Yttira-stabilized Zirconia Research Articles

Effects of Nanoscale PEALD YSZ Interlayer for AAO Based Thin Film Solid Oxide Fuel Cells

Performance of thin film solid oxide fuel cells (TF-SOFCs) were improved by inserting plasma enhanced atomic layer deposition (PEALD) of yttira-stabilized zirconia (YSZ) interlayers. By controlling the ratio between Y2O3 and ZrO2 in YSZ deposition supercycles, Y2O3 mol.% in YSZ films were changed. High Y2O3 contained PEALD YSZ interlayered cell showed higher maximum power density (123.6 mW/cm2 at 500 °C, 286.1 mW/cm2 at 550 °C) and smaller polarization resistance compared with reference Y2O3 concentrated PEALD YSZ interlayer cell. (108.5 mW/cm2 at 500 °C, 181.1 mW/cm2 at 550 °C) Exchange current densities of TF-SOFCs at 500 °C also improved ~ 59.3% at high Y2O3 concentrated PEALD YSZ interlayer cells. These phenomena caused by high density of oxygen vacancies in high Y2O3 concentrated PEALD YSZ interlayer, which helps oxygen incorporation reactions at cathode-interlayer interface. Therefore, cathodic polarization loss related with oxygen reduction reactions was decreased, and then, performance and exchange current density were improved. Results of this study imply that insertion of simple PEALD YSZ interlayer at cathode-electrolyte interface efficiently improves performance and electrochemical characteristics of TF-SOFCs.

Synthesis of Ni/YSZ based anode and investigation of effect of PVA as pore-former upon porosity, microstructure and thermal behavior for potential use in solid oxide fuel cells (SOFCs)

Abstract This study focuses on development of Nickel–Yttira Stabilized Zirconia (Ni/YSZ) based anode material by solid-state synthesis using polyvinyl alcohol (PVA) in varying weight concentrations (3 wt.% and 6 wt.%) both as a binder and as a pore-former. Scanning electron microscopy and X-ray diffraction based structural and morphological analyses were performed and the synthesis of a homogeneous NiO/YSZ phase was confirmed after sintering at 1 000 °C. Microscopic study investigating effects of PVA concentration upon pore density in pellets pressed under the same conditions was conducted. Thermo-gravimetric/differential thermal analysis of the powders pre-dried at around 120 °C was carried out up to 1 000 °C to investigate the phase change and the thermal decomposition behavior. Findings on reduction of NiO/YSZ in a mixed atmosphere of H2/Ar at 550 °C are also discussed in detail. The results demonstrated that areal porosity of 12 % and 24 % was achieved with 3 wt.% and 6 wt.% of PVA, respectively.

Cathode Optimization for an Inert-Substrate-Supported Tubular Solid Oxide Fuel Cell

Inert-substrate-supported tubular solid oxide fuel cells with multi-functional layers were fabricated in this work. The tubular single cells consisted of a porous yttira-stabilized zirconia inert-substrate supporting layer, a Ni anode current collecting layer, a Ni-Ce0.8Sm0.2O1.9 anode electrochemical layer, a yttira-stabilized zirconia/Ce0.8Sm0.2O1.9 bi-layer electrolyte, and a La0.6Sr0.4Co0.2Fe0.8O3-δ cathode. Thickness of the La0.6Sr0.4Co0.2Fe0.8O3-δ cathode layer could be varied from 2.5 to 25.0 μm by controlling the number of dip-coatings in the single cell fabrication process. Electrochemical performance of the tubular single cells was investigated as a function of cathode thickness. Electrode resistance and maximum power density of the single cell were significantly affected by the thickness of the cathode. Increasing the cathode thickness to 15 μm was effective in reducing the sheet resistance of the layer and the electrode resistance of the single cell. Further increasing the cathode thickness induced a higher electrode polarization loss, which originated from insufficient gas diffusion and transport processes. Therefore, the optimum thickness of the La0.6Sr0.4Co0.2Fe0.8O3-δ cathode layer was determined to be 15 μm. At 800 °C, the tubular single cell with the optimum cathode thickness displayed the highest observed maximum power density of 559 mWcm-2 under the hydrogen/air operation mode. Additionally, the tubular single cell exhibited good thermal cycling stability between 800 and 25 °C for five cycles. These results illustrate the advantages of this system for future applications of the inert-substrate-supported tubular single cells in repeated startup and shut down conditions.

NiMo‐Ceria‐Zirconia Catalyst for Inert‐Substrate‐Supported Tubular Solid Oxide Fuel Cells Running on Model Gasoline

AbstractA NiMo‐ceria‐zirconia catalyst was developed for an inert‐substrate‐supported tubular single cell running on isooctane fuel. The tubular single cell consisted of an inert porous yttira‐stabilized zirconia (YSZ) supporter, Ni‐Ce0.8Sm0.2O1.9 (Ni‐SDC) anode, YSZ/SDC bilayer electrolyte and La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) cathode. The NiMo‐ceria‐zirconia catalyst was packed in the tubular channel of the single cell as a reforming unit. With application of the catalyst in the tubular cell, the mixture of isooctane and air can be efficiently reformed to simple syngas (a mixture of hydrogen and carbon monoxide) at 750 °C. Compared to electrochemical performance of the single cell without the catalyst, applying the catalyst increased maximum power density of the single cell from 240 to 306 mW cm−2, and the single cell exhibited significantly improved stability during the 18 h performance test. The results suggest the feasibility of applying the NiMo‐ceria‐zirconia catalyst for operating the tubular single cell using isooctane fuel.

