Electrical conductivity of stabilized zirconia with ytterbia and scandia

SOLID STATE ELSEWIER

Solid State Ionics 79 (1995) 137-142

IONICS

Electrical conductivity of stabilized zirconia with ytterbia and scandia Osamu Yamamoto a, Yoshinori Arati a, Yasuo Takeda a, Nobuyuki Imanishi a, Yasumobu Mizutani b, Masayuki Kawai b, Yasuhisa Nakamura b a Department of Chemistry, Faculty of Engineering, Mie University, 1515 Kamihama, Tsu 514, Japan b Technical Research Institute, Toho Gas Co. Lid, Tokai, Aich 476, Japan

Abstract The electrical conductivity change with annealing at 1000°C in the Yb,O,-ZrO, (YhSZ) and Sc,O,-ZrO, (ScSZ) systems have been examined. The conductivity change depended on the composition of the dopant. In the YhSZ system, the sintered sample with 10 mol% Yb,O, showed only slight conductivity decrease with annealing, while those with 8 mol% Yh,O, and 12 mol% Yb,O, a significant decrease. The ScSZ with 8 mol% Sc,O, (8ScSZ) exhibited the cubic phase and those with 11 and 12 mol% Sc,O, (11ScSZ and 12ScSZ) the rhombohedral phases at room temperature. 8ScSZ showed a significant aging effect with annealing at 1000°C. 11ScSZ and 12 ScSZ showed no conductivity change with annealing. Keywords:

Stabilized

zirconia;

Ionic conductivity;

Solid oxide fuel cell (SOFC)

1. Introduction High temperature solid oxide fuel cells (SOFC) have exceptional potential for electric power generation system, because of the high energy conversion efficiency and the simplicity of system design [l]. However, there are many materials problems remaining to be solved to obtain a high performance fuel cell, arising from the high operating temperature as high as 1000°C. The operating temperature is manly due to the limitation of the electrical conductivity of the acceptable solid oxide electrolyte [2]. At this stage, yttria-stabilized zirconia (YSZ) is extensively used as the electrolyte, because of the high oxide ion and low electronic conductivity, and the stability under reducing and oxidizing atmospheres. The con0167-2738/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0167-2738(95)00044-5

ductivities of YSZ with 8 mol% Y203 are 0.1 S/cm at 1000°C and 0.03 S/cm at 800°C. In the course of research activity in the last decade, it has been stated that a lower operating temperature, like at 800°C or lower, is better from the long term stability of cells [3]. To develop SOFC operating at an intermediate temperature, a new type solid oxide ion conductor with a high conductivity of more than 0.1 S/cm at the operating temperature should be developed. Ytterbia-stabilized (YbSZ) and scandia-stabilized zirconia (ScSZ) have been reported to have a higher conductivity than YSZ. However, the long term stability of these oxide ion conductors has not been clarified. Especially, ScSZ has been reported to exhibit an aging effect in annealing at high temperature [4]. In this study, the conductivity change by aging at

138

0. Yamamoto et al. /Solid

a high temperature has been examined and discussed as has the possibility for using the electrolyte in SOFC.

State Ionics 79 (1995) 137-142 Table 1 Observed

phases in the Yb,O, -ZrO,

Composition

Yb,O,

and Sc,O, -ZrO,

systems

Phase monocl.

tetrag.

cubic

0 0 0 0

100 25 0 0

0 75 100 100

0 0 0 0

0 0 0 0 0 0

100 89 67 0 0 0

0 11 33 100 0 0

0 0 0 0 100 100

0

0

100

0

rhomboh.

mol%

2. Experimental The ytterbia-stabilized zirconia powder was prepared by a co-precipitation method. An aqueous solution of ammonia was added into the mixture of ytterbium chloride solution and ZrOCl, solution. The precipitate obtained was filtered and washed before drying at 100°C. The powder was heated at 800°C. Most of the ScSZ samples were prepared by a sol-gel method [5]. The scandium solution, which was prepared by dissolving Sc,O, into nitric acid, was added into the aqueous solution of ZrO (NO,), in the proper proportion. To the solution with scandium and zirconium were added formic acid and ethylene glycol. This was heated at about 120°C with stirring, and then fired at 800°C for 12 h. The YbSZ and ScSZ powders were compacted into bars(for conductivity measurements) and tablets(for XRD measurements) at a low pressure. These were then isostatically pressed at 100 MPa followed by sintering at 1700°C for ScSZ and at 1500°C for YbSZ for 15 h. The relative density of the sintered samples was more than 90%. X-ray diffraction (XRD) patterns of sintered disks were obtained with monochromated CuKa radiation and a scintillation detector. Silicon powder was used as an internal standard. The electrical conductivity measurements were carried out with cylindrical samples of about 0.5 cm in diameter and about 3 cm long with platinum paint electrodes sintered at 1000°C for 1 h. The ac conductivity was measured with the two-probe method using a frequency-response analyzer (Solartron FRA-1250) over a frequency range 10-l to 6.5 X lo4 Hz with an applied potential of 0.2 V and a temperature range 250 to 1000°C. All conductivity measurements were performed in air.

