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PREPARATION OF SEMICONDUCTING CERAMICS(NTC THERMISTORS) BY CHEMICAL METHOD

E. Jabry, G. Boissier, A. Rousset, R. Carnet, A. Lagrange

To cite this version:E. Jabry, G. Boissier, A. Rousset, R. Carnet, A. Lagrange. PREPARATION OF SEMICONDUCTINGCERAMICS (NTC THERMISTORS) BY CHEMICAL METHOD. Journal de Physique Colloques,1986, 47 (C1), pp.C1-843-C1-847. <10.1051/jphyscol:19861129>. <jpa-00225526>

JOURNAL DE PHYSIQUE Collogue CI, supplement au n"2. Tome 47, fevrier 1986 page cl-843

PREPARATION OF SEMICONDUCTING CERAMICS (NTC THERMISTORS) BY CHEMICAL METHOD

E. JABRY, G. BOISSIER, A. ROUSSET, R. CARNET* and A. LAGRANGE*

Laboratoire de Chimie des Matériaux Inorganiques, Université Paul Sabatier, 118 Route de Narbonne, F-31062 Toulouse Cedex, France 'Société L.C.C.-CI.CE. , (Groupe Thomson-CSF), Avenue du Colonel Prat, St Rpollinnaire, F-21100 Dijon, France

Résumé - La décomposition en atmosphère contrôlée de précurseurs oxaliques mixtes (Mn, Ni )C„04, 2H20 permet de préparer directement et à basse tempé­rature (600-700°C) des poudres de manganites de nickel Mn3 Nix0.((kx<l) pré­sentant une morphologie contrôlable. Ces poudres, frittées à 1160°C seulement; conduisent à des céramiques semi-conductrices très bien densifiées (96 % de la densité théorique) et présentant des résistivités électriques nettement plus faibles que celles observées jusque là pour de tels composés.

Abstract - The decomposition in controlled atmosphere of mixed oxalic precur­sors (Mi^ Ni )C204, 2HgO allows at low temperature (600-700°C), direct prepa­ration of powders of nickel manganites Mn3_xNix04(0<x<l)producing a con­trolled morphology. These powders sintered at only 1160°C lead to highly den-sified semiconducting ceramics (96 % of the theoretical density) and have electrical resistivities clearlylower than those observed up until now for such compounds.

I - INTRODUCTION

Thermistors with a Negative Temperature Coefficient (N.T.C.) are semiconducting ce­ramics, most often constituted by transition metals manganites. In these solids the electrical conductivity is due to the phenomenon of transfer of electrons (hopping)

between the Mn and Mn ions in octahedral sites /I/. The resistivity of these

phases follows the classical law of semiconductors : P = PQe with B = £ | ,AE is

the activation energy of the hopping mechanism. The necessity for improving the electrical properties of these electronic compo­nents necessitates the modification of the preparation process notably through the reaching of an optimized microstructure /2//3/. Therefor recent investigations have stressed the importance that must be given to the morphology of the powders submitted to sintering /3//4//5/. In this study we will reveal the interest of preparing powders of nickel manganites at low temperature which,as a result of this,have a controlled morphology. The sin­tering operation is then much simplified, it leads to very dense and well optimized ceramics as far as the electrical properties are concerned. The nickel manganites M n3_ x

N ix°4 (0<x<l) are phases with a spinel structure which

are very difficult to obtain in the pure state /6//7//8//9Z. Rather than using the conventional method of solid-solid reaction between Mn,0„ and NiO we have preferred

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19861129

JOURNAL DE PHYSIQUE

t o use the decomposition o f a mixed organic precursor i n order t o ob ta in d i r e c t l y powders o f n i cke l manganites. These powders may be s in te red a t r e l a t i v e l y low tem- peratures wich appears as a great advantage as the n icke l manganites are unstable a t h igh temperature /I 0//11/. From a s t r u c t u r a l p o i n t of view, d i f f e r e n t d i s t r i b u t i o n s have been proposed i n part i - c u l a r f o r the compound NiMn204 ( x = 1 ) ; we w i l l adopt the model based on the s tab i -

l i s a t i o n energies o f the manganese and n icke l ions i n t h e c r y s t a l l i n e f i e l d , namely 2 t 3+ Mn IMn2-2x ~ n i ~ ~ i : ' 1 0:- (O<x<l) /12/.

