Spinel protective layers

Thin film electrode/ spinel protection layer... 

X. Montero, F. Tietz et al., MnC01.9Feo.1O4 spinel protection layer on commercial ferritic steels for interconnect applications in solid oxide fuel cells. J. Power Sources 184, 172-179 (2008)... 

When I is larger than Iqr, the cubic Equation 21 produces no answers. I R means the mass of CO gas that diffuses through the porous protective layer. As the protective spinel layer is more porous than zirconia layer, Ilr>IcR-... 

To build up functional or protective layers, various methods from other technical industries have been adapted to the specific needs of ceramic sensor materials. Important criteria for the selection of the various technologies are the shape of the ceramic substrate (e.g., thimble or planar design), if it is co- or post-fired, if the layer material is expensive (e.g., noble metals) or inexpensive (e.g., alumina, magnesia spinel), and if the geometry and thickness need to be controlled precisely. 

Pt of the electrode material is deposited on the ceramic coating layer so as not to lose the electric characteristics. The coating layer is deposited on the sensor element with plasma jet coating of the spinel powder. It needs to meet the following conditions as an important protection layer (i) it should not be peeled off easily and (ii) it should have the porosity such that it is not plugged easily by poisonous substances in the exhaust gas. Therefore, specification are such that it should not be too thick, or too thin, and must be less dense. 

LiNio.sMni 5O4 (LNM) is considered as the most promising cathode to increase the density energy, as it operates at 4.7 V [65]. Unfortunately, its use is still postponed, mainly because of a significant capacity loss at elevated temperature T>50 °C, which is a critical environment for HEV and EV applications. As for the LiMn204 spinel, the remedy envisioned for canceling the electrode-electrolyte reactions is the same for both materials, namely coat the particles with a protective layer. 

Extensive work has been dedicated to the development of protective coatings for SOFC interconnects (see Future Fuel Cells ). Mitigation of Cr evaporation and excessive growth of oxide scale can be achieved with the use of a protective layer and a contact layer deposited on the dense alloy. The protective layer is obtained with the deposition of Co-Mn oxides to form (Mn,Co,Cr)304 spinel layer [4-6]. 

Fig. 7 Schematic representation of the effects of the protective layer, e.g., (Cr,Mn)304 spinel, and the contact layer on ions diffusion/evaporation for SOFC interconnects...

The transition from non-protective internal oxidation to the formation of a protective external alumina layer on nickel aluminium alloys at 1 000-1 300°C was studied by Hindam and Smeltzer . Addition of 2% A1 led to an increase in the oxidation rate compared with pure nickel, and the development of a duplex scale of aluminium-doped nickel oxide and the nickel aluminate spinel with rod-like internal oxide of alumina. During the early stages of oxidation of a 6% A1 alloy somewhat irreproducible behaviour was observed while the a-alumina layer developed by the coalescence of the rodlike internal precipitates and lateral diffusion of aluminium. At a lower temperature (800°C) Stott and Wood observed that the rate of oxidation was reduced by the addition of 0-5-4% A1 which they attributed to the blocking action of internal precipitates accumulating at the scale/alloy interface. At higher temperatures up to 1 200°C, however, an increase in the oxidation rate was observed due to aluminium doping of the nickel oxide and the inability to establish a healing layer of alumina. 

The oxide film formed in dry air at room temperature consists of a spinel phase, probably a solid solution of magnetite and maghemite. Such films form on magnetic tapes. They are around 1.5-2.0 nm thick, and in a dry atmosphere, can provide indefinite protection (e.g. the Delhi pillar). Ali and Wood (1969) found that with time and at a relative humidity of 46%, some hematite developed as well. At higher temperatures (200-300 °C) well defined duplex films with an inner layer of magnetite... 

Arranged in layered fashion on the alumina substrate are the zirconia underlayer, the platinum reference electrode, the zirconia solid electrolyte stabilized with 5.1 mole % Y2O3, the platinum measurement electrode, and finally,the protective spinel (A203 Mg0) layer. The zirconia layer is Umm long, 1+mm wide and 30pm thick. 

As a result, the zirconia solid electrolyte, two electrodes and protective spinel layer become porous. 

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