Advanced SnO2-Based Ceramics: Synthesis, Structure, Properties

Abstract

Tin dioxide presents specific optical and electrical properties and a good chemical stability which confers special characteristics to the SnO2 based materials. SnO2 belongs to the important class of transparent conductor oxide materials that combine low electrical resistance with high optical transparency in the visible range of the electromagnetic spectrum. These properties are requiered for optoelectronic applications i.g light emitting diodes, electrode materials in solar cells flat panel displays, transparent field effect transistors[Wagner, 2003; Presley et al., 2004]. Tin dioxide is aslo an oxidation catalyst and its activity and selectivity can be substantially improved by incorporation of various additives [Mihaiu et al., 2002]. Another field in which tin dioxide plays a dominant role is in solid state gas sensors. A wide variety of oxides exhibit sensitivity towards oxidizing and reducing gases by a variation of their electrical properties, but SnO2 was one of the first considered, and still is the most frequently used, material for these applications [Caldararu et al., 1999]. The sensor properties of SnO2 depend not only on such factors as the oxide’s surface stoichiometry, the methodology used to prepare the powder, temperature and atmosphere of calcination but also, and mainly, on the high specific area deriving from the low densification of this oxide. Nowadays, SnO2 is certainly one of the main polycrystalline ceramic candidates to compete with the traditional multicomponent ZnO-based varistors (voltage-dependent resistor-VDR’s), especially because of high electrical stability and its more simple microstructure [Bueno et al., 2007]. High density in polycrystalline ceramics is essential for high varistor properties, since the phenomena involved for good varistor properties occur in the region of the material’s grain boundaries. The main restriction in a wider use of this type of material is related to poor sintering ability of the SnO2-based compositions. This behaviour is related to the low diffusivity of the SnO2 structure and predominance of the nondensifying mechanisms (surface diffusion and evaporationcondensation). The latter result in grain and pore growth, thus limiting the final density [Varela et al., 1990]. The sintering problem is further complicated by the formation of deleterious intermediate phases above 1273K and high vapor pressurre of SnO [Dolet et al., 1992]. Dense SnO2-based materials have been obtained either by using sintering additives (e.g.,CuO) to promote densification by liguid-phase mechanism or by applying high pressures (e.g., in hot isostatic pressing technique).