Effect of sintering aid and repeated sol infiltrations on the dielectric and piezoelectric properties of a PZT composite thick film

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Abstract

Thick PZT films have been produced using a combination of spin coating of a composite slurry and subsequent infiltration of PZT producing sol. The effect of adding a Cu2O–PbO sintering aid and repeated sol infiltrations have been studied with the aim of producing dense PZT films. Relative permittivity has been shown to increase with the addition of sintering aid and increased levels of sol infiltration. Measurements of piezoelectric properties indicate that sol infiltrations have no effect on d33 once a critical density has been exceeded. A sample with approximately 10% closed porosity was obtained following the incorporation of sintering aid and four infiltration steps per layer. This resulted in a mean relative permittivity of approximately 700 and a d33 of 62 pC/N (poling conditions: 8 V/μm for 5 min at 200 °C).

Introduction

Sol-gel technology allows thin oxide films (<1 μm) to be deposited onto a variety of substrates at temperatures well below those conventionally used for bulk ceramic processing. Typically temperatures as low as 600–700 °C are used, so allowing the ceramic deposition to be incorporated into the silicon processing stages.1, 2 In addition, the lowering of the processing temperature reduces the interdiffusion of atomic species between the different layers and the vaporisation of PbO.2, 3 Thin PZT films have been deposited to produce devices such as membrane sensors, accelerometers and micromotors.4 However, devices requiring larger actuation forces (i.e. high frequency transducers, vibration control devices) require thicker (>10 μm) piezoelectric films.5 It is not practical to produce thick PZT coatings using the standard sol-gel technique because of the increased risk of cracking due to shrinkage and the time required to produce very thick layers by repeated layering.6, 7

In a study of bulk sintering of PZT ceramics Villegas et al.8 found that the addition of 1–2 wt.% colloidal sol derived PZT powder resulted in a lowering of the sintering temperature which in turn lead to an increase the piezoelectric properties due to lower lead volatilisation. An alternative approach used by Barrow et al.6 was to mix the PZT powder with the PZT sol and form the colloidal PZT powder in situ. Higher levels of sol were utilised in the in situ technique resulting in the formation of a composite structure where the sol derived PZT formed an interconnecting network. The thick film was built up by depositing a series of composite layers. Each layer was spin coated and fired at an intermediate temperature designed to pyrolise the sol. Once the required number of coatings had been applied the whole film was subjected to a crystallisation stage where the sample was sintered at higher temperatures designed to develop the perosvkite phase. Using this technique, thick, crack free PZT films have been deposited. Barrow et al.6 attributed the crack free nature of the films to a) the presence of large amounts of powder which resulted in a decrease in the level of sol present and hence lower shrinkage; (b) strong bonding between the sol-gel and the PZT particles making cracking less likely.

The incorporation of PZT powder into the sol may also have an additional benefit. Wu et al.2 showed that the addition of 1 wt.% PZT powder lowered the perovskite formation temperature by 50 °C and increased the dielectric and ferroelectric properties of the film. The incorporation of the PZT micro-powder should promote heterogeneous nucleation of the perovskite phase from the sol and may, therefore, result in randomly orientated nano PZT.

Using the composite layering technique Barrow et al.6 obtained thick films and quoted properties comparable to those of bulk ceramics. However, in a later paper4 lower values of relative permittivity were reported. This was attributed to the presence of the sol derived PZT with a low relative permittivity of approximately 400 compared to 1300 for the bulk PZT. The different value of relative permittivity obtained for the sol derived PZT and bulk PZT is probably due to the small size of the sol derived PZT grains.7 Similar low values of relative permittivity have also been reported by other investigators working on thick films.5, 9 In these cases the decrease was attributed to the presence of significant levels of porosity. In reality thick films are likely to be affected by both of these factors indicating that relative permittivity of the thick films will always be lower that that expected for bulk ceramics.

The effect of various processing parameters on the relative permittivity of composite films between 0.1 and 1.5 μm thick was studied by Ohno et al.1 It was found that the major factors affecting the film performance were powder loading and sol concentration. High powder loadings (maximum of 70 wt.%) and high sol concentrations (maximum of 0.6 M) were shown to result in thicker films with high polarisation and coercive fields. However, high powder loading and sol concentrations were also found to result in lower values of dielectric constant due to the formation of pores. A subsequent sol infiltration and annealing stage resulted in an increase in the relative permittivity of the material through a reduction in the level of porosity. Improvements in relative permittivity of PZT composite films (from 200 to 1350) were also observed by Kholkin et al.10 following repeated sol infiltration.

It is also possible to reduce the level of porosity through increased sintering. Corker et al.11 showed that the addition 5 wt.% Cu2O–PbO sintering aid improved the density of PZT compacts sintered at 710 °C. The same level of the Cu2O–PbO sintering aid was later incorporated into a composite slurry to produce thick films (>15 μm).12 The resultant films exhibited a density gradient (higher density closer to substrate) with a relative permittivity and d33 of approximately 680 and 52 pC/N respectively.

The aim of this work was to combine these two approaches to improved sintering and so produce thick composite films with improved structural homogeneity and electrical properties. It is envisaged that this combined approach will also significantly reduce the shrinkage on sintering and hence the stresses generated within the constrained film.

The work examined the variation in relative permittivity and piezoelectric properties of 10 μm thick hard doped PZT composite films with increasing levels of sol infiltration and the addition of 4.7 wt.% (the actual amount added by Corker et al.12) Cu2O–PbO sintering aid. A high powder loading (1.5 g PZT/ml of sol) was selected to maximise the ferroelectric properties of the thick film and the thickness of the deposited layers. Repeated sol infiltration and pyrolysis of the individual composite layers was then conducted in an attempt to maximise the dielectric properties of the film. The variation in the film properties was recorded as a function of sol infiltration.

Section snippets

Composite slurry production

A hard doped PZT [Pb1.05(Zr0.46Ti0.48Nb0.02Sb0.02Mn0.02)O3] sol was produced using the route described in Fig. 1.12 A composite slurry was then made by mixing the PZT producing sol with a hard doped PZT powder (mean diameter 0.5 μm, Ferroperm PZ26) to produce a powder loading of 1.5 g/ml. 2 wt.% (relative to the PZT powder mass) of a dispersant (Kenrich Petrochemicals, KR55) was also added to ensure thorough dispersion of the PZT powder. To examine the effect of sintering aid addition 4.7 wt.%

Dielectric constant

XRD analysis of the films following the 710 °C treatment showed the films to be composed only of perovskite phase PZT. Fig. 2, Fig. 3 show the variation in relative permittivity and dielectric loss for films produced using the (C+XS)4 procedure where X was varied between 0 and 4. It can be seen that the relative permittivity is increased by both sol infiltration and the addition of sintering aid. Dielectric loss decreases slightly with increased sol infiltration but is largely independent of

Conclusion

It has been shown that the incorporation of a series of sol infiltration stages gives rise to an increase in the relative permittivity. This is primarily due to the reduction in the level of porosity.

Incorporation of 4.7 wt.% 0.2Cu2O–0.8PbO sintering aid leads to an additional increase in density and relative permittivity for a given level of sol infiltration.

For thick films produced with 4.7 wt.% sintering aid there was no change in the piezoelectric strain coefficient d33 with increased

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