I 1 - PREPARATION OF NICKEL MANGANITES POWDERS

The powders r e s u l t from t h e decomposition o f o x a l i c precursors Mnl-v Niy(C204)2,

2H 0 (O<y<1/3) prepared by c o p r e c i p i t a t i o n i n aqueous so lu t ion . ~ i f T e r e n t composi- t i g n s have been studied as shown i n t a b l e I which a lso g ives the compositions o f the corresponding manganites Mn3-x N ix O4 w i t h x = 3y and O<x<l.

Table I --

1. Decomposition o f the mixed oxalates i n a i r

Thermal decomposition processing i s described e l sewhere /13/. This process does no t lead d i r e c t l y t o the requ i red sp ine l s t ruc tu re . I n fac t , beyond 420°C, t h a t i s t o say a f t e r the end o f the decomposition, XRD analys is reveals a mixture o f phases c o n s t i t u t e d o f cubic Mnp03 and o f rhomboedral ferromagnetic NiMn03.

Taking i n t o account the h i b h r e a c t i v i t y o f the oxides, a thermal- treatment o f se- vera l hours a t 800°C a l lowsa simultaneous c r y s t a l l i s a t i o n o f these oxides i n an un i - uue so inel ohase accordinu t o the reac t ion . .

x ~ i i n 0 ~ + (3/2 -x ) Mn203 + NixMn3-x04 i- 1/4 O2 f o r a1 1 values 0.50<x<1. Thus sp inel phase i s obtained a t 800°C ins tead o f a t 1100°C w i t h a conventional pro- cessing. The morphologic analys is o f the manganites thus prepared show t h a t t h e powders are formed o f r a t h e r s t r o n g l y conglomerate p a r t i c l e s w i t h a la rge range o f s izes shapes and a low surface area (1-2 m2 g-l 1. Such c h a r a c t e r i s t i c s which der ived from t h e thermal treatment done dur ing several hours a t 800°C, are no t necessar i ly the best t o opt imize the m ic ros t ruc tu re i n a f u r t h e r s i n t e r i n g operat ion. Be t te r control- l e d p roper t ies would be hoped f o r i n the case o f these manganites i f they were ob- ta ined d i r e c t l y a t lower temperature. I n t h i s way i t i s essen t ia l t o avoid the f o r - mation o f t h e phase Ni Mn O3 in lower ing t h e oxygen p a r t i a l pressure dur ing the pre- cursor decomposition.

2. Decomposition o f the mixed oxalates i n c o n t r o l l e d atmosphere

Due t o the conventional atmosphere being too ox id iz ing , we have worked i n mix- tu res o f N2 + 02, poorer i n oxygen. Af ter several tes ts , we have succeeded i n ob ta i -

n ing d i r e c t l y and a t low temperature (600- X)O°C) monophasic manganites.The condi- t i o n s we have used are t h e f o l l o w i n g : a) Deco_mposition o f t h e s a l t i n a mixture o f N7/07 whose oxygen p a r t i a l pressure i s - - 7.5 10' Pascal (Pa). b) Heat treatment i n neu t ra l atmosphere from 42o0~(s0ak ing t ime : two hours) t o 7w0C. C ) Quenching Pure phases have been obtained f o r t h e compounds Mn3-, N ix O4 such t h a t x>0.50 i.e. fo r a l l the compounds concerning the N.T.C. thermistors . For h igh manganese contents

i t had been imposs ib le , whatever t h e o p e r a t i n g cond i t i ons , t o end up w i t h monophasic oxides. These r e s u l t s c o n f i r m t h o s e a o f SARKAR / 9 / and WALCH / l o / who admi t a non m i s c i b i l i t y o f t h e ox ides Mn304 and NiO.

F ig . 1

The n i c k e l manganites prepared i n t h i s way a t r e l a t i v e l y low temperature a r e c o n s t i - t u t e d of v e r y weakly agglomerated p a r t i c l e s w i t h a r e g u l a r s i z e and shape ( sma l l oc- tahedra 4-5 um)(P igure no 1 ) . T h e i r su r face area evo lves w i t h t h e heat t r ea tmen t and can be c o n t r o l l e d b e f o r e s i n t e r i n g .

