Surface and grain boundary analysis of doped zirconia ceramics studied by AES and XPS

Theunissen, G.S.A.M.; Winnubst, Louis; Burggraaf, A.J.

doi:10.1007/bf01105274

Cited by 149 publications

(109 citation statements)

References 20 publications

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“…Specific information on the atomic structure of ZrO 2 ⅐10 mol % Y 2 O 3 grain boundaries has recently been obtained by analytical high-resolution transmission electron microscopy (23,24). Making use of Z-contrast imaging and electron energy-loss spectroscopy (EELS) on a symmetric 24°[001] tilt boundary an increase of the yttrium concentration in the grain boundary core by about a factor of 2 has been found, as anticipated by early auger electron spectroscopy͞x-ray photoelectron spectroscopy (25) and energy dispersive x-ray spectroscopy (22) studies. A concomitant increase in the concentration of oxygen vacancies exceeding the value expected from the enhanced yttrium concentration has been observed in agreement with atomic simulations of the grain boundary (23).…”

Section: Figmentioning

confidence: 99%

Enhanced oxygen diffusivity in interfaces of nanocrystalline ZrO ₂ ⋅Y ₂ O ₃

Knöner

Reimann

Röwer

et al. 2003

Proc. Natl. Acad. Sci. U.S.A.

120

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First measurements of oxygen grain boundary diffusion coefficients in nanocrystalline yttria-doped ZrO2 (n-ZrO2⅐6.9 mol % Y2O3) are presented. The 18 O diffusion profiles measured by secondary ion mass spectroscopy are much deeper in the nanocrystalline specimens than in single crystals. An oxygen diffusivity, D B, in the grain boundaries can be deduced, which is Ϸ3 orders of magnitude higher than in single crystals. From the present data the temperature variation of the oxygen grain boundary diffusivity, D B ‫؍‬ 2.0 ؋ 10 ؊5 exp (؊0.91 eV͞kBT) m 2 ͞s, and the oxygen surface exchange coefficient, k ‫؍‬ 1.4 ؋ 10 ؊2 exp (؊1.13 eV͞kBT) m͞s, are derived. Y ttria stabilized zirconia (ZrO 2 ⅐Y 2 O 3 ) is widely used as an electrolyte material in solid oxide fuel cells or oxygen sensors. The technological importance of the material arises from its extraordinary oxygen transport properties, which are attributed to an oxygen vacancy mechanism. The replacement of quadrivalent Zr by trivalent Y gives rise to a high concentration of structural oxygen vacancies for charge compensation and stabilizes the cubic phase (1). The structural vacancies dramatically increase the mobility of oxygen ions, and the highest ionic conductivities were reported for Y 2 O 3 concentrations of 7-10 mol % (2).There is a broad interest in reducing the high operating temperatures of solid oxide fuel cells, typically 900-1,000°C, and resulting degradation effects by increasing the oxygen ion conductivity of the electrolyte. In undoped monoclinic nanocrystalline ZrO 2 it was found that the diffusion of oxygen in interfaces is 10 3 to 10 4 times faster than in the bulk of the crystallites (3). The question arises as to whether the high oxygen diffusivities of ZrO 2 ⅐Y 2 O 3 upon doping can be further enhanced by the introduction of a large number of crystallite interfaces. A reason for an enhanced oxygen diffusion could be a loosely packed grain boundary structure, which is well established for pure metals (4), or easy formation of vacancy-type free volumes within the grain boundaries (5). On the other hand, there are reports that oxygen transport across grain boundaries could be impeded by a blocking effect due to space charge layers in the grain boundary region (6) or silica-containing grain boundary phases (7). In the present study, we present direct measurements of the grain boundary diffusion coefficients in nanocrystalline ZrO 2 ⅐Y 2 O 3 , which are accomplished by using 18 O as a tracer and secondary ion mass spectroscopy for depth profiling.Fully dense nanocrystalline ZrO 2 ⅐Y 2 O 3 bulk specimens were synthesized by dc sputtering of a ZrY metal target and crystallite condensation in an argon atmosphere at 240 Pa. Subsequent oxidation by slow exposure to an oxygen atmosphere (2,000 Pa) and in situ uniaxial compaction at high pressures (2 GPa) yielded disk-shaped specimens 5 mm in diameter with a relative mass density ͞ 0 Ϸ89% of the green body and a mean crystallite size Ͻ5 nm. From the sputter target with the composition of Zr 84 Y 16 , speci...

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Section: Figmentioning

confidence: 99%

Enhanced oxygen diffusivity in interfaces of nanocrystalline ZrO ₂ ⋅Y ₂ O ₃

Knöner

Reimann

Röwer

et al. 2003

Proc. Natl. Acad. Sci. U.S.A.

