Advanced Materials Research Vols. 105-106

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: WC-Co tapes with different Co contents were prepared by rolling the mixtures of WC-Co powders and polymer binders, and layered WC-Co green compacts with discontinuous distribution of Co contents were obtained by laminating the green WC-Co tapes together. After burning off the binders and other organic composition in H2 atmosphere, the green compacts were sintered in vacuum. Microstructure of both the green and the sintered tapes were observed by scanning electron microscope. An electron probe microanalyser was used to measure the linear distribution of Co element in the layered WC-Co cemented carbides. Sintering densification and mechanical property of the multilayered WC-Co alloys were also investigated and compared with those of conventional WC-Co alloys with nominal Co contents of 8% and 12%, respectively. It was shown that by controlling sintering temperature and time, a continuous graded distribution of Co composition can be realized in the laminated WC-Co alloys, with more Co-rich phase in the original high Co content side (12%). Furthermore, the graded WC-Co cemented carbides can realize a combination of high hardness and high strength.

Abstract: A series of BST(Ba0.65Sr0.35TiO3) sol with different viscosity were prepared by using Ba(CH3COOH)2, Sr(CH3COOH)2 and Ti(OC4H9)4 as raw materials. The BST thin films were fabricated by sol-gel method and spin-coating process on Si(100) substrates. The effect of sol viscosity on crystallization, microstructure and dielectric properties of BST thin films were analyzed by using X-ray diffractometry, scanning electron microscopy and impedance phase analyzer. The results show that the BST films coated by sol with viscosity of 3.0cp and annealed at 750°C for 1h are basically non-crystalline. The crystallization degree increases with the increase of sol viscosity at the same annealing temperature. The ABO3 perovskite structure is formed when the sol viscosity increases to 3.5cp. With further increasing sol viscosity to 6.0cp, the crystal grains of the film grow well and range very compact, the surface become smooth, and the films have optimal dielectric properties.

Abstract: A novel rapid prototyping technology for glass-alumina functionally gradient materials (G-A FGMs) based on the quick solidification of wax was proposed. The feature of the technology came from its layer-by-layer fabrication of the wax-glass/alumina composite layer. With the help of orthogonal experiment method, the influence of different parameters (sintering temperature, component ratio (the glass and the alumina), and number of layers) on the flexural strength of FGMs was discussed. Optimum conditions obtained from the above experiments were applied to prepare the G-A FGM with three layers. The G-A FGM was analyzed in detail, by observing it under an environmental scanning electron microscope (ESEM) coupled with an X-ray energy dispersive spectrometer (X-EDS). The results of orthogonal experiment show that number of layers is the principal parameter to the flexural strength of FGMs, component ratio is the secondary factor, and sintering temperature may be not an important factor. The special microstructure appears in the cross section of the G-A FGM, observed by ESEM. It is from the X-EDS graph concluded that alumina and glass probably react to form the special microstructure with part removal of wax.

Abstract: The dynamic stress field under the SH-waves at the moving crack tip of functionally graded materials is analyzed, and the influences of parameters such as graded parameter, crack velocity, the angle of the incidence and the number of the waves on dynamic stress intensity factor are also studied. Due to the same time factor of scattering wave and incident wave, the scattering model of the crack tip can be constructed by making use of the displacement function of harmonic load in the infinite plane. The dual integral equation of moving crack problem subjected to SH-waves is obtained through Fourier transform with the help of the exponent model of the shear modulus and density, then have some process on the even and odd term of the integral kernel. The displacement is expanded into series form using Jacobi Polynomial, and then the semi-analytic and numerical solutions of dynamic stress intensity factor are derived with Schmidt method.

