This is an update on previouswork and it appears that they have successfully reached the holy grail ofsuperconductivity research. We have roomtemperature effects that could easily be fitted into a framework that neededmodest cooling at most. Of course,turning all this into something that is commercially useful is quite anothermatter as we have seen with the materials finding their way into cryogenicsystems today. Yet it has been done eventhere.
This is a classic example ofscience progressing at its best. It hastaken over a century to reach this point from the initial recognition of thepossibility. Hundreds, if not thousands,worked toward this day. It is still not particularlycomplete but we are glimpsing the end of the tunnel.
As I have posted, the principlerequirement of a MFEV (magnetic field exclusion vehicle) is a thin layer ofsuperconducting material at as high a temperature as possible. This is getting us there. Simply producing hexagonal chips of suchmaterial could easily be assembled into such a structure.
Superconductivity Near 20 Celsius - Superconductivity Approaches RoomTemperature
17 March 2011
Superconductors.ORG herein reports the observation of superconductivitynear 20 C.
In eight magnetization testsa small amount of the compound (Tl5Pb2)Ba2MgCu10O17+ consistentlyproduced sharp diamagnetic transitions (the Meissner effect) near 20 Celsius(see above graphic), and resistive transitions that appeared near 18.5C (seebelow right). These temperatures are believed accurate +/- 2 degrees.
Resistance-v-temperature tests of this material were performed using a4-point probe. Four significant bits of data resolution were necessary toresolve the 18.5C critical transition temperature (Tc) due to a lowsignal-to-noise ratio (S/N). A sharp transition appeared across just 1.5 uA ofa 220 uA signal. This suggests a superconductive volume fraction less than 1%of the bulk.
This extraordinarily high Tc wasachieved by engineering a theoretical D223 structure (where D=11 hex) thatpushes the limit of the longest C-axis lattice that will superconduct, whilesimultaneously establishing near-optimum Pb-doping of the Tl-Cu-O blockinglayers (see structure at left).
In 2008 a Sn-In-Pb-Tm cuprateproduced superconductivity near 195K . That material had a C-axis latticeconstant around 33 angstroms. Attempts to go beyond 33 Å within that systemfailed to produce signs of superconductivity. That fact pointed to 33 Å being arough upper size limit for a superconductive unit cell within this family ofcopper 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 onincreasing the unit cell parameters.
The first material attempted was(Tl5Pb)Ba2MgCu9O15+ which had a C axis length near 30 Å, but a reducedpercentage of Pb doping - around 16%, sted 20% (see below 7C graphic). (Tl5Pb)Ba2MgCu9O15+ displayeda magnetic Tc near 9C (282K) and resistive Tc near 7C (280K). This was animprovement. However, within its magnetization plot were signs of still highersuperconductivity being generated by a minority phase.
(Tl5Pb2)Ba2MgCu10O17+, with aunit cell near 32.7 Å, was then synthesized, displaying an unambiguousdiamagnetic transition near 20 Celsius.
These 18C and 7C structures areshown below alongside the previous world record holders for comparison.
With two atoms of divalent Pb, theinsulating layer of the 18C superconductor is hole-doped 28.5%. This increaseddoping level approaches the optimum of 25% discovered empirically within theSn-In-Pb copper-oxide family in 2008. Additionally, 28.5% is near a 30% optimumlevel for Pb-doping found by Shao, et al, in 1995(1).
Dots have been placed within theC1 plot below, depicting where D223(Tl5Pb2-Mg) and B223(Tl5Pb-Mg) lie relativeto the other high performance thallium-cuprates. Temperatures plotted (inKelvin) are resistive. The correlation between planar weight ratio and Tc isunmistakable.
As with prior discoveries thatadvanced high Tc through asymmetry along the C axis, this material does notform stoichiometrically (by conventional mixing of chemicals). It must besynthesized using the layer cake method, as shown below. The prototype pelletseach had roughly 20 to 22 layers. And, even using this technique, the volumefraction is low. Thus, commercialization will have to wait for a refinementmethod to be developed.
This discovery is being releasedinto the public domain without patent protection in order toencourage additional research. Synthesis was by the solid state reactionmethod.
The below stoichiometricratios were used for the ODD layers:
PbO 99.99% (Alfa Aesar) 2.88 moles (gr.)
Tl2O3 99.99% (Alfa Aesar) 7.366moles
BaCO3 99.95% (Alfa Aesar) 2.546 moles
CuO 99.995% (Alfa Aesar) 4.106moles
...and the below ratios forthe 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 werepelletized at 70,000 PSI and sintered for 35-36 hours at 865C. The pellet wasthen annealed for 10 hours at 500C in flowing O2. The magnetometer employedtwin Honeywell SS94A1F Hall-effect sensors with a tandem sensitivity of 50mv/gauss. The 4-point probe was bonded to the pellet with CW2400 silver epoxyand 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 withSubstitution 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