Precursors to Nonoxide Macromolecules and Ceramics - American

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Precursors to Nonoxide Macromolecules and Ceramics C. K. Narula, R. T. Paine, and R. Schaeffer Department of Chemistry, University of New Mexico, Albuquerque, N M 87131 There is great interest in developing molecular precursors for boron-nitrogen polymers and boron nitride solid state materials, and one general procedure is described in this report. Combinations of B-trichloroborazene and hexamethyldisilazane lead to formation of a gel which, upon thermolysis, gives hexagonal boron nitride. The BN has been characterized by infrared spectroscopy, x-ray powder diffraction and transmission electron microscopy. Non-oxide ceramic m a t e r i a l s are t y p i c a l l y prepared from simple s t a r t i n g reagents by using c l a s s i c a l e n e r g e t i c r e a c t i o n c o n d i t i o n s ( h i g h t e m p e r a t u r e , high p r e s s u r e , CVD, e t c . ) . Although these r o u t e s p r o v i d e u s e f u l m a t e r i a l s f o r many a p p l i c a t i o n s , t h e r e i s i n c r e a s i n g need f o r a l t e r n a t e low energy procedures which p e r m i t more f l e x i b l e p r o c e s s i n g of t h e ceramic e n d - p r o d u c t . One s y n t h e t i c r o u t e which has a t t r a c t e d r e c e n t a t t e n t i o n i n v o l v e s t h e combination of simple reagents i n t o an adduct c o n t a i n i n g a l l t h e elements of i n t e r e s t f o r the f i n a l s o l i d s t a t e product. F o l l o w i n g thermal o r photochemical a c t i v a t e d i n t r a m o l e c u l a r e l i m i n a t i o n - c o n d e n s a t i o n c h e m i s t r y , an o l i g o m e r i c or p o l y m e r i c ceramic p r e c u r s o r i s o b t a i n e d which can be e a s i l y handled and converted by p y r o l y s i s t o t h e d e s i r e d ceramic e n d - p r o d u c t . This concept has been s u c c e s s f u l l y a p p l i e d i n t h e p r o d u c t i o n of s i l i c o n c a r b i d e and s i l i c o n n i t r i d e (1). R e l a t i v e l y l i t t l e a t t e n t i o n has been g i v e n , on t h e o t h e r hand, t o t h e development of m o l e c u l a r p o l y m e r i c p r e c u r s o r s f o r Group 13-15 b i n a r y c e r a m i c s . The p o t e n t i a l importance of p o l y m e r i c p r e c u r s o r s i n t h i s area can be i l l u s t r a t e d by a b r i e f overview of s e l e c t e d c h e m i s t r y known t o p r o v i d e t e c h n o l o g i c a l l y i m p o r t a n t boron nitride. Boron n i t r i d e may be o b t a i n e d i n t h r e e p r i m a r y c r y s t a l l i n e modifications ( 2 ) : α , β, and χ . The most commonly encountered α form has a g r a p h i t i c s t r u c t u r e (hexagonal c e l l , a = 2.504 Â, ç = 6.661 Â ) . For many y e a r s , t h i s m o d i f i c a t i o n has been prepared from combinations of cheap boron and n i t r o g e n c o n t a i n i n g r e a g e n t s , e . g . B ( 0 H ) and (NH )C0, B ( 0 H ) , C and N o r KBH and NH C1 ( 3 - 5 ) . More 3

2

3

2

4

4

0097-6156/88/0360-0378$06.00/0

© 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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30. N A R U L A E T A L .

