**3.1 Structural and microstructural characterization of the infiltrate PZT thick films**

There are practically no differences between the structural and microstructural characterizations of thin and thick PZT films. However, it should be taken into account that PZT thick film phase formation, grain growth, crack formation, etc., are highly conditioned by the precursor powder and the powder agglomerate formation, while in PZT thin films they are mainly conditioned by the substrate structure, the characteristic of the PZT precursor solution and the preparation conditions.

#### **3.1.1 Structural characterization of the precursor powder and the infiltrated PZT thick films (X-ray analysis)**

Prior to the structural characterization of the PZT thick films it is important to carry out the precursor powder characterization. Figure 3 a) shows the X-ray diffraction pattern of the PZT TRS600 precursor powder used in the preparation of infiltrated PZT thick films (Pérez et al., 2007, Pérez, 2004). The diffraction pattern of this PZT powder shows the presence of a pure perovskite PZT phase and two marginal extra diffraction peaks (26.8o and 33.12o) that are associated with the lead excess and other additives (such as Nb2O5) used in the preparation of the commercial PZT powders (TRS, 1998, Kholkin et al., 2000). Normally, the additive compounds used in the preparation of commercial PZT powder have the objective of improving its dielectric and piezoelectric properties. However, they also show extra attributes when the PZT powders are used in the preparation of PZT thick films. For instances, the lead oxide excess present in the PZT precursor powder guarantee that the PZT thick films remain stoichiometric after the annealing process.

Figure 3b) shows the X-ray diffraction patterns of PZT thick films prepared with different number of top infiltrations. The analysis of the (110) PZT diffraction peak shows a decrease of the width and a small shift of the peak position, relatively to the TRS600 PZT commercial powder, as the number of infiltration steps increases. It is evident that the infiltration process reduces the PZT thick film surface roughness, which results in a better relation between incident and diffraction angle of the films. For that reason, the decrease in the width of the (110) PZT diffraction peak is mainly attributed to the decrease of the surface roughness.

In contrast, the shift in the peak position of the maximum as the number of top infiltration layers increase, has been associated with two factors: i) a small difference between PZT powder and PZT solution compositions and ii) a change in the stress of the films provoked by the infiltration cycle, which somehow compacts the film structure.

In the next section, one analyzes the effect of the number of top infiltrations in structural, dielectric and piezoelectric properties of a intermediate infiltrated PZT thick film. Moreover, the dielectric properties of the infiltrated PZT thick films will be simulated based on 0-3 and cube ceramic/ceramic composite models, where the numbers 0 and 3 describe the connectivity of the two phases of the material (i.e., the sol gel matrix interconnected in the three directions (3) whereas PZT powder particles are not connected in any direction (0)) (Newnham et al., 1978). Finally, the structural and electrical results will be compared with the ones reported by Dorey *et. al.,* and Ohno *et al.,* in PZT thick film infiltrated with a high

**3. Structural, electric and piezoelectric characterization of the PZT thick films** 

**3.1.1 Structural characterization of the precursor powder and the infiltrated PZT thick** 

Prior to the structural characterization of the PZT thick films it is important to carry out the precursor powder characterization. Figure 3 a) shows the X-ray diffraction pattern of the PZT TRS600 precursor powder used in the preparation of infiltrated PZT thick films (Pérez et al., 2007, Pérez, 2004). The diffraction pattern of this PZT powder shows the presence of a pure perovskite PZT phase and two marginal extra diffraction peaks (26.8o and 33.12o) that are associated with the lead excess and other additives (such as Nb2O5) used in the preparation of the commercial PZT powders (TRS, 1998, Kholkin et al., 2000). Normally, the additive compounds used in the preparation of commercial PZT powder have the objective of improving its dielectric and piezoelectric properties. However, they also show extra attributes when the PZT powders are used in the preparation of PZT thick films. For instances, the lead oxide excess present in the PZT precursor powder guarantee that the PZT

