Tetragonal YSZ Research Articles

Mitigating low-temperature hydrothermal degradation of 2 mol% yttria stabilised zirconia and of 3 mol% yttria stabilised zirconia/nickel oxide by calcium oxide co-doping and two-step sintering

Hydrothermal degradation deteriorates the fracture toughness and strength of tetragonal stabilised zirconia. The phase transformation to the monoclinic phase is particularly critical for materials with lower stabiliser content such as 2 mol% and 3 mol% yttria stabilised zirconia (2YSZ and 3YSZ). In this work, two routes in two-step sintering and co-doping with calcium oxide are analysed to mitigate the degradation. Both strategies show a reduction in average grain size of the 2YSZ while maintaining a comparable densification. The achieved smaller grains suppress the hydrothermal degradation rates. In addition, a mitigating effect beyond the reduction in grain size of YSZ is found for CaO doping. 1.6 mol% CaO co-doped 2YSZ shows less than 4% of monoclinic phase after 50 h in an autoclave with H2O at 134 °C. Pure 2YSZ contrarily reaches 100% monoclinic phase after 20 h at the same conditions. A suppression of degradation by CaO doping was also observed for the composite of nickel oxide and 3 mol% yttria stabilised zirconia. Hence, CaO co-doping can be an interesting strategy to increase resistivity against hydrothermal degradation for both biomedical and renewable energy applications. The findings further outline a route to achieve tetragonal YSZ with lower yttria contents than 2 mol%.

Investigation of phase and structural states in nanocrystalline powders based on zirconium dioxide

The behavior of a polycrystalline body during sintering ZrO2 is determined not so much by the properties of individual particles but by the properties of aggregates, and the strength of the bonds between powder particles significantly affects the sintering process and the final properties of the ceramic composite materials. An increase in density and a uniform porosity distribution is achieved at such a pressure and sintering temperature when the process of intensive recrystallisation has not yet begun. Conventional sintering at a temperature of 1500 °C with a holding time of 1 h of ZrO2 and ZrO2–20 wt. % Al2O3 samples with an initial particle size of 120 nm and 40 nm, respectively, allowed us to obtain a material with a relative density of 81% (ZrO2) and 86% (ZrO2–20 wt. % Al2O3). The method of electroconsolidation was used to produce bulk samples of submicron ceramics of ZrO2–n wt. % Al2O3 (n = 10; 20; 30) compositions with a density of up to 99% of the theoretical one and a microstructure scale of about 270 nm. It was established that the addition of metastable Al2O3 nanopowders causes a shift in the onset of shrinkage of tetragonal YSZ to high temperatures and affects the reduction of the grain size of tetragonal YSZ to 170–200 nm.

Immobilization of radiotoxic elements with Y‐stabilized zirconia: The thorium case

AbstractY‐stabilized zirconia (YSZ) phases were found to incorporate Th atoms, and the corresponding solubility ranges depend on the relative Y content. For the tetragonal phases with a lower Y concentration of 14 at.%, a maximal possible Th intake on the Zr/Y metal site reached ca. 10.3 at.%. Cubic phases with higher Y content could dissolve 11 at.% Th in an equilibrium state and up to ca. 12.3 at.% Th under nonequilibrium conditions. Larger Th–Zr/Y solubility range for phases with higher Y concentration was found to be related to the associated symmetry increase, as concluded from synchrotron radiation powder diffraction data. Specifically, the introduction of Th into tetragonal YSZ induces evolution toward higher cubic symmetry via flattening of the Zr/YO8 polyhedra. In addition, a tetragonal YSZ crystal lattice exhibits strongly anisotropic expansion with a concomitant decrease in tetragonality upon the intake of Th. This results in an easier accommodation of bigger Th atoms via a structural stabilization of longer Zr/Y–O bonding distances which yields more symmetrical coordination of central Zr/Y metal ions by surrounding O atoms. Cubic symmetry is, therefore, more favorable to the incorporation of large tetravalent actinide elements.

