Yittria-stabilized Zirconia Research Articles

Influence of Alkali Metal Oxide on CMAS Damage Progression in Air Plasma Sprayed Thermal Barrier Coatings

Thermal barrier coatings (TBCs), which typically comprised of a yittria-stabilized zirconia (YSZ) ceramic top coat, a bond coat and a metallic substrate, have been widely used to protect the hot-section metallic components of gas turbine engines. More recently at high temperatures exceeding 1200℃, the TBCs can be damaged by calcium-magnesium-alumino-silicates (CMAS) resulting from the ingestion of siliceous minerals with the intake air and from unclean fuels. The chemical composition of CMAS is mainly comprised of CaO, MgO, Al2O3 and SiO2. In addition to that, alkali metal oxides can be interblended into CMAS constituents under some operating environments. In this study, the effect of alkali metal oxide (K2O) on CMAS damage progression on air plasma sprayed (APS) YSZ TBC specimens was investigated through simulating the CMAS damage at lab-scale employing synthetic CMAS products. Two types of synthetic CMAS products both with and without K2O were prepared. The synthetic CMAS was spread on the APSed TBC specimen, followed by isothermal exposure at different temperatures up to 1300℃ for various times. It was found that the infiltration of CMAS into the top coat was observed in the both CMAS compositions. However, the infiltration rate was significantly decreased due to the presence of K2O. In the case of CMAS without K2O, the reaction between CMAS and YSZ was significant resulting in the change in microstructural morphology, whereas it was little occurred on the CMAS including K2O. These results suggest that the addition of K2O to YSZ top coat can mitigate the CMAS damage progression.

Porosity and pore size distribution influence on thermal conductivity of yttria-stabilized zirconia: Experimental findings and model predictions

Porous yittria-stabilized zirconia is an important advanced ceramic material for technological applications. One of the most important characteristics of this material is low thermal conductivity, which is greatly influenced by the presence of pores into the microstructure. In fact, air trapped in the pores represents a better thermal insulator. The role of the pore volume fraction on porous material characteristics has been extensively studied. On the other hand, the influence of the structure disorder, the pore size range and pore size distribution have been studied much less. In this study, an intermingled fractal model capable of relating thermal properties of ceramic materials and their pore microstructure has been proposed. Model predictions are found confirming the experimental data fairly well, even better than the others models available in the literature.

Mechanical and Dielectric Relaxation in 8 Mol% YSZ

8 mol% Yittria Stabilized Zirconia (8YSZ) with high ionic conductivity is currently essential material for highly-efficient and environmentally-friendly energy technologies, such as solid oxide fuel cells (SOFCs). Mechanical properties, as well as electrochemical properties, of 8YSZ are of great importance for reliability and durability of SOFCs which work under harsh environment including high temperature, mechanical stress, electrical field, oxidizing/reducing environment, etc. In this study, mechanical behavior of 8YSZ was studied by cycling compression testing and Dynamic Mechanical Analysis (DMA) in the 25 – 600°C temperature range. Both testing methods demonstrate significant frequency dependent mechanical losses in the 50 – 500°C temperature range that can be attributed to anelastic reorientation of oxygen vacancy – dopant clusters. In addition, electrical polarization and impedance spectroscopy were carried on in the same temperature range, and the results are compared to the results of mechanical spectroscopy. Underplaying mechanisms for electro-mechanical coupling in 8YSZ were discussed in more details in this paper.

Optimization of the rheology and sintering behavior of electrolyte and anode pastes to be used in co-extrusion of solid oxide fuel cells

Typical electrolyte paste including Yittria Stabilized Zirconia (YSZ) and anode paste containing YSZ, NiO, starch/activated carbon (pore former) were prepared. In order to perform co-extrusion process the pastes must have nearly same rheological properties. So, the prepared pastes unified in order to have same rheological characteristics using the Benbow–Bridgwater model. The unification results showed a linear relationship between the required liquid content for paste preparation and weight percentage of NiO powder in unified pastes. Such a linear relationship can be used to predict the required liquid content of the intermediate anode pastes. The unified pastes were extruded as single layer tubes and sintered at various experimental conditions. Micro-structural features of the sintered tubes were investigated using Scanning Electron Microscope (SEM) and optimum sintering temperature and time were found as 1450 °C and 2 h, respectively for full electrolyte densification. Moreover, 30 wt% of carbon may be regarded as the optimum carbon content for creating enough porosity in the anode layers.

