THE ROLE OF HIGH-PRESSURE SINTERING IN PREPARING HIGH-QUALITY N-TYPE ND2-XCEXCUO4-Y SUPERCONDUCTORS

High-pressure high-temperature sintering (HHS) under 1.0, 2.0, 4.0, and 6.0 GPa for a short time was directly used to reduce n-type Nd1.85Ce0.15CuO4-y. Superconductors with higher critical temperature were obtained. The samples display lower resistivity, and more metalliclike resistance behavior at normal state with increasing sintering pressure. A small increase of lattice parameter and cell volume was observed via electron and x-ray diffraction. Transmission electron microscopy studies also show that there is a preferential orientation along the c axis among the neighboring grains. This helps get well-matching grain boundaries in the HHS samples. A Cu2O high-pressure phase was found via energy-dispersive x-ray analysis in the sample treated above 5.0 GPa, which is thought to be a deleterious factor for superconductivity. Sintering under suitable pressure not only enhances the intragranular conductivity, but also effectively improves the intergranular links. Finally, via the assumed interstitial oxygen ordering in the T' structure, a possible mechanism of n-type reduction is proposed based on the change in tolerance factor under reduction atmosphere and high pressure.