Low-temperature sintered lead-free piezoelectric ceramics and their applications for vibratory gyrosensors

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Low-temperature sintered lead-free piezoelectric ceramics and their applications for vibratory gyrosensors


Author: Wong, Mau Tak
Title: Low-temperature sintered lead-free piezoelectric ceramics and their applications for vibratory gyrosensors
Degree: M.Phil.
Year: 2014
Subject: Piezoelectric ceramics
Lead abatement.
Piezoelectric devices.
Lead-free electronics manufacturing processes.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Physics
Pages: xiv, 124 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2747302
URI: http://theses.lib.polyu.edu.hk/handle/200/7495
Abstract: The main objectives of the present work are to prepare low-temperature sintered lead-free piezoelectric ceramics and to fabricate piezoelectric vibratory gyro-sensors using the lead-free piezoelectric ceramics. For environmental protection reasons, lead-free piezoelectric ceramics have been extensively studied for replacing the widely used piezoelectric ceramics such as PbZrxTi1-xO₃ (PZT), which contain a large amount of toxic lead. K₀.₀₅Na₀.₀₅NbO₃ (KNN) is one of the most promising candidates for the lead-free piezoelectric ceramics. Besides the saving of energy, low-temperature sintering is always an important topic for the fabrication of ceramic components, in particular for co-fired multi-layered components. The low sintering temperature allows the use of relatively cheap inner electrodes, such as Pd/Ag rather than Pt. In this work, two methods, high-energy ball milling and liquid phase sintering, have been evaluated for lowering the sintering temperature of 0.91K₀.₄₇Na₀.₄₇Li₀.₀.₆NbO₃-0.09NaSbO₃ (KNLN-NS) ceramics which have been shown to possess good piezoelectric properties. The high-energy ball milling technique has been shown to be effective in reducing the grain size and lowering the sintering temperature of a number of compounds and oxides, including perovskite oxides. Our results show that the grains of the KNLN-NS ceramics are reduced from ~3 μm to ~1 μm after the high-energy ball milling process at 200 rpm for 2 h. However, there is no significant change in the sintering temperature that remains at a high temperature of 1080°C. On the other hand, probably due to the smaller grain size, the piezoelectric properties of the ceramics decrease by about 30%.
A sintering aid comprising of Cu and Ba in a molar ratio of 71.5/28.5 has been used to lower the sintering temperature of the KNLN-NS ceramics. The sintering aid is effective in decreasing the sintering temperature, improving the densification and reducing the dielectric loss of the KNLN-NS ceramics. For the ceramic added with 2.5 wt% of the sintering aid (i.e., KNLN-NS-Cu/Ba-2.5), the sintering temperature is decreased to 980°C, the density is increased to ~4.4 g/cm³ and the dielectric loss is reduced to 1.0%. As the sintering aid becomes molten at a eutecture temperature of 890°C, the densification of the ceramics is promoted via liquid-phase sintering. However, partly due to the secondary phase and hardening effect, the piezoelectric properties of the KNLN-NS-Cu/Ba-2.5 ceramics are decreased, giving a d₃₃ of 134 pC/N. If less sintering aid is used, the ceramics become porous, the density decreases and the dielectric loss at low frequencies is sensitive to moisture. However, they can still be effectively poled at high temperatures and exhibit relatively good and stable piezoelectric properties (d₃₃ = 153 pC/N). Owing to the low sintering temperature and dielectric loss, the KNLN-NS-Cu/Ba ceramics are promising for practical applications, in particular for multi-layered components. Bimorph-type piezoelectric vibratory gryo-sensors have been fabricated using both the KNLN-NS (sintered at 1080°C) and KNLN-NS-Cu/Ba (sintered at 1000°C) ceramics, and an electronic readout circuit has been developed to detect the small signals induced in the gyro-sensors subjected to a rotation and to give a dc output voltage proportional to the rotation speed. The piezoelectric vibratory gyro-sensor is made up of a stack of two oppositely poled ceramic plates and is electrically driven to vibrate at its bending resonance frequency. Subjected to a rotation, a Coriolis force is induced, bending it in a direction normal to both the vibrating and rotation directions. The charges induced on the gyro-sensor by the induced bending are detected by the electronic readout circuit that comprising of charge amplifiers, differential amplifier and demodulator. All the gyro-sensors exhibit a linear relationship between the dc output voltage and rotation speed. Our results also show that the high dielectric loss at low frequencies of the KNLN-NS-Cu/Ba ceramic arisen from moisture absorption does not affect the performance of gyro-sensors that operated at high frequencies such as 20-30 kHz. A good compromise between the dimension and sensitivity is obtained for a gyro-sensor of size 4 mm x 0.6 mm x 0.6 mm. The sensitivity of such a gyro-sensor fabricated using the KNLN-NS-Cu/Ba ceramic reaches a high value of 0.57-0.84 mV/dps in the range of -360 dps to +360 dps.

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