Patent Publication Number: US-2009218087-A1

Title: Thermal conduction structure, composite material, and method of producing the material

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on Japanese Patent Application No. 2008-50155 filed on Feb. 29, 2008, and Japanese Patent Application No. 2008-50156 filed on Feb. 29, 2008, the disclosure of which are incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a thermal conduction structure, a composite material, and a method of producing the composite material. 
     2. Description of Related Art 
     A material usually has a constant thermal conductivity or a constant electric conductivity as a physical property. However, JP-A-5-23900, JP-A-11-236636, JP-A-9-329290 (corresponding to U.S. Pat. No. 5,156,087), and JP-A-2000-274976 disclose a structure having a thermal conductivity, which is variable without an appearance change of the structure. The thermal conductivity or the electric conductivity is required to be variable based on a use of the structure and an environment of the structure. 
     However, a structure disclosed in JP-A-5-23900 is deformed as a whole. A method of fixing the structure onto a heat receiving portion or a heat releasing portion may be limited. A variation of a thermal conductivity of a structure disclosed in JP-A-11-236636 is small due to a material used for the structure. A structure disclosed in JP-A-9-329290 includes two gases having different thermal conductivities. A variation of a thermal conductivity of the structure is limited to a range of the thermal conductivities of the two gases. A variation of a thermal conductivity of a structure disclosed in JP-A-2000-274976 is small. The structure includes a fluid, and a variation of a thermal conductivity of the fluid cannot be large. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing and other problems, it is an object of the present invention to provide a thermal conduction structure, a composite material, and a method of producing the composite material. 
     According to a first example of the present invention, a thermal conduction structure includes a heat receiving portion, a heat releasing portion, and a plurality of actuators disposed between the heat receiving portion and the heat releasing portion. A path is defined from the heat receiving portion to the heat releasing portion through the actuator. The actuator is movable between a first position and a second position so as to correspond to an energy supplied from outside. The actuator is contact with the heat receiving portion and the heat releasing portion, when the actuator is in the first position. The path has a non-contact part between the heat receiving portion and the heat releasing portion, when the actuator is in the second position. 
     Accordingly, a variation of a thermal conductivity or an electric conductivity of the thermal conduction structure can be large. The thermal conduction structure can be used for a variety of heat receiving portions and a variety of heat releasing portions. 
     According to a second example of the present invention, a composite material includes a mix of a first portion made of a first material and a second portion made of a second material. The second material has at least one of a thermal conductivity and an electric conductivity, which is lower than that of the first material, and has a thermal expansion coefficient lower than that of the first material. The first portion has a contact rate to be contact with each other. The contact rate of the first portion is changed when the composite material has a temperature equal to or higher than a predetermined value, such that at least one of a thermal conductivity and an electric conductivity of the composite material is changed. 
     Accordingly, a variation of the thermal conductivity or the electric conductivity of the composite material can be large. The composite material can be used for a variety of heat receiving portions and a variety of heat releasing portions. 
     According to a third example of the present invention, a method of producing a composite material includes a mixing of a first material and a second material, and a solidifying the mix of the first material and the second material such that a first portion made of the first material is continuously contact with each other. 
     Accordingly, a variation of a thermal conductivity or an electric conductivity of the composite material can be large. The composite material can be used for a variety of heat receiving portions and a variety of heat releasing portions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
         FIG. 1  is a schematic side view showing a principle of a thermal conduction structure according to first and second embodiments; 
         FIG. 2  is a graph showing a relationship between a surface temperature and a thermal conductivity of the thermal conduction structure; 
         FIG. 3  is a schematic perspective view showing a construction of the thermal conduction structure; 
         FIG. 4  is a schematic side view showing the thermal conduction structure having a non-contact part; 
         FIG. 5  is a schematic side view showing the thermal conduction structure not having the non-contact part; 
         FIG. 6  is a graph showing a relationship between a temperature of a heat receiving portion of the structure and a temperature of a heat releasing portion of the structure; 
         FIG. 7  is a schematic side view showing a thermal conduction structure according to a third embodiment; 
         FIG. 8  is a schematic side view showing a thermal conduction structure according to a fourth embodiment; 
         FIG. 9  is a schematic side view showing a thermal conduction structure according to a fifth embodiment; 
         FIG. 10  is a schematic side view showing a thermal conduction structure according to a sixth embodiment; 
         FIG. 11  is a schematic side view showing a thermal conduction structure according to a seventh embodiment; 
         FIG. 12A  is a schematic view showing a composite material according to an eighth embodiment at a high temperature, and  FIG. 12B  is a schematic view showing the composite material at a low temperature; 
         FIG. 13A  is a schematic view showing another composite material according to an eighth embodiment at a low temperature, and  FIG. 13B  is a schematic view showing the another composite material at a high temperature; 
         FIG. 14  is a schematic cross-sectional view showing a composite structure made of the composite material; 
         FIG. 15  is a view showing thermal conductivities and linear expansion coefficients of materials used for producing the composite material; and 
         FIG. 16  is a graph showing a relationship between a temperature of a heat receiving portion of the composite structure and a temperature of a heat releasing portion of the composite structure. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     As shown in  FIG. 1 , a thermal conduction structure  1  includes a heat receiving portion  7 , a heat releasing portion  9  and actuators  3  disposed between the heat receiving portion  7  and the heat releasing portion  9 . The heat receiving portion  7  may be a heat receiving face, and the heat releasing portion  9  may be a heat releasing face. As shown in  FIG. 5 , a path is formed from the heat receiving portion  7  to the heat releasing portion  9  through the actuator  3 . The actuator  3  is deformable and movable between a first position shown in  FIG. 5  and a second position shown in  FIG. 4 , for example. The actuator  3  is deformable corresponding to an energy supplied from outside. 
