Yttria-stabilized Zirconia Grains Research Articles

Fabrication and Microstructure of Self-Supporting Thin Ceramic Electrolytes Prepared by Laser Machining

Self-supporting thin Yttria Stabilized Zirconia (YSZ) ceramics electrolytes have been prepared by laser machining. They are carved from a sintered YSZ plate to shape a 20 µm thick and 8 mm in diameter central region, surrounded by an unprocessed 150 µm thick supporting zone. Scanning Electron Microscopy (SEM) and Electron BackScattering Diffraction (EBSD) studies confirmed that the strains produced by the laser processing are small and limited to only one or two layers of YSZ grains (~5 µm). SEM and Transmission Electron Microscopy (TEM) have been also used to characterize the surface of the membrane. It is corrugated and coated with YSZ nanoparticles as a result of the laser plasma deposition. Electrochemical characterization by Impedance Spectroscopy (EIS) showed that this surface morphology improves the electrical performance of the membrane slightly but clearly, reducing the cathode polarization resistance by about 5% in the 650-850 ºC range.

Investigations of soot combustion on yttria-stabilized zirconia by environmental transmission electron microscopy (ETEM)

Environmental transmission electron microscopy (ETEM) equipped with an aberration corrector was used to get further insights into the mechanism of soot oxidation on yttria-stabilized zirconia (YSZ), an O2− ionic conductor. In situ observations, with a resolution of around 1nm, of a “tight” YSZ/real soot mixture were performed at 550°C in the presence of a few mbar of oxygen (2 and 3mbar). Preliminary experiments were conducted to validate that moderate electron beam illumination conditions do not control the soot combustion. Video sequences recorded at 550°C clearly confirmed that bulk YSZ oxygen atoms are the active species for soot oxidation at the soot/YSZ interface as no oxidation takes place without contact. The number of soot/YSZ contact points and their evolution with time are the key parameters for soot oxidation. The contact length between the soot and the YSZ grains could be estimated to be around 20–40nm only.

Thermal Shock Induced Phases Transformation and Microstructural Changes in a Novel Hydrogen Transport Membrane

ABSTRACTBulk samples of a novel cermet (ceramic/metal composite) hydrogen transport membrane (HTM) were subjected to thermal cycling in the temperature range between 25-850°C to study phase transformations and microstructural changes under thermal shock. Scanning electron microscopy (SEM) and electron probe micro analyzer (EPMA) with energy dispersive spectroscopy (EDS) were used to characterize the microstructural and chemical changes in the membrane upon thermal cycling. SEM & EPMA analyses indicated that the temperature gradient during thermal cycling produced more micro-cracks inside the HTM disc, whereas, the chemical reaction between Pd and oxygen to form PdO disturbed the continuity of the metal palladium (Pd) - Yttria Stabilized Zirconia (YSZ) dual phases interconnection system from surface down. The agglomerates of un-crystallized YSZ grains found to be the inherent in the cracks of the as-received HTM. A combination of trans-granular and inter-granular crack propagation results around the YSZ grains and the new precipitates. Based on the electron fractography analyses by both SEM and EPMA, the micro voids coalescence develops ahead of the crack tips in the cross-section of the HTM after 500 thermal cycles.

Significant reduction in hydration energy for yttria stabilized zirconia grain boundaries and the consequences for proton conduction.

Using well-established atomistic techniques, we investigate the defect chemistry, structural effects, and energetics of proton incorporation at the Σ5(310)/[001] and Σ5(210)/[001] symmetrical tilt grain boundaries of yttria-stabilized zirconia. Building upon past work, we consistently show a dramatic decrease (∼4-5 eV) in the proton incorporation and hydration energies in and around the grain boundary structures compared to values obtained for the bulk material and undoped ZrO2 grain boundaries. This decrease is prevalent in both Y segregated grain boundaries and grain boundaries where the distribution of Y is completely random. The results presented here strongly support the argument that proton conduction in this system is primarily interfacially driven, as reported by numerous experimental studies. Redox properties are also presented for grain boundaries structures both with and without defect segregation. The methodology and results presented here can also be applied to a wide range of proton conductors and will prove essential in any future assessment of the effects of grain boundaries on the defect chemistry of protons in these systems.

