Abstract:
A Stirling cryocooler includes a displacer having a displacer base portion disposed on a center axis and a displacer tip portion aligned along the center axis, extending from the displacer base portion to a working-gas expansion space, and a regenerator disposed surrounding the displacer tip portion such as to guide reciprocating travel of the displacer along the center axis. The displacer tip portion includes a plurality of platelike components arranged along the center axis, with each of the plurality of platelike components being furnished with a component side surface defining a portion of the outer surface of the displacer tip portion. The plurality of platelike components form working gas layers between pairs of adjoining components, and/or are formed of a synthetic resin material.

Description:
RELATED APPLICATIONS 
       [0001]    Priority is claimed to Japanese Patent Application No. 2015-015718, filed Jan. 29, 2015, the entire content of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    Certain embodiments of the invention relate to a Stirling cryocooler, and particularly, to an expander of a Stirling cryocooler. 
         [0004]    2. Description of Related Art 
         [0005]    A displacer is provided in an expander of a Stirling cryocooler. One end of the displacer is disposed on a “low temperature” portion of the expander, and the other end is disposed on a “normal temperature” portion thereof. Accordingly, a temperature gradient is produced in the displacer when the cryocooler is used. Consequently, via the displacer heat from the normal temperature portion invades the low temperature portion. Heat having intruded this way decreases the refrigerating capacity of the Stirling cryocooler. 
       SUMMARY 
       [0006]    According to an aspect of the present invention, there is provided a Stirling cryocooler, including: a displacer that includes a displacer base portion disposed on a center axis and a displacer tip portion aligned along the center axis, extending from the displacer base portion to a working-gas expansion space; and a regenerator disposed surrounding the displacer tip portion such as to guide reciprocating travel of the displacer along the center axis. The displacer tip portion includes a plurality of platelike components arranged along the center axis, each of the plurality of platelike components being furnished with a component side surface defining a portion of the outer surface of the displacer tip portion. The plurality of platelike components are arranged to form working gas layers between each of where two of the platelike components are adjacent to each other. 
         [0007]    According to another aspect of the present invention, there is provided a Stirling cryocooler, including: a displacer that includes a displacer base portion disposed on a center axis and a displacer tip portion aligned along the center axis, extending from the displacer base portion to a working-gas expansion space; and a regenerator disposed surrounding the displacer tip portion such as to guide reciprocating travel of the displacer along the center axis. The displacer tip portion includes a plurality of platelike components arranged along the center axis, each of the plurality of platelike components being furnished with a component side surface defining a portion of the outer surface of the displacer tip portion. The plurality of platelike components are formed of a synthetic resin material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a view schematically showing a Stirling cryocooler according to an embodiment of the present invention. 
           [0009]      FIG. 2  is a sectional view schematically showing an expander of the Stirling cryocooler according to an embodiment of the present invention. 
           [0010]      FIG. 3  is a sectional view schematically showing a portion of a displacer of the expander according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    It is desirable to decrease heat transmitted through a displacer in an expander of a Stirling cryocooler. 
         [0012]    In addition, certain embodiments of the invention include arbitrary combinations of the above-described components, or components or expressions of the present invention that may be interchangeable with each other between methods, devices, systems, or the like. 
         [0013]    According to certain embodiments of the invention, it is possible to decrease heat transmitted through the displacer in the expander of the Stirling cryocooler. 
         [0014]    Hereinafter, certain embodiments of the invention will be described in detail with reference to the drawings. In addition, in descriptions thereof, the same reference numerals are assigned to the same elements, and overlapping descriptions are appropriately omitted. In addition, configurations described below are exemplified, and do not limit a scope of the present invention. 
         [0015]      FIG. 1  is a view schematically showing a Stirling cryocooler  10  according to an embodiment of the present invention. The Stirling cryocooler  10  includes a compressor  11 , a connection pipe  12 , and an expander  13 . 
