A low-temperature thermal conductivity unsteady-state testing system and method

Abstract

This application discloses a low-temperature thermal conductivity unsteady-state testing system and method, relating to the field of low-temperature thermal conductivity testing technology. The system includes a refrigerator, a vacuum chamber, a vacuum chamber flange cover, a primary cold shield, a secondary cold shield, a sample testing assembly, an air inlet assembly, an air outlet assembly, a sample holder, and a test sample. The method is applicable to this system. Using the low-temperature thermal conductivity unsteady-state testing system and method of this application, rapid measurement of low-temperature thermal conductivity can be achieved, with no temperature difference on the test sample surface, greatly reducing radiative heat leakage. Furthermore, based on this transient measurement principle, a correction method for the test data is proposed, thereby improving the final test accuracy. Simultaneously, when performing transient thermal conductivity measurements using this application, the measurement time is from tens to hundreds of seconds, significantly improving measurement efficiency. Moreover, when it is necessary to change the test sample, only the test rod along with the test sample needs to be removed from the container for replacement, without disassembling the entire system.

Description

Technical Field

[0001] This application relates to the field of low-temperature thermal conductivity testing technology, specifically a low-temperature thermal conductivity unsteady-state testing system and method. Background Technology

[0002] Currently, thermal insulation materials are widely used in cryogenic technologies such as liquid hydrogen and liquid helium. The thermal insulation performance of cryogenic insulation materials affects the operating efficiency of large-scale cryogenic refrigeration systems, as well as the storage and transportation efficiency of liquid cryogenic working fluids. Thermal conductivity is a crucial indicator for characterizing the quality of its thermal insulation performance, making its accurate and efficient measurement extremely important.

[0003] The thermal conductivity at low temperatures is mainly measured using the steady-state method. Patent CN115266814A proposes a low-temperature thermal conductivity testing device and measurement method, as shown in the attached figures of the specification. Figure 2 As shown (the reference numerals in this figure differ from those in this text, and the reference numerals are detailed in the specific technical text of this patent), the secondary cold shield has a secondary cold plate. One end of the sample to be tested is mounted on the secondary cold plate, and a copper plate is mounted on the other end of the sample. A first temperature sensor and a first heating block are mounted on the copper plate, and a second temperature sensor and a second heating block are mounted on the secondary cold plate. The secondary cold shield is installed inside the primary cold shield, which is installed inside the vacuum chamber. The vacuum chamber and the refrigerator are both fixedly mounted on a bracket. The primary and secondary cold heads of the refrigerator are connected to the primary and secondary cold shields, respectively. During the experiment, the two ends of the sample to be tested are brought into contact with the copper plate and the cold plate, respectively, and temperature is measured and controlled. Finally, the thermal conductivity is calculated using Fourier's steady-state thermal conductivity law.

[0004] However, this technology measures thermal conductivity using a steady-state method, which requires establishing a stable temperature difference, thus consuming a considerable amount of time, typically on the order of hours. Furthermore, during steady-state measurements, a body temperature exists on the upper and lower surfaces of the test sample before the heater is turned on, and the sample itself exhibits a temperature gradient during the test, leading to heat loss through radiation. In terms of temperature control, this technology uses a heater and temperature sensor at the cold head, resulting in a simplistic and inaccurate control method. Summary of the Invention

[0005] The purpose of this application is to provide a low-temperature thermal conductivity unsteady-state testing system and method to solve the technical problems mentioned in the background art.

[0006] To achieve the above objectives, this application discloses the following technical solutions:

[0007] In a first aspect, this application discloses a low-temperature thermal conductivity unsteady-state testing system, including a refrigerator, a vacuum chamber, a vacuum chamber flange cover, a primary cold shield, a secondary cold shield, a sample testing component, an air inlet component, and an air outlet component.

[0008] The refrigeration unit is fixed to the vacuum hood flange cover plate;

[0009] The top side of the vacuum hood is fixed to the vacuum hood flange cover plate via the vacuum hood flange;

[0010] The top side of the primary cold shield is fixed to the refrigerator via a primary cold shield flange cover plate. The primary cold shield is installed inside the vacuum chamber, and the primary cold shield is in contact with the primary cold head of the refrigerator.

