A model describing the pyrolysis of an organo-silicate polymer (Blackglas polymer) to form a silicon oxycarbide glass was developed based on the known chemistry and architecture of the polysiloxane precursor. The objectives were to develop a plausible reaction pathway to explain the pyrolysis process, rate, and product spectrum, and to study the effect of various heating protocols on the pyrolysis process. The model successfully predicted the evolution rates of the major gases as a function of process temperature and the overall pressures reached during the process. The effects of various heating protocols on the outgassing kinetics were studied to develop an optimum protocol for a rapid pyrolysis process that gives a composite with desirable mechanical properties. Overall, the model appears to capture the essential characteristics of the process and is in good agreement with experimental results.

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Introduction

The development of pyrolysis routes to form ceramic materials is attractive because the polymeric precursors can be molded easily at relatively low temperatures and then fired to produce near net shapes. These systems can also be adapted for rapid prototyping operations. One such process uses a polysiloxane precursor to form Blackglas, a refractory silicon oxycarbide, invented at Allied Signal (Leung et al., 1994; Annamalai et al., 1996). The Blackglas silicon oxycarbide offers the ease of fabrication of a polymer and the high temperature stability and capabilities of a ceramic. It is also envisioned to be an excellent candidate for use as a composite matrix to operate in the 260-1,200 deg C regime. A generic fabrication process for converting Blackglas or similar polysiloxane precursors to ceramics is shown in Figure 1. In these fabrication processes, the pyrolysis step is the bottleneck due to the slow diffusion and the low saturation concentrations of evolved gases in the resin (Dente and Ranzi, 1983). Proper control of the rate of pyrolysis and postcure temperature may enable one to minimize microcracks and pores and to relate changes in the composition of the matrix to the physical and mechanical properties of the final product (Hurwitz et al., 1995; Meador et al., 1996; Wang et al., 2000).

A lumped parameter model was previously developed (Annamalai et al., 1995, 1996) to enable one to control the rate

of mass loss as a function of time or temperature for a given heating protocol. However, the mechanical and thermal properties of the product and the composition of the evolved gases are functions of the processing variables and the chemistry of the precursor. The lumped model did not address these issues. This article describes how a mechanistic approach that considers the chemistry of the precursors, the chemical pathways for the dissociation and redistribution reactions leading to gas evolution, and the microstructure and properties of the product can be useful in designing a pyrolysis protocol for producing a useful ceramic part in minimal time.

Acknowledgments

This work was partially supported by ARPA Technology Development Agreement No MDA 972-93-2-0007 administered by the USAF Wright Laboratory Materials Directorate.

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Manuscript received Apr. 25, 2001, and revision received Mar. 25, 2002.

Joel L. Plawsky, Feng Wang, and William N. Gill

Dept. of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180