Figure 3b) shows the X-ray diffraction patterns of PZT thick films prepared with different number of top infiltrations. The analysis of the (110) PZT diffraction peak shows a decrease of the width and a small shift of the peak position, relatively to the TRS600 PZT commercial powder, as the number of infiltration steps increases. It is evident that the infiltration process reduces the PZT thick film surface roughness, which results in a better relation between incident and diffraction angle of the films. For that reason, the decrease in the width of the (110) PZT diffraction peak is mainly attributed to the decrease of the surface

In contrast, the shift in the peak position of the maximum as the number of top infiltration layers increase, has been associated with two factors: i) a small difference between PZT powder and PZT solution compositions and ii) a change in the stress of the films provoked

**3.1 Structural and microstructural characterization of the infiltrate PZT thick films**  There are practically no differences between the structural and microstructural characterizations of thin and thick PZT films. However, it should be taken into account that PZT thick film phase formation, grain growth, crack formation, etc., are highly conditioned by the precursor powder and the powder agglomerate formation, while in PZT thin films they are mainly conditioned by the substrate structure, the characteristic of the PZT

molecular weight prepared PZT solution (Ohno et al., 2000; Pérez et al., 2007).

precursor solution and the preparation conditions.

thick films remain stoichiometric after the annealing process.

by the infiltration cycle, which somehow compacts the film structure.

**films (X-ray analysis)** 

roughness.

Fig. 3. X-ray diffraction patterns of a) TRS600 PZT precursor powder and b) PZT thick films prepared with different number of top infiltrations (0, 2, 4, 6, and 8) (Pérez et al., 2007). (Copyright Elsevier)

Finally, a decrease in the intensity of the extra TRS600 X-ray diffraction peaks (visible at 26.8o and 33.12o) is observed, showing that as the number of infiltrations increase the factional volume of the formed sol-gel PZT phase is more palpable. This fact emphasizes the idea that as the number of infiltrations increases the number of pores decreases due to a complete coverage by the sol-gel solution. Moreover, the decrease in the 26.8o X-ray diffraction peak could be also associated with the possible evaporation of the lead oxide during the PZT thick film heat treatment process.

Piezoelectric Thick Films: Preparation and Characterization 361

infiltrations, while film without infiltration shows a porous structure mainly in the last deposited layer. It is also visible in the cross section images that the infiltration process results in a decrease of the film porosity; however, there is some remaining porosity that cannot be eliminated. It is called *internal or closed porosity* and results from a premature

Dorey et al., report a similar behavior in infiltrated PZT thick films using a high molecular weight precursor solution, as shown in Fig. 6 (Dorey et al., 2002). We can see that in films with four infiltrations of this solution, the internal porosity is higher than those reported by Pérez et. al., (Pérez, 2004). It is believed that the higher closed porosity observed in these films result from the increase of the density and viscosity of the precursor solution. Nevertheless, other factors like average grain size and shape and tortuosity of the pore

Fig. 6. SEM cross section images of the PZT thick films after 0 (left), 2 center and 4 (right) infiltrations using a high molecular weight precursor solution (Dorey et al., 2002).

Surface roughness and porosity can be responsible for the possible deterioration of the dielectric constant and polarization values and the increase dielectric losses in PZT thick film. As mentioned, they are highly affected by the infiltration process. For this reason, in the next section one presents the dielectric, ferroelectric and piezoelectric behavior of the

**4. Electrical properties of the PZT thick films as a function of the number of** 

We have already seen that in PZT thick films the structural and microstructural properties are highly dependent on the number of infiltrations, but what about the dielectric, ferroelectric and piezoelectric properties? It is well-known that in bulk materials the dielectric, ferroelectric and piezoelectric properties are highly dependent on grain size, porosity, phase formation, stoichiometric, crystallographic orientation, amongst others. However, in PZT thick films this dependence may be notably different because the electrical properties are also affected by substrate clamping, surface powder agglomeration and the mixture of phases coming from the precursors PZT powder and

closing up of the pores channels.

(Copyright Elsevier)

**infiltrations** 

PZT sol-gel solution, etc.

channel should be also taken into account.

PZT thick films as a function of the number of infiltrations.
