Precursors to Nonoxide Macromolecules and Ceramics - American

Chapter 30

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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.


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

380

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


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

+

M e

s i c l 3

( ) 4

3 CH CI C1

B

N

H

3 3 3 3 * (

M e

s i 3

)

N M e 2

—

2

2

> [(

N M e

B

N

H

)3 3 3 3l

+

M e

n

3

S i C 1

(5

>

4 CH CI C 1

B

N

M e

3 3 3 3

+

(

M e

s i 3

)

N H

2

2

2

N H

B

N

M e

»[( ) 3 3 3^ 3

+

M e

S i C 1 3

6

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



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 .



382


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.



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.



384 4. 5. 6. 7. 8.


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.

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1,

1987


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