         [0016]    The compressor  11  includes a compressor case  14 . The compressor case  14  is a pressure vessel, which is configured so as to hold a high-pressure working gas in an airtight manner. For example, the high-pressure working gas is helium gas. In addition, the compressor  11  includes a compressor unit, which is accommodated in the compressor case  14 . The compressor unit includes a compressor piston and a compressor cylinder. The compressor piston or the compressor cylinder can be either a movable member  15 , which is configured so as to reciprocate in the compressor case  14 ; the other is a stationary member, which is fixed to the compressor case  14 . The compressor unit includes a drive source for driving the movable member  15 , with respect to the compressor case  14 , in the axial direction along the center axis of the movable member  15 . The compressor  11  includes a support portion  16 , which supports the movable member  15  in the compressor case  14  such that the movable member  15  can reciprocate movement. The movable member  15  vibrates, with respect to the compressor case  14  and the stationary member, at certain amplitude and a frequency. As a result, working gas pressure inside the compressor  11  is changed by specific amplitude and a specific frequency. 
         [0017]    A working gas chamber is formed between the compressor piston and the compressor cylinder. The working gas chamber is connected to one end of the connection pipe  12  via communication passages, which are formed in the above-described stationary member and the compressor case  14 . The other end of the connection pipe  12  is a working gas chamber of the expander  13 . Accordingly, the working gas chamber of the compressor  11  and the working gas chamber of the expander  13  are connected to each other by the connection pipe  12 . 
         [0018]    As described below with reference to  FIG. 2 , the expander  13  includes an expander main body  20 , a displacer  22 , and at least one support portion  40 . 
         [0019]      FIG. 2  is a view schematically showing the expander  13  according to the embodiment of the present invention.  FIG. 2  schematically shows an inner structure of the expander  13 . 
         [0020]    The expander main body  20  is a pressure vessel, which is configured so as to hold a high-pressure working gas in an airtight manner. The pressure vessel may be configured with a plurality of vessel portions that are connected to each other so as to hold the inner portion of the pressure vessel in an airtight manner. The displacer  22  is a movable member, which is configured so as to reciprocate movement in the expander main body  20 . The support portion  40  supports the displacer  22  to the expander main body  20  such that the displacer  22  can reciprocate movement. 
         [0021]    The expander main body  20  includes a first section  24  and a second section  26 . The first section  24  is an expansion space  28  of working gas that is formed between the expander main body  20  and the displacer  22 . A cooling stage  29  for cooling an object is provided on the portion of the expander main body  20  adjacent to the expansion space  28 . The second section  26  is configured so as to support the displacer  22  to the expander main body  20  via elastic members  30 . 
         [0022]    In the expander main body  20 , a portion of the first section  24  side is accommodated in a vacuum vessel (not shown). A vacuum layer inside the vacuum vessel and an atmospheric layer outside the vacuum vessel are separated from each other by a flange  47 . 
         [0023]    The second section  26  is adjacent to the first section  24 , in a reciprocating direction (shown by arrow C in  FIG. 1 ) of the displacer  22 . A seal portion  25  is provided between first section  24  and the second section  26 . Accordingly, the second section  26  is partitioned from the first section  24 . Accordingly, pressure variation of the working gas in the first section  24  is not transmitted to the second section  26  and/or does not substantially influence the pressure of the working gas in the second section  26 . In addition, the same kind of gas as the working gas is sealed in the second section  26  such that the second section  26  has the same pressure as an average pressure of the working gas fed from the compressor  11 . Accordingly, the second section  26  forms an average pressure chamber  27  in the inner portion of the second section  26 . 
         [0024]    The displacer  22  includes a displacer head  32 , which is accommodated in the first section  24 , and a displacer rod  34 , which extends from the displacer head  32  to the second section  26  through the seal portion  25 . The displacer rod  34  is a shaft portion that is thinner than the displacer head  32 . The displacer  22  has a center axis (shown by a chain line A in  FIG. 1 ) that is parallel to the reciprocal direction, and the displacer head  32  and the displacer rod  34  are coaxially provided in the center axis of the displacer  22 . 
