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Hot axial and hot isostatic pressing was applied for single-core MgB 2 /Ti tapes. Differences in transport current density, n -exponents and critical current anisotropy are discussed and related to the grain connectivity influenced by pressing. The magnetic Hall probe scanning measurements allowed observing the isolated regions for axially hot pressed sample attributed to the longitudinally oriented cracks introduced by pressing. The highest current densities were measured for the tape subjected to hot isostatic pressing due to improved connectivity.

From the beginning of the Mg–B system investigations, pressure was an important factor in procedures obtaining the bulk superconductors for application purposes: at hot isostatic pressing (HIP-ing), annealing under inert or active gas medium, i.e. argon with hydrogen and nitrogen, or for extrusion methods for wires manufacturing. MgB 2 reveals interesting properties. We present the current state of our technological efforts in improving the microstructure and superconducting properties of this material, with use of the high gas pressure and high temperature treatment of samples crystallized in crucibles made of BN and SiC. As the precursor we mainly use the commercial MgB 2 powder. The results of microstructure investigations and XRD analyses are presented.

A superconducting Hg0.7Re0.3Ba2Ca2Cu3O8+δ thick film on a polycrystalline LaAlO3 (LAO) substrate was obtained by a two-step method by screen-printing a Re0.3Ba2Ca2Cu3Ox precursor coating on the substrate followed by high-pressure gas firing in Hg vapour. Morphological observations have revealed that the adherent to the ceramic substrate film exhibited highly porous microstructure composed of plate-like crystallites characterized by a superconducting transition temperature of about 110 K. The major phases identified were a high-Tc (Hg,Re)-1223 phase and a low-Tc (Hg,Re)-1212 phase. Non-superconducting secondary impurity phases were also identified. Energy dispersion X-ray spectroscopy analyses at the substrate/film boundary showed a thin continuous layer of BaLa2O4 compound, the presence of which confirmed chemical interaction between the LAO substrate and (Hg,Re)–Ba–Ca–Cu–O film.

Superconducting phases of 2212-type, with the general formula Tl2−xHgxSr2Ca1Cu2Oz, were synthesized from metal oxides in closed silica tubes at 880°C. The most phase-pure sample was obtained for x=0.7. A mixture with the nominal composition Tl1.3Hg0.7Sr2Ca1Cu2Oz, used in a single-crystal growth in a high-temperature high-pressure gas trap furnace (HTHP), gave plate-like crystals with a maximum diameter of 500 μm, having unit cell parameters which could be refined to a=3.796 A and c=28.433 A (space group I4/mmm). The structural details were investigated by X-ray single-crystal analysis (Rw=0.025). Both alkaline-earth metal sites in the structure are probably mixed, but with different Ca/Sr ratios. As previously reported, some copper entered the (Tl,Hg) site in the high-pressure synthesis, giving a resulting composition of (Tl0.8Hg0.7Cu0.5)(Sr1.7Ca0.3)(Ca0.7Sr0.3)Cu2O7.5 according to energy dispersive spectrometry (EDS) analysis and structural refinement. Tc (onset) for both powder and single-crystal samples, as prepared, was about 45 K, as determined by AC susceptibility.

We report the first measurements of the in-plane transport properties of single crystals of the four-layer superconductor HgBa 2 Ca 3 Cu 4 O 10+σ with a transition temperature of 130 K. In the normal state the temperature dependence of the electrical resistivity and the Hall coefficient indicates that these crystals are underdoped. For the cotangent of the Hall angle we found a T 2 dependence. In the mixed state, the following form holds for a universal correlation between hall resistivity ϱ H and electrical resistivity ϱ : ϱ H ≈ ϱ n , with n = 1.75.

Crystals of Hg0.5Pb0.5Ba2Ca4Cu5O12+δ were measured on an X-ray four-circle single-crystal diffractometer and the crystal structure was refined to R = 0.033. The onset of the superconducting transition of the crystals ranges from 101 K-115 K. The space group is P4/mmm and lattice parameters of the five-layer material are a = 3.8529(3) A, and c = 22.172(2) A . Hg is partially substituted for Pb and heavy atoms are shifted away from the origin by ∼0.38 A . Stacking faults of 1234 or 1256 material are manifest in the Fourier maps, and the amount was refined to 4.4%. These stacking faults induce an additional electron density at the ideal O(4) site and mask a possible oxygen occupancy at ( 1 2 , 1 2 , 0 ). However, electron density is observed at a distance of 0.6 A from ( 1 2 , 1 2 , 0 ), suggesting a shift of the excess oxygen atom from its ideal position. Bondlengths and distances show systematic changes with increasing number n of CuO2 layers. Changes occur mainly in the rock-salt type layer, whereas the CaCuO2 units remain stable over the whole H-12(n−1)n series.

