Silicon and Germanium

SECTION 12

Silicon and Germanium

p. w . SCHENK

Silico n Si

T h e s t a r t i n g m a t e r i a l for p r e p a r a t i o n of h i g h - p u r i t y s i l i c o n i s e i t h e r c o m m e r c i a l s i l i c o n , p u r i f i e d by r e c r y s t a l l i z a t i o n f r o m m o l t e n a l u m i n u m , o r s i l i c o n p r o d u c e d b y Von W a l l e n b e r g ' s m o d i - fication of K u h n e ' s m e t h o d , e x p l a i n e d b e l o w .

T h e r e a c t i o n v e s s e l i s a H e s s i a n c e r a m i c c r u c i b l e o r a w a t e r - c o o l e d c o p p e r ingot m o l d , w h i c h i s f i r s t p a c k e d with finely p o w d e r e d c o r u n d u m in t h e following m a n n e r : T h e b o t t o m i s filled to a h e i g h t of about 1 c m . w i t h c o r u n d u m p o w d e r ( s i e v e d t h r o u g h a 0 . 1 5 - m m . s c r e e n ) , an a l u m i n u m t u b e w i t h a d i a m e t e r of about 8 c m . (with a s s m o o t h l y p o l i s h e d a s u r f a c e a s p o s s i b l e ) i s i n s e r t e d v e r t i c a l l y , and t h e s p a c e b e t w e e n t h e A l t u b e and t h e c r u c i b l e w a l l i s tightly p a c k e d with p o w d e r e d c o r u n d u m . T h e r e a c t i o n m i x t u r e , c o n s i s t i n g of one p a r t of A l p e l l e t s (99.995%), one p a r t of (very p u r e sulfur (purified b y t h e B a c o n and F a n e l l i o r b y the Von W a r t e n b e r g m e t h o d , s e e p . 342), and one p a r t of t h e p u r e s t a v a i l a b l e q u a r t z s a n d ( r o c k c r y s t a l p o w d e r s u c h a s u s e d f o r m a k i n g t r a n s - p a r e n t fused q u a r t z i t e m s — i t i s e s p e c i a l l y low in b o r o n ) , i s t h e n p a c k e d i n s i d e t h e t u b e to a h e i g h t of about 5 c m . T h e Al t u b e i s t h e n pulled u p w a r d 4 c m . and t h e p r o c e s s i s r e p e a t e d . After t h r e e r e p e t i t i o n s , t h e c r u c i b l e i s full. It s h o u l d not b e filled a l l at o n c e b e c a u s e t h e f r i c t i o n would m a k e it i m p o s s i b l e to w i t h d r a w t h e Al t u b e . F i n a l l y , a n o t h e r 0 . 5 - c m . l a y e r of Al p e l l e t s i s added and t h e m a s s i s ignited w i t h a n H²- 0² t o r c h in o r d e r to p r e c l u d e any c o n t a m i n a t i o n of t h e c h a r g e . A f t e r c o o l i n g , t h e s l a g and t h e p r o d u c t block a r e e a s i l y d u m p e d out; t h e h a r d w h i t e s l a g i s r e a d i l y b r o k e n up w i t h a h a m m e r . T h e b l o c k w i t h t h e p r o d u c t c o n t a i n e d in it i s b r o k e n down w i t h w a t e r , t h e Al (OH) ³ i s d i s s o l v e d w i t h h y d r o c h l o r i c a c i d , and t h e a l u m i n a ( c o r u n d u m ) t h u s o b t a i n e d i s f i l t e r e d off a n d w a s h e d . It m a y t h e n b e r e u s e d . T h e Si b l o c k i s e x t r a c t e d f o r s e v e r a l d a y s with b o i l i n g 1:2 h y d r o c h l o r i c a c i d , w a s h e d and boiled with H F - H²S 0⁴. T h e r e s i d u e i s t h e n m e l t e d

676

1 2 . S I L I C O N AND G E R M A N I U M 677

Si i s at

for half a n h o u r with K H S 0⁴ and i s t h e n t h o r o u g h l y w a s h e d . F r o m e a c h 350 g. of A l - S i 0² c h a r g e , 120 g, of c r u d e p r o d u c t i s o b t a i n e d . T h i s y i e l d s 75 g. of p u r e s i l i c o n , w h i c h i s t h e n s m e l t e d in a s t r e a m of a r g o n and c h l o r i n e (as shown below) to r e m o v e Al and o t h e r m e t a l t r a c e s . T h e Si r e c r y s t a l l i z e d f r o m m o l t e n Al m a y a l s o b e p u r i f i e d by t h e following p r o c e d u r e :

A q u a r t z c r u c i b l e 12 c m . long and 2 c m . in d i a m e t e r (a p u r e q u a r t z t u b e fused a t one end) i s filled w i t h 20 to 30 g. of p o w d e r e d T h i s c r u c i b l e i s i n s e r t e d into a n o t h e r fused q u a r t z t u b e , w h i c h 60 c m . long and 3 c m . in d i a m e t e r , and h a s a m a l e g r o u n d joint t h e t o p (Fig. 219). T h e 5 - m m . a r g o n inlet t u b e i s fused at t h e b o t t o m t o a 2 - c m . - d i a m e t e r o u t e r t u b e , w h i c h in t u r n i s s e a l e d to t h e top of a c a p e q u i p p e d w i t h a f e m a l e g r o u n d joint.

T h e a r g o n t h u s r e t u r n s t h r o u g h t h e a n n u l u s b e t w e e n t h e 2 - c m . t u b e and t h e q u a r t z t u b e and m a y t h e r e f o r e b e r e - c o v e r e d f o r r e u s e . T h e exit g a s e s flow to t h e hood t h r o u g h a b u b b l e c o u n t e r filled w i t h H^sS 0⁴. T h e q u a r t z t u b e i s s l i p p e d into a 4 0 - c m . - l o n g p l a t i n u m - r h o d i u m - r i b b o n f u r n a c e ; a s h i e l d e d t h e r m o c o u p l e i s i n s e r t e d i m m e d i a t e l y u n d e r n e a t h t h e t u b e . T h e f u r n a c e i s b r o u g h t t o a t e m p e r a t u r e of 1470°C, a s m e a s u r e d at t h e b o t t o m of t h e r e a c t i o n t u b e . An o x y g e n - f r e e s t r e a m of a r g o n (purified in t h e s a m e m a n n e r a s N², s e e p . 458) i s fed t o t h e inlet t u b e v i a a feed l i n e c o n s t r u c t e d e x c l u s i v e l y of g r o u n d - j o i n t - c o n n e c t e d e l e m e n t s . S o m e p u r e c h l o r i n e m a y b e m i x e d with a r g o n by m e a n s of a t e e j o i n t . F i r s t , a l l a i r i s d i s p l a c e d f r o m t h e a p p a r a t u s u n t i l no o x y g e n c a n b e d e t e c t e d a t t h e o u t l e t ( s e e p . 3 3 6 , s e c t i o n on oxygen). H e a t i n g i s t h e n b e g u n and, w h e n t h e Si i s m e l t e d , t h e c h l o r i n e s t r e a m i s s t a r t e d at a r a t e w h i c h will a l l o w 2 to 3 l i t e r s of C l² to flow t h r o u g h t h e a p p a r a t u s in t h e c o u r s e of a b o u t 3 h o u r s . B e s i d e s S 1 C I 4 , a l l i m p u r i t i e s e x c e p t b o r o n a r e t h u s v o l a t i l i z e d . T h e c h l o r i n e flow i s t h e n s h u t off, t h e t e m p e r a t u r e a l l o w e d to d r o p to 1300°C, and t h e f u r n a c e r e m o v e d . T h e r e a c t i o n t u b e m a y b e r e m o v e d f r o m t h e s t a n d a f t e r about half a n h o u r , and t h e c r u c i b l e (which i s a l w a y s c r a c k e d ) m a y t h e n b e d u m p e d out.

T h e p r o d u c t b l o c k i s left in p u r e c o n c e n t r a t e d h y d r o f l u o r i c a c i d f o r 24 h o u r s s o t h a t a l l t h e a d h e r i n g q u a r t z p a r t i c l e s m a y b e

F i g . 2 1 9 . S m e l t i n g of s i l i c o n u n d e r c h l o r i n e a c c o r d i n g to Von W a r -

t e n b e r g .

6 7 8 P. W . S C H E N K

r e m o v e d . Silicon p r e p a r e d in t h i s m a n n e r h a s a r e s i s t i v i t y of 15-17 o h m - c m .

F u r t h e r p u r i f i c a t i o n i s p o s s i b l e by t h e z o n e m e l t i n g t e c h ­ n i q u e of Pfann. In o r d e r t o avoid c o n t a m i n a t i o n f r o m t h e w a l l s of t h e c r u c i b l e d u r i n g t h e m e l t i n g , a t e c h n i q u e s i m i l a r t o t h a t of V e r n e u i l - M i e t h e s i s r e c o m m e n d e d .

Other preparative methods:

T h i s i s b a s i c a l l y an old p r o c e s s . A s u s e d b y D u P o n t , it i s a r r a n g e d a s f o l l o w s :

T h e q u a r t z g l a s s t u b e A ( F i g . 220), 180 c m . long and 20 c m . in d i a m e t e r , i s h e a t e d a l o n g i t s e n t i r e length by an e l e c t r i c f u r n a c e .

