Electrical properties of ZnO stabilized beta″-alumina

Solid State Ionics 14 (1984) 41-44 North-Holland, Amsterdam

ELECTRICAL PROPERTIES OF ZnO STABILIZED BETA"-ALUMINA W. JAKUBOWSKI and S. BIELA Institute o f Physics, Warsaw Technical University, ul. Chodkiewicza 8, 02-524 Warsaw, Poland

Received 9 January 1984 Revised manuscript received 27 March 1984

The electrical conductivity of ZnO doped (1-5 wt%) beta"-alumina ceramic samples has been measured by the complex admittance method. The smallest value of electrical resistivity was in the case of the samples doped with 3 wt% ZnO. The bulk conductivity dependence (in In oT - l I T coordinates) is characterized by two slopes, with a bend occurring at about 250°C. The low temperature activation energy of bulk conductivity is not influenced by the doping level. The high temperature activation energy decreases with increasing ZnO content. The change in the slope of the Arrhenius plot of the beta"-alumina bulk conductivity is in agreement with what should be expected from Wang's model.

1. Introduction It is well known that the beta"-alumina phase obtained as a result of the high temperature solid state reaction in the N a 2 0 - A 1 2 0 3 system is not stable and transforms irreversibly to the beta phase [ 1 ]. The beta"-alumina phase stability could be improved by doping substrates (Na20 and a or 3' A1203) with certain stabilizing agents. Lithium oxide or magnesium oxide or combination of both are frequently used for this purpose [ 2 - 5 ]. High value o f the ionic conductivity and duplex microstructure (small, micrometer size grains and needle-like ~100 #rn grains) are characteristic features of the lithia-stabilized beta"alumina. The MgO-stabilized material has a more homogeneous microstructure than that of Li20 stabilized, but its ionic conductivity is not so high. Doping with combination MgO and Li20 produces good conductive ceramic but many o f these compositions are unstable in liquid sodium. The ionic conductivity of beta"-alumina samples doped with CoO, NiO, CuO and ZnO is also lower than that of the Li20 stabilized isomorph [6]. The beta-alumina phase is stable and thus needs no stabilizing dopants. However, it is a less conductive phase than beta"-alumina. Since both materials are used as electrolytes in reversible electrochemical cells [5 ] a high ionic conductivity and good stability

are of the greatest importance. This is the reason for the investigation of the stabilization of the beta"alumina phase, and of the role the stabilizing agents play in conduction processes. In this respect, the investigation of the influence of zinc as stabilizing dopant on electric conductivity of beta"-alumina seems to be very interesting. Zinc ions could be accommodated, not only in spinel blocks where they substitute for aluminum ions, but also in the conduction planes [7,8]. In the latter case, they can directly interact with mobile sodium ions, modifying the mechanism of charge transport.

2. Experimental Samples o f beta"-alumina were prepared in a typical calcination-sintering ceramic procedure of mechanically prepared mixtures of substrates. The details of the preparation method are given elsewhere [4]. The mixtures contained 9 wt% Na20, 1 - 5 wt% ZnO and AI203powder to balance to 100%. Sintering was performed in two steps: 15 min at 1600°C and 3 h at 1420°C. Rectangular samples about 2 X 3 X 5 mm 3 in size were then cut from sintered discs with a diamond saw. The opposite smaller faces o f the rectangular samples were, after polishing, coated with Pt deposited elec-

W. Jakubowski, S. Biela/ZnO stabilized ~"-alumina

42

trodes. The impedance of the samples was measured on a Tesla BM 507 impedance meter in the frequency range of 5 - 5 × 105 Hz and in the range of 20-500°C. The readings were taken while cooling down the samples from 500°C.

