EAGER: Is Na-NH3 a High Temperature Superconductor?

【非技术内容】在特定温度以下，电流可以在某些材料中以零电阻流动，因此无需任何能量输入。这些材料被称为超导体，目前正在使用，例如，为磁共振成像仪提供强大而稳定的磁场。为了使超导体在技术和减少能源消耗方面产生更大的影响，发现超导性的温度需要从目前被称为高温超导体的陶瓷材料中发现的-135摄氏度（或-211F）的记录显著提高。有趣的是，1946年和1973年进行的实验报告了在-93摄氏度（或-135华氏度）以下的氨钠速冻溶液中存在超导性的证据，比高温超导体高出40多摄氏度。然而，许多其他的实验小组都不能证实这个系统中超导性的存在。本项目在实验条件大大改善的情况下，系统地尝试澄清这一重要的未解决问题。参与该项目的研究生和博士后学者将以独特的探索模式进行研究，获得独特的体验。1946年Ogg关于超导性的报告是基于在玻璃毛细管中快速冷冻的钠-氨溶液样品的一小部分电阻从10,000下降到10欧姆的观察。从100个淬火冷却样品中有7个样品的小磁场观察中推断出的持续电流，当样品从永磁体中取出时也可以看到。在大多数样品中观察到这些特征的失败归因于样品在冷却过程中的开裂。虽然其他六个实验小组也曾试图复制Ogg的结果，但只有两个小组成功了。研究小组使用今天改进的实验条件来直接澄清这个重要的未解决的科学难题。Ogg解释说，他的样品中的超导性是快速冷却速度的结果，这样可混溶的钠-氨溶液在冻结时绕过液-液相分离区。课题组的钠氨溶液在高纯度手套箱中制备，而不是在实验室环境中，以消除任何污染。样品池由薄的不锈钢毛细管制成，将冷却速度加快到0.01 s，比Ogg的样品快两个数量级。在金属电池中，超导转变不能被破裂的固体样品所掩盖。在第二组实验中，研究小组将钠氨溶液渗透到内径为150nm的中空玻璃纤维和孔径为7nm的多孔Vycor玻璃筒中。这些宿主内部的液态金属和二元流体混合物没有表现出相分离的迹象。钠-氨溶液的相分离可能同样受到抑制，从而为无需淬火冷却的超导性提供了理想条件。

Non-technical AbstractBelow a specific temperature, electricity can flow in some materials with zero resistance and hence without any input of energy. These materials are known as superconductors and are currently in use, for example, to provide strong and stable magnetic fields for magnetic resonance imaging (MRI) machines. In order for superconductors to have an even larger impact in technology and in the reduction of energy consumption, the temperature where superconductivity is found needs to be raised significantly from the current record of -135C ( or -211F) found in some ceramic materials known as high temperature superconductors. Interestingly, experiments carried out in 1946 and 1973 reported evidence of superconductivity in rapidly frozen solutions of sodium in ammonia below -93C (or -135F), more than forty degrees Celsius higher than the high temperature superconductors. A number of other experimental groups however were not able to confirm the existence of superconductivity in this system. This project, with greatly improved experimental conditions is a systematic attempt in clarifying this important unresolved issue. Graduate students and post-doctoral scholars working on this project will receive unique experience in carrying research in an unique exploratory mode.Technical AbstraactThe 1946 report of superconductivity by Ogg was based on the observations of a dramatic drop in resistance from 10,000 down to 10 ohms in a fraction of the rapidly frozen samples of sodium-ammonia solution confined in glass capillary. Persistent current, deduced from the observation of a small magnetic field in 7 out of the 100 quenched cooled samples were also seen when the samples were removed from a permanent magnet. The failure of observing such signatures in the majority of the samples were attributed to the cracking the samples in the cooling process. While six other experimental groups had attempted to replicate the results of Ogg, only two were successful. The research team uses today's improved experimental conditions to directly clarify this important unresolved scientific puzzle. Ogg interpreted the superconductivity in his samples is the consequence of fast cooling rate so that the miscible sodium-ammonia solution bypasses the liquid-liquid phase separation region when it freezes. The sodium-ammonia solutions of the research team are prepared in high purity glove box instead of ambient laboratory environment to eliminate any contamination. Sample cells are made from thin stainless steel capillary to speed up the cooling rate to 0.01 s, ~two orders of magnitude faster than Ogg's samples. With a metallic cell, a superconducting transition cannot be masked by a cracked solid sample. In the second set of experiments, the research team infiltrates the sodium-ammonia solutions into hollow glass fibers of 150 nm inner diameter and into porous Vycor glass cylinders with pore diameter of 7 nm. Liquid metal and binary fluid mixtures inside these hosts show no evidence of phase separation. It is possible that phase separation of the sodium-ammonia solutions are similarly suppressed thus providing an ideal condition for the onset of superconductivity without quench-cooling.