LOCAL RESISTIVITY AND CRYSTALLOCHEMISTRY OF GRAIN BOUNDARIES IN SEMICONDUCTING CERAMICS

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LOCAL RESISTIVITY AND

CRYSTALLOCHEMISTRY OF GRAIN BOUNDARIES IN SEMICONDUCTING CERAMICS

M. Berger, J. Laval

To cite this version:

M. Berger, J. Laval. LOCAL RESISTIVITY AND CRYSTALLOCHEMISTRY OF GRAIN BOUND-

ARIES IN SEMICONDUCTING CERAMICS. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-

965-C1-970. �10.1051/jphyscol:19901150�. �jpa-00230063�

C o l l o q u e C l , s u p p l 6 m e n t a u n o l , T o m e 51, j a n v i e r 1990

LOCAL RESISTIVITY AND CRYSTALLOCHEMISTRY OF GRAIN BOUNDARIES IN SEMICONDUCTING CERAMICS

M.H. BERGER a n d J.Y. LAVAL

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 , CNRS-ESPCI, l0 R u e V a u q u e l i n , F-75231 Paris C e d e x 05, France

Resume : Une caracterisation locale chimique, geometrique et electrique de la structure intergranulaire dans les cbramiques semiconductrices conduit distinguer deux types de joints : les joints speciaux et les joints genbraux.

Dans les ferrites de MnZn, les joints generaux donnent lieu A une forte segregation de Cat+ accompagnee d'une dbpletion en F e e + , et possedent une resistivite &levee. Les joints de coincidence rbvelent une faible segregation de Cae' et sont moins rbsistifs. On peut bviter la percolation des joints speciaux (12%) et augmenter la hauteur des barrieres Blectriques en ajustant le taux de CaO.

Abstract : A local chemical, geometrical and electrical characterization of semiconducting ceramics allows two types of grain boundaries to be distinguished : general and special boundaries. In MnZn ferrites, general boundaries give rise to a large segregation of Ca2'coupled with a depletion of FeZ' and are highly resistive. Coincidence boundaries do not exhibit a strong segregation and are less resistive. It is possible to maintain the amount of conducting boundaries lower than the percolation rate (12 %) and to increase the height of electrical barriers, by adjusting the CaO amount.

Introduction

The electrical properties of polycrystalline MnZn ferrites are controlled by the high intrinsic resistivity of its grain boundaries. This provides the possibility for varying the electrical characteristics of the ceramics by controlling the sintering additives, and thermomechanical treatments. For instance. a small amount of CaO together with a slightly oxidizing atmosphere increases the resistivity of the ceramic by a factor of 50 / l / . The more open structure of boundaries favours Cat' segregation which augments the electrical intragranular resistivity. This leads to semiconducting grains with a high magnetic permeability separated by insulating boundaries. These boundaries act as electrical barriers for eddy currents and result in lower magnetic losses at a medium frequency range. However, the analysis of the intergranular structure of ferrite$ shows a large diversity of boundaries which parallel variations found in their electrical behaviour. It is then necessary, in order to optimize these materials, to define the crystallochemistry of less resistive boundaries. This implies the need to undertake a triple local characterization namely a geometrical, chemical and electrical study of the grain boundary. The variations of the bulk electrical properties are then explained according to the distribution of the different types of boundaries. The way to change this distribution will then be considered.

Experimental

The intergranular structure has been observed on a Jeol 100 CX TEM-STEM. The chemical analyses were carried out by EDX (Tracor) and by EELS (Gatan) on a VG dedicated STEM. The geometry of boundaries has been determined from electron diffraction patterns and with a program which calculates the rotation relating the lattices of the adjacent grains / 2 / . The electrical measurements on boundaries have been carried out with a novel in situ electrical device designed in our laboratory. The measurements were performed in TEM on a thin foil by applying a current of a few mAmps. A 0.1 pm tungsten microelectrode was successively positioned across the boundary with a precision of 0.1 pm, fig.1, to obtain the voltage drop at the boundary /3/. We have chosen areas transparent to the electron beam but far enough from the edge to ensure that the current lines remain parallel to the movement of the electrode.

