This is an update on previous
work and it appears that they have successfully reached the holy grail of
superconductivity research. We have room
temperature effects that could easily be fitted into a framework that needed
modest cooling at most. Of course,
turning all this into something that is commercially useful is quite another
matter as we have seen with the materials finding their way into cryogenic
systems today. Yet it has been done even
there.
This is a classic example of
science progressing at its best. It has
taken over a century to reach this point from the initial recognition of the
possibility. Hundreds, if not thousands,
worked toward this day. It is still not particularly
complete but we are glimpsing the end of the tunnel.
As I have posted, the principle
requirement of a MFEV (magnetic field exclusion vehicle) is a thin layer of
superconducting material at as high a temperature as possible. This is getting us there. Simply producing hexagonal chips of such
material could easily be assembled into such a structure.
Superconductivity Near 20 Celsius - Superconductivity Approaches Room
Temperature
17 March 2011
Superconductors.ORG
Superconductors.ORG herein reports the observation of superconductivity
near 20 C.
In eight magnetization tests
a small amount of the compound (Tl5Pb2)Ba2MgCu10O17+ consistently
produced sharp diamagnetic transitions (the Meissner effect) near 20 Celsius
(see above graphic), and resistive transitions that appeared near 18.5C (see
below right). These temperatures are believed accurate +/- 2 degrees.
Resistance-v-temperature tests of this material were performed using a
4-point probe. Four significant bits of data resolution were necessary to
resolve the 18.5C critical transition temperature (Tc) due to a low
signal-to-noise ratio (S/N). A sharp transition appeared across just 1.5 uA of
a 220 uA signal. This suggests a superconductive volume fraction less than 1%
of the bulk.
This extraordinarily high Tc was
achieved by engineering a theoretical D223 structure (where D=11 hex) that
pushes the limit of the longest C-axis lattice that will superconduct, while
simultaneously establishing near-optimum Pb-doping of the Tl-Cu-O blocking
layers (see structure at left).
In 2008 a Sn-In-Pb-Tm cuprate
produced superconductivity near 195K . That material had a C-axis lattice
constant around 33 angstroms. Attempts to go beyond 33 Å within that system
failed to produce signs of superconductivity. That fact pointed to 33 Å being a
rough upper size limit for a superconductive unit cell within this family of
copper perovskites. Since the 3 Celsius superconductor discovered in December 2010,
had a C axis length under 28 Å, the next attempt to advance high Tc focused on
increasing the unit cell parameters.
The first material attempted was
(Tl5Pb)Ba2MgCu9O15+ which had a C axis length near 30 Å, but a reduced
percentage of Pb doping - around 16%, sted 20% (see below 7C graphic). (Tl5Pb)Ba2MgCu9O15+ displayed
a magnetic Tc near 9C (282K) and resistive Tc near 7C (280K). This was an
improvement. However, within its magnetization plot were signs of still higher
superconductivity being generated by a minority phase.
(Tl5Pb2)Ba2MgCu10O17+, with a
unit cell near 32.7 Å, was then synthesized, displaying an unambiguous
diamagnetic transition near 20 Celsius.
These 18C and 7C structures are
shown below alongside the previous world record holders for comparison.
With two atoms of divalent Pb, the
insulating layer of the 18C superconductor is hole-doped 28.5%. This increased
doping level approaches the optimum of 25% discovered empirically within the
Sn-In-Pb copper-oxide family in 2008. Additionally, 28.5% is near a 30% optimum
level for Pb-doping found by Shao, et al, in 1995(1).
Dots have been placed within the
C1 plot below, depicting where D223(Tl5Pb2-Mg) and B223(Tl5Pb-Mg) lie relative
to the other high performance thallium-cuprates. Temperatures plotted (in
Kelvin) are resistive. The correlation between planar weight ratio and Tc is
unmistakable.
As with prior discoveries that
advanced high Tc through asymmetry along the C axis, this material does not
form stoichiometrically (by conventional mixing of chemicals). It must be
synthesized using the layer cake method, as shown below. The prototype pellets
each had roughly 20 to 22 layers. And, even using this technique, the volume
fraction is low. Thus, commercialization will have to wait for a refinement
method to be developed.
This discovery is being released
into the public domain without patent protection in order to
encourage additional research. Synthesis was by the solid state reaction
method.
The below stoichiometric
ratios were used for the ODD layers:
PbO 99.99% (Alfa Aesar) 2.88 moles (gr.)
Tl2O3 99.99% (Alfa Aesar) 7.366
moles
BaCO3 99.95% (Alfa Aesar)
2.546 moles
CuO 99.995% (Alfa Aesar) 4.106
moles
...and the below ratios for
the EVEN layers.
MgO 99.95% (Alfa Aesar)
1.04 moles (4x stoichiometric)
CuO 99.995% (Alfa Aesar) 4.10 moles
(4x stoichiometric)
The chemical precursors were
pelletized at 70,000 PSI and sintered for 35-36 hours at 865C. The pellet was
then annealed for 10 hours at 500C in flowing O2. The magnetometer employed
twin Honeywell SS94A1F Hall-effect sensors with a tandem sensitivity of 50
mv/gauss. The 4-point probe was bonded to the pellet with CW2400 silver epoxy
and used 7 volts on the primary. Temperature was determined using an Omega type
"T" thermocouple and precision OP77 DC amplifier.
RESEARCH NOTE: The copper-oxides are strongly hygroscopic.
All tests should be performed immediately after annealing.
E. Joe Eck
© 2011 Superconductors.ORG
All rights reserved.
1. The Synthesis and Characterization of HgBa2Ca2Cu3O8+ Superconductors with Substitution of Hg by Pb, by H.M. Shao, C.C. Lam, P.C.W. Fung, X.S. Wu, J.H. Du, G.J. Shen, J.C.L. Chow, S.L. Ho, K.C. Hung, and X.X. Yao, Physica C Volume: 246, 1995 Page(s): 207-215
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