[0011] The top side of the secondary cold shield is fixed to the refrigeration unit via a secondary cold shield flange cover plate. The secondary cold shield is installed inside the primary cold shield, and the secondary cold shield is in contact with the secondary cold head of the refrigeration unit.

[0012] The secondary cold head of the refrigeration unit is connected to the heater;

[0013] The sample testing assembly includes a test chamber neck tube, a sample testing chamber, a sample testing rod, a sample holder, a temperature sensor, and a sample chamber heater. The test chamber neck tube is fitted over the upper part of the sample testing rod and is fixed to the top of the vacuum chamber flange cover. A test chamber upper flange is installed on the top side of the test chamber neck tube. The middle part of the sample testing chamber is fixed to the primary cold shield flange cover and the secondary cold shield flange cover via positioning flanges, and the bottom of the sample testing chamber extends into the secondary cold shield. The top of the sample testing rod is fixed to the test rod flange, the test rod flange is fixed to the test chamber upper flange, and the bottom of the sample testing rod extends between the secondary cold shield flange cover and the corresponding positioning flange. The sample holder is installed at the bottom of the sample testing rod, and the test sample is installed on the sample holder. The temperature sensor is installed inside the sample holder to monitor the peripheral temperature of the test sample. The sample chamber heater is installed at the bottom of the sample testing rod to control the peripheral temperature of the test sample.

[0014] The air intake assembly includes an air intake neck and an air intake pipe; the air intake neck is sleeved on the upper part of the air intake pipe, and the bottom of the air intake neck is fixed to the top side of the vacuum hood flange cover; the top of the air intake pipe extends to the outside of the top side of the air intake neck, and the bottom of the air intake pipe extends into the secondary cold shield and communicates with the sample testing chamber for introducing low-pressure helium and rewarmed gas into the sample testing chamber.

[0015] The air outlet assembly includes an air outlet port, which is installed on the top side of the vacuum hood flange cover and communicates with the interior of the vacuum hood.

[0016] In one embodiment, a shock-absorbing bellows is provided between the upper part of the refrigerator and the top side of the vacuum hood flange cover.

[0017] In one embodiment, the sample holder includes a lifting component, an upper clamping plate, a lower clamping plate, and connecting bolts;

[0018] The two ends of the lifting component are respectively fixed to the top of the upper clamping plate and the bottom of the sample testing rod;

[0019] The upper clamping plate and the lower clamping plate are spaced apart, the test sample is placed between the upper clamping plate and the lower clamping plate, and the upper clamping plate and the lower clamping plate are fixed together by the connecting bolts.

[0020] In one embodiment, two test samples are disposed between the upper clamping plate and the lower clamping plate, and a planar heat source is clamped between the two test samples. The planar heat source is connected to the sample chamber heater.

[0021] In one embodiment, the temperature sensor is mounted on the upper clamp and the lower clamp.

[0022] In one embodiment, the temperature rise model of the planar heat source is: in, For the temperature rise result, ΔT c λ is the temperature rise constant caused by the contact thermal resistance between the planar heat source and the test sample, λ is the measured thermal conductivity, D(τ) is the dimensionless time, and r is the temperature coefficient of resistance.

[0023] In one embodiment, the rewarming gas is helium.

[0024] Secondly, this application discloses a method for unsteady-state testing of low-temperature thermal conductivity, including the aforementioned unsteady-state testing system for low-temperature thermal conductivity, the method comprising the following steps:

[0025] Step 1: Evacuate the vacuum chamber through the outlet port and vacuum machine, then fill the sample testing chamber with low-pressure helium through the inlet pipe and helium delivery machine, then evacuate the vacuum chamber again through the outlet port, and repeat 2 to 3 times.

[0026] Step 2: Evacuate the vacuum chamber until the internal pressure reaches 10. -2 ~10 -3 Pa;

[0027] Step 3: Turn on the refrigerator for pre-cooling. After pre-cooling is completed, the temperature around the test sample is controlled by the refrigerator's cold head, heater, and sample chamber heater.

