Patent Publication Number: US-7722923-B2

Title: Process for producing multifunctional material

Description:
TECHNICAL FIELD 
     The present invention relates to a method for producing a multifunctional material. More particularly, the present invention relates to a method for producing a multifunctional material which can readily adsorb even volatile organic compounds (VOCs), which has a large surface area and thus exhibits high photocatalytic activity, which has high coating hardness, and which exhibits excellent heat resistance, corrosion resistance, exfoliation resistance, and wear resistance. 
     BACKGROUND ART 
     Hitherto, titanium dioxide (TiO 2 ) (referred to simply as “titanium oxide” in the present specification and appended claims) has been known to be a substance exhibiting a photocatalytic function. Since the 1970&#39;s, attempts have been made to form a titanium oxide film on the surface of metallic titanium. Known titanium oxide film formation methods include a method in which a titanium oxide film is formed on the surface of metallic titanium through anodization; a method in which a titanium oxide film is thermally formed on the surface of a metallic titanium sheet in an electric furnace supplied with oxygen; and a method in which a metallic titanium sheet is heated in the combustion flames of city gas at 1,100 to 1,400° C., to thereby form a titanium oxide film on the surface of the metallic titanium sheet (see Non-Patent Document 1). 
     In order to produce a photocatalytic product exhibiting deodorant, antibacterial, antifog, or antifouling effect by virtue of its photocatalytic function, generally, a substrate is coated with titanium oxide sol through spray coating, spin coating, dipping, or a similar technique, thereby forming a film (see, for example, Patent Documents 1 to 3). However, the formed coating film is likely to undergo exfoliation or wear, and therefore the thus-produced product is difficult to use over a long period of time. Meanwhile, there has been known a method for forming a photocatalytic coating through sputtering (see, for example, Patent Documents 4 and 5). 
     As has also been known, when crystal nuclei produced through any process (e.g., CVD or PVD) are placed in a sol containing an inorganic or organic metal compound, to thereby grow titanium oxide crystals from the crystal nuclei, or when a sol is applied to the crystal nuclei, followed by solidification and thermal treatment, to thereby grow titanium oxide crystals from the crystal nuclei, the thus-grown titanium oxide crystals assume a columnar shape, and exhibit high photocatalytic activity (see, for example, Patent Documents 6 to 8). However, in such a case, columnar crystals are grown only from seed crystals provided on a substrate, and thus the thus-grown columnar crystals exhibit insufficient bonding strength to the substrate. Therefore, the thus-produced photocatalyst is not necessarily satisfactory in terms of durability (e.g., wear resistance). 
     Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 09-241038 
     Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 09-262481 
     Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 10-053437 
     Patent Document 4: Japanese Patent Application Laid-Open (kokai) No. 11-012720 
     Patent Document 5: Japanese Patent Application Laid-Open (kokai) No. 2001-205105 
     Patent Document 6: Japanese Patent Application Laid-Open (kokai) No. 2002-253975 
     Patent Document 7: Japanese Patent Application Laid-Open (kokai) No. 2002-370027 
     Patent Document 8: Japanese Patent Application Laid-Open (kokai) No. 2002-370034 
     Non-Patent Document 1: A. Fujishima, et al., J. Electrochem. Soc. Vol. 122, No. 11, pp. 1487-1489, November 1975 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     An object of the present invention is to provide a method for producing a multifunctional material which exhibits high photocatalytic activity, which can readily adsorb even VOCs, which has a high coating hardness, and which exhibits excellent heat resistance, corrosion resistance, exfoliation resistance, and wear resistance. 
     Means for Solving the Problems 
     In order to achieve the aforementioned object, the present inventor has conducted extensive studies. As a result, the present inventor has found that when a substrate in which at least a surface layer thereof is formed of titanium, titanium oxide, a titanium alloy, or a titanium alloy oxide is thermally treated under specific conditions by applying a combustion flame of an any desired fuel directly to a surface of the substrate, or when the surface of the substrate is thermally treated in an oxygen-containing gas atmosphere under specific conditions, a layer with forest microcolumns (hereinafter such a layer may be referred to as a “layer formed of forest microcolumns”) which are formed of titanium oxide or a titanium alloy oxide is provided in the interior of the surface layer; that when the layer formed of the forest microcolumns is cut in parallel with the surface layer, there are produced a material including the substrate and a layer formed of the titanium-oxide- or titanium-alloy-oxide-formed forest microcolumns exposed on at least a portion of the substrate, as well as a material including a thin film, numerous continuous small-width protrusions which are provided thereon, and forest microcolumns formed on the protrusions, wherein the protrusions and microcolumns are formed of titanium oxide or a titanium alloy oxide and exposed on the thin film; and that both the thus-produced materials are useful multifunctional materials. The present invention has been accomplished on the basis of these findings. 
