Study of the Local Atomic Order in Si/C/N Nanopowders by X-ray Photoelectron Spectroscopy
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A.GHEORGHIU-de LA ROCQUE, G.DUFOUR, P.A.BONNEFONT, C.SENEMAUD Laboratoire de Chimie-Physique, URA CNRS 176, Universit6 Pierre et Marie Curie II rue Pierre et Marie Curie, F-75231 Paris Cedex 05 M.CAUCHETIER CEA Saclay, Service des Photons, Atomes et Mol6cules F-91191 Gif Sur Yvette Cedex
ABSTRACT A study by X-ray photoelectron spectroscopy (XPS) of the local atomic environment in laser-synthetized Si/C/N nanopowders is presented. The Si 2p, C Is, and N Is core levels as well as the valence band distribution are investigated. The attention is focused on samples with C/N atomic ratio around 0.6 which exhibit interesting properties concerning annealing treatments. The presence of C-N bonds after 1500'C annealed samples is evidenced.
INTRODUCTION In recent years a wide interest has been paid to Si-based nanosized powders due to their possible applications in various fields and particularly in the range of ceramics with improved thermomechanical properties (1). The production of such nanopowders by laser irradiation of an organic precursor appears as a powerful method which allows to obtain nanosized particles with high chemical purity and with controlled size, chemical composition and crystallinity. For example, ultrafine powders have been synthesized by laser spray pyrolysis of a liquid precursor hexamethyldisilazane (WADS) (2). Samples with atomic ratio C/N in the range 0.34 to 1.34 have been obtained by adjusting the experimental conditions (3). We present in this paper a study of the local atomic and electronic structure in Si/C/N nanopowders by X-ray photoelectron spectroscopy (XPS). By this method valuable informations concerning the local bonding around each atomic site are obtained from the study of core levels (4); our study have concerned the Si 2p, N Is and C Is core levels. Moreover the total valence band distributions which give complementary information on the chemical bonding in the samples have been analyzed (5). We have focused our attention on samples with intermediate compositions, close to C/N equal 0.6, and on both as-formed and heated (1500'C and 1600'C) samples. In this compositional range, the C/N value remains approximately constant under annealing treatments and the crystallisation of the samples occurs at higher temperature as compared to lower or higher C/N values. As-formed powders are amorphous. The evolution of the local order with annealing treatments under nitrogen up to 1600'C strongly depends on the C/N value. The presence of CN bonds around both C and N atoms is evidenced after 1500'C annealing, from the study of both C Is and N Is core levels; C-C bonds are also detected. C-N bonds are still observed after a 1600'C annealing and in this case both carbide and nitride phases are present. Preliminary results concerning the effect of incorporating Al compounds in the nanometric scale powders in order to obtain premixed Si/C/N containing the in situ sintering aids are presented (6). EXPERIMENTAL PROCEDURE The nanopowders were synthesized as described previously (2,3). The method is
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