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One significant application of donor doped barium titanate (BaTiO3) is in the manufacture of Positive Temperature Coefficient of Resistance (PTCR) thermistors. Combined synthesis, phase diagram and electrical studies were undertaken on donor doped barium titanate with a view to understanding the factors responsible for PTCR phenomena. A range of materials, both commercial PTCR devices and in-house lanthanum-doped barium titanate samples, have been studied. All three commercial PTCR samples measured were found to be electrically inhomogeneous with two PTCR-exhibiting regions and a conductive grain core. The conductive core resistance had a characteristic temperature dependence, with a minimum occurring in the vicinity of the tetragonal to cubic phase transition (Tc) of barium titanate. The phase relations and electrical behaviour of two joins in the lanthanum-doped BaTiO3 system, join A (Ba4-4xLa4xTi4-xO12) and join B (Ba1-yLayTiO3+), were also studied. Compositions on joins A and B for 0x0.195 and 0y0.1 respectively, crystallised as single phase barium titanate. Charge compensation on both joins (at these concentrations) was achieved by a mixture of both titanium vacancies and free electrons. The electron compensation mechanism, Ba_1-yLa_yTi. 4+_1-yTi. 3+O_3, significantly complicates determination of phase relations in this system, since it occurs off the BaO-TiO_2-La_2O_3 ternary phase diagram. Ac impedance measurements indicated that samples on join A were electrically inhomogeneous resulting in the presence of different regions with variable Tx values; furthermore, the phase transitions in each region were themselves complex. The tetragonal to cubic phase transition was studied by a combination of x-ray diffraction, ac impedance and by varying dopant concentration. The resulting behaviour was complex, with both first order and continuous transitions occurring. There was also evidence of a two phase, (i.e. both cubic and tetragonal barium titanate) region associated with a distribution in particle size: small particles (&60 2m) were cubic; larger ones were tetragonal. It is apparent that the combined, complicating effects of grain size and segregation phenomena make it inappropriate to give an explanation for the phase relations and electrical behaviour of lanthanum-doped barium titanate in terms of classical phase equilibria and phase transition theories.

Donor doped barium titanate ceramics are well known for their Positive Temperature Coefficient of Resistance (PTCR) characteristic above the crystallographic transition temperature, T° '130°C, where the material changes from the ferroelectric state to paraelectric. The shape and magnitude of the PTCR characteristic are known to be dependent on the composition and preparation of the ceramic, the presence of impurities, particularly donor dopant concentration and acceptor ions, and the sintering conditions. Thirty years ago Heywang proposed a model based on the presence of two-dimensional resistive grain boundary layers consisting of discrete electron traps located in energy between the conduction and valence bands, to explain the PTCR effect. Donor doped barium titanate samples were prepared in a number of different ways: the variation of donor concentration, the addition of impurity acceptor ions, reduction of the sintering temperature and variation of the sintering atmosphere. These samples were investigated by examining their microstructure and their electric and dielectric properties, both at room temperature and above the transition. Theoretical analysis of the experimental results, based on the Heywang model, was then performed to investigate the effects of preparation on the grain boundary layer characteristics. Resistivity - temperature measurements were carried out to find the effect of composition and sintering conditions on the PTCR characteristic and capacitance - temperature measurements demonstrated the effects of donor and acceptor incorporation on the dielectric properties of barium titanate. Grain boundary and grain bulk resistance were separated by means of a. c. impedance methods at room temperature, where the effects of composition and sintering on each were observed. Finally, current - voltage measurements between TT and the resistivity maximum were made for samples containing different donor concentrations, to examine the current conduction mechanism. Detailed analysis of the electric and dielectric measurements permitted the effects of composition and sintering on the grain boundary layer characteristics to be determined. Acceptor state densities were estimated using the resistivity - temperature measurements and capacitance - temperature results, between TT and the resistivity maximum. Resistivity - temperature measurements above the maximum enabled acceptor energies to be estimated. Analysis of the dielectric properties showed that neither the composition nor sintering atmosphere affected the dielectric properties of the grain boundary layers, which were found to obey the Curie-Weiss law above the transition temperature in the same way as the grain bulk. The observed effects of the changes in the preparative conditions to the electric and dielectric properties were explained in terms of the Heywang model and microstructural development, resulting from modifications to the grain boundary layers. The conduction mechanism was examined by means of current - voltage measurements above the transition temperature and below the resistivity maximum. In contrast to the prediction of Heywang. this was found to be predominantly diffusion limited.

