In the present work, we study the effect of quenching and annealing on the ferroelectric and piezoelectric properties at room temperature and elevated temperatures of a new ternary BiFeO₃-PbTiO₃-Li_0.5Bi_0.5TiO₃ bulk piezo ceramic. While sacrificing part of the maximally obtainable piezoelectric constant value, using an optimal heat treatment, a quasi-stable value for the piezoelectric constant of 65 pC/N was obtained irrespective of the annealing temperature. All experimental results point to the direction of unusual defect behavior in this novel ternary system leading to a well-defined metastable state. The quenching and annealing process are completely reversible and can be used in combination with additional chemical modifications to tailor the properties of this new high-temperature piezoelectric ceramic to the intended use conditions.

@en

The main objective of the research as described in this thesis was to enhance the piezo- and electrical properties of bismuth ferrite-based (BiFeO3 or BFO) piezoceramics at room and elevated temperatures, using both chemical and physical approaches.@en

In this work, we investigated the piezoelectric properties of BiFeO3-rich (1 − (y + x)) BiFeO3–y PbTiO3–x SrTiO3 (0.1 ≤ x ≤ 0.35; 0.1 ≤ y ≤ 0.3) bulk piezoceramics, as this system could potentially lead to the development of bulk piezoelectric ceramics that are suitable for high-temperature applications (>200 °C). Samples with various levels of PbTiO3 and SrTiO3 were prepared via a conventional solid-state route. X-ray diffraction confirmed a pure perovskite phase for the compositions, which was explored without secondary phases. It was found that the addition of comparable levels of PbTiO3 and SrTiO3 to the BiFeO3 ceramic resulted in higher piezoelectric properties compared to those of the pure BiFeO3 and binary systems. The Curie temperature was significantly reduced by dual doping, with SrTiO3 and PbTiO3 additions resulting in comparable Curie temperature depressions. The locations of the phase boundaries between the cubic, pseudocubic, and rhombohedral crystal structures were determined. The highest piezoelectric properties, including a d33 value of 250 pC/N at room temperature, were obtained for the samples with the composition x = 0.3, y = 0.25, which was close to the cubic–pseudocubic phase boundary in the phase diagram. The temperature dependence of the piezoelectric properties varied depending on the previous thermal history, yet an appropriate heat treatment resulted in an almost temperature-stable d33 value. The ceramic with the lowest temperature sensitivity and a high Curie temperature of 350 °C was found for x = 0.1, y = 0.2 with a d33 value of 60 pC/N at RT and 71 pC/N at 300 °C (after poling at 60 kV/cm and a stabilizing heat treatment). However, the materials developed were still unsuitable for applications at high temperatures due to a rapidly increasing electrical conductivity with increasing temperature.
@en

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%.

@en

BiFeO ₃ is a multiferroic material with a perovskite structure that has a lot of potential for use in sensors and transducers. However, obtaining pure single-phase BiFeO ₃ ceramic with a low electrical conductivity via solid-state reactions remains a problem that limits its application. In this work, the suppression of secondary phases in BiFeO ₃ was studied by varying the compositional parameters and the sintering temperature. The addition of 1% Bi ₂O ₃ to the stoichiometric precursor mixture prevented the formation of secondary phases observed when sintering stoichiometric precursors. The pure phase ceramic had a p-type conductivity and a three-decade lower electrical conductivity as measured by impedance spectroscopy. Annealing of optimally synthesized material at different partial pressures of oxygen in an oxygen–nitrogen gas atmosphere showed that the reason for this type of conductivity lies in the high concentration of defects associated with oxygen. By annealing in various mixtures of nitrogen and oxygen, it is possible to control the concentration of these defects and hence the conductivity, which can go down another two decades. At a pO ₂  (Formula presented.) the conductivity is determined by intrinsic charge carriers in the material itself.

@en

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.

@en

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.

@en