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Effectofnano-Al2O3additiononthedensificationofYSZelectrolytesARTICLEinJOURNALOFNANORESEARCHJANUARY2009ImpactFactor:0.52DOI:10.4028/www.scientific.net/JNanoR.6.115Source:OAI

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Effect of nano-Al2O3 addition on the densification of YSZ electrolytes

Dachamir Hotza1,a, Adrian Leo2,b, Jaka Sunarso2,c, Joo C. Diniz da Costa2,d

1 Group of Ceramic and Glass Materials (CERMAT), Department of Chemical Engineering (ENQ), Federal University of Santa Catarina (UFSC), 88040-900 Florianpolis, SC, Brazil

2 Films and Inorganic Membrane Laboratory (FIMLab), Division of Chemical Engineering (ChemEng), The University of Queensland (UQ), Brisbane, Qld 4067, Australia

a [email protected] (corresponding author), b [email protected], c [email protected], d

[email protected],

Submitted: 3.02.209 Revised: 20.04.2009 Accepted: 29.4.2009

Keywords: SOFC; Yttria; Zirconia; Nano-Alumina; Electrolyte.

Abstract. This work investigates the effect of nanosized Al2O3 addition on the sinterability of YSZ

electrolyte. (1x)YSZ + Al2O3 ceramics with compositions x = 0 to 0.01 were prepared by the conventional mixed oxide route from a commercial powder suspension (particle size

mol%) at 1300C. It was also found that Al2O3 added up 0.3 wt.% enhanced the densification and grain growth. A further increase in Al2O3 content resulted in decreased densification and grain

growth.

Many studies [7-9] have reported that Al2O3 addition to YSZ not only assisted sintering but also

lowered the overall and grain-boundary resistivity of YSZ, which may be explained by scavenging

of the resistive siliceous phase at the grain boundary. Butler and Drennan [3] examined the

microstructure of 4Y2O3/4Yb2O3-stabilized ZrO2 with additions of 0.98 and 2.68 mol% of Al2O3.

They found that most of the intergranular Al2O3 particles contained inclusions rich in Zr or Zr+Si.

Moreover, they also observed that the intergranular particles were frequently associated with

amorphous cusps rich in Si and Al. Their observation confirmed the scavenging effect of Al2O3 on

the siliceous secondary phase. Miyayama et al. [7] suggested that Al2O3 additions have a dual role

according to their concentration level. Below the solubility limit of Al2O3 in YSZ (0.5 mol%), grain

and grain-boundary resistivities increased, and grain growth was promoted. In the case of Al2O3

concentration above the solubility limit the trend was reversed as the grain-boundary resistivity

decreased and grain growth was inhibited. Mori et al. [8] added up to 30 wt.% Al2O3 to YSZ,

providing significant improvements in mechanical strength, thermal shock resistance and thermal

conductivity. However, the electrical conductivity of the composites at 1000C decreased for Al2O3 content higher than 1 wt.% (1.29 mol%). Feighery and Irvine [9] also investigated the addition of 1

wt.% Al2O3 in YSZ and reported that the grain boundary conductivity substantially increased at

high temperatures. The conductivity then remained constant up to 10 wt.% (12.9 mol%) Al2O3

added to YSZ, at which point the conductivity rapidly decreased. Yang et al. [10] examined the

effect of Al2O3 addition in yttria-stabilized tetragonal zirconia polycrystals (YTZP). Within the

solubility limit of Al2O3, at around 0.2 wt.% (0.25 mol%), densification increased as the amount of

Al2O3 increased.

One of the major impediments for production of these materials is associated with the need of

two or three firing steps. A single step co-firing would increase production throughput and decrease

energy consumption in the manufacturing process of planar SOFCs. However, a successful one-step

cofiring of the multilayered structure requires a lower sintering temperature to minimize the

distortion of the structure and the chemical interaction between the electrodes and the electrolyte

[8]. The lower bound for the cofiring temperature is dictated by the YSZ electrolyte since it has the

highest sintering temperature.

Although researchers generally considered Al2O3 as a sintering aid to YSZ, only a limited

number have actually used nanosized Al2O3 powders [11-13]. In fact, in the literature no special

attention was given to the particle size of raw materials, being the initial particle size not even

mentioned in many cases [9,10]. Navarro et al. [11,12] prepared nanosized YSZ and Al2O3 powders

using a colloidal coprecipitation route to mixing powders in order to achieve a uniform distribution

and homogeneous microstructure [11]. They found that up to 10 wt.% (~12 mol%) Al2O3 enhanced

the YSZ densification process, though above this value the densification process was retarded [12].

