I. Halász

Thermoanalytical and x-ray diffraction investigations of Ba2Cu3O5+d for preparation of Y-Ba-Cu-O superconductors

Abstract Thermogravimetric (TG) measurements and differential thermoanalysis (DTA) were carried out connected with X-ray powder diffraction investigations of Ba 2 Cu 3 O 5+ d samples prepared at different temperatures. A low temperature tetragonal phase with higher oxygen content and a high temperature tetragonal phase could be distinguished with lattice constants a = 16.19 A and 12.96 A, and c = 12.38 A and 9.16 A, respectively. A possible mechanism for selective preparation of superconductors from high temperature phase has also been proposed assuming a structure with space group P4 2 /mcm.

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.

Comparison of Y-Ba-Cu-O compounds prepared from BaCuO2 and Ba2Cu3O5+?

Series of Y-Ba-Cu-O compositions were prepared from barium cuprates and Y2O3, using a two-step synthesis route. It has been shown that Ba2Cu3O5+δ is essential in formation of the YBa2Cu3O7−δ superconducting phase while BaCuO2 is not an appropriate starting material. A wide composition range fromx=2 to 17 was prepared from Ba2Cu3O5+δ in the YBaxCu1.5xOz series without disappearance of macroscopic superconductivity atTc>77 K. Resistivity measurements hint at the existence of two superconducting phases. ESR investigations revealed a baseline hysteresis, depending on the actual value ofx.

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.

Non-bulk superconductivity induced by neutron irradiation in multiphase YBa2Cu3O7 and Tl2Ca1Ba2Cu2O8

Abstract Irradiation effects of low-dose (10 14 n/cm 2 ) neutrons on the resistance and AC susceptibility of multiphase YBa 2 Cu 3 O 7 and Tl 2 Ca 1 Ba 2 Cu 2 O 8 have been studied. Experimental results show that dispersed local superconducting regions in the samples may be unified to form a continuous percolating network using neutron irradiation. If the concentration of oxygen vacancies and interstitials is below or above an optimum value, the irradiation increases or decreases, respectively, the superconductive volume fraction and - via Cu-O bond length - the T c value.

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.

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.

Magnetic properties of TlCaBaCuO4.5 compounds prepared from different starting materials

It has been shown, that different starting materials result in Tl−Ca−Ba−Cu−O superconductors of different type even at the same heat treatment process. This manifests itself in the developing structure, in the number, share-rate and superconducting properties of the occurring phases as well. Microcrystals, phase-compositions, crystallinities, unit cells, onset and critical temperatures, magnetic flux exclusions, critical currents, magnetizations and critical magnetic fields essentially unlike from each other have been observed, depending on the starting materials. The presence of a starting compound having low melting point, as Ba(NO3)2 can cause a more compact, better superconducting final product, implying better connection between the grains.