Abstract:
A portable electronic device includes a heat source which generates heat in association with generating, charging, or consuming electric power. A housing base member is disposed in proximity to the heat source, and a radiation film is disposed on at least a portion of an external surface of the housing base member. The radiation film has an emissivity that is higher than an emissivity of the housing base member.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is based upon and claims the benefit under 35 USC 119 of Japanese Patent Application No. 2006-312666 filed on Nov. 20, 2006, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a portable electronic device. 
         [0004]    2. Description of the Related Art 
         [0005]    In recent years, the power consumption of portable electronic devices has tended to increase. For example, the power consumption of mobile personal computers has increased as the speeds of central processing units (CPU) thereof have increased. In addition, the power consumption of cellular phones has also tended to increase as the amount of information transmitted and received by the cellular phones has increased and as the number of functions thereof has also increased. Moreover, the power consumption of digital cameras has also tended to increase as the number of pixels that digital cameras are capable of photographing has increased and as the length of moving images that digital cameras are capable of recording has lengthened. Consequently, the amount of heat emitted from electric parts of these portable electronic devices has also tended to increase. 
         [0006]    Several mechanisms for preventing the temperature of a device from rising have been proposed. For example, Japanese Patent Application Laid-Open Publication No. 2001-75677 discloses providing a fan motor to suck air into a laptop-type personal computer to cool the personal computer through a suction port, and enhancing the efficiency of the suction of the air by decreasing the loss of the sucked-in air due to a turbulent flow in the neighborhood of the suction port of the fan motor. 
         [0007]    Moreover, Japanese Patent Application Laid-Open Publication No. 2000-31676 describes cooling an electronic device by using natural convection by a light metal having a high thermal conductivity without performing forced cooling with a fan. 
         [0008]    Furthermore, Japanese Patent Application Laid-Open Publication No. 2000-253115 proposes radiating heat to the outside of an electronic device through a housing a magnesium alloy having a high thermal conductivity. 
         [0009]    However, performing forced cooling with a fan has problems such as an installation location in order to reduce noises, electric power consumption, maintenance, and the like, which are related to the fan necessary to perform forced cooling. Moreover, performing cooling with a fan is not suitable for a mobile device having little extra space, such as a cellular phone or a digital camera. 
         [0010]    Moreover, even if a metal having a high thermal conductivity is used as the housing of the portable electronic (or information) device, a user frequently feels discomfort when the user touches the housing because the heat radiation of the housing is not sufficient. 
       SUMMARY OF THE INVENTION 
       [0011]    It is, therefore, an object of the present invention to enhance heat radiation from a portable electronic device. 
         [0012]    According to one aspect of the present invention, a portable electronic device is provided which includes: a heat source which generates heat in association with generating, charging, or consuming electric power; a housing base member disposed in proximity to the heat source; and a radiation film disposed on at least a portion of an external surface of the housing base member, wherein the radiation film having an emissivity that is higher than an emissivity of the housing base member. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The above and other objects, advantages and features of the present invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are provided for illustration only, and thus are not intended as a definition of the limits of the present invention. In the drawings: 
           [0014]      FIG. 1  is a sectional view showing a portable electronic device  1 , to which the present invention is applied; 
           [0015]      FIG. 2  is a graph showing relationships between wavelengths of black body radiation and energy densities of the radiation; 
           [0016]      FIG. 3  is a graph showing a relationship between temperatures (° C.) of a black body and heat radiation amounts (W) from the surface (10 cm 2 ) of the black body when the ambient temperature is set at 23° C.; 
           [0017]      FIG. 4  is a graph showing relationships between amounts of heat (W) emitted from a heater and surface temperatures (° C.) of a housing according to the existence of a radiation film  22  in case of using an Al plate as a housing base member  21 ; 
           [0018]      FIG. 5  is a graph showing relations between amounts of heat (W) emitted from the heater and surface temperatures (° C.) of the housing according to the existence of the radiation film  22  in case of using a SUS plate as the housing base member  21 ; 
           [0019]      FIG. 6A  is a front view showing a first application of the portable electronic device of the present invention; 
           [0020]      FIG. 6B  is a rear elevation showing the first application of the portable electronic device of the present invention; 
           [0021]      FIG. 6C  is a sectional view taken along line VIC-VIC in  FIG. 6B ; 
           [0022]      FIG. 7A  is a front view showing an alternative structure of the first application of the portable electronic device of the present invention; 
           [0023]      FIG. 7B  is a rear elevation showing the alternative structure of the first application of the portable electronic device of the present invention; 
           [0024]      FIG. 7C  is a sectional view taken along line VIIC-VIIC in  FIG. 7B ; 
           [0025]      FIG. 8A  is a front side perspective view of a second application of the portable electronic device of the present invention; 
           [0026]      FIG. 8B  is a rear side perspective view of the second application of the portable electronic device of the present invention; 
           [0027]      FIG. 9A  is a front side perspective view of an alternative structure of the second application of the portable electronic device of the present invention; 
           [0028]      FIG. 9B  is a rear side perspective view of the alternative structure of the second application of the portable electronic device of the present invention; 
           [0029]      FIG. 10  is a perspective view showing a third application of the portable electronic device of the present invention; and 
           [0030]      FIG. 11  is a partially sectional view taken along line XI-XI in  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]    In the following, the best modes for implementing the present invention are described with reference to the attached drawings. While various technically preferable features are described below, the scope of the invention is not limited to the following embodiments and illustrated examples. 
