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SYNTHESIS AND PROPERTIES OF CONDUCTING MATERIALS IN THE TiO2 - SnO2 SYSTEM
M. Hennaut, P. Duvigneaud, E. Plumat
To cite this version:
M. Hennaut, P. Duvigneaud, E. Plumat. SYNTHESIS AND PROPERTIES OF CONDUCTING
MATERIALS IN THE TiO2 - SnO2 SYSTEM. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-
13-C1-17. �10.1051/jphyscol:1986102�. �jpa-00225486�
SYNTHESIS AND PROPERTIES OF CONDUCTING MATERIALS IN THE TiOz - SnO,
SYSTEM
M.F. HENNAUT, P . H . DUVIGNEAUD and E . PLUMAT
U n i v e r s i t e de B r u x e l l e s , 5 0 , Avenue F . Roosevelt, B - 1 0 5 0 Bruxell e s , Belgium
Resume.- Oes poudres submicroniques de T i 0 2 o n t 6 t 6 s y n t h e t i s 6 e s p a r h y d r o l y - iF%Griag@e de 1 ' i s o ~ r o p o x y d e de t i t a n e . L ' i n f l u e n c e de l a c o n c e n t r a t i o n des r e a c t i f s a StP @ t u d i @ e . L ' a d d i t i o n de Niobium c o n d u i t 5 des p a r t i c u l e s sphe- r i q u e s . D ' a u t r e p a r t des oxydes m i x t e s Ti02-Sn02 o n t e t e i l a b o r @ s p a r l ' h y - d r o l y s e d ' u n e s o l u t i o n de c h l o r u r e s d ' 6 t a i n e t d ' a n t i m o i n e c o n t e n a n t une sus- c e n s i o n de T i 0 2 dope p a r Nb. Les s o l u t i o n s s o l i d e s Sn02-Ti02 se f o r m e n t a i s e - ment dPs llOO°C rnais l ' i n c o r p o r a t i o n des dopants s p e c i f i q u e s dans ces s o l u - t i o n s e s t l i m i t 6 e . La c o n d u c t i b i l i t e 6 l e c t r i q u e e s t une f o n c t i o n n o n - l i n h a i - r e de l a c o m ? o s i t i o n .
A b s t r a c t . - T i 0 2 s u b m i c r o n i c po:rders have been s y n t h e s i s e d by c o n t r o l 7 e d hydro- fiFi's-Cf t i t a n i u m t e t r a i s o p r o p o x i c i e . The i n f l u e n c e o f r e a g e n t s c o n c e n t r a t i o n has been i n v e s t i ~ a t e d . A d d i t i o n s o f Niobium r e s u l t i n s p h e r i c a l p a r t i c l e s . Mixed powders o f Ti@-SnC2 have been produced by h y d r o l y s i s o f SnC14-SbC13 s o l u t i o n s c o n t a i n i n g t h e Nb doped T i 0 2 p a r t i c l e s . The SnO2-Ti02 s o l i d s o l u - t i o n s a r e e a s i l y formed f r o m llOO°C. However, t h e i n c o r p o r a t i o n o f t h e s?e- c i f i c dopants i s l i m i t e d . The e l e c t r i c a l c o n d u c t i v i t y i s a n o n - l i n e a r f u n c - t i o n o f t h e c o m p o s i t i o n .
I - IMTRODUCTIOPI.
T i n and t i t a n i u m d i o x i d e s have t h e same r u t i l e s t r u c t u r e and f o r m complete s o l i d so- l u t i o n s a t 1430 " C 1 1 . - 21. On t h e ocher hand, b o t h o x i d e s a r e o x y g e n - d e f i c i e n t and n - t y p e semiconductors. The e l e c t r i c a l c o n d u c t i v i t y i s enhanced by a p p r o p r i a t e do- p i n g , e.g. antimony f o r Sn02 and n i o b i u m f o r T i 0 2 . The complete i o n i s a t i o n o f donor l e v e l s i n t r o d u c e d by t h e s e dopants o c c u r s a t l o w e r t e r r p e r a t u r e i n SnO2 t h a n i n T i 0 2 so t h a t a t room temperature, t h e e l e c t r i c a l c o n d u c t i v i t y o f Sb-doped Sn02 ( - 1 0 & - 1 cm-1 i n t h e f i l ~ s ) i s s e v e r a l o r d e r s o f magnitude g r e a t e r t h a n t h a t o f Nb-doped T i 0 2 . The l o w t e m 7 e r a t u r e c o e f f i c i e n t o f c o n d u c t i v i t y o f Sb-doped SnO? f r o m room terrpera- t u r e u3 t o llOO°C, p e r m i t s a p p l i c a t i o n s as h e a t i n g elements and e l e c t r o d e s . Mowever,
