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  • Chapter 32

    Boron Nitride and Its Precursors

    K. J. L. Paciorek 1, W. Krone-Schmidt 1, D. H. Harris 1, R. H. Kratzer 1, and Kenneth J. Wynne 2

    1Ultrasystems Defense and Space, Inc., Irvine, CA 92714 2Office of Naval Research, Arlington, VA 22217

    Stepwise condensation of a series of borazines was investigated. The compounds studied were B-tri-chloro-N-triphenylborazine, B-triamino-N-triphenyl-borazine, B-trianilinoborazine, B-tris[di(trimethyl-silyl)amino]-borazine, B-trichloro-N-tris(trimethyl-silyl)borazine, and B-triamino-N-tris(trimethyl-silyl)borazine. The formation of preceramic polymers occurs by ring opening mechanism which leads to singly and doubly joined borazine rings. This process requires the presence of a combined total of six protons on the ring and exocyclic nitrogens. Compounds where this arrangement is absent, B-trichloro-N-triphenylborazine, B-tris[di(trimethylsilyl)amino]borazine, and B-trichloro-N-tris(tri-methylsilyl)borazine, failed to undergo condensation to any significant degree. Materials obtained from the phenyl-substituted borazines gave essentially nonprocessible polymers on pyrolysis; from B-tri-amino-N-tris(trimethylsilyl)borazine, preceramic polymers amenable to fiber manufacture were obtained. Investigation of potential linear precursor to boron-nitrogen polymers resulted in the synthesis of -imido-bis[bis(trimethylsilyl)-aminotrimethylsilylamino]borane (I) and -x-bis-[bis(trimethylsilyl)aminotrimethylsilylamino]borane (II). The crystal structures of the two compounds were essentially identical with the exception of the bridging atoms and their immediate environments. The same applied to mass spectral breakdown patterns wherein the major fragments differed by one amu, with the exception of the ion 332+ which is characteristic to the oxygen-bridged compound.

    R e f r a c t o r i e s such as boron n i t r i d e , s i l i c o n n i t r i d e , s i l i c o n carbide, and boron carbide are of great importance f o r the product i o n or pr o t e c t i o n of systems which can be operated i n very high

    0097-6156/88/0360-0392506.00/0 1988 American Chemical Society

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    In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

  • 32. PACIOREK ET AL. Boron Nitride and Its Precursors 393

    temperature environments. The a p p l i c a b i l i t y of these materials as coatings, f i b e r s , as well as bulk items depends on the a b i l i t y to produce a r e a d i l y p r o c e s s i b l e polymer, which can be formed in t o the desired shape of the f i n a l item p r i o r to exhaustive p y r o l y s i s and transformation into a ceramic. The technology to provide such precursors f o r s i l i c o n carbide, s i l i c o n n i t r i d e , and graphite, i n p a r t i c u l a r the l a t t e r , has been developed and although work i s p r o c e e d i n g on improvements, the premises are r e l a t i v e l y well e s t a b l i s h e d Q ) . The s i t u a t i o n i n the case of boron n i t r i d e i s e n t i r e l y d i f f e r e n t . The extensive work performed i n the s i x t i e s was di r e c t e d at thermally s t a b l e polymers. The products were poorly characterized and the l i t e r a t u r e data are c o n f l i c t i n g (2). More recently, claims have been made to the synthesis of BN preceramic polymers amenable to f i b e r production using the p y r o l y s i s of B-triamino-N-triphenylborazine (3). Unfortunately, t h i s work could not be reproduced (4,5).

    Boron n i t r i d e , i n view of i t s unique p r o p e r t i e s , namely absence of e l e c t r i c a l c o n d u c t i v i t y , oxidation r e s i s t a n c e , o p t i c a l transparency, and high neutron capture c r o s s - s e c t i o n f o r s p e c i a l a p p l i c a t i o n s , o f f e r s advantages over other ceramics. Thus, f o r the past several years the group at Ultrasystems has been a c t i v e l y involved i n i n v e s t i g a t i n g routes leading to preceramic BN polymers using both c y c l i c and l i n e a r s t a r t i n g materials. Some of the find i n g s generated w i l l be discussed.

    Results and Discussion

    Borazine synthesis. The ease of borazine r i n g condensation would be expected to depend on the nature of the substituents. To i n v e s t i -gate these aspects, a s e r i e s of compounds were synthesized, namely B - t r i c h l o r o - N - t r i phenyl b o r a z i n e , B-triamino-N-triphenylborazine, B - t r i a n i l i n o b o r a z i n e , B - t r i s [ d i ( t r i m e t h y l s i l y l ) a m i n o ] b o r a z i n e , B - t r i c h l o r o - N - t r i s ( t r i m e t h y l s i l y l ) b o r a z i n e , and B-triamino-N-tris-( t r i m e t h y l s i l y l ) b o r a z i n e .