Study of microstructure, electronic structure and thermal properties of Al2O3-doped tetragonal YSZ coatings

Al2O3-doped tetragonal yttria-stabilized zirconia (t-YSZ) coatings were prepared by plasma spraying under various spraying parameters. The analysis of variance reveals that only Al2O3 addition significantly affects thermal properties of coatings. The electronic structures and microstructures results show that the increase in number of defects and boundary density with the increasing addition of Al2O3 enhances phonon scattering resulting in a decrease in the thermal conductivity, which suggests that the Al2O3 addition can improve the insulation performance of the YSZ coating. The increase in boundary density with the increase in Al2O3 addition is due to recrystallization resulting from the decreasing melting point of Al2O3–YSZ composites, lattice distortion, and the presence of amorphous structure and monoclinic Al2O3 pinning at grain boundaries. The change in the coefficient of thermal expansion with Al2O3 addition is attributable to the variation in the bonding strength of the Al2O3-doped t-YSZ caused by the change in lattice constants.

Modification of NiCoCrAlY with Pt: Part II. Application in TBC with pure metastable tetragonal (t′) phase YSZ and thermal cycling behavior

A thermal barrier coating system comprising Pt-modified NiCoCrAlY bond coating and nanostructured 4 mol.% yttria stabilized zirconia (4YSZ, hereafter) top coat was fabricated on a second generation Ni-base superalloy. Thermal cycling behavior of NiCoCrAlY-4YSZ thermal barrier coatings (TBCs) with and without Pt modification was evaluated in ambient air at 1100 °C up to 1000 cycles, aiming to investigate the effect of Pt on formation of thermally grown oxide (TGO) and oxidation resistance. Results indicated that a dual layered TGO, which consisted of top (Ni,Co)(Cr,Al)2O4 spinel and underlying α-Al2O3, was formed at the NiCoCrAlY/4YSZ interface with thickness of 8.4 μm, accompanying with visible cracks at the interface. In contrast, a single-layer and adherent α-Al2O3 scale with thickness of 5.6 μm was formed at the interface of Pt-modified NiCoCrAlY and 4YSZ top coating. The modification of Pt on NiCoCrAlY favored the exclusive formation of α-Al2O3 and the reduction of TGO growth rate, and thus could effectively improve overall oxidation performance and extend service life of TBCs. Oxidation and degradation mechanisms of the TBCs with/without Pt-modification were discussed.

Spectroscopic investigation of the electronic structure of yttria-stabilized zirconia

The electronic structure and optical properties of yttria-stabilized zirconia are investigated as a function of the yttria content using multiple experimental and theoretical methods, including electron energy-loss spectroscopy, Kramers-Kronig analysis to obtain the optical parameters, photoelectron spectroscopy, and density functional theory. It is shown that many properties, including the band gaps, the crystal field splitting, the so-called defect gap between acceptor $({\mathrm{Y}}_{\mathrm{Zr}}^{{}^{\ensuremath{'}}})$ and donor $({\mathrm{V}}_{\mathrm{O}}^{\ifmmode\bullet\else\textbullet\fi{}\ifmmode\bullet\else\textbullet\fi{}})$ states, as well as the index of refraction in the visible range exhibit the same ``zig-zag-like'' trend as the unit cell height does, showing the influence of an increased yttria content as well as of the tetragonal--cubic phase transition between $8\phantom{\rule{4pt}{0ex}}\mathrm{mol}%$ and $20\phantom{\rule{4pt}{0ex}}\mathrm{mol}%\phantom{\rule{0.16em}{0ex}}{\mathrm{Y}}_{2}{\mathrm{O}}_{3}$. Also, with \ifmmode \check{C}\else \v{C}\fi{}erenkov spectroscopy (CS), a new technique is presented, providing information complementary to electron energy-loss spectroscopy. In CS, the \ifmmode \check{C}\else \v{C}\fi{}erenkov radiation emitted inside the TEM is used to measure the onset of optical absorption. The apparent absorption edges in the \ifmmode \check{C}\else \v{C}\fi{}erenkov spectra correspond to the energetic difference between the disorder states close to the valence band and the oxygen-vacancy-related electronic states within the band gap. Theoretical computations corroborate this assignment: they find both, the acceptor states and the donor states, at the expected energies in the band structures for diverse yttria concentrations. In the end, a schematic electronic structure diagram of the area around the band gap is constructed, including the chemical potential of the electrons obtained from photoelectron spectroscopy. The latter reveal that tetragonal YSZ corresponds to a $p$-type semiconductor, whereas the cubic samples exhibit $n$-type semiconductor properties.

