INTERGRANULAR STRUCTURE AND ELECTRICAL BEHAVIOUR IN A MIXED OXIDE-MnZn FERRITE

(1)

HAL Id: jpa-00225578

https://hal.archives-ouvertes.fr/jpa-00225578

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.

INTERGRANULAR STRUCTURE AND ELECTRICAL BEHAVIOUR IN A MIXED

OXIDE-MnZn FERRITE

J. Laval, M. Pinet

To cite this version:

J. Laval, M. Pinet. INTERGRANULAR STRUCTURE AND ELECTRICAL BEHAVIOUR IN A MIXED OXIDE-MnZn FERRITE. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-329-C1-333.

�10.1051/jphyscol:1986148�. �jpa-00225578�

(2)

JOURNAL DE PHYSIQUE

Colloque C 1 , supplgment

au

n 0 2 ,

Tome

47, f b v r i e r 1986 page cl-329

INTERGRANULAR STRUCTURE AND ELECTRICAL BEHAVIOUR IN A MIXED OXIDE-MnZn FERRITE

J.Y. LAVAL

and

M.H. PINET

L a b o r a t o i r e des M i c r o s t r u c t u r e s , U A 450, CNRS-ESPCI,

1 0 ,

R u e Vauquelin, F-75231 P a r i s Cedex 05, F r a n c e

Rdsm@ : Les propriBtds Glectriques des oxydes mixtes ont dtd ddtennin@es lo- calement par l a m6thode des m i c r ~ l e c t r c d e s de tungstsne e t r e l i k s aux

ana-

lyses cristallqraphiques e t chimiques en microscopic dlectronique en transmission e t a m mesures Glectriques

in

s i t u . Les chutes

de

p t e n t i e l aux joints ont a i n s i dt6 relides directement 5 l a structure e t c o w s i t i o n inter- granulaire. I1 apparaet que

p u r

l e s f e r r i t e s de Mn-Zn l a r 6 s i s t i v i t 6 e s t surtout contr616e par l e mode de s-%ation du calcim. aux joints.

Abstract : The e l e c t r i c a l properties of mixed oxides have been determined by local voltage drop off m e a s u r ~ n t s with tungsten microelectrodes and correlated with crystallographical and chemical analyses

i n

transmission electron m i c r o s c o ~ a s well a s in-situ e l e c t r i c a l experiments. Fran these

data

it was possible t o directly r e l a t e the grain boundary v o l b g e drop off

to

i t s structure and composition, It was shown t h a t i n

the

case of Mn-Zn f e r r i t e s the intergranular r e s i s t i v i t y is mainly controlled by the mode of segregation of calcium a t grain boundaries.

The e l e c t r i c a l properties of p l y c r y s t a l l i n e oxides depend strongly on the interqranular structure. For instance, the specific e l e c t r i c a l properties of grain boundaries i n Mn-Zn f e r r i t e s explain why

this

material

is s t i l l

one of the best choices for high frequency (kHz) technology. The grain boundary being highly resis- t i v e compared

to

the medium range r e s i s t i v i t y (a few

R-cm)

of the grain, the eddy currents accross the ain boundaries a r e kept negligible. Magnetic losses, which a r e praportional

to to%,

w i l l remain very lcw and the material w i l l be highly suit- able for high frequency applications /1/. Proofs for Ca segregation a t grain

b u n -

daries (G.B.) have long

been

given f r m chemical selective dissolution and from 4 5 ~ a autoradiography /2,11/. Electrical data have been obtained a t G.B. while seve- r a l observations of the interqranular microstructure have been reported separately /3/,/4/,/5/,/6/. Up

t o

now, the very few correlated data t o be published have been mainly on p l y c r y s t a l l i n e silicon /7/ and zinc oxide /8/ and they did not correspond t o any systematic d i r e c t procedure but t o a few specific indirect correlations. This paper describes a f i r s t attempt

t o

directly r e l a t e structure and e l e c t r i c a l proper- t i e s of G.B. i n a mixed oxide, i.e. Mn-Zn ferrites-We

shcw

t h a t such

a

material i s particularly appropriate f o r t h i s goal and t h a t the results can be successfully applied

to

other semiconductor oxides.