3. Results and discussion In Table 1, the room temperature XRD results obtained in this study for YbSZ and ScSZ are sum-

3 5 8 12 SczO,

mol%

2.9 4.9 7.8 8.0 11.0 12.0 12.0+0.3wt%

Al,o,

marized. The ratio of the tetragonal phase and cubic phase was determined by the X-ray diffraction intensity ratio of the (1 1 1) plane of the monoclinic and cubic phases. The pure tetragonal phase of the YbSZ and ScSZ was observed in the composition of around 3 mol% Yb,O, and Sc,O,. The tetragonal phase of ZrO, was stabilized by taking advantage of fine-particle technology and minor doping with Y203 by Gupta et al. [6]. The conductivity of the tetragonal phase was reported to be 6.5 X lo-’ S/cm at 1000°C. The tetragonal phase with Yb,O, and Sc,O, were also stabilized by the fine-particle technique of Yamamoto et al. [7]. In the system of YbSZ, the cubic phase is observed in the composition range 8-12 mol% Yb,O,. The phase study suggested that the cubic phase is stable in the range 7-46 mol% Yb,O, [8]. On the other hand the phase relation of the Sc,O,-ZrO, system is quite complicated and the observed phase depended on the preparation method [9]. The sample with 8 mol% Sc,O, (8ScSZ) prepared from the oxides heated at 1700°C showed the presence of monoclinic ZrO, and a fluorite-related phase [lo]. In our case, a single cubic phase of ZrO, with 8 mol% Sc,O, was observed by sol-gel methods. In the composition range of 11-15 mol% Sc,O,; the rhombohedral phase (P-phase) was obtained. The sample with 10 mol% Sc,O, (1OScSZ) showed a mixture of P-phase and cubic phase. High temper-

0. Yamamoto et al. /Solid

ature XRD at 1000°C indicated that the P-phase transformed to the cubic phase. The p-phase is known to be a rhombohedral distortion of the fluorite-type structure. A small addition of Al,O, (0.3 wt% or more) to ZrO,-(11-12) mol% Scr,O, had the effect of stabilizing the high temperature cubic phase at room temperature. It is surprising that the presence of a mere 0.3 wt% Al,O, should have such an effect. The XRD patterns of 12 mol% Sc,O,ZrO, (12ScSZ) with 0.3 wt% Al,O, (12ScSZO.3A) sintered at 1000°C for more than 1000 h showed no formation of P-phase. Fig. 1 shows the temperature dependence of the electrical conductivity of YbSZ as sintered. The highest conductivity of 0.22 S/cm at 1000°C is obtained in 8 mol% Yb,O, (8YbSZ) in the Yb,O,ZrO, system. The conductivity value was about 50% higher than that of 8 mol% Y,O,-ZrO, (8YSZ). In Fig. 2, the temperature dependence of electrical conductivity of ScSZ are shown as a function of the content of Sc,O,. In this system, the highest conductivity of about 0.3 S/cm at 1000°C was found in the composition range 8 to 12 mol% Sc,O,. In the YSZ system, the highest conductivity is found at the composition of the lowest solubility limit of Y203 in ZrO,, because of the interaction of the positive charged oxygen vacancies and the negative charged

lOOOT’(K-‘) Fig. 1. Temperature Yhsz.

dependence

of the electrical

conductivity

of

139

State Ionics 79 (1995) 137-142 I

I

1

u AA

0’0

6

*

I

1

I

2.9scsz

A.:

4.9scsz

5

BSCSZ 0’ 12scsz

0:

Oo’ 6 .,’ . 6

0. 0

-2

7

I

0:

E Cx

“* . o

rk 0 6

b

AU

.

3

Cl .

-4 .

I 0.8

I

I 1

L

. 1.2

.

