HAL Id: jpa-00225530
https://hal.archives-ouvertes.fr/jpa-00225530
Submitted on 1 Jan 1986
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Influence of processing parameters on the electrical properties of Zn1-x CoxO ceramics (0 � x � 0.10)
B. Tanouti, Roger Salmon, Jean-Pierre Bonnet
To cite this version:
B. Tanouti, Roger Salmon, Jean-Pierre Bonnet. Influence of processing parameters on the electrical
properties of Zn1-x CoxO ceramics (0 � x � 0.10). Journal de Physique Colloques, 1986, 47 (C1),
pp.C1-861-C1-864. �10.1051/jphyscol:19861132�. �jpa-00225530�
INFLUENCE OF PROCESSING PARAMETERS ON THE ELECTRICAL PROPERTIES OF Znx xC oxO CERAMICS (0 < x < 0.10)
B. TANOUTI, R. SALMON and J.-P. BONNET
Laboratoire de Chimie du Solide du C.N.R.S., 351, Cours de la Libération, F-33405 Talence Cedex, France
Résumé - L'influence de la concentration en cobalt et de la durée du frittage sur la conductivité électrique de céramiques de la solution solide Zn, Co 0 ( O ^ x ^ 0.10) mesurée ^entre 400 et 1300 K, xa x été étudiée.
L'interprétation des résultats obtenus associe la dissolution du _cobalt dans la structure wurtzite de l'oxyde de zinc à la formation de deux niveaux d'énergie dans la bande interdite.
Abstract - The electrical conductivity of Zn, Co 0 solid solution has been studied as a function of composition (0gx£ 0.10) and of sintering time. The behavior observed is probably due to the appearance of two energy levels in the forbidden band.
I - INTRODUCTION
Zinc oxide is a n-type semiconductor with a large energy gap ( «^ 3.2 eV) . This is generally attributed to the presence of an excess of metal in the wurtzite structure /l/. The electrical properties are thus controlled by the departure from stoichiometry which depends itself of the elaboration process / 2 / . They may also be influenced by the dissolution of small amounts of transition metals able to create energy levels in the forbidden band / 3 / . Such effects are particularly important in the case of cobalt-doped zinc oxide, which is the major phase in varistor materials / 4 / . In order to understand the influence of their processing parameters on the final properties, we have undertaken the study of the electrical conductivity of Zn Co o ceramics as a function of sintering time and of composition
(0.$. x 4: 0.10) . II - EXPERIMENTAL
Ceramics samples were prepared from mixtures of reagent-grade ZnO and cobaltous carbonate. The powders were suspended in ethanol and stirred for 4 h. in an agate jar mill. After drying, they were ground and bound with polyvinylic alcohol, then pressed under 200 MPa, The pellets obtained were set in an alumina boat and fired at 1300°C in pure oxygen for the times_|elected. In all cases the heating and cooling rates were 600°C h
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19861132
JOURNAL DE PHYSIQUE
The ceramic itïicrostructures were examined on polished samples by scanning electron microscopy
.
X-ray dispersive energy analysis does not show the existence of cobalt oxide precipitates.ni ne electrical conductivity was neasured in the 1300-400 K range in air using the four-probe method /5/. For electrical contacts, platinum wires and paint were used. For each tenpera-ture the equilibriun value repor-ted is the ordinate of the horizontal asymptote to the log o= f(tirne) cuzve.
III - XESULTS
1
-
Influence of the sintering time on the electrical properties of Z n 0 ~ 9 9 C 0 0 ~ 0 1 0 solid solution.- -
This study was performed on ceramics having the same composition, but sintered at 1300°C for various times from 3 up to 63 hours.
Figure la) illustrates the temperature dependence of the electrical conducéiyity of the samples studied. Al1 the
curves l o g o = f(10 T ) show two linear parts separated by a transition temperature T L _ determined with an accuracy of 30 K.
To each linear part canL'be associated an activation energy E
log o (Q-' cm-')
1
1 (KI IO00 800,
3 600 400 ,Pig.1
-
Temperature dependence of the electrical conductivity of Znl-xCo O ceranics sintered a.: 1300°C in air.X
a) influence of the sintering time b) influence of the composition (composition of sample : (sintering time : 24 h.)
.99C00 .OlO)
Eht r e s p e c t i v e l y
-
a r e l i s t e 6 i n T a b l e 1.E v o l u C i o n o f T,._, Z l t a n d Y h t p a r a m e t e r s w i t h s i n t e r i n g t i m e
L -
1
S i n L e r i n g i i m e ( h )/
T L r i 30 K1
E 1; ( e V ) / E h t ( e V )1
From L h e s e r e s u l t s i é a p p e a r s t h a t t h e e l e c t r i c a l c o n d u c t i v i k y a n d t h e t r a n s i t i o n t e m p e r a t u r e b o t h i n c r e a s e w i t h . t h e s i n t e r i n g C i m e . The o p p s i t e b e h a v i o r i s o b s e r v e d f o r t h e h i g h t e m p e - a t u r e a c t i v a t i o n e n e r g y E w h e r e a s t h e low k e a p e r a t u r e a c t i v a t i o n e n e r g y El. seems t o h b e i n s e n s i t i v e i o k h e s i n i e r i n g t i n e . The p e c u l i a r c a s e o f t h e s a m p l e s i m t e r e d f o r 63 h w i l l b e j u s t i f i e 6 l a t e r ( s e e D i s c u s s i o n ) .
