Y2O3-doped Zirconia Research Articles

Indentation-induced stress distribution and pressure effect on the resistivity of YSZ

Ionic conductivities measured under GPa-order high pressure provide various information about ion hopping mechanisms such as the activation volume (ΔV). Traditionally, anvil cells have been used for high-pressure measurements. We previously reported a new method for high-pressure impedance measurements, up to a few GPa, employing an indentation-induced local stress field. In this method, both mechanical and electrical (Young's modulus and high pressure impedance) properties can be obtained simultaneously. However, in this method, high pressures are induced only around the tip of the indenter, and such stress distribution should be considered for the estimation of ΔV accurately. In the present study, employing a finite element method (FEM) calculation, the stress distribution around the tip of the indenter, and effects of such GPa-order high pressures on the O2− ion conduction of Y2O3-doped zirconia (YSZ) are shown.

High-pressure (GPa) impedance measurements based on an indentation-induced local stress field

Electrical measurements of conducting and dielectric materials under high pressures (in the order of GPa) reveal important information regarding orbital overlaps, electronic states, changes in transition temperatures, and activation volumes (Delta V). In this study, we demonstrate a new method for high-pressure impedance measurements, up to 4 GPa, utilizing an indentation-induced local stress field. The current system does not require any pressure mediums or pressure calibrations. The Delta V for O2- ion conduction in 10 mol\% Y2O3-doped zirconia at 500 degrees C was estimated to be 3.0 cm(3) mol(-1). Delta V increased with increasing temperatures from 500 to 600 degrees C. The technique also allows the concurrent determination of the effective elastic modulus by fitting the experimental data obtained from the indentation load-depth profile curve with the Hertzian elastic model. The experimental values were consistent with the theoretical values. (C) 2013 Elsevier B.V. All rights reserved.

Accelerated degradation of 8.5 mol% Y2O3-doped zirconia by dissolved Ni

This study focuses on the clarification of the accelerated degradation of the ionic conductivity of Ni-containing 8.5mol% Y2O3-doped ZrO2 (8YDZ) under reducing atmosphere, a transition that proceeds 50 times faster in comparison to pure 8YDZ. A comparative study of high-performance anodes and NiO/YDZ thin-film specimens (NiO thin film on thick 8YDZ and 10YDZ electrolytes) has been carried out to characterize the microstructural and chemical evolution of the YDZ under operating conditions (reducing atmosphere). The extremely deep indiffusion of Ni (>100μm) into microcrystalline YDZ was revealed after sintering at 1400°C for 5h (air). Hence, such Ni-containing 8YDZ is expected to form whenever NiO and 8YDZ are co-sintered at high temperature, e.g., during half-cell (anode with thin-film electrolyte) fabrication. Moreover, the spinodal decomposition of the Ni-containing 8YDZ on the short time scale was identified and studied by conventional and analytical transmission electron microscopy (TEM). This comprises both the microstructural coarsening as well as the strong chemical decomposition on the nanoscale. Since the decomposition rate is directly related to the mobilities of the host cations these have to be enhanced by the dissolved Ni. The explanation therefore is based on a localized strain field that is expected to arise in the vicinity of individual Ni species due to electron capture under reducing atmosphere. It is proposed that the coulomb interactions are weakened (due to the expansion of the YDZ lattice) while lattice vibrations are enhanced leading to lower potential barriers for ion hopping and thus to higher cation mobilities.

Surface proton conductivity of dense nanocrystalline YSZ

Ionic transport in nanocrystalline solid oxides is of considerable current interest. Several studies have reported room-temperature proton conductivity in nanoscaled 8 mol% Y2O3-doped zirconia (YSZ), although the location of the transport species is not clear. In this study, nanocrystalline YSZ with a grain size of ∼50 nm was prepared by spark-plasma sintering of nanoscaled commercial powder to a density of >97% of the theoretical value. Impedance spectroscopy was employed to analyze the electrical behavior in the temperature range 25–600 °C in H2O- and D2O-wetted atmospheres (air, O2 and 10% H2 : 90% N2). Transport in wet conditions below 50 °C is limited to the sample surface and occurs via proton hopping (Grötthus mechanism), as demonstrated by a conductive H+/D+ isotope effect. The impedance in these conditions is dominated by a single arc which can be modelled with parallel paths for proton transport on the lateral sample surface and a capacitance with values of the order of those of the grain interior. Surface proton transport is considerable, exhibiting a resistance at 26 °C in wet atmospheres comparable to that obtained at ∼300 °C. In contrast, transport by volumetric processes (grain, grain boundary or nanopores) was demonstrated to be insignificant by experiments involving conductivity measurements with a coated lateral surface, with electrode configurations of different areas, and emf measurements in a water-vapour concentration cell.