I 1 1 - PREPARATION AND CHARACTERISATION OF THE CERAMICS

1. P repa ra t i on

The powders f o rme ly ob ta ined f rom 600 t o 800°C,either t h e ones prepared i n a i r o r es- pec i a1 l y t h e ones prepared i n c o n t r o l l e d atmosphere, were p u t i n a m a t r i x a f t e r an o r - gan i c b i n d e r had been added, app l y i ng a pressure o f 4 kb . The r e s u l t i n g d i scs , w i t h a d iameter 0 = 0.5 cm and th i ckness e = 0.1 cm, were submi t ted t o d i f f e r e n t heat t rea tments i n a i r w i t h d i f f e r e n t p r o f i l e s ( t a b l e 11).

2. Cha rac te r i s a t i o n

Tab le I 1 g i v e s XRD a n a l y s i s and d e n s i t i e s .

Table I1

D i f f r a c t i o n a n a l y s i s revea led t h e presence o f s p i n e l s t r u c t u r e and t h e absence o f any o t h e r c r y s t a l l i n e compounds (NiO + Mng04). The ceramics prepared f rom powders ob ta ined

i n c o n t r o l l e d atmosphere have a h ighe r d e n s i t y t han those t r e a t e d i n a i r atmosphere ( f i g u r e 2 ) . High d e n s i t i e s a r e ob ta ined f o r r e l a t i v e l y low s i n t e r i n g temperature (1160°C). The g r a i n s i z e i s about 2OPm. F i n a l l y , i t i s necessary t o p o i n t o u t t h a t t h e s i n t e r i n g of t h e powders Mn304 ana NiO heat t r e a t e d i n t h e same c o n d i t i o n s (1160°C)

l e a d t o i ncomp le te s o l i d e - s o l i d - r e a c t i o n s and t h a t t h e d e n s i t i e s remain low, about 4.2. g.cm-3. The f i g u r e s no 3 and 4 a l l ows us t o d e f i n e t h e d i f f e r e n c e s o f m i c r o s t r u c t u r e observed. F i g u r e n o 4 shows a more heterogeneous g r a i n s s i z e and an impor tan t p o r o s i t y .

c1-846 30URNAL DE PHYSIQUE

density

1 F i g . ~ : Change o f d e n s i t y w i t h t h e n i c k e l - conten t S i n t e r i n g a t 1160°C, 2 H,coo l ing a t 30°C/h

s t a r t i n g powders ob ta ined a t 70 C

P sca l (Pa) Y i n c o n t r o i l e d atmosphere,7,5 10

---- s t a r t i n g po lde rs ob ta ined a t 800°C 4 H i n a i r .

-.-ceramics ob ta ined by convention_al method -(TT%Fring a t 1200°C, 2 H)

F i g . 4 -10~m

I V - ELECTRICAL PROPERTIES OF THE N.T.C. THERMISTORS

To deter i i l ine t h e e l e c t r i c c h a r a c t e r i s t i c s , t h e samples were e l e c t r o d e d w i t h s i l v e r p r i n t and f i r e d a t 800°C. The r e s i s t i v i t y i s determined by r e s i s t a n c e measurements&

RS 25 + U.05OC and u s i n g t h e r e l a t i o n s h i p 0 = (R = r e s i s t a n c e i n ohms, S = su r face "

i n cmL, e = t h i ckness i n cm). A second measurement done a t 85 + 0.05OC a l l ows deduc- t i o n o f t h e thermal s e n s i t i v i t y c o e f f i c i e n t B /I/. The r e s u l t s ob ta ined a r e shown i n f i g u r e 5 .

r e s i s t i v i t y p .7000 (n.cm)

-60ao A 1 '\,

- 5000 1 '.\. 4000 \.

' I

. 3000

F i g . 5 : Change of r e s i s t i v i t y w i t h t h e n i c k e l con ten t .

S i n t e r i n g a t 1160°C, 2 H c o o l i n g at30°C/h

s t a r t i n g powders ob ta ined a t 700°C i n c o n t r o l l e d atmosphere, 7,5 l o 3 (Pasca l ) Pa.

