**3.2.1 One-step deposition (influence of solvent)**

Variations of the solvent used in the precursor solution cause changes in the solution characteristics. A solvent mixture that has, for example, a lower boiling point, leads to a faster evaporation of the solvent for the same temperature. This stage of the study aimed to observe the morphological changes associated to the use of different solvents for the same temperature.

Figure 6 shows the film obtained at 350 °C, using ethanol and propylene glycol as solvent in the precursor solution. The morphology obtained showed a large amount of cracked plates. Locally, the plates are fairly homogeneous, with little porosity, as evidenced in detail showed in Figure 6b. This morphology may be associated to the high viscosity of the solvent, which hinders the spread of the solution on the substrate surface, resulting in a thicker solution film rich in solvent. When the solvent evaporates, the cohesion and adhesion to the substrate throughout the film generate a lot of residual stresses, causing the oxide film to crack.

Fig. 6. (a) SEM image of the film obtained from ethanol and propylene glycol on the substrate at 350 °C, without heat treatment; (b) magnification of (a).

The films deposited from ethanol and propylene glycol solution presented clefts (Figure 6) and after heat treatment, the cracks increased, not only in number but also in intensity (Figure 7). The plates, which were well bonded to the surface, suffered a major influence of the contraction during the zirconia crystallization, contributing to the increase in cracking and detachment of the film as shown in Figure 7. This mechanism has been proposed by (Φstergård, 1995).

Depositions from the solution of ethanol and 2-methoxy, 1-propanol showed, at first, the formation of an apparently continuous film instead of plaque formation. This can be seen in Figure 8a. However, as this solution has lower boiling point and greater fluidity, there is a very high scattering on the surface. This phenomenon and the rapid evaporation of the solvent after the scattering, lead to a cracked film.

It is also possible to observe the deposition of some precipitates distributed over the layer of the film (Figure 8). These are related to the evaporation during the transport of the droplets toward the substrate, forming YSZ powder that clings to the surface of the film.

Variations of the solvent used in the precursor solution cause changes in the solution characteristics. A solvent mixture that has, for example, a lower boiling point, leads to a faster evaporation of the solvent for the same temperature. This stage of the study aimed to observe the morphological changes associated to the use of different solvents for the same

Figure 6 shows the film obtained at 350 °C, using ethanol and propylene glycol as solvent in the precursor solution. The morphology obtained showed a large amount of cracked plates. Locally, the plates are fairly homogeneous, with little porosity, as evidenced in detail showed in Figure 6b. This morphology may be associated to the high viscosity of the solvent, which hinders the spread of the solution on the substrate surface, resulting in a thicker solution film rich in solvent. When the solvent evaporates, the cohesion and adhesion to the substrate throughout the film generate a lot of residual stresses, causing the

Fig. 6. (a) SEM image of the film obtained from ethanol and propylene glycol on the

The films deposited from ethanol and propylene glycol solution presented clefts (Figure 6) and after heat treatment, the cracks increased, not only in number but also in intensity (Figure 7). The plates, which were well bonded to the surface, suffered a major influence of the contraction during the zirconia crystallization, contributing to the increase in cracking and detachment of the film as shown in Figure 7. This mechanism has been proposed by

**(a) (b)**

Cracked plates

Depositions from the solution of ethanol and 2-methoxy, 1-propanol showed, at first, the formation of an apparently continuous film instead of plaque formation. This can be seen in Figure 8a. However, as this solution has lower boiling point and greater fluidity, there is a very high scattering on the surface. This phenomenon and the rapid evaporation of the

It is also possible to observe the deposition of some precipitates distributed over the layer of the film (Figure 8). These are related to the evaporation during the transport of the droplets

toward the substrate, forming YSZ powder that clings to the surface of the film.

substrate at 350 °C, without heat treatment; (b) magnification of (a).

solvent after the scattering, lead to a cracked film.

**3.2 Morphology characterization** 

temperature.

oxide film to crack.

(Φstergård, 1995).

**3.2.1 One-step deposition (influence of solvent)** 

Fig. 7. (a) SEM image of the film obtained from ethanol and propylene glycol on the substrate at 350 °C, after heat treatment at 700 °C for 2 hours; (b) magnification of (a).

On the other hand, the film formed (Figure 8b), is not continuous and suggests the overlapping of non-homogeneous rough boards, which are formed during the rapid evaporation of the solvent. This type of morphology is undesirable to use in SOFC electrolyte, because it forms deep cracks and discontinuities.

Fig. 8. (a) SEM image of the film obtained from ethanol and 2-methoxy, 1-propanol on the substrate at 350 °C, without heat treatment; (b) magnification of (a).

After heat treatment (Figure 9), there seems to be a softening of the cracks seen after the deposition, possibly related to the contraction during the zirconia crystallization. Which, in this case, tends to reduce overlapping plates, softening the final morphological structure. However, this effect is not enough to homogenize the surface, and the cracks are not completely eliminated (Figure 9b).

The films obtained from ethanol and diethylene glycol monobutyl ether showed a considerable reduction in the amount of cracks and discontinuities (Figure 10a). After a soft scattering of the solution onto the substrate surface, the solvent evaporates properly, thus reducing internal stresses in the film and reducing the number of cracks, making it more homogeneous (Neagu et al., 2006).

However, the magnification of the image (Figure 10b) revealed the presence of small cracks distributed throughout the film. These cracked regions jeopardize the homogeneity of the

Fuel Cell: A Review and a New Approach

zirconia cubic phase.

for 2 hours.

700 °C for 2 hours.

**3.2.2 Multi-layer deposition** 

depositions with intermediate heat treatment.

discontinuities distributed throughout the film.

About YSZ Solid Oxide Electrolyte Deposition Direct on LSM Porous Substrate by Spray Pyrolysis 155

The films have undergone little morphological changes after heat treatment (Figure 11a). There was a slight increase of the cracks, which can be best shown in Figure 11b, that may be associated to the changes in the volume of the film during the crystallization of the

From the results obtained in depositions with different solvents, the solution (ethanol and diethylene glycol monobutyl ether) was chosen to use in the protocol of multi-layer

Figure 12 shows the film after the first layer deposition, where the surface is completely covered. After heat treatment (Figure 12b) there is a homogenization of the surface morphology, however, this homogenization is accompanied by the appearance of some

Fig. 12. SEM images of the film obtained from intermittent deposition at substrate

Fig. 13. SEM images of the film obtained from intermittent deposition at substrate

temperature of 350 °C: (a) after the second deposition; (b) after the second heat treatment at

temperature of 350 °C: (a) after the first deposition; (b) after the first heat treatment at 700 °C

**(a) (b)**

**(a) (b)**

Fig. 9. (a) SEM image of the film obtained from ethanol and 2-methoxy, 1-propanol on the substrate at 350 °C after heat treatment at 700 °C for 2 hours; (b) magnification of (a).

film and show that the use of this solvent alone is not enough for complete densification of the deposited layer.

Fig. 10. (a) SEM image of the film obtained from ethanol and diethylene glycol monobutyl ether on the substrate at 350 °C, without heat treatment; (b) magnification of (a).

Fig. 11. (a) SEM image of the film obtained from ethanol and diethylene glycol monobutyl ether on the substrate at 350 °C, after heat treatment at 700 °C for 2 hours; (b) magnification of (a).

The films have undergone little morphological changes after heat treatment (Figure 11a). There was a slight increase of the cracks, which can be best shown in Figure 11b, that may be associated to the changes in the volume of the film during the crystallization of the zirconia cubic phase.