Precursors to Nonoxide Macromolecules

379

r e c e n t l y , chemical vapor d e p o s i t i o n (CVD) t e c h n i q u e s u t i l i z i n g m i x t u r e s o f BCI3 and NH3, BF3 and NH3 o r B2H6 and NH3 have been employed f o r t h e s y n t h e s i s o f h-BN ( 3 , 6 - 1 0 ) . P e r t i n e n t t o o u r own s t u d i e s , s e v e r a l i n v e s t i g a t o r s have a l s o used s u b s t i t u t e d borazenes as p r e c u r s o r s i n c l a s s i c a l t h e r m o l y s i s o r CVD p r e p a r a t i o n s o f h-BN. For example, Constant and Feurer (H) p y r o l y z e d h e x a c h l o r o borazene, (ClBNC1)3 a t 900-950°C on s i l i c o n s u b s t r a t e s and observed t h e d e p o s i t i o n o f amorphous BN which was subsequently converted t o h-BN by e l e c t r o n bombardment. I n thorough, systematic t h e r m o l y s i s s t u d i e s r e p o r t e d by Rice and coworkers (12) and Paciorek and coworkers ( 1 3 ) , i t was found t h a t a v a r i e t y o f s u b s t i t u t e d borazenes (XBNYJ3 produced boron r i c h BN s o l i d s t a t e materials. At t h e i n i t i a t i o n o f o u r s t u d i e s s e v e r a l years ago, t h e r e had been r e l a t i v e l y few w e l l documented r e p o r t s on t h e f o r m a t i o n o f p o l y m e r i c B-N compounds. An e x p l a n a t i o n f o r t h i s s i t u a t i o n may be found i n t h e h i s t o r i c a l development o f m o l e c u l a r b o r o n - n i t r o g e n chemistry (14-16). I n "the p e r i o d 1950-1970, a g r e a t deal o f a t t e n t i o n was g i v e n t o t h e e v o l u t i o n o f syntheses f o r monomeric aminoboranes, iminoboranes and borazenes and progress made was impressive. For a v a r i e t y o f reasons, advances i n B-N polymer c h e m i s t r y were f r u s t r a t e d . A f t e r t h a t p e r i o d i n t h e USA, f u r t h e r e f f o r t s t o e x p l o r e p o l y m e r i c m a t e r i a l s were few and t h i s area attracted l i t t l e attention. Nonetheless, s e v e r a l r e p o r t s i n t h e p a t e n t and open l i t e r a t u r e suggested t h a t borazene compounds c o u l d be employed as monomers f o r modest m o l e c u l a r w e i g h t oligomers (13,11-24). F u r t h e r m o r e , a t a n t a l i z i n g p a t e n t r e p o r t i n 1976 by Taniguchi and coworkers ( 2 5 ) suggested t h a t s p i n n a b l e f i b e r s c o u l d be o b t a i n e d from a borazene (H NBHPh)3 based polymer which were, i n t u r n , converted t o boron n i t r i d e . More r e c e n t l y , Paciorek and coworkers have r e p o r t e d on t h e f o r m a t i o n o f boron n i t r i d e m a t e r i a l s from s e v e r a l condensed s u b s t i t u t e d borazenes ( 2 3 , 2 4 ) . Recognizing t h e need f o r fundamental s y n t h e t i c c h e m i s t r y l e a d i n g t o b o r o n - n i t r o g e n preceramic p o l y m e r s , we embarked on a program t o develop improved condensation r o u t e s f o r aminoboranes, iminoboranes and borazenes. I n p a r t i c u l a r , borrowing from t h e p r i n c i p l e s o f o r g a n i c polymer s y n t h e s e s , c h e m i s t r y was sought which would r e s u l t i n t h e c r o s s l i n k i n g o f borazene monomer b u i l d i n g blocks. I t was r a t i o n a l i z e d t h a t c r o s s l i n k i n g t h r o u g h amide b r i d g e s would produce preceramic o l i g o m e r s w i t h "excess" n i t r o g e n ( e . g . N:B = 1 . 5 ) w h i c h , upon p y r o l y s i s , might r e t a i n more n i t r o g e n than found i n p y r o l y s i s p r o d u c t s from monomeric borazenes. I n a d d i t i o n , i t was a n t i c i p a t e d t h a t t h e c r o s s l i n k i n g might r e s u l t i n boron n i t r i d e m a t e r i a l s w i t h a porous m a c r o s t r u c t u r e . 2

Results I t i s w e l l known t h a t some s i l y l a m i n e s , i n combination w i t h h a l o b o r a n e s , p r o v i d e a f a c i l e r o u t e t o aminoboranes ( E q u a t i o n 1) (26). M e l l e r and F u l l g r a b e (19) have p r e v i o u s l y r e p o r t e d t h a t a R BC1 + R NSiMe 2

2

3

-> R BNR 2

v a r i a t i o n of t h i s reaction w i l l

(1)

