Loop differential thermal expansion combined air gap type thermal switch and manufacturing method thereof

Abstract

The invention discloses a loop differential thermal expansion combined air gap type thermal switch for a dilution refrigerator and a manufacturing method of the loop differential thermal expansion combined air gap type thermal switch for the dilution refrigerator. The device is composed of a cold-end base, a hot-end base, a supporting shell, a cold-end conductor, a hot-end conductor, a concentric staggered cold-end cylinder group and a concentric staggered hot-end cylinder group. By means of the structural arrangement and the difference of thermal expansion coefficients of different materials, extremely small air gaps are naturally formed between conductors at low temperature, and harsh machining precision is not needed. Meanwhile, the design of the concentric staggered cylinder group greatly increases the heat exchange area, so that the heat exchange efficiency of the gas working medium is enhanced; high-reliability sealing is realized by adopting a sealing structure of an annular sealing groove and a high-purity indium wire, and an initial gap is ensured to be adjustable by matching with bolt pretightening force control. And positive significance is achieved on performance improvement and production and manufacturing of the dilution refrigerating machine.

Description

Technical Field

[0001] This invention relates to the technical field of refrigeration and cryogenic engineering, to dilution refrigerators, and particularly to a loop differential thermal expansion combined air gap thermal switch for dilution refrigerators and its manufacturing method. Background Technology

[0002] As a refrigeration device capable of continuously providing mK-level ultra-low temperature environments with low magnetic fields and low vibrations, dilution refrigerators play an irreplaceable role in fields such as superconducting quantum computing, quantum cryogenic transport properties, quantum fundamental research, and condensed matter physics.

[0003] The efficiency, final substrate temperature, and stability of the cooling process in a dilution refrigerator are significantly affected by the pre-cooling stage. The thermal switch is a key component in the pre-cooling stage of the dilution refrigerator; its core function is to achieve thermal conduction or thermal isolation by changing its own thermal resistance. Thermal switches can be classified according to their working principle into mechanical, superconducting, and air-gap types, among others.

[0004] Mechanical thermal switches utilize a drive mechanism to achieve contact or separation between the hot and cold ends, thereby enabling or blocking the flow of heat. Theoretically, they have an almost infinite breaking ratio, but due to their structural complexity, they not only generate significant additional heat loss but also have poor robustness, which makes it difficult to expand their application range.

[0005] Superconducting thermal switches utilize the abrupt change in thermal conductivity during the "normal state-superconducting state" phase transition of superconducting materials to achieve heat flow interruption. They possess characteristics such as high switching ratio and no mechanical wear in extremely low temperature environments. However, the operating temperature of superconducting thermal switches must be significantly lower than their own superconducting transition temperature to function properly, which cannot cover the wide temperature range requirements of the pre-cooling stage of dilution refrigerators.

[0006] Air gap thermal switches change the thermal conductivity by filling or extracting a gaseous working medium between the heat-conducting plates, thereby opening and closing the heat conduction path. They have advantages such as simple structure, no wear, wide temperature range, high switching ratio, and reliable operation, and have become the mainstream choice for the pre-cooling stage of dilution refrigerators.

[0007] Ideally, an air-gap thermal switch needs to achieve the following three functions:

[0008] 1) Efficient heat conduction during the precooling stage to achieve rapid precooling. In the precooling stage of the dilution refrigerator, the thermal switch needs to have efficient heat transfer capabilities. This can be achieved by reducing the air gap distance, optimizing the state of the heat transfer medium in the air gap, and increasing the heat exchange area, thereby giving itself efficient heat transfer characteristics and quickly transferring the cold energy of the cold head to each cold stage of the dilution refrigerator, thus shortening the precooling time and improving the start-up efficiency of the dilution refrigerator.

[0009] 2) Efficiently cut off heat after reaching the pre-cooling temperature to eliminate parasitic heat load. When the dilution chiller reaches the preset pre-cooling temperature, the adsorption pump should be able to quickly adsorb the heat transfer medium in the air gap and switch to the heat cut-off state to block the heat transfer path between stages, thereby minimizing the transfer of external heat to the low-temperature load and enabling the low-temperature load to be further reduced to an even lower temperature.

[0010] 3) Stable and controllable. Based on the operating stage and temperature requirements of the dilution refrigerator, flexible and reliable switching between heat conduction and heat insulation states is achieved. Under complex operating conditions of extremely low temperatures and high vacuum, the stability of the switching function needs to be maintained over a long period to avoid performance degradation of the dilution refrigerator or experimental interruption due to component failure.

[0011] Currently, the core performance of air-gap thermal switches, such as thermal conductivity in the on-state and heat leakage in the off-state, highly depends on parameters such as "air gap spacing" and "heat transfer area." The air gap spacing typically needs to be controlled within the range of 10-100 μm to improve gas thermal conductivity efficiency. However, such a small spacing is extremely difficult to maintain during assembly, welding, and long-term operation. The dimensional errors of conventional manufacturing processes are already close to the width of the air gap itself. Once assembly deviations or deformation of the supporting structure occur, it is very easy to cause direct contact between the hot and cold ends of the air-gap thermal switch. In this case, even vacuuming cannot achieve thermal isolation, causing the thermal switch to fail.

[0012] Therefore, existing air-gap thermal switches used in dilution refrigerators have a core technical challenge: the requirement for extremely small air gaps (μm level) and ultra-large heat exchange areas to achieve efficient heat conduction is inherently contradictory to the requirement for low machining and assembly precision. The technical problem that needs to be solved is how to enable air-gap thermal switches to simultaneously achieve efficient heat conduction and rapid pre-cooling during the pre-cooling stage, efficient heat insulation and parasitic heat load blocking after reaching the pre-cooling temperature, and ensure stable and controllable operation without the need for stringent machining precision. Summary of the Invention

[0013] The purpose of this invention is to provide a loop differential thermal expansion combined air-gap thermal switch for dilution refrigerators and its manufacturing method. This invention addresses the core technical pain point of existing air-gap thermal switches for dilution refrigerators, which have inherently conflicting requirements for achieving efficient heat conduction (such as a μm-level extremely small air gap and ultra-large heat exchange area) and low machining and assembly precision. The invention also addresses how to enable air-gap thermal switches to simultaneously achieve efficient heat conduction and rapid pre-cooling during the pre-cooling stage, efficient heat insulation and parasitic heat load blocking after reaching the pre-cooling temperature, and ensure stable and controllable operation without requiring stringent machining precision.

[0014] To achieve the above objectives, the technical solution adopted by the present invention is: a loop differential thermal expansion combined air gap thermal switch for a dilution refrigeration machine, characterized in that: the air gap thermal switch includes a cold end base, a hot end base, a cold end conductor, a hot end conductor, a concentric interlaced cold end cylinder assembly, a concentric interlaced hot end cylinder assembly, and a supporting shell, wherein the end face of the cold end conductor away from the cold end base is the cold end conductor contact surface, and the end face of the hot end conductor away from the hot end base is the hot end conductor contact surface;

[0015] The cold-end conductor located in the concentric interlaced cold-end cylindrical assembly is connected to the cold-end base, and the concentric interlaced cold-end cylindrical assembly is connected to the cold-end conductor; the hot-end conductor located in the concentric interlaced hot-end cylindrical assembly is connected to the hot-end base, and the concentric interlaced hot-end cylindrical assembly is connected to the hot-end conductor.

[0016] The cold end base and the hot end base are respectively sealed to both ends of the supporting shell, and the three together form a sealed cavity. The cold end conductor, the hot end conductor, the concentric interlaced cold end cylinder group and the concentric interlaced hot end cylinder group are all located in the sealed cavity, and the cylinder walls of the concentric interlaced cold end cylinder group and the concentric interlaced hot end cylinder group are arranged concentrically and interlaced.

[0017] The coefficients of thermal expansion of the cold end base, hot end base, and supporting shell are smaller than those of the cold end conductor and hot end conductor. When cooled to a specific temperature, the cold end conductor and hot end conductor shrink more significantly than the cold end base, hot end base, and supporting shell, resulting in a very small gap between the contact surfaces of the cold end conductor and the hot end conductor, which were originally in contact or close proximity.

[0018] Furthermore, the size of the gap is less than or equal to 50 μm; the size of the gap depends on the difference in the coefficients of thermal expansion between the cold end conductor, the hot end conductor and the supporting shell, the initial lengths of the cold end conductor and the hot end conductor, and the temperature at which the cold end conductor and the hot end conductor are cooled.

[0019] Furthermore, the cold-end conductor is an integrally formed structure, comprising three cylinders of different diameters, which, from top to bottom, are the cold-end conductor assembly column, the cold-end conductor transition column, and the cold-end conductor main body column. The lower end face of the cold-end conductor main body column is the cold-end conductor bonding surface.

