1.  Field of the Invention
The present invention generally relates to a substance separation structure and a method of preparing the same, and more specifically, it relates to a substance separation structure provided with a permeable membrane formed on a porous base material having continuous holes and a method of preparing the same. 
2.  Description of the Prior Art
A gas permeation membrane is put into practice in the field of oxygen enrichment, nitrogen enrichment, carbon dioxide separation, hydrogen separation and the like.  An oxygen enrichment membrane or a nitrogen enrichment membrane concentrating oxygen or nitrogen contained in the air is employed for combustion or medical application.  A carbon dioxide separation membrane separating methane and carbon dioxide contained in natural gas from each other is employed for recovering carbon dioxide.  A hydrogen separation membrane is used for separating and recovering gaseous hydrogen employed for desulfurizing petroleum. 
Gaseous hydrogen, employed as the fuel for a fuel cell or the like, is industrially prepared by denaturation of gaseous fuel or the like.  According to the denaturation of gaseous fuel, for example, gaseous hydrogen is prepared by reforming steam, while the reformed gas contains carbon monoxide, carbon dioxide and the like as subcomponents in addition to the main component of hydrogen.  When the reformed gas is applied to the fuel for a fuel cell as such, for example, the performance of the cell is deteriorated.  Therefore, the reformed gas must be purified for removing the subcomponents other than hydrogen and obtaining high-purity gaseous hydrogen.  The reformed gas may be purified by a method utilizing the characteristic of a hydrogen-permeable membrane selectively permeating only hydrogen. 
Japanese Patent Laying-Open No.  11-267477 (1999) proposes a method of forming a hydrogen-permeable metal film such as a Pd film or an Nb film of about 0.1 to 20 xcexcm in thickness on a surface of a porous support of stainless steel or ceramics such as alumina or silicon nitride by ion plating, in order to prepare a hydrogen-permeable membrane having no pinholes. 
The hydrogen permeability of such a hydrogen-permeable membrane is in inverse proportion to the thickness thereof, and hence the thickness of the hydrogen-permeable membrane must be reduced to the utmost in order to improve the hydrogen permeability.  When a hydrogen-permeable membrane of not more than 1 xcexcm in thickness is formed on a surface of a porous base material by ion plating, however, it is impossible to form a dense membrane having no pinholes.  Therefore, a hydrogen-permeable membrane sufficiently improved in hydrogen permeability cannot be prepared. 
An object of the present invention is to provide a substance separation structure capable of forming a hydrogen-permeable membrane of not more than 1 xcexcm in thickness on a surface of a porous base material as a dense membrane having no pinholes with high hydrogen permeability and durability, and a method of preparing the same. 
The inventor has made various studies on membrane forming methods, to find out that a dense membrane having no pinholes can be prepared by polishing a surface of a porous base material with abrasive grains containing a porous material and thereafter forming a permeable membrane. 
On the basis of this recognition, a substance separation structure according to an aspect of the present invention comprises a base material including a porous material having a continuous hole with an opening of the hole formed on at least one surface, a porous layer, formed to fill up the opening formed on at least one surface of the base material, having a hole smaller than the hole of the base material, and a permeable membrane of not more than 1 xcexcm in thickness formed on at least one surface of the base material formed with the porous layer to selectively permeate ions or neutral elements or molecules, while the surface roughness of at least one surface of the base material formed with the porous layer is not more than 0.3 xcexcm in Rmax. 
In the substance separation structure according to the present invention, the surface of the base material is flattened to the surface roughness of not more than 0.3 xcexcm in Rmax while the opening of the hole formed on the surface of the base material is filled up with the porous layer, whereby the permeable membrane of not more than 1 xcexcm in thickness can be formed on the surface of the base material in a dense state with no pinholes.  Thus, permeability of the permeable membrane can be improved. 
In the substance separation structure according to the present invention, the ratio (Tb/Ta) of the mean thickness Tb of a portion of the porous layer formed on the surface of the base material outside the hole of the base material to the mean thickness Ta of a portion of the porous layer formed in the hole of the base material is set to at least 0 and not more than 1, thereby improving adhesion between the permeable membrane and the surface of the base material.  Thus, durability of the substance separation structure can be improved. 
Preferably, the porous material forming the base material is at least one material selected from a group consisting of ceramics, a metal and a complex of the ceramics and the metal. 
More preferably, the porous material forming the base material is porous silicon nitride having porosity of at least 30% and not more than 70%. 
Further preferably, the porous material forming the base material includes at least one material selected from a group consisting of iron and nickel, and has porosity of at least 60% and not more than 95%. 
In the substance separation structure according to the present invention, the permeable membrane may not be porous.  Preferably, the permeable membrane is made of a metal, an alloy or a compound containing at least one material selected from a group consisting of palladium (Pd), platinum (Pt), gold (Au), silver (Ag), niobium (Nb), tantalum (Ta), vanadium (V) and zirconium (Zr).  More preferably, the permeable membrane is formed by a single layer or a plurality of layers.  Further preferably, the permeable membrane is made of a compound having a zeolite structure or a perovskite structure. 
Preferably, the surface roughness of at least one surface of the base material not yet formed with the porous layer is not more than 0.3 xcexcm in Rmax.  More preferably, the surface roughness of at least one surface of the base material formed with the porous layer and the permeable membrane is not more than 0.3 xcexcm in Rmax. 
A method of preparing a substance separation structure according to another aspect of the present invention comprises steps of polishing a surface of a base material including a porous material having a continuous hole with an opening of the hole formed on at least one surface with abrasive grains containing a porous material to be capable of filling up the opening with a porous layer and forming a permeable membrane of not more than 1 xcexcm in thickness by plating or ion plating on the surface of the base material formed with the porous layer by the polishing. 
Preferably in the method according to the present invention, the porous material forming the base material is at least one material selected from a group consisting of ceramics, a metal and a complex of the ceramics and the metal. 
Preferably, the abrasive grains containing the porous material contain at least one material selected from a group consisting of porous aluminum oxide and titanium oxide.  More preferably, the average diameter of the abrasive grains containing the porous material is smaller than the average diameter of the opening formed on at least one surface of the base material. 
In the method according to the present invention, arc ion plating is preferably employed as the ion plating. 
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.