Patent Publication Number: US-2022229215-A1

Title: Radiation suppression film and radiation suppression structure

Description:
TECHNICAL FIELD 
     The present invention relates to a radiation suppression film and a radiation suppression structure that suppress radiation of a long wavelength infrared ray radiated from a surface of an object. 
     BACKGROUND ART 
     With the progress of detection techniques, an object such as a vehicle or a flying object can be detected using electromagnetic waves of various wavelengths. For example, a detection technique using a long wavelength infrared ray having a wavelength of 8 to 14 micrometers (μm) radiated from a heat source having a temperature around room temperature has been developed. By using the detection technique using a long wavelength infrared ray, even an object having enhanced stealth to radio waves can be detected. Meanwhile, there is a demand for stealth techniques and disturbing techniques against the detection technique using a long wavelength infrared ray. For example, if long wavelength infrared radiation radiated from an object can be suppressed, it becomes difficult to detect the object even if a long wavelength infrared ray is used. 
     PTL 1 discloses a thermal camouflage laminate. The thermal camouflage laminate of PTL 1 has a structure in which a layer containing metal and polyethylene is laminated on a surface of a fabric or the like. According to the thermal camouflage laminate of PTL 1, emissivity of a mid-infrared ray in a wavelength region of 3 to 5 μm and emissivity of a long wavelength infrared ray in a wavelength region of 8 to 14 μm can be suppressed to a range of 0.4 to 0.95. 
     PTL 2 discloses a camouflage combat jacket having a surface of a fabric on which a metal material is processed, and a camouflage print of three or more colors applied to the processed surface. The camouflage combat jacket of PTL 2 is characterized in that area-weighted average radiation power of a clothing surface is 0.4 to 0.85, and a difference in maximum radiation power between the colors is 0.1 to 0.6. 
     In the techniques disclosed in PTLs 1 and 2, metal having high reflectance and low emissivity is used in a long wavelength infrared region. Since the metal has high reflectance in the long wavelength infrared region, thermal radiation from an object or a human body covered with a fabric is suppressed. Further, since the metal has small emissivity in its own long wavelength infrared region, the thermal radiation is suppressed. 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] U.S. Pat. No. 4,529,633 
         [PTL 2] JP 2004-053039 A 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     Since an object of the techniques disclosed in PTLs 1 and 2 is to cause an object serving as a heat source to be blended in nature, it is sufficient that the emissivity of the surface of the object is equal to or more than 0.4. However, when the emissivity is equal to or more than 0.4, there is a possibility that the surface of the object heated by sunlight to have a high temperature is detected. If the emissivity can be suppressed to less than 0.4, the stealth with respect to the detection technique using a long wavelength infrared ray can be improved but since suppression of the emissivity to less than 0.4 is difficult, there is a possibility that detection is performed by the detection technique using a long wavelength infrared ray. 
     An object of the present invention is to solve the above-described problem and provide a radiation suppression film that suppresses infrared radiation having a wavelength within a long wavelength infrared region and is less easily detected by a detection technique using a long wavelength infrared ray. 
     Solution to Problem 
     A radiation suppression film according to one aspect of the present invention includes a porous body containing a material transparent to a long wavelength infrared ray as a base material. 
     A radiation suppression structure according to one aspect of the present invention includes a substrate and a radiation suppression film including a porous body in which holes are dispersed in a base material formed on at least a part of a surface of the substrate and containing a material transparent to a long wavelength infrared ray. 
     Advantageous Effects of Invention 
     According to the present invention, a radiation suppression film that suppresses infrared radiation having a wavelength within a long wavelength infrared region and is less easily detected by a detection technique using a long wavelength infrared ray can be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual diagram illustrating an example of a cross section of a radiation suppression structure including a radiation suppression film according to a first example embodiment of the present invention. 
         FIG. 2  is a conceptual diagram illustrating an example of a cross section of a radiation suppression structure including a radiation suppression film according to a second example embodiment of the present invention. 
         FIG. 3  is a conceptual diagram illustrating an example of a cross section of a radiation suppression structure including a radiation suppression film according to a third example embodiment of the present invention. 
         FIG. 4  is a conceptual diagram illustrating an example of a cross section of a radiation suppression structure including a radiation suppression film according to a fourth example embodiment of the present invention. 
         FIG. 5  is a conceptual diagram illustrating an example of a cross section of a radiation suppression structure including a radiation suppression film according to a fifth example embodiment of the present invention. 
         FIG. 6  is a conceptual diagram illustrating an example of a cross section of a radiation suppression structure including a radiation suppression film according to a sixth example embodiment of the present invention. 
     
    
    
     EXAMPLE EMBODIMENT 
     Hereinafter, forms for implementing the present invention will be described with reference to the drawings. The example embodiments to be described below have technically favorable limitations for implementing the present invention. However, the scope of the invention is not limited to below. In all the drawings used in the following description of the example embodiments, the same reference numerals are given to the same parts unless there is a particular reason. In the following example embodiments, repeated description of similar configurations and operations may be omitted. 
     First Example Embodiment 
     First, a radiation suppression film according to a first example embodiment of the present invention will be described with reference to the drawing. The radiation suppression film of the present example embodiment suppresses radiation of infrared light (hereinafter also referred to as a long wavelength infrared ray) in a long wavelength infrared region. In particular, the radiation suppression film of the present example embodiment suppresses radiation of a long wavelength infrared ray having a wavelength of 8 to 14 micrometers (μm). When a surface of an object is covered with the radiation suppression film of the present example embodiment, leakage of the long wavelength infrared ray radiated from the object to an outside is suppressed. Hereinafter, a configuration in which the radiation suppression film of the present example embodiment is laminated on a surface of an object (hereinafter referred to as a substrate) will be described. 
