Mesopore Size Dependence of Protonic and Lithium Ionic Conductivity of Porous Alumina
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Mesopore Size Dependence of Protonic and Lithium Ionic Conductivity of Porous Alumina
Hideki Maekawa,1,2,3 HangYan Shen,1,2 Yutaka Fujimaki,1 Kotaro Kawata,1 Kaoru Shibata,4 Masayosi Kawai,5 and Tsutomu Yamamura1,3 1
Department of Metallurgy, Tohoku Univ, 980-8579, Sendai, Japan PRESTO, Japan Science and Technology Corporation, Kawaguchi, 332-0012, Japan 3 Center for Interdisciplinary Research, Tohoku Univ, 980-8578, Sendai, Japan 4 Neutron Scattering Research, JAERI, Tokai-mura, 319-1195, Japan 5 KENS, Institute of Materials Structure Science, Tsukuba, 305-0801, Japan 2
e-mail:[email protected]
ABSTRACT Ordered-mesoporous-Al2O3 was synthesized by the sol-gel route using neutral copolymer surfactants as templates. The pore size was controlled over the range of 3~22 nm by using different surfactant copolymers under different synthetic conditions. Protonic conductivity of hydrated mesoporous alumina was examined for CIP treated mesoporous alumina pellet. The highest protonic conductivity was obtained as 0.004 S·cm-1 at 25 oC under 80% RH for the pellet with 15 nm averaged pores. The conductivity increased monotonically with increasing pore size under constant humidity. Quantitative determination of the concentration and the mobility of protons in hydrated mesoporous alumina was performed by 1H NMR, quasi-elastic neutron scattering (QENS) and TG/DTA measurements. Both the mobility and the concentration increased with increasing pore size. On the other hand, composites composed of synthesized mesoporous-Al2O3 and lithium iodide (LiI) was prepared. Dc electrical conductivity of 50LiI·50(mesoporous-Al2O3) was 2.6×10-4 S cm-1 at room temperature, which was considerably higher than the previously reported LiI-alumina composites. A systematic dependence of conductivity upon pore size was observed, in which the conductivity increased with decreasing the pore size except for pore size=3 nm. The 7Li diffusion constant of the composite was investigated by QENS as well as pulsed field gradient nuclear magnetic resonance (PFG-NMR). The dc conductivities showed reasonable agreement with an Einstein equation using measured diffusion constants. INTRODUCTION Fabrication of ionic conductor composites having mesoscale interface and restricted geometry by artificial structure-directing methods is attractive approach for the development of high ionic conductor both in fundamental and application point of view. The enhancement of conductivity is expected by using interfacial phenomena such as space-charge effect and formation of surface specific conduction paths. A dispersion of insoluble dielectric oxide particles such as Al2O3 in certain ionic conductors is known to increase total electrical conductivity [1-4]. One explanation of this effect has been the increase of cationic defect concentration at a space-charge region near the insulator-ionic conductor interfaces [1-4]. The width of the space charge is characterized by the Debye length which is in the range of several
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10 nm for certain ionic conductors
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