Microstructural and Thermal properties of Plasma Sprayed YSZ Nano-Clusters Thermal Barrier Coatings

In the present study, a novel durable three layered thermal barrier coating (TBCs) were prepared using atmospheric plasma spray (APS) on Ni718 superalloy substrate consisting of the YSZ nano-clusters. In order to develop a functionally graded coating system, the non-transformable (t′) tetragonal YSZ nano-clusters (40 nm) were synthesized by a sol–gel process and characterized at the temperature 1200 °C for 100 h. NiCrAlY was used as bond coat. The developed coating system introduces a protective top layer of MoSi2 (top coat) for preventing diffusion of oxygen, oxidation of the bond coating, provides thermal insulation and protection against corrosion and high temperature erosion. Microstructural, thermal oxidation resistance, thermal shock and adhesion strength of TBCs were analyzed. Different properties of as-sprayed TBCs have no significant effect on thermal oxidation property. The TBCs have shown better thermal shock resistance but lower adhesion strength than the TBCs made of without MoSi2 layer.

The effect of ZnO addition on the phase transformation, microstructure, and ionic conductivity of 8YSZ ceramics

In this paper the effect of zinc oxide (ZnO) addition on the phase transformation and ionic conductivity of 8 mol % Yttria-stabilised Zirconia (8YSZ) is investigated. Pure 8YSZ and ZnO doped YSZ ceramics are prepared using the solid state reaction method sintered at 1550°C for 2 hours. The X-ray Diffraction (XRD) results reveal the presence of tetragonal, cubic and monoclinic phase of Zirconium dioxide (ZrO 2 ) for all undoped and doped YSZ sintered samples. The phase stability of tetragonal YSZ was found to be increased with the increase in ZnO addition. Minor fraction of monoclinic phase was found in pure YSZ sintered sample and the amount of monoclinic phase decreased with the increasing amount of Zn after sintering at 1550°C for 2 hours. The fraction of cubic phase was also found to decrease with the increase in Zn concentration. The highest ionic conductivity of 1.03×10 −3 S cm −1 was obtained at room temperature for samples with 3 mol% ZnO. Pure 8YSZ sintered sample on the other hand, yielded 9.88x10 −4 S cm −1 .

The effect of sintering temperature on the microstructure and phase transformation in tetragonal YSZ and LZ/YSZ composites

In this paper, the effect of sintering temperature on the microstructure and phase transformation in lanthanum zirconate/yttria stabilized zirconia (LZ/YSZ) composites was studied. The phase stability of tetragonal YSZ was found to be decreased with the increase in sintering temperature along with the increase in volume fraction of LZ in LZ/YSZ composites. The addition of LZ in YSZ resulted in the phase transformation from tetragonal YSZ to monoclinic YSZ at high temperatures. Minor fraction of monoclinic phase was found in YSZ matrix of 30LZ and 50LZ composites after sintering at 1473K for 200h. The fraction of cubic phase was also found to increase with the increase in sintering time and temperature. However, no correlation of cubic phase was found to exist with the addition of LZ content. Coherent growth between LZ/YSZ interface was observed using transmission electron microscopy. Due to different expansion coefficients of LZ and YSZ, the largest thermal stress appeared (during cooling process) across the LZ/YSZ interface. Thermal stress promoted the phase transformation (tetragonal to monoclinic) in YSZ and distorted the grains. In LZ/YSZ composites, the average tensile stress increased with the increase in LZ content and temperature. At all sintering temperatures, the YSZ exhibited tensile stress while a compressive stress exited in LZ matrix. In all three (30LZ, 50LZ and 70LZ) composites, the maximum average stress was present in the LZ/YSZ matrix at sintering temperature of 1675K.