Abstract: Composites consisting of Ce0.8Sm0.2O1.9 (SDC)-carbonate were developed as electrolytes for low temperature solid oxide fuel cells (LTSOFC). The SDC power was prepared by sol-gel method. The carbonates were binary eutectics of (Li/Na)2CO3, (Li/K)2CO3 and (K/Na)2CO3. Conductivity measurements showed that the conductivities were depended on the type of carbonates. Discontinuities were found in the Arrhenius plots for both SDC-(Li/Na)2CO3 and SDC-(Li/K)2CO3. For SDC-(Na/K)2CO3 composite electrolyte, the conductivity increased as temperature rose following one slope. Single cells based on various composites were fabricated by a uniaxial die-press method and tested at 450-600 oC. The results showed all cells exhibited improved performances upon that of pure SDC-based cell. The best power density of 532 mW cm-2 at 600 oC was achieved for LTSOFC using composite of SDC and (Li/Na)2CO3. Conductivity mechanism was also discussed.

Abstract: A two-phase flow, multi-component model has been optimized for a PEM (Proton Exchange Membrane) Fuel Cell. The modeling domain consists of the membrane, two catalyst layers, two diffusion layers, and two channels. Both liquid and gas phases are considered in the entire cathode and anode, including the channel, the diffusion layer and the catalyst layer. The Gravity effect on liquid water was considered in channels. Typical two-phase flow distributions in the cathode gas channel, gas diffuser and catalyst layer are presented. Source term and porosity term were optimized. Based on the simulation results, it is found that two-phase flow characteristics in the cathode depend on the current density, operating temperature, and cathode and anode humidification temperatures. Water mass fraction for the fuel cell with anode upward is higher than that the case with cathode-upward. Liquid water with the case of cathode-upward blocks pores in the gas diffuser layer leading to increasing the concentration polarization. Gravity of liquid water exerts the effect on the water mass fraction in the cathode.

Abstract: The porous NiO/ samaria doped ceria (SDC) cermets, which have been used as the anode-supported for solid oxide fuel cell (SOFC), are fabricated via the tape casting process. In this article using different kinds of slurry system, prepared the large size (60×60mm) anode substrates for SOFC successfully. Effects of solvent, dispersant, plasticizer, binder and the sintering temperature program were investigated. Experimental results show that thicknesses and pore rates of anode substrates are 0.6 ~ 1.5mm and 20% ~ 40% respectively. And all of the complete and flat anode substrates have highly support intensity, for their homogeneous microstructure and pore distribution. Sintering the green pieces of anode substrates at the temperature of 1300°C for 2h, different green pieces have different shrinkages that could be from 20% to 30% measuring with thermal dilatometer. Using this kind of anode substrate, could find out a perfect one to matching the electrolyte film easily.

Abstract: Cathode catalyst layer plays an important role in PEMFC. Electrochemical reaction in cathode catalyst layer is a control process for the performance in PEMFC. In this paper, oxygen reduction reaction (ORR) is studied by molecular simulations based on a series pathway which consist of four steps. We calculated the free energy of four steps respectively by molecular simulations. Comparing free energy of our steps, we found that the fourth step can release more energy than the other steps. At the same time, we found that the energy released in ORR is decreased with the increase of temperature. The process of the first step in the series pathway release less energy than that of other steps. The results are very helpful for optimization of construction in the cathode and improving performance of PEM fuel cell.

Abstract: Interest in fuel cells in recent years has promoted the development of hydrogen sources. Methane (the main composition of natural gas) is an optimal fuel for hydrogen production due to its rich resource and its high ratio of hydrogen to carbon. In this work, several hydrogen processes, such as steam reforming of methane, partial oxidation, and auto thermal reforming, were reviewed. Different processes exhibit different importance for hydrogen production due to their diversity on usages. In this paper the special method of natural hydrogen production from natural gas with super adiabatic rich combustion is depicted in details. Some problems of this method were analyzed and discussed. In view of the existing problems, a new method was developed to be used for conversion of natural gas to hydrogen. The method can solve the problems of flame drift, heat preservation, product cooling, and low transform efficiency. Due to its simple and compact structure, it is attractive for distributing hydrogen production system and solving the transportation and storage problems of hydrogen.