[0020] The concentric interlocking cold-end cylinder group consists of 2-5 thin-walled cylinders of different diameters nested coaxially. One end of each thin-walled cylinder is connected to a cold-end conductor transition post, and the main cold-end conductor post is located inside all the thin-walled cylinders.

[0021] Furthermore, the hot-end conductor is an integrally formed structure, comprising three cylinders of different diameters, which, from bottom to top, are the hot-end conductor assembly column, the hot-end conductor transition column, and the hot-end conductor main body column. The upper end surface of the hot-end conductor transition column is the hot-end conductor bonding surface.

[0022] The concentric interlaced hot-end cylinder group consists of 1-4 thin-walled cylinders of different diameters nested coaxially. One end of each thin-walled cylinder is connected to a hot-end conductor transition column, and the main hot-end conductor column is located inside all the thin-walled cylinders.

[0023] Furthermore, the main body of the cold end base is a cylindrical T-shaped block with a through first assembly hole in the center. Multiple first threaded connection through holes for connecting the cold plate of the dilution refrigeration unit to the supporting shell are evenly opened at the end along the circumference. A first annular sealing groove for embedding the sealing indium wire is opened at one end near the supporting shell. The lower end of the cold end base has a boss structure.

[0024] The cold-end conductor assembly post is inserted into the first assembly hole of the cold-end base; the upper end face of the cold-end conductor transition post is tightly attached to the lower end face of the cold-end base and sealed and welded around the circumference.

[0025] Furthermore, the hot end base and the cold end base are completely symmetrical. The main body of the hot end base is a cylindrical T-shaped block with a through second assembly hole in the center. Multiple second threaded connection through holes for connecting the cold plate of the dilution refrigeration unit and the supporting shell are evenly opened along the circumference at the end. A second annular sealing groove for embedding the sealing indium wire is opened at one end near the supporting shell. The upper end of the hot end base has a boss structure.

[0026] The hot-end conductor assembly post is inserted into the second assembly hole of the hot-end base; the lower end face of the hot-end conductor transition post is tightly attached to the upper end face of the hot-end base and sealed and welded around the circumference.

[0027] Furthermore, in the concentric interlaced cold-end cylinder group, the wall thickness of each thin-walled cylinder is 0.5-1.0 mm, and the radial distance between the smallest thin-walled cylinder and the cold-end conductor and the hot-end conductor is 0.1-0.33 mm; the upper end face of all thin-walled cylinders in the concentric interlaced cold-end cylinder group is vertically aligned with the lower end face of the cold-end conductor transition column and welded to ensure that there is no relative displacement between the thin-walled cylinders;

[0028] In the concentric interlaced hot-end cylindrical assembly, the wall thickness of each thin-walled cylinder is 0.5-1.0 mm. The lower end face of all the thin-walled cylinders is vertically aligned with the upper end face of the hot-end conductor transition column and welded to ensure that there is no relative displacement between the thin-walled cylinders.

[0029] When the cylinder walls of the concentric staggered cold-end cylinder group and the concentric staggered hot-end cylinder group are arranged concentrically and staggeredly, the radial distance between adjacent thin-walled cylinders is 0.1-0.33 mm, and the cylinder walls do not contact each other.

[0030] Furthermore, the main body of the support shell is cylindrical, with an inner diameter larger than the maximum outer diameter of the concentric staggered cold end cylindrical assembly, and a gap of 1-2.5 mm is reserved to avoid contact. The length of the support shell is adapted to the distance between the two stages of the dilution refrigeration unit.

[0031] A process through hole with a diameter of 3-6mm is opened near the middle of the outer wall of the support shell for welding stainless steel conduits. The stainless steel conduits connect the adsorption pump and the gas working fluid source to realize the introduction and discharge of gas.

[0032] The end faces of the supporting shell are provided with annular bosses that are adapted to the annular sealing grooves of the cold end base and the hot end base respectively. The height and width of the annular bosses are matched with the corresponding annular sealing grooves.

[0033] The end faces of both ends of the supporting housing are also provided with multiple third threaded connection through holes, which are respectively adapted to the first threaded connection through hole of the cold end base and the second threaded connection through hole of the hot end base;

[0034] To ensure tight contact between the cold-end conductor bonding surface and the hot-end conductor bonding surface, the third threaded connection through hole of the supporting shell is fastened with bolts to the first threaded connection through hole of the cold-end base and the second threaded connection through hole of the hot-end base, respectively. The high-purity sealing indium wire in the first annular sealing groove and the high-purity sealing indium wire in the second annular sealing groove are both pressed by the corresponding annular bosses. The supporting shell, the cold-end base, and the hot-end base together form a sealed cavity.

[0035] The first and second annular sealing grooves have a depth of 0.8-1.2 mm and a width of 0.7-0.85 mm, with tolerances controlled within ±0.01 mm.

[0036] The height of the cold-end conductor main column is adapted to the length of the supporting shell, and the height of the hot-end conductor main column is adapted to the length of the supporting shell.

[0037] A first threaded hole is opened at the center of the far end of the cold end conductor, and a second threaded hole is opened at the center of the far end of the hot end conductor. Both the first and second threaded holes are used for installation and fixation with the cold plate of the dilution refrigeration unit.

[0038] The cold end conductor bonding surface and the hot end conductor bonding surface were originally in a close proximity state, which means that the distance between the cold end conductor bonding surface and the hot end conductor bonding surface is less than or equal to 20um.

[0039] A method for manufacturing a loop differential thermal expansion combined air gap thermal switch for a dilution refrigerator, as described above, comprises the following steps:

[0040] Step 1: Select stainless steel for machining the cold-end and hot-end bases. Machin the cylindrical T-shaped main body using a lathe, and drill a through-hole with a diameter of 10-17 mm and a tolerance of H7 at the center. Drill 8-16 first threaded connection through holes along the circumference at the end of the cold-end base and 8-16 second threaded connection through holes along the circumference at the end of the hot-end base, with the hole position tolerance controlled within ±0.1 mm. Machin a first annular sealing groove and a second annular sealing groove with a depth of 0.8-1.2 mm and a width of 0.7-0.85 mm, with a tolerance controlled within ±0.01 mm, at the end of the cold-end and hot-end bases closest to the supporting shell using a lathe. Finally, polish the inner wall surfaces of the cold-end and hot-end bases, as well as the first and second assembly holes, to reduce radiative heat leakage during shutdown.

[0041] Step 2: High-purity oxygen-free copper is selected for processing the cold-end and hot-end conductors. Oxygen-free copper has high thermal conductivity and its coefficient of thermal expansion differs significantly from that of stainless steel. The cold-end conductor is machined in one piece on a lathe, with the following steps processed from top to bottom: a 10-17 mm diameter cold-end conductor assembly column, a 24.4-28 mm diameter cold-end conductor transition column, and a 20-24 mm diameter cold-end conductor main column with a height adapted to the length of the supporting shell. The processing of the hot-end conductor is completely symmetrical to that of the cold-end conductor. It is necessary to ensure that the mating surfaces of the hot-end conductor and the cold-end conductor are flat and mated, and both mating surfaces need to be polished to Ra≤0.4 μm. First and second threaded holes are machined at the far center of the cold-end and hot-end conductors, respectively, for mounting and fixing with the cold plate of the dilution refrigeration unit.

[0042] Step 3: Select high-purity oxygen-free copper, consistent with the cold-end and hot-end conductors, and process individual thin-walled cylinders using slow wire EDM. The wall thickness is 0.5-1.0 mm, and the length matches the height of the main columns of the cold-end and hot-end conductors. Design thin-walled cylinders of different diameters according to the radial spacing requirement of 0.1-0.33 mm. Remove burrs from the inner wall of the thin-walled cylinders by polishing to reduce radiative heat leakage during shutdown. When assembling, select 2-5 thin-walled cylinders to form a concentric staggered cold-end cylinder group and 1-4 thin-walled cylinders to form a concentric staggered hot-end cylinder group, ensuring uniform radial spacing between adjacent thin-walled cylinders. The radial spacing between the smallest thin-walled cylinder in the concentric staggered cold-end cylinder group and the cold-end and hot-end conductors is 0.1-0.33 mm.

