Aloysius J.A. Winnubst

Effect of impurities on sintering and conductivity of yttria-stabilized zirconia

The effect of low concentrations of Fe2O3, Al2O3 and Bi2O3 on the sintering behaviour of (ZrO2)0.83 (YO1.5)0.17, made by alkoxide synthesis, has been investigated. The best results are achieved with Bi2O3 as a sinter agent and a relative density of 95% is obtained at 1200 K. The effects of these impurities on the electrical conductivity of the bulk and the grain boundaries has been investigated using frequency dispersion analysis (101-106 Hz). All investigated impurities have a negative influence on both the bulk and grain-boundary conductivity. For Fe2O3 and Al2O3 grain-boundary segregation factors of about two are calculated.

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

The surface- and grain boundary composition of Y, Ce and Ti doped zirconia were studied by X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy/Scanning Auger Microscopy. The grain boundaries and free surfaces showed the same enrichment levels. After heat treatment ≥ 1000 ‡C all yttria doped samples showed yttrium enrichment. In the ZrO2-Y2O3 system the yttrium enrichment did not depend on the bulk concentration and amounted 30–34 mol% YO1.5 in all cases. As a consequence the segregation factor increases with decreasing solute concentration in the bulk. The thickness of the segregation layer was about 2–4 nm. In the ternary Y doped systems yttrium is the main segregant. In ceria-doped tetragonal zirconia polycrystals (Ce-TZP) systems significant segregation of cerium starts atT≥1300‡C and is mainly attributed to Ce3+. In Y,Ti-TZP systems also strong segregation of Ti4+ occurs. The absolute value of the increase of the surface concentration in fine grained material is smaller than in coarse grained material. This is mainly due to depletion of the bulk.

Sintering kinetics and microstructure development of nanoscale Y-TZP ceramics

Zirconia samples doped with 3 mol% yttria were prepared by gel precipitation from a metal chloride solution and their sintering behaviour compared with that of a commercial powder. Dense (relative density 97%) nanoscale ceramics with a mean grain size of 60 nm are obtained after sintering at 1050°C for 7h. Important densification mechanisms in the initial sintering stage are grain boundary sliding and grain boundary diffusion. Grain growth in the final sintering stage seems to be impurity drag controlled. Extremely low activation energies are obtained for both densification and grain growth in the initial sintering stages. Special attention has been paid to the effect of aggregate size of the precursor powder on the final grain size.

Powder preparation and compaction behaviour of fine-grained Y-TZP

Two wet chemical preparation methods are described for yttria-doped tetragonal zirconia powders. Both methods yield powders with an extremely small crystallite size (8 nm) and a narrow size distribution. The agglomerate and aggregate structure of these powders have been investigated by several techniques. Gel precipitation from an alkoxide solution in water (“alkoxide” synthesis) results in a ceramic powder with irregular-shaped weak and porous agglomerates, which are built up from dense aggregates with a size of 18 nm. Gel precipitates formed from a metal-chloride solution in ammonia (“chloride” synthesis) do not contain aggregates. Both types of agglomerate are fractured during isostatic compaction. Hydrolysis and washing under (strong) basic conditions probably decrease the degree of aggregation. The aggregate morphology and structure are key parameters in the microstructure development during sintering of a ceramic. Several characteristics of these powders are compared with those of a commerical one (Toyo Soda TZ3Y).

Microstructural development, electrical properties and oxygen permeation of zirconia-palladium composites☆

Yttria-stabilized cubic zirconia (YSZ)-palladium dual phase composites have been investigated. The percolative composite containing 40 vol% Pd (ZYPd40) showed a much larger oxygen permeability than that of the non-percolative composite containing 30 vol% Pd (ZYPd30). For a 2.0 mm thick percolative composite, an oxygen flux of 4.3 × 10−8 mol/cm2/s was measured at 1100 °C with oxygen partial pressures at the feed and permeate sides being 0.209 and 0.014 atm, respectively. This value is two orders of magnitude larger than that observed for a 2.0 mm thick non-percolative composite at the same temperature with the oxygen partial pressures at the feed and permeate sides being 0.209 and 1.5 × 10−4 atm, respectively. From the dependence of the oxygen permeation on the temperature and on the oxygen partial pressures, it was concluded that the transport of the oxygen ions through the YSZ phase in the percolative system was the rate limiting step.

