**3.2. SnO2 microwave sintering**

*3.1.5. Sintering model for thick films*

38 Advanced Ceramic Processing

substrates have been studied.

Eq. 17 [38,41]:

sintering [52].

18, 19, and 20 [52,53]:

mechanism, as with glass.

Most of the kinetic studies of SnO2-based ceramic are developed to oxide mixed synthesis compressed into pellets, where significant amounts of mass are used. However, the appearance of thick and thin films makes possible the integration of smaller electric devices, and thus new techniques for the synthesis and deposition of powders on conductive and insulating rigid

The sintering of films has been increasingly used for applications in sensors, fuel cells, or photo catalysis that requires porous films [45,46]. This application is based on the fact that sintering occurs on rigid substrates such as viscous flow, wherein the voltage-limiting densification of the material is the force of attraction between the substrate and the deposited material particles [47,48]. The model used for understanding the sintering of thin films is based on Scherer and Garino's studies where the rate of densification of the film is delayed by the substrate, as in

( )

The sintering mechanisms remain the same; however, the densification rate is retarded by tension caused by the substrate, like as the system would be sintered followed viscous sintering

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

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.

"

"

2

2

*SnO*

*SnO*

*p*

*v v*

*f*

& & (17)

*<sup>X</sup> ZnO Zn V O Sn O O* ® ++ ·· (18)

*<sup>X</sup> CoO Co V O Sn O O* ® ++ ·· (19)

e

1 3 3 1

*c p*

é ù æ ö <sup>+</sup> ê ú ç ÷ = - ê ú è ø - ë û

r

r

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

Sintering mechanisms at Coble initial stage were adjusted to SnO2-based ceramic inserts with 0.95 mol% of ZnO sintered in a microwave oven and compared with results obtained in a conventional oven. The results showed that samples were sintered in a microwave oven to reach 87% after 30 min of sintering at 1050°C and grain size, while in a conventional oven, the density is 67%. It can be seen in Figure 10 by which the sample (a) is in the initial stage of sintering grain size, while in (b) indicating the morphology of the grains is already in inter‐ mediate sintering mechanism.

**Figure 10.** SEM of sintered samples in (a) conventional oven and (b) microwave oven, at 1050°C/30 min (by authors).

The sample sintered in a conventional oven showed a linear shrinkage of 5% and had an activation energy of 325 kJ/mol with predominant mechanisms at this early stage: structural rearrangement of particles, diffusion via crystal lattice, and surface diffusion, while samples sintered in microwave oven showed an activation energy of 111 kJ/mol and mechanisms as broadcast via crystalline reticulum. Figure 11 shows that there was a change sintering mechanisms for conventional sintering since there is a rate change in linear shrinkage rate of the material, whereas for microwave sintering the heating rate was rapid and lower temper‐ ature which does not inhibit sintering mechanisms densifying.

**Figure 11.** Curves of Ln(Y) *versus* Ln(t) with temperature as a parameter for obtaining the coefficients of sintering at initial stage, for SnO2 samples (doped with 0.95 mol% of ZnO) sintered in (a) oven conventional and (b) microwave oven (by authors).

The direct relationship between the grain growth and the increasing density for the samples subjected to microwave and conventional heating are shown in Figure 12. With their respective error bars, it may be said that for about the same density of 88% of the samples, the mean grain size for the sintered sample in a microwave oven at 1050°C for 30 min is 1.2 μm, while that for the samples sintered in a conventional oven at 1300°C/30 min is 1.8 μm, and this difference increases even more because it enters the final sintering stage, which is when the grains grow more sharply, so the grain size is increased to about 3 μm. The reduced grain samples sintered in a microwave oven results in more grain boundaries to increase the mechanical strength and modifying the electrical properties of the material.