[0028] Step 4: Based on the temperature sensor's detection results, after the ambient temperature around the test sample reaches the test temperature, the transient thermal conductivity test begins. The relationship between the temperature rise and dimensionless time is: ΔT s (τ)=P0(π 3 / 2 rλ) -1 D(τ), where ΔT s (τ) is the temperature rise, D(τ) is the dimensionless time, and λ is the thermal conductivity being measured.

[0029] Step 5: After the time test is completed, turn off the refrigerator, turn on the heater, and introduce nitrogen gas through the air inlet pipe to reheat the unit.

[0030] In one implementation, the relationship between temperature rise and dimensionless time is modified, and the modified relationship is as follows: Where, ΔT c The constant for temperature rise caused by the contact thermal resistance between the planar heat source and the test sample.

[0031] In one embodiment, the corrected power P0 caused by the heat absorption of the planar heat source is expressed as: P0 = P(t) - CΔT(t) / t, where the compensation caused by the change in the resistance of the planar heat source is: P(t) = I 2 R(t), the temperature rise model of the planar heat source is obtained as follows:

[0032] Beneficial Effects: The low-temperature thermal conductivity unsteady-state testing system and method of this application places the test sample in a specially structured container consisting of a vacuum chamber, a primary cold shield, a secondary cold shield, and a sample testing assembly. A refrigerator is used to obtain the low temperature. The temperature of the sample testing chamber is coupled and controlled through the refrigerator's cold head and the sample chamber heater, in conjunction with the cooling provided by the refrigerator. The thermal conductivity of the material at low temperatures is measured using the transient plane heat source method. Finally, based on the transient temperature distribution gradient data, the thermal conductivity of the material in different temperature zones is calculated. This low-temperature thermal conductivity unsteady-state testing system and method enables rapid measurement of low-temperature thermal conductivity, with no temperature difference on the test sample surface, significantly reducing radiative heat leakage. Furthermore, based on this transient measurement principle, a correction method for the test data is proposed, thereby improving the final test accuracy. Simultaneously, when performing transient thermal conductivity measurements using this application, the measurement time is from tens to hundreds of seconds, greatly improving measurement efficiency.

[0033] Furthermore, compared with the prior art, when the test sample needs to be replaced, this application only requires removing the test rod along with the test sample from the container for replacement, without disassembling the entire system. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 This is a three-dimensional schematic diagram of the unsteady-state test system for low-temperature thermal conductivity in an embodiment of this application;

[0036] Figure 2 The attached figures are from existing patent CN115266814A;

[0037] Figure 3 This is a three-dimensional schematic diagram of the unsteady-state test system for low-temperature thermal conductivity of the hidden vacuum cover in an embodiment of this application;

[0038] Figure 4 This is a three-dimensional schematic diagram of the low-temperature thermal conductivity unsteady-state test system with a hidden vacuum chamber and a primary cold shield in an embodiment of this application.

[0039] Figure 5 This is a schematic diagram of the sample holder in an embodiment of this application.

[0040] Reference numerals: 1. Refrigeration unit; 2. Vacuum hood; 3. Vacuum hood flange cover; 4. Vacuum hood flange; 5. Primary cold shield; 6. Primary cold shield flange cover; 7. Secondary cold shield; 8. Secondary cold shield flange cover; 9. Heater; 10. Test chamber neck tube; 11. Sample test chamber; 12. Sample test rod; 13. Sample holder; 14. Test rod flange; 15. Test sample; 16. Inlet neck tube; 17. Inlet pipe; 18. Vibration damping bellows; 19. Sample chamber heater; 20. Outlet port. Detailed Implementation

[0041] The technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0042] In the description of this disclosure, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," "outer," "top," and "bottom," etc., indicating the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, are only for the convenience of describing this disclosure and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this disclosure. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. It is understood that, unless otherwise specified, the fixing method described below can be a screw connection commonly used in the prior art.