2

p r o v i d e coupled

substituted

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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borazenes. Indeed, t h e f o l l o w i n g model r e a c t i o n s were examined i n our work and b i s - b o r a z i n y l amines 1 and 2 were i s o l a t e d i n high y i e l d (Equations 2 and 3). Compounds"! and~2 were f u l l y c h a r a c t e r i z e d by elemental a n a l y s i s and s p e c t r o s c o p i c techniques (27). (Cl)(Me) B N Me 2

3

3

+ ( M e ^ i ^ N H -> [ M e ^ N g M e ^ N H

3

(2)

ι

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C 1 ( M e )

B

N

M e

2 3 3 3

+

(

M e

S i ) 3

2

N M e

"* [

M e

B 2

N

M e

]

3 3 3 2

N M e

( 3 )

2 The Β NMR s p e c t r a f o r these compounds show resonances a t 37.4 and 28.0 (1) and 37.2 and 27.6 (2), r e s p e c t i v e l y . Extending t h e model c h e m i s t r y t o t h e f o r m a t i o n of preceramic p o l y m e r s , we found t h a t o l i g o m e r i c g e l s are o b t a i n e d by r e l a t e d r e a c t i o n s i n v o l v i n g B - t r i c h l o r o b o r a z e n e s as shown i n Equations 4-6. I n each case, a t r a n s p a r e n t gel formed which took t h e shape of i t s c o n t a i n e r . Some s p e c i f i c e x p e r i m e n t a l d e t a i l s f o r t h e f o r m a t i o n of 3 are p r o v i d e d , and a summary of t h e p r o c e s s i n g of t h e o l i g o m e r i s g i v e n i n F i g u r e 1. The g e l s may be formed i n a v a r i e t y 1 1

CH CI C1

B

N

H

3 3 3 3

+

(

M e

s i ) 3

2

N H

1 [ ( H N )



3 3 3 3 n B

N

]

H

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( ) 4

3 CH CI C1

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3 3 3 3 * (

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)

N M e 2



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2

> [(

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M e

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3

S i C 1

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)

N H

2

2

2

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5 of o t h e r s o l v e n t s , and t h e p r o c e s s i n g of these m a t e r i a l s i s described separately ( 2 7 ) . T r i c h l o r o b o r a z e n e , CI3B3N3H3 (50 mmol) and (Me3Si)2NH (75 mmol) were combined a t -78°C i n CH2C12 and t h e m i x t u r e s t i r r e d f o r 30 min a t -78°C and then s l o w l y warmed t o 25°C over two h o u r s . A c o l o r l e s s gel formed which was i s o l a t e d by vacuum e v a p o r a t i o n of the v o l a t i l e s . The r e s u l t i n g c o l o r l e s s g l a s s y s o l i d was p y r o l y z e d i n vacuo a t 900°C f o r 24 hours i n a q u a r t z tube and t h e evolved v o l a t i l e s i d e n t i f i e d as NH and NH4CI. The remaining s o l i d was b r i e f l y (2 hours) heated i n a i r a t 1200°C i n o r d e r t o remove minor carbon i m p u r i t i e s and t o improve c r y s t a l l i n i t y . This s o l i d was then t r e a t e d a t room temperature w i t h 40% aqueous HF t o remove b o r i c a c i d and s i l i c a formed i n small q u a n t i t i e s . The s o l i d o b t a i n e d a t 900°C was i d e n t i f i e d as boron n i t r i d e ; however, t h e m a j o r i t y of t h e m a t e r i a l was amorphous. A f t e r t r e a t m e n t a t 1200°C, w h i t e c r y s t a l l i n e b o r o n - n i t r i d e was o b t a i n e d i n about 55% y i e l d . 3

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

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30. N A R U L A E T A L .

Precursors to Nonoxidt Macromolecules

900 C VAC. -EtjO

1200 C AJR -NH

-NH

-NKQ

e

Figure 1 .

381

e

3

WHITE S0LI0

Summary o f borazene gel p r o c e s s i n g .

