Space-charge conduction in yttria and alumina codoped-zirconia 1

__ E!B

SOLID SrATE

2% r

__

ELSEVIER

lcmlcs

Solid State Ionics 96 (1997) 247-254

Space-charge

conduction

in yttria and alumina codoped-zirconia’ Xin Guo

for

State Key Lcrboratoty

Synthesis and Processing

of Advanced

430070,

Received

People’s

Materials,

Wuhan University

Republic

19 June 1995; accepted

of

of

Technology, Wuhan, Hubei Province

China

I 1 November

1996

Abstract The grain-boundary resistances of 9 mot% Y,O, and 1.5 mol% Al,O, codoped-ZrO, after sinterings of 1520-1600°C were measured by the complex impedance approach. And the microstructure of the specimen was analyzed by means of SEM, EPMA and TEM. According to a space-charge theory analysis, the space-charge potential of the Y,O, and Al,O,-codoped ZrO, is negative, which is the result of an A& segregation and a Vi depletion in the space-charge layers. As Al;, has a stronger tendency to form associates with Vi than Ylr, the Al& segregation further reduces the concentration of mobile Vi in the space-charge layers, thus increasing the grain-boundary resistance contributed from the space-charge layers. A higher Vi concentration in the space-charge layers of the Al,O,/ZrO, is suggested by the space-charge theory, but due to the pores and the amorphous phases covering the AI,O, particles, the effect of the space-charge layers with high Vi; concentration will not be very obvious. Keywords:

Space-charge;

Zirconia;

Yttria; Alumina

1. Introduction The grain-boundary resistivity of stabilized-ZrO, is usually 100-1000 times larger than that of the bulk [ 1,2]. According to previous works of the author, this is due to the Vo depletion in the spacecharge layers resulting from solute segregation there [3,4]. However, the solute segregation will change lattice-defect concentrations in the space-charge layers; the grain-boundary resistivity is thus changed. It has been established that a certain amount of Al,O,, e.g., >0.6 mol%, can decrease the grainboundary resistivity [5-91. The effect of Al,O, has

’ Part Ii of “Space-charge conduction in stabilized-zirconia”. Project supported by the Natural Science Foundation of China and the Natural Science Foundation of Hubei Province. 0167-2738/97/$17.00 PII

0

SO167-2738(96)00593-O

1997 Elsevier Science

B.V. All rights reserved

been explained by the scavenging of SO, located at the grain boundaries [lo] and the formation of highly conductive grain-boundary phases [6]. However, the possible influence of Al,O, on the space-charge layers of the stabilized-ZrO, has been ignored. Therefore, this influence will be the subject of the present work.

2. Experimental 2. I. Preparation

of specimens

ZrO, specimens with 9 mol% Y,O, and 1.5 mol% A&O, were used in the present work (denoted as 15AYZ). Powder used to produce the specimens was prepared from ZrOC1,.8H,O, YCl, and AlCl, by a

248

X. Guo I Solid State lonics 96 (1997)

coprecipitation method. The powder was pressed into pellets (22 mm in diameter by 4 mm thick) at 200 MPa, then sintered at 1520, 1540, 1580 and 1600°C for 2 h. Platinum electrodes were applied to the ZrO, specimens by the decomposition of chloroplatinic acid at 1000°C. 2.2. Measurements

and analyses

The specimens were subjected to the following measurements and analyses: ( 1) Average grain-size measurements were obtained from the scanning electron micrographs of the surfaces of as-sintered specimens, and calculations of the grain size were based on a method described by Fullman [ 111, (2) resistances were measured by the complex impedance approach in a frequency range of 20 Hz-l MHz with a HP4284A precision LCR meter, and the bulk and grain-boundary resistances were separated out by the analyses of the complex impedance plots, (3) aluminium distribution information inside the specimens was examined by electron probe microanalysis (EPMA, type JCXA-733) on the specimens polished and coated with carbon, (4) microstructure studies were carried out by SEM (type JSM-35C) and TEM (type Philips-CMlZ/STEM).

