Multilayer epitaxial structures YBCO-interlayer-YBCO were formed by pulsed laser deposition (PLD) method on the cube textured Ni-W tapes. Standard buffer architecture Y₂O₃-YSZ-CeO₂ was applied. Use of SrTiO₃ interlayers enabled to stop YBCO film overgrowth: morphology and microstructure degradation and maintain high critical current density (Jc). Jc in 1 μm thick multilayer film composed of four 250 nm thick YBCO layers is comparable to Jc in single layer 250 nm thick YBCO film. Current carrying capacity of the introduced method amounted to 220 А/cm on 1 μm thick YBCO. Application of this method allows overcoming such fundamental problem as degradation of Jc in thick films and multiplying critical current (Ic) with number of YBCO layers.High Jc value (more than 2 MA/cm²) demonstrates free current flowing through interlayers and potentiates to form YBCO films without substantial Jc degradation in separate layers.

In order to apply coated conductors operated at boiling temperature of liquid nitrogen (77.3 K) to various fields, an improvement of the superconducting properties are required. In the case of REBa₂Cu₃O_y (RE123; RE: rare earth elements) coated conductors, a biaxial orientation of RE123 crystals on substrate and the improvement of a critical current density (J_c) have been greatly conducted. However, this RE123 coated conductor needs a high growth temperature during film fabrication that causes degradation of superconducting properties due to impurity diffusion from metallic tape substrate. Moreover, for the achievement of high critical current (I_c), a particular technique for the fabrication of thick RE123 film is needed. In order to establish the fabrication method of high performance RE123 films by feasible simple process, we endeavored to fabricate the RE123 films on single crystalline substrate by liquid phase epitaxy method using KOH flux at low temperature. Obtained films of Nd123 fabricated in 1-atm air showed bi-axial orientation of 123 single phase completely above 425°C. However, the T_c^onset of Nd123 film fabricated at 525°C denoted 67.1 K which was drastically lower compared with previous reports of Nd123 single crystal prepared by Ba-Cu-O flux at high fabrication temperature, and then T_c^onset degraded with decreasing fabrication temperature. It is well known that the T_c degrades with increasing amount of RE substitution for Ba site and the RE/Ba substitution is suppressed by lowering of oxygen partial pressure in ambient atmosphere. From these facts, we succeeded in the fabrication of high-T_c (> 90 K) Nd123 film using KOH flux method at low temperature by controlling of oxygen partial pressure.

This research was partially supported by “Japan Society for the Promotion of Science (JSPS): Grant-in-Aid for Scientific Research (26870382)”

Abstract

One of the main obstacles towards the realization of YBa₂Cu₃O_7-x (YBCO) wires by the powder-in-tube method is the presence of grain boundaries (GB) which strongly limit the performances of the cables. These come from the misorientation of adjacent grains and result in an array of weak links that limit the critical current density. To this date many progresses have been made but due to the complexity of the task and the practical impossibility to reach a perfect alignment between all the grains, research groups have largely moved to coated conductors where only the in-plane orientation is an issue. For this last concern, it seems reasonable to think that part of the effort should be devoted to the improvement of the properties of the grain boundaries themselves.

This work was motivated by the possibility that the degradation of the electrical transport properties at GB is caused by a local depletion of the charge carrier density. In this context, we used electric field effect to modify the charge carrier density in YBCO with the final aim of tuning the transport properties of GB. This technique has the advantage to induce a change in the doping of the system without introducing disorder, as for example in the case of chemical doping.

Due to the high carrier density brought by each atomic plane of YBCO, electric field effect is only effective on ultra-thin YBCO layers. Hence we restricted our study to ~10 nm films deposited on single crystalline SrTiO₃ substrates by Pulsed Laser Deposition. We checked the morphological and structural quality of the films by a series of techniques including X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). I-V curves at different temperatures (down to 4.2 K) and in high magnetic field (up to 20 T) were studied as a function of the hole doping.

We formed multilayer epitaxial structures YBa₂Cu₃O_x- SrTiO₃ by pulsed laser deposition (PLD) method on textured Ni-W tapes in order to overcome such fundamental problem as degradation of critical current density along with YBCO layer thickness. Critical current in four-layer structure (250 nm, 1 μm thick in total) of 22.0 А indicates multiplying critical current with number of YBCO layers. High critical current values in multilayer films demonstrate free current flowing through 10-50 nm thick dielectric interlayers that creates an opportunity to pass current flowing through whole structure. We carried out thorough transmission electron microscopy measurements to understand and reason about such effect. It could be explained by RABiTS tape grain misorientation to high angles and interlayer discontinuities near grain boundaries. Inhomogenuities in interlayers serve as channels for necessary oxygen diffusion into the lower YBCO layers, so that critical characteristics of each YBCO layer remain high and comparable to ones of monolayer YBCO films.

Authors would like to thank M.U. Presnyakov, A.L. Vasiliev and V.V. Guriev for help with measuring.

This work was supported in part by the Engineering and Physical Sciences Research Council, U.K.

