Dulcina P. F. de Souza

Structural studies of a ZrO2–CeO2 doped system

Abstract The local structure around Zr, Ce and dopant atoms (Fe and Ni) in the ZrO2–CeO2 system investigated by X-ray absorption spectroscopy (XAS) is reported to better understand the tetragonal phase stabilization process of zirconia. The first coordination shell around Zr atoms is not sensitive to the introduction of dopants or to an increase in the ceria content (from 12 to 20 mol%). Ce ions maintain the eight-fold coordination as in CeO2, but with an altered bond distance. The formation of vacancies resulting from reduction of Ce atoms can be discarded, because XANES spectra clearly show that Ce ions are preferentially in a tetravalent state. XANES and EXAFS experiments at the Fe K-edge evidence that the local order around Fe is quite different from that of the Fe2O3 oxide. On the one hand, ab initio EXAFS calculations show that iron atoms form a solid solution with tetragonal ZrO2. The EXAFS simulation of the first coordination shell around iron evidences that the substitution of zirconium atoms by iron ones generates oxygen vacancies into the tetragonal network. This is a driven force for the tetragonal phase stabilization process. For Ni doped samples, EXAFS results show that Ni–O mean bond length is similar to the distance found in the oxide material, i.e., NiO compound. Besides this result, no evidence of similar solid solution formation for Ni-doped systems has emerged from the EXAFS analysis.

Electrical properties of zirconia–alumina composites

Abstract Sintered zirconia–alumina composites, prepared in a wide range of compositions, are studied in terms of their electrical response. Both grain conductivity and dielectric constant show the typical characteristics expected from the percolation theory, with v c =0.14±0.2 as the critical zirconia volume fraction for the onset of conduction. When the conducting zirconia phase is calcined prior to forming the composite, the whole system still shows a strongly reduced conduction response even for zirconia volume fractions ( v ) in the range of 0.4–0.5, after which it is considerably enhanced for v =0.7. These results are discussed in terms of (i) the influence of the materials microstructure and (ii) the effect of stress resulting from the alumina sintering on the calcined zirconia grains on the overall electrical response of the composite.

Microstructural Evolution of the Al2O3-ZrO2 Composite and its Correlation with Electrical Conductivity

The Al2O3-ZrO2 composite was studied by impedance spectroscopy, a non destructive technique that was found to be sensitive to the composites microstructure. The observed decrease in the zirconia grain and grain boundary conductivities points to compression on zirconia grain by alumina matrix. This effect increased with decreased concentration of zirconia in the composite. Measurements were taken of composites above the percolation threshold for vacancy conduction along the zirconia grains. The effect of densification and grain growth on the composites conduction was measured. The changes in the zirconia grain and grain boundary specific conductivities were found to be correlated.

XPS characterisation of ceria-stabilised zirconia doped with iron oxide

Abstract The addition of iron oxide on ceria-stabilised zirconia was studied by X-ray photoelectron spectroscopy (XPS). Zirconia presents a monoclinic to tetragonal phase transition at 1443 K, which is accompanied by a volume variation of approximately 3–5%, preventing the formation of a dense polycrystalline monoclinic ceramic. Ceria-stabilised tetragonal zirconia presents high toughness and can be applied as a structural material. However, CeO 2 –ZrO 2 has low sinterability, so it is important to investigate the effect of sintering dopants, such as iron, copper and manganese ions, which could improve the sinterability and the mechanical properties of the ceramic. In previous studies, it was shown that the addition of 0.3 mol% of Fe 2 O 3 helped in sintering the ceramic, and enhanced the electrical conductivity, although it was not determined if this enhancement was due to ionic or electronic contribution. In this work, we employed XPS to characterise ZrO 2 + x mol% CeO 2 +0.3 mol% Fe 2 O 3 , x =12 and 20, ceramics sintered at 1450 and 1600°C in order to better understand the influence of iron in the stabilisation of the tetragonal phase and electrical conductivity.

