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High- superconducting ceramic samples of the form (x = 0.3, 0.5, 0.7, 0.9 and 1) are prepared in a closed quartz tube at C. The electrical resistance versus temperature is measured in the temperature range between 15 K and room temperature. The effect of magnetic field and applied electric current on the second stage of the transition is studied. It is found that the transition does not occur at a definite temperature but in a temperature range which may approach 50 K or even more. This is believed to be due to the very short coherence length ( Å) of HTSCs or to the random motion of the magnetic flux. The results obtained in our work can give some information about superconducting grains and weak superconducting boundaries.

Superconductor samples of nominal composition Hg 1−x Tl x Ba 2 Ca 3 Cu 4 O 10+δ (0.3 x 1.0) are prepared at ambient pressure using a closed quartz tube at 850 • C. X-ray powder diffraction for the prepared samples is carried out and the results show that the structure of this system is a tetragonal. The lattice parameters a and c are calculated, and the data indicate that a is not affected by the substitution of Tl 3+ into Hg 2+ , whereas c decreases as the Tl content increases. The superconductivity is investigated using the electrical resistance and ac magnetic susceptibility measurements. All the samples have a superconducting behaviour except when x = 0.3. To improve the superconducting properties of this system, the cooling rate for the sample with x = 0.7 is studied; also the replacement of Ca by Sr for both x = 0.3 and x = 0.5 have been investigated. The excess conductivity for x = 0.3 shows that the dimensionality is changed from two-dimensional in the high-temperature region to three-dimensional in the mean field region.

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