Fabrication and Characterization of a Gd0.2Ce0.8O2−δ–Sm0.5Sr0.5CoO3-δ Composite Cathode for an Intermediate-Temperature Solid Oxide Fuel Cell

The anode supported solid oxide fuel cell of yittria-stabilized zirconia (YSZ) based electrolyte with cell structure of NiO-YSZ | YSZ | Gd0.2Ce0.8O2-δ (GDC) | GDC-Sm0.5Sr0.5CoO3 − d (SSC), is investigated. The SEM results show that the YSZ thin film is dense and the thickness is 5 mm. The maximum power densities are 608 and 104 mWcm-2 at the temperatures of 800 and 600 oC, respectively. The cell has been operated for long term test and the results show that the anode porosity is critical for the stability of the cell performance. The results of the variation of the contact pressure between the current collectors and the cell show that its effect on the performance efficiency is intensively enhanced at the high current density region.

(Invited) In Situ Optical and Electrochemical Studies of SOFC Carbon Tolerance

High operational temperatures needed for solid oxide fuel cells (SOFCs) are required by the large activation energy of the oxide diffusion through the electrolyte. These high temperatures, in the range of 800-1000 °C, accelerate cell degradation and severely reduce the number of materials suitable for construction. Employing newly developed, more highly conducting electrolytes such as scandia-stabilized zirconia (ScSZ) enable SOFCs to operate at lower temperatures without sacrificing power (1). Intermediate temperature (IT) solid oxide fuel cells, operating in the 500-750 °C range, permit non-ceramic and non-metallic components to be used in the fuel cell stack, reducing costs and enabling a larger variety of stack geometries (2). However, the high catalytic activity of nickel still causes unwanted carbon deposition on the anode surface (3). This carbon buildup can cause premature cell degradation as well as reduce the power density of the fuel cell. Chronopotentiometry, a technique that couples real time electrochemical monitoring of cell voltage with synchronous in situRaman spectroscopy, provides insight into the mechanisms of carbon deposition and removal as well as the effect deposited carbon has on fuel cell performance (3). This technique can quantify the carbon growth with < 2 second temporal resolution and 1 μm spatial resolution across the anode surface.Figure 1: Raman “G” peak growth at 675 °C observed in situfrom a cell operating under methane with a ScSZ electrolyte supported membrane electrode assembly.In this work, we examine qualitative differences in carbon buildup (Fig. 1) between yittria-stabilized zirconia (YSZ) and ScSZ supported solid oxide fuel cells with a Ni-YSZ cermet anode exposed to methane at 675 °C and 725 °C. We also discuss the differences in the linear sweep voltammetry (LSV) between cells running under methane and hydrogen. (Fig. 2)Figure 2: (Top) Comparison of carbon growth at different polarizations of YSZ and ScSZ SOFCs operating at 725 °C under methane fuel. (a) ScSZ OCV (b) ScSZ 75% IMax (c) YSZ OCV (d) YSZ 75% IMax (Bottom) Comparison of LSV traces running under H2 and CH4. The “swing” in the CH4 trace, indicating a change in carbon removal mechanism can be seen at 150 mA (4).Observed differences between the amounts of carbon formed for YSZ and ScSZ fuel cells at high and low temperatures and different polarizations (Fig. 2) implicate a role played by the electrolyte in the formation of carbon on the anode. Different carbon removal mechanisms have fundamentally different Nernst potentials, and when coupled with Raman spectroscopy, allow us to identify not only the relative amounts, but also the type of carbon formed.

Freestanding Micro‐SOFCs with Electrodes Prepared by Electrostatic Spray Deposition

AbstractWe report a freestanding micro solid oxide fuel cell with both the anode and cathode deposited using electrostatic spray deposition (ESD) technique. The cell is consisted of dense yittria‐stabilized zirconia (YSZ) electrolyte (100 nm thick), porous lanthanum strontium manganite (LSM)–YSZ cathode (∼3 μm thick), and porous NiO‐YSZ anode (∼3 μm thick). LSM‐YSZ and NiO‐YSZ composite powders were initially prepared by glycine nitrate process and super‐critical fluid processes, respectively, and both cathode and anode layers were deposited by the ESD. The resulting freestanding micro cell exhibited an open circuit voltage close to the theoretical value of 1.09 V, and a maximum power density of 41.3 mWcm–2 at 640 °C.