Effect of Ce Substitution on Mechanical and Thermal Properties of Tetragonal YSZ: First-Principles Calculations

The effect of impurity Ce on the mechanical and thermal properties of tetragonal ZrO2 stabilized by rare earth element Y (YSZ) have been studied using first principles density functional theory within generalized gradient approximation (GGA) for the exchange correlation potential. The predicted elastic constants indicate that YSZ and Ce doped YSZ (CeYSZ) are mechanically stable structures. And then the numerical estimates of bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, sound velocity and minimum thermal conductivity were performed using the calculated elastic constants and analyzed for the first time. The values of sound velocity from different orientations are also reported. The agreement between the results of the available experiments and our calculations was satisfactory. Our calculated results indicate that Young’s modulus, hardness, mean sound velocity and minimum thermal conductivity of YSZ can be decreased by Ce substitution. The reasons are from the “softened” Ce-O bond strength using bond population and relative volume change under external hydrostatic pressure. Chemical bonding nature was also analyzed from the density of states and electron density difference.

Upconversion luminescence and blackbody radiation in tetragonal YSZ co-doped with Tm(3+) and Yb(3+).

Lanthanide doped inorganic nanoparticles with upconversion luminescence are of utmost importance for biomedical applications, solid state lighting and photovoltaics. In this work we studied the downshifted luminescence, upconversion luminescence (UCL) and blackbody radiation of tetragonal yttrium stabilized zirconia co-doped with Tm(3+) and Yb(3+) single crystals and nanoparticles produced by laser floating zone and laser ablation in liquids, respectively. The photoluminescence (PL) and PL excitation (PLE) were investigated at room temperature (RT). PL spectra exhibit the characteristic lines in UV, blue/green, red and NIR regions of the Tm(3+) (4f(12)) under resonant excitation into the high energy (2S+1)LJ multiplets. Under NIR excitation (980 nm), the samples placed in air display an intense NIR at ∼800 nm due to the (1)G4→(3)H5/(3)H4→(3)H6 transitions. Additionally, red, blue/green and ultraviolet UCL is observed arising from higher excited (1)G4 and (1)D2 multiplets. The power excitation dependence of the UCL intensity indicated that 2-3 low energy absorbed photons are involved in the UCL for low power levels, while for high powers, the identified saturation is dependent on the material size with a enhanced effect on the NPs. The temperature dependence of the UCL was investigated for single crystals and targets used in the ablation. An overall increase of the integrated intensity was found to occur between 12 K and the RT. The thermally activated process is described by activation energies of 10 meV and 30 meV for single crystals and targets, respectively. For the NPs, the UCL was found to be strongly sensitive to pressure conditions. Under vacuum conditions, instead of the narrow lines of the Tm(3+), a wide blackbody radiation was detected, responsible for the change in the emission colour from blue to orange. This phenomenon is totally reversible when the NPs are placed at ambient pressure. The UCL/blackbody radiation in the nanosized material exhibits non-contact pressure colour-based sensor characteristics. Moreover, tuning the color of the blackbody radiation in the nanoparticles by harvesting the low energy photons into the visible spectral region was found to be possible by adjusting the excitation power, paving the way for further developments of these nanoparticles for lighting and photovoltaic applications.

