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INTERGRANULAR PHASES IN ELECTROCERAMICS
R. Metz, M. Brieu, R. Legros, A. Rousset
To cite this version:
R. Metz, M. Brieu, R. Legros, A. Rousset. INTERGRANULAR PHASES IN ELECTROCERAMICS.
Journal de Physique Colloques, 1990, 51 (C1), pp.C1-1003-C1-1008. �10.1051/jphyscol:19901156�.
�jpa-00230252�
COLLOQUE DE PHYSIQUE
Colloque Cl, suppl6ment au n o l , Tome 51, janvier 1990
INTERGRANULAR PHASES IN ELECTROCERAMICS
R. METZ"), M. BRIEU, R. LEGROS and A. ROUSSET
Laboratoire de Chimie des Materiaux Inorqaniques, CNRS U R A 1311,
Universite Paul Sabatier, 118 Route de Narbonne, F-31062 Toulouse Cedex, France
Resume - La stabilisation des proprietes electriques des thermistances i base de manganites de metaux de transition a et6 obtenue en faisant precipiter des phases isolantes (NiO, CuMnO,, BaMnO ) a cat& de la phase spinelle. L'etude par microscopic electronique conf irme que je baryum n'entre pas dans les grains du spinelle, ces derniers sont enrobes par les phases isolantes. Ces phases isolantes constituent des barrigres de diffusion pouvant jouer un r6le important dans les phenomenes de veillissement.
Abstract - The stability of the resistivity of N.T.C. thermistors is improved by the presence of precipitated phases in the vicinity of the spinel phase. The electron microscopy study allows one to make sure that the barium does not enter the spinel spinel structure but rather the grains of spinel phase are coated with insulating phase. It can be suggested that the insulating phase could built-up diffusion barriers.
INTRODUCTION
Transition metal manganite ceramics are used in thermally sensitive resistors (N.T.C.
thermistors). In these passive components the electrical conductivity results from electron transfer (hopping) between Mn3' and Mn4' ions / l / . Low resistivity (less than 100 n.cm) can be achieved by substituting copper ions to manganese ions /1,2/.
However, copper or mixed nickel-copper manganites show unstable electrical properties under thermal constraint that can hamper their practical use. The electrical stability of these ceramics depends on the cooling procedure which follows the sintering process : for instance air-quenched samples show better stability than slowly cooled samples.
Furthermore, the electrical stability is even improved when starting phases are doped with barium ; this has been related to the presence of precipitated phases such as Ba Mn 0, /3,4/.
Besides the importance of such results from an industrial point of view, a better knowledge of the mechanisms of stabilization of the electrical properties requires precise identification and location of the precipitated phases in the ceramics. This communication reports on electron microscopy and energy dispersive X-ray radiation (EDAX) studies of nickel-copper manganites with general formula :
Mn,,,,Nio,,,Cuo,4,0, doped, or not, with barium.
(1) Present address : L.0.E.-C.N.R.S., 29, rue Jeanne-Marvig,
B.P. 4347, 31055-TOULOUSE-CEDEX.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901156
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EXPERIMENTAL : preparation and characterization
The nickel copper manganites have been prepared by the oxalate precursor method / barium ions were introduced during the oxalate coprecipitation. Details of the si procedure have been described previously 151.
Three different batches were studied and characterized by X-ray diffraction analysis i - Slowlv cooled ceramics : X-ray diffraction diagrams indicate a single spinel pha stability of electrical properties is characterized by a relative resistance drift o 12 % after 1 000 hours at 125°C.
i i - Quenched ceramics : Ni 0 and Cu Mn 0, phases appear along with the spinel phas electrical stability is improved : about 7 % resistance drift.
i i i
-
Ba-dooed Quenched ceramics : The Ba Mn 0, phase is found in addition to the NiCu Mn 0, phases along with the predominant spinel phase. The electrical stabi strongly improved : about 2 % resistance drift.
RESULTS
1") Electron microsco~v
1-1. Scanning electron microscopy
Scanning eiectron microscopy is well suitable as it makes it possible to observe gra grain-boundaries without chemical etching or mechanical polishing. Microstr differences depending on the cooling procedure have been observed.
