Initial stages of SiO x deposition on graphite
- PDF / 9,452,883 Bytes
- 7 Pages / 576 x 792 pts Page_size
- 51 Downloads / 190 Views
J. B. Danner and J. M. Vohs Department of Chemical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6272
D. A. Bonnell Department of Materials Science, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6272 (Received 2 July 1993; accepted 16 December 1993)
The reaction of tetraethoxysilane (TEOS) and the subsequent deposition of SiO* on the basal plane and edges of highly oriented pyrolytic graphite (HOPG) were studied. Interfacial bonding and surface morphologies resulting from different reaction conditions were probed using scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), Rutherford backscattering spectroscopy (RBS), temperature programmed desorption (TPD), and high resolution electron energy loss spectroscopy (HREELS). The initial reaction of TEOS was found to occur at surface defects. STM images indicated that SiO;,: films do not grow layer-by-layer, confirming earlier indirect observations to that effect.
I. INTRODUCTION Although carbon has been successfully used in a number of structural applications, its potential as a structural material has not yet been fully realized. This is particularly true in high temperature applications where oxidation is often the limiting factor, in spite of the use of rather sophisticated protection strategies. The fundamental issues involved with oxidation protection of carbon have been reviewed by Buckley,1 and Strife and Sheehan.2'3 Regardless of whether oxidation protection involves diffusion barriers, chemical inhibition, or a combination of the two, atomic bonding in the coating and at the carbon interface dictates its efficacy. There are numerous reports in the literature on various aspects of the use of coatings to inhibit the oxidation of carbon and its composites. The effectiveness of carbides, nitrides, silicides, and noble metals as oxygen diffusion barriers has been well documented.2"6 The optimum choice of a coating material for a particular application depends on its vapor pressure, melting point, oxygen permeability, and coefficient of thermal expansion. Borate-based glass formers are used at temperatures below 1500 °C, while silicon-based systems (Si3N4 and SiC) are limited to 1800 °C. For applications above 1800 °C refractory oxides and titanates have been investigated and found to be appropriate for short time, single-cycle applications. Phosphates, borates, and chlorides are thought to be chemical inhibitors which poison reactive sites by bonding to undercoordinated carbon atoms.2'5'7'8 In order to accommodate thermal expansion J. Mater. Res., Vol. 9, No. 4, Apr 1994 http://journals.cambridge.org
Downloaded: 15 Mar 2015
mismatch, strategies for oxidation protection above 1800 °C usually include layers of several compounds, for example, refractory oxide/silicate glass/refractory oxide/carbide. Optimization of current methods and development of new approaches of oxidation inhibition require a systematic understanding of reactions of carbon with potential coating materials. The reactions of oxides, n
Data Loading...