Fuel and Operation Dependence of Interfacial Nanostructure from the Anode and Cathode of Solid Oxide Fuel Cells

In Solid Oxide Fuel Cells (SOFCs), electrochemical reactions take place at the triple-phase boundaries (TPBs) between the electrolyte, the electrode and the gaseous reactant. Small changes in TPB structure or chemistry, and the interface between electrode and electrolyte can drastically affect SOFC performance and lifetime. However, very limited experimental work has been reported on the nanostructure and chemistry evolution of the interface and TPBs in the SOFC upon cell operation. The present work employed Transmission Electron Microscopy (TEM) to investigate the nanostructure and chemistry of the grain boundaries and interfaces including TPB in both anode and cathodes of SOFC.For the Ni/YSZ anodes of SOFC, the evolution of interface nanostructure and chemical composition were systematically examined with respect to the thermodynamic state of cell operation. The fuels evaluated in this present study include H2, synthesis gas, and fuels with trace (ppm) phosphine. Previous study shows the existence of an interface ribbon phase of NiO in the Ni/YSZ interfaces, but not along the Ni/Ni grain boundaries. Previous study also indicates that the thickness of the NiO ribbon layer is dependent upon cell operation time, with thicker NiO ribbon phases developing at longer operation times. The present study further demonstrates that evolution of the NiO ribbon phase is also fuel dependent. For the cells operated for an equivalent duration of 196 hours, the thickness of the NiO layer is about 10 nm under 97.5% H2 (with balance of N2), and the thickness of the NiO layer is about 50 nm under 25% H2 (balance N2). The interaction of trace (ppm) phosphine with Ni/YSZ anode of commercial SOFCs has also been investigated and evaluated for both synthesis gas and hydrogen fuels. Experiments indicate that degradation rates and mechanisms are fuel dependent. Degradation of cells operated in synthesis gas (syngas) with phosphine is more severe than that from cells operated in hydrogen with phosphine. In the cell operated in syngas containing 10 ppm phosphine, significant microstructure degradation was observed within both the Ni phase and the YSZ phase. In addition to the formation of Ni-P phases on the outer layer of the anode, significant pitting corrosion was observed in the Ni grains. At the YSZ side, a previously undetected YPO4 phase is observed at the YSZ/YSZ/Ni triple phase boundaries, and tetragonal YSZ and cubic YSZ domains with sizes of several tens of nanometers are observed along the Ni/YSZ interface. These observations contrast with data obtained for the cell operated in hydrogen with phosphine, where no YPO4 phase is observed and the alternating tetragonal YSZ and cubic YSZ domains at the Ni/YSZ interface are smaller with typical sizes of 5-10 nm.For the cathode of commercial cells, cathode infiltration of nanomaterials has been proven to be effective in introducing eletro-catalyst to the intrinsically functional electrodes and enhancing the performance of the as-prepared SOFCs. However, limited fundamental research has focused on study of the stability of such infiltrated nano-particles upon long-term operation. In the present study, the nanostructure and chemistry of the infiltrated La0.6Sr0.4CoO3 electro-catalyst and its evolution upon operation were analyzed by TEM for cells that were electrochemically characterized under industrially relevant conditions.

Synthesis and characterization of non-transformable tetragonal YSZ nanopowder by means of Pechini method for thermal barrier coatings (TBCs) applications

A simple soft chemical method of synthesizing tetragonal yttria stabilized zirconia nanopowders is described here. Zirconium oxy-chloride octahydrate and yttrium nitrate hexahydrate were taken as a source of zirconium, citric acid was taken as a chelating agent, and ethylene glycol was used as a polysterification agent. The synthesized powders were characterized by X-ray diffraction, thermogravimetric analysis and differential scanning calorimetry, transmission electron microscopy, field emission scanning electron microscope, Fourier transform infrared spectroscopy and Raman spectroscopy. Furthermore, precise cell parameters were calculated in order to exactly determine non-transformable tetragonal crystal structure, which was the best zirconia phase for thermal barrier coatings applications. In this process, tetragonal yttria stabilized zirconia nanopowders could be prepared at a temperature of 1,000 °C and the process was simple and cost-effective.