Slowlv cooled ceramics
-
They are made of 20 pm plateled-like grains with shape (figure l-if. The grain size distribution is narrow and no precipitated p observed.Quenched ceramics - Additional phases are found to be precipitated surfaces. These phases yield more secondary electrons than the matrix and thus ap brighter phases an the micrographs (figure l-ii). Their shape is irregular and t preferentialy located between grains but surround them only partially. The ceramic p size is slightly decreased ; about 1 5 pm an average, but the grain size distribution remains narrow. A weak porosity between grains is observed.
Figure 1 : Comparison between microstructure of the three ceramics :
i - Slowly cooled ceramics i i
-
Quenched ceramicsiij - Ba-doped quenched ceramics
Ba-doped Quenched ceramics
-
The grains look more spherical ; the mean size is about 7 pm i.e. three times smaller than for the single-phased ceramics. A significant amount of precipitated phases is observed on the free surfaces (figure l-i i i). The volumic fraction of segregated phases i s very large, about fourty percent of the total volume. With better resolution (figure 2-i) two precipitated phases should be distinguish : spherical noduls and pourly defined forms.i i i
A Zone
..- -
\-
Figure 2 : The same image obtained on a rough surface of Ba-doped ceramic by :
i - secondary electrons ;
i i - backscattered electrons (inverse image is presented) ;
i i i - EDAX diagrams from the three A , B and C zones defined by electron microscopy.
Cl-1006 COLLOQUE DE PHYSIQUE
1-2. Backscattered electrons
---
Analysis of images obtai'ned with backscattered electrons a1 low to dist i n g ~ barium, a heavy element that easily releases electrons, from lighter elements as m;
nickel and copper.
Figure 2 compares images obtained by secondary electrons (figure i backscattered electrons (figure 2-i i ) on a rough surf ace of the barium-doped ceramic easy comparison inverse backscattered electron micrograph is presented).
One can distinguish tree zones noted A, B and C from their different colours and The spherical noduls (A) with black colour (in inverse image of backscattered elc are likely to be constituted of a barium-rich phase.
These tree phases are even more distinguishable from each others on tk section of the ceramic after polishing (figure 3, direct image). The white c particles ( A zone) is a barium rich phase. The dark grey phase with large equiaxed (B zone) the more important can finally be attributed to the spinel phase. Final phase with light grey colour and pourly defined shapes (C zone) can be attributed t and Cu Mn 0, oxides, the presence of which has been revealed by powder X-ray diff analysis.
It is worthy to note that the barium-rich phase retains an almost sp morphology for several pm from the surface but that in the bulk this form progre turns columnar. The columns can reach a length of a few tens of pm (figure 3). T
size and shape of the precipitated phases in the bulk of the ceramic lead to a coating of the grains of the spinel phase.
Figure 3 : Image obtained by backscattered electrons on the cross section of the Ba-doped quenched ceramic after polishing.
2") - EDAX study
To support the above interpretation of the electron microscopy photographs, local elemental analysis was performed using energy dispersive absorption X-ray analysis (EDAX).
It is to be reminded that EDAX is a semi-quantitative method for elementary surface analysis ; the volume of the probe is of the order of magnitude of one pm3.
The relatively large volume of the grains of the B zone (= 10 pm3) is quite suitable for this kind of analysis ; on the contrary the phases precipitated at the grain boundaries display much smaller dimensions (A zone) or fuzzy limits (C zone). The A and C zones are thus more difficult to analyse individually.