Deterioration of yttria-stabilized zirconia by boron carbide alone or mixed with metallic or oxidized Fe, Cr, Zr mixtures

In the frame of severe accident conditions (PHEBUS FPT3 test), different experiments were carried out on the interactions of 20% yttria-stabilized zirconia (YSZ) and 20% ceria-stab zirconia with boron carbide or its oxidation products (B2O3): either tests under steam between 1230° and 1700°C with B4C alone or B4C mixed with metals, either tests under Ar with boron oxide present in a mixture of iron and chromium oxides. In all cases an interaction was observed with formation of intergranular yttrium borate. At 1700°C boron oxide is able to “pump out” the Y stabiliser from the YSZ grains but also some trace elements (Ca and Al) and to form a eutectic containing YBO3 and yttrium calcium oxy-borate (YCOB). At the same time a substantial swelling (“bloating”) of the zirconia happens, qualitatively similar to the foaming of irradiated fuel in contact with a Zr-melt. In all samples the lowering of the Y (or Ce)-content in the YSZ grains is so sharp that in the interaction layers zirconia is no longer stabilized. This is important when YSZ is envisaged as simulant of UO2 or as inert matrix for Am-transmutation.

Synthesis and characterization of nanocrystalline yttria-stabilized zirconia for an electrolyte in a solid-oxide fuel cell

We synthesized various sizes of nanocrystalline yttria-stabilized zirconia (YSZ) for an electrolyte in solid-oxide fuel cell (SOFC) by using the sol-gel method. The surface morphology and the thermo-mechanical properties for different nano-grain sizes were characterized. The YSZ grains were found to grow non-linearly, as well as non-logarithmically, with increasing sintering temperature. The surface morphology shows that the grains of nano-sized YSZ are well dispersed in the measured area, with a narrow size distribution and without significant aggregation. The values of the thermal expansion coefficient of nano-grain YSZ obtained in this study are close to those of the other components of the electrodes and the interconnects used in the SOFC. The effect of the grain size on the thermo-mechanical property is also found to be negligible.

Electrochemical decomposition of CO2 and CO gases using porous yttria-stabilized zirconia cell

Electrochemical decomposition of CO2 and CO gases using a porous cell of Ru-8mol% yttria-stabilized zirconia (YSZ) anode/porous YSZ electrolyte/Ni–YSZ cathode system at 400–800°C was studied by analyzing the flow rate and composition of outlet gas, current density, and phases and elementary distribution of the electrodes and electrolyte. A part of CO2 gas supplied at 50ml/min was decomposed to solid carbon and O2 gas through the cell at the electric field strengths of 0.9–1.0V/cm. The outlet gas at a flow rate of 3ml/min included 61–63% CO2 and 37–39% O2 at 700–800°C and the outlet gas at a flow rate of 50ml/min included 73–96% (average 85%) CO2 and 4–27% (average 15%) O2 at 800°C. On the other hand, the supplied CO gas was also decomposed to solid carbon, O2 and CO2 gases at 800°C. The fraction of outlet gas at a flow rate of 50ml/min during the CO decomposition at 800°C for 5h was 11–36% CO, 59–81% O2 and 2–9% CO2. The detailed decomposition mechanisms of CO2 and CO gases are discussed. Both Ni metal in the cathode and porous YSZ grains under the DC electric field have the ability to decompose CO gas into solid carbon and O2− ions or O2 gas.

Evaluation of Microstructural Evolution of Nanostructured Yttria-stabilized Zirconia During Sintering Using Impedance Spectroscopy

A non-destructive evaluation (NDE) technique of impedance spectroscopy (IS) was employed for studying the sintering process of nanostructured yttria-stabilized zirconia (YSZ) after exposure at 1100 °C in air for different times. The variations within microstructure of YSZ were correlated with the EIS parameters. The results showed that the resistance and capacitance of YSZ grains (g) and grain boundaries (gb) varied with the sintering time. The resistance of YSZ g and gb increased significantly in 10 h, which may correspond to closure of pores. While during the stage from 10 to 200 h, the resistance of the g was basically consistent, and the gb decreased with the growth of g, which indicated that the gb resistance was more sensitive to grain size. The change of porosity and pore shape could be interpreted through impedance parameters. The porosity decreased and the shape of pores became smaller and rounder with the increasing sintering time.