? r o l o n q e l t h e r m a l t r e a t m e n t s a t temperatures h i g h e r t h a n 1100°C r e s u l t i n t h e break- dovn o f t h e e l e c t r i c a l p r o p e r t i e s because o f t h e v o l a t i l i s a t i o n o f t h e Sn and Sb o x i d e s and t h e r e d u c t i o n o f ~ b 5 + i n t o Sb3+ 1 3 - 4 /. On t h e o t h e r hand, i t i s known t h a t T i 0 2 do?ed by 1-3 a t . % Nb e x h i b i t s h i g h and s t a b l e v a l u e s o f e l e c t r i c a l conduc- t i v i t y (-1-5 cm-1) up t o 1350°C / 6 /. The q u e s t i o n o f whether T i 0 2 can p a r t i a l l y r e p l a c e SnOp f o r h i g h - t e m p e r a t u r e e l e c t r i c a l a p p l i c a t i o n s i s o f g r e a t i n t e r e s t es- p e c i a l l y i n SnOz-coated ceramics f o r which p r e v i o u s i n v e s t i g a t i o n s have been c a r r i e d o u t / 4 - 5 1
An o b v i o u s ' a p ? r o a c h t o answer t h e above q u e s t i o n i s t o s i n t e r n i x t u r e s o f doped Sn02 and Ti0,particles and i n v e s t i g a t e t h e r e l e v a n t e l e c t r i c a l p r o p e r t i e s as a f u n c t i o n o f phase c o m p o s i t i o n a t h i g h temperature.
T h i s communication d e s c r i b e s :
i - The s y n t h e s i s o f submicronic l4b-doped T i 0 2 p a r t i c l e s by c o n t r o l l e d a l k o x i d e hydro- l y s i s . T h i s method enables us t o s i n t e r packed powder compacts w i t h e x c e l l e n t r e l a - t i v e d e n s i t i e s ( >95 %) a t temperatures as l c w as 1100"C, as shown by t!.Kent Bowen
and a1./7-9/.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986102
J O U R N A L DE PHYSIQUE
ii- The synthesis o f doped TiOp-SnO2 pcwders r i x t u r e s by p r e c i ? i t a t i o n o f SnC14-SbC13 s o l u t i o n s c o n t a i n i n g t h e Ti02 p a r t i c l e s .
iii- The study o f s i n t e r i n g , phase f o r m a t i o n and e l e c t r i c a l c o n d u c t i v i t y o f compacted powders between 1100°C and 1.900°C.
I I . EXPERIMENTAL.
The T i 0 2 powders were prepared by t h e c o n t r o l l e d h y d r o l y s i s o f a d i l u t 6 d i s o p r o p y l - a l c o h o l s o l u t i o n o f t i t a n i u m t e t r a i s o p r o p o x i d e (Janssen). l l a t e r was d i s s o l v e d i n separate p o r t i o n s o f a l c o h o l . The concentrations ran7e between 0.3 t o 1.3 M H20.
Niobium was added as ethoxide (Ventron). The powders were c e n t r i f u ~ e d and d r i e d a t 80°C. P a r t i c l e sizes, y i e l d o f p r e c i p i t a t i o a and delay t i m e f o r t u r b i d i t y were de- termined as a f u n c t i o n o f t h e reagents concentrations. Phase t r a n s f o r m a t i o n and s i n t e r i n g o f these powders were s t u d i e d by d i l a t o m e t r i c , t h e r n o g r a v i m e t r i c and X-ray d i f f r a c t i o n measure~ents.
The Ti02 powders doped w i t h 1 a t . % Nb were disoersed i n a l c o h o l i c s o l u t i o n s c o n t a i - n i n g SnCIE and SbCl (Sb/Sn = 0.1) / 9/. T i n and antimony hydroxides were p r e c i p i t a - t e d on t h e TiOZ p a r g i c l e s w i t h ammonia. The powders were c a l c i n e d a t 500°C, then pressed i n t o d?scs (0 = 13 mm) under a l o a d o f 8000 ~g/crnz and s i n t e r e d between 1100°C and 1400°C.The e l e c t r i c a l measurements have been described elsewhere /lo/.
