Papers by Author: Ce Wen Nan

Abstract: In this paper, to improve the elevated temperature performance of spinel LiMn2O4 as cathode materials, the cation/anion co-doping and surface modification together were adopted. The SiO2 coated Li1.02Co0.1Mn1.9O4−xSx spinels were synthesized by the solid-state reaction method and Sol-gel coating process. The samples are characterized by XRD, SEM, galvanostatic charge-discharge. The results show that the Li1.02Co0.1Mn1.9O3.98S0.02 exhibits the best initial discharge capacity of 122mAh/g, and capacity retention rate gets to 92% after 100 cycles at room temperature (25 °C). The substitution of Co and S for Mn and O in LiMn2O4 can enhance the crystal structure stability and overcomes the Jahn-Teller distortion, but cannot resolve the elevated temperature cycling issue of the spinel cathode materials. The capacity loss of Li1.02Co0.1Mn1.9O3.98S0.02 without SiO2 coating gets to 38% after 50 cycles, whereas the 2.0wt.% SiO2-coated Li1.02Co0.1Mn1.9O3.98S0.02 cathode material has only 5.0% capacity loss after 50 cycles at elevated temperature (55°C). It indicates that nano SiO2 coating could suppress Mn dissolution in the electrolyte during cycles. So combining cation/anion co-doping and surface modification is best way to improve the elevated temperature cycling performance of spinel LiMn2O4 as cathode materials.

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: 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: CaCu3Ti4O12 (CCTO) ceramic thick films have been prepared on Copper substrate, using ceramic powders by an aerosol deposition method (ADM). The ceramic powders are prepared by traditional solid state reaction processing at 1100 oC for 5 h. X-ray diffraction and scanning electron microscopy are used to investigate the microstructure and the phase composition of the deposited films. The results indicate that thick films are pure CCTO phase and homogenous. The dielectric impedance spectra indicate that the dielectric constant of CCTO thick film can reach 3×103.

Abstract: We present Mg and Ti co-doped NiO-based polycrystalline ceramic with high-permittivity dielectric properties. Analysis of the ceramic’s microstructure and composition proves that obvious grain boundaries NiTiO3 are formed. The measurements of frequency and temperature dependence of impedance and dielectric permittivity indicate that the grain and grain boundaries have remarkable influence on the dielectric properties due to the various activation energies.

Abstract: PCS (particle composite system) is a kind of dry coating equipment. The PCS process involves encapsulating UHMWPE (ultra-high molecular polyethylene) particles with a layer of SCB (super conductive carbon black) or ACB (acetylene black), and subsequently compacting these CB-encapsulated UHMWPE powders by compression molding to manufacture conductive polymer composites respectively. Morphologies of these composite particles were investigated by SEM. By SEM, we can see the conductive networks of CB in polymer composites. Coating-molding process with PCS can be used to form network structure in ceramics and metal as well.

Abstract: Ni-Si-O thin films on Si substrate have been prepared by a sol-gel method. The microstructure and phase composition of the films were investigated by XRD, SEM, FTIR. The XRD results showed that the films are amorphous while annealed at 800oC for 10 min, the SEM imagines proved that the films are smooth and thickness is about 190 nm. Electrical property of the film indicated that the leakage current was as low as 10-6A/cm2 at an electric field of 1MV/cm. The dielectric properties of Ni-Si-O thin films can be improved as increasing the annealed temperature.

Abstract: ZnO-Pr6O11-Dy2O3-based varistor ceramics doped with 0~1.5 mol% La2O3 were fabricated by a conventional ceramic method. All the samples were sintered at 1350 oC
for 2 h. The phase composition and microstructure of the ceramic samples have been investigated by XRD, SEM and EDS. The results of SEM micrographs indicated that the La2O3 additives can promote ZnO grain’s growth, and the rare earth elements dispersed mainly in the intergranular phase observed by EDS. The electrical properties of the samples determined by the V-I curves revealed that the breakdown voltage of samples decreases from 508 V/mm to about 100 V/mm with the increase of La2O3, and the nonlinear exponent also decreases from 20.2 to 13.2. The typical leakage current is about 10.2 μA for the sample doped with 0.5 mol% La2O3.

Abstract: The precursor powder of lead-zirconate-titanate (PZT) and NiFe2O4 (NFO) was prepared using one step sol-gel method. The dry powders got after a series of processes were sintered via a conventional ceramic solid-state reaction technique. All the diffraction peaks in the XRD patterns of the composites can be readily indexed to NFO and PZT, respectively. The SEM micrograph shows two different phases of PZT and NFO distinctly and the particle size of NFO was about 2~4 μm. The magnetoelectric (ME) coefficient, dielectric constant, piezoelectric constant, magnetic hysteresis loops of the samples were measured. The dc magnetic Hdc resulting in the maximum ME coefficient varies from 0.65kOe to 1.0kOe with the increase of the NFO volume fraction. The maximum magnetoelectric coefficient of 6.15Vcm-1Oe-1 was obtained at resonance frequency of 289.8kHz, dc magnetic intensity of 0.8kOe and NFO volume fraction of 0.25.

Abstract: NiO-NiTiO3 composite ceramics were prepared by the conventional solid state reaction process and dielectric properties were investigated as a function of frequency and temperature. The results indicate that all samples showed high dielectric constant ~103 at 0.1 kHz, but a remarkable decrease of the dielectric loss appeared when the volume fraction of NiTiO3 phase reached 42.4%. Relaxor-like peaks were observed in the temperature dependence of the dielectric constant with ceramics having low composition of NiTiO3 phase (≤21.2%). The possible response mechanism associated with the microstructure and the associated defects of the composites was discussed.