Ye et al. [13] employed commercially available powders and reported that best results were

observed by adding 0.2 mol% of nano-Al2O3 to submicron-YSZ. In this case, the relative density

increased from 73 to 93% at a sintering temperature of 1250C, and no adverse effect on the electrical conductivity was reported.

Nevertheless, studies of the effect of the addition of nanosized powder are still limited in the

literature. Hence, in this paper we systematically investigate the effect of nanosized Al2O3 particles

and their effect as a sintering aid for promoting densification of YSZ at firing temperatures from

1200 to 1500C. The resultant materials are characterized using measurements of firing shrinkage and bulk density for evaluation of densification behaviour, XRD for phase determination, and

SEM/EDS for microstructural analysis.

116 Journal of Nano Research Vol. 6

Experimental

Sample Preparation. Mixtures with compositions (1x)YSZ + xAl2O3 (x = 0, 0.001, 0.002, 0.005 and 0.010) were prepared by the conventional mixed-oxide method from YSZ (ZrO2, submicron

powder, 5.3 mol% Y2O3, Sigma Aldrich, USA), and aluminum hydroxide oxide (AlO(OH), 20

wt.% dispersion in water,

increased as a function of the sintering temperature. However, there was some overlapping of

curves, notably at higher temperatures due to experimental variation of density measurements (

0.5% RD). However, it is observed that a small amount of nano-Al2O3 doping sharply increased the

density for samples sintered at 1300C. Moreover, at this sintering temperature, densities higher

than 95% were achieved with addition of at least 0.5 mol% Al2O3. At 1500C specimens doped with >0.2 mol% Al2O3 provided relative densities >95%. The enhanced density achieved with

minute amounts of nano-Al2O3 was also observed elsewhere [13].

80%

85%

90%

95%

100%

0 0.2 0.4 0.6 0.8 1

nano-Al2O3 (mol%)

Re

lati

ve

De

ns

ity

(%

)

1500

1400

1300

Temperature (C)

Fig. 2. Relative density of nano-Al2O3-added YSZ samples sintered at 1300-1500C.

Further increase in nano-Al2O3 content led to an increase in sintered density up to the highest

level added in this experiment which is 1 mol%. Nevertheless, samples were not required to be

sintered at higher temperatures to achieve 95% density. This level of densification was reached

already at 1300C by adding 0.5 mol% nano-Al2O3. These results are in agreement with those of Tekeli and Demir [15], whose microstructural analysis of Al2O3-dopped YSZ showed that the

specimens with up to 1 mol% Al2O3 were very dense and free from porosities in the grain interiors.

They suggested that the small amount of Al2O3 addition located at the grain boundaries of YSZ was

sufficient for improving the densification. However, in their case, when specimens were doped with

higher amounts of Al2O3, porosity occurred in the grain interior and along the grain boundaries

[15]. The porosity hindered the sintered density and grain growth.

The X-ray diffraction profiles of pure and Al2O3-added YSZ samples sintered at 1300C is shown in Fig. 3. The single structure identified was cubic ZrO2, independently of the amount of

added Al2O3. The peaks corresponding to the samples sintered at higher temperatures (not shown)

are sharper, indicating that cubic ZrO2 was better crystallized at higher temperatures. According to

Yang et al. [10], when more than 0.5 wt.% (0.62 mol%) of Al2O3 was added to ZrO2, diffraction

peaks from a second phase appeared, which were associated to -alumina. In our case, even for 1 mol% added Al2O3 no peak of corindon could be detected. On the other hand, our observations are

consistent with nanopowders of zirconia doped with alumina [16] suggesting that the Al2O3 content

was below the detection limit or that the Al3+

ions segregate at the interfaces on a molecular level as

very small clusters, or formed an amorphous phase [16]. Thus, Al2O3 nanoparticles may have

diffused more efficiently into the ZrO2 lattice.


25 30 35 40 45 50 55 60 65

2-Theta (degrees)

Inte

nsity (

arb

itra

ry u

nits)

c

c

c

c

c

pure YSZ, x=0

YSZ/nano-Al2O3, x =0.010

YSZ/nano-Al2O3, x =0.002

Fig. 3. XRD patterns (25 to 65 degrees) of (1x)YSZ + xAl2O3 sintered at 1300C for 2 hours,

where (c): cubic zirconia.