     Accordingly, the present invention provides a method for producing multifunctional materials, the method being characterized by comprising thermally treating a substrate in which at least a surface layer thereof is formed of titanium, titanium oxide, a titanium alloy, or a titanium alloy oxide by applying a combustion flame directly to a surface of the substrate so that the temperature of the surface layer is raised to 600° C. or higher, or thermally treating a surface of the substrate in an oxygen-containing gas atmosphere so that the temperature of the surface layer is raised to 600° C. or higher, to thereby provide, in the interior of the surface layer, a layer formed of forest microcolumns of titanium oxide or a titanium alloy oxide; and subsequently cutting the layer formed of the forest microcolumns in parallel with the surface layer through application of, for example, thermal stress, shear stress, or tensile force, to thereby yield a material including the substrate and a layer formed of the titanium-oxide- or titanium-alloy-oxide-formed forest microcolumns exposed on at least a portion of the substrate, as well as a material including a thin film, numerous continuous small-width protrusions which are provided thereon, and forest microcolumns formed on the protrusions, wherein the protrusions and microcolumns are formed of titanium oxide or a titanium alloy oxide and exposed on the thin film. 
     The present invention also provides a method for producing multifunctional materials, the method being characterized by comprising thermally treating a substrate in which at least a surface layer thereof is formed of titanium, titanium oxide, a titanium alloy, or a titanium alloy oxide by applying a combustion flame directly to a surface of the substrate so that the temperature of the surface layer is raised to 600° C. or higher, or thermally treating a surface of the substrate in an oxygen-containing gas atmosphere so that the temperature of the surface layer is raised to 600° C. or higher, to thereby provide, in the interior of the surface layer, a layer formed of forest microcolumns of titanium oxide or a titanium alloy oxide; and subsequently cutting the layer formed of the forest microcolumns in parallel with the surface layer through application of thermal stress due to a difference in temperature between the surface and a back surface of the substrate, to thereby yield a material including the substrate and a layer formed of the titanium-oxide- or titanium-alloy-oxide-formed forest microcolumns exposed on at least a portion of the substrate, as well as a material including a thin film, numerous continuous small-width protrusions which are provided thereon, and forest microcolumns formed on the protrusions, wherein the protrusions and microcolumns are formed of titanium oxide or a titanium alloy oxide and exposed on the thin film. 
     EFFECTS OF THE INVENTION 
     The production method of the present invention produces the following multifunctional materials: a material including a substrate and a layer formed of forest microcolumns of titanium oxide or a titanium alloy oxide, the microcolumns being exposed on at least a portion of the substrate; and a material including a thin film, numerous continuous small-width protrusions which are provided thereon, and forest microcolumns formed on the protrusions, wherein the protrusions and microcolumns are formed of titanium oxide or a titanium alloy oxide and exposed on the thin film. Each of these materials exhibits high photocatalytic activity, can readily adsorb even VOCs, has high coating hardness, and exhibits excellent heat resistance, corrosion resistance, exfoliation resistance, and wear resistance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a micrograph of a multifunctional material obtained in Example 1. 
         FIG. 2  is a micrograph showing the state of the surface of a thin film of a fragmented material  3 , as viewed from the side opposite the thin film surface having thereon numerous continuous white small-width protrusions formed of titanium oxide on which forest microcolumns are exposed. 
         FIG. 3  is a micrograph showing the state of the surface of the fragmented material  3 , as viewed from the side where the thin film has thereon the numerous continuous white small-width protrusions formed of titanium oxide on which the microcolumns are exposed. 
         FIG. 4  is a micrograph showing the state of a layer  2  formed of white forest microcolumns of titanium oxide. 
         FIG. 5  is a micrograph showing the state of a titanium sheet surface  1  from which the layer  2  formed of the forest microcolumns has been removed. 
         FIG. 6  is a micrograph showing the state of a layer formed of white forest microcolumns of titanium oxide, the layer constituting a material obtained in Example 5, wherein the layer formed of the forest microcolumns is exposed on the major portion of a titanium sheet surface. 
         FIG. 7  is a graph showing the results obtained in Test Example 4 (antifouling test). 
         FIG. 8  is a graph showing the results obtained in Test Example 5 (crystal structure and bond characteristics). 
         FIG. 9  is an SEM micrograph showing a test piece heated for 120 seconds in Example 8. 
         FIG. 10  is an SEM micrograph showing a test piece heated for 180 seconds in Example 8. 
         FIG. 11  is an SEM micrograph showing a test piece heated for 480 seconds in Example 8. 
     
