The absence of an allotropie transformation
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PROCEDURE
AND
with the p r y o m e t e r . Once the t u b e s w e r e c a l i b r a t e d ( t e m p e r a t u r e v s f i l a m e n t c u r r e n t c u r v e s o b t a i n e d ) , high v o l t a g e s w e r e a p p l i e d in o r d e r to o b s e r v e the e m i s s i o n p a t t e r n s . A s e x p e c t e d , p a t t e r n s c o r r e s p o n d i n g to b c c s t r u c t u r e w e r e o b t a i n e d . T h e f i r s t two f i g u r e s show the v a n a d i u m f i e l d - e m i s s i o n p a t t e r n a s the s u r f a c e i s c l e a n e d by h e a t i n g f r o m r o o m t e m p e r a t u r e , F i g . 1, to about 1250 K, F i g . 2. T h r e e of the four t u b e s e v a c u a t e d d u r i n g the f i r s t v a c u u m " r u n " showed an a l l o t r o p i c t r a n s f o r m a t i o n on e a r l y a t t e m p t s to o p e r a t e them at a t e m p e r a t u r e of about 1820 K which is a p p r o x i m a t e l y that r e p o r t e d in Ref. 1. U n f o r t u n a t e l y , e v e r y a t t e m p t to d e t e r m i n e v i s u a l l y o r to p h o t o g r a p h the high t e m p e r a t u r e s t r u c t u r e was u n s u c c e s s f u l b e c a u s e e a c h t i m e the p h a s e b o u n d a r y was s e e n to m o v e a c r o s s the p a t t e r n , an a r c d e s t r o y e d the n e e d l e b e f o r e t h e r e w a s t i m e to p h o t o g r a p h the p a t t e r n (which t a k e s about 30 s) o r to identify the h i g h temperature crystal structure visually.
RESULTS
Field-emitter needles were fabricated from 99.5 pct p u r e v a n a d i u m w i r e and p l a c e d i n s i d e c o n v e n t i o n a l e v a c u a t e d f i e l d - e l e c t r o n e m i s s i o n t u b e s .8 The e m i t t e r t e m p e r a t u r e a s a function of e m i t t e r - f i l a m e n t c u r r e n t was obtained by m e a n s of an o p t i c a l p y r o m e t e r b e f o r e an a t t e m p t was m a d e to o b s e r v e f i e l d - e m i s s i o n p a t t e r n s . T h i s p r o c e d u r e e n a b l e s an o p e r a t o r to r e a d f i l a m e n t c u r r e n t s , which can be done quickly f r o m a m e t e r , r a t h e r than a t t e m p t i n g to d e t e r m i n e e a c h t e m p e r a t u r e GEORGE BARNES is Professor, Department of Physics, University of Nevada, Reno, Nev. 89507. Manuscript submitted December 29, 1971. METALLURGICAL TRANSACTIONS
Fig. 1 - - F i e l d - e l e c t r o n e m i s s i o n pattern f r o m a vanadium 0aec) e m i t t e r just above room t e m p e r a t u r e . VOLUME 4, FEBRUARY 1973-549
Fig. 2 - - E m i s s i o n p a t t e r n of the e m i t t e r of Fig. 1 when the t e m p e r a t u r e is r a i s e d to a b o u t 1250 K.
Fig. 3 - - E m i s s i o n p a t t e r n of an e m i t t e r m a d e of z o n e - r e f i n e d v a n a d i u m at 1325 K. The p a t t e r n c o r r e s p o n d s to a bcc s t r u c t u r e , but with a d i f f e r e n t o r i e n t a t i o n f r o m that in F i g s . 1 and 2. 5 5 0 - V O L U M E 4 , FEBRUARY 1973
Fig. 4 - - E m i s s i o n p a t t e r n of the s a m e e m
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