Single Yttria-stabilized Zirconia Research Articles

In-Operando Observations of Ni-YSZ Patterned Fuel Electrodes Under SOFC and SOEC Operations

In-operando observations and electrochemical measurements of patterned nickel (Ni) film electrodes under both solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) operations are conducted. Confocal laser scanning microscopy (CLSM) is applied to acquire the optical and laser microscopy images of patterned Ni film electrodes under polarization to study the Ni mobility on the yttria-stabilized zirconia (YSZ) surface. The Ni film dynamically spreads and breaks up only under SOFC operation, while such a phenomenon is not observed during SOEC operation. Ni coarsening occurs regardless of the operation mode. The morphological evolution of Ni qualitatively correlates with the changes of the current density. The Ni evolution in a single YSZ open window is well correlated with the total current density variation, which means that Ni movements of most of the YSZ windows are synchronized on the whole cell.

Performance of single YSZ, Gd2Zr2O7 and double-layered YSZ/Gd2Zr2O7 thermal barrier coatings in isothermal oxidation test conditions

Oxidation is an inevitable failure mechanism under the operating temperature in gas turbines. To avoid negative effects of oxidation, ceramic-based materials having low thermal conductivity and high stability should be used to hot section components. In accordance with this purpose, thermal barrier coatings (TBCs) are used in order to increase the lifetime of gas turbine engine components that have not reached to desired levels yet. Yttria stabilized zirconia (YSZ) has been used as a conventional top coat material in TBCs. Increased the turbine inlet temperatures (TIT) promote to researchers to try higher stable material such as rare earth zirconates. In this study, CoNiCrAlY metallic powders were sprayed using a new emerging technique as called cold gas dynamic spray (CGDS) on Inconel 718 substrates. Single layer YSZ, Gd2Zr2O7 and double-layered YSZ/Gd2Zr2O7 were deposited by electron beam physical vapor deposition (EB-PVD) technique as top coat materials. In high temperature furnace, both TBC samples were isothermally oxidized at 1000 °C under different time periods. TBCs were examined as microstructural before and after oxidation tests. Thermally grown oxide (TGO) layer forming at the interface during oxidation were investigated and compared for each TBC systems. Oxidation and TGO growth behaviors were discussed.

Using the macroscopic scale to predict the nano-scale behavior of YSZ thin films

In this work, Yttria-stabilized zirconia (YSZ) thin films were deposited using dual reactive magnetron sputtering. By varying the deposition conditions, the film morphology and texture of the thin films are tuned and biaxial alignment is obtained. Studying the crystallographic and microstructural properties of the YSZ thin films, a tilted columnar growth was identified. This tilt is shown to be dependent on the compositional gradient of the sample. The variation of composition within a single YSZ column measured via STEM–EDX is demonstrated to be equal to the macroscopic variation on a full YSZ sample when deposited under the same deposition parameters. A simple stress model was developed to predict the tilt of the growing columns. The results indicate that this model not only determines the column bending of the growing film but also confirms that a macroscopic approach is sufficient to determine the compositional gradient in a single column of the YSZ thin films.

In Situ Analytical Electron Microscopy Studies of Redox Reactions at a YSZ/Pt Interface

Redox reactions were studied at a single yttria-stabilized zirconia (YSZ)/Pt electrode interface, in parallel with pure YSZ with no catalyst electrode, by in situ analytical electron microscopy at elevated temperatures and in an oxygen atmosphere. In situ electron holography showed that the oxide underwent reduction at elevated temperatures in a vacuum and was consequently reoxidized upon exposure to an oxygen flux at the same temperature. In situ energy loss spectroscopy measurements were in agreement with in situ electron holography observations and indicated that the oxidation state of the host cation zirconium was altered in the reduced state of the YSZ to the metastable state Zr(3+).

Novel microstructure of EB-PVD double ceramic layered thermal barrier coatings

New ceramic candidate materials for thermal barrier coating (TBCs) application and modifications of yttria stabilized zirconia (YSZ) have been increasingly studied, aiming at improving thermal cycling resistance and thermal insulation of TBCs. A novel Gd 2O 3–Yb 2O3 doped YSZ/YSZ double ceramic layered TBC was prepared by means of electron beam physical vapor deposition (EB-PVD), where the undoped YSZ top layer is featured with clusters of columns with sub-grains growing around columns, while the doped YSZ bottom layer feather-like structure. The TBC showed structure stability after 190 h heat treatment at 1373 K. The double layered TBC exhibited thermal cycling lifetime of more than 360 h, much higher than that of the single YSZ TBC.

Microstructural Features of RF-sputtered SOFC Anode and Electrolyte Materials

Thin-film samples of yttria-stabilized zirconia (YSZ), nickel oxide (NiO)-YSZ, and YSZ/nickel (Ni)-YSZ bilayer were fabricated by RF-sputtering. The single YSZ layer and YSZ/Ni-YSZ bilayer samples were annealed while the NiO-YSZ layer remained as-deposited. Cross-section transmission electron microscopy (TEM) samples of these thin-films were then prepared, which allowed detailed chemical and structural characterization of these thin-films on the nanometer-scale. Both YSZ and NiO-YSZ layers were fully dense and exhibiting equiaxed grain morphologies. Selected area electron diffraction (SAED) showed the YSZ crystal structure to be predominantly cubic in the annealed samples and amorphous in the as-deposited NiO-YSZ sample. It was found that YSZ film was 70 nm thick and dense, with equiaxed grains ranging from 12–20 nm. Surface roughness of the YSZ in the bilayer fell in the range of 5–20 nm. The Ni-YSZ film in the bilayer was 230 nm thick and porous, which consisted of columnar grains 13–75 nm in length and 9–22 nm in width. The bilayer sample showed no delamination or cracking along the YSZ/Ni-YSZ interface. It is believed that the nano-sized grains, minimal surface roughness and thin layers found in these films are desirable microstructural features for the anode and electrolyte in micro-solid oxide fuel cells (SOFCs). Correlation between microstructural features and electrochemical performance will be reported in a separate study.

Interaction of NiO with yttria-stabilized zirconia

As-prepared and heat treated plasma-produced 8 and 10 mol% yttria-stabilized zirconia (YSZ) powders doped with 0, 5, 10 and 75 mol% NiO have been investigated by XRD. The as-prepared powders are mixtures of metastable tetragonal and cubic phases but they transform to a single YSZ phase upon heat treatment above 1200 °C. The solubility of NiO in yttria-stabilized zirconia is less than 2 mol% at 1600 °C and varies little within the investigated yttria concentration range. The solubility increases with increasing temperature and the total NiO concentration. The activation energy of the solubility is about 0.25 eV.

A wide range air-fuel ratio sensor with a plate-type structure

A new wide range air-fuel ( A F ) ratio sensor has been constructed with a plate-type structure using a single YSZ (Yttria Stabilized Zirconia) solid electrolyte plate shared by the electrochemical oxygen storage, pumping, and sensing cells. The sensing characteristic was tested in fuel lean (O 2CO 2N 2) and fuel rich (COCO 2N 2) condition and compared with that of simulation. The operation pumping voltage could be set beyond the limiting current region when the reference voltage was fixed at 450 mV. This improper sensing characteristic was a result of the fact that the electrochemical oxygen sensing and pumping cells share a single electrolyte and one of the electrodes. A model to explain this sharing effect was set up and a new operating method for this sensor was suggested, which varied the reference voltage in proportion to the pumping voltage. The detecting range of this A F ratio sensor extended up to very high concentrations of oxidizing and reducing gases by adopting variable reference voltage.