    
    
     BEST MODES FOR CARRYING OUT THE INVENTION 
     The substrate employed in the production method of the present invention, in which at least a surface layer thereof is formed of titanium, titanium oxide, a titanium alloy, or a titanium alloy oxide, may be entirely formed of titanium, titanium oxide, a titanium alloy, or a titanium alloy oxide. Alternatively, the substrate may include a top-surface-forming layer formed of titanium, titanium oxide, a titanium alloy, or a titanium alloy oxide, and a base formed of a material other than such a titanium material. No particular limitation is imposed on the form of the substrate for providing a final product exhibiting photocatalytic activity, and the substrate may have any form (e.g., a flat sheet form or three-dimensional form). 
     In the case where the substrate, in which at least a surface layer thereof is formed of titanium, titanium oxide, a titanium alloy, or a titanium alloy oxide, includes a top-surface-forming layer formed of titanium, titanium oxide, a titanium alloy, or a titanium alloy oxide, and a base formed of a material other than such a titanium material, the thickness (amount) of the top-surface-forming layer may be equal to that of a layer formed of forest microcolumns of titanium oxide or a titanium alloy oxide (i.e., the entirety of the top-surface-forming layer serves as the layer formed of the titanium-oxide- or titanium-alloy-oxide-formed forest microcolumns), or may be greater than that of the layer formed of the titanium-oxide- or titanium-alloy-oxide-formed forest microcolumns (i.e., a portion of the top-surface-forming layer (in a thickness direction) serves as the layer formed of the forest microcolumns, and the remaining portion does not undergo any change). No particular limitation is imposed on the material of the base, so long as the material does not burn, melt, or deform during thermal treatment in the production method of the present invention. The base may be formed of, for example, iron, an iron alloy, a nonferrous alloy, glass, or ceramic material. Such a substrate including a thin-film-form surface layer and a base may be prepared through, for example, a process in which a coating of titanium, titanium oxide, a titanium alloy, or a titanium alloy oxide is formed on the surface of a base through sputtering, vapor deposition, spraying, or a similar technique, or a process in which a commercially available titanium oxide sol is applied onto the surface of a base through spray coating, spin coating, or dipping, to thereby form a coating on the base surface. The thickness of the thus-formed surface layer is preferably 0.5 μm or more, more preferably 4 μm or more. 
     In the production method of the present invention, no particular limitation is imposed on the titanium alloy employed, and a variety of known titanium alloys may be employed. Examples of titanium alloys which may be employed include Ti-6Al-4V, Ti-6Al-6V-2Sn, Ti-6Al-2Sn-4Zr-6Mo, Ti-10V-2Fe-3Al, Ti-7Al-4Mo, Ti-5Al-2.5Sn, Ti-6Al-5Zr-0.5Mo-0.2Si, Ti-5.5Al-3.5Sn-3Zr-0.3Mo-1Nb-0.3Si, Ti-8Al-1Mo-1V, Ti-6Al-2Sn-4Zr-2Mo, Ti-5Al-2Sn-2Zr-4Mo-4Cr, Ti-11.5Mo-6Zr-4.5Sn, Ti-15V-3Cr-3Al-3Sn, Ti-15Mo-5Zr-3Al, Ti-15Mo-5Zr, and Ti-13V-11Cr-3Al. 
     The production method of the present invention employs a combustion flame or a high-temperature oxygen-containing gas atmosphere. The combustion flame may originate from, for example, a solid fuel, hydrogen, or a gas predominantly containing a hydrocarbon. As used herein, the term “gas predominantly containing a hydrocarbon” refers to a gas containing a hydrocarbon in an amount of 50 vol. % or more; for example, a gas containing, in an amount of 50 vol. % or more, natural gas, LPG, a hydrocarbon (e.g., methane, ethane, propane, butane, ethylene, propylene, or acetylene), or an appropriate gas mixture thereof, and containing, for example, air, hydrogen, or oxygen in an appropriate amount. No particular limitation is imposed on the components (other than oxygen) of the oxygen-containing gas atmosphere, so long as the atmosphere contains oxygen. The gas atmosphere may be, for example, exhaust gas generated through combustion of the aforementioned fuel, unburned gas of the aforementioned fuel, or a gas not containing the aforementioned fuel. In the production method of the present invention, when the surface layer of the substrate which is thermally treated is formed of titanium or a titanium alloy, oxygen for oxidizing the surface layer is required, and the combustion flame or combustion exhaust gas employed must contain air or oxygen in an amount required for oxidizing the surface layer. 
