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SCANNING AUGER MICROPROBE INVESTIGATION OF GRAIN BOUNDARY SEGREGATION IN SOME ELECTROCERAMICS
J. Tanaka, H. Haneda, S. Hishita, F. Okamura, S. Shirasaki
To cite this version:
J. Tanaka, H. Haneda, S. Hishita, F. Okamura, S. Shirasaki. SCANNING AUGER MICROPROBE INVESTIGATION OF GRAIN BOUNDARY SEGREGATION IN SOME ELECTROCERAMICS.
Journal de Physique Colloques, 1990, 51 (C1), pp.C1-1055-C1-1060. �10.1051/jphyscol:19901164�.
�jpa-00230270�
SCANNING AUGER MICROPROBE INVESTIGATION OF GRAIN BOUNDARY SEGREGATION IN SOME ELECTROCERAMICS
J. TANAKA, H. HANEDA, S. HISHITA, F.P. OKAMURA and S. SHIRASAKI National Institute for Research in Inorganic ~aterials, Namiki 1-1, Tsukuba-shi, Ibaraki 305, Japan
ABSTRACT Spatial distributions of elements near the grain boundaries of both ZnO varistors and NiO ceramics were inves- tigated by a scanning Auger microprobe. In ZnO varistors doped with Bi and CO, the content of the additive Bi segre- gatcd at every grain boundaries measured in the range from 2 to 10 nm in thickness. In many segregation layers, a total molar fraction of catlons. Zn and Bi, was higher than that of oxygen, suggesting that the segregated Bi was in a state with a deficit of oxygen. In NiO ceramics doped with Li, the content of the additive Li segregated at grain boundaries in the range from 10 to 20 nm in thickness. In this material, contrary to the cases of the varistor, the segregated Li was in a state with a surplus of oxygen.
1
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INTRODUCTIONA zinc oxide varistor doped with B1 is an n-type semiconductor, which exhlbits strongly nonlinear current-voltage characteristics. The origin responsible for this nonlinear characteristics are electronic barriers formed at grain boundaries. It has been known that a small amount of the additive ingredient. B1203. has a most remarkable influence on forming the barriers. In order to elucidate the roles of the ingredient in this material, the spatial distribution of elements near grain boundaries has been investigated up to datell2.3).
A nickel oxide doped with Li is a typical p-type s e m i c o n d ~ c t o r ~ * ~ ) . A most important difference between p- and n-type semiconductors is that the p-type semlconductor becomes conductive upon oxidation, whereas the n - type becomes conductive upon reduction. We measured scanning Auger spec- troscopies on cleft surfaces of ZnO doped with Bi and NiO doped with Li.
It was found that the additive ingredient Bi in ZnO segregated at the grain boundary in the range from 2 to 10 nm in thickness which was in a state with a deficit of oxygen, while the additive ingredient Li in NiO segregat- ed at the grain boundary in the range from 10 to 20 nm in thickness which was in a state with a surplus of oxygen.
This paper reports about the spatial distribution of elements near grain boundaries in ZnO-RipO3, Zn0-Bi203-COO and pure ZnO systems and Li20-NI0 systems.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901164
Cl-1056 COLLOQUE DE PHYSIQUE
2.1 - Sample Preparation
ZnO varistors containing Bi2O3 (denoted as ZB) were synthesized b coprecipitation method. A mixed sol.ution of ZnC12(99m%) and BiC13(
was added to a large excess of 1N diethylamine to coprccipitate Zn ant as hydroxides. The resultant powder was calcined at 500°C for 2 ho pressed into disks under a pressure of 200MPa and sintered at 1250°C fl hour in the air. Pure ZnO samples (pure-Z) and ZnO varistors contai Bi(lm%) and Co(lm%) (ZBC) were also prepared by the similar method.
specimens obtained were 8mm in diameter and 1.5mm in thickness. The g size was 20 to 40 pm.
NiO ceramics doped with Li (NL) were synthesized from nickel carbo.
mixed with ethanol solution containing 0.2 mol% of lithium nitrate.
mixture was dried and calcined, and the obtained powder was fired at 12 for 2 hours to be sintered to disks with the same dimensions as ZnO.
grain size was 10 to 20 pm.
All the samples used here were rapidly cooled after the sintering.
2.2 - Measurements and Results
Current-voltage characteristics of ZB and ZBC were measured at temperature by a two-probe method using indium-gallium alloy electrol The results are shown in Fig.1. A nonlinear electrical behavior observed for ZR and ZBC. Values of a(=logI/logV) were a26.5 and for ZB and ZBC, respectively.
Impedance vs. frequency dcpendences of NL were measured at room peraturc between 1IIz and lOOkHz using a frequency responsc analyzer(1 5050 of NF Co. Ltd.). The result is shown in Fig.2.
Spatial distributions of elements were measured by a scanning AI microprobe(SAM; JAMP-30 of JEOL Co. Ltd.). The exciting voltage of electron beam was 5kV and the beam current 7.5nA. The beam diameter about 0.5pm. which was small enough in comparison with the grain S
Depth profiles were obtained by successive measurements of SAM after in mittent sputtering the sample surface with Ar ions. The acceleral voltage of the ion beam was 2kV and the etching area of about 0.04mm2 bc much wider than the grain size. Under these conditions, the etching I
was about 3nm/min for Si02.
