Characteristics And Mechanism Analysis Of Heat Transfer Across Contact Surfaces At Cryogenic Temperatures Under Vacuum

The advantages of the high-temperature superconducting technology which cool thesuperconductor by refrigerator directly consist of reducing the size of equipment; saving therefrigerant; reducing the cost of cooling the superconductor. The Thermal ContactResistance （TCR） influences significantly the heat transport at the contact surfaces in highheat flux and cryogenic fields. Therefore reducing the TCR plays a critical role during thedesign or optimization of the equipment. Moreover the influence factors of the TCR aremultitudinous, such as temperature, contact pressure, roughness and the variation of thethermal parameters with the temperature, so the measurement of TCR is important. Thevalue of the TCR will change when the type of the specimen or the testing environmentchange. So the measured value is invalid. Above all the data of the TCR are inadequate,especially at cryogenic applications. This paper developed an experimental setup accordingto the principle of Laser Photothermal Method （LPM）. This method can complete themeasurement during several seconds. Oxygen-Free Copper （OFC）, Aluminum Nitride （AlN）ceramic and Stainless Steel304（SS304） were employed. In the temperature range of70K-300K the TCR between above materials were measured using the LPM. And using theinfluence of temperature on the TCR was analyzed. At last the relationship between theTCR between smooth processing surfaces and the thermal conductivity of contactingmaterials was analyzed.The main work and conclusion are as followings:LPM is a transient method and can complete the measurement during a very short period,so the measurement efficiency has been greatly improved. The principle of LPM was firstlygiven when the TCR and thermal diffusivity are measured. Then the experimental systemwas set up and the experimental apparatus was introduced. Then the heat leak of theexperimental setup was analyzed. This paper emphasized that the choice of the heatinglaser modulation frequency would influence the measurement accuracy. At last thethickness and machining method of the specimen were introduced. In order to improve themeasurement accuracy, a carbon film was placed on the detection surface and a gold filmwas placed on the heating surface. The value of the optimal modulation frequency for different materials were given, whenthe thermal diffusivity or TCR was measured using the LPM. Then the thermal diffusivitiesof OFC, AlN ceramic and SS304were measured in the temperature range of30K-300K.This paper firstly measured the thermal diffusivity of OFC and the thermal diffusivitydecreased with the increasing of the temperature and at lower temperature the thermaldiffusivity changed severely. The average relative error between experimental data and thereference values is6.7%. The thermal diffusivity of AlN ceramic increased with theincreasing of the temperature from30K to70K, and then decreased with the increasing ofthe temperature. The peak value is0.00135m²·s^-1and the temperature was70K. Thethermal diffusivity of SS304decreased with the increasing of temperature which wassimilar with that of OFC. Finally, the variation of the thermal diffusivity with thetemperature was analyzed according to the definition of the thermal diffusivity.The TCR of OFC-OFC, AlN-AlN ceramic and SS304-SS304was measured using LPMfrom70K to300K and at the contact pressure of0.20MPa-0.75MPa. The TCR of OFCincreased with the increasing of the temperature. The TCR of AlN ceramic firstlydecreased and then increased with the increasing of the temperature. The TCR of SS304decreased with the increasing of the temperature. The TCR OFC-OFC, AlN-AlN ceramicand SS304-SS304decreased linearly with the increasing of the contact pressure. In addition,the TCR increased with the increasing of surface roughness. The results showed that thechange of the temperature can influence the thermal parameters of the contact materials andcause the real contact area to change.At a certain contact pressure, the TCR of OFC-SS304, SS304-ALN ceramic decreasedwith the increasing of the temperature. The TCR of OFC-AlN ceramic firstly decreased andthen increased with the increasing of the temperature and lower the temperature was, moreobvious the impact of the contact pressure on the TCR. At the same temperature the TCRdecreased linearly with the increasing of the contact pressure. At different contact pressurethe relationship between the TCR and the temperature was simulated. And the formulascould well describe the relationship between the temperature and the TCR.For the smoothing surface, when the intrinsic thermal conductivity is high, the TCR ofthe same contact materials is large, such that the TCR of OFC was the smallest, and theTCR of SS304was the largest. When the contact materials were different, the change of the TCR was dependent on the material of which the thermal conductivity was smaller. Forexample the TCR of SS304-OFC was smaller than that of SS304-AlN ceramic andmeanwhile the TCR of SS304-SS304was smallest.At last, the influence of intrinsic thermal conductivity on the TCR was analyzed. At acertain contact pressure, the TCR decreased with the increasing of the intrinsic thermalconductivity. When the temperature rose from85K to280K, the TCR of OFC-OFCpresented an exponential function of intrinsic thermal conductivity. When the temperaturerose from75K to275K, the TCR of SS304-SS304was a linear relationship with intrinsicthermal conductivity. When the temperature rose, the change of the TCR between AlNceramic with intrinsic thermal conductivity appeared “return phenomenon”. The TCR ofOFC-SS304and harmonic mean thermal conductivity was a exponentiation relationship.The TCR of OFC-AlN ceramic was an exponential function of harmonic mean thermalconductivity.