Support layers microstructure

The microstructure, properties, and performance of Ni-YSZ anodes depend sensitively on the microscopic characteristics of the raw materials (e.g., particles size and morphology of NiO and YSZ powders). The particle sizes of the starting YSZ powders vary usually from 0.2 to 0.3 pm, whereas those for the NiO powders are 1 pm. The Ni to YSZ volume ratio usually varies from 35 65 to 55 45. For example, the reported Ni to YSZ volume ratios include 34 66 [20, 21], 40 60 [24], 43 57 [22], and 55 45 [23], For a bilayer anode, the functional anode layer in contact with the electrolyte contains 45 to 50 vol% Ni, whereas the anode support layer has 35 to 40 vol% Ni [25, 26], A pore former is usually added to tailor the shrinkage (for the cofiring) and to achieve sufficient porosity (>30 vol%) in the anode or the anode support layer. 

SOFC electrodes are commonly produced in two layers an anode or cathode functional layer (AFL or CFL), and a current collector layer that can also serve as a mechanical or structural support layer or gas diffusion layer. The support layer is often an anode composite plate for planar SOFCs and a cathode composite tube for tubular SOFCs. Typically the functional layers are produced with a higher surface area and finer microstructure to maximize the electrochemical activity of the layer nearest the electrolyte where the reaction takes place. A coarser structure is generally used near the electrode surface in contact with the current collector or interconnect to allow more rapid diffusion of reactant gases to, and product gases from, the reaction sites. A typical microstructure of an SOFC cross-section showing both an anode support layer and an AFL is shown in Figure 6.4 [24],... 

In addition to bilayered electrodes with a functional layer and a support layer, electrodes have also been produced with multilayered or graded structures in which the composition, microstructure, or both are varied either continuously or in a series of steps across the electrode thickness to improve the cell performance compared to that of a single- or bilayered electrode. For example, triple-layer electrodes commonly utilize a functional layer with high surface area and small particle size, a second functional layer (e.g., reference [26]) or diffusion layer with high porosity and coarse structure, and a current collector layer with coarse porosity and only the electronically conductive phase (e.g., reference [27]) to improve the contact with the interconnect. 

Once the structural support layers have been fabricated by extrusion or EPD for tubular cells or by tape casting or powder pressing for planar cells, the subsequent cell layers must be deposited to complete the cell. A wide variety of fabrication methods have been utilized for this purpose, with the choice of method or methods depending on the cell geometry (tubular or planar, and overall size) materials to be deposited and support layer material, both in terms of compatibility of the process with the layer to be deposited and with the previously deposited layers, and desired microstructure of the layer being deposited. In general, the methods can be classified into two very broad categories wet-ceramic techniques and direct-deposition techniques. 

Finally, it has been shown by de Lange et al. [43,61] that ageing of the calcined membranes changes their microstructure. Long-term ageing under ambient conditions (i.e. 16 months at 20°C) or 80 days at 40°C and 60% RH results in a few percent loss of porosity of non-supported layers. Ageing at 350°C for 10 days in an air atmosphere containing 1.7% H2O vapour caused a porosity decrease from 27 to 25%. 

Summarizing progress in the field thus far, the book describes current materials, future advances in materials, and significant technical problems that remain unresolved. The first three chapters explore materials for the electrochemical cell electrolytes, anodes, and cathodes. The next two chapters discuss interconnects and sealants, which are two supporting components of the fuel cell stack. The final chapter addresses the various issues involved in materials processing for SOFC applications, such as the microstructure of the component layers and the processing methods used to fabricate the microstructure. 

The porous membranes consist of a porous metal or ceramic support with porous top layers which can have different morphologies and microstructures. Their essential structural features are presented in Figures 2.1 and 2.2 and are discussed later (Section 2.2). 

The fabrication of polyimide based microstructures with a single metallization layer is described. The silicon wafer is merely used as a supporting structure to perform the photolithographic steps and thin film procedures in the cleanroom. The wafer is removed at the end of the process. 

Parylene C plastic microstructures were formed by an additive process. A sacrificial layer of photoresist was used to define the channel regions. The structures were supported on a PC substrate [139,231] or Si substrate [231,691]. 

Microstructure of commercial alumina ceramic used as a support in this work is shown in Fig. 1. According to XRD, atomic emission spectroscopy and SEM data this membrane consists of a coarse a- A1203 - tube coated with finer layer of a- A1203 - tube (thickness of 40-60 pm). Traces of B, Ca and Si are detected by atomic emission spectroscopy. Pore sizes in the barrier layer of these tubes vary in the rather wide range from 0.05 to 0.5 micron, mean pore diameter being 0.2... 

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