Below 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 Abstraact
The 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.
