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Scripta Materialia 58 (2008) 215–218

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Codoping of zirconia with yttria and scandia

Daniel Meyer,a,b,* Ulrich Eisele,a Raphaelle Sateta and Jurgen Rodelb

aRobert Bosch GmbH, Corporate Sector Research and Advance Engineering, Applied Research 1 – Materials (CR/ARM),

Robert Bosch Platz 1, 70839 Gerlingen-Schillerhohe, GermanybTechnische Universitat Darmstadt, Institute of Materials Science, Petersenstrasse 23, 64287 Darmstadt, Germany

Received 2 August 2007; accepted 22 September 2007Available online 25 October 2007

A new zirconia material with increased strength for application as exhaust gas sensor has been developed. This material is cod-oped with yttria and scandia and has been compared to a reference material doped only with yttria. Both mechanical strength andtoughness of the codoped material are enhanced in comparison to the reference material, while the ionic conductivity of the twomaterials is adjusted to be of same magnitude.� 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Ceramics; Electrical properties; Mechanical properties; Fracture; Corrosion

The technology of exhaust gas sensors relies on thetransport of oxygen ions in zirconia materials. Zirconiastabilized with yttria is used as solid electrolyte [1] forthis application as it exhibits both high ionic conductiv-ity and high mechanical strength. Ionic conductivity andmechanical properties of zirconia, however, are con-trarily influenced by the yttria dopant content. The ionicconductivity of zirconia increases with increasing yttriacontent up to a concentration of about 9–10 mol.%yttria. With higher contents of yttria the ionic conduc-tivity decreases [2,3].

Pure zirconia at room temperature has a monocliniccrystal structure, but can be stabilized in the tetragonalphase by the addition of 2–2.5 mol.% yttria [4]. Undermechanical stress the tetragonal phase transforms intothe monoclinic phase accompanied by a volume expan-sion. This leads to transformation toughening of zirco-nia [5,6], providing high-toughness and high-strengthzirconia materials.

The yttria content in exhaust gas sensors is a compro-mise between high ionic conductivity and high mechan-ical strength and is adjusted at about 4–5 mol.% yttriacontent [1]. This yttria content is thus not optimizedfor high strength. For higher applied stresses in exhaust

1359-6462/$ - see front matter � 2007 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2007.09.025

* Corresponding author. Address: Robert Bosch GmbH, CorporateSector Research and Advance Engineering, Applied Research 1 –Materials (CR/ARM), Robert Bosch Platz 1, 70839 Gerlingen-Schillerhohe, Germany. Tel.: +49 711 811 35083; e-mail addresses:dkmeyer@gmx.net; daniel.meyer3@de.bosch.com

gas sensors in the future a higher mechanical strength isrequired.

In the search for other doping concepts for zirconia,alkaline earth elements may also be considered asdopants. The ionic conductivity for zirconia doped withalkaline earth elements, however, is low due to forma-tion of oxygen vacancy clusters [3] and the mechanicalstrength is also low [7]. Codoping zirconia with yttriaand magnesia produces a linear decrease in ionicconductivity from the yttria-doped zirconia [8] and amechanical strength comparable to that of magnesia-stabilized zirconia, which is lower than the strength ofyttria-doped zirconia [9]. As mentioned above, yttria-doped zirconia displays a high ionic conductivity andhigh mechanical strength. In contrast, the highest ionicconductivity amongst the rare earth elements is providedby scandia-doped zirconia. The strength of these materi-als is comparable to that of yttria-doped zirconia[2,3,7,10,11]. The amount of scandia required to achievehigh strength is about 3.5 mol.%, while only 2–2.5 mol.% yttria are required to achieve high-strengthzirconia [5,7,11,12]. This is due to the lower stabilizationof scandia compared to yttria [4], which is also reflectedin a higher hydrothermal degradation rate at the samedopant content [13]. The high ionic conductivity of scan-dia-doped zirconia is due to the ionic radius of the scan-dium, which is close to the ionic radius of zirconium.Hence the deformation of the crystal lattice throughthe introduction of scandium into the lattice of zirconiais low. This leads to a limited formation of associatedoxygen vacancies, which inhibit the ionic conductivity

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216 D. Meyer et al. / Scripta Materialia 58 (2008) 215–218

[3,14–16]. On the other hand, the cost of scandia is quitehigh compared to that of yttria, thus providing a moti-vation to limit the amount of scandia used.