     [Structure] 
       FIG. 1  is a conceptual diagram illustrating an example of a cross section of a radiation suppression structure  1  including a radiation suppression film  10  according to the present example embodiment. The radiation suppression film  10  is formed on a surface of a substrate  100 . The radiation suppression film  10  has a porous body  11  in which holes  112  are dispersed in a base material  111 . A plurality of holes  112  is dispersed in the porous body  11 . The radiation suppression structure  1  is a structure in which the radiation suppression film  10  is formed on a surface portion of the substrate  100 . In the present example embodiment, since the porous body  11  itself in which the holes  112  are dispersed in the base material  111  constitutes the radiation suppression film  10 , the porous body  11  itself corresponds to the radiation suppression film  10 . 
     The base material  111  contains a material transparent to the long wavelength infrared ray. The material transparent to the long wavelength infrared ray is a material having high transmittance of the long wavelength infrared ray. 
     For example, a chalcogenide compound such as zinc selenide (ZnSe) or zinc sulfide (ZnS) can be used as the base material  111 . Further, for example, germanium (Ge) can be used as the base material  111 . Further, for example, polyethylene can be used as the base material  111 . The base material  111  may be a single material or a combination of a plurality of materials. Further, an additive that is not transparent to the long wavelength infrared ray may be mixed with the base material  111 . The transmittance of the long wavelength infrared ray of the material of the base material  111  is favorably equal to or more than 40%. The transmittance of the long wavelength infrared ray of the material of the base material  111  is more favorably equal to or more than 60% at a thickness of 5 mm. 
     Emissivity ε is calculated by the following equation 1 using a measured value of black-body radiation intensity I b  and a measured value of radiation intensity I e  of a sample (λ: wavelength and T: absolute temperature): 
       ε(λ, T )= I   e (λ, T )/ I   b (λ, T )  (1)
 
     Absorptivity α is calculated by the following equation 2 using reflectance R and transmittance T: 
       α(λ, T )=1−the reflectance  R (λ, T )−the transmittance  T (λ, T )   (2)
 
     The emissivity ε is equivalent to the absorptivity α. Therefore, the emissivity ε is calculated by the following equation 3: 
       ε(λ, T )=1−the reflectance  R (λ, T )−the transmittance  T (λ, T )   (3)
 
     For example, ZnSe is used as a window material for the infrared ray, and the transmittance T of the long wavelength infrared ray of a bulk material not including holes is about 70% (0.7) at a thickness of 5 mm. The window material for ZnSe to which surface treatment has not been applied has high reflectance R and high transmittance T on the surface. When the reflectance R and the transmittance T of ZnSe are applied to the equation 2, the emissivity ε is smaller than 0.3 because the transmittance T is 0.7. 
     In the techniques disclosed in PTL 1 (U.S. Pat. No. 4,529,633) and PTL 2 (Japanese Patent Application Laid-Open No. 2004-053039), a lower limit value of the emissivity ε is 0.4. In the case where the emissivity ε is equal to or more than 0.4, there is a possibility that the surface of the object is heated by sunlight to have a high temperature, and the object is detected using the long wavelength infrared ray. Therefore, the emissivity ε is favorably less than 0.4, but it is difficult to make the emissivity ε less than 0.4 with the techniques of PTLs 1 and 2. 
     In contrast, by using the radiation suppression film  10  including the porous body  11  in which the holes  112  are dispersed in the base material  111  containing a material transparent to the long wavelength infrared ray, the long wavelength infrared ray radiated from the surface of the substrate  100  is scattered by the holes  112 . A part of the long wavelength infrared ray scattered by the holes  112  is reabsorbed by the surface of the substrate  100  and converted into heat. Therefore, the long wavelength infrared ray radiated from the surface of the substrate  100  to the outside is reduced. Further, since the base material  111  has high transparency in the long wavelength infrared region, thermal radiation of the base material  111  itself in the long wavelength infrared region is small. Since these effects are synergized, the emissivity of the long wavelength infrared ray from the surface of the substrate  100  can be made less than 0.4 by covering the surface of the substrate  100  with the radiation suppression film  10 . For example, when ZnSe is used as the base material  111  of the radiation suppression film  10 , the emissivity ε of bulk ZnSe not including holes is less than 0.3, and thus it is not difficult to make the emissivity of the radiation suppression film  10  less than 0.4. 
     The thickness of the base material  111  is set to be larger than the wavelength in the long wavelength infrared region. The thickness of the base material  111  is favorably 10 times or more the wavelength in the long wavelength infrared region. Further, the thickness of the base material  111  is more favorably equal to or more than 500 μm. 
     The hole  112  is a gap formed inside the base material  111 . The hole  112  may be formed not only inside the base material  111  but also in the surface of the base material  111 .  FIG. 1  illustrates the hole  112  as being spherical but the actual shape of the hole  112  is not particularly limited. The shapes of the holes  112  may be uniform or may be various shapes. The size of the hole  112  is not particularly limited but the hole is favorably formed into a size large enough to scatter the long wavelength infrared ray. 