3. Results The values of the bulk and grain boundary conductivities were derived on the base of admittance plots. The dependence of the bulk specific resistivity at 300°C on composition of the samples is presented in fig. 1. As it is seen, the smallest electrical resistivity corresponds to the samples doped with 3 wt% ZnO. The samples with this composition were characterized by a density of 3.19 g/cm 3 , and they contained about 80% beta" phase (the rest was the beta phase). The relative proportion of beta" and beta phases were determined from peak heights of selected reflections on X-ray diffractograms [4]. It should be mentioned that our samples of other compositions than 3 wt% ZnO were less dense and also contained less beta" phase. Nevertheless, it seems that the conclusions drawn from our results still concern the pure beta" phase, since the role of beta phase in electrical charge transport should not be significant both because of its

much higher resistivity and its smaller volume fraction in the samples. The more so that the application of the simple mixing rule to the beta and beta" phase mixture indicated that the low temperature slope of Arrhenius plots is also dependent on phase composition. This is not the case for our samples. The low temperature activation energy is independent on ZnO concentration and has the same value as t\~r the 100% beta" phase sample (Li20 stabilized [6]). We assume that the conclusions derived from our results are related only to the beta" phase. This should be so at least for the conclusions concerning the activation energies. When absolute value of the electrical conductivity is calculated for beta" phase its volume fraction and percent of theoretical density should be taken into account. The absolute value of conductivity will not be discussed in what follows, only the slope of its temperature dependence (or activation energy). The temperature dependences of bulk and grain boundary electrical conductivities (the values of o not corrected for fraction of beta phase) are presented in fig. 2, as Arrhenius plots (ln aT versus 1/T). The bulk conductivity dependence is characterized by two slopes with activation energies 0.27 eV and 0.21

V . j . Egb= 0,67 eV

16

14

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I

I

2

3

4

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5

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%ZnO

1,if-

1,5

I

I

I

3,o

1] /

Fig. 1. T h e dependence of the electrical resistivity (at 300°C) of the ZnO stabilized Na + beta"-alumina on dopant concentration. These crosses and circles represent two sintering processes m a d e under the same conditions.

Fig. 2. The temperature dependence o f the bulk (a) and (b) and grain b o u n d a r y (c) electrical conductivities of the Na + beta"-alumina ceramic samples stabilized with ZnO: (b) 1 wt%, (a) and (c) 3 wt%.

W. Jakubowski, S. Biela/ZnO stabilized f-alumina

43

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Fig. 3. The dependence of the activation energy on ZnO concentration at (a) lower and (b) higher temperatures.

eV in low and high temperature ranges respectively. The activation energy for grain boundary conductivity amounts to 0.67 eV. The low temperature activation energy of the bulk conductivity was constant and did not depend on the ZnO concentration. In the same samples the high temperature activation energy decreased with increasing ZnO content (fig. 3). It is worth to note that the value of high-temperature activation energy extrapolated to zero ZnO concentration was close to that of the low temperature activation energy (fig. 3). It seems to indicate that in pure samples, if such could be obtained, the temperature dependence should be represented on an Arrhenius plot by a single-slope line over the whole temperature range of the measurements. The observed change in the slope of the Arrhenius plot of the beta"-alumina bulk conductivity taking place in temperature range of 2 0 0 - 3 0 0 ° C seems to be due to stabilizing dopants. If dopant ions are not only built into the spinel blocks but also exist in the conduction planes, their removal from the conduction planes by dipping the samples in molten NaNO 3 for enough time should make the temperature dependence of the bulk conductivity a single straight line on the Arrhenius plot over the whole temperature range. This is really the case for our samples heated in molten sodium nitrate with sodium chloride (7 : 1 ratio) during 188 h - one activation energy for all the samples regardless o f the ZnO concentration is observed after such a treatment. The value o f this activation energy is equal to that of low temperature activation energy (fig. 4). Since the washing out of the Zn 2+ ions from the beta"-

-2 -4

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I

I

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1,5

2,0

2,5

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Fig. 4. The temperature dependence of the bulk conductivity for ZnO doped Na÷ beta"-alumina (3 wt%), (a) before and (b) after heating in molten sodium nitrate with sodium chloride: (o) measurements in air, (+) in dried argon, (c) Ag+ beta"-alumina sample.

alumina conduction plane changes two-slope dependence of In oT versus 1/T to the single-slope we assume that the influence of those Zn 2+ ions bound into spinel blocks on the activation energy is much smaller and may be neglected. It is worth noting that silver beta"-alumina is also characterized by a single-slope Arrhenius line for the bulk conductivity. It is possible that, when it is produced from Na beta"-alumina by ion exchange in molten AgNO3, not only sodium but also doping ions from the conduction planes are replaced by silver.