It has been shown that by replacing yttria with scan-dia the ionic conductivity increases linearly from theoverall yttria-doped zirconia conductivity to the overallscandia-doped zirconia conductivity in a 9 mol.% dopedzirconia ceramic [2,17,18]. This has also been demon-strated for an 11 and 8 mol.% doped zirconia ceramic,which displays an irregularity only when the dopantconsists of 50% yttria and 50% scandia [19,20]. The fol-lowing equation gives the ionic conductivity of zirconiaceramic codoped with yttria and scandia:

riðCodopedÞ ¼ riðz mol:% Y2O3Þþyz� riðz mol:% Sc2O3Þ

� riðz mol:% Y2O3Þ ð1Þx mol.% Y2O3 + y mol.% Sc2O3 = z mol.% total dopantcontent; ri: ionic conductivity.

Furthermore, it was demonstrated, that the mechan-ical strength of zirconia can be enhanced by replacingyttria with scandia in the range of 8 mol.% dopant con-tent [18]. In a zirconia material codoped with yttria andscandia the monoclinic phase on the fracture surface wasfound to be enhanced compared to a purely yttria-dopedzirconia, thus demonstrating an increase in transforma-tion toughening [21].

Mechanical strength and ionic conductivity havebeen evaluated for two compositions. One zirconiamaterial has a certain dopant content of yttria (refer-ence). The dopant content of a second material isreduced compared to the first material and is a mixtureof yttria and scandia (codoped). The ionic conductivityaccording to Eq. (1) was adjusted to be the same forthe reference and the codoped material.

Undoped zirconia powder (CS02, Fa. Zirpro), Y2O3

(AMR, 99.99% purity) and Sc2O3 (Auer Remy,99.95% purity) were used as raw materials. Yttria and/or scandia (depending on composition) were dissolvedin nitric acid on a heating plate, and the nitric acidwas then evaporated on the heating plate (RCT basic,IKA Labortechnik). The crystallized rare earth nitratesand hydroxides then were dissolved in deionized waterand the zirconia powder dispersed (Dispermat, VMAGetzmann GmbH). The dispersion was dried in a dryingfurnace (LUT6050, Heraeus) and the dried pieces thencrushed with pestle and mortar. After calcination at650 �C (LHT16/R17, Naber Industrieofenbau) thepowders were annealed at 1600 �C for 5 h (LHT16/R17).

Figure 1. Microstructure of reference (a) and codoped (b) material.

The material obtained was then crushed in a mortarand attrition milled with ZrO2 milling balls and a poly-amide can and stirred for 18 h in water with 1 wt.%Dispex A 40. The milled powder was heated again to400 �C to burn the abraded plastic material of theattritor (LHT16/R17). Round samples with a diame-ter of 25 and 10 mm were uniaxially pressed at 255and 102 MPa, respectively (Elektrohydraulische Presse,P/O/Weber). Pressure-less sintering was performed at1385 �C for 5.5 h (LHT16/R17). The resulting sampleswere about 21 and 8 mm in diameter, respectively, andhad a thickness of about 1 mm. A reference materialwith 3.4 mol.% yttria and a codoped material with1.6 mol.% yttria and 0.8 mol.% scandia, measured byICP-OES (Optima 3300 DV, Perkin Elmer) wereproduced.

Particle size distributions of the powders were mea-sured by laser deflection (Mastersizer 2000, Malvern).The reference and the codoped material have a D50 of0.34 and 0.26 lm, respectively. Density was measuredby the Archimedes method, yielding 98.6% (reference)and 97.9% (codoped) of theoretical density. Microstruc-ture and grain size characterization was performed usinga scanning electron microscope (Supra 35VP, GeminiLEO). Samples were first ground and polished using aRotopol-31 polishing machine by Struers, followed bythermal etching at 1250 �C for 2 h (LHT16/R17). Repre-sentative microstructures are provided in Figure 1. Thegrain size is estimated from the micrographs using quan-titative microstructure analysis software (Carl ZeissKS400) in combination with the linear line interceptmethod (ASTM0). At least 1000 grains were measured.With a constant factor of 1.56 the average three-dimen-sional grain size can be specified from the linear lineintercept length after Mendelson [22]. Both referenceand codoped material show similar average grain sizesof 360 ± 36 and 390 ± 39 nm, respectively. The tetrago-nal phase content of the two materials was assessed byX-ray diffraction (Siemens D5000, quantification withRietveldfit, TOPAS 2.1 software, Bruker-AXS). With96.2 wt.% the tetragonal phase content in the codopedmaterial is about 20% higher than in the reference(79.2 wt.%). For the evaluation of the ionic conductiv-ity, impedance spectroscopy (Solartron SI1260) wasperformed at 500 �C. Round electrodes (diameter5 mm) of silver paste were painted onto the two oppositeflat sides of the 8 mm diameter samples after sinteringand the paste hardened at 817 �C for 8 min. Figure 2shows the results for the impedance analysis. While

Figure 2. Impedance spectra of the reference and the codoped samples. Figure 3. Strength vs. pores identified as critical defects for referenceand codoped zirconia material.

