Thin Yttria-stabilized Zirconia Layer Research Articles

Functional coatings made of eco-friendly materials

Thermal barrier coatings protect alloys (for example, in turbine blades) from extreme temperatures. There is a pressing need to find better materials than currently used yttria-stabilized zirconia (i.e. materials with better thermal stability, lower thermal conductivity, higher thermal expansion coefficient matching that of the alloy). Here we explore gehlenite (Ca2Al2SiO7) for possible application as a material for thermal barrier coatings. We found that gehlenite can indeed be used in next-generation thermal barrier coatings, as a top coat on a thin layer of yttria-stabilized zirconia.

Influence of compatibility between sol and intermediate layer on the performance of yttria-stabilized zirconia nanofiltration membrane

This paper reports the synergistic effect of the sol and intermediate layer on the performance of yttria-stabilized zirconia (YSZ) nanofiltration (NF) membranes. We have focused on the characterization of the microstructure, pure water permeance, and molecular weight cut-off (MWCO) of the NF membranes derived from zirconia sols of different precursor concentrations on two types of supported ZrO2 ultrafiltration (UF) membranes. We found that the performance of YSZ membranes strongly depends on the sol concentration and the pore size of the intermediate layer. In addition, YSZ gel membrane formation was found to follow the filtration process. Therefore, it is essential to maintain the compatibility between the sol and intermediate layer to fabricate high-performance NF membranes. A crack-free thin YSZ layer with an MWCO of 816 Da (pore size: 1.4 nm) and a water permeance of 25 L m-2 h-1 bar-1 was fabricated using a precursor concentration of 0.03 mol/L, on ZrO2 UF membrane with a pore size of 5.5 nm. The YSZ NF membrane exhibited a relatively high retention rate towards MgCl2 (71%), whereas a lower retention rate was observed for NaCl (35%).

Enhancing the efficiency benefit of thermal barrier coatings for homogeneous charge compression ignition engines through application of a low-k oxide

Prior experiments reported by the authors have proven the hypothesis that achieving a dynamic temperature swing on the combustion chamber surface will lead to improved thermal and combustion efficiencies of the homogeneous charge compression ignition engine. A thin layer of yttria-stabilized zirconia, roughly 150 μm, was plasma sprayed on the piston top. It led to markedly advanced ignition and heat release in the gasoline homogeneous charge compression ignition engine, accompanied with reduced unburnt hydrocarbon and carbon monoxide emissions, improved combustion efficiency, and a higher thermal efficiency. A related computational study highlighted the critical role of coating thermal conductivity in achieving a desired dynamic response; hence, the second phase of experimental investigations focused on introducing structured porosity in the yttria-stabilized zirconia coating, as a means of reducing effective conductivity. Indeed, additional incremental improvements were observed, as well as limitations related to adverse effects of the surface roughness and the fuel interactions with the surface roughness and open pores. Erosion can also be a problem in a direct injection engine. Therefore, the third round of investigations focused on a material with a natively low conductivity (low-k), sprayed on the top of an Al piston in a relatively dense form, and in a way that yields a smooth surface. The objective was to capitalize on the low conductivity, while avoiding the pitfalls accompanying high-porosity formulations. The heat-storage capacity was limited by keeping the thickness relatively low. The results verify the paramount importance of thermal conductivity in the context of high “temperature swing” behavior and indicate a potential to improve the homogeneous charge compression ignition engine’s combustion efficiency roughly 1.5%, with the overall indicated efficiency improvement on the order of 5%, on a relative basis. In addition, the low-k oxide thermal barrier applied to the piston extended significantly the low-load homogeneous charge compression ignition operability limit.

Femtosecond Laser Additive Manufacturing of Multi-Material Layered Structures

Laser additive manufacturing (LAM) of a multi-material multi-layer structure was investigated using femtosecond fiber lasers. A thin layer of yttria-stabilized zirconia (YSZ) and a Ni–YSZ layer were additively manufactured to form the electrolyte and anode support of a solid oxide fuel cell (SOFC). A lanthanum strontium manganite (LSM) layer was then added to form a basic three layer cell. This single step process eliminates the need for binders and post treatment. Parameters including laser power, scan speed, scan pattern, and hatching space were systematically evaluated to obtain optimal density and porosity. This is the first report to build a complete and functional fuel cell by using the LAM approach.

Behavior of yttria-stabilized zirconia (YSZ) during laser direct energy deposition of an Inconel 625-YSZ cermet

