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Now showing 1 - 5 of 5
  • (2021) Kong, Scarlet
    Piezoelectric single crystals provide a large electro-mechanical response that is desired for sensor and actuator applications. However, the complexity and cost of single crystal growth inhibits their widespread use. Alternatively, polycrystalline piezoelectrics can be easily fabricated at low cost. Due to the anisotropic piezoelectric response, the electro-mechanical response of polycrystals is largely limited by the elastic coupling of randomly aligned grains. Microstructural engineering via crystallographic texturing of polycrystalline ceramics offers an alternative method that both enhances the electro-mechanical response while maintaining an economical fabrication process. There is currently limited understanding on how the changes in the microstructure from the texturing process, such as the addition of heterogenous templates and changes in grain orientation distribution, impacts the electro-mechanical response mechanisms. Understanding the structural and electro-mechanical mechanism that enhances the piezoelectric response will benefit sensor and actuator applications such as sonar systems and medical imaging instruments. In this thesis, a range of techniques were used to study the microstructural changes and the strain mechanisms from crystallographic texturing, and its impact on the electro-mechanical response. The local response of electro-mechanical phase-change ceramics was investigated by theoretical calculations to see how texturing changed the strain heterogeneity at the grain-scale. Textured ceramics of Pb(Mg1/3Nb2/3)O3− x%PbTiO3 (PMN-PT) were fabricated by tape casting and templated grain growth, using BaTiO3 platelet templates. The structure of these textured ceramics was characterised using synchrotron x-ray diffraction to gain a more comprehensive understanding on the texture that develops. In situ electric-field dependent diffraction measurements were then used to study how texture affects the electro-mechanical response mechanism. And lastly, the piezoelectric non-linearity and response stability over time was analysed in the direct piezoelectric response mode. Changes in electric-field-induced phase transformation local strain response in polycrystalline piezoelectrics of varying grain orientation distribution was studied through theoretical calculations. The March-Dollase function was used to generate texture distributions in a model polycrystal, and strains associated with phase transitions from pseudo-cubic to tetragonal, rhombohedral and orthorhombic symmetries were calculated. In the tetragonal system, the overall strain response was improved by 60% with crystallographic texture, at very strong texture (of March Dollase distribution r = 0.05); and the local strain heterogeneity was similar to the random system. However, moderate increases in texture had a negative impact on the strain heterogeneity. In crystallographic symmetries with higher numbers of possible polarization (and thus spontaneous strain) directions, the magnitude of the strain response increased, and the heterogeneity of the system decreased. Using high energy x-ray diffraction and in situ electric-field dependent measurements, the microstructural modifications due to crystallographic texturing and the resulting electro-mechanical response mechanisms were investigated. Structural analysis of textured ceramics, by calculating the orientation distribution function (ODF), showed that sheet-like texture developed in the piezoelectric. Texture characterisation, by calculating the ODF, was able to provide a more comprehensive quantification of texture than the Lotgering factor, where multiple samples with a 95% Lotgering factor showed different ODFs and maximum Multiples of Random Distribution (MRD) values of 12.8 and 16.54. In addition to the crystallographic texture observed in the diffraction pattern, a ferroelastic texture was also seen. Termed the ‘self-poling effect’, this unique structural distortion reduced the remanent strain of the textured piezoelectric to 0.13% after poling to 2 kV/mm, while the random (untextured) ceramic produced a remanent strain of 0.18% after poling to 2 kV/mm. On a subsequent unipolar strain cycle, however, the textured ceramic achieved a larger response of 0.16% compared to 0.096% of random ceramic. A new strain mechanism model was proposed to explain the relationship between the self-poling effect and the observed strain behaviour. Finally, the direct piezoelectric response in textured ceramics was measured to understand the piezoelectric non-linearity and response stability under application-like conditions. By applying a static offset and dynamic sinusoidal compressive loading, the d33 and d32 response were measured. The piezoelectric non-linear behaviour increased with crystallographic texturing. In particular, the piezoelectric property in the d33 mode, deteriorated faster and was more unstable, losing around 60% of its original d33 value once the offset load was released. On the other hand, the d32 response of texture ceramics was able to recover 90% of its original response after loading. Furthermore, the stability of the piezoelectric response in textured piezoelectric was observed to be compositionally dependent, where the d32 response decayed over time faster in PMN-31PT than in PMN-32PT. The results from this thesis shows that crystallographic texturing alters the local strain environment, affecting the electro-mechanical response mechanisms, both enhancing the response but also potentially degrading the expected lifetime and performance of these ceramics. Further work in understanding the microstructural interaction with the electro-mechanical response will help to optimise texture fabrication of piezoelectric materials for industrial applicability.

