For the purpose of lowering the sintering temperature of alumina with a CuO-TiO 2-Nb 2 O 5 additive, effects induced by changes in the firing atmosphere, in particular, oxygen partial pressure (pO 2), were studied. It was confirmed that sintering was promoted in regions where the pO 2 was lower than that in the air atmosphere, 0.21 atm. Our analysis of the additive using TG-DTA and XRD methods indicated that both the melting temperature and compositions of the produced compound vary with the change in firing atmosphere. These changes were presumed to affect the sintering performance. By lowering the pO 2 to 0.05 atm, a dense sintered sample with good dielectric properties was obtained at a firing temperature of 925 °C.
Transport properties (electrical resistance and diamagnetism) of composites consisting of the cuprates YBa 2 Cu 3 O 7−δ and PrBa 2 Cu 3 O 7−δ (YBCO and PBCO) have been found to be considerably governed by the presence of the third phase Y 1−x Pr x Ba 2 Cu 3 O 7−δ (YPBCO) yielded around the grain boundaries of initial ingredients YBCO and PBCO. The transport characteristics are mainly featured by a drop of the resistivity ρ (its amplitude ρ) appearing at the onset temperature T c1 and a shoulder in the resistivity ρ-T curve. The resistivity drop describes the granular superconductivity of the existing YBCO grains, and the shoulder depicts emerging YPBCO. These two features correlate with each other, through the formation of YPBCO. Owing to the presence of YPBCO, the textural structure of the composites can easily be controlled by changing the sintering temperature T s and sintering time t s to yield a large variety of the electrical and diamagnetic behaviours.
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