--- s t a r t i n g powders ob ta ined a t 800°C, 4 H i n a i r --- ceramics ob ta ined by convent ional met hod ( s i n t e r i n g a t 1200°C, 2 H)

For s i m i l a r s i n t e r i n o and composi t ion, we can observe s e n s i b l e d ivergences o f r e s i s t i v i t y which can be a t t r i b u e d t o t h e d i f f e r e n c e s o f m i c r o s t r u c t u r e . Ceramics w i t h t h e h ighe r d e n s i t i e s e x h i b i t lower r e s i s t i v i t i e s . The minimum r e s i s t i v i t y (1030 a.cm) i s t h e lowest o f a l l t h e ones we have found i n t h e b i b l i o g r a p h y concer- n i n g t h e n i c k e l manganites. It means t h a t t he e l e c t r i c a l p r o p e r t i e s o f these compounds were n o t op t im ized and t h a t i t i s p robab l y s t i l l p o s s i b l e t o o b t a i n i n f e r i o r va lues b y i n c r e a s i n g t h e d e n s i f i c a t i o n .

The s i n t e r i n g temperatures above 1160°C d i d not a l l o w improvement inthe e l e c t r i c a l p e r - formances f rom t h e powders produced by chemical methods. On t h e o t h e r hand, f o r t h e ceramics prepared f rom s o l i d - s o l i d r e a c t i o n s between Mng04 and NiO, t h e bes t r e s u l t s are observed a t a s i n t e r i n g temperature o f 1300°C, w h ~ c h a l l ows a minimum o f r e s i s t i v i t y a t 2200 a.cm o n l y . F i g u r e 2 shows t h e r e s i s t i v i t y versus n i c k e l con ten t . Fo r t h e ceramics ob ta ined by conven t i ona l method and s i n t e r e d a t 1200°C, i .e. i n c o n d i t i o n s r a t h e r c l o s e t o those prepared by chemical method, we observe h i g h values o f r e s i s t i v i t y as w e l l a s v e r y i r r e g u l a r v a r i a t i o n s w i t h t h e n i c k e l content , which shows l i t t l e coherence w i t h t h e model proposed. Such c h a r a c t e r i s t i c s can be exp la ined by c o n s i d e r i n g t h a t t h e subs- t i t u t i o n o f n i c k e l i n t h e s p i n e l l a t t i c e i s n o t c o n t r o l l e d as conf i rmed by t h e XRD s t u d i e s On t h e o t h e r hand, whatever t h e s i n t e r i n g temperature, t h e ceramics prepared by che mica1 method show a minimum of r e s i s t i v i t y . Th i s appears f o r a n i c k e l con ten t o f x = 0.80 whereas t h e o r e t i c a l l y t h i s minimum was expected f o r x = 0.66. Th i s d i v e r - gence can be t h e r e s u l t o f a d i s t r i b u t i o n s l i g h t l y d i f f e r e n t f r om t h e one proposed

i n i t i a l l y , t a k i n g i n t o account t h e presence o f Ni2+ i o n s i n t h e t e t r a h e d r a l s i t e s f o r example and i n t h e case o f t h e compound NiMn204 i t has a l r e a d y been D ~ O D O S ~ ~ /I 1 //14/.

V - CONCLUSION

The p r e p a r a t i o n b y chemical process a t r e l a t i v e l y low temperature o f pure p o w d e r s d n i c k e l manganites w i t h a c o n t r o l l e d morphology has a b e n i f i c i a l e f f e c t on t h e d e n s i t y o f N.T.C. ceramics. Besides lower s i n t e r i n g temperature, c l e a r l y lower r e s i s - t i v i t e s can be ob ta ined and a b e t t e r c o r r e l a t i o n 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 w i t h t h e n i c k e l con ten t can be &served I n c o n t r a s t t o t h e convent iona l process based on t h e r e a c t i o n a t h i g h temp-erature o f m i x t u r e o f manganese and n i c k e l oxides, t h i s me t h d allows us t o break away f rom t h e problems o f p u r i t y b u t a l s o f r o m problems o f morphology o f raw m a t e r i a l s , which th rough t h e m i c r o s t r u c t u r e l a r g e l y i n f l uences t h e e l e c t r i c a l p r o p e r t i e s . I n t h i s way u s i n g chemical process makes the ~ p t i m i s a t i ~ n of e l e c t r i c a l p r o p e r t i e s of n i c k e l based manganites eas ie r .

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