SOVIET PHYSICS J O U R N A L
COLORING
AND
BREAKDOWN
35
OF
BARIUM
TITANATE*
No Io S h e f e r and E . V. B u r s i a n I z v e s t i y a VUZ. F i z i k a , Vol. 10, No. 8, ppo 60--64, 1967 Spontaneous increase in current in strong fields is found to be related to motion of color centers. Electron injection increases the number of f centers, which reduces the electrical strength. Some t i t a n i u m c o m p o u n d s at high t e m p e r a t u r e s show a c o m p a r a t i v e l y slow i n c r e a s e in e l e c t r i c a l c o n d u c t i v i t y in s t r o n g f i e l d s , w h i c h i s not due to h e a t i n g by the c u r r e n t , a s for p o l y e r y s t a l l i n e b a r i u m t i t a n a t e [1]; the e f f e c t h a s a l s o b e e n o b s e r v e d [ 2 - 1 0 ] for a l k a l i h a l i d e s and for o t h e r t i t a n i f e r o u s c e r a m i c s . T h i s c u r r e n t r i s e i s due to a g i n g and e n d s in breakdown~ A c o l o r i s p r o d u c e d in t r a n s p a r e n t b a r i u m t i t a n a t e s i n g l e c r y s t a l s u n d e r t h e s e c o n d i t i o n s [11]; ( c e n t e r s p r o p a g a t e f r o m the c a t h o d e , and v ones f r o m the anode. It i s u s u a l l y d i f f i c u l t to e x a m i n e b r e a k d o w n and a g i n g b e c a u s e b r e a k d o w n i s v e r y f a s t and aging i s e x t r e m e l y slow; but t h e l o s s in e l e c t r i c a l s t r e n g t h c a n be f o l l o w e d at e l e v a t e d t e m p e r a t u r e s [1, 3, 12], it b e i n g a s s u m e d t h a t h e a t i n g a l t e r s o n l y the r a t e s of t h e p r o cesses.
T h i s a p p r o a c h h a s b e e n e x a m i n e d f o r b a r i u m tit~-. n a t e c e r a m i c [22], and i t w a s found t h a t the c o n d u c t i v i t y c h a n g e i s due to p r o p a g a t i o n of c o l o r e d r e g i o n s f r o m the c a t h o d e and a n o d e . No c o n d u c t i v i t y c h a n g e o r c o l o r i n g w a s o b s e r v e d f o r s i n g l e c r y s t a l s , but o n l y a few e x p e r i m e n t s w e r e done, with o n l y one t y p e of c r y s t a l (made f r o m K F s o l u t i o n by R e m e i k a ' s method)~ I,rnA
50
0
~Q
0
b
5
6
g
~ rain
Fig. I~ Current as a function of time for 2 • 2 • 2 mm 3 BaTiO 3 crystals at 280 ~ C: a) cathode point, anode plane, U = 1800 V; b) planar electrodes of silver paste, 5 kV/cm.
[
train
5
It is stated [6] that the electrical conductivity is unstable in the single crystals. We have examined the time dependence of the elec trical conductivity for barium titanate single crystals and have correlated the effects with motion of the color center s ~ l,mA 80
f
/ %/
60
R e a s o n s f o r l o s s of s t r e n g t h b e f o r e b r e a k d o w n m a y be c o l o r - c e n t e r f o r m a t i o n and c o m p l i c a t e d p r o c e s s e s a s s o c i a t e d w i t h the m i g r a t i o n of t h e s e c e n t e r s . It t h u s a p p e a r s r e a s o n a b l e to c o r r e l a t e c o l o r i n g with c o n d u c t i v i t y i n c r e a s e . The r e s u l t s of t h i s a p p r o a c h a r e of c o n s i d e r a b l e i n t e r e s t , in t h a t t h e d a t a on t h e b r e a k down m e c h a n i s m s of t i t a n i f e r o u s c e r a m i c s a r e often c o n f l i c t i n g , a s for p o l y c r y s t a l l i n e b a r i u m t i t a n a t e , w h i c h h a s b e e n u n d e r d i s c u s s i o n for o v e r a d e c a d e [1, 3, 1 3 - 2 0 ] , w h i l e the b r e a k d o w n of s i n g l e c r y s t a l s h a s not b e e n e x a m i n e d , e x c e p t t h a t i t i s known [21] that the breakdown voltage has a negative temperature coefficient.