         [0025]    The displacer rod  34  is supported by the expander main body  20  in the second section  26  such that the displacer  22  can reciprocate movement. For example, the above-described seal portion  25  may be a rod seal that is formed between the displacer rod  34  and the expander main body  20 . 
         [0026]    The first section  24  forms a cylinder portion  42  that surrounds the displacer head  32 . The expansion space  28  is formed between a bottom surface (that is, an inner surface of the cooling stage  29 ) of the cylinder portion  42  and a tip surface of the displacer head  32 . The expansion space  28  is formed on a side opposite to a joining portion between the displacer head  32  and the displacer rod  34  in the reciprocal direction of the displacer  22 . A gas space  36  serving as a compression space of the working gas in the expander  13  is formed between the joining portion and the seal portion  25 . As described above, the gas space  36  is connected to the connection pipe  12 . 
         [0027]    In the expander  13 , the expander main body  20  supports the displacer  22  at a plurality of different positions in the reciprocal direction of the displacer  22  such that the displacer  22  can reciprocate movement. Accordingly, the expander  13  includes two support portions  40 . The two support portions  40  are provided in the second section  26 . Therefore, it is possible to prevent tilting of the displacer  22  with respect to the center axis. 
         [0028]    The support portion  40  includes the above-described elastic member  30 . The elastic member  30  is disposed between the displacer rod  34  and the expander main body  20  such that an elastic restoring force is applied to the displacer  22  when the displacer  22  is displaced from a neutral position. Accordingly, the displacer  22  reciprocates movement at a natural frequency, which is determined from an elastic coefficient of the elastic member  30 , an elastic coefficient due to the pressure of the working gas, and the weight of the displacer  22 . 
         [0029]    For example, the elastic member  30  has a spring mechanism that includes at least one plate spring. The plate spring is a spring referred to as a flexure spring, and the plate spring is flexible in the reciprocal direction of the displacer  22  and is rigid in a direction perpendicular to the reciprocal direction. Therefore, the elastic member  30  allows the axial movement of the displacer  22  along the direction along the center axis. The elastic member, however, regulates the movement of the displacer  22  in the radial direction, orthogonal to the center axis. The displacer rod  34  is fixed to the elastic member  30  via an elastic member attachment portion  49 . 
         [0030]    Through the aforementioned, a vibration system, including the displacer  22  and the elastic member  30 , is configured. The vibration system is configured such that the displacer  22  vibrates so as to have the vibration and the phase difference at the same frequency as the vibration of the movable member  15  of the compressor  11 . 
         [0031]    The displacer head  32  includes a displacer base portion  50  and a displacer tip portion  52 . The displacer base portion  50  is axially disposed on the center axis of the displacer  22 . The displacer tip portion  52  extends from the displacer base portion  50  to the expansion space  28  of the working gas along the center axis of the displacer  22 . The end portion of the displacer rod  34  is attached to the center of the displacer base portion  50 . 
         [0032]    The displacer base portion  50  is a cylindrical hollow member that extends along the center axis of the displacer  22 . The displacer rod  34  is fixed to a rear surface of the displacer base portion  50 . A cavity  64  is formed inside the displacer base portion  50 . The cavity  64  is sealed in an airtight manner from the gas space  36 . For example, the displacer base portion  50  is formed of a metal material. 
         [0033]    The displacer rod  34  is a hollow tube in which both ends are opened. In the inner space of the displacer rod  34 , one end of the displacer rod  34  communicates with the cavity  64  of the displacer base portion  50 , and the other end communicates with the average pressure chamber  27 . Accordingly, the cavity  64  of the displacer base portion  50  has the average pressure similar to the inner portion of the second section  26 . For example, the displacer rod  34  is formed of a metal material. 