NiMo-Ceria-Zirconia Catalyst for Inert-Substrate-Supported Tubular Solid Oxide Fuel Cells Running on Model Gasoline

A novel NiMo-ceria-zirconia catalyst was developed for an inert-substrate-supported tubular single cell running on isooctane fuel. The catalyst with composition of 11 wt.% Ni, 3 wt.% Mo and 86 wt.% ceria-zirconia showed high activity for reforming of isooctane/air mixture and excellent coking tolerance. This catalyst was applied in the tubular single cell consisted of an inert porous yttira-stabilized zirconia (YSZ) supporter, Ni-Ce0.8Sm0.2O1.9 (Ni-SDC) anode, YSZ/SDC bilayer electrolyte and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathode. By packing the catalyst into the tubular channel of the single cell, the mixture of isooctane and air can be efficiently reformed to simple syngas (a mixture of hydrogen and carbon monoxide) at 750 °C. The carbon conversion reached 75% and the hydrogen and carbon monoxide yields were over 50%, which were much higher compared with the cell with no catalyst (carbon conversion: <50%, syngas yields: <35%). As a result, the maximum initial power density of the single cell increased from 240 to 306 mW cm-2 by applying the catalyst. Moreover, the single cell displayed significantly improved stability during the 18 h performance test. The results suggest the feasibility of applying the NiMo-ceria-zirconia catalyst for operating the tubular single cell using isooctane fuel.

Fast initializing solid state electrochemical carbon dioxide sensor fabricated by a tape casting technique using yttria stabilized zirconia and sodium beta alumina heterojunction

Solid-state bi-electrolyte carbon dioxide sensors with an yttira stabilized zirconia (YSZ) and sodium beta alumina (NBA) heterojunction were fabricated using the tape casting method to measure CO2 concentrations in the air. The enhanced interface between NBA, expressed by (1+x) Na2O·11 A12O3 and YSZ was made by the liquid phase sintering method while varying the ratio of Na2O/Al2O3 from x=0 to x=0.6. YSZ/Pt electrode was used as an air reference, maintaining a constant oxygen concentration of 21%, and Na2CO3 was used as a CO2 sensing electrode. The responses of the sensors to various concentrations of CO2 were evaluated at temperatures ranging from 550°C to 650°C. The electric potential difference of the sensors exhibited Nernstian behavior with sensitivity greater than 99% of the theoretical value at 650°C. The activity of Na2O at the bi-electrolyte junction remained constant, showing invariance with the CO2 concentrations tested. The time for the initial stabilization was as short as 66s.

Characteristics of Bulk and Coating in Gd2−xZr2+xO7+0.5x(x = 0.0, 0.5, 1.0) System for Thermal Barrier Coatings

Gadolinium zirconate, Gd₂Zr₂O 7 , is one of the most versatile oxides among the new thermal-barrier-coating (TBC) materials for replacing conventional yttira-stabilized zirconia (YSZ). Gd₂Zr₂O 7 exhibits excellent properties, such as low thermal conductivity, high thermal expansion coefficient comparable with that of YSZ, and chemical stability at high temperature. In this study, bulk and coating specimens with Gd 2-x Zr 2+x O 7+0.5x (x = 0.0, 0.5, 1.0) compositions were fabricated in order to examine the characteristics of this gadolinium zirconate system with different Gd content for TBC applications. Especially, coatings with Gd₂-xZr₂+xO 7+0.5x (x = 0.0, 0.5, 1.0) compositions were produced by suspension plasma spray (SPS) with suspension of raw powder mixtures prepared by planetary milling followed by ball milling. Phase formation, microstructure, and thermal diffusivity were characterized for both sintered and coated specimens. Single phase materials with pyrochlore or fluorite were fabricated by normal sintering as well as SPS coating. In particular, coated specimens showed vertically-separated columnar microstructures with thickness of 400 ~ 600 μm.