Crystals of Hg0.8Pb0.2Ba2Ca3Cu4O10.14 were measured on an X-ray four-circle single-crystal diffractometer and the crystal structure was refined to R = 0.023. The space group is P4/mmm and the lattice parameters are a = 3.8530(5) A , and c = 18.968(3) A . Lead is incorporated at the Hg site, but it is shifted slightly from the origin in the z = 0 plane. A model of Pb4+ clusters in the basal plane is proposed, which is in agreement with recent TEM results. A reduction of electron density at the Hg site is attributed to the substitution by Cu or CO3 groups. Stacking faults of 1223 or 1245 material are manifested in the Fourier maps, and it was possible to refine the amount to 3.8%. However, these stacking faults induce additional electron density at the O(4) site ( 1 2 , 1 2 0 ), and cause an overestimation of the excess oxygen content, if it is not corrected carefully. The bondlengths are distances are in agreement with those of other members of the Hg-12(n−1)n series, and show systematic changes with increasing number n of CuO2 layers. The effect of stacking faults on the diffraction of all members of the Hg-12(n−1)n family is discussed, and a method for their determination is given.

Single crystals of the Hg 1− x Pb x Ba 2 Ca n −1 Cu n O 2 n +2+ δ (x = 0 and 0.2) and infinite-layer CaCuO 2 compounds have been grown using a high gas pressure. The Hg 0.8 Pb 0.2 -1234 single crystals of a size up to 0.5×0.5×0.05 mm 3 have a T c onset of 129 K. Sing le crystals of the CaCuO 2 of a size up to 2×1×0.05 mm 3 have been obtained without stabilizing additives of Sr or Li. The T c onset varies between 70 and 100 K. X-ray structural refinements have been performed on the CaCuO 2 and Hg-1234 single crystals. HRTEM investigations proved a high quality of the crystal structure of the Hg-12 ( n -1) n crystals.

A comparison of fast-neutron irradiation effects on the irreversibility line (IL) and the intragrain critical current densities ( J c ) in the 90 K and the 60 K phase of YBa 2 Cu 3 O 7- x (123) and in YBa 2 Cu 4 O 8 (124) ceramics is presented. The 124 compound shows a transition into the reversible regime at distinctly lower temperatures and fields than the 90 K phase of 123. However, the IL of 124 lies above the one of the 60 K phase. Upon irradiation we observe a pronounced shift of the position of the IL to higher fields and temperatures for the 60 K phase of 123 and for the 124 compound. The IL remains almost unchanged in the 90 K phase of 123 at the relatively low fluences employed so far. We also observe an increase of J c after irradiation. The values of the J c enhancement as well as their temperature and field dependence vary considerably among these compounds.

Tl and Hg based HTS layers are the key materials for electronic devices applications. We have reported fabrication method of thin films on SrTiO 3 and LaAlO 3 substrates with T c up to 134 K and J c over 10 6 A/cm 2 at 77 K, [1]. An important progress in optimization and reproducibility of the growth process was achieved by the knowledge of the phase equilibrium acting during high gas pressure annealing, realized by precise DTA. The thermodynamic data related to oxygen content in the system allowed us to choose the optimum process conditions. We have observed large differences in between Hg and Tl family crystallization process, related to the liquid phase composition and especially to their oxygenation state. For the Hg compounds the crystallization takes place on the surface between the liquid and vapor, while for Tl the whole growth is completed in the liquid phase. Therefore, the Tl compounds are much likely to be grown by the liquid phase epitaxy method. In the case of the Hg family superconductors growth, the effect of Mo substitutions was investigated in order to eliminate the surface growth problem, and to reduce the high Hg partial pressure in the crucible.

Synthesis and properties of high-T c superconducting (Hg 1−x Re x )Ba 2 Ca 2 Cu 3 O 8+δ (x=0.05÷0.2) single crystals and films obtained by high pressure gas method are presented. The influence of Re-doping on their microstructure and superconductivity is discussed. The powder of Ba 2 Ca 2 Ca 3 O y and ReO 2 was used to prepare the precursor material, which was mixed with HgO to prepare pellets for high pressure synthesis of single crystals. After homogenization at 860°C for 1 h crystallization from the melt occurred at temperatures between 1060 and 950°C with the cooling rate of 3–5 °C/h and 50 °C/min below 950°C. The Hg 1−x Re x Ba 2 Ca 2 Cu 3 O 8+δ films were prepared by depositing the precursor film by laser ablation and by liquid phase epitaxy (LPE) on the single-crystalline substrates of SrTiO 3 and LaAlO 3 . Precursor films were made by the deposition of 1–3 μm thick amorphous layers of Ba 2 Ca 2 Cu 3 O y + ReO 2 .

Single crystals of the HgBa 2 Ca n−1 Cu n O 2n+2+δ compounds have been grown using a high gas pressure. The Hg1234 single crystals have a T c of 129 K. X-ray and TEM investigations proved the high quality of the crystal structure. Single crystals of the CaCuO 2 infinite layer compound of a size up to several mm 2 have been obtained under a high gas pressure without stabilizing additives of Sr or Li. The T c of the as grown crystals is 70–80 K

The HgBa 2 Ca n−1 Cu n O 2n+2+δ ceramics have been synthesized for n=2 and 3 using a gas pressure technique. The kinetics of the formation reaction of Hg1223 has been investigated at high gas pressure. The X-ray and magnetization measurements confirmed the synthesis of Hg1223 and Hg1212 compounds with T c of 134 and 128K respectively.