     The heat receiving portion  7  is contact with the actuator  3 , and the heat releasing portion  9  is also contact with the actuator  3 , when the actuator  3  is in the first position shown in  FIG. 5 . The path has a non-contact part  11  between the heat receiving portion  7  and the heat releasing portion  9 , when the actuator  3  is in the second position shown in  FIG. 4 . 
     In  FIG. 5 , the actuator  3  is contact with both of the heat receiving portion  7  and the heat releasing portion  9 . Further, a first contact part between the actuator  3  and the heat receiving portion  7 , and a second contact part between the actuator  3  and the heat releasing portion  9  may be integrated with each other. 
     In  FIG. 4 , the actuator  3  is not contact with the heat releasing portion  9 . Alternatively, the actuator  3  may not be contact with the heat receiving portion  7  in place of the heat releasing portion  9 . Alternatively, a first contact part between the actuator  3  and the heat receiving portion  7 , and a second contact part between the actuator  3  and the heat releasing portion  9  may be separated from each other. 
     Thus, a variation of a thermal conductivity between the heat receiving portion  7  and the heat releasing portion  9  can be large. That is, the thermal conductivity is large when the actuator  3  is in the first position shown in  FIG. 5 , and the thermal conductivity is small when the actuator  3  is in the second position shown in  FIG. 4 . 
     As shown in  FIG. 1 , the actuator  3  has a first end  3   a  and a second end  3   b . The first end  3   a  of the actuator  3  is continuously contact with one of the heat receiving portion  7  and the heat releasing portion  9 , and the second end  3   b  of the actuator  3  is contact with other of the heat receiving portion  7  and the heat releasing portion  9 , when the actuator  3  is in the first position. The second end  3   b  of the actuator  3  is not contact with other of the heat receiving portion  7  and the heat releasing portion  9 , when the actuator  3  is in the second position. 
     The first end  3   a  of the actuator  3  is continuously contact with the heat receiving portion  7 , as shown in  FIG. 1 . Thus, a movement of the actuator  3  can quickly correspond to a temperature change of the heat receiving portion  7 . 
     The first end  3   a  of the actuator  3  may be continuously contact with the heat releasing portion  9 . Thus, a timing of the movement of the actuator  3  can be delayed compared with a case in which the first end  3   a  of the actuator  3  is continuously contact with the heat receiving portion  7 . 
     The thermal conduction structure  1  may further include a support member  5  disposed between the heat receiving portion  7  and the heat releasing portion  9 . The support member  5  supports the heat receiving portion  7  and the heat releasing portion  9 . The actuator  3  may include a bimetal and/or a shape-memory alloy deformable to correspond to a thermal energy supplied from outside of the actuator  3 . 
     As shown in  FIG. 7 , the actuator  3  may include a sack member  12  made of an elastic material. The sack member  12  is sealed to have a gas inside, such that the sack member  12  expands or contracts so as to be deformable to correspond to a thermal energy supplied from outside. 
     The actuator  3  is in the first position without the non-contact part  11 , when the actuator  3  has a temperature equal to or higher than a predetermined value, for example. The actuator  3  is in the second position with the non-contact part  11 , when the actuator  3  has a temperature lower than the predetermined value. 
     In this case, the heat receiving portion  7  may be an engine of an automotive vehicle, and the heat releasing portion  9  may be an exhaust pipe or a water-cooled radiator. When a temperature of the engine is low during a warm-up, heat releasing from the engine can be reduced. When the temperature of the engine becomes high, heat of the engine can be quickly released to the exhaust pipe or the water-cooled radiator. 
     A temperature for changing the actuator  3  from the first position to the second position can be made different from a temperature for changing the actuator  3  from the second position to the first position by changing characteristics of the bimetal or the shape-memory alloy, for example. In this case, as shown in  FIG. 2 , hysteresis is generated between a surface temperature of the heat receiving portion  7  or the heat releasing portion  9 , and a thermal conductivity between the heat receiving portion  7  and the heat releasing portion  9 . 
     When the actuator  3  has a temperature equal to or higher than a predetermined value, the actuator  3  may be in the second position with the non-contact part  11 . When the actuator  3  has a temperature lower than the predetermined value, the actuator  3  may be in the first position without the non-contact part  11 . In this case, the thermal conduction structure  1  may be used for a keep-warm pot having a double structure, for example. That is, an inner wall of the keep-warm pot may be the heat receiving portion  7 , and an outer wall of the keep-warm pot may be the heat releasing portion  9 . When the keep-warm pot has a high-temperature liquid inside, a thermal conductivity can be made low. Thus, the high-temperature liquid can be kept hot. 