1 1.4

1 O3 T-’ / K-’ Fig. 2. Temperature scsz.

dependence

of the electrical

conductivity

of

Y 3+ ions substituted for Zr4’ ion and the distortion of the cubic lattice by the ionic radius difference of Zr4+ and Y3+. In the case of the ScSZ system, The phase diagram of the system by Spiridonov [9] suggested that the lowest solubility limit of Sc,O, at 1000°C is around 8 mol% like in the YSZ system. However, the highest conductivity was not found at the lowest solubility limit, but the samples with higher content of Sc,O, also showed high conductivity. The difference between the YSZ and ScSZ systems may be the difference of ionic radii of Y3+ and Sc3+ ions. The ionic radius of Sc3+ is more close to that of Zr4+ and the distortion of the lattice by the substitution of Sc3+ for Zr4+ does not severely affect the migration of oxygen ions. The conductivity of ScSZ showed the highest value in the ZrO,-based oxide ion conductors. The activation energy for conduction of the ScSZ system (about 60 KJ/mol) was lower than that of the YSZ and YbSZ systems. A conductivity jump near 580°C is observed for 12ScSZ. According to the Sc,O,-ZrO, phase diagram, this temperature corresponds to the phase transition of P-phase to cubic phase. In spite of the high conductivity of the ScSZ system, the electrolyte is not often used in SOFCs, mainly because of its unfavorable aging characteristics and the cost of scandium compounds. The cost of it may be reduced by an economy of scale merit,

State Ionics 79 (1995) 137-142

0. Yamamoto et al. /Solid

140

I

I

I

I

I

I

I 0

0

0.2 -

0

1OYbSZ

- A

12YbSZ

l

3j

000

b O.l-

a0

0

0’

8YbSZ

0 n I

I 0

O

I

I

I

2000

I

I

6000

4000 Time (hr)

Fig. 3. Conductivity

change of YbSZ on annealing

because the natural abundance of scandium is not so low, and few technological applications of the compound have been developed as yet. Inozemtsev et al. [ll] reported a 50% decrease in conductivity of 9 mol% Sc,O, at 800°C after annealing for 200 h. The decrease was attributed to the formation of an ordered rhombohedral phase. A similar aging effect was also reported for the YSZ system. The conductivity decrease on annealing at 1000°C depended on the composition of Y203. 8YSZ showed a more significant conductivity decrease and 9YSZ and 1OYSZ not significant decrease. In Fig. 3, the conductivity changes of YbSZ at 1000°C are shown as a function of annealing time at 1000°C. The conductivity of 8YbSZ and 12YbSZ decrease with time, and the value after 2000 h annealing was 0.1 S/cm for 8YbSZ; after 300 h it was 0.07 S/cm for 12YbSZ. On the other hand, the conductivity of 1OYhSZ

I

I

o.3 z e

?L

B

g

0

b

shows no change after annealing for 6000 h. In Fig. 4, the conductivity changes of ScSZ at 1000°C are shown. In this case, the conductivity of 0.24 S/cm of as sintered 8ScSZ decrease with time to 0.13 S/cm after 2000 h and then show no change on further annealing. Similar conductivity decreases with aging time at 1000°C were reported in 7.8 ScSZ and 7 mol% Sc,O,-1 mol% Y,O,-ZrO,. In the case of 7.8ScSZ, the conductivity of 0.32 S/cm before aging decreased to 0.18 S/cm after aging for 300 h at 1000°C. The samples with higher Sc,O, content show no significant conductivity decrease with annealing time. The conductivity of 0.29 S/cm at 1000°C for 1lScSZ showed no change after annealing for 6000 h. The aging process for stabilized zirconia was first investigated by Carter and Roth [12] for the CaOZrO, system and it was concluded that defect order-

I

I

I

o

0

3

q

0.2-

I

I

at 1000°C in air.

0

cl fJ

cl

0

- q

8ScSZ

- .

1oscsz

- 0 1lScSZ - n

0.1 -

I o

0

I

I

I

I

I

4000

2000

I

GO00

Time(hr) Fig. 4. Conductivity

change of ScSZ on annealing

at 1000°C in air.

12scSz

0. Yamamoto et al./Solid

Stare Ionics 79 (1995) 137-142

141

A

.I” I

I

1

20

I

40

I

I

60

I

1

h

Oh

1OOh

I

I

I

20

40

I

I

I

60

I

I

80

28ldegree Fig. 6. XRD patterns 1000°C in air.

I

80

for 11 ScSZ+O.7

wt% Al,O,

annealed

at

2Wdegree Fig. 5. XRD patterns for 8 ScSZ annealed at 1000°C in air.

ing processes must be taking place. A square-root law for the conductivity change of 8YSZ at 875°C was found by Kleitz et al. [13], indicating a diffusion-controlled process for segregation of oversaturated impurities at the grain boundaries. Moghandam and Stevenson [14] have reported the influence of annealing on the conductivity of 4.5 YSZ at 1000°C. The initial decrease of conductivity was attributed to precipitation of tetragonal zirconia