고온 산소분리막용 고강도 다공성 지지체 제조 및 기계적 특성 연구

Porous YSZ ceramics are fabricated using 3 mol% yittria-stabilized zirconia (3YSZ) and NiO with different particlesizes (0.6 and 7 μm). Nickel oxide (NiO) is added to the YSZ powder as a pore former with different amounts(40, 50, and 60 vol%) and at different sintering temperatures (1350 and 1400℃) are applied in order to evaluate the temperature effects on the pore and mechanical properties. Heat treatment is conducted after sintering at 700℃ in H₂ for the NiO reduction process; then, Ni is removed using a HNO₃ etchant solution. According to the NiO contentand sintering temperatures, 41-67% porous YSZ ceramic is obtained and the flexural strength increases, while the porosity decreases with an increasing sintering temperature. The optimum flexural strength (136.5 ± 13.4MPa) and porosity (47%) for oxygen transport porous YSZ membrane can be obtained with 40 vol% of 7 μm NiO particle at a sintering temperature of 1350℃.

Parametric Appraisal of Process Parameters for Adhesion of Plasma Sprayed Nanostructured YSZ Coatings Using Taguchi Experimental Design

This paper presents the application of the Taguchi experimental design in developing nanostructured yittria stabilized zirconia (YSZ) coatings by plasma spraying process. This paper depicts dependence of adhesion strength of as-sprayed nanostructured YSZ coatings on various process parameters, and effect of those process parameters on performance output has been studied using Taguchi's L16 orthogonal array design. Particle velocities prior to impacting the substrate, stand-off-distance, and particle temperature are found to be the most significant parameter affecting the bond strength. To achieve retention of nanostructure, molten state of nanoagglomerates (temperature and velocity) has been monitored using particle diagnostics tool. Maximum adhesion strength of 40.56 MPa has been experimentally found out by selecting optimum levels of selected factors. The enhanced bond strength of nano-YSZ coating may be attributed to higher interfacial toughness due to cracks being interrupted by adherent nanozones.

Coupled Metal Oxide-Doped Pt/SnO2 Semiconductor and Yittria-Stabilized Zirconia Electrochemical Sensors for Detection of VOCs

Yttria-stabilized zirconia (YSZ)-based sensors using Pt/SnO2-based sensing electrodes were fabricated and applied to detect several volatile organic compounds (VOCs) including acetaldehyde, acetone, ethanol and toluene. The whole sensor was exposed to the VOCs-contaminated air, i.e. without air reference. YSZ was prepared by coprecipitation, calcined, pelleted, and sintered at 1560°C for 12 h. The YSZ pellet was coated with two layers of porous Pt electrode, on one of which as the sensing electrode, a layer of Pt/SnO2 doped with SiO2, MoO3, CeO2 or Sm2O3 was screen printed. The oxide-doped Pt/SnO2 samples were prepared by a sol-gel method and characterized by XRD, SEM, and BET. As 5 wt% SiO2 or Sm2O3 is doped onto the Pt/SnO2, the crystallite size decreases from 17 to 6 or 8 nm, respectively. An automated flow system was used for evaluating the sensors performances in presence of various VOCs of different concentrations at 350–600°C. Response/recovery times of all VOCs sensors decrease with the oxide doping. Pt/SnO2 doped with 5 wt% CeO2, compared to other oxides, shows the highest EMF responses and lowest response and recovery times to the VOCs. By increasing the Pt/CeO2/SnO2 sensing layer thickness, EMF response is enhanced and maximum response shifts to higher temperatures.

Investigation of the microstructural effect of Ni–yttria stabilized zirconia anode for solid-oxide fuel cell using micro-beam X-ray absorption spectroscopy analysis

The microstructural effect of the solid-oxide fuel cell (SOFC) anode on fuel cell performance has been investigated using X-ray absorption spectroscopy (XAS). Anodes were examined after the fuel cell had operated at 600–700 °C using H2 fuel. The microstructure of micro tubular anodes consisting of conventional Ni–yittria stabilized zirconia (YSZ) has been controlled by altering the co-sintering temperature with the zirconia electrolyte. Impedance analysis has clearly shown that the overpotentials for the gas transport and the electrochemical reactions improve for the anode prepared at lower co-sintering temperatures. Using data from the X-ray absorption near edge structure (XANES) energy region, the metallic Ni fraction as a function of distance from the electrolyte has also been measured for those samples. XANES observation has shown the oxidation status of Ni in the anode is quite sensitive to position relative to the anode/electrolyte interface. These results may be an indication of the positions of the reaction sites for oxide ions and the fuel: assuming the oxidation state of nickel (Ni–O) designates the active triple phase boundary.