0
[
2
J
~ rain
Fig. 3. Current as a function of time for 2 • 2 x 2 mm 3 BaTiO 3 crystals at 1400 V and various temperatures: i) +260 ~ C, 2) 280 ~ C, 3)+320 ~ C, 4) +360 ~ C. Cathode point, anode plane. RESULTS
*Read at the All-Union Conference on Recent Insulator and Semiconductor Techniques, Leningrad~
2
Fig. 2. Current as a function of time for 2 x 2 x 2 mm 3 BaTiO 3 crystals at 300 ~ C and voltages (V) of: i) 600, 2) i000, 3) 1400, 4) 1800. Cathode point, anode plane.
I, mA ~ - - . f
y!
AND
DISCUSSION
The crystals were grown by the Blattner-Mattiaa method from BaCI 2 and were used near the breakdown
36
IZVESTIYA VUZ. FIZIKA
field; Fig. 1 shows an example. The electrode shape had no marked effect on the general form of the current curves, as there was always a r i s e (1) (often identifiable with aging), then a phase of regeneration (2), and finally predominant aging (3)~ The use of the t e r m s regeneration and aging in these c a s e s is confusing, because the p r o c e s s e s are not actually aging, although they are related to the latter. I, m A
tit
IMO MC MO
,/iM y<",., Z'
J I,
4M
20E
Z
9
~
/0 / f t m m
Fig~ 4. 1) Rise in c u r r e n t at 200 ~ C, 2) effect of field r e v e r s a l , 3) effect of shortt e r m removal of field for: a) barium titanate c e r a m i c , 8 k V / c m , b) single crystal, 6 kV/cm~ Planar s i l v e r paste electrodes. The electrical m e a s u r e m e n t s were accompanied by color cinematography to r e c o r d the patterns o f f and v c e n t e r s . Rise (1) is due to entry of f centers from the cathode, while fall (2) is due to entry of v centers from the anode, and the r i s e (3) is due to further injection o f f c e n t e r s . The total c u r r e n t given by a point cathode is less, but the p r o c e s s e s associated with the f centers are much m o r e pronounced than those for planar electrodes, while the rate of entry of v centers is almost unaffected. Electrons may enter the conduction band of the c r y s t a l f r o m that of the metal by tunneling through the forbidden band of the c r y s t a l . Several factors facilitate this in a real crystal: electrons may enter via defect levels in the forbidden band, Tamm surface levels may distort the band structure at the interface, and the local fields (due to inhomogeneity) may greatly exceed the mean field. This is why the tunnel mechanism is not ruled out [23, 25] regarding onset of breakdown in semiconductors and insulators with relatively narrow forbidden bands, as for barium titanate. Stages (2) and (3) are accelerated by r a i s i n g T and E, no m a t t e r what the electrode shape (Figs. 2 and 3); the f and v c e n t e r s are also observed to move f a s t e r . The apparent absence of a time dependence for single c r y s t a l s [22] led to the view that grain boundaries play a p a r t i c u l a r part in the r i s e in conductivity. Our r e s u l t s would show that this is not so. Entry o f f c e n t e r s c a u s e s the r i s e in parts (1) and (3) (Fig. 1), which lead on to breakdown. The nature of the p r o c e s s e s causing loss of electrical strength is used in defining the type of breakdown (thermal or electrical), not the consequences [24, 25].