The fluorine-free metal organic decomposition (FF-MOD) is one of the promising methods to prepare REBa₂Cu₃O_y (RE123, RE:rare earth elements) thin films because of the simple and fast chemical reaction to form highly homogeneous RE123 phase without generating any harmful gases. However, FF-MOD thin films show lower critical current properties in the magnetic fields due to lack of effective pinning centers. Our preliminary study revealed that Cl doping to the starting solution resulted in generation of columnar oxychloride crystals, Ba₂Cu₃O₄Cl₂ (Ba2342), which assist c-axis alignment of Y123 and apparently enhance in-field J_c [1]. In the present study, we have studied magnetic angular dependence of critical current density, J_c(θ), for Cl-free, Cl-doped and Cl and dilute impurity metal co-doped Y123 films as well as microstructures of these films. In particular, Cl and Sn co-doped Y123 films showed unprecedented J_c(θ) properties, suggesting the existence of c-axis correlated pinning, which are usually the case with nanorods-introduced films prepared by PLD method or heavy-ion irradiated films. This J_c(θ) properties have never been reported in unirradiated films prepared by the MOD method. Detailed studies on the mechanism are now underway. Also, effects of Cl-doping on the synthesis conditions of Y123 films will be reported. By doping of Cl, c-axis aligned Y123 films can be obtained under synthesis conditions of 760°C, P_O2 = 10 Pa, 30 min, which is 40°C lower than the conditions for Cl-free films.
[1] T. Motoki et al., Supercond. Sci. Technol. 27 (2014) 095017.

Cube texture formation of Cu-33 at.%Ni alloy substartes and CeO2 buffer layer prepared by chemical solution deposition on the textured substrate were investigated by electron back scattered diffraction (EBSD) and XRD technics systematically. The results shown that a strong cube textured Cu-33at.%Ni alloy substrate with the cube texture fraction of 99.8 % (< 10°) was obtained after annealing at 1000°C for 1 h. The full width half maximum (FWHM) values for the X-ray (111)phi-scan and (002) omega-scan in this substrate were 7.31° and 5.51°, respectively. Furthermore, the cube texture fraction of epitaxially grown CeO2 buffer layer was 95 % (< 10°), and the FWHM values of phi-scan and omega-scan being 6.98° and 5.92°, respectively.

Gd-Ba-Cu-O tapes with the starting composition, GdBa₂Cu₃O_7-δ (Gd1.00) and Gd_1.18Ba_2.12Cu_3.09O_7-δ (Gd1.18), was fabricated by a suspension spinning method. The suspension composed with Gd1.00 and Gd1.18 powder, dimethyl sulfoxide, hexamethylphospheric triamide and poly(vinyl alcohol) was pushed into methyl alcohol using a syringe. This chemical fabrication process is just suitable for fast and low-cost formation of superconducting tapes. Samples thus prepared were heated at temperature range of 980-1040 °C in oxygen controlled atmosphere. The superconducting properties of samples were strongly dependent on both the starting composition and heat treatment condition, such as partial melting temperature and the cooling rate for preferable peritectic reaction. A high Jc value of 1.2×10⁴ A/cm² with good reproducibility under 0T at 77.3K was obtained for the Gd1.18 sample heat-treated under 1%O2+Ar atmosphere with cooling rate of 10 °C/h from 1000 °C. Furthermore, Ag₂O doping into this Gd1.18 tape was found to be effective a crystal growth of Gd123 phase, the refinement of the size of the Gd211 particle and the reduction of voids and cracks in microstructure. The degradation of samples was also examined by soaking the tapes in ultrapure water. The Gd1.18 doped with Ag₂O sample was also found to exhibit more water resistant property compared with the Ag free sample.

REBa₂Cu₄O₈ (RE=Y, Nd, Sm, Gd, Er, Yb, etc.; RE124) exhibits characteristic critical temperature (T_c) behavior which is not observed in REBa₂Cu₃O₇ (RE123), and T_c higher than 90K is obtained by Ca substitution for RE or applying pressure. To understand T_c mechanism and for further control of T_c, T_c in RE124 films should be investigated, and RE124 epitaxial films are needed for this kind of study. However, the phase diagram of Y-Ba-Cu-O indicates that RE124 single phase formation is difficult since other phases such as RE123 coexist depending on temperature and composition, RE124 becomes unstable with decrease in oxygen pressure, and RE124 dissolves into RE123 at high temperature. Therefore, understanding of phase formation mechanism of RE124 is needed to control phases and structure in the films. In the present study, temperature and annealing time dependence of phase formation from Yb124 solution in metal organic deposition (MOD) was discussed in detail. Yb124 films were prepared by annealing calcined films in 740-840 ºC for 3-24 hours. Structure of the films was evaluated using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Yb124 was formed without Yb123 at 760ºC, Yb123 and Yb124 coexisted at 780-800 ºC, and Yb123 was formed at 820-840ºC together with slight amount of Yb124. Although Yb124 started to form after 9 hours annealing, less than 3 hours annealing resulted in Yb123 phase formation. This shows that phase formation of Yb124 is much slower than that in Yb123 due to large volume of unit cell in Yb124. Based on the phase diagram, phase formation kinetics and surface and interface energy, the Yb124 phase formation mechanism and condition for Yb124 single phase films are discussed.

This work was supported by the ALCA project of the Japan Science and Technology Agency.

