Influence of grain size and stoichiometry on the electrical behaviour of YBa2Cu3O7-δ

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Abstract
The role of the microstructure for the electrical properties of polycrystalline YBa2Cu3O7-δ is examined. Experimental data are presented showing the effect of (i) variation in grain size and (ii) deviation from stoichiometry for YBa2Cu3O7-δ on the critical current density in « zero » field at 77 K and the room temperature resistivity. Firstly a strong decrease in the critical current density has been observed when an average grain size of 15 μm is exceeded. Large grains promote microcracks due to anisotropic volume changes during cooling from the sintering temperature. This causes a reduction in the effective current carrying cross-section of the material. Ultrasonic measurements were used to confirm an increase in the microcrack density for large grained samples. Secondly, a significant modification of the electrical properties was achieved for deviations from stoichiometry greater than 0.5 mole %. Previous work has shown that the presence of minor phases due to incomplete calcination or poor mixing leads to reduced critical current density. Deviations from stoichiometry of the initial powder will also promote minor phases. These two aspects must be taken into account in the choice of a suitable ceramic processing route