The conductivity r i s e preceding breakdown is not a consequence of heating by the current either for cer a m i c s or single c r y s t a l s ; Fig. 4 shows a r i s e in section 1, which is halted by polarity r e v e r s a l in part 2, where the current falls to nearly the initial value. The rise starts again, as in 3, if the voltage is switched off for a few seconds and then switched on again. The c u r r e n t increment is evidently determined by the field direction, not by the current, i . e . , by the amount of heat released. Calculations from the data of [11] show that the c u r rent increment due to the f centers is far in excess of that due to the temperature r i s e until t h e / - c o l o r a t i o n is complete. Of course, this does not rule out the p r o duction of a considerable amount of heat in the last stage of breakdown, which leads on to thermal or m e chanical damage to the specimen. Other evidence that the loss of electrical strength is not of thermal origin comes from preliminary m e a s u r e ments of the potential distribution in these single c r y s tals. The potential difference a c r o s s the cathode half of the specimen steadily falls as the f centers enter, and the increased electrical conductivity in the f - c o l ored region p e r s i s t s for a long time in the absence of an electric field. The r i s e in conductivity is therefore not due to t e m p e r a t u r e r i s e but to i n c r e a s e in conductivity in the f region near the cathode. CONC LUSIONS 1. Barium titanate single c r y s t a l s show a complicated conductivity variation, which is due to competing p r o c e s s e s of f and v coloring. It is i n c o r r e c t to conclude [22] that grain boundaries are responsible for the spontaneous r i s e in conductivity in a c e r a m i c . 2. The f - c e n t e r coloring is responsible for the loss of electrical strength in barium titanate in this t e m p e r ature range (20-360 ~ C). REFERENCES 1. N. I. Shefer, Dissertation, Herzen Pedagogic Institute, Leningrad, 1954. 2. No I. Shefer, Uch. Zap. LGPI im. Gertsena, 148, 67, 1958. 3. N. I. Shefer, Summaries of P a p e r s at the Third All-Russian Conference on the P h y s i c s of Insulators and Semiconductors [in Russian], Krasnodar, 1963. 4. M. S. Kosman and V. F. Pisarenko, DAN SSSR, 115, 693, 1957; DAN SSSR, 115, 898, 1957. 5. V. Ya. Kunin and A. N. Tsikin, FTT, 2, 2358, 1960; FTT, 3, 214, 1961; FTT, 5, 2771, 1963. 6. A . Branwood and H. Tredgold, Proco P h y s . , Soe., 76, 93, 1960. 7. Ya. N. P e r s h i t s , FTT, 5, 1348, 1963. 8. F. Cardon, P h y s . St. Sol., 3, 1415, 1963. 9. M. G. Harwood, Brit. J . Applo P h y s . , 16, 1493, 1965. 10. J. A. Raalte, J . Appl~ P h y s . , 36, 3365, 1965o 11. M. S. Kosman and E. V. Bursian, DAN SSSR, 115, 483, 1957; Izv. AN SSSR, s e t . f i z . , 22, 1459, 1958.
SOVIET
PHYSICS
JOURNAL
12. I. E. Balygin and K. S. Porovskii, ZhTF, 26, 1714, 1956. 13. B. M. Vul, I. M. GolTdman, and R. L. Razbash, ZhETF, 20, 465, 1950o 14. V~ Io Sarafanov, ZhTF, 27, 590, 1954. 15. E. A. Konorova, V. Vo Krasnopevtsev, and G. I. Skanavi, Izv. AN SSSR, s e r . f i z . , 22, 408, 1958. 16. P. Fang and S. Brower, P h y s . R e v . , 113, 456, 1959 o 17. S. V. Bogdanov, FTT, 4, 2179, 1962. 18. I. Ueda, M. Takiucbi, and S. I k e g a m i , J . Phys. Soc. J a p . , 17, 1679, 1962. 19. P. Fang, J o P h y s . Soe. J a p . , 18, 1698, 1963. 20. I~ Ueda, M. Takiuehi, and S. Ikegami, J. Phys. Soc~ Japo, 19, 1267, 1964o
37
21. A. Branwood, I. Hurd, and A. Tredgold, Brit. J . Appl. P h y s . , 13, 528, 1962. 22. K. Lehovec and G. Shirn, J . Appl. P h y s . , 33, 2036, 1962. 23. A. A. Vorob~ev and V. Go VorobVev, Electrical Breakdown and Failure of Solid Insulators [in Russian], Vysshaya shkola, Moscow, 1966. 24. W. Franz, Insulator Breakdown [Russian t r a n s lation], IL, Moscow, 1961. 25. G. I. Skanavi, P h y s i c s of Insulators in Strong Fields [in Russian], Moscow, 1958.
22 October 1966 Revised 3 April 1967
Orenburg Pedagogic Institute Herzen Leningrad Pedagogic Institute