Superconductivity

Certain materials, known as superconductors (hereafter SC's), when cooled below a certain critical temperature (Tc), exhibit extraordinary properties including resistance-less electrical conductivity and reflection of magnetic fields. Unfortunately, known SC's must be cooled far below room temperature. Usually liquid nitrogen is used for the cooling. If room-temperature SC's could be created it would be a huge economic and energy boon.

In high school I made it all the way to the state science fair with a project that was, at heart, stupidly simple, but which used some uncommon materials and machinery.

For the project I asked, "How much longer can the life of a superconductor be extended by insulating it in a vacuum?" I got to play around with lots of fun equipment: a SC, a wicked powerful magnet, liquid nitrogen, and a bell jar and vacuum pump, but in essence, the vacuum pump and bell jar and a thermometer would have done the trick. All I was really testing was rate of heat transfer into a vacuum.

If I had not used an overly complicated setup, however, I would never have stumbled upon an interesting discovery. As I repeatedly cooled the SC beyond Tc (the point of superconductivity), let it warm, measured the amount of time it remained superconductive, then repeated, I found that the SC remained superconductive for shorter and shorter intervals.

However, warming the SC all the way back up to room temperature (instead of just above Tc), before cooling it again "reinvigorated" the superconductor. By reinvigorated, I mean that the SC retained its superconductivity longer when it was warmed up before cooling. Assuming that the SC warmed at the same rate each time after it was cooled, then the Tc, must have risen slightly, resulting in the prolonged superconductivity. This leads to much more interesting questions than my vacuum insulation question:

• Can a period of heating above room temperature before cooling raise the Tc? By how much?
• What is the optimal temperature to heat the SC to before cooling? How high can the Tc be raised?
• Does the rate of heating/cooling affect the Tc?
• Is the assumption that the Tc is being affected accurate or could the heat conductivity of the material be affected by heating?
• What is structurally occurring inside the material due to the heating and cooling?

If anyone has answers to any of these questions, please post them at the bottom of this page or email me.

I will dig up my old science fair report and attach it or copy it here as well.

These questions were re-raised in my mind when I read this fascinating New York Times article on the structure of glass and how much science does not understand about glass. I read: "The final structure of the glass also depends on how slowly it has been cooled." and thought immediately of superconductors. Surely there is a connection here.

Addendum

I've added some excerpts from my science fair report below. Forgive me if these are rough. I did this when I was much younger. The two images below show the decline in the superconductivity interval without reheating. I know there are very few datapoints, but I am convinced that the effect is real.

The BSCCO superconductor which was used has a critical temperature of 110 Kelvin (-163 degrees Celsius). Liquid nitrogen, with a temperature of 77 Kelvin, was used to produce these temperatures. The length of time that the superconductor retained its superconductive properties was measured by timing how long a magnet floated above it. This property is called the Meissner effect and will only occur when the superconductor is below its critical temperature.


Superconductors are materials that conduct electricity without any measurable loss of current. They were first discovered by Heike Kamerlingh Onnes in 1911. He found that mercury exhibited no loss of electrical current below 4.2 K.

The superconductor used in this experiment is Bismuth-based, its chemical formula is as follows: BiiCaSriCuiOv. Normal conductors, such as copper, lose some of their current as electrons collide with atoms. These collisions give off heat and sometimes light. The effects of these collisions are put to good daily use in toasters, electric ovens, and light bulbs. However, the loss of current that is welcomed in light bulbs is a nuisance in copper wire. This is where superconductors come in.

According to the theory of Cooper Pairs, superconductors do not lose any current because the electrons in superconductors form pairs (Cooper Pairs) and move together through the material without colliding. Superconductors, however, are impractical because they must be kept at very low temperatures or they will lose their superconductive properties. A vacuum chamber could be used to insulate a superconductor, slowing the transfer of heat and thus allowing the superconductor to retain its properties longer than would be normally possible, such as when exposed to air.

The vacuum pump forced much of the air out of the bell jar. The bell jar is not completely airtight but a layer of Vaseline around the rim helps to create a controlled environment. A piston pulls air out of the bell jar through the hole in the middle of the plate on the vacuum pump. The air pressure is now greatly lowered inside the bell jar. The air pressure outside is pushing in on the bell jar with 101 kPa (14.7 lbs) of pressure per square inch. Because the small amount of air inside the bell jar is pushing out with considerably less force, it is difficult and almost impossible to move the bell jar.

Liquid nitrogen is excellent to use for cooling the superconductor because of its low boiling point (77 K). Also, since it makes up 78% of the air we breathe, it is very cheap and easy to obtain.

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