Characterization of laser synthesized silicon nitride powders with nanoscale by solid-state NMR

Abstract

Interest in the preparation of nano-powders has increased since •anoscale powders can be used to produce fine-grained homogeneous materials with unique properties [1, 2]. It has been shown recently that nanoscale Si-based powders can be formed by laser-induced reactions from silica or mixture gaseous [3, 4]. Silicon nitride nano-powder, having many applications in microelectronics and the mechanical and chemical industries, has been prepared from SiH4 and gaseous NH3 by this method [5]. Conventional analysis techniques such as X-ray diffraction, electron microscopy, IR spectroscopy, etc., have been used to characterize the laser synthesized powders. These techniques, giving important structural and morphological information on the long-range order, are not efficient in studying the short-range order in the case of amorphous or poorly crystalline materials. Solid state NMR using magic angle spinning (MAS) appears to be a powerful non-destructive tool for the characterization of amorphous or poorly crystalline materials such as nano-powders, since the frequencies of the resonance line or chemical shifts are characteristic of the environment of a given atom and neighbouring groups [6]. The experiment of cross-polarization (CP) from 1H to rare spins is used to enhance the signal intensity and to improve the accumulation efficiency [7]. In this letter we present the characterization of nano-powder silicon nitride laser-prepared by using solid state MAS NMR techniques. The process of silicon nitride powder preparation has been described in previous work [5]. Conventional analysis techniques have been used for the products. The results indicate that the powders are superfine (8-50 rma), quite uniform, nearly monodispersed in size. The experiments of solid state NMR were performed on a Bruker MSL-400 NMR spectrometer. The NMR spectra were recorded at a resonance frequency of 79.4MHz for 29Si and 400.13 MHz for ~H, corresponding to a magnetic field of 9.4 T. The sample spinning speeds were 4 kHz. The experiment of cross-polarization from proton to silicon nuclear was carried out under proper matching conditions. Tunnelling Mass Spectroscopy (TMS) was used as reference for chemical shifts. 29Si MAS and CP/MAS NMR spectra of laser-