For years superconductor research has focused on improving the performance of YBCO, the most widely used industrial superconductor. Doping YBCO with calcium can improve its current-carrying capability across grain boundaries by as much as 35 percent - but at a sacrifice. Doping lowers its 91K critical temperature (Tc) to a level uncomfortably close to that of the 77K liquid nitrogen coolant.
To date there have been just two variants of YBCO discovered that will accommodate calcium without Tc loss: Y2CaBa4Cu7O16+ (Tc ~96K) and Y3CaBa4Cu8O18+ (Tc ~99K). Superconductors.ORG today reports a third calcium-doped YBCO formulation has been found that produces a Tc considerably higher than even those two compounds. YCaBa3Cu5O11+ displays a resistive Tc near 107K.
The 96K and 99K discoveries were both intergrowth
structures with much larger unit cells than pristine YBCO. This new discovery, YCa-235 has a unit cell just slightly larger
than normal YBCO and is not an intergrowth. The 2323C structure (shown at left) achives a higher Tc by establishing planar
weight disparity (PWD) between the Cu-O atoms in the upper C1 axis and the single copper atom in the lower. Also,
by establishing PWD between the 3 heavy barium atoms in the upper C2 and the lighter yttrium and calcium atoms in the
In addition to a resistive Tc near 107K, the R-T plot at page top also shows a rogue phase near 135K. This was likely produced by a 3423C structure that formed as a minority phase. This conclusion was reached by examining the results achieved with a 177K YBCO variant in March 2009. The 3423C has one more barium atom and one more copper atom in the upper part of the structure, further increasing planar weight disparity.
Note that the resistance plot at page top has more noise than the 96K and 99K plots. This suggests the volume fraction may not be as great. However, having both 107K and 135K superconductive phases helps to mitigate this fact.
Synthesis of this material was by the solid state reaction method. Stoichiometric amounts of the below precursors were mixed, pelletized and calcined for 9 hours at 775C; sintered at 890C for 12 hours; then finally annealed for 10 hours at 500C in flowing O2.
The 4-point probe was bonded to the pellet with CW2400 silver epoxy and used 7 volts on the primary. The copper-oxides are strongly hygroscopic. All tests should be performed immediately after annealing. This discovery is being released into the public domain without patent protection to encourage additional research.