In 1986, a cuprate superconductor (Ba-La-Cu-O system) having a critical temperature which goes over the BCS limit (~30 K) was discovered and then a cuprate superconductor (Y-Ba-Cu-O system) with a critical temperature higher than 77 K was discovered. Furthermore, a Hg-based cuprate with a critical temperature of 133 K was found. The 133 K is still the highest critical temperature of conventional superconductors under atmospheric pressure
They found a possible room-temperature superconductor material obtained by bringing alkanes into contact with the graphite materials. We showed that ring current in a ring-shaped container into which n-octane-soaked thin graphite flakes were compressed did not decay for 50 days at room temperature, suggesting that the material is capable of conducting electricity without energy loss at room temperature.
But the critical temperatures of these materials have not been measured. Therefore, in this study, they attempt to measure the critical temperature of the room temperature superconductor obtained by bringing alkane into contact with graphite material. However, since the above-mentioned material to be measured is an inhomogeneous material, the four-probe method cannot be applied to its resistance measurement. The reason is that there is a possibility that the measurement current path does not necessarily pass through a voltage-measurement terminal in inhomogeneous materials and therefore even if the potential difference between the two voltage-measurement terminals becomes zero, it does not necessarily mean that the resistance becomes zero.
Thus, the resistance measurement using four-probe method for inhomogeneous materials causes misunderstanding. The transition from the normal to the
superconducting state or from the superconducting to the normal conducting state is accompanied by abrupt change in resistance. Although the result obtained by the resistance measurement using the two-probe method includes contact resistance, sudden jump in resistance at critical temperature can be discriminated by the two-probe method.
It has been confirmed that the mixture obtained by bringing the alkane into contact with the graphite material has almost zero resistance at room temperature. If the mixture obtained by bringing alkane into contact with graphite material is gradually heated from room temperature, when it reaches the critical temperature, the resistance of the mixture will jump suddenly. In this study, the critical temperatures of mixtures consisting of graphite materials and alkanes are measured by the two-probe method.
In this research, a pitch-based graphite fiber was used as the graphite material. The sample for critical temperature measurement was prepared by packing the graphite fiber in a polytetrafluoroethylene (PTFE) tube and then injecting alkane into the tube with a syringe. Since the pitch-based graphite fiber is brittle, the fiber is sometimes broken into pieces when packing it in the PTFE tube. Therefore, the resistance of packed pitch-based graphite fibers before injecting alkane into the PTFE tube has a wide range of values.
They have observed a sharp rise in resistance of mixture of graphite fibers and alkanes inserted in the PTFE tube during heating them, using the two-probe-resistance measurements. This observation shows that a phase transition from the superconducting to the normal conducting state occurred in the mixture of graphite fibers and n-alkane. In other words, this indicates that the mixture of graphite fibers and n-alkane packed in the PTFE tube remains superconductive until its resistance rapidly rises, and the temperature at which the resistance suddenly rises is the critical temperature. They have noticed that when the pitch-based graphite fibers are packed in the PTFE tube, they may be broken into pieces depending on how they are packed into the tube. The finer the
graphite fibers break, the greater the resistance of the sample before injecting alkane into the PTFE tube packed with the graphite fiber. From the relationship between the amount of change in resistance at the phase transition and the critical temperature, they found that the critical temperature decreases as the fiber is finely divided. That is, the critical temperature decreases, as the ratio of the basal plane surface to the edge plane surface decreases. This fact suggests that the basal plane plays an important role in superconductivity. Furthermore, we have found that the greater the carbon number of alkane, that is, the higher the boiling point of alkane, the higher the critical temperature. They have demonstrated that superconductors having critical temperatures exceeding 500 K can be obtained by using n-alkanes having 16 or more carbon atoms.