     When the long wavelength infrared ray radiated from the substrate  100  and traveling inside the base material  111  collides with the hole  112 , the long wavelength infrared ray is scattered at an interface between the base material  111  and the hole  112 . When increasing the opportunity of scattering of the long wavelength infrared ray radiated from substrate  100 , the long wavelength infrared ray radiated from the surface of radiation suppression film  10  can be reduced. Therefore, it is favorable to increase the opportunity to backscatter the long wavelength infrared ray toward the substrate  100  by causing the long wavelength infrared ray to collide with the holes  112 . 
     A ratio of a volume of the holes  112  to a total volume of the base material  111  is porosity. The porosity is not particularly limited as long as the long wavelength infrared ray radiated from the surface of the radiation suppression film  10  can be suppressed. However, when the porosity is too small, a frequency at which the long wavelength infrared ray radiated from the substrate  100  is scattered by the holes  112  decreases. Therefore, when the porosity is too small, the long wavelength infrared ray radiated from the substrate  100  is radiated from the surface of the radiation suppression film  10  without being scattered, and a sufficient radiation suppression effect cannot be obtained. Meanwhile, when the porosity is too large, mechanical strength of the radiation suppression film  10  becomes weak and becomes brittle. Therefore, the porosity is favorably set within a specific range. For example, when the porosity is set to 20 to 70%, the sufficient radiation suppression effect and mechanical strength can be obtained. 
     The substrate  100  is an object having the radiation suppression film  10  formed on its surface. The material of the substrate  100  is not particularly limited as long as the radiation suppression film  10  can be formed on the surface. For example, metal, ceramic, plastic, or the like can be applied to the substrate  100 . 
     The substrate  100  is a surface portion of an object to be concealed with respect to detection using the long wavelength infrared ray. For example, a surface portion of an object such as a vehicle or a flying object corresponds to the substrate  100 . When the surface portion (substrate  100 ) of the object such as a vehicle or a flying object is covered with the radiation suppression film  10 , the long wavelength infrared ray radiated from the surface of the object can be reduced, so that the object can be concealed from detection using the long wavelength infrared ray. 
     [Manufacturing Method] 
     Next, a method for manufacturing the radiation suppression film  10  will be described with an example. For example, the radiation suppression film  10  can be manufactured using an aerosol deposition method, a cold spraying method, a plasma spraying method, a sol-gel method, or the like. 
     In the case of using the aerosol deposition method, the radiation suppression film  10  can be formed by blowing aerosolized fine particles of the base material  111  onto the surface of the substrate  100  at a high speed and performing room temperature impact consolidation. The porosity and a hole size of the radiation suppression film  10  can be controlled by adjusting a particle diameter of fine particles of the base material  111  and a blowing speed. The aerosol deposition method is suitable for the base material  111  containing a hard material such as ZnSe or ZnS. 
     For example, when fine particles of ZnS are formed into an aerosol and blown onto the surface of the stainless steel substrate  100  and room temperature impact consolidation is performed, the radiation suppression film  10  constituted by the ZnS porous body  11  can be formed on the surface of the stainless steel substrate  100 . 
     Further, the radiation suppression film  10  may be formed by fixing a block formed by sintering fine particles of the base material  111  to the surface of substrate  100  with an adhesive or the like. In this case, the porosity and the hole size can be controlled by adjusting the particle diameter of the fine particles, a sintering temperature, and a sintering time. 
     The above is the description of the radiation suppression film  10  of the present example embodiment. The structure of  FIG. 1  is an example, and the structure of the radiation suppression film  10  is not limited to the form as in  FIG. 1 . For example, the radiation suppression film  10  may be formed not on a flat surface but on a curved surface. The radiation suppression film  10  may be formed not on a smooth surface but on a surface having irregularities. The radiation suppression film  10  may be continuously or discontinuously formed on at least one surface of substrate  100 . The radiation suppression film  10  may be formed on surfaces of the different substrates  100  so as to straddle a boundary between the substrates  100 . The radiation suppression film  10  may be formed on the surface of the substrate of not only metal but also ceramic or plastic. 
     For example, when the radiation suppression film  10  is formed on the surface of the object such as a vehicle or a flying object, concealment of the object with respect to a search using the long wavelength infrared ray is improved. Further, when the radiation suppression film  10  is formed on an upper surface of a radio wave absorber or a radio wave scatterer, scattering of radio waves can be prevented, so that the concealment can be further improved. The radiation suppression film  10  of the present example embodiment is not limited to the above application, and can be used for any application intended to prevent the long wavelength infrared ray radiated from the object from leaking to the outside. 
     As described above, the radiation suppression film according to the present example embodiment includes the porous body containing the material transparent to the long wavelength infrared ray as the base material. In other words, the radiation suppression film of the present example embodiment includes the porous body in which the holes are dispersed in the base material containing a material transparent to the long wavelength infrared ray. In one aspect of the present example embodiment, the material of the base material contains at least one of materials selected from the group of ZnSe, ZnS, and Ge. In one aspect of the present example embodiment, the material of the base material contains polyethylene. In one aspect of the present example embodiment, the radiation suppression film is constituted by a layer of the porous body. 
     Furthermore, the radiation suppression structure according to one aspect of the present example embodiment includes the radiation suppression film including the porous body in which the holes are dispersed in the base material formed on at least a part of the surface of the substrate and containing a material transparent to the long wavelength infrared ray, and the substrate. 
