Hong Lim Lee

Oxygen-ion conductivity of BaO- and MgO-doped LaGaO3 electrolytes

Abstract The oxygen-ion conductivity of LaGaO 3 electrolytes doped with BaO and MgO is studied. Curved behaviour of the oxygen ion conductivity observed in the Arrhenius plot. Therefore, the activation energy in the low-temperature region is higher than that in the high-temperature region. The dopant cation-oxygen vacancy association, especially the Mg Ga ′−V ·· cluster, may contribute to increase the activation energy in the low-temperature region

Oxygen ion conductivity and cell performance of La0.9Ba0.1Ga1-xMgxO3-δ electrolyte

Abstract La 0.9 Ba 0.1 Ga 0.8 Mg 0.2 O 3− δ , an alternative electrolyte candidate for the ceramic fuel cell, exhibits an oxygen ion conductivity higher than 0.1 S/cm at 800°C. The maximum power density of the single cell Ni/electrolyte composite anode/La 0.9 Ba 0.1 Ga 0.8 Mg 0.2 O 3− δ electrolyte/Sm 0.5 Sr 0.5 CoO 3− δ cathode system was measured as 0.21 W/cm 2 at 1000°C.

Preparation of Al2TiO5 from alkoxides and the effects of additives on its properties

Al2TiO5 was prepared by the sol-gel method from alkoxides and its mechanical and thermal properties measured. The prepared Al2TiO5 powder was very fine and had a narrow particle-size distribution. The addition of mullite and Al2O3 to the prepared Al2TiO5 inhibited the grain growth during sintering, resulting in a decrease of microcracking and an increase of fracture strength. Al2TiO5/mullite composite exhibited a higher fracture strength than Al2TiO5/alumina composite. The thermal expansion coefficient of Al2TiO5 increased with the addition of mullite and alumina, and also increased with temperature up to 1000°C; however, it decreased in the temperature range between 1000 and 1200°C during heating, due to decomposition of Al2TiO5. The addition of mullite inhibited the decomposition of Al2TiO5, but the addition of Al2O3 accelerated it. Al2TiO5 prepared from metal alkoxides was also more stable than that prepared from the commercial alumina and titania powders.

Proton conduction in La0.6Ba0.4ScO2.8 cubic perovskite

Abstract The proton conduction in single phase La 0.6 Ba 0.4 ScO 2.8 with a cubic perovskite structure was studied. The endothermic peak in differential thermal analysis (DTA) curve and the weight loss in thermal gravimetric analysis (TGA) one for as-sintered sample were observed between 340 and 410 °C during the heating process in dry N 2 atmosphere. These results were due to the dehydration of the water, which might have been incorporated into oxygen vacancies during the cooling process after sintering. In wet N 2 atmosphere ( p H 2 O =6.1 hPa), a plateau of the conductivity data was observed between 450 and 300 °C in the plot of log( σT ) versus the reciprocal temperature while the conductivity data measured in dry atmosphere decreased linearly over the measured temperature range, implying that proton conduction was dominant below 450 °C.

Electrical Conduction Behavior of BaO-doped LaInO3 Perovskite Oxide

In the La1-xBaxInO3-δ system, the compositions of x=0.1–0.3 were analyzed to consist of cubic and orthorhombic structures. The composition of x=0.4, viz., La0.6Ba0.4InO3-δ, was analyzed to be a single phase of a simple cubic. Under a dry atmosphere, La0.6Ba0.4InO3-δ exhibited a mixed conduction behavior of oxide ion and p-type electronic conductions at high oxygen partial pressures. However, in a nitrogen atmosphere, it showed pure oxide ion conduction. Proton conduction was observed in the presence of water vapor and the activation energy for proton conduction was measured as 0.84 eV.