         [0032]      FIG. 1  is a sectional view of the principal part of a portable electronic device  1 , to which the present invention is applied. As shown in  FIG. 1 , the portable electronic device  1  includes a heat source  10  and a housing  20 . The heat source  10  is, for example, electronic parts that generate heat as they generate, charge, or consume electric power such as, for example, elements (e.g., a CPU) on a circuit board, and/or internal power sources, such as batteries. 
         [0033]    The housing  20  includes a housing base member  21  and a radiation film  22  formed on the external surface of the housing base member  21   
         [0034]    The housing base member  21  houses the heat source  10 , (such as the circuit board controlling the electronic device  1  and the power source). It is preferable that the housing base member  21  be contacted to the internal heat source  10  in order to conduct heat from the internal heat source  10  to the housing base member  21  efficiently. It is preferable to use a metal having a high thermal conductivity as the housing base member  21  such as stainless steel (SUS). In particular, it is preferable to use a metallic material containing any of Al, Mg, and Ti as the principal component, such as an Al alloy, a Mg alloy, or a Ti alloy as the housing base member  21 . Each one of pure metals such as Al, Mg and Ti is also preferably used. The use of such metallic materials enables the housing base member  21  to conduct the heat from the internal heat source  10  efficiently. 
         [0035]    Because such metals each having a high thermal conductivity also have a high reflectance, the emissivity (=1-reflectance) of such materials is low. Consequently, using a metal having a high thermal conductivity for the housing base member  21  can prevent radiation from the external surface of the housing base member  21 . 
         [0036]    The properties of the housing base member  21  are examined in more detail below. 
         [0037]      FIG. 2  is a graph showing the relationships between wavelengths (μm) and energy densities of radiation (J/m 3 ) by a black body when the temperature of the black body (emissivity=1) is set at 0° C., 20° C., 40° C., 60° C., and 80° C. As shown in  FIG. 2 , the radiation wavelength range in the temperature range from 0° C. to 80° C. is from about 5 μm to about 100 μm, and the peaks of the energy densities of radiation are in the wavelength range from about 10 μm to about 25 μm. If the housing base member  21  were a black body, then it is conceivable that infrared rays in almost the same wavelength ranges as those shown in  FIG. 2  would be generated from the housing base member  21  at surface temperatures of the housing base member  21  in the range from 0° C. to 80° C. 
         [0038]      FIG. 3  is a graph showing a relationship between the surface temperatures (° C.) of a black body ranging from 0° C. to 100° C. and the heat radiation amounts (W) from 10 cm 2  of the surface of the black body when the ambient temperature is set at 23° C. For example, when the temperature of the black body is 50° C., the radiation amount from 10 cm 2  of the surface of the black body is a little less than 2 W. If the housing base member  21  were a black body, then it is conceivable that almost the same quantities of heat as the quantities shown in  FIG. 3  would be radiated from the housing base member  21  at the surface temperatures of the housing base member  21  ranging from 0° C. to 100° C. 
         [0039]    However, because the housing base member  21  is not a black body, the actual emissivity of the housing base member  21  is lower than 1. 