     The actuator  3  may include a solenoid, which is operable to correspond to an electric energy supplied from outside. The actuator  3  may include a magnet, which is operable to correspond to a magnetic energy supplied from outside. 
     Embodiments are described with reference to  FIGS. 1-16 . 
     First Embodiment 
     A method of manufacturing a thermal conduction structure  1  will be described with reference to  FIGS. 3 and 4 . A bimetal is processed to have a U-shape so as to produce an actuator  3 . The bimetal is processed such that the U-shape is to be open when the actuator  3  is heated. The actuator  3  is a cuboid having a bottom face of 2 mm×2 mm, and a height of 1 mm. A plurality of the actuators  3  is manufactured. 
     Further, a support member  5  is an epoxy resin coated with a hot-melt having a bottom face of 2 mm×2 mm, and a height of 1.1 mm. A plurality of the support members  5  is manufactured. 
     A heat receiving portion  7  is a copper board having an area of 100 mm×100 mm, and a thickness of 0.5 mm. The actuators  3  are mounted on a whole top face of the copper board through a braze  6  shown in  FIG. 4 . The actuators  3  are arrayed with a predetermined interval in each of a left-and-right direction and a front-and-rear direction of the copper board of  FIG. 3 . At this time, a dummy epoxy resin (not shown) having a bottom face of 2.5 mm×2.5 mm, and a height of  1 mm is disposed in a space in which the support member  5  is to be located. After the dummy epoxy resin is removed, the support member  5  is mounted in the space. 
     The heat releasing portion  9  is a copper board having an area of 100 mm×100 mm, and a thickness of 0.5 mm. The heat releasing portion  9  covers the actuator  3  and the support member  5  disposed on the heat receiving portion  7 , and is pressurized with a temperature of 150° C. Thus, the heat receiving portion  7  and the heat releasing portion  9  are connected to each other through the support member  5 . A periphery of the heat receiving portion  7  and the heat releasing portion  9  is sealed with a silicon resin. As shown in  FIG. 4 , when the heat receiving portion  7  is not heated, a clearance corresponding to the non-contact part  11  is generated between the first end  3   a  of the actuator  3  and the heat releasing portion  9 . 
     An operation of the thermal conduction structure  1  will be described. The thermal conduction structure  1  is disposed on a hot plate with the heat receiving portion  7  to be a bottom side. The thermal conduction structure  1  is heated in this state. A temperature of the heat receiving portion  7  is gradually increased from a room temperature to 250° C. When the temperature of the heat receiving portion  7  becomes equal to or higher than a predetermined value, as shown in  FIG. 5 , the U-shape of the actuator  3  opens such that the first end  3   a  of the actuator  3  contacts the heat releasing portion  9 . At this time, a path extending from the heat receiving portion  7  through the actuator  3  to the heat releasing portion  9  is disabled to have the non-contact part  11 . This state continues until when the temperature of the heat receiving portion  7  becomes 250° C. The temperature of the heat receiving portion  7  is decreased after the heating of the hot plate is stopped. When the temperature of the heat receiving portion  7  becomes lower than the predetermined value, the U-shape of the actuator  3  closes such that the path is able to have the non-contact part  11  between the first end  3   a  of the actuator  3  and the heat releasing portion  9 . 
     A temperature variation of the heat releasing portion  9  is shown in a y-axis of a solid line of  FIG. 6 , while a temperature of the heat receiving portion  7  is increased, which corresponds to an x-axis of the solid line of  FIG. 6 . As a comparison example, a temperature variation of a single copper plate is shown in a dashed line of  FIG. 6  while the single copper plate is heated on the hot plate with the same condition. An increasing rate of the temperature of the heat releasing portion  9  is small until when the temperature of the heat receiving portion  7  becomes about 150° C. This is because the first end  3   a  of the actuator  3  does not contact the heat releasing portion  9 . That is, a thermal conductivity from the heat receiving portion  7  to the heat releasing portion  9  is low. After the temperature of the heat receiving portion  7  exceeds 150° C., the increasing rate of the temperature of the heat releasing portion  9  becomes large. This is because the first end  3   a  of the actuator  3  contacts the heat releasing portion  9 . The thermal conductivity from the heat receiving portion  7  to the heat releasing portion  9  becomes large, because a thermal conduction path is generated from the heat receiving portion  7  through the actuator  3  to the heat releasing portion  9 . In contrast, an increasing rate of a temperature of the single copper plate of the comparison example is constant. 
     Effect of the heat conduction structure  1  will be described. As shown in  FIG. 6 , the variation of the thermal conductivity of the heat conduction structure  1  can be made large. The heat conduction structure  1  can be used for a heat receiving portion or a heat releasing portion having a variety of sizes or shapes by adjusting a height or size of the actuator  3 . The heat conduction structure  1  can have a simple construction. 