F i g . 220. P r e p a r a t i o n of s i l i c o n by t h e D u P o n t m e t h o d . Zinc v a p o r , p r o d u c e d f r o m h i g h - p u r i t y z i n c , flows in t h r o u g h R⁹ p r e h e a t e d S i C l⁴ v a p o r e n t e r s at 0 t h r o u g h t h e c o i l D. T h e a p p a ­ r a t u s i s p r e h e a t e d for 5 h o u r s b e f o r e t h e r e a g e n t v a p o r s a r e a l l o w e d to e n t e r . T h e two b a f f l e s , K^x a n d ϋΓ², a r e p i e r c e d by o p e n i n g s ( 5 - c m . d i a m e t e r ) and b r e a k u p t h e r a t h e r f a s t flow of r e a g e n t v a p o r s . T h e y i e l d i s about 3 t o 4 k g . of s i l i c o n o v e r a p e r i o d of about 24 h o u r s . T h e p r o d u c t i s s u p p o s e d t o b e s p e c t r o - s c o p i c a l l y p u r e a f t e r w a s h i n g with w a t e r . T h i s pilot p l a n t a p p a ­ r a t u s h a s b e e n m i n i a t u r i z e d for l a b o r a t o r y p u r p o s e s by Von W a r t e n b e r g . F u r t h e r p u r i f i c a t i o n i s a c c o m p l i s h e d by h i s m e t h o d of m e l t i n g u n d e r C l², given a b o v e .

PROPERTIES:

F r i a b l e , m i r r o r l i k e (when polished) s u b s t a n c e with a b l u i s h l u s t e r . I n s o l u b l e in a c i d s , i n c l u d i n g H F . Soluble in s o d i u m h y d r o x i d e , e v o l v i n g H². M . p . 1423°C, b . p . 2630°C; d 2 . 4 . L a m e l l a e o r o c t a h e d r a l c r y s t a l s . D i a m o n d s t r u c t u r e .

REFERENCES:

A. Kiihne, Z. a n o r g . a l l g . C h e m . 99, 123 (1917); G e r m a n P a t e n t 147 871 CI. 13 i, C h e m . Z e n t r . 1904 I, 64; D . W. L y o n , J . E l e c t r o - c h e m . Soc. 96, 359 (1949); H. von W a r t e n b e r g , Ζ. a n o r g . a l l g . C h e m .

SiCl⁴ + 2 Z n = Si + 2 ZnCl² 169.9 130.8 28.1 272.6

asbestos asbestos

1 2 . S I L I C O N AND G E R M A N I U M 679 265, 186 (1951); 2 8 3 , 372 (1956); 286, 247 (1957); F . B . L i t t o n and H. C. A n d e r s o n , J . E l e c t r o c h e m . Soc. 1 0 1 , 287 (1954); H. C.

T h e u e r e r , Bell L a b . R e c . 3 3 , 327 (1955), C h e m . Z e n t r . 1956, 9938.

F o r l i t e r a t u r e on z o n e m e l t i n g , s e e W. G. Pfann, C h e m . E n g . N e w s 3 4 , 1440 (1956); H. K l e i n k n e c h t , N a t u r w i s s . 3 9 , 400 (1952);

S. M u l l e r , Z. N a t u r f o r s c h g . 9b, 504 (1954); F . T r e n d e l e n b u r g , Angew. C h e m . 66, 520 (1954), F o r m e l t i n g without c r u c i b l e s , s e e P . H. K e c k , S. B . L e v i n , J . B r o d e r and R. L i e b e r m a n n , P h y s . R e v . (2) 92, 847; B u l l . A m e r . P h y s . Soc. 2 8 , 11 (1953), C h e m . Z e n t r . 1954, 7362; C. P . K e m p t e r and C. A l v a r e z - T o s t a d o , Z.

a n o r g . a l l g . C h e m . 290, 238 (1957).

Silane s SiH⁴ (Si²H^e, Si³H⁸)

Silicon h y d r i d e s a r e p r e p a r e d e i t h e r by a c i d d e c o m p o s i t i o n of m a g n e s i u m s i l i c i d e o r b y r e d u c t i o n of S i C l⁴w i t h L i A l H⁴. Only a r e l a t i v e l y m o d e s t y i e l d of s i l a n e (20-30%) i s o b t a i n e d by t h e a q u e o u s a c i d d e c o m p o s i t i o n of s i l i c i d e , but t h e p r o p o r t i o n of t h e h i g h e r s i l a n e s i s s o m e w h a t g r e a t e r . If t h e r e a c t i o n i s c a r r i e d out in liquid N H³ o r N³H⁴ i n s t e a d of w a t e r , a p p r e c i a b l y h i g h e r y i e l d s ( o v e r 80%) a r e o b t a i n e d ; b e s i d e s , u p t o 90% of t h e p r o d u c t c o n - s i s t s of S i H⁴, if one n e g l e c t s t h e e a s i l y s e p a r a b l e H^s.

SiMg² + 4 NH⁴Br = SiH⁴ + 2MgBr² + 4 N H³

67,7 391.8 32.1 368.3 68.1

T h e r e a c t i o n v e s s e l ( F i g . 221) i s filled to a b o u t two t h i r d s of i t s h e i g h t w i t h d r y liquid N H³ ( s e e p . 460). E n o u g h N H⁴B r i s d i s s o l v e d in t h e N H³ s o t h a t a n a p p r o x i m a t e l y 50% e x c e s s i s

F i g . 2 2 1 . P r e p a r a t i o n of s i l a n e .

680 P . W . S C H E N K

p r e s e n t ; i . e . , 8 5 - 9 0 g. N H⁴B r s h o u l d b e u s e d for e a c h 10 g. of MggSi. T h e finely d i v i d e d s i l i c i d e ( s e e p . 921) i s p l a c e d in t h e s i d e f l a s k b. A slow s t r e a m of N H³ i s i n t r o d u c e d t h r o u g h t u b e ο and

\llowed t o b u b b l e t h r o u g h t h e liquid t o p r o v i d e a g i t a t i o n ; p o r t i o n s of t h e s i l i c i d e a r e slowly a d d e d by r o t a t i n g t h e s m a l l f l a s k in t h e joint g. T h e g a s evolved flows t h r o u g h t h e c o l l e c t i n g t u b e β, w h i c h i s filled w i t h w a t e r in o r d e r t o r e m o v e t h e e x c e s s N H³. T h e w e l l - b o i l e d w a t e r , acidified with HCl in o r d e r to p r e v e n t d e c o m ­ p o s i t i o n of t h e s i l a n e , m a y b e r e p l e n i s h e d a s r e q u i r e d w i t h t h e aid of t h e l e v e l i n g b u l b s i and j . F r o m t h e c o l l e c t i n g t u b e , t h e g a s flows t h r o u g h t u b e k⁹ w h i c h i s p a c k e d w i t h P²0⁵ t o r e m o v e t h e w a t e r , and i s t h e n c o n d e n s e d in a t r a p l^$ w h i c h i s c o o l e d w i t h liquid N². It i s finally f r a c t i o n a t e d by S t o c k ' s p r o c e d u r e (see P a r t I, p . 66), u s i n g high v a c u u m e q u i p m e n t .

LiAlH⁴ + SiCLi = SiH⁴ + LiCl + A1C1,

38.0 169.9 32.1 42.4 133.4

A s o l u t i o n of 0.348 g. of L i A 1 H⁴ (9.19 m m o l e s ) in 12.5 g. of p e r o x i d e - f r e e e t h e r i s p l a c e d in a r e a c t i o n v e s s e l , w h i c h i s a t t a c h e d to a v a c u u m p u m p t h r o u g h a g r o u n d g l a s s j o i n t . T h e s o l u t i o n i s t h e n f r o z e n and 1.33 g. of S i C l⁴ (7.82 m m o l e s ) i s c o n ­ d e n s e d on t h e solidified s o l u t i o n (the S i C l⁴ i s b e s t m e a s u r e d out in t h e g a s e o u s s t a t e ) . T h e r e a c t o r i s now p e r m i t t e d to w a r m slowly. At a b o u t 0°C, a v i g o r o u s g a s e v o l u t i o n c o m m e n c e s . T h e p r o d u c t s a r e c o n d e n s e d in a l i q u i d - n i t r o g e n - c o o l e d t r a p , t h e n d i s t i l l e d a t — 159°C into a t r a p , a l s o cooled with liquid N². T h e y i e l d i s 175 m l . (99%) of p u r e S i H *

When LiH (even in v e r y l a r g e e x c e s s ) i s u s e d i n s t e a d of L i A l H⁴, t h e r e a c t i o n g i v e s p o o r e r y i e l d s . See P e a k e , N e b e r g a l l and Yun T i C h e n for an a p p a r a t u s u s e d in p r e p a r a t i o n of l a r g e r q u a n t i t i e s of S i H⁴ by t h a t m e t h o d .

T h e s t a r t i n g m a t e r i a l for p r e p a r a t i o n of S i²H⁶ i s S i^sC l⁶. An L i A l H⁴ s o l u t i o n i s a d d e d slowly, d r o p by d r o p , to S i²C l⁶ in e t h e r s o l u t i o n . T h e L i A l H⁴ e x c e s s should b e about 15%. T h e S i³H⁶ y i e l d i s about 8%.

PROPERTIES:

S i H⁴: M . p . - 1 8 5 ° C , b . p . -111.9°C; d (at m.p.) 0 . 6 8 .

S i²H⁶: M . p . - 1 3 2 . 5 ° C , b . p . - 1 4 . 5 ° C ; d (-25) 0 . 6 8 5 . V e r y r e ­ a c t i v e . Ignites in a i r .

