X-ray and neutron scattering and solid state NMR investigations on precursor-derived B-C-N ceramics using isotopic substitution

Amorphous B-C-N ceramics were produced by thermolysis of poly(borosesquicarbodiimide) of idealized formula [B-2(NCN)(3)](n) at 1100 degreesC in an argon atmosphere. Their structure was investigated by X-ray and neutron diffraction. Wide-angle diffraction showed that the ceramics consist of an amorphous structure with hexagonal planar near-range ordered atomic arrangements similar to graphite. Additionally, magic-angle spinning nuclear magnetic resonance (MAS NMR) studies demonstrated that boron is preferentially trigonally bonded to nitrogen. Small-angle scattering (SAS) experiments revealed that the as-thermolyzed ceramics are inhomogeneous. By combined small-angle X-ray and neutron scattering experiments the medium-range order and its temperature dependency were investigated. Contrast variation studies using isotopic substitution (N-nat vs N-15) suggest that the ceramics contains sub-microscopic pores. The Guinier radius (R-G) of the pores increases upon annealing. Positron annihilation investigations of the ceramics also revealed the presence of nanopores.