| Popis: |
Previous studies of the polycarbosilane, "HPCS", and its partially allyl-substituted derivative, AHPCS, have indicated a hyperbranched molecular structure consisting of a mixture of terminal -CH 2 SiR 3 , and internal -CH 2 SiR n (CH 2 -) 3 - n (n = 0,1,2) units ([R 3 SiCH 2 -] x [-SiR 2 (CH 2 -) y [-SiR(CH 2 -) 1 . 5 ] z [-Si(CH 2 -) 2 ] l (R = H or allyl (-CH 2 CH=CH 2 )). This hyperbranched, liquid, polycarbosilane undergoes crosslinking above ca. 250°C via loss of H 2 from the -SiH n (n = 2,3) groups and, in the case of AHPCS, through hydrosilation of the allyl (-CH 2 CH=CH 2 ) groups to form a hard, glassy solid. Further heating to 1000°C produces an amorphous "SiC" which then undergoes conversion on further heating to 1600°C to produce a partially (nano)crystalline β-SiC ceramic. The conversion of the amorphous "Sic x H y " material obtained from AHPCS by partial pyrolysis to 800-1000°C has now been studied by means of DSC/TGA, solid state NMR, Raman, XRD, and TEM. Moreover, the polymer has been processed into monolithic notched-beam specimens suitable for hardness and toughness measurements. The results of some of these studies are summarized, and examples of composite ceramic objects obtained by using AHPCS as a SiC-matrix source are provided. |