     In the production method of the present invention, the substrate, which has a surface layer formed of titanium, titanium oxide, a titanium alloy, or a titanium alloy oxide, is thermally treated at high temperature by applying a combustion flame directly to a surface of the substrate; or the surface of the substrate is thermally treated at high temperature in an oxygen-containing gas atmosphere. The thermal treatment may be performed by means of, for example, a gas burner, or may be performed in a furnace. 
     The temperature and time for the thermal treatment must be regulated so that a layer formed of forest microcolumns of titanium oxide or a titanium alloy oxide is provided in the interior of the surface layer formed of titanium, titanium oxide, a titanium alloy, or a titanium alloy oxide; and subsequently the layer formed of the forest microcolumns is cut in parallel with the surface layer through application of, for example, thermal stress, shear stress, or tensile force, to thereby yield a material including the substrate and a layer formed of the titanium-oxide- or titanium-alloy-oxide-formed forest microcolumns exposed on at least a portion of the substrate, as well as a material including a thin film, numerous continuous small-width protrusions which are provided thereon, and forest microcolumns formed on the protrusions, wherein the protrusions and microcolumns are formed of titanium oxide or a titanium alloy oxide and exposed on the thin film. Preferably, the thermal treatment is performed at 600 to 1,500° C. for more than 200 seconds, or at a temperature higher than 1,500° C. More preferably, the thermal treatment is performed at 600 to 1,500° C. for more than 400 seconds, or at a temperature higher than 1,500° C. 
     The thermal treatment under the aforementioned conditions forms an intermediate piece including a layer formed of forest microcolumns (layer height: about 1 to about 20 μm, average diameter of the microcolumns: about 0.2 to about 3 μm), and a thin film provided on the layer and having a thickness of about 0.1 to about 10 μm. Thereafter, the layer formed of the forest microcolumns is cut in parallel with the surface layer through application of, for example, thermal stress, shear stress, or tensile force, to thereby yield a material including the substrate and a layer formed of the titanium-oxide- or titanium-alloy-oxide-formed forest microcolumns exposed on at least a portion of the substrate (i.e., the entirety of the thin film, which is provided on the layer formed of the forest microcolumns on the substrate, may be exfoliated from the layer, or a portion of the thin film may fail to be exfoliated and remain on the layer), as well as a material including the thin film, numerous continuous small-width protrusions which are provided thereon, and forest microcolumns formed on the protrusions, wherein the protrusions and microcolumns are formed of titanium oxide or a titanium alloy oxide and exposed on the thin film. 
     When the layer formed of the forest microcolumns is cut in parallel with the surface layer through application of thermal stress, for example, either of a surface and a back surface of the substrate is cooled or heated, to thereby provide a difference in temperature between these surfaces. In this case, cooling is performed by, for example, bringing the surface or the back surface of the aforementioned hot intermediate piece into contact with a cooling object (e.g., a stainless steel block), or blowing cooling air (air of ambient temperature) to the surface or the back surface of the aforementioned hot intermediate piece. When the hot intermediate piece is left to cool, thermal stress is generated, but the stress is small. 
     When the layer formed of the forest microcolumns is cut in parallel with the surface layer through application of shear stress, for example, friction force is applied to the surface and the back surface of the aforementioned intermediate piece in relatively opposite directions. When the layer formed of the forest microcolumns is cut in parallel with the surface layer through application of tensile force, for example, tensile force is applied to the surface and the back surface of the aforementioned intermediate piece in opposite directions perpendicular to the surfaces by means of, for example, vacuum suction pads. In the case where only a material in which the layer formed of the titanium-oxide- or titanium-alloy-oxide-formed forest microcolumns is exposed on at least a portion of the substrate is separated from the entirety, there may be removed, through grinding, sputtering, or a similar technique, a portion of the aforementioned intermediate piece, the portion corresponding to a material including the thin film, numerous continuous small-width protrusions which are provided thereon and formed of titanium oxide or a titanium alloy oxide, and forest microcolumns exposed on the protrusions. 