#### **3.3. Thick films varistor obtained by electrophoretic deposition**

Lustosa et al. [55] conducted a study on thick films of SnO2-based nanoparticles and their electrical properties. The ceramic powder with composition 98.95 mol% SnO2 + 1 mol% ZnO + 0.05 mol% Nb2O5 was synthesized by Pechini method, calcined in a muffle furnace, submitted to milling in the Attritor mill and to the separation of particles by gravimetry. After separation for use of the smaller particles, one ethylic aliquot containing SnO2 powder was taken to an electrophoretic deposition system (Figure 13) for obtain the films. In sequence, the films were submitted to sintering in a microwave oven at 1000°C/40 min. In order to improve the varistor New Approaches to Preparation of SnO2-Based Varistors — Chemical Synthesis, Dopants, and Microwave Sintering http://dx.doi.org/10.5772/61206 41

The sample sintered in a conventional oven showed a linear shrinkage of 5% and had an activation energy of 325 kJ/mol with predominant mechanisms at this early stage: structural rearrangement of particles, diffusion via crystal lattice, and surface diffusion, while samples sintered in microwave oven showed an activation energy of 111 kJ/mol and mechanisms as broadcast via crystalline reticulum. Figure 11 shows that there was a change sintering mechanisms for conventional sintering since there is a rate change in linear shrinkage rate of the material, whereas for microwave sintering the heating rate was rapid and lower temper‐

**Figure 11.** Curves of Ln(Y) *versus* Ln(t) with temperature as a parameter for obtaining the coefficients of sintering at initial stage, for SnO2 samples (doped with 0.95 mol% of ZnO) sintered in (a) oven conventional and (b) microwave

The direct relationship between the grain growth and the increasing density for the samples subjected to microwave and conventional heating are shown in Figure 12. With their respective error bars, it may be said that for about the same density of 88% of the samples, the mean grain size for the sintered sample in a microwave oven at 1050°C for 30 min is 1.2 μm, while that for the samples sintered in a conventional oven at 1300°C/30 min is 1.8 μm, and this difference increases even more because it enters the final sintering stage, which is when the grains grow more sharply, so the grain size is increased to about 3 μm. The reduced grain samples sintered in a microwave oven results in more grain boundaries to increase the mechanical strength and

Lustosa et al. [55] conducted a study on thick films of SnO2-based nanoparticles and their electrical properties. The ceramic powder with composition 98.95 mol% SnO2 + 1 mol% ZnO + 0.05 mol% Nb2O5 was synthesized by Pechini method, calcined in a muffle furnace, submitted to milling in the Attritor mill and to the separation of particles by gravimetry. After separation for use of the smaller particles, one ethylic aliquot containing SnO2 powder was taken to an electrophoretic deposition system (Figure 13) for obtain the films. In sequence, the films were submitted to sintering in a microwave oven at 1000°C/40 min. In order to improve the varistor

ature which does not inhibit sintering mechanisms densifying.

modifying the electrical properties of the material.

**3.3. Thick films varistor obtained by electrophoretic deposition**

oven (by authors).

40 Advanced Ceramic Processing

**Figure 12.** Evolution of grain size as a function of the calculated density of the samples sintered in a conventional oven and a microwave oven at a temperature of 800 °C to 1050 °C [by authors].

property, a Cr3+ ion deposition was carried out (also by electrophoresis) on films surface, and then the samples were submitted to different heat treatment for the diffusion of cations in grain boundary region. Figure 14 shows the sintered film, which had a low porosity, homogeneous thickness to the full extent of the film. The chromium addition is known to improve the properties of a varistor system by acting on defect formation at grain boundary region and increase the potential barrier parameter.

**Figure 13.** Electrophoretic system for deposition of SnO2-based particles (by authors).

After the heat treatment for Cr3+ diffusion, the films were taken to the electrical characteriza‐ tion. From the varistor responses, shown in Figure 15, it was observed that the heat treatment used after the chromium deposition influenced the improvement of the nonlinear coefficient of the samples. All films had lower rupture voltage less than 65 V and a low leakage current.

**Figure 14.** SEM of the film deposited by electrophoresis and sintered at 1000 °C/40 min: (a) top vision; (b) and (c) dif‐ ferent magnifications of cross-sectional vision [55].

**Figure 15.** Graphs of current density *versus* electric field: (a) for films without Cr3+ and films thermally treated at 900 °C and (b) films thermally treated at 1000 °C after the Cr3+ deposition [55].
