Yttria-stabilized Zirconia Grains Research Articles

Characterization of Yttria-Stabilized Zirconia (YSZ) following high temperature hydrogen exposures of molybdenum-YSZ composites

Molybdenum matrix ceramic-metal (cermet) composites containing 40–70 vol% yttria stabilized zirconia (YSZ) particles (as a surrogate for ceramic fuel particles) were produced via spark plasma sintering (SPS) and exposed to flowing hydrogen at high temperature (2000–2630 K). Both steady state and thermally cycled (4 cycles with intermediate cooling to room temperature) conditions were examined for a constant total hot testing time of 80 min. Due to the high hydrogen permeability in Mo at these temperatures, pronounced microstructural degradation was observed in YSZ particles located many millimeters below the sample surface. Observed changes to the YSZ particles from the hot hydrogen testing include partial matrix debonding, thermal expansion, redeposition of material, phase changes to a monoclinic phase and cracking/bubble formation on the YSZ grain boundaries. Overall, the YSZ particles exhibited acceptable tolerance to this extreme operating environment.

Generation of nanopore structures in yttria-stabilized zirconia by femtosecond pulsed laser irradiation

Femtosecond pulsed laser irradiation of yttria-stabilized zirconia (YSZ) was performed to investigate the feasibility and fundamental characteristics of nanopore structure fabrication. Numerous nanopores were successfully generated on the YSZ surface with only a single laser pulse shot by controlling laser power near the ablation threshold. The nanopore generation mechanism involves light focusing by the convex shape of a crystal grain and ablation inside the grain. By increasing laser power, the number of nanopores increased. Cross-sectional observation revealed that the depth of nanopores was about 500 nm, and nanopores were generated inside the YSZ grains of the top layer while keeping the grains' shapes unchanged. It was found that higher scanning speed and a small number of scans enabled nanopores fabrication without changing the original surface shape, avoiding the removal of the surface layer. Laser-irradiated surfaces of YSZ were characterized by Raman spectroscopy, and it was found that there was no significant phase change in the specimen after nanopore generation compared with the original YSZ surface, confirming that there was no thermal damage to the bulk material. The surface with nanopores showed an enhancement in surface hydrophilicity. These findings demonstrated the possibility of selectively generating nanopores in the top surface of YSZ for use in functional surfaces without serious thermal damage to the workpiece. It is expected that the nanopore generation will lead to new applications of YSZ.

Elucidating the performance benefits enabled by YSZ/Ni-YSZ bilayer thin films in a porous anode-supported cell architecture.

Increasing the performance and improving the stability of solid oxide cells are critical requirements for advancing this technology toward commercial applications. In this study, a systematic comparison of anode-supported cells utilizing thin films with those utilizing conventional screen-printed yttria-stabilized zirconia (YSZ) is performed. High-resolution secondary ion mass spectrometry (SIMS) imaging is used to visualize, for the first time, the extent of Ni diffusion into screen-printed microcrystalline YSZ electrolytes of approximately 2-3 μm thickness, due to the high temperature (typically >1300 °C) used in the conventional sintering process. As an alternative approach, dense YSZ thin films and Ni(O)-YSZ nanocomposite layers are prepared using pulsed laser deposition (PLD) at a relatively low temperature of 750 °C. YSZ thin films exhibit densely packed nanocrystalline grains and a remarkable suppression of Ni diffusion, which are further associated with some reduction in the ohmic resistance of the cell, especially in the low temperature regime. Moreover, the use of a Ni-YSZ nanocomposite layer resulted in improved contact at the YSZ/anode interface as well as a higher density of triple phase boundaries due to the nanoscale Ni and YSZ grains being homogeneously distributed throughout the structure. The cells utilizing the YSZ/Ni-YSZ bilayer thin films show excellent performance in fuel cell operation and good durability in short-term operation up to 65 hours. These results provide insights into ways to improve the electrochemical performance of SOCs by utilizing innovative thin film structures in conjunction with commercially viable porous anode-supported cells.

Space charge layer evolution at yttria-stabilized zirconia grain boundaries upon operation of solid oxide fuel cells

Long-lasting solid oxide fuel cells (SOFC) rely upon an intact ionically conducting network in both the electrolyte and electrode. Understanding the impact of the high-temperature electrochemical operations on the possible nanostructure evolutions of ionic conductors is indispensable for the development of durable SOFC. Here we present possibly the first experimental evidence of space charge layer evolution of grain boundaries (GBs) of ionic conducting yttria-stabilized zirconia (YSZ) in a Ni/YSZ composite anode of SOFCs. Cells with identical Ni/YSZ composite anodes are either thermally treated or electrochemically operated using either dry or humidified H2 fuel for up to 3,349 h. When operating under humidified H2 fuel, a discrete core-shell structure is found to develop exclusively along the YSZ/YSZ GB planes. The core-shell structure is elliptically shaped elongated along the GB planes. The core has a lower mass with a Y-depleted ZrO2-x with a cubic structure. The shell has a constant thickness of ∼1 nm and is Y-enriched. The nucleation and growth mechanisms of the core-shell structure are proposed in terms of the space charge layer evolution at YSZ GBs. The effect of the humidity in the fuel, the cell operating temperatures, and the current density on the formation of the core-shell structure is investigated.