Figure 3 shows a derivative Auger spectrum of a cleft grain bounc of ZBC: the spectrum of ZB was almost the same as ZBC. Figures 4 ar show the depth profiles at the grain boundaries and their vicinities of The ordinate is molar fraction, normalized by relative sensitive facl
(RSF's) which will be given below. The depth profile of pure-Z is il trated in Fig.6.
In Fig.7 and 8 are shown the Auger spectrum of a cleft grain bounc of NL and its depth profile, respectively.
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DISCUSSION3.1 - Grain Boundary in ZnO Ceramics
(1) Auger Spectrum A majority of cleft surfaces were flat without (
continuous step for ZB and ZBC, which implies that the samples were cl along grain boundaries. As can be seen in Fig.3, some intense AL signals were observed on the cleft surface of ZBC, which were respecti~
attributed to Bi. Zn and 0 as shown in 'the figure. A weak signal duc carbon was sometimes observed. The signal grew with the exposure timc the air after cleavage, and disappeared upon the Ar ion sputtering for 10 seconds. The origin of C was considered to be a hydrocarbon or cal dioxide adsorbed on the surface.
In the present work, the SAM measurements were performed on 48 bol aries of seven ZB's and 8 ones of two ZBC's. The Bi was found at el grain boundaries measured.
As seen in Fig.3, the CO ion as an additive ingredient was not fc at any grain boundary. As reported by ~ a t s u o k a ~ ) and Kim and ~ i m , ~ ) CO ions were considered to be dissolved in the grains of ZnO so as enhance the electrical nonlinearity by forming localized levels grains7).
obtained from six grains were 20 for Zn(LMM) and 30 for O(KLL), respective- ly. The HSF of Bi(NVV) was estimated to be 70 from BigOg.
As can be seen from Figs.4 and 5 , the content of Bi decreases with etching time, finally reaching to zero in the graln. Corresponding to this decrease of Bi, the content of Zn gradually increases and becomes constant in the grain, approximately the same value as the content of 0.
The content of segregated Bi varied from boundary to boundary; howev- er, in many cases, the content of Bi at the grain boundary was around 10%.
Kingery et a1.3) reported that the molar ratio of Bi to Zn was about 0.04 at a precipitation-free boundary and about 0.055 at a small precipitation particle. The segregated Bi estimated by SAM seems tc be slightly higher
than that estimated by STEM.
The thickness of the segregation layer was estimated from the etching time; however, the accuracy of the estimation is not high because of sur- face irregularities, impurity knock-in effects and long escape length of the Auger electron of Zn which is about three times larger than that of Bi.
Thus. the thickness estimated by this method is in the range from 2 to 10 nm: about more than 2 nm in the cases of Figs.4 and 5. The thickness estimated by SAM is in substantial agreement with that estimated by STEM ( 5 nm)3)
The sample doped with Bi and CO (ZBC) showed almost the same behaviors as Z B , which suggests that the CO ions have very little effect on the segregation process of other ions.
(3) Role o f Additive As shown in Fig.5, the content of 0 decreases near the grain boundary, being approximately parallel to the content of Zn.
Assuming that the valences of Zn, Bi and 0 ions are + 2 , + 3 and -2, respec- tively, the mo7.ar fraction of 0 is expected to increase so as to compensate the excess plus charges due to the introduction of Bi ions with the valence of 3 + substituting Zn ions with the valence of 2+ at the grain boundaries and their vicinities. However, in the present study, the content of 0 is found to bc a little less than that of the stoichiometric ZnO. This suggests that the segregated Bi ions form as a state in which there is a little deficit of 0 ion in comparison with the 0 content in the stoichio- metric ZnO.
Similar tendencies in the spatial distribution of elements was ob- served also in samples (ZB) fired in a high oxygen pressure of lOOatm at 600°c and 800°C for 2 hours, which had almost the same or slightly small a-value compared to that of the original ZB. On the other hand, as shown in Fig.6, the contents of Zn and 0 were flat being equal to one another in the sample without Bi, pure-Z, corresponding to the fact that there exists no state with a deficit of oxygen at pure boundaries. Therefore, i t is considered that the state with a deficit of oxygen is produced by the segregated Bi
.
Kingery et a13). pointed out that the Bi ions enter Zn0 lattice by substituting the Zn site. As the ionic radius of Bi is far larger than that of Zn, the substitution of Zn by Bi produces the lattice strain near the grain boundary. It is plausible to consider that a new state may have an cffcct to reduce the lattice strain, and this effect can be conversely another origin of this state.
3.2
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Grain Boundary in NiO Ceramics(1) Spatial Distribution
of
Elements The samples were cleft along grain boundaries similarly to the case of ZnO. As can be seen from Fig.7, some intense Auger signals are observed on the cleft surface and attributed to Li. Ni and 0, respectively. The segregation of Li was found at every grain boundaries measured, more than 20 boundaries of three samples.The RSF's of Ni and 0 were estimated from intra-grains. The RSF of Li was estimated by extrapolating RSF's of B and Be relative to atomic number. The estimated RSF of Li was between 15 and 20 and so we adopted 20.