The Effect of Co and Zn Addition on Densification and Electrical Properties of Ceria-Based Nanopowders

In this work, cobalt and zinc-doped Ce0.8Gd0.2O1.9 samples were prepared starting from a commercial nanopowder and compared to the undoped material. The powder samples were pressed and afterwards sintered by a two-step procedure, before characterization by X-Ray Diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Impedance Spectroscopy (IS) in air. Cobalt or zinc additions are effective as sintering aid, allowing peak sintering temperatures around 1000°C to reach densifications above of 93% of theoretical density, showing no evidence for the presence of secondary phases. The total conductivity at 800 °C of pressed Zn-doped samples (6.7x10-2 S/cm) and Co-doped samples (7.5x10-2 S/cm) is similar for undoped samples (7.2x10-2 S/cm) showing that Zn and Co has a positive effect on densification without compromising the electrical conductivity.

Microstructural and Electrical Features of Yttrium Stabilised Zirconia with ZnO as Sintering Additive

Adding ZnO reduces sintering temperature of yttria stabilized zirconia. Adding up to 0.5 wt% of ZnO is possible to densify to 8 mol% yttria stabilized zirconia (TZ8Y) to 95% of relative density at 1300 °C, besides, the electrical conductivity increases about 30% at 800 °C when compared to pure TZ8Y with the same relative density and average grain size. These results show that TZ8Y co-doped with ZnO can be a potential electrolyte to solid oxide fuel cells and electrolyzer cells.

Propriedades elétricas e microestrutura de Céria dopada com Gd+3 e Y+3 para aplicação como eletrólitos em pilhas a combustível

Clean renewable alternative energy source is a great issue of work nowadays. In a near future fuel cells will be the most promising answer of this issue. 8 mol% Yttria stabilized zirconia (8YSZ) is the most used material as electrolyte because of its ionic conduction and chemical stability under reducing and oxidant atmosphere, which allows that this device shows good efficiency at operating temperatures around 1000 oC. Rare earth doped ceria has received special attention due to its high electrical conductivity that permits SOFCs to operate into a range of 500 to 800oC. In this work Y doped ceria and Gd doped ceria electrical conduction are discussed as function of its microstructures obtained in different sintering conditions. 8YSZ results are also presented as reference. The grain electrical conductivity of rare earth doped ceria is bigger than 8YSZ. However, the grain boundary conductivity is mostly affected by the sintering conditions, specially sintering temperature and soaking time. It can be more than 1 order of magnitude smaller than grain conductivity. The grain boundary sensibility is related to the second phase formation along the grain boundary during sintering. The formation of this phase depends on the kind of dopants and it is clearly favored by the presence of Y2O3.

GRAIN BOUNDARY CONDUCTIVITY OF ZIRCONIA YTTRIA AND THE SPACE CHARGE CONCEPT

Abstract Liquid phase sintering of YSZ ceramics containing a small part of Er2O3 displays one transient increase in the grain boundary conductivity after sintering for one hour. This transient increase has been measured for short additional time intervals between one and two hours and found to last for 30 min. The transient effect is attributed to changes in the space charge due to Er+3 ion diffusion to the grains from the glass phase. The driving force for the inversion of the segregation behavior of the stabilizing ions is attributed to a phase separation in the sintering glass phase. The effective thickness of the space charge and its conductivity have been obtained.

Glass phase expelling during liquid phase sintering of YSZ

Expelling of the liquid phase during sintering of ZrO2-6.5 mol% Y2O3- 0.5 mol% Pr2O3 ceramic was observed as a result of grain coarsening. ZrO2- 7.0 mol% Y2O3 samples, without Pr2O3 addition, do not show this effect under the same sintering conditions. The expelling process is caused by surface tension forces and attracting van der Waals forces between the grains, coupled with the existence of two glass phases on the grain boundaries. The amount of expelled glass phase increases with grain growth, but saturates above 16 mm average grain size.

Alumina purification by carbothermal reduction

Sodium, silicon, iron, calcium and gallium as impurities in alumina were removed by carbothermal reduction from alumina particles between 1100 and 1400 °C. Reduction of calcium and silicon oxides starts as low as 1200 and 1100 °C, respectively. Thermodynamics and rate of the alumina cleaning process are discussed.