Nanoindentation of single-crystal and polycrystalline yttria-stabilized zirconia: A comparative study by experiments and molecular dynamics simulations

A combination of nanoindentation experiments and molecular dynamics (MD) nanoindentation simulations is carried out in this paper to study the mechanical properties and deformation behaviors of single-crystal and polycrystalline yttria-stabilized zirconia (YSZ). Different Young’s moduli are found in the nanoindentation experiments of single-crystal and polycrystalline YSZ samples, though these two YSZ materials almost have the same hardness. In addition, with the aid of atomic force microscopy, different deformation behaviors are observed in single-crystal and polycrystalline YSZ. The different Young’s moduli and deformation behaviors of single-crystal and polycrystalline YSZ observed in experiments are explained by MD simulations. The simulation results show that during the indentation process, the slip occurs inside single-crystal YSZ but along the grain boundaries of polycrystalline YSZ. These different deformation mechanisms between single-crystal and polycrystalline YSZ account for the lager Young’s modulus and more significant pile-up phenomenon observed in single-crystal YSZ. In addition, our MD simulations also reveal that the mechanical properties of YSZ are almost independent of the temperature but can be significantly affected by the number of YSZ grains. Specifically, both the Young’s modulus and hardness of polycrystalline YSZ are found to decrease as the number of grains grows.

A Study on YSZ Abradable Seal Coatings Prepared by Atmospheric Plasma Spray and Mixed Solution Precursor Plasma Spray

The yttria-stabilized zirconia (YSZ) ceramic has been proved to be a promising material for abradable seal coatings. In this research, abradable seal coatings named C-YSZ and M-YSZ were prepared by applying the conventional plasma spraying method with improved nanostructured powders and by the mixed solution precursor plasma spraying technique, respectively. A study on the microstructure, mechanical properties, oxidation resistance and hot corrosion resistance was conducted on both C-YSZ and M-YSZ coatings by using scanning electron microscopy, transmission electron microscopy, muffle furnace and x-ray diffractometer. The results show that the C-YSZ coating presents larger-size pores with denser YSZ structures and grains showing size larger than 100 nm distributed around the pores compared with M-YSZ, in which the pores are much smaller and the grain size is 20-30 nm. As a result, the M-YSZ coating displays lower hardness and bond strength compared with C-YSZ. The oxidation tests show that the M-YSZ coating possesses better oxidation resistance due to the nanostructure topcoat compared with C-YSZ. However, the hot corrosion tests indicate that both the C-YSZ coating and the M-YSZ coating fail to present ideal hot corrosion resistance.

Space Charge Layer Evolution at Yttria-Stabilized Zirconia Grain Boundaries Upon Operation of Solid Oxide Fuel Cells

Space Charge Layer Evolution at Yttria-Stabilized Zirconia Grain Boundaries Upon Operation of Solid Oxide Fuel Cells

(Invited) Additive Dual Nano Catalysts to Accelerate Oxygen Reduction Reaction in Cathode of Solid Oxide Fuel Cells

The sluggish oxygen reduction reaction (ORR) in the cathode is hindering the power density of the solid oxide fuel cells (SOFCs). Infiltration of catalyst into the cathode of SOFCs is promising to accelerate the ORR. However, the degradation associated with the coarsening of infiltrated nano-catalyst is intense. To stabilize the catalyst, a ultra-thin ~5 nm Atomic Layer Deposition (ALD) is employed to coat a dual electrocatalyst into the porous lanthanum strontium manganite (LSM)/yttria-stabilized zirconia (YSZ) cathode. After 504 h operation at 750 °C, the ALD coating resulted in the SOFC peak power density enhancement by ~200 %. Remarkably, the nano-catalysts with ~ 10 nm in dimensions, are uniformly distributing on YSZ grain surface, triple phase boundaries and at LSM/LSM surface grain boundaries after 504 h operation at 750 °C. This study demonstrates an effective approach of stabilizing the nano-sized and minute amount of catalysts for enhancing ORR activity at elevated temperatures.

