Structure and Property Evolution Induced by the Phase Transitions in Several Antiferroelectric Materials

Antiferroelectric (AFE) materials are an important group of functional materials showing unique properties such as double polarization-electric field (P-E) hysteresis loop, and charge release under the pressure and temperature. These performances are strongly connected to the structural phase transitions induced by external conditions. Many investigations were carried out to optimize their properties but the structure-property relationship of such AFE materials still remain unclear. With this bearing in mind, in this thesis, I firstly investigate the crystal structure, domain structure and properties evolution under the external stimuli such as electric-field (E-field), mechanical force, and temperature in the typical PbZrO3-based AFE materials. Secondly, a systematic study was conducted on the doped silver niobate ceramics to understand the impacts of the chemical composition and E-field cycling on these novel lead-free AFE materials. The targeted materials of typical PbZrO3-based samples were selected as La/Nb doped Pb(Zr, Sn, Ti)O3 ternary systems with the composition Pb0.97La0.02(Zr0.56Sn0.33Ti0.11)O3 (PLZST1), Pb0.99(Nb0.02Zr0.73Sn0.21Ti0.04)O3 (PNZST1) and Pb0.99(Nb0.02Zr0.65Sn0.28Ti0.05)O3 (PNZST2). These three compositions are representative, supplying diverse phase transition behaviours for studying. The in situ neutron powder diffraction (NPD) of the PLZST1 material reveals that the pseudo-tetragonal AFE phase is transferred into the rhombohedral FE phase with an application of the sufficient E-field, and recovers after withdrawal of the external field. The resultant average structure change as a function of the E-field is in accordance with the reversible AFE-FE phase transition. However, the ω dependent NPD patterns suggest this process is not fully reversible: in the induced FE state, the strain exhibits an elliptical distribution, which in turn leads to significant preferred orientation in the final AFE state. The formation of this preferred orientation provides an explanation for the properties variation appearing in AFE materials after exposure to the sufficiently high E-field. X-ray diffraction pattern of PNZST1 sample indicates the orthorhombic AFE phase while the result of NPD contradicts this conclusion with a rhombohedral FE phase. After careful characterization of the surface and bulk properties, it is found that the near surface and bulk regions show different phases. Additionally, the surface processing such as polishing and heat-treatment can induce an AFE/FE phase transition within micrometres of the surface. The in-situ hydrostatic-pressure neutron diffraction proves that the mechanical force helps stabilize the AFE phase of this composition. Therefore, the surface processing induced phase transitions can be attributed to the change of states of residual stress. The in-situ NPD studies of PNZST2 material describe its structural variation as a function of E-field and temperature. Through the mode decomposition approach, the relationships between AFE/FE modes and octahedral rotation mode were systematically investigated. At room temperature, the pristine AFE phase can be poled into the meta-stable FE phase by applying the external E-field. At this stage, both AFE and FE phases consist of modes associated with octahedral rotation and A-site ionic displacements. The temperature-induced phase transition indicates that the octahedral rotation and ionic displacements are weakly coupled in the room-temperature FE phase and decoupled in the high-temperature FE phase. Furthermore, both temperature and E-field-induced phase transitions between the AFE and high-temperature FE phase demonstrate the critical role of coupling between the octahedral rotation and A-site ionic displacements in AFE structure stabilization. The evolution of structure and electrical properties with composition in (1-x)AgNbO3-xLiTaO3 (ANLT100x) (0 ≤ x ≤ 0.09) ceramics have been systematically investigated by diffraction techniques, complemented by dielectric and polarisation measurements. The symmetry mode decomposition and Rietveld refinement of distortive modes were firstly used to analyse the origin of the anti/ferroelectricity observed. The in/out phase octahedral tilting around the a-axis (H2 mode) and the antiparallel ionic displacements (Λ3 mode), present large amplitudes in the pure AgNbO3. These two modes vanish progressively with increasing x and their amplitudes experience a sudden drop when x = 0.053. Accompanied by the disappearance of these two modes, a new phase with R3c symmetry appears and grows with further increasing LiTaO3 content. The composition dependent amplitudes of the primary modes, and R3c phase fractions, lead to a comprehensive understanding of the dielectric and ferroelectric properties affected by LiTaO3. For the composition located around the phase boundary, x = 0.045 and 0.06, FE wake-up effects were detected. The refinement of neutron diffraction patterns after different electric cyclicity describe an increase of ferroelectricity associated with the R3c phase fraction increments i.e., field-cycling-induced phase transition from Pmc21 to R3c. The local probes such as the electron diffraction and piezoresponse microscopy (PFM), show that the in/antiphase octahedral rotation around the p and the local strain state are the decisive factors for this field-cycling-induced phase transition. In summary, the wake-up effects can be regarded as the nucleation and growth of the R3c phase with increasing number of electric cycles

Moment matching technique for fast and robust uncertainty quantifications of complex systems

Despite the growing popularity of reinforced concrete (RC) core walls, robust uncertainty quantification of their global and local performances remains an intractable challenge. A combination of a random sampling technique in conjunction with a simplified structural analysis has been widely used, but substantial ambiguity remains in the simulation accuracy and the resultant uncertainties. As a fast and robust alternative, this study suggests a moment matching (MM) technique. MM can dramatically reduce required sample points to only a few, which enables a direct use of a high-precision parallel multiscale finite element analysis (PM-FEA) for uncertainty quantification. This study demonstrates how to apply the combination of MM and PM-FEA to quantify uncertainties behind complex RC core walls, notably in terms of global force-resisting capacity and microscopic progressive bar buckling of five U-shaped walls. Results suggest that the combination of MM and PM-FEA will serve as a promising method to improve our understanding of uncertainties behind complex RC structure

FFTPL: An Analytic Placement Algorithm Using Fast Fourier Transform for Density Equalization

We propose a flat nonlinear placement algorithm FFTPL using fast Fourier transform for density equalization. The placement instance is modeled as an electrostatic system with the analogy of density cost to the potential energy. A well-defined Poisson's equation is proposed for gradient and cost computation. Our placer outperforms state-of-the-art placers with better solution quality and efficiency

AGGLOMERATION DURING FLUIDIZED-BED COMBUSTION OF BIOMASS

Wheat stalk is tested to investigate the formation of bed agglomeration. The results show that defluidization time decreases with the combustion temperature increasing. The minimum fluidization velocity of the bed material after the test increases. The K, Ca and Si elements play the most important role in bed defluidization