  • (2023) Ireland, Jake
    Pluripotent stem cell-derived cardiomyocytes (hPSC–CM) have great importance for predicting safety parameters for pharmaceutical compounds and models of healthy versus disease states of the human heart. In recent years, there has been an insistence that all new pharmaceutical products are tested on in vitro models for potential proarrhythmic effects and the increased demand for improved biomimetic hPSC-CM in pharmaceutical safety assays such as the Comprehensive in vitro Proarrhythmic Assay (CiPA). In addition, hPSC-CM are being utilised in cell therapies to treat and reverse the effects of ischaemic heart disease, offering potential cures for cardiovascular diseases instead of treatments for delaying progressive heart failure. In the first part of this thesis, I will examine how purified extracellular matrix proteins (ECMPs) can influence pluripotent stem cell (PSC) behaviour and how we may use this to precondition cardiac progenitor lineage specifications. I use array-based techniques to investigate how protein combinations affect proliferation, pluripotency, germ layer, and cardiac progenitors. This method allows us to visualise how individual proteins can affect cells' behaviour in a larger array whilst highlighting how specific combinations can precondition pluripotent cells towards a cardiomyocyte lineage. This combinatorial approach led to the identification of several unique matrices that promote differentiation, which will aid efforts at producing therapeutically useful cell types with greater efficiency. In the second part of this thesis, I demonstrate a novel bioreactor that attenuates a magnetic field to dynamically modulate the stiffness of magnetoactive hydrogel to look at how biomimetic dynamic stiffening of a substrate can influence cardiomyocyte lineage specification. We investigate how biomimetic in vivo mechanics may influence cell fate by following the expression profiles of cells in different dynamic environments. Non-invasive electromagnetic signals affect substrate stiffness when combined with magnetic particles and magnetic fibres and how this can help direct cell orientation and accompanying lineage specification Finally, I investigate how variability in cell phenotypes and expression patterns are influenced by biomimetic cues and how these variabilities could be utilised in future safety assessment protocols and cell therapy treatments for cardiovascular disease.

  • (2022) Paull, Oliver
    This thesis represents an effort to understand the structure of anisotropically strained Bismuth Ferrite (BiFeO3 BFO). This is executed by using anisotropic epitaxy and exploring the structure, magnetism, and electromechanical response in anisotropically strained BFO at various levels of average in-plane strain. This includes in the vicinity of the strain-induced morphotropic phase boundary where large enhancements to the electromechanical performance are identified. Bismuth ferrite (BFO) is a room-temperature magnetoelectric material that is able to easily adapt its crystal structure to accomodate any strain that is applied to it. By utilising high-index crystallographic substrates the effect anisotropic epitaxial strain has been explored using three different substrate materials (SrTiO3, (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT), and LaAlO3) each with four orientations. The unit cell parameters of the BFO films behave linearly when weakly compressively strained on SrTiO3, and become more non-linear on LSAT. The strain-driven morphotropic phase boundary in BFO films grown on tilted LaAlO3(310) surfaces is able to stabilise a low-symmetry bridging phase between the well known M_A and M_C symmetries of BFO when deposited on SrTiO3 and LaAlO3 respectively. The anisotropic strain conditions of the substrate miscut force the BFO film to maintain strain along a high-symmetry in-plane direction whilst partially relaxing in the orthogonal low-symmetry in-plane direction. Interferometric displacement sensor (IDS) measurements indicate that the intrinsic piezoresponse of this new phase of BFO is double that of the R'-like version. Moreover we see spectroscopic indications through IDS and band-excitation frequency response measurements that there is a field-induced phase transition occurring under electric field wherein the low-symmetry phase is reversibly interconverted into the tetragonal-like phase creating a giant effective electromechanical response. These observations are fully supported by density functional theory and effective Hamiltonian calculations. When growing thicker films of this soft low-symmetry phase, a rich and detailed phase coexistence between the R', T', and bridging phase arise that is reminiscent of a highly tilted mixed-phase BFO. The topography of these samples also exhibit domain-like periodic stripes that evolve with the crystallography and are intimately linked together. \\ At the end of this thesis a number of neutron scattering experiments are presented on BFO films on YAlO3, LaAlO3, LSAT, and SrTiO3 substrates. Despite calculations and some experimental hints of a C-type antiferromagnetic phase in T'-BFO, there appears to be no evidence of this magnetic phase in BFO//YAO and BFO//LAO. Additionally, a cycloid model has been developed and implemented in order to fit ambiguous cycloidal peaks with a constrained model. This model is applied to two different systems of BFO with the results and interpretations discussed.

  • (2023) Zaman, Tasmia
    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.

  • (2023) Wen, Haotian
    Luminescent nanoparticles have shown wide applications ranging from lighting, display, sensors, and biomedical diagnostics and imaging. Among these, fluorescent nanodiamonds (FNDs) containing nitrogen-vacancy (NV) color centers are posed as emerging materials particularly in biomedical and biological imaging applications due to their room-temperature emission, excellent photo- and chemical- stability, high bio-compatibility, and versatile functionalization potentials. The shape variation of nanoparticles has a decisive influence on their fluorescence. However, current relative studies are limited by the lack of reliable statistical analysis of nanoparticle shape and the difficulty of achieving a precise correlation between shape/structure and optical measurements of large numbers of individual nanoparticles. Therefore, new methods are urgently needed to overcome these challenges to assist in nanoparticle synthesis control and fluorescence performance optimization. In this thesis a new correlative TEM and photoluminescence (PL) microscopy (TEMPL) method has been developed that combines the measurements of the optical properties and the materials structure at the exact same particle and sample area, so that accurate correlation can be established to statistically study the FND morphology/structure and PL properties, at the single nanoparticle level. Moreover, machine learning based methods have been developed for categorizing the 2D and 3D shapes of a large number of nanoparticles generated in TEMPL method. This ML-assisted TEMPL method has been applied to understand the PL correlation with the size and shape of FNDs at the single particle level. In this thesis, a strong correlation between particle morphology and NV fluorescence in FND particles has been revealed: thin, flake-like particles produce enhanced fluorescence. The robustness of this trend is proven in FND with different surface oxidation treatments. This finding offers guidance for fluorescence-optimized sensing applications of FND, by controlling the shape of the particles in fabrication. Overall the TEMPL methodology developed in the thesis provides a versatile and general way to study the shape and fluorescence relationship of various nanoparticles and opens up the possibility of correlation methods between other characterisation techniques.