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.