Aluminum-27 and Silicon-29 Solid-State Nuclear Magnetic Resonance Study of Silicon Carbide/Aluminum Nitride Systems:  Effect of Silicon/Aluminum Ratio and Pyrolysis Temperature

Abstract

Three series of SiC/AlN samples were prepared by pyrolysis of “poly(aluminosilazane)”, the product of the reaction between hexamethylcyclotrisilazane and triethylaluminum. The Si/Al ratio was varied for one series (with constant pyrolysis temperature) and the pyrolysis temperature was varied for the other two series, with Si/Al = 1 and 3. 27Al and 29Si magic angle spinning NMR were used to analyze these samples in terms of coordination number, nearest neighbor atoms, and crystallinity. NMR results indicate that higher crystallinity and phase separation into discrete SiC and AlN crystallites accompany an increase in pyrolysis temperature. Disorder and increased heterogeneity in connectivity are observed with increasing Si/Al ratio of the initial reactants.

Pyrolytic conversion of an AlSiNC precursor prepared via hydrosilylation between [Me(H)SiNH]4 and [HAlN(allyl)]m[HAlN(ethyl)]n

“…39 The spectrum of ISP reveals that the allyl : ethyl ratio is 0.7 : 9. The 27 Al NMR spectrum of ISP demonstrates one broad signal centered at 135 ppm, which is assignable to the HAlN 3 environment, 32 and the chemical shift of ISP in the 27 Al NMR spectrum is similar to that of AE-alane. 39 Since the SiH and NH groups in TCS, the C C bonds in the allyl groups and the AlH groups in AE-alane are reactive groups, hydrosilylation between the SiH groups and C C bonds 43 and dehydrocoupling between the NH and AlH groups 24 could occur.…”

“…The solid-state NMR spectra of the pyrolyzed residues were obtained at 100.54 MHz ( 13 C) and 79.43 MHz ( 29 Si) with cross polarization and magic angle spinning (CP/MAS) techniques. The solid-state 27 Al NMR (104.17 MHz) spectra were recorded with the MAS technique only. The residues were packed in sample tubes in a nitrogen-filled glove box and kept under nitrogen flow during the measurements.…”

“…• C, the solid-state 27 Al and 29 Si NMR spectra of the pyrolyzed residue were recorded on the same instrument with the MAS technique only. Five hundred scans were accumulated with a pulse delay of 100 s ( 29 Si) or 1 s ( 27 Al) employing the spinning rate described above.…”

Preparation of Si-Al-N-C Ceramic Composites by Pyrolysis of Blended Precursors

“…The 27 Al NMR spectrum of this precursor exhibited a broad signal at 100 ppm 28,29 assignable to the AlN 4 environment, 30 which was formed by dehydrocoupling between the NH bonds and the AlH bonds in the HAlN 3 environment. Since only the HAlN 3 resonance at 135 ppm is observed and no AlN 4 resonance at 100 ppm is observed in the 27 Al NMR spectra of the blended precursors, dehydrocoupling between the NH bonds in PHPS and the AlH bonds in PEIA occurs only to a very limited extent; essentially no Al-N-Si linkages are formed during the preparation of the blended precursors.…”

“…2B exhibits a broad signal at ࣽࢬ44 ppm, which is assignable to the SiN 4 environment, 30 and no Si 3 N 4 reflection is observed in the XRD pattern of SA11 pyrolyzed at 1000c C. These results suggest the presence of amorphous Si 3 N 4 in the residue pyrolyzed at 1000c C. As described above, the results for the solid-state 29 Si NMR spectrum and XRD pattern indicate the presence of crystalline SiC in the residue pyrolyzed at 1600c C. Based on thermodynamic considerations, the transformation from Si 3 N 4 into SiC can occur above 1440c C via the following reaction:…”