Microstructure degradation of YSZ in Ni/YSZ anodes of SOFC operated in phosphine-containing fuels

The interaction of trace (ppm) phosphine with the nickel/yttria stabilized zirconia (YSZ) anode of commercial solid oxide fuel cells has been investigated and evaluated for both synthesis gas and hydrogen fuels in an effort to examine P–Y reactions. The Ni poisoning effects reported in literature were confirmed and degradation was examined by electrochemical methods and post-test microstructural and chemical analyses. The results indicate that P-induced degradation rates and mechanisms are fuel dependent and that degradation of cells operated in synthesis gas (syngas) with phosphine is more severe than that of cells operated in hydrogen with phosphine. As reported in published literature, a cell operated in syngas containing 10ppm phosphine demonstrated significant microstructural degradation within the Ni phase, including formation of Ni–P phases concentrated on the outer layer of the anode and significant pitting corrosion in the Ni grains. In this research, a previously undetected YPO4 phase is observed at the YSZ/YSZ/Ni triple grain junctions located at the interface with the YSZ electrolyte. Tetragonal YSZ (t-YSZ) and cubic-YSZ (c-YSZ) domains with sizes of several tens of nanometers are also newly observed along the Ni/YSZ interface. These observations contrast with data obtained for a cell operated in dry hydrogen with phosphine, where no YPO4 phase is observed and the alternating t-YSZ and c-YSZ domains at the Ni/YSZ interface are smaller with typical sizes of 5–10nm. The data imply that electrolyte attack by P is a potentially debilitating mode of degradation in SOFC anodes, and that the associated reaction mechanisms and rates are worthy of further examination.

Microstructural and chemical evolution near anode triple phase boundary in Ni/YSZ solid oxide fuel cells

In this study, we report the micro-structural and chemical evolution of anode grain boundaries and triple phase boundary (TPB) junctions of Ni/YSZ anode supported solid oxide fuel cells. A NiO phase was found to develop along the Ni/YSZ interfaces extending to TPBs in the operated cells. The thickness of the NiO ribbon phase remains constant at ~ 5 nm in hydrogen for operating durations up to 540 h. When operating on synthesis gas, an increase in interphase thickness was observed from ~ 11 nm for 24 h of operation to ~ 51 nm for 550 h of operation. YSZ phases are observed to be stable in H 2 over 540 h of operation. However, for the cell operated in syngas for 550 h, a 5–10 nm tetragonal YSZ ( t-YSZ) interfacial layer was identified that originated from the Ni/YSZ interfaces. Yttrium species seem to segregate to the interfaces during operation, leading to the formation of t-YSZ in the Y-depleted regions.

Red and infrared luminescence from tetragonal YSZ:Pr 3+ single crystal fibres grown by LFZ

Single crystal fibres of Pr 3+-doped tetragonal yttria stabilized zirconia were grown by the laser floating zone method. The fibres show strong red luminescence at room temperature under ultraviolet optical pumping, which is due to the transition between the 1D 2 and 3H 4 multiplets of the Pr 3+ ion. Additionally, in the infrared spectral range main transitions were found at ∼1140 nm and 1560 nm. Their intensity ratio was shown to be dependent on the excitation wavelength, suggesting the presence of multiple Pr 3+-related centres.

Eight mole percent yttria stabilized zirconia powders by organic precursor route

An organic precursor-mixing route has been developed for preparation of 8 mol% yttria stabilized zirconia (8YSZ) ceramics. Polymeric salt of succinic acid with yttrium and zirconium has been prepared separately by treating sodium succinate with yttrium chloride and zirconyl chloride followed by washing with water and drying at 120 °C. Thorough mixing of the two salts in stoichiometric proportions by planetary ball milling followed by calcination at 850 °C resulted in a precursor powder containing nanocrystalline (∼40 nm) monoclinic zirconia, tetragonal YSZ, cubic YSZ and yttria. Compacts prepared after deagglomeration of powder by planetary ball milling produce 8YSZ ceramics having density 99.3% TD on sintering at 1550 °C for 2 h.