The samples were sintered from ,a mixture of 53.5 % Fe,O,, 25.8 % MnO, 20.7 $ ZnO

+

0.012 % SiO, + X % CaO where X = 0.016, 0.2, 0.38 % for the samples A, B, C respectively. These materials are dense and mainly single phase.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901150

Cl-966 COLLOQUE DE PHYSIQUE

Glasslcrystal interface

A residue of the liquid phase formed during the sintering may subsist at tri' junctions in the form of triangular vitreous pockets. They correspond to silica containing mostly Ca and Mn. The crystal interfaces with the glassy phase are maii planar or facetted. The most frequent planes are 11111 and 11101 /4/. During sinterii planes of high atomic density have developed in the liquid phase.

At high temperature, this liquid phase can wet the boundaries but it usua.

withdraws on cooling as vitreous films are rarely observed in these materials. Films :

mostly found when one of the interfacial plane is tlll). One the other h;

crystal/crystal interfaces admitting one {110} plane, which are frequently observed, i

not wetted by the glassy phase. The charge accommodation between an interfacial plan<

which contain either cations or anions, with a plane which contain both types of it should lead to a higher energy of the boundary which can be minimized by the conservat:

of the vitreous film during cooling. This explains why the {l111 planes which h;

alternated charges are frequently observed adjacent to vitreous films contrary to 1

t1101 planes with mixed charges.

Crystal/crystal interfaces

The grain boundary structure depends upon the misorientation of the grains and 1 orientation of the interface plane. The study of many ceramic compounds like SrTiO, /!

YBa,Cu,O,-, /6/, VC / l / , SiAlON / 8 / , A1,0, /9/ has shown that a geometric characterization based on the coincidence site lattice concept permits one to explain t special physical and chemical properties of some boundaries by the existence of a comn order at the interface. Therefore we will characterize the grain boundaries by the index of coincidence between lattices /10/ (where X is inversely proportional to t density of coincidence sites) and by the orientation of the interfaces separating t grains.

Coincidence Boundaries

Three types of interfaces are found : planar, facetted and curved. The plar interfaces correspond mostly to dense planes of the crystal lattice (11111,, 11101 13111,) or of the coincidence lattice. During the sintering, the curved interfaces c rearranged into facets. One of them at least is a dense plane, fig.2. This facetting c occur at different scales. Fig. 3 shows facets with lengths varying from 1.5 pm down 20 nm. These data suggest that high coincidence boundaries with curved interfaces may fact be microfacetted.

General boundaries

The general boundaries have mostly random interfaces. However, a few of th correspond to {11111 or {1101, planes.

Intergranular segregation

Calcium is not detected inside the grains, but it segregates towards the boundari with a coefficient which depends on the interface geometry. General boundaries enclose high amount of Ca (fig. 4a) which can reach 15 at %. Calcium is only slightly detected high coincidence boundaries, whatever the interface is, fig. Qb, and in coinciden boundaries where the interface is a mirror plane (ex X 13b : 1510},,,), or is close simple planes (ex X 2 15 : {110),, (1111,). The microfacetting of high coinciden boundaries with random interface plane explains why only a very small amount of calci is found in these boundaries.

Relation between amounts of Ca2' segregation and FeL' at qrain boundaries

Ue have evidenced that the Fe2'IFe3+ ratio between grains and boundaries vari according to the degree of Cae' segregation. On the EELS spectra a shift of the Fe and L, peaks towards higher energy and a reduction of the ratio of the L, and L, pe, intensity (I,,/I,,) are related to a diminution of Fee'/Fe3' /11/. A shift of 3 e V and decrease in I,,/I,, from 3.8 (Fig. 5c) to 2.9 (Fig. 5b) is found when the electron be, is displaced from the grain to the boundary containing Ca2'. On the other hand, I

modifications were found at the boundary where no calcium is detected (fig. 5e-f). Th.

hishlights the combined effects of Cae+ segregation and Fe2' depletion at boundaries.

extinction contours.