     According to the radiation suppression film of the present example embodiment, the long wavelength infrared ray radiated from the surface of the object is scattered by the holes, and a part of the long wavelength infrared ray is reabsorbed by the surface of the object and converted into heat. Therefore, the long wavelength infrared ray radiated from the surface of the object to the outside is reduced. Further, since the base material has high transparency in the long wavelength infrared region, thermal radiation of the base material itself in the long wavelength infrared region is small. Since these effects are synergized, when the surface of the object is covered with the radiation suppression film of the present example embodiment, the emissivity of the long wavelength infrared ray from the surface of the object can be reduced to less than 0.4. 
     That is, according to the radiation suppression film of the present example embodiment, infrared radiation having the wavelength within the long wavelength infrared region is suppressed and the radiation suppression film can be made less easily detected by a detection technique using a long wavelength infrared ray. 
     Second Example Embodiment 
     Next, a radiation suppression film according to a second example embodiment of the present invention will be described with reference to the drawing. The radiation suppression film of the present example embodiment has a structure in which the porous body included in the radiation suppression film of the first example embodiment is dispersed inside a resin. Hereinafter, description of structures, functions, and the like similar to those of the first example embodiment may be omitted. 
     [Structure] 
       FIG. 2  is a conceptual diagram illustrating an example of a cross section of a radiation suppression structure  2  including a radiation suppression film  20  according to the present example embodiment. The radiation suppression film  20  is formed on a surface of a substrate  200 . The radiation suppression film  20  has a structure in which porous bodies  21  in each of which holes  212  are dispersed inside a base material  211  are dispersed inside a resin  23 . The radiation suppression structure  2  is a structure in which a radiation suppression film  20  is formed on a surface portion of the substrate  200 . In the present example embodiment, the radiation suppression film  20  is constituted by the porous bodies  21  in each of which the holes  212  are dispersed inside the base material  211  and the resin  23  in which the porous bodies  21  are dispersed. 
     The base material  211  contains a material transparent to a long wavelength infrared ray, similar to the base material  111  of the first example embodiment. The material transparent to the long wavelength infrared ray is a material having high transmittance of the long wavelength infrared ray. Characteristics such as the material and physical properties of the base material  211  are similar to those of the base material  111  of the first example embodiment. The base material  211  is dispersed inside the resin  23 . The base material  211  may be not only dispersed inside the resin  23  but also exposed to a surface of the radiation suppression film  20 . The size and shape of the base material  211 , the dispersion state in the resin  23 , and the like are not particularly limited, but it is favorable to form the base material  211  and the resin  23  such that the long wavelength infrared ray can be easily scattered. 
     The hole  212  is a gap formed inside the base material  211 , similarly to the hole  112  of the first example embodiment. The properties of the hole  212  are similar to those of the hole  112  of the first example embodiment. 
     The resin  23  is a base including the porous bodies  21  each having the dispersed holes  212  inside the base material  211 . The resin  23  contains a material transparent to the long wavelength infrared ray. For example, polyethylene can be used as the resin  23 . The thickness of the resin  23  is made larger than a wavelength in a long wavelength infrared region. The thickness of the resin  23  is favorably 10 times or more the wavelength in the long wavelength infrared region. Further, the thickness of the resin  23  is more favorably equal to or more than 500 μm. 
     When the long wavelength infrared ray radiated from the substrate  200  and traveling inside the resin  23  collides with the porous body  21  or the hole  212  inside the porous body  21 , the long wavelength infrared ray is scattered at an interface between the resin  23  and the porous body  21  or an interface between the base material  211  and the hole  212 . When increasing the opportunity of scattering of the long wavelength infrared ray radiated from substrate  200 , the long wavelength infrared ray radiated from the surface of radiation suppression film  20  can be reduced. Therefore, it is favorable to increase the opportunity to backscatter the long wavelength infrared ray toward the substrate  200  by causing the long wavelength infrared ray to collide with the porous bodies  21  and the holes  212 . 
     A ratio of a volume of the porous bodies  21  to a total volume of the resin  23  (hereinafter referred to as a ratio of the porous bodies  21 ) is not particularly limited as long as the long wavelength infrared ray radiated from the surface of the radiation suppression film  20  can be suppressed. However, when the ratio of the porous bodies  21  is too small, a frequency at which the long wavelength infrared ray radiated from the substrate  200  is scattered by the ratio of the porous bodies  21  decreases. Therefore, when the ratio of the porous bodies  21  is too small, the long wavelength infrared ray radiated from the substrate  200  is radiated from the surface of the radiation suppression film  20  without being scattered, and a sufficient radiation suppression effect cannot be obtained. Meanwhile, when the ratio of the porous bodies  21  is too large, the amount of the resin is insufficient, and the porous bodies cannot maintain a film structure. Therefore, the ratio of the porous bodies  21  is favorably set within a specific range. 
     The substrate  200  is an object having the radiation suppression film  20  formed on its surface, similar to the substrate  100  of the first example embodiment. 
     [Manufacturing Method] 
     Next, a method for manufacturing the radiation suppression film  20  will be described with an example. For example, the radiation suppression film  20  can be manufactured by applying the resin  23  in which the porous bodies  21  each having the holes  212  dispersed inside the base material  211  are dispersed. 