         [0040]    In more detail, the emissivity on the longer wavelength side of a material is generally expressed by the following formula (I) based on the Hagen-Rubens&#39; formula: 
         [0000]    
       
         
           
             
               
                 
                   2 
                    
                   
                     
                       
                         2 
                          
                         ɛω 
                       
                       σ 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where ∈ is a dielectric constant, ω is an angular frequency (ω=2πν), and σ is an optical conductivity (where ω=0). 
         [0041]    As can be seen from formula (1), the emissivity of the material becomes smaller as the frequency ω becomes smaller, that is, as the wavelength λ (=c/ν) of the radiation becomes longer. Consequently, the longer the wavelength is in the wavelength range, the lower the emissivity is. 
         [0042]    Moreover, as can also be seen from formula (1), the larger the optical conductivity a of the material is, the smaller the emissivity of the material becomes. Consequently, all of the metals and the conductors that have high optical conductivities have small emissivities. 
         [0043]    The limit of the longest wavelength of the optical conductivity is equal to that of the electric conductivity. Consequently, all of the conductors having electrical conducting properties have low emissivities in a long wavelength range. Therefore, a radiation material in the long wavelength range is preferably an electrical insulator, in contrast to the material preferably used to form the housing base member  21 . 
         [0044]    According to the present embodiment, the radiation film  22  is provided on (for example, formed on) the external surface of the housing base member  21 . The radiation film  22  may be a radiation material having a high emissivity (defined as an emissivity of 0.9 or more in the infrared region of wavelengths of 10 μm or more). The basic requirement of the radiation material having a high emissivity is that it be an electrical insulator. Therefore, any electrical insulator material that can easily be produced may be selected as the material for the radiation film  22 . Various oxides such as SiO 2  and alumina (Al 2 O 3 ), clay minerals, such as kaolin, and the like, can be used as the radiation material. For example, one or more of SiO 2 , Al 2 O 3 , kaolin, RFeO 3  (R is a rare earth element), or the like, can be used. 
         [0045]    Because the emissivity of a material having an electrical conducting property, such as an ordinary metal or graphite, which may appear to be black in the visible light region, is low in the long wavelength range, such a material cannot be used as the radiation material. 
         [0046]    The radiation film  22  of SiO 2 , Al 2 O 3 , or kaolin can be formed in a sheet by, for example, applying an emulsion liquid containing the high radiation material (e.g., SiO 2 , Al 2 O 3 , or kaolin) onto a board drying the high radiation material on the board. The radiation film  22  can also be prepared by forming RFeO 3  (R is a rare earth element) on the housing base member  21  by a dip method using a nitrate thermal decomposition method. 
         [0047]    It is preferable that the radiation film  22  be opaque, the wavelength range wherein the radiation film is opaque or not is 3000 nm to 50000 nm, in order to avoid the influences of the housing base member  21 , which is made of a metallic material, on the radiation by the radiation film  22 . For example, by forming Al 2 O 3  as a porous body on the external surface of the housing base member  21  using a technique such as anodization, an opaque radiation film  22  can be formed. Alternatively, a cloth using thin glass fibers can be used as the radiation film  22 . 
         [0048]    Incidentally, if the radiation material is transparent, the wavelength range wherein the radiation material is transparent or not is 3000 nm to 50000 nm, then the thickness of the radiation film  22  is preferably made to be 100 μm or more in order to avoid the influences of the housing base member  21  on the radiation by the radiation film. 
         [0049]    In the following, advantages achieved by providing the radiation film  22  are shown by describing a specific example. 
       EXAMPLE 
     1. Housing Base Member 
       [0050]    A housing base member  21  having the dimensions 87 mm×54 mm×9 mm was made of an Al plate or a SUS plate having a thickness of 1.5 mm, and a heating element of dimensions 48 mm×33 mm×4.5 mm having a heater therein as the heat source  10  was housed in the housing base member  21 . 
       2. Radiation Film 
       [0051]    An emulsion liquid containing SiO 2 , Al 2 O 3  and kaolin as a high radiation material (having an emissivity of 0.9 or more in the infrared region of wavelengths of 10 μm or more) was applied on the housing base member  21 , and the emulsion liquid was dried to form a sheet-shaped radiation film  22  on the housing base member  21 . 
       3. Measurement of Surface Temperature of Housing 
       [0052]    The surface temperature (° C.) of the housing was measured at when various electric powers (W) were applied to the heater. 
