Life Prediction of Tungsten Filaments in Incandescent Lamps
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G. W. KING
T h e p h e n o m e n o n of g r a i n b o u n d a r y s l i d i n g with d i f f u s i o n a l a c c o m m o d a t i o n has b e e n ap p l i ed to the p r o b l e m of i n c a n d e s c e n t l a m p b u r n - o u t with good r e s u l t s . A c c o r d i n g l y , the r e s u l t s s u g g e s t that the e x t e n t to which the g r a i n b o u n d a r y shape d e v i a t e s f r o m a p l a n a r i n t e r f a c e p e r p e n d i c u l a r to the w i r e a x i s is the c r i t i c a l f a c t o r in d e t e r m i n i n g the life of the f i l a m e n t . In addition, a method f o r the o p t i m i z a t i o n of the c o i l g e o m e t r y , m i c r o s t r u c t u r e , and t e m p e r a t u r e f o r i n c r e a s i n g the f i l a m e n t life a r e s u g g e s t e d .
I. INTRODUCTION W UNGSTEN filaments when used in incandescent lamps experience high temperatures and relatively low stresses. The temperatures, on the homologous scale, are in the range 0.7 to 0.8 T m while the stresses vary from G/(108 to 104). The dominant mechanism of creep in this range of stress and temperature is diffusional creep. I We can expect, therefore, that the grain size will have a strong influence on creep resistance since the creep rate, in diffusional creep, varies as d -3 or d -2 depending on the relative importance of grain boundary and lattice self diffusion. 4"~ The importance of grain size has been recognized in the empirical development of the tungsten filament. As shown in Fig. I, the grain size and shape has been optimized to reduce diffusional creep to a minimum. The best grain structure is shown in Fig. l(c). The idea is to get the largest grain size possible while avoiding a bamboo type of grain boundary. The result is an e l o n g a t e d g r a i n shape with a l a r g e a s p e c t r a t i o and j a g g e d g r a i n e d g e s . T h i s g r a i n s t r u c t u r e is obt a i n e d by p r o p e r doping s and heat t r e a t m e n t of tungsten wire. Th e c r e e p and f a i l u r e of t u n g s t e n f i l a m e n t c o i l s with the m i c r o s t r u c t u r e shown in F i g . l(c) is the s u b j e c t of t h i s p a p e r . Since this m i c r o s t r u c t u r e is not the t y p i c a l e q u i a x e d p o l y c r y s t a l l i n e g r a i n s t r u c t u r e , the u s u a l e q u a t i o n s f o r d i f f u s i o n a l c r e e p 5,6 cannot be applied. We s h a l l d e v e l o p the e q u a t i o n s for the c r e e p of a w i r e with this s p e c i a l m i c r o s t r u c t u r e and apply it to the d e f o r m a t i o n of a c o i l c o n s t r u c t e d f r o m the w i r e and hung b e t w e e n two p o s t s as shown in F i g . 1. The i m p o r t a n t f e a t u r e s of the m i c r o s t r u c t u r e in F i g . l(c) a r e the e l o n g a t e d g r a i n s and the jagged g r a i n e d g e s . In fact the g r a i n s a r e v e r y long, e q u a l to s e v e r a l g r a i n d i a m , so that, on the a v e r a g e , t h e r e is l e s s than one g r a i n b o u n d a r y of the type shown i
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