111. RESULTS AND DISCUSSION.
The i n d u c t i o n times necessary f o k t h e t i t a n i u m hydroxide n u c l e a t i o n have been p l o t t e d as a f u n c t i o n o f i n i t i a l water c o n c e n t r a t i o n f o r two concentration o f Ti(OC3H7)4, e.g. 0.1 and 0.15H/1 on a l o g a r i t h m i c diagram (Rg. l a ) . The r a t e o f nucleatior, i s
~ n v e r s e i y p r o p o r t i o n a l t o t h e i n d u c t i o n t i m e so t h a t t h e slopes o f Fig.1, c l o s e t o -3, i n d i c a t e t h a t t h e r a t e = k ( ~ ~ 0 ) 3 . Pence, t h e t h i r d step o f t h e h y d r o l y s i s reac- t i o n , e.g. :
Ti(OC3E7)* + 3 H20 -. Ti(OC3H7) (OH)3 + 3 C3H70H
appears t o be t h e l i m i t i n g process o f t h e p a r t i c l e formation. This r e s u l t i s i n agreement w i t h t h a t o f Garringer concernin? t h e h y d r o l y s i s o f t i t a n i u v e t h o x i d e / 3 / . F i g . 1 b shows t h a t t h e y i e l d o f p r e c i p i t a t i o n becomes appreciable (75 % and more) o n l y when (H20/(Ti (OC3H7)4)> 3. This r e s u l t confirms t h e i n t e r p r e t a t i o n o f Fig. 1 a.
The c o n d i t i o n s compromising a good m i x i n g o f reagents p r i o r t o t h e n u c l e a t i o n and a good y i e l d o f p r e c i p i t a t i o n are achieved f o r 0.15M Ti(OC3ti7)4 and 0.5 t o Q.6M H2O/11/
The p a r t i c l e s p r e c i p i t a t e d w i t h G.5 H Hz0 a r e shown i n Fig. 2 a. The mean rain
s i z e i s 0.3 um. These p a r t i c l e s a r e n o t r e a l l y equiaxed b u t a r e r a t h e r m u l t i n u c l e a r as p r e v i o u s l y observed / 7 / .
t
1 0 0 -
o Ti10C3HJL 0 1 M
+ nrogq,~, o l s n
OITIIOGH,~, + 0 0 1 a t % Ybl OISaw
0 IS 0.5 1 rn,olr M I
-
I :toc.2.~ , ,
o 3n
---
1 TI OC><' 0 ? il1 .
0 k5 LA?31 ( M I
~ i g . 1 a - l o g a r i t h m o f delay t i r e before t u r b i d i t y versus i n i t a l water c o n c e n t r a t i o n
b - y i e l d o f p r e c i p i t a t i o n versus i n i t i a l water c o n c e n t r a t i o n .
F i g . 2a - Undoped T i 0 2 p a r t i c l e s - 0.5,U-m F i g 2b - Nb-doped T i 0 2 p a r t i c l e s 0.1 p m
When Nb e t h o x i d e i s added t o t h e s o l u t i o n so t h a t Mb/Ti=O.Ol, t h e behavior i s q u i t e d i f f e r e n t : t h e i n d u c t i o n times a r e m o d i f i e d ( F i g . l a ) ; th e p a r t i c l e shape becomes e- q u i axed and s h e r i c a l (Fig.2b). The niobium precursor which i s probably l e s s s t a b l e than Ti(0C H P appearr t o enhance t h e n u c l e a t i o n and t h e growth o f t h e p a r t i c l e s so t h a t smal 1 3 p g r ? i c l e f l o c c u l a t i o n i s impeded and t h e g r a i n s are mononuclear.
Di 1 a t o m e t r i c measurements(Fig. 3) performed on an amorphous T i 0 compact show t h a t
most o f t h e shrinkage occurs d u r i n g t h e a l l o t r o p i c transformatqons: amorphous ac
anatase 75O0c,ruti l e . The shrinkage i n v o l v e d i n t h e l a s t t r a n s f o r m a t i o n (1 1%) i s g r e a t e r than t h a t p r e d i c t e d by t h e c r y s t a l lo g r a p h i c t r a n s i t i o n a n a t a s e - r u t i l e (2.5%).
A rearrangement o f t h e p a r t i c l e s occurs and induces a s u i t a b l e packing f o r t h e s i n - t e r i n g step.'h's step has a maximum speed around llOOOc so t h a t appreciable densi- t i e s (>go%) can \ e reached from t h i s temperature.
The Sb-doped t i n oxide p r e c i p i t a t e d on amorphous and r u t i l e T i 0 c o n s i s t s o f f i n e c r y s t a l l i t e s as shown by t h e X-ray peak broadening observed up
20800°c. These pow- ders are v e r y r e a c t i v e and form s o l i d s o l u t i o n s a t llOOoc. A t 1400°c, t h e l a t t i c e parameters measured from t h e (220), (2001, (1 01 ) a r d (21 1 ) X-ray d i f f r a c t i o n peaks i n - d i c a t e t h a t complete s o l i d s o l u t i o n s e x i s t f o r SnO concentrations l e s s than 35% and more than 65%. This i s c o n s i s t e n t w i t h t h e e q u i l i b ? i u m diagram r e p o r t e d by Park/2/
( ~ i g . 5 ) . The r e l a t i v e d e n s i t i e s o f these s o l i d - s o l u t i o n s ( F i g . 6 ) decrease w i t h i n c r e a - s i n g Sn02 content. I n f a c t , t h e d e n s i f i c a t i o n o f pure and Sb-doped Sn02 i s d i f f i t u l t t o perform as p r e v i o u s l y r e p o r t e d /12/.
