**2. Influence of synthesis methods on SnO2 electrical properties**

The processing by mixing oxides is widely used at the industrial scale for the production of varistor ceramics mainly due to its low cost, consisting basically of an initial powder mix and wet milling followed by drying, deagglomeration of powder, forming pellets/bulks, and sintering. The varistor synthesis with large amounts of chemical additives and/or impurities resulting from the process can lead to non-densifying sintering mechanisms. This mean that impurities may accumulate on the material surface and increase the mass flow on the surface or forming more unstable compounds that can evaporate and condense on the surface, favoring grain growth without decreasing pore size. The advancement in ceramic materials process technology aims to find low-cost methods and the viability of the process on an industrial scale. Among the processes available in the literature for the production of ceramics, techniques can be cited as coprecipitation, sol-gel, dehydration by rapid cooling (freeze drying), combustion method, and polymeric precursor method known as the Pechini method [28–31].

*1.3.1. Schottky type*

30 Advanced Ceramic Processing

describes this behavior is [24,25]

*1.3.2. Poole–Frenkel type*

of Schottky type [26]:

be considered [27].

In this model, the electrons are emitted and pass through the potential barriers particularly due to the action of temperature distorting the energy band diagram, near the interface. This distortion modifies the potential barrier favoring the thermal emission. The equation that

\* ½

where *A*\* is the Richardson constant, *φ*b is the potential barrier height, *E* represents the electric field, *T* is the ambient temperature in Kelvin, and *β* is a constant related to the width of the

1/2

The emission of the Poole–Frenkel type assumes the formation of coulombian centers in the grain–intergranular layer interface region. The relationship that describes this type of emission is on Equation 9, where the external electric field variations are more relevant than for issue

> é ù f

 b

<sup>½</sup> 2 .. *<sup>b</sup>*


where *c* is a constant of the material, *T* is the room temperature, *E* is the electric field, *k* is the Boltzmann constant, and *φ*b is the height of the potential barrier. The thermionic emission cannot explain the high nonlinear coefficients observed in varistors. In the post rupture zone with the presence of high electric fields, the possibility that distortion of the energy levels and, therefore, the possibility that electrons pass through the potential barrier by tunneling must

The processing by mixing oxides is widely used at the industrial scale for the production of varistor ceramics mainly due to its low cost, consisting basically of an initial powder mix and wet milling followed by drying, deagglomeration of powder, forming pellets/bulks, and

*<sup>E</sup> J c E exp kT*

**2. Influence of synthesis methods on SnO2 electrical properties**

 w


é ù f b

ë û (7)

() *n* - = (8)

ë û (9)

2 .. *<sup>b</sup>*

b

where *n* is the grain number per unit length and *ω* is the width of the barrier.

*<sup>E</sup> J A T exp kT*

*S*

potential barrier in accordance with the following equation [25]:

*P*

**Figure 3.** Schematic representation of reactions developed in the polymeric precursor method (Pechini method) [31].

The polymeric precursor method involves a complexation reaction of metal ions by an organic complexing agent as carboxylic acid. The metal ions are complexed into carboxylic sites forming a metal carboxylate, which is sequentially polymerized with ethylene glycol, as shown in Figure 3, citric acid is often used as the complexing agent. This process shows advantages such as low temperature of synthesis and high control of stoichiometry, and allows the obtention of powder with nanometric particles. The immobilization of metal ion in organic matrix reduces the segregation of the metal during the decomposition of the polymer at high temperatures, thus ensuring a homogeneous composition [31]. The ceramic powders are obtained by controlled calcination of the resin until total oxide formation.

Another method widely used for controlled synthesis of multifunctional ceramics is the solgel, that is used for the synthesis of a colloidal suspension where the dispersed phase is a solid and the dispersion medium is liquid, and is called sol. Therefore, there is the formation of a dual phase material: a solid body that is occupied by a solvent, i.e., moist gel. The initiator compounds, commonly called precursors, consist of a metal surrounded by many connections and typically are inorganic salts or organic compounds. The two precursors undergo two chemical reactions at sol preparation: hydrolysis and condensation, which resulted from the addition of an acid or base catalyst to form small solid particles or clusters in a liquid (aqueous solvent) [32,33]. The sol-gel method provides homogenous mixtures of cations on an atomic scale and also allows the preparation of ceramic powders with high surface area and films or gels fibers, which have high technological importance. The method has advantages over other conventional methods such as high purity, resin calcination at low temperatures, and synthesis of oxides with defined and controlled properties [32–34].

Also, the controlled precipitation method (CPM) can be used to prepare precursor powders. In this case, the solution containing the cation of interest is added to another solution contain‐ ing a precipitating agent that can be a base or anion (ammonia, urea, and oxalic acid). In this way, the final product precipitate is separated by filtration, washed, dried, and calcined to obtain the oxide. The precipitation process has a complex mechanism, which is dependent on the degree of saturation of the ion to be used. The process starts by formation of cluster from chemical species in the solution, known as nucleation process. Reaching the ion solubility limits the growth stage of formed centers and finally the formation of precipitates [35].

To check the influence of the chemical synthesis route the electrical properties of the SnO2 based varistors, Mosquera et al. [36] carried out the synthesis of tin oxide by controlled precipitation and polymeric precursor (Pechini) methods that's offering the strict control of the chemical purity and the particle size of the raw material. The system SnO2.Co3O4.Nb2O5.TiO.Al2O3, with 1 mol% Co3O4, 0.05 mol% Nb2O5, and 1 mol% TiO2 and variations of 0.05 (named SCNT05A), 0.1 (named SCNT1A), and 0.2 mol% (named SCNT2A) of Al2O3 were prepared. Following synthesis, the materials were submitted to heat treatment at 600°C/1 h (controlled precipitation method, CPM) and 600°C/2 h (Pechini method, PCH) to eliminate organic matter and obtain the full formation of the oxide. The use of dopants in both methods resulted in no change in the SnO2-crystal structure or formation of secondary phases due to have been added small amounts of dopants (Figure 4). The SEM micrographs indicated the influence of the addition of the aluminum grain growth control. The Pechini method showed smaller grains and more porous samples.

**Figure 4.** SEM for sintered samples at 1350°C, obtained by CPM and PCH (a) 0.05% Al2O3 and (b) 0.1% Al2O3. XRD for varistor system whit 0.2% Al2O3 synthesized by CPM and PCH [36].

The aluminum concentration also influenced on the electrical properties, as shown in Figure 5, mainly in the breakdown electric field variation that had been related to decreasing of grain size. The samples showed nonlinear coefficient (*α*) of similar values, but the sample prepared by Pechini method and with 0.2% Al2O3 had the highest value for *α* (21.7) and the breakdown electric field (due to the smaller grain size).

**Figure 5.** log J *versus* log E curves of samples synthesized by (a) CPM and (b) PCH, sintered at 1350°C [36].
