I N T R O D U C T I O N
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IIT Kharagpur
2 CHAPTER - 1
INTRODUCTION
An e x c e p t i o n a l l y l a r g e i n c r e a s e i n e l e c t r i c a l r e s i s t i v i t y w ith in a small tem perature i n t e r v a l i s termed a s th e P o s i t i v e Tem
p e r a t u r e C o e f f i c i e n t o f R e s i s t i v i t y (PTCR) w h ile th e d e c r e a s e , a s N egative Temperature C o e f f i c i e n t o f R e s i s t i v i t y (NTCR). Both th e PTCR and NTCR e f f e c t s have been s u c c e s s f u l l y u t i l i z e d i n manufac
t u r i n g t h e r m i s t o r s which are " b a s ic a l ly t h e r m a lly s e n s i t i v e r e s i s t o r s ( i . e . a r e s i s t i v e d e v i c e p o s s e s s i n g a l a r g e tem p era tu re c o e f f i c i e n t o f r e s i s t a n c e o v e r an exten ded tem p era tu re r a n g e ). These d e v i c e s are made up o f se m ic o n d u c tin g m a t e r i a l s i n v a r i o u s shapes and s i z e s c o v e r in g a wide range o f te m p e ra tu re c o e f f i c i e n t from - 6 . 5 $ t o 70$ p e r °C. A com prehensive o u t l i n e on th e d i f f e r e n t t y p e s o f t h e r m i s t o r s h as been p r e s e n t e d i n a book by Macklen .-1
The NTC t h e r m i s t o r s g e n e r a l l y c o n s i s t o f a s i n t e r e d ceram ic body from a m ixture o f o x i d e s ch osen from t h e elem en ts Mn, Ni, Co, Cu and Fe. By changing th e c o m p o s i t i o n and geom etry, t h e i r room tem p era tu re r e s i s t a n c e can be v a r i e d from 1 t o 10 6 ohm. T h e ir tem p era tu re c o e f f i c i e n t l i e s between - 2 and - 6 . 5 $ p e r °C.
On th e b a s i s o f th e p r o p e r t i e s o f s e m ic o n d u c tin g m a t e r i
a l s used i n PTC t h e r m i s t o r s , th ey may be b r o a d l y d i v i d e d i n t o two c a t e g o r i e s . I n t h e f i r s t c a t e g o r y , a c o n v e n t i o n a l e le m e n ta l semi
c o n d u c t o r , u s u a l l y S i, i s u sed w ith e l e c t r o d e s and t e r m i n a l s o n th e o p p o s i t e f a c e s . PTC r e g i o n f o r b o th n - and p - t y p e s i l i c o n e x t e n d s o v e r a tem p era tu re range from s u b -z e r o t o 150°C o r above w ith a tem p era tu re c o e f f i c i e n t o f th e o r d e r o f 0 . 8 $ p e r °C a t room tem per
a t u r e . The second c a t e g o r y c o n t a i n s t h e s w it c h in g PTC t h e r m i s t o r ,
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IIT Kharagpur
R e s is ta n c e (o h m s )
-100 0 100 200 300
T e m p e r a t u r e (*C)
FIG. 1.1 R E S I S T A N C E - T E M P E R A T U R E C H A R A C T E R I S T I C S OF D I F F E R E N T T Y P E S OF T H E R M I S T O R M A T E R I A L S AND T H A T OF A P L A T I N U M R E S I S T A N C E
T H E R M O M E T E R ( r ef . 1)
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IIT Kharagpur
4
f o r example*doped se m ico n d u ctin g BaTiO^ which p o s s e s s e s a l a r g e PTCR anomaly ( ~ 7 0 ^ p e r °C) o v e r a r e s t r i c t e d tem p erature r e g i o n .
A com parison o f r e s i s t a n c e -t e m p e r a t u r e c h a r a c t e r i s t i c s between v a r i o u s t y p e s o f t h e r m i s t o r s and a p la tin u m r e s i s t a n c e thermometer, i s g iv e n i n P ig . 1 .1 . T h is shows th e tremendous fundamental a d van ta ges t h a t t h e r m i s t o r s p o s s e s s o v e r one o f t h e i r m a jo r r i v a l s , th e p la tin u m thermometer, as tem p era tu re s e n so r . The wide range o f a v a i l a b l e p h y s i c a l form s from su b -p in h ea d ed s iz e d
bead throu gh d i s c , rod and f l a t f i l m shapes to m etal sheathed p r o b e s and p l a s t i c e n c a p s u la te d a s s e m b lie s add v e r s a t i l i t y to th e advant
a g e s o f t h e t h e n n i s t o r s a s a tem p era tu re s e n s o r . O th e r m ajor a p p l i c a t i o n , b e s i d e s i t s u s e a s tem p erature s e n s o r , i s a s pow er s e n s o r s i n e l e c t r o n i c c i r c u i t s . The w o r ld -w id e p r o d u c t i o n o f such th e r m is t o r s , a t p r e s e n t , i s ~ 1 5 0 m i l l i o n p i e c e s p e r y e a r .