     When the temperature of the heat receiving portion  7  is low, the thermal conductivity of the thermal conduction structure  1  can be made low. When the temperature of the heat receiving portion  7  is high, the thermal conductivity of the thermal conduction structure  1  can be made high. For example, the heat receiving portion  7  may be an engine of an automotive vehicle, and the heat releasing portion  9  may be an exhaust pipe or a water-cooled radiator. When a temperature of the engine is low during a warm-up, heat releasing from the engine can be reduced. When the temperature of the engine is increased, heat of the engine can be quickly released to the exhaust pipe or the water-cooled radiator. 
     Second Embodiment 
     Operation of an actuator  3  of a thermal conduction structure  1  of a second embodiment is different from that of the first embodiment. When the actuator  3  is not heated, a U-shape of the actuator  3  opens such that a first end  3   a  of the actuator  3  contacts a copper board corresponding to a heat releasing portion  9 . In contrast, when the actuator  3  is heated, the U-shape of the actuator  3  closes such that a non-contact part  11  is generated between the first end  3   a  of the actuator  3  and the heat releasing portion  9 . Therefore, the thermal conduction structure  1  of the second embodiment has a high thermal conductivity when the heat receiving portion  7  is not heated. The thermal conduction structure  1  of the second embodiment has a low thermal conductivity when the heat receiving portion  7  is heated. The actuator  3  of the second embodiment is manufactured by converting a direction of a bimetal between an inner side and an outer side from the first embodiment. 
     The thermal conduction structure  1  may be used for a keep-warm pot having a double structure, for example. That is, an inner wall of the keep-warm pot may be the heat receiving portion  7 , and an outer wall of the keep-warm pot may be the heat releasing portion  9 . When the keep-warm pot has a high-temperature liquid inside, a thermal conductivity can be made low. Thus, the high-temperature liquid can be kept hot. 
     Third Embodiment 
     A thermal conduction structure  1  of a third embodiment has a construction shown in  FIG. 7 . A heat receiving portion  7  and a heat releasing portion  9  are connected to each other through a support member  5 , similar to the first embodiment. A plurality of actuators  3  is mounted on a whole top face of the heat receiving portion  7 , similar to the first embodiment. 
     The actuator  3  is constructed with a tube  10  made of phosphor bronze and a sack member  12 , which has air inside and is made of silicon rubber. The tube  10  has a cross-section of an oval shape. A side face of the tube  10  is brazed to the heat receiving portion  7 . At this time, a major axis direction of the cross-section of the tube  10  is parallel to the heat receiving portion  7  and the heat releasing portion  9 . 
     The sack member  12  is made of silicon rubber, and has a sphere shape. Air is sealed in the sack member  12 . The sack member  12  is disposed inside of the tube  10 . Both of an upper end and a bottom end of the sack member  12  contact an inner wall of the tube  10 . A clearance is generated between a left-and-right end of the sack member  12  and the inner wall of the tube  10 , as shown in  FIG. 7 . 
     When the heat receiving portion  7  is not heated, a clearance is generated between a top end  10   a  of the tube  10  and the heat releasing portion  9 . At this time, the heat receiving portion  7  and the heat releasing portion  9  are not contact with each other through the actuator  3 . Therefore, a thermal conductivity is low between the heat receiving portion  7  and the heat releasing portion  9 . 
     When the heat receiving portion  7  is heated, the sack member  12  expands. Then, the top end  10   a  of the tube  10  is pushed upward to be contact with the heat releasing portion  9 . At this time, a thermal circuit is defined to connect the heat receiving portion  7 , the tube  10  and the heat releasing portion  9 . Therefore, the thermal conductivity becomes high between the heat receiving portion  7  and the heat releasing portion  9 . When the heating of the heat receiving portion  7  is stopped, the sack member  12  contracts. Again, a clearance is generated between a top end  10   a  of the tube  10  and the heat releasing portion  9 . 
     Thus, the actuator  3  of the thermal conduction structure  1  is activated by a thermal energy supplied from outside such that a thermal conductivity can be changed between the heat receiving portion  7  and the heat releasing portion  9 . 
     Fourth Embodiment 
     A thermal conduction structure  1  of a fourth embodiment has a construction shown in  FIG. 8 . Similar to the first embodiment, a heat receiving portion  7  and a heat releasing portion  9  are connected to each other through a support member  5 . Actuators  3  are mounted on a whole top face of the heat receiving portion  7 . 
     The actuator  3  is constructed with an iron core  13 , a solenoid  15 , and an iron foil  17 . The iron core  13  has a diameter of 2 mm, and a length of 3 mm. An end of the iron core  13  is brazed to the heat receiving portion  7 . A clearance is defined between the iron core  13  and the heat releasing portion  9 . The solenoid  15  is an enamel wire winded around the iron core  13 , and is connected to an external energy source (not shown). When a current passes through the solenoid  15 , the iron core  13  and the solenoid  15  operate as an electromagnet. 
     A first end  17   a  of the iron foil  17  is fixed between the support member  5  and the heat releasing portion  9 . Thus, the iron foil  17  is always contact with the heat releasing portion  9 . A second end  17   b  of the iron foil  17  is located between the iron core  13  and the heat releasing portion  9 . 
