Extending the investigations on Bi-based perovskite solid solutions for high-temperature piezoelectric ceramics, this paper considers the binary solid-solution system (1−x)Bi(Ni1∕2Ti1∕2)O3–xPbTiO3 [(1−x)BNT–xPT] for 0.39⩽x⩽1.00. High-density polycrystalline ceramics were fabricated using conventional solid-state processing methods. These ceramics are then taken for structural and electrical properties and differential scanning calorimetry measurements. A morphotropic phase boundary (MPB) was found at the composition 0.51BNT–0.49PT, with a corresponding paraelectric-ferroelectric phase transition TC≈400°C. The electrical poled ceramics demonstrated piezoelectric d33 coefficients≈260pC∕N at room temperature for the MPB BNT–PT compositions. Experimental data are also given for the influence of MnO2 doping to the BNT–PT system close to the MPB compositions, noting an improvement in dielectric losses but a reduction in d33≈180pC∕N. Tricritical behavior is also identified in the tetragonal phase field, with a TC enhancement above TC∼495°C found similar to other Bi(Me′Me″)O3–PbTiO3 systems with high MPB transitions.
The ferroelectric transition temperature (Tc) behavior of perovskite solid solutions based on PbTiO3–Bi(Me′Me″)O3 (Me′=Fe3+, Zn2+, Sc3+, In3+, Mg2+, Ni2+, etc., and Me″=Ti4+, Nb5+, W6+) was considered. Trends in the Tc compositional dependence near the PbTiO3 end member could be described with a geometrical polynomial expression. Three main cases were observed: Case 1, a continued increase in transition temperature above the end member PbTiO3 (495°C); case 2, an increase and then decrease of the transition temperature; and case 3, a continuous decrease in the transition temperature with Bi(Me′Me″)O3 additions. It was noted that for all case 2 examples the enhancement of ΔTc=Tc(max)−Tc(PT) increased as the distribution of B-site ionic radii increased. A correlation was therefore proposed between the maximum enhancement in transition temperature and the spread of tolerance factor (Δt) and∕or variance in B-site ionic radius (σ2). Finally, it was proposed that these observations are consistent with random-field effects created by local strain-field fluctuations within the perovskite lattice.
The recent discovery of high-temperature piezoelectric ceramics based on Bi(Me′Me″)O3-PbTiO3 solutions have also permitted the development of high-temperature relaxor ferroelectrics with ternary solid solutions. One of the high-temperature compositions based on the xBiScO3-yPb(Mg1∕3Nb2∕3)O3-zPbTiO3 (xBS-yPMT-zPT) ternary system exhibited a high permittivity maxima of ∼17 000 and a Tmax of 500–550 K. These materials are compared to complex lead perovskite relaxor ferroelectrics by determination of the activation energy, EA, and freezing temperature, Tf, from the Vögel-Fulcher relationship and also a high-temperature deviation temperature, TD, from the Curie-Weiss behavior. It was found that these parameters scale within the perovskite relaxor systems and from the self-consistent trends of these defining parameters. It is suggested that a general comparison for relaxor ferroelectrics may exist. The highest EA, Tf, and TD values all exist within the xBS-yPMT-zPT ternary system within the relaxor ferroelectric systems compared in this study. Furthermore, given the compositional trends within these comparisons, insights into the nature of the dynamic polarization behavior in the perovskite relaxor ferroelectrics may be gained.
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