         [0034]    A regenerator  38  is provided in the expander main body  20 . The regenerator  38  is disposed around the displacer tip portion  52  so as to guide the reciprocation of the displacer  22  in the axial direction. The regenerator  38  includes a regenerator material vessel, which is a portion of the expander main body  20 , and a regenerator material, which is accommodated in the regenerator material vessel. The regenerator material vessel is an annular or doughnut-shaped vessel that extends in the axial direction so as to be coaxial with the displacer  22 , and forms an accommodation space of the regenerator material. The inner tube of the regenerator material vessel functions as the cylinder portion  42 . For example, the regenerator material has a laminated structure of wire meshes. The regenerator  38  allows the working gas to flow between the gas space  36  and the expansion space  28 . 
         [0035]    A water cooling type heat exchanger  37  can be provided so as to be adjacent to the regenerator  38  between the regenerator  38  and the gas space  36  in the axial direction. Similar to the regenerator  38 , the water cooling type heat exchanger  37  is also formed in an annular shape or a doughnut shape. Similar to the regenerator  38 , an inner wall portion of the water cooling type heat exchanger  37  functions as the cylinder portion  42 . 
         [0036]    The water cooling type heat exchanger  37  cools the working gas supplied from the compressor  11  and realizes heat exchange for discharging the heat from the working gas to the outside of the expander  13 . In general, since the working gas that is supplied from the compressor  11  to the gas space  36  has a higher temperature than the room temperature, the water cooling type heat exchanger  37  cools the high temperature gas so as to be the room temperature. 
         [0037]    The water cooling type heat exchange  37  is disposed around the displacer base portion  50 . The side surface of the displacer base portion  50  can slide with respect to the inner wall portion of the water cooling type heat exchanger  37 . Accordingly, the water cooling type heat exchanger  37  can guide the reciprocation of movement of the displacer head  32  in the axial direction. The seal portion interfering with the flow of the working gas between the gas space  36  and the expansion space  28  may be formed between the side surface of the displacer base portion  50  and the inner wall portion of the water cooling type heat exchanger  37 . 
         [0038]    In addition, a low temperature heat exchanger  39  is attached so as to be adjacent in the axial direction to the regenerator  38  between the regenerator  38  and the cooling stage  29 . The low temperature heat exchanger  39  is disposed around the displacer tip portion  52 . A working gas flow passage, which connects the gas space  36  and the expansion space  28 , is formed by the water cooling type heat exchanger  37 , the regenerator  38 , and the low temperature heat exchanger  39 . 
         [0039]    The flange  47  is provided at a position in the axial direction corresponding to the boundary between the regenerator  38  and the water cooling type heat exchanger  37 . That is, the flange  47  forms a boundary between a normal temperature portion and a low temperature portion of the Stirling cryocooler  10 . A portion of the first section  24 , specifically, the gas space  36  and the water cooling type heat exchanger  37 , are provided in the normal temperature portion of the Stirling cryocooler  10 . The seal portion  25  and the second section  26  are also provided in the normal temperature portion of the Stirling cryocooler  10 . Remaining portions of the first section  24 , specifically, the regenerator  38 , the low temperature heat exchanger  39 , the expansion space  28 , and the cooling stage  29  are provided in the low temperature portion of the Stirling cryocooler  10 . 
         [0040]    In addition, the displacer rod  34  and the displacer base portion  50  are provided in the normal temperature portion of the Stirling cryocooler  10 . The displacer tip portion  52  is provided in the low temperature portion of the Stirling cryocooler  10 . 
         [0041]    The displacer  22  is driven by pulsation of a working gas pressure generated by the vibration of the movable member  15  of the compressor  11 . A reverse Stirling cycle is formed between the expansion space  28  and the working gas chamber of the compressor  11  by reciprocation of movement of the displacer  22  and the movable member  15  of the compressor  11 . Accordingly, the low temperature heat exchanger  39  adjacent to the expansion space  28  is cooled. The cooling stage  29  is cooled by the low temperature heat exchanger  39 , and the object can be cooled by the Stirling cryocooler  10 . 