[0043] Step 4: Select stainless steel material consistent with both the cold-end and hot-end bases for the support shell to ensure matching thermal expansion characteristics and low thermal conductivity requirements; machine the cylindrical body on a lathe, with an inner diameter reserved with a 1-2.5 mm gap based on the maximum outer diameter of the concentric staggered cold-end cylindrical assembly, a thickness of 0.65-0.8 mm, and a length adapted to the distance between the two cold plates of the dilution refrigeration unit, generally 122-242 mm; machine annular bosses with height and width matching the first annular sealing groove of the cold-end base and the second annular sealing groove of the hot-end base on both end faces; drill 8-16 threaded connecting through holes along the circumference at the ends of both end faces, with hole position tolerance controlled within ±0.1 mm; drill process through holes with a diameter of 3-6 mm at the centerline of the outer wall surface, and subsequently weld stainless steel conduits at the process through holes using clean laser welding technology to connect the adsorption pump and the gas working fluid source;

[0044] Step 5: After the parts are processed, they are combined and welded, then assembled. Finally, the leakage rate of the entire cavity is detected by a helium mass spectrometer leak detector, thus forming a loop differential thermal expansion combined air gap thermal switch for dilution refrigerators.

[0045] Furthermore, in step 5, the specific steps are as follows:

[0046] The first step is pre-welding of the core components: The cold-end conductor assembly post is inserted into the first assembly hole of the cold-end base, with an interference fit, so that the upper end face of the cold-end conductor transition post is in contact with the lower end face of the cold-end base. Vacuum brazing is used to seal the circumference. The hot-end conductor assembly post is then inserted into the second assembly hole of the hot-end base, with an interference fit, so that the lower end face of the hot-end conductor transition post is in contact with the upper end face of the hot-end base. Vacuum brazing is used to seal the circumference. Next, the upper end faces of all the thin-walled cylinders in the concentric interlaced cold-end cylinder assembly are vertically aligned with the lower end faces of the cold-end conductor transition posts, and fixed using vacuum brazing. Similarly, the lower end faces of all the thin-walled cylinders in the concentric interlaced hot-end cylinder assembly are vertically aligned with the upper end faces of the hot-end conductor transition posts, and fixed using vacuum brazing. After welding, the temperature of each component must be allowed to drop to room temperature before proceeding to the next step to avoid the thermal expansion and contraction of the cold-end and hot-end conductors affecting subsequent assembly.

[0047] The second step is the assembly of the sealed cavity: First, high-purity sealing indium wires with diameters matching the groove width are embedded in the first annular sealing groove of the cold-end base and the second annular sealing groove of the hot-end base; second, all the thin-walled cylinders of the concentric interlaced cold-end cylinder group and all the thin-walled cylinders of the concentric interlaced hot-end cylinder group are inserted into the support shell from both the top and bottom directions, forming a coaxial nested state. Then, ensuring that the contact surfaces of the cold-end conductor and the hot-end conductor are precisely aligned, the support shell is placed between the cold-end base and the hot-end base, so that the annular bosses at both ends of the support shell are respectively embedded in the first annular sealing groove of the cold-end base and the second annular sealing groove of the hot-end base; then, using bolts, the first threaded connecting holes evenly distributed along the circumference of the cold-end base are connected to the hot-end base. The second threaded connection through hole, which is evenly distributed around the circumference, and the third threaded connection through hole, which is evenly distributed around the circumference of the end faces of the supporting shell, are used to connect and fix the cold end base, the hot end base and the supporting shell. On the one hand, the annular bosses at both ends of the supporting shell press the high-purity sealing indium wire in the corresponding annular sealing groove to achieve cavity sealing. On the other hand, screws are screwed into the first threaded hole and the second threaded hole at the center of the far end of the cold end conductor and the hot end conductor respectively. The preload is controlled by a torque wrench to adjust the initial gap between the cold end conductor and the hot end conductor, eliminate the initial gap between the cold end conductor and the hot end conductor or control the initial gap between the cold end conductor and the hot end conductor to be less than or equal to 20um, so that the cold end conductor and the hot end conductor are initially in close contact or close distance.

[0048] In view of the above technical features, the present invention has the following beneficial effects:

[0049] 1. This invention discloses a loop differential thermal expansion combined air gap thermal switch for a dilution refrigeration unit, comprising a cold end base, a hot end base, a supporting shell, a cold end conductor, a hot end conductor, a concentric interlaced cold end cylindrical assembly, and a concentric interlaced hot end cylindrical assembly. By combining the differential thermal expansion structure with the concentric interlaced thin-walled cylindrical assembly, and using a central solid conductor as the main bearing to receive assembly preload and control cold contraction displacement, the mechanical support function and the large-area heat exchange function are completely decoupled. This effectively prevents contact short circuits caused by stress or thermal contraction deformation of the thin-walled cylinders, allowing the large-area heat exchange advantage of the concentric interlaced cylindrical assembly to be fully realized. This combination not only naturally forms a μm-level gap at low temperatures through the difference in the thermal expansion coefficients of the materials, completely eliminating the dependence on ultra-high precision machining for traditional air-gap thermal switches, but also adds a direct, high-efficiency solid-state heat conduction path that traditional air-gap thermal switches lack. In the initial pre-cooling stage, ultra-high-efficiency solid-state heat transfer is achieved through the tight fit of the central conductors at the hot and cold ends. Simultaneously, the large heat exchange area of ​​the concentric staggered cylindrical assembly completely compensates for the core shortcoming of insufficient heat exchange area in a single differential thermal expansion structure, achieving continuous and efficient heat conduction across the entire pre-cooling temperature range. This significantly shortens the pre-cooling time of the dilution refrigerator, thus realizing efficient heat conduction during the pre-cooling stage of the air-gap thermal switch. Under extremely low-temperature vacuum disconnection conditions, this structure can completely cut off parasitic heat leakage through a stable physical gap. Furthermore, a highly reliable seal is achieved using an annular sealing groove and high-purity indium wire, combined with bolt pre-tightening force control to ensure an adjustable initial gap, thereby ensuring that the dilution refrigerator achieves a lower final substrate temperature and higher system operational stability. This has significant positive implications for the performance improvement and manufacturing of dilution refrigerators. Attached Figure Description

[0050] Figure 1 These are sectional views and isometric views of the cold-end base, cold-end conductor, and concentric interlaced cold-end cylindrical assembly in specific embodiment 1;

[0051] Figure 2 These are sectional views and isometric views of the hot-end base, hot-end conductor, and concentric interlaced hot-end cylindrical assembly in specific embodiment 1;

[0052] Figure 3 This is a cross-sectional view of the overall structure of a loop differential thermal expansion combined air gap thermal switch for a dilution refrigerator, as shown in Specific Embodiment 1. Figure 3 (a) is an enlarged view of a portion of the gap between the concentric interlaced cold-end cylindrical assembly 5 and the cold-end conductor 3 and the hot-end conductor 4. Figure 3 (b) is a magnified view of the partial gap between the concentric staggered cylindrical group 5 and the concentric staggered hot end cylindrical group 6;

[0053] Figure 4 This is a cross-sectional view of the overall structure of a loop differential thermal expansion combined air gap thermal switch (with gap) for a dilution refrigerator in specific embodiment 1;

[0054] Figure 5 This is an overall structural diagram of a loop differential thermal expansion combined air gap thermal switch (with adsorption pump) for a dilution refrigeration unit in specific embodiment 1;

[0055] Figure 6 This is a schematic diagram of the supporting shell structure in specific embodiment 1.

[0056] In the diagram: 1. Cold end base; 1-1. First threaded connection through hole; 1-2. First assembly hole; 1-3. First annular sealing groove; 2. Hot end base; 2-1. Second threaded connection through hole; 2-2. Second assembly hole; 2-3. Second annular sealing groove; 3. Cold end conductor; 3-1. First threaded hole; 3'. Cold end conductor main body column; 4. Hot end conductor; 4-1. Second threaded hole; 4'. Hot end conductor main body column; 5. Concentric staggered cold end cylinder assembly; 6. Concentric staggered hot end cylinder assembly; 7. Cold end conductor transition column; 8. Hot end conductor transition column; 9. Cold end conductor assembly column; 10. Hot end conductor assembly column; 11. Cold end conductor mating surface; 12. Hot end conductor mating surface; 13. Support shell; 13-1. Third threaded connection through hole; 13-2. Annular boss; 14. Gap; 15. Adsorption pump; 16. Stainless steel conduit. Detailed Implementation

[0057] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0058] See Figures 1 to 6 Specific embodiment 1: This embodiment 1 provides a loop differential thermal expansion combined air gap thermal switch for a dilution refrigeration machine, including a cold end base 1, a hot end base 2, a cold end conductor 3, a hot end conductor 4, a concentric interlaced cold end cylinder group 5, a concentric interlaced hot end cylinder group 6, and a supporting shell 13. The end face of the cold end conductor 3 away from the cold end base 1 is the cold end conductor bonding surface 11, and the end face of the hot end conductor 4 away from the hot end base 2 is the hot end conductor bonding surface 12.

