High-temperature superconducting (HTS) power equipment and tests at present require operating temperatures higher than the liquid nitrogen (LN2). LN2 is an important cryogenic refrigerant and liquid insulation material, which is widely used in HTS equipment. Although pressurized LN2 can achieve temperatures higher than 77 K, it requires the use of sealed pressure vessels. Thus, a liquid cryogen that can offer temperatures higher than LN2 at atmospheric pressure, excellent electrical insulation performance, and high thermal conductivity has to be explored. Tetrafluoromethane (CF4) is another cryogenic refrigerant and electrical insulation material. The mixture of LN2 and liquid CF4 has excellent thermal properties. When mixed in appropriate proportions, it has a melting point of 50 K and a boiling point of 145 K. The LN2/CF4 mixture is used in the insulating layer of the superconducting energy pipeline to provide a low-temperature environment (85–100 K) for the HTS dc cable. However, the insulation characteristics of the LN2/CF4 mixture at cryogenic temperatures have not been studied, quantitatively. In this article, a test platform is developed to measure the breakdown voltage of the liquid mixture. The dc breakdown voltages of LN2/CF4 liquid mixture under varying mixing proportions are obtained at cryogenic temperature. The insulation failure probability is estimated using a two-parameter Weibull statistical method. The results reveal that the breakdown strength of the LN2/CF4 mixture is lower than that of pure LN2. As the proportion of CF4 increases, the breakdown field strength of liquid mixture first decreases and then increases. When the mole fraction of CF4 is 60%, the average breakdown strength decreases to the lowest value (14.07 kV/mm), about half of that of pure LN2 (28.26 kV/mm). The results of this article can provide a reference for the applications of LN2/CF4 liquid mixture in HTS power equipment and a scheme to expand the working temperature range of superconducting applications.

中文翻译：

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LN2/CF4 混合物在低温下的电绝缘特性


目前高温超导（HTS）电力设备和测试要求的工作温度高于液氮（LN2）。 LN2是一种重要的低温制冷剂和液体绝热材料，广泛应用于高温超导设备中。虽然加压液氮可以达到高于 77 K 的温度，但它需要使用密封压力容器。因此，必须探索一种能够在大气压下提供高于液氮的温度、优异的电绝缘性能和高导热率的液态制冷剂。四氟甲烷 (CF4) 是另一种低温制冷剂和电绝缘材料。 LN2和液体CF4的混合物具有优异的热性能。按适当比例混合时，熔点为50 K，沸点为145 K。LN2/CF4混合物用于超导能源管道的绝缘层，提供低温环境（85-100 K） ) 对于 HTS 直流电缆。然而，LN2/CF4 混合物在低温下的绝缘特性尚未得到定量研究。在本文中，开发了一个测试平台来测量液体混合物的击穿电压。在低温下获得了不同混合比例的 LN2/CF4 液体混合物的直流击穿电压。使用二参数威布尔统计方法估计绝缘故障概率。结果表明，LN2/CF4 混合物的击穿强度低于纯 LN2。随着CF4比例的增加，液体混合物的击穿场强先减小后增大。当CF4摩尔分数为60%时，平均击穿强度降至最低值（14.07 kV/mm)，约为纯液氮 (28.26 kV/mm) 的一半。本文研究结果可为LN2/CF4液体混合物在高温超导电力设备中的应用提供参考，并为扩大超导应用的工作温度范围提供方案。