[0043] In this document, the term "comprising" is intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0044] Please see Figure 1 and Figure 3-5

[0045] This embodiment discloses a low-temperature thermal conductivity unsteady-state testing system in the first aspect, including a refrigerator 1, a vacuum chamber 2, a vacuum chamber flange cover 3, a primary cold shield 5, a secondary cold shield 7, a sample testing component, an air inlet component, and an air outlet component.

[0046] The refrigerator 1 is fixed to the vacuum chamber flange cover 3. Specifically, the refrigerator 1 is a GM refrigerator with a primary cold head of 35W@50K and a secondary cold head of 1.5W@4.2K. Other specifications can also be selected. Its main function is to achieve low temperature acquisition and maintenance. A vibration damping bellows 18 is installed between the upper part of the refrigerator 1 and the top side of the vacuum chamber flange cover 3. The vibration damping bellows 18 is made of stainless steel, and its main function is to reduce the transmission of vibration from the GM refrigerator to the sample testing chamber 11 (described later), achieving vibration control within ±10μm.

[0047] The top side of the vacuum chamber 2 is fixed to the vacuum chamber flange cover plate 3 via the vacuum chamber flange 4. The vacuum chamber 2 is made of stainless steel and its function is to maintain the vacuum level of the cryogenic thermostat in the low-temperature thermal conductivity measurement system, reducing gas heat leakage. Its vacuum level is 10. -2 ~10 -3 Pa.

[0048] The top side of the primary cold shield 5 is fixed to the refrigerator 1 via the primary cold shield flange cover 6. The primary cold shield 5 is installed inside the vacuum chamber 2, and the primary cold shield 5 is in contact with the primary cold head of the refrigerator 1. The primary cold shield 5 is made of oxygen-free copper. It is in contact with the refrigerator 1 and, as described below, in contact with the sample testing chamber 11 for heat conduction, thereby achieving the transfer of cold energy.

[0049] The top side of the secondary cold shield 7 is fixed to the refrigerator 1 via the secondary cold shield flange cover plate 8. The secondary cold shield 7 is installed inside the primary cold shield 5, and the secondary cold shield 7 is in contact with the secondary cold head of the refrigerator 1. The secondary cold shield 7 is made of oxygen-free copper. It is in contact with the refrigerator 1 and, as described below, in contact with the sample testing chamber 11 for heat conduction, thereby achieving the transfer of cold energy.

[0050] The secondary cold head of the refrigeration unit 1 is connected to the heater 9. The heater 9 can control the temperature of the cold head by working together with the refrigeration unit 1.

[0051] The sample testing assembly includes a test chamber neck 10, a sample test chamber 11, a sample test rod 12, a sample holder 13, a temperature sensor, and a sample chamber heater 19. The test chamber neck 10 is fitted over the upper part of the sample test rod 12 and is fixed to the top of the vacuum chamber flange cover 3. A test chamber upper flange is installed on the top side of the test chamber neck 10. The middle part of the sample test chamber 11 is fixed to the primary cold shield flange cover 6 and the secondary cold shield flange cover 8 via positioning flanges, and the bottom of the sample test chamber 11 extends into the secondary cold shield 7. The top of the sample test rod 12 is fixed to the test rod flange 14, which is fixed to the test chamber upper flange. The bottom of the sample test rod 12 extends between the secondary cold shield flange cover 8 and the corresponding positioning flange. The sample holder 13 is installed at the bottom of the sample test rod 12, and the test sample 15 is installed on the sample holder 13. A temperature sensor is installed inside the sample holder 13 to monitor the temperature around the test sample 15. The sample chamber heater 19 is installed at the bottom of the sample test rod 12 and is used to control the temperature around the test sample 15. The sample test chamber 11 is filled with low-temperature helium gas to maintain the low-temperature environment required for the experiment, and there is a certain temperature gradient between its upper and lower parts during the experiment.