A detailed XRD pattern in the range from 29 to 36 is further depicted in Fig. 4 for samples

sintered at 1300C. It can be observed that pure YSZ presented a main peak at 30.5 corresponding to cubic zirconia. When nano-Al2O3 was added, a shift to the left of the main peak occurred, which

was proportional to the amount of added alumina. The same characteristic resulted for the

secondary peaks found at 35.0 and 35.5. These features suggest that an interaction between Al2O3 and YSZ occurred, although no new crystalline phase could be detected.

29 30 31 32 33 34 35 36

2-Theta (degrees)

Inte

nsity (

arb

itra

ry u

nits)

YSZ/nano-Al2O3, x=0.010c

c

c

pure YSZ, x =0

YSZ/nano-Al2O3, x=0.002

Fig. 4. XRD patterns (29 to 36 degrees) of (1x)YSZ + xAl2O3 sintered at 1300C for 2 hours,

where (c): cubic zirconia.

Journal of Nano Research Vol. 6 119

Fig. 5 shows an SEM micrograph of a typical cross section sintered YSZ sample, in addition to

the distribution of Al along the surface as detected by EDS for samples containing 0.1 and 0.5

mol% nano-Al2O3. A major problem in the processing of nanosized powders is the formation of

agglomerates, which occurs spontaneously due to van der Waals forces. In the present case, Al2O3

agglomerates or second phases were not visible from the SEM micrographs or EDS mapping, even

for higher amounts of added nano-Al2O3. The matrix composition was apparently homogeneous, in

agreement with the XRD results.

(a) (b)

(c) (d)

Fig. 5. Typical micrographs of samples sintered at 1200C for 2 hours: (a) SEM: 0.999YSZ +

0.001Al2O3 and (b) EDS: Al distribution; (c) SEM: 0.995YSZ + 0.005Al2O3 and (d) EDS: Al

distribution.

Fig. 6 shows the microstructures obtained for pure and nano-Al2O3-added YSZ samples sintered

at different temperatures. After sintering at 1200C the grain sizes of pure YSZ (a) and YSZ with 0.5 mol% of Al2O3 (c) remained similar. For both samples, the grain size distribution was observed

to be narrow with an average surface grain size estimated at 0.40 m. However, the nano-Al2O3-

added YSZ microstructure was more densely packed. Samples sintered at 1400C, either pure YSZ (b) or YSZ with 0.5 mol% of Al2O3 (d) presented a coarser grain size. The latter showed the lowest

surface porosity compared to the other samples. All these results were in agreement with density

and shrinkage measurements.


(a) (b)

(c) (d)

Fig. 6. Typical SEM micrographs of (1x)YSZ + xAl2O3 sintered at different temperatures for 2 hours: (a) x = 0, 1200C; (b) x = 0, 1400C; (c) x = 0.005, 1200C; and (d) x = 0.005, 1400C.

The effect of nano-Al2O3 on the YSZ was clearly evidenced by the above results. Whilst no

phase changes were observed in the XRD, YSZ cubic phase was attained at high sintering

temperatures. Nevertheless, a shift occurred in the range 30 to 31 by increasing the amount of nano-Al2O3, suggesting that an interaction occurred with the YSZ phase, though the YSZ

crystallinity seemed not to be altered. Higher sintering temperatures led to denser samples, and the

density slightly increased as a function of the amount of nano-Al2O3.

Conclusions

The addition of nano-Al2O3 to YSZ increased the density of the electrolyte samples sintered at

1300C or reduced the temperature required for sintering to densities higher than 95%. The optimum amount of added nano-Al2O3 for enhancing the sinterability of YSZ electrolyte depends

on the sintering temperature. The nano-Al2O3 addition had no remarkable effect either on grain size

of YSZ or on segregation of a second phase in the YSZ for the range of Al2O3 amounts and

sintering temperatures used. Cubic zirconia was the single crystalline phase detected, although

XRD features suggest some chemical interactions dependent on the amount of added Al2O3.

References

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(1994) 157-164.

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behaviour and microstructural development, J. Mater. Sci. 30 (1995) 1939-1948.

[13] G. Ye, F. Ju, C. Lin, S. Gopalan, U. Pal, Single-step co-firing technique for SOFC fabrication

Ceram. Eng. Sci. Proc. 26 (2005) 25-32.

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Volume 7, Parthenon Press, Carnforth, 1986.

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alumina-doped cubic zirconia, Ceram. Int. 31 (2005) 973-980.

[16] V.V. Srdic, M. Winterer, H. Hahn, Sintering behavior of nanocrystalline zirconia doped with

alumina prepared by chemical vapor synthesis, J. Am. Ceram. Soc. 83 (2000) 1853-1860.