Results obtained for each of the A, B and C zones are listed in table I and pictured in figure 3-iii. Almost all the Ba is located in the A zone as expected from the interpretat ion of the backscattered electron photographs. From the X-ray powder, analysis, the barium is mainly present as Ba Mn 0, ; in the A zone this phase is mixed with a spinel phase of formula Mn,,, Ni,,, Cu,
,
0, very close to that of the studied spinel phase i.e.Mn
,,,,
Ni,,,, Cu,,,, 0,. The analysis of the B zone, which is expected to be the pure spinel phase, yields the formula Mn,,, Ni,,,
Cu,,,
0, very close to the previous one within the limits of accuracy of the method. Finally the C zone features Ni/Mn and Cu/Mn ratios much higher than expected for the pure spinel phase ; this is in agreement with the presence of Ni 0 and Cu Mn 0, as indicated by X-ray powder analysis.Table I - EOAX weight percentage of elements for each of the A, B and C zones Elements Starting Ba-doped A zone B zone
spine1 spine1
C zone
DISCUSSION
The above electron microscopy study makes sure that the barium does not enter the spinel structure but rather surrounds the grains. The large amount of precipitated phases NiO/CuMnO, and BaMnO, along with the high electrical stability of the quenched samples suggest a correlation between such microstructure and the conduction mechanisms.
(i) Potential barriers are generated by precipitated phases at the grain boundaries.
It is to be reminded that, in these manganite-based ceramics, the electrical conductivity relies on a hopping effect between Mn3' and Mn4' ions. It can be assumed that an increasing amount of insulating phases precipitated at the grain boundaries leads to going smoothly from a grain-governed hopping mechanism (chemical composition, ionic distribution in the spine1 network) to a grain boundaries-governed conduction mechanism giving rise to potential barrier set-up. By analogy to the ZnO-based varistances where the conduction mechanism is governed by the potential barrier formed by the contact of two grains, the I(V) curve should be non-linear. To check this assumption comparative studies of the electrical conductivity of monophased and polyphased ceramics have been carried out (5). However the so-called "Varistor effect" could not be detected. It appears that for both the single- and multiphased ceramics the conduction mechanism can be explained by the hopping model (1).
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( i i ) The precipitated phases at the grain boundaries can give rise to intergrain diffu!
barriers.
The instability of the electrical properties under thermal constraint ma) attributed to the migration of ions (Mn3', Mn", Cu') from one grain to another, resull in variations in the number of couples of carriers Mn3+/Mn4+. It can be suggested that insulating phases could build up diffusion barriers which could hamper ion diffusion. ! barriers could explain the improvement of the electrical stability.
A deeper knowledge is to be sought for through further experimer investigations involving, along with microstructure characterization, a study of cht carriers mobility.
CONCLUSION
The stability of the resistivity of N.T.C. thermistors is improved by the presenc precipitated phases in the vicinity of the spinel phase, but simultaneously an increase this resistivity is observed. The above electron microscopy study allows one to make that the barium does not enter the spinel structure but rather surrounds the grains.
cross-section observation indicates, on the one hand, a modif icatin of the morphologj the precipitated phases (spherical noduls close to the surface and columnar in the bu and on the other hand, that the grains of the spinel phase are coated with insula phases (Ni 0, Cu Mn 0, and Ba Mn 0,). In a first approach, it can be suggested that insulating phases could build-up 'diffusion barriers thus explaining the raise of resistivity and the better electrical stability insofar as it is assumed that agein connected with ionic or electronic diffusion phenomena.
The above study proves, once again, the industrial importance of multiph ceramics by emphasizing the relationship between microstructure and electrical properti REFERENCES
/l/ MACKLEN, E.D., Electrochemical publications limited, (1979)
/2/ CAFFIN, J.P., Thesis Toulouse (1986).
131 CAFFIN, J.P., ROUSSET, A., CARNET, R., and LAGRANGE, A., Nigh Tech. Ceramics, edited by P. Vincenzini.
Elsevier Science Publishers B.V., Amsterdam (1987).
/ 4 / ROUSSET, A., LEGROS, R., CAFFIN, J.P., and LAGRANGE, A., Proceeding, 2" Colloque International sur les Composants Passifs, Paris (1987).
/ 5 / METZ, R., LEGROS, R., ROUSSET, A., CAFFIN, J.P.,
LOUBIERE, A., and BUI AI, Journal of Silicates Industriels, Ceramic Science and Technology (accept6 sous presse).
ACKNOWLEDGEMENTS
The authors are thankful to Or. A. Rocher for providing facilities for the use Cambrigde microscope, and they wish to thank J. Daste and J. P. Bernadou for perform the many necessary X-ray dispersive radiation analysis.
