【作者】 毕冬梅；

【导师】 陈焕新；

【作者基本信息】 华中科技大学 ， 制冷及低温工程， 2013， 博士

【摘要】 在高温超导技术领域，利用GM制冷机的冷头直接冷却超导体，可以减小设备的体积、提高冷却效率，并节省大量冷媒，降低了实现低温环境的成本。由于在低温或高热流密度下，固体间的接触热阻会对界面热输运特性产生很大影响，因此降低接触热阻对提高冷却效率起着非常关键的作用。另外，在半导体发光管（LED）散热、航天器仪表热控制等导热作为热量传递主要方式的领域，接触热阻的存在对热传递过程影响很大，必须要加以考虑。影响界面接触热阻大小的因素较多，包括温度、加载压力、表面粗糙度及两侧接触材料的物性参数随温度变化等，因此不同样品间的接触热阻一般靠实验进行测定，当样品的材料、测试环境变化时，之前得到的数据往往不能直接加以利用，所以造成目前许多材料间的接触热阻数据非常欠缺，特别是在低温下的数据。为此，根据激光光热法（Laser Photothermal Method，LPM）测量原理，设计了一套样品间接触热阻快速测量装置，当测量温度环境达到后，就可以在几秒内完成测量过程。选用无氧铜（Oxygen-Free Copper, OFC）、AlN （Aluminum Nitride）陶瓷及SS304（Stainless Steel304）作为测试样品，在70K-300K温度区间内，在0.2MPa-0.75MPa下，测量了样品间的接触热阻，并对接触热阻随温度变化趋势进行了详细分析。最后，对光滑处理界面下样品间的接触热阻与两侧样品的热导率关系进行了分析。论文主要工作和结论如下：1) LPM方法是一种非稳态测试方法，可以在较短的时间内完成整个测量过程，大大提高了测量效率。首先详细介绍了LPM方法测量样品间接触热阻和样品热扩散系数的原理，并设计了一套LPM测量实验平台，对测量系统的实验装置进行了介绍，并对系统漏热进行了分析计算。然后强调了加热激光调制频率的选择对测量精度的影响，并介绍了样品的厚度选择及机械加工方法，为得到可靠的测量信号，在样品的加热面和探测面分别进行镀碳膜和镀金膜处理。2)给出了当利用LPM方法测量样品间接触热阻和样品热扩散系数时，针对不同样品选用的调制频率数值。进而测量了OFC、AlN陶瓷和SS304在30K-300K温度区间内的热扩散系数值，发现在整个测量温度区间内，OFC的热扩散系数随温度升高而不断减小，并且温度越低热扩散系数的变化速率越大。测量结果与已报道的数据非常接近，两者间平均相对误差为6.7%，说明LPM测量方法比较可靠。AlN陶瓷的热扩散系数先随温度升高而增大，并在约70K时出现最大值(0.00135m²·s^-1)，而后随温度升高而减小。SS304的热扩散系数也随温度升高而减小。从热扩散系数的定义式出发，分别解释了这三种样品的热扩散系数出现上述变化趋势的原因。3)在70K-300K温度区间内，在0.20MPa-0.75MPa下，利用LPM方法分别测量了OFC-OFC、AlN-AlN陶瓷和SS304-SS304间的接触热阻。其中OFC-OFC间的接触热阻随温度升高而不断增大；AlN-AlN陶瓷间的接触热阻先随温度升高而不断减小，而后再随温度的升高而不断增大；SS304-SS304间的接触热阻随温度升高而一直减小。上述三种同种材料间的接触热阻都会随加载压力的不断增大而减小，在实验条件下两者间近似于直线关系。样品表面粗糙度大小会明显改变接触热阻值，并且温度的改变不仅会影响两侧材料的物性参数，还会影响界面的实际接触面积大小。4)在相同加载压力下，对于不同材料样品间的接触热阻，其中SS304-OFC和SS304-AlN陶瓷间的接触热阻都是随温度升高而不断减小；OFC-AlN陶瓷间的接触热阻先随着温度升高而减小，而后再随温度升高而增大，并且在低温下的变化速率较大。在相同测量温度下，加载压力越大，上述三种组合间的接触热阻都会减小，并且两者间近似于直线关系。对不同加载压力下，样品间的接触热阻与温度变化间的关系进行拟合，可以较好地描述两者间的关系。5)在光滑处理表面相同加载压力下，同种材料相接触时，材料固有的导热能力越强，那么样品间的接触热阻也越小，如OFC-OFC间的接触热阻<AlN-AlN陶瓷间的接触热阻<SS304-SS304间的接触热阻。而不同材料相接触时，接触热阻变化主要依赖于导热能力较差的样品，并与其热导率的变化趋势相反，如在相同温度和加载压力下，SS304-OFC间的接触热阻<SS304-AlN陶瓷间的接触热阻<SS304-SS304间的接触热阻。利用ANSYS模拟了在稳态传热过程中，接触热阻值的变化对界面温差的影响。6)最后，分析了样品间的接触热阻与其固有热导率间的关系。对于同种材料件的接触热阻而言，在一定加载压力下，材料的固有热导率越大，则接触热阻值越小。在85K-280K的温度区间内，OFC-OFC间的接触热阻与其固有热导率之间呈指数关系变化；在75K-275K温度区间内，SS304-SS304间的接触热阻与其固有热导率间呈直线关系变化；而随着温度的升高，AlN-AlN陶瓷间的接触热阻与其固有热导率间的变化存在“回转现象”。OFC-SS304间接触热阻与其调和平均热导率之间呈乘幂关系；OFC-AlN陶瓷间的接触热阻与其调和平均热导率间呈指数函数。更多还原

【Abstract】 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.更多还原