| Ferroelectricity involves a complex interplay of electrical, mechanical, and thermal effects. Ferroelectric materials have been extensively studied for nearly a century due to their unique properties such as piezoelectricity, pyroelectricity, ferroelectricity, photoelectricity, and electrostriction effects. In addition, ferroelectric materials usually show good structural compatibility when integrated into electronic devices. As a result, ferroelectric materials can be successfully utilized as capacitors, transducers, sensors, in areas ranged from national defense, biotechnology, precise mechanical components, and information technology. However, there are still many technical challenges remained to be solved for ferroelectrics.The physical properties of crystal materials are largely determined by the behaviors of electrons. However, the existence of lattice defects such as point defects, dislocations, and twins, could significantly change their properties. Classical ferroelectric phenominonical theory indicates that ferroelectric properties are closely related to the state of stress conditions. It has been extensively demonstrated that the Curie temperature, polarization, piezoelectric coefficients, ferroelectric switching behaviors, and phase transition can be changed when the state of stress is changed. As a result, one would expect that the stress field associated with microstructural defects will also change ferroelectric properties. For example, ferroelectric materials suffer from electrical fatigue with repetitive cycling, polarization imprint, and dead layer, all of which are closely linked to defects. Furthermore, defects should be considered when some particular behaviors such as ferroelectric size effect, conductivity, and spectroscopy features are analyzed. The effects of defects could be enhanced with the miniaturization of devices and applications of multilayer heterostructure. As a result, the study of microstructural defects in ferroelectrics is significant to analyze ferroelectric behaviors and design materials and devices with controlled properties. The main defects in ferroelectric materials include dislocations, twins, and point defects. Fristly, polarization can be induced by the stress field of dislocations. Dislocations are closely linked to ferroelectric size effect and other properties related to the size of a ferroelectric heterostructures. Since the properties of solids are controlled largely by spatial arrangment of atoms, dislocated atoms have to be carefully treated to understand the relationship between dislocation size effect and ferroelectric size effect. Secondly, twinned structures can influence the formation and motion of ferroelectric domains. Furthermore, the aggregation of other kind of defects and nonstoichiometric structure on the twin plane is unavoidable in material. Thirdly, the aggregation of point defects can greatly influence the ferroelectric properties, in particular domain dynamics, switching kinetics. In addition, point defects, i.e. oxygen vancancies also play an important role on conductivity, band structure, and magnetism properties. As a result, it is both scientifically and technologically of importance to systematically study defect structures of ferroelectric materials.As a model ferroelectric perovskite, BaTiO3 has benn extensively studied due to its simple structure. In this dissertation, BaTiO3 crystallites of various sizes are successfully prepared. The properties and structures of defects are studied systematically with emphasis on dislocations,{111} twins, and point defects. Several methods for the controlled synthesis of BaTiO3 with different defect structures were developed. The particular physical properties induced by defects are discussed. The main topics of this dissertation are as follows:1) The preparation and characterization of BaTiO3 nanocubes.The BaTiO3 nanocubes are prepared through Composite-Hydroxides-Mediated (CHM) approach and characterized by using XRD, TEM, and SEM etc. XRD and Raman results indicate that the product is tetragonal BaTiO3. The size of the as synthesized particles is-40-80 nm. They are cubic or cuboid in shape with sharp edges. The exposed surfaces are (100)pc facets (subscript pc denotes pseudocubic and will be omitted for convenience hereafter). The growth of BaTiO3 nanocubes in molten hydroxides follows the dissolution-crystallization process and it is controlled by the combination of Ti-O octahedra and the diffusion of Ba2+cations. 