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

Response behaviour of oxygen sensing solid electrolytes

The response time (t r) after a step change in oxygen partial pressure was investigated for some solid electrolytes used in Nernst type oxygen sensors. The electrolyte as well as the (porous) electrode material affect the value oft r. Stabilized Bi2O3 materials exhibit slower response rates (largert r values) than stabilized ZrO2. Introduction of Bi2O3 in stabilized ZrO2 increases the response time. Gold electrodes show a higher response rate than platinum in the oxygen partial pressure and temperature region used.\u

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

Electrochemical characterisation of 3Y-TPZ-Fe2O3 composites

The influence of the addition of ferric oxide to 3Y-TZP on the conductivity and microstructure of sintered Y-stabilised tetragonal zirconia ceramics (3Y-TZP) was investigated. A comparison was made between two different dense 3Y-TZP¿¿-Fe2O3 composites. Compacts were made by pressureless sintering at 1150 °C or by sinterforging at 1000 °C and 100 MPa. The sinterforging process resulted in smaller zirconia and hematite grains and a higher monoclinic zirconia content as compared to the compact that was sintered pressureless. The high monoclinic content led to loss of ionic conductivity. The addition of ferric oxide caused electronic conductivity. The sinterforging resulted in a high concentration of metastable defects in the zirconia¿hematite composite, leading to a relatively high electronic conductivity. Heating above 380 °C caused irreversible loss of these defects and a large decrease in electronic conductivity

The luminescence of yitria stabilized zirconia doped with Bi2O3

Yttria stabilized zirconia doped with Bi2O3 shows an efficient luminescence due to the Bi3+ ions. Two luminescent centres were observed which are ascribed to BiO8 and BiO7 groups

Sinter forging of zirconia toughened alumina

Sinter forging experiments have been carried out on powder compacts of zirconia toughened alumina (ZTA) Ceramics Alumina-15 wt% zirconia was prepared by a gel precipitation method and calcined at temperatures of 900 or 1100°C. Full densification of ZTA ceramics was obtained within 15 min at 1400°C and 40 MPa. A homogeneous microstructure can be observed with an alumina grain size of 0.7 mgrm and a zirconia grain size of 0.2 mgrm. Almost no textural evolution occurred in the microstructure. During sinter forging the densification behaviour of the compacts was improved by an effective shear strain, for which values of more than 100% could be obtained. As a result of the shear deformation the densification of ZTA in the agr alumina phase stage shifted to lower temperature. During pressureless sintering the gamma to agr alumina transformation temperature was dependent of the preceding calcination temperature, while during sinter forging this phase transformation was independent of calcination temperature and took place at a lower temperature

Improvement of mechanical properties of zirconia-toughened alumina by sinter forging

ZTA powder with a composition of 85 wt% alumina/15 wt% zirconia was prepared by a gel precipitation method. Sinter forging was performed with this powder to enhance the mechanical properties of ZTA materials. The influence of processing flaws on mechanical properties of sinter forged materials and pressureless sintered materials was investigated. Sinter forging at 40 MPa effectively decreases process flaw sizes resulting in a homogeneous microstructure and improves the grain boundary structure because of large shear applied in this process. Sinter forging resulted in an increase in strength and toughness by a factor of 1.5–2 when compared with pressureless sintered compacts. The fracture energy is enhanced by a factor of two. The predominate mechanism for improvement of mechanical properties of these sinter-forged ZTA materials is grain boundary strengthening

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 (ldquoalkoxiderdquo 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 (ldquochloriderdquo 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)

Plasticity of nanocrystalline zirconia ceramics and composites

The deformation strain rate of nanocrystalline Y-TZP shows an increase by a factor 4 if the grain size decreases from 200 to 100 nm. Real superplastic deformation (strain rate > 10−4 s−1) is observed in these materials at relative low temperature (1100–1200 °C). Grain-boundary analysis indicates (partial) removal of an ultra-thin (1 nm), yttrium-rich grain boundary layer after deformation.\ud \ud Uniaxial pressure-assisted sintering techniques (=sinter-forging) provide the opportunity of large shear strains during densification. Sinter-forging experiments on zirconia-toughened alumina (15 wt% ZrO2/85 wt% Al2O3) resulted in a dense composite within 15 min at 1400 °C and 40 MPa, with effective shear strains up to 100%. Sinter-forging of Y-TZP and ZTA gives an increase in strength, reliability and fracture toughness. These improvements are caused by the large shear strains that result from the removal of processing flaws. Also, the number of microcraks at the grain boundaries and the interatomic spacing between the grains are reduced by the forging techniques, resulting in a strengthening of the grain boundaries if compared with pressureless sintering. K1C values of 10 MPa√m are obtained for Y-TZP, while no classical stress-induced phase transformation toughening is observed. Sinter-forged ZTA samples showed a better wear resistance than free sintered ones.\u