Development and electro-acoustic properties of BNKLT-based piezoelectric ceramic
Creators: name: Taghaddos, Elaheh
Publisher: name: No Publisher Supplied
Year: 2019

description: The functionality of piezoelectric materials to interconverting the mechanical and electrical energy leads to a huge market of piezoelectric materials in a wide range of applications from daily use, such as a speaker, musical instrument, humidifier to more specialized applications in military, medical and automotive industries. The market of piezoelectric is dominated by Pb(ZrxTi1-x)O3 compositions with outstanding electromechanical properties. However, it is essential to replace lead-containing piezoelectric materials with non-toxic alternatives. Bismuth sodium titanate and its solid solution are a promising lead-free candidate for transducer and actuators applications. The ternary solid solution of 0.88BNT-0.08BKT-0.04BLT (BNKLT88) exhibits high coercive field (Ec ~ 47 kV.cm-1), mechanical quality factor (Qm ~ 420), depolarization temperature (Td ~ 220 °C), and low dielectric loss (tan ~ 1.2%). These desired high power properties make BNT-based composition a potential lead-free candidate to replace for hard commercial PZT. In this dissertation, an attempt has been made to study the effect of B-site Mn and Fe acceptor dopants on the mechanical quality factor of BNKLT88 ceramics and improve the high power performance. The samples were processed by conventional and flash sintering methods. The effect of sintering parameters on the crystal structure, piezoelectric, dielectric and electrical properties have been comprehensively evaluated. The electromechanical properties were measured by the IEEE-Standard procedures. The impedance spectroscopy technique was used to characterize the electrical conductivity and impedance relaxation of the specimen. The optimum electromechanical properties were obtained at 1.5 mol% of Mn or Fe-doped composition processed by the conventional sintering method. The functionality of the (BiNa0.88 K0.08Li0.04)0.5Ti0.985Mn0.015O3 ceramic for high power application has been evaluated by prototyping of low frequency transducer. In the conventional sintering method, the effect of both sintering temperature and the particle size on the electromechanical properties showed improved mechanical quality factor up to 1200 in calcined powder with a median particle size of 535 nm. The evaluation of the mechanical quality factor as a function of vibration velocity for high power application showed lower heat generation resulting in stable Qm at higher vibration velocity of 0.6 m.s-1 and reduction of input power by 70-75%. X-ray photoelectron microscopy study of the specimen indicated coexistence of Mn 2+/Mn+3 or Fe2+/Fe+3 in Mn or Fe-doped ceramics. In flash sintering (FS), the optimum sintering parameters to fully densify BNKL88-1.5Mn composition were achieved at 1 KHz alternating current, 100 V·cm-1 initial electric field and the preset maximum current limit of 1.5 A·cm-2. The uniform distribution of elements was observed via energy-dispersive spectroscopy. Ceramics with finer grain size of 10-15 µm, similar electromechanical properties, more symmetric butterfly shape strain, and lower resistivity have been achieved by the FS method at a preset furnace temperature of 880 °C. The grain boundary contribution in the conduction mechanism was more pronounced in FS specimen.Two unfocused single-element transducers with the same center frequency of 3.5 MHz were designed and fabricated based on BNKLT88-1.5 Mn and commercial PZT. The pulse-echo response showed that -6dB bandwidth of 20% and 18% for BNT-based and PZT transducer, respectively. The acoustic pressure of BNT-based and PZT-based transducers was linear up to 105 V and 70 V peak to peak voltage with maximum rarefaction acoustic pressure of 1.1 MPa and 1.01 MPa, respectively. Spatial peak pulse average intensity, spatial peak temporal average intensity, and output power were characterized. The results confirmed that the properties of BNT-based transducer are comparable with lead-based PZT counterpart. The electrical conductivity and impedance relaxation of BNKLT-based ceramics were systematically explored in the temperature range from 450 to 600 °C via impedance spectroscopy technique. The bulk and grain boundary characteristics were extracted from simulating the impedance spectrum with equivalent circuits. Interestingly, even though Mn and Fe had similar oxidation state with similar dielectric and piezoelectric properties, their electrical behavior was completely different. Considerably higher resistivity in the bulk, a modest increase in the grain boundary activation energy and a decrease in the bulk activation energy were caused by Mn-doping. On the other side, Fe doping resulted in remarkably higher conductivity, lowering the bulk activation energy, and a modest increase in grain boundary activation energy. The conductivity as high as 0.01 S.cm-1 was achieved in Fe-doped piezoceramic at 600 °C.