[0052] The air intake assembly includes an air intake neck 16 and an air intake pipe 17. The air intake neck 16 is fitted onto the upper part of the air intake pipe 17, and the bottom of the air intake neck 16 is fixed to the top side of the vacuum hood flange cover 3. The top of the air intake pipe 17 extends to the outside of the top side of the air intake neck 16, and the bottom of the air intake pipe 17 extends into the secondary cold shield 7 and communicates with the sample testing chamber 11 for introducing low-pressure helium and rewarming gas into the sample testing chamber 11. Helium may be used as the rewarming gas.

[0053] The exhaust assembly includes an exhaust port 20, which is installed on the top side of the vacuum chamber flange cover 3 and communicates with the interior of the vacuum chamber 2. The exhaust port 20 is connected to corresponding devices such as vacuum pumps and exhaust pumps to perform vacuuming and exhausting functions. Specifically, it is used as an exhaust port before the experiment begins and as an exhaust port for reheating after the experiment.

[0054] Based on the above, the cooling source is the refrigerator 1. Through heat conduction, the cooling energy is transferred to the test sample 15 via the secondary cold shield 7, the sample testing chamber 11, and helium gas, thereby controlling the ambient temperature around the test sample 15. The heater 9 at the secondary cold shield 7 serves to coarsely adjust the temperature control. The cooling energy, controlled jointly by the secondary cold shield 7 and heater 9 of the refrigerator 1, is transferred to the helium atmosphere in the sample testing chamber 11. Then, it is electrically heated by the sample chamber heater 19 at the bottom of the sample testing rod 12, thus performing a secondary fine-tuning of the temperature control. Finally, the temperature at the test sample 15 is controlled by the coupling of the cooling energy of the refrigerator 1 and the heat from the heater 9 and sample chamber heater 19, achieving a temperature control accuracy of ±10 mK.

[0055] In this embodiment, the sample holder 13 includes a lifting component 1301, an upper clamping plate 1302, a lower clamping plate 1303, and connecting bolts 1304. The lifting component 1301 can be a metal rod structure, and there can be three or more of them. The two ends of the lifting component 1301 are fixed to the top of the upper clamping plate 1302 and the bottom of the sample testing rod 12, respectively. The upper clamping plate 1302 and the lower clamping plate 1303 are spaced apart, and the test sample 15 is placed between the upper clamping plate 1302 and the lower clamping plate 1303, and the upper clamping plate 1302 and the lower clamping plate 1303 are fixed together by the connecting bolts 1304. The connecting bolts 1304 can be epoxy resin bolts, and there can be two or more of them.

[0056] In this embodiment, a transient measurement method is used for testing. Therefore, two test samples 15 are disposed between the upper clamping plate 1302 and the lower clamping plate 1303, and a planar heat source is clamped between the two test samples 15. The planar heat source is connected to the sample chamber heater 19. The planar heat source can be a hot wire, which is equipped with a heating test probe. A temperature sensor is mounted on the upper clamping plate 1302 and the lower clamping plate 1303.

[0057] Based on the above configuration, the temperature rise model of the planar heat source is as follows: in, For the temperature rise result, ΔT c λ is the temperature rise constant caused by the contact thermal resistance between the planar heat source and the test sample 15, λ is the measured thermal conductivity, D(τ) is the dimensionless time, and r is the temperature coefficient of resistance.

[0058] In a second aspect, this embodiment also discloses a method for testing the unsteady-state thermal conductivity at low temperatures, applicable to the aforementioned unsteady-state thermal conductivity testing system. Specifically, the method includes the following steps:

[0059] Step 1: Vacuum the vacuum chamber 2 through the outlet port 20 and the vacuum machine, then fill the sample test chamber 11 with low-pressure helium through the inlet pipe 17 and the helium delivery machine, then evacuate the vacuum through the outlet port 20 again, and repeat the process 2 to 3 times.

[0060] Step 2: Evacuate vacuum chamber 2 to achieve a pressure of 10⁻² to 10⁻³ Pa inside vacuum chamber 2;

[0061] Step 3: Turn on the refrigerator 1 for pre-cooling. After pre-cooling is completed, the temperature around the test sample 15 is controlled by the cold head, heater 9, and sample chamber heater 19 of the refrigerator 1.

