B. Andrzejewski

The effect of MgO addition on the formation and the superconducting properties of the Bi2223 phase

Abstract MgO particles were introduced into (Bi,Pb)2Sr2Ca2Cu3O10+δ powder in various weight fractions. Bulk samples were made from the Bi2223/MgO powder and heat-treated at various temperatures. The phase formation and the microstructure were studied for different amounts of MgO and for different sintering conditions. It was found that, below 845 °C, the ratio of Bi2223 phase decreased when the amount of MgO increased. The addition of MgO caused the Bi2201 phase content to decrease via an increase of the liquid viscosity. The presence of MgO influenced the microstructure of the samples and decreased the mean size of Bi2223 grains. The effects of the MgO addition on the intergranular critical current density, Jc as a function of magnetic field was investigated by magnetisation hysteresis measurements. MgO presence significantly expands the magnetisation hysteresis loop and thus increases Jc for amount of MgO, which does not exceed 15 wt.%.

The effect of Bi2201 phase on the intergranular critical field and current density in Bi2223 superconductors

Abstract The phase equilibria between Bi2201, Bi2212 and Bi2223 were established relative to the sintering temperature. By improving the sintering conditions (temperature, intermediate grindings), a procedure was determined to prevent the formation of Bi2201 phase and also to obtain nearly pure Bi2223 powder. It was found that the residual Bi2201 phase is an important limiting factor of superconducting properties—samples rich in the Bi2201 phase exhibited almost no intergranular critical currents and fields, while the highest superconducting values were found in the samples free of the phase. Magnetic and transport measurements showed that the transport behaviour of the grain boundaries becomes semiconducting in samples sintered at high temperature, in which an increase of the Bi2201 phase content is observed.

Lower Josephson critical field in granular HTS measured by microwave absorption

The method of magnetically modulated microwave absorption (MMMA) has been used here to measure the lower Josephson critical field in a YBaCuO superconductor. The measurements based on the changes in the width of the hysteresis of the MMMA signal which were observed while increasing the range of the magnetic field sweep were carried out. The effects observed by us are explained using the Clem model of the cubic array of superconducting grains. Also, a simple interpretation of the influence of the magnetic modulation on the measured value of the lower Josephson critical field is provided.

Periodic microwave absorption in superconducting whiskers

The microwave absorption and structural studies of BiSrCaCuO (2212) whiskers have been performed. The low-angle boundaries in the (a, b) and (a, c) planes have been observed. The periodic microwave absorption signal indicated the flux quantization in a system of weak links and superconducting loops formed on the boundaries. Regular character of this signal suggests possible applications in SQUID magnetometry. Lower Josephson critical fieldHclJ=1.6 Oe and the critical temperatureTc=89 K have been determined. The Josephson penetration depth was estimated from the Ginzgurg-Landau, and Clem models.

Inter- and intragrain critical fields in HTS measured by microwave absorption

Abstract For the first time the method of magnetically modulated microwave absorption (MMMA) has been used here to measure the Josephson critical fields and the lower critical field of the grains in a YBaCuO superconductor.

Influence of microwave power on magnetically modulated microwave absorption in a granular high-temperature superconductor

Abstract In this paper we present the influence of microwave power and temperature on magnetically modulated microwave absorption (MMMA) in the granular ceramic superconductor YBa 2 Cu 3 O 6.9 (Y-123 phase). A simple Josephson junction local-temperature model describing observed changes of MMMA linewidths as a function of microwave power is proposed. The agreement between experimental results and the proposed model proves that in high-temperature superconductors absorption of microwave power occurs mainly in intergrain Josephson junctions of a ceramic sample.

The effect of the YBCO-PST composite composition on the superconducting carrier concentration determined by microwave studies under high pressure

Abstract The effect of hydrostatic pressure ( p T c in YBa 2 Cu 3 O 7− δ –Pb(Sc 0.5 Ta 0.5 )O 3 (YBCO–PST) composite is measured by the method of magnetically modulated microwave absorption (MMMA). The T c dependence on the PST fraction in weight x (0, 0.25, 0.5 and 0.75) is approximated by an inverted parabola function whereas the influence of pressure on T c is represented by the equation: d T c /d p =0.61(2)−1.72(6) x . The result may be explained assuming that PST phase in YBCO–PST composite influences the superconducting carrier concentration similar to the chemical substitution in YBa 2 Cu 3 O 7 [J.J. Neumeier, H.A. Zimmermann, Phys. Rev. B 47 (1993) 8385]. It is suggested that ions from PST diffuse to YBCO cell during the sintering of the composite.

Demagnetising effect in HTS in GHz frequency fields

In the present paper, we investigate the demagnetising effect in granular superconductors by applying the method of magnetically modulated microwave absorption (MMMA). The quantitative description is based on microwave measurements of demagnetising factors in cylindrical high-temperature superconductors (HTS). The results prove to be in good agreement with the calculations of Chen et al. [D.-X. Chen, J.A. Brug, R.B. Goldfarb, IEEE Trans. Magnetics 27 (1991) 3601] of demagnetising factors for cylindrical samples. The shielding effect caused by the sample as well as by individual grains, whose influence is especially noticeable for highly porous materials, is also discussed. The final results point to the necessity of allowing for the demagnetising effect in microwave absorption studies of superconductors.

The effects of growth conditions on superconducting properties of Nd1Ba2Cu3O7−δ single crystals

Abstract The influence of thermal treatment conditions on the structure and superconducting properties of single crystal Nd 1 Ba 2 Cu 3 O 7− δ growths has been studied. The crystals were grown in air by the flux method. The microstructure of the crystals was studied using scanning electron microscopy and scanning tunnelling microscopy. To examine the superconducting properties of the crystals measurements of the DC magnetic moment in applied magnetic fields were performed. A superconducting transition was not observed in any of the samples. This may have been a result of Nd/Ba or Cu/Al substitution.

Magnetic study of YBCO-PST composites

Abstract The magnetic properties of granular superconductor–insulator composites were investigated. The composites were intimate mixture of the YBa 2 Cu 3 O 7− δ (YBCO) high-temperature superconductor and Pb(Sc 0.5 Ta 0.5 )O 3 (PST) ferroelectric insulator. The observed decay of superconductivity, lowering of the critical temperature and increasing of the number of paramagnetic centres as PST content increases are explained assuming interdiffusion among YBCO and PST elements and formation of an additional phase, which did not exist in the initial powders.