REFERENCES:

A. Stock, H y d r i d e s of B o r o n and Silicon (Ithaca, 1933). A d d i t i o n a l r e f e r e n c e s to t h e l i t e r a t u r e a r e found t h e r e . A. Stock, Z. E l e c t r o - c h e m . 32, 341 (1926); W. C. J o h n s o n and S. I s e n b e r g , J . A m e r . C h e m . Soc. 5 7 , 1 3 4 9 (1935); A. E . Finholt, A. C. Bond, Κ. E . W i l z b a c h

12. S I L I C O N AND G E R M A N I U M 681 and H. J . S c h l e s i n g e r , J . A m e r . C h e m . S o c . 6 9 , 2692 (1947); F . F e h r e r a n d W. T r o m m , Z . a n o r g . a l l g . C h e m . 2 8 2 , 29 (1955); J . S.

P e a k e , W. H. N e b e r g a l l a n d Yun T i C h e n , J , A m e r . C h e m . S o c . 7 4 , 1526 (1952); H. S. G u t o w s k y a n d E . O. S t e j s k a l , J . C h e m . P h y s i c s , 2 2 , 939 (1954).

Polysilane s (SiH)x, (SiH²)x

2SiHBr^{: i} + 3Mg = 2 (SiH) + 3MgBr²

537,7 73.0 58.2 552.5

M a g n e s i u m t u r n i n g s (6 t o 8 g.) a r e p l a c e d i n t h e r e a c t i o n c h a m b e r α ( F i g . 2 2 2 ) , w h i c h i s s e p a r a t e d b y a p e r f o r a t e d p l a t e b f r o m t h e f i l t e r g\ t h e r e a c t i o n c h a m b e r i s e v a c u a t e d a n d filled w i t h p u r e N^s ( s e e p . 458). V e s s e l a i s filled w i t h a 30 v o l . % s o l u t i o n of S i H B r³ in a b s o l u t e e t h e r . W i t h s t o p ­ c o c k s e a n d / s t i l l c l o s e d , s t o p c o c k d i s o p e n e d t o a d m i t j u s t enough S i H B r³ s o l u t i o n t o c o v e r t h e Mg t u r n i n g s . T h e h e a t of t h e i n s t a n t l y c o m m e n c i n g r e a c t i o n c a u s e s t h e e t h e r t o b o i l . T h e M g i s c o v e r e d w i t h a y e l l o w i n ­ c r u s t a t i o n w h i c h i s b r o k e n u p b y t h e a c t i o n of t h e h a n d - o p e r a t e d a g i t a t o r ? ! . T h e r e a c t i o n p r o d u c t c o l l e c t s on t h e f i l t e r g. A f t e r a b o u t 20 t o 30 m i n u t e s , t h e e t h e r a n d t h e m a g n e s i u m b r o m i d e e t h e r a t e a r e d r a w n b y s u c t i o n into i a n d a r e t h e n r e l e a s e d into k. A b s o l u t e e t h e r f r o m v e s s e l ο i s now l e t in v i a s t o p c o c k d a n d t h e a p p a r a t u s i s w a s h e d s e v e r a l t i m e s u n t i l t e s t s a m p l e s d r a w n out t h r o u g h t h e t w o - w a y s t o p c o c k I a r e f r e e of B r . T h e r e a c t i o n p r o d u c t i s now s u s p e n d e d in e t h e r a n d , b y t i p p i n g t h e a p p a r a t u s , it i s m a d e t o o v e r f l o w into f l a s k m; t h e e t h e r i s now p u m p e d out t o o b t a i n t h e d r y p r o d u c t . Y i e l d i s a b o u t o n e g r a m .

F i g . 2 2 2 . P r e p a r a t i o n of ( S i H )^x

CaSi

68.2

+ 2HC1

72.9

(SiH²) + CaCl²

30.1 111.0

T h e a p p a r a t u s i s s i m i l a r t o t h a t i l l u s t r a t e d in F i g . 2 2 2 , e x c e p t t h a t s t o p c o c k s β a n d / a r e o m i t t e d a l o n g w i t h t h e r e f l u x c o n d e n s e r

682 ^P. W . S C H E N K

and the p e r f o r a t e d p l a t e b. A 5-g. q u a n t i t y of CaSi (see p . 946 for p r e p a r a t i o n ) i s p o u r e d f r o m f l a s k m (by r o t a t i n g t h e f l a s k in t h e joint) into 300 m l . of H C l - s a t u r a t e d a b s o l u t e a l c o h o l , w h i c h c o v e r s t h e g l a s s f i l t e r p l a t e g. T h e p r o c e d u r e m u s t b e c a r r i e d out in an a t m o s p h e r e of d r y C O^s with good a g i t a t i o n . T h e addition of CaSi t a k e s about an h o u r . Good cooling m u s t b e p r o v i d e d . After s t a n d i n g o v e r n i g h t w h i l e t h e evolution of H^s c e a s e s , t h e b r o w n p r e c i p i t a t e i s s e p a r a t e d in a C 0² a t m o s p h e r e f r o m t h e s u p e r n a t a n t liquid.

T h e p r e c i p i t a t e i s f i r s t w a s h e d w i t h i c e - c o l d a b s o l u t e a l c o h o l and t h e n with a b s o l u t e e t h e r .

T h e p r e p a r a t i o n of t h e h i g h e r u n s a t u r a t e d Si h y d r i d e s m a y b e a c c o m p l i s h e d by c l e a v i n g t h e s a t u r a t e d h y d r i d e s with e l e c t r i c a l d i s c h a r g e .

PROPERTIES:

( S i H )^x: l e m o n - y e l l o w s u b s t a n c e . H a r d , a m o r p h o u s ; o x i d i z e s slowly in a i r .

(SiH;g)^x: b r o w n i s h s u b s t a n c e . T h e d r y m a t e r i a l i g n i t e s s p o n ­ t a n e o u s l y in a i r , l e a v i n g a S i 0² r e s i d u e w h i c h m a y be g r a y b e c a u s e of t h e p r e s e n c e of i r o n s i l i c i d e . E v o l v e s Η²w i t h a l k a l i h y d r o x i d e s .

REFERENCES:

G. Schott and W. H e r r m a n n , Z . a n o r g . a l l g . C h e m . 2 8 8 , 1 (1956); G. Schott and E . H i r s c h m a n n , Z. a n o r g . a l l g . C h e m . 2 8 8 , 9 (1956); A. Stock and K. S o m i e s k i , B e r . d t s c h . c h e m . G e s . 54, 524 (1921); 5 6 , 247 (1923); R. S c h w a r z and F . H e i n r i c h , Z. a n o r g . a l l g . C h e m . 221, 277 (1935).

Silico n Tetrachlorid e

SiCl4

Si + 2 C 1² = S i C l⁴ 28.1 141.8 169.9

Silicon ( p r e p a r e d a s shown above) o r c o a r s e l y g r o u n d f e r r o - s i l i c o n (which should c o n t a i n a s m u c h Si a s p o s s i b l e ) i s p l a c e d in a b o a t i n s e r t e d into a P y r e x t u b e a b o u t 60 c m . long a n d 2 - 3 c m . in d i a m e t e r , t h r o u g h w h i c h a s t r e a m of C l² i s p a s s e d ( F i g . 223). T h e CI ² i s p r e d r i e d o v e r c o n c e n t r a t e d Η ²S 0⁴. A c o n d e n s e r i s a t t a c h e d t o t h e o t h e r end of t h e t u b e by m e a n s of a n a d a p t e r ; t h e t u b e i t s e l f i s h e a t e d in an e l e c t r i c f u r n a c e . T h e t u b e s h o u l d i n c l i n e t o w a r d t h e c o n d e n s e r to p r e v e n t t h e S i C l⁴ f r o m b a c k i n g u p .

1 2 . S I L I C O N AND G E R M A N I U M 683

F i g . 2 2 3 . P r e p a r a t i o n of s i l i c o n t e t r a c h l o r i d e .

T h e c o n d e n s e r e n d of t h e r e a c t i o n t u b e should p r o j e c t o u t of t h e f u r n a c e f o r s o m e d i s t a n c e s o t h a t t h e i n v a r i a b l e b y p r o d u c t F e C l³ m a y d e p o s i t without p l u g g i n g t h e c o n d e n s e r . T h e c o n - d e n s e r d i s c h a r g e s into a d i s t i l l i n g f l a s k w h i c h i s s e t in a n i c e b a t h . A l l j o i n t s m u s t b e tight a n d t h e e n t i r e a p p a r a t u s c a r e f u l l y d r i e d b e f o r e t h e s t a r t of t h e r u n . A C a C l ³ t u b e i s a t t a c h e d a t t h e end of t h e s i d e a r m of t h e d i s t i l l i n g f l a s k . If t h i s p r e c a u t i o n i s o v e r l o o k e d , t h e s i d e a r m will i m m e d i a t e l y b e plugged with s i l i c i c a c i d p r o d u c e d b y r e a c t i o n with a t m o s p h e r i c m o i s t u r e . T h e r e a c t i o n tube i s h e a t e d t o about 400°C a n d t h e C l² flow i s t h e n s t a r t e d .