Synergetic Interaction of Additive Dual Nanocatalysts to Accelerate Oxygen Reduction Reaction in Fuel Cell Cathodes

The sluggish oxygen reduction reaction (ORR) in the cathode is hindering the power density of solid oxide fuel cells (SOFCs). Infiltration of catalyst into the cathode of SOFCs is promising to accelerate the ORR. However, the degradation associated with the coarsening of the nanocatalyst is intense. To stabilize the catalyst, atomic layer deposition (ALD) is employed to coat a dual electrocatalyst consisting of a superjacent 2 nm CoOx layer and superjacent 3 nm discrete Pt particles into the porous lanthanum strontium manganite (LSM)/yttria-stabilized zirconia (YSZ) cathode. After 504 h of operation at 750 °C, the ALD coating resulted in the peak power density enhancement by ∼200%, while CoOx becomes Mn-enriched (MnCo)Ox nanograins coupling with nano-Pt. The Pt/(MnCo)Ox nanocouplings are uniformly distributed on the YSZ grain surface, triple-phase boundaries, and at LSM/LSM surface grain boundaries. This study demonstrates an effective approach of stabilizing the minute amount of catalyst for enhancing ORR activity at elevated temperatures.

Microstructural evolution, mechanical properties and degradation mechanism of PS-PVD quasi-columnar thermal barrier coatings exposed to glassy CMAS deposits

Thermal barrier coatings (TBCs) applied in aero-engines tend to be attacked by molten calcia-magnesia-alumino-silicate (CMAS) at high operating temperatures. Yttria-stabilized zirconia (YSZ) coatings with quasi-columnar microstructure were fabricated by plasma spray physical vapor deposition (PS-PVD) technique. The chemical changes, microstructural transformation, mechanical properties and degradation mechanisms of the CMAS-interacted TBCs in the thermal cycling tests were investigated. Feathered YSZ grains were dissolved in the CMAS melts, and then the ZrO2 grains were reprecipitated with spherical shape, accompanying with phase transformation from tetragonal (t) to monoclinic (m). The thermal cycling tests reveal that the YSZ coating fails at the early stage due to the attack of CMAS. The fractures in intra-columns lead to partial spallation of the coatings. The failure of the coating occurs at the interfaces between thermally grown oxides (TGO) layer and YSZ topcoat; especially, the hardness and Young’s modulus of the YSZ coatings increase intensively, as the coatings were infiltrated by the CMAS for a long time.

Localization of Yttrium Segregation within YSZ Grain Boundary Dislocation Cores

Ionic conductivity blocking at grain boundaries in polycrystalline electrolytes is one of the main obstacles that need to be overcome in order to improve the performance of solid state fuel cells and batteries. To this aim, harnessing the physical properties of grain boundaries in ionic conducting materials such as yttria‐stabilized zirconia (YSZ) down to the atomic scale arises as a greatly important task. Here a structural and compositional analysis of a single grain boundary in a 9 mol.% yttria content YSZ bicrystal by means of aberration‐corrected scanning transmission electron microscopy is presented. Studies here combine strain and compositional atomic resolution analysis with density‐functional‐theory calculations in order to find a preferential segregation of yttrium to the expansive atomic sites at the grain boundary dislocation cores. These results address a crucial step toward the understanding of the physical properties of grain boundaries down to atomic dimensions.

Temperature induced structure degradation of yttria-stabilized zirconia thermal barrier coatings

The structural aspects of yttria-stabilized zirconia (YSZ) thermal barrier coatings grown on a polycrystalline alumina substrate have been studied as a function of annealing time at 1250 °C with the use of electron microscopy, X-ray diffraction, and Raman spectroscopy. Upon the high temperature exposure, the feathery morphology of the columnar grains of the as-deposited coatings smoothens out with the concurrent development of surface undulations and grain coarsening. It is shown that the grain coarsening results in significant voiding along the merged grain boundaries and inside YSZ grains. The nucleation and growth of the high density of voids is related to the clustering of vacancies supplied from the merged gaps between columnar grains and subgrains within the columnar grains. The continued development of the voids leads to the formation and propagation of large cracks within the columnar grains, which weakens the overall mechanical integrity of the coating.