[0059] The cold end conductor 3 located in the concentric interlaced cold end cylindrical group 5 is connected to the cold end base 1, and the concentric interlaced cold end cylindrical group 5 is connected to the cold end conductor 3; the hot end conductor 4 located in the concentric interlaced hot end cylindrical group 6 is connected to the hot end base 2, and the concentric interlaced hot end cylindrical group 6 is connected to the hot end conductor 4.

[0060] The cold end base 1 and the hot end base 2 are respectively sealed to both ends of the supporting shell 13, and the three together form a sealed cavity. The cold end conductor 3, the hot end conductor 4, the concentric interlaced cold end cylindrical group 5 and the concentric interlaced hot end cylindrical group 6 are all located in the sealed cavity, and the cylindrical walls of the concentric interlaced cold end cylindrical group 5 and the concentric interlaced hot end cylindrical group 6 are arranged concentrically and interlaced.

[0061] The coefficients of thermal expansion of the cold-end base 1, the hot-end base 2, and the supporting shell 13 are less than those of the cold-end conductor 3 and the hot-end conductor 4. When cooled to a specific temperature, the cold-end conductor 3 and the hot-end conductor 4 shrink more significantly than the cold-end base 1, the hot-end base 2, and the supporting shell 13. This results in a very small gap 14 forming between the cold-end conductor contact surface 11 and the hot-end conductor contact surface 12, which were originally in contact or close proximity. (See [reference]) Figure 4 .

[0062] The size of the gap 14 is less than or equal to 50 μm; the size of the gap 14 depends on the difference in the coefficients of thermal expansion between the cold end conductor 3, the hot end conductor 4 and the supporting shell 13, the initial lengths of the cold end conductor 3 and the hot end conductor 4, and the temperature at which the cold end conductor 3 and the hot end conductor 4 are cooled.

[0063] The cold end conductor 3 is an integrally formed structure, including three cylinders of different diameters, which are, from top to bottom, the cold end conductor assembly column 9, the cold end conductor transition column 7, and the cold end conductor main body column 3'. The lower end surface of the cold end conductor main body column 3' is the cold end conductor bonding surface 11.

[0064] The concentric interlocking cold-end cylinder group 5 consists of 2-5 thin-walled cylinders of different diameters nested coaxially. One end of each thin-walled cylinder is connected to the cold-end conductor transition post 7, and the main cold-end conductor post 3' is located inside all the thin-walled cylinders. For example, the concentric interlocking cold-end cylinder group 5 consists of 2 thin-walled cylinders of different diameters nested coaxially, see [reference needed]. Figure 1 .

[0065] The hot-end conductor 4 is an integrally formed structure, including three cylinders of different diameters, which are, from bottom to top, the hot-end conductor assembly column 10, the hot-end conductor transition column 8, and the hot-end conductor main body column 4'. The upper end surface of the hot-end conductor transition column 8 is the hot-end conductor bonding surface 12.

[0066] The concentric interlocking hot-end cylinder group 6 consists of 1-4 thin-walled cylinders of different diameters nested coaxially. One end of each thin-walled cylinder is connected to the hot-end conductor transition post 8, and the main hot-end conductor post 4' is located inside all the thin-walled cylinders. For example, the concentric interlocking hot-end cylinder group 6 may consist of a single thin-walled cylinder. See [reference needed]. Figure 2 .

[0067] The main body of the cold end base 1 is a cylindrical T-shaped block with a through first assembly hole 1-2 in the center. Multiple first threaded connection through holes 1-1 are evenly opened along the circumference at the end for connecting the cold plate of the dilution refrigeration unit to the supporting shell 13. A first annular sealing groove 1-3 for embedding sealing indium wire is opened at the end near the supporting shell 13. The lower end of the cold end base 1 has a boss structure. This "end" refers to the part of the cylindrical T-shaped block that is close to the outer edge of the large-diameter cylindrical head.

[0068] The cold-end conductor assembly post 9 is inserted into the first assembly hole 1-2 of the cold-end base 1; the upper end face of the cold-end conductor transition post 7 is tightly attached to the lower end face of the cold-end base 1 and sealed and welded around the circumference.

[0069] The hot end base 2 is completely symmetrical to the cold end base 1. The main body of the hot end base 2 is a cylindrical T-shaped block with a through second assembly hole 2-2 in the center. Multiple second threaded connection through holes 2-1 are evenly opened along the circumference at the end for connecting the cold plate of the dilution refrigeration machine to the supporting shell 13. A second annular sealing groove 2-3 for embedding sealing indium wire is opened at the end near the supporting shell 13. The upper end of the hot end base 2 has a boss structure. This "end" refers to the part of the cylindrical T-shaped block that is close to the outer edge of the large-diameter cylindrical head.

[0070] The hot-end conductor assembly post 10 is inserted into the second assembly hole 2-2 of the hot-end base 2; the lower end face of the hot-end conductor transition post 8 is tightly attached to the upper end face of the hot-end base 2 and sealed and welded around the circumference.

[0071] In the concentric interlaced cold-end cylindrical assembly 5, the wall thickness of each individual thin-walled cylinder is 0.5-1.0 mm, for example, 0.8 mm. The radial distance between the smallest thin-walled cylinder and the cold-end conductor 3 and hot-end conductor 4 is 0.1-0.33 mm, for example, 0.3 mm. Figure 3 (a) shows a magnified view of the local gap between the concentric interlaced cold-end cylindrical group 5 and the cold-end conductor 3 and the hot-end conductor 4. The upper end face of all the thin-walled cylinders in the concentric interlaced cold-end cylindrical group 5 is vertically aligned with the lower end face of the cold-end conductor transition column 7 and welded to ensure that there is no relative displacement between the thin-walled cylinders.

[0072] In the concentric interlaced hot-end cylindrical group 6, the wall thickness of each thin-walled cylinder is 0.5-1.0mm, for example, the wall thickness of each thin-walled cylinder is 0.8mm. The lower end face of all the thin-walled cylinders is vertically aligned with the upper end face of the hot-end conductor transition column 8 and welded and fixed to ensure that there is no relative displacement between the thin-walled cylinders.

[0073] When the concentric staggered cold-end cylindrical group 5 and the concentric staggered hot-end cylindrical group 6 are arranged concentrically and staggeredly, the radial distance between adjacent thin-walled cylinders is 0.1-0.33mm, for example, 0.3mm (e.g.) Figure 3 (b) shows a magnified view of the local gap between the concentric interlocking cylindrical group 5 and the concentric interlocking hot-end cylindrical group 6. The walls of each cylinder remain in contact with each other.

[0074] The main body of the support shell 13 is cylindrical, with an inner diameter larger than the maximum outer diameter of the concentric interlaced cold-end cylindrical group 5 (that is, the outer diameter of the thin-walled cylinder with the largest diameter in the concentric interlaced cold-end cylindrical group 5). A gap of 1-2.5mm is reserved, such as a gap of 1.5mm, to avoid contact. The length of the support shell 13 is adapted to the distance between the two stages of the cold plates of the dilution refrigeration machine. This means that the total length of the support shell 13, together with the cold-end base 1 and the hot-end base 2 at both ends, can meet the distance requirements between the two stages of the cold plates of the dilution refrigeration machine, ensuring that the two ends of the support shell 13 can be connected to the cold plates of the dilution refrigeration machine. For example, the length of the support shell 13 is 218mm.

[0075] A process through hole with a diameter of 3-6 mm, such as a 6 mm diameter process through hole, is opened on the outer wall of the supporting shell 13 near the middle position for welding the stainless steel conduit 16. The stainless steel conduit 16 is connected to the adsorption pump 15 (e.g., Figure 5 (as shown in the figure) and a gaseous working fluid source (not shown in the figure) to achieve gas introduction and discharge;

[0076] The end faces of the supporting shell 13 are provided with annular bosses 13-2 that are adapted to the annular sealing grooves of the cold end base 1 and the hot end base 2 respectively. The height and width of the annular bosses 13-2 are matched with the corresponding annular sealing grooves (i.e., the first annular sealing groove 1-3 and the second annular sealing groove 2-3).

[0077] The end faces of both ends of the supporting housing 13 are also provided with a plurality of third threaded connection through holes 13-1, which are respectively adapted to the first threaded connection through hole 1-1 of the cold end base 1 and the second threaded connection through hole 2-1 of the hot end base 2.

[0078] When ensuring tight contact between the cold end conductor contact surface 11 and the hot end conductor contact surface 12, the third threaded connection through hole 13-1 of the supporting shell 13 is fastened with bolts to the first threaded connection through hole 1-1 of the cold end base 1 and the second threaded connection through hole 2-1 of the hot end base 2. The high-purity sealing indium wire in the first annular sealing groove 1-3 and the high-purity sealing indium wire in the second annular sealing groove 2-3 are both pressed by the corresponding annular boss 13-2. The supporting shell 13, the cold end base 1, and the hot end base 2 together form a sealed cavity.