3. Results 3.1. Properties

of the specimens

The complex, impedance approach has been used extensively in the examination and development of solid electrolytes after an initial report by Bauerle [12]. This approach can effectively probe the bulk resistance as well as the resistance across grain boundaries and phase interfaces. As the present work

Table

247-254

concerns only the space-charge conduction, only the grain-boundary resistances will be analyzed. The sintering temperatures, average grain sizes d,, and the grain-boundary resistances R,, measured at 440 and 500°C are given in Table 1. In order to explain further the measured results, the resistances per unit surface area of the grain boundaries pih were calculated from [ 131 Pill = R,,(sll)lD

(1)

where s/l is the cross-section/length ratio of the specimens and D the grain boundary density. It is astonishing to compare R,, and P:,,. As the microstructural effects are eliminated from the p$, values, they can be considered to represent the real grainboundary property.

3.2. Microstructure The solubility of Al,O, in ZrO, is very low, only 0.5 mol% Al,O, can be dissolved in Y,O,-stabilized ZrO, sintered at 1700°C and cooled at 220”C/h [5], and the solubility of Al,O, is about 0.1 mol% when sintered at 1300°C [9]. It is thus obvious that the A120, addition in the present work is far beyond the Al,O, solubility. Fig. 1 shows the scanning electron micrographs of the as-sintered surface and the polished surface of the specimen 1.5AYZ. AlTO, particles can be observed as bright or dark spots in Fig. la, and most of them are situated intergranularly. The composition of a particle was measured by EDAX, the result is given in Table 2, some ZrO, has been dissolved in the Al,O, particles. Because there are quite large differences in the elastic modulus and the thermal expansion coefficient between Al,O, and ZrO,,

I

Sintering temperature, grain size and electrical data for ISAYZ Sintering temp.

d,

(“Cl

(pm)

500°C

440°C R,, (a,

pih (fi cm*)

Rrb @I

pi, CR cm*)

1520

2.4

360

0.55

105

0.16

1540

5.9

350

1.31

100

0.37

1580

16.5

345

3.61

72

0.75

1600

30.1

215

4.1

50

0.96

X. Guo I Solid State lottics 96 (1997) 247-254

249

small dark spots shown in Fig. 2a and Fig. 2c are Al,O, particles and pores. No apparent segregation of yttrium was observed. Thus the yttrium segregation is subdued by the preferential segregation of aluminium, which has also been observed in [4]. Fig. 3 shows the transmission electron micrographs of a ZrO, grain boundary and an intergranular Al,O, particle. Some amorphous grain boundary phases were observed both at the ZrO, grain boundary (Fig. 3a) and Al,03/Zr0, interface (Fig. 3b). The composition of the phases measured by EDAX is given in Table 3, a high proportion of silicon in the phases are detected. This result is in accordance with most of the previous works carried out by various researchers [8,10,14-161. A kind of crystal phase has also been observed in the previous works of the author [6,7]. However, the grain boundaries without the grain-boundary phases are quite clean [6,7,10]. According to Butler and Drennan [lo], the clean grain boundaries are the result of the fact that Al,O, can attract and remove SiO, from the grain boundaries. The most important results of the microstructural/ compositional characterization of the YzO, and Al,O,-codoped ZrO, can now be summarized as fo11ows:

Fig. I. Scanning electron micrographs of the as-sintered (a) and the polished surface (b) of the specimens. Table 2 Composition

of an A120,

particle

measured

surface

by EDAX

Element

Wt.%

At.%

Al Y Zr

38.73 9.9 I 51.35

68.04 S.29 26.68

many Al?O, particles are accompanied by pores, which can be seen from Fig. lb. Fig. 2 shows the EPMA analysis result; (a) and (c) are scattered electron images, and (b) and (d) show the aluminium distribution. From these photographs, it can be seen that aluminium mainly segregates at the grain boundaries (including the grain-boundary interfaces and the space-charge layers adjacent to the interfaces), and the aluminium segregation is more serious when sintered at higher temperatures. Some

1. Al?O, particles are present, some of the particles are accompanied by pores and amorphous phases. 2. ZrO, grain-boundary interfaces are generally clean and free from the grain-boundary phases. 3. Some grain-boundary phases are present.