120

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“…XPS measurements of segregation in YCe-TZP at 1000-1400~ to the external surface [39,403 or the grain boundaries [5] showed, however, that no such segregation of cerium occurs during sintering. Therefore, the segregation of cerium to the external surface as observed by Hernandez et al must have taken place during ageing.…”

Section: Ageing Of Tetragonal Zirconiamentioning

confidence: 99%

The effect of ceria co-doping on chemical stability and fracture toughness of Y-TZP

Boutz

Winnubst

Langerak

et al. 1995

JOURNAL OF MATERIALS SCIENCE

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The fracture toughness and ageing resistance of yttria, ceria-stabilized tetragonal zirconia polycrystals (Y, Ce-TZP) were evaluated as a function of grain size and ceria content. Very fine grained, fully dense materials could be produced by sinter forging at relatively low temperatures (11 50-1200 ~ The ageing resistance in hot water (185 ~ of 2 mol % Y203 -stabilized TZP is strongly enhanced by alloying with ceria. The ceria content necessary to avoid degradation completely, decreases with grain size. The toughness of fully dense Y, Ce-TZP is 7-9 MPa m 1/2 for grain sizes down to 0.2 ~tm. No or very little transformation took place during fracturing and no clear variation with grain size was observed for the toughness at grain sizes up to 0.8 pm. Reversible transformation and crack deflection may explain the observed toughness values.

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“…Yttria segregation to the external surface has been observed for YSZ containing an Y 2 O 3 doping between 2 and 19 mol %. [2][3][4] The process of segregation is driven by the reduction of the surface free energy of the electrolyte. Strain relaxation and the formation of a space-charge region associated with defects and aliovalent cations are considered to be the main contributions in the reduction.…”

Section: Introductionmentioning

confidence: 99%

Subsurface segregation of yttria in yttria stabilized zirconia

Ridder

Welzenis

Gon

et al. 2002

Journal of Applied Physics

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The segregation behavior in 3 and 10 mol % polycrystalline yttria stabilized zirconia ͑YSZ͒, calcined at temperatures ranging from 300 to 1600°C, is characterized using low-energy ion scattering ͑LEIS͒. In order to be able to separate the Y and Zr LEIS signals, YSZ samples have been prepared using isotopically enriched 94 ZrO 2 instead of natural zirconia. The samples are made via a special precipitation method at a low temperature. The segregation to the outermost surface layer is dominated by impurities. The increased impurity levels are restricted to this first layer, which underlines the importance of the use of LEIS for this study. For temperatures of 1000°C and higher, the oxides of the impurities Na, Si, and Ca even cover the surface completely. The performance of a device like the solid oxide fuel cell which has an YSZ electrolyte and a working temperature around 1000°C, will, therefore, be strongly hampered by these impurities. The reduction of impurities, to prevent accumulation at the surface, will only be effective if the total impurity bulk concentration can be reduced below the 10 ppm level. Due to the presence of the impurities, yttria cannot accumulate in the outermost layer. It does so, in contrast to the general belief, in the subsurface layer and to much higher concentrations than the values reported previously. The difference in the interfacial free energies of Y 2 O 3 and ZrO 2 is determined to be Ϫ21Ϯ3 kJ/mol.

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Surface and grain boundary analysis of doped zirconia ceramics studied by AES and XPS

Cited by 149 publications

References 20 publications

Enhanced oxygen diffusivity in interfaces of nanocrystalline ZrO ₂ ⋅Y ₂ O ₃

Enhanced oxygen diffusivity in interfaces of nanocrystalline ZrO ₂ ⋅Y ₂ O ₃

The effect of ceria co-doping on chemical stability and fracture toughness of Y-TZP

Subsurface segregation of yttria in yttria stabilized zirconia

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Surface and grain boundary analysis of doped zirconia ceramics studied by AES and XPS

Cited by 149 publications

References 20 publications

Enhanced oxygen diffusivity in interfaces of nanocrystalline ZrO 2 ⋅Y 2 O 3

Enhanced oxygen diffusivity in interfaces of nanocrystalline ZrO 2 ⋅Y 2 O 3

The effect of ceria co-doping on chemical stability and fracture toughness of Y-TZP

Subsurface segregation of yttria in yttria stabilized zirconia

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Enhanced oxygen diffusivity in interfaces of nanocrystalline ZrO ₂ ⋅Y ₂ O ₃

Enhanced oxygen diffusivity in interfaces of nanocrystalline ZrO ₂ ⋅Y ₂ O ₃