冷压与热压处理对YBa2Cu3O7-δ的结构相变和超导电性的影响

 本文研究了冷压（2.2~5.2 GPa）与热压（2.2 GPa，2.50~950 ℃）对YBa₂Cu₃O_7-δ的正交→四方相转变区和超导电性的影响。冷压保持其正交结构，但破坏了超导性，在98 K（失超点）到室温呈半导体特性，再在空气中烧结，可恢复液氮温区的零电阻状态。以Cu锅密封样品，热压处理至950 ℃，不发生Cu的析出。热压处理后，98 K（失超点）到室温呈半导体特性，再经通氧烧结，于86 K出现零电阻。热压处理过程，从450~950 ℃为正交→四方转变区。950 ℃为转变结束温度，大大高于氧气、空气、氮气、真空状态的温度。400 ℃附近为起始转变温度，低于氮气、氧气气氛的温度。因此相变区加宽。T_0-t结束温度升高，与Cu锅的抑制还原作用有密切关系。如降低高压淬火速率（<<10² ℃/s），或随后辅以氧气氛中的后处理，将有利于获得既有高密度又有高T_c的YBa₂Cu₃O_7-δ超导体。

The Influences of Cool- and Hot-Pressure Treatment on the Phase Transition and Superconductivity of YBa2Cu3O7-δ

In this paper, the influence of cool- and hot-pressure treatment were studied on the phase transition region from or thorhombic to tetragonal and superconductivity in YBa₂Cu₃O_7-δ. After the cool pressure treatment of 2.2~5.2 GPa, the samples remained the orthorhombic structure, but lost their superconductivities and kept up the semiconductivity in the region from 98 K to room temperature. If the samples are sintered in air, their zero resistance states will be regained. When the samples were sealed hermetically into a "Cu bowl", there was no separation of Cu in the samples within the region of hot-pressure temperature from 250 ℃ up to 950 ℃. After the hot-pressure treatment (2.2 GPa, 250~950 ℃), the samples were in semiconductiviting state in the region of 98 K to room temperature. If the samples are followed by an after-treatment in oxygen, the zero resistance state can be observed (at about 86 K). In the process of hot-pressure treatment, it was found that the transition region of orthorhombic to tetragonal is in the 450~950 ℃, 950 ℃ is the end temperature of transition, which is higher than that of samples obtained by sintering in oxygen, air, nitrogen and vacuum. And about 400 ℃ is the onset temperature of transition, which is lower than that in nitrogen, air and oxygen. Therefore the phase transition region is widened, and the end temperature of T_0-t raises. These are related to that the "Cu bowl" restrains the reduction role under high pressure. If the quenching rate under high pressure is slowed (<<10² ℃/s), or the samples are followed by an after-treatment in O₂, the YBa₂Cu₃O_7-δ both with high density and T_c can be obtained. The deficiency and hole of oxygen are also discussed here.