Development of super high temperature-type proton conductor based on alpha alumina.

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Firstly, the most appropriate dopant for incorporation of hydrogen was studied using single crystal of alpha alumina. With the measurements of IR absorption of OH stretching vibration, the magnesium is found most effective dopant among Mg,Ni,Co, and Ti. In order to improve the low yield of crystal growth due to the thermal stress, the additions of Cr,Y,B,Sm,Nd,La,Ce as the second dopant were examined. However, the any explicit improvements were not obtained.The location of the incorporated hydrogen in the crystal structure of alpha-alumina was investigated with the Mg doped samples. The polarization angle dependency of the absorption of OH stretching vibration was examined for several sets of the directions of the incident beam and crystal faces. The comparison between the measurement and the theoretically calculated results showed that hydrogen is located between one of the 0-0 bond constituting the edges of oxygen octahedron in the corundum structure with making the hydrogen bond. Am … More ong the four 0-0 bonds of different spacing, one whose spacing is 272 pm is found preferred for hydrogen location. The same tendency was observed for Ti doped alpha-alumina.The H/D isotope effect of electrical conductivity was examined for Mg doped single crystal samples. The proton was confirmed to be the most dominant charge carrier in the low oxygen potentials region. The anisotropy was observed also in the electrical conduction. The conductivity was higher when the direction of the electric field is perpendicular to c-axis than pararell, which was reasonably explained by the preference of hydrogen location among the O-O bonds.As the Mg was found to be the most appropriate dopant, the electrochemical properties were studied with the commercially supplied polycrystalline- alumina tube whose main impurities were magnesium. The emf of the hydrogen concentration cells using these tubes as the electrolyte were measured for various hydrogen and oxygen potential gradients. These values were analyzed theoretically based on the model of the crystal defects and the equation between the emf of the cell and the potentials difference on the electrodes was derived. The parameter appeared in the equation was determined as a function of the temperature.The electrochemical properties of magnesium-aluminate spinel which is almost always the co-existing phase of Mg-doped polycrystalline -alumina were also studied to know the effect of second phase on the properties of the proton conduction. It was confirmed that hydrogen might be incorporated into magnesium-aluminate spinel as proton and that the it was movable. However, the conductivity due to magnesium ion was found far higher and the contribution of proton to the electrical conductivity could be ignored. Less