M Majoros

Superconductivity of powder-in-tube MgB2 wires

A new class of `powder-in-tube Mg-B superconducting conductors has been prepared using two different methods: an in situ technique where an Mg + 2B mixture was used as a central conductor core and reacted in situ to form MgB2, and an ex situ technique where fully reacted MgB2 powder was used to fill the metal tube. Conductors were prepared using silver, copper and bimetallic silver/stainless steel tubes. Wires manufactured by the in situ technique, diffusing Mg to B particles experienced ~25.5% decrease in density from the initial value after cold deformation, due to the phase transformation from Mg + 2(β-B)→MgB2 all with hexagonal structure. A comparative study of the intergranular current and grain connectivity in wires was conducted by AC susceptibility measurements and direct four point transport measurements. Using a SQUID magnetometer, magnetization versus magnetic field (M-H) curves of the round wires before and after sintering and reactive diffusion were measured at 5 K and in magnetic fields up to 5 T to define the Jcmag. The direct current measurements were performed in self field at 4.2 K. A comparison between zero-field-cooled (ZFC) and field-cooled (FC) susceptibility measurements for sintered Ag/MgB2, and reacted Cu/Mg + 2B conductors revealed systematic differences in the flux pinning in the wires which is in very good agreement with direct high transport current measurements.

AC losses in striated YBCO coated conductors

We measured AC losses in RABiTS nonstriated as well as striated coated conductors provided by the US Air Force Research Laboratory. In addition, samples with different kinds of well defined inter-filament bridging have been measured and their losses compared with those of samples with unbridged filaments and with mono-layer samples. This bridging is designed to aid current sharing between filaments, which is advantageous, but it is necessary to determine if the effect on AC losses is detrimental. It was found that AC losses of striated samples with multiple bridges are higher than those of the samples with no bridges due to significant filament coupling but even so the losses are still substantially lower than those of a monolayer sample.

Transport ac losses and screening properties of Bi-2223 multifilamentary tapes covered with magnetic materials

Abstract DC current–voltage characteristics and transport ac losses in Bi-2223 multifilamentary tapes covered with different magnetic materials were measured. The results are compared with those without the magnetic cover. Also the measurements of screening properties of the magnetically covered tape exposed to a magnetic field generated by another parallel Bi-2223 tape with different orientations of the current were performed. The iron powder was found to be an effective magnetic screen to decouple the tapes. Transport ac losses of the magnetically screened tape were not affected by the current in the neighbouring tape. It was found that the presence of a continuous magnetic cover decreases the critical current I c of Bi-2223 tape by about 20%. In the range of currents 0.5 I c − I c , the total losses in the Bi-2223 tape covered with iron powder were increased 2.5 times, including the losses in the iron.

Magnetic screening as a possible way to decrease transport AC losses in multifilamentary superconductors — basic theoretical considerations

The proposed method to decrease transport AC losses in multifilamentary superconductors is based on principle of magnetic de-coupling of the filaments using magnetic screening. From a simple theoretical model it was found that while for a superconducting filament with circular cross-section the presence of a magnetic surrounding does not influence the magnetic field distribution within its cross-section nor the transport AC losses, for a filament with an elliptical cross-section the magnetic surrounding affects both of these parameters. The slight increase of the losses in the superconducting filaments caused by the presence of the magnetic material and the losses in the magnetic material itself and also some reduction of the transport current is minor in comparison with reduction of AC losses in the composite by magnetic de-coupling of the filaments. This reduction is more pronounced if the number of filaments is high.

Pulsed laser deposition of epitaxial YBa2Cu3O7−y / oxide multilayers onto textured NiFe substrates for coated conductor applications

Pulsed laser depositions of double-buffer and triple-buffer YBa2Cu3O7−y (YBCO)/Y2O3(YSZ)/CeO2 heterostructures have been performed in situ onto commercially available biaxially textured NiFe 50%/50% tape. The deposition in the forming gas (4% H2/Ar) from a CeO2 target and the deposition in vacuum from a CeO2:Pd composite target have been explored as two possible routes for cube-on-cube growth of the first buffer layer. The influence of the critical processing parameters on the texture is investigated and some of the issues involved in the reduction of NiO (111) and the formation of cube-on-cube NiO (200) growth are discussed. X-ray diffraction has been used for texture evaluation of the substrate and subsequent deposited layers. The substrate–buffer interface region has been studied by focused ion beam cross section electron microscopy. Both the buffers and YBCO layers show biaxial alignment with ω and scans having optimum YBCO full width at half maximum (FWHM) values of 4.3° and 8.8°, respectively. The morphology has been characterized using atomic force microscopy and scanning electron microscopy. The value of Tc (onset) has been measured at 90 K (ΔTc = 10 K). The critical current density, Jc, has been measured by transport measurements and magnetic measurements performed in a dc SQUID magnetometer.