     First, fine particles of the base material  211  are sintered to produce a sintered body of the porous body  21 . Porosity and hole size of the sintered body of the porous body  21  can be controlled by adjusting a particle diameter of the fine particles, a sintering temperature, and a sintering time. Next, the sintered body of the porous body  21  is pulverized to produce particles of the porous body  21 . Next, the particles of the porous body  21  and the resin  23  are mixed to produce a coating material in which the particles of the porous body  21  are dispersed in the resin  23 . Then, the coating material in which the particles of the porous body  21  are dispersed in the resin  23  is applied to the surface of the substrate  200  by a flow immersion method or the like and consolidated, so that the radiation suppression film  20  can be formed on the surface of the substrate  200 . 
     For example, a ZnS porous body  21  is produced by pulverizing a porous sintered body obtained by sintering fine particles of ZnS. Further, a coating material is prepared by mixing the ZnS porous body  21  with the polyethylene resin  23 . The radiation suppression film  20  in which the ZnS porous bodies  21  are dispersed in the polyethylene resin  23  can be formed on the surface of the substrate  200  made of stainless steel by applying the coating material to the surface of the substrate  200  made of stainless steel using a flow immersion method and consolidating the coating material. 
     The above is the description of the radiation suppression film  20  of the present example embodiment. The structure of  FIG. 2  is an example, and the structure of the radiation suppression film  20  is not limited to the form as in  FIG. 2 . For example, the radiation suppression film  20  may be formed not on a flat surface but on a curved surface. The radiation suppression film  20  may be formed not on a smooth surface but on a surface having irregularities. The radiation suppression film  20  may be continuously or discontinuously formed on at least one surface of substrate  200 . The radiation suppression film  20  may be formed on surfaces of the different substrates  200  so as to straddle a boundary between the substrates  200 . 
     Since the radiation suppression film  20  can be easily formed even on a surface having a complicated shape because of using the resin  23  as the base material. When the radiation suppression film  20  is formed into a film shape, the radiation suppression film  20  does not need to be brought into close contact with the surface of the substrate  200 . Therefore, a radiation suppression effect of the long wavelength infrared ray can be obtained even in the case where it is difficult to bring the radiation suppression film  20  into close contact with the surface of substrate  200 . 
     Further, a material that is not transparent to the long wavelength infrared ray may be added to the resin  23  as long as transmittance of the long wavelength infrared ray is not significantly reduced. For example, to improve moldability, a material that is not transparent to the long wavelength infrared ray may be mixed with the resin  23 . For example, to obtain an effect other than the radiation suppression effect of long wavelength infrared ray, a material that is not transparent to the long wavelength infrared ray may be mixed with the resin  23 . 
     As described above, the radiation suppression film of the present example embodiment has the structure in which the porous body having a material transparent to the long wavelength infrared ray as the base material is dispersed inside the resin containing the material transparent to the long wavelength infrared ray. In one aspect of the present example embodiment, the material of the resin contains polyethylene. In one aspect of the present example embodiment, the radiation suppression film is formed in a film shape. 
     According to the radiation suppression film of the present example embodiment, the long wavelength infrared ray radiated from the surface of an object is scattered by the porous bodies and holes, and a part of the long wavelength infrared ray is reabsorbed by the surface of the object and converted into heat. Therefore, the long wavelength infrared ray radiated from the surface of the object to the outside is reduced. Further, since the resin and the base material have high transparency in the long wavelength infrared region, thermal radiation of the resin itself and the base material itself in the long wavelength infrared region is small. Since these effects are synergized, when the surface of the object is covered with the radiation suppression film of the present example embodiment, the emissivity of the long wavelength infrared ray from the surface of the object can be reduced to less than 0.4. 
     That is, according to the radiation suppression film of the present example embodiment, infrared radiation having the wavelength in the long wavelength infrared region can be suppressed. Moreover, since the radiation suppression film of the present example embodiment contains the resin as a base, the radiation suppression film can be more easily formed on the surface of the substrate than the radiation suppression film of the first example embodiment. 
     Third Example Embodiment 
     Next, a radiation suppression film according to a third example embodiment of the present invention will be described with reference to the drawing. The radiation suppression film of the present example embodiment includes an infrared-ray absorption layer. Hereinafter, description of structures, functions, and the like similar to those of the first example embodiment may be omitted. 
     [Structure] 
       FIG. 3  is a conceptual diagram illustrating an example of a cross section of a radiation suppression structure  3  including a radiation suppression film  30  according to the present example embodiment. A radiation suppression film  30  including a porous body  31  and an infrared-ray absorption layer  35  is formed on a surface of a substrate  300 . The porous body  31  has a structure in which holes  312  are dispersed inside a base material  311 , similarly to the porous body  11  of the first example embodiment. The radiation suppression structure  3  has a structure in which the infrared-ray absorption layer  35  is formed on a surface portion of the substrate  300 , and the porous body  31  is formed on a surface of the infrared-ray absorption layer  35 . In the present example embodiment, the radiation suppression film  30  is constituted by the porous body  31  having the holes  312  dispersed inside the base material  311  and the infrared-ray absorption layer  35 . 
     The base material  311  contains a material transparent to a long wavelength infrared ray, similar to the base material  111  of the first example embodiment. The material transparent to the long wavelength infrared ray is a material having high transmittance of the long wavelength infrared ray. Characteristics such as the material and physical properties of the base material  311  are similar to those of the base material  111  of the first example embodiment. 
     The hole  312  is a gap formed inside the base material  311 , similarly to the hole  112  of the first example embodiment. The properties of the hole  312  are similar to those of the hole  112  of the first example embodiment. 