110 cm long bi-axially textured Sm0.2Ce0.8O1.9-x (SCO) single buffer tape has been prepared on textured NiW(200) alloy substrate by using the Polymer Assisted Chemical Solution Deposition (PACSD) method combined with slot-die coating technique for REBCO coated conductors. Smooth, uniform, and crack-free SCO tape with sharp (200) c-axis orientation and highly texture has been obtained by controlling the viscosity of solution, the moving speed of tape and the release rate of solution, bi-lateral texture of tape has be more than 90%. The influence factors, i.e. the viscosity of solution, the moving speed of tape, and release rate of solution on the surface morphologies and textures of SCO buffer prepared by slot-die coating method have been studied in this paper.

This work was supported by the National Natural Science Foundation of China (Nos. 51202202, 51271155), National Magnetic Confinement Fusion Science Program (2011GB112001), Program of International S&T Cooperation (S2013ZR0595).

The implementation of YBa₂Cu₃O_7-δ coated conductors in large scale applications generally requires an improvement of the superconductors’ in-field performance. The incorporation of nanostructured secondary phases has proven to be an adequate approach to achieve flux pinning, hereby preventing the drastic decrease of the critical current density J_c at moderate-to-high magnetic fields as well as its angular dependency on the magnetic field. The most effective pinning occurs when the nanometer-sized defects are homogeneously distributed throughout the superconductor. To achieve this, we have produced nanocomposite thin films starting from preformed nanocrystals (PNC) in combination with low fluorine chemical solution deposition. The use of PNC’s generally offers a better control of the final microstructural properties of the nanocomposite films when compared to self-assembled nanocrystal formation during YBCO processing.
In this work, small ZrO₂ nanoparticles with a diameter of 3-4 nm were synthesized and subsequently stabilized in the low fluorine YBCO precursor solution via a ligand exchange procedure. Afterwards, the YBCO-PNC solutions were deposited on single crystal LaAlO₃ substrates via both spin-coating and ink-jet printing. The main focus of this research pointed to understanding the factors which control the microstructure development of the nanocomposite thin films using X-ray diffraction and electron microscopy. By strict optimization on both the precursor and processing level, we achieved nanocomposite thin films exhibiting J_c > 4 MA/cm² at 77 K in self-field and a strong enhancement of the pinning force and the superconducting properties over the complete magnetic field and temperature range.
This newly developed approach delivers scalable and high quality superconducting films, capable of meeting the strict requirements for the successful implementation and distribution of coated conductors throughout energy market.

In the chemical solution deposition process of YBCO superconducting films, fluorine is of significant importance in avoiding the formation of BaCO₃, which hinders the growth of high-quality YBCO films below 800 ℃. On the other hand, great efforts have been made to decrease the fluorine content in the precursor solution due to the environmental problems of the gaseous fluorinated by-products. In this study, we demonstrated that a F/Ba atomic ratio of 2 in the precursor solution, which corresponds to a 10% fluorine content of the conventional TFA-MOD solution, is sufficient since ATR-FT-IR characterizations revealed that BaCO₃ in the pyrolyzed film was remarkably suppressed. The intermediate phase evolution prior to the formation of YBCO was systematically investigated. During the pyrolysis process, in-situ FT-IR showed that no fluorinated gaseous by-products were generated. Thus all fluorine atoms were supposed to remain in the pyrolyzed film as fluorides. By performing XRD theta/2theta scan, BaF₂ but neither YF₃ nor CuF₂ was detected in the films quenched at 400 - 800 ℃. The formation priority of BaF₂ over YF₃ and CuF₂ was in accordance with the chemical equilibrium calculation of the potentially involved reactions. Moreover, the intermediate BaF₂ phase was found to be highly textured when processing temperature is above 700 ℃, which was supposed to be the nucleation templates of YBCO. As a result, YBCO films with J_c of 5 and 1.8 MA cm^-2 (77 K, 0 T) could be obtained on LAO single crystal substrates and buffered RABiTS tapes, respectively.

The work presents the growth of YBa₂Cu₃O_7-x (YBCO) films from a precursor solution with low-fluorine content for coated conductors applications. The low fluorine precursor solution was prepared starting from yttrium acetate, barium trifluoroacetate, copper acetate and propionic acid. The total content of fluorine in this solution is about 30% with respect to the all TFA solution. The kinetics of the thermal decomposition of the precursor powder was studied under air, oxygen and humid oxygen atmosphere from ambient temperature up to 1000 ^oC. The coated solution was deposited on (100) SrTiO₃ single crystalline substrates by the spin coating. The as-obtained precursor YBCO film was pyrolyzed at 400 ^oC for 1h in humid oxygen atmosphere. The final films were annealed at 845 ^oC for 2 h in humidified nitrogen mixed with oxygen, then were cooled to 450 ^oC and kept at this temperature for 1 h in dry oxygen atmosphere.
The phase evolution during the thermal treatment of the YBCO films, were studied by grazing incidence X-ray diffraction techniques, and surface morphology by atomic force microscopy. The chemical composition and the depth compositional profiling of the YBCO films were performed using auger electron spectroscopy. The as-deposited YBCO films have a high epitaxial degree with full Width at Half Maximum of ω-scan of 0.2^o and a dense morphology with Tc = 91.5 K (R=0) and Jc = 3.6 MA/cm², respectively.

