Chapter 32
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Boron Nitride and Its Precursors K. J. L. Paciorek , W. Krone-Schmidt , D. H. Harris , R. H. Kratzer ,and Kenneth J. Wynne 1
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Ultrasystems Defense and Space, Inc., Irvine, CA 92714 Office of Naval Research, Arlington, VA 22217
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Stepwise condensation of a series of borazines was investigated. The compounds studied were B-trichloro-N-triphenylborazine, B-triamino-N-triphenylborazine, B-trianilinoborazine, B-tris[di(trimethylsilyl)amino]-borazine, B-trichloro-N-tris(trimethylsilyl)borazine, and B-triamino-N-tris(trimethylsilyl)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(trimethyl silyl)amino]borazine, and B-trichloro-N-tris(trimethylsilyl)borazine, failed to undergo condensation to any significant degree. Materials obtained from the phenyl-substituted borazines gave essentially nonprocessible polymers on pyrolysis; from B-triamino-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 μ-imido-bis[bis(trimethylsilyl)aminotrimethylsilylamino]borane (I) and μ-οxο-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 charac teristic to the oxygen-bridged compound. +
Refractories such as boron nitride, silicon nitride, silicon c a r b i d e , and boron c a r b i d e a r e o f g r e a t importance f o r t h e p r o d u c t i o n o r p r o t e c t i o n o f systems which c a n be o p e r a t e d i n v e r y h i g h
0097-6156/88/0360-0392506.00/0 © 1988 American Chemical Society In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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32. PACIOREK ET AL.
Boron Nitride and Its Precursors
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temperature e n v i r o n m e n t s . The a p p l i c a b i l i t y o f these m a t e r i a l s as c o a t i n g s , f i b e r s , as w e l l as b u l k items depends on t h e a b i l i t y t o 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 i n t o t h e d e s i r e d shape o f t h e f i n a l item p r i o r t o e x h a u s t i v e p y r o l y s i s and transformation i n t o a ceramic. The t e c h n o l o g y to provide such p r e c u r s o r s f o r s i l i c o n c a r b i d e , s i l i c o n n i t r i d e , and g r a p h i t e , i n particular the l a t t e r , has been developed and a l t h o u g h work i s proceeding on i m p r o v e m e n t s , t h e premises are r e l a t i v e l y well established Q) . The s i t u a t i o n i n t h e case o f boron n i t r i d e i s entirely different. The e x t e n s i v e work performed i n t h e s i x t i e s was d i r e c t e d a t t h e r m a l l y s t a b l e polymers. The p r o d u c t s were p o o r l y c h a r a c t e r i z e d and t h e l i t e r a t u r e d a t a a r e c o n f l i c t i n g ( 2 ) . More r e c e n t l y , c l a i m s have been made t o t h e s y n t h e s i s o f BN p r e c e r a m i c polymers amenable to fiber production using the p y r o l y s i s of B-triamino-N-triphenylborazine ( 3 ) . U n f o r t u n a t e l y , t h i s work c o u l d not be r e p r o d u c e d (4,5). Boron nitride, i n view o f i t s unique p r o p e r t i e s , namely absence o f e l e c t r i c a l conductivity, oxidation resistance, optical transparency, and h i g h n e u t r o n capture cross-section f o r special a p p l i c a t i o n s , o f f e r s advantages over o t h e r c e r a m i c s . Thus, f o r t h e past several years t h e group a t U l t r a s y s t e m s has been a c t i v e l y i n v o l v e d i n i n v e s t i g a t i n g r o u t e s l e a d i n g t o p r e c e r a m i c BN polymers u s i n g both cyclic and l i n e a r starting materials. Some o f t h e f i n d i n g s g e n e r a t e d w i l l be d i s c u s s e d . R e s u l t s and D i s c u s s i o n Borazine synthesis. The ease o f b o r a z i n e r i n g c o n d e n s a t i o n would be expected t o depend on t h e n a t u r e o f t h e s u b s t i t u e n t s . To i n v e s t i gate t h e s e a s p e c t s , a s e r i e s o f compounds were s y n t h e s i z e d , namely B-tr i c h l o r o - N - t r i phenyl borazine , B-triamino-N-triphenylborazine, B-trianilinoborazine,B-tris[di(trimethylsilyl)amino]borazine, B-trichloro-N-tris(trimethylsilyl)borazine, and B - t r i a m i n o - N - t r i s (trimethylsilyl)borazine. B-trichloro-N-triphenylborazine, mp 290-292°C, was o b t a i n e d i n 86* y i e l d f o l l o w i n g t h e p r o c e d u r e o f G r o s z o s and S t a f i e j ( 6 ) . This m a t e r i a l was then transformed into B-triamino-N-triphenylb o r a z i n e i n 67% y i e l d u s i n g t h e method o f T o e n i s k o e t t e r and H a l l (1) · B - t r i a n i l i n o b o r a z i n e and t h e n o v e l B - t r i s [ d i ( t r i m e t h y l s i l y l ) aminojborazine (mp, 131.5-132°C.; c h a r a c t e r i z e d by GC/MS, molecular i o n 558 amu, and e l e m e n t a l a n a l y s i s ) were s y n t h e s i z e d i n 76 and 71* yields, 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 hexamethyld i s i l a z a n e with c h l o r o b o r a z i n e i n the presence o f t r i e t h y l a m i n e . The synthesis of B-trichloro-N-tris(trimethylsilyl)borazine was much more c o m p l i c a t e d than t h a t o f t h e o t h e r b o r a z i n e s . The o v e r a l l scheme i s g i v e n below: BC1