     The infrared-ray absorption layer  35  is formed on the surface of the substrate  300 . The porous body  31  is formed on an upper surface of the infrared-ray absorption layer  35 . The infrared-ray absorption layer  35  is an absorption layer that absorbs the long wavelength infrared ray. For example, a material having high long wavelength infrared absorptivity such as a black body coating material or a carbon material is used for the infrared-ray absorption layer  35 . The material of the infrared-ray absorption layer  35  is not particularly limited as long as the material can absorb the long wavelength infrared ray radiated from the substrate  300  and the long wavelength infrared ray backscattered from the porous body  31 . 
     The substrate  300  is an object having the radiation suppression film  30  formed on its surface, similar to the substrate  100  of the first example embodiment. The infrared-ray absorption layer  35  is formed on the surface of the substrate  300 . 
     [Manufacturing Method] 
     Next, a method for manufacturing the radiation suppression film  30  will be described with an example. For example, the radiation suppression film  30  can be manufactured by forming a layer of the porous body  31  on the surface of substrate  300  on which the infrared-ray absorption layer  35  is formed. Hereinafter, an example of laminating the porous body  31  on the infrared-ray absorption layer  35  using an aerosol deposition method will be described. 
     First, a coating material containing a material that absorbs the long wavelength infrared ray, such as a black body coating material, is applied to the surface of the substrate  300  to form the infrared-ray absorption layer  35 . Then, the radiation suppression film  30  can be formed by blowing aerosolized fine particles of the base material  311  onto the surface of the substrate  300  on which the infrared-ray absorption layer  35  has been formed at a high speed and performing room temperature impact consolidation. Porosity and hole size of the porous body  31  can be controlled by adjusting a particle diameter of the fine particles of the base material  311  and a blowing speed. 
     For example, the infrared-ray absorption layer  35  is formed by applying a black body coating material to the surface of the stainless steel substrate  300 . Then, when fine particles of ZnS are formed into an aerosol and blown onto the surface of the infrared-ray absorption layer  35  and room temperature impact consolidation is performed, the radiation suppression film  30  constituted by the ZnS porous body  31  and the infrared-ray absorption layer  35  can be formed on the surface of the stainless steel substrate  300 . 
     Further, the radiation suppression film  30  may be formed by fixing a block formed by sintering fine particles of the base material  311  to the surface of substrate  300  on which the infrared-ray absorption layer  35  has been formed with an adhesive or the like. In this case, the porosity can be controlled by adjusting the particle diameter of the fine particles, the sintering temperature, and the sintering time. 
     The above is the description of the radiation suppression film  30  of the present example embodiment. The structure of  FIG. 3  is an example, and the structure of the radiation suppression film  30  is not limited to the form as in  FIG. 3 . For example, the radiation suppression film  30  may be formed not on a flat surface but on a curved surface. The radiation suppression film  30  may be formed not on a smooth surface but on a surface having irregularities. The radiation suppression film  30  may be continuously or discontinuously formed on at least one surface of substrate  300 . The radiation suppression film  30  may be formed on surfaces of the different substrates  300  so as to straddle a boundary between the substrates  300 . 
     As described above, the radiation suppression film of the present example embodiment includes the infrared-ray absorption layer that absorbs the long wavelength infrared ray. As one aspect of the present example embodiment, the infrared-ray absorption layer is formed between the layer formed on at least a part of the surface of the substrate that radiates the long wavelength infrared ray and including the porous body, and the substrate. 
     The radiation suppression structure according to one aspect of the present example embodiment includes the infrared-ray absorption layer formed between the layer including the porous body and the substrate, and which absorbs the long wavelength infrared ray. 
     The radiation suppression film of the present example embodiment absorbs the long wavelength infrared ray radiated from the object surface by the infrared-ray absorption layer. The long wavelength infrared ray absorbed by the infrared-ray absorption layer is converted into heat or reradiated in any direction. The long wavelength infrared ray reradiated from the infrared-ray absorption layer is reradiated in the direction of the substrate or the direction of the porous body. The long wavelength infrared ray reradiated in the direction of the substrate is mainly converted into heat. The long wavelength infrared ray reradiated in the direction of the porous body is scattered by the holes, and a part of the long wavelength infrared ray is reabsorbed by an infrared-ray absorption film or the surface of the object and converted into heat. Therefore, the long wavelength infrared ray radiated from the surface of the substrate to the outside is reduced. Further, since the base material has high transparency in the long wavelength infrared region, thermal radiation of the base material itself in the long wavelength infrared region is small. Since these effects are synergized, when the surface of the object is covered with the radiation suppression film of the present example embodiment, the emissivity of the long wavelength infrared ray from the surface of the object can be reduced to less than 0.4. 
     Fourth Example Embodiment 
     Next, a radiation suppression film according to a fourth example embodiment of the present invention will be described with reference to the drawing. The radiation suppression film of the present example embodiment has a structure in which the radiation suppression film of the second example embodiment is formed on the surface of the infrared-ray absorption layer of the third example embodiment. Hereinafter, description of structures, functions, and the like similar to those of the first to third example embodiments may be omitted. 
