Effect of rare earth (Er, Gd, Eu, Nd and La) and bismuth additives on the mechanical and piezoelectric properties of lead zirconate titanate ceramics

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Abstract

Mechanical, piezoelectric and dielectric properties of PZT ceramics (Pb0.98M0.02[(Zr0.535Ti0.465)0.995]O3, M=Er, Gd, Eu, Nd, La and Bi) have been studied. The fraction of A sites occupied by the different additives is estimated and is found to increase with the ionic radius of the additive ion. The density is high in Bi-, Gd- and Eu-PZT samples. Small amounts of the pyrochlore phase are present in Gd- and Eu-PZT. A maximum strength of 80–100 MPa and KIC of ∼1.5 MPa m1/2 are obtained in Gd- and Eu-PZT. The R-curve behavior becomes more pronounced in Gd- and Eu-PZT, and it is observed that the strength and KIC increase with the slope of the KIC versus c1/2 plot. Very good piezoelectric properties, i.e. d33 of 410 PCN−1, dielectric constant of 1005, κp of 0.49 and the hydrophone figure of merit, dhgh, of 2760 are obtained for Nd-PZT. The piezoelectric properties in Bi-PZT are also higher than for the undoped composition, while for the Gd-PZT, these are slightly inferior. For all other compositions, the piezoelectric properties are not affected much.

Introduction

The strengthening of metals by the incorporation of solute atoms in solid solution is well known [1], [2], [3], [4], [5]. The resistance offered by the solute to the dislocation motion causes such strengthening.

Not much has been said regarding the strengthening of ceramics by solid solution. In general, in ceramics the dislocation motion at room temperature is not significant owing to the small number of operating slip systems and the low density of mobile dislocations. The exceptions are some halides, where appreciable plastic deformation can take place at relatively low temperatures. A good example is a small addition of SrCl2 to KCl which increases the yield strength by a factor of 10 [6].

In other ceramics the mechanisms responsible for the changes in the mechanical properties because of solid solution additives are found to vary. Thus, the addition of Cr2O3 to Al2O3 was reported to increase the bulk modulus of Al2O3 owing to a systematic decrease in the lattice parameter [7]. Solid solutions of MgO, TiO2 and MnTiO4 in Al2O3 change the hardness as well as the grain size and density [8]. AlN as a solute in SiC has also received considerable attention [9] because AlN inhibits the phase changes taking place in SiC, making it more sinterable and leading to an improvement in the flexural strength. The stabilization of the tetragonal phase of ZrO2 at room temperature by the addition of Y2O3, CeO2, etc. and the increase in toughness because of transformation toughening is well known.

While the effect of solid solution additives on the piezoelectric properties and the microstructure of PZT ceramics was well studied, their effect on the mechanical properties has received much less attention. Apart from the effect of the amounts of the constituent ions (i.e. the Zr/Ti ratio and Pb excess or deficiency), the effects of only a few solid solution additives on the mechanical properties have been reported. Hase et al. [13] introduced Mn in PZT ceramics by diffusion after the preparation of the samples. With the diffusion of Mn2+ ions from the grain boundary location to the inside, the fracture mode changed from intergranular to transgranular and the flexural strength (strength in three- or four-point bending) increased from 100 to 140 MPa. On the other hand, the addition of Cr2O3 reduced the strength substantially by about 25–45 MPa. Cr3+ segregated to the grain boundaries and the fracture mode was completely intergranular. Durans et al. [14] studied the effect of some rare earth (RE) dopants (Pb0.88RE0.08(Ti0.98Mn0.02)O3, RE=La, Nd, Sm) on lead titanate ceramics. The KIC (critical stress intensity factor in mode I or fracture toughness) was found to decrease from 2.0 MPa m1/2 for La, to 1.9 MPa m1/2 for Nd and 1.7 MPa m1/2 for Sm. Owing to large tetragonality, the undoped lead titanate samples undergo cracking during cooling from the sintering temperature and have very poor mechanical properties. This was attributed to the increasing tetragonality in the samples. Storz and Dungan [11] found KIC of a 95/5 PZT to decrease with an increasing Nb5+ content.

In the present work, we have studied the effect of some RE additives on the strength, toughness and hardness of PZT ceramics. Their dielectric, piezoelectric and electromechanical properties have also been studied. In addition to RE's Bi, a non-rare earth additive with its ionic radius falling in the center of the range of the ionic radii of the REs has also been chosen. For convenience, results have been presented as a function of the ionic radius of the additive cation. The additives used were Er, Gd, Eu, Bi, Nd and La in order of their increasing ionic radius (Table 1) [15].

Section snippets

Experiments

All the compositions were prepared based on the following general formula: Pb1-xMx[(Zr0.535Ti0.465)1-z/4x/4]O3 where M is the trivalent additive used. Concentration (x) of M was fixed at 0.02. The formula mentioned above assumes that the additive goes to site A (Pb) and that the charge compensation is brought about by vacancies at B (Zr, Ti) sites. However, as will be discussed later this may not be completely true in all the cases.

To compensate for PbO volatility during calcinations and

Weight loss, density, microstructure and phases

Fig. 1 shows the percent weight loss on sintering with different additives. The weight changes are expected to be almost exclusively owing to the PbO loss or gain except in the case of samples containing Bi where the weight loss would also be owing to the loss of Bi2O3.

Fig. 2 shows the sintered density of the various samples. The relative densities or the compactness could not be determined as it was complicated because of the presence of three different phases (tetragonal and rhombohedral

Distribution of the additive cations between A and B sites

When an additive oxide is added to a ceramic with a perovskite structure (ABO3), the added cation distributes itself between the A and B sites in a ratio that depends upon the cation. Therefore, to discuss the results, it is helpful if some estimate on the distribution of the cation between the two sites can be made. We have made such an estimate using a procedure analogous to that of Troccaz et al. [24], [25].

The details are given in Ref. [26] and the results are shown in Fig. 9 which also

Conclusions

In the present work the effect of adding a fixed amount of (2%) of certain RE additives (Er, Gd, Eu, Nd and La) and Bi on the mechanical and piezoelectric properties of MPB composition of PZT were studied. The primary aim was to see the effect of the solid-solution additives on the mechanical and piezoelectric properties of PZT. The main conclusions can be summarized as follows:

  • 1.

    Largest enhancements in KIC and strength are obtained in Gd-PZT and Eu-PZT. For Eu-PZT the strength increases from ∼60

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