[0079] The first annular sealing groove 1-3 and the second annular sealing groove 2-3 have a depth of 0.8-1.2 mm and a width of 0.7-0.85 mm, with a tolerance controlled within ±0.01 mm. In this embodiment 1, the first annular sealing groove 1-3 and the second annular sealing groove 2-3 have a depth of 1.2 mm and a width of 0.8 mm.

[0080] The height of the cold end conductor main body post 3' is adapted to the length of the supporting shell 13, and the height of the hot end conductor main body post 4' is adapted to the length of the supporting shell 13. When cooled to a specific temperature, the cold end conductor contact surface 11 and the hot end conductor contact surface 12 can change from being in contact or close to each other to forming a very small gap 14 between them.

[0081] A first threaded hole 3-1 is provided at the center of the distal end of the cold end conductor 3, and a second threaded hole 4-1 is provided at the center of the distal end of the hot end conductor 4. Both the first threaded hole 3-1 and the second threaded hole 4-1 are used for installation and fixation with the cold plate of the dilution refrigeration unit. Here, "distal end of cold end conductor 3" refers to the end of cold end conductor 3 that is away from the cold end conductor contact surface 11, and "distal end of hot end conductor 4" refers to the end of hot end conductor 4 that is away from the hot end conductor contact surface 12.

[0082] A method for manufacturing a loop differential thermal expansion combined air gap thermal switch for a dilution refrigerator, comprising the following steps:

[0083] Step 1: Select stainless steel (e.g., 304 or 316 stainless steel) to machine the cold-end base 1 and the hot-end base 2. Machine the cylindrical T-shaped platform body on a lathe, drilling a through-hole with a diameter of 10-17mm (e.g., 17mm) and a tolerance of H7. Drill 8-16 first threaded connection through holes 1-1 (e.g., 16 M4 threaded connection through holes) along the circumference of the end of the cold-end base 1, and drill 8-16 second threaded connection through holes 2-1 along the circumference of the end of the hot-end base 2, controlling the hole position tolerance to ±0.1mm. Machine a groove with a depth of 0.8-1.2mm (e.g., 1.2mm) and a width of 0.7-0.85mm (e.g., 0.8mm) at the end of the cold-end base 1 and the hot-end base 2 closest to the supporting shell 13 using a lathe. The first annular sealing groove 1-3 and the second annular sealing groove 2-3 are controlled within ±0.01 mm. Finally, the inner wall surfaces of the cold end base 1 and the hot end base 2, as well as the first assembly hole 1-2 and the second assembly hole 2-2, are polished to reduce radiative heat leakage during shutdown. Here, "inner wall surface of cold end base 1" refers to the side wall surface of cold end base 1 facing the supporting shell 13, and "inner wall surface of hot end base 2" refers to the side wall surface of hot end base 2 facing the supporting shell 13.

[0084] Step 2: Select high-purity oxygen-free copper for processing the cold-end conductor 3 and the hot-end conductor 4. Oxygen-free copper has high thermal conductivity and its coefficient of thermal expansion differs significantly from that of stainless steel. The cold-end conductor 3 is machined in one piece using a lathe. From top to bottom, the following components are machined sequentially: a cold-end conductor assembly column 9 with a diameter of 10-17 mm (e.g., 17 mm), a cold-end conductor transition column 7 with a diameter of 24.4-28 mm (e.g., 27.5 mm), and a cold-end conductor main column 3' with a diameter of 20-24 mm (20 mm) and a height adapted to the length of the supporting shell 13. The processing of the hot-end conductor 4 is completely symmetrical with that of the cold-end conductor 3. It is necessary to ensure that the hot-end conductor mating surface 12 and the cold-end conductor mating surface 11 are flat and mated. Both mating surfaces need to be polished to Ra≤0.4 μm. At the far center of the cold-end conductor 3 and the hot-end conductor 4, a first threaded hole 3-1 and a second threaded hole 4-1, for installation and fixation with the cold plate of the dilution refrigeration unit, are machined respectively. For example, M5 threaded holes are used.

[0085] Step 3: Select high-purity oxygen-free copper, consistent with cold-end conductor 3 and hot-end conductor 4, and process individual thin-walled cylinders using slow wire EDM. The wall thickness is 0.5-1.0 mm (e.g., 0.8 mm), and the length matches the height of the cold-end conductor main body column 3' and the hot-end conductor main body column 4'. Design thin-walled cylinders of different diameters according to the radial spacing requirement of 0.1-0.33 mm (e.g., 0.3 mm). Remove burrs from the inner wall of the thin-walled cylinders by polishing to reduce radiative heat leakage during shutdown. When assembling, select 2-5 (e.g., 2) thin-walled cylinders to form a concentric staggered cold-end cylinder group 5, and 1-4 (e.g., 1) thin-walled cylinders to form a concentric staggered hot-end cylinder group 6, ensuring uniform radial spacing between adjacent thin-walled cylinders. The radial spacing between the smallest thin-walled cylinder in the concentric staggered cold-end cylinder group 5 and the cold-end conductor 3 and hot-end conductor 4 is 0.1-0.33 mm (e.g., 0.3 mm).

[0086] Step 4: Select the same stainless steel material (e.g., 304 or 316 stainless steel) as the cold end base 1 and hot end base 2 to process the supporting shell 13, to ensure matching thermal expansion characteristics and low thermal conductivity requirements; process the cylindrical body on a lathe, with an inner diameter that reserves a 1-2.5mm gap based on the maximum outer diameter of the concentric staggered cold end cylindrical assembly 5 (e.g., a 1.5mm gap), a thickness of 0.65-0.8mm (e.g., 0.71mm), and a length adapted to the distance between the two stages of the dilution refrigeration unit, generally 122-242mm, e.g., a length of 218mm. mm; Annular bosses 13-2 with height and width matching the first annular sealing groove 1-3 of the cold end base 1 and the second annular sealing groove 2-3 of the hot end base 2 are machined on both end faces; 8-16 third threaded connecting through holes 13-1 are drilled along the circumference at the end of both end faces, such as 16 M4 threaded connecting through holes, with hole position tolerance controlled within ±0.1mm; Process through holes with a diameter of 3-6mm (such as process through holes with a diameter of 6mm) are drilled at the centerline of the outer wall surface, and stainless steel conduits 16 are subsequently welded at the process through holes using clean laser welding technology to connect the adsorption pump 15 to the gas working fluid source;

[0087] Step 5: After the parts are processed, they are combined and welded, then assembled. Finally, the leakage rate of the entire cavity is detected by a helium mass spectrometer leak detector, thus forming a loop differential thermal expansion combined air gap thermal switch for dilution refrigerators.

[0088] Specifically, the first step is the pre-welding of the core components: The cold-end conductor assembly post 9 is inserted into the first assembly hole 1-2 of the cold-end base 1, with an interference fit, so that the upper end face of the cold-end conductor transition post 7 is in contact with the lower end face of the cold-end base 1. Vacuum brazing technology is used to weld and seal the circumference. The hot-end conductor assembly post 10 is then inserted into the second assembly hole 2-2 of the hot-end base 2, with an interference fit, so that the lower end face of the hot-end conductor transition post 8 is in contact with the upper end face of the hot-end base 2. Vacuum brazing technology is used to weld and seal the circumference. First, perform a ring weld seal. Then, align the upper surfaces of all thin-walled cylinders in the concentric interlaced cold-end cylinder assembly 5 with the lower surfaces of the cold-end conductor transition pillars 7, and fix them using vacuum brazing technology. Next, align the lower surfaces of all thin-walled cylinders in the concentric interlaced hot-end cylinder assembly 6 with the upper surfaces of the hot-end conductor transition pillars 8, and fix them using vacuum brazing technology. After welding, wait for each component to cool to room temperature before proceeding to the next step to avoid the thermal expansion and contraction of the cold-end conductor 3 and hot-end conductor 4 affecting subsequent assembly.