Enhanced transport currents in Cu-sheathed MgB2 wires

Copper-sheathed MgB2 wires, prepared by an in-situ process, were exposed to neutron radiation in order to introduce defects into the superconductor. The high level of disorder (4.6 × 10−2 dpa) leads to a decrease of the transition temperature by more than 4 K, but to an increase of the slope of the irreversibility line, thus resulting in higher irreversibility fields at low temperatures. The transport currents are significantly enhanced at 4.2 K for fields above 2 T.

Niobium based intermetallics as a source of high-current/high magnetic field superconductors

Abstract The article is focused on low temperature intermetallic A15 superconducting wires development for nuclear magnetic resonance and nuclear magnetic imaging, MRI, magnets and also for cryogen-free magnets. There are many other applications which would benefit from new development such as future large Hadron collider to be built from A15 intermetallic conductors. This paper highlights the current status of development of the niobium based intermetallics with special attention to Nb 3 (Al 1− x ,Ge x ). Discussion is focused on the materials science aspects of conductor manufacture, such as β-phase (A15) formation, with particular emphasis on the maximisation of the superconducting parameters, such as critical current density, J c , critical temperature, T c , and upper critical field, H c2 . Many successful manufacturing techniques of the potential niobium–aluminide intermetallic superconducting conductors, such as solid-state processing, liquid–solid processing, rapid heating/cooling processes, are described, compared and assessed. Special emphasis has been laid on conditions under which the J c ( B ) peak effect occurs in some of the Nb 3 (Al,Ge) wires. A novel electrodeoxidizing method developed in Cambridge whereby the alloys and intermetallics are produced cheaply making all superconducting electromagnetic devices, using low cost low temperature superconductors, more cost effective is presented. This new technique has potential to revolutionise the existing superconducting industry enabling reduction of cost orders of magnitude.

MgB2 superconductors for applications

Abstract The paper presents up to date results on MgB 2 filamentary conductors and coated conductors from the point of view of their possible applications. Basic physical parameters of MgB 2 compound relevant to conductor design are introduced. The correlation between the slope of the irreversibility line, critical temperature and critical current density is discussed with respect to the conductor performance and applicability. The possible origin of the observed anomalous decrease of ac susceptibility at 50 K in copper clad in situ powder-in-tube MgB 2 wires is presented. Some numerical results on critical currents, thermal stability and ac losses of the MgB 2 multifilamentary and coated conductors with magnetic cladding of their filaments are given.

Composite Cu/Fe/MgB2 superconducting wires and MgB2/YSZ/Hastelloy coated conductors for ac and dc applications

We discuss the results of a study of MgB2 multifilamentary conductors and coated conductors from the point of view of their future dc and ac applications. The correlation between the slope of the irreversibility line induced by neutron irradiation defects and in situ structural imperfections and the critical temperature and critical current density is discussed with respect to the conductor performance and applicability. We debate the possible origin of the observed anomalous decrease of ac susceptibility at 50 K in copper clad in situ powder-in-tube MgB2 wires. Different conductor preparation methods and conductor architectures, and attainable critical current densities are presented. Some numerical results on critical currents, thermal stability and ac losses of future MgB2 multifilamentary and coated conductors with magnetic cladding of their filaments are also discussed.

YBa2Cu3O7−δ coated conductor deposited onto non-magnetic ternary alloy NiCrW RABiTS tape by in situ pulsed laser deposition

Abstract Pulsed laser deposition of YBa 2 Cu 3 O 7− δ (YBCO)/buffer (Y 2 O 3 , YSZ, CeO 2 ) heterostructures have been performed in situ onto recently developed non-magnetic oxygenation resistant NiCrW tape. The influence of the critical processing parameters on texture development are investigated and the issues involved in NiO formation and relation to the substrate surface quality are discussed. The roles of Ni poisoning YBCO as well as local cation disorder are considered as possible current limiting factors. X-ray diffraction has been used for macro-texture evaluation. Both buffers and YBCO layers show good biaxial alignment with ω and ϕ scans having best YBCO FWHM values of 4.0° and 6.5° respectively. A comparison is made with results achieved on industrial Ni 50 Fe 50 tape. The film morphology has been characterized using atomic force microscopy and scanning electron microscopy. The cation disorder has been studied by Raman spectroscopy. Critical temperatures of 90 K (Δ T c =5 K) have been measured. Direct transport as well as magnetic measurements shows the critical current density J c is 0.2 MA/cm 2 in self-field at liquid nitrogen temperatures.