4. Discussion It was reported that, for Li20 stabilized beta"alumina samples, the decrease in the activation energy was found for increasing concentration o f lithia [9]. If we take the results of the work of Bugden and Duncan [9] and make the graph of activation energies as a function o f Li20 concentration, quite good linear dependence (fig. 5) similar to our results for ZnO doping is observed. What is more sur-

W. Jakubowski, S. Biela/ZnO stabilized ~",qlumina

44

I

0,2

L _ _

0,4

I

I

0,6

0,8

_ _

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Fig. 5. The dependence of activation energy on Li20 concentration (results of ref. [9]).

prising is that the extrapolation o f the line to zero L i 2 0 concentration gives an activation energy equal to the activation energy observed for zero ZnO concentration. The influence o f the stabilizing ions on the mechanism o f conductivity exists not only for ZnO but also for L i 2 0 doped samples. In case o f ZnO doped beta"-alumina single crystals Bates et al. [7] have found that Zn 2+ ions enter during crystal growth not only spinel blocks but also the conduction layers (opposite to the MgO stabilized single crystals where all the Mg 2+ ions enter the positions in spinel blocks). After treatment o f samples in molten NaNO 3 they found, from X-ray fluorescence measurements, a decrease in the Zn content. They assume that during the treatment the Zn 2+ ions are removed from the conduction layers. We think that similar situation exists in our samples. The treatment o f ZnO stabilized beta"-alumina ceramic samples in molten NaNO3/NaC1 mixture causes the change in the character o f the temperature dependence o f the conductivity. It becomes one straight line on the In oT versus 1/T plot over the whole temperature range of the measurements. Disappearance of the straight-line segment corresponding to lower value o f the activation energy existing before that treatment (fig. 4) may be explained by the fact that Zn 2÷ ions are removed from the conduction layers. The dependence o f the activation energy on ZnO content (fig. 3) is an additional evidence that the Zn 2+ ions enter the conduction layers while sintering. This behaviour o f the electrical conductivity could

be explained in frame of Wang's model [10]. The fraction o f the Zn 2+ ions entering the conduction layers while the samples were sintered, produces the additional number of sodium vacancies. The higher number o f vacancies does not enter into the vacancy superstructure and thus the activation energy for their migration should be lower than that for those o r d e r e d It is just the case for our samples at higher (above 200°C) temperatures. At lower temperatures, vacancy superstructure mechanism is dominant and activation energy is higher than for partially disordered situation, existing at higher temperatures. The existence o f the similar dependence of the activation energy on doping level for the Li20 doped samples, seems however hardly fit to such a simple explanation since Li + ions have the same valency as the mobile sodium ions, and so they should not influence the concentration of sodium vacancies. Further investigations o f the interaction between mobile and stabilizing ions built into the spinel blocks and conduction layers are needed.

Acknowledgement The authors gratefully acknowledge financial support from Polish Academy of Sciences, Research Project No. 0310.

References [1] J.D. Hodge, J. Am. Ceram. Soc. 66 (1983) 166. [2] J.H. Kennedy, in: Topics in applied physics, Vol. XXI, ed. S. Geller (Springer, Berlin, 1977). [3] G.E. Youngblood, A.V. Virkar, W.R. Cannon and R.S. Gordon, Am. Ceram. Soc. Bull. 5 (1977) 206. [4] W. Jakubowski and D.H. Whitmore, J. Am. Ceram. Soc. 62 (1979) 381. [5] I. Wynn Jones, Electrochim. Acta 22 (1977) 681. [6] M. Wasiucionek, J. Garbarczyk and W. Jakubowski, Solid State Ionics 7 (1982) 283. [7] J.B. Bates, H. Engstrom, J.C. Wang, B.C. Larson, N.J. Dudney and W.E. Brundage, Solid State Ionics 5 (1981) 159. [8] E.E. Hellstrom and R.E. Benner, Solid State Ionics 11 (1983) 125. [9] W.G. Bugden and J.H. Duncan, Sci. Ceram. 9 (1977) 348. [10] J.C. Wang, Phys. Rev. B26 (1982) 5911.