         [0042]    The cold, which is generated in the expansion space  28 , is accumulated in the regenerator  38 . While a boundary region between the low temperature heat exchanger  39  and the regenerator  38  is cooled so as to be the lowest temperature, a boundary region between the water cooling type heat exchanger  37  and the regenerator  38  has the room temperature. Accordingly, a temperature gradient is generated in the axial direction in the regenerator  38 . As described above, the regenerator  38  surrounds the displacer tip portion  52  so as to guide the reciprocation in the axial direction of the displacer  22 . Accordingly, similar to the regenerator  38 , a temperature gradient is also generated in the axial direction in the displacer tip portion  52 . 
         [0043]    Hereinafter, for convenience of description, the side close to the expansion space  28  in the regenerator  38  and the displacer head  32  is referred to as a “low temperature side”. The side close to the gas space  36  in the regenerator  38  and the displacer head  32  is referred to as a “high temperature side”. 
         [0044]    The displacer tip portion  52  is formed of a plurality of platelike components, which are arranged in the axial direction. The plurality of platelike components are arranged such that a working gas layer  62  is formed between two platelike component adjacent to each other. Each of the plurality of platelike components has a component side surface, which defines a portion of the outer surface of the displacer tip portion  52 . 
         [0045]    The plurality of platelike components include a first platelike component  54   a  that faces the expansion space  28 , and at least one intermediate platelike component that connects the first platelike component  54   a  to the displacer base portion  50 . In the embodiment shown in  FIG. 2 , the displacer tip portion  52  includes three intermediate platelike components. Accordingly, the displacer tip portion  52  includes four platelike components. 
         [0046]    The first platelike component  54   a  is a solid member which has a hemispherical shape or a semi-ellipsoidal sphere shape. The outer shape of the first platelike component  54   a  is defined by a first front surface  56   a  that is exposed to the expansion space  28 , and a first rear surface  58   a  that faces the side (that is, displacer rod  34  side) opposite to the first front surface  56   a . The first front surface  56   a  is a side surface (or front surface) of the first platelike component  54   a , which is curved in a hemispherical shape or a semi-ellipsoidal sphere shape. The first front surface  56  defines the tip surface of the displacer head  32 . The first rear surface  58   a  is an approximately flat surface. In addition, if necessary, irregularities such as slits may be formed on the first front surface  56   a . Irregularities or holes for connecting the first rear surface  58   a  to the intermediate platelike component may be also formed on the first rear surface  58   a.    
         [0047]    Hereinafter, three intermediate platelike components are referred to as a second platelike component  54   b , a third platelike component  54   c , and a fourth platelike component  54   d . The second platelike component  54   b  is disposed on the expansion space  28  side, and the fourth platelike component  54   d  is disposed on the displacer base portion  50  side. The third platelike component  54   c  is disposed between the second platelike component  54   b  and the fourth platelike component  54   d.    
         [0048]    Each of the second platelike component  54   b , the third platelike component  54   c , and the fourth platelike component  54   d  is a solid member which has a disk shape. Each of the second platelike component  54   b , the third platelike component  54   c , and the fourth platelike component  54   d  has the same shape as each other. 
         [0049]    The outer shape of the second platelike component  54   b  is defined by a second front surface  56   b , a second rear surface  58   b , and a second side surface  60   b.    
         [0050]    The second front surface  56   b  and the second rear surface  58   b  are approximately flat surfaces. However, if necessary, irregularities or holes may be also formed on the second front surface  56   b  and the second rear surface  58   b.    
         [0051]    A portion of the second front surface  56   b  comes into contact with the first rear surface  58   a  of the first platelike component  54   a , and the remaining portion of the second front surface  56   b  does not come into contact with the first rear surface  58   a . The working gas layer  62  is formed on the non-contact portion. For example, as shown in  FIG. 2 , the second front surface  56   b  includes the outer circumferential portion that comes into contact with the first rear surface  58   a  of the first platelike component  54   a , and a center concave portion that does not come into contact with the first rear surface  58   a  and faces the first rear surface  58   a.    
