Properties and Structural Characterization of the Laser Synthesized Nano-Composite Si-N-C Powders
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ABSTRACT The properties and structure characteristics of the ultrafine Si-N-C powders synthesized from laser induced hexamethyldisilazane and ammonia gas phase reactions were characterized by multiple techniques. Result shows that the Si-N-C particles are uniform in diameter with narrow size distributions and less agglomerated. The powders exhibited a short distance ordered structure which was distorted at high temperature annealing. HTEM observation shows that the particles are amorphous or have nano-composite structure with SiC particles dispersed in the amorphous Si-N-C matrix. The nucleation and growth of the composite particles occurred by the polymerization of the ligand formed from the decomposition of SiCH 3 and N-H bonds in hexamethyldisilazane accompanied by the release of small hydrocarbon gases.
1 INTRODUCTION Recent interests in achieving high mechanical performance structural ceramic materials lead to the development of those so called "Nano Composite" Si 3N4-SiC (NCSNC) materials [1-5]. This NCSNC materials are generally fabricated by conventional sintering of amorphous Si-N-C powders [1], Si3N4-SiC composites [3], and the mixture of monolithic nano or submicro Si3N4 and SiC powders [5]. The NCSNC structure can not be obtained by sintering the mechanically mixed traditional coarse Si 3N4 and SiC powders, which only yields a macrosize composite of Si 3N4-SiC bodies without predominant mechanical performance improvement [6-7]. As a required precursor powders for formation of the NCSNC materials, the Si-N-C and Si 3N4-SiC nano powders were studied extensively on their synthesis [8-10]. We previously reported the formation of Si-N-C nano powders from laser pyrolysis of hexamethyldisilazane and ammonia gases [11-12]. The studies showed that the laser synthesis process can produce the nano Si-N-C powders with high yield and wide composition range depending on processing parameters. These powder characteristics were attributed to the unique design of the reactant flow and the laser heating sources in the process forming a steady small reaction zone , and the facility process control of powder characteristics by processing parameters [13]. In the present paper, we have studied the properties and structural 269 Mat. Res. Soc. Symp. Proc. Vol. 363 01995 Materials Research Society
characteristics of the composite Si-N-C ceramic powders synthesized by the laser induced HMDS+NH 3 vapor phase reactions.
2 EXPERIMENTAL The nano Si-N-C powders were synthesized by CO 2laser pyrolysis of HMDS+NH 3 gases. A detailed powder synthesis process and principles were reported elsewhere [14]. The chemical compositions and characteristics of the Si-N-C powders used for the present study are lined in Table. Table Chemical compositions and characteristics of Si-N-C powders Compositions (wt%) C Si N 51
29
20
TEM Diameter (nm)
SAXD Diameter (nm)
BET (m2/g)
20
26.3
162.4
Fourier Transform Infrared (FTIR) spectra were carried out on Niclet-510P FT-IR spectrometer by pressing the mixture of Si-N-C powders (0.1mg) and the
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