Impact of Ni on Accelerated Degradation of 8.5 Mol% Y2O3-Doped Zirconia

This study focuses on the investigation of the influence of solved nickel on the accelerated degradation of the ionic conductivity of 8YDZ (8.5 mol% Y2O3-doped zirconia) under reducing atmosphere. In comparison to pure 8YDZ, the degradation of Ni-containing 8YDZ proceeds 50 times faster and is completed after 100 h at 950 oC. The impact of Ni-incorporation into 8YDZ on microstructure and chemistry was studied by light microscopy (LM) and transmission electron microscopy (TEM). After thermal treatment under reducing atmosphere, a characteristic colour change of 8YDZ is observed by LM that is attributed to the in-diffusion (> 50 µm) of Ni into the 8YDZ substrate. TEM dark-field imaging reveals the coarsening of nano-scaled precipitates of the metastable t'-YDZ in cubic 8YDZ which is correlated with the degradation of the ionic conductivity. Since the decomposition rate of 8YDZ is determined by the mobility of Y and Zr, the dissolved Ni must have a strong impact thereon. An explanation based on the reduction of solved Ni under reducing operation conditions is proposed. It is based on the strain field which are expected to arise in the vicinity of the Ni-atoms after reduction.

Impact of Ni on Accelerated Degradation of 8.5 Mol% Y2O3-Doped Zirconia

not Available.

Internal Friction of Some Ceria Stabilized Zirconia Ceramics

The internal friction of some ceria stabilized zirconia ceramics were measured in the temperature range of 30-450°C in air using two method, piezoelectric resonance method and forced torsion method. The internal friction was observed for 12%CeO2 doped ZrO2 which was reduced in flowing hydrogen. The doping of CeO2 to cubic ZrO2-Y2O3 suppressed the amplitude of internal friction. The result suggested the lowing effect of CeO2 to the mobility of oxygen in cubic Y2O3-doped zirconia ceramics.

Sintering kinetics at constant rates of heating: effect of GeO2 addition on the initial sintering stage of 3 mol% Y2O3-doped zirconia powder

The sintering behavior of 3 mol% Y2O3-doped zirconia powders with and without a small amount of GeO2 was investigated to clarify the effect of GeO2 addition on the initial sintering stage. The shrinkage of powder compact was measured under constant rates of heating (CRH). The sintering rate was accelerated by GeO2 addition, and increased with increasing GeO2 content. The mechanism, apparent activation energy (nQ), and apparent frequency factor (β0n) of diffusion at the initial sintering stage were estimated using the sintering-rate equations that are applicable to the CRH data. The sintering mechanism changed from grain-boundary (GBD) to volume diffusions (VD) by GeO2 addition, and both nQ and β0n of diffusion increased with increasing GeO2 content. It is, therefore, concluded that the enhanced sintering mechanism by GeO2 addition is explained by the GBD→VD change and increases in both nQ and β0n of diffusion at the initial sintering stage.

Mechanical properties of ytterbia stabilized zirconia ceramics (Yb-TZP) fabricated from powders prepared by co-precipitation method

2.0, 2.4, 2.6, 3.0 and 3.5 mol% Yb2O3-doped tetragonal zirconia polycrystals (Yb-TZP) were fabricated and their mechanical properties [three-points bending strength (σf), Vickers hardness (Hv) and fracture toughness (K1C)] were measured. The value of σf was the highest for the 2.6 mol% Yb2O3-doped sample. The Hv-value increased and the K1C-value decreased, as the Yb2O3 content increased. The comparison of these mechanical properties with those of the corresponding Y2O3-doped zirconia (Y-TZP) revealed the excellence of Yb-TZP, though slight.