Caracterização microestrutural de materiais cerâmicos utilizando o programa de processamento digital de imagens Image J

Neste trabalho foram quantificados o tamanho de grão e a porosidade de uma amostra de zircônia estabilizada com ítria (ZEI) sinterizada a 1500 ºC/ 2 h e a fração volumétrica das fases ZEI, NiO e poros de um compósito de ZEI/NiO sinterizado a 1500 ºC/ 2 h. Utilizando o programa de análise de imagens ImageJ foi possível tratar as imagens removendo ruídos e evidenciando as regiões de interesse de análise. Após as imagens serem analisadas, foi feito o tratamento estatístico dos dados como sugerido nas normas ASTM E 562-02 e ASTM E 1382-97. As frações volumétricas obtidas desta maneira foram 50 ± 7% de NiO, 32 ± 2% ZEI e 17 ± 7% poros. O tamanho médio de grãos da ZEI foi 4,6 ± 0,2 µm e sua porosidade 1,5 ± 0,2%. Os resultados de porosidade foram comparados com os valores obtidos utilizando o método baseado no princípio de Arquimedes. A diferença entre os valores permitiu por este método quantificar somente a porosidade aberta, enquanto a análise de imagens quantifica também os poros fechados.

Influence of Microstructure on the Thermal Diffusivity of Sintered Porous YSZ

The microstructure of sintered porous yttria‐stabilized zirconia (YSZ) pellets was controlled by adjusting the sintering temperature and adding appropriate amounts of polymethyl methacrylate powder to produce pores. The thermal diffusivity of the pellets was measured using the laser flush technique. The influence of pellet microstructure on its thermal diffusivity was investigated. At both high and low temperatures, the influence of the density, pore size, YSZ grain size, grain‐boundary concentration, and the ratio of closed pores on the phonon diffusion and photon heat radiation was clarified.

The separation of grain and grain boundary impedance in thin yttria stabilized zirconia (YSZ) layers

An improved electrode geometry is proposed to study thin ion conducting films by impedance spectroscopy. It is shown that long, thin, and closely spaced electrodes arranged interdigitally allow a separation of grain and grain boundary effects also in very thin films. This separation is shown to be successful for yttria stabilized zirconia (YSZ) layers thinner than 20nm. In a series of experiments it is demonstrated that the extracted parameters correspond to the YSZ grain boundary and grain bulk resistances or to grain boundary and substrate capacitances. Results also show that our YSZ films produced by pulsed-laser deposition (PLD) on sapphire substrates exhibit a bulk conductivity which is very close to that of macroscopic YSZ samples.

Investigation of aluminosilicate as a solid oxide fuel cell refractory

Aluminosilicate represents a potential low cost alternative to alumina for solid oxide fuel cell (SOFC) refractory applications. The objectives of this investigation are to study: (1) changes of aluminosilicate chemistry and morphology under SOFC conditions, (2) deposition of aluminosilicate vapors on yttria stabilized zirconia (YSZ) and nickel, and (3) effects of aluminosilicate vapors on SOFC electrochemical performance. Thermal treatment of aluminosilicate under high temperature SOFC conditions is shown to result in increased mullite concentrations at the surface due to diffusion of silicon from the bulk. Water vapor accelerates the rate of surface diffusion resulting in a more uniform distribution of silicon. The high temperature condensation of volatile gases released from aluminosilicate preferentially deposit on YSZ rather than nickel. Silicon vapor deposited on YSZ consists primarily of aluminum rich clusters enclosed in an amorphous siliceous layer. Increased concentrations of silicon are observed in enlarged grain boundaries indicating separation of YSZ grains by insulating glassy phase. The presence of aluminosilicate powder in the hot zone of a fuel line supplying humidified hydrogen to an SOFC anode impeded peak performance and accelerated degradation. Energy dispersive X-ray spectroscopy detected concentrations of silicon at the interface between the electrolyte and anode interlayer above impurity levels.