Journal of Nano Research Vol. 6 doi:10.4028/www.scientific.net/JNanoR.6

Effect of Nano-Al2O3 Addition on the Densification of YSZElectrolytes doi:10.4028/www.scientific.net/JNanoR.6.115 References[1] M. Mori, T. Abe, H. Itoh, O. Yamamoto, Y. Takeda, T. Kawahara, Cubic-stabilizedzirconia and alumina composites as electrolytes in planar type solid oxide fuel-cells, SolidState Ionics 74 (1994) 157-164.doi:10.1016/0167-2738(94)90206-2

[2] A.A.E. Hassan, N.H. Menzler, G. Blass, M.E. Ali, H.P. Buchkremer, D. Stver, Influenceof alumina dopant on the properties of yttria-stabilized zirconia for SOFC applications, J.Mater. Sci. 37 (2002) 3467-3475.doi:10.1023/A:1016563123018

[3] E.P. Butler, J. Drennan, Microstructural analysis of sintered high-conductivity zirconiawith Al2O3 additions, J. Amer. Ceram. Soc. 65 (1982) 474-478.doi:10.1111/j.1151-2916.1982.tb10336.x

[4] K.C. Radford, R.J. Bratton, Zirconia electrolyte cells Part 1: Sintering studies, J. Mater.Sci. 14 (1979) 59-65.doi:10.1007/BF01028328

[5] H. Bernard, Sintered Stabilized Zirconia Microstructure and Conductivity, Report R-5090, Commissariat l' Energie Atomique, CEN-Saclay, France, 1981.

[6] S. Tekeli, The solid solubility limit of Al2O3 and its effect on densification andmicrostructural evolution in cubic-zirconia used as an electrolyte for solid oxide fuel cell,Mater. Design 28 (2007) 713-716.

[7] M. Miyayama, H. Yanagida, A. Asada, Effect of Al2O3 addition on resistivity andmicrostructure of yttria-stabilized zirconia, Am. Ceram. Soc. Bull. 64 (1986) 660-664.

[8] M. Mori, T. Abe, H. Itoh, O. Yamamoto, Y. Takeda, T. Kawahara, Cubic-stabilizedzirconia and alumina composites as electrolytes in planar type solid oxide fuel cells, SolidState Ionics 74 (1994) 157-164.doi:10.1016/0167-2738(94)90206-2

[9] A.J. Feighery, J.T.S. Irvine, Effect of alumina additions upon electrical properties of 8mol% yttria-stabilized zirconia, Solid State Ionics 121 (1999) 209-216.doi:10.1016/S0167-2738(99)00015-6

[10] S.Y. Yang, J.J. Lee, J.J. Kim, J.S. Lee, Sintering behavior of Y-doped ZrO2 ceramics:


The effect of Al2O3 and Nb2O5 addition, Solid State Ionics 172 (2004) 413-416.doi:10.1016/j.ssi.2004.03.026

[11] L. M. Navarro, P. Recio, P. Duran, Preparation and properties evaluation of zirconiabased/Al2O3 composites as electrolytes for solid oxide fuel cell systems Part 1: Powderpreparation and characterization, J. Mater. Sci. 30 (1995) 1931-1938.doi:10.1007/BF00353015

[12] L. M. Navarro, P. Recio, P. Duran, Preparation and properties evaluation of zirconiabased/Al2O3 composites as electrolytes for solid oxide fuel cell systems Part 2: Sinteringbehaviour and microstructural development, J. Mater. Sci. 30 (1995) 1939-1948.doi:10.1007/BF00353016Can't connect to PubMed

[13] G. Ye, F. Ju, C. Lin, S. Gopalan, U. Pal, Single-step co-firing technique for SOFCfabrication Ceram. Eng. Sci. Proc. 26 (2005) 25-32.doi:10.1002/9780470291245.ch3

[14] S. Wu, Sintering Additives for Zirconia Ceramics, Research Reports in MaterialsScience, Volume 7, Parthenon Press, Carnforth, 1986.

[15] S. Tekeli, U. Demir, Colloidal processing, sintering and static grain growth behavior ofalumina-doped cubic zirconia, Ceram. Int. 31 (2005) 973-980.doi:10.1016/j.ceramint.2004.10.011

[16] V.V. Srdic, M. Winterer, H. Hahn, Sintering behavior of nanocrystalline zirconia dopedwith alumina prepared by chemical vapor synthesis, J. Am. Ceram. Soc. 83 (2000) 1853-1860.

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