| It was well known that lead oxide based ferroelectrics, represented by lead zirconate titanate (Pb(Zr, Ti)O3 or PZT) were the most widely used materials for piezoelectric actuators, sensors and transducers. However, lead toxicity was a subject of environmental concern within electronic manufacturing. Considering lead toxicity during processing, the use, recycling and disposal of devices containing PZT, important efforts have been carried out during the last years to develop lead-free ferroelectric compositions that can replace PZT.Sodium bismuth titanate (Bi0.5Na0.5TiO3, abbreviated as BNT), was a kind of perovskite-type ferroelectric with a relatively large remnant polarization (Pr=38μC/cm2) at room temperature and a relatively high curie temperature (Tc=320℃). For its strong ferroelectricity at room temperature, BNT has been considered to be a promising candidate material for lead-free piezoelectric ceramics. However, the large coercive field (Ec～7.3 kV/mm), low depoling, and high conductivity during poling were barriers for practical applications. It was note worthy that BNT-based piezoelectric showed high strain when the A-site was slightly substituted by Ba2+and the addition of BaTiO3 to BNT may improve the piezoelectric and sintering properties. In this thesis, BNT based ceramic/thin films were prepared and characterized. The primary conclusions in the separated parts were as follows:(Ⅰ) lead-free (1-x)Bio.5Na0.5Ti03-xBaTi03 (abbreviated as BNT-xBT) powders were synthesized by a modified citrate acid method. Furthermore, the prepared powders were pressed and calcined to form the corresponding ceramics. The structural and ferroelectric properties of BNT-based ceramics have been investigated. The analytic results showed that all the BNT-xBT samples were homogeneous with a pure perovskite crystal structure. The ferroelectric measurements showed that BNT-based ceramics shows the typical ferroelectric polarization hysteresis loops and the BNT-based ceramics exhibit lower coercive field compare to the BNT ceramic (Ec=73 kV/cm). Furthermore, BNT-0.1BT, BNT-0.06BT, and BNT-0.05BT exhibited the best ferroelectric properties.(Ⅱ) BNT thin films were prepared with indium-tin-oxide (ITO) coated glass as substrates by using an improved sol-gel spin coating process with (CH3COO)3Bi, C16H36O4Ti, and CH3COONa as the bismuth, titanium, sodium resources, respectively. The influence of different film thickness on the structure and ferroelectric properties of the BNT films are investigated in details. The results showed that the prepared BNT films were crystallized in the well perovskite phase and the film with 6 coatings exhibited the best ferroelectric properties with Pr=22.02μC/cm2.(Ⅲ) 0.90Bio.5Nao.5Ti03-0.10BaTi03 thin film with indium-tin-oxide (ITO) coated glass as substrates were prepared by using an improved sol-gel spin coating process for the first time. A new solution process was developed for the preparation of precursor solution which was stable in air, presenting a low gelation and trend to the formation of precipitates. The as-prepared thin film was crystallized into single perovskite phase and indicated good ferroelectric properties. In this thesis, the variations of remnant polarization Pr and saturation polarization Ps with measurement temperatures for the 0.90BNT-0.10BT thin film were detected at different setting voltages, respectively.Both the observed Pr and Ps values increased with increased sweeping electric fields under the present conditions. On the other hand, when the temperature was below 62℃, both the Pr and Ps values had not obvious changes. However, as temperature further increased, both the values increased obviously to some extent. But it is not always so. When the temperature increased furthermore, both the values exhibited the opposite behavior or changed slightly. The Pr reached the largest values when the temperature was 74℃. The largest value was about 0.059μC/cm,0.077μC/cm2, and 0.122μC/cm2 when the voltage was 20 V,30 V, and 40 V, respectively. It is remarkable that the thin films show excellent heat-to-electrical energy conversion properties. The results demonstrated the feasibility of using lead-free BNT-BT films for pyroelectric energy harvesting. |
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