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The poling procedure has always been the key issue in producing piezoelectric actuators with optimised performance. This is also true with the relatively new category of pre-stressed bender actuators, where mechanical bias achieved with a passive layer is introduced in the actuators during manufacturing. Due to these factors, the behaviour of the actuator under poling is different compared to its bulk counterparts. In this paper, two different thicknesses of commercial PZT 5A and PZT 5H materials were used in bulk actuators and pre-stressed benders realised by new method. Pre-stress was introduced by using a post-fired biasing layer utilising sintering shrinkage and difference in thermal expansion. The hysteresis loop of the actuators was measured under 0.5-7.0 MV/m electric fields at 25-125 • C temperatures, providing information about their remnant polarisation and coercive field before poling. The results showed that high electric field and 25 • C temperatures in poling provided higher remnant polarisation and coercive electric field than using 125 • C temperature at poling. Difference was especially significant in coercive electric field values where up to 114.8% difference was obtained for PZT 5H bulk actuator and 65.9% for pre-stressed actuators. Higher coercive fields can be utilized as increased operating voltage range of piezoelectric devices. The differences in results obtained here and by others can be explained by the different pre-stress level, stronger clamping of the thicker passive layers of the RAINBOW and THUNDER actuators and passive ring area introducing high tensile stresses. The same conditions were used to pole the actuators, after which the displacement and dielectric constant of the actuators were measured. The displacement measurements showed that remnant polarisation has good correlation with displacement. This fact can be used in estimating pre-stressed actuator performance before actual poling. The dielectric constant measurements with a small signal after poling gave even better correlation than the remnant polarisation.

In this paper, the effect of structural gradients in monolithic piezoelectric actuators is investigated. Different cross-section profiles were micro-machined with a laser into commercial PZT 5H bulk discs with thicknesses of 375 µm and 500 µm (∅ 25 mm). Profiles and curvatures of the actuators were measured which showed both concave and convex structures, thus indicating pre-stress of the actuators. After poling, the distribution of out-of-plane displacement was scanned by a fibre-optic laser vibrometer. Maximum displacements of ∼6.3 µm and ∼24.8 µm were obtained from a freely moving and clamped ∼375 µm thick actuator, respectively, in a ±0.5 V/µm electric field at 10 Hz frequency without load. Furthermore, deflection in the centre of the actuators was measured up to 184 mN load using the same electric field and frequency. Bending of the bulk actuators without any additional layer was a consequence of the gradient in poling and driving electric field via thickness variation of the material. Hence, different regions produced strain distribution and bending in a similar fashion to other benders. Actuators with the highest arch height exhibited the highest displacement and load bearing capabilities derived from the increased area moment of inertia and enhanced piezoelectric response due to pre-stress. The results show that the monolithic bending actuators can be realised by simple structural designing of the actuator. Such structural gradients can be one reason contributing to the higher displacement of RAINBOW actuators compared to other pre-stressed actuators. In a further development, the structural gradients can be utilized in high displacement pre-stressed actuators and in miniaturized monolithic piezoelectric devices.

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