[0089] The second step is the assembly of the sealed cavity: First, high-purity sealing indium wire with a diameter matching the groove width is embedded in the first annular sealing groove 1-3 of the cold end base 1 and the second annular sealing groove 2-3 of the hot end base 2; second, all the thin-walled cylinders of the concentric interlaced cold end cylinder group 5 and all the thin-walled cylinders of the concentric interlaced hot end cylinder group 6 are inserted into the support shell 13 from both the top and bottom directions to form a coaxial nested state. Then, while ensuring that the cold end conductor contact surface 11 and the hot end conductor contact surface 12 are precisely aligned, the support shell 13 is placed between the cold end base 1 and the hot end base 2, so that the annular protrusions 13-2 at both ends of the support shell 13 are separated. Do not embed the first annular sealing groove 1-3 of the cold end base 1 and the second annular sealing groove 2-3 of the hot end base 2; then use bolts (such as M4 stainless steel bolts) to connect and fix the cold end base 1, hot end base 2 and supporting shell 13 through the first threaded connection through holes 1-1 evenly distributed around the circumference of the cold end base 1 and the second threaded connection through holes 2-1 evenly distributed around the circumference of the hot end base 2, and simultaneously cooperate with the third threaded connection through holes 13-1 evenly distributed around the circumference of the end faces of the supporting shell 13. On the one hand, the annular bosses 13-2 at both ends of the supporting shell 13 press the high-purity sealing indium wire in the corresponding annular sealing groove to achieve The cavity is sealed. On the other hand, screws (such as M5 copper screws) are screwed into the first threaded hole 3-1 and the second threaded hole 4-1 at the far center of the cold-end conductor 3 and the hot-end conductor 4, respectively. By controlling the preload with a torque wrench, the initial gap between the cold-end conductor 3 and the hot-end conductor 4 can be adjusted to some extent, eliminating the initial gap and ensuring that the cold-end conductor 3 and the hot-end conductor 4 are initially in close contact. "Close contact" means that the cold-end conductor contact surface 11 of the cold-end conductor 3 and the hot-end conductor contact surface 12 of the hot-end conductor 4 are tightly fitted together, but without compression or deformation. The initial gap between the cooling end conductor 3 and the hot end conductor 4 is extremely important. On the one hand, it avoids the gap from being too large at low temperatures, which would affect the thermal conductivity in the conducting state. On the other hand, it avoids the initial gap from being too small (i.e., the cooling end conductor contact surface 11 and the hot end conductor contact surface 12 are squeezed and deformed after they are attached). Because the surfaces of the cooling end conductor contact surface 11 and the hot end conductor contact surface 12 are not parallel and smooth enough, the switch cannot be completely disconnected at low temperatures, causing the thermal switch to fail. Finally, the leakage rate of the entire cavity is detected by a helium mass spectrometer leak detector, thus forming a loop differential thermal expansion combined air gap thermal switch for dilution refrigerators.

[0090] In this embodiment 1, a loop differential thermal expansion combined air-gap thermal switch for a dilution refrigerator utilizes the difference in thermal expansion coefficients between the conductor material and the supporting shell 13 material. At low temperatures, a very small gap 14 can naturally form between the cold end conductor 3 and the hot end conductor 4, achieving the shut-off of the air-gap thermal switch without requiring extremely high machining precision. Simultaneously, the concentric staggered cylindrical assembly design results in a larger internal heat exchange area for the thermal switch, significantly increasing the heat exchange area and thus enhancing the heat exchange efficiency of the gaseous working fluid. The smaller gap, larger heat exchange area, and lower machining precision requirements optimize the core performance of the air-gap thermal switch while reducing manufacturing difficulty. Furthermore, a highly reliable seal is achieved using an annular sealing groove and a high-purity indium wire sealing structure. Combined with bolt preload control, the initial gap is adjustable, ensuring that the dilution refrigerator achieves a lower final base temperature and higher system operational stability.

[0091] Ideally, at room temperature, the cold-end conductor 3 and the hot-end conductor 4 are initially in close contact, and gaps of varying sizes are pre-set between the conductors and the adjacent cylindrical walls, between the concentric interlaced cold-end cylindrical group 5 and the concentric interlaced hot-end cylindrical group 6, and between the supporting shell 13 and the adjacent cylindrical walls. After the cooling process is initiated, helium is injected into all the aforementioned gaps in a timely manner through the stainless steel conduit 16 connected to the supporting shell 13. During this stage, the heat transfer path of the thermal switch mainly includes direct heat conduction between the cold-end conductor 3 and the hot-end conductor 4, indirect heat conduction between the cold-end conductor 3, helium, and the adjacent cylindrical wall, indirect heat conduction between the concentric interlaced cold-end cylindrical group 5, helium, and the concentric interlaced hot-end cylindrical group 6, heat conduction within the supporting shell 13, and a small amount of heat transfer through radiation. As the temperature continues to decrease, due to the difference in thermal expansion coefficients between the conductors (i.e., cold-end conductor 3 and hot-end conductor 4) and the supporting shell, the conductors shrink more than the supporting shell 13, naturally forming a minimal gap 14 in the near-end regions of the cold-end conductor 3 and the hot-end conductor 4. At this time, the heat transfer path between the cold-end conductor 3 and the hot-end conductor 4 changes from direct heat conduction to indirect heat conduction via cold-end conductor 3-helium gas-hot-end conductor 4, while the other heat transfer paths remain unchanged. When the thermal switch temperature drops to a predetermined threshold, the adsorption pump 15 is activated to completely adsorb and remove the helium gas from the sealed cavity. At this time, the minimal gap 14 between the cold-end conductor 3 and the hot-end conductor 4 remains unchanged, but the helium-mediated heat conduction path is cut off, leaving only the heat conduction of the low thermal conductivity supporting shell 13 and a small amount of thermal radiation as heat transfer pathways. The thermal switch thus enters the thermally isolated disconnected state.

[0092] In other words, it offers highly efficient heat conduction: unlike single differential thermal expansion structures that rely solely on end-face heat exchange and have extremely limited heat exchange area, and traditional fixed air gap structures that cannot simultaneously handle room-temperature solid-state heat conduction and low-temperature air gap heat conduction, the loop differential thermal expansion combined air gap thermal switch in this embodiment 1 for a dilution refrigerator achieves ultra-high efficiency direct solid-state heat conduction during the initial pre-cooling stage through the tight fit between the cold-end conductor 3 and the hot-end conductor 4, a feature not found in traditional air gap thermal switches. This is further enhanced by the use of concentric interlaced cylindrical assemblies (i.e., concentric interlaced...). The ultra-large heat exchange area of ​​the cold-end cylindrical group 5 and the concentric staggered hot-end cylindrical group 6 forms a parallel air gap heat transfer path through the helium working fluid; the micron-level working air gap (i.e., gap 14) naturally formed on the cold and hot end faces during the cooling process is still in the optimal range of gas efficient heat exchange, with no abrupt attenuation of thermal conductivity throughout the process, achieving continuous and efficient heat conduction throughout the pre-cooling stage, greatly shortening the pre-cooling time of the dilution refrigerator, and the core working gap (i.e., gap 14) is naturally formed through the thermal expansion characteristics of the material, without relying on ultra-high precision machining.

[0093] High-efficiency thermal insulation: Unlike existing fixed air gap structures that are prone to short circuits due to contact with the cylinder wall and unstable air gap size, the loop differential thermal expansion combined air gap thermal switch in this embodiment 1 for a dilution refrigerator completely removes the helium working fluid in the sealed cavity by adsorption pump 15 after pre-cooling. The micron-level air gap that is naturally formed at low temperature can stably maintain its original size, completely cutting off the gas heat conduction path mediated by helium. Only a small amount of solid heat leakage and a very small amount of radiative heat exchange remain in the low thermal conductivity stainless steel supporting shell, achieving an ultra-high breaking ratio thermal isolation effect and efficiently cutting off the parasitic heat load of the dilution refrigerator. At the same time, there are no moving parts and no risk of contact short circuit, ensuring long-term stable operation of the thermal switch under extremely low temperature and high vacuum conditions, ultimately helping the dilution refrigerator achieve a lower base temperature and higher operational stability.

[0094] Specific embodiment 2, this embodiment 2 provides a loop differential thermal expansion combined air gap thermal switch. The difference between embodiment 2 and embodiment 1 is that: the cold end conductor bonding surface 11 and the hot end conductor bonding surface 12 were originally in a close distance state (i.e., the initial gap) means that the distance between the cold end conductor bonding surface 11 and the hot end conductor bonding surface 12 is less than or equal to 20um.

[0095] When cooled to a specific temperature, the cold-end conductor 3 and the hot-end conductor 4 shrink more significantly than the cold-end base 1, the hot-end base 2, and the supporting shell 13. This results in a very small gap 14 forming between the cold-end conductor contact surface 11 and the hot-end conductor contact surface 12, which were originally in close proximity. In the initial stage of pre-cooling, the thermal conductivity of the helium gas gap between the end faces of the cold-end conductor 3 and the hot-end conductor 4 can reach more than 80% of the solid contact thermal conductivity (i.e., the contact between the cold-end conductor contact surface 11 and the hot-end conductor contact surface 12) in Specific Embodiment 1. This is almost no different from the pre-cooling efficiency of the zero-gap scheme in Embodiment 1, thus achieving the technical effect of efficient heat conduction and efficient heat insulation.