The research leading to these results has received funding from the European Union Seventh Framework Programme [FP7/2007- 2013] EUROTAPES under grant agreement n° NMP-LA-2012-280432.

The use of low-cost techniques such as chemical solution deposition (CSD) for the growth of high performance YBa2Cu3O7-d (YBCO) coated conductors (CCs) is one of the major requirements in obtaining a widespread use of superconductivity in large scale applications. CSD has become a very competitive cost-effective and scalable methodology to produce epitaxial thin films and so extensive analysis of all the relevant processing steps is worthwhile.
The last processing step in any CC manufacturing process is devoted to achieve optimal oxygen content. Achieving optimal oxygen content in YBa2Cu3O7-d films is actually a critical issue in the development of high critical current superconductors. On the other hand, it is well known that microstructural and morphological features of the films can have a strong influence on the oxygenation kinetics. For that reason, it is very attractive to use in-situ monitoring tools of the oxygenation process to achieve a full control of the final carrier concentration in the films and coated conductors.
In this work we will show that in-situ electrical resistivity measurements is a very useful tool to analyze the kinetic effects in the oxygenation process of YBCO films.. We have particularly concentrated in elucidating the mechanism of oxygen incorporation and excorporation in standard and nanocomposite YBCO films grown from TFA precursors by CSD and we have determined its dependence on several processing parameters. From the analysis of the influence of PO2, T and gas flow we conclude that in our films the rate determining step in oxygenating YBCO thin films is actually the surface reaction instead of the usual believe that bulk diffusion is the most relevant parameter. After achieving this conclusion we demonstrate that advanced strategies and methodologies can be used to fasten oxygenation kinetics and to finely tune the oxygen content and so of the final carrier concentration in the YBCO films. Hall effect measurements have been used to monitor the carrier concentration and we have analysed its correlation with the film microstructure and the superconducting properties, particularly Tc and the critical currents.

The research leading to these results has received funding from EU-FP7 NMP-LA-2012-280432 EUROTAPES project and MAT2014-51778-C2-1-R national project.
 

We found that the magnetic field dependence of the surface resistance (Rs) of YBCO films in high magnetic field was quite different by the applied magnetic field direction. The Rs(90) when magnetic field was applied perpendicular to the a-b plane increased with increasing the applied magnetic field, however, the Rs(0) when magnetic field was applied parallel to the a-b plane did’nt change. We can explain above the results by the movement of the vortices, because the Rs depends on the movement of the vortices. When a magnetic field is applied parallel to the a-b plane blocking layers in YBCO crystal act strongly as intrinsic pinning centers, therefore vortices cannot move, and magnetic field dependence of the Rs(0) is quite small. On the other hand, the case of applying a magnetic field perpendicular to the a-b plane, there is nothing to work as strong pins in the film, therefore the magnetic field dependence of the Rs (90) is large. For the blocking layer work as a pin, its size should be less than about twice the coherence length. Thus, the magnetic field dependence of the Rs(0) is expected to be larger in higher temperature region. In fact, the dependency measured at 70 K was larger.

This research was partially supported by the Promotion of Innovative Research and Development
(Development of next-generation NMR technology using HTS materials)

With the improvements in nanofabrication techniques, studies with materials patterned on micro and nanometric scales have been intensified. At these sizes, superconducting materials exhibit a variety of mesoscopic phenomena that arise due to confinement effects. Latter have been utilized in devices such as single electron detectors, amplifiers, resonator cavities, single photon detectors (SPDs) and other. The presence of magnetic fields and/or transport currents can affect the performance of such devices: induce the penetration of vortices, their motion, hence cause dissipation and ruin the sensibility of the device. Therefore a detailed study of the dissipation mechanisms in mesoscopic superconductors is very important for future advances and improved design of superconducting devices. In the present work, we analyzed the contributions and interplay of the dissipative mechanisms due to the locally induced electric field and an intrinsic relaxation of the superconducting order parameter, ψ, in mesoscopic geometries, using the time-dependent Ginzburg-Landau theory. Although often neglected, we show that for very small superconductors, the dissipation due to the relaxation of ψ during the flux penetration is as important as the dissipation generated by electric field.

This work was supported by Brazilian agencies São Paulo Research Foundation (FAPESP), grant 2013/17719-8 and CAPES.

A large number of vortices in field-cooled isothermal distributions were examined using scanning SQUID microscopy on YBa₂Cu₃O_7-x thin films grown by pulsed laser deposition. An isotropic vortex glass state was found for all fields measured in the $\mu$T range. This glass showed no orientational ordering but retained some short range positional order. Groups of abnormally closely spaced vortices were also observed above a threshold field.
microTesla and Sub-microTesla fields were maintained throughout cooling by magnetic shielding and actively offsetting the Earth's field.
A weak short-range positional order was found through autocorrelation on the vortex distribution. Almost all vortices were found to have six nearest neighbours, however Delaunay triangulation showed that the lattice had very little orientational order.
A small but significant portion of the vortices were found to have an intervortex spacing around half of the average spacing, and these were seen to form groups of closely positioned vortices. The origin of this grouping will be discussed, but is ultimately unclear and may be the subject of further work.

