Enhanced Ionic Conduction Observed for Ordered-Mesoporous Alumina-Ionic Conductor Composites
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Enhanced Ionic Conduction Observed for Ordered-Mesoporous Alumina-Ionic Conductor Composites Hideki Maekawa,1,2 Ryo Tanaka,1 and Tsutomu Yamamura1 1 Tohoku Univ, Dept of Metallurgy, Sendai, Japan 2 PRESTO, Japan Science and Technology Corporation, Japan. ABSTRACT Ordered-mesoporous Al2O3 was synthesized by the sol-gel method using neutral surfactants as templates. The pore size can be controlled over the range of 2.8~12.5 nm by using different surfactant copolymers and by different synthetic conditions. By utilizing cyclohexane as a co-solvent, mesoporous Al2O3 having relatively mono-dispersed particle size was obtained. Composites composed of the synthesized mesoporous Al2O3 and the lithium ion conductor (LiI) was prepared. The dc electrical conductivity of 50LiI ·50(mesoporous Al2O3) was 2.6×10-4 S cm-1 at room temperature, which is more than 100 times higher than that of pure LiI. The pore size dependence of the conductivity of LiI-mesoporous Al2O3 composite was examined. A systematic dependence of conductivity upon pore size was observed, in which the conductivity increased with decreasing the pore size. INTRODUCTION A dispersion of insoluble dielectric oxide particles such as Al2O3 in certain ionic conductors is known to increase total electrical conductivity [1-3]. This effect was observed for the first time on the LiI-Al2O3 system [1]. The mechanism of the conductivity enhancement was suggested to be the increase of cationic defect concentration at a space-charge region near the insulator-ionic conductor interfaces [1-3]. The width of the space charge is characterized by the Debye length which is in the range of several 10 nm for certain ionic conductors [4]. In the case where the distance between two interfaces is comparable to the Debye length, a further conductivity enhancement due to a “nano-size” effect was predicted, as a result of an overlapping space-charge layer [5]. The purpose of the present investigation is to prepare the LiI-mesoporous Al2O3 composites with various pore sizes and to investigate the channel size dependence on the conductivity. EXPERIMENTAL DETAILS Preparation and Characterization of the Mesoporous Al2O3 Powdered mesoporous Al2O3 was prepared by the sol-gel method utilizing neutral surfactants as templates [6-8]. Surfactants used were Span80, Span85 and Triton X114 (Aldrich). Surfactants, Als(OBt)3 (Kanto Chemicals), and LaCl3·7H2O (Nacalai Tesque) were dissolved in the solvent (cycohexane, Wako Chemicals) and stirred for 30 minutes. To this solution, water was added dropwise and the solution was let stirred for 15 h at 318 K. The resulting sol was filtered and dried at 373 K. Mesoporous-Al2O3 was obtained by calcination of the sol at 773 K for 6 h. Pore size, pore volume and BET surface area were obtained by N2 adsorption and desorption isotherms at 77 K by using Micromeritics ASAP2010. X-ray powder diffraction
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Table 1. Synthetic conditions and physical properties of the mesoporous Al2O3. Sample
Surfactants
A B C D E
Span 80 Span 85 Triton X-114 Triton X-1
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