published_or_final_version / abstract / Mechanical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

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Efficiency of particulate semiconductors for driving photocatalytic reactions is impractically low due to the recombination of excited carriers and intermediate species at the interface. In the literature it has been demonstrated internal depolarisation fields in ferroelectric materials separate electron and hole carriers, this gives rise to spatially distinct reduction and oxidation processes. It is hypothesised this property can supress the rate of back reactions and carrier recombination to improve photocatalytic efficiency. In this thesis the properties of ferroelectric particulates for driving photocatalytic reactions are investigated. Lithium niobate and barium titanate powders were suspended in aqueous solutions of acid black 1 or rhodamine b dye. Adsorption studies compared retention of dye in the double layer by the different powders. Under UV or simulated solar illumination barium titanate or lithium niobate powders photocatalytically decolourised the dye solutions. Powders of lithium niobate powder doped with magnesium or iron showed altered reaction rates and structural selectivity of decolourisation reactions. Photochemical deposition of silver nanoparticles at the surface of the barium titanate or lithium niobate powders increased the rate of photocatalytic decolourisation of rhodamine b solutions under UV or simulated solar illumination. Photochemical reduction of carbon dioxide to form formic acid and formaldehyde over lithium niobate powder was studied under UV illumination. Solid-liquid phase reactions were carried out using aqueous suspensions of powder bubbled with carbon dioxide gas. Solid-gas phase reactions were investigated using a purpose built reaction vessels loaded with carbon dioxide gas and water vapour. Under solid-gas phase conditions the rate of formation of products over lithium niobate powder was greater than over titanium dioxide powder.

by Lee, Kai Kwong Dennis = 一激光束對在光折變晶體中之另一激光束的影響 / 李啓光. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2000. / Includes bibliographical references (leaves 106-109). / Text in English; abstracts in English and Chinese. / by Lee, Kai Kwong Dennis = Yi ji guang shu dui zai guang zhe bian jing ti zhong zhi ling yi ji guang shu de ying xiang / Li Qiguang. / Acknowledgments --- p.i / Abstract --- p.ii / Table of Content --- p.iv / Chapter Chapter 1 --- Introduction --- p.1 / Chapter Chapter 2 --- Theory --- p.4 / Chapter 2.1 --- Two-wave Coupling --- p.4 / Chapter 2.2 --- Four-wave Mixing --- p.10 / Chapter 2.3 --- Fanning --- p.13 / Chapter 2.4 --- Phase-conjugation --- p.16 / Chapter 2.5 --- Spatial Solitons --- p.19 / Chapter Chapter 3 --- "Experiments,Results and Discussion" --- p.26 / Chapter 3.1 --- Measurements of Optically Induced Refractive Index Change --- p.26 / Chapter 3.2 --- Optical Control Experiments --- p.65 / Chapter 3.3 --- Soliton Experiments --- p.80 / Chapter Chapter 4 --- Concluding Remarks and Suggested Further Studies --- p.99 / References --- p.105

Lead-based and lead-free ferroelectric ceramic materials were prepared at low sintering temperatures with particular regard to their applications in thick film piezoelectric components. This project is focused on the development of processing methods and novel compositions to be used for thick film production by electrophoretic deposition (EPD) on heat-resistant alloys. Lead-based glasses and an oxide mixture (LiCO3, Bi2O3 and CuO), denoted LBCu, with low melting points were used as sintering aids for lanthanum-doped lead zirconate titanate (PLZT) ceramics. The required temperature to achieve dense ceramics was reduced from 1250 to 950 °C. It was found that the highest ferroelectric properties were obtained by the use of LBCu in comparison with those incorporating glass additives due to the shielding effect of the glass phase between the ferroelectric grains. However, the results of thick film preparation shown that the samples with glass additives were much smoother and relatively free of cracks up to 1000 ÂoC. In terms of lead-free ceramics, novel compositions were prepared, based on (Ba,Ca)(Zr,Ti)O3-(K0.5Bi0.5)TiO3 (BCZT-KBT) solid solutions having various Ca and Zr contents. The new solid solutions exhibited interesting features comprising core-shell type microstructures and relaxor ferroelectric behaviour in addition to reduced sintering temperatures and higher Curie point compared with BCZT ceramics. The required sintering temperature reduced to 1125 °C at 65% KBT, in comparison with 1500 °C for pure BCZT. The results showed that the compositional heterogeneity in the shell regions was reduced by air quenching, relative to that of the slow-cooled state, due to the retention of the more chemically-homogeneous high temperature state by the quenching process. The improvements were evident in increased polarisation, piezoelectric coefficient and depolarisation temperature values. However, the slow-cooled samples exhibited high reversible strain levels due to the presence of polar nanoregions (PNRs) in the ergodic state within the shell regions. Comparing the results obtained for two BCZT compositions, it was demonstrated that the stability of the ferroelectric tetragonal phase in slow-cooled BCZT-KBT samples was improved for the ceramic with lower Ca and Zr concentrations, denoted x=0.06, in comparison with that for higher levels, denoted x=0.15. Moreover, the electric field-induced ferroelectric state in the quenched ceramic with x=0.06 was found to be more stable during heating, giving rise to an enhanced depolarisation temperature.