4. Discussion The grain-boundary resistance consists of the contributions from the grain-boundary phases and the space-charge layers [3,17], thus the pz,, values given in Table 1 reflect the comprehensive effects of the grain-boundary phases and the space-charge layers. The amount of the amorphous grain-boundary phases is determined by the amount of impurities (especially SiO, in the case of the present work, according to the formation mechanism of the aluminium and silicon rich grain-boundary phases proposed by Butler and Drennan [lo]) in the specimen which should be constant, the amount of the phases should

250

X. Guo I Solid State lonics

Fig. 2. EPMA photographs

96 (1997)

247-254

of the specm~ens sintered at 1520°C (a,b) and 1580°C (c.d).

also be constant. During sinterings from low to high temperatures, the amorphous grain-boundary phases have been observed to change progressively from being a uniformly dispersed film of several atoms thick through to becoming relatively large isolated pockets along the boundaries and in triple points, while the amount of the phases were considered to be constant [14]. The uniformly dispersed film will totally block the ionic transport across the grain boundaries, while the isolated pockets allow some direct grain to grain contacts, only partly blocking

the ionic transport across the boundaries, it is thus obvious that the morphology change of the amorphous grain-boundary phases will not increase pi,,. The formation of the crystal grain-boundary phase was considered to reduce the pib, thus the increasing p$, during sinterings from low to high temperatures (Table 1) surely is not the result of the crystal phase. However, during the sinterings from low to high temperatures, the aluminium segregation was enhanced (Fig. 2) because the grain-boundary interfaces are generally clean, this enhancement can be

251

X. Cue I Solid State lonics 96 (1997) 247-254

ZrO, interfaces tion is used.

(a)

will be assessed.

4.1. The space-charge

Kroger-Vink

nota-

layers in the ZrO, grain

boundaries

In thermodynamic equilibrium the grain-boundary interfaces of an ionic crystal may carry an electrical potential resulting from the presence of excess ions of one sign, this potential is compensated by a space-charge potential of the opposite sign [ 18-201. The intrinsic space-charge phenomenon of Y,O,stabilized ZrO, has been discussed in [3,4], the space-charge potential was found to be negative which corresponds to an YL, segregation and a Vi; depletion in the space-charge layers. However, the Yh, segregation and the Vi depletion in the spacecharge layers can be modified by the segregations of other solutes with aliovalences [4,21]. Al,O, may be dissolved in ZrO, in a substitutional way, because it is obviously impossible for it to be interstitially dissolved when considering the relatively large radius of Al” with respect to interstices in ZrO, lattice. Then the defect chemistry equations for Y,O, and Al,O, can all be written as

Fig.

3. Transmission

boundary (a) and the

electron A120,/Zr01

micrographs

of the ZrOz

grain

interface (b).

Table 3 Composition

of amorphous grain-boundary

phases measured by

EDAX Element

Wt.%

Oxide (wt.%)

Al

9.26

17.50

SI

29.40

62.91

Ca

8.00

II.20

Y

4.43

5.63

Zr

2.48

3.35

Y,O, -9 2Yh, + v;; + 30;,

(la)

A&O, + 2Al;, + V, + 30;.

