Heat treatment temperatures, perovskites

To control the extent of intermixing and stoichiometry of the cation species, there have been a number of investigations to synthesize stoichiometric mixed-metal precursors with structures similar to the perovskite crystal structure. The motivation behind these efforts is that stoichiometric precursors with structures similar to the desired crystaUine phase should undergo crystallization at lower heat treatment temperatures. Most attempts in this area, however, have resulted in mixed-metal species with a cation stoichiometry different than perovskite." " The formation of such compounds indicates the importance of thermodynamic sinks in the synthesis of mixed-metal precmsors. ... 

Metal oxides are usually prepared by calcinations of suitable precursors such as hydroxides, nitrates, carbonates, carboxylates, etc. This process usually gives oxides with pseudomorphs of the starting materials. When large amounts of thermal energy are applied for the decomposition of the precursors, it facilities sintering of the product particles and therefore aggregated particles are obtained. When mixed oxides such as spinel, perovskite, and pyrochlore are the desired products, heat treatment at higher temperatures is required. 

Other manganites LnMn03, where Ln = Nd, Sm, Dy, Y, Er, were found to exhibit oxidative nonstoichiometry (90, 91). Wachowski et al. (92) prepared a perovskite of starting composition LaFe03.22. However, Tofield and Scott (79) did not find nonstoichiometry in LaFe03. The different behavior of these compounds should be related to the significant difference in the temperatures used for the final heat treatment in air (500°C for LaFe03.22 1100°C for the stoichiometric compound). This... 

Cd2Nb207) crystals transform themselves into the main crystal phase of the perovskite-type when the temperature of the heat treatment of the glass-ceramic increases. Therefore, the main characteristics of the glass-ceramics at 1050 C are similar to those of the Si02-Al203-Ti02-Pb0 system. 

Hirano, T., Tosho, T., Watanabe, T., and Akiyama, T. (2009) Self-propagating high-temperature synthesis with post-heat treatment of Lai Sr Fe03 (x = 0-l) perovskite as catalyst for soot combustion. /. Alloy. Compd., 470, 245-249. 

By XRD analysis, pure LSM is foimd for heat treatments greater than 770°C. The symmetry of the perovskite is the same for all three La-precursors, rhombohedral. The crystal structure depends strongly on the preparation procedure, especially the firing temperature and atmosphere. In contrast, the level of substitution fixes the concentration of Mn cations. While the bulk conductivity in this composition should be electronic, the small grain size and large grain boundary area contribute significantly to the conductivity of the LSM porous electrodes (Kuo, 1990). 

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