3. RESULTS AND DISCUSSION The mechanical behavior of the individual layers SiO2, SiO2-TiPt, SiO2-TiPt-PZT and SiO2-TiPt-PZT-Pt was reported in our previous work [7]. The stress-strain plot of the PZT thin film stack including an elastic loadingunloading segment and the final reloading to fracture is shown in Figure 3. The composite modulus for SiO2-TiPtPZT-Pt was 94.7 GPa with the failure strain being 0.65%. The stress vs. strain curves showed nonlinear behavior after approximately 0.35% strain. E = 94.7 GPa 0 100 200 300 400 500 600 700 0 0.2 0.4 0.6 0.8 Strain (%) Stress (MPa) Figure 3. Stress vs. strain of a freestanding PZT composite film including the loading-unloading and final reloading segments. The piezoelectric behavior of this PZT material was evaluated using the same freestanding composite specimens. Optical images of the beam’s profile taken at bias voltages between 1.5V and 6V, shown in Figure 4, were used for this purpose. The stress mismatch between the PZT layer and the attached Pt and SiO2 layers resulted in composite beam deflection up to 480 µm at bias voltages of 6V. A first estimate for piezoelectric coefficient d31 was calculated as 176±27 pm/V by using multilayer beam bending equations developed by Ballas et al [16]. This d31 value is higher than that reported for bulk PZT [17] (93.5 pm/V) and is in the upper range of d31 values reported in literature for sol-gel prepared PZT thin films. This may be attributed to the presence of fewer defects, lack of microcracking, perfect bonding between layers, the influence of fabrication conditions in the formation of PZT with high ferroelectric properties, and the use of freestanding films as opposed to PZT films attached to a rigid substrate that were used in previous studies. Table 1. Mechanical properties of the PZT composite and the component layers Material Modulus (GPa) Failure Stress (MPa) Failure Strain (%) SiO2 72.3±2 1,060±200 1.4±0.2 SiO2-TiPt 87.9±1 780±70 0.8±0.1 Pt [13] 173 1,876±10 3.84±0.26 SiO2-TiPt-PZT 87.9±1 412±50 0.5±.0.05 SiO2-TiPt-PZT-Pt 93.5±6 511±50 0.6±0.05 PZT (Extracted) 84±2 510±35 0.5±0.1 263
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