Mixed Metal Oxide Nanoparticles
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Mixed metal oxide (MMO) nanoparticles (also called heterometal oxide nanoparticles) can play an appreciable role in many areas of chemistry and physics. The unique electronic and magnetic properties obtained when combining two metals in an oxide matrix have been well studied. However, the most common use for MMOs has been in the area of catalysis, and here they have found use both as the catalyst and as catalyst supports. Specifically, MMO containing aluminum has found many opportunities in catalysis.
Mixed metal oxides are also used in many applications in the electronic industry as passive or active components in devices. These exhibit high dielectric, and ferro- or pyroelectric properties, e.g., BaTiO3, LiNbO3, KTaO3, Pb1 −xLaxTiyZr2O3, etc. The most remarkable MMO materials are surely the thermal-conducting superconductors based on YBa2Cu3O7 −x. The technological interest in MMOs such as silica–titania and silica–zirconia arises from their chemical resistance and their thermomechanical or optical properties. SiO2–TiO2 glasses and zircon, SrZrO4, are characterized by very low thermal expansion, which confer them a high thermal-shock resistance. SiO2–TiO2 and SiO2–ZrO2 glasses have high refractive indices and are wonderful catalysts or catalyst supports. Owing to their refractoriness, these MMOs are difficult to produce by conventional melting techniques; thus, sol–gel technique is used for their preparation. The homogeneity of binary oxide has great influence on structural evolution of the gels during the heat treatment. Magnesium aluminate spinel (MgAl2O4) is expected to have applications as a structural material at high temperatures because it has a high melting point and superior thermal and chemical properties. It is also of technological interest as a refractory materials at elevated temperatures. It is important to realize that improved properties are recorded for ceramic materials at nanoscale and these materials are expected to find the most important place in technology in the coming years.
In this article, we have covered the most important and recent methods of preparation of nanocrystalline MMOs. Special emphasis has been given on sol–gel technique because of its popularity among researchers, resulting in homogeneous and pure MMO nanoparticles. Among physical methods, perhaps flame spray pyrolysis is gaining more importance in recent time. Structure, properties, and applications of MMO nanoparticles have also been discussed. This area of research is wide open and offers scope for development in synthetic and application fields.