If, in a d d i t i o n t o S i C l⁴, t h e h i g h e r Si c h l o r i d e s a r e d e s i r e d ( S i³C l⁶ a n d S i³C l⁸) , t h e t e m p e r a t u r e of t h e t u b e should b e a s low a s p o s s i b l e , b u t no l o w e r t h a n j u s t b e l o w 400°C. W h e n t h e r e a c t i o n i s w e l l e s t a b l i s h e d , h e a t i n g m a y b e s h a r p l y r e d u c e d s i n c e t h e r e a c t i o n i t s e l f e v o l v e s c o n s i d e r a b l e h e a t . T h e c r u d e c h l o r i d e , w h i c h i s a c c u m u l a t e d in t h e d i s t i l l i n g flask, m a y b e p u r i f i e d b y f r a c t i o n a l d i s t i l l a t i o n . If a n a b s o l u t e l y C l - f r e e p r e p a r a t i o n i s d e s i r e d , a s e c o n d d i s t i l l a t i o n o v e r Cu t u r n i n g s i s p e r f o r m e d . Both d i s t i l l a t i o n s m u s t be r u n in a b s o l u t e l y d r y e q u i p m e n t . T h e a m p o u l e s into w h i c h t h e S i C l⁴ i s d i s t i l l e d should b e fused to t h e d i s t i l l a t i o n a p p a r a t u s , s i n c e it i s i m p o s s i b l e t o o b t a i n a n o n - t u r b i d p r o d u c t if t h i s i s not d o n e . Yield i s q u a n t i t a t i v e .

PROPERTIES:

C l e a r , c o l o r l e s s liquid; f u m e s h e a v i l y upon e x p o s u r e t o a i r . R a p i d l y h y d r o l y z e s in w a t e r t o f o r m a S i 0³ g e l . M i s c i b l e with b e n z e n e , e t h e r , c h l o r o f o r m a n d s a t u r a t e d h y d r o c a r b o n s . F o r m s e s t e r s of s i l i c i c a c i d w i t h a l c o h o l s . B . p . 57.5°C, m . p . -68°C;

d 1.52.

6 8 4 P . W . S C H E N K

Highe r Silico n Chloride s

If t h e p r e p a r a t i o n of S i C l⁴ i s c a r r i e d out at t e m p e r a t u r e s b e l o w 400°C, it i s p o s s i b l e t o i s o l a t e v e r y s m a l l q u a n t i t i e s of S i g C ^ and S i³C l⁸ f r o m t h e r e s i d u e of t h e final d i s t i l l a t i o n .

C h l o r i d e s u p to S i g C ^ a r e b e s t p r e p a r e d by c h l o r i n a t i o n of c a l c i u m s i l i c i d e . T h e p r o c e d u r e i s a s f o l l o w s : C l², d r i e d o v e r c o n c e n t r a t e d HgSC^, i s p a s s e d t h r o u g h a v e r t i c a l g l a s s t u b e ( 3 4 - m m . d i a m e t e r , about one m e t e r long), half filled with b e a n - s i z e l u m p s of C a s i l i c i d e (about 3 0 - 3 5 % C a ) . T h e t u b e s h o u l d not b e too full, for t h e n it m i g h t plug d u r i n g t h e r u n ; about 2 0 0 - 2 5 0 g. i s u s e d . T h e t u b e i s p l a c e d i n s i d e a s h o r t , m o v a b l e e l e c t r i c h e a t i n g c o i l . It i s i m p o r t a n t t h a t t h e r e a c t i o n t a k e p l a c e at t h e l o w e s t p o s s i b l e t e m p e r a t u r e and t h a t only a s h o r t s e c t i o n of t h e t u b e b e h e a t e d at a n y t i m e . T h e r e a c t i o n s t a r t s at 250°C. T h e t e m p e r a t u r e m u s t t h e n b e i m m e d i a t e l y l o w e r e d t o 150°C. T h e CI ² flow s h o u l d not e x c e e d 100 b u b b l e s / m i n u t e . T h e r e a c t i o n p r o d u c t s a r e a c c u - m u l a t e d in a c o o l e d r e c e i v e r v i a a n a t t a c h e d c o n d e n s e r . A f t e r 12 to 14 d a y s , d u r i n g w h i c h t h e h e a t i n g c o i l i s s l o w l y m o v e d a l o n g t h e e n t i r e length of t h e t u b e , a l l t h e s i l i c i d e i s r e a c t e d , and about 700 m l . of c h l o r i d e m i x t u r e i s c o l l e c t e d . T h e h i g h e r c h l o r i d e s a r e o b t a i n e d f r o m t h i s m i x t u r e by f r a c t i o n a t i o n . If a low t e m - p e r a t u r e and a s l o w flow r a t e of C l² a r e u s e d , t h e n about 35% of t h e p r o d u c t m i x t u r e b o i l s at a h i g h e r t e m p e r a t u r e t h a n S i C l⁴. About 30% of t h i s i s S i²C l⁶, 4 % , S i³C l⁸, and 1% r e p r e s e n t s c h l o r i d e s up to S i⁶C l i 4 . After e v a p o r a t i o n of t h e S i C l⁴, t h e r e s i d u e i s f r a c t i o n a t e d at r e d u c e d p r e s s u r e . Addition of 2 t o 5% of a n a l k a l i c h l o r i d e , a l k a l i n e e a r t h c h l o r i d e o r a m m o n i u m c h l o r i d e o r dilution of t h e c h l o r i d e s (e.g., with S i C l⁴) should i m p r o v e t h e y i e l d .

PROPERTIES:

B . p . : S i²C l⁶, 147°C; S i³C l⁸, 216°C; S i⁴C l^{1 0}, 150°C (at 15 m m . ) ; S i⁵C l^{1 2}, 190°C (15 m m . ) ; S i^eC l^{1 4} s u b l i m e s in v a c u u m a t 200°C.

A c c o r d i n g to R, S c h w a r z , t h e h i g h e r Si c h l o r i d e s , s u c h a s Si^Clgg, c a n b e p r e p a r e d by t r e a t m e n t of S i C l⁴ in t h e a p p a r a t u s i l l u s t r a t e d in F i g . 224, which t h e a u t h o r c a l l s a " q u e n c h i n g t u b e . "

T h e p r o c e d u r e i s a s follows.

A s i l i c o n c a r b i d e r o d , h e l d in p l a c e by two e l e c t r o d e c o n n e c t o r c l a m p s , i s fitted into a L i e b i g c o n d e n s e r . T h e e l e c t r o d e c l a m p s a r e s e a l e d into P y r e x c a p s , w h i c h fit o v e r t h e g r o u n d - g l a s s m a l e j o i n t s of t h e c o n d e n s e r .

If the e l e c t r i c a l t e r m i n a l s of t h e s e c l a m p s c o n s i s t of w a t e r - cooled c o p p e r t u b i n g , t h e n t h e s e a l to t h e g l a s s c a p s i s m a d e with r u b b e r tubing. A c l a m p i s m a d e by c l o s i n g one end of t h e c o p p e r t u b e , welding on a p i e c e of t h i n w a l l s t a i n l e s s s t e e l tube and

1 2 . S I L I C O N A N D G E R M A N I U M 685

F i g . 2 2 4 . P r e p a r a t i o n of t h e h i g h e r s i l i c o n c h l o r i d e s .

s p l i t t i n g t h e end of t h e l a t t e r with two l e n g t h w i s e s a w c u t s . T h e SiC r o d f i t s into t h a t end. T h e o p e n e n d of t h e c o p p e r t u b e i s c l o s e d with a c a p w h i c h c a r r i e s t h e i n l e t a n d o u t l e t t u b e s for t h e c o o l i n g w a t e r (the inlet t u b e e x t e n d s w e l l i n s i d e t h e c l o s e d t u b e ) .

A l t e r n a t i v e l y , i r o n e l e c t r o d e c l a m p s , s e a l e d to t h e P y r e x c a p s with a s b e s t o s - w a t e r g l a s s m i x t u r e , a r e u s e d with no c o o l i n g . In t h a t c a s e , t h e P y r e x c a p s m a y h a v e t o be wound with cotton s t r i n g , w e t t e d with w a t e r t o r e m o v e t h e h e a t .

T h e ^aq u e n c h i n g t u b e " i t s e l f c a n be m a d e of c o p p e r . In t h i s c a s e , the g l a s s c a p s a r e s e a l e d on with r u b b e r t u b i n g ( p i e c e s of b i c y c l e i n n e r t u b e ) .

T h e a p p a r a t u s i s f i r s t t h o r o u g h l y d r i e d by h e a t i n g in a s t r e a m of i n e r t g a s , with t h e c o o l i n g w a t e r off. After b r i e f c o o l i n g , a s t r e a m of h y d r o g e n i s i n t r o d u c e d a t a and t h e a i r i s d i s p l a c e d . T h e n 25 m l . of S i C l⁴ i s p l a c e d in f l a s k a, t h e h y d r o g e n s a t u r a t e d with t h e S i C l⁴ and t h e h e a t i n g r e s u m e d [the h y d r o g e n i s f i r s t p a s s e d o v e r P d a b e s t o s to r e m o v e 0 ² and o v e r P²0⁵f o r d r y i n g

(see s e c t i o n on H y d r o g e n ) ] .

T h e f i r s t r u n in a n e w tube should be m a d e with H²; Si ^{1 0}C 1^{2 0}H² i s f o r m e d u n d e r t h e s e c o n d i t i o n s . If o x y g e n - f r e e a r g o n a n d S i C l⁴ a r e u s e d in a l a t e r r u n , Si ^{1 0} CI 22 i s f o r m e d . The t e m p e r a t u r e of t h e SiC r o d should be b e t w e e n 1000°C and 1100°C. A t r a p b⁹ cooled with liquid n i t r o g e n , i s c o n n e c t e d to t h e d i s t i l l i n g f l a s k which s e r v e s a s t h e r e c e i v e r .