       4. Results 
       [0053]      FIG. 4  is a graph showing measured relationships between the amounts of heat (W) emitted from the heater and the surface temperatures (° C.) of the housing when the housing  20  included an Al housing base member  21  and a radiation film  22  (lower line), and when the housing included only an Al housing base member without a radiation film (upper line).  FIG. 5  is a graph showing measured relationships between the amounts of heat (W) emitted from the heater and the surface temperatures (° C.) of the housing when the housing  20  included an SUS housing base member  21  and a radiation film  22  (lower line), and when the housing included only an SUS housing base member without a radiation film (upper line). The amounts of heat (W) emitted from the heaters are electric powers here. 
         [0054]    If the Al plate was used as the housing base member  21 , the surface temperature was 47° C. at the electric power of 2 W if no radiation film  22  was provided, whereas the surface temperature lowered to 39° C. at the electric power of 2 W if the radiation film  22  was provided. 
         [0055]    On the other hand, if the SUS plate was used as the housing base member  21 , the surface temperature was 52° C. at the electric power of 2 W if no radiation film  22  was provided, whereas the surface temperature was lowered to 47° C. at the electric power of 2 W if the radiation film  22  was provided. 
         [0056]    Accordingly, with the structure of the present embodiment, the heat from the heat source  10  can be conducted by the housing base member  21  and can be efficiently emitted by radiation from the radiation film  22  by using a material having a high thermal conductivity as the housing base member  21 , and by using the radiation film  22  made of a radiation material having a high emissivity (an emissivity of 0.9 or more in the infrared region of wavelengths of 10 μm or longer) on the external surface of the housing base member  21 . 
         [0057]    Incidentally, a fuel cell device may be used as the internal power source of the portable electronic device  1  in place of the battery. Although the amounts of heat emitted from the fuel cell device increases more than that of the conventional battery at the time of power generation by the fuel cell device, a temperature rise of the housing  20  can be suppressed because the radiation film  22  is formed on the external surface of the housing base member  21 . 
         [0058]    Moreover, instead of a metal, a resin, such as a plastic, may be used as the housing base member  21 , and the radiation film  22  may be formed on the external surface of the housing base member  21  using the resin such as the plastic. 
       [First Application] 
       [0059]      FIGS. 6A-6C  are three orthographic views showing a cellular phone  30  as a first application of the portable electronic device to which the present invention is applied.  FIG. 6A  is the front view,  FIG. 6B  is the rear elevation, and  FIG. 6C  is a sectional view taken along a line VIC-VIC in  FIG. 6B . The cellular phone  30  includes a first housing  40  and a second housing  50 , and the first housing  40  and the second housing  50  are coupled with each other through a hinge section  43  so as to be foldable with respect to each other. 
         [0060]    Operation keys  41  are provided on the front surface of the first housing  40  (the surface that is opposed to the second housing  50  when the first and second housings are folded together). A main board  44 , a keypad (not shown), and the like, are housed inside the first housing  40 . 
         [0061]    Moreover, a concave portion  46 , in which a battery pack  45  as the internal power source of the cellular phone  30  is housed, is formed in the back surface of the first housing  40 , and a battery cover  42  is provided to cover the concave portion  46  (e.g., when the battery pack is housed therein). 
         [0062]    The second housing  50  is provided with liquid crystal display sections  51  and  52 , and a lens section  53  for a built-in camera. Moreover, liquid crystal display apparatuses  54  and  55  and a lens driving section  56  are housed inside the second housing  50 . 
         [0063]    The first housing  40  and the second housing  50  include housing base members  40   a  and  50   a , respectively, all or a part of each of which is thin-walled and made of a metal having a high thermal conductivity, and radiation films  40   b  and  50   b  respectively formed on the external surfaces of the housing base members  40   a  and  50   a . The radiation films  40   b  and  50   b  are shown only in  FIG. 6C . Moreover, the radiation film  40   b  is formed on substantially the whole external surface of the outside of the first housing  40 , except for the portions where the operation keys  41  are provided, and the radiation film  50   b  is formed on substantially the whole surface of the outside of the second housing  50 , except for the portions where the liquid display sections  51  and  52  and the lens section  53  of the built-in camera are provided. 
