**1.5.1 Spray pyrolysis**

A typical setup for spray pyrolysis consists of an atomizer, the precursor solution, the heated substrate and a temperature controller. Three types of atomizers are commonly used in spray pyrolysis: compressed air (the solution is exposed to a beam of air) (Balkenende et al., 1996); ultrasound (short wavelengths are produced by ultrasonic frequencies, generating a very fine spray) (Arya & Hintermann, 1990); electrostatic (the solution is exposed to a high electric field) (Chen et al., 1996).

Films prepared by spray pyrolysis have been used in several devices, such as solar cells, sensors, anti-reflective coatings, thermal barriers, solid oxide fuel cells, among others.

The deposition of thin films via spray pyrolysis involves spraying a metallic salt solution on a heated substrate. The solution droplets reach the substrate surface, where solvent evaporation and the decomposition of the metal salt occurs, forming a film. The film morphology and thickness depend on the volume of solution sprayed and the substrate temperature. The film formed is usually a metal salt, which is converted to oxide in the heated substrate.

Many processes occur sequentially or simultaneously during the formation of a thin film by spray pyrolysis: atomization of the solution, transport and evaporation of solution drops, solution spread on the substrate, evaporation of the solvent and, finally, drying and decomposition of the precursor salt. Understanding these processes helps to improve the quality of the films obtained, facilitating the production scale and reproducibility of the process.

The initial parameter in the preparation of a film with certain desired characteristics, using the technique of spray pyrolysis, is the definition of the precursor solution to be used. This solution should provide the ions, needed to form the desired film, and should contain a solvent, with adequate evaporation rate and stability in the process conditions.

In the aerosol, the drops of solution are transported and eventually evaporate. The gravitational, electrical or thermophoretic forces, have influence on the trajectory of droplets and their evaporation, so the modeling of film growth should be taken into account.

When the formation of dense films is wanted, it is important that the maximum amount of drops reaches the substrate without the formation of particles along the path. Sears et al. investigated the mechanism of SnO2 film growth (Sears & Gee, 1988). In case of excessively rapid evaporation of the solvent along the way, the droplet size decreases and precipitates precursor salt on the edges of the drop, causing the deposition of precipitates on the substrate surface. This phenomenon is extremely deleterious to obtain dense and homogeneous films, since the particles formed in the atomizer-substrate path add to the substrate surface, forming a porous crust (Perednis, 2003).

On the other hand, if the drops are sprayed against the substrate with sufficiently high strength, spread lightly, maintain an evaporation rate equivalent to the solute precipitation rate, the solute nucleates and precipitates homogeneously, creating a dense and continuous film (Yu & Liao, 1998).

A model of the possible transport situations of aerosol from the atomizer, toward the heated substrate can be seen in Figure 2. In zone I, the droplet is too large, has a very slow solvent evaporation rate and results in the formation of a brittle precipitate. In the second case, the drops have size and strength suitable for spraying, forming homogeneously aggregates of precipitates (zone II). And finally, in zone III, the drops are too small and not strong enough to reach the substrate, causing particles to appear before reaching the substrate.

Fig. 2. Aerosol transport model.

146 Electrochemical Cells – New Advances in Fundamental Researches and Applications

CVD Applicable to ceramic coatings Thin, non-uniform coatings,

Screen printing Simple Non-uniform, porous coating

Table 1. Advantages and disadvantages of some techniques used to obtain SOFC materials.

A typical setup for spray pyrolysis consists of an atomizer, the precursor solution, the heated substrate and a temperature controller. Three types of atomizers are commonly used in spray pyrolysis: compressed air (the solution is exposed to a beam of air) (Balkenende et al., 1996); ultrasound (short wavelengths are produced by ultrasonic frequencies, generating a very fine spray) (Arya & Hintermann, 1990); electrostatic (the solution is exposed to a high

Films prepared by spray pyrolysis have been used in several devices, such as solar cells, sensors, anti-reflective coatings, thermal barriers, solid oxide fuel cells, among others.

The deposition of thin films via spray pyrolysis involves spraying a metallic salt solution on a heated substrate. The solution droplets reach the substrate surface, where solvent evaporation and the decomposition of the metal salt occurs, forming a film. The film morphology and thickness depend on the volume of solution sprayed and the substrate temperature. The film formed is usually a metal salt, which is converted to oxide in the

Many processes occur sequentially or simultaneously during the formation of a thin film by spray pyrolysis: atomization of the solution, transport and evaporation of solution drops, solution spread on the substrate, evaporation of the solvent and, finally, drying and decomposition of the precursor salt. Understanding these processes helps to improve the quality of the films obtained, facilitating the production scale and reproducibility of the