As shown in Fig.8, the contents of Ni and 0 were equal to each other
Cl-1058 COLLOQUE DE PHYSIQUE
at intra-grains far from grain boundaries. However, near the grain bour aries, the content of 0 was larger than that of Ni. From the etchi time, it was estimated that the 0-enriched layer was in the range from to 20 nm in thickness.
At the grain boundary, the Li segregated by about 5 mol%. The thic ness of the Li-segregation layer seems to be less than that of the enriched layer, being about 2/3 of the later thickness.
(2)
of
Additive I As illustrated in Fig.2, the resistance s phase are almost independent of frequency. This suggests that the gr2 boundary has no component electrically capacitive, and thus, the Li-segr gated and 0-enriched layers are conductive.In NiO doped with Li, the Li(l+) ions enter NiO structure by subs1 tuting the Ni sites, while to maintain the charge neutrality, the Ni i c correspondingly change their valences from 2+ to 3 + , i.e. Niii, which 2
the hole carrier of electrical transport .4) Therefore. the segregated causes the increase of the carrier concentration near the grain boundary.
On the other hand, the excess oxygen ions near the grain boundary pr duce nickel vacancies by the reaction
1/202(g)
+ 06
+ vgi.Where, Vfii is the nickel vacancy trapping two holes. From the Ni vaca cies, all or a part of holes are thermally emitted as
Vfii
+
Vki + h. andV&i ? Vki + h' , respectively.
The emitted holes are localized carriers as N i G i .
Thus, both the additive Li and the excess 0 produce the carriers ne the grain boundary for the segregation layer to be more conductive than t intra-grain.
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SUMMARYThe depth profiles of Auger spectra on the grain boundaries in the Z and NiO ceramics were measured. The segregations of the additive ingred ent Bi and Li were found at every grain boundaries measured in respecti cases.
In the case of ZnO, the amount of Zn decreased corresponding to t increase of the amount of Bi near the grain boundary. The amount of was around 10 m% and the thickness of the segregation layer was about 2 10 nm. It was also found that the total amount of cations, Zn and Bi. H
higher than that of anion, 0, in most of the segregation layers. Th fact suggests the possibility that the segregated Bi is in a state with deficit of oxygen.
In the case of NiO, the amount of Ni decreased corresponding to t increase of the amount of L1 at the grain boundary. The amount of Li W
around 5 m% and the thickness of the segregation layer was about 10 to nm. It was found that the total amount of cations, Ni and Li, was le than that of anion. 0, in many segregation layer. This suggests t possibility that the segregation layer is in a state with a surplus oxygen.
It is worth noting that, in comparison with the amount of 0 at t stoichiometric intra-grain, the grain boundary is in a state with le oxygen in the case of ZnO which becomes an n-type semiconductor upon t reduction, whereas the grain boundary is in a state with excess oxygen the case of NiO which becomes a p-type semiconductor upon the oxidation.
REFERENCES
1. D.R.Clarke, J.Appl.Phys., 49(1978)2407.
2. H.Kanai, M.Imai and T.Takahashi, J.Mater.Sci..20(1985)3957.
3. W.D.Kingery, J.B.Vander Sand and T.Mitamura, J.Amer.Ceram.Soc., 62 (1979)221.
4. D.Adler, "Treatise on Solid State Chemistry, Vo1.2 Defects in Solids"
Edited by N.B.Hannay(l975 Plenum)p292.
5. C.M.Osburn and W.R.Vest, J.Phys.Chem.Solids. 32(1971)1343.
6. M.Matsuoka, Advances in Ceramics, vo1.1(1981)290.
7. E.D.Kim and C.H.Kim, J.Appl.Phys.. 58(1985)3231.
- 1 00 1 0 0 V / v o l t
Fig.1 - Current-voltage characteris- Fig.2 - Impedance and Phase vs. fre- tics of ZnO-Bi203 ( 0 ) and ZnO-Bi203- quency of NiO-Li2O ceramics.
COO ( 0 ) varistors.
0 500 1000
K I N E T I C ENERGY ( e V ) 0 50 100 150 2 0 0 250
SPUTTER I NG TIME (5)
F i g . 3
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A u g e r s p e c t r u m o n c l e f t Fig.4-
Depth profile near grain surface of Zn0-Bi203-COO varistor. boundary in ZnO-Big03 varistor.COLLOQUE DE PHYSIQUE
0 50 100 150 200 250 0 50 103 150 200 2'
SPUTTERING TIME ( S ) SPUTTERING TIME (5)
F i g . 5 - D e p t h p r o f i l e n e a r g r a i n F i g . 6 - D e p t h p r o f i l e n e a r g r boundary in ZnO-Bi2Og varistor. boundary in pure Z n O ceramic.
F i g . 7 - A u g e r s p e c t r u m o n c l e f t F i g . 8 - D e p t h p r o f i l e n e a r g r s u r f a c e o f NiO-Li2O c e r a m i c . boundary in NiO-Li2O ceramic.
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