Dielectric Constant Confirmed as a Robust Tc-Enhancement Mechanism

16 February 2025
Superconductors.ORG

        In 2014 Superconductors.ORG found signs that the dielectric constant (relative permittivity K) of substructures within a superconductor lattice correlated directly with Tc, the critical transition temperature. But those signs were in a low-volume-fraction superconductor. In a low-VF superconductor which atomic sites are being occupied by which atoms can often be the subject of much debate. Now for the first time the link between Kappa and Tc has been confirmed in a high-VF superconductor, where the structure is well established.

       A superconductor discovered some 20 years ago turned out to be an ideal testbed for confirmation of a correlation between Kappa and Tc. Above right, in a plot of Sn2Ba2(Ca0.5Tm0.5)Cu3O8+, there are tin atoms in the insulating cation layers of the 3212 structure. Tin-oxide (SnO) nanowire has a Kappa of only 2.3. But, when alloyed with antimony, SnSb has a Kappa of 147 — more than 63 times as great. Additionally, tin and antimony have nearly identical ionic radii that should allow them to occupy the same atomic sites. This makes a 50-50 mix of tin and antimony suitable for boosting the sub-structure Kappa of this material without radically altering the planar-weight-ratio (PWR) — which has also been found to promote Tc.

       The above right magnetization test shows diamagnetic (Meissner) transitions reaching minimum at 112 Kelvin in the original Sn2-3212 formulation. Above left, the Meissner transition in the improved SnSb-3212 upgrade reaches minimum near 145 Kelvin - an increase in Tc of 33 degrees. Both formulations show a diamagnetic transition of greater than 140 milli-Gauss, confirming that the volume fraction is high. In tests done back in 2005 three distinct steps were observed in the transition, suggesting it was multi-phasic. There is also a diamagnetic step midway in the new plots, made with a different magnetometer.




       The above graph shows the Kappa-Tc correlation in the low-VF superconductor Tl7Sn2Ba2(Si,Ti,Mn)Cu10O20+, which was improved by increasing the dielectric constant of its anion. This seems to suggest that Tc can be increased through permittivity adjustments to either the cation or anion layers.

       Stoichiometric amounts of the below chemicals were used in the synthesis of these compounds:

BaCO3   99.95%   (Alfa Aesar)   (decomposes to BaO during calcination)
Tm2O3   99.99%   (Stanford Materials)
CaCO3   99.95%   (Alfa Aesar)   (decomposes to CaO during calcination)
SnO   99.9%   (Alfa Aesar)
Sb2O3   99.999%   (Alfa Aesar)
CuO   99.995%   (Alfa Aesar)

       The chemical precursors were mixed thoroughly and pelletized. Since antimony-oxide has such a low melting point, the mix was pre-sintered for 12 hours at 640C. Then the pellet was calcined at 770C for 2 hours and sintered overnight at 880C. Lastly, it was annealed for 10+ hours at 500C in flowing O2. Testing temperatures were determined using an Omega type "T" thermocouple. The magnetometer employed twin Honeywell SS94A1F Hall-effect sensors with a tandem sensitivity of 50 mv/Gauss.


RESEARCH NOTE: The copper-oxides are strongly hygroscopic. All tests should be performed immediately after annealing.


E. Joe Eck
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