The e q r i m e n t s have been performed on s o f t Mn-Zn f e r r i t e s with a basic

mlar

composition close

to

52 % Fe2O3, 30 % M, 17 % ZnO

+

1 % additives. Hcwever the samples d i f f e r by the nature and proportion of additives a s

w11

a s by the t h m mechanical treatments (temperature, atmosphere, pressing). Microstructural obsenr- ations and chemical analyses have been performed on a J W L 100

CX

=-STEM electron mi~?33sCOpS €quipped with

an

EDAX X-ray selective energy analyzer. Voltage d m Off

a t grain boundaries has been measured locally by the microelectrodes technique. The samples are submitted

to

a D.c. vo1taqebhil.e being observed by optical microscopy.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986148

(3)

JOURNAL

DE

PHYSIQUE

The tungsten electrodes a r e prepared by electrcchenical thinning and are micro&- pulated with a pneumatic system (Fig. 1) /9/.

Fig. 1 : Schemtic mounting used for local AV measurements

FESULTS

lo) Intergranular microstructure

A l l samples a r e characterized by vitreous phases (V.P.) a t t r i p l e qrain junctions (Fig. 2a)*. The glassy nature of these V.P. can be revealed by typical diffuse halo i n m i c d f f r a c t i o n o r by electron induced e f f e c t s which lead

to

oxygen bubbles formation limited

to the

V.P.

/lo/.

The intergranular microstructure of

the

samples d i f f e r s mainly by the s i z e of the V.P. a s well a s by the p s s i b l e existence of a vitreous interfacial film (V.1 .F. )

,

^{(Fig. 3)}

,

a t grain boundaries,

its

extent and distribution. The determination of the distribution of V.P. Along grain boundaries requires a very careful observation of the sanples i n order t o make sure t h a t each G.B. has been seen end on. It

is to be

noticed t h a t facets

are

observed m t only a t G.B.'s (Fig. 4a) where they probably minimize the energy for a given orientation, but also a t glass-crystalline interfaces between the tetrahedra configuration

i n

the V.P. close

to

the interface and the crystalline structure nearby (Fig. 4b).

X-ray microanalysis i n STEM (Fig. 5a) does not show up Ca o r S i within the grain whereas these elements a r e clearly detected i n the glassy phase (Fig. 5b).

mreover only calcium appears when G.B.'s are deprived of glassy phases (Fig. 5c).

2 ) Electrical Easurements

Fig. 6a shows the voltage profile obtained by positioning the

two

electrodes across a G.B. and scanning one of them away. By e x t r a p l a t i o n

to

d = 0 the exact G.B. voltage drop off can

be

deduced. I f one

assums

a 1

m

thick G.B. a practical r e s i s t i v i t y ~G.B. can be determined which ranges from 103

up to

105

a-cm.

The intergranular r e s i s t i v i t y

is

usually < 1 S2-an. Fig. 6b shows t h a t AV

i s

proportional

to

the n-r of grain boundaries traversed i.e. inversely proportional

to

the qrain size.

This rrethcd gives good quantitative r e s u l t s but they can hardly be referred

to

a typical type of microstructure since thinning

the

tested area f o r electron microscop observation i n order

to

observe and analyse the SJIE interfaces i s an arduous task with no

quarantee

of success.

I n order t o circumvent such a difficulty

we

have started in-situ electricdl expsrirrents i n the electron microscope which enable

us to

directly correlate the microstructure and the voltage drop off a t grain boundaries. The r e s u l t s are i n good agreement with the previous

data

and a l l m one

to

begin classification of intergranular microstructures according

to

t h e i r e l e c t r i c a l behaviour a s shmm i n

the

discussion.

t Some secondary crystalline phases can also be found a t t r i p l e grain junctions a s sham i n Fig. 2b.

(4)

Fig. 2a : Vitreous phase (v.P.) a t a triple-grain junction characterized

by

a halo in microdif

f

raction

2b : Secondary crystalline phase a t a t r i p l e jmction characterized

by

-

electron diffraction contrast

Fig. 3 : Vitreous intergranular film (V.I.F.)

Fig. 4 : Faceted interfaces

i n

f e r r i t e a)

on

a typical grain

boundary

b) a t crystal-glass interface.

(5)

JOURNAL

DE PHYSIQUE

i l - s u n - a 5 ~ 9 . 3 5 ~ 9E D R X R E R D Y

RRTE 3 C P S T I n E SBLSEE

B e - * # K E Y 2BEYCCH PRST 6 8 L S E C A F302RR)i B

F 5 1 9 9 ME8 R FS' 1 2

1% 2 ta ⁴

1

CURSOR < K E V > = 1 3 . 7 8 8 EDflX

l i - J U N - 8 3 1 9 ' 2 0 11 EDRX RERbY R A T E S C p 5 T I N E 9BL,SEC I @ - + B K E V 2 8 B V / C H P R S T 6 8 L S t C R t 3 0 l E R 4 0 B

F S - 2 6 MEM F S = 1 2

C 2

I

1

CURSOR < K E Y > - 0 3 . 7 8 1 EDRX

i % - J U h - I S 1 9 . 2 4 iS EDRX RERDY RRTE: 1 C P 9 T I U E ' 9 0 L S E C 0 0 - 4 0 K E V . Z ~ ~ V / C H P R S T 6 8 L S E C R . F I B P 6 k b b - -- B

F S - i40 MEW. R F S = 1 2

6

1

CURSOR < K E Y ) - 8 3 . 6 6 0 EDnX

5b 5c

Fiq.

