Silicon −29 Solids NMR-MAS Characterization of Non-Oxide Powders and Fibers

Abstract

Powder x-ray diffraction has been the most common method for rapid structural analysis and identification of crystalline ceramic raw materials.However, new structural ceramic raw materials often have linebroadened or featureless XRD patterns due to structural strain or small crystallite size. Characterization of various silicon carbide and silicon nitride powders and ceramic fibers by Si-29 NMR-MAS spectroscopy has revealed structural details and differences in these materials previously indistinguishable by XRD or other routine methods.Unique variables intrinsic to a given ceramic synthesis process are reflected in the NMR-MAS spectra of the resulting products.

A solid state 29Si and 13C NMR study on the synthesis of thin silicon–oxycarbide glass sheets by a sol–gel route

“…From the 29 Si NMR spectra it follows that during the heat treatment of the starting gels at 400 ³C, the less condensed units (T 1 , T 2 and Q 3 ) condense further thus resulting, as expected, in more completely condensed T 3 and Q 4 units and a higher degree of condensation. This trend of increasing condensation as a function of temperature was also found to continue in the samples heat treated at 600 ³C.…”

“…DTA±TG studies under synthetic air showed that the removal of ±CH 3 groups begins around 400 ³C in such gels containing colloidal silica 24 and thus this temperature was chosen as the upper temperature for the heat treatment in air. 29 Si NMR spectra of the samples, containing 0 or 0.25 mol of colloidal silica per mol of alkoxide, heat treated at 400 ³C in air (type A) or nitrogen atmosphere (type N), are shown in Fig. 3.…”

“…4. Table 2 shows the percentage signal intensities of each SiC x O 42x site, determined using integration and deconvolution of the 29 Si NMR spectra for different samples. An increase in the amount of Q units and a decrease in the amount of M, D and T units is observed on heating the glasses to temperatures of w900 ³C.…”

“…The presence of such amounts of hydrogen for temperatures up to 1000 ³C is consistent with literature data. 20 The above 29 Si NMR and chemical analysis results are MTC00-80 5 25 62 Ð 4 4 100/9 MTC00-400A Ð 4 9 Ð 53 34 100/658 MTC00-400N 1 15 77 Ð Ð 8 100/8 MTC25-80 4 30 46 1 2 17 100/25 MTC25-400A 5 11 41 Ð 9 35 100/79 MTC25-400N 3 11 66 3 4 13 100/25 The stoichiometric formulae derived from 29 Si NMR (Table 2) are in fairly good agreement with those derived from chemical analysis ( Table 3). The differences are more marked for samples heat treated at 900 ³C as compared to those sintered at higher temperatures.…”

“…It was a curiosity, however, to check whether it is this change in porosity which is responsible for the above behaviour during sintering, or if there is any further structural modi®cation induced because of the incorporation of colloidal silica. The objective of the present study was to follow the structural evolution of oxycarbide phases during heat treatment of gel sheets up to 1450 ³C by DTA±TG, solid state 29 Si and 13 C MAS NMR and elemental analysis, in order to understand the above effects. Since the amount of colloidal silica had no signi®cant effect on the mechanical properties of the glasses, glasses containing 0.25 mol of colloidal silica per mol alkoxide and those containing no colloidal silica only, have been studied in the present work.…”