Thermal barrier coatings (TBC) are regularly used today to protect and extend the service life of several superalloys which are extensively used in high temperature applications. The existing TBCs are typically between 0.1 to 0.5 mm in thickness, are deposited on metal substrates using plasma spray or electron beam vapor deposition, and can reduce temperatures at the substrate surface by up to 300 °C. For greater temperature reductions there is a need for thicker TBCs. The building of thick TBCs has to date been stymied by poor adhesion, and cracking during deposition. It has been suggested that a functionally graded approach may reduce the residual stresses which result in these defects. To date there have been few reports on the deposition of ceramic or cermet coatings using laser AM and none have reported on the phase stability of ceramic particles post-deposition. This paper is a first report on the phase stability of ceramic particles following the compositional segregation of elements during deposition using a powder feed additive manufacturing process. Functionally graded (FG), thick TBCs (>3 mm) consisting of Inconel 625 (IN625) and yttria-partially stabilized zirconia (8YSZ) were deposited on an A516 steel substrate via laser direct energy deposition (LDED). Good interfaces were observed between the bond coat (BC) and first cermet layer and between the graded cermet layers. However, cermet layers deposited with 10 wt.% or more YSZ developed a thin layer of YSZ on the surface. The thin layer of YSZ greatly hindered additional deposition of new cermet layers. In cermet layers that did exhibit good interfaces, fine, re-solidified, YSZ particles were homogenously distributed within the Inconel 625 matrix. The YSZ particles exhibited a tetragonal lattice structure and were depleted of yttrium. In contrast, the thin YSZ layer formed on a cermet surface showed no yttrium depletion.

An aqueous sol–gel process for the fabrication of high-flux YSZ nanofiltration membranes as applied to the nanofiltration of dye wastewater

Ceramic nanofiltration (NF) membranes are becoming increasingly important for the recovery and purification of dyes and salts (e.g., NaCl) in high salinity waste streams. This work reports an aqueous sol–gel process for the preparation of high-flux yttria-stabilized-zirconia (YSZ) NF membranes derived from zirconium oxychloride, and the procedure is completely free of organic solvents. The use of zirconium oxychloride as a precursor dissolved in deionized water was expected to contribute to environmental protection and reduce cost. A crack-free thin YSZ layer was fabricated on α-Al2O3 supported ZrO2 ultrafiltration membranes. The YSZ NF membrane exhibited a great water permeability of 28Lm−2h−1bar−1 and a low molecular weight cut-off of 800Da. Furthermore, the desalination and recovery were examined with fluorescent brightener wastewater in different operation parameters by YSZ NF membranes. The filtration results revealed that the fluorescent brightener retention increased slightly with increasing transmembrane pressure and cross-flow velocity. When the filtration was operated at 60°C, the fluorescent brightener recovery value of 99% was obtained while the NaCl removal was greater than 98%, meanwhile the flux was up to 89Lm−2h−1 under suitable conditions, indicating that ceramic NF membranes were the technology of choice in treatment of high temperature dye wastewater.

Reversible Fuel Cells with Tri-Layer Electrolytes

Reversible solid oxide fuel cells (SOFC) are operated as solid oxide fuel cells and as solid oxide electrolyzer cells (SOEC). In SOFC mode, hydrogen is consumed at the fuel electrode to convert chemical energy of fuel oxidation into water vapor. In SOEC mode, water vapor is decomposed at the fuel electrode to generate hydrogen under an applied DC voltage. Much of the reported work on reversible solid oxide cells is based on yttria-stabilized zirconia (YSZ) electrolyte, Ni + YSZ fuel electrode, and a perovskite-based oxygen electrode. At high operating voltages in the SOEC mode, cell degradation can occur which involves oxygen electrode delamination, formation of cracks within the electrolyte, and also reduction of the electrode near the fuel electrode. The higher the applied voltage, the greater is the propensity to degradation. These observations are consistent with the chemical potential of oxygen in the electrolyte exceeding both electrode bounds under an applied voltage. It has been shown that a small amount of electronic conductivity can minimize degradation. The principal role of electronic conductivity is to smoothen out the variation in oxygen chemical potential through the electrolyte such that it does not exceed electrode bounds. YSZ-based electrolyte if doped with sufficient amount of CeO2 can create significant electronic conductivity such that the electrolyte becomes a mixed ionic electronic conductor. Doping with ceria also lowers the ionic transference number of the electrolyte. A possible bi-layer electrolyte consists of a thin layer of YSZ (or another purely oxygen ion conductor) deposited on a mixed ionic electronic conducting material. As long as electronic conduction through the blocking layer is negligible, the overall ionic transference number of the bi-layer electrolyte can approach unity. In this paper, theoretical analysis of the bi-layer electrolyte for reversible cells is presented. Two configurations of the bi-layer electrolyte are investigated. (1) The electron blocking layer adjacent to the oxygen electrode. (2) The electron blocking layer adjacent to the fuel electrode. For both configurations, the spatial distribution of oxygen chemical potential through the bi-layer electrolyte in both the SOFC and the SOEC modes are evaluated. The ionic transference number of the MIEC part of the bi-layer electrolyte is varied over a wide range. Theoretical analysis shows that such bi-layer electrolyte cells should exhibit good performance characterstics and excellent stability. The net variation of the oxygen chemical potential through the bi-layer electrolyte is determined as a function of transport properties of both layers, electrode polarizations and interfacial resistances to electron transfer.

