Percolation and electrical resistivity in the YBa2Cu3O7−Ag composite system

We have measured the resistivity of the YBa₂Cu₃O₇–Ag composite system with Ag volume fraction,p _Ag ranging from 0–100%. The percolation behavior and the normal-state resistivity are studied. We find a three-dimensional threshold for the Ag matrix occurring atp _Ag 20% and a threshold for the superconducting matrix occurring at 1–p _Ag30%. The electrical resistivity is interpreted in terms of a resistivity model for granular YBa₂Cu₃O₇ proposed by Halbritter et al. [Z. Phys. B —Condensed Matter71, 411 (1988)]. Based on this model, we show that the resistivities between the YBa₂Cu₃O₇ grains and flaws in the grains are greatly diminished in the samples, indicating significantly improved grain growth behavior of the YBa₂Cu₃O₇ phase resulting from the addition of Ag. With the diminishing of the grain boundary resistivities, an intrinsic value for the temperature coefficient of resistivity, (1/gr)(d/dT), can be inferred, which is 3.3×10^–3/K at 300 K. This value is very close to that observed in the Cu–O₂ planes in single crystals and epitaxial films of YBa₂Cu₃O₇.     

The increments of partial sums of a dependent sequence when the moment generating functions do not exist

In the paper, we give the almost sure limiting behaviour of the increments of partial sums of a-mixing sequence when the moment generating functions do not exist. The results obtained are close to those for an independent sequence provided that the mixing coefficients tend to zero rapidly enough.The Project Supported by National Natural Science Foundation of China.     

配合物的生成热和配体的质子化热之间的直线焓关系——Ⅶ.Ni(Ⅱ)-2,2′-联吡啶-N-(间位取代苯基)氨基乙酸三元体系

有关Ni(Ⅱ)-N-(间位取代苯基)氨基乙酸二元体系的直线焓和直线熵关系,前文已有报道,本文报告Ni(Ⅱ)-2,2′-联吡啶-N-(间位取代苯基)氨基乙酸(Ni(Ⅱ)-biPy-m-RPhG:R=CH_3,H,CH_3O,Cl)三元体系的生成热研究,发现在此三元体系中亦存在良好的直线焓和直线熵关系。