Guo Bin = 光折變摻銅鉀鈉鈮酸鍶鋇和摻鐵鈮酸鋰晶體中的二波耦合及摻鐵鈮酸鋰晶體中的亮光伏空間孤子 / 郭斌. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references. / Text in English; abstracts in English and Chinese. / Guo Bin = Guang zhe bian shan tong jia na ni suan si bei he shan tie ni suan li jing ti zhong de er bo ou he ji shan tie ni suan li jing ti zhong de liang guang fu kong jian gu zi / Guo Bin. / Acknowledgements --- p.i / Abstract --- p.ii / Table of Contents --- p.v / Chapter Chapter 1 --- Introduction --- p.1 / References --- p.6 / Chapter Chapter 2 --- Background --- p.7 / Chapter 2.1 --- Photorefractive Effect Based on Band Transport Model --- p.7 / Chapter 2.2 --- Photovoltaic Effect --- p.15 / Chapter 2.3 --- Surface Charge Recombination in Fe:LiNb〇3 Crystals --- p.17 / References --- p.19 / Chapter Chapter 3 --- Noise of Two-Wave Mixing in Cu:KNSBN Crystal --- p.20 / Chapter 3.1 --- Introduction --- p.20 / Chapter 3.2 --- Experimental Setup --- p.22 / Chapter 3.3 --- Results and Discussion --- p.23 / Chapter 3.4 --- Conclusion --- p.29 / References --- p.30 / Chapter Chapter 4 --- Bright Photovoltaic Spatial Soliton in Fe:LiNb03 Crystals / Chapter 4.1 --- Introduction --- p.32 / Chapter 4.2 --- Theory --- p.33 / Chapter 4.3 --- Experimental Setup --- p.36 / Chapter 4.4 --- Results and Discussion --- p.39 / Chapter 4.5 --- Conclusion --- p.58 / References --- p.59 / Chapter Chapter 5 --- Two-Wave Mixing of Narrow Laser Beams in a Fe:LiNbO3 Crystal --- p.60 / Chapter 5.1 --- Introduction --- p.60 / Chapter 5.2 --- Theory of Two-Wave Mixing --- p.62 / Chapter 5.3 --- A Simplified Analysis of Two-Wave Mixing of Narrow Beams --- p.66 / Chapter 5.4 --- Experimental Setup --- p.70 / Chapter 5.5 --- Results and Discussion --- p.71 / Chapter 5.6 --- Conclusion --- p.86 / References --- p.87 / Chapter Chapter 6 --- Two-Wave Mixing of Narrow Laser Beams with an Incoherent Background Beam --- p.89 / Chapter 6.1 --- Theory --- p.89 / Chapter 6.2 --- Experimental Setup --- p.93 / Chapter 6.3 --- Results and Discussion --- p.94 / Chapter 6.4 --- Conclusion --- p.105 / References --- p.106 / Chapter Chapter 7 --- Conclusion and Future Outlook --- p.107