Effect of MgO particle size on the microstructure, mechanical properties and wear performance of ZTA–MgO ceramic cutting inserts

The mechanical properties, microstructure and wear performance of zirconia-toughened alumina (ZTA) cutting inserts with Magnesia (MgO) in different particle sizes as additives was investigated. The MgO particle sizes were varied from 80 nm to 7000 nm. The alumina (Al 2O 3), yittria stabilized zirconia (YSZ) and MgO powders were mixed, compacted and sintered at 1600 °C using the solid-state sintering method. The mechanical and physical properties of the samples such as wear resistance, Vickers hardness, fracture toughness, microstructure and density were analyzed. Commercially available stainless steel (316L) was used as the workpiece for the wear resistance study. It was observed that smaller MgO particle sizes induce better wear performance and mechanical properties for the cutting inserts. Wear resistance analysis showed that the cutting insert with nano-sized MgO (particle size 80 nm) had the lowest wear area of 0.019 mm². The same cutting insert also possessed the highest Vickers hardness value of 1740 Hv compared to the other samples. Furthermore, microstructural observations show that the Al 2O 3 grain size depends on the particle size of MgO, and is directly related to its hardness property.

Material Science and Applications of Vector Materials

We define a vector material as one can manipulate surroundings or circumstance of oneself. Especially, the interaction effect between surface charges and materials based on the electric force is called termed as electrovector effect. In this report, we demonstrated that fabrication and development of electrets which have electrovector effect. The electrets with hydroxyapatite (HAp) have the electrovector effect which increases the ability to promote bone regeneration (osteoanagenesis). In order to reveal the working and mechanism of electrovector effect on biomineralization in vivo, we study electrovector effect of crystal growth on the electret substrates. We found that surface charges on the substrates promoted precipitation of HAp in simulated body fluid (SBF), and influenced the morphology of calcite thin film growth on substrates with HAp and yittria stabilized zirconia (YSZ) .

Fabrication and evaluation of the electrochemical performance of the anode-supported solid oxide fuel cell with the composite cathode of La 0.8Sr 0.2MnO 3−δ–Gadolinia-doped ceria oxide/La 0.8Sr 0.2MnO 3−δ

The anode-supported single cell was constructed with porous Ni–Yittria-stabilized zirconia (YSZ) as the anode substrate, an airtight YSZ as the electrolyte, and a screen-printed La 0.8Sr 0.2MnO 3−δ (LSM)–Gadolinia-doped ceria (GDC)/LSM double-layer cathode. The SEM results show that the YSZ thin film is highly integrated, fully dense with a thickness of 13 μm, and exhibits excellent compatibility between cathode and electrolyte layers. The effects of feed rates of the reactants, temperature, and contact pressure between the current collector and the unit cell were systematically investigated. The results are based on the assumption that the anode contribution to the polarization resistance is negligible. Our analysis showed that the electrochemical reaction is limited by mass transfer control when the airflow rate is decreased to 500 ml min −1. The maximum power density is 204.6 mW cm −2 at 800 °C with H 2 and air at flow rates of 800 and 2000 ml min −1, respectively. According to the AC-impedance data, the resistances of charge transfer at the electrode/electrolyte interface are 3.79 and 1.90 Ω cm 2. The resistances of oxygen-reduction processes are 3.63 and 1.01 Ω cm 2 at 700 and 800 °C, respectively. The results from the sensitivity analysis of the variation of contact pressure between current collectors and membrane electrode assembly (MEA) show that the influence is enhanced at the regions of the high current density.

Production of Synthetic Fuels by Co-Electrolysis of Steam and Carbon Dioxide

Co-electrolysis of H2O and CO2 was studied in solid oxide cells (SOCs) supported by nickel-/yittria-stabilized zirconia (Ni/YSZ) electrode. Polarization characterization indicates that electrochemical reduction of both CO2 and H2O occurs during co-electrolysis. In parallel with the electrochemical reactions, the equilibrium of the water–gas shift reaction is reached, and moreover, CO is produced via the water–gas shift reaction. The degradation observed when performing co-electrolysis in these SOCs occurs mainly at the Ni/YSZ cathode and may be a consequence of impurities in the gas stream, adsorbing on active sites in the SOC. The low degradation is most likely acceptable for long-time operation.

Debye-type relaxation in yittria stabilized zirconia

Dielectric relaxation and ac conductivity behavior were investigated for yittria stabilized zirconia (YSZ) compounds with yittria concentration varying from 8 to 12 mol%. Dielectric loss spectrum shows that two types of Debye-type relaxation exist for the YSZ materials with yittria concentration varying in the range of 10–12 mol%. Observed data indicates that both the Debye-type relaxation and ceramics/metallic interface polarization are responsible for very high relative dielectric constant in the low frequency and high temperature range. Debye-type peaks get suppressed due to the predominance of the dielectric loss; this dielectric loss is attributed to the loss current conductivity. Electric modulus spectrum shows that grain and grain boundary offer same capacitance to the ion conduction at high temperatures. The frequency range of dc conductivity extends to higher frequency side as temperature increases.