     When a current does not pass through the solenoid  15 , the electromagnet constructed with the iron core  13  and the solenoid  15  is not activated. The second end  17   b  of the iron foil  17  does not contact either the heat releasing portion  9  or the iron core  13 . At this time, the heat receiving portion  7  and the heat releasing portion  9  are not contact with each other through the actuator  3 . Thus, a thermal conductivity is low between the heat receiving portion  7  and the heat releasing portion  9 . 
     When a current passes through the solenoid  15 , the electromagnet constructed with the iron core  13  and the solenoid  15  is activated. The second end  17   b  of the iron foil  17  contacts the iron core  13  by a magnetic force. A thermal circuit is defined to connect the heat receiving portion  7 , the iron core  13 , the iron foil  17 , and the heat releasing portion  9 . Thus, a thermal conductivity becomes high between the heat receiving portion  7  and the heat releasing portion  9 . 
     Thus, the actuator  3  of the thermal conduction structure I is activated by an electric energy supplied from outside such that a thermal conductivity can be changed between the heat receiving portion  7  and the heat releasing portion  9 . 
     Fifth Embodiment 
     A thermal conduction structure  1  of a fifth embodiment has a construction shown in  FIG. 9 . Similar to the first embodiment, a heat receiving portion  7  and a heat releasing portion  9  are connected to each other through a support member  5 . Actuators  3  are mounted on a whole top face of the heat receiving portion  7 . 
     The actuator  3  is constructed with a hollow bobbin  19 , an enamel wire  21 , an iron core  23  and a spring  25 . The hollow bobbin  19  has an inner diameter of 2 mm, and a length of 2 mm. The enamel wire  21  is winded around the hollow bobbin  19 , and is connected to an external energy source (not shown). When a current passes through the enamel wire  21 , the hollow bobbin  19  and the enamel wire  21  operate as an electromagnet. The spring  25  is disposed in the hollow bobbin  19 , and a first end of the spring  25  is fixed to the heat receiving portion  7 . Further, a second end of the spring  25  is connected to the iron core  23  such that the iron core  23  is biased toward the heat releasing portion  9 . 
     When a current does not pass through the enamel wire  21 , the hollow bobbin  19  and the enamel wire  21  does not operate as the electromagnet. The iron core  23  contacts the heat releasing portion  9  due to a biasing force of the spring  25 . At this time, a thermal circuit is defined to connect the heat receiving portion  7 , the spring  25 , the iron core  23 , and the heat releasing portion  9 . Thus, a thermal conductivity becomes high between the heat receiving portion  7  and the heat releasing portion  9 . 
     When a current passes through the enamel wire  21 , the electromagnet constructed with the hollow bobbin  19  and the enamel wire  21  is activated. The iron core  23  is pulled into the hollow bobbin  19 , and a clearance is generated between the iron core  23  and the heat releasing portion  9 . At this time, the heat receiving portion  7  and the heat releasing portion  9  are not contact with each other through the actuator  3 . Thus, a thermal conductivity is low between the heat receiving portion  7  and the heat releasing portion  9 . 
     Thus, the actuator  3  of the thermal conduction structure  1  is activated by an electric energy supplied from outside such that a thermal conductivity can be changed between the heat receiving portion  7  and the heat releasing portion  9 . 
     Sixth Embodiment 
     A thermal conduction structure  1  of a sixth embodiment has a construction shown in  FIG. 10 . Similar to the first embodiment, a heat receiving portion  7  and a heat releasing portion  9  are connected to each other through a support member  5 . Actuators  3  are mounted on a whole top face of the heat receiving portion  7 . 
     The actuator  3  is constructed with a spring board  27  made of phosphor bronze, and a permanent magnet  29 . Further, an electromagnet  31  is mounted on a top face of the heat releasing portion  9 . 
     The spring board  27  has an approximately Z-shape, and a bottom face  27   a  of the spring board  27  is brazed to the heat receiving portion  7 . The permanent magnet  29  is mounted on a low side of a top face  27   b  of the spring board  27 . 
     When the electromagnet  31  is not activated, the top face  27   b  of the spring board  27  contacts the heat releasing portion  9 . At this time, a thermal circuit is defined to connect the heat receiving portion  7 , the spring board  27 , and the heat releasing portion  9 . Thus, a thermal conductivity becomes high between the heat receiving portion  7  and the heat releasing portion  9 . 
     When the electromagnet  31  is activated, the electromagnet  31  and the permanent magnet  29  repel each other. Then, the spring board  27  is deformed due to a repulsion between the electromagnet  31  and the permanent magnet  29 . The top face  27   b  of the spring board  27  is separated from the heat releasing portion  9 . At this time, the heat receiving portion  7  and the heat releasing portion  9  are not contact with each other through the actuator  3 . Thus, a thermal conductivity is low between the heat receiving portion  7  and the heat releasing portion  9 . 
     Thus, the actuator  3  of the thermal conduction structure  1  is activated by a magnetic energy supplied from outside such that a thermal conductivity can be changed between the heat receiving portion  7  and the heat releasing portion  9 . 
     Seventh Embodiment 
     A thermal conduction structure  1  of a seventh embodiment has a construction shown in  FIG. 11 . Similar to the first embodiment, a heat receiving portion  7  and a heat releasing portion  9  are connected to each other through a support member  5 . Actuators  3  are mounted on a whole top face of the heat receiving portion  7 . 
