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The 9th International Workshop on Processing and Applications of Superconducting (RE)BCO Large Grain Materials (PASREG 2015) was held at the University of Liege, Belgium on September 02- 04, 2015, as a satellite conference of the European Conference on Applied Superconductivity (EUCAS 2015). The Chairman was Philippe Vanderbemden; Benoît Vanderheyden, David Cardwell, and Herbert C. Freyhardt served as Co-Chairmen. PASREG 2015 was the ninth in a series of international workshops previously held in Cambridge, UK (1997), Morioka, Japan (1999), Seattle, USA (2001), Jena, Germany (2003), Tokyo, Japan (2005), Cambridge, UK (2007), Washington D.C., USA (2010), and Tainan, Taiwan (2012).

PASREG 2015 covered novel and important processing aspects of bulk High Temperature Superconductors (HTS) as well as bulk MgB₂. Besides processing, the workshop highlighted advanced material characterization technologies and activation techniques, which revealed the superior superconducting properties of bulk HTS and MgB₂. Novel developments and innovative applications involving bulk superconductors complemented the discussion and exchange of information. The workshop was attended by 63 participants from 19 different countries. A total of 50 contributions were presented, of which 31 were presented in oral form and 19 were presented as posters.

Together with a 'Special Issue' of Superconductor Science and Technology, these proceedings gather a selection of original research papers presented at the workshop (including both oral and poster presentations). We greatly acknowledge all the effort of the authors.

Philippe Vanderbemden

Benoît Vanderheyden

Herbert Freyhardt

David Cardwell

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

The relevant pinning centers of Abrikosov vortices in MgB₂-based materials are oxygen-enriched Mg-B-O inclusions or nanolayers and inclusions of MgB_x (x>4) phases. The high critical current densities, j_c, of 10⁶ and 10³A/cm² at 1 and 8.5 T, respectively, at 20 K can be achieved in polycrystalline materials (prepared at 2 GPa) containing a large amount of admixed oxygen. Besides, oxygen can be incorporated into the MgB₂ structure in small amounts (MgB_1.5O_0.5), which is supported by Auger studies and calculations of the DOS and the binding energy. The j_c of melt textured YBa₂Cu₃O_7-δ (or Y123)-based superconductors (MT-YBaCuO) depends not only on the perfectness of texture and the amount of oxygen in the Y123 structure, but also on the density of twins and micro-cracks formed during the oxygenation (due to shrinking of the c-lattice parameter). The density of twins and microcracks increases with the reduction of the distance between Y2BaCuO5 (Y211) inclusions in Y123. At 77 K j_c=8·10⁴ A/cm² in self-field and jc=10³ A/cm2 at 10 T were found in materials oxygenated at 16 MPa for 3 days with a density of twins of 22–35 per µm (thickness of the lamellae: 45-30 nm) and a density of micro-cracks of 200–280 per mm. Pinning can occur at the points of intersection between the Y123 twin planes and the Y211 inclusions. MTYBaCuO at 77 K can trap 1.4 T (38×38×17 mm, oxygenated at 0.1 MPa for 20 days) and 0.8 T (16 mm in diameter and 10 mm thick with 0.45 mm holes oxygenated at 10 MPa for 53 h). The sensitivity of MgB₂ to magnetic field variations (flux jumps) complicates estimates of the trapped field. At 20 K 1.8 T was found for a block of 30 mm in diameter and a thickness of 7.5 mm and 1.5 T (if the magnetic field was increased at a rate of 0.1 T) for a ring with dimensions 24×18 mm and a thickness of 8 mm.

YBa₂Cu₃O_y (YBCO) foam samples show an open, porous foam structure, which may have benefits for many applications of high-T_c superconductors. As the basic material of these foams is a pseudo-single crystalline material with the directional growth initiated by a seed crystal similar to standard melt-textured samples, the achieved texture of the YBCO is a very important parameter. We analyzed the local texture and grain orientation of the individual struts forming the foam by means of atomic force microscopy and electron backscatter diffraction (EBSD). Furthermore, the magnetic properties of a foam strut are evaluated by means of SQUID measurements, from which the flux pinning forces were determined. A scaling of the pinning forces in the temperature range between 60 K and 85 K was performed. These data and the details of the microstructure are compared to IG-processed, bulk material.

Bulk (RE)BCO superconductors are able to trap record magnetic fields and can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings. When such superconducting (SC) "trapped field magnets" are combined to a ferromagnetic (FM) disc, the total magnetic moment is increased with respect to that of the superconductor alone. In the present work, we study experimentally the magnetic behaviour of such hybrid FM/SC structures when they are subjected to cycles of applied field that are orthogonal to their permanent magnetization, i.e. a "crossed-field" configuration. Experimental results show that the usual "crossed-field demagnetization" caused by the cycles of transverse field is strongly reduced in the presence of the ferromagnet.

A series of disk-shaped, bulk MgB₂ superconductors (sample diameter up to 4 cm) was prepared in order to improve the performance for superconducting super-magnets. Several samples were fabricated using a solid state reaction in pure Ar atmosphere from 750 to 950^oC in order to determine the optimum processing parameters to obtain the highest critical current density as well as large trapped field values. Additional samples were prepared with added silver (up to 10 wt.-%) to the Mg and B powder. Magneto-resistance data and I/V-characteristics were recorded using an Oxford Instruments Teslatron system. From Arrhenius plots, we determine the TAFF pinning potential, U₀. The I/V-characteristics yield detailed information on the current flow through the polycrystalline samples. The current flow is influenced by the presence of pores in the samples. Our analysis of the achieved critical currents together with a thorough microstructure investigation reveals that the samples prepared at temperatures between 775°C and 805°C exhibit the smallest grains and the best connectivity between them, while the samples fabricated at higher reaction temperatures show a reduced connectivity and lower pinning potential. Doping the samples with silver leads to a considerable increase of the pinning potential and hence, the critical current densities.

