Transcript Development of an Automated System to Measure Critical

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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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

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New superconducting materials are still
being found. (MgB2)
The record critical temperature
continues to increase with new
discoveries. (current max = 138K)
Making Resistance Measurements
Four-Point Measurement

Known current applied through
two outermost wires

Voltage drop measured
between two innermost wires
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Resistance calculated using
Ohm’s Law:
V
R
I
Probe
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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

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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
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All inputs are entered
in initialization phase.
Resistance or
resistivity can be
calculated.
Either temperature or
current can be varied
for each run
Program Front Panel
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
Resistance vs. Temperature
Plots of Temperature and Voltage
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
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Add desired features.
Begin investigation of HTS samples.