Further investigation was conducted on the PZT composites to understand their nonlinear and hysteretic behavior. Uniaxial tension experiments were carried out on PZT composite specimens to deduce the origins of the non-linear behavior shown in Figure 3, by performing multiple loading (beyond 0.35% strain) and unloading to zero strain cycles with a final reloading to failure. The resulting multiple stress vs. strain curves of SiO2-TiPt-PZTPt composites did not deviate from a single curve indicating a nonlinear elastic behavior of the PZT composite stacks. In literature, it has been reported that PZT exhibits nonlinear elastic behavior attributed to 90° domain switching upon applying stress [18,19], which is a possible explanation for the non-linear mechanical behavior recorded in the present stress vs. strain curves. The mechanical behavior of the thin films made of SiO2, SiO2TiPt, SiO2-TiPt-PZT-Pt, and SiO2-TiPt-PZT reported in our previous work [7] are tabulated in Table 1. Using simple lamination theory, the mechanical behavior of PZT was extracted from the properties of individual layers of SiO2, Pt and PZT composites. Figure 4. PZT beam deflection at bias voltages between 1.5V and 6V. Additionally, the effect of applied pre-stress on the electric field induced stress hysteresis of the PZT composite films was measured. Our in-situ microscale tensile testing apparatus was used to preload the specimens that were then biased by DC voltage varying between -10 V and 10 V. The force on the specimens was recorded by the loadcell attached at one end of the specimen while the other end was firmly attached to an actuator. The electric field induced stress is plotted against applied voltage in Figure 5. The field induced stress hysteresis loops were asymmetric at small applied stresses becoming of equal magnitude as the applied stresses larger than ~300 MPa, before accounting for residual stresses in the PZT layer. Residual and mechanical stresses do affect domain switching in piezoelectric ceramics [17]: at higher stresses, the domains that could easily undergo 90° switching in the direction that causes hysteresis on the right half of butterfly loops were mechanically constrained. This led to reduced hysteresis in the right half of the butterfly curve thereby making the hysteresis loops equal in magnitude. Similarly, the intersection of the hysteresis loops shifted from negative to positive electric field values at stresses larger than 150 MPa. This intersection point compares the relative population of domains that remained polarized in one direction vs. the opposite direction upon removal of the electric field. The remnant polarization exhibited by 1.5 V 3 V 4.5 V 6 V 264
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