     In the thus-obtained material in which the layer formed of the titanium-oxide- or titanium-alloy-oxide-formed forest microcolumns is exposed on at least a portion of the substrate, height of the layer formed of the forest microcolumns varies depending on the level of the microcolumns at which the layer formed of the forest microcolumns is cut in parallel with the surface layer. However, generally, the height of the layer formed of the forest microcolumns is about 1 to about 20 μm, and the average diameter of the microcolumns is about 0.5 to about 3 μm. This material is a multifunctional material which can readily adsorb VOCs, which has a large surface area and thus exhibits high photocatalytic activity, which has high coating hardness, and which exhibits excellent heat resistance, corrosion resistance, exfoliation resistance, and wear resistance. 
     Meanwhile, the above-obtained material including a thin film, numerous continuous small-width protrusions which are provided thereon and formed of titanium oxide or a titanium alloy oxide, and forest microcolumns exposed on the protrusions is in the form of small fragments, and protrusions of each of the fragments have a height of about 2 to about 12 μm. Height of the microcolumns varies depending on the level of the microcolumns at which the layer formed of the forest microcolumns is cut in parallel with the surface layer, but generally, the height of the microcolumns is about 1 to about 5 μm, and the average diameter of the microcolumns is about 0.2 to about 0.5 μm. However, under some conditions for cutting the layer formed of the forest microcolumns in parallel with the surface layer, virtually no microcolumns may be present on numerous continuous small-width protrusions, and the protrusions may be exposed. Also, this material can adsorb VOCs, has a large surface area, and thus exhibits high photocatalytic activity. This material may be employed as it is, or may be pulverized before use. The thus-pulverized product can also readily adsorb VOCs, has a large surface area, and thus exhibits high photocatalytic activity. 
     In the production method of the present invention, through control of thermal treatment conditions (temperature and treatment time), microcolumns of titanium oxide or a titanium alloy oxide, numerous continuous small-width protrusions, or forest microcolumns provided on the protrusions can be doped with carbon, and the carbon doping amount can be regulated. In the case of carbon doping, for example, a gas predominantly containing a hydrocarbon is employed as a fuel for generating a combustion flame or a combustion exhaust gas. The hydrocarbon-containing gas preferably contains an unsaturated hydrocarbon in an amount of 30 vol. % or more. More preferably, the hydrocarbon-containing gas contains acetylene in an amount of 50 vol. % or more. Most preferably, the hydrocarbon-containing gas contains 100% acetylene. When an unsaturated hydrocarbon (in particular, acetylene, having a carbon-carbon triple bond) is employed, during combustion of the hydrocarbon, intermediate carbon radicals are formed through decomposition of such an unsaturated bond particularly in a reducing flame. The thus-formed carbon radicals, which exhibit strong activity, are likely to cause carbon doping, and the thus-doped carbon is contained in the form of Ti—C bond. Thus, when titanium-oxide- or titanium-alloy-oxide-formed microcolumns, numerous continuous small-width protrusions, or microcolumns provided on the protrusions are doped with carbon, these microcolumns or protrusions exhibit enhanced hardness, and therefore the resultant multifunctional material exhibits improved mechanical properties (e.g., coating hardness and wear resistance), as well as improved heat resistance and corrosion resistance. 
     When a layer formed of forest microcolumns of titanium oxide or a titanium alloy oxide, numerous continuous small-width protrusions, and microcolumns provided on the protrusions are doped with carbon, the resultant material responds to visible light having a wavelength of 400 nm or more (not to mention UV rays), and is particularly effective as a photocatalyst. That is, the material can be employed as a visible-light-responsive photocatalyst, and exhibits photocatalytic function even indoors (not to mention outdoors). 
