G. Zsolt

High Tc superconductivity of a TlBaCaCuO compound

Abstract A TlBaCaCuO 4.5+ x compound has been investigated from the point of view of superconductivity. Depending on the heat treatment, one part of the samples exhibits superconductivity with an onset of 121 K and a zero resistivity of 106 K, while the other part of them shows only a sharp drop in resistivity at 130 K which hints at the existence of superconducting domains. This picture was confirmed by magnetic and ESR measurements giving possibility for estimation of the critical magnetic fields.

Preparative chemistry consequences on YBaCuO superconducting compounds

Abstract A new way of two-stage synthesis has been elaborated for preparation of Y Ba Cu O superconductors. In the first step, a new Ba 2 Cu 3 O 5 double-metal oxide has been established from which a wide composition range of Y 1 Ba 2 Cu 3 O 7−d Y 1 Ba 17 Cu 25.5 O x (x ≈ 44±d) superconductors have been prepared by addition of Y 2 O 3 . Resistivity and microwave measurements hint at an existence of two superconducting phases.

Preparation dependent superconductivity in TlCaBa2Cu3O7.5±d above 100 K

Different heat treatment procedures were applied during sample preparation, which resulted in different superconducting properties in samples of the same nominal composition, TlCaBa2Cu3O7.5±d. This manifests itself in different critical temperatures: 104 K, 107 K and 93 K. The highestTc belongs to the best-crystallized samples in which the tetragonal Tl2Ca1Ba2Cu2O8±d phase is responsible for the superconductivity. The effect of heat treatments is reflected in the structural and magnetic properties as well.

Structure and superconductivity of variously prepared Tl-Ca-Ba-Cu-O compounds

Abstract Starting materials, compositions and heat treatments were varied to study the influence of preparation conditions on the structure and superconductivity of Tl-Ca-Ba-Cu-O compounds. Twenty-four different groups of samples were synthesized, and their phases, structures, superconducting parameters and inter-relations were investigated. Most of the samples are superconducting above 100 K. The critical temperatures occupy a rather wide interval of 61–121 K. A few specimens have no superconductivity down to liquid helium temperatures. The number, type, quantity and proportion of phases produced strongly depend on the chemical quality and mixture of the starting materials and on the heat treatment. We searched for all known phases which may be responsible for superconductivity. It has been observed that better superconductivity generally occurs for the more highly crystalline structures. In our experiments, the presence of non-reacted barium cuprates in the samples does not obstruct the establishment of superconductivity, but Tl2O3, CaO, BaO, CuO and other impurities disturb and limit the quality of superconductivity. The superconducting parameters are also affected by the oxygen stoichiometry. The resistivity-temperature curves of some samples indicate the presence of a two-phase system. A change with time of the microstructure of some samples was observed, and was reflected in macroscopic change of the superconducting parameters. This is supported by NMR investigations. Scanning tunneling microscopy indicates an oriented surface structure of the samples.

Construction and parameters of the first high Tc, superconducting ceramic magnets for small fields

Abstract High T c superconducting magnets have been constructed and built of individual ceramic rings. For the elimination of problems of power dissipation on the resistance of electrical contacts, energy was supplied by a contactless magnetic induction method. The magnetic fields produced by two systems of rings were 16.1 and 20.7 mT. Other types of high T c magnets have been built based on superconducting ceramic spirals supplied by d.c. current, which produces a magnetic field of 0.15 and 3.2 mT.

Effect of starting materials on the critical parameters of superconducting Tl Ca Ba Cu O compounds

Abstract Using different batches of starting materials (samples I, TINO 3 , CaO, Ba 2 Cu 3 O 5 ; II, TINO 3 , CaO, BaO, CuO; III, TINO 3 , CaO, Ba(NO 3 ) 2 , CuO; IV, TINO 3 , CaO, BaCuO 2 ; V, TINO 3 , CaO, Ba(NO 3 ) 2 , CuO, O 2 ) and the same heat treatment process, high T c superconducting systems of nominal composition TICaBaCuO 4.5 were prepared. Different superconducting phases were formed depending on the starting compounds used. Specimens I and IV had a single-phase (2,1,2,2) character. In samples II and III the dominant (2,1,2,2) phase was accompanied by (2,0,2,1) and by (1,2,2,3) and (2,2,2,3) phases in small quantity, respectively, while specimens V can be characterized by the existence of CuO and Cu 2 O beside the chief (2,1,2,2) phase. The superconducting properties were affected by starting materials, which manifests itself in the difference in onset and critical temperatures, lower and upper critical magnetic fields, paramagnetic limits, coherence length, penetration depth and Ginzburg - Landau parameter of specimens of different kinds. Due to the weak coupling created by interstitial material, different intergrain, intragrain and direct transport critical currents were determined, depending on the type of samples. The main aim of this work was to compare the experimental data with the corresponding theories to draw conclusions on the role of the starting compounds.

Observation of a diamagnetic signal up to 132 K in a Tl−Ca−Ba−Cu−O compound

Tl−Ca−Ba−Cu−O compounds of the nominal composition of (1, 1, 1, 1) have been prepared from different starting materials and with the same heat treatment process. During the investigation of their properties a diamagnetic signal has been found up to the temperature of 132 K. This hints at the existence of a certain material content distributed into discrete superconducting domains. The parameters of it reflect a superstructure of the (2, 2, 2, 3) and (2, 1, 2, 2) compositions having a time-dependent behaviour. On the basis of the signal levels at 132 K and 4.2 K, the quantity of the high temperature superconducting material is about 0.01% of the total one at liquid He temperatures.

High-T/sub c/ superconducting magnets based on thick film arrangements

High-T/sub c/ superconducting magnets built with Y-Ba-Cu-O or Bi(Pb)-Sr-Ca-Cu-O thick films are discussed. Critical current densities of the samples prepared by an oxalate route are between 7000 and 23000 A/cm/sup 2/ at helium temperatures depending on the details of the preparation. The self-magnetic field of the individual layer rings are 0.5-1.7 mT which can produce field strength of the magnets of 55-178 mT, corresponding to the experimental critical currents. If the specimens of oxide-nitrate reactions can provide critical current densities of 5000-15000 A/cm/sup 2/ at nitrogen temperatures they lead to the number of ampere-turns of 550-22500 A/cm and magnetic field intensity of 69 mT -2.82 T. Since the applicable techniques of the film preparation are very flexible concerning the shape and size of the products, magnetic field profiles of different character can be obtained for various purposes. >

Difference of Thermoelectric Power in Single and Multiple Phase Y-Ba-Cu-O Superconductors

Since the discovery of superconductivity above 30 K by Bednorz and Mullerl and above 90 K by Wu et al.2, an ever increasing effort was made to investigate the physical properties of high-Tc ceramics. Most of the experiments report the resistivity drop and the diamagnetism as fundamental characteristics. The conclusions drawn from these effects do not, however, characterize a true physical property because of the effect of contacts and/or the superposition of simultaneous phenomena. The thermoelectric power (TEP) measurement seems to be one of the most sensitive tools to investigate the transport properties of high-Tc superconductors.

Comparison of superconducting parameters of variously prepared and treated thin films

The interaction of Y-Ba-Cu-O and Pr-Ba-Cu-O or Y-Pr-Ba-Cu-O double layers has been studied. The investigation of the electrical properties has been served for the characterization of the quality of samples. The experimental results of a.c. susceptibility and d.c. magnetization investigations can be evaluated on the basis of the proximity effect.