V. Štrbík

Tl-based superconductors for high-current, high-field applications

We have reviewed the convenience and possibility of utilizing thallium-based superconductors for power applications. Some basic properties of Tl-based systems such as the crystal structure, phase stability, weak-links, pinning problems and irreversibility lines are discussed first. Two basic approaches to conductor preparation are then described, the closed and the open one. Special attention is focused on the fabrication of Tl-1223 and Tl-2223 superconductors. In the case of the more successful open approach a two-step technological procedure is usually chosen: first, the basic precursor material without Tl is synthesized, then second, the precursor is thallinated in a one- or two-zone reaction chamber. The most important precursor preparation techniques are described including aerosol deposition from either solution or ink, electro-deposition, sol-gel methods such as spin- and dip-coating and screen-printing and painting. Finally, some properties of the produced conductors important for power applications are presented.

The role of MgO content in ex situ MgB2 wires

An experimental study of the effect of MgO content in the MgB2 powder used for ex situ made composite wires was carried out. Two single-core MgB2/Fe/Cu wires were made using commercial MgB2 powders from Alfa Aesar containing different fraction of MgO. Critical temperature and critical currents of as-deformed and heat-treated wires were measured. The differences between the wires are discussed and correlated with the MgO content. It was found that by increasing the amount of MgO, the inter-grain connectivity worsens, but well distributed and low size MgO particles improve flux pinning.

Thin YBCO films prepared by low-temperature spray pyrolysis

Thin YBCO films were prepared from aerosol by a low-temperature deposition process consisting of two steps-the deposition at atmospheric pressure and firing in vacuum degraded by partial pressure of oxygen-both at temperatures not exceeding 600 degrees C. A stoichiometric 1-2-3 aqueous nitrate solution of Y, Ba and Cu constituents was used as a source of aerosol. MgO and Al2O3 substrates were heated to 140 to 160 degrees C during deposition. The obtained films were 1 to 10 mu m thick with Tc>80 K and Jc=103 to 104 A cm-2. However, detailed TG, DTG and DTA studies performed by the authors and others showed that thermal decomposition of the nitrates used only starts at temperatures higher than 200 degrees C. The use of low substrate temperatures during deposition (140 to 160 degrees C in this case) is apparently the reason for rather low Jc values. Optimization of the preparation process is suggested, which could lead to higher Jc values at processing temperatures not exceeding 600 to 700 degrees C. Such temperatures are still interesting for substrates with elevated diffusivity into prepared films.

Hysteresis of transport critical currents in high-temperature Y1Ba2Cu3O7-x superconductors: bulk samples and thin films

The current-voltage characteristics and transport critical currents at T=4.2 K and 77.3 K in Y1Ba2Cu3O7-x bulk and thin film samples in magnetic fields up to 9 T have been measured. Large hysteresis of resistively measured critical currents was observed. This is caused by the magnetic field history as well as by the transport current history. An additional explanation of the observed hysteresis based on the different surface and volume pinning mechanisms is proposed.

MgB2 composite wires with Fe, Nb and Ta sheaths

Single-core MgB2 composite wires have been made by the powder-in-tube method using commercial Mg, B and MgB2 powders (Alfa Aesar) in Fe, Nb and Ta tubes and both in situ and ex situ processes. Prepared wires were subjected to annealing at temperatures ranging from 600 °C up to 950 °C for 30 min in argon atmosphere. Resistive (R(T)) and transport current (Ic(μ0H)) measurements have shown how the sheath material that was used influences the critical temperature and critical current density. Inter-diffusion and reaction has been observed only for the iron sheath. Niobium has appeared as the best sheath material for MgB2 wires made by the in situ process but as the worst sheath material for the ex situ method. The reason is mainly due to the large transversal cracks generated in the fully Nb sheathed ex situ wires during the deformation, which are not healed by the subsequent heat treatment.

