Synthesis Of Three-dimensional Dense Silicon Oxycarbide Ceramics By Pyrolysis Of Polysiloxanes

The synthesis of ceramics via polymer pyrolysis is a new method that has been developed in recent years. The method, in comparing to the conventional powder sintering process, has the advantages of enabling design of the ceramics at the molecular level, allowing near net-shape forming and having a lower ceramic formation temperature. The ceramics made along this route possess superior high temperature properties that are not reached by the conventional sintered ceramics. Of the limited number of ceramic precursor systems, polysiloxanes are available in rich physical and chemical structures, inexpensive and insensitive to air and moisture, which thus are the promising precursors for conversion to ceramics, especially on consideration of their large scale applications in the future. Although researchers have been done on the ceramic transformation from polysiloxanes abroad for more than ten years, the work was concentrated mainly on the process of the ceramic transformation and characterization of the structures of the products. Up to now, there has been limited amount of domestic work relevant to ceramization of polysiloxanes. The highest ceramic yield was 76wt%.The present work based on molecular design to increase the ceramic yield of the polysiloxanes precursors. According to the hydrosilylation reaction of polysiloxanes, we reacted the hydrogen-containing polysiloxane (HPSO) and the 1,3,5,7-tetramethyl- 1,3,5,7-tetravinylcycletetrasiloxane (D4Vi) under catalyzing of vinylsiloxane platinum complex, to form highly cross-linked network of polysiloxanes. Through controlling the cross-linking state of the polysiloxanes and pyrolysis process, we achieved three dimensional dense SiOC ceramics.We developed the"two-step cross-linking"process, first cross-linking at lower temperature to form a elastics, and then further heated at 80°C , by this strategy, the cracking of the shaped polysiloxanes being successfully avoided. The following pyrolysis of the shaped polysiloxanes with slow heating and cooling rate provided dense SiOC ceramics. The highest ceramic yield was 81.4wt.%, when the mass ratio of HPSO/D4Vi was 1:1. It was found that part of Si-H groups was left in the cross-linked bodies, and it was this remaining groups actively ceramized during pyrolysis to form the dense and crack-free amorphous SiOC ceramics. It was stable up to 1200°C. Further annealing the amorphous ceramics at 1400°C led to the partial decomposition of the ceramics and generated a number of micro pores in the bodies. This was accompanied by the formation of white fiber-like products in the surface of the ceramics.