         [0052]    The second side surface  60   b  is a cylindrical surface. The second side surface  60   b  defines a portion of a cylindrical side surface of the displacer tip portion  52 . 
         [0053]    The working gas layer  62  is formed between the center concave portion of the second front surface  56   b  and the first rear surface  58   a . The width (that is, the height of the center concave portion) of the working gas layer  62  in the axial direction is smaller than the thickness of the platelike component in the axial direction. The width of the working gas layer  62  may be smaller than 1/10, 1/50, or 1/100 of the thickness of the second platelike component  54   b . The width of the working gas layer  62  may be less than 1 mm, 0.5 mm, or 0.1 mm. 
         [0054]    A microscopic gap  67  is generated in a contact region between two platelike components adjacent to each other (refer to  FIG. 3 ). The gap  67  is formed between the outer circumferential portion of the second front surface  56   b  and the first rear surface  58   a . The working gas layer  62  is not sealed in an airtight manner with respect to a clearance  66  between the displacer tip portion  52  and the cylinder portion  42 , and the expansion space  28 . The working gas layer  62  communicates with the clearance  66  through the microscopic gap  67 . Since the working gas pressure in the clearance  66  is changed according to the pressure of the expansion space  28 , the working gas pressure in the working gas layer  62  is also changed according to the pressure of the expansion space  28 . 
         [0055]    The third platelike component  54   c  and the fourth platelike component  54   d  are configured similar to the second platelike component  54   b . The side surface of each of the third platelike component  54   c  and the fourth platelike component  54   d  defines a portion of the side surface of the displacer tip portion  52 . The second working gas layer  62  is formed between the second platelike component  54   b  and the third platelike component  54   c , and the third working gas layer  62  is formed between the third platelike component  54   c  and the fourth platelike component  54   d . In addition, a second gap is formed between the second platelike component  54   b  and the third platelike component  54   c , and a third gap is formed between the third platelike component  54   c  and the fourth platelike component  54   d . Similar to the above-described gap  67 , the gaps cause the working gas layer  62  to communicate with the clearance  66 . The rear surface of the fourth platelike component  54   d  is connected to the front surface of the displacer base portion  50 . 
         [0056]    The first front surface  56   a  of the first platelike component  54   a  faces the low temperature heat exchanger  39  and the cooling stage  29  in a state where the expansion space  28  is interposed therebetween. The second side surface  60   b  of the second platelike component  54   b  faces the regenerator  38  in a state where the clearance  66  is interposed therebetween. Similarly, the side surfaces of the third platelike component  54   c  and the fourth platelike component  54   d  also face the regenerator  38  in a state where the clearance  66  is interposed therebetween. 
         [0057]    A position of a boundary between the first platelike component  54   a  and the second platelike component  54   b  in the axial direction is closer to the expansion space  28  relative to a position of a boundary between the regenerator  38  and the low temperature heat exchanger  39  in the axial direction. That is, the first rear surface  58   a  and the second front surface  56   b  are surrounded by the low temperature heat exchanger  39 . In addition, a boundary between the second platelike component  54   b  and the third platelike component  54   c  is surrounded by the regenerator  38 . A boundary between the third platelike component  54   c  and the fourth platelike component  54   d  is also surrounded by the regenerator  38 . A position of a boundary between the fourth platelike component  54   d  and the displacer base portion  50  in the axial direction corresponds to a position of a boundary between the water cooling type heat exchanger  37  and the regenerator  38  in the axial direction, and is surrounded by the flange  47 . 
         [0058]    The plurality of platelike components are connected to each other using screw portions (not shown). For example, the plurality of platelike components are connected to each other by long screws penetrating the platelike components. The screw head of each of the long screws may be provided on the displacer base portion  50  side, or may be provided on the first platelike component  54   a  side. 