     [Structure] 
       FIG. 4  is a conceptual diagram illustrating an example of a cross section of a radiation suppression structure  4  including a radiation suppression film  40  according to the present example embodiment. The radiation suppression film  40  including an infrared-ray absorption layer  45  and a resin  43  in which porous bodies  41  are dispersed is formed on a surface of a substrate  400 . The porous body  41  has a structure in which holes  412  are dispersed inside a base material  411 , similarly to the porous body  21  of the second example embodiment. The porous bodies  41  are dispersed inside the resin  43 , similar to the resin  23  of the second example embodiment. The radiation suppression structure  4  is a structure in which the radiation suppression film  40  is formed on a surface portion of the substrate  400 . In the present example embodiment, the radiation suppression film  40  is constituted by the porous bodies  41  in each of which the holes  412  are dispersed inside the base material  411 , the resin  43  in which the porous bodies  41  are dispersed, and the infrared-ray absorption layer  45 . 
     The base material  411  contains a material transparent to a long wavelength infrared ray, similar to the base material  211  of the second example embodiment. The material transparent to the long wavelength infrared ray is a material having high transmittance of the long wavelength infrared ray. Characteristics such as the material and physical properties of the base material  411  are similar to those of the base material  211  of the second example embodiment. 
     The hole  412  is a gap formed inside the base material  411 , similarly to the hole  212  of the second example embodiment. The properties of the hole  412  are similar to those of the hole  212  of the second example embodiment. 
     The resin  43  is a base including the porous bodies  41  each having the dispersed holes  412  inside the base material  411 . The resin  43  contains a material transparent to the long wavelength infrared ray. For example, polyethylene can be used as the resin  43 . The thickness of the resin  43  is made larger than a wavelength in a long wavelength infrared region. The thickness of the resin  43  is favorably 10 times or more the wavelength in the long wavelength infrared region. Further, the thickness of the resin  43  is more favorably equal to or more than 500 μm. 
     An infrared-ray absorption layer  45  is formed on a surface of a substrate  400 . A layer of the resin  43  in which the porous bodies  41  are dispersed is formed on an upper surface of the infrared-ray absorption layer  45 . The infrared-ray absorption layer  45  is similar to the infrared-ray absorption layer  35  of the third example embodiment. 
     The substrate  400  is an object having the radiation suppression film  40  formed on its surface, similar to the substrate  100  of the first example embodiment. The infrared-ray absorption layer  45  is formed on the surface of the substrate  400 . 
     [Manufacturing Method] 
     Next, a method for manufacturing the radiation suppression film  40  will be described with an example. For example, the radiation suppression film  40  can be manufactured by applying the resin  43  in which the porous bodies  41  each having the holes  412  dispersed inside the base material  411  are dispersed to the surface of the substrate  400  on which the infrared-ray absorption layer  45  has been formed. 
     First, fine particles of the base material  411  are sintered to produce a sintered body of the porous body  41 . The porosity and the hole size of the sintered body of the porous body  41  can be controlled by adjusting the particle diameter of the fine particles, the sintering temperature, and the sintering time. Next, the sintered body of the porous body  41  is pulverized to produce particles of the porous body  41 . Next, a coating material containing a material that absorbs the long wavelength infrared ray, such as a black body coating material, is applied to the surface of the substrate  400  to form the infrared-ray absorption layer  45 . Next, the particles of the porous body  41  and the resin  43  are mixed to produce a coating material in which the particles of the porous body  41  are dispersed in the resin  43 . Then, the coating material in which the particles of the porous body  41  are dispersed in the resin  43  is applied to the surface of the infrared-ray absorption layer  45  by a flow immersion method or the like and consolidated, so that the radiation suppression film  40  can be formed on the surface of the substrate  400 . 
     For example, the infrared-ray absorption layer  45  is produced by applying a black body coating material to the surface of a stainless steel substrate  400 . Further, a ZnS porous body  41  is produced by pulverizing a porous sintered body obtained by sintering fine particles of ZnS. A coating material is prepared by mixing the ZnS porous body  41  with a polyethylene resin  43 . The coating material is applied to the surface of the stainless steel substrate  400  on which the infrared-ray absorption layer  45  has been formed by a flow immersion method and consolidated. The radiation suppression film  40  in which the ZnS porous bodies  41  are dispersed in the polyethylene resin  43  is laminated on the infrared-ray absorption layer  45  can be formed on the surface of the substrate  400  made of stainless steel. 
     The above is the description of the radiation suppression film  40  of the present example embodiment. The structure of  FIG. 4  is an example, and the structure of the radiation suppression film  40  is not limited to the form as in  FIG. 4 . For example, the radiation suppression film  40  may be formed not on a flat surface but on a curved surface. The radiation suppression film  40  may be formed not on a smooth surface but on a surface having irregularities. The radiation suppression film  40  may be continuously or discontinuously formed on at least one surface of substrate  400 . The radiation suppression film  40  may be formed on surfaces of the different substrates  400  so as to straddle a boundary between the substrates  400 . 
     Fifth Example Embodiment 
     Next, a radiation suppression film according to a fifth example embodiment of the present invention will be described with reference to the drawing. The radiation suppression film of the present example embodiment has a structure in which a protective layer is formed on a surface of a base material. In the present example embodiment, an example in which the protective layer is formed on the surface of the base material of the first example embodiment will be described, but the protective layer may be formed on the surface of the base material of the third example embodiment or on the surface of the resin of the second or fourth example embodiment. Hereinafter, description of structures, functions, and the like similar to those of the first example embodiment may be omitted. 
