Bismuth ferrite is a potentially interesting lead-free piezoelectric material for use in high-temperature applications due to its high Curie temperature. However, the high coercive field and high leakage currents of pure BiFeO3 (BFO) prevent reaching its theoretical performance level. The classic approach to tailoring piezoceramic properties to their desired use conditions is the use of doping. In this work, we produce bulk BFO piezoceramic by the conventional sintering method with single element doping with cobalt (0.125-3 at. %) or titanium (1-5 at. %) and dual doping (Co and Ti added simultaneously). Cobalt doping reduces the required field for poling and also increases the leakage currents. Titanium doping reduces the leakage currents but destroys the piezoelectric properties as the coercive field strength cannot be reached. However, when both elements are used simultaneously at their appropriate levels (0.25 at. % each), a piezoelectric ceramic material is obtained, requiring a low field for full poling (9 kV/mm) and showing excellent room temperature performance such as a d33 = 40 pC/N, a dielectric constant in the region of 100 and dielectric losses less than 1%.

In this work, we present the impact of using Na₂CO₃ as a sintering aid and grain growth agent on the crystal structure, microstructure, and piezoelectric properties of (Bi_0.5Na_0.5)TiO₃ ceramics. The addition of Na₂CO₃ leads to a substantial increase in the grain size and density even at a reduced sintering temperature of 1025 °C. However, at the same time, the value of the piezoelectric constant d₃₃ drops dramatically. Using high-resolution x-ray diffraction analysis, we demonstrate that the decrease in piezoelectric constant is due to a change in the chemical composition of the (Bi_0.5Na_0.5)TiO₃ base material rather than due to the change in the grain size. High Na₂CO₃-addition levels lead to the formation of Bi₂O₃ as a secondary phase during sintering too.

BiFeO₃ is an interesting multiferroic material with potential use in sensors and transducers. However, the high coercive field and low dielectric strength of this material make the poling process extremely difficult. Poling becomes a lot easier if the ceramic particles are incorporated in a non-conductive polymer with comparable dielectric properties. In this work, unstructured composites consisting of BiFeO₃ particles in a non-piezoactive PVDF terpolymer matrix are made with a ceramic volume fraction ranging from 20% to 60%. The highest piezoelectric charge and voltage constant values (d₃₃ = 31 pC/N and g₃₃ = 47 mV m/N) are obtained for a BiFeO₃-PVDF terpolymer composite with a volume fraction of 60%. The Poon model is chosen to analyse the volume fraction dependence of the dielectric constant while the modified Yamada model is used to analyse the piezoelectric charge constant data. It is concluded that the maximum possible piezoelectric constant for bulk BiFeO₃ can be as high as 56 pC/N.

A new concept of the formation of charge transfer (CT) complexes between an intrinsically electron-donating conjugated microporous polymer and a small molecule acceptor is reported. Spirobifluorene-based mesoporous organic polymers with high porosity and Brunauer–Emmett–Teller surface area are synthesized by the Suzuki-coupling reaction of spirobifluorene and pyrene monomers. The simple doping of the synthesized mesoporous, electron-rich, conjugated polymer with 7,7,8,8-tetracyanoquinodimethane as an acceptor leads to efficient CT complexation in the electron-donating mesoporous spaces. This results in a high-speed synthesis (within 5 s), thermally stable compound (up to about 200 °C), and good control of the concentration of donor–acceptor pairs in the CT complex.

Polymer-piezoceramic composites have drawn a lot of attention for sensor and energy harvesting applications. Poling such materials can be difficult due to the electric field getting mostly distributed over the low dielectric constant matrix. During this process, the electrical matrix conductivity plays a vital role. This work shows how two different polymer materials, loaded with various piezoelectric ceramic fillers, have very different poling efficiencies simply due to their intrinsic matrix conductivity. It is shown how temperature increases the matrix conductivity, and hence, increases the piezoelectric charge constant of the composites. By choosing the proper matrix material under the proper conditions, piezoelectric composites can be poled at electric fields as low as 2 kV mm^-1, which is identical to that of bulk ceramic fillers. In addition, the matrix conductivity can be altered by aging the composites in a high humidity atmosphere, which can increase the piezoelectric charge constant in similar fashion. This is a simple method to increase the matrix conductivity, and hence the piezoelectric charge constant, without the need to add any conductive fillers into the composites, which increase complexity, and leads to an increased dielectric losses.

BiFeO₃ is a multiferroic material with the perovskite structure which is promising for use in sensors and transducers. Single phase production of BiFeO₃ remains a challenge, however. In this study, the optimal calcination temperature to obtain close to single phase powder was determined to be 750°C. The sintering temperature of 775°C was also found to obtain high density ceramics (≈ 95 % of theoretical density). It is shown that off-stoichiometry of bismuth oxide in precursors effects the content of secondary phase. Impedance spectroscopy indicates that the content of secondary phases has a large effect on the electrical conductivity BiFeO₃.

While most of the work on piezoelectric composites focuses on methods to reduce the dielectric constant of the composite (for better sensor and energy harvesting performance), for haptic feedback and actuator applications the opposite is desirable. We present here a study of the effect of adding a second ceramic phase (BaTiO3 nanoparticles) to composites of (K_0.5N_0.5)_1-xLi_xNbO₃ (x = 0.03) in PVDF-TrFE-CFE in order to increase the dielectric constant of the composite. Adding small amounts of these nanoparticles to the composites results in an increase in the dielectric constant and, at high total ceramic loadings, an increase in the density of the composite. Furthermore, while adding larger amounts of nanoparticles leads to agglomeration and reduced densities, it also allows access to higher loadings of ceramic than normally attainable.