Chemical Reactions During the Thermal Processing of Borazene Polymers
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CHEMICAL REACTIONS DURING THE THERMAL PROCESSING OF BORAZENE POLYMERS R. Rye , T. T. Borek , D. A. Lindquist , and R. T. Paine ,Jandia National Laboratories, Albuquerque, NM 87185 Department of Chemistry, University of New Mexico, Albuquerque,
NM
Abstract A class of borazene polymers has been developed which consist of a twodimensional array of six-membered borazene rings with the borons of adjacent borazene rings separated by -NH- groups. Pyrolysis of these polymers above =1000 C leads to crystalline graphite-like boron nitride (h-BN). The thermal chemistry of thin films of one polymer deposited on KOH etched Al has been examined by thermal decomposition mass spectroscopy (TDMS) and thermal gravimetric analysis (TGA), and the gas evolution chemistry is found to be essentially complete by temperatures of less than 400 C. All products desorb with the same temperature profile and the major desorbing species are NH_ and N consistent with a loss of excess nitrogen and hydrogen in the polymer, ang HCI from decomposition of by-products of the synthesis step. Isotope labeling shows that complete exchange occurs below 400 C between the ring and amino bridge nitrogens. Since the formation of ordered crystalline h-BN films requires heating to temperatures of the order of 1000 C, whereas the gas evolution and ring opening chemistry is complete by roughly 400 C, it is concluded that gas evolution chemical Processes are not rate limiting in BN ceramic production.
Introduction.
B. sorazinyl Ame P,, iotope Labeled
Soluble or fusible polymers offer benefits over powder processing and gas phase deposition schemes for the formation of ceramic materials, and they furnish a systematic framework with which to probe the mechanistic aspects of ceramic formation. In addition, polymers provide access to forms such as fibers, foams and some
coatings
-',-.
insert in Fig. 1.
N."N
4
not available from classical
synthesis routes (1-3). Several groups have recently reported that preparations of polymers suitable for production of boron nitride (4-12) and polyborazinyl amines (5-7), prepared by cross-linking borazenes, are particularly useful. The polyborazinyl amine used here is schematically illustrated by the
"ct
A. DBomnyl AminePolymer
H:
0
... o AtomieMo
so
10s
Units (am-)
Figure 1. Mass spectra at the In the present report thermal maximum decomposition rate. Spectrum decomposition mass spectroscopy A: 1 borazene amine polymer. Spectrum (TDMS), thermal gravimetric analysis B: N labeled polymer. (TGA) and X-ray photoelectron spectroscopy (XPS) are used to identify the major thermal decomposition
Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
1042
the products produced from thin polyborazinyl amine coatings and to explore The pyrolysis chemistry is apparently complete below pyrolysis mechanism. 400 C and this implies that the gas evolution processes are not rate in the thermal conversion of borazene polymers to the graphitelimiting like form of BN (noted here as h-BN). Experimental. The Al substr
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