Determination of phase composition of silicon nitride powders by silicon-29 magic angle spinning nuclear magnetic resonance spectroscopy

Abstract

absolute number densities of neutral atom and ion a t the ground states by using the absorption or fluorescence technique (1 0, 51, 52). Registry No. Ar, 7440-37-1; Mg, 7439-95-4. LITERATURE CITED (1) Demers, D. R.; Allemand, C. 0. Anal. Chem. 1981, 53, 1915. (2) Fassel. V. A.; Knlseley, R. N. Anal. Chem. 1974, 46, 1llOA. (3) Houk, R. S.; Fassel, V. A.; Flesch, G. D.; Svec, H. J.; Gray, A. L.; Taylor, C. E. (IO) Nojlri, Y.; Tanabe, K.; Uchida, H.; Haraguchl, H.; Fuwa, K.; Wlnefordner, J, D.The use o f aolkhtate %I NMR spectroscopy to analyze q N 4 powders b demonstrated for a series of commercial and research samples. I n particular, two dlfferent commercial powders contaln 20% and 30% amorphous Si3N4 content, as identtfied by the applicatlon of methods described herein.These same materlals had been analyzed by X-ray dmtaction as only crystalline SiaNp NYR spectroscopy is capable of dlstlngulohlng among dlfferent amorphous silicon specks normally found In preparatlons of S13N4 powders. These Include amorphous S13N4, sHlcon oxynltrldes, sllicates, and elemental slllcon. The measurement of concentration of these species Is made for correlatlon wHh properties of powder sinterability.Sinterable silicon nitride (Si3N4) has been intensely investigated since 1974 because of potential applications as a tough, refractory ceramic material (1). The production of reliable and cost-effective structural Si3N, ceramic by sintering of powders must begin with powders that have, among other qualities, an a-phase content in excess of 85% (2). Excessive @-Si3N4 in the powder interfers with microstructure changes that accompany sintering. On the other hand, some a-Si3N4 (amorphous) can aid densification of the final material (3).Oxygen and elemental silicon can also aid sintering, although concentrations in excess of 2% can deteriorate mechanical properties at elevated temperatures. Oxygen normally appears as amorphous silicon oxynitrides and silicates.In view of what is known about the effects of the powder's phase and purity on sinterability and fiial ceramic mechanical properties, it is necessary to have rapid, reliable methods for the determination of the crystallinity and purity of batches of Si3N4 powder before carrying out final product formation and sintering. Traditionally, X-ray powder diffraction has been used to determine the presence of @-Si3N4, silicon oxynitrides, silicates, and silicon in the a-Si,N4 powders. The powder diffraction technique fails, however, in the identification of amorphous species. All amorphous species contribute to the background diffraction signal whose intensity is difficult

Solid-state NMR investigations of Si-29 and N-15 enriched silicon nitride

“…The 29 Si chemical shifts and peak-widths are collected in Table 1: the NMR resonance positions obtained in our work are very similar to those previously observed in Refs. [3,6,[15][16][17] and incidentally identical to those reported for each Si 3 N 4 modification by Carduner et al [15,16].…”

“…We further note that overall slower T 1 relaxation is observed in our work (and in Ref. [17]) compared to earlier reports [6,16]. However, it is difficult to draw conclusions between different samples and studies, considering that the presumably sole relaxation mechanism in these materials is stemming from (possibly varying) paramagnetic impurities.…”

“…Note that for the Si 3 15 N 4 sample having 29 Si at n.a., the spin-0 isotopes 28 Si and 30 Si constitute the sole neighboring nuclei for E95% of all 15 N spins. This accounts for the very narrow 15 N peak-widths (p27 Hz) from Si 3 15 N 4 ; they are most likely dominated by spin-spin (T 2 ) relaxation-stemming homogeneous broadenings, and possibly also small shift-dispersion contributions [16] due to the slight variations in Si-N bond-lengths at each 15 N site [14]. However, in a fully 29 Si-enriched sample, each 15 N site experiences J-couplings to three 29 Si neighbors.…”

“…The two modifications of Si 3 N 4 have been characterized by 29 Si MAS NMR in several investigations [3,4,6,[15][16][17]. The 29 Si site of b-Si 3 N 4 produces a single 29 Si NMR peak around À49 ppm.…”

A study of nitridation process of polycarbosilane fibers by solid-state high-resolution NMR

“…More definitive changes are observed in the spectra of polycarbosilane fibers heated at £176 The intensities of the signals from SiC3H and SiC4 are very small compared with that of 580~ The --45 ppm signal is assigned to the signal of Si--N bonds coming from amorphous Si3N 4 (--48 ppm in [8]). The signal near --100 ppm is assigned to surface oxides.…”