[0062] Step 4: Based on the temperature sensor's detection results, after the ambient temperature around the test sample 15 reaches the test temperature, the transient thermal conductivity test begins. The relationship between the temperature rise and dimensionless time is: ΔT s (τ)=P0(π 3 / 2 rλ) -1 D(τ), where ΔT s (τ) is the temperature rise, D(τ) is the dimensionless time, and λ is the thermal conductivity being measured.

[0063] Step 5: After the time test is completed, turn off the refrigerator 1, turn on the heater 9, and introduce nitrogen gas through the air inlet pipe 17 for rewarming.

[0064] To improve the accuracy of the measurement results, the following modifications are made in this embodiment. Specifically, the relationship between temperature rise and dimensionless time is modified, and the modified relationship is as follows: Where, ΔT c Let P be the temperature rise constant caused by the contact thermal resistance between the planar heat source and the test sample 15. The corrected relationship for the power P0 caused by the heat absorption of the planar heat source is: P0 = P(t) - CΔT(t) / t, where the compensation caused by the change in the resistance of the planar heat source is: P(t) = I 2 R(t).

[0065] Based on the above corrections, the temperature rise model for the planar heat source is obtained as follows:

[0066] Finally, it should be noted that the above description is only a preferred embodiment of this application and is not intended to limit this application. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A low temperature thermal conductivity unsteady state testing system, characterized in that, Includes a refrigeration unit (1), a vacuum chamber (2), a vacuum chamber flange cover (3), a primary cold shield (5), a secondary cold shield (7), a sample testing assembly, an inlet assembly, and an outlet assembly; The refrigeration unit (1) is fixed to the vacuum hood flange cover plate (3); The top side of the vacuum hood (2) is fixed to the vacuum hood flange cover plate (3) via the vacuum hood flange (4); The top side of the primary cold shield (5) is fixed to the refrigerator (1) through the primary cold shield flange cover plate (6). The primary cold shield (5) is installed inside the vacuum hood (2), and the primary cold shield (5) is in contact with the primary cold head of the refrigerator (1). The top side of the secondary cold shield (7) is fixed to the refrigeration unit (1) through the secondary cold shield flange cover plate (8). The secondary cold shield (7) is installed inside the primary cold shield (5), and the secondary cold shield (7) is in contact with the secondary cold head of the refrigeration unit (1). The secondary cold head of the refrigeration unit (1) is connected to the heater (9); The sample testing assembly includes a test chamber neck tube (10), a sample test chamber (11), a sample test rod (12), a sample holder (13), a temperature sensor, and a sample chamber heater (19); the test chamber neck tube (10) is sleeved on the upper part of the sample test rod (12), and the test chamber neck tube (10) is fixed to the top of the vacuum hood flange cover plate (3), and a test chamber upper flange is installed on the top side of the test chamber neck tube (10); the middle part of the sample test chamber (11) is fixed to the primary cold screen flange cover plate (6) and the secondary cold screen flange cover plate (8) respectively through positioning flanges, and the bottom of the sample test chamber (11) extends into the secondary cold screen (7); The top of the sample test rod (12) is fixed to the test rod flange (14), the test rod flange (14) is fixed to the upper flange of the test chamber, and the bottom of the sample test rod (12) extends to the space between the secondary cold shield flange cover plate (8) and the corresponding positioning flange; the sample holder (13) is installed at the bottom of the sample test rod (12), and the test sample (15) is installed on the sample holder (13); the temperature sensor is installed inside the sample holder (13) to monitor the temperature around the test sample (15); the sample chamber heater (19) is installed at the bottom of the sample test rod (12) to control the temperature around the test sample (15); The air intake assembly includes an air intake neck (16) and an air intake pipe (17); the air intake neck (16) is sleeved on the upper part of the air intake pipe (17), and the bottom of the air intake neck (16) is fixed to the top side of the vacuum hood flange cover (3); the top of the air intake pipe (17) extends to the outside of the top side of the air intake neck (16), and the bottom of the air intake pipe (17) extends into the secondary cold screen (7) and communicates with the sample testing chamber (11) for introducing low-pressure helium and rewarmed gas into the sample testing chamber (11); The air outlet assembly includes an air outlet (20), which is installed on the top side of the vacuum hood flange cover (3) and communicates with the interior of the vacuum hood (2).