     The actuator  3  is constructed with a pair of iron boards  33 ,  35  and a magnetic fluid  37 . Further, an electromagnetic  39  is mounted on a top face of the heat releasing portion  9 . 
     An end of the iron board  33  is brazed to the heat receiving portion  7  such that a longitudinal direction of the iron board  33  corresponds to a longitudinal direction of the support member  5 . An end of the iron board  35  is brazed to the heat releasing portion  9  such that a longitudinal direction of the iron board  35  corresponds to a longitudinal direction of the support member  5 . A clearance  41  having a dimension of 1 mm is located between the iron boards  33 ,  35  in the longitudinal direction of the iron boards  33 ,  35 . The magnetic fluid  37  has a volume equal to or larger than a volume of the clearance  41 . 
     When the electromagnet  39  is not activated, the magnetic fluid  37  is not located in the clearance  41 . Therefore, the iron boards  33 ,  35  are not connected to each other. At this time, the heat receiving portion  7  and the heat releasing portion  9  are not contact with each other through the actuator  3 . Thus, a thermal conductivity is low between the heat receiving portion  7  and the heat releasing portion  9 . 
     When the electromagnet  39  is activated, the magnetic fluid  37  is pulled toward the clearance  41  so as to connect the iron boards  33 ,  35 . At this time, a thermal circuit is defined to connect the heat receiving portion  7 , the iron board  33 , the magnetic fluid  37 , the iron board  35 , and the heat releasing portion  9 . Thus, a thermal conductivity becomes high between the heat receiving portion  7  and the heat releasing portion  9 . 
     Thus, the actuator  3  of the thermal conduction structure  1  is activated by a magnetic energy supplied from outside such that a thermal conductivity can be changed between the heat receiving portion  7  and the heat releasing portion  9 . 
     For example, a shape-memory alloy may be used in place of the bimetal as a material of the actuator  3  in the first and second embodiments. 
     Such changes and modifications are to be understood as being within the scope of the present invention as defined by the appended claims. 
     Eighth Embodiment 
     As shown in  FIG. 12A , a composite material  50  is a mix of a first portion  101  made of a first material and a second portion  103  made of a second material. The second material has at least one of a thermal conductivity and an electric conductivity lower than that of the first material. The second material has a thermal expansion coefficient lower than that of the first material. When the composite material  50  has a high temperature, expansion of the first material is large relative to that of the second material. As shown in  FIG. 12A , the first portion  101  has a contact rate to be contact with each other, and the contact rate is increased when the composite material  50  has a high temperature. Thus, the first portion  101  is continuously connected to each other such that a high-conduction path  109  can be easily defined from a heat receiving portion  105  to a heat releasing portion  107 . The heat receiving portion  105  may be a heat receiving face, and the heat releasing portion  107  may be a heat releasing face. Here, the first material has at least one of a thermal conductivity and an electric conductivity higher than that of the second material. Therefore, at least one of a thermal conductivity and an electric conductivity of the composite material  50  becomes high due to the high-conduction path  109 . 
     In contrast, when the composite material  50  has a low temperature, contraction of the first material is larger than that of the second material. As shown in  FIG. 12B , the contact rate of the first portion  101  is decreased when the composite material  50  has a low temperature. Thus, the high-conduction path  109  is not generated. Therefore, at least one of the thermal conductivity and the electric conductivity of the composite material  50  becomes low. 
     Accordingly, at least one of the thermal conductivity and the electric conductivity of the composite material  50  can be widely changed based on a temperature of the composite material  50 . Further, the composite material  50  can be used between a heat receiving portion or a heat releasing portion having a variety of sizes or shapes, by only changing a shape or size of the composite material  50 . 
     As shown in  FIG. 13A , a composite material  60  is a mix of a first portion  201  made of a first material and a second portion  203  made of a second material. The second material has at least one of a thermal conductivity and an electric conductivity lower than that of the first material. The second material has a thermal expansion coefficient lower than that of the first material. When the composite material  60  has a low temperature, contraction of the second material is larger than that of the first material. As shown in  FIG. 13A , the first portion  201  has a contact rate to be contact with each other, and the contact rate is increased when the composite material  60  has a low temperature. Thus, the first portion  201  is continuously connected to each other such that a high-conduction path  209  can be easily defined from a heat receiving portion  205  to a heat releasing portion  207 . Here, the first material has at least one of a thermal conductivity and an electric conductivity higher than that of the second material. Therefore, at least one of a thermal conductivity and an electric conductivity of the composite material  60  becomes high due to the high-conduction path  209 . 
     In contrast, when the composite material  60  has a high temperature, expansion of the second material is larger than that of the first material. As shown in  FIG. 13B , the contact rate of the first portion  201  is decreased when the composite material  60  has a high temperature. Thus, the high-conduction path  209  is not generated. Therefore, at least one of the thermal conductivity and the electric conductivity of the composite material  60  becomes low. 
     Accordingly, at least one of the thermal conductivity and the electric conductivity of the composite material  50 ,  60  can be widely changed based on a temperature of the composite material  50 ,  60 . Further, the composite material  50 ,  60  can be used between a heat receiving portion or a heat releasing portion having a variety of sizes or shapes, by only changing a shape or size of the composite material  50 ,  60 . 
