*3.1.6. Microwave × conventional sintering of SnO2-based ceramic*

One of the ceramic materials that have been very exploited for its great technological and industrial interest is the SnO2. Its applications are widely focused on sensors, solar cells, and catalysts, i.e., requiring high porosity, since its sintering process is limited to nondensifying mechanisms such as surface diffusion at low temperatures and evaporation–condensation at high temperatures [49–51]. Accordingly, what has been done to induce densifying sintering mechanisms is to cause solid substitution reactions that decrease the free energy by the formation of substitutional defects and vacancies that facilitate material transport during sintering [52].

It is possible to increase the densification of SnO2 by the addition of small amounts of lower valence densifying agents that generate substitutional defects and oxygen vacancies, such as ZnO, CoO, and MnO2, that promote the mass diffusion by solid solution, according to Eqs. 18, 19, and 20 [52,53]:

$$\stackrel{\text{SnO}\_2}{\rightarrow} \stackrel{\text{SnO}\_2}{\rightarrow} \stackrel{\text{O}\_2}{\rightarrow} + \stackrel{\text{O}\_2}{V\_O^{\bullet\bullet}} + \stackrel{\text{O}\_2}{\bullet} \tag{18}$$

$$\text{CoO} \xrightarrow{\text{SnO}\_2} \text{Co}^\*\_{\text{Su}} + \text{V}^{\bullet \bullet}\_{\text{O}} + \text{O}^{\times}\_{\text{O}} \tag{19}$$

New Approaches to Preparation of SnO2-Based Varistors — Chemical Synthesis, Dopants, and Microwave Sintering http://dx.doi.org/10.5772/61206 39

$$\text{MnO}\_2 \xrightarrow{\text{SnO}\_2} \text{Mn}^\*\_{\text{Su}} + \text{V}^{\bullet \bullet}\_{\text{O}} + \text{O}^X\_{\text{O}} \tag{20}$$

Also, there is the densification by CuO, Fe2O3, and MnO doping that promotes liquid solution formation [51]. Another way to improve the densification of SnO2-based varistors is to use the microwave as a source of power in the sintering process. According to Hao et al. [53], while conventional sintering occurs as a consequence of surface energy reduction, microwave sintering not only reduces the surface energy but also creates vacancies in the neck [53]. As a consequence of the increase in vacancies in grain necks, the mass flow also enhances in this region, promoting densification. In the case of dielectric materials, the oscillation of the electric field is the only external factor that will cause the internal heating of the material. Thus, the response of the oscillating electric field to the dielectric is determined by *ε* = *ε* ′ + *i ε* ″, where *ε* ′ is a dielectric constant that depends on the medium, and *ε*″ is the dielectric loss factor; when the material exhibits high dielectric loss, i.e., a high value *ε*″, the microwave energy is absorbed and converted into heat within the material [54]. When a material has high dielectric loss, the microwave can be directly applied to it; however, a susceptor material must be used. The susceptor absorbs microwave radiation and heats up the first piece so that it reaches its critical temperature, which consists of 40% to 50% of the melting temperature of the material above which has high dielectric losses.
