Synthesis and characterization of polyureasilazane derived SiCN ceramics

Samples of SiCN ceramics were synthesized by thermal treatment of commercially available CERASET (TM) polyureasilazane, used as liquid-polymer precursor, at pyrolysis temperatures of 1000, 1050, 1100, 1150, and 1200 degrees C. Electron paramagnetic resonance (EPR) signals due to sp(2)-hybridized carbon-related dangling bonds were recorded over the 4-300 K temperature range at X band (9.6 GHz), and the spectra showed the presence of an intense EPR line with g=2.0027 at room temperature for all samples; at liquid helium temperature an additional line was seen present as a shoulder to main line. These two signals are due to carbon-related dangling bonds present as (i) defects on the free-carbon phase and (ii) within the bulk of SiCN ceramic network. The value of the antiferromagnetic exchange constant between dangling bonds in the various samples, not hitherto available in the literature, was estimated from the temperature variation of the EPR linewidth to be anywhere from J=-12 to J=-15 K in the samples synthesized at 1000, 1100, and 1150 degrees C. The EPR linewidth of the samples decreased with increasing pyrolysis temperature of a sample, being 0.07 mT at X band at room temperature for the samples synthesized at 1150 and 1200 degrees C, which can be used potentially as stable g markers, due to their very narrow linewidth. The W-band (95 GHz) room-temperature spectrum of the sample pyrolyzed at 1100 degrees C exhibited only one line consistent with the X-band spectrum. At higher frequency of G band (170 GHz) the EPR spectra are better resolved, clearly showing the presence of a strong (g=2.0027) and a weak (g=2.0032) EPR line at room temperature, and the latter corresponds to the line present at X band as shoulder at liquid-helium temperature. In addition, nuclear magnetic resonance measurements were carried out to confirm the existence of free-carbon phase and oxygen atoms in the samples. (c) 2006 American Institute of Physics.