2. MATERIALS AND EXPERIMENTAL METHODS The materials used in this study were thin film PZT composites fabricated by a combination of SiO2, Ti, Pt, and PZT. Uniaxial tension experiments were conducted on PZT composite specimens with gauge length of 1,000 µm and widths of 50 and 100 µm as shown in Figure 1. The microscale tension specimens were fabricated by a combination of chemical vapor deposition, physical vapor deposition and chemical solution deposition on silicon substrate followed by various etching techniques to obtain freestanding dog bone thin film specimens as shown in Figure 1. Though the nominal thickness of the individual layers could be calculated from the deposition time for each layer, the actual thickness of each layer was measured from SEM micrographs. The PZT composite specimens were made of a 1-µm thick PZT layer sandwiched between 100 nm thick Pt electrodes with a 300 nm thick sacrificial layer of SiO2 that served as thermal and electrical insulator. The uniaxial tension tests were conducted by using an in-situ setup that had a PZT actuator to generate loading and a loadcell to measure the load in the sample, identical to one reported before [13-15]. The test apparatus was placed under an optical microscope to capture photos of the specimen gauge section having a fine and dense speckle pattern as shown in Figure 1. These pictures were later analyzed by using Digital Image Correlation (DIC) to compute the full-field strain on the specimen surface and the stress-strain curves were constructed with the use of the loadcell measurements. The mechanical response of PZT was then derived from the stress strain plots of the individual materials comprising the PZT composites and the composite behavior with the aid of basic lamination theory [7]. The PZT composite specimens used to evaluate piezoelectric behavior were identical to the dog bone shaped specimens shown in Figure 1. The SiO2-TiPt-PZT-Pt composite was used to measure the d31 coefficient by biasing the Pt electrodes that were firmly bonded on either side of PZT. Due to their freestanding nature, these specimens underwent out-of-plane bending upon biasing the electrodes with very fine electrical probes. The die containing specimens was mounted vertically under an optical microscope as shown in Figure 2. Optical images of the specimen’s sidewalls were recorded to measure the beam deflection upon application of electric field. The optical images of the specimen’s sidewall at different bias voltages, ranging between 0-6V (see later Figure 4), were processed to calculate the deflection of the beam and fit it to an analytical model for a piezoelectric beam subjected to bending, which was developed by Balls et al [16], to compute the d31 piezoelectric coefficient. Figure 1. A freestanding SiO2-TiPt-PZT-Pt specimen with a fine speckle pattern deposited to measure full field mechanical strains. Figure 2. Schematic of a die containing PZT specimens positioned vertically under an optical microscope for measuring out of plane deflection of PZT composite beams under an applied bias. Speckle pattern on sample’s surface 100 µm Optical microscope Specimen die Microscale specimens 262
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