Ceramic powder processing wetting

The powder-forming processes are similar in many ways to those nsed for powder metallurgy described in the previons section. For example, pressing is a common method for processing ceramics however, ceramic powders can be pressed in either dry or wet form. In wet form, they can also be extended, just like metals, and cast in a variety of process variations. The nominal forming pressures and shear rates associated with some of these processing methods are snmmarized in Table 7.3. Yon may want to refer back to this table when each of the varions processing techniques is described in more detail. 

In the wet process a slip carrying the ceramic powder is laid down, by screenprinting for example, onto a temporary carrier such as a glass tile. The process can be repeated to build up the required thickness of the dielectric onto which the electrodes are screen-printed. The next dielectric layer is then laid down and the process repeated. The mutlilayer structure is diced as described above, and the individual chips removed from the tile for the subsequent stages, as for the dry process. 

Chemical decomposition is usually observed in solid reactions, such as carbonate, hydroxides, nitrate, acetate, oxalates, alkoxides and so on, when they are heated at a certain temperature. The decomposition leads to the formation of a new solid product, together with one or more gaseous phases, which is usually used to produce powders of simple oxides in most cases and complex oxides sometimes. Although this method has not been widely reported for the synthesis of transparent ceramic powders, it could be a potential technique for such a purpose, due to its various advantages, such as simple processing, inexpensive raw materials, and capability of large scale production. In fact, the calcination step involved in most wet-chemical processing routes, especially chemical precipitation or co-precipita-tion, is chemical decomposition, either from carbonates or hydroxides, as discussed later. 

Nanosize particles with diameters in the range of 1 -- 100 nm are of interest for fabrication of advanced ceramics and semiconductors. Typical ways of synthesizing nanosized powders e wet chemical processes and aerosol processes. The later are advantageous for manufacture of particulates since they do not involve the tedious steps, high liquid volumes and surfactants of wet chemistry processes (i). As a result, they can produce materials of high purity at high yields and provide a molecular level mixing for composite powders. 

Ceramic powders of BaCeo.9Yo.1O2.95 (BCYIO) have been prepared by the sol-gel method [115]. Barium and yttriimi acetate and cerium nitrate were used as ceramic precursors in a water solution. The reaction process studied by DTA-TG and XRD showed that calcination of the precursor powder at r>1000°C produces a single perovskite phase. The densification behavior of green compacts studied by constant heating rate dilatometry revealed that the shrinkage rate was maximal at 1430 °C. Sintered densities higher than 95% of the theoretical one were thus obtained below 1500 °C. The bulk and additional blocking effects were characterized by impedance spectroscopy in an wet atmosphere between 150 and 600 °C. Proton conduction behavior was clearly identified. 

PLS allows near net shaping of ceramic components with complex geometries using standard powder-processing methods, which results in reduction of the process cost. It will be of special interest for the sintering of the UHTC complex shapes and/or porous architectures in combination with wet forming techniques that are the main object of this chapter. 

Lee, S.-H., Sakka, Y, Kagawa, Y. (2008). Corrosion of ZrB2 powder during wet processing-analysis and control. Journal of the American Ceramic Society, 97(5), 1715-1717. doi 10.1111/j.l551-2916.2008.02343.x. 

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. 

Both wet-ceramic techniques and direct-deposition techniques require preparation of the feedstock, which can consist of dry powders, suspensions of powders in liquid, or solution precursors for the desired phases, such as nitrates of the cations from which the oxides are formed. Section 6.1.3 presented some processing methods utilized to prepare the powder precursors for use in SOFC fabrication. The component fabrication methods are presented here. An overview of the major wet-ceramic and direct-deposition techniques utilized to deposit the thinner fuel cell components onto the thicker structural support layer are presented below. 

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