It h a p p e n s o c c a s i o n a l l y t h a t t h e r e a c t i o n f a i l s to s t a r t . In t h a t c a s e , t h e g a s i s e i t h e r not c o m p l e t e l y w a t e r - f r e e o r it c o n t a i n s

6 8 6 P . W . S C H E N K

oxygen. W h e n e v e r a n e w SiC r o d i s u s e d , t h e f i r s t r u n should b e m a d e with H2 a s t h e c a r r i e r g a s , for o t h e r w i s e t h e r e a c t i o n will not s t a r t . F r o m 112 g. of S i C l4 t h e y i e l d i s about 35 g. c r u d e p r o d u c t , f r o m w h i c h t h e l o w e r b o i l i n g f r a c t i o n s a r e s t r i p p e d off.

T h e Si ^{1 0} C I²oΗ² c a n n o t b e d i s t i l l e d without d e c o m p o s i n g . T h e p r o d u c t p r e p a r e d with a r g o n i s f r a c t i o n e d u n d e r high v a c u u m . T h e S i1 0C l22 c o m e s o v e r a s a highly v i s c o u s oil b e t w e e n 215 and 220°C.

T h e p r e p a r a t i o n of t h e h i g h e r Si c h l o r i d e s by p a s s a g e of S i C l⁴ o v e r Si at 1000°C i s d e s c r i b e d by Rochow a n d D i d t s c h e n k o ; t h e p r e p a r a t i o n by m e a n s of a glow d i s c h a r g e i s d e s c r i b e d by H e r t w i g a n d W i b e r g .

PROPERTIES:

V i s c o u s , w i t h a c o n s i s t e n c y r a n g i n g f r o m oily to h o n e y l i k e . F l a m m a b l e . H y d r o l y z e d by w a t e r .

REFERENCES:

A. B e s s o n and L . F o u r n i e r , C o m p t e s R e n d u s Hebd. S e a n c e s A c a d . S c i . , 152, 603 (1911); C. F r i e d e l , C o m p t e s R e n d u s Hebd. S e a n c e s A c a d . Sci. 7 3 , 1011 (1871); C. F r i e d e l a n d A. L a d e n b u r g , L i e b i g s Ann. C h e m . 2 0 3 , 253 (1880); L. G a t t e r m a n a n d E . E l l e r y , B e r . d t s c h . c h e m . G e s . 32, 1114 (1899); L. G a t t e r m a n and K. W e i n l i g , B e r . d t s c h . c h e m . G e s . 27, 1943 (1894); J . W. M e l l o r , C o m p r e ­ h e n s i v e T r e a t i s e on I n o r g . C h e m . , VI, p . 9 7 1 ; Int. C r i t . T a b l . , Vol. I, p . 162; G. M a r t i n , J . C h e m . Soc. (London) 105, 2 8 3 6 , 2860 (1914); B e r . d t s c h . c h e m . G e s . 4 5 , 2097 (1912); 4 6 , 2 4 4 2 , 3 2 8 9 (1913); L. T r o o s t a n d P . H a u t e f e u i l l e , Ann. C h i m . P h y s . (5) 7, 459 (1871); R. S c h w a r z and H. M e c k b a c h , Z . a n o r g . a l l g . C h e m . 2 3 2 , 241 (1937); E. G. Rochow a n d R. D i d t s c h e n k o , J . A m e r . C h e m . Soc. 74, 5545 (1952); H. S. G u t o w s k i and E . O. S t e j s k a l , J . C h e m . P h y s . 2 2 , 939 (1954); D. F . S t e d m a n n , U . S . P a t e n t 2 , 6 2 1 , 1 1 1 ; W. J. W a l t o n , U . S . P a t e n t 2,602,728 (1952); H. S c h a e f e r , Z . a n o r g . a l l g . C h e m . 274, 265 (1953); C. F . W i l k i n s , J . C h e m . Soc. (London), 1 9 5 3 , 3409; K. A. H e r t w i g and E . W i b e r g , Z. N a t u r f o r s c h . , 6b, 336 (1951); W. C. Schumb a n d E. L. G a m b l e in H. S. Booth, I n o r g . S y n t h e s e s New Y o r k and London, 1939 , Vol. I, p . 4 2 .

Silico n Tetrabromid e SiBr4

Si + 2 B r² = S i B r⁴ 28.1 319.7 347.8

A b o a t , p l a c e d in a V y c o r t u b e , i s h e a t e d in a n e l e c t r i c f u r n a c e t o about 600°C. T h e b o a t c o n t a i n s finely p o w d e r e d Si (which i s

1 2 . S I L I C O N AND G E R M A N I U M 687 b e s t m i x e d w i t h about 4% c o p p e r d u s t ) . An Cfe-free s t r e a m of N^s ( f r o m a " c o p p e r t o w e r , " s e e p . 4 5 8 ) , s a t u r a t e d w i t h B r^s v a p o r in a w a s h b o t t l e , i s p a s s e d o v e r t h e s i l i c o n . A c o n d e n s e r w i t h a r e c e i v e r i s a t t a c h e d to t h e o t h e r end of t h e t u b e , a s in t h e p r e p a - r a t i o n of S i C l * . T h e c r u d e p r o d u c t i s p u r i f i e d by f r a c t i o n a l d i s - t i l l a t i o n .

PROPERTIES:

C o l o r l e s s liquid. E a s i l y h y d r o l y z e d . F u m e s in a i r . M . p . 5.2°C;

b . p . 152.8°C; d 2 . 7 8 9 . In c o n t r a s t to S i C l⁴ it r e a c t s v i o l e n t l y with m e t a l l i c p o t a s s i u m .

REFERENCE:

L. G a t t e r m a n , B e r . d t s c h . c h e m . G e s . , 2 2 , 189 (1889).

Silico n (II) Bromid e SiBr²

SiBr⁴ + Si = 2SiBr²

347,8 28.1 375.9

A s shown in F i g . 2 2 5 , an u n g l a z e d p o r c e l a i n b o a t i s c h a r g e d w i t h 20 g. of v e r y p u r e s i l i c o n and p l a c e d in a p o r c e l a i n o r q u a r t z t u b e ( 3 0 - 4 0 m m . d i a m e t e r , 800 m m . l o n g ) , p l a c e d in a G l o b a r t u b u l a r f u r n a c e . T h e b o a t i s p u s h e d to t h e h o t t e s t p a r t of t h e f u r n a c e . T h e t e m p e r a t u r e s h o u l d b e a b o u t 1150°C at t h i s p o i n t . T h e e n d s of t h e t u b e , w h i c h e x t e n d b e y o n d t h e f u r n a c e , a r e wound w i t h l e a d c o o l i n g c o i l s a n d a r e c l o s e d w i t h w e l l - s e a t e d r u b b e r s t o p p e r s c a n d d. T h e s t o p p e r s a r e p a i n t e d w i t h q u i c k - s e t t i n g c h l o r i n a t e d r u b b e r c e m e n t to e n s u r e a b e t t e r s e a l . T h e f l a s k a, w h i c h c o n t a i n s S i B r⁴, i s f i r s t c o o l e d t o a t l e a s t -80°C and i s t h e n v e r y s l o w l y

F i g . 2 2 5 . P r e p a r a t i o n of s i l i c o n (II) b r o m i d e .

688 P . W . S C H E N K

e v a c u a t e d v i a t r a p e and s t o p c o c k s b a n d / . C a u t i o n i s n e c e s s a r y s i n c e t h e Si p o w d e r i s e a s i l y e n t r a i n e d . When a high v a c u u m h a s b e e n a t t a i n e d , t h e f u r n a c e i s t u r n e d on. A s s o o n a s t h e t e m p e r a ­ t u r e r e a c h e s 1100°C^f t r a p e i s c o o l e d in D r y I c e o r liquid n i t r o g e n and flask a i s s e t in i c e w a t e r . C r y s t a l s of SigBr⁶ f o r m i m m e ­ d i a t e l y at g and u n r e a c t e d S i B r⁴ c o l l e c t s in β. In s i x t o s e v e n h o u r s , about 4 0 - 6 0 m l . S i B r⁴ d i s t i l l s o v e r , of w h i c h about 10 m l . i s r e c o v e r e d u n r e a c t e d in e . T h e a p p a r a t u s i s a l l o w e d to c o o l u n d e r v a c u u m , d r y N² i s i n t r o d u c e d t h r o u g h / , s t o p p e r d i s r e m o v e d , and t h e t u b e i s quickly w i t h d r a w n f r o m t h e f u r n a c e . A s u i t a b l e w i d e - m o u t h flask, w h i c h i s k e p t r e a d y , i s p u s h e d o v e r t h e o p e n end of t h e t u b e and, a f t e r flask α i s d e t a c h e d , a s t r e a m of d r y N^s i s i n t r o d u c e d v i a b and ο into t h e now i n c l i n e d t u b e . T h e p r o d u c t i s m e l t e d by c a r e f u l h e a t i n g with a n open f l a m e (the t u b e i s fanned with t h e flame) and a l l o w e d t o flow into t h e w i d e - m o u t h f l a s k . T h i s o p e r a t i o n , w h i c h t a k e s about 15 m i n u t e s , i s c a r r i e d out in an

a t m o s p h e r e of d r y N². T h e p r o d u c t a c c u m u l a t e d f r o m s e v e r a l r u n s i s t r a n s f e r r e d into an Anschiitz d i s t i l l a t i o n f l a s k and i s h e a t e d in high v a c u u m for half an h o u r at 170-180°C in o r d e r to s e p a r a t e t h e Si^Brs. About 80% of t h e p r o d u c t r e m a i n s b e h i n d a s a d a r k b r o w n s u b s t a n c e , m i x e d with p o w d e r e d Si. T h i s i s d i s s o l v e d in t h r e e to four t i m e s i t s v o l u m e of d r y b e n z e n e a n d f i l t e r e d w h i l e p r o t e c t e d f r o m m o i s t u r e in a n a p p a r a t u s s i m i l a r t o t h a t i l l u s t r a t e d in F i g . 4 9 , p . 7 1 ; t h e a p p a r a t u s , h o w e v e r , h a s no h e a t i n g j a c k e t and t h e h o r i z o n t a l o u t l e t s ο and d⁹ t h i s t i m e p r o v i d e d with s t o p c o c k s , a r e fused i n s t e a d to t h e m i d d l e s e c t i o n . T h e c l e a r , d a r k y e l l o w s o l u ­ t i o n i s f r e e d f r o m b e n z e n e u n d e r v a c u u m and i s t h e n h e a t e d f o r a n o t h e r h o u r in high v a c u u m at 170°C. D r y a i r i s a d m i t t e d a f t e r c o o l i n g a n d t h e f l a s k c o n t a i n i n g t h e r e s i d u e i s b r o u g h t t o a t e m ­ p e r a t u r e of - 8 0 ° C At t h i s t e m p e r a t u r e , t h e S i B r² i s s o b r i t t l e t h a t it c r u m b l e s e a s i l y on s l i g h t s h a k i n g .