         [0059]    In addition, the plurality of platelike components are formed of the same resin material. For example, the resin material is polycarbonate. The resin material may be glass fiber reinforced plastic (GFRP). Alternatively, the resin material may be general plastic such as Bakelite. 
         [0060]    The first platelike component  54   a  among the plurality of platelike components is disposed on the lowest temperature side. The second platelike component  54   b  among the plurality of intermediate platelike components is disposed on the lowest temperature side. The fourth platelike component  54   d  among the plurality of platelike components is disposed on the highest temperature side. 
         [0061]    Accordingly, during the operation of the Stirling cryocooler  10 , the fourth platelike component  54   d  is cooled to a lower temperature than that of the displacer base portion  50 . The third platelike component  54   c  is cooled to a lower temperature than that of the fourth platelike component  54   d . The second platelike component  54   b  and the first platelike component  54   a  are cooled to a lower temperature than that of the third platelike component  54   c.    
         [0062]    A displacer of an expander of a typical Stirling cryocooler is configured of a single solid member. The solid member is formed of a metal material. As described above, since the temperature gradient is formed in the displacer and in general, the metal has high thermal conductivity, heat quantity input from the normal temperature portion to the low temperature portion through the displacer is large. The heat loss exerts an adverse effect to refrigeration capacity of the Stirling cryocooler. Accordingly, a displacer of an expander configured of a single hollow member may be adopted. Therefore, the heat input to the low temperature portion due to the heat conduction of the displacer member is suppressed. However, convection of gas in a cavity portion of the hollow member is generated, and the convection generates heat loss. In order to prevent the convection, a method in which a partition plate or a porous body is installed in the cavity portion may be considered. However, even when this method is used, it is not possible to sufficient prevent the convection. 
         [0063]    According to the present embodiment, the displacer tip portion  52  is divided into the plurality of platelike components. The plurality of platelike components are laminated to form the displacer tip portion  52 . Compared to an integral component, in this divided lamination structure, it is possible to decrease the heat conduction between platelike components adjacent to each other. In addition, since each of the platelike components is a solid member, convection of gas is not generated in the inner portion of the platelike component. Accordingly, it is possible to decrease heat transfer through the displacer  22  in the expander  13  of the Stirling cryocooler  10 . Heat loss decreases, and it is possible to improve the refrigeration capacity of the Stirling cryocooler  10 . 
         [0064]    The plurality of platelike components are formed of a resin material. In general, the resin material has thermal conductivity that is significantly lower than that of a metal material. For example, the thermal conductivity of polycarbonate is 0.2 W/m·K to 0.3 W/m·K. Therefore, compared to the case in which the displacer is formed of a metal material, in the present embodiment, it is possible to improve the refrigeration capacity of the Stirling cryocooler  10 . 
         [0065]    The working gas layer  62  is formed between the platelike components adjacent to each other. In general, the thermal conductivity of gas is significantly lower than the thermal conductivity of the metal and is lower than the thermal conductivity of the resin material. For example, the thermal conductivity of helium gas in the low temperature portion is approximately 0.15 W/m·K and is lower than the thermal conductivity of the polycarbonate. Accordingly, by forming the working gas layer between the platelike components formed of a resin material, it is possible to further decrease heat transfer through the displacer  22 . 
         [0066]    In addition, since the width of the working gas layer  62  in the axial direction is sufficiently small, a temperature gradient is not substantially generated in the working gas layer  62 . In other words, a rear surface of a platelike component and a front surface of an adjacent platelike component are cooled to substantially the same temperature. Accordingly, convection of the working gas in the working gas layer  62  is substantially prevented. 
         [0067]    The plurality of platelike components are connected to each other using screw portions. Accordingly, since it is possible decrease thermal contact between the platelike components, it is possible to decrease the heat transfer through the displacer  22 . 