Superconducting shielding current is excited when external field is applied to superconductor. In case for field cooling of bulk superconductor, shielding current is an origin of strong trapped field. When external field is changed to a properly arranged bulk HTS array, various magnetic field distribution can be formed by an excited shielding current in each bulk HTS. This paper presents a simple intuitively method to design magnetic field distribution using supercurrents in bulk high-temperature superconductor (HTS) array. In this method, an ideal current path for intended field distribution is represented by shielding currents in bulk HTS array. Expected performance can be roughly estimated by using Biot-Savart law. As examples, Maxwell coil pair and helical field generator are designed. This method can be applied to design various magnet devices using bulk HTS array.

The specific heat C_p(T) and entropy S(T) properties of the orthorhombic PrBa₂Cu₃O_6+x compounds in two states of oxygen concentration x, an over doped (OV) with x = 1 and an optimally doped (OP) with x = 0.95 are reanalyzed below the Néel temperature of the antiferromagnetic ordering of the Pr sublattice T_N = 17.5 and 14 K, respectively. Two simultaneous anomalies for both states are observed. The first one occurs near the previous spin reorientation phase transition temperature T₂ ∼ 11.5 and ∼ 9-10 K, respectively whereas the second one remains close to the so called low-critical temperature T_cr ∼ 4-5 K for the OV state as it has been reported before for the OP state. By fitting the C_p(T)/T data to A{T²}^-3/2 + γ + C{T²}¹ + D{T¹}² for T < T_cr the four coefficients obtained with the best adjusted A-squared values are compared with previous findings. Reduced values for y are confirmed in this work. The results which are well described by the contribution of the DT⁵ term to C_p(T) can be connected with the previous Pr-Cu(2) magnetic coupling that is sufficiently enough to cause a modest spin reorientation phase transition at T₂ and a critical magnetic behaviour below T_cr.

During pulsed field magnetization of melt-grown HTS flux jumps can occur and the shielding current falls by 10-20 times. As the duration of pulse is shorter than the temperature relaxation time (<< 1 s), the circular current remains small during the field falling. The residual trapped field in the hole of the annulus has a direction opposite to that of the pulsed field. Small circular current and high critical current density are explained by the fact that flux moves through narrow regions of the annulus body. The angle of the sector with "soft flux" (i.e. a low J_c region) is estimated to be ∼ 7 deg.

Applications of the extended critical state model are considered. The trapped magnetic field, the penetration field and the field dependence of the critical current density are analysed. The critical current density and the trapped field in superconducting grains depend on the grain size. Asymmetry of the hysteresis curves relative to the M = 0 axis is related to the scale of the current circulation.

For the past 10 years, we have studied high-temperature superconducting (HTS) bulk magnets for use in electromagnetic rotating machines. If the magnetic field effectively magnetizes the HTS bulk, then the size of the motor and generator can be reduced without a reduction in output. We showed that the melt-textured Gd-Ba-Cu-O HTS bulk effectively traps a high magnetic field using waveform control pulse magnetization (WCPM). WCPM makes it possible to generate any pulsed magnetic field waveform by appropriately changing the duty ratio of the pulse width modulation. By chopping so that the pulsed magnetic field has a period of about 1ms, the WCPM technology enables active control of the rise time and suppresses magnetic flux motion that decreases magnetization efficiency. This method is also useful for any HTS bulk magnet, and the high magnetic flux density is trapped in the HTS bulk by a single pulse magnetic field. We developed a magnetizer that has a feedback system from the penetrated magnetic flux density to realize WCPM. In this research, using only a single pulse magnetic field of WCPM method at 77K, an HTS bulk with a 45mm diameter and 19mm thickness trapped a maximum magnetic field of 1.63T, which is more than 90% of the trapped magnetic flux density by FC magnetization. This result suggests that the pulse magnetizing method can replace the conventional field-cooled method and promote the practical use of HTS magnets for electromagnetic power applications.

The single-grain YBCO samples were prepared by TSMG process. Two nominal compositions: 1 mol.Y123 + 0.25 mol.Y₂O₃ + 1.0 wt.% CeO₂ (Y123/Y₂O₃) or 70 wt.% Y-123 + 30 wt.% Y-211 (Y123/Y211) and 1.0 wt.% CeO₂ and two different types of substrates were tested. It is shown that the growth rate in both systems is influenced by segregation of CuO in to the nonsolidified part of the sample what causes stop of isothermal growth. Measured melt losses are lower for ZrO₂ substrate than for Y₂O₃/Yb₂O₃ substrate what influence final microstructure of prepared bulk superconductors.

The conditions for single-grain growth of YBCO bulk superconductors by top seeded infiltration growth were tested. It is shown that the interaction of melt formed from BaCuO₂ + CuO + Y₂O₃ precursor with the Sm123 seed causes dissolution of the seed at maximum melting temperature 1045 °C. Experiments with low weight Y211 pellet confirmed that the low concentration of Y in the infiltration melt is responsible for this effect. The most effective way suppressing the seed dissolution was shown to be the insertion of Y123 + Y211 buffer layer between the seed and the Y211 pellet. This buffer layer possesses the melt which is saturated with yttrium what prevents dissolution of the seed.