2) The selective etching of BaTiO3 nanocubes and analysis of dislocation size effect.The hydrothermal etching process of BaTiO3 nanocubes in acid solution is studies. It is indicated that the etching process contains dissolution of Ba2+ and Ti-O octahedra and the re-crystallization of Ti-O octahedra. The BaTiO3 nanocubes can be totally etched and form spindle liked TiO2 particles with either anatase or rutile phases. However, with controlled experimental conditions, nanosized cavities can be formed in the exposed faces of BaTiO3 nanocubes and finally to form a hollow structure. Since the stress field of dislocation increase the reaction activity around the dislocation core area, localized preferential etching is expected and etch pits will be created on nanocubes with dislocations. A statistic analysis of the etch pits indicated that they only formed in the BaTiO3 crystallites with size greater that-60 nm. The etching of smaller particles undergoes the Ostwald process controlled mainly by thermodynamic stability so that the edges and corners of the nanocubes are preferentially etched. HRTEM results indicate that distorted surface lattices with deficiency of Ba2+can be formed in the BaTiO3 nanocubes. It can also be investigated from the HRTEM images that there are no dislocations in particles smaller that-60 nm. It is consistent with the etching results and proved that the critical size of dislocation in BaTiO3 nanocubes is about 60 nm.The dislocation critical size of tetragonal BaTiO3 is 22 nm as calculated by using the classical elastic theory of dislocations. The calculated result is different from the experimental result (60 nm). The reasons of the difference are as follows.①The BaTiO3 is assumed as a sphere in the model of calculation.②The elastic anisotropy of BaTiO3 (with anisotropy factor-2.5).③The surface layer of Ti-O structures. The first-principles calculated shear modulus of cubic BaTiO3 is 116.2 GPa, and the calculated dislocation critical size of cubic BaTiO3 is 46.5 nm which indicated that the phase transition of ferroelectric is correlated with dislocation critical size.3) Controlled growth of{111}pc twinned BaTiO3 crystallites.The{111} twinned BaTiO3 crystallites can be synthesized through a two step CHM approach. The studies on amorphous TiO2 indicate that the short range order of Ti-O octahedra is important to the formation of{111} twin in BaTiO3.The face-shared Ti-O octahedra is the elementary unit of{111} twin plane. The{111} twinned BaTiO3 crystallites are characterized by using XRD, Raman, and SEM etc. The size of the {111} twinned BaTiO3 crystallites is-10-40μm. These particles show morphology of penetrated cubes. The study indicates that BaTiO3 is formed after 4 hours reaction and the penetrated structures can be investigated after 8 hours reaction. The product is BaTiO3 microsized crystallites after react for 20 days with the decrease of supersaturation.SEM and AFM results indicate that the growth of{111} twinned BaTiO3 crystallites follows the 2D layered growth model. The nucleus is preferentially formed at Twin Plane Reentrant Edges (TPRE) since the lower nucleate barrier. That is why the BaTiO3 particles with{111} twin can grow much bigger. The asymmetrical penetrated morphology is induced by the multiple twins and localized thermodynamic instability. The 90°domain can be investigated through etching or decoration method. The width of the domain ribbon is about 300 nm. The experimental conditions can greatly influence the morphology of{111} twinned BaTiO3. The product becomes irregular with more H2O added to the reaction system. Experiments also indicate that doping can slower the growth of{111} twinned BaTiO3.4) The control of point defects and related properties of BaTiO3 crystallites.Photochromic effect is demonstrated with UV irradiation on{111} twinned BaTiO3. This photochromic effect is confirmed to be reversible under near infrared irradiation or heat treatment. The red shift of absorption edge of as synthesized{111} twinned BaTiO3 indicates that the sample has defect energy levels. The red shift of absorption after UV irradiation indicates that new defect energy levels can be formed during UV irradiation. The UV induced point defects could be Ti3+, oxygen vacancies, and complex defects. Large amount of point defects could be aggregated after UV irradiation. The lone pair electrons of Ti3+and trapped by oxygen vacancies can lead to ferromagnetism. It is shown that the saturated magnetism is greatly enhanced by UV irradiation. Raman and hysteresis loop results indicated that the BaTiO3 crystallites are still ferroelectric phase. It is expected that the control of point defects could be an effective method to bring ferromagnetism to typically non-magnetic materials.In conclusion, BaTiO3 crystallites with regular and uniform morphology can be successfully prepared through CHM approach. The defect structures and properties are carefully studied. The dislocation structures are investigated by hydrothermal etching method and HRTEM measurements. It is proposed that etching is size selective. The dislocation critical size is discussed and its influence to ferroelectric size effect is expected. The multiple{111} twinned BaTiO3 with penetrated structure is controllably synthesized. The TPRE promoted growth is studies. The photochromic effect of{111} twinned BaTiO3 is investigated and the produce of point defects under photo irradiation is discussed. The UV irradiation enhanced ferromagnetism in BaTiO3 is also discussed. It is expected that these properties can be used in photoswitch, sensors, and magnetoelectric devices.
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