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Single crystalline α-Si 3 N 4 nanowhiskers were synthesized by direct nitridation of the NH 3 -pretreated silicon powders under N 2 /H 2 atmosphere without adding any catalysts. The phase composition and microstructure of the as-obtained products were investigated. It is shown that the NH 3 pretreatment has a great influence on the phase and microstructure of the products. With NH 3 pretreatment, single crystalline α-Si 3 N 4 nanowhiskers were obtained. Whereas, the dominant β-Si 3 N 4 particles were formed without NH 3 pretreatment. The isolated α-Si 3 N 4 crystal nuclei and the relatively thick oxide layer were formed on the surface of the silicon particles after pretreating by NH 3. The growth of the α-Si 3 N 4 nanowhiskers is obeyed the vapor-solid (VS) mechanism. [ABSTRACT FROM AUTHOR]

Silicon oxynitride nanopowders were successfully prepared by thermal explosion of the Si and SiO 2 mixed powders with the different particle sizes in various molar ratio (x / y = 2.0, 1.5) under N 2 , supplemented by the subsequent gravity separation strategy. The effects of Si/SiO 2 molar ratio, preheating temperature and insulation time on the phase composition of the as-oxynitrided products in the thermal explosion strategy were investigated. Meanwhile, the effects of sedimentation time on the phase composition and oxygen content of the as-obtained products in the gravity separation strategy were also discussed. It is shown that the by-products are absent in the as-oxynitrided products with a 1.5:1 M ratio. Prolonging the insulation time or preheating temperature is beneficial to improve the content of Si 2 N 2 O phase with a 2.0:1 M ratio. All the by-product phases are disappeared in the as-oxynitrided products with a 2.0:1 M ratio after sedimentation treatment of 120 s. The as-obtained silicon oxynitride nanopowders with 58% of recovery and 16.2% of oxygen content are close to the theoretical stoichiometric ratio of Si 2 N 2 O. The preparation mechanism of silicon oxynitride nanopowders was proposed. [ABSTRACT FROM AUTHOR]

In this work, millimeter-scale Si 3 N 4 @SiO x core-shell structure nanowires were synthesized using the direct nitridation method without the use of catalysts. The silicon powders used as raw materials were pretreated at 1300 °C for 3 h under ammonia. The ammonia was then converted into a nitrogen/hydrogen gas mixture and heated at 1350 °C for another 5 h. The morphology and structure of the as-obtained nanowires were investigated in detail. The Si 3 N 4 @SiO x nanowires with a core-shell structure were composed of a crystalline α-Si 3 N 4 core and an amorphous SiO x shell. The formation of the α-Si 3 N 4 core nanowire is due to the reaction of the N 2 gas with the byproduct SiO gas. However, the formation of the amorphous SiO x shell is related to the deposition location of the samples. For the inner part of the samples, the SiO x shell derives from the deposition of the byproduct SiO gas at 1350 °C, while the SiO x shell of the external part of the samples likely derives from the oxidation of the silicon nitride core during the cooling process. [ABSTRACT FROM AUTHOR]

In this work, the quasi-spherical β-Si3N4 powders were synthesized via an efficient direct nitridation strategy with CaF2 as the catalytic material under NH3 atmosphere. The effect of CaF2 on phase composition and crystalline morphology was studied. CaF2 additive can accelerate the nitridation of silicon powders, and the particles of nitridation products tend to have an equiaxed structure with the CaF2 additive increasing. When 4 wt% CaF2 additive or more was added, submicron β-Si3N4 particles with quasi-spherical morphology and eminent crystal integrity were obtained. In contrast, irregular α-Si3N4 particles appear as the main phase with less than 4 wt% CaF2 additive. The growth mechanism of Si3N4 particles was also discussed. CaxSiyOz liquid phase is crucial in the nitridation of silicon powders with CaF2 additive. [ABSTRACT FROM AUTHOR]