    B - t r i c h l o r o - N - t r i p h e n y l b o r a z i n e , mp 290-292C, was obtained i n 86* y i e l d f o l l o w i n g the procedure of Groszos and S t a f i e j (6). T h i s m a t e r i a l was then transformed i n t o B-triamino-N-triphenyl-borazine i n 67% y i e l d using the method of Toeniskoetter and Hall (1) B - t r i a n i l i n o b o r a z i n e and the novel B - t r i s [ d i ( t r i m e t h y l s i l y l ) -aminojborazine (mp, 131.5-132C.; characterized by GC/MS, molecular ion 558 amu, and elemental a n a l y s i s ) were synthesized i n 76 and 71* y i e l d s , r e s p e c t i v e l y , by i n t e r a c t i o n of a n i l i n e and hexamethyl-d i s i l a z a n e with chloroborazine i n the presence of triethylamine.

    The s y n t h e s i s of B - t r i c h l o r o - N - t r i s ( t r i m e t h y l s i l y l ) b o r a z i n e was much more complicated than that of the other borazines. The o v e r a l l scheme i s given below:

    BC1 3 + E t 3 N B C l 3 ' N E t 3 CI

    Me QSiN NSiMe Q 3 I I 3 CIB^

    HN(SiMe.) 2 +

    NEt 3 ^ B C l P y r o l y s i s

    I SiMe

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    In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

  • 394 INORGANIC AND ORGANOMETALLIC POLYMERS

    Boron t r i c h l o r i d e - t r i e t h y l a m i n e adduct, mp 89-91C, was obtained i n 80* y i e l d f o l l o w i n g e s s e n t i a l l y the procedure of Ohashi, et a l . (8). The product was washed w i t h methanol, but not c r y s t a l -l i z e d from i t . I t was found that c r y s t a l l i z a t i o n from d i l u t e e t h a n o l r e p o r t e d by G e r r a r d , L a p p e r t , and Pearce (9) caused extensive degradation . B i s ( t r i m e t h y l s i l y l ) a m i n o d i c h l o r o b o r a n e was prepared f o l l o w i n g the procedure of Wells and C o l l i n s (10).

    The p y r o l y s i s of b i s ( t r i m e t h y l s i l y l )aminodichloroborane to B - t r i c h l o r o - N - t r i s ( t r i m e t h y l s i l y l ) b o r a z i n e , contrary to l i t e r a t u r e (11^, d i d not take place i n b o i l i n g xylene. Temperatures above 150 C were necessary f o r t r i m e t h y l c h l o r o s i l a n e e l i m i n a t i o n . The highest y i e l d of the r e l a t i v e l y pure product was around 20*. The transformation of the B - t r i c h l o r o - N - t r i s ( t r i m e t h y l s i l y l J b o r a z i n e to B - t r i a m i n o - N - t r i s ( t r i m e t h y l s i l y l Jborazine proceeded r e a d i l y using l i q u i d ammonia.

    B o r a z i n e P y r o l y s i s Studies. In a s i m p l i s t i c manner, one can v i s u a l i z e the formation of boron n i t r i d e v i a stepwise e l i m i n a t i o n s between r i n g s , i . e . ,

    R I

    X MONOMER PRECERAMIC POLYMER

    \

    BORON NITRIDE CROSSLINKED POLYMER

    g i v i n g i n i t i a l l y the " l i n e a r " preceramic polymer, then the l i g h t l y c r o s s l i n k e d s t r u c t u r e B, followed by a p a r t i a l B-N system, C, and f i n a l l y pure boron n i t r i d e . The p y r o l y s i s techniques u t i l i z e d were f u l l y described p r e v i o u s l y (4J and thus w i l l not be r e i t e r a t e d here. The p o t e n t i a l borazine candidates can be broadly d i v i d e d i n t o two c l a s s e s of m a t e r i a l s , one where the s u b s t i t u e n t on the r i n g boron i s n i t r o g e n - f r e e , e.g., a halogen, and the second where the su b s t i t u e n t i s an amino moiety. With respect to the s u i t a b i l i t y of any given borazine, the important aspects are the ease of formation of the l e a v i n g molecule and i t s v o l a t i l i t y . In t h i s respect, B - t r i c h l o r o - N - t r i s ( t r i m e t h y l s i l y l ) b o r a z i n e would seem an i d e a l candidate. Unfortunately, p y r o l y s i s at ,^60 C l i b e r a t e d only 6.1* of the expected t r i m e t h y l c h l o r o s i l a n e ; 75* of the s t a r t i n g m a t e r i a l was recovered. Heating at 360C f o r 19 h r e s u l t e d i n 56.0* y i e l d of t r i m e t h y l c h l o r o s i l a n e and a gl a s s y residue. Further heating at

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