Figure 4. Fracture strength vs. ionic conductivity for reference andcodoped zirconia.

D. Meyer et al. / Scripta Materialia 58 (2008) 215–218 217

the half circle of the grains is flat, the half circle of thegrain boundary can be clearly discerned. The ionic resis-tivity of the material is provided by the real part of thediameter for the grain and the grain boundary (seeFig. 2). The specific ionic conductivity of the materialcan be estimated from the geometry of the sample andthe electrodes:

ri ¼1

qi

tA

ð2Þ

where ri is the ionic conductivity, qi is the ionic resistiv-ity from impedance measurements, t is the thickness ofthe sample and A is the surface of one electrode.

The ionic conductivities of reference and cod-oped material were determined as 4.8 · 10�3 ± 1 · 10�3

and 4.3 · 10�3 ± 0.9 · 10�3 S cm�1, respectively. Thestrength of 27 and 24 specimens which were ground par-allel (325/400) was measured using the ring-on-ring-test(inner diameter 15 mm, outer diameter 30 mm). Thethicknesses of the samples after grinding were about0.6 and 0.7 mm. As these samples and test configurationlead to a non-linearity in stress and strain, the strength ofthe samples was estimated from the breaking force by fi-nite-element analysis. The volume of the tested sampleswas also considered by numerical calculations and thestrengths are given with an effective volume of 1 mm3.The average fracture strength rf was obtained as530 ± 11 MPa for the reference and 570 ± 11 MPa forthe codoped material. A Weibull analysis was carriedout with the values of strength, and the Weibull moduliwere determined with the maximum likelihood methodand a confidence interval of 90% [23]. The Weibull mod-ulus was 9.9 [7.7;12.9] for the reference and 11.2[8.5;14.9] for the codoped material. Fractography on15 broken samples was conducted using the scanningelectron microscope (Supra 35VP, Gemini LEO). A crit-ical defect size was determined using the procedureaccording to Fett and Munz [24]. Mostly pores wereidentified as critical defects. In Figure 3 strength is plot-ted vs. the critical defect size of fracture-causing pores.Figure 3 demonstrates that the strength of the codopedmaterial is higher than that of the reference for a specificdefect size. This indicates a higher toughness for the cod-oped material than for the reference, as shown by theGriffith equation for estimating fracture toughness fromfractography:

rf ¼KIcffiffiffi

ap �

ffiffiffi

pp ð3Þ

where a is the critical defect size and KIc is the fracturetoughness.

Finally, the key result is provided in Figure 4, wherefracture strength and ionic conductivity for both materi-als are contrasted. The fracture strength for the newmaterial is considerably improved while both ionic con-ductivities are comparable.

The electrical conductivity of the codoped material isat the same level as the reference material. Applicationof Eq. (1) leads to good results. The amount of tetrago-nal phase was increased by codoping. This is due to thelower dopant content [5] and the lower stabilization ofzirconia by scandia [4]. The strength of the codopedmaterial is about 10% higher than the strength of thereference material. This is correlated to the highertetragonal phase content in the codoped material, whichis responsible for transformation toughening. A higherdegree of tetragonal to monoclinic transformation incodoped zirconia was also found by Jang [21]. The Wei-bull modulus of the codoped material is high. Fractog-raphy suggests that a higher fracture toughness of thecodoped material is obtained as compared to the refer-ence material, in accordance with the higher tetragonalphase content and the resulting enhanced transforma-tion toughening.

Two zirconia compositions were produced and ionicconductivity and mechanical properties investigated.Higher mechanical strength and fracture toughnesswas achieved for zirconia codoped with yttria and scan-dia compared to a simply yttria-doped reference mate-rial, while the ionic conductivity of the two materialswas adjusted to be the same.

218 D. Meyer et al. / Scripta Materialia 58 (2008) 215–218

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