Fabrication of low-temperature solid oxide fuel cells with a nanothin protective layer by atomic layer deposition

Anode aluminum oxide-supported thin-film fuel cells having a sub-500-nm-thick bilayered electrolyte comprising a gadolinium-doped ceria (GDC) layer and an yttria-stabilized zirconia (YSZ) layer were fabricated and electrochemically characterized in order to investigate the effect of the YSZ protective layer. The highly dense and thin YSZ layer acted as a blockage against electron and oxygen permeation between the anode and GDC electrolyte. Dense GDC and YSZ thin films were fabricated using radio frequency sputtering and atomic layer deposition techniques, respectively. The resulting bilayered thin-film fuel cell generated a significantly higher open circuit voltage of approximately 1.07 V compared with a thin-film fuel cell with a single-layered GDC electrolyte (approximately 0.3 V).

Low-Power and Controllable Memory Window in Pt/Pr<SUB>0.7</SUB>Ca<SUB>0.3</SUB>MnO<SUB>3</SUB>/Yttria-Stabilized Zirconia/W Resistive Random-Access Memory Devices

Yttria-stabilized zirconia (YSZ) layers of various thicknesses were designed and introduced before Pr0.7Ca0.3MnO3 (PCMO) film was deposited on W bottom electrodes with a submicron via-hole structure. By changing the thickness of the YSZ barrier layer (3, 5, 9, and 13 nm), a tunable memory window can be realized while low power consumption (P(max) < 4 microW) is maintained. Resistive switching (RS) in a Pt/PCMO/YSZ/W stack with a thin YSZ layer can be ascribed to an oxidation/reduction reaction caused by a ring-type PCMO/W contact, while RS with a thick YSZ layer may be related to oxygen migration across the YSZ layer between the PCMO film and the W bottom electrode and the increase (decrease) of the effective tunnel barrier height of the YSZ layer. Excellent RS behavior characteristics, such as a large R(HRS)/R(LRS) ratio (> 10(3)), die-to-die uniformity, sweeping endurance, and a retention time of more than 10(3) s, can be obtained by optimizing the thickness of YSZ layer.

Preparation of a 3DOM Ni–YSZ Anode With a Dense YSZ Thin Electrolyte Layer for Solid Oxide Fuel Cells

A three-dimensionally ordered macroporous (3DOM) Ni–yttria stabilized zirconia (YSZ) anode with a YSZ thin electrolyte layer was developed by a colloidal crystal templating method in order to lower the operating temperature of solid oxide fuel cells due to a high and uniform electrochemical interface that originated from a 3DOM structure. In the present study, 2μm polystyrene beads were used as the template. The anode structure was strongly influenced by the pH of the suspension, and it was optimized to 1.8 according to the zeta-potential measurement and scanning electron microscope observation. The 3DOM anode and thin electrolyte layer were simultaneously formed by sintering the deposit that was obtained by filtering two kinds of suspensions by turns. Here, we successfully obtained a 3DOM anode with high porosity of 72% and a 3μm thin YSZ layer on it, which was favorable in reducing both electrolyte and polarization resistances.

A simple bilayer electrolyte model for solid oxide fuel cells

A simple electrolyte model for bilayer ion-conducting membrane used in solid oxide fuel cell (SOFC) was developed. The model considers yttria-doped bismuth oxide (YDB) and gadolinia-doped ceria (GDC) as the substrate electrolyte, both coated with a thin layer of yttria-stabilized zirconia (YSZ) on the anode side. Migration of oxygen ions and diffusion of free electrons and electron holes through the bilayer electrolyte form the basis of modeling in this study. Results showed that depositing a very thin layer of YSZ onto the substrate electrolyte in the thickness ratio of 1 to 10,000 could increase the interfacial oxygen partial pressure significantly in the bilayer electrolyte and reduce the penetration of electronic current. The improved interfacial oxygen partial pressure by YSZ coating, higher than the equilibrium partial pressure of Bi/Bi 2O 3, made bismuth oxide a possible candidate for use in SOFC. An important result of the electronic blocking effect with bilayer electrolyte SOFC is to improve the open circuit voltage and thus to enhance the output power density of the cell.

A Controlled Solid State Diffusion Process to form Cermet Anodes for Solid Oxide Fuel Cells

ABSTRACTThe interfacing of thin film vapor deposition technologies and solid state electrochemistry has led to the recent development of polarized electrochemical vapor deposition (PEVD). In this study, PEVD was applied to deposit a thin layer of yttria stabilized zirconia (YSZ) over a porous metallic electrode to form the cermet anode of a solid oxide fuel cell (SOFC). During PEVD, oxygen ions are transported through the solid electrolyte of an SOFC under an electrical potential gradient provided by an external dc source. At the metallic electrode (anode) surface, oxygen ions react electrochemically with ZrCl4and YCl3 in the vapor phase to deposit YSZ. The growth of YSZ resembles the mechanisms illustrated in Wagner's tarnishing theory. However, modification has been made to the initial growth of YSZ at both electronically and ionically shorted paths along the metallic electrode and solid electrolyte surfaces, respectively. The initial experimental results in the present study showed that PEVD is capable of depositing a thin layer of YSZ on a porous metallic electrode to form a cermet anode. This layer not only provides a continuous ionic conducting path in the anode to reduce the overpotential loss, but also protects the metallic electrode from further sintering, vapor loss and poisoning in the harsh SOFC operating conditions.