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Barium titanate (BaTiO₃), a known ferroelectric material, is of great interest as a future lead-free piezoelectric if appropriately doped to tailor the electric properties for specific applications. This work focuses on the study and rationalisation of the dielectric properties of a series of A-site/B-site co-doped compositions: SrZrO3-BaTiO₃, CaZrO₃-BaTiO₃, LaScO₃-BaTiO₃ and GdScO₃-BaTiO₃. The effect of sintering conditions and microstructure on ceramics is shown to have a significant impact on the physical properties of these materials. Pellet inhomogeneity, air sensitivity in pre-calcined powders and the presence of parasitic grain boundary capacitances are all shown to have adverse effects on properties, including the magnitudes of relative permittivity and TC calculated from total capacitance data. These can be overcome by careful control of synthesis conditions. Dielectric spectroscopy measurements on the optimised materials show that increasing addition of SrZrO₃, CaZrO₃ or LaScO₃ causes the phase transitions between the various polymorphs of BaTiO₃ to coalesce. In each case TC is reduced whilst each of the other phase transitions is shifted to higher temperatures, until the coalescence temperature is reached. When doped with GdScO₃ TC is observed to fall, but so too are the rhombohedral/orthorhombic and orthorhombic/tetragonal transitions, resulting in a stabilisation of the tetragonal polymorphic phase. This is suggested to result from an antipolar displacement of small Gd₃₊ ions, resulting in 8-coordinate ion and stabilisation of the tetragonal polymorph. The addition of dopant species is shown to result in two different high temperature conduction regimes. Both mechanisms are observed within single compositions over different temperature ranges. It is suggested that this is due to a change between n- and p-type electronic conduction processes or mixed ionic/electronic processes. Finally, it is shown that trends observed in changes to TC cannot be accounted for by simple and widely used size-based arguments alone, but requires consideration of cation size variance and charge dilution effects in order to fully understand the impact on TC of dopant addition.

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The development of high pressure-high temperature oil wells in the U.K.'s north sea province has led to the application of high-strength, corrosion resistant alloys for production tubulars. One such alloy is super-duplex stainless steel (SDSS). The cold-worked material combines high strength with excellent corrosion resistance properties as a result of its chromium, molybdenum, tungsten and nitrogen content. The material's macro properties are a function of its two-phase microstructure made up of roughly equal parts austenite and ferrite. Recent in-service failures of SDSS tubulars have identified gaps in the understanding of the material's properties. The study investigated the link between the microstructure of the material and its mechanical and corrosion behaviours. The results revealed that the microstructure of the material was highly anisotropic and varied through the pipe-wall as a result of manufacturing techniques. A method of measuring the crack resistance of the material was developed, with the results showing that the spatial arrangement of the microstrucure determined the crack resistance of the fracture plane. The properties of ferrite were identified as the limiting factor on the crack resistance of the material. The investigation also revealed that a small difference in chemical composition between the two phases led to the preferential dissolution of ferrite in aqueous oxygen bearing chloride environments.

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