     In a layer formed of forest microcolumns of titanium oxide or a titanium alloy oxide on a substrate, which layer is formed through the production method of the present invention, as shown by micrographs of  FIGS. 1 ,  4 , and  6 , each of the microcolumns has, for example, a prismatic form, a cylindrical form, a pyramidal form, a conical form, an inverted pyramidal form, or an inverted conical form. For example, each of the microcolumns extends straight in a direction perpendicular or oblique to the surface of the substrate, extends in a curved or bending form, extends in a branched form, or extends in a form of combination thereof. The entire form of the microcolumns can be represented by various expressions, including a frost crystal form, a raised carpet form, a coral form, a colonnaded form, and a form of columns built with building blocks. The width, height, bottom size, etc. of the microcolumns vary depending on, for example, thermal treatment conditions. 
     In a material including a thin film, numerous continuous small-width protrusions which are provided thereon and are formed of titanium oxide or a titanium alloy oxide, and forest microcolumns exposed on the protrusions, which material is formed through the production method of the present invention, as shown by a micrograph of  FIG. 3 , the numerous continuous small-width protrusions have an appearance of the exterior of walnut shell or an appearance of pumice stone, and each of the continuous small-width protrusions has an appearance of a bent wrinkle-like or crinkle-like pattern. The form of the microcolumns exposed on the protrusions is similar to that of the microcolumns constituting the layer provided on the substrate. However, since most of the microcolumns are cut at binding portions between the microcolumns and the thin film, generally, the density of the microcolumns exposed on the protrusions is smaller than that of the microcolumns constituting the layer provided on the substrate. 
     EXAMPLES 
     The present invention will next be described in more detail with reference to Examples and Comparative Examples. 
     Examples 1 to 5 
     A surface of a titanium sheet (thickness: 0.3 mm) was thermally treated with a combustion flame of a fuel shown in Table 1 at a surface layer temperature shown in Table 1 for a period of time shown in Table 1. Thereafter, when the surface subjected to the combustion flame was brought into contact with a flat surface of a stainless steel block (thickness: 30 mm) for cooling the sheet, the resultant product was separated into a material including the titanium sheet, and a layer formed of white forest microcolumns of titanium oxide, the layer exposed on the major portion of the titanium sheet surface; and fragmented materials, each including a thin film, numerous continuous white small-width protrusions provided thereon, and forest microcolumns formed on the protrusions, wherein the protrusions and microcolumns are formed of titanium oxide and exposed on the thin film. Specifically, the layer formed of the titanium oxide forest microcolumns, which had been formed in the interior of the surface layer of the titanium sheet through thermal treatment, was cut in parallel with the surface layer through cooling after the thermal treatment. Thus, multifunctional materials of Examples 1 to 5 were obtained. 
       FIG. 1  is a micrograph of the multifunctional material obtained in Example 1.  FIG. 1  shows the state where a layer  2  formed of white forest microcolumns of titanium oxide is exposed on a titanium sheet surface  1 ; and a fragmented material  3  remains on a portion of the layer  2 , the fragmented material  3  including a thin film, numerous continuous white small-width protrusions provided thereon, and forest microcolumns formed on the protrusions, wherein the protrusions and microcolumns are formed of titanium oxide and exposed on the thin film. Such a titanium sheet surface is not generally exposed in the case where a multifunctional material is produced through the production method of the present invention; i.e., the micrograph of  FIG. 1  shows the state where a portion of the layer  2  formed of the forest microcolumns is removed from the titanium sheet surface  1 .  FIG. 2  is a micrograph showing the state of the surface of the thin film of the fragmented material  3 , as viewed from the side opposite the thin film surface having thereon the numerous continuous white small-width protrusions formed of titanium oxide on which the microcolumns are exposed.  FIG. 3  is a micrograph showing the state of the surface of the fragmented material  3 , as viewed from the side where the thin film has thereon the numerous continuous white small-width protrusions formed of titanium oxide on which the microcolumns are exposed.  FIG. 4  is a micrograph showing the state of the layer  2  formed of the white forest microcolumns of titanium oxide.  FIG. 5  is a micrograph showing the state of the titanium sheet surface  1  from which the layer  2  formed of the forest microcolumns has been removed.  FIG. 6  is a micrograph showing the state of the layer formed of white forest microcolumns of titanium oxide, the layer constituting the material obtained in Example 5, wherein the layer formed of the forest microcolumns is exposed on the major portion of the titanium sheet surface. 
