Yttria-stabilized zirconia substrates

Yttria stabilized zirconia formed by this reaction fills the air electrode pores. The dynamics of this CVD stage has been modeled by Carolan and Michaels [120], who observed that films produced in this manner penetrate the substrate no more than 2-3 pore diameters from the chloride face of the substrate. It has also been shown that the penetration depth is independent of water concentration. The second step of this method is the EVD step. Once pore closure is achieved, the reactants are not longer in contact. Electrochemical semipermeability related to the existence of small electronic conductivity and large gradient of oxygen partial pressure leads to oxygen transport from the steam side to the chloride side through the deposited electrolyte. The electrochemical reactions involved are ... 

Fig. 34 SEM SE image of the anode/solid electrolyte interfacial region. (A) Substrate (ytterbia and yttria stabilized zirconia) (B) Pt metailic electrode (C) PEVD product (yttria stabilized zirconia).

Electrochemical vapor deposition has been explored for making gas-tight, dense, solid electrolyte films on porous substrates, - and the most smdied system has been the yttria-stabilized zirconia films on porous alumina substrates for solid... 

In EVD, a modified form of chemical vapor deposition (CVD), an electrochemical potential gradient is used to grow a thin, dense layer of the ionic conducting oxide (e.g., yttria-stabilized zirconia) on a porous substrate. EVD is either a single-step or a two-step process depending on the nature of the substrate. For a porous substrate, the first step involves pore closure by CVD (i.e., deposition from the vapor of an oxide layer by reaction of a chloride gas precursor compound with water vapor or oxygen) ... 

Several recent reports describe using clay or other inorganic fillers to form CP composites. Polyani-line-polypyrrole composite coatings containing clay or yttria stabilized zirconia were electrodeposited onto AA 2024-T3 [158], with improved corrosion resistance of the substrate. Similarly, particulate-filled polyaniline and polypyrrole films on AA 2024-T3 were prepared electrochemically using a variety of fillers, including clay, carbon black, short carbon fiber, zirconia, and silica [159]. Again, enhanced corrosion performance for these composites was observed. 

Inaba M, Mineshige A, Maeda T, Nakanishi S, loroi T, Takahshi T, et al. Growth rate of yttria-stabilized zirconia thin films formed by electrochemical vapour-deposition using NiO as an oxygen source II. Effect of the porosity of NiO substrate. Solid State Ionics 1997 104(3/4) 303-10. 

The electrochemical cell, as shown schematically in Fig. 1, consists of gas-tight ceramic electrolyse in the form of tubes, discs, planar substrates, or thick films which is sandwiched by precious metal like platinum or silver as electrodes. Both electrodes are in close contact with two separate gas compartments. Such cells often modified by other electrode materials like electric conducting oxides (chromites, manganites) are commonly used as gas sensors, fuel cells, and electrolysis cells. Yttria-stabilized zirconia (YSZ Zro.84To.i6< i.92) as oxide ionic conductor is widely used as an electrolyte. 

These devices feature the use of micro-fabrication techniques adapted from the micro-electronics industry. These encompass substrate etching, thin-fihn deposition, lithography, and film-etching steps. This field has recently been reviewed by Evans et al. [11], and all devices exhibit the beautiful structural quality resulting from the micro-fabrication techniques. An example of a micro-planar SOFC fabricated on a silicon substrate is illustrated in Fig. 19.4 [12]. Figure 19.4a shows the sequence of fabrication steps used to make the edge-supported SOFC membrane which spans an aperture with dimensions 600 x 600 pm. The yttria stabilized zirconia (YSZ) electrolyte, which is only 70-nm thick, was deposited by... 

Each of the three functional layers of the cell, anode, electrolyte, and cathode, are deposited onto the substrate using screen printing. In practice, each of the functional components requires multiple layers to provide the necessary functionality and manufacturability. Unlike other SOFC designs and similar to the metal-supported cell, the IP-SOFC uses neither the electrolyte nor electrodes to provide stmctural support. This allows very thin cells with consequently very small material quantities (cf. previous section). The cell functional layers again are made of conventional SOFC materials yttria-stabilized zirconia electrolyte, nickel cermet anode, and rare earth... 

A unique proposed applieation for an yttria-stabilized zirconia is in carbon monoxide detection. A platinum electrode is attached on both sides of a zirconia electrolyte. One side is covered with a platinum eatalyst on a porous alxunina substrate and the Pt electrode is not in direct contact with the sample gas. Platinum on the substrate acts as a catalyst for CO oxidation. A cross-sectional view of the CO sensor is shown in fig. 15 (Okamoto et al. 1980) (the operating temperature is around 300°C). When carbon monoxide exists in the atmosphere, most will be eatalytically oxidized by the oxygen in air during difiusion through the porous substance. Therefore, the gas that reaches the Pt electrode is not CO but a CO2-O2 mixture. On the other hand, on the surface of the platinum electrode without the catalyst, carbon monoxide is oxidized to CO2 and causes an anomalous EMF. This potential shows a one-to-one correspondence to the CO concentration. The typical performance of the CO sensor in air at 300°C is shown in fig. 16 (Okamoto et al. 1980). The EMF output increases with the CO content, but the slope of the curve decreases gradually. This sensor can operate at temperatures between 260 and 350 C and no speeial O2 reference gas is necessary. 

An anode supported type SOFC was recently proposed as a design which can operate at lower temperatures <900 °C. An extremely high power density, over 1 W/cm, was reported for a single cell by Visco et al. (1999) at Lawrence Berkeley National Laboratory, and University of California in US, and now the stack developments are promoted by Pacific Northwest National Laboratoiy (PNNL) and Delphi Co. for auxiliary power unit in heavy duty mobiles (Mukeqee et al., 2003). In an anode supported SOFC, a thin, dense electrolyte film (thickness 10 (um) is prepared on a porous composite substrate of nickel metal and yttria stabilized zirconia (YSZ) as the anode, and the porous cathode is attached to the dense electrolyte. The thin electrolyte and planar cell stack decreases the Ohmic loss, and results in a high power... 

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