         [0064]    The housing base members  40   a  and  50   a  and the radiation films  40   b  and  50   b  can be formed from materials that are the same as or similar to the materials used to form the housing base member  21  and the radiation film  22  as described above, and the radiation films  40   b  and  50   b  can be formed on the housing base members  40   a  and  50   a  in a manner that is the same as or similar to the way in which the radiation film  22  is formed on the housing base member  21  as described above. In this regard, the housing base members  40   a  and  50   a  and the radiation films  40   b  and  50   b  can be formed of the same or similar materials, however, can also be formed of different materials. 
         [0065]    With this structure of the first housing  40  and the second housing  50 , heat from the main board  44 , the liquid display apparatuses  54  and  55 , the battery pack  45 , and the like, can be efficiently radiated from the cellular phone  30 . 
         [0066]    Incidentally, the radiation film may be formed on only a part of the external surfaces of the housing base members  40   a  and  50   a . For example, as shown in  FIGS. 7A-7C , the battery cover  42  may include base member  42   a  (as a removable portion of the housing base member  40   a ) and a radiation film  42   b  formed on the housing base member  42   a , and the radiation film  42   b  may be the only radiation film provided to the cellular phone  30  (that is, no radiation film is provided on the housing base members  40   a  and  50   a  except at the base member  42   a  in this alternative structure). The radiation of heat from the battery pack  45 , which has the largest amounts of heat emitted from the components of the cellular phone  30 , can still be efficiently performed in this structure in which the radiation film  42   b  is formed only on the housing base member  42   a  of the battery cover  42 . With this structure, moreover, the amount of the radiation film becomes the minimal, whereby the amount of material needed to form the radiation film on the cellular phone  30  can be reduced. 
       [Second Application] 
       [0067]      FIGS. 8A and 8B  are perspective views showing a digital camera  60  as a second application of the portable electronic device to which the present invention is applied.  FIG. 8A  shows the front side of the digital camera  60 , and  FIG. 8B  shows the back side of the digital camera  60 . 
         [0068]    The digital camera  60  has a housing  70 . A lens  71  projects from the front section of the housing  70 . A shutter key  72  and a finder  73  are provided at the upper part of the housing  70 , and the finder  73 , operation keys  74 , a liquid crystal display section  75 , and the like, are provided at the rear surface of the housing  70 . A housing section (not shown) of the lens  71 , a lens driving mechanism  76 , an imaging device  77 , a control circuit  78 , an internal power source  79 , and the like, which are heat sources, are housed inside the housing  70 . Incidentally, the lens driving mechanism  76  is housed in the neighborhood of the lens  71 . 
         [0069]    The housing  70  includes a housing base member  70   a , all or a part of which is thin-walled and made of a metal having a high thermal conductivity, and a radiation film  70   b  provided on the external surface of the housing base member  70   a . The radiation film  70   b  is provided on substantially the whole external surface of the housing base member  70   a , except for the parts where the lens  71 , the shutter key  72 , the finder  73 , the operation keys  74 , the liquid display section  75 , and the like, are provided. 
         [0070]    The housing base member  70   a  and the radiation film  70   b  can be formed from materials that are the same as or similar to the materials used to form the housing base member  21  and the radiation film  22  as described above, and the radiation film  70   b  can be formed on the housing base member  70   a  in a manner that is the same as or similar to the way in which the radiation film  22  is formed on the housing base member  21  as described above. In this regard, the housing base member  70   a  and the radiation film  70   b  can be formed of the same or similar materials, however, can also be formed of different materials. 
         [0071]    With the structure of the housing base member  70 , the heat from the lens driving mechanism  76 , the imaging apparatus  17 , the control circuit  78 , the internal power source  79 , and the like, can be efficiently radiated from the digital camera  60 . 
         [0072]    Incidentally, as shown in  FIGS. 9A and 9B , the radiation film  70   b  may be provided on the housing base member  70   a  only at a portion of the housing  70  at the periphery of the lens  71 . In this structure, the remaining portion of the housing  70  is formed by only the housing base member  70   a . In other words, in this alternative structure, the radiation film  70   b  is provided only in the neighborhood of the part where the lens driving mechanism  76  is built in. By providing the radiation film  70   b  only in the neighborhood of the part where the lens driving mechanism  76  is built in, the heat from the lens driving mechanism  76 , which radiates a large amount of heat, can still be efficiently radiated. With this structure, moreover, the amount of the radiation film becomes minimal, whereby the amount of the material needed to form the radiation film on the digital camera  60  can be reduced. 
