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Abstract
Piezoelectric materials are smart materials widely used in the fields of information and communications, industrial automation, medical diagnostics and so on. However, lead-based piezoelectric materials have dominated the market in the past decades, resulting in environmental and human health problems worldwide. In this project, a high performance lead-free ceramics (1-y)(Na0.55K0.45)1-xLix(Nb,Ta)O3–yAgNbO3 have been successfully developed by spark plasma sintering technique.
(K,Na)NbO3 (KNN) based ceramics shows strong temperature dependent characteristics. It would be a feasible strategy if the temperature stability could be improved through appropriate chemical doping and sintering routes. In this work, the effects of sintering temperature and post annealing temperature were studied for ceramic 0.82(Na0.55K0.45)0.94Li0.06(Nb0.85Ta0.15)O3–0.18AgNbO3. It was found that the ceramic sintered at 980 ℃ and post annealed at 1050 ℃ possesses the best properties, i.e. remnant polarization Pr of 22.33 μC/cm2, dielectric constant εr of 598.8 (at 10 kHz) and piezoelectric coefficient d33 of 101 pC/N.
The doping effect of Ag on the properties of 1-y(Na0.55K0.45)0.94Li0.06(Nb0.85Ta0.15)O3–yAgNbO3 (y=0, 0.06, 0.12, 0.18) was investigated. It was confirmed that orthorhombic – tetragonal transition temperature (TO-T) was shifted to a higher temperature as the increase of Ag doping level. It showed relatively better piezoelectric and dielectric properties when 6 mol% Ag doped, which have d33 of 127 pC/N, kt of 0.27 and εr of 676 (at 10 kHz). Besides, a higher TO-T of 65 ℃ was achieved at 18 mol% Ag content, which shows a weaker piezoelectric response.
The doping effect of Li on the properties of 0.94(Na0.55K0.45)1-xLix (Nb0.85Ta0.15)O3–0.06AgNbO3 (x=0, 0.02, 0.04, 0.06, 0.08, 0.10) was also studied. The Li dopant has shown a great impact on both Curie temperature Tc and TO-T. 4 mol% Li-doped sample shows relatively high TO-T of 95 ℃ and have a reliable dynamic piezoelectric coefficient (d33*) around 240 pm/V at a broad temperature range from 25 ℃ to 105 ℃. Moreover, 2 mol% Li-doped sample have a high d33* of 320 pm/V at a high temperature of 135 ℃, which has a great potential for high temperature applications.
The KNNLT based ceramics in this thesis shows relatively high temperature stability and good ferroelectric and piezoelectric properties comparable to other KNNLT ceramics. The study demonstrates the success in developing high performance lead-free KNN based ceramics with better temperature stability, and their potential applications in piezoelectric sensors, actuators and ferroelectric memory etc. that could work at higher temperature.