[0096] In the manufacturing process of a loop differential thermal expansion combined air gap thermal switch, controlling the initial gap between the cold end conductor 3 and the hot end conductor 4 is extremely important. This is to prevent an excessively large initial gap (greater than 20 μm) from causing the gap to widen at low temperatures, thus affecting thermal conductivity during operation. For example, it is essential to ensure that the initial gap plus the low-temperature shrinkage is less than or equal to 50 μm, and that the size of the gap 14 is less than or equal to 50 μm. Here, "low-temperature shrinkage" refers to the total low-temperature shrinkage of the cold end conductor 3 and the hot end conductor 4.

[0097] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A loop differential thermal expansion combined air gap thermal switch for a dilution refrigeration unit, characterized in that: The air gap thermal switch includes a cold end base (1), a hot end base (2), a cold end conductor (3), a hot end conductor (4), a concentric interlaced cold end cylinder assembly (5), a concentric interlaced hot end cylinder assembly (6), and a supporting shell (13). The end face of the cold end conductor (3) away from the cold end base (1) is the cold end conductor bonding surface (11), and the end face of the hot end conductor (4) away from the hot end base (2) is the hot end conductor bonding surface (12). The cold end conductor (3) located in the concentric interlaced cold end cylinder group (5) is connected to the cold end base (1), and the concentric interlaced cold end cylinder group (5) is connected to the cold end conductor (3); the hot end conductor (4) located in the concentric interlaced hot end cylinder group (6) is connected to the hot end base (2), and the concentric interlaced hot end cylinder group (6) is connected to the hot end conductor (4); The cold end base (1) and the hot end base (2) are respectively sealed to both ends of the supporting shell (13), and the three together form a sealed cavity. The cold end conductor (3), the hot end conductor (4), the concentric interlaced cold end cylinder group (5) and the concentric interlaced hot end cylinder group (6) are all located in the sealed cavity, and the cylinder walls of the concentric interlaced cold end cylinder group (5) and the concentric interlaced hot end cylinder group (6) are arranged concentrically and interlaced. The coefficients of thermal expansion of the cold end base (1), the hot end base (2) and the supporting shell (13) are smaller than those of the cold end conductor (3) and the hot end conductor (4). When cooled to a specific temperature, the cold end conductor (3) and the hot end conductor (4) shrink more significantly than the cold end base (1), the hot end base (2) and the supporting shell (13), resulting in a very small gap (14) between the cold end conductor contact surface (11) and the hot end conductor contact surface (12) which were originally in contact or close proximity.

2. The loop differential thermal expansion combined air gap thermal switch for a dilution refrigerator according to claim 1, characterized in that: The size of the gap (14) is less than or equal to 50 μm; the size of the gap (14) depends on the difference in the coefficient of thermal expansion between the cold end conductor (3), the hot end conductor (4) and the supporting shell (13), the initial length of the cold end conductor (3) and the hot end conductor (4), and the temperature at which the cold end conductor (3) and the hot end conductor (4) are cooled.

3. The loop differential thermal expansion combined type air gap thermal switch for a dilution refrigerator according to claim 2, characterized in that: The cold end conductor (3) is an integrally formed structure, including three cylinders of different diameters, which are, from top to bottom, the cold end conductor assembly column (9), the cold end conductor transition column (7) and the cold end conductor main body column (3'), and the lower end surface of the cold end conductor main body column (3') is the cold end conductor bonding surface (11). The concentric interlocking cold-end cylinder group (5) is composed of 2-5 thin-walled cylinders of different diameters nested coaxially. One end of the thin-walled cylinder is connected to the cold-end conductor transition column (7), and the cold-end conductor main column (3') is located inside all the thin-walled cylinders.

4. The loop differential thermal expansion combined air gap thermal switch for a dilution refrigeration unit according to claim 3, characterized in that: The hot end conductor (4) is an integrally formed structure, including three cylinders of different diameters, which are, from bottom to top, the hot end conductor assembly column (10), the hot end conductor transition column (8) and the hot end conductor main body column (4'). The upper end surface of the hot end conductor transition column (8) is the hot end conductor bonding surface (12). The concentric interlocking hot-end cylinder group (6) is composed of 1-4 thin-walled cylinders of different diameters nested coaxially. One end of the thin-walled cylinder is connected to the hot-end conductor transition column (8), and the hot-end conductor main column (4') is located inside all the thin-walled cylinders.

5. A loop differential thermal expansion combined air gap thermal switch for a dilution refrigerator according to claim 4, characterized in that: The main body of the cold end base (1) is a cylindrical T-shaped block with a through first assembly hole (1-2) in the center. Multiple first threaded connection through holes (1-1) are evenly opened at the end along the circumference for connecting the cold plate of the dilution refrigeration machine to the support shell (13). A first annular sealing groove (1-3) for embedding the sealing indium wire is opened at one end near the support shell (13). The lower end of the cold end base (1) has a boss structure. The cold end conductor assembly post (9) is inserted into the first assembly hole (1-2) of the cold end base (1); the upper end face of the cold end conductor transition post (7) is closely attached to the lower end face of the cold end base (1) and sealed and welded around the circumference.

6. A loop differential thermal expansion combined air gap thermal switch for a dilution refrigerator according to claim 5, characterized in that: The hot end base (2) is completely symmetrical with the cold end base (1). The main body of the hot end base (2) is a cylindrical T-shaped block with a through second assembly hole (2-2) in the center. Multiple second threaded connection through holes (2-1) for connecting the cold plate of the dilution refrigeration machine and the supporting shell (13) are evenly opened at the end along the circumference. A second annular sealing groove (2-3) for embedding the sealing indium wire is opened at one end near the supporting shell (13). The upper end of the hot end base (2) has a boss structure. The hot end conductor assembly column (10) is inserted into the second assembly hole (2-2) of the hot end base (2); the lower end face of the hot end conductor transition column (8) is closely attached to the upper end face of the hot end base (2) and sealed and welded around the circumference.

7. A loop differential thermal expansion combined air gap thermal switch for a dilution refrigerator according to claim 6, characterized in that: In the concentric interlaced cold-end cylinder group (5), the wall thickness of a single thin-walled cylinder is 0.5-1.0 mm, and the radial distance between the smallest thin-walled cylinder and the cold-end conductor (3) and the hot-end conductor (4) is 0.1-0.33 mm; the upper end face of all thin-walled cylinders in the concentric interlaced cold-end cylinder group (5) is vertically aligned with the lower end face of the cold-end conductor transition column (7) and welded to ensure that there is no relative displacement between the thin-walled cylinders; In the concentric interlaced hot-end cylindrical group (6), the wall thickness of each thin-walled cylinder is 0.5-1.0 mm. The lower end face of all the thin-walled cylinders is vertically aligned with the upper end face of the hot-end conductor transition column (8) and welded to ensure that there is no relative displacement between the thin-walled cylinders. After the concentric and staggered cold-end cylindrical group (5) and the concentric and staggered hot-end cylindrical group (6) are arranged in a concentric and staggered manner, the radial distance between adjacent thin-walled cylindrical groups is 0.1-0.33 mm, and the cylindrical walls do not contact each other.

8. A loop differential thermal expansion combined air gap thermal switch for a dilution refrigerator according to claim 7, characterized in that: The main body of the supporting shell (13) is cylindrical, with an inner diameter larger than the maximum outer diameter of the concentric interlaced cold end cylindrical group (5), and a gap of 1-2.5 mm is reserved to avoid contact. The length of the supporting shell (13) is adapted to the distance between the two stages of the dilution refrigeration unit. A process through hole with a diameter of 3-6 mm is opened on the outer wall of the supporting shell (13) near the middle position for welding stainless steel conduit (16). The stainless steel conduit (16) connects the adsorption pump (15) and the gas working fluid source to realize the introduction and discharge of gas. The end faces of the supporting shell (13) are provided with annular bosses (13-2) that are adapted to the annular sealing grooves of the cold end base (1) and the hot end base (2) respectively. The height and width of the annular bosses (13-2) are matched with the corresponding annular sealing grooves. The end faces of both ends of the supporting shell (13) are also provided with a number of third threaded connection through holes (13-1). These third threaded connection through holes (13-1) are respectively adapted to the first threaded connection through hole (1-1) of the cold end base (1) and the second threaded connection through hole (2-1) of the hot end base (2). When ensuring that the cold end conductor contact surface (11) and the hot end conductor contact surface (12) are in close contact, the third threaded connection through hole (13-1) of the supporting shell (13) is fastened with bolts to the first threaded connection through hole (1-1) of the cold end base (1) and the second threaded connection through hole (2-1) of the hot end base (2). The high-purity sealing indium wire in the first annular sealing groove (1-3) and the high-purity sealing indium wire in the second annular sealing groove (2-3) are pressed by the corresponding annular boss (13-2). The supporting shell (13), the cold end base (1) and the hot end base (2) together form a sealed cavity. The first annular sealing groove (1-3) and the second annular sealing groove (2-3) have a depth of 0.8-1.2 mm and a width of 0.7-0.85 mm, with a tolerance controlled within ±0.01 mm. The height of the cold end conductor main column (3') is adapted to the length of the supporting shell (13), and the height of the hot end conductor main column (4') is adapted to the length of the supporting shell (13); The cold end conductor (3) has a first threaded hole (3-1) at the center of its far end, and the hot end conductor (4) has a second threaded hole (4-1) at the center of its far end. The first threaded hole (3-1) and the second threaded hole (4-1) are used to install and fix the cold plate of the dilution refrigeration unit. The cold end conductor bonding surface (11) and the hot end conductor bonding surface (12) were originally in a close-range state, meaning that the distance between the cold end conductor bonding surface (11) and the hot end conductor bonding surface (12) was less than or equal to 20um.

