The absence of an allotropie transformation
3
The Absence of an AIIotropic Transformation in Pure Vanadium at High Temperatures GEORGE BARNES Field-electron emission techniques show that an allotropic transformation does not occur in vanadium at high temperatures if the samples are of high purity. Such purity can be ob- tained by heating for a sufficiently long time in a high vacuum or by zone refining. THE literature indicates that there have been differ- ences in opinion as to whether an allotropic form of vanadium existed at high temperatures,l-4 although a low temperature transition at about 230 K from bcc to tetragonal structure has been discovered. 5 This article confirms the results of the more recent studies on va- nadium at high temperatures which show that no high temperature allotrope exists in vanadium of high purity. The field-electron microscope 6 is an extremely use- ful tool in the study of the crystalline structure of metals because it presents the observer with a visual image of an electron-emission pattern from the single- crystal tip of a needle of the material in question. Cor- relations between the light and dark areas of the pattern and the crystal directions within the single crystal can be made.7'a To an experienced operator, the general appearance of the pattern can furnish information as to whether or not there is much impurity on the surface. In general, if the pattern changes smoothly and gradually from the dark areas (holes) to the bright areas, the surface is relatively free of impurities, but if the intensity of the light from the pattern changes sharply or if light areas are set off by sharp, dark borders, as in Fig. 1, one can be sure that there are surface, if not volume, im- purities. Pankow 9 has reported the effects of some sur- face impurities on the field-emission patterns of vanadium. The field-electron and field-ion emission microscope not only enable the observer to identify the crystal structure of the emitter material, but they permit him to see visually, by means of the field emission pattern, any changes in the crystal structure while they are taking place.X~ If a sufficiently fine control of emitter temperature is possible, the transition from one clio- tropic phase to another can be observed in the form of a sharp line or phase boundary moving across the pat- tern as reported in Ref. 11. with the pryometer. Once the tubes were calibrated (temperature vs filament current curves obtained), high voltages were applied in order to observe the emission patterns. As expected, patterns corresponding to bcc structure were obtained. The first two figures show the vanadium field-emission pattern as the surface is cleaned by heating from room temperature, Fig. 1, to about 1250 K, Fig. 2. Three of the four tubes evacuated during the first vacuum "run" showed an allotropic transformation on early attempts to operate them at a temperature of about 1820 K which is approximately that reported in Ref. 1. Unfortunately, every attempt to determine visu- ally or to photograph the high temperature structure was unsuccessful because each time the phase boundary was seen to move across the pattern, an arc destroyed the needle before there was time to photograph the pat- tern (which takes about 30 s) or to identify the high- temperature crystal structure visually. EXPERIMENTAL PROCEDURE AND RESULTS Field-emitter needles were fabricated from 99.5 pct pure vanadium wire and placed inside conventional evacuated field-electron emission tubes .8 The emitter temperature as a function of emitter-filament current was obtained by means of an optical pyrometer before an attempt was made to observe field-emission pat- terns. This procedure enables an operator to read fila- ment currents, which can be done quickly from a meter, rather than attempting to determine each temperature GEORGE BARNES is Professor, Department of Physics, University of Nevada, Reno, Nev. 89507. Manuscript submitted December 29, 1971. Fig. 1--Field-electron emission pattern from a vanadium 0aec) emitter just above room temperature. METALLURGICAL TRANSACTIONS VOLUME 4, FEBRUARY 1973-549
-
Upload
george-barnes -
Category
Documents
-
view
218 -
download
0
Transcript of The absence of an allotropie transformation
The Absence of an AIIotropic Transformation
in Pure Vanadium at High Temperatures GEORGE BARNES
F i e l d - e l e c t r o n e m i s s i o n techniques show that 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 does not occur in vanad ium at high t e m p e r a t u r e s if the s a m p l e s a r e of high pu r i t y . Such pur i ty can be ob- ta ined by hea t ing for a su f f i c ien t ly long t ime in a high vacuum o r by zone re f in ing .
T H E l i t e r a t u r e i nd i ca t e s that t h e r e have been d i f f e r - ences in opinion as to whe ther an a l l o t r o p i c fo rm of vanad ium ex i s t ed at high t e m p e r a t u r e s , l - 4 al though a low t e m p e r a t u r e t r a n s i t i o n at about 230 K f rom bcc to t e t r a g o n a l s t r u c t u r e has been d i s c o v e r e d . 5 This a r t i c l e c o n f i r m s the r e s u l t s of the m o r e r e c e n t s tud ies on v a - nadium at high t e m p e r a t u r e s which show that no high t e m p e r a t u r e a l l o t r o p e e x i s t s in vanad ium of high pur i ty .