Fig. 2 : Facetted GBs a) C = 11 : 50°.5 [Oil] b) 15O <Ill>

Fig. 3 : Facets of different lengths along the same boundary.

b) is a magnification of the white frame of a).

Fig. 4 : X-ray microanalyses on a general GB a) and on X3

till),,,

Cl-968 COLLOQUE DE PHYSIQUE

Fig. 5 : Coupled X-ray and EELS analyses. On a Ca containing boundary a), IL,/IL, is lower b) than within the adjacent grain c). When Ca is not detected d) IL,/IL shows no variation at the GB e ) and within the adjacent grain f )

Fig. 6 : In situ electrical measurements on a special GB (AV = 5 mV) (a) and two genera boundaries (AV = 45 and 15 mV) (b and c).

S

ample A sample B sample C

Fig. 8 : Crystallographical maps from samples A , B and C.

Fig. 9 : Percentage of grain boundaries with a given potential drop.

a) sample A, b) sample B.

Cl-970 COLLOQUE DE PHYSIQUE Interaranular resistivity

Fig. 6 gives representative examples of consecutive measurements of voltage drc across different boundaries on sample B. The 5 mv drop is associated to a coincider boundary, whereas the other values (15, 45 mV) correspond to general boundaries; 1 variation of the barrier heights for these general boundaries is probably related to 1 different types of interfaces : dense planes or random interfaces. Here,the resistivj of the general boundaries is 2 to 10 times higher than that for special boundaries.

Discussion

Vitreous pockets at triple junctions usually do not wet the boundaries. This me:

that less energy is generally required to form a solid/solid interface than 1 glass/solid interfaces. However, this rule is not valid for interfaces with a pl:

containing only one type of ions (anion or cation). The resistive behaviour of boundari is not strictly related to the existence of an intergranular phase but to the structl of the boundaries. The intergranular structure, which is not as dense as the grz favours the segregation of calcium. This induces the Fet' migration towards grains whj reduces electron transfer between Fee' and Fe3'.

The geometrical, chemical and electrical characterization of GBs reveals that t presence of common sites at the interface favours a denser structure which lowers t tendency for segregation. In such case the Fee+ concentration remains practical constant. Such GBs do not exhibit an electric barrier high enough to retain eddy currc inside grains. Thus it is mandatory to avoid the percolation of such boundaries acrc the sample in order to maintain a high bulk resistivity. Moreover, it is necessary limit local heating effects due to eddy current flux across several grains. Percolatj concepts applied to model structures, fig. 7 , allows one to predict different variatior parameters. Thus, from theoretical calculations /12/, /13/, /14/ the variation of t

size of grain clusters linked by conductive boundaries prior to the percolation can found as well as the evolution of the conductivity after percolation versus the p rat of conductive boundaries. For a dense 3-dimensional structure where a grain admits neighbours the percolation starts for p = p .= 0,12. These calculations show that t percolation of less resistive boundaries occurs for a very small amount of sc boundaries and the cluster size diverges when nearing p,. The conductivity increaa slowly beyond p,.

Therefore, in order to obtain low loss materials, one needs to control the rate formation of special boundaries and to keep it under 15%. Moreover the amount coincidence boundaries can be modified by changing the calcium content. Different sets boundaries were systematically analyzed on samples A , B, C with a calcium content 0.016, 0.2, and 0.38 wt % respectively. The corresponding rate of special boundaries 15, 10 and 5 %, fig. 8. Thus the effect of calcium is two-fold since it reduces the ra of coincidence boundaries and at the same time it increases the height of the electric barriers at general grain boundaries (fig.9).

/l/ M. Paulus, Mat. Science Research

2

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We are grateful to J. Chazelas (CSF-Thornson) for his friendly contribution concerni X-ray microanalysis and electron energy loss spectroscopy on the V. G. dedicated STEM.