9. A method for manufacturing a loop differential thermal expansion combined air gap thermal switch for a dilution refrigerator as described in claim 8, comprising the following steps: Step 1: Select stainless steel material to process the cold end base (1) and the hot end base (2). Machin the cylindrical T-shaped table body on a lathe, and drill a through-hole with a diameter of 10-17 mm and a tolerance of H7 in the center. Drill 8-16 first threaded connecting through holes (1-1) along the circumference at the end of the cold end base (1) and 8-16 second threaded connecting through holes (2-1) along the circumference at the end of the hot end base (2), with the hole position tolerance controlled within ±0.1 mm. Machin a groove with a depth of 0.8-1.2 mm and a width of 0.7-0.85 mm on a lathe at the end of the cold end base (1) and the hot end base (2) near the supporting shell (13), with the tolerance controlled within ±0.01 mm. The first annular sealing groove (1-3) and the second annular sealing groove (2-3) are mm in diameter. Finally, the inner wall surfaces of the cold end base (1) and the hot end base (2), as well as the first assembly hole (1-2) and the second assembly hole (2-2), are polished to reduce radiative heat leakage during shutdown. Step 2: Select high-purity oxygen-free copper for processing the cold end conductor (3) and the hot end conductor (4). Oxygen-free copper has high thermal conductivity and its thermal expansion coefficient is significantly different from that of stainless steel. The cold end conductor (3) is processed by lathe integral forming. From top to bottom, the cold end conductor assembly column (9) with a diameter of 10-17 mm, the cold end conductor transition column (7) with a diameter of 24.4-28 mm, and the cold end conductor main column (3') with a diameter of 20-24 mm and a height adapted to the length of the supporting shell (13) are processed in sequence. The processing of the hot end conductor (4) is completely symmetrical with that of the cold end conductor (3). It is necessary to ensure that the hot end conductor mating surface (12) and the cold end conductor mating surface (11) are flat and mated. The two mating surfaces need to be polished to Ra≤0.4 μm. The first threaded hole (3-1) and the second threaded hole (4-1) for mounting and fixing with the cold plate of the dilution refrigeration machine are processed at the far center of the cold end conductor (3) and the hot end conductor (4), respectively. Step 3: Select high-purity oxygen-free copper consistent with the cold end conductor (3) and hot end conductor (4), and process a single thin-walled cylinder using slow wire cutting technology. The wall thickness is 0.5-1.0 mm, and the length is adapted to the height of the cold end conductor main column (3') and the hot end conductor main column (4'). Design thin-walled cylinders of different diameters according to the requirement of radial spacing of 0.1-0.33 mm. Remove the burrs on the inner wall of the thin-walled cylinder by polishing to reduce radiative heat leakage during shutdown. When assembling, select 2-5 thin-walled cylinders to form a concentric staggered cold end cylinder group (5) and 1-4 thin-walled cylinders to form a concentric staggered hot end cylinder group (6) to ensure that the radial spacing of adjacent thin-walled cylinders is uniform. The radial spacing between the smallest thin-walled cylinder in the concentric staggered cold end cylinder group (5) and the cold end conductor (3) and the hot end conductor (4) is 0.1-0.33 mm. Step 4: Select stainless steel material consistent with cold end base (1) and hot end base (2) for processing the supporting shell (13) to ensure matching thermal expansion characteristics and low thermal conductivity requirements; process the cylindrical body by lathe, with an inner diameter reserved with a gap of 1-2.5 mm according to the maximum outer diameter of the concentric interlaced cold end cylindrical group (5), a thickness of 0.65-0.8 mm, and a length adapted to the distance between the two cold plates of the dilution refrigeration machine, generally 122-242 mm; process annular bosses (13-2) on both end faces, with height and width matching the first annular sealing groove (1-3) of the cold end base (1) and the second annular sealing groove (2-3) of the hot end base (2); drill 8-16 threaded connecting through holes along the circumference at the end of both end faces, with hole position tolerance controlled within ±0.1 mm; drill a diameter of 3-6 mm at the center line of the outer wall. A process through hole of mm is made, and a stainless steel conduit (16) is subsequently welded at the process through hole using clean laser welding technology to connect the adsorption pump (15) and the gas working fluid source. Step 5: After the parts are processed, they are combined and welded, then assembled. Finally, the leakage rate of the entire cavity is detected by a helium mass spectrometer leak detector, thus forming a loop differential thermal expansion combined air gap thermal switch for dilution refrigerators.

10. A method for manufacturing a loop differential thermal expansion combined air gap thermal switch for a dilution refrigerator according to claim 9, characterized in that: In step 5, the specific steps are as follows: The first step is pre-welding of the core components: The cold-end conductor assembly post (9) is inserted into the first assembly hole (1-2) of the cold-end base (1), and the two are interference-fitted so that the upper end face of the cold-end conductor transition post (7) is in contact with the lower end face of the cold-end base (1). Vacuum brazing technology is used to weld and seal around the circumference. The hot-end conductor assembly post (10) is inserted into the second assembly hole (2-2) of the hot-end base (2), and the two are interference-fitted so that the lower end face of the hot-end conductor transition post (8) is in contact with the upper end face of the hot-end base (2). Vacuum brazing technology is used. Weld and seal around the circumference; then align the upper surfaces of all thin-walled cylinders of the concentric interlaced cold-end cylinder group (5) with the lower surfaces of the cold-end conductor transition column (7) and fix them by vacuum brazing; align the lower surfaces of all thin-walled cylinders of the concentric interlaced hot-end cylinder group (6) with the upper surfaces of the hot-end conductor transition column (8) and fix them by vacuum brazing; after welding, wait for the temperature of each component to drop to room temperature before proceeding to the next step to avoid the thermal expansion and contraction of the cold-end conductor (3) and hot-end conductor (4) affecting subsequent assembly; The second step is the assembly of the sealed cavity: First, high-purity sealing indium wire with a diameter matching the groove width is embedded in the first annular sealing groove (1-3) of the cold end base (1) and the second annular sealing groove (2-3) of the hot end base (2); then, all the thin-walled cylinders of the concentric interlaced cold end cylinder group (5) and all the thin-walled cylinders of the concentric interlaced hot end cylinder group (6) are inserted into the supporting shell 13 from the upper and lower directions to form a coaxial nested state, and then the cold end conductor is ensured to be in contact with the surface (1 1) When precisely aligned with the hot-end conductor contact surface (12), place the support shell (13) between the cold-end base (1) and the hot-end base (2), so that the annular bosses (13-2) at both ends of the support shell (13) are respectively embedded in the first annular sealing groove (1-3) of the cold-end base (1) and the second annular sealing groove (2-3) of the hot-end base (2); then, using bolts, the first threaded connection through holes (1-1) evenly distributed along the circumference of the cold-end base (1) and the hot-end base (2) are connected. The second threaded connection through hole (2-1) is evenly distributed around the circumference, and at the same time, the third threaded connection through hole (13-1) is evenly distributed around the circumference of the end face of the support shell (13) to realize the connection and fixation between the cold end base (1), the hot end base (2) and the support shell (13). On the one hand, the annular boss (13-2) at both ends of the support shell (13) presses the high-purity sealing indium wire in the corresponding annular sealing groove to achieve cavity sealing. On the other hand, screws are screwed into the first threaded hole (3-1) and the second threaded hole (4-1) at the far center of the cold end conductor (3) and the hot end conductor (4) respectively. The preload is controlled by the torque wrench to adjust the initial gap between the cold end conductor (3) and the hot end conductor (4), eliminate the initial gap between the cold end conductor (3) and the hot end conductor (4) or control the initial gap between the cold end conductor (3) and the hot end conductor (4) to be less than or equal to 20um, so that the cold end conductor (3) and the hot end conductor (4) are initially in close contact or close distance.

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