서스펜션 플라즈마 용사로 제조된 란타눔/가돌리늄 지르코네이트 열차폐코팅의 구조와 열전도도 특성

High Temperature Materials Unit, National Institute for Materials Science, Tsukuba, 305-0047, Japan(Received December 3, 2014 ; revised December 24, 2014 ; accepted December 24, 2014)AbstractWith increase in demand for higher operating temperatures of gas turbines, extensive research efforts havebeen carried out to enhance the performance of thermal barrier coatings (TBCs) in the field of coating pro-cessing as well as materials. In this study, thermal barrier coatings in lanthanum/gadolinium zirconate system,which is one of the most promising candidates for replacing yttira-stabilized zirconia (YSZ) in thermal barriercoating applications, are fabricated via suspension plasma spray. Dense, 300 ~ 400 µm thick coatings of fluorite-phase zirconate with modest amount of segmented microstructures are obtained by using suspension plasmaspray with suspensions of planetary-milled mixture between lanthanum and/or gadolinium oxide and nanozirconia. These coatings exhibit thermal conductivities of 1.6 ~ 1.7 W/mK at 1000

열차폐코팅용 GdO1.5-ZrO2계 희토류 지르코네이트 세라믹스의 상형성과 열물리 특성

Gadolinium zirconate, Gd₂Zr₂O 7 , is one of the most promising candidates for replacing yttira-stabilized zirconia (YSZ) in thermal barrier coating (TBC) applications due to its low thermal conductivity and chemical stability at high temperature. In this study, rareearth zirconate ceramics in the GdO 1.5 -ZrO₂ system with reduced gadolinia contents were fabricated via solid-state reaction as well as hot-pressing at 1800℃. The phase formation, microstructure, and thermo-physical properties of these oxides were examined. The potential application of GdO 1.5 -ZrO₂ ceramics for TBC was also discussed.

Performance and long term stability of large area anode supported solid oxide fuel cells (SOFCs)

A long term stability test was conducted on a large area anode supported cell. The cell consisted of anode support, NiO (Nickel Oxide)+YSZ (Yttira Stabilized Zirconia); an anode functional layer, NiO+YSZ; electrolyte, YSZ; a cathode functional layer, LSCF (La0.6Sr0.4Co0.2Fe0.8O3-d)+GDC (Gd doped ceria); and a cathode current collector, LSCF. For a single cell stack test set-up, FeCr alloy end plates were machined for a co-flow of fuel and oxidant gases through internal manifolds and gas channels. The cell voltage was measured in a constant current mode (0.4A/cm2) for ~4400h at 750°C. During the long term test, I–V curve and impedance spectra were frequently measured to trace an increase in ohmic and non-ohmic resistance as a function of time. It was found that the dominant factor in the single cell stack degradation was an increase of non-ohmic resistance (activation polarization) rather than ohmic resistance. After the long term test, a post-material analysis was conducted by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning Auger microscopy (SAM), energy dispersive spectroscopy (EDS), etc. The main degradation mechanism was determined to be the change of anode microstructure, which was caused by the high pressure of water vapor, especially at the gas outlet region.

Simulation model for an EB-PVD coating structure using the level set method

A comprehensive modeling approach to link machine dynamics, deposition, and substrate kinematics in an electron beam physical vapor deposition (EB-PVD) is presented in this paper. The machine dynamics in EB-PVD process are captured by finite element models, resulting in the prediction of evaporation rate and vapor distribution. The deposition process is modeled using the level set method, which is one of the computational techniques for tracking topographic evolution. The proposed simulation model is implemented in Matlab and is compared with experimental results published by other researchers. Results indicate that the proposed simulation model can be used to predict microstructure features such as zigzag and helical columnar shapes. The pitch of a zigzag microstructure can be predicted within 20% at the 0.3 to 6 μm level for Yttira-stabilized Zirconia (YSZ) coating.

Electrochemical NOx sensors based on stabilized zirconia: comparison of sensing performances of mixed-potential-type and impedancemetric NOx sensors

A tubular sensor was fabricated by using yttira stabilized zirconia (YSZ) and ZnCr2O4 sensing-electrode (SE) aiming for detection of NOx at high temperature. The sensing characteristics of this YSZ-based sensor were evaluated at 700∘C by means of potentiometric (mixed-potential) and impedancemetric methods with the variation of thickness of SE. A correlation between the thickness and the sensing performances was obtained for both types of NOx sensors. The mixed-potential-type sensor using ZnCr2O4-SE exhibited high NOx sensitivity when the SE thickness was small (4 μm). On the other hand, the impedancemetric sensor, employing the same oxide-SE, provided almost equal sensitivity to NO and NO2 when the SE thickness was large (39 μm). In this case, the total concentration of NOx can be measured. The comparison of sensing mechanisms for the both sensors was briefly discussed.

The effect of colonies of aligned grains on critical current in high-temperature superconductors

X-ray microdiffraction has been used to map the orientational distribution of grains in a 4 × 5mm2 polycrystalline film of Tl1−yBa2Cu2Cu3Ox (Tl-1223) grown on yttira-stabilized zirconia (YSZ). The film consists of “colonies”, each containing ∼ 104 grains with their a-axes aligned; the c-axes of all grains are aligned normal to the film, but there is no overall a-axis alignment. While the grain boundaries within a colony are small angle, colonies are separated by large-angle tilt boundaries. The typical colony size is 0.4 mm. Measured critical currents are compared with simulations based on observed distributions of grain alignment; we find that variations in grain alignment account for the observed variations in the critical current density. Simulations over longer distances indicate that a colony structure can substantially increase the critical current, especially in magnetic fields.