# Why is my small Bi-2223 superconductor not levitating or locking above magnets?

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• Goliatbagge
Goliatbagge
I am a supervisor for a student's high school project. He has acquired a small superconductor of Bi-2223, specifically this one: https://shop.can-superconductors.com/hts-demo-parts/11-superconducting-bi-2223-bar

The superconductor is just a couple of centimeters long and a few millimeters wide and seems to have a critical temperature of -165 °C. However, when he cools it down with liquid nitrogen, he still can't get it to levitate or lock above magnets.

We have tried different variations and with different types of magnets ("ordinary" bar magnets and neodymium magnets). I have no prior experience with this. What could we be doing wrong?

(At first, I thought it got too warm when we lifted it out of the liquid nitrogen to place on the magnets, but then I tried letting it lie in an aluminum mold with liquid nitrogen and instead approached it with a magnet to achieve the locking effect, but that also did not succeed.)

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Goliatbagge said:
am a supervisor for a student's high school project. He has acquired a small superconductor of Bi-2223, specifically this one: https://shop.can-superconductors.com/hts-demo-parts/11-superconducting-bi-2223-bar

The superconductor is just a couple of centimeters long and a few millimeters wide and seems to have a critical temperature of -165 °C. However, when he cools it down with liquid nitrogen, he still can't get it to levitate or lock above magnets.
It looks like the ad says 77K, which is more like -195C:

But if you have access to liquid nitrogen, it seems like that should be cold enough. Can you check the resistance instead?

berkeman said:
Can you check the resistance instead?
Given a resistivity at room temperature, of 1 milliOhm.cm, the resistance based on the dimensions given will be about 30 milliohm.

Notice that the device has two silver contacts at each end.
Start by soldering it to four twisted insulated wires about 1 metre long.
Connect the two inner terminals to a millivolt meter.
Connect the two outer terminals to a 1 amp current source.

The current source can be as simple as a 12V car battery, with a 10 watt filament lamp to limit the current.

When you lower the device on the wires into LN2, you should see a gradual reduction in the voltmeter reading from about 30 mV, followed by a sudden drop to zero volts, if or when it becomes a superconductor.

If the device was a pure metal, then before it becomes superconducting, the resistance would be proportional to absolute temperature, but being an alloy, we cannot be sure of the bumps that may be present in the relationship.

Notice that the voltmeter will very quickly reach the LN2 temperature, a technique that I have used with a wire-wound resistor, to find the level of LN2 in a Dewar.

berkeman
Can we back up a bit? Levitation is not absolute - a superconductor doesn't go flying off into space as it excludes the earth's magnetic field. You have an upward force, and it needs to exceed the gravitational pull for an object to levitate. That depends on the magnetic field and its gradient.

It may be easier to do this the other way - levitate a small (small!) neodymium magnet over a plane of superconductor.

Mayhem
Maybe the metal is from a bad batch, and is impure. Try returning the bismuth superconducting alloy, and buy from a different LOT or batch number. :)

Can we back up a bit? Levitation is not absolute - a superconductor doesn't go flying off into space as it excludes the earth's magnetic field. You have an upward force, and it needs to exceed the gravitational pull for an object to levitate. That depends on the magnetic field and its gradient.

It may be easier to do this the other way - levitate a small (small!) neodymium magnet over a plane of superconductor.

I agree, you need a fairly strong magnet so a small neodymium magnet would be ideal.
Also, do note that the order of operation does matter; if you cool the thing down with the magnet already in place it should indeed "lock" but the effect might be difficult to detect unless the magnet is strong. If you instead FIRST cool down the SC and then move the magnet closes your should feel some resistance dues to the meissner effect. If you can' detect that, then there is something wrong.
Baluncore said:
If the device was a pure metal, then before it becomes superconducting, the resistance would be proportional to absolute temperature, but being an alloy, we cannot be sure of the bumps that may be present in the relationship.

it is not an alloy; it is an perovskite (think of it as a metal-oxide) meaning in the normal state the resistance can be very high (depending on the exact stoichiometry) . It should indeed be possible to measure the resistive transition; but be very, very careful about applying high currents. If you have 1 amp going through a SC that suddenly goes normal and becomes highly resistive things can get a bit iffy.

f95toli said:
If you have 1 amp going through a SC that suddenly goes normal and becomes highly resistive things can get a bit iffy.
Read the data sheet, it says 1 amp is OK.
Room temp resistance is very low at about 30 mΩ. You might check that.
The device was made with four terminals for measuring changes of resistance, in the presence of magnetic fields.

No, it says "recommended measuring current up to 60 mA".
1 amp (in fact many amps) will be fine as long as it is superconducting state; but I would suggest avoiding using as current that high in the normal state.
I would certainly not use a current that high to measure the resistive transition.

If you exceed the critical current density jc, the superconductor will go normal. If you have a non-uniform superconductor, like many HTSCs, as soon as one spot goes normal (and one spot is always he worst) the supercurrent will flow around the bad spot, pushing the boundary region normal, making the spot bigger, and diverting more current, and so on and so on.

This is best avoided.

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