SOLID SOLUBILITY MECHANISMS, DEFECT CHEMISTRY, PHASE EQUILIBRIA, AND ELECTROMECHANICAL PERFORMANCE OF TIN-DOPED BCTZ

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Copyright: Zaman, Tasmia
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Abstract
Piezoelectrics are used widely in the microelectronics sectors as sensors, actuators, transducers, capacitors, tunable microwave devices, electrocaloric coolers, etc. The presence of lead in these materials represents an environmental driving force to develop lead-free alternatives. In the present work, the effects of Sn4+ on the solid solubility mechanisms, defect chemistry, phase equilibria, piezoelectric properties, and electrocaloric and energy-storage performances of lead-free (Ba0.85Ca0.15)([Ti0.92 xSnx]Zr0.08)O3 (x = 0.00 0.10) have been examined. The samples were synthesized by solid-state sintering at 1500°C for 12 h in air. The materials were characterized in terms of RT XRD, HT XRD, DSC/TGA, SEM/EDS, XPS, pycnometry, firing shrinkage, Raman, PL, dielectric measurements, and piezoelectric measurements. Despite the widespread assumption of substantial solid solubility, with Sn4+ as a neutral dopant, integration of X-ray photoelectron spectroscopy data and defect equilibria showed that interstitial solid solubility occurred at all Sn4+ doping levels for the three polymorphs obtained, which were orthorhombic Pmm2, tetragonal P4mm, and cubic Pm-3m; simultaneous substitutional solid solubility in tetragonal P4mm was deduced. It also was possible to distinguish the inferred solute distributions as ordered or disordered. The fundamental materials data were interpreted in terms of three solid solubility mechanisms, which were clarified further by the elucidation of four property and performance regions as a function of Sn4+ doping level. The former were separated by two stress-induced phase transformations at x = 0.04 and x = 0.08, which resulted in inflections in the sigmoidal trends in properties and performance as a function of Sn4+ doping level, which were chemical-induced. These trends were directly attributable to the dominant solid solubility mechanism, thus highlighting the importance of knowledge of this factor. While these trends were exemplified by fundamental materials characterization parameters, a higher level of understanding was revealed through the electromechanical characterization, which allowed inference of the order vs disorder of the solid solutions. Integration of these two types of data enabled a comprehensive understanding of the chemical and structural features of these materials and their critical roles in properties and performance. This synthesis also enabled the preparation of a new morphotropic phase diagram, a behavioral phase diagram illustrating the effects of temperature and Sn4+ dopant level on the phase stabilities and invariant points, and a revised BaO-TiO2 equilibrium phase diagram. The present work highlights the importance of the solid solubility mechanism to materials selection, processing, properties, performance, and interpretation of the relevant mechanisms.
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Publication Year
2023
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Thesis
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PhD Doctorate
UNSW Faculty
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