       [Third Application] 
       [0073]      FIG. 10  is a perspective view showing a notebook personal computer  80  as a third application of the portable electronic device to which the present invention is applied. 
         [0074]    The personal computer  80  includes a lower housing  81 , an upper housing  82 , a hinge  83  which couples the lower housing  81  and the upper housing  82 , and a power source section go. The lower housing  81  and the upper housing  82  are configured to be capable of being folded together (i.e., in a stack) using the hinge  83 . 
         [0075]    The lower housing  81  has an arithmetic processing circuit including a CPU, a random access memory (RAM), a read only memory (ROM), and other electric parts therein, and a keyboard (not shown) is provided on the surface thereof opposed to the upper housing  82 . The upper housing  82  is provided with a liquid crystal display (not shown) on the surface thereof opposed to the lower housing  81 . 
         [0076]    An installing section  84  is formed at the rear part of the hinge  83  on the lower housing  81 . A power source section  90  is freely attachable to and detachable from the installing section  84 . Thus, the upper housing  82  and lower housing  81 , including the hinge  83  and installing section  84 , form a main body that the power source section  90  is attachable to and detachable from. 
         [0077]      FIG. 11  is a partial sectional view taken along line XI-XI in  FIG. 10 , in which only the right half portion of the main body section  91  is shown by a cutaway view. The power source section  90  includes a main body section  91  and fuel cartridges  92 , which are freely attachable to and detachable from the main body section  91  at installing sections  94  of the main body  91 . An interface  93  to supply electric power to the lower housing  81  when the power source section  90  is connected with the lower housing  81  is provided on the surface of the main body section  91  that is opposed to the lower housing  81 . Moreover, interfaces  95  to be connected to the fuel cartridges  92  are formed on the installing sections  94 . 
         [0078]    Two fuel cell devices  100  are provided in the housing of the main body section  91 . One fuel cell device  100  is provided for each of the installing sections  94 . The fuel cell device  100  is a device which converts reaction energy of fuel and air into electric power energy, and includes, for example, a pump  101  which supplies fuel and water from inside one of the fuel cartridges  92 , a vaporizer  102  which vaporizes the fuel, a reformer  103  which generates a gas containing hydrogen (reformed gas) by a reforming reaction of the fuel, a carbon monoxide remover  104  which removes carbon monoxide, which is a by-product of the reforming reaction, a power generation cell  105  which converts reaction energy of the hydrogen in the reformed gas and oxygen in the air into electric power energy, and the like. 
         [0079]    The housing of the main body section  91  is made by forming a radiation film  91   b  on the external surface of a housing base member  91   a , all or a part of which is thin-walled and made of a metal having a high thermal conductivity. 
         [0080]    The housing base member  91   a  and the radiation film  91   b  can be formed from materials that are the same as or similar to the materials used to form the housing base member  21  and the radiation film  22  as described above, and the radiation film  91   b  can be formed on the housing base member  91   a  in a manner that is the same as or similar to the way in which the radiation film  22  is formed on the housing base member  21  as described above. In this regard, the housing base member  91   a  and the radiation film  91   b  can be formed of the same or similar materials, however, can also be formed of different materials. 
         [0081]    With this structure of the housing main body section  91 , the heat from the fuel cell devices can be efficiently radiated. 
         [0082]    The cellular phone  30 , the digital camera  60 , and the notebook personal computer  80  have been described above as portable electronic devices to which the present invention is applicable. The devise to which the present invention is applicable are not limited to such devices. For example, the present invention can be applied to other portable electronic devices, such as a personal digital assistant (PDA), an electronic personal organizer, a wrist watch, an electronic cash register, and a projector. 
         [0083]    Incidentally, fuel cell devices may be used as the internal power sources of the cellular phone  30  and the digital camera  60 . Although the amounts of heat emitted from the fuel cell device at the time of power generation increases more than that of a conventional battery, the rise of the temperature of the housing can be suppressed because according to the present invention a radiation film is formed on the external surface of a housing base member of the housing. 
         [0084]    Although various exemplary embodiments have been shown and described, the invention is not limited to these embodiments. Therefore, the scope of the invention is intended to be limited solely by the scope of the claims that follow.