On 21 July, 1983, the Vostok Research Station in Antarctica logged the coldest temperature ever measured on earth at -89.2 C. This is equivalent to 183.95 Kelvin. On March 6, 2008, Superconductors.ORG measured signs of superconductivity just over 185 Kelvin in an optimized 1223/1212 intergrowth, marking the first observation of superconductivity at earth ambient temperatures.

       Like the 181K superconductor reported in January of 2008, the 185K superconductor appeared as a minority phase in a 1223/1212 host that was doped with extra Tm and Cu (see structure types at page bottom). Through trail and error Tc was found to peak with slightly more Lead and slightly less Indium than the 181K formulation. Eight separate tests of the compound (Sn_1.0Pb_0.5In_0.5)Ba₄Tm₅Cu₇O₂₀₊ produced an average Tc of 185.6K. Interestingly, the 3-to-1 ratio of 4A to 3A metals in the insulating layer is also the ratio that produces the highest transition temperatures among binary alloy superconductors. 
 

The structure type responsible for this record high Tc is believed to be a 1245/1212 intergrowth (shown at left). This structure does not form stoichiometrically. It results as a byproduct only. So, commercial prospects of this discovery will hinge on manufacturers developing a method of mass producing and refining it into a pure form.         The graphs at page top show a resistive transition just above 185K and a Meissner transition just above 186K*. The volume fraction of the 1245/1212 phase is less than 1% of the bulk, which would normally require multiple plots to be summed together to improve the signal-to-noise ratio. However, in this instance both signals were strong enough to be readily observed above the background noise with a single pass.         Another minority phase resistive transition was observed near 163K (shown above). This is believed to be the 1234/1212 structure, which produced a stronger signal due to a larger volume fraction.         The structure progression below depicts how the 163K and 185K minority phases result from adding additional Tm and Cu to the host 1223/1212 parent. The 1234/1212 and 1245/1212 structures have a greater amount of planar weight disparity (PWD) by virtue of their asymmetry along the C (vertical) axis. And greater PWD has previously been found to promote a higher Tc (more on PWD).

     ^*The magnetometer employed twin Honeywell SS94A1F Hall-effect sensors with a tandem sensitivity of 50 mv/gauss.

       Synthesis of the 1223/1212 host was by the solid state reaction method. Stoichiometric amounts of the below precursors were mixed, pelletized at 70,000 psi and sintered for 36 hours at 830C. The pellet was then annealed for 10 hours at 500C in flowing O2.
SnO   99.9%   (Alfa Aesar)   1.17 moles (gr.)
PbO   99.99%   (Alfa Aesar)   0.97 mole
In₂O₃   99.9%   (Alfa Aesar)   0.60 mole
Tm₂O₃   99.99%   (Stanford Materials)   5.135 moles (includes an extra .06 atom of Tm)
BaCuO_x   99.9%   (Alfa Aesar)   7.54 moles
CuO   99.995%   (Alfa Aesar)   0.73 mole (includes an extra .06 atom of Cu)

RESEARCH NOTE: Tm-copper-oxides have been found to be strongly hygroscopic. All tests should be performed immediately after annealing.

- E. Joe Eck
© 2008 Superconductors.ORG
Patent Pending. All rights reserved. 
 

BACK to "News" page at Superconductors.ORG