(lb)

Thus the predominant defects in the specimen should be Vi, Y& and Al;,. According to the spacecharge formalism developed by Kliewer and Koehler [20] and Yan et al. [22], after the introduction into a perfect crystal of Vi, YL, and Al:,, if the dopant concentrations meet the requirement of dilute solution, the crystal free energy becomes r F =

i

dx [n,(x)&, + n&N,

: +(X)@(X)]

considered to mainly occur in the space-charge layers. As a result, the change of p’& values should reflect the property change of the space-charge layers. Within this framework, the space-charge layers in the ZrO, grain boundaries and the Al,O,/

- TS,

+ n,,(x)r/,, (2)

where x is the distance from the grain-boundary interfaces, at the interfaces x=0, while in the bulk x=x; Q(x) is the spatially varying static potential, which is referenced to zero at the grain-boundary interfaces and reaches a bulk value of @= far from the interfaces; n,(x), nv(x) and n,,(x) are the number

252

X. Guo I Solid Stute lonics 96 (1997) 247-254

densities of Vi, Y& and Al;,, respectively, F,, is the formation free energy of Vi, U, and U,, the elastic interaction energies between the grain-boundary interfaces and YL,, Al;,; in the bulk, U, = U,, =O. T is the absolute temperature, 5, the configurational entropy, which has been discussed in [20], p(x) is the charge density, given by P(X) = e]2n,(x)

- n&)

- ~*,(-a

(3)

In the equilibrium state, the free energy F should be minimum. We minimize the free energy by setting 8,. = 0.

(4)

The equilibrium defect concentration function of distance from the interfaces is

n,(x)

[V,(X)] = 7

of Vi as a thus obtained

1

=

(5)

where N is the number of cation or anion sites per unit volume, the factor 2 in Eq. (5) results from the presence of 2N oxygen sites per unit volume. In the bulk, the electrostatic potential is GX, Eq. (5) thus becomes

The potential difference between the bulk and the grain-boundary interfaces (e @=- e@(O) = e @%j,, is determined by applying the bulk electroneutrality condition [p(m)=O], which is 2&l,

= [-f;,lx + Wl;,l,.

(7)

Equating Eqs. (6) and (7), the potential given by r&l,

difference

is

+

W;,lx

4

>

(8) If the effects of Y& and Al& dominate over the intrinsic defects, we have (e@X)zro, < 0, thus the space-charge potential in the specimens is negative, which corresponds to a V& depletion in the spacecharge layers, while the potential of the grain-boundary interfaces should be positive. According to [3], this is the result of the accumulations of Zr&,,, Y&b and A1i;,gh in the interfaces. In the case of the present work, 9 mol% Y,O, and 1.5 mol% A120,

are doped in ZrO,, the dilute solution requirement is no longer met, but because of the negative effective charge of Y& and Al;,, the nature of the spacecharge potential will not be altered. In Y,O,-stabilized ZrO,, the Y,O, segregation has been observed by Winnubst et al. [23]. However, in the specimen l.SAYZ, Y,O, segregation is subdued by the preferential segregation of Al,O,, thus the major solute in the space-charge layers should be Al;,. An A&O, grain-boundary segregation factor of about two has been obtained by Verkerk et al. [24]. The preferential segregation of A&O, may be due to the substantial size misfit between A13+ and Zr4+, thus the substantial elastic misfit strain energy. When a minority solute has a large elastic misfit strain energy and segregates strongly to the boundaries, the segregation of the majority solute will be significantly suppressed. Such a case has been proved by Yan et al. in KC1 [22]. YL, and Al;, have a strong tendency to form defect associates with Vo [25,26], when the concentration of Y& or Al;, is high, the association tendency is enhanced. In the space-charge layers, the effect of the formation of defect associates should not be neglected, the formation of defect associates further reduce the concentration of mobile Vo in the space-charge layers, thus the grain-boundary resistance is enhanced. Mackrodt et al. [26] have calculated the association energies for various kinds of associates between Vo and Y& or Al& (Table 4), according to the results, Al;, has a much higher tendency to form associates with V, than Ygr. During the sinterings from low to high temperatures, the Al;, segregation increases, the concentration of mobile Vo in the space-charge layer is reduced, then the pi,, values increase. As shown in Table 1, this is just the case. And according to the above discussion, the pi,, of YSZ with high-purity should be smaller than that of the specimen 1.5AYZ. Ioffe et al.