     For example, a metal material such as silver, copper, invar and so on may be used as the first material. An inorganic oxide such as silica may be used as the second material. A polymer compound such as epoxy resin may be used as the second material. A composite of the inorganic oxide and the polymer compound may be used as the second material. The second material may have a thermal conductivity lower than that of the first material. The second material may have an electric conductivity lower than that of the first material. The second material may have a thermal conductivity lower than that of the first material, and may have an electric conductivity lower than that of the first material. 
     For example, the first material may have a particle shape. In this case, the first portion  101 ,  201  made of the first material may be constructed with a single particle or plural particles. For example, the second material may have a particle shape. Alternatively, as shown in  FIGS. 12A and 13A , the second portion  103 ,  203  made of the second material may be integrated. 
     The composite material  50 ,  60  may be produced by mixing the first material and the second material, and solidifying the mix of the first material and the second material such that the first portion  101 ,  201  is continuously contact with each other. 
     At this time, the first portion  101 ,  201  may be made to continuously contact with each other by pressurizing the mix of the first material and the second material. Further, the first portion  101 ,  201  can be made to continuously contact with each other by settling the mix of the first material and the second material, and precipitating the first material having a specific gravity larger than that of the second material. In the precipitating, the first material may be exposed from a surface of the composite material  50 ,  60 . Thus, the first portion  101 ,  201  can be secured to continuously contact with each other In the producing of the composite material  50 , a temperature of the composite material  50  is set to be higher than the predetermined value in the solidification, when the thermal expansion coefficient of the second material is smaller than that of the first material. 
     In the producing of the composite material  60 , a temperature of the composite material  60  is set to be lower than the predetermined value in the solidification, when the thermal expansion coefficient of the second material is larger than that of the first material. Thereby, the thermal conductivity or the electric conductivity of the composite material  50 ,  60  can be widely changed at a boundary of the predetermined value. 
     As shown in  FIG. 14 , a composite structure  70  is constructed with a first copper foil  51 , a second copper foil  53 , and the composite material  50 ,  60 . The composite material  50 ,  60  is disposed between the first copper foil  51  and the second copper foil  53 . 
     At least one of the thermal conductivity and the electric conductivity can be widely changed between the first copper foil  51  and the second copper foil  53  based on a temperature of the composite structure  70 . 
     A producing of the composite material  50  and the composite structure  70  is specifically described with reference to  FIGS. 15 and 16 . 
     Silver powder, silica binder, and silica sphere are mixed at a weight ratio of 5:5:1. The silver powder has an average size of 20 μm. The silica binder includes siloxane. The silica sphere has an average diameter of 100 μm. Here, the silver powder, the silica binder, and the silica sphere have each thermal conductivity and linear expansion coefficient shown in  FIG. 15 . The linear expansion coefficient corresponds to a thermal expansion coefficient. 
     As shown in  FIG. 15 , the thermal conductivity and the linear expansion coefficient of the silica binder is smaller than those of silver. The silver powder corresponds to the first material, and the silica binder corresponds to the second material. Further, the silica sphere is used to control a thickness of the composite material  50 . 
     The mix of the silver powder, the silica binder, and the silica sphere is cast on the first copper foil  51 , and sandwiched between the first copper foil  51  and the second copper foil  53 . The mix of the silver powder, the silica binder, and the silica sphere is pressed with a pressure of 40 g/cm 2  until when the copper foils  51 ,  53  are electrically contact with each other. Then, the mix of the silver powder, the silica binder, and the silica sphere is calcined with a temperature of 600° C. in the pressed condition. Thus, as shown in  FIG. 14 , the composite material  50  can be formed between the first copper foil  51  and the second copper foil  53 . The composite structure  70  can be used as a sheet having a thermal conductivity, which is variable. 
     The composite structure  70  constructed with the copper foils  51 ,  53  and the composite material  50  is placed on a hot plate in a condition that the first copper foil  51  is to be a bottom side. The composite structure  70  is heated in the situation such that a temperature of the first copper foil  51  is gradually increased from a room temperature to 450° C. 
     A temperature variation of the second copper foil  53  is shown in a y-axis of a solid line of  FIG. 16 , while the temperature of the first copper foil  51  is increased, which corresponds to an x-axis of the solid line of  FIG. 16 . As a comparison example, a temperature variation of a single copper board is shown in a dashed line of  FIG. 16  when the single copper board is similarly heated on the hot plate. The temperature of the second copper foil  53  is slowly increased until when the temperature of the first copper foil  51  becomes around 200° C., which corresponds to a predetermined temperature. This is because a thermal expansion of the silver powder is not sufficient when the temperature of the first copper foil  51  is low. The silver powder is not contact with each other. Therefore, a thermal conductivity is low from the first copper foil  51  to the second copper foil  53 . 