Mechanical properties of ultra-fine grained zirconia ceramics

The mechanical properties of tetragonal zirconia (TZP) materials doped with Y, Ce or Ti were studied as a function of temperature and grain size. Fine grained Y-TZP (grain size < 0.3 μm) shows values for fracture toughness and strength at room temperature, which are comparable with the coarse grained transformation toughened materials, despite lacking transformation toughening. The morphology of the fracture surface points to crack deflection as the most important toughening mechanism. At 800 °C fracture toughness and strength are higher than in coarse grained Y-TZP materials. Doping Y-TZP with Ce or Ti results in a similar trend in mechanical properties, for fine grained material, as for the Y-TZP materials.

AES/STEM grain boundary analysis of stabilized zirconia ceramics

Semiquantitative Auger Electron Spectroscopy (AES) on pure monophasic (ZrO2)0.83(YO1.5)0.17 was used to determine the chemical composition of the grain boundaries. Grain boundary enrichment with Y was observed with an enrichment factor of about 1.5. The difference in activation energy of the ionic conductivity of the grain boundary compared with the bulk can be explained by the Y segregation. When Bi2O3 is introduced into this material and second phase appears along the grain boundaries of the cubic main phase. Energy dispersive X-ray analysis (EDS) on a scanning transmission electron microscope (STEM) shows an enrichment of bismuth at the grain boundaries of this second phase.

Yttria-Ceria stabilized tetragonal zirconia polycrystals: Sintering, grain growth and grain boundary segregation

An analysis is presented of grain growth and densification of yttria-ceria stabilized tetragonal zirconia polycrystals (Y, Ce-TZPs) using both isothermal and non-isothermal techniques. The characteristics of Y, Ce-TZPs are compared to those of Y-TZP and Ce-TZP and the effect of increasing ceria concentration at constant yttria content is evaluated. During non-isothermal sintering two regimes are distinguished: below 900–1000°C the neck area increases strongly by surface diffusion accompanied by only very little densification and grain growth, in the temperature interval 900–1000°C to 1200°C the materials densify to 95% of the theoretical density via a grain boundary diffusion mechanism and grain growth accelerates. Dense materials with grain sizes of 0·15–0·20 μm can be prepared by isothermal sintering at 1100–1150°C. In Y, Ce-TZP it is yttrium that segregates to the grain boundaries at 1150–1400°C. The yttrium content of the grain boundaries in Y, Ce-TZP is independent of temperature and ceria-concentration under the investigated experimental conditions. Grain growth in dense TZP is controlled by a solute drag mechanism at elevated temperatures (>1200°C); this drag is highest for Y-TZP, absent for Ce-TZP and moderate for Y, Ce-TZP.

The electrode resistance of ZrO2---Y2O3(-Bi2O3) solid electrolytes with Pt electrodes

The electrode resistance (Re1) at low over-voltages has been determined for Bi2O3 free and Bi2O3 doped yttria-stabilized zirconia with sputtered platinum electrodes. The anode and cathode resistances are measured separately and are equal. Bi2O3 causes a decrease of the electrode resistance at small overvoltage and with 5 × 10−4 < PO2 less-than-or-equals, slant 1 atm. The rate determining step for the electrode reaction under the measuring conditions probably is a charge-transfer process in which oxygen adsorbs dissociatively on the bismuth free and associatively on the bismuth doped samples. The complex impedance behaviour of the electrode reaction can be interpreted in terms of R-C circuits in the PO2 range of 10−2−1 atm.

Effect of Dopants on the Sintering Behaviour and Stability of Tetragonal Zirconia Ceramics

The microstrueture development during non-isothermal and isothermal sintering has been studied for tetragonal zireonia ceramics (TZP) containing carious amounts of Y, Ce and Ti. Smaller grain sizes were obtained when Ce-TZP was doped with yttrium. This could he attributed to segregation of yttrium to the grain boundaries, thus causing an impurity drag. With increasing temperature the grain growth in the Ce-TZP samples became faster, which could be attributed to the absence of a dragging force. The slow grain growth at low temperature in the Ce-TZP samples could he attributed to the slow diffusion kinetics of the diffusing species (trivalent and tetravalent cerium). The critical grain size for retainment of the tetragonal phase at room temperature is larger in the ECe-TZP systems compared to the Y-TZP amt Ce-TZP systems. The chemical stability increased by doping Y-TZP with cerium or titanium.