The boron n i t r i d e o b t a i n e d i n t h i s study was c h a r a c t e r i z e d by i n f r a r e d s p e c t r o s c o p y , powder x - r a y d i f f r a c t o m e t r y and t r a n s m i s s i o n e l e c t r o n microscopy. Trace elemental analyses were a l s o performed by energy d i s p e r s i v e x - r a y a n a l y s i s and carbon a r c emission spectroscopy. R e p r e s e n t a t i v e s p e c t r a a r e d i s p l a y e d i n Figures 2 - 4 . The i n f r a r e d spectrum o f 3 ( F i g u r e 2) c o n t a i n e d i n a KBr p e l l e t shows a broad a b s o r p t i o n i n t h e r e g i o n 1425-1065 c n H , a sharp a b s o r p t i o n a t 710 c n H , and t h e spectrum c l o s e l y resembles s p e c t r a r e p o r t e d i n t h e l i t e r a t u r e ( 6 , 8 , 2 8 ) . The broad band c e n t e r e d a t 3300 cm-1 suggests t h e presence o f r e t a i n e d N-H f u n c t i o n a l i t i e s ; however, t h i s f e a t u r e may a l s o be seen i n some commercial samples o f h-BN. The x - r a y powder p a t t e r n ( F i g u r e 3) i s a l s o s i m i l a r t o p a t t e r n s r e p o r t e d i n t h e l i t e r a t u r e (29.). The l i n e a t 2Θ ~26°corresponds t o t h e 002 r e f l e c t i o n , a l i n e a t 2Θ - 4 2 - 4 4 ° corresponds t o o v e r l a p o f t h e 100 and 101 r e f l e c t i o n s , and a l i n e a t 2Θ - 5 4 ° i s t h e 004 r e f l e c t i o n . The l i n e s a r e broader than t y p i c a l l y observed from h i g h l y c r y s t a l l i n e commercial h-BN samples, and t h i s l i n e broadening may r e s u l t from small c r y s t a l l i t e s i z e o r from t h e presence o f t h e p r e v i o u s l y observed t u r b o s t r a t i c structure modification (30)· The measured spacing d(002) = 3 . 3 5 - 3 . 3 6 A agrees very w e l l w i t h t h e r e p o r t e d d(002) = 3.330 A f o r h-BN prepared by c o n v e n t i o n a l methods. Most t u r b o s t r a t i c m o d i f i c a t i o n s show a g r e a t e r d e v i a t i o n i n d(002) = 3 . 5 - 3 . 6 A ; t h e r e f o r e , i t i s assumed t h a t any t u r b o s t r a t i c d i s o r d e r i s s m a l l . The m a c r o s t r u c t u r e o f t h e boron n i t r i d e o b t a i n e d here i s porous w i t h pores ~2 urn i n d i a m e t e r . There i s no evidence f o r m i c r o p o r o s i t y and t h e BET s u r f a c e area i s ~35 m2 g - 1 . Transmission e l e c t r o n micrographs ( F i g u r e 4) show r e g i o n s o f w e l l developed crystallinity. The c r y s t a l l i n g g r a i n s a r e - 5 - 1 0 nm on a s i d e and 30-40 nm l o n g . The BN (002) l a t t i c e f r i n g e s a r e c l e a r l y v i s i b l e .

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

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7oT

4000

3590

3180

2770

2360

1950

1540

1130

720

310

cm"

Figure 2 .

Figure 3.

I n f r a r e d spectrum o f BN.

X - r a y powder d i f f r a c t i o n a n a l y s i s o f BN.

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

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30. NARULA ET AL.

Figure 4 .

Precursors to Nonoxide Macromolecules

Transmission e l e c t r o n micrograph o f

383

BN.

Conclusion The r e s u l t s of t h i s study c l e a r l y r e v e a l t h a t c r o s s l i n k i n g of borazene r i n g s may be achieved w i t h s i l y l a m i n e s and t h e r e s u l t i n g o l i g o m e r i c g e l s may be e f f i c i e n t l y processed t o g i v e hexagonal boron n i t r i d e i n good y i e l d s and w i t h good c r y s t a l u n i t y . The c h a r a c t e r i s t i c s of t h e g e l s formed by t h i s s y n t h e t i c r o u t e lend themselves t o a v a r i e t y of p r o c e s s i n g modes, e . g . s o l - g e l and aerogel p r o c e s s i n g , not a v a i l a b l e i n t h e s y n t h e t i c procedures n o r m a l l y a p p l i e d i n boron n i t r i d e p r e p a r a t i o n s . Additional studies are i n progress which are designed t o t a k e advantage of these features. Acknowledgment i s made t o Sandia N a t i o n a l L a b o r a t o r y f o r of t h i s r e s e a r c h . Literature 1. 2. 3.

support

Cited

Wynne, K . J . ; R i c e , R.W. Ann. Rev. Mater. S c i . 1984, 14, 297. Hoard, J.L.; Hughes, R.E. I n The Chemistry of Boron and Its Compounds; M u e t t e r i e s , E.L. Ed. J . Wiley and Sons, NY 1967. M e l l e r , A. Gmelin Handbuck der Anorganische Chemie. Boron Compounds Second Supplement 1983, 1, 304.