High tunability and low loss tangent of ferroelectric thinfilms offer unique opportunity for the development of variousmicrowave devices. Silver tantalate niobate, which showsexcellent microwave properties, was selected for this study.Ag(Ta,Nb)O3(ATN) showed week dielectric dispersion in a widefrequency range from 1 kHz up to 100 GHz, negligible losses upto 30 GHz, and ease to tailor paraelectric state in a widetemperature range by Ta:Nb ratio. This thesis is mainly based on the synthesis andcharacterization of niobate ferroelectric ATN thin films. Thinfilms for various measurements were prepared by pulsed laserdeposition and rf-magnetron sputtering techniques. X-ray diffraction (XRD) pattern show that ATN/Pt80Ir20films have been found to be (001) preferentiallyoriented, while the epitaxial quality of ATN/LaAlO3heterostructures have been ascertained. Dielectricproperties were analyzed by measuring the relationship betweendielectric permittivity and frequency as well as dielectricpermittivity and temperature. Reliable tracing of theferroelectric hysteresis polarization versus electric loopsindicate the ferroelectric state in ATN films at temperaturebelow 125 K and yields the remanant polarization of 0.4µC/cm2@ 77 K. The fundamental current-voltage behavior in Ag(Ta,Nb)O3ferroelectric films was measured usingMe/Ag(Ta,Nb)O3/Pt80Ir20, Me = Pd, Au, Cr, and Al, vertical capacitivecell structures with different top electrodes. Various kinds ofconduction mechanisms such as Schottky emission, Poole-Frenkel,Fowler-Nordheim, and ionic conduction were classified. Finally, by fabricating interdigital capacitors on the oxidesubstrates, the characteristics and performances of Ag(Ta,Nb)O3varactors were examined. Au/Cr/ATN/LaAlO3interdigital capacitors exhibited loss tangent aslow as 0.0033 @ 1 MHz, weak frequency dispersion of 5.8 % in 1kHz to 1 MHz range, tunability as high as 16.4 %,K-factor (tunability/tanδ) higher than 48.

In recent years multiferroic materials have great application potential in the ferroelectric random access memory and emerging spintronics development setting off the boom of the multiferroic materials research. It was assume that the ferroelectric and magnetic properties cannot coexist at the same temperature range or their coupling is weak making the searching for multiferroics in dream. Multiferroic materials have been discovered in recent years. Multiferroics may even be induced due to the strong coupling between artificial layers by growing multilayer technique. [1] [2] Among of which the BiFeO3 compound contains of antiferromagnetic and ferroelectric properties at room temperature. The BiFeO3 thin films manifests a larger spontaneous porlarization than that of the bulk and is highly capable of industrial application. In this study, Pb doped BFO (BPFO) this films were grown on top the conductive SrRuO3 layers. By varying BPFO¡¦s thickness, the strain effect the relative physical properties were studied. It is found that the lattice constants of SRO is totally different to that of STO substrate indicating strain relaxation. Similar phenomena are also discovered for BPFO thin film which is believed due to the tilting effect on the SRO/STO interface.

In this study, the reactive rf magnetron sputtering was used to deposit Ba(Zr0.1,Ti0.9)O3 (BZT) ferroelectric thin films on ITO/Glass substrate, and MFM structure was fabricated. The effects of various sputtering parameters on the characteristics of thin films, such as the oxygen concentrations, substrate temperature, rf power and chamber pressure were discussed, and then the optimal sputtering parameters were determined. The annealing process of rapid thermal annealing (RTA) and conventional thermal annealing (CTA) were used to promote the ferroelectric characteristics. The physical characteristics of BZT thin films were obtained by the analyses of SEM, XRD patterns and AFM morphologies. The surfaces, cross-section, crystallization and surface roughness of thin films were discussed. To investigate the electrical properties, the capacitance-voltage (C-V), current-voltage (I-V) and P-E hysteresis characteristics of BZT thin films were measured by the HP4284A impedance analyzer, HP4156C semiconductor parameter analyzer and computerized radiant technology (RT66), respectively. Furthermore, characteristics of the MFM structure were discussed. From the experimental results, the dielectric constant with optimal sputtering parameters was about 84, and the leakage current of thin film was about 6 ¡Ñ 10-8 A/cm2 when the applied electrical field of thin film was at 0.4 MV/cm. The remanent polarization (Pr) and coercive field (Ec) were 2.87 £gC/cm2 and 259 kV/cm from the P-E hysteresis loops, respectively. In addition, the ferroelectric characteristics of the thin film could be improved after rapid thermal annealing at 550¢J for 3 minutes. The remanent polarization (Pr), coercive field (Ec) and saturated polarization (Ps) were 3.42 £gC/cm2, 266 kV/cm and 6.99 £gC/cm2, respectively.