PIP process greatly influencing the microstructure and electrical conductivity of polymer-derived SiCN ceramics

Abstract

Precursor infiltration and pyrolysis (PIP) technique is widely used to densify porous ceramics. In this paper, PIP technique was used to fabricate dense polymer-derived SiCN ceramic bulks from porous SiCN preforms. Electrical conductivity and microstructure of dense SiCNs and the evolution with pyrolysis temperature were studied. Results were compared with that of porous SiCNs which were prepared by powder consolidation route without PIP process. Raman analysis showed that graphitization level of free-carbon in dense SiCNs is higher than that in porous SiCNs. As a consequence, the electrical conductivity of dense SiCNs was much higher than that of porous SiCNs. Free-carbon in porous SiCNs underwent a sp3-to-sp2 transition with increasing pyrolysis temperature throughout the temperature range of 1000 °C–1400 °C, whereas the sp3-to-sp2 transition was completed before 1200 °C in dense SiCNs. Main structural evolution of free-carbon in dense SiCNs was the growth of nanocrystalline graphite (nc-graphite) into ordered graphite when pyrolyzed at temperatures higher than 1200 °C. The growth activation energy of sp2 cluster size vs. pyrolysis temperature in the nc-graphite growth stage was found to be higher than that in the sp3-to-sp2 transition stage. The activation energy of conductivity increasing vs. pyrolysis temperature of the porous SiCNs was higher than that of the dense SiCNs, indicating that the electrical conductivity is more sensitive to the sp3-to-sp2 transition process than the sp2 cluster growth. This paper indicates that processing route can have great influence on the microstructure and electrical property of PDCs.