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

INORGANIC AND ORGANOMETALLIC POLYMERS

384 4. 5. 6. 7. 8.

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9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

Kalyoncu, C.S. Ceram. Eng. Sci. Proc. 1985, 1356. Archer, N.J. Chem. Soc. (London) Spec. Pub. No. 30 1977, 167. Rand, M.J.; Roberts, J.F. J. Electro. Chem. Soc. 1968, 115, 423. Pierson, H.O. J. Composit. Mat. 1975, 9, 228. Takahashi, T.; Itoh, H.; Takeuchi, A. J. Cryst. Growth 1979. 47, 245. Sano, M.; Aoki, M. Thin Solid Films 1981, 83, 247. Chopra, K.L.; Agarwal, V.; Vankar, V.D.; Deshpandey, C.V.; Bunshah, R.F. Thin Solid Films 1985, 126, 307. Constant, G.; Feurer, R. J. Less Common Metals 1981, 82, 113. Bender, B.A.; Rice, R.W.; Spann, J.R. Ceram. Eng. Sci. Proc. 1985, 6, 1171. Paciorek, K.J.L.; Harris, D.H.; Kratzer, R.H. J. Polym. Sci. Polym. Chem. 1986, 24, 173. Lappert, M.F. In Developments in Inorganic Polymer Chemistry; Lappert, M.F. Ed., Elsevier Publ. Co., New York, 1962. Gaines, D.F.; Borlin, J. In Boron Hydride Chemistry; Muetterties, E.L. Ed., Academic Press, NY 1975. Steinberg, H.; Brotherton, R.J. In Organoboron Chemistry; Vol. 2, J. Wiley, NY 1966. Wagner, R.I. U.S. Pat. 3,288,726 Nov. 29, 1966; Chem. Abstr. 1977, 66 38349W. Wagner, R.I.; Bradford, J.L. Inorg. Chem. 1962, 1, 99. Meller, Α.; Füllgrabe, H.J. Z. Naturforsch. 1978, 33b, 156. Gerrand, W.; Mooney, E.F.: Pratt, D.E. J. Appl. Chem. 1963, 13, 127. Aubrey, D.W.; Lappert, M.F. J. Chem. Soc. 1959, 2927. Gerrand, W.; Hudson, H.R.; Mooney, E.F. J. Chem. Soc. 1962, 113. Paciorek, K.J.L.; Kratzer, R.H.; Harris, D.H.; Smythe, M.E.; Kimble, P.F.U.S. Pat 4,581,468 Apr. 8, 1986; Chem Abstr. 1986, 105, 80546g. Paciorek, K.J.L.; Kratzer, R.H.; Harris, D.H.; Smythe, M.E. Polym. Preps. 1984, 25, 15. Taniguchi, I.; Koichi, H.; Takayoshi, M. Jpn. Kokai 76 53,000. Chem Abstr. 1976, 85, 96588. Nöth, H. Z. Naturforschg. 1961, 16b, 618. Narula, C.K.; Paine, R.T.; Schaeffer, R. manuscript in preparation. Brame, E.G. Margrave, J.L.; Meloche, V.W. J. Inorg. Nucl. Chem. 1957, 5, 48. Pease, R.S. Acta Cryst. 1952, 5, 356. Biscoe, J.; Warren, B.E. J. Appl. Phys. 1942, 13, 364; Thomas, J.; Weston, N.E.; O'Connor, T.E. J. Am. Chem. Soc. 1963, 84. 4619; Economy, J.; Anderson, R. Inorg. Chem. 1966, 5, 989; Matsuda, T.; Uno, N.; Nakae, H.; Hirai, T. J. Mat. Sci. 1986, 21, 649.

RECEIVED

September

1,

1987

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