The Grain Size Effect on the Yttria Stabilized Zirconia Grain Boundary Conductivity

Conductivity of yttria stabilized zirconia produced from weakly agglomerated nanopowder was investigated by ac impedance spectroscopy. Dense ceramic samples with the grain size from 90 to 800 nm were prepared at the variation of both pressing and sintering conditions. It was found that the bulk conductivity is not affected by grain size, while grain boundary conductivity is dependent on this factor. Observed grain boundary resistance increases with grain size. This relationship is contrary to the previous results obtained for the range from 1 to 18 microm where grain boundary resistance decreased with grain size. Maximum of grain boundary resistance versus ceramics grain size is observed at 450 degrees C for the grain size about 270 nm.

In situ redox cycle of a nickel–YSZ fuel cell anode in an environmental transmission electron microscope

Environmental transmission electron microscopy is used in combination with density functional theory calculations to study the redox stability of a nickel/yttria-stabilized zirconia solid oxide fuel cell anode. The results reveal that the transfer of oxygen from NiO to yttria-stabilized zirconia triggers the reduction reaction. During Ni reoxidation, the creation of a porous structure, due to mass transport, accounts for the redox instability of the Ni-based anode. Both the expansion of NiO during a redox cycle and the presence of stress in the yttria-stabilized zirconia grains are observed directly. Besides providing an understanding of the Ni–YSZ anode redox degradation, the observations are used to propose an alternative anode design for improved redox tolerance.

The Effect of Transition-Metal Doping on Chromium Deposition at Pt/YSZ Cathode Interfaces

Understanding compositional effects, especially at solid oxide fuel cell operating conditions, is important for the selection of materials that minimize the impact of chromium deposition. In this study, the effect of yttria-stabilized zirconia (YSZ) doping on Cr deposition at a Pt electrode/YSZ electrolyte interface was investigated. A comparison between undoped and doped YSZ electrolyte surfaces after cathodic polarization in moist air at in the presence of chromia indicated that the presence of Mn, Co, or Fe in the electrolyte significantly promotes Cr deposition at the electrode/electrolyte interface. Electron microscopy of the Co-doped YSZ electrolyte surface after cathodic polarization for 10–45 h clearly showed that with Co doping, the deposition of Cr occurs preferentially at YSZ grain boundaries. Impedance spectroscopy results showed that chromium deposition led to a decrease in resistance with Mn- and Fe-doped YSZ electrolyte, which was attributed to the increase in the three-phase boundary length. However, this beneficial effect of Cr deposition was not observed with Co doping, which may be related to the preferential deposition of chromia at the YSZ grain boundary, resulting in fewer Cr deposits at the cathode/electrolyte interface.

Influence of asymmetric electrode geometry on an impedance spectrum of a plasma-sprayed thermal barrier coating system

Influence of asymmetric electrode geometry on an impedance spectrum of a plasma-sprayed thermal barrier coating (TBC) system was investigated. The impedance spectrum of the TBC system included impedance of the yttria stabilized zirconia (YSZ) grains, YSZ grain boundaries (negligible), the thermally grown oxide (TGO) and the electrode reaction. In the TBC system with a continuous TGO layer, the impedance of the YSZ grain was measured without influence of asymmetric electrode geometry above 100 kHz. In a frequency range below 100 kHz, asymmetric electrode geometry induced the spread of an electrical conduction region outside an electrode attached on the top coating surface. The impedance of the TGO and the electrode reaction was significantly affected by the asymmetric electrode geometry. The precise interpretation of an impedance spectrum of TBC systems measured under the condition of asymmetric electrode geometry requires further studies on the spread of an electrical conduction region due to decrease in frequency, increase in asymmetry of electrode geometry.

