| Bi0.5Na0.5Ti O3(BNT, for short) lead free piezoelectric material possesses excellent ferroelectric and piezoelectric properties and has been considered as one of the most promising candidates to replace PZT. Analogous to PZT, BNT can form solid solutions with morphotropic phase boundary(MPB) with other perovskite materials. Around MPB, BNT-based solid solution has a mixture of rhombohedral and tetragonal symmetry leading to the increase of polarization orientation. The polarization can be easily oriented under the external mechanical stress or electric field, which can result in a significant improvement of piezoelectric properties. This thesis mainly focused on the improvement of electric properties and phase transition behavior of BNT-based ceramics and structure and electric properties of BNT-based thin films.Firstly, lead-free(1–x)Bi0.5Na0.5Ti O3-x Bi Co O3(BNT-BC, for short)(x=0, 0.015, 0.025, 0.03, 0.035, 0.04 and 0.06) piezoelectric ceramics have been fabricated using the conventional solid state reaction method and their structure and electric properties have been studied systemically. The morphotropic phase boundary(MPB) between rhombohedral and tetragonal phases locates near x=0.025-0.035. Around MPB, BNT-BC ceramics exhibit the improved electric properties. For the ceramics with x=0.025, the remnant polarization(Pr), saturated polarization(Ps), coercive field(Ec), bipolar total strain(SB), piezoelectric constant(d33), unipolar strain(SU), thickness electromechanical coupling factor(kt) and mechanical quality factor(Qm) are 40.6μC/cm2, 35.4μC/cm2, 5.25 k V/mm, 0.181%, 107 p C/N, 0.094%, 0.45 and 257, respectively. With the increasing x, the temperature for the maximum relative dielectric constant Tm changes slightly and the depolarization temperature Td decreases first and then increases.Secondly,(1-x)Bi0.5Na0.5Ti O3-x Ba Zr0.055Ti0.945O3(BNT- BZT, for short)(x=0, 0.01, 0.03, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.12) ceramics have been fabricated using modified solid state reaction method and their structure and electric properties and phase transition behavior of 0.94BNT-0.06 BZT ceramics have been studied. The MPBs are confirmed to be x=0.06-0.09 and electric properties reach to the maximum values for these compositions. At room temperature, for the composition with x=0.07, piezoelectric constant(d33), planar electromechanical coupling factor(kp) and pyroelectric coefficient(p) are 173 p C/N, 0.27 and 57 n C·cm-2°C-1, respectively. The depolarization temperature Td for this composition is 82°C and its electric properties change slightly before Td. The Td shows strong dependence with BZT content. At the rhombohedral side, Td shifts to higher temperature then shifts greatly to lower temperatures near the MPB composition. At the tetragonal side, the Td tends to increase again. 0.94BNT-0.06 BZT ceramics exhibit a phase transition from ferroelectric to antiferroelectric at Td. At Tm, the phase changes from mixture of rhombohedral and tetragonal to tetragonal phase. The dielectric peak between Td and Tm does not involve any phase change.Lastly, 0.975Bi0.5Na0.5Ti O3-0.025 Bi Co O3(BNT-BC) thin films have been successfully deposited on(111) Pt/Ti/Si O2/Si(100) substrates by pulse laser deposition and their dielectric, ferroelectric, local piezoelectric properties, fatigue and temperature dependent leakage current behaviors have been studied systematically. X-ray diffraction indicates the thin films are single phase and <001> oriented. Compared with other BNT polycrystal thin films, BNT-BC thin films exhibit excellent ferroelectric and fatigue properties. The remnant polarization(2Pr) is about 27μC/cm2 and polarization only decreases by 10% after 5×109 switching cycles. At frequency with 1 k Hz, the dielectric constant of the thin films is about 500 and dielectric loss is around 0.22. The tunability of the dielectric constant can reach to 12% at 20 k V/mm. At low electric field, the ohmic conduction is dominant leakage current conduction mechanism and at high electric field, Schottky emission is dominant. The thin films have some polar nanoregions(PNR’s) that are on the order of 100-300 nm in length and a few tens of nanometers in width. The piezo-phase response hysteresis loop and piezo-amplitude response butterfly curve are observed by switching spectroscopy mode of piezoelectric force microscope(SS-PFM). The strain is about 0.12% and the piezoelectric coefficient d33 calculated from piezo-amplitude response butterfly curve is distributed between 19-63 pm/V. |
