A new compositional family of relaxor ferroelectrics was investigated based on the high-temperature Bi(Me)O-3-PbTiO3 ferroelectric perovskite family. Compositions were fabricated near an estimated morphotropic phase boundary (MPB) of the xBiScO(3) -yPb(Mg1/3Nb2/3)O-3-zPbTiO(3) (xBS-yPMN-zPT) ternary system exhibiting high-temperature relaxor properties of T-max similar to 250 degrees-350 degrees C and epsilon(max) similar to 10000-24000 at 1 kHz. Analysis of the low-field a.c. permittivity by a Vogel-Fulcher type dependence enabled key parameters of activation energy, EA, and freezing temperature, T-f, to be determined. The remanent polarization was studied over a broad temperature range and was observed to show classical ferroelectric square loop hysteresis behavior at temperatures T < T-f while slim loop hysteresis behavior was observed at temperatures T-f < T < T (max). Pyroelectric current measurements were made and integrated to determine macroscopic polarization-temperature dependence, and were in excellent agreement with the hysteresis data. The macroscopic polarization was found to drop off rapidly near the freezing temperature, T-f. At temperatures T > T-max the deviation temperature, T-D, was obtained from Curie-Weiss analysis and found to be similar to 600 degrees C. A comparison of characteristic electrical properties was made between the high-temperature perovskite relaxors and the classical complex lead relaxor compound, Pb(Mg1/3Nb2/3)O-3 (PMN).

Through analysis of low-field AC susceptibility data in relaxor ferroelectric and magnetic spin glass solids, the key parameters of activation energy, EA, and freezing temperature, Tf, were extracted from the Vögel–Fulcher dependence of the maxima. At higher temperatures the deviation temperature, TD, was obtained from departure of the high temperature Curie–Weiss behavior. Collectively, these parameters were found to scale across different compounds and solid solutions for both the magnetic spin glasses and relaxor ferroelectrics.

Compositions near the morphotropic phase boundary (MPB) of the BiScO 3 –Pb(Mg 1/3 Nb 2/3 )O 3 –PbTiO 3 ternary system showed high‐temperature relaxor properties (1 kHz) of T max ∼250°C and a permittivity maximum of ∼16 000. Transmission electron microscopy (TEM) was used to investigate the nature of the microstructure under ambient conditions and with in situ heating studies for samples with different composition and electric field‐temperature histories. A mottled domain structure was observed with diffraction contrast TEM techniques and associated with frozen in polar micro‐domains. These observations are consistent with the low field permittivity temperature measurements made under various frequencies (100 Hz–100 kHz) with a freezing temperature, T f ∼160°C. Field‐cooled (FC) samples exhibited a macro‐domain structure similar to normal ferroelectric behavior. On heating the FC samples to approximately the T max and above, the domain contrast was no longer apparent. When subsequently cooled to room temperature conditions, a micro‐domain structure was observed, similar to the zero FC samples. The results are discussed with respect to permittivity measurements and phenomenological mechanisms contributing to the dispersion in the permittivity below the Curie maximum.

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.

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 dielectric and piezoelectric properties for bismuth-based perovskite system (1−x)Bi(Ni2∕3Nb1∕3)O3–xPbTiO3 (BNN–PT100x) with x=0.6–0.95 were investigated. High-density polycrystalline ceramics were fabricated using a conventional solid-state reaction method. Morphotropic phase boundary (MPB) at x=0.625–0.65 composition was observed by x-ray-diffraction measurements, separating rhombohedral and tetragonal phases. Analogous to Pb(ZrTi)O3 ceramic, enhanced dielectric and piezoelectric activities were associated with the composition near the MPB. The dielectric constant (K33T) and piezoelectric coefficient (d33) for BNN–PT65 composition were found to be 1100 and 140pC∕N, respectively, with a Curie temperature (Tc) around 273 °C. The addition of manganese (Mn) resulted in lowering the dielectric loss and increasing the mechanical quality factor Q when compared with the pure counterpart. The coercive field was found to increase to 30.7kV∕cm when BNN-PT65 was modified with magnesium (Mg). These results demonstrate the BNN–PT system to be a competitive piezoelectric material with a reduced lead content.

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 industrial and scientific communities have expressed the need for sensing and actuation over a broad temperature range. This review presents high temperature piezoelectric materials that are commercially available and those that are under development. Key materials, in order of increasing Curie temperature (T/sub C/), are Pb(Zr,TiO)/sub 3/ (PZT), PbTiO/sub 3/, (Pb,Ba)Nb/sub 2/O/sub 6/, Na/sub 0.5/Bi/sub 4.5/Ti/sub 4/O/sub 15/, and LiNbO/sub 3/. The maximum operation temperature is limited by T/sub C/ and dielectric loss combined with the level of electrical resistivity. With increased T/sub C/ also comes the expense of reduced piezoelectric coefficient (d), being further reduced in non-morphotropic phase boundary (MPB) systems. Recently new high T/sub C/ systems with MPBs analogous to PZT have been developed. Predicted by a perovskite crystal structure tolerance factor relationship, compositions based on Bi(Me)O/sub 3/-PbTiO/sub 3/, where Me=Sc/sup +3/,(Mg/sup +2/,Ti/sup +4/), etc., exhibit piezoelectric activity compared to PZT, with T/sub C/s greater than 100/spl deg/C higher, making them promising candidates for high temperature applications.

Building on the ferroelectric family based on the Bi(Me+3)O3–PbTiO3 solid solutions, the complex solid solution (1−x)Bi(Mg3∕4W1∕4)O3–xPbTiO3 [(1−x)BMW–xPT] was investigated. This system was found to exhibit a broad morphotropic phase boundary at x∼0.48mol% PbTiO3 with a corresponding Curie temperature of 205°C separating pseudocubic and tetragonal ferroelectric phases. Based on dielectric, x-ray diffraction (XRD), and calorimetric data, a simple dielectric phase field diagram was established. On further structural analysis with diffraction contrast transmission electron microscopy along with XRD, evidence of B-site chemical ordering was found for the (1−x)Bi(Me′Me″)O3–xPbTiO3 perovskite family.