PROPERTIES:

F o r m u l a w e i g h t 1 8 7 . 9 2 . B r o w n , f r i a b l e s u b s t a n c e , s o m e w h a t l i k e s h e l l a c ; t r a n s p a r e n t in t h i n s h e e t s . B e g i n s to soften a t about 60°C; f i l a m e n t s m a y b e d r a w n at 100-110°C; at 160° t o 180°C, it a c q u i r e s t h e v i s c o s i t y of a h e a v y g r e a s e . D e c o m p o s i t i o n s t a r t s at 200°C a n d SigBrg i s f o r m e d . I g n i t e s s p o n t a n e o u s l y in a i r at 120°C. M o l e c u l a r weight f r o m b e n z e n e , 3 , 0 0 0 - 3 , 6 0 0 . H y d r o l y z e s in w a t e r , f o r m i n g s u b s i l i c i c a c i d s . S t r o n g r e d u c i n g a g e n t . Soluble in b e n z e n e , x y l e n e , C C 1⁴, e t c . ; s l i g h t l y s o l u b l e in e t h e r . R e a c t s e x p l o s i v e l y with s t r o n g o x i d i z e r s s u c h a s H N O^a.

REFERENCE:

M. S c h m e i s s e r , p r i v a t e c o m m u n i c a t i o n , 1958.

1 2 . S I L i C O N AND G E R M A N I U M 689 Silico n Tetraiodid e

Sii⁴

Si + 2 I² = Sil⁴

28.1 507.7 535.8

T h e p r e p a r a t i o n i s c a r r i e d out in t h e a p p a r a t u s shown in F i g . 2 2 6 . It c o n s i s t s of a q u a r t z t u b e a , w h i c h i s 500 m m . long a n d 30 m m . in d i a m e t e r a n d h a s a m a l e g r o u n d - g l a s s joint on b o t h e n d s . A t t a c h e d t o t h e m a i n t u b e a t a r i g h t a n g l e i s t u b e b⁹ a p p r o x i ­ m a t e l y 50 m m . long, w i t h a f e m a l e g r o u n d joint a t t h e e n d . T h e a p p a r a t u s i s filled in t h e following m a n n e r w i t h a m i x t u r e c o n ­ s i s t i n g of c o m m e r c i a l g r a d e p o w d e r e d s i l i c o n p l u s 4% of c o p p e r d u s t : a 2 5 - c m . - l o n g s t r i p of c o p p e r i s f a s h i o n e d into a s p i r a l w h i c h i s h e l d in p l a c e in t h e q u a r t z t u b e b y a l o o s e p l u g of a s ­ b e s t o s w o o l . T h e c o p p e r s p i r a l i s p l a c e d in s u c h a m a n n e r t h a t one e n d of it e x t e n d s into t h e joint a n d t h e a s b e s t o s p l u g 1 i s l o c a t e d about 5 t o 6 c m . a w a y f r o m t h e fitting. T h e t u b e i s now c h a r g e d w i t h t h e s i l i c o n a n d c o p p e r p o w d e r m i x t u r e , w h i c h i s p a c k e d a s l o o s e l y a s p o s s i b l e . T h e t u b e i s h e l d v e r t i c a l l y d u r i n g t h e filling a n d t h e a d d i t i o n i s c o n t i n u e d u n t i l t h e c o p p e r s p i r a l i s j u s t c o v e r e d . T h e c h a r g e i s t h e n h e l d in p l a c e b y t h e l o o s e a s b e s t o s wool p l u g 2 . T h e c h a r g e d t u b e i s i n s e r t e d in a t u b u l a r e l e c t r i c f u r n a c e , w h i c h i s e q u i p p e d w i t h a t h e r m o c o u p l e , in s u c h a w a y t h a t t h e u p p e r e d g e of t h e c h a r g e i s e v e n w i t h t h e end of t h e f u r n a c e ; t h e a p p a r a t u s i s t i l t e d about 4 0 ° f r o m t h e h o r i z o n t a l . A r e c e i v e r d w i t h a g l a s s o u t l e t t u b e i s a t t a c h e d a t t h e l o w e r j o i n t . T h e u p p e r e n d of t u b e α i s c l o s e d b y a fitting with a s t o p c o c k , t h r o u g h w h i c h a s t r e a m of o x y g e n - f r e e n i t r o g e n i s i n t r o d u c e d a t a r a t e of f o u r b u b b l e s / s e c . T h e N⁸i s p r e d r i e d o v e r c o n c e n t r a t e d H²S 0⁴, s i l i c a g e l a n d PgOg. F l a s k k⁹ c o n ­ t a i n i n g t h e iodine, i s a t t a c h e d t o t h e a r m b. T h e a p p a r a t u s i s f i r s t t h o r o u g h l y h e a t e d in a s t r e a m of N^a at 600 t o 700°C f o r o n e t o two h o u r s . T h e f i r s t addition of iodine i s t h e n m a d e b y r o t a t i n g t h e iodine flask k in t h e joint; m o d e r a t e h e a t i s a p p l i e d a t t h e u p p e r a s b e s t o s p l u g . C r u d e S i l⁴, c o n t a m i n a t e d w i t h i o d i n e , i m m e ­ d i a t e l y flows t o t h e l o w e r e n d w h e r e it c o l l e c t s in t h e r e c e i v e r . C a r e m u s t b e t a k e n t h a t s o l i d S i l⁴ d o e s n o t p l u g t h e t u b e . If n e c e s s a r y , t h e SiLi m u s t b e m e l t e d down b y fanning with a n open f l a m e . About 150 g. of c r u d e S i ^ c a n t h u s b e o b t a i n e d o v e r s e v e r a l h o u r s . T h e y i e l d , b a s e d o n i o d i n e , i s 70%. U n r e a c t e d Si c a n b e r e u s e d in l a t e r r u n s . F o r p u r i f i c a t i o n , t h e r e c e i v e r i s b r o k e n off a t g⁹ quickly c r u s h e d in a m o r t a r a n d t h e c r u s h e d m a s s p o u r e d into a w e l l - h e a t e d d i s t i l l i n g f l a s k ( s e e F i g . 2 2 7 ) , equipped w i t h a n a p p r o x i m a t e l y 2 0 - m m . - d i a m e t e r s i d e a r m . T h e c h a r g e a i s c o v e r e d with a n a p p r o x i m a t e l y 5 - m m . - t h i c k l a y e r of

690 P . W . S C H E N K

F i g⁰ 226. P r e p a r a t i o n of s i l i c o n t e t r a i o d i d e .

F i g . 2 2 7 . D i s t i l l a t i o n of s i l i c o n t e t r a i o d i d e . [^J p o w d e r e d c o p p e r b and a 2 - c m . - l o n g l o o s e plug of c o p p e r wool d i s p u s h e d u p into t h e s i d e a r m . T h e d i s t i l l i n g flask i s now h e a t e d in a liquid m e t a l b a t h and e v a c u a t e d with an oil p u m p . Some S i l⁴, t o g e t h e r with I^a, b e g i n s to s u b l i m e at a b a t h t e m p e r a t u r e of 1 5 0^UC ; t h e r e a c t i o n b e t w e e n Cu and I² b e g i n s at 180°C, a n d t h e m e l t b e g i n s t o b o i l at 2 0 0 ° C T h e s t o p c o c k on t h e l i n e to t h e p u m p i s now c l o s e d , and t h e S i l⁴ i s d i s t i l l e d o v e r . Since s o m e I² c o m e s o v e r at t h e s t a r t , t h e s o l i d S i l⁴ i s r o s e - c o l o r e d . After t h r e e d i s - t i l l a t i o n s , p u r e w h i t e , s o l i d S i l⁴ ( l e m o n y e l l o w a s a liquid) i s o b t a i n e d . B e c a u s e s o m e c o p p e r d u s t i s o c c a s i o n a l l y e n t r a i n e d d u r i n g t h e d i s t i l l a t i o n , t h e final d i s t i l l a t i o n of c o l o r l e s s S i ^ i s b e s t done without t h e addition of c o p p e r . T h e p r o d u c t s h o u l d b e s t o r e d u n d e r an N^s b l a n k e t in s e a l e d a m p o u l e s , w h i c h a r e k e p t in a d a r k p l a c e .

PROPERTIES:

M . p . 122°C, b . p . 287.5°C. W h i t e c r y s t a l l i n e s u b s t a n c e w h i c h m e l t s t o a l e m o n - y e l l o w liquid. V e r y s e n s i t i v e to m o i s t u r e and light; d e c o m p o s e s r e a d i l y on h e a t i n g into i t s e l e m e n t s .

