Voltage-current characteristics of bulk YBa2Cu3 O7-x superconductors

V-I characteristics of the sintered bulk YBa₂Cu₃O_7-x superconducting samples with J_c ranging from 30 to 1000 A/cm² were measured in zero applied magnetic fields and at liquid-nitrogen temperatures by using a microcomputer-controlled I_c measuring system. The results show that the V-I characteristics of these oxide superconductors can be divided into four stages. In the first stage, where voltage starts to go upwards, the voltage can be expressed as an exponential function of the transport current, which is different from the power relation often seen in the cases of traditional superconductors such as NbTi alloys. In the second stage, the voltage is a linear function of the current, but its slope is much smaller than the normal-state resistance of the sample. Finally, an unstable stage characterized by a sharply rising voltage appears before the V-I characteristic is turned into a normal-state one. These features of the V-I characteristics are closely related to the inhomogeneity existing in the sintered oxide superconductors