We have observed periodic magnetoresistance oscillations around 80 K in a 5 microns wide channel with a100 nm-size hole fabricated on a film of the multiband superconductor (Tl_xCu_1-x)(Ba_ySr_1-y)₂Ca₂Cu₃O_z. The period of magnetoresistance oscillations is consistent with h/2e, where h is the Plank constant and e is the electron charge. In the oscillations, a phase shift is shown around 200 mT, which is similar to previous observations [1] in a mesoscopic superconducting ring with two order parameters. We discuss the implication of the phase slip for multiband superconductivity. In the session, we will report the details of the Little-Parks experiment and the experimental results.
[1] H. Bluhm et al. Phys. Rev. Lett. 97, 237002 (2006)

This study was supported by Nanofabrication platform in Nanotechnology project sponsored by Manistry of Education, Culture, Sports, Science, and Technology (MEXT), Japan

In YBCO films doped with artificial pinning centers (APC), such as BaZrO₃ or BaCeO₃ nanorods or nanodots, the critical current density, J_c, is usually described with the form J_c ~ B^-alpha even though the shape of the curve does not really allow this. In the field region just above the low field plateau, the shape of the J_c(B)-curve (in log-log scale) is rounded and not straight as in undoped films. The exponent alpha is found to decrease from 0.5 to around 0.2 in BaZrO₃ doped films and 0.4 in BaCeO₃ doped films. The incompatibility with the curved data and the linear fit has lead to publication of alpha-parameters which are not comparable to each other due to different fitting limits. In this paper we show that it is better to do the fit of alpha with the Dew-Hughes pinning force F_p(x) = F_p0(B/B_irr)^p(1-B/B_irr)^q, where alpha = p-1, and that the alpha and the roundness of the curve depend on the diameter of the pinning centers, but not e.g. temperature. This is shown from measurements of differently doped thin YBCO films and from large scale Ginzburg-Landau simulations. The result is to be expected since the diameters of the dopants are roughly the same size as the coherence length and it has been shown earlier that pinning centers much smaller than the coherence length lead to alpha = 0.5 and those much larger lead to alpha = 1.

The present research in the improvement of vortex pinning in practical applications of YBa₂Cu₃O_7-x (YBCO) superconductors focusses on the introduction of nanoprecipitates in the YBCO matrix. Pulsed Laser Deposition (PLD) of YBCO with BaZrO₃ is a technique known to introduce elongated structures (nanorods), which turn out to be extremely efficient pinning centers [1].
In this work we have grown YBCO and YBCO/BZO PLD films on metallic templates, suitable for the growth of so-called coated conductors. Despite the complexity of the structure, after the optimization of the deposition procedure the YBCO/BZO showed only a moderate reduction of T_c with respect to the pure YBCO (reference) sample [2].
We use a very-high-frequency (~50 GHz) dielectric resonator microwave technique in order to induce very-short-distance vortex shaking. The technique was found useful to determine pinning in YBCO/BZO samples grown on single-crystal dielectric substrates [3]. Using a generalized model for vortex dynamics [4] we determine the pinning parameter r and the spring constant (Labusch parameter) k_p, as a function of the temperature T >60 K and applied field µ₀H < 0.8 T. We find that the improvement of pinning by BZO is reduced with respect to the effect observed in films grown on single crystals. In particular, the reduction of the critical temperature must be taken into account in order to appreciate the results. We discuss the vortex pinning regimes on the basis of the field dependences of the pinning parameters.
[1] J. L. Macmanus-Driscoll et al., Nature Mater. 3, 439 (2004).
[2] E. Silva et al., IEEE Trans. Appl. Supercond. 25, 6601205 (2015)
[3] N. Pompeo et al., Appl. Phys. Lett. 91, 182507 (2007); J. Appl. Phys. 105, 013927 (2009)
[4] N. Pompeo and E. Silva, Phys. Rev. B 78, 094503 (2008)

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

Nahed Moutalbi1,2, Jacques Noudem3, Ali M’Chirgui1,2
1Systems and Applied Mechanics Laboratory LASMAP, Polytechnic School of Tunisia, Rue El Kawarezmi La Marsa 743, Tunis, Tunisia
2Departement of Physics Faculty of Sciences Jarzouna Bizerte
3CRISMAT - LUSAC / CNRS, UCBN / ENSICAEN, 6 Bd Maréchal Juin, 14050 Caen, France
Abstract

In the present paper we investigate the influence of La_0.66Sr_0.33MnO₃ (LSMO) nanostructures on the transport and magnetic properties of YBa₂Cu₃O₇ (YBCO) thin films. The LSMO nanostructures were obtained by polymer assisted deposition (PAD). The polymer assisted deposition technique is a promising alternative for the deposition of buffer layers for high temperature superconducting thin films, in coated conductor applications. By choosing the appropriate deposition conditions nanoislands having a mean diameter of 27 ± 11 nm were grown on MgO (001) single crystal substrates, with a highly uniform substrate coverage and a particle density of 500 µm^-2. No short length ordering of the particles was observed. The as-obtained surface decorated substrates were used as templates for the subsequent growth of YBCO films. The superconducting layers were grown by co-evaporation. The aim of the study is to characterize the pinning landscape produced by the nanostructures. Due to the ferromagnetic behaviour of LSMO below its Curie temperature, a magnetic pinning component in addition to the normal core pinning contribution is expected. Improved transport properties were recorded for the YBCO films grown on the decorated substrates in terms of the J_c(B) dependence as well as the pinning force, Fp. The average pinning potential was evaluated from magnetic relaxation measurements.