Vacancy Ordering in the ZrO2Rich Part of the Systems Ca–Zr–O–N, Mg–Zr–O–N, and Y–Zr–O–N

CaO, MgO, and Y2O3doped zirconia samples were nitrided at temperatures between 1400 and 2000°C in a graphite heated resistance furnace in a nitrogen atmosphere. Depending on the content of nitrogen and aliovalent oxides, different types of ordered (β type) and disordered (tetragonal, cubic) phases were observed. Nitridation of pure ZrO2results in the formation of the β″ phase. With increasing content of the aliovalent oxides, other β type phases (β′, β) are detectable. The stability fields of the individual β type phases are determined by the ratio ofVN/Vtotaland by the kind of dopant oxide. In the system Y–Zr–O–N, the transition tetragonal–cubic was observed for higher Y2O3concentrations. In this case, the “effective” total concentration of vacancies is the dominant factor which controls the transition.

耐火物中の不純物の高温蒸発がジルコニア焼結体表面の着色に及ぼす影響

Coloration is an important effect in zirconia ceramics. We have observed a phenomena of coloration on the surface of 3mo1%Y2O3-doped zirconia in sintered processing used some of new refractory. XPS, TG and IR analyses have proved that the problem is caused by the influence of residual impurities in the refractory.The authors however have found a interested results that the phenomena was improved by reheating the refractory in the high temperature for several times before using, and the doping of Y203 or A1203 content into ZrO2 ceramics(PSZ) has improved the coloration.

Hysteresity effects in 3 mol% yttria-doped zirconia (t'-phase)

Single-crystal and polycrystal samples of 3 mol % Y2O3-doped zirconia (t'-phase) were subjected to uniaxial compression tests at 1000°C in order to separate the effects of phase transformation (tetragonal-to-monoclinic) from ferroelastic domain switching. Plastic deformation was observed after an elastic regime, with attributes characteristic of domain switching. X-ray diffraction traces at room and high temperatures before and after the compression test verified that there was, indeed, a variant reorientation within each sample. Deformation bands were observed on single crystals, and Raman spectroscopy revealed that no monoclinic phase was present. These results verify the existence of ferroelastic domain switching phenomenon in this material.

Effects of organic binders on the sintering of isostatically compacted zirconia powders

Certain processing-related flaws in cold isostatically pressed ceramic powder compacts may arise from the delayed burn-out of organic binders until the sintering temperature is approached, although the isostatic compaction technique usually gives a higher and much more uniform green density than the conventional die compaction technique. For the 3 mol% Y2O3-doped zirconia powder in which 3 wt% PEG 1500 was introduced, the sintered density and sintering shrinkage were found to decrease in a near linear manner with increasing isostatic compaction pressure. The processing-related defects were identified as intergranular pores (1–5 μm). It is considered that these processing-related defects are a consequence of incomplete organic burn-out at low and intermediate temperatures in the heating-up period and the swelling of intergranular pores associated with the burn-out of residual organic binders at temperatures close to the sintering temperature. A higher calcination temperature and an extended calcination dwell time may be required to eliminate the organic residuals in the isostatically pressed ceramic powder compacts than in the conventional die-pressed samples.

Effects of ZrO2 and Y2O3 dissolved in zyttrite on the densification and the?/? phase transformation of Si3N4 in Si3N4-ZrO2 composite

In Si3N4-ZrO2 composite, the effects of zirconia and Y2O3 dissolved in zyttrite on the densification and theα/β phase transformation of Si3N4 were studied using hot-pressing of Si3N4 with the addition of pure, 3, 6, and 8 mol% Y2O3-doped zirconia. Reaction couples between Si3N4 and ZrO2 of zyttrite were made to observe the reaction phenomena. The addition of pure zirconia was not effective to obtain full density of the Si3N4-ZrO2 composite. However, Y2O3 diffused from the added zyttrite promoted densification; the density of Si3N4 with 5 vol% pure ZrO2 composite was 71% theoretical, and nearly full density (>97%) could be obtained in Si3N4 with 5 vol% 6, 8 mol% Y2O3-doped ZrO2 composite. On the basis of observations of the Si3N4-pure ZrO2 reaction couple, the reaction between Si3N4 and ZrO2 resulted in the formation of Si2N2O phase, and theα/β phase transformation of Si3N4 occurred via this Si2N2O phase. From the XRD analysis of the reaction layer between Si3N4 and zyttrite, it is suggested that the reaction products, Y2Si2O7 and Y2Si3N4O3 phases, play an important role in the densification of Si3N4-zyttrite composite.