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

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 the grain boundaries [1,56,57].

The addition of bivalent metals such as CoO [58], ZnO [59], and CuO [60,61] is made to enhance the densification because these cations act as acceptors of electrons and replace the tin ions in crystal lattice, creating oxygen vacancy defects that promote mass diffusion in the network and promoting densification, according to Eq. 21 [58]:

$$\text{MO} \xrightarrow{\text{SnO}\_2} \text{M}^\*\_{\text{Sn}} + \text{V}^{\bullet \bullet}\_{\text{O}} + \text{O}^{\times}\_{\text{O}} \tag{21}$$

The *MSn* '' defect types present in the grain boundary region trap the electrons released by other types of modifiers and create a potential barrier in the grain boundary region.

The electrical conductivity of the varistor system can be improved with the addition of pentavalent ions as Sb2O5 [62], Nb2O5 [1], and V2O5 [63], which act as electron donors to the crystal lattice, resulting in electron concentration and tin vacancies, as demonstrated in Eq. 22 [1,62]:

$$2D\_2O\_g \xrightarrow{SnO\_2} 2D\_{su}^\bullet + 2e^\cdot + 4O\_O^X + \ \forall \mathcal{O}\_{O(g)}^X \tag{22}$$

Trivalent cations that act as acceptors of electrons are added to SnO2 crystal lattice, such as chromium [63–65], ytterbium [67], and scandium [68], which were used to improve the varistor properties of the system. The segregation of these ions in the grain boundary potential barrier increases the resistivity values and causes the improvement of nonlinear coefficient due to the higher adsorption of electron acceptor species on the grain boundary surface, increasing the barrier height potential and decreasing the conductivity, as demonstrated in Eq. 23 [57,58,67]:

$$\rm M\_2O\_3 \xrightarrow{SnO\_2} \rm 2M\_{Su}^\cdot + V\_O^{\bullet \bullet} + \rm 3O\_O^X \tag{23}$$

There are many papers available in the literature [56–58,61–63,66–70], which studied the influence of doping agent into the tin oxide matrix ceramic. The possible microstructural, morphological, and varistor property changes that may occur with the addition of certain elements are searched.