The research leading to these results has received funding from the European Community's Seven Framework Programme (FP7/2007-2013) under Grant Agreement n.280432, EUROTAPES project and from UEFISCDI, Romania under Grant PCCA Nos. 138/2012.

The development of 2G YBCO tape has led to the availability of practical conductors suitable both for medium/high field applications at liquid helium temperature and at intermediate temperatures for lower fields. The E - J characteristics of these tapes are essential to the understanding of flux dynamics and flux pinning, to discern the optimum material processing as well as for magnet design. A collective pinning (CP) mechanism has been shown to be predominant in YBCO single crystal and explains the consistent non-power-law scaling of E - J characteristic of YBCO tapes observed above 40 K. The E - J curves of a 4 mm wide YBCO Bruker tape with promising enhanced properties at high field, have been measured in a wider range of temperatures (< 40 K) and fields (up to 5 T), where a change in the pinning mechanisms is expected. Our measurements show an appreciable change in the scaled E - J characteristics shape as the temperature is lowered below T ~ 35 K in self-field. Additionally T ~ 35 K has been observed to be the temperature at which an unexpected inversion in the apparent n-number field dependence takes place (e.g. n ~ 17 and n ~ 22 respectively at 1 T and 5 T at 30 K). The analysis presented in this work shows how the CP model gives a satisfactory quantitative description of the E - J characteristics and, through the determination of the CP potential u₀ = U/k_BT and of the CP exponent µ, allows to identify when a change in pinning regime takes place and provides essential information about the flux bundles size and the pinning centre nature and dimensions.

The authors acknowledge financial support from EUROTAPES, a collaborative project funded by the European Union's Seven Framework Programme (FP7/2007-2013) under Grant Agreement no. 280432.

The improvement in the fabrication techniques of iron-based superconductors have made these materials real competitors of High Temperature Superconductors and MgB₂. In particular, iron-chalcogenides have proved to be the most promising for the realization of high current carrying tapes. But their use on a large scale cannot be able to leave the knowledge of the current stability mechanisms in these compounds out of consideration. Indeed, we have recently observed the presence of flux flow instabilities (FFI) in Fe(Se,Te) thin films grown on CaF₂. Here we present the results of current-voltage characterizations at different temperatures, applied magnetic fields up to 16 T and field orientations on Fe(Se,Te) thin films grown on different substrates. These results will be analyzed from the point of view of the most validated models with the aim to identify the FFI nature (i.e. thermal or electronic) and its dominant microscopic mechanisms, in order to give a further advance to the high current carrying capability of iron-chalcogenide superconductors.

Since their discovery, iron based superconductors (IBS) have attracted the attention of the scientific community, especially for investigating the pairing mechanism and for applications because of the high critical temperature, the short coherence length resulting in high upper critical field, and the metallic behavior. From first experiments with high energy particle irradiation, it was clear that IBS are more robust to irradiation with respect to cuprates and that irradiation induces efficient defect for vortex pinning, thus improving the critical current density. In particular, high-energy heavy-ions (HEHI) form correlated defects along the irradiation direction [1], while proton irradiation should produce more isotropic damage in the crystal lattice [2]. Here we present the study of the vortex pinning anisotropy in (Ba, K)Fe₂As₂ crystals, irradiated with 250 MeV Au ions and with 5.5 MeV protons, along the c-axis. On the same crystal, we irradiated three zones in a different way: with Au ions, with protons, with both particles. A fourth zone is left as-grown. The samples were characterized by the magneto-optical technique with an indicator film. The magnetic field distribution and the local current density values were measured at different temperatures, for several applied fields, after zero field cooling. Then we studied the anisotropy of vortex pinning by analyzing the critical current density dependence on the local magnetic field, with the help of an empirical model [3]. We found that the combined effect of HEHI and proton irradiation is additive for what concerns vortex pinning and that changes of the relaxation rate of the critical current density, during magnetic flux penetration, are due to the anisotropy of vortex pinning by HEHI.

[1] Y. Nakajima et al., Phys. Rev. B 80 (2009) 012510
[2] Y. Nakajima et al., Phys. Rev. B 82 (2010) 220504(R)
[3] F. Laviano et al., Supercond. Sci. Technol. 27 (2014) 044014

Authors gratefully acknowledge support from INFN staff for irradiations.

Engineered nanoscale defects within REBa2Cu3O7- (REBCO) based coated conductors are of great interest for enhancing vortex-pinning, especially in high-applied magnetic fields. We have conducted extensive research to optimize vortex-pinning and enhance Jc via controlled introduction of various types of nanoscale defects ranging from simple rare-earth oxides and Ba-based perovskites to double perovskite rare-earth tantalates and niobates (Ba2RETaO6 and Ba2RENbO6). This talk will provide an overview on how density, morphology, and composition of these engineered nanoscale defects affects vortex-pinning in different temperature, field and angular regimes. Detailed microstructural and superconducting properties coated conductors with these engineered defects will be presented. It will be shown that certain nanodefect configurations that provide the best performance at high-operating temperatures also provide the optimal properties at low operating temperatures out to high-applied magnetic fields.

