Papers by Author: Helen Lai Wah Chan

Abstract: A new kind of multiferroic three-ply-structured nanoceramic composites consisting of nanosized alloy-based magnetostrictive (MS) and ferroelectric powders (FE) were fabricated successfully by a conventional solid state reaction in air or argon ambient in our previous investigation. But there are still some problems needed to be solved, for example, the magnetic aging and decrease of saturated magnetization phenomena occurred in samples are perhaps related to the harsh preparation conditions such as storage duration and sintering atmosphere and so forth. In this paper, we will review the characteristics of the novel nanocomposites by means of the microstructure and magnetism existed in the samples with nanosized MS powders (i.e. (Tb_0.3Dy_0.7)_0.75Pr_0.25Fe_1.55, Sm_0.88Nd_0.12Fe_1.93) as fillers. Also, we hope some reasonable explanations and deeper understandings can be concluded about the proposed ageing and magnetism reduction exhibited when the samples were subjected to co-firing in air or argon ambient. Finally, the potential improvement and multifunctional applications in the future are also suggested.

Abstract: (Ba,Sr)TiO3 thin film has been deposited on Si (001) wafer with the SiO2 layer as the block layer through laser molecular-beam epitaxy using an ultra thin Sr layer as template. X-ray diffraction measurements and the cross-sectional observations under transmission electron microscope indicated that BST was well crystallized. This deposition of Sr layer is considered to remove the thin SiO2 layer to produce a layer, which is crystallized and has a lattice structure matching with that of perovskite BST. The maximum in-plane dielectric tunability is calculated to be 50% at 1 GHz under a moderate DC bias field of 13.3 V/µm. This BST/Si structure is believed to be a promising candidate in the development of ferroelectric BST-based microwave devices.

Abstract: Phlogopite with layered silicate structure had been firstly chemically modified via an in situ intercalation method, and phlogopite-polymer nanocomposite films were prepared from 2,2'-bis (3,4-dicarboxyphenyl) hexafluropropane dianhydride (6FDA) and oxydimethyl aniline (ODA) in N,N-dimethylacetamide as a solvent by using in-situ polymerization process combined with ultrasonic dispersion and multi-step curing. The structure of phlogopite minerals and its polymer nanocomposites were characterized by X-ray diffraction (XRD) and infrared spectra (FTIR) respectively. The experimental results indicated that the phlogopites with layered nanostructure had lost their ordered structure and had been exfoliated or intercalated. Thereafter, they were dispersed randomly in the polyimide matrix. The dependence of dielectric properties and thermal stabilities of the nanocomposite films on the phlogopite content and frequency were studied.

Abstract: The carbon fiber (CF) reinforced butadiene rubber (BR) electromagnetic shielding composite has been prepared by mechanical agitation and roll mixing methods. The composites have been characterized and tested. As shown in the experimental results, the mechanical properties was increased with the increase of CF, and the electromagnetic shielding effectiveness (SE) of BR based composite with 15 wt% CF content has been tested, the maximum SE was about 60 dB.

Abstract: A sol-gel process was used to prepare polyimide-silica hybrid films from the polyimide precursors and TEOS in N,N- dimethyl acetamide, then the hybrid film was treated with hydrofluoric acid to remove the dispersed silica particles, leaving pores with diameters between 80nm to 1µm, depending on the size of silica particles. The structure and dielectric constant of the hybrid and porous films were characterized by FTIR,SEM. The porous films displayed relatively low dielectric constant compared to the hybrid polyimide-silica films.

Abstract: Porous barium strontium titanate−titania (BST−TiO2) nanocomposite was developed by the combination of electrochemical anodization and hydrothermal synthesis. Self organized titania nanotube arrays were first fabricated by electrochemical anodization of titanium foil in 0.2 wt.% HF. The as-anodized titania was then used as a template and was subject to a hydrothermal treatment in the solution mixture of barium hydroxide and strontium hydroxide with different molar ratios. Well crystallized barium strontium titanate was developed directly from the titania nanotubes. Annealing was carried out to transform residual amorphous titania to crystallized anatase, resulting in the porous BST−TiO2 nanocomposite. The surface morphology and structure of the nanocomposite were characterized. The photoelectrochemical response of the BST−TiO2 composite was investigated and the photocatalytic property was evaluated through the photo-decomposition of an organic dye solution. The effect of hydrothermal parameters on the surface morphology and the photocatalytic activity of the nanocomposite was studied.

Abstract: A comparison experiment was conducted in which some lead zirconate titanate (PZT) piezoelectric ceramic rings were simply immersed in a 0.01 M NaOH solution while other PZT rings were immersed in the solution with a 50 Hz AC voltage applied between the electrodes of the rings and a counter electrode in the solution. Though the simple immersion showed no noticeable influence on the PZT rings, those PZT rings treated with the application of the AC voltage were obviously degraded in their piezoelectric properties. It was proposed that the degradation resulted from the collaborated reactions of atomic hydrogen and oxygen generated in the AC voltage-induced electrolysis of water. Water may be an important origin for degradation of piezoelectric ceramic devices operating under AC voltages.

Abstract: Barium titanate with A-site substituted by various amount of bismuth oxide (Ba1-x BixTiO3, abbreviated as BBT, x=0.05, 0.1, 0.15) were prepared by solid-state reaction. The effect of bismuth substitution on crystallographic phase, dielectric and ferroelectric properties was studied. The X-ray diffraction shows that the samples were crystallized into pure perovskite structure when x=0.05 and 0.1, while for x=0.15 sample, second phase appeared in the dominant perovskite phase. The temperature dependence of dielectric permittivity of the ceramics was investigated and the evolution from normal ferroelectrics to relaxor ferroelectric sates was observed. In the range 0≤x≤0.1, the temperature of dielectric peak Tm is independence of the frequency, indicating the normal ferroelectrics behavior. At x=0.15, dielectric relaxation process with a broadening distribution of the permittivity dielectric is observed.

Abstract: Lead-free piezoelectric ceramics based on bismuth sodium titanate (BNT) -barium hafnate titanate (BHT) were prepared by a two-step synthesis process. The final BNT-BHT ceramics sintered at 1180oC for 2 h in air showed a perovskite structure with high density. The morphotropic phase boundaries (MPB) were found in BNT based piezoelectric ceramics with 8~10 wt% BHT in composites. In the case of Bi0.5Na0.5TiO3-0.08BaHf0.05Ti0.95O3 ceramics, a maximum piezoelectric coefficient d33 of 122.6 pC/N was obtained. The remnant polarization (Pr) and coercive field (Ec) were measured and the relationship between ferroelectricity and the BHT fraction in the compounds was investigated. The BNT-BHT ceramics were expected to be a new and promising candidate for lead-free piezoelectric device applications.