5 :

X-ray selective energy analysis s p c t r a a ) within the grain

b) on a vitreous intergranular film

(T7. I.F. )

c) on a glass-free grain boundary.

Fig.

6 :

Voltaqe drop off

AV

k a s u r e d

w i t h

tunqsten micrcelectrodes)versus electrode spacing

d

a)

across one grain boundary

b) within two areas

_Aand B

of the

sane

samnle

where

the grain s i z e

is

respectively

@ = 3 and @ = 7

w

(6)

DISCUSSION

From electron microscopy observations and microanalyses, three main types of microstructure have been found :

(1) Ca-free

grain

boundaries (2) Ca-segregated qrain boundaries

( 3 ) Grain boundaries coated with a vitreous film (V.1.F'. )

By combining voltage drop off

measurerrents

by the rnicrcelectrodes technique with i n s i t u e x p e r i ~ n t s ,

it

was possible t o a t t r i b u t e a typical value AV for each of these cases unambiguously. I t was found thaet the G.B. r e s i s t i v i t y increases

in

the following order 1 + 3 -t 2.

According t o these results one may i n f e r t h a t

Ca

plays a major role i n creating e l e c t r i c a l barriersat the grain boundaries. ~ a 2 + segreqation a t G.B. leads t o the counter-migration of ~ etoward the qrain, i n order t o preserve ch- ~ + neu- t r a l i t y a t the interface /11/. Thus the electron exchanqe between ~ e 3 + and Fe2+ on octahedral

sites

cannot

occur

and the conductivity viiich essentially corresponds i n f e r r i t e s t o electron transport

w i l l

be

very law. ^V.I.F.

creates also

an

electri- c a l barrier but smaller than i n case 2 owing probably t o the presence of ~ e

in

~ + appreciable mounts ( = 1 %) i n the glassy phase.

Such a mechanism is applicable t o other charged cations searegating a t grain boundaries

and to

other oxide semiconductors such a s ZnO o r NiO.

CONCLUSICPJ

The corrbination of voltage drop off masurem?nts a t grain boundaries with observation by transmission electron microscopy a s well a s i n s i t u local chemical and e l e c t r i c a l signals enables

us to

correlate the r e s i s t i v i t y of the qrain bornday t o i t s microstructure. It apFears that i n order

to lower

maqnetic losses i n those materials it

is

necessary

t o

control the nature and cornpsition

of

the interfaces as well a s t h e i r macroscopical distribution. Segregation o r co-seqreqation a t i n t e r - faces seems

to

be the phenomenon influencing e l e c t r i c a l p r o p r t i e s and special- l y the eddy current intensity a t grain boundaries.

It i s

to

be emphasized t h a t the described mthcdology can be

used

t o eluci- date the conductivity mechanism for various polycrystalline simple o r mixed oxides.

/1/ WESS E., Int. Conf. on Ferrites, Japan (1970) 187

/2/ GUIUAUD C., PAULUS M. and VUI'IER R., C.R. Acad. Sci. Fr. 23(242) : 2712- 2715 (1956)

/3/ CHIAhG Y., KINGZFU W.D., Adv. Ceramics

6

(1983) 303

/4/ FWNKEN P.E.C., STACY W .T.

,

J. Am. Ceram. Scc.

63

⁽¹⁹⁸⁰⁾

,

³¹⁵

/5/ MISHRA R.K., E.K., THOMAS G., M i i t . Science Res.

2

(1981) 199

/ 6 / SUM)AHL R.C., WATE J.B.B., HOLWS R.J., PASS C.E., Adv. Ceramics

1

(1981) 502 /7/ MAURICE J.L., LAW J.Y., J. Phys.

2

(1982) C1

/8/ EINZINGER R. Grain boundaries

i n

semiconductors (1982) 343 /9/ HAMELIN A. thesis Orsay (L972)

/lo/

L A W J.Y., WESTMACOTT K.H., AMWlRA M.C., J. Phys.,

43

⁽¹⁹⁸²⁾^C9^123-126

/11/ PAULUS M., M a t . Science Res.,

3

(1966) 31