Angela Adams and Kenneth Purcell Advisor: Dr. Doug Harper Solid State Laboratory

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Development of an Automated System to Measure Critical Temperature of High Temperature Superconductors. Angela Adams and Kenneth Purcell Advisor: Dr. Doug Harper Solid State Laboratory Western Kentucky University. Definitions.

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### Development of an Automated System to Measure Critical Temperature of High Temperature Superconductors

Solid State Laboratory

Western Kentucky University

Definitions
• Superconductor -- conducts electricity with no resistance below a certain temperature
• Critical Temperature (Tc)-- temperature at which resistivity goes to zero

Courtesy of ORNL

Why Investigate Superconductors?
• There is still no all encompassing theory to explain superconductor behavior.
• BCS Theory describes standard superconductors
• No Theory explains HTS
• New superconducting materials are still being found. (MgB2)
• The record critical temperature continues to increase with new discoveries. (current max = 138K)
Four-Point Measurement

Known current applied through two outermost wires

Voltage drop measured between two innermost wires

Resistance calculated using Ohm’s Law:

Making Resistance Measurements
Probe
• Sample is attached to bottom of probe
• Four wires are connected to the sample for the resistance measurement
• Temperature is measured by a semiconductor
Cryostat
• Probe placed in center of cryostat
• Surrounding reservoirs filled with Liquid Nitrogen
Getting Started
• Equipment on Hand
• Janis Cryostat
• Lakeshore Temperature Controller
• Equipment Needed
• Keithley Sourcemeter
• Keithley Nanovoltmeter
Need for Automated System
• Three parameters must be observed at one time.
• For good resolution of the resistance change, data must be collected approximately every 500 ms.
• Large amounts of data will be collected and stored for each run.
Program Initialization Window
• All inputs are entered in initialization phase.
• Resistance or resistivity can be calculated.
• Either temperature or current can be varied for each run
Preliminary Tests

Sample -- YBCO, Tc = 93K

• Test 1
• Taken with sample in Cryostat
• Realized solder joints failed at 140K.
• Test 2
• Taken with sample submerged in liquid nitrogen
• Realized method of voltage data acquisition is too noisy
Future Work
• Improve solder joints in order to make measurements at liquid nitrogen temperatures.
• Reduce noise by:
• Using a filter
• Implementing the Current Reversing Technique