**4.1. Effect of Ca, Ba, Sr addition on Co, Sb-doped SnO2 varistors**

**Figure 15.** Graphs of current density *versus* electric field: (a) for films without Cr3+ and films thermally treated at 900 °C

**Figure 14.** SEM of the film deposited by electrophoresis and sintered at 1000 °C/40 min: (a) top vision; (b) and (c) dif‐

The addition of crystal lattice modifiers to SnO2 matrix is required because in the SnO2 sintering process, there is a predominance of mass transport mechanisms (evaporation and condensa‐ tion), which leads to coalescence and grain growth, which hinder densification. Densification is a precondition to obtain the varistor properties since the phenomena involved in the formulation of non-ohmic properties occur in the grain boundary region. Thus, the studies are carried out to understand the doping effect on the sintering and densification, electrical conductivity, and non-ohmic properties of SnO2-varistor. The defects generated by modifying agents are of Frenkel type (generators of interstitial ion) and Schottky type (generators of vacancies) and are responsible for the formation and modification of the potential barrier in

**4. Network modifiers that promote properties of SnO2-based varistor**

and (b) films thermally treated at 1000 °C after the Cr3+ deposition [55].

ferent magnifications of cross-sectional vision [55].

42 Advanced Ceramic Processing

the grain boundaries [1,56,57].

Aguilar-Martínez et al. [69] investigated the effect of calcium (sample named SCa), barium (sample named SBa), and strontium (sample named Sr) additions on the microstructure and electrical properties of SnO2-Co3O4-Sb2O5 ceramic varistors.

By XRD analysis, it should be noted that the concentrations of dopants added (SbO, CaO, Ba, and SrO) were too small to be detected by the X-ray equipment. The microstructure of the samples was characterized by scanning electron microscopy. As shown Figure 16, it was found that the addition of strontium and calcium promotes densification and grain growth. The addition of BaO leads to a significant alteration of microstructure, changing the grain size and the morphology of grains from a nearly round shape to smaller and elongated grains. Barium addition causes increase of porosity, reduction of grain size, and changes in the grain mor‐ phology (from approximately equiaxed to elongated grains) [69].

**Figure 16.** SEM images of the as-sintered surfaces of SnO2-based varistors: (a) S, (b) SCa, (c) SSr, and (d) SBa [69].

Since electrical conduction in SnO2-based varistor ceramics is controlled by the grain-boundary barriers, the observed fact (the significant grain growth in a SnO2-system with SrO and CaO added) suggests that Sr and Ca materials are more suitable for low-voltage varistor prepara‐ tion. The current–voltage curves of all prepared ceramic samples are nonlinear behavior. Figure 17 shows graphs of current density versus electric field for ceramics with and without additions sintered at 1350 °C [69].

Ceramics with calcium addition exhibit the lowest electric field at a fixed current density (10– 3 A cm–2). The addition of strontium shows a similar effect on microstructure and current– voltage characteristics. However, the BaO addition showed that low-field conductivity is slightly lower with respect to the reference material, but the high-field part remains un‐ changed. This behavior may be attributed to the resulting microstructure. Despite the grain morphology and porosity, the samples S (only Co an Sb as dopants), SCa, and SBa showed New Approaches to Preparation of SnO2-Based Varistors — Chemical Synthesis, Dopants, and Microwave Sintering http://dx.doi.org/10.5772/61206 45

**Figure 17.** J *versus* E characteristic plots for all samples [69].

that the addition of strontium and calcium promotes densification and grain growth. The addition of BaO leads to a significant alteration of microstructure, changing the grain size and the morphology of grains from a nearly round shape to smaller and elongated grains. Barium addition causes increase of porosity, reduction of grain size, and changes in the grain mor‐

**Figure 16.** SEM images of the as-sintered surfaces of SnO2-based varistors: (a) S, (b) SCa, (c) SSr, and (d) SBa [69].

additions sintered at 1350 °C [69].

3

Since electrical conduction in SnO2-based varistor ceramics is controlled by the grain-boundary barriers, the observed fact (the significant grain growth in a SnO2-system with SrO and CaO added) suggests that Sr and Ca materials are more suitable for low-voltage varistor prepara‐ tion. The current–voltage curves of all prepared ceramic samples are nonlinear behavior. Figure 17 shows graphs of current density versus electric field for ceramics with and without

Ceramics with calcium addition exhibit the lowest electric field at a fixed current density (10–

 A cm–2). The addition of strontium shows a similar effect on microstructure and current– voltage characteristics. However, the BaO addition showed that low-field conductivity is slightly lower with respect to the reference material, but the high-field part remains un‐ changed. This behavior may be attributed to the resulting microstructure. Despite the grain morphology and porosity, the samples S (only Co an Sb as dopants), SCa, and SBa showed

phology (from approximately equiaxed to elongated grains) [69].

44 Advanced Ceramic Processing

nonlinear coefficients of 5.7, 5.0, and 4.9, respectively, higher than the value for sample SSr (nonlinear coefficient of 2.7) [69].
