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Title: Study of bismuth layer-structured ferroelectrics for high-temperature applications
Authors: Hu, Tiantian
Keywords: Hong Kong Polytechnic University -- Dissertations
Ferroelectric crystals
Issue Date: 2006
Publisher: The Hong Kong Polytechnic University
Abstract: With the trends in environmental protection, there are growing interest and demand in developing lead-free ferroelectric materials for replacing the currently used lead-based materials in various piezoelectric and pyroelectric applications. In the present work, bismuth titanate-based ferroelectric materials Bi₃.₅-x/₃Nd₀.₅Ti₃-xNbxO₁₂ (BNTN), both in the forms of ceramic and film, with good piezoelectric and pyroelectric properties have been successfully fabricated and have been shown to be potential candidates for high-temperature sensor applications. BNTN ceramics were prepared using a conventional mixed-oxide technique. The ceramics were sintered well into a single phase of bismuth layered perovskite structure at 1000℃ for 4 hours. The effects of the niobium (Nb) dopant on the electrical, ferroelectric, piezoelectric and pyroelectric properties have then been investigated and discussed. No significant effect of the Nb dopant on the Curie temperatures (Tc) is observed; all the BNTN ceramics exhibit similar high values of Tc (~ 540℃). Concluding from the observations on the temperature and frequency dependences of the dielectric loss, the Nb dopant can effectively reduce the oxygen vacancies in the BNTN ceramics, while the oxygen vacancies are responsible for the high conductivity of the bismuth tianate-based ceramics. Our results also reveal that after the doping with Nb, the dielectric constant e, coercive field Ec and leakage current J of the Bi₃.₅Nd₀.₅Ti₃O₁₂ (BNT) ceramic decrease, while its remanent polarization Pr, piezoelectric (charge) coefficient d₃₃ and pyroelectric coefficient p increase. At the optimum doping level (~ 6 mol%), the ceramic exhibits the largest Pr (15 μC/cm²), largest d₃₃ (22 μC/N), largest p (129 μC/m²K), and the lowest J (3x10⁻⁸ A/cm²). Since the ceramics also have a relatively low value of ε (~ 99), their piezoelectric (voltage) coefficient and figure of merit for pyroelectricity are large and comparable to those of a lead-based ferroelectric ceramic. Together with the high Curie temperature, the BNTN ceramics therefore should be good candidates for various high-temperature sensing applications. For MEMS application studies, Bi₃.₅Nd₀.₅Ti₃O₁₂ (BNT) thin films of thickness 900 nm have been successfully fabricated on platinized silicon substrates using a sol-gel method. The dielectric, ferroelectric as well as the piezoelectric and pyroelectric properties of the films were investigated. Our results reveal that the BNT thin films can crystallize well into a single phase of bismuth layered perovskite structure at 650℃ and 700℃. However, a higher annealing temperature results in larger grains and better properties. For the BNT thin film annealed at 700℃, the observed ε, tanδ, Pr, p and e₃₁,f are 132, 0.034, 22 μC/cm², 148 μC/m²K and 1 .45 C/m², respectively. Similar to the cases for ceramics, owing to the low ε, the piezoelectric (voltage) coefficient and figure of merit for pyroelectricity of the films are large and comparable to those of a lead-based ferroelectric thin film. Therefore, the BNT thin films could be employed in various sensor applications.
Description: 1 v. (various pagings) : ill. ; 30 cm.
PolyU Library Call No.: [THS] LG51 .H577M AP 2006 Hu
Rights: All rights reserved.
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