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Title: Study of BNT-BKT-BT lead-free piezoelectric ceramics and their application in piezoelectric devices
Authors: Choy, Siu-hong
Degree: Ph.D.
Issue Date: 2007
Abstract: Perovskite lead-free ceramics bismuth sodium titanate (Bi0.5Na0.5TiO3, BNT) modified with potassium and barium to form a new compound, 0.90Bi0.5Na0.5TiO3-0.05Bi0.5K0.5TiO3- 0.05BaTiO3 (BNKBT-5), have been fabricated by a solid-state reaction method. The dielectric, piezoelectric and ferroelectric properties of the ceramics have been measured and the microstructures studied by X-ray diffraction and scanning electron microscopy. In the ferroelectric hysteresis loop measurements, a large remanent polarization (Pr ~ 28.5 uC/cm2 ) and coercive field (Ec) ~3.5 MV/m have been observed. The electromechanical coupling coefficients kp and kt are 0.31 and 0.46, respectively. Those properties are comparable to that of lead-based ceramics such as lead zirconate titanate (PZT). BNKBT-5 has relatively high dielectric loss (dissipation factor, tan d ~2.7%), low mechanical quality factor (QM ~ 65 for radial mode) and low piezoelectric charge coefficient (d33 ~160 pC/N). To improve the properties of BNKBT-5, three different compounds have been used as additives/dopants. These include various amounts of cerium oxide (CeO2), dicalcium ferrite (Ca2Fe2O5) and bismuth lithium titanate (Bi0.5Li0.5TiO3 (BLT)). All the samples with different compositions have been characterized. The measured dielectric, piezoelectric and ferroelectric properties are compared with that of undoped BNKBT-5. It has been found that the BNKBT-5 doped with 1.5 mole % of BLT, namely BNKLBT-1.5, has the best performance. It can enhance kp, kt, QM, Pr, and can reduce the dielectric loss tan d but do not lower the depolarization temperature. Two different types of devices have been fabricated using BNKBT-5 and BNKLBT-1.5 ceramic rings. The first type of device is compressive type accelerometers. A PZT accelerometer with similar structure has also been fabricated for comparison. The accelerometers are calibrated using a back-to-back calibration method against a standard reference accelerometer (Briiel & Kjaer standard reference accelerometer Type 8305). Within the +-2.5% tolerance, the PZT accelerometer has a mean sensitivity value of 4.34 pC/ms-2 from 50 Hz to 8.24 kHz. The BNKBT-5 accelerometer has a mean value of 2.24 pC/ms-2 from 50 Hz to 10.1 kHz and the BNKLBT-1.5 accelerometer has a mean value of 2.97 pC/ms-2 from 50 Hz to 12.45 kHz. Although the PZT accelerometer has the highest sensitivity, but it has the lowest working frequency range. The BNKLBT-1.5 accelerometer has a reasonably high sensitivity and the broadest sensing frequency range which would be the most preferable choice for structural health monitoring applications. The second type of device is ultrasonic wirebonding transducers for microelectronic packaging. It has been found that if titanium is used as the metal parts in the transducer, the BNKLBT-1.5 transducer has similar axial displacement (~1.6 um) to that of PZT/stainless steel (which is the state-of-the-art transducer) transducer presumably because the lead-free ceramic has an acoustic impedance close to that of titanium metal. The lateral displacement of the BNKLBT-1.5 is much smaller than that of PZT transducer thus can improve the bonding quality. It shows that lead-free ceramics has the potential to replace PZT in certain transducer designs.
Subjects: Hong Kong Polytechnic University -- Dissertations.
Piezoelectric ceramics.
Piezoelectric devices.
Pages: xvii, 199, vi leaves : ill. ; 30 cm.
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