There a re two m a jo r c l a s s i f i c a t i o n schemes o f th e PTC
t h e r m i s t o r s d ep en d in g on w hether t h e d e v i c e i s u sed as a s e n s o r o r a s a h e a t - d i s s i p a t o r . The PTC jump, i . e . th e s w it c h in g , i s th e im p o rta n t d e v i c e p r o p e r t y f o r th e fo r m e r purpose* w h ile th e s t a t i c I - V c h a r a c t e r i s t i c s w ith i t s c u r r e n t l i m i t i n g p o t e n t i a l , f o r th e l a t e r . T a b le 1.1 shows a wide v a r i e t i e s o f a p p l i c a t i o n s o f th e t h e r m i s t o r s . T y p i c a l p r o p e r t i e s and c h a r a c t e r i s t i c s o f commerci
a l l y a v a i l a b l e BaTiO^ t h e r m i s t o r s a r e shown i n T a b le 1 .2 .
R e c e n t l y , t h e p o t e n t i a l o f PTC t h e r m i s t o r h a s tou ch ed a new h e i g h t i n e n g in e e r i n g a p p l i c a t i o n , as an e n ergy s a v e r '’ ( e l e c t r o c a l o r i c f l o w - m e t e r s ) . D o s t e r t ^ has r e p o r t e d sim p le and e a s i l y c a l i b r a t e d e v a l u a t i o n c i r c u i t f o r f l o w - m e t e r s based o n t h e e l e c t r o c a l o r i c p r i n c i p l e . T h is can le a d t o c o s t e f f e c t i v e flo w m e t e r s f o r
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IIT Kharagpur
T a b le 1.1 } A p p l i c a t i o n o f S w itch in g PTC T h e r m isto r s ( A f t e r Saburi and Wakino^)
Mode A p p l i c a t i o n
Thermal i n d i c a t o r , r e g u l a t o r ; tem p erature com pensating o f e l e c t r o n i c a p p a ra tu s; l e v e l m eter, flo w m e te r, thermometer, hu m id ity m eter; v o l t a g e s t a b i l i z e r ; o v e r h e a t p r o t e c t o r ; n o - c o n t a c t sw itc h .
Current l i m i t o r and s t a b i l i z e r ; v o l t a g e s t a b i l i z e r ; a u to m a tic volume c o n t r o l ; r e l a y p r o t e c t i o n ; t im e r ; therm al chamber.
Table 1 .2 i Im portant P r o p e r t i e s o f Commercial Barium T it a n a t e T h e r m is t o r s ( A f t e r A n d r i c h ' ^ )
S p e c i f i c w eight
S p e c i f i c r e s i s t i v i t y ( (*25° (P S p e c i f i c h ea t
Curie tem p era tu re, TQ S w it c h in g te m p e ra tu re , T
s D i s s i p a t i o n i n s t i l l a i r , D
(Samples w ith a volume o f a p p r o x im a te ly 0 . 5 cm^)
Emax /E min (nc load) / R*dn ( l o a d e d >
I n h e r e n t shunt c a p a c i t a n c e
Maximum tem p era tu re c o e f f i c i e n t Maximum momentary o v e r l o a d ( ^ 0 . 1 5 )
5. 6 gm/ cm-^
20-50 ohm-cm 0 . 1 2 c a l / g °C
1 20°C 110°C
1 0 - 1 5 mw/ °C
104
-105
1 02 - 105 Approx. 1 up A pprox. 60$ / °C Approx. 1 KW 1. Sensor
( S e n s in g element)
2. D i s s i p a t o r (Heat d i s s i p a t i n g elem ent)
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IIT Kharagpur
nO
v a r i o u s a p p l i c a t i o n s , such as th e measurement o f f u e l consum ption i n m otor v e h i c l e s o r o f the q u a n t i t i e s o f the h e a t consumed i n c e n t r a l h e a t in g systems s u p p lie d w ith h o t l i q u i d , f o r example, w ater.
The PTCR e f f e c t i s a d i r e c t con sequen ce o f t h e d is a p p e a r ance o f f e r r o e l e c t r i c p o l a r i s a t i o n n e a r th e C u rie tem p era tu re o f a se m ico n d u ctin g f e r r o e l e c t r i c m a t e r i a l . BaTiO^, one o f th e most
common f e r r o e l e c t r i c c e r a m ic s , p r o d u c e s t h e s t r o n g e s t PTCR e f f e c t around i t s f e r r o e l e c t r i c t r a n s i t i o n tem p era tu re ( * ^120° C ) . The
s e m i c o n d u c t i v i t y i n BaTiO^ i s n o r m a lly in t r o d u c e d by changing the v a l e n c y o f a p a r t o f th e T i - i o n s from 4+ to 3+. T h is v a l e n c y
change may be e f f e c t e d by two d i f f e r e n t mechanisms.
