17 O MAS and static NMR spectra were obtained for lithium, sodium, potassium, cesium, and rubidium disilicate crystals and glasses. Fitting of the 17 O NMR parameters for the crystals including two bridging oxygen (BO) atoms and one nonbridging oxygen (NBO) atom reasonably reproduced the observed spectra. The 17 O NMR nuclear-quadrupole coupling constant (ν Q ) e 2 qQ/h), asymmetry parameter (η) and the isotropic chemical shift were obtained from the line shape simulation. Among these three oxygen atoms, the 17 O NMR chemical shift of the BO(2) and NBO atoms strongly depends on a variety of alkali metal cations, whereas the BO(1) atom did not. The 17 O NMR chemical shift of the O(2) and NBO atoms increased with an increase in the ionic radius of the alkali metal cation. The present 17 O NMR results for crystals, together with those from the literature, provide a revised relationship between the Si-O-Si angles and ν Q . An empirical relationship between the cosine of the Si-O-Si angles and ν Q was found. 17 O NMR spectra for the glasses were fitted with one BO and one NBO atom in terms of a Gaussian distribution of ν Q . The 17 O NMR chemical shifts of both the BO and the NBO atoms depend on the ionic radius of the alkali metal cations in the same direction as the BO (2) and the NBO atoms in the crystals. The ν Q of glass samples was interpreted using the above relation between the Si-O-Si angles and ν Q obtained from the crystalline samples. The estimated average Si-O-Si angles decrease with increasing ionic radius of the alkaline cations. The narrowest distribution was obtained for the potassium-disilicate glass centered at 139°. Li glass has a distribution centered at around 144.°I ntroduction Characterization of the structure and understanding the relationships between the structure and properties of silicate melts and glasses are important in both the earth science and material science fields: the chemical and physical behaviors of magma dominate many geological processes, and most technological glasses and glass ceramics begin in the molten state. The structure and properties of silicate glasses and melts have been investigated by glass scientists and steel making metallurgists using many physical property measurements and structural analysis which have been combined in some review papers. 1,2 Spectroscopic studies are playing an increasingly important role in the understanding of atomic scale bonding and the structure of silicate melts and glasses. Among the many spectroscopic techniques applied to this system, NMR has an advantage for studying the local structure of ions composing the glass network structure. Multinuclear NMR, such as 17 O, 23 Na, 27 Al, and 29 Si, have been successfully used in the structural studies of silicate glasses and melts, because of its sensitivity to the differences in the coordination environments for the nucleus of interest. [3][4][5][6][7][8][9][10][11][12][13] In the 29 Si NMR studies of alkali metal silicate glasses, the local structure around Si has been understood by the ...

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Synthesis of boron nitride spheres (BNS) was achieved by vapor phase pyrolysis of ammonia borane (BH 3 NH 3 ) using two independently temperature-controlled furnaces in a glove box filled with N 2 . The BNS were heated at 13001-17001C in flowing NH 3 , N 2 , or Ar by multistep heat treatment. The sizes of the BNS could be controlled by heating BH 3 NH 3 at different rates (11C/min: 300-800 nm, 51C/min: 300 nm-1.2 lm and 101C/ min: 300 nm-1.8 lm). The microstructures of BNS heated in different ambient gases were observed using transmission electron microscopy with selected area electron diffraction. NH 3 gas produced BNS with well-crystallized surface shells and amorphous cores whereas N 2 and Ar gases crystallized the entire BNS. It is evident that of these three ambient gases (NH 3 , N 2 , and Ar), N 2 gas significantly enhances the crystallization of BN with randomly oriented grains.

Local structure around silicon and nitrogen atoms in Na-Si-O-N oxynitride glasses has been studied by me ans of 29Si MAS NMR spectroscopy. The NMR spectra have been analyzed on the assumption that the resonances of singly nitrified silicate tetrahedra shift towards less shielded direction by about 1.5ppm com pared with those of non-nitrided ones. It has been esti mated that the number of silicon atoms around nitro gen is about 2.5, suggesting that about a half of the total nitrogen atoms may be bonded to two silicon atoms.

The oxidation of α­Si3N4 and β-sialon false(Si6−zAlzOzN8−zfalse) powders with the different z values false(z=1, 2, 3) in a dry atmosphere of normalAr/O2 (80/20 kPa) at 1000-1300°C was studied by X-ray diffraction, 29normalSi and 27normalAl magic-angle spinning-nuclear magnetic resonance (MAS-NMR), transmission electron microscopy (TEM), and thermogravimetric analysis. The oxidation products of the β-sialons consisted of amorphous SiO2 and aluminosilicate, the latter crystallizing to mullite at T⩾1200°C. Very fine acicular mullite grains were formed in the oxidized particles as confirmed by TEM. The initial oxidation kinetics can be described by a two-stage linear law for a degree of reaction less than 10%, followed by parabolic oxidation at 10-60% reaction. In both the linear and parabolic kinetic regions, the oxidation rate constants were very similar for the three sialons but were smaller for α­Si3N4. The activation energies for the linear kinetic regions were 260, 260, 300, and 270 kJ mol−1 for α­Si3N4 and the β-sialons with z=1, 2, and 3, respectively, whereas the corresponding values for the diffusion process were 410, 430, 410, and 380 kJ mol−1. The oxidation mechanism is discussed on the basis of the X-ray diffraction, 29normalSi and 27normalAl MAS-NMR, and kinetic results. © 2001 The Electrochemical Society. All rights reserved.

Early-stage thermal oxidation (below 1100°C) of carbothermally synthesized ␤-sialon powder was monitored by X-ray powder diffraction, solid-state 29 Si and 27 Al MAS NMR spectroscopy, and thermogravimetry. No crystalline oxidation products were detected by XRD but 29 Si and 27 Al MAS NMR indicated the early formation of amorphous silica, followed by the formation of an amorphous aluminosilicate with an atomic environment similar to that of mullite. The initial oxidation was described by a linear kinetic law with an activation energy of 170 kJ⅐mol −1 , suggesting the rate-limiting step to be due to dissolution of O 2 in an amorphous silica surface layer on the ␤-sialon particles.