Highly oriented Y1Ba2Cu3O7 thin films prepared in situ by vacuum co-evaporation

The thin films YBa2Cu3O7 have been prepared on single crystal (100)-SrTiO3 and (200)-MgO substrates by vacuum co-evaporation of Y, BaF2 and Cu. The substrate temperature and total film thickness have been 650-700°C and 150 nm, respectively. The X-ray diffraction, scanning electron microscopy and low-temperature electrical measurements show that prepared films are highly c-axis oriented perpendicular to the substrate, the onset critical temperature Ton ≈ 90 K, zero-resistance critical temperature is TCO = 85 K, and critical current density jC ≈ 3x105 A/cm2 at 4.2 K and zero magnetic field.

Thin BSCCO films prepared by deposition from aerosol

Thin BSCCO 2212 films were prepared by deposition of aerosol generated by a pneumatic nebulizer from stoichiometric aqueous nitrate solutions. A two-step procedure was used: first deposition onto MgO substrate at about 150 degrees C, then firing at T>800 degrees C, both at atmospheric pressure. Detailed TG, DTG and DTA studies of individual constituents as well as their homogenized mixtures have shown, however, that the thermal decomposition of nitrates deposited on a substrate as a precursor film starts only at temperatures higher than 150 to 200 degrees C, and it is completed only during the final annealing step at T>800 degrees C. Principles of the aerosol deposition and thermal decomposition of individual constituents and their stoichiometric mixture are discussed and the use of ultrasonic aerosol generation is suggested.

Basic properties of rectangular MgB2/FeNiCo and MgB2/Fe wires made in situ

Single-core MgB2 wires have been made by the powder-in-tube method using commercial Mg and B powders (Alfa Aesar) and two-axial rolling deformation in FeNiCo alloy and pure iron sheaths. Deformed wires were subjected to annealing at temperatures ranging from 500 up to 750 °C for 30 min in argon atmosphere. Resistive (R(T)) and transport current (Ic(B)) measurements have shown how the sintering temperature influences the critical temperature and critical current densities. Strong inter-diffusion and reaction especially between the Mg + B core and FeNiCo sheath has been observed. This reaction is mostly responsible for the presented differences in the critical parameters of FeNiCo and iron sheathed wires.

The effect of hydrostatic extrusion on the Jc(B) characteristic of ex situ MgB2 wires

MgB2/Fe(Cu) wires fabricated by the PIT method with involvement of the hydrostatic extrusion (HE) process show a noticeable core homogeneity and refined microstructure which result in high transport Jc values. The highest Jc for sintered wire were measured for HE SiC doped wire 104 A cm−2 (Fe clad) and 0.5 × 104 A cm−2 (Cu/Fe clad), both at 4.2 K and 5.5 T. Combination of HE with two-axial rolling (TAR) has led to Jc = 0.4–0.5 × 104 A cm−2 at 4.2 K and 5.5 T. High n-exponent values (between 21–110 at 4.2 K and 5.5 T) prove significant grain refinement (individual grains below 200 nm), homogeneity and density within the hydrostatically extruded cores. After deformation by hydrostatic extrusion the enhanced Jc at higher fields for pure MgB2 and MgB2 SiC nano-doped wires in comparison to only TAR wires were measured. Wires sheathed with Fe gave better results than those sheathed with a bimetallic Cu/Fe sleeve. The beneficial effect of hydrostatic extrusion on Jc(B) characteristics can be attributed to very high core density in comparison to other deformation routes.

Time-resolved carrier dynamics in Hg-based high-temperature Superconducting photodetectors

We present fabrication and optical time-resolved photoresponse characterization of Hg-based high-temperature superconducting (HTS) thin films. The films were prepared from RF-magnetron-sputtered precursor films by an ex-situ mercuration process. Femtosecond, time-resolved, optical pump-probe spectroscopy measurements gave insight into the details of the Cooper-pair breaking and formation processes in our Hg-based films at temperatures far below the materials critical temperature. We observed that the Cooper-pair dynamics is not limited by the phonon bottleneck, which makes this HTS compound the material-of-choice for ultrafast optoelectronics applications.