The f i r s t mechanism i n v o l v e s c r e a t i o n o f oxygen v a c a n c i e s i n t h e l a t t i c e by a n n e a lin g th e m a t e r i a l i n a r e d u c in g atmosphere a t e l e v a t e d te m p e r a tu r e s . To s a t i s f y th e e l e c t r i c a l n e u t r a l i t y c o n d i t i o n , two Ti^+ i o n s change t h e i r v a l e n c y from 4+ t o 3+ to compensate th e l o s s o f each 0 o i o n s as f o l l o w s :
Ba T i 4+0 3 - - f - 0 2 — ^ Ba T i > d . D
The second mechanism i n v o l v e s d o p in g o f e i t h e r t r i v a l e n t
7, 7, 7,
i o n e . g . La , Sb , Gd"^ and o t h e r r a r e - e a r t h i o n s at th e B a - s i t e o r p e n t a v a l e n t i o n s l i k e Nb^+ , Ta^+ e t c . a t th e T i - s i t e and th u s fo r m in g an r e t y p e s e m ic o n d u c tin g BaTiO^ w ith a lo w room tem per
a t u r e dc r e s i s t i v i t y o f t h e o r d e r o f 10 - 200 ohmr-cm.
Two t y p i c a l s u b s t i t u t i o n r e a c t i o n s , one o n B a - s i t e (La^*) and th e o t h e r on T i - s i t e (Ta^+ ) , can be i l l u s t r a t e d a s f o l l o w s :
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IIT Kharagpur
Ba2+Ti4+0 + ^ 3 * _ xBa2+ — > Ba?+ L a ^ T i J'Ti^* 0 , (1.2)
j 1"*X X X i-"*X j
Ba2 f T i4+0 3 + xTa5+ - xTj>+ — Ba2+Ta^+ T* ° 3 ( 1 * 3)
I n e i t h e r c a s e , th e s u b s t i t u t i o n r e s u l t s i n r e s t o r a t i o n o f e l e c t r i -
k+ 3+
c a l n e u t r a l i t y by t h e v a le n c y change o f Ti t o Ti and th u s i n -
jtj, ^i,
t r o d u c i n g h op p in g c o n d u c t io n between Ti and T i ^ . Normally, an optimum d o p in g c o n c e n t r a t i o n ( ~ 0 . 2 t o 0 . 4 atom <fi) c o r r e s p o n d s to th e lo w e s t r e s i s t i v i t y above which i t i n c r e a s e s .
A f t e r th e f i r s t r e p o r t o f PTCR e f f e c t i n se m ic o n d u c tin g
7 R
BaTiO^ by Haaijman e t a l . , Sauer and F la s c h e n have a l s o o b s e r v e d th e same e f f e c t i n La-doped BaTiO^ i n th e y e a r 1956. An e x t e n s i v e amount o f r e s e a r c h e f f o r t s has gone i n t o t h e u n d e r s t a n d in g o f th e mechanism and o t h e r a s p e c t s o f t h i s e f f e c t . A d e t a i l e d r e v ie w i s p r e s e n t e d i n th e n e x t c h a p te r . Eventhen, a number o f q u e s t i o n s
s t i l l remain unanswered and s e v e r a l c o n t r o v e r s i e s have been b rough t o u t i n r e c e n t y e a r s .
I n t h i s i n v e s t i g a t i o n , an a ttem p t h as been made to d i s t i n g u is h between th e e l e c t r i c a l c h a r a c t e r i s t i e s o f t h e g r a in b o u n d - a r i e s from t h a t o f th e g r a in s by c o m p le x -p la n e impedance a n a l y s i s , adopted f o r th e f i r s t tim e, t o study th e F T C -m a te r ia l s. The
t e c h n iq u e h as been e x p l o i t e d t o stud y th e e f f e c t s o f v a r i o u s p r o c e s s in g p a ra m e te rs l i k e s i n t e r i n g te m p e ra tu re and t im e , c o o l i n g r a t e and c o m p o s i t i o n . The same h as a l s o been s u c c e s s f u l l y u sed to stud y th e e l e c t r o d e e f f e c t and th e e x t e n t o f s u r f a c e r e o x i d a t i o n . C-V c h a r a c t e r i s t i c s o f th e specim ens have been s t u d ie d and e x p la in e d on th e b a s i s o f t h e p h y s i c s o f MIS j u n c t i o n . The e l e c t r i c a l measure
ments have been c o r r o b o r a t e d w ith a d e t a i l e d m i c r o s t x u c t u r a l
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IIT Kharagpur
s t u d i e s supported by e l e c t r o n - p r o b e m icro a n a l y s i s . The i n v e s t ! g a t i o n has l e d to t h e s u c c e s s f u l in d ig e n o u s developm ent o f PTC t h e r m i s t o r s .