2. The low-temperature thermal conductivity unsteady-state testing system according to claim 1, characterized in that, A shock-absorbing bellows (18) is provided between the upper part of the refrigeration unit (1) and the top side of the vacuum cover flange (3).

3. The transient test system for thermal conductivity of cryogens as recited in claim 1, wherein The sample holder (13) includes a lifting component (1301), an upper clamping plate (1302), a lower clamping plate (1303), and connecting bolts (1304); The two ends of the lifting component (1301) are respectively fixed to the top of the upper clamping plate (1302) and the bottom of the sample testing rod (12); The upper clamping plate (1302) and the lower clamping plate (1303) are spaced apart, the test sample (15) is placed between the upper clamping plate (1302) and the lower clamping plate (1303), and the upper clamping plate (1302) and the lower clamping plate (1303) are fixed together by the connecting bolt (1304).

4. The transient test system for thermal conductivity of cryogens as recited in claim 3, wherein, Two test samples (15) are disposed between the upper clamping plate (1302) and the lower clamping plate (1303), and a planar heat source is clamped between the two test samples (15). The planar heat source is connected to the sample chamber heater (19).

5. The transient test system for thermal conductivity of cryogens as recited in claim 3, wherein, The temperature sensor is mounted on the upper clamping plate (1302) and the lower clamping plate (1303).

6. The low temperature thermal conductivity unsteady state test system of claim 4, wherein, The temperature rise model of the planar heat source is as follows: in, For the temperature rise result, ΔT c λ is the constant of temperature rise caused by the contact thermal resistance between the planar heat source and the test sample (15), λ is the measured thermal conductivity, D(τ) is the dimensionless time, and r is the temperature coefficient of resistance.

7. The low temperature thermal conductivity unsteady state test system of claim 1, wherein, The rewarming gas is helium.

8. A method for testing the unsteady thermal conductivity of cryogenic materials, comprising the system for testing the unsteady thermal conductivity of cryogenic materials according to any one of claims 1 to 7, characterized in that, The method includes the following steps: Step 1: Vacuum the vacuum chamber (2) through the outlet port (20) and the vacuum machine, then fill the sample test chamber (11) with low-pressure helium through the inlet pipe (17) and the helium delivery machine, and then evacuate through the outlet port (20) again, and repeat 2 to 3 times. Step 2: Evacuate the vacuum chamber (2) until the internal pressure reaches 10. -2 ~10 -3 Pa; Step 3: Turn on the refrigerator (1) to pre-cool down. After the pre-cooling is completed, the temperature around the test sample (15) is controlled by the cold head, heater (9) and sample chamber heater (19) of the refrigerator (1). Step 4: Based on the temperature sensor's detection results, after the temperature around the test sample (15) reaches the test temperature, the transient thermal conductivity test begins. The relationship between the temperature rise and the dimensionless time is: ΔT s (τ)=P0(π 3 / 2 rλ) -1 D(τ), where ΔT s (τ) is the temperature rise, D(τ) is the dimensionless time, and λ is the thermal conductivity being measured. Step 5: After the time test is completed, turn off the refrigerator (1), turn on the heater (9), and introduce nitrogen gas through the air inlet pipe (17) for rewarming.

9. The transient test method for thermal conductivity of cryogens as recited in claim 8, wherein, The relationship between temperature rise and dimensionless time is modified, and the modified relationship is: where ΔT c is a temperature rise constant caused by the contact thermal resistance between the planar heat source and the test sample (15).

10. The transient test method for thermal conductivity of cryogens as recited in claim 9, wherein, The corrected relationship for the power P0 caused by the heat absorption of the planar heat source is: P0 = P(t) - CΔT(t) / t, where the compensation caused by the change in the resistance of the planar heat source is: P(t) = I 2 R(t), the temperature rise model of the planar heat source is obtained as follows:

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