Performance and durability of nanostructured (La 0.85Sr 0.15) 0.98MnO 3/yttria-stabilized zirconia cathodes for intermediate-temperature solid oxide fuel cells

In this communication we report the fabrication of nanostructured (La 0.85Sr 0.15) 0.98MnO 3 (LSM)/yttria-stabilized zirconia (YSZ) composite cathodes consisting of homogeneously distributed and connected LSM and YSZ grains approximately 100 nm large. We also investigate for the first time the role of the cathode nanostructure on the performance and the durability of intermediate-temperature solid oxide fuel cells. The cathodes were fabricated using homogenous LSM/YSZ nanocomposite particles synthesized by co-precipitation, using YSZ nanoparticles of 3 nm as seed crystals. Detailed microstructural characterization by transmission electron microscopy with energy-dispersive X-ray spectroscopy revealed that many of the LSM/YSZ junctions in the cathode faced the homogeneously connected pore channels, indicating the formation of a considerable number of triple phase boundaries. The nanostructure served to reduce cathodic polarization. As a result, these anode-supported solid oxide fuel cells showed high power densities of 0.18, 0.40, 0.70 and 0.86 W cm −2 at 650, 700, 750 and 800 °C, respectively, under the cell voltage of 0.7 V. Furthermore, no significant performance degradation of the cathode was observed during operation at 700 °C for 1000 h under a constant current density of 0.2 A cm −2.

Kinetics of Cathodic Reaction at Porous Composite Electrode under Mixed Control of Migration and Oxygen Exchange

The oxygen reduction kinetics was investigated at the porous (LSM)–yttria-stabilized zirconia (YSZ) composite electrodes with various YSZ grain sizes by employing ac impedance spectroscopy and the potentiostatic current transient (PCT) technique. For this purpose, LSM-YSZ composite electrodes were coated on a YSZ pellet by sintering at in air. The grain size of YSZ was controlled by varying the initial particle size of YSZ. From analyses of the ac impedance spectra and the cathodic PCTs measured on the electrodes as functions of temperature and ionic conductivity, it was first recognized that the overall cathodic reaction is mainly controlled by the oxygen-exchange reaction coupled with ion migration at low operating temperatures below . Second, from the discrete Fourier transformation analysis of impedance spectra, it was confirmed that the distribution of relaxation times for oxygen-ion migration becomes wider with increasing YSZ grain size in the range from due to the larger effective migration length . Finally, from the theoretical and experimental analyses of the cathodic PCTs based upon the equivalent circuit in consideration of the relaxation time distribution for ion migration, it is suggested that as the YSZ grain size decreases, the higher density of the three-phase-boundary (TPB) length leads to an increase in the overall current density and a reduction in the time to reach the steady-state current density , thus causing high power density.

The properties of YSZ electrolyte sintering at 1300 °C

The properties of yttria stabilized zirconia(YSZ) related to the sintering process were discussed. YSZ nano-powders about 40–100 nm as raw material, the sub-micrometer grain sizes such as 0.4–3 μm in YSZ were gotten by sintering process at 1300 °C, which was performed at 1000 °C for 2 h, then raised the temperature at the rate of 50 °C/h to 1400 °C, then decreased directly to 1300 °C in 30 minutes, finally at 1300 °C for 5–20 hours. The ratio of bigger grain size becomes larger as the holding time increasing at 1300 °C. The grains less than 1 μm are about 50%, eg, 43.2%, 52.2% and 51.1% related to 1300 °C holding 5 hours, 8 hours and 10 hours, respectively. As YSZ grain size became small, the electrical conductivities did not decrease, even increased, about 0.20 s/cm at 1000 °C. The reduced sintering temperature and time were benefited to co-fire with the electrodes in electrode-supported SOFCs.

Space Charge‐Mediated Ionic Transport in Yttria‐Stabilized Zirconia–Alumina Composite Membranes

This paper reports ionic conductivity of yttria‐stabilized zirconia (YSZ)–Al2O3 composite membranes. The tape cast specimens were subjected to binder burnout (500°C) and sintering (1550°C) processes to obtain 200–300 μm thick membranes. The ionic conductivity and microstructure of the membranes were characterized and are discussed in this paper. The ionic conductivity of the composite specimens was enhanced and was correlated with the number of charge carrier and their mobility. The solubility of Al2O3 in YSZ was minimal and nanosize Al2O3 of the batch sintered into microsize and existed as a distinct phase. The scanning electron microscopy micrographs revealed that YSZ and Al2O3 grains were strained.