long, the probability for positroN [5 G. Brauer, w. Anwand, E.-M. Nicht, J. Kuriplach, M. Sob, N. re-emission by the 3 ev work function Wagner, P.G. Coleman, M.J. Puska, T Korhonen, Phys. Rev. B 54(1996)2512 small--at most 3% of that for a single-crystal 6H- [6]J Stormer, A. Goodyear, w. Anwand, G. Brauer, P G. Cole- man,w. Triftshauser, J Phys. Condens Matter 8(1996)L89to within statistical uncertainty. This supports the view that there may be low-level work-function re￾emission from the sintered sample at 0.5 keV. However, the unavoidable experimental scatter in the data may obscure a small epithermal tail to higher energies (on the right of the plot). The work function suggested by these measurements is F+ = 3.0 0.3 eV, close to the previously measured values [6]. SPIS data of the SiC/SiC sample and a 6H–SiC sample, taken just for comparison at the Bath, UK positron beamline, give diffusion lengths of 186 and 45 nm for the composite and crystalline samples, respectively, when analysed by VEPFIT. These diffusion lengths are consistent with the curves drawn on Fig. 5 for work-function re-emission, although for the composite sample epithermal re-emission dom￾inates at low energies. The bulk S value is found to be higher for the SiC/SiC sample, in agreement with results presented in Fig. 3. In summary, the positron re-emission measure￾ments (Figs. 5 and 6) – in combination with co￾mparative SPIS studies – suggest that, although the diffusion length for thermalized positrons in the composite is rather long, the probability for positron re-emission by the 3 eV work function is very small—at most 3% of that for a single-crystal 6H– SiC sample. 4. Conclusions It has been experimentally demonstrated by XRD that a macroscopic SiC/SiC composite sintered from nano-crystalline 3C–SiC consists exclusively of 3C–SiC containing still some graphite inclusions. AFM measurements on different areas of the composite sample reveal that micrometer sized 2D plies of SiC fibres are embedded in a matrix of 3D crystallites with diameters in the range between 30 and 90 nm. SPIS investigations underline the perfectness of the composite at an atomic size level due to the sintering process used, and demonstrate that depth profiling of defects is not hindered by a large surface roughness. From positron affinity calculations it becomes clear that graphite embedded in 3C–SiC is attractive to positrons. Positron lifetime measurements indicate also the presence of graphite in the SiC/SiC composite studied. Appreciable re-emission from the SiC/SiC sample is only observed for incident positron energies below 1 keV, characteristic of epithermal positron emission. Acknowledgement The authors express their gratitude to Dr. V. Heera (FZ Rossendorf) for valuable discussions of various aspects of this work. References [1] L.L. Snead, R.H. Jones, P. Fenici, A. Kohyama, J. Nucl. Mater. 233–237 (1996) 26. [2] Advanced SiC/SiC ceramic composites: developments and applications in energy systems, in: A. Kohyama, M. Singh, H.-T. Lin, Y. Katoh (Eds.), Ceramic Transactions, vol. 144, American Ceramic Society, Westerville, OH, 2002. [3] V. Heine, C. Cheng, R. Needs, J. Am. Ceram. Soc. 74 (1991) 2630. [4] M.J. Rutter, V. Heine, J. Phys.: Condens. Matter 9 (1997) 8213. [5] G. Brauer, W. Anwand, E.-M. Nicht, J. Kuriplach, M. Sob, N. Wagner, P.G. Coleman, M.J. Puska, T. Korhonen, Phys. Rev. B 54 (1996) 2512. [6] J. Sto¨rmer, A. Goodyear, W. Anwand, G. Brauer, P.G. Cole￾man, W. Triftsha¨user, J. Phys.: Condens. Matter 8 (1996) L89. 3350 G. Brauer et al. / Applied Surface Science 252 (2006) 3342–3351 Fig. 6. Re-emitted positron spectra from the SiC/SiC composite and a crystalline 6H–SiC sample as a function of stopping potential. Solid line: fit to the data. The two arrows indicate zero and maximum positron energies, yielding a value for the positron work function = 3.0 0.3 eV.