     After the temperature of the first copper foil  51  exceeds 200° C., the temperature of the second copper foil  53  is quickly increased. This is because the silver powder thermally expands when the temperature of the first copper foil  51  becomes high. The silver powder is continuously contact with each other. The thermal conduction path  109  of the silver powder is generated from the heat receiving portion  105  corresponding to the first copper foil  51  to the heat releasing portion  107  corresponding to the second copper foil  53 . Thus, the thermal conductivity becomes high from the heat receiving portion  105  to the heat releasing portion  107 . In contrast, the temperature of the copper board of the comparison example is constantly increased. Further, an electric conductivity of a sheet made of the composite material  50  has the same characteristic as the thermal conductivity of the sheet made of the composite material  50 . 
     The thermal conductivity of the composite material  50  can be low when the first copper foil  51  corresponding to the heat receiving portion  105  has a low temperature. The thermal conductivity of the composite material  50  can be high when the first copper foil  51  corresponding to the heat receiving portion  105  has a high temperature. For example, the heat receiving portion  105  may be an engine of an automotive vehicle, and the heat releasing portion  107  may be an exhaust pipe or a water-cooled radiator. When a temperature of the engine is low during a warm-up, heat releasing from the engine can be reduced. When the temperature of the engine is increased, heat of the engine can be quickly released to the exhaust pipe or the water-cooled radiator. 
     Ninth Embodiment 
     A producing of a composite material  60  and a composite structure  70  will be described. 
     Invar powder, epoxy resin, and silica sphere are mixed at a weight ratio of 5:5:1. The invar powder is an iron-nickel alloy, and has an average size of 20 μm. The silica sphere has an average diameter of 1000 μm. Here, the invar powder, the epoxy resin, and the silica sphere have each thermal conductivity and linear expansion coefficient shown in  FIG. 15 . The linear expansion coefficient corresponds to a thermal expansion coefficient. 
     As shown in  FIG. 15 , the thermal conductivity of the epoxy resin is smaller than that of the invar, and the linear expansion coefficient of the epoxy resin is larger than that of the invar. The invar powder corresponds to the first material, and the epoxy resin corresponds to the second material. Further, the silica sphere is used to control a thickness of the composite material  60 . 
     The mix of the invar powder, the epoxy, and the silica sphere is cast on a first copper foil  51 , and sandwiched with a second copper foil  53 . The mix is pressed with a pressure of 40 g/cm 2  at a room temperature until when the copper foils  51 ,  53  are electrically contact with each other. Thus, as shown in  FIG. 14 , the composite material  60  can be formed between the first copper foil  51  and the second copper foil  53 . The composite structure  70  can be used as a thermal conductivity variable sheet. 
     The composite structure  70  constructed with the copper foils  51 ,  53  and the composite material  60  is placed on a hot plate in a condition that the first copper foil  51  is to be a bottom side. The composite structure  70  is heated in the situation such that a temperature of the first copper foil  51  is gradually increased from a room temperature to 450° C. At this time, the thermal conductivity of the composite material  60  is high until when the temperature of the first copper foil  51  becomes around 100° C., which corresponds to a predetermined temperature. This is because the epoxy resin does not expand at a low temperature. Continuous contact among the invar powder is maintained, and a thermal conduction path of the invar powder is generated from the first copper foil  51  to the second copper foil  53 . Thus, the thermal conductivity becomes high from the first copper foil  51  to the second copper foil  53 . 
     The thermal conductivity of the composite material  60  is quickly decreased when the temperature of the first copper foil  51  exceeds 100° C. This is because the epoxy resin thermally expands. The thermal conduction path of the invar powder is not provided. The invar powder is not contact with each other. Further, an electric conductivity of the composite material  60  has the same characteristic as the thermal conductivity of the composite material  60 . 
     The thermal conductivity of the composite material  60  can be high at a low temperature. The thermal conductivity of the composite material  60  can be low at a high temperature. For example, the composite material  60  may be used for a keep-warm pot having a double structure. That is, an inner wall of the keep-warm pot may be the heat receiving portion  205 , and an outer wall of the keep-warm pot may be the heat releasing portion  207 . When the keep-warm pot has a high-temperature liquid inside, a thermal conductivity can be made low. Thus, the high-temperature liquid can be kept hot. 
     Tenth Embodiment 
     A producing of a composite material  50  will be described. 
     Silver powder having an average size of 20 μm, and silica binder including siloxane are mixed at a weight ratio of 5:5. The mix is disposed in a heat-resistant container having a flat bottom, and left for one day. At this time, the silver power is precipitated because a gravity of the silver powder is larger than that of the silica binder. The precipitated silver powder is continuously contact with each other. Then, the mix is calcined at a temperature of 600° C. so as to produce the composite material  50 . The composite material  50  is taken from the container, and is cut to have a thickness of  1  mm. In the cutting, it is checked if silver is exposed from both of the heat receiving portion  105  and the heat releasing portion  107  of the composite material  50 . If silver is not exposed, the surface of the composite material  50  is ground so that silver is exposed. Thus, the composite material  50  is used as a thermal conduction variable sheet. The sheet has a property approximately equal to that of the sheet produced in the eighth embodiment. 
     The composite material  50 ,  60  may be produced by using three or more materials. The composite material  50 ,  60  may be produced by using the same materials having different particle sizes. 
     Such changes and modifications are to be understood as being within the scope of the present invention as defined by the appended claims.