A c c o r d i n g t o G o m b e r g ( s e e P a r t III, s e c t i o n on A d s o r p t i v e and C a t a l y t i c a l l y Active M a t e r i a l s ) , S i²I^e i s f o r m e d when S i l⁴ i s h e a t e d t o g e t h e r with v e r y d r y , p o w d e r e d s i l v e r at 280°C for s i x h o u r s in a s e a l e d , e v a c u a t e d t u b e .

REFERENCES:

R. S c h w a r z and A. P f l u g m a c h e r , B e r . d t s c h . c h e m . G e s . 7 5 , 1062 (1941); U. W a n n a g a t a n d R. S c h w a r z , Z . a n o r g . a l l g . C h e m . 277,

12. S I L I C O N AND G E R M A N I U M 6 9 1

82 (1954); U. W a n n a g a t a n d E . R i n g e l , p r i v a t e c o m m u n i c a t i o n ; F . B. Litton and H. C A n d e r s o n , J . E l e c t r o c h e m . Soc. 1 0 1 , 287 (1954).

Chlorosilane s SiHCl³, SiH²Cl², SiH³Cl

S e v e r a l m e t h o d s a r e a v a i l a b l e f o r t h e p r e p a r a t i o n of t h e t h r e e c h l o r o s i l a n e s . In o r d e r to o b t a i n S i H C l³ a s t h e p r i n c i p a l p r o d u c t , t h e b e s t w a y i s t o r e a c t Si w i t h H C l in t h e p r e s e n c e of c o p p e r .

Si + 3HC1 = S1HCI3 + H²

28.1 109.4 135.4 22.41

A p o r c e l a i n boat i s filled w i t h finely p o w d e r e d s i l i c o n , w h i c h h a s b e e n p u r i f i e d by b o i l i n g with h y d r o c h l o r i c a c i d and d i l u t e h y d r o f l u o r i c a c i d . About 10% C u C l^s i s m i x e d in with t h e s i l i c o n . T h e b o a t i s p l a c e d in a P y r e x g l a s s t u b e fitted with an a d a p t e r and a c o n ­ d e n s e r , a s d e s c r i b e d in t h e p r e p a r a t i o n of S i C l⁴. In t h i s c a s e , h o w e v e r , t h e end of t h e c o n d e n s e r s h o u l d e x t e n d into t h e m i d d l e of t h e d i s t i l l i n g f l a s k . T h e a p p a r a t u s i s f i r s t c a r e f u l l y h e a t e d . H y d r o g e n c h l o r i d e g a s i s p r o d u c e d f r o m NaCl and H⁸S 0⁴, without a d d i t i o n of h y d r o c h l o r i c a c i d , s i n c e it m u s t b e a b s o l u t e l y d r y . T h e f u r n a c e a r o u n d t h e t u b e w h i c h c o n t a i n s t h e b o a t i s h e a t e d to 300°C a n d a s l o w s t r e a m of HC1 i s i n t r o d u c e d . T h e r e c e i v e r i s c o o l e d w i t h a c e t o n e - D r y Ice m i x t u r e . F i n a l l y , t h e c r u d e p r o d u c t i s d i s t i l l e d d i r e c t l y f r o m t h e r e c e i v e r . T h e f i r s t f r a c t i o n i s HC1.

T h e S i H C l³ c o m e s o v e r at 36.5°C. With a c a r e f u l f r a c t i o n a t i o n of t h e f o r e r u n , S i H^sC l³ m a y a l s o b e r e c o v e r e d . T h e y i e l d of S i H C l³ i s about 50%. In o r d e r t o i m p r o v e t h e y i e l d of S i H³C l³, it i s r e c ­ o m m e n d e d t h a t h y d r o g e n b e a d d e d t o t h e HC1 in t h e r a t i o of H³: H C 1 = 4 : 1 .

Other preparative possibilities:

SiH⁴ + HC1 = SiH³Cl + H² (A. S t ο c k) SiCl⁴ + H² = S1H3CI + HC1 ( D . T . H u r d )

PROPERTIES:

W a t e r - c l e a r , m o b i l e , f l a m m a b l e l i q u i d s . V e r y v o l a t i l e . H y d r o - l y z e d by w a t e r . F u m e in a i r . R e m a r k a b l y i n e r t t o w a r d m e t a l s , e v e n s o d i u m .

S i H C l³: Synonym: s i l i c o c h l o r o f o r m . M . p . -134°C, b . p . 36.5°C;

d 1.35. In t h e p r e s e n c e of A l C l³d i s p r o p o r t i o n a t e s a t t e m p e r a t u r e s a b o v e 300°C to S i C l⁴ a n d S i H²C l³.

S i H³C l³: M . p . -122°C, b . p . 8.5°C; d (-122°C) 1.22.

S i H³C l : M . p . -118°C, b . p . - 3 0 . 5 ° C ; d (-113°C) 1.15.

6 9 2 P . W . S C H E N K REFERENCES:

S t a n d a r d p o w d e r e d Si ( 1 0 0 - m e s h , about 97.5% Si) c a n b e u s e d for t h e p r e p a r a t i o n ; t h e a d d i t i o n of s o m e C u C l i s a d v i s a b l e . T h e a p p a r a t u s shown in F i g . 228 should b e u s e d . H y d r o g e n , d r i e d with fused KOH in d r y i n g t o w e r s a , flows t h r o u g h a b u b b l e r b w h i c h i s filled with B r² and h e a t e d to 45°C on a w a t e r b a t h . T h e B r²- l a d e n g a s t h e n flows t h r o u g h t u b e c⁹ filled with P t - a s b e s t o s and h e a t e d to about 200°C, w h e r e it i s c o n v e r t e d t o H B r . E i t h e r F e B r² ( d r i e d at 100°C) o r m o i s t r e d p h o s p h o r u s i s p l a c e d in d. T h e t o w e r s e a r e

Tribromosilan e SiHBr<

Si + 3 HBr = SiHBr3 + H2

28.1 242.8 268.8 22.41

Η OH Br

α δ ^c

\h

F i g . 2 2 8 . P r e p a r a t i o n of t r i b r o m o s i l a n e .

L. G a t t e r m a n , B e r . d t s c h . c h e m . G e s . 2 2 , 1 9 0 (1889); O. Ruff a n d K. A l b e r t , B e r . d t s c h . c h e m . G e s . 3 8 , 2226 (1905); C. C o m b e s , C o m p t e s R e n d u s Hebd. S e a n c e s A c a d . S c i . , 122, 531 (1896); G. H.

W a g n e r , U. S. P a t e n t s 2,657,114; 2 , 6 2 7 , 4 5 1 ; A. Stock and C. S o m i e s k i , B e r . d t s c h . c h e m . G e s . 52, 695 (1919); D. T. H u r d , J . A m e r . C h e m . Soc. 67, 1545 (1945); G. H. W a g n e r , U. S. P a t e n t 2,499,009;

s e e G. F r i t z , Z . a n o r g . a l l g . C h e m . , 280, 134 (1955), f o r q u a n t i ­ t a t i v e d e t e r m i n a t i o n of t h e S i - Η bond e n e r g i e s .

1 2 . S I L I C O N AND G E R M A N I U M 693 filled with C a B r². T h e c h a r g e s in d a n d e m u s t b e r e p l e n i s h e d f r e q u e n t l y . T u b e / c o n t a i n s t h e Si; it i s h e a t e d to 3 6 0 - 4 0 0 ° C . F l a s k g i s p l a c e d in a cooling b a t h at - 3 0 ° C . If l o w e r t e m p e r a t u r e s a r e u s e d , t h e b y p r o d u c t S i B r⁴ t e n d s t o c o n d e n s e and plug t h e t u b e .

B e f o r e a r u n , t h e a i r i s d i s p l a c e d w i t h H² and t h e a p p a r a t u s i s t h o r o u g h l y d r i e d . T h e f u r n a c e s a r e t h e n t u r n e d on and B r³ i s a d d e d t o t h e b u b b l e r f r o m t h e d r o p p i n g funnel. T h e g a s s t r e a m i s s o r e g u l a t e d t h a t it p a s s e s t h r o u g h t h e B r² a t t h e r a t e of 3 - 4 b u b b l e s p e r s e c o n d . U n d e r t h e s e c o n d i t i o n s (45°C), a b o u t 60 g.

v a p o r i z e s in 5 h o u r s . U n r e a c t e d H B r and H² l e a v e t h e a p p a r a t u s t h r o u g h t h e C a C l² t u b e t. At t h e end of t h e r u n , t h e p r o d u c t i s d i s t i l l e d d i r e c t l y f r o m f l a s k g into r e a g e n t b o t t l e s I p r o v i d e d w i t h fusible n e c k s . When s u c h a b o t t l e i s full, t h e s t o p c o c k h i s c l o s e d and t h e b o t t l e i s s e a l e d . When a f r e s h b o t t l e i s a t t a c h e d , it i s f i r s t e v a c u a t e d a n d t h e n filled by o p e n i n g ft. T h e s e s a f e t y m e a s u r e s a r e i m p o r t a n t s i n c e S i H B r³ i s highly f l a m m a b l e and r e a d i l y h y d r o l y z e d . T h e d i s t i l l a t i o n p r o d u c e s t w o f r a c t i o n s : I, boiling u p t o 125°C and H, boiling f r o m 125 t o 154°C.