One of the authors (N. Margiani) acknowledges Shota Rustaveli National Science Foundation financial support for participation in the 12th European Conference on Applied Superconductivity EUCAS 2015.

Since the discovery of superconductivity in Bi-O-S system[1], various layered superconductors with rock-salt type BiS₂ layer have been reported. Simlar to iron-based superconductor REFeAs(O,F), superconductivity in RE(O,F)BiS₂ (RE = La ~ Nd[2,3]) appeared by doping fluorine into oxygen site. In general, conventional solid-state reaction has been usually adopted for introduction of fluorine to oxides, where metal fluorides and other raw materials are mixed at the stoichiometric composition of the objective substance and reacted at high temperatures. On the other hand, various topotactic fluorination reactions have also been reported to date, mainly on transition metal oxides having perovskite-type structures, where precursor oxides reacts on fluorinating agents, such as XeF₂, NH₄F, poly (vinylidene fluoride) and so on. Synthesis of several oxyfluorides, such as CrOF₃ and RbLaNb₂O₆F were reported via this route, but any superconductors have never been discovered through this method.
In the presentation, synthesis and superconducting properties of F-doped Bi₂OS₂ and RE(O,F)BiS₂ are reported. Bi2(O,F)S₂ was successfully synthesized by topotactic reaction between undoped Bi2OS₂ and XeF₂, and showed superconductivity with T_c = 5.1 K. This is the first topotactic reductive fluorination resulting in an electron-doped superconductor. Superconductivity in RE(O,F)BiS₂ synthesized by topotactic reaction as well as effects of high pressure annealing will be presented.

[1] Y. Mizuguchi et al., Phys. Rev. B 86 220510(R) (2012) [2] Y. Mizuguchi et al., J. Phys. Soc. Jpn 81 114725 (2012), [3] S. Demura et al., J. Phys. Soc. Jpn 82 033708 (2013)

We consider a proximity effect for systems consisting of a superconductor (S) and nonuniform ferromagnetic (F). We focus on the influence of magnetic inhomogeneities on the critical temperature of the superconducting transition T_c. The experimental data show a strong influence of magnetic inhomogeneities on the critical temperature for a specific choice of the multilayer geometry [1]. The effect is significant in case when the characteristic size of magnetic inhomogeneities is an order of the correlation length in the superconductor. We work with usual Hamiltonian of magnetic superconductor in the BCS approximation. Using local unitary rotation, we diagonalize Hamiltonian term that corresponds to an interaction with the effective exchange field. It leads to an appearance of effective tensor field [2]. Then we use model representation of domain wall when two homogeneous ferromagnet layers with antiparallel magnetizations, separated by magnetic interlayer in which the magnetization is rotated by 180 degrees in linear manner. As shown by Houzet and Buzdin [3] the problem of finding the critical temperature in SF bilayer can be reduced to the T_c determination in the magnetic superconductor with renormalized effective exchange field under certain strict conditions. We study the influence of the domain wall thickness on the critical temperature in such a system. When the thickness of the domain wall is comparable with the coherence length, T_c increases significantly with respect to the homogeneous case due to suppression of "depairing" effect of exchange field. The impact increases at reducing the full thickness of the system. This allows qualitatively describing the experimental behavior of T_c for the system F/S/F/AF/F [1] depending on the thickness of the antiferromagnetic (AF) layer of chromium.
1. Goryunov Yu.V. arXiv cond-mat. 0305428 (2003)
2. Proshin, Yu.N., Tumanov V.A. J. Phys.: CS. 568, Part 2, 022042 (2014)
3. Houzet M., Buzdin A.I. Phys. Rev. B 74, 214507 (2006)

The work is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University. YP is also thankful to the RFBR (13-02-01202) for partial support.

For applications of superconductors, transition temperature T_c is important. A bulk superconductor has its own T_c, however T_c of a nano-structured superconductor depends on its size. Peeters et al. showed that T_c of an atomic scale nano-film shows a sawtooth behavior with thickness of the film, using the BdG equation under a parabolic band approximation [1]. Experimentally, Nishizaki et al. found a bulk nano-structured metal prepared by high-pressure torsion (HPT) shows T_c enhancement [2], because nano-scaled grains are made by the HPT. We studied size and shape effects on T_c of nano-structured superconducting square plates, numerically [3]. We found T_c shows a sawtooth behavior as a function of the size of the superconductor, which is similar to that of the nano-film [1], and increases with decreasing the size. Also we found T_c of a rectangular superconductor is higher than that of a square superconductor when areas of both of superconductors are same [4].
In this study, we investigate the origin of enhancement of T_c, the sawtooth behavior of T_c and impurity effects on T_c. We use the Gor’kov equations to determine T_c, because we treat impurity effects on the nano-sized superconductors using the Green’s functions and the finite element method. We solve the Gor’kov equations with a number conservation equation self-consistently and obtain the T_c’s and the order parameter distribution for various sized superconducting square and rectangular plates. We will present size and shape dependences of T_c and also impurity effects on T_c.

