Key Engineering Materials Vols. 336-338

Abstract: YSZ-NiCr powder was synthesized by gel-reduction process at 850°C for 2 h. YSZ-NiCr cermet was fabricated by hot-press sintering at 1350°C for 1 h. The powder has a mean particle size of 42 nm and the sintered specimen has a fine and homogeneous microstructure with a mean crystalline size of 0.2 &m. The conductivity has a tendency to decrease with increasing temperature. This behavior can be accounted for that there are two conduction paths through the cermet, an electronic path through the Ni/Cr metal phase and an ionic path through the ZrO2-Y2O3 phase. The objective of this work is to give a possible improvement in the cermet anode of SOFC.

Abstract: Gelcasting and dry pressing are used to produce anode composites for SOFC. In this paper a composite material Ni/Ce-YSZ containing Ni and Ce-doped Y2O3-stabilized ZrO2 (Ce-YSZ) based on Ce-YSZ electrolyte was fabricated via sintering infiltration of NiO at the temperature of 1400°C, followed Ni/Ce-YSZ materials are successfully produced by reduction- sintering process. Hydrogen reduction of NiO in the NiO/Ce-YSZ anode material was accomplished successfully through pore channels of debinded carbon volume. XRD results suggest that a solid solution of Ce-YSZ was produced. The microstructure of NiO/Ce-YSZ, and Ni/Ce-YSZ material was observed by SEM. This study confirms that sintering infiltration of NiO is a viable process for fabricating anode materials based on zirconia electrolyte and there are a good interface between electrolyte and anode.

Abstract: Mechanical mixtures of zirconia xerogel with variable content of crystalline Y2O3 up to 25 mol%, were hydrothermally treated by microwave route at 110 °C for 2 hours in the presence of 0.2 M solution of (KOH+K2CO3) mineralizer. The resulting amorphous hydrated zirconia-yttria solid solutions with a maximum solubility of Y2O3 content between 20 ~ 25 mol%, showed a remarkable reduction of the surface area at the increasing Y2O3 content of the starting mixture. The as-synthesized products and the corresponding calcined powders at 400 °C were uniaxially pressed into pellets (10 x 7 x 2 ~ 4 mm, in width) at 150 MPa. Conductivities were measured at 25 °C by AC impedance method with a frequency range from 10 Hz to 1 MHz with the pellets equilibrated either under silica gel or under increasing relative humidity (RH) up to ~90 %. The effects of composition, surface area, calcination temperature and relative RH on the proton conductivity of the amorphous solid solutions are discussed.

Abstract: Ceria electrolytes co-doped with Gd3+ and Y3+ (Ce0.8Gd0.2-xYxO1.9) were synthesized by GNP. The crystal structures, lattice parameters and crystallite sizes were determined by XRD. The morphology of the powders was observed by TEM. Sintered at 1250oC for 4 h, all die-pressed ceria pellets achieved densities higher than 95% of theoretical values. The ionic conductivities were measured using AC impedance technique and Ce0.8Gd0.05Y0.15O1.9 showed a higher ionic conductivity than those of other compositions at 500-750oC, indicating that it is an attractive electrolyte material for IT-SOFC.

Abstract: Sm0.1Ce0.9O1.95 (SDC) films, as promising electrolyte materials, have been successfully prepared on Al2O3 ceramic substrates by RF magnetron sputtering growth. The relationship between sputtering parameters and film microstructure was discussed. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) were used to characterize the morphology and crystal structure of the SDC films. The SEM images show that the crystallinity becomes better and better with the increase of sputtering power from 100W to 300W. X-ray diffraction patterns indicate that the thin SDC electrolyte film on the Al2O3 ceramic substrate grows preferentially along the (111) compact plane with a pure fluorite structure when the RF power reaches 300W. After annealing treatment at 600°C and 800°C, respectively, it can be seen that SDC film becomes denser with few pinholes at the annealing temperature of 800°C. High oxygen ion conductivity (1.46×10-2 Scm-1) of the SDC film was obtained when the RF sputtering power and annealing temperature were 300W and 800°C, respectively.

Abstract: The structure, thermal expansion coefficients, and electric conductivity of Ce1-xGdxO2-x/2 (x = 0 ~ 0.6) solid solutions, prepared by gel-combustion method, were investigated. The uniform small particle size of the gel-combustion prepared materials allows sintering of the samples into highly dense ceramic pellets at 1300°C, a significantly lower temperature, compared to that of 1600~1650°C required for ceria solid electrolytes prepared by traditional solid state techniques. XRD showed that single-phase solid solutions have been formed in all investigated range. The maximum conductivity, σ600°C = 5.26×10-3S/cm, was found at x = 0.2. The thermal expansion coefficient, determined from high-temperature X-ray data is 8.125×10-6 K-1 at x = 0.2.

Abstract: Well-shaped octahedral crystals of NaX zeolite of a large size of 30μm were synthesized by a hydrothermal method in a mother solution having a 3.5Na2O : Al2O3 : 2.1SiO2 : 593~2000H2O composition. Thermal treatment of NaX zeolite crystals results in the formation of an intermediate amorphous phase (T < 1000oC) and crystalline phase of aluminium silicate at temperature above 800 and 900oC. Environmental scanning electron microscopy (ESEM), High resolution Transmission Electron Microscopy (HRTEM), X-ray powder diffraction (XRD), Fourier transform infrared(FT-IR) spectroscopy, DTA/TGA and BET analysis were used to characterize the initial materials and the obtained products after various heat treatments.

Abstract: The solid-state reaction method and the liquid citrate method were used respectively to synthesize SrCe0.9Y0.1O3-α (SCY10) powders. SCY10 membranes were prepared from each kind of the powders by sintering at 1450°C for 10 hours. XRD was employed to examine the phase of the powders and the membranes. The microstructure of the membranes was observed with SEM. The sintering ability and electrical properties of the membranes were investigated. The results indicate that the membrane prepared from SCY10 powder synthesized by the liquid citrate method has higher relative density and nearly the same conductivity, compared with the membrane prepared from SCY10 powder synthesized by solid-state reaction.

Abstract: A kind of novel composite electrolyte, consisting of zinc doped ceria and carbonates, was developed. The phase and morphology of the composite material were characterized. Ceramic fuel cells based on this composite electrolyte were constructed with two techniques. The hydrogen-air fuel cell fabricated with anode-supported technique showed relative low open circuit voltages (OCVs) and high output performance, but the electrolyte-supported one exhibited relative high OCVs and low output performance. During the fuel cell operation, water was observed at both anode and cathode sides, indicating hybrid ions conduction lies in the composite electrolyte under fuel cell environment.

Abstract: A new type of limiting current oxygen sensor which uses yttria (8%mol) stabilized zirconia (YSZ) as oxygen ion conducting solid electrolytes and dense La0.8Sr0.2FeO3 (LSF) as diffusion barrier was developed successfully. The oxygen sensor shows excellent performance at oxygen concentrations range of from 0 to 21%. The advantages of the sensor are simple construction, low cost and potential long term stability.