Fabrication and characterization of a Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ—Gadolinia-doped ceria cathode for an anode-supported solid-oxide fuel cell

Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ (BSCF) and gadolinia-doped ceria (GDC) were synthesized via a glycine-nitrate process (GNP). A cubic perovskite of BSCF was observed by X-ray diffraction (XRD) at a calcination temperature above 950 °C. An anode-supported solid-oxide fuel cell was constructed from the porous NiO + YSZ as the anode substrate, the yittria-stabilized zirconia (YSZ) as the electrolyte, and the porous BSCF-GDC layer as the cathode with a GDC barrier layer. For the performance test, the maximum power density was 191.3 mW cm −2 at a temperature of 750 °C with H 2 fuel and air at flow rates of 335 and 670 sccm, respectively. According to the AC-impedance data, the charge-transfer resistances of the electrodes were 0.10 and 1.59 Ω cm 2, and the oxygen-reduction and oxygen-ion diffusion resistances were 0.69 and 0.98 Ω cm 2 at 750 and 600 °C, respectively. SEM microstructural characterization indicated that the fuel cell as fabricated exhibited good compatibility between cathode and electrolyte layers.

H2-H2O Effective Binary Diffusivity Measurement in Solid Oxide Fuel Cell Anode Using an Electrochemical Cell

The effective binary diffusivity of H2 and H2O in a Ni and yittria-stabilized zirconia (YSZ) anode of the solid oxide fuel cells (SOFCs) was measured between 650 and 800oC using an electrochemical cell consisting of an oxygen pump, an oxygen sensor, and a porous SOFC anode pellet. The effective binary diffusivity was obtained from the relationship between the current density across the oxygen pump, and the H2 partial pressure gradient across the anode sample measured using the oxygen sensor. The anode limiting current density and concentration polarization were estimated using the experimental results.

Out-of-cell measurements of H 2–H 2O effective binary diffusivity in the porous anode of solid oxide fuel cells (SOFCs)

The effective binary diffusivity of H 2 and H 2O in a Ni and yittria-stabilized zirconia (YSZ) anode of the solid oxide fuel cells (SOFCs) was measured between 650 and 800 °C using an electrochemical cell consisting of an oxygen pump, an oxygen sensor, and a porous SOFC anode pellet. The effective binary diffusivity was obtained from the relationship between the current density across the oxygen pump, and the H 2 partial pressure gradient across the anode sample measured using the oxygen sensor. The anode limiting current density and concentration polarization were estimated using the experimental results.

Hip-simulator wear studies of an alumina-matrix composite (AMC) ceramic compared to retrieval studies of AMC balls with 1–7 years follow-up

Wear in large diameter ceramic-on-ceramic bearings were compared in both simulator and patient-retrieval studies. Four combinations of 36 mm diameter alumina matrix composite bearings (AMC: alumina–zirconia) were run in a hip simulator using alumina (ALX) bearings as the historical control. The hip simulator was run in microseparation test mode to 5-million cycles (5Mc) duration. Run-in and steady-state wear, mapping of wear-stripes, monoclinic transformations, surface roughness and wear debris morphology were analyzed sequentially. The balls and cups were inspected at every event and surface changes recorded. For AMC bearings, the zirconia transformation was studied by confocal Raman microprobe spectroscopy (RMS). Bearing surfaces were analyzed by SEM imaging and surface roughness was measured by white-light interferometry. The same techniques were used on the retrieved AMC balls with the exception that zirconia phase transformation was measured by X-ray diffraction (XRD). All balls and cups revealed stripe wear within 0.1Mc duration. Stripes expanded over 5Mc duration to span a 45–90° arc on the balls and covered an area of 150–350 mm 2. The narrow wear scar on cup rims expanded circumferentially and radially with duration. The AMC/AMC performance represented a 5–12-fold wear reduction compared to controls; hybrid ALX/AMC combinations showed a 3-fold reduction. Tetragonal to monoclinic transformation was detected in all AMC bearings and peaked at less than 30% by end of study. In non-wear zones, monoclinic content averaged less than 20%. SEM and roughness mapping at 5Mc showed that AMC surfaces had superior resistance to stripe wear. The stripe wear and monoclinic transformations in the two retrieved AMC balls with alumina liners (3–7 years use) appeared similar to the laboratory study. Thus the monoclinic transformation was anticipated by the laboratory performance. It is therefore hypothesized that once the concentration of monoclinic reaches a certain level, further zirconia transformation is suppressed until the compressive alumina constraint can be overcome by additional wear. This appears to be a very different process compared to that seen with the yittria-stabilized zirconia balls used from year 1985.