E a c h of t h e s e f r a c t i o n s u n d e r g o e s a final f r a c t i o n a l d i s t i l l a t i o n in a c o l u m n . T h e a p p a r a t u s i s shown in F i g . 2 2 9 . T h e s e a l e d b o t t l e s i a r e a r r a n g e d on p a d s of g l a s s wool in r e c e i v i n g v e s s e l s . On t o p of t h e v i a l s r e s t g l a s s - e n c a s e d i r o n h a m m e r s k; t h e s e a r e l a t e r lifted w i t h m a g n e t s a n d u s e d t o s h a t t e r t h e t i p s of t h e r e a g e n t b o t t l e s ( s e e P a r t I, p . 63). T h e a p p a r a t u s i s e v a c u a t e d a n d gently h e a t e d by fanning with a n o p e n f l a m e . T h e r e a g e n t b o t t l e s a r e now o p e n e d , f l a s k b i s c o o l e d in a c o l d b a t h t o - 4 0 ° C , and t h e c o n t e n t s of t h e b o t t l e s a r e d i s t i l l e d into b. T h e r e c e i v i n g v e s s e l s c o n t a i n i n g t h e e m p t y r e a g e n t b o t t l e s a r e s e a l e d off. T h e a p p a r a t u s i s now filled with d r y N² and t h e d i s t i l l a t i o n i s s t a r t e d in a s t r e a m of N².

F i g . 2 2 9 . F r a c t i o n a t i o n of c r u d e o b t a i n e d in t h e p r e p a r a t i o n of t r i b r o m o s i l a n e .

6 9 4 P . W . S C H E N K

T h e j a c k e t of t h e c o l u m n i s m a i n t a i n e d at a t e m p e r a t u r e b a r e l y u n d e r t h e boiling point by m e a n s of a s t r e a m of a i r p r e h e a t e d in I . T h e f r a c t i o n s b o i l i n g at 64°C, 111.8°C and 153.4°C a r e c o l l e c t e d . They contain, r e s p e c t i v e l y , S i H²B r², S i H B r³ and S u 3 r⁴. A t o t a l of about 57 g. of t h e c r u d e i s o b t a i n e d f r o m 60 g. of B r . After f r a c t i o n a t i o n , about 6 0 - 7 0 % of t h e c r u d e i s r e c o v e r e d a s S i H B r³.

P u r i t y m a y b e d e t e r m i n e d c o n v e n i e n t l y by g a s o m e t r y , a c c o r d ­ ing to t h e e q u a t i o n : S i H B r³ + 5 NaOH = 3 N a B r + N a³S K )³ + 2 H²0 + H² o r by m e l t i n g - p o i n t and b o i l i n g - p o i n t d e t e r m i n a t i o n s .

PROPERTIES:

C o l o r l e s s , m o b i l e liquid. B.p. 111.8°C. m . p . - 7 3 . 5 ° C . I g n i t e s e a s i l y in a i r . Vap. p . (0°C), 8.8 m m . Log ρ = - 1 8 1 9 . 5 / Τ + 7.6079.

H y d r o l y z e d in cold w a t e r to H²S i²0³ and H B r .

REFERENCES:

A. B e s s o n a n d L . F o u r n i e r , C o m p t e s R e n d u s H e b d . S e a n c e s A c a d . S c i . , 1 5 1 , 1055 (1911); H. Buff and F . W o e h l e r , L i e b i g s Ann.

C h e m . 104, 99 (1857); C . C o m b e s , C o m p t e s R e n d u s H e b d . S e a n c e s A c a d . S c i . , 122, 531 (1896); B u l l . Soc. C h i m . F r a n c e (3), 7, 242 (1892); L. G a t t e r m a n , B e r . d t s c h . c h e m . G e s . 22, 193 (1889);

W . C. Schumb and R. C. Young, J . A m e r . C h e m . Soc. 52, 1464 (1930); W. C. Schumb in H. S. Booth, I n o r g . S y n t h e s i s , Vol. I (New Y o r k and London, 1939), p . 38; G. Schott and W. H e r r m a n n , Z.

a n o r g . a l l g . C h e m . 288, 4 (1956).

Dimethyldichlorosilan e (CH³)²SiCl²

2CH³C1 + Si = (CH³)²SiCl²

101.0 28.1 129.1

C o m m e r c i a l g r a d e p o w d e r e d Si (170 g.) i s r i n s e d w i t h h y d r o ­ c h l o r i c a c i d , p l a c e d in a P t d i s h and w a s h e d w i t h w a t e r . It i s t h e n h e a t e d w i t h h y d r o f l u o r i c a c i d a n d w i t h s u l f u r i c a c i d , a g a i n w e l l w a s h e d with w a t e r and t h o r o u g h l y d r i e d . It i s t h e n m i x e d with 30 g. of c o p p e r (I) c h l o r i d e . T h e r e s u l t i n g p o w d e r m a y b e u s e d a s i s . H o w e v e r , it i s p r e f e r a b l e to t a b l e t it b e f o r e p l a c i n g it in a 4 0 - c m . - l o n g and 3 - c m . - d i a m e t e r P y r e x t u b e . T h e t u b e i s w r a p p e d with an a s b e s t o s l a y e r , on t o p of w h i c h t h e r e i s a h e a t i n g c o i l . T h i s a s s e m b l y i s t h e n w r a p p e d w i t h a double l a y e r of a s b e s t o s t a p e . One end i s c l o s e d w i t h a t w o - h o l e r u b b e r s t o p p e r . A g l a s s t u b e i s i n s e r t e d t h r o u g h o n e h o l e of t h e s t o p p e r and c o n n e c t e d

1 2 . S I L I C O N AND G E R M A N I U M 695 t h r o u g h a b u b b l e r filled w i t h c o n c e n t r a t e d H^sS 0⁴ t o a c y l i n d e r of m e t h y l c h l o r i d e . T h e o t h e r h o l e of t h e s t o p p e r i s fitted with a t h e r m o m e t e r . T h e o t h e r e n d of t h e t u b e i s d r a w n out a n d s l i g h t l y b e n t d o w n w a r d . It i s a t t a c h e d t o a L i e b i g c o n d e n s e r p r o v i d e d with a r e c e i v e r . A t r a p , m a i n t a i n e d a t -80°C, i s a t t a c h e d t o t h e r e - c e i v e r ; a n d , finally, a C a C l³ t u b e i s c o n n e c t e d t o t h e o u t l e t s i d e of t h e t r a p . T h e e n t i r e a p p a r a t u s i s c a r e f u l l y d r i e d p r i o r t o t h e r u n . T h e m e t h y l c h l o r i d e i s i n t r o d u c e d t h r o u g h t h e b u b b l e r a t a r a t e of 2 b u b b l e s / s e c o n d ; t h e t e m p e r a t u r e of t h e t u b e i s c o n t r o l l e d at 300°C. C o n d e n s a t e i m m e d i a t e l y a p p e a r s in t h e r e c e i v e r . T h e r u n i s t e r m i n a t e d a f t e r about 70 h o u r s ; t h e c o n d e n s a t e s in t h e t r a p and in t h e r e c e i v e r a r e c o m b i n e d a n d t h e m i x t u r e i s f r a c t i o n a l l y d i s t i l l e d , u s i n g a good c o l u m n . T h e d e s i r e d p r o d u c t c o m e s o v e r a t 70°C. About 30 t o 40% of t h e c r u d e i s ( C H³)²S i C l³; about t h e s a m e a m o u n t of C H³S i C l³ ( b . p . 65.7°C) i s a l s o o b t a i n e d .

Analysis:

F o r t h e d e t e r m i n a t i o n of Si in l o w - b o i l i n g o r g a n i c c o m p o u n d s , s e e B. Smith, A c t a C h e m . S c a n d . , 1 1 , 579 (1957).

PROPERTIES:

C o l o r l e s s l i q u i d . M . p . - 7 6 . 1 ° C , b . p . 70°C; d (25°C) 1.06.

H y d r o l y z e s in m o i s t a i r o r w a t e r with t h e f o r m a t i o n of p o l y m e r i c m e t h y l s i l o x a n e s .

REFERENCES:

F . K r a u s a n d A. v o n G r o s s e , Die C h e m i e d e r m e t a l l o r g a n i s c h e n V e r b i n d u n g e n ( B e r l i n , 1937); E . G. Rochow, J . A m e r . C h e m . S o c . 67, 963 (1945); M. J . H u n t e r , J . F . Hyde, E . L . W a r r i c k a n d H. J . F l e t c h e r , J . A m e r . C h e m . Soc. 6 8 , 667 (1946); W. P a t n o d e a n d D. F . W i l c o c k , J . A m e r . C h e m . S o c . 6 8 , 358 (1946); W . P a t n o d e , U . S . P a t e n t 2,380,997; E . G. Rochow in L. F . A u d r i e t h , I n o r g . S y n t h e s e s , Vol. Ill (New Y o r k - T o r o n t o - L o n d o n , 1950), p . 5 6 .

Chlorosiloxane s

Si⁴0⁴Cl⁸, Si^uOn—iCl²n + 2

P o w d e r e d s i l i c o n i s h e a t e d in a 2 : 1 m i x t u r e (by v o l u m e ) of C l² and Qg, u s i n g t h e s a m e a p p a r a t u s a s f o r t h e p r e p a r a t i o n of S i C l⁴. Both t h e g a s e s a n d t h e a p p a r a t u s m u s t b e c a r e f u l l y d r i e d . T h e 0² i s i n t r o d u c e d f i r s t , a n d t h e n t h e C l² flow i s s t a r t e d . It i s b e s t n o t to r e l y upon t h e o b s e r v a t i o n of g a s b u b b l e s in t h e w a s h b o t t l e s f o r