EPR Studies of SiBNC Preceramic Polymers and Ceramic Employing Isotope Labeling

摘要

The magnetic properties of amorphous preceramic polymer and ceramic samples with a composition of silicon, boron, nitrogen, and carbon were investigated. The incorporation of carbon atoms into the ternary SiBN system leads to an increase of the concentration of unpaired electrons. In the present article, this paramagnetism is intensively studied. EPR and SQUID measurements show that the concentration of paramagnetic centers is increased when the sample is pyrolyzed at higher temperatures. Previously performed solid-state NMR experiments demonstrate that the carbon structure undergoes a transformation during pyrolysis. This process may be directly related to the amount of unpaired electrons in the sample. In order to identify the location of the unpaired electrons, EPR experiments on isotope labeled SiBNC ceramics were compared with experiments on unlabeled material. Different pyrolysis temperatures and isotope labeling lead to a variation of the intensity and the shape of the EPR line. The isotope enrichment causes a broadening of the EPR resonance line, which is consistent with the idea that the small polycyclic aromatic carbons with corresponding nitrogen bridges act as sinks for the unpaired electron. As a consequence, unresolved hyperfine couplings to ~(13)C and ~(15)N nuclei lead to a broader line.