Microstructural evolution of plasma spray physical vapor deposited thermal barrier coatings at 1150 °C studied by impedance spectroscopy

Plasma spray physical vapor deposition (PS-PVD) is a new promising technology for processing thermal barrier coatings (TBCs). In this study, the microstructural evolution of PS-PVD yttria-stabilized zirconia (YSZ) TBCs at 1150 °C was investigated by scanning electron microscopy, electron backscattered diffraction, and impedance spectroscopy. Bridging is observed between the secondary columns of the coatings during the first 8 h of heat treatment, while sintering necks are observed between the quasi-columns over the heat-treatment interval of 12–20 h. As the heat-treatment time increases to 100 h, the nanogaps between the secondary columns disappear and a large number of shrinkage pores are observed in the columns, leading to a typical cellular structure. Besides, the average grain size of the YSZ coatings increases from 0.20 (as-deposited) to 0.55 µm (after 50 h of heat treatment). The resistance of the YSZ grains increases while that of the YSZ grain boundaries decreases because of grain coarsening. During the first 4 h of heat treatment, the resistance of the thermally grown oxide (TGO) scale increases because of its thickening. However, its resistance decreases during the next 4 h of heat treatment mainly because of the change in its composition (from pure alumina to mixed oxides).

Ion transfer in Ni-containing composite anodes of solid oxide fuel cells: A microstructural study

High-resolution transmission and scanning electron microscopy of the solid oxide fuel cell (SOFC) anodes made of Ni and yttria-stabilized zirconia (YSZ), revealed a substantial microstructural reconstruction induced by the anodic current. These microstructural alterations, primarily the growth of NiO inclusions at the Ni| YSZ interface, the resultant orientation relationship between the NiO and YSZ grains, and Ni grain fragmentation on subsequent reduction, provide a straightforward evidence for the so-called oxygen spillover mechanism of the anodic reaction.

Calcium-rich CMAS corrosion induced microstructure development of thermal barrier coatings

Ca50Mg10Al10Si30 (CMAS) corrosion induced microstructure evolution and composition development in yttria-stabilized zirconia (YSZ) thermal barrier coatings prepared by electron-beam physical vapor deposition (EB-PVD), have been investigated by means of analytical electron microcopy, Raman spectroscopy and X-ray diffraction. The corroded YSZ still mainly consists of t-ZrO2 and no harmful monoclinic zirconia formation occurs; however, the feathery and columnar morphology of the as-deposited YSZ coating transforms into an equiaxed grain structure with dramatic grain coarsening and significant CMAS infiltration. The CMAS corrosion induces dissolution of YSZ into the molten CMAS, spinel MgAl2O4 formation both in the intergranular gaps and inside YSZ grains, and anorthite (CaAl2Si2O8) precipitates in the intergranular gaps with reduced Mg and Ca content in the infiltrated amorphous CMAS. These results show that CMAS with a high content of calcium can induce significant grain coarsening and microstructure evolution that has a detrimental effect on primary function of the YSZ coating.

Advanced analysis on growth mechanisms of thermally grown oxide at elevated temperature for thermal barrier coatings

Yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were prepared by plasma spraying. The as prepared YSZ TBCs were characterized by scanning electron microscopy (SEM), Electron backscatter diffraction (EBSD) and focused ion beam (FIB) systems before and after heat treatment. A thermally grown oxide (TGO) layer was formed between YSZ topcoat and bond coat after heat treatment. EBSD results revealed that the TGO layer was composed of α-Al2O3 and cubic Al2NiO4 layers. Mechanism controlling the growth of the TGO layer were analyzed in detail. It was found that the AlNi phase gradually disappeared from bond coat during heat treatment, which provided Al and Ni sources for the formation of TGO. Meanwhile, EBSD results confirmed that the growth of the TGO layer was dominated by the diffusion of Al and Ni rather than O penetration. Furthermore, direct evidence on the diffusion pathways of Al and Ni was also found, indicating that Al and Ni proceeded along the pores, cracks and YSZ grain boundaries to promote the further growth of TGO layer with increased heat treatment time.

Oxygen vacancy ordering induced displacements of cations in yttria-stabilized zirconia

Using scanning transmission electron microscopy, we report direct observation of oxygen vacancy ordering induced atomic displacements of the cation sub-lattice in yttria-stabilized zirconia (YSZ). We find that the cation lattice adopts a zigzag configuration along the [100] direction with alternately narrow and wide lattice spacings equivalent of 0.85 and 1.15 times of the (200) inter-planar distance of the cubic YSZ. Using atomistic simulations, we show that the cation displacements are induced by the alternate presence of oxygen vacancies at the (1/4, 1/4, 1/4) and (1/4, 3/4, 1/4) sites of the unit cells in the [001] direction. The results demonstrate that significant enrichment of yttrium atoms can occur within individual YSZ grains in addition to the typical surface or grain boundary segregation of dopant atoms.