3
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Et N 3
CI
Me SiN I CIB^ Q 3
NSiMe I ^ B C l « Pyrolysis Q
•BCl 'NEt 3
3
HN(SiMe.) + NEt
2
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I SiMe„
In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
INORGANIC AND ORGANOMETALLIC POLYMERS
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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 a d d u c t , mp 89-91°C, was o b t a i n e d 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 t h e procedure o f Ohashi, e t 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 t h a t c r y s t a l l i z a t i o n from d i l u t e ethanol r e p o r t e d by G e r r a r d , Lappert, 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 t h e procedure o f W e l l s and C o l l i n s ( 1 0 ) . The p y r o l y s i s o f 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 p l a c e i n b o i l i n g x y l e n e . Temperatures above 150 C were n e c e s s a r y 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 h i g h e s t y i e l d o f the r e l a t i v e l y pure p r o d u c t 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 J b o r a z i n e proceeded r e a d i l y u s i n g l i q u i d ammonia. Borazine P y r o l y s i s Studies. I n a s i m p l i s t i c manner, one can v i s u a l i z e the f o r m a t i o n o f boron n i t r i d e v i a s t e p w i s e 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
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BORON NITRIDE
CROSSLINKED POLYMER
g i v i n g i n i t i a l l y t h e " l i n e a r " p r e c e r a m i c polymer, then t h e 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, f o l l o w e d 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 t e c h n i q u e s u t i l i z e d were f u l l y d e s c r i b e d p r e v i o u s l y (4J and thus w i l l n o t be r e i t e r a t e d here. The p o t e n t i a l b o r a z i n e c a n d i d a t e s can be b r o a d l y d i v i d e d i n t o two c l a s s e s o f m a t e r i a l s , one where t h e s u b s t i t u e n t on t h e r i n g boron i s n i t r o g e n - f r e e , e.g., a halogen, and t h e second where t h e s u b s t i t u e n t i s an amino moiety. With r e s p e c t t o t h e s u i t a b i l i t y o f any g i v e n b o r a z i n e , t h e i m p o r t a n t a s p e c t s a r e t h e ease o f f o r m a t i o n 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 r e s p e c t , 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. U n f o r t u n a t e l y , p y r o l y s i s a t ,^60 C l i b e r a t e d o n l y 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* o f t h e s t a r t i n g m a t e r i a l was r e c o v e r e d . H e a t i n g a t 360°C 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 g l a s s y r e s i d u e . Further heating at
In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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360°C f o r 72 h a f f o r d e d o n l y an a d d i t i o n a l 7.3% o f t h e expected trimethylchlorosilane. T h i s r e s u l t shows c l e a r l y t h a t t h i s type o f b o r a z i n e does not l e a d t o a d e s i r a b l e preceramic polymer, inasmuch as a t 260 C t h e condensation process proceeds o n l y t o a l i m i t e d degree, w i t h the s t a r t i n g m a t e r i a l b e i n g l a r g e l y r e c o v e r e d and t h e polymeric product f o r m e d b e i n g b o t h i n s o l u b l e and i n f u s i b l 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 would not be expected t o undergo condensation r e a d i l y inasmuch as t h e f o r m a t i o n o f chlorobenzene i s not a favored process. The e x p e r i m e n t a l data supported this p o s t u l a t i o n ; e s s e n t i a l l y no d e g r a d a t i o n o c c u r r e d a t 300°C. B - t r i a m i n o - N - t r i p h e n y l b o r a z i n e , and i n p a r t i c u l a r i t s isomer, B - t r i a n i l i n o b o r a z i n e , t h e l a t t e r e i t h e r alone o r i n c o - r e a c t i o n w i t h B - t r i a m i n o - N - t r i p h e n y l b o r a z i n e , gave f u s i b l e ,