The f i e l d - e l e c t r o n m i c r o s c o p e 6 i s an e x t r e m e l y u s e - ful tool in the s tudy of the c r y s t a l l i n e s t r u c t u r e of m e t a l s b e c a u s e i t p r e s e n t s the o b s e r v e r with a v i s u a l i m a g e of an e l e c t r o n - e m i s s i o n p a t t e r n f rom the s i ng l e - c r y s t a l t ip of a needle of the m a t e r i a l in ques t ion . C o r - r e l a t i o n s be tween the l ight and d a r k a r e a s of the p a t t e r n and the c r y s t a l d i r e c t i o n s within the s ingle c r y s t a l can be made.7 'a
To an e x p e r i e n c e d o p e r a t o r , the g e n e r a l a p p e a r a n c e of the p a t t e r n can fu rn i sh i n fo rma t ion as to whe the r o r not t h e r e i s much i m p u r i t y on the s u r f a c e . In g e n e r a l , if the p a t t e r n changes smooth ly and g r a d u a l l y f rom the d a r k a r e a s (holes) to the b r igh t a r e a s , the su r f ace is r e l a t i v e l y f r ee of i m p u r i t i e s , but if the in tens i ty of the l ight f rom the p a t t e r n changes s h a r p l y o r if l ight a r e a s a r e s e t off by s h a r p , d a r k b o r d e r s , as in F ig . 1, one can be s u r e that t he re a r e s u r f a c e , if not vo lume, i m - p u r i t i e s . P a n k o w 9 has r e p o r t e d the e f fec t s of some s u r - face i m p u r i t i e s on the f i e l d - e m i s s i o n p a t t e r n s of vanad ium.
The f i e l d - e l e c t r o n and f i e l d - i o n e m i s s i o n m i c r o s c o p e not only enable the o b s e r v e r to ident i fy the c r y s t a l s t r u c t u r e of the e m i t t e r m a t e r i a l , but they p e r m i t him to s ee v i s u a l l y , by m e a n s of the f ie ld e m i s s i o n p a t t e r n , any changes in the c r y s t a l s t r u c t u r e while they a r e taking place.X~ If a suf f ic ien t ly fine con t ro l of e m i t t e r t e m p e r a t u r e i s p o s s i b l e , the t r a n s i t i o n f rom one c l i o - t r op i c phase to ano ther can be o b s e r v e d in the fo rm of a s h a r p l ine o r phase boundary moving a c r o s s the pa t - t e rn a s r e p o r t e d in Ref. 11.
with the p r y o m e t e r . Once the tubes we re c a l i b r a t e d ( t e m p e r a t u r e vs f i l amen t c u r r e n t c u r v e s obta ined) , high vo l t ages we re app l ied 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 . As expec ted , p a t t e r n s c o r r e s p o n d i n g to bcc s t r u c t u r e we re obta ined . The f i r s t two f igu re s show the vanad ium 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 leaned by hea t ing f rom r o o m t e m p e r a t u r e , F ig . 1, to about 1250 K, F ig . 2.
T h r e e of the four tubes evacua ted dur ing the f i r s t vacuum " 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. Unfor tuna te ly , e v e r y a t t emp t to d e t e r m i n e v i s u - a l ly o r to photograph 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 each t ime the phase boundary was seen to move 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 needle b e f o r e t h e r e was t ime to pho tograph the pa t - t e rn (which t akes about 30 s) o r to ident i fy the h igh- t e m p e r a t u r e c r y s t a l s t r u c t u r e v i sua l l y .
EXPERIMENTAL PROCEDURE AND RESULTS
Field-emitter needles were fabricated from 99.5 pct pure vanad ium wi r e and p laced ins ide convent iona l evacua ted f i e l d - e l e c t r o n e m i s s i o n tubes .8 The e m i t t e r t e m p e r a t u r e as a funct ion of e m i t t e r - f i l a m e n t c u r r e n t was obta ined by m e a n s of an op t i ca l p y r o m e t e r be fo re an a t t emp t was made to o b s e r v e f i e l d - e m i s s i o n pa t - t e r n s . This p r o c e d u r e enab le s an o p e r a t o r to r e a d f i l a - men t c u r r e n t s , which can be done quickly f rom a m e t e r , r a t h e r than a t t empt ing to d e t e r m i n e each 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. Fig. 1- -Fie ld-e lec t ron emiss ion pat tern f rom a vanadium 0aec) emi t t e r just above room temperature .
METALLURGICAL TRANSACTIONS VOLUME 4, FEBRUARY 1973-549
Fig. 2 - - E m i s s i o n pa t t e rn 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 about 1250 K.
Fig. 4 - - E m i s s i o n pa t t e rn of the s a m e e m i t t e r at 1630 K.
Fig. 3 - - E m i s s i o n pa t t e rn of an e m i t t e r made of z o n e - r e f i n e d va- nadium at 1325 K. The pa t t e rn 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 fe ren t o r i en ta t ion f r o m that in Figs . 1 and 2. Fig, 5 - -Emis s ion pa t t e rn of the s a m e e m i t t e r at 1900 K.
