Papers by Author: Wan Ping Chen

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Authors: Wang Xiang, Wan Ping Chen, Wen Chao You, Helen Lai Wah Chan, Long Tu Li
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.
Authors: Yan Hong Gu, Wan Ping Chen, Ming Jian Ding, Jian Quan Qi
Abstract: BiFeO3, BiFe0.9Ti0.1O3 and BiFe0.9Ti0.05O3 ceramics were prepared by solid state reactions and were compared in electrical and dielectric properties. The resistivity of BiFe0.9Ti0.05O3 was 1.3×1012 Ω cm, which was about two and a half orders of magnitude higher than that of BiFeO3. and three times higher than that of BiFe0.9Ti0.1O3. The dielectric loss of BiFe0.9Ti0.05O3 was 0.1 and 0.02 at 100Hz and 1MHz, respectively. These phenomena can be explained base on the decrease of oxygen vacancies VO•• and defect complexes between the ferrum vacancies VFe and oxygen vacancies VO•• in the ceramics.
Authors: Yuan Meng, Wan Ping Chen
Abstract: Ba4Nb2O9 ceramics were synthesized via solid state reaction and sintered in air. Annealing in 5%H2 + 95%N2 at 900°C for 2 h resulted in an increase in leakage current by several orders of magnitude, and obvious increases in low-frequency capacitance and dielectric loss, while re-oxidation in air at 900°C for 2 h led to a partial recovery. It is proposed that oxygen vacancies and Nb4+ were formed through annealing in the reducing atmosphere while inverse process occurred in the re-oxidation treatment. Re-oxidation is helpful to improve the properties of barium niobates ceramics sintered or heat-treated in reducing atmospheres and further investigations are desirable.
Authors: Hu Yong Tian, Wan Ping Chen, D.Y. Wang, Y. Wang, J.T. Zeng, Helen Lai Wah Chan
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.
Authors: Wan Ping Chen, Hu Yong Tian, J.Q. Qi, Y. Wang, Z.J. Shen, X.F. Ruan, Helen Lai Wah Chan
Authors: Le Wei Qin, Ke Feng He, Wan Ping Chen
Abstract: Nanostructured AgBr/TiO2 visible light photocatalyst was prepared through deposition-precipitation method with AgNO3, KBr and Ti(OBu )4 as the precursors. Though the photocatalyst showed a relatively high photocatalytic efficiency for degradation of methyl orange in water under visible light irradiation in the first cycle, its efficiency was obviously decreased in the subsequent cycles. X-ray diffraction indicated that there existed a dramatic crystalline growth of AgBr in the AgBr /TiO2 nanocomposite after visible light irradiation, which must increase the recombination rate of photo-generated electrons and holes and be responsible for the observed decrease in photocatalytic efficiency. Photo-induced crystalline growth of AgBr may be a limiting factor for the lifetime of AgBr/TiO2 visible light photocatalyst and more attention should be paid to it.
Authors: Ming Jian Ding, Wan Ping Chen, Yan Hong Gu, Yu Wang
Abstract: Nb2O5 ceramic pellets were prepared through sintering and hydrogen was introduced into Nb2O5 pellets through a process known as electrochemical hydrogen charging, in which Nb2O5 acted as the cathode in a 0.01 M NaOH solution. The color of Nb2O5 was found dramatically darkened after the treatment while X-ray diffraction indicated that there was no change in the crystal structure. I-V and R-T measurements showed that the resistivity was greatly decreased after hydrogen charging and the resistivity exhibited a negative-temperature-coefficient (NTC). It was proposed that hydrogen can be incorporated into Nb2O5 lattice and occupies at some interstitial site. Nb5+ of Nb2O5 is partially reduced to Nb4+ by hydrogen and the NTC-type conduction results from electron transfer between Nb5+ and Nb4+ through a hopping mechanism.
Authors: Lu Feng, Ke Feng He, Wan Ping Chen
Abstract: Nanostructured AgI/TiO2 photocatalysts exhibit highly efficient visible light photocatalytic activity. Though there have been extensive investigations on AgI/TiO2 photocatalysts, study on their stablity is still very limited. So in this paper, such a study is designed and conducted. AgI/TiO2 nanostructured photocatalysts were prepared and separately immersed in HCl solutions, DI water and NaOH solutions with pH value of 0, 4, 7, 10 and 14, respectively. X-ray diffraction and diffusive reflectance UV-vis spectroscopy analyses reveal that the immersion in acidic solutions led to the formation of AgCl and a blue shifting to 450nm for the absorption edge of AgI/TiO2, while the alkaline treatment had no significant effect on the composite structure of AgI/TiO2 or on the UV-vis absorption edge. Photocatalytic tests show that immersion in acidic solutions had more significant influence on AgI/TiO2 photocatalytic degradation of methyl orange (MO). At pH = 4, photocatalytic efficiency was dropped nearly 30%. At pH = 10, no significant change was observed. AgI/TiO2 nanostructured photocatalysts have thus been found to display a higher stability in alkaline solutions than in acidic solutions.
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