Solid‐State NMR Studies on Precursor‐Derived Si‐B‐C‐N Ceramics

Effect of Precursor on Speciation and Nanostructure of SiBCN Polymer‐Derived Ceramics

“…This shoulder on the SiN 4 peak is indicative of the presence of small concentrations of SiC 4 tetrahedra in these HN1B samples. 26 This structural assignment correlates well with previous observation of SiC nanocrystallites in this ceramic by TEM and selected area electron diffraction (SAED). 16 At a spinning speed of 6 KHz, the Si 3 N 4 peak at~À48 ppm contains spinning side bands that indicate nonzero chemical shift anisotropy (CSA).…”

“…The SiN 4 peaks in these 29 Si MAS NMR spectra are asymmetric with a tail in the low‐field region that becomes visible as a shoulder at a pyrolysis temperature of 1400°C. This shoulder on the SiN 4 peak is indicative of the presence of small concentrations of SiC 4 tetrahedra in these HN1B samples . This structural assignment correlates well with previous observation of SiC nanocrystallites in this ceramic by TEM and selected area electron diffraction (SAED) .…”

“…16,23,24 Several structural studies of polyborosilazane-derived SiB-CN PDCs have also been reported in the literature. 19,21,22,25,26 The structure is generally believed to consist of amorphous nanodomains of SiCN, BN, and C. A nuclear magnetic resonance (NMR) spectroscopic study of SiBCN ceramics derived from a boron-modified polyhydridovinylsilazane precursor showed that, after pyrolysis to 1050°C, these ceramics contain a SiCN matrix, where Si atoms are bonded to both N and C atoms in SiC x N 4Àx tetrahedral units. 15 The incorporation of these mixed bonding environments in the SiCN network is proposed to be one of the factors that leads to a thermodynamically stable ceramic at high temperatures.…”

Single-Source-Precursor Synthesis of Hafnium-Containing Ultrahigh-Temperature Ceramic Nanocomposites (UHTC-NCs)

“…The 29 Si NMR spectra (Figure ) show signals for Si sites with bonds to C and N, i.e., SiC x N 4– x (0 ≤ x ≤ 4) tetrahedra . The chemical shift and the fractions of the different Si sites were evaluated upon deconvolution of the 29 Si NMR spectra and are summarized in Table .…”

“…Please note that, upon the hydroboration process of the hafnium-modified polysilazane, first, BC 3 sites are expected, which obviously undergo rearrangement reactions already at temperatures as low as 400 °C to convert into BCN 2 and BN 3 . Considering the evolution of the Si sites with temperature (SiC 2 N 2 sites get consumed and the amount of SiCN 3 decreases as the synthesis temperature increases; see Table and Figure ), we assume that the BC 3 units undergo exchange reactions with SiC 2 N 2 sites to generate BCN 2 and SiC 4 units as follows: BC 3 + SiC 2 N 2 = BCN 2 + SiC 4 . Similarly, the formation of BN 3 can be explained as 2BC 3 + 3SiC 2 N 2 = 2BN 3 + 3SiC 4 and BC 3 + SiCN 3 = BN 3 + SiC 4 .…”