Table 4 Solute-oxygen

vacancy

association

energies

Associate

Energy (kJ/mol)

(Ai;,V,). (AI;,V,AI;,)”

- 143 -253 -39 -43

(y;4’0)’ (Yk~VX,^

[26]

3.3

X. Guo I Solid Srure Ionics 96 (1997) 247-2.54

obtained a pi,=O.72 R cm* for 5.7 mol% Y,O,stabilized ZrO, with high-purity and a grain size of 18.0 pm at 450°C [27], this pii, value is much smaller than 3.61 fl cm’ for l.SAYZ with a grain size of 16.5 km at 440°C. It is now clear that Al,O, can increase the resistance of the space-charge layers. As R,, =R,,+Rgb,, Rg,,, R,, and R,,i are the resistances of the grain boundaries, the space-charge layers and the grain-boundary interfaces, in the cases with grain-boundary phases, Rgbi is the resistance of the phases. Previous works [5-lo] reported that Al,O, can decrease the grain-boundary resistances of ZrO>, this is mainly the result of the reduction of R,,, caused by A120,. Now it is strikingly found that Al,O, has two opposite effects on the grain-boundary resistance of ZrO,: Al,O, can increase the grainboundary resistance contributed from the spacecharge layers; and at the same time, A&O, can decrease the grain-boundary resistance contributed from the grain-boundary impurity phases. In ZrOz with high-purity, the first effect is more obvious, however, in ZrOz with impurities, the second effect is more obvious. 4.2. The space-churge interjkes

layers in the A12031Zr02

Space-charge layers with high defect concentrations were usually considered to exist in the interfaces between LiI/Al?O, [28] and AgCl/Al,O, [29,30]. Maybe the same effect can also be expected for the Al,O,/ZrO, interfaces. First we consider the free surface of the Al,O, particles. When TiO, is doped in Al,O,, it was experimentally proved that Vl, is the charge-compensating defect for Tik,, and the electroneutrality condition is 3[VT,] = [Tik,] [31]. We assume that for the case of Al,O,-ZrO, the same charge-compensating mechanism holds true, thus we have 3Zr02 -+ 3Zr,, + Vr, + 60:). The electroneutrality

condition

(9) in the bulk should be

.?]VT,], = [ZrA,lx. According

(10)

to the above discussion, FA, - 3edj(x) KT

we readily get

1

(11)

and (e@J,,+l

= f

FA, + KT lnw

>.

(12)

In the case of the present work, (e@%),,:ol >O, which corresponds to a segregation of Zr,, and a depletion of Vi, in the space-charge layers, while the potential of the Al,O, particle surface should be negative, which may be the result of the accumulation of VT,. When the Al?O, particles are put into the grain boundaries of ZrO,, at the A120,/Zr0, interfaces, the following defect reactions may occur (13a) (13b) VI, + Zr&,,

# Zri,.

(13c)

And the formations of defect associates between VT, and A1i;,gh, Yiigh and Zr&,i, are also possible. The net effect of ali these reactions is that the A120,/ ZrO, interface potential is lower than the grainboundary interface potential of ZrO,. It is now impossible to anticipate the sign and magnitude of the actual potential of the Al,O,/ZrO, interfaces, however, one point is certain that the reduced potential will increase the V, concentration in the space-charge layers of ZrO,, thus the resistance of the A120,/Zr0, interfaces is reduced. But as shown in Fig. lb and Fig. 3b, the A120, particles are covered by the pores and the amorphous phases, then the resistance-reducing effect will not be very obvious. If the pores and the amorphous phases are removed, this effect can be significantly enhanced.

5. Conclusion Al,O, segregation reduces the concentration of mobile Vi in the space-charge layers of ZrO,, thus increasing the grain-boundary resistance contributed from the space-charge layers. However, space-charge layers of higher Vi concentration may exist adjacent to the A120,/ZrOz interfaces, but due to the pores and the amorphous phases covering the Al,O, particles, the effect of such kind of space-charge layers is not very obvious. In a summary, the effect

254

of Al,O, satisfying.

X. Guo I Solid State Ionics 96 (1997) 247-254

on the resistance

of ZrO,

is not very

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