Development of lead-free piezoelectric ceramic resonators for high-frequency oscillator applications

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Development of lead-free piezoelectric ceramic resonators for high-frequency oscillator applications


Author: Wong, Ho-yan
Title: Development of lead-free piezoelectric ceramic resonators for high-frequency oscillator applications
Degree: M.Phil.
Year: 2009
Subject: Resonators
Piezoelectric ceramics
Oscillators, Electric
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Physics
Pages: 1 v. (various pagings) : ill. ; 30 cm.
InnoPac Record:
Abstract: The main objective of the present work is to develop lead-free piezoelectric ceramic resonators for high-frequency oscillator applications. Lead-free piezoelectric ceramics have been extensively studied recently for replacing the widely used lead-based piezoelectric materials for environmental protection reasons. CaBi₄Ti₄O₁₅ (CBT) is one of the most important lead-free piezoelectric ceramics. It has a very high Curie temperature Tc (790 °C), a high resistivity (10¹² Ω·cm) and a low dielectric loss tanδ (0.2 %). Hence it has been widely studied for high-temperature and high-frequency applications. Recently, CeO-modified [(Na₀.₅K₀.₅)₀.₉₄Li₀.₀₆]₀.₅Bi₄.₅Ti₄O₁₅ (MBT) has been developed. The piezoelectric properties of the ceramic have been improved significantly, giving a large increase in the piezoelectric coefficient d₃₃ (28 vs 18 pC/N). In the present work, CuO and a complex of Cu/Ba (in a molar ratio 71.5/28.5) have been used to further improve the physical properties of the CBT and MBT ceramics for the high-frequency resonator applications. Our results show that the doping of CuO is effective in improving the densification, enhancing the piezoelectric properties and reducing the dielectric loss tanδ. For the CBT ceramic doped with 0.3 wt% CuO, the density increases from 6660 to 6942 kg/m³, d₃₃ increases from 10.1 to 12.4 pC/N, and tanδ decreases from 0.27 to 0.12 %. On the other hand, the Cu/Ba complex is effective in decreasing the sintering temperature of the CBT and MBT ceramics. The Cu/Ba complex becomes molten at a eutectic temperature of 890°C. As a result, because of the liquid-phase which promoting the densification, the ceramics can be well-sintered at a lower temperature. For the CBT ceramic added with 0.30 wt% of the Cu/Ba complex (CBT-Cu/Ba-0.30), the sintering temperature is decreased from 1200 °C to 990 °C, while the other physical properties are remained almost unchanged or even improved slightly. It has a high ρ (6756 kg/m³), a large d₃₃ (11.6 pC/N), a large planar-mode electromechanical coupling coefficient (kp = 3.6 %) and a high resistivity (ρr = 2.4×10¹³ Ω·cm). Unlike CBT, there is degradation in the piezoelectric and dielectric properties of the MBT ceramics, even at a very low addition level of Cu/Ba. Nevertheless, besides the low sintering temperature (1000°C), the MBT ceramic added with 0.4 wt% Cu/Ba (MBT-Cu/Ba-0.40) has a relatively large d₃₃ (24.5 pC/N) and a high ρr (2.9×10¹³ Ω·cm).
Because of the good piezoelectric properties and low sintering temperature, CBT-Cu/Ba-0.30 and MBT-Cu/Ba-0.40 have been used to fabricate high-frequency double-layered ceramic resonators. The resonator has a pair of top and bottom electrodes and an inner electrode in between them; all of them are of the same dimensions (2 mm and 3 mm in diameter) and smaller than the lateral dimension of the resonator (~5.4mm × 4.5 mm). As the sintering temperature for CBT-Cu/Ba-0.30 and MBT-Cu/Ba-0.40 are below 1000°C, Ag70/Pd30 inner electrode, instead of Pt, can be used for cofiring with the ceramics, and hence there is a great saving in the production cost. Our results reveal that the spurious vibrations of the resonators have been successfully suppressed. This should be party due to the smaller electrodes which is one of the typical approaches in the energy trapping technique to confine the vibrations in the electroded region and hence to reduce the standing waves formed by the reflected waves from the sample edge. This should also partly due to the second harmonic thickness extension mode vibrations. Our results also reveal that although the MBT-Cu/Ba-0.4 ceramic has better piezoelectric properties, its performance as a resonator is not as good as CBT-Cu/Ba-0.30. For the CBT-Cu/Ba-0.30 double-layered resonator, it has a relatively "clean" resonance response and a lower temperature coefficient of frequency (TCF =-39.3 ppm/°C). It also has a relatively low impedance at resonance (14 and 30 Ω for 3-mm and 2-mm electrodes, respectively), which is very close to the value for a commercially available PZT resonator. These suggest that the CBT-Cu/Ba-0.30 lead-free double-layered resonators have good potential to replace the lead-based resonators.

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