     [Structure] 
       FIG. 5  is a conceptual diagram illustrating an example of a cross section of a radiation suppression structure  5  including a radiation suppression film  50  according to the present example embodiment. The radiation suppression film  50  including a porous body  51  and a protective layer  57  is formed on a surface of a substrate  500 . The porous body  51  has a structure in which holes  512  are dispersed inside a base material  511 , similarly to the porous body  11  of the first example embodiment. The radiation suppression structure  5  has a structure in which the porous body  51  is formed on a surface portion of the substrate  500 , and the protective layer  57  is formed on a surface of the porous body  51 . In the present example embodiment, the radiation suppression film  50  is constituted by the porous body  51  having the holes  512  dispersed inside the base material  511  and the protective layer  57 . 
     The base material  511  contains a material transparent to a long wavelength infrared ray, similar to the base material  111  of the first example embodiment. The material transparent to the long wavelength infrared ray is a material having high transmittance of the long wavelength infrared ray. Characteristics such as the material and physical properties of the base material  511  are similar to those of the base material  111  of the first example embodiment. 
     The hole  512  is a gap formed inside the base material  511 , similarly to the hole  112  of the first example embodiment. The properties of the hole  512  are similar to those of the hole  112  of the first example embodiment. 
     The protective layer  57  contains a material transparent to a long wavelength infrared ray. For example, the protective layer  57  is a thin film of an oxide or a fluoride having high transmittance of the long wavelength infrared ray. The protective layer  57  protects the base material  511  from deterioration due to wind and rain and high temperature. Examples of the material of the protective layer  57  include Al 2 O 3 , Y 2 O 3 , HfO 2 , SiO 2 , WO 3 , TiO 2 , ZrO 2 , ZnO, CeO 2 , Cr 2 O 3 , Ga 2 O 3 , Y 2 O 3 , CeF 3 , LaF 3 , YF 3 , and ThF 4 . Among the above materials, Y 2 O 3 , CeF 3 , LaF 3 , and YF 3  have high transparency in the long wavelength infrared ray and are suitable. For example, the protective layer  57  can be formed by sputtering, vacuum vapor deposition, a sol-gel method, a thermal spraying method, an aerosol deposition method, or the like. If the protective layer  57  is too thick, thermal radiation of the protective layer  57  itself interferes with stealth of the base material  511 . The thickness of the protective layer  57  is not limited, but the thickness of the protective layer  57  is desirably equal to or less than 3 μm. In the case where the base material  511  contains a high refractive index material such as ZnS or ZnSe, the protective layer  57  favorably contains a low refractive index material. When the refractive index of the protective layer  57  is smaller than that of the base material  511 , the protective layer  57  functions as an antireflection film and can suppress reflection of light from a surrounding environment on the surface of the radiation suppression film  50 . Therefore, the stealth of the base material is improved. Examples of the low refractive index material include YF 3 , ThF 4 , LaF 3 , CeF 3 , Al 2 O 3 , and Y 2 O 3 . 
     The substrate  500  is an object having the radiation suppression film  50  formed on its surface, similar to the substrate  100  of the first example embodiment. The radiation suppression film  50  is formed on the surface of the substrate  500 . 
     The above is the description of the radiation suppression film  50  of the present example embodiment. The structure of  FIG. 5  is an example, and the structure of the radiation suppression film  50  is not limited to the form as in  FIG. 5 . For example, the radiation suppression film  50  may be formed not on a flat surface but on a curved surface. The radiation suppression film  50  may be formed not on a smooth surface but on a surface having irregularities. The radiation suppression film  50  may be continuously or discontinuously formed on at least one surface of substrate  500 . The radiation suppression film  50  may be formed on surfaces of the different substrates  500  so as to straddle a boundary between the substrates  500 . 
     As described above, the protective layer containing the material transparent to the long wavelength infrared ray is formed on the outermost surface of the radiation suppression film of the present example embodiment. According to the present example embodiment, the base material is protected by the protective layer from deterioration due to wind and rain and high temperature. For example, the protective layer has a smaller refractive index than the base material. When the refractive index of the protective layer is smaller than that of the base material, the protective layer functions as an antireflection film and can suppress reflection of light from a surrounding environment on the surface of the radiation suppression film. 
     Sixth Example Embodiment 
     Next, a radiation suppression film according to a sixth example embodiment of the present invention will be described with reference to the drawing. The radiation suppression film of the present example embodiment has a configuration in which the radiation suppression films of the first to fifth example embodiments are simplified. 
       FIG. 6  is a conceptual diagram illustrating an example of a cross section of a radiation suppression film  60  of the present example embodiment. The radiation suppression film  60  includes a porous body  61  containing a material transparent to a long wavelength infrared ray as a base material. 
     According to the present example embodiment, the radiation suppression film that suppresses infrared radiation having a wavelength within a long wavelength infrared region and is less easily detected by a detection technique using a long wavelength infrared ray can be provided. 
     While the present invention has been described with reference to the example embodiments, the present invention is not limited to these example embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims. 
     This application is based upon and claims the benefit of priority from Japanese patent application No. 2019-100505, filed on May 29, 2019, the disclosure of which is incorporated herein in its entirety by reference. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  2 ,  3 ,  4 ,  5  Radiation suppression structure 
           10 ,  20 ,  30 ,  40 ,  50 ,  60  Radiation suppression film 
           11 ,  21 ,  31 ,  41 ,  51 ,  61  Porous body 
           23 ,  43  Resin 
           35 ,  45  Infrared-ray absorption layer 
           100 ,  200 ,  300 ,  400 ,  500 ,  600  Substrate 
           111 ,  211 ,  311 ,  411 ,  511 ,  611  Base material 
           112 ,  212 ,  312 ,  412 ,  512  Hole