2
-
I n f l u e n c e o f t h e coba12c c o n c e n t r a t i o n o n t h e e l e c - k r i c a l c o n d u c t i v i t y o f t h e Znl-xgxO s o l i d s o l u t i o n ( O ( x ( 0 . 1 0 ) .- -
The s t u d y was p e r f o r m e d on s a m p l e s o f v a r i o u s c o b a l t c o n t e n t ( O < x , < 0 . 1 0 ) a l 1 s i n t e r e d i n t h e same c o n d i t i o n s ( 2 4 h . a-t 1300°C i n a S t r e a m o f p u r e o x y g e n ) . T h e t e m p e r a - t u z e d e p e n d e n c e o f t h e e l e c t r i c a l c o n d u c , i v i t y o f t h e s a m p l e s s t u d i e d i s r e p r e s e n t e d o n F i g u r e l b
.
A s i n t e p r e v i o u s s-:udy i t a p p e a r s t h a t a l 1 t h e c u r v e s l o g 0- = f ( 103T") c o n s i s t o f t w o l i n e a r p a r t s . s e p a r a t e d by a t r a n s i - t i o n t e m p e r a t u r e T.
The v a l u e s o f Tt- a n d o f t h e a c t i v a t i o n e n e r g i e s E ht ansZElt a r e l i s t e d f o r e v e r yc o m p o s i t i o n i n T a b l e I I .
From t h e s e r e s u l t s i t a p p e a r s t h a t t h e e l e c t r i c a l c o n d u c t ï v i é y o f Znl-xCoxO c e r a m i c s d e c r e a s e s when t h e f r a c t i o n o f c o b a l t o x i d e i s i n c r e a s e d . The p e c u l i a r b e h a v i o r o b s e r v e d f o r t h e s o l i d s o l u t i o n o f c o m p o s i k i o n x = 0 . 0 0 5 w i l l b e d i s c u s s e d l a t e r .
TABLE ïI
I V
-
DISCUSSIONF o r a l 1 t h e s a m p l e s s t u d i e d , 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 was E v o l u t i o n o f T t r , E l t a n d E h t p a ï a m e t e r s wiLh c o m p o s i t i o n C o b a l t c o n c e n t r a t i o n x
0.005 0.010 0.020 0.025 0.030 0 . 0 4 0 0.100
TLr ? 30 K 625 900 890 780 680 680 680
E l c ( e V ) O . 1 0 0 . 2 1 O . 22 O . 24 O . 23 O . 40 O . 62
Xh.t(eV) 0 . 2 0 0 . 2 6 O . 3 1 O . 47 0.64 O . 7 4 O . 78
JOURNAL DE PHYSIQUE
independent on the applied voltage which indicates that no effective potential barrier is localized at the grain boundaries in the 1300 - 400 K temperature range.
Comparing Figure la) and lb) shows that the electrical conducti- vity can be lowered by either increasing the cobalt content of the solution or decreasing the sintering time. This can mean that the diffusion of cobalt ions in the matrix is a rather slow process. Then, for short sintering times, the distribution of co- balt is not even, its concentration being the highest in the outer core of the zinc oxide grains. Indeed, if such a similar configuration exists in a sample sintered for a long time -due to the evaporation of zinc oxide £rom the outer core of the grains for instance- then it would explain the lowering of the elec- trical conductivity observed for the sample sintered for 63 h.
At low temperature and for x < 0.04 the activation energy E does not depend on the cobalt concentration and is close to &at of pure ZnO. The low temperature conductivity must then be control- led mainly by the oxide own donor states which lie 0.3 eV below the conduction band 6 The decrease in the conductivity observed when x is increased could then be the result of a lowered carriers mobility (Fig. lb).
On the opposite, the high temperature activation energy E ht ' observed when x$ 0.10, and consequently the Fermi level, depends on the cobalt concentration. The lowering of the transition tem- perature Ttr when x increases (Table II) could be due to a decrease in the Zn0 level concentrations, and to a contribution O£ deeper levels created by the introduction of CO'+ ions in the wurtzite structure which would lie 1.6 eV below the conduction band (7 )
.
The increase of the electrical conductivity observed when x = 0.005 is assoicted with a sharp increase in density / 7 / . The dissolution could then involve the formation of intersti- tial zinc and of donor levels according to :
As the concentration of cobalt is increased, the evolution of the density suggests that a substitutional mechanism would take over the previous one, i.e. :
(1-x) Zn0
+
xcoo
+ (1-x) znZn + XCO&+
:Owhich would involve the existence of deep energy levels.
REFERENCES
/1/ Thomas, D.G., J. Phys. Chem. Solids, 3 (1957) 229.
/2/ Heiland
,
G., Mollwo, E. and ~toFkman, F., Solid State Physics, 8, 191, Academic Press, New York 1959.
/3/ ~ u ~ o n t - ~ a v l o v s k y , N:, Caralp, F., Delahes, P. and Amiell,J., phys. Stat. Solid (a)
2
(1976) 615/4/ Salmon, R., Graciet, M., Le Flem, G., Hagenmuller, P., Hildebrandt, M. and Buchy, F., Revue de Physique Appliquée,
(1978) 67
/5/ Laplume, J., l'Onde électrique,
335
(1955) 113./6/ Tanouti, B., Bonnet J.P. and Salmon, R., C.R. Acad. Sc.,
297
(1983) 335
/7/ Tanouti, B., Bonnet, J.P. and Salmon, R