Effect of SiC on the mechanical properties of 3Y-TZP/SiC composites

Although yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) have high strength and high fracture toughness at room temperature, it cannot be generally used for high-temperature applications of structural ceramics because its mechanical properties drastically degrade at high temperatures [1]. To overcome this obstacle TZP composites dispersed with the second phase such as SiC whiskers have been studied [2, 3]. However, there are serious problems with SiC whisker-reinforced TZP composites because of whisker agglomeration due to inhomogeneous dispersion. This study has been carried out in order to enhance the mechanical properties of Y-TZP ceramics not only at room temperature but also at high temperatures by dispersing SiC powder into the Y-TZP matrix, since SiC powder has a high elastic modulus, high microhardness and high strength even at high temperatures. The mechanical properties and microstructures of Y-TZP/SiC composites have been studied. 3Y-TZP powder (Tosho Corp., Japan) and/3-SIC powder (H. C. Stark, Germany), with average particle diameters of 0.5 and 0.62/zm, respectively, were used as the starting materials. Up to 30 vol % /3-SIC powder was homogeneously mixed into 3Y-TZP powder matrices by wet-milling with ethanol. The mixtures were dried at 120 °C and hotpressed at 1500 °C for 60 min in Ar. The densities of the hot-pressed specimens were measured by electronic densimeter (ED-120T, Mirage Trading Co., Japan) and relative densities were determined by comparing with theoretical densities which were calculated using the mole fraction of Y203 and ZrO2 unit cell size [4]. The microstructures of 3Y-TZP/SiC composites were observed by scanning electron microscope. Phase analysis was conducted by X-ray diffractometer for the specimens before and after flexural strength testing [5]. Flexural strength was measured by a bend tester (Model 4024, Instron, Co., USA) and fracture toughness was measured by the single edge notched beam method. 3Y-TZP ceramics showed a relative density higher than 99.9% when hot-pressed at 1500 °C for i h in Ar. The density of the 3Y-TZP/SiC composites, however, slightly decreased as SiC content increased and was observed to be higher than 98% even with 30 vol % SiC powder addition. Fig. la and b are scanning electron micrographs (SEMs) of the polished surfaces of 3Y-TZP and 3Y-TZP/30 vol % SiC ceramics. The grains of the SiC-reinforced

Phase Formation and Electrical Conductivity of Ba-Doped LaScO3

Phase formation and conduction behavior in BaO-doped LaScO3 with the perovskite structure have been investigated. The structures of La1-xBaxScO3-δ were analyzed to be cubic for La0.6Ba0.4ScO3-δ and La0.5Ba0.5ScO3-δ, a mixture of cubic and orthorhombic for La0.9Ba0.1ScO3-δ, La0.8Ba0.2ScO3-δ and La0.7Ba0.3ScO3-δ, and orthorhombic for La0.95Ba0.05ScO3-δ. La1-xBaxScO3-δ showed a pure oxide-ion conduction below intermediate oxygen partial pressures under dry atmosphere. At high oxygen partial pressures, it exhibited mixed oxide-ion and p-type electronic conduction. In the presence of water vapor, proton conduction was observed due to the formation of protonic defects and the bulk conductivity by protons was highest in the composition of La0.5Ba0.5ScO2.75. The apparent activation energy for proton conduction was almost the same in each compound, 0.66–0.68 eV, despite the differences in crystal structure and A-site dopant concentration.

Phase relationship of barium and magnesium doped LaGaO3 perovskite oxides

Abstract The formation of phases in Ba2+ and Mg2+ ions doped perovskite system and the relationship among the formed phases were investigated by the powder X-ray diffraction method. Ba2+ ion was hardly dissolved into La site in the La1−xBaxGaO3−δ system so that BaLaGa3O7 and BaLaGaO4 were formed as the secondary phases. In the LaGa1−yMgyO3−δ system, the LaGaO3 single phase was produced in the composition region y≤0.25 and La4Ga2O9 was produced at the composition y=0.3 as the secondary phase. In the La1−xBaxGa1−yMgyO3−δ system, Ba2+ ion could be dissolved into La site of the LaGaO3 structure together with dissolution of some Mg2+ ion into Ga site. A new compound La3GaMgO7 of a tetragonal structure was found in the compositions doped by large amount of Mg2+ ion.

Effect of environmental factors on thermal shock behaviour of polycrystalline alumina ceramics

The effect of the environmental factors on thermal shock behaviour of polycrystalline alumina ceramics was studied by quenching the alumina specimens into various quenching media. The environment factors of quenching media were controlled by changing the temperature of water and changing the concentration of the propylene glycol/water solution. The convection heat transfer coefficient and thermal stress increased as the temperature of cooling water increased and decreased as the concentration of the propylene glycol in water increased. The critical thermal stress which makes the cracks grow catastrophically was found to be generated by the critical cooling rate, and the critical cooling rate of alumina ceramics was found to be a certain value (550 °C/s) and same for all cooling liquids. Therefore, cooling rate was found to be the most influential of the environmental factors in thermal shock.

Submicron Al2O3/SiC composite powder preparation by SHS technique

The main purpose of the present study is to prepare Al 2 O 3 -SiC composite powder by SHS technique using the thermite reaction of Al to supply the heat required for the synthesis of SiC. SiO 2 , Al and C (graphite) powders were used as starting materials, the average particle sizes of which were 1.6μm, 3.7μm and 0.6μm, respectively.