Notes Bull. Korean Chem. Soc. 2001, Vol. 22, No. 10 1149 High-T c Superconducting Membrane: (I) Theoretical Consideration for Gas Separation Joo-Byoung Yoon, Eue-Soon Jang, Andre Ayral, † Louis Cot, † and Jin-Ho Choy * National Nanohybrid Materials Laboratory, School of Chemistry & Molecular Engineering, Seoul National University, Seoul 151-747, Korea † Laboratoire de Matériaux et Procédés Membranaires (LMPM), ENSCM, 8 Rue de l'Ecole Normale, F34296, Montpellier, France Received November 30, 2000 Keywords : Gas separation, Superconducting membrane, Magnetic field. Magnetic separation of paramagnetic or ferromagnetic materials from diamagnetic ones has been widely studied in the context of various technologies. A selective separation of iron oxide impurity from ceramic row materials is an old subject. In the present study, a new route to paramagnetic and diamagnetic gas separation is suggested using the mag- netic properties of membranes. In general, the cryogenic process is a well-established industrial technology to separate nitrogen and oxygen gases. 1 However, it is known to be inadequate for producing a small quantity of highly pure gases. This is the reason why many researchers have been exploring a new membrane process as an alternative, which should be very simple in equipment, not be energy consuming and, therefore, quite economic to separate gases. 2-4 In many cases, the main driving force to separate gases is the molecular sieving effect using micro- porous membranes such as zeolite, silica, and alumina ones. Such a sieving technique is efficient to separate gases with a large difference in molecular size, but not with similar molecular size like nitrogen, argon, and oxygen as listed in Table 1. Recently, various materials have been developed and ap- plied to gas separation procedures, based on the difference of chemical or physical properties. The polymer membranes made of polyaniline or polyamide, for example, can be used for gas separation, which prefers oxygen molecules to nitro- gen ones. 5-6 In general the oxygen/nitrogen selectivity is often over 6 in polymer membranes, but unfortunately it suffers from low permeability. Even though the most permeable polymers developed so far show particular selectivity only, no better material is known beyond the upper bound of permeability square selectivity. On the other hand, inorganic membranes made of perovskites and molecular sieves like zeolite with alkali metal have also been extensively studied. 7-8 The former is based on the oxygen ionic conductivity and the separation factor of oxygen/nitrogen often reaches infinity, while the latter uses the quadruple moment difference bet- ween oxygen and nitrogen. But both should also suffer from low permeation and/ or low selectivity. More recently, gas separation methods have been tried involving superconducting membranes and ferromagnetic ones. 9-10 Reich et al. reported that a superconducting mem- brane could be applied to gas separation using the diamag- netic property of a superconductor, based on the idea that the permeation of diamagnetic nitrogen molecules is preferable to that of paramagnetic oxygen molecules. 9 A ferromagnetic gauze was also used in magnetic gas separation, but it needs a strong magnetic field to induce a high selectivity. Even under a 10 T magnetic field induced by a superconducting magnet, however, the (O 2 /N 2 ) selectivity does not exceed 2. More recently, K. Ishizaki et al. have demonstrated an ex- ample of a superconducting magnetic filter being applied to separate oxygen and argon gases, using a magnetic field gradient concept. That is, paramagnetic oxygen molecules can accumulate at the pore entrance, where the field gradient is the highest, and consequently permeate preferentially through the superconducting membrane, compared to diamagnetic nitrogen molecules. Also they theoretically calculated the magnetic force for oxygen molecules induced by the Meissner effect of a superconductor. A good model membrane in part of the geometrical shape, the applied field strength, and the pore size were postulated. In this study, our primary attention has been paid to comparing the present results with the previous ones, in such a way that the solution to improving the separation effici- ency in a superconducting membrane could be suggested. In 1986, Bednorz and Muller discovered superconductivity for the first time in a layered perovskite, La2-x Ba x CuO 4 , * To whom all correspondence should be addressed. Tel: +82-2- 880-6658, Fax: +82-2-872-9864, e-mail: jhchoy@plaza.snu.ac.kr Table 1. Physical properties of various gas molecules Gas molecules Magnetic property at boiling temperature [χm (10 -6 cm 3 /mol)] Boiling temperature (K) Kinetic diameter (Å) Oxygen (O2) Paramagnetic (+3402 g) (+7667 liq) 90.1 3.46 Nitrogen (N2) Diamagnetic (-12.05) 77.4 3.64 Argon (Ar) Diamagnetic (-6.99) 87.3 3.35 Nitric acid (NO) Paramagnetic (+1461) 121.4 ~3.8 Carbon dioxide (CO2) Diamagnetic (-20) 194.7 ~4.0