Microstructure sintered

Figure 8. Polarization curves (IR-free. measured in humidified Hj) at various SDC anodes at Tca = 800 C.O original one (sintered at 1050 °C, without polymer) O the optimized microstructure (sintered at 1150°C with 0.5 wt% polymer) Ru-catalyzed (0.1 mg-Ru ern loaded on the optimized one) Ni-catalyzed [0.75 mg-Nicnf (8 vol%) loaded on the optimized one, see Section 11.3).

Characteristics of microstructure sintered by spark plasma sintering... 

Fig. 5.62 Grain size and technology effects in the strength and toughness of pressureless sintered alumina. Each data point represents the averages of grain size and strength measured for a group of specimens sintered at the same temperature. The numbers that distinguish different gel casting routes refer to Table 5.2. Different data points for one casting route refer to microstructures sintered at different temperatures [20]. With kind permission of Elsevier...

Figure 9.11. Microstructures of porous sintered alumina prepared undoped (right) and when doped with magnesia (left). Optical micrographs, originally 250x (after Burke 1996).

The difficulty of obtaining pure / "-material for the electrolyte has been tackled in many production processes worked out in the past. Unless precautions are taken, sintering of a -alumina-derived / "-alumina compositions invariably results in the duplex microstructure and a low-strength ceramic. Therefore a balance has to be struck between conductivity and strength. The problem arises because the conversion from —alumina to / " -alumina is slow... 

As can be seen from this figure, the heat-resistance was remarkably improved by the drastic changes in the microstructure from amorphous to polycrystalline structure. Another type of SiC-based fiber, SA fiber (2), has a sintered SiC polycrystalline structure and includes very small amounts of aluminum. This fiber exhibits outstanding high temperature strength, coupled with much improved thermal conductivity and thermal stability compared with the Nicalon and Hi-Nicalon fibers. The fabrication cost of the SA fiber is also reduced to near half of that of the Hi-Nicalon Type S [ 17]. The SA fiber makes SiC/SiC composites even more attractive to the many applications [18]. In the next section, the production process, microstructure and physical properties of the SA fiber are explained in detail. 

Milnes and Mostaghaci [5.5] compared the consequences of different drying methods on the density, the sinter rate and micro structures of sublimated TiO-, suspensions. Evaporation of water in a micro-oven and by radiation heating leds to strongly bound agglomerates, while freeze drying resulted in softly bound secondary clusters. The freeze dried powder reached in 2 h of sintering 98 % of the theoretical density, while differently dried powders needed twice as much time and had a less fine microstructure. 

Most of the literature focuses on the aspects of sinterability and microstructure, but limited data on the electrical properties is available. Tok [152] reported a conductivity of 18.3 x 10-3 Scm-1 at 600°C for Gd0 jCeo.gOj 95, and we measured a high conductivity of 22 x 10-3 scm-1 for Sm0 2Cc08O 9 at the same temperature. Their activation energies are relatively low—less than 0.7 eV. Although conductivity data reported for doped ceria prepared with carbonate precipitation is varied from different authors [153-155], the conductivity is generally high and the activation energy is usually low for ceria electrolytes fabricated with this method. 

In addition to the chemical interactions described above, microstructural changes in the electrode can lead to performance degradation after long operation time. For example sintering of the porous structure can degrade electrode performance. In the case of LSM cathodes, the sintering ability is found to be related to the strontium dopant level and stoichiometric composition of (La, Sr)xMn03. In addition, LSM with A-site deficient compositions (x < 1) sinters more readily than their B-site deficient counterparts (x > 1) [198, 199],... 

In addition to the use of composite anodes and cathodes, another commonly used approach to increase the total reaction surface area in SOFC electrodes is to manipulate the particle size distribution of the feedstock materials used to produce the electrodes to create a finer structure in the resulting electrode after consolidation. Various powder production and processing methods have been examined to manipulate the feedstock particle size distribution for the fabrication of SOFCs and their effects on fuel cell performance have also been studied. The effects of other process parameters, such as sintering temperature, on the final microstructural size features in the electrodes have also been examined extensively. 

FIGURE 6.10 Microstructures of Sm2 ISr[Ni04 cathodes sintered at three firing temperatures (a) 900°C, (b) 1000°C, and (c) 1100°C, and (d) of a test cell [67]. Reprinted from [67] with permission from Elsevier. 

Corbin SF, Lee J, and Qiao X. Influence of green formulation and pyrolyzable particulates on the porous microstructure and sintering characteristics of tape cast ceramics. J. Am. Cer. Soc. 2001 84 41 17. 

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