Effect of Silicon Exposure on Pyrolysis of Preceramic Polysilazane
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EFFECT OF SILICON EXPOSURE ON PYROLYSIS OF PRECERAMIC POLYSILAZANE HELEN N. HAN, DAVID A. LINDQUIST, JOHN S. HAGGERTY AND DIETMAR SEYFERTH Massachusetts Institute of Technology, Cambridge, MA 02139 ABSTRACT Preceramic polysilazane was added to high purity Si powders as a reactive binder during synthesis of reaction bonded Si3 N4 parts to improve the defect structures in both the unfired and nitrided parts. This study examined how the presence of Si powder affected the polymer-to-ceramic transformation of the polysilazane. Pyrolysis of both polysilazane and polysilazane+Si samples were characterized by TGA. Also, the gaseous volatiles evolved during pyrolysis were characterized by infrared and mass spectrometry. Results show that between 1005500C, silicon powder inhibits the loss of silazane oligomers from the polysilazane. However, for temperatures above 5500C, the weight loss differences between the pure polysilazane and the polysilazane+Si samples were less pronounced. Specific solvents used in the preparation of the polysilazane+Si mixtures affected the quantity and type of species in the volatiles during pyrolysis. INTRODUCTION A potentially improved means of fabricating complex net-size, net-shape Si3 N4 pieces results from combining polymer-pyrolysis and reaction-bonding processes. In this approach, a preceramic polymer is used as a reactive binder for the silicon powder. During the forming process, the polymer acts as a traditional binder by holding the powder together, thus providing green strength for the unfired part. However, unlike conventional organic binders which must be completely volatilized in a nonoxidizing atmosphere, often leaving carbon residues, the inorganic preceramic polymers may be pyrolyzed directly into Si 3 N4 . Direct conversion of this type of binder can decrease the impurity content of the resulting Si3 N4. In addition, the preceramic polymer binder should form necks between nearly touching particles in the Si powder compact, thereby improving connectivity and healing flaws during nitridation. Modeling this duplex process requires investigating both the effects of the preceramic polymer on nitridation of the Si powders, and the effects of the Si on the polymer during reactive pyrolysis. Our previous research has investigated the effect of polysilazanes on the nitriding kinetics of high purity Si powders [1]. These results showed that, although nitridation was inhibited by silazane binders through their effect on Si 3N4 nucleation, complete conversion was achieved in N2 in 10 h at 14000C. This research investigates the effects of the Si particles on the pyrolysis process. EXPERIMENTAL The polysilazane utilized in this study was donated by the Ethyl Corporation, Baton Rouge, LA with designation: Type A (synthesized on 12-19-89). It was synthesized by a modification of the Seyferth-Wiseman process [2-4] which uses a three step process involving ammonolysis of methyldichlorosilane to form a mixture of cyclic silazanes followed by a treatment of the latter with a catalytic amount of KH,
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