Lead-free piezoceramics with the composition (1Àx)(K 1Ày Na y )NbO 3 -x(Bi 1/2 Na 1/2 ) ZrO 3 (KNyN-xBNZ) were prepared using a conventional solid-state route. X-ray diffraction, Raman spectroscopy, and dielectric measurements as a function of temperature indicated the coexistence of rhombohedral (R) and tetragonal (T) phase, typical of a morphotropic phase boundary (MPB) as the BNZ concentration increased and by adjusting the K/Na ratio. High remnant polarization (P r =24 lC/cm 2 ), piezoelectric coefficient (d 33 =320 pC/N), effective piezocoefficient ({d_{33}^*}=420 pm/V), coupling coefficient (k p =48%), and high strain (S=0.168%) were obtained at room temperature, but significant deterioration of P r , {d_{33}^*}, and k p were observed by increasing from room temperature to 160°C (17.5 lC/cm 2 , 338 pm/V, and 32%, respectively) associated with a transition to a purely T phase. Despite these compositions showing promise for roomtemperature applications, the deterioration in properties as a function of increasing temperature poses challenges for device design and remains to be resolved.
Lead-free ceramics with the composition 0.91K1/2Bi1/2TiO3–0.09(0.82BiFeO3-0.15NdFeO3-0.03Nd2/3TiO3) were prepared using a conventional solid state, mixed oxide route. The ceramics exhibited a high strain of 0.16% at 6 kV mm−1, stable from room temperature to 175 °C, with a variation of <10%. The materials were fabricated into multilayer structures by co-firing with Pt internal electrodes. The prototype multilayer actuator exhibited constant strains up to 300 °C with a variation of ∼15%. The composition showed fatigue resistant behaviour in both monolithic and multilayer form after bipolar loading of 106 cycles.
Potassium sodium niobate (KNN) is a potential candidate to replace lead zirconate titanate in sensor and actuator applications but there are many fundamental science and materials processing issues to be understood before it can be used commercially, including the influence of composition and processing atmosphere on the conduction mechanisms and functional properties. Consequently, KNN pellets with different K/Na ratios were sintered to 95% relative density in air and N2 using a conventional mixed oxide route. Oxygen vacancies (VO ) played a major role in the semi-conduction mechanism in low p(O2) for all compositions. Impedance spectroscopy and thermo-power data confirmed KNN to be n-type in low p(O2) in contradiction to previous reports of p-type behaviour. The best piezoelectric properties were observed for airrather than N2-sintered samples with d33=125 pC/N and kp=0.38 obtained for K0.51Na0.49NbO3.
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