EXPERIMENT #3

Determination of Critical Current

There is a certain maximum current that superconducting materials can be made to carry, above which they stop being superconductors. If too much current is pushed through a superconductor, it will revert to the resistive mixed state even though it is below its transition temperature. The value of Critical Current Density (J) is a function of temperature, the colder you keep the superconductor the more current it can carry.

Objective:

1. To determine the critical current of a YBCO superconductor.

Materials:

• YBCO superconductor with four point probe and thermocouple

• millivolt meter

• 10 watt power supply capable of a 10 ampere output (for use with a disk superconductor) or a 1 watt supply capable of a 1 ampere output (for use with a superconductor coil).
• ammeter

• liquid nitrogen

Procedure:

1. Attach current leads, (C,C) to the power supply (Figure 17).
2. Attach millivolt meter to leads V and V.
3. Attach thermocouple to millivolt meter and calibrate.
4. Carefully immerse the superconductor into liquid nitrogen.
5. When the boiling stops carefully adjust the current to 0.1 ampere.
6. Record voltage and current.
7. Keep the superconductor in the liquid nitrogen at all times during this experiment to prevent it from heating up.
8. Increase the current in 0.5 ampere intervals (for disk) or .05 ampere intervals (for coil) and record the voltage drop and current for each trial. Repeat this step until the sample heating produces noticeable liquid boiling.
9. Do not remove the superconductor from the liquid nitrogen until you have turned off the power supply.

• Make a graph plotting the voltage drop on the Y axis and the current on the X axis.

• Determine the critical current for your superconductor at 77 K

• Calculate the critical current density, (A/mm), for your superconductor

CAUTION !!! Excessive electrical heating can damage samples even while they are immersed in liquid nitrogen. Teacher must try this experiment and set a clear limit to electrical current students can use.

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Date posted 04/01/96 (ktb)