Papers by Author: Yuan Hua Lin

Abstract: The Cu_1.8Se/CuAgSe nanostructure was synthesized by a simple two-step process. Starting with the template of cubic Cu_1.8Se nanoplate by precipitation method, Cu_1.8Se/CuAgSe nanostructure and ternary CuAgSe were prepared through a rapid ion exchange reaction using various amount of AgNO₃ at room temperature. The as-prepared samples were analyzed by XRD, SEM and DRS. It was found that Cu_1.8Se/CuAgSe heterostructure and the pure CuAgSe phase were formed without changing the morphology, and these samples had efficient light absorption from UV light to near-infrared light region. Photocatalytic properties of these samples were evaluated by the degradation of Congo red under visible and near-infrared light. The Cu_1.8Se/CuAgSe nanostructure showed enhanced photocatalytic activity due to the lower recombination of charge-carrier in the photodegradation process.

Abstract: A series of Na₅La(WO₄)₄:M³⁺ (M = Eu, Sm, Bi) phosphors have been synthesized successfully by the solid-state reactions. The corresponding crystal structure and lattice parameters were refined by the Rietveld method, and the impact of doped metal ions upon the structure were also studied. The photoluminescent properties were investigated at room temperature. All the samples were observed a strong red emission under 395nm or 465 nm excitation, and the CIE chromaticity coordinates (x = 0.67, y = 0.33) was found to be close to the National Television Standard Committee (NTSC) standard red, implying high color purity. This kind of tungstate phosphor is a promising candidate as the red phosphor for white light-emitting diodes (WLED).

Abstract: High density BaTiO₃ ceramics have been prepared by the addition of CuO-Bi₂O₃-B₂O₃ mixed oxide. The results indicate that the sintering temperature of BaTiO₃ bulk can be decreased by about 300°C and high dielectric behavior can be remained. The CuO-Bi₂O₃-B₂O₃ additives can increase the breakdown strength, and leads to an obvious enhancement of the energy density, which is nearly 2 times of pure BaTiO₃.

Abstract: Cathode materials Li-[MnxNi1/2-x/2Co1/2-x/2]-O (x=0.333, 0.4, 0.6, 0.8, 0.9) were fabricated by lithium salts (LiNO3) and manganese-nickel-cobalt hydroxide precursors using the required amounts of Mn, Ni, Co for a given value of x. When Mn content is between 0.5 and 0.8, layered and spinel structure co-existed. Meanwhile the rate Li/M is also a key factor for synthesis of layered-spinel intergrowth structure cathode materials without other impurities. Li-[Ni0.2Co0.2Mn0.6]-O and Li-[Ni0.1Co0.1Mn0.8]-O were synthesized by a Two-step Co-precipitation method.

Abstract: The CaCu3Ti4O12 samples with slight amount of doped bismuth were prepared and tested in this research. No second phase with bismuth was found in the doped samples. It was inferred that the bismuth ions has entered the lattice and take the place of the calcium ions. The grain size was diminished with the increase of the bismuth content. The bismuth atoms can inhibit the grains from growing large, and it could be used to modify the microstructure of CaCu3Ti4O12 ceramic. The reduction of the grain size resulted in the decrease of the relative dielectric constant, according to the “internal barrier layer capacitance (IBLC)” theory. The impedance measurements showed that the doped samples have less conductivity and lower potential barrier at the grain boundaries, and the substitution of the bismuth ions on the calcium cites might be the reason for it. As a result of the lowered potential barrier, the non-ohmic I-V property of the CaCu3Ti4O12 ceramic almost disappears in the doped ones.

Abstract: The precursor of Ca2Co2O5 was prepared by coprecipitation method. The bulk Ca2Co2O5 samples were prepared by conventional sintering and Spark Plasma Sintering (SPS), respectively. The relative density of bulk Ca2Co2O5 ceramic, which was prepared by conventional sintering is about 75%; while the samples prepared by SPS has a density of 98%. The thermoelectric properties were enhanced by SPS, compared with samples prepared with conventional sintering. The maximum powerfactor of the conventional sintering and SPS samples are 2.70×10-4W∙m-1∙K-2 and 3.85×10-4 W∙m-1∙K-2, respectively.

Abstract: The cooling rate effect on the grain boundaries of the CaCu3Ti4¬O12 ceramic was investigated in this research. The resistance at the grain boundaries is found to be reduced as the cooling rate rises. The activation energies at the grain boundaries do not change with the cooling rate, suggesting that the conduction mechanism keeps the same in different cooling conditions. There is no significant difference in the permittivity of the samples. The dielectric loss can be lowered by using a smaller cooling rate. These results give clues to comprehend the conductive mechanism of the CCTO ceramic. The variation of the cooling rate may affect the re-oxidation process and cause the changes in electric properties.

Abstract: Ca2Co2O5 ceramic powders were synthesized by the coprecipitation method and consolidated by spark plasma sintering (SPS) technique.The observation by the SEM indicated that the ceramics body showed fine and layer microstructure.The density of the body increased with an increase of the fritting temperature.XRD patterns showed that the ceramics sample exhibits inpure phase with increasing the sintering temperayure to 850°C and that the grain was preferentially oriented.The electrical conductivity was reduce and the Seebeck coefficient slightly increased with an increase of the sintering temperature.As a result,the thermoelectric properties of the sample prepared by SPS at 800 °C for 5 min was better than the sample prepared by SPS at 750 °C for 5 min at high temperature.The sample treated by the SPS process under the condition of 800 °C,40 MPa and 5 min showed the maximum power factor (PF) of 3.85×10-4W∙m-1∙K-2 at 923 K.

Abstract: Nanometer La2CuO4 particles have been prepared using chemical precipitation method. The reaction mechanism, microstructure and phase composition of the samples were investigated by TG-DTA, XRD, SEM. The XRD results showed that pure La2CuO4 phase can be obtained by calcining the La-Cu-O-H precursor powders at 900 oC for 2 h, and the impurity phase will appear in the La2CuO4 ceramic as increasing the sintering temperature over 1200 oC. La2CuO4 ceramic samples sintered at 950 oC for 6 h exhibit good thermoelectric properties. Seebeck coefficient can reach 714 μV/K, and power factor is about 2.33×10-5 W/mK2.

Abstract: ZnO varistors with higher voltage gradients are expected in the power system, and the key to this property is to diminish the grain size. In this paper, nano-sized ZnO particles are used as the raw material in order to obtain small grains. Pellets are fabricated by two sintering methods: the conventional pressureless sintering and the spark plasma sintering (SPS). For the samples fabricated by the conventional sintering, the average grain size varies from 1 μm to more than 10 μm when the sintering temperature changes from 900 oC to 1200 oC. The voltage gradient can reach as high as 2020 V/mm. The grain size of the SPS samples is less than 1 μm, and the voltage gradients can reach 963 V/mm.