Novel polysilazanes as precursors for silicon nitride/silicon carbide composites without “free” carbon

Polysilazanes with chemical composition Si4N4CHx (x = 12–14) were obtained on two different reaction pathways. In the first route three equivalents of dichlorosilane, H2SiCl2, were mixed with one equivalent of methyldichlorosilane, H3CSiHCl2, and reacted with ammonia to yield [(SiH2–NH)3(H3CSiH–NH)]n (1) after appropriate work-up. In the second route, dichlorosilane was reacted with methylamine and ammonia in a 3:1 ratio to deliver [(SiH2–NH)3(SiH2–NCH3)]n (2). The raw products were further cross-linked by base-catalyzed dehydrocoupling reactions involving SiH and NH units to yield products 1a and 2a, respectively. The molecular structure of 1 and 2 was investigated by means of high resolution 1H, 13C{1H}, and 29Si{1H} NMR spectroscopy in C6D6 solution as well as by IR spectroscopy. Because of the insolubility of the cross-linked products 1a and 2a solid state 1H, 13C, and 29Si CP-MAS NMR in combination with IR spectroscopy were applied for their spectroscopic characterization. Chemical compositions were determined using elemental analysis. lysis in an argon atmosphere up to 1400 °C of the cross-linked products delivered Si3N4/SiC ceramics in ∼94% yield. The absence of “free” carbon was revealed by mass spectra-coupled thermogravimetric analysis (TGA). These investigations indicated a one-step decomposition in the 250–700 °C range with predominant evaporation of hydrogen. In contrast, elimination of methane and ammonia (1) or methane and methylamine (2) was below 0.5 mass%. Additionally, neutron wide angle scattering proved the absence of a “free” carbon phase. High-temperature properties were investigated using high temperature TGA as well as XRD, EDX and TEM of annealed ceramic samples. The onset of crystallization in both materials was around 1500 and 1300 °C (1 atm N2), respectively, and α- as well as β-Si3N4 formed besides α/β-SiC. Above 1900 °C ceramics derived from both 1a and 2a decomposed to α/β-SiC/β-Si3N4/α-Si composites.