The initial parameter in the preparation of a film with certain desired characteristics, using the technique of spray pyrolysis, is the definition of the precursor solution to be used. This solution should provide the ions, needed to form the desired film, and should contain a

In the aerosol, the drops of solution are transported and eventually evaporate. The gravitational, electrical or thermophoretic forces, have influence on the trajectory of droplets

When the formation of dense films is wanted, it is important that the maximum amount of drops reaches the substrate without the formation of particles along the path. Sears et al.

solvent, with adequate evaporation rate and stability in the process conditions.

and their evaporation, so the modeling of film growth should be taken into account.

high cost

materials

Many experimental variables

Thin, non-uniform coatings

Complex for ceramic

**Technique Advantage Disadvantage** 

Spray pyrolysis Simple, applicable to ceramic

Sol-gel Simple, applicable to ceramic coatings

Electrodeposition Simple, effective for complex shapes

**1.5.1 Spray pyrolysis** 

heated substrate.

process.

electric field) (Chen et al., 1996).

coatings, low cost

The substrate temperature can be considered the most important factor to determine the solvent evaporation rate. It is directly linked to the time the solvent takes to spread over the surface of the substrate and the speed at which it evaporates after being spread. In addition, the substrate temperature should be high enough so that the salt decomposition reactions occur, in order to form the desired final material.

It is known that low substrate temperatures provide an excellent scattering of the solution, however, the film layer that is formed is very rich in solvent, taking too much time to evaporate. After stopping the solution spraying, the film is still wet and the local rise in temperature causes the solvent to evaporate, diminishing the volume and contributing to the formation of tension in the film. The strong adhesion between film and substrate prevents free contraction of the film, promoting the formation of cracks (Neagu et al., 1981).

Fuel Cell: A Review and a New Approach

**2. Materials and methods** 

solvents used in this work.

Fig. 3. Spray pyrolysis experimental apparatus.

Table 2. Boiling temperatures of the solvents used.

reason, in this chapter, a single temperature was studied, 350 °C.

About YSZ Solid Oxide Electrolyte Deposition Direct on LSM Porous Substrate by Spray Pyrolysis 149

The spray pyrolysis setup consisted mainly of the following parts: a spraying unit, a liquid feeding unit, and a temperature control unit (Figure 3). The spray unit consisted of an airbrush (Campbell Hausfeld) using an air blast atomizer. The liquid feeding unit is the precursor solution, constituted by yttrium chloride (YCl3.6H2O) (Aldrich Chemicals) and zirconium acetylacetonate (Zr(C6H7O2)4) (Aldrich Chemicals) dissolved in three different solvents: (1) mixture of ethanol (C2H5OH) (FMaia) and propylene glycol (C3H8O2) (Proton) (1:1 vol.%); (2) mixture of ethanol and 2-methoxy, 1-propanol (C4H10O2) (Aldrich Chemicals) (1:1 vol.%); (3) mixture of ethanol and diethylene glycol monobutyl ether (C8H18O3) (Aldrich Chemicals) (1:1 vol.%). The Table 2 shows the boiling temperatures of any individual

> Solvent Boiling Point [°C] ethanol 78.4 propylene glycol 188.2 2-methoxy, 1-propanol 120.0 diethylene glycol monobutyl ether 230.4

The solutions were prepared according to the stoichiometry required to the films (ZrO2)0.92(Y2O3)0.08 (Perednis & Gauckler, 2004) and adopting a final concentration of salts in solution of 0.1 mol.L-1. The precursor solution was maintained under stirring and heating at 50 ºC in a hotplate stirrer (Fisaton), in order to obtain the complete dissolution of the salts and decrease the heat loss of the substrate. Finally, the temperature control unit consisted in a hotplate, used for heating the substrate. A thermostat controlled the hotplate temperature and the substrate temperature was monitored by an infrared pyrometer. The precursor solution was sprayed on the heated LSM porous substrate, in order to obtain the YSZ films. The substrate temperature is determining in the morphology of films obtained, since it is directly related to the solvent evaporation rate, and a quick evaporation of the solvent promotes the formation of particles instead of forming a continuous film. On the other hand, the slow evaporation of the solvent, promotes crack formation in the film. Previous studies were made to determine the optimum temperature to obtain continuous films. For this

However, if the substrate temperature is high, it will reduce the time of solvent evaporation to a point when the solvent partially evaporates before touching the substrate, inhibiting the spread. In cases where the temperature is too high, the droplets of solution not even touch the substrate, all the solvent is evaporated on the way between the atomizer and the surface and only particles are deposited on the substrate (Perednis & Gauckler, 2004).

The choice of an intermediate temperature is necessary to obtain a film with the desired characteristics. This should consider the solvent used, so it evaporates after a light scattering of the solution on the substrate.