The preen p e l l e t s o f SnO - T i 0 mixed powdcv's exhi b i t e l e c t r i c a l c o n d u c t i v i t y values ranging from IO-?A-~ cm21 (160% SnO ) t o 3.5 1 0 - ~ ? - 1 cm-1 (50% SnO ) a t r om tem- perature, whereas t h e Nb-doped T i 0 2 G e l l e t s are i n s u l a t i n g ( <I 0-7&-1 cm-?).
0 500 1000 Tl'cl
Fig.3 - D i l a t o m e t r i c curve o f an amorphous compacted sample o f doped TiOp
I4eating r a t e : 3 c/min.
J O U R N A L DE PHYSIQUE
I I I I
Tc =
1430°
1400
- -
-
-
T10z 55 + SnOz ss
-
-
Sn02 20 40 60
Mol % 80 T102
F i g . 4 - E v o l u t i o n o f l a t t i c e parameters o f Sn02-Ti02 system a f t e r s i n t e r i n g a t 1400°c ( I h o u r )
F i g . 5 - System Sn02-Ti02 a f t e r /2/.
A f t e r f i r i n g a t l l O O O c and 1400°c, t h e Nb-doped T i 0 samples become c o n d u c t i n g ( 6 3 1 0-4&1 c m - I ) . The Sb-doped SnO remains c o n d u $ t i n g a f t e r t h e s e t r e a t m e n t s . On t h e o t h e r hand, t h e doped SnO - ~ i 8 m i x t u r e s a r e l e s s c o n d u c t i v e t h a n t h e s i n g l e doped o x i d e s i n t h e o v e r a l l rang& o f $ompositions. The a d d i t i o n o f Sb-doped SnO t o Nb-doped TiOp i n t h e one-phase r e g i o n (up t o 35% Sn02) l o w e r s t h e c o n d u c t i v i t y
$0a l a r g e minimum c o r r e s p o n d i n g a p p r o x i m a t e l y t o t h e two-phases r e g i o n . It i s l i k e l y t h a t i/ t h e r e l e v a n t l a t t i c e exp?ns.ion i m e d e s t h e charae t r a n s f e r betweer t h e
T i and Nb c a t i o n s
ii/ t h e s o l u b i l i t y o f sb5+ i n t h i s phase i s poor
On-the o t h e r hand, t h e a d d i t i o n o f T i 0 t o Sb-doped SnO p e r t u r b s t h e band conduc- t i o n o f t h e l a s t o x i d e t o a s i g n i f i c a n ? e x t e n t so t h a t $he donnor a c t i o n o f t h e do- pants i s l e s s e f f i c i e n t .
I
I 20
. . -
_ _ - o ' / /0 25 50 75 100 ./' SnO,
F i g . 6 - R e l a t i v e d e n s i t y and e l e c t r i c a l c o n d u c t i v i t y versus c o m p o s i t i o n o f t h e
Nb-Ti02 / Sb-Sn02 system.
Nb and Sb-do7ed Sn02-Ti02 s o l i d s o l u t i o n s a r e e a s i l y formed ( a t T > llOO°C) by t h e l i q u i d m i x t e c h n i q u e d e s c r i b e d above. The s p e c i f i c donnor e f f e c t s o f ~ b 5 + and sb5+
decrease as Ti02-Sn02 s o l i d s o l u t i o n s a r e formed. The e l e c t r i c a l c o n d u c t i v i t y has a minimum i n t h e two-phases r e g i o n . The b e s t c o n d i t i o n s compromising t h e e l e c t r i c a l p r o p e r t i e s and t h e chemical s t a b i l i t y a r e t o b e f o u n d i n t h e SnO2 r i c h r e g i o n o f t h e b i n a r y diagram f o r T i 0 2 c o n t e n t l e s s t h a t 20 %. Such c o m p o s i t i o n s can a l s o be depo- s i t e d by vapor p y r o l y s i s and some p r e l i m i n a r y experiments show t h a t such f i l m s a r e more s t a b l e a t 1200°C t h a n t h e Sb-doped s i n g l e SnOp f i l m s .
REFERENCES.
/1/ D. GARCIA-D. SPEI3EL. J . Am.Ceram.Soc. 55 ( 6 ) pp.322 (1972)
/2/ F". PARK- T. E. MITCHELL-A. H. HEUER. J .AmXeram. Soc. 58 (1-2) pp.43-47 (1975) /3/ C.P.. VINCENT, D.C.G. \IESTON. J . E l e c t r . S o c . 119 ?p.5f5-521 (1972)
/C-/