Table 2 Electrical Electrolyte

3Y-TZP SYSZ 9YSZ 3YbTZP

8YbSZ 1oYbsz 12Ybsz 2.9Sc-TZP 8ScSZ 1lScZ 12scsz

conductivity a

from the cubic matrix and the further decrease to ordering in the cubic phase. Fig. 5 shows the XRD patterns of 8ScSZ as sintered and after annealing at 1000°C. After annealing for 10000 h, the monoclinic and tetragonal phases are not observed and only the cubic phase is observed. Slight relative X-ray intensity changes from each plane are detected. The conductivity decrease of 8ScSZ could not be explained by the second phase formation, but is due to the impurity segregation or ordering in cubic phase. In Fig. 6, the XRD patterns of 1lScSZ with 0.7 wt% Al,O, are shown. No significant change of the

of zirconia based electrolytes Conductivity at 1000°C

Bending strength

(S/cm) at 800°C

Thermal expan. coeff. (l/K X 104)

Ref.

WI 1161 [161 1151

as sintered

after aging b

as sintered

(MPa)

0.056 0.13 0.13 0.063 0.20 0.15 0.10 0.09 0.30 0.03 0.26

0.050 0.09 0.12 0.04 0.15 0.15

0.018 0.03

1200 230

10.8 10.5

275

10.7

0.063 0.012 0.30 0.29

a 3Y-TZP:3 mol% Y,O,-ZrO,, 2.9 SC-TZP: b After aging at 1000°C for 1000 h.

0.014 0.063 0.02 0.031 0.13 0.12 0.12 2.9 mol% Sc,O3-Zr02.

255

142

0. Yamamoto et al. /Solid

patterns is observed. The cubic phase is stable after annealing at 1000°C for 200 h.

4. Conclusion In Table 2, the conductivity data of the zirconiabased oxide ion conductors are summarized along with previously reported data. For an application in SOFC, the long term stability of the conductivity for operating peiods of 50000 h or more is important. Additionally, good mechanical properties of the electrolyte are required for the planar type SOFC. A major advantage of the planar SOFC is that it has a capability of high current drain as high as 0.5 A/cm2 or more. To reduce the contribution of the electrolyte resistance, the conductivity of the electrolyte should be less than 0.1 S/cm at operating temperature. As shown in this table, acceptable electrolytes for SOFC, which have the conductivity of higher than 0.1 S/cm at lOOO”C,are 10 YSZ, 10 YbSZ and 8-12 ScSZ. The highest conductivity of 0.3 S/cm at 1000°C is found in 11 ScSZ. The conductivity of 11 ScSZ and 12 ScSZ at 800°C is 0.12 S/cm. This means that these electrolytes could be used for an intermediate temperature SOFC. The three-point bending strength and the thermal expansion coefficient of 12 ScSZ are almost comparable to that of YSZ.

State Ionics 79 (1995) 137-142

References [l] B.C.H. Steele, Ceramic Electrochemical Reactors(Ceram ionic, London, 1987). [2] N.Q. Mirth, J. Am. Ceram. Sot. 76 (1993) 563. [3] B.C.H. Steele, Science and Technology of Zirconia V (Am. Ceram. Sot., Columbus, OH, 1993) p. 713. [4] S.P.S. Badwal, J. Mater. Sci. 22 (1987) 1435. [5] M.P. Pechini, U.S. Pat. 3, 330, 679 (1967). [6] T.K. Gupta, J.H. Bechtold, R.C. Kuznicki, L.H. Cadoff and B.R. Rossing, J. Mater. Sci. 12 (1977) 2421. [7] 0. Yamamoto, Y. Takeda, R. Kanno, and K. Kohno, J. Mater. Sci. 25 (1990) 2805. [S] T.H. Etsell and S.N. FIengas, Chem. Rev. 70 (1970) 339. [9] F.M. Spirdonov, L.N. Popova and R. Ya Porilskii, J. Mat. Sci. Lett. 2 (1970) 430. [lo] S.P. Badwal, J. Mater. Sci. 18 (1983) 3117. [ll] M.V. Inozemtser, M.V. Perfiles and V.P. Gorelov, Sov. Electra-Chem. 12 (1976) 1128. [12] R.E. Carter and W.L. Roth, in: EMF Measurements in high temperature systems, ed. C.B. AIcock (Am. Ceram. Sot., Columbus, OH, 1968) p. 125. 1131 M. KIeitz, H. Bernard, E. Fernandez and E. Schouler, Science and Technology of Zirconia II, (Am. Ceram. Sot., Columbus, OH, 1981) p. 310. [14] F.K. Moghadam and D.A. Stevenson, J. Am. Ceram. Sot. 65 (1982) 213. [15] 0. Yamamoto, Y. Takeda, R. Kanno, K. Kohno and T. Kamiharai, J. Mat. Sci. Lett. 8 (1989) 198. [16] E. Eldre, Science and Technology of Zirconia II (Am. Ceram. Sot., Columbus, OH, 1984) p. 685.