     Example 6 
     A surface of a Ti-6Al-4V alloy sheet (thickness: 0.3 mm) was thermally treated with a combustion flame of a fuel shown in Table 1 at a surface layer temperature shown in Table 1 for a period of time shown in Table 1. Thereafter, when the surface subjected to the combustion flame was brought into contact with a flat surface of a stainless steel block (thickness: 30 mm) for cooling the sheet, the resultant product was separated into a material including the titanium alloy sheet, and a layer formed of forest microcolumns of a titanium alloy oxide, the layer exposed on the major portion of the titanium alloy sheet surface; and fragmented materials, each including a thin film, numerous continuous small-width protrusions provided thereon, and forest microcolumns formed on the protrusions, wherein the protrusions and microcolumns are formed of a titanium alloy oxide and exposed on the thin film. 
     Example 7 
     A titanium thin film (thickness: about 3 μm) was formed on a stainless steel sheet (SUS316) (thickness: 0.3 mm) through electron beam deposition. A surface of the thin film was thermally treated with a combustion flame of a fuel shown in Table 1 at a surface layer temperature shown in Table 1 for a period of time shown in Table 1. Thereafter, when the surface subjected to the combustion flame was brought into contact with a flat surface of a stainless steel block (thickness: 30 mm) for cooling the sheet, the resultant product was separated into a material including the stainless steel sheet, and a layer formed of white forest microcolumns of titanium oxide, the layer exposed on the major portion of the stainless steel sheet surface; and fragmented materials, each including a thin film, numerous continuous white small-width protrusions provided thereon, and forest microcolumns formed on the protrusions, wherein the protrusions and microcolumns are formed of a titanium alloy oxide and exposed on the thin film. 
     Comparative Example 1 
     A commercially available titanium oxide sol (STS-01, product of Ishihara Sangyo Kaisha, Ltd.) was applied onto a titanium sheet (thickness: 0.3 mm) through spin coating, followed by heating, to thereby form, on the titanium sheet, a titanium oxide coating exhibiting enhanced adhesion. 
     Test Example 1 
     Scratch Hardness Test: Pencil Method 
     Each of the materials obtained in Examples 1 to 7 (test pieces), in which a layer formed of forest microcolumns is exposed on the surface of a base sheet, was subjected to pencil scratch hardness test according to JIS K 5600-5-4 (1999) by use of pencils (Uni 1H to 9H, products of Mitsubishi Pencil Co., Ltd.). The test was performed on the surface of the material on the side of the microcolumns. The results are shown in Table 1. As shown in Table 1, even when a 9H pencil was employed, no damage was observed in all the test pieces. 
     Test Example 2 
     Chemical Resistance Test 
     Each of the materials obtained in Examples 1 to 7 (test pieces), in which a layer formed of forest microcolumns is exposed on the surface of a base sheet, was immersed in a 1M aqueous sulfuric acid solution or a 1M aqueous sodium hydroxide solution at room temperature for one week, followed by washing with water, and drying. Thereafter, the thus-treated material was subjected to pencil scratch hardness test in a manner similar to that described above. The results are shown in Table 1. As shown in Table 1, even when a 9H pencil was employed, no damage was observed in all the test pieces; i.e., the materials of Examples 1 to 7 exhibited high chemical resistance. 
     Test Example 3 
     Heat Resistance Test 
     Each of the materials obtained in Examples 1 to 7 (test pieces), in which a layer formed of forest microcolumns is exposed on the surface of a base sheet, was placed in a tubular furnace; the material was heated in an air atmosphere from room temperature to 500° C. over one hour; the material was maintained at 500° C. for two hours; and the material was left to cool to room temperature over one hour. Thereafter, the thus-treated material was subjected to pencil scratch hardness test in a manner similar to that described above. The results are shown in Table 1. As shown in Table 1, even when a 9H pencil was employed, no damage was observed in all the test pieces; i.e., the materials of Examples 1 to 7 exhibited high heat resistance. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Surface layer 
                 Heating 
                 Scratch 
                 Chemical 
                 Heat 
               