550-VOLUME4, FEBRUARY 1973 METALLURGICAL TRANSACTIONS
I t w a s s o o n d i s c o v e r e d t h a t v a n a d i u m n e e d l e s w h i c h w e r e d u l l e d by a n a r c c o u l d b e r e s h a r p e n e d w h i l e s t i l l i n t h e i r t u b e s b y h e a t i n g to a t e m p e r a t u r e o v e r 1800 K, and s e v e r a l v a n a d i u m e m i t t e r s w e r e s o t r e a t e d and r e - u s e d . H o w e v e r , a f t e r the e m i t t e r s h a d b e e n h e a t e d a f ew t i m e s a b o v e t h e t e m p e r a t u r e a t w h i c h t he t r a n s f o r - m a t i o n h a d o c c u r r e d , no f u r t h e r c h a n g e in t he c r y s t a l s t r u c t u r e cou ld be o b s e r v e d . O ne s u c h e m i t t e r w a s t a k e n to o v e r 1900 K ( j u s t b e l o w the m e l t i n g t e m p e r a - t u r e ) a n d a n o t h e r h e l d a t 1900 K f o r 7 r a i n w i t h o u t u n - d e r g o i n g a c h a n g e .
I t a p p e a r e d a s i f , w h a t e v e r i m p u r i t y h a d b e e n p r e s e n t a n d h a d c a u s e d a t r a n s i t i o n in t he v a n a d i u m b e f o r e h e a t - i n g , had b e e n r e m o v e d by e v a p o r a t i o n , l e a v i n g t he v a n a - d i u m in a s t a t e of h i g h e r p u r i t y . I t f u r t h e r a p p e a r e d t h a t e x t r e m e l y p u r e v a n a d i u m p r o b a b l y would not u n d e r g o a t r a n s i t i o n a t h i g h t e m p e r a t u r e s .
A s a m p l e of t he v a n a d i u m w i r e , p u r i f i e d i n a z o n e r e - f i n e r by m a k i n g s e v e r a l p a s s e s o v e r i t , d i d not show any t e n d e n c y to u n d e r g o a t r a n s f o r m a t i o n , e v e n t h o u g h two f i e l d - e m i t t e r n e e d l e s m a d e of i t w e r e h e a t e d to 1900 K a n d a n o t h e r to i t s m e l t i n g p o i n t . F i g . 3 s h o w s t he b c c p a t t e r n of a z o n e - r e f i n e d n e e d l e a t 1325 K, F i g . 4 , a t 1630 K, a n d F i g . 5 a t 1900 K. A s c a n b e s e e n , t he p a t - t e r n s h o w s no c h a n g e in t he c r y s t a l s t r u c t u r e of the v a n a d i u m in t h i s s e q u e n c e . I t s h o u l d b e n o t e d t h a t h e a t - i n g a n e m i t t e r c a u s e s i t to e m i t e l e c t r o n s by t h e r m i o n i c
e m i s s i o n a s w e l l a s by f i e l d e m i s s i o n . T h e a d d e d c u r - r e n t t e n d s to f i l l i n o r e v e n c o m p l e t e l y m a s k s o m e of t he d a r k h o l e s , b u t , a s c a n b e s e e n in F i g s . 3, 4, a n d 5, t h e u n d e r l y i n g s t r u c t u r e i s the s a m e . A t r o o m t e m p e r - a t u r e , t h i s e m i t t e r s t i l l h a d t h e s a m e c r y s t a l s t r u c t u r e .
CONC LUSION
T h e r e i s no a U o t r o p i c t r a n s f o r m a t i o n a t h i g h t e m p e r - a t u r e s in p u r e v a n a d i u m .
REFERENCES
1. A. U. Seybolt and H. T. Sumsion: AIME Trans., 1953, vol. 197, p. 292. 2. J. O. McCaldin andP. Dune: J. Metals, 1954, vol. 6, p. 619. 3. Handbook of the Rare Elements, M. A. Filyand and E. I. Semenova, eds.,
vol. II, p. 348, Boston Technical Publishers, Inc., Cambridge, Mass., 1970. 4. B. M. Vasyutinskiy, G. N. Kartmazov, Y. M. Smimov, and V. A. Finkel: Phys.
Metals Met., 1966, vol. 21, p. 134. 5. V. A. Finkel, V. I. Glamazda, and G. P. Kortun: Zhur. Eksper. i Teoret. Fiz.,
1969, vol. 57, p. 1065. Also Soy. Phys. JETP, 1970, vol. 30, p. 581. 6. E. W. Muller: Ergeb. Exakt. Naturwiss., 1953, vol. 27, p. 290. 7. M. H. Nichols: Phys. Rev., 1940, vol. 57, p. 297. 8. G. Barnes: Phys. Rev., 1955, vol. 97, p. 1579. 9. G. Pankow: Vierter Internationaler Kongress Fur Electronenmikroskipie,
Berlin, 10-17 Sept. 1958, Band 1 Physikalisch-Technischer Tell, Springer- Verlag, Berlin, 1960.
10. G. N. Bates and G. Barnes: Appl. Phys. Lett., 1967, vol. 11, p. 75. 11. G. Barnes: Proc. of the Colloque International sur L'etude des Transformations
Crlstallines, a Haute Temperature (au-dessus de 2000 K), to be published.
METALLURGICAL TRANSACTIONS VOLUME 4, FEBRUARY 1973-551,