High performance zirconia-bismuth oxide nanocomposite electrolytes for lower temperature solid oxide fuel cells

We develop a novel nanocomposite electrolyte, consisting of yttria-stabilized zirconia (YSZ) and erbia-stabilized bismuth oxide (ESB). The 20 mol% ESB-incorporated YSZ composite (20ESB-YSZ) achieves the high density (>97%) at the low sintering temperature of 800 °C. The microstructural analysis of 20ESB-YSZ reveals the characteristic nanocomposite structure of the highly percolated ESB phase at the YSZ grain boundaries (a few ∼ nm thick). The ionic conductivity of 20ESB-YSZ is increased by 5 times compared to that of the conventional YSZ due to the fast oxygen ion transport along the ESB phase. Moreover, this high conductivity is maintained up to 580 h, indicating high stability of the ESB-YSZ nanocomposite. In addition, the oxygen reduction reaction at the composite electrolyte/cathode interface is effectively enhanced (∼70%) at the temperature below 650 °C, mainly due to the fast dissociative oxygen adsorption on the ESB surface as well as the rapid oxygen ion incorporation into the ESB lattice. Thus, we believe this ESB-YSZ nanocomposite is a promising electrolyte for high performance solid oxide fuel cells at reduced temperatures.

Molecular Dynamic Study of Gadolinium-Doped Ceria and Yttria Stabilized Zirconia Grain Boundaries

Gadolinium-doped ceria (GDC) and yttria-stabilised zirconia (YSZ) are very well known materials, used as components for solid-oxide fuel cells (SOFC). They both are very good electrolytes, and they have been vastly used in this sense. The main difference between GDC and YSZ when used as electrolytes is that GDC shows higher oxygen conduction at lower temperatures, leading to the possibility of developing intermediate temperatures SOFC (IT-SOFC). But their versatility allow us to use them also as anode materials, like YSZ, which in combination of nickel particles, is used to catalyse oxygen reduction. In addition, YSZ combined with nickel (Ni-YSZ) is also used as anode in SOFC, in combination with GDC as electrolyte. In any case, the main role of both is related to oxygen migration, i.e. ionic conductivity. In GDC and YSZ oxygen migration undergoes via a vacancy hopping mechanism. Vacancy mobility depends on several aspects, like the association energy with respect to the dopants, dopants concentration, or temperature, whose effects are very well studied. Grain boundaries (GBs) are also known to have an important impact in conductivity, despite that little is known about their influence. From an experimental point of view, results suggests that the presence of these GBsoffers faster migration channels. On the other hand, dopants tend to segregate to GBs, thus trapping oxygen vacancies and reducing the ionic conductivity of the materials. To clearly understand the role of GBs in the oxygen conductivity, it is necessary to study the system at the atomic level. One of the main issues, however, is that the model must include all the chemical and structural complexity inherent to GBs, which force us to generate very large models. In this work, and based on interatomic potential and molecular dynamic techniques, we present a strategy based on the work of Sayle et al. (2007) referred as Amorphisation and Recrystallization (A&R). With this methodology, consisting on melting and recrystallizing the system at high pressures, we are able to obtain accurate models that allow us to describe realistic GBs including dislocations and translocations. After generating different models, we have performed a series of molecular dynamic simulations in order to see the effect of the temperature in the structure,ionic mobility and also oxygen diffusion and therefore its conductivity. Results seem to indicate that indeed, the presence of the grain boundaries are beneficial for the enhancement of the oxygen diffusion in the system.

In-situ electron microscopy investigation of reduction-induced microstructural changes in NiO

The structural characteristics of NiO–YSZ (YSZ: Yttria-stabilized zirconia) composites and the redox stability of NiO have been explored by in-situ and high-resolution transmission electron microscopy (TEM). NiO has been reduced on alumina support while doing in-situ microscopy, and the observations on redox instability and microstructural changes have been extrapolated to understand the behavior of NiO/Ni–YSZ system. In the composite, the NiO and YSZ grains are strain free and no reaction product is observed at the grain boundaries after long term annealing. Under a low partial pressure of oxygen, NiO may begin to reduce to metallic Ni at temperatures as low as ~200°C. The volume change associated with such a reduction process is ~40.3%. Although metallic Ni nucleates in NiO coherently, such a huge volume contraction may lead to porosity generation and mechanical instability. Long term exposure of metallic Ni to high temperatures leads to coarsening, which reduces the available surface area. This in turn has a direct, negative consequence on the catalytic properties of the metallic Ni particles. The coarsened particles may even undergo faceting, and the facets may not be the most active catalytically. Synthesis of finely dispersed and percolatively connected Ni–YSZ anodes for SOFCs and core–shell or compositionally graded electrodes for batteries may be viable solutions to enhance the lifetime and structural integrity of such electrodes.