PALS study on the defect structure of yttria-stabilized zirconia

Powders of stabilized ZrO 2–8 mol% Y 2O 3 (YSZ) have been obtained by mechanical milling in zirconia vials. The samples were characterized by X-ray diffraction (XRD). Positron annihilation lifetime (PALS) measurements were performed to investigate the lattice defects originated by the incorporation of yttria and those mechanically induced. The XRD results indicate the formation of tetragonal YSZ solid solution. PALS results indicate that positron trapping occurs at different kinds of defects such as vacancy-like defects, grain boundary and associated defects.

In situ X-ray Rietveld analysis of Ni–YSZ solid oxide fuel cell anodes during NiO reduction in H2

A synthesis and characterization of solid oxide fuel cell (SOFC) anodes of nickel with 8%mol yttrium stabilized zirconia (Ni–YSZ) is presented. Attention was focused on the kinetics and phase composition associated with the transformation of NiO–YSZ to Ni–YSZ. The anodes were prepared with an alternative synthesis method that includes the use of nickel acetylacetonate as an inorganic precursor to obtain a highly porous material after sintering at 1400°C and oxide reduction (NiO–YSZ → Ni–YSZ) at 800°C for 8 h in a tubular reactor furnace using 10% H2/N2. The obtained material was compressed by unidirectional axial pressing into 1 cm-diameter discs with 15–66 wt% Ni and calcinated from room temperature to 800°C. A heating rate of 1°C min−1 showed the best results to avoid any anode cracking. Their structural and chemical characterization during the isothermal reduction were carried out by in situ time-resolved X-ray diffraction, refined with the Rietveld method (which allowed knowing the kinetic process of the reduction), scanning electron microscopy and X-ray energy dispersive spectroscopy. The results showed the formation of tetragonal YSZ 8%mol in the presence of nickel, a decrement in the unit cell volume of Ni and an increment of Ni in the Ni–YSZ anodes during the temperature reduction. The analysis indicated that the Johnson–Mehl–Avrami equation is unable to provide a good fit to the kinetics of the phase transformation. Instead, an alternative equation is presented.

Morphological Changes at the Interface of the Nickel‐Yttria Stabilized Zirconia Point Electrode

The , Ni/YSZ point electrode has been investigated using long‐term potential step measurements and impedance spectroscopy at 1273 K. Morphological and structural changes at the electrode interface were evaluated by electron microscopy, energy dispersive X‐ray analysis, and Raman spectroscopy ex situ. The anodic current was found to induce a self‐catalytic effect on the electrode, and the anodic “steady state” current increased to more than twice the initial value with a time constant of about 40 h. In contrast, cathodic polarization reduced the performance of the electrode, and the cathodic current decreased significantly with a time constant of about 20 h. Redistribution of material in the reaction zone is suggested to control most of the changes in electrode activity. At anodic overpotentials it was observed that Ni was transported to the electrolyte surface, forming a “necklace” of Ni particles around the electrode/electrolyte contact. This is believed to increase the three‐phase boundary (TPB) length and account for the higher activity of the electrode. At cathodic overpotentials the transfer of Ni to the YSZ was found to be restricted, and it is proposed that agglomeration of dispersed metal particles reduced the TPB length, and accordingly the cathodic current. In addition to the morphological modifications, the catalytic properties of the surfaces were significantly altered as the electrode was polarized. Transformation from cubic to tetragonal YSZ, due to segregation of the material, was observed on the surface of the electrolyte when the sample was kept at working conditions for long periods of time (135 days). The passage of current was not found to generate any permanent phase transformation in the YSZ.