         [0068]    The position of the boundary between the first platelike component  54   a  and the second platelike component  54   b  in the axial direction is closer to the expansion space  28  relative to the position of the boundary between the regenerator  38  and the low temperature heat exchanger  39  in the axial direction. Accordingly, since the first platelike component  54   a  and the second platelike component  54   b  are divided, it is possible to decrease heat input from the first platelike component  54   a  to the second platelike component  54   b . It is possible to decrease heat input from the cooling stage  29  to the boundary region between the low temperature heat exchanger  39  and the regenerator  38  through the displacer tip portion  52 . 
         [0069]    In addition, the microscopic gap  67  between the platelike components generated due to the division structure is adjacent to the clearance  66  (refer to  FIG. 3 ). Abrasion powder generated due to sliding between the displacer head  32  and the cylinder portion  42  can be received by the gap. Accordingly, it is possible to prevent staying or accumulation of the abrasion power on the sliding surface. Since the abrasion powder changes sliding resistance, the abrasion powder influences the reciprocation of the displacer  22 . Accordingly, in the division structure of the displacer  22  according to the present embodiment, stable reciprocation of the displacer  22  is obtained. 
         [0070]    Since the displacer base portion  50  and the displacer rod  34  are hollow, it is possible to decrease weight of the displacer  22 . 
         [0071]    Hereinbefore, the embodiment of the present invention is described. The present invention is not limited to the embodiment, various design modifications can be performed, various modification examples can be performed, and a person skilled in the art understands that the modifications examples are included in the scope of the present invention. 
         [0072]    In an embodiment, a first screw portion may be formed in one of two platelike components adjacent to each other, and a second screw portion, which engages with the first screw portion may be formed on the other. As shown in  FIG. 3 , the first second portion may be formed on the first rear surface  58   a  of the first platelike component  54   a , and a second screw portion may be formed on the second front surface  56   b  of the second platelike component  54   b . In this way, the plurality of platelike components may be connected to each other using a screw portion  68 . 
         [0073]    In an embodiment, the plurality of platelike components may be formed of different materials. One end or both ends of the displacer tip portion  52  may be formed of a first resin material, and other portions of the displacer tip portion  52  (for example, an intermediate portion in the axial direction) may be formed of a second resin material. A liner expansion coefficient of the first resin material may be smaller than that of the second resin material. The thermal conductivity of the first resin material is higher than that of the second resin material. In a material having a small liner expansion coefficient, it is possible to prevent change of the clearance  66  due to cooling. A material having a small linear expansion coefficient may be used on at least an end portion of the clearance  66 . For example, the second platelike component  54   b  and the fourth platelike component  54   d  may be formed of GFRP, and the third platelike component  54   c  may be formed of polycarbonate. 
         [0074]    In an embodiment, a reflective surface may be formed on the front surface and/or the rear surface of the platelike component facing the working gas layer  62 . A metal coating layer may be formed on the front surface and/or the rear surface of the platelike component. In this way, radiant heat between the platelike components may be decreased. 
         [0075]    The position of the boundary between the regenerator  38  and the low temperature heat exchanger  39  in the axial direction may be closer to the expansion space  28  relative to the position of the boundary between the first platelike component  54   a  and the second platelike component  54   b  in the axial direction. That is, the first rear surface  58   a  and the second front surface  56   b  may be surrounded by the regenerator  38 . 
         [0076]    In an embodiment, at least one platelike component may be a hollow platelike component. In this case, the working gas or other gas may be sealed in the platelike component. Alternatively, the inner portion of the platelike component may be maintained in a vacuum state. 
         [0077]    In an embodiment, at least one platelike component may be formed of a plurality of portions, which are divided into a direction (for example, in the radial direction or the circumferential direction) different from the axial direction. 
         [0078]    It should be understood that the invention is not limited to the above-described embodiment, but may be modified into various forms on the basis of the spirit of the invention. Additionally, the modifications are included in the scope of the invention.