Meso-porous alumina capillary tube as a support for high-temperature gas separation membranes by novel pulse sequential
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A meso-porous anodic alumina capillary tube (MAAC) having highly oriented radial meso-pore channels with a minimum diameter of 3 nm has been successfully synthesized using a novel pulse sequential anodic oxidation technique at 100 Hz of pulse frequency. A value resulting in a high channel-pore formation rate at 1 V of the pulse sequential voltage was determined to be the optimum pulse frequency for the anodization. Transmission electron microscopy observation and N2 sorption analysis revealed that controlling the minimum pore channel diameter at 3 nm was possible by the voltage of 1 V. The gas permeance according to Knudsen’s diffusion mechanism was demonstrated at 500 °C, by evaluating gas permeation properties through the meso-porous anodic alumina capillary tube with radial meso-pore channels with minimum diameter of 3 nm, achieving hydrogen permeance of 1.8 × 10−6 mol/m2 s Pa.
I. INTRODUCTION
To develop high-temperature gas separation membranes, a great effort has been made to fabricate a membrane as a thin layer on a porous support, such as an amorphous silica membrane on a porous alumina support.1–15 To minimize the defects in the membrane, meso-porous ␥-alumina is often used as an intermediate thin layer between the membrane and the macro-porous support.7–15 Generally, the mean pore size in the ␥-alumina intermediate layer is controlled to be about 4 nm. However, a small number of larger macro-pores sometimes exist in the intermediate layer, which causes the formation of pinholes or cracks in the membranes. To develop high-performance gas separation membranes, it is important to develop new technologies for fabricating a porous support having an intermediate layer with precisely controlled meso-pores of single-nanometer size. An amorphous alumina membrane synthesized through the anodic oxidation of metal aluminum is wellknown for its highly oriented channel-pore structure.16 It was reported by several researches that the channelpore diameter (dp) primarily depended on the anodizing voltage (Va), and that the dp decreased linearly with decreasing the Va.17–19 Then, Ono et al. reported that the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0016 114
J. Mater. Res., Vol. 20, No. 1, Jan 2005
dp decreased to be 7 nm as a minimum at 5 V, followed by increase at 2V.20 One possible explanation for such variation of the channel-pore diameters formed at the lower voltages was the low efficiency of anodized film growth, that is, channel-pore formation rate (Rf) at low current density (Ja), as the Ja decreased exponentially with decreasing Va,20 and the Rf decreased with decreasing Ja.21 At the lower voltages below 5 V, it was impossible to keep the Rf much higher than the chemical etching rate (Rc)22 of alumina in the acid electrolyte solution, which could lead to the observed increase in dp at 2 V. Recently, we have demonstrated that a novel pulse sequential anodic oxidation technique has some advantages for increasing the current density under anodizing voltages belo
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