               
                   
                 Fuel 
                 temperature 
                 time 
                 hardness 
                 resistance 
                 resistance 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Example 1 
                 Acetylene 
                 1100° C. 
                 10 
                 minutes 
                 Over 9H 
                 Over 9H 
                 Over 9H 
               
               
                 Example 2 
                 Acetylene 
                 1200° C. 
                 7 
                 minutes 
                 Over 9H 
                 Over 9H 
                 Over 9H 
               
               
                 Example 3 
                 Acetylene 
                 1220° C. 
                 8 
                 minutes 
                 Over 9H 
                 Over 9H 
                 Over 9H 
               
               
                 Example 4 
                 Acetylene 
                 1250° C. 
                 10 
                 minutes 
                 Over 9H 
                 Over 9H 
                 Over 9H 
               
               
                 Example 5 
                 Methane 
                 1100° C. 
                 10 
                 minutes 
                 Over 9H 
                 Over 9H 
                 Over 9H 
               
               
                 Example 6 
                 Acetylene 
                 1100° C. 
                 10 
                 minutes 
                 Over 9H 
                 Over 9H 
                 Over 9H 
               
               
                 Example 7 
                 Acetylene 
                 1100° C. 
                 8 
                 minutes 
                 Over 9H 
                 Over 9H 
                 Over 9H 
               
               
                   
               
            
           
         
       
     
     Test Example 4 
     Antifouling Test 
     There were employed, as samples, the material (surface area: 8 cm 2 ) obtained in Example 4, in which a layer formed of forest microcolumns is exposed on the surface of a base sheet, and the titanium-oxide-coated titanium sheet (surface area: 8 cm 2 ) obtained in Comparative Example 1. Each of the samples was subjected to deodorant test. Specifically, each of the samples was immersed in an aqueous methylene blue solution (80 mL, methylene blue concentration: about 12 μmol/L). After the effect of initial adsorption on a decrease in methylene blue concentration became negligible, the aqueous solution was irradiated with visible light by means of a fluorescent lamp with a UV cutoff filter (product of Matsushita Electric Industrial Co., Ltd.), and absorbance of the aqueous methylene blue solution at 660 nm was measured at a predetermined interval of irradiation time by means of a water quality inspection apparatus (DR/2400, product of HACH). The results are shown in  FIG. 7 . 
     As is clear from  FIG. 7 , the material obtained in Example 4, in which a layer formed of forest microcolumns is exposed on the surface of a base sheet, degrades methylene blue at a higher rate, as compared with the case of the titanium-oxide-coated titanium sheet obtained in Comparative Example 1; i.e., the material of Example 4 exhibits high antifouling effect. 
     Test Example 5 
     Crystal Structure and Bond Characteristics 
     A sample collected from microcolumns of the material obtained in Example 3, in which a layer formed of forest microcolumns is exposed on the surface of a base sheet, was subjected to X-ray diffractometry (XRD), and as a result the sample was found to have a rutile crystal structure. 
     Microcolumns of the material obtained in Example 3, in which a layer formed of forest microcolumns is exposed on the surface of a base sheet, were subjected to analysis by means of an X-ray photoelectron spectrometer (XPS) under the following conditions: acceleration voltage: 10 kV, target: Al, Ar ion sputtering: 2,700 seconds. When the sputtering rate is 0.64 Å/s, which corresponds to that of SiO 2  film, the depth is about 173 nm. The results of XPS analysis are shown in  FIG. 8 . The highest peak is observed at a binding energy of 284.6 eV. This peak is considered to correspond to a C—H(C) bond, which is generally observed through Cls analysis. The second highest peak is observed at a binding energy of 281.6 eV. The binding energy of a Ti—C bond is 281.6 eV, and thus the microcolumns of the material of Example 3 are considered to be doped with C in the form of Ti—C bond. XPS analysis was performed on 14 points at different height levels of the microcolumns, and as a result a peak similar to that described above was observed at 281.6 eV or thereabouts in an XPS spectrum corresponding to each of the points. 
     Example 8 
     Titanium circular sheets, each having a diameter of 32 mm and a thickness of 0.3 mm, were employed as test pieces. The surfaces of the test pieces were heated by means of a combustion flame of acetylene so that the surface temperature was maintained at about 1,150° C. A first test piece was heated for 120 seconds, and then was left to cool. A second test piece was heated for 180 seconds, and then was left to cool. A third test piece was heated for 480 seconds, and immediately thereafter, the surface subjected to the combustion flame was brought into contact with a flat surface of a stainless steel block (thickness: 30 mm) for cooling the test piece. Through this cooling, a thin film is exfoliated from the surface of the titanium sheet, to thereby yield a material in which a layer formed of white forest microcolumns of titanium oxide is exposed on the titanium sheet surface. Holes, each having a size of 3 μm×12 μm and a depth of 10 μm, were provided in the surface of each of these three test pieces by means of an FIB-SEM apparatus (SMI8400SE, product of Seiko Instruments Inc.), and the side surfaces and bottom surface of each of the holes were observed by means of an SEM apparatus (VE7800, product of Keyence Corporation).  FIG. 9  is an SEM micrograph showing the test piece heated for 120 seconds;  FIG. 10  is an SEM micrograph showing the test piece heated for 180 seconds; and  FIG. 11  is an SEM micrograph showing the test piece heated for 480 seconds. As shown in  FIG. 10 , in the test piece heated for 180 seconds, a microcolumnar structure begins to be formed at the lower portion of a coating. Therefore, conceivably, further continuous treatment with a flame extends microcolumns, to thereby form a microcolumnar structure as intended by the present invention.