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Ferroelectric materials have been an area of keen interest for researchers due to their useful electro-mechanical coupling properties for a range of modern applications, such as sensing, precision actuation, or energy harvesting. The distribution of the piezoelectric coefficients, which corresponds to the piezoelectric properties, in traditional crystalline ferroelectric materials are determined by their inherent crystalline structure. This restriction limits the tunability of their piezoelectric properties. In the present work, ferroelectric lattice materials capable of a wide range of rationally designed piezoelectric coefficients are achieved through lattice micro-architecture design. The piezoelectric coefficients of several lattice designs are analyzed and predicted using an analytical volume-averaging approach. Finite element models were used to verify the analytical predictions and strong agreement between the two sets of results were found. Select lattice designs were additively manufactured using projection microstereolithography from a PZT-polymer composite and their piezoelectric coefficients experimentally verified and also found to be in agreement with the analytical and numerical predictions. The results show that the use of lattice micro-architecture successfully decouples the dependency of the piezoelectric properties on the material's crystalline structure, giving the user a means to tune the piezoelectric properties of the lattice materials. Real-world application of a ferroelectric lattice structure is demonstrated through application as a multi-directional stress sensor. / Master of Science / Ferroelectric materials have been an area of keen interest for researchers due to their useful electro-mechanical coupling properties for a range of modern applications, such as sensing, precision actuation, or energy harvesting. However, the piezoelectric properties of traditional materials are not easily augmented due to their dependency on material crystalline structure. In the present work, material architecture is investigated as a means for designing new piezoelectric materials with tunable sets of piezoelectric properties. Analytical predictions of the properties are first obtained and then verified using finite element models and experimental data from additively manufactured samples. The results indicate that the piezoelectric properties of a material can in fact be tuned by varying material architecture. Following this, real-world application of a ferroelectric lattice structure is demonstrated through application as a multi-directional stress sensor.

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Magnetic and ferroelectric materials have both had a very important impact in our society, not only because of the fascinating science behind the two phenomena, but also as a result of their use in many technological applications. The coupling and coexistence of these two order parameters within the same material opens the door to exiting new functional devices. Materials where magnetism and ferroelectricity coexist are known as multiferroic materials. In this thesis, muon spectroscopy and other complementary experimental techniques, including neutron scattering and resonant ultrasound spectroscopy, are used to investigate two di↵erent multiferroics. Muon and total neutron scattering studies have been performed on BiFeO3, one of the most studied multiferroic materials. Muon measurements reveal an anomaly in the temperature region of 200 - 220 K with a sudden and abrupt change in the muon’s precession frequency that corresponds to a process of muon di↵usion throughout the entire sample. The pair distribution function, calculated from total neutron scattering experiments on the compound, suggest that a change in the local structure of the material involving the bismuth-oxygen bond, in the same temperature region as the muon di↵usion sets in, is a strong indicative that there is a link between two in terms of the muon di↵usion being triggered by these local changes. Also, an extensive analysis and characterisation of the magnetic and ferroelectric properties of Ba4Dy0.87Nb10O30, an entirely new tetragonal tungsten bronze magnetoelectric material, is given. Neutron scattering and dielectric measurements are used to show that this material becomes ferroelectric below 470 K. We use muon spectroscopy and magnetic susceptibility measurements to investigate the magnetic properties of the material. Muon measurements under an applied electric field indicate that there is a strong coupling between the magnetism and ferroelectricity in the material. Resonant ultrasound spectroscopy is use to investigate whether the source of this coupling could be related to strain e↵ects. Magnetic neutron scattering measurements show that there is no long range ordering in the material.

The relatively recent discovery of the giant electrocaloric effect in ferroelectric ceramics may lead to new solid state cooling technologies that are energy efficient, reliable, portable, and environmentally friendly. This phenomenon, along with many other novel field-coupled properties of ferroelectrics, such as piezoelectricity, pyroelectricity, the electro-optic effect, phase changes, and polarization switching, make these materials useful for a wide range of technological applications including sensors, ultrasound, infrared cameras, sonar, diesel engine fuel injectors, ferroelectric random access memory, electro-optic modulators, vibration control, and electrocaloric cooling devices. Most of world’s current cooling and refrigeration technology is based upon the vapor-compression cycle of a refrigerant. Refrigeration systems that are based on this technology are bulky, require moving parts in the compressor and some of them have a less than optimal environmental impact. Thin film devices that utilize the electrocaloric effect could have a significant impact on refrigeration, heat pumps, air conditioning, energy scavenging, and computer cooling systems. Especially for the latter ones, the fan-based solutions are not likely to be able to keep up with the increases in computing power and the resulting current densities in integrated circuits. The ability to make quantitative predictions of the behavior of ferroelectric structures is of significant importance given the experimental efforts on the synthesis of barium titanate nanodots, nanorods, nanowires, and nanotubes, and lead zirconate titanate (PZT) thin films, and nanoparticles, and the potential for technological applications of these structures. The research contained herein implements a full thermo-electro-mechanical continuum framework and numerical methods based on phase-field modeling to study the domain and phase structure evolution associated with the electrocaloric effect in barium titanate ferroelectric nanodots. / text