High temperature porosity in tungsten

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forty y e a r s it has been known that p o r o s i t y o c c u r s in doped t u n g s t e n f i l a m e n t s exposed to high t e m p e r a t u r e s . 1 T h e o r i g i n of this p o r o s i t y w a s obs c u r e . While the p o r e s w e r e an a p p a r e n t h a z a r d to long f i l a m e n t life, t h e i r technological i m p o r t a n c e was not a p p r e c i a t e d s i n c e the f a i l u r e of o r d i n a r y i n c a n d e s cent l a m p s was a s c r i b e d to e v a p o r a t i o n . 2 R e c e n t l y , it has b e c o m e c l e a r that s e v e r a l kinds of p o r e s a r e p r e s ent in t u n g s t e n . The p r e s e n c e of the p o r e s can be e i t h e r s a l u t a r y o r d e s t r u c t i v e . F o r c o n v e n i e n c e in this r e p o r t , p o r e s g r e a t e r than about 2 p m in d i a m e t e r a r e t e r m e d " m i c r o s c o p i c p o r e s " . T h e i r d i a m e t e r can be c o m f o r t a b l y d e t e r m i n e d by light m i c r o s c o p y . S m a l l e r p o r e s which r e q u i r e ~ s c a n n i n g e l e c t r o n m i c r o s c o p i c o r t r a n s m i s s i o n electron microscopic methods for detailed observation are termed "submicroscopic p o r e s " . V e r y s m a l l s u b m i c r o s c o p i c p o r e s in l i n e a r a r r a y s play a d e c i s i v e r o l e in inhibiting r e c r y s t a l l i z a tion s-7 a n d t h e r e b y p e r m i t the l a t e r d e v e l o p m e n t of the i n t e r l o c k i n g g r a i n s t r u c t u r e n e c e s s a r y for s a g - r e s i s t ant f i l a m e n t s . On the o t h e r hand, at t e m p e r a t u r e s of o p e r a t i o n of c u r r e n t t u n g s t e n - h a l o g e n l a m p s (2900~ and above) l a r g e m i c r o s c o p i c p o r e s develop which can grow to s i z e s a p p r o a c h i n g that of the f i l a m e n t d i a m e t e r . s'9 The r e s u l t a n t local d i m i n u t i o n in c r o s s - s e c t i o n of the f i l a m e n t is a p o s s i b l e m e c h a n i s m f o r f i l a m e n t f a i l u r e which r e s t r i c t s f i l a m e n t o p e r a t i o n at u l t r a h i g h t e m p e r a t u r e s w h e r e g r e a t e r lighting e f f i c i e n c i e s a r e p o s s i b l e . T h i s p a p e r a t t e m p t s to c l a r i f y the o r i g i n s and d e v e l o p m e n t of the m i c r o s c o p i c - s i z e d p o r e s in doped t u n g s t e n . PROCEDURE The e x p e r i m e n t s w e r e conducted on a v a r i e t y of 0.025-in. diam t u n g s t e n w i r e s obtained f r o m G T E Sylvania, Towanda, P a . i n c l u d i n g a l u m i n a - s i l i c a doped w i r e (AW), s o m e s p e c i a l l y processed w i r e i n t e n d e d to y i e l d low nickel and i r o n content (H and J), a 3 pct Re a l l o y (RW) and undoped w i r e (WT). A d d i t i o n a l l y , highp u r i t y doped t u n g s t e n was p r o c e s s e d by a s p e c i a l fabr i c a t i o n s~hedule which led to an e x t r e m e l y c o a r s e J. BRETT is Head of Metal Physics Group, General Telephone and Electr

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