[1] A. A. Shanenko, M. D. Croitoru, and F. M. Peeters, Phys. Rev. B 75 (2007) 014519 (1-9).
[2] T. Nishizaki, S. Lee, Z. Horita, T. Sasaki, N. Kobayashi, Physica C 493 (2013) 132.
[3] H. Suematsu, M. Kato and T. Ishida, J. Phys: Conf. Ser. 150 (2009) 052250.
[4] M. Umeda, M. Kato and O. Sato, submitted to Physica C.

We thank for, T. Ishida, T. Nishizaki, Y. Nagai, T. Tamegai, K. Hirata, for useful discussion. This work was supported by KAKENHI Great Number 26400367, and program for leading graduate schools of ministry of education, culture, sports, science and technology-Japan.

[1] A.M. Datesman, J.C. Schultz, T.W. Cecil, C.M. Lyons, and A.W. Lichtenberger, Gallium Ion Implantation into Niobium Thin Films Using a Focused-Ion Beam, IEEE Trans. Apl. Supercond. 15, p. 3524-3527 (2005).

The samples were fabricated at NanoFacility Piemonte, INRIM, a laboratory supported by Compagnia di San Paolo. The authors thank Dr. E. Enrico and Mr. R. Rocci for their technical support.

Non-superconducting compounds PrBa₂Cu₃O_6+x (PBCO) remain until now in magnetism, the most interesting and challenging cuprate materials. During the last recent years, the question about the origin of the transitions below the Néel temperature, (T_N^Pr) of the antiferromagnetic ordering (AFM) of the Pr sublattice has not been well clarified [1, 2]. The specific heat, C_p(T) data, obtained in the 1.5-30 K temperature range on polycrystalline samples in orthorhombic phase with two oxygen concentration states, an optimally doped (OP; x = 0.95) and an over doped (OV; x = 1) are reanalyzed and the corresponding pure magnetic contribution of the Pr moments, ΔC_p(T) deduced using the C_p data of YBCO in the OP state which are considered as the phonon and electronic contributions. For temperatures below 17.5 K, which is confirmed as T_N^Pr for the OV state in the temperature dependencies of the following curves, ΔC_p(T)/T, ΔC_p(T)/T², ΔS(T)/T and ΔS(T)/T², where S(T) is the total entropy, the so-called low-critical temperature, (T_cr) is clearly observed at ~ 4-5 K with the occurrence at T₂ ~ 11.5 K of the earlier reported spin reorientation phase transition (SRPT) [3]. For PBCO in the OP state, the SRPT at T₂ is now well identified near 10.25 K, just below T_N^Pr ~ 14.0 K. The curves for both states, ΔC_p(T)/T versus T² are well fitted below T_cr using a phenomenological development up to the second order (T²)² contribution (i.e., ΔC_p containing the T⁵ term). Some modest non-linear magnetic effects are then obtained. They are correlated to the previous Pr-Cu(2) magnetic coupling [4] at the SRPT which provides a small perturbation near T₂^Pr-Cu of the AFM ordering of both Pr and Cu(2) sublattices while a magnetic reordering of the Pr subsystem is suggested below T_cr. Both magnetic transition temperatures are apparently weakly independent of the oxygen concentration contrary to T_N^Pr.
[1] W. Younsi, M. Lahoubi, M.-L. Soltani, D. Schmitt, J. Hejtmánek: J. Phys.: Conf. Ser., 150 (2009) 052290.
[2] W. Younsi, M. Lahoubi, M.-L. Soltani, J. of Low Temp. Phys. 166 (2012) 212.
[3] S. Uma, W. Schnelle, E. Gmelin, G. Rangarajan, S. Skanthakumar, J. W. Lynn, R. Walter, T. Lorenz, B. Büchner, E. Walker, A. Erb, J. Phys.: Condens. Matter 10 (1998) L33.
[4] A. T. Boothroyd, A. Longmore, N. H. Andersen, E. Brecht, Th. Wolf, Phys. Rev. Lett. 78 (1997) 130.

The author thanks Dr. J. Hejtmánek (Institute of Physics of ASCR, Praha, Czech Republic) for providing some PBCO specimens. The author is indebted to Dr. G. Hilscher (Institut für Experimentalphysik, Technische Universität Wien, Vienna, Austria) for his encouragements during the last icfe-8 Conference to continue the research on these compounds.

In order to achieve high J_c, the introduction of effective pinning center is essential. In this study, we have investigated superconducting properties and microstructure of (Sm, Gd, Dy)_1.18Ba_2.12Cu_3.09O_y(SGD123) filament doped with Co as a candidate for act effective pinning center.
SGD123 filament doped with Co (0, 0.05, 0.10, 0.15at%) have been fabricated by a solution spinning method. Sample were partially melted in 1%O₂+Ar atmosphere gas at 1030°C, and then oxygenated in 100%O₂ gas.
The Co-free SGD123 sample showed high J_c value of 3.6×10⁴A/cm² at 77K and 0T. Among Co-doped samples, the sample doped with 0.05at%Co showed the highest J_c values of 5.1×10⁴A/cm². From these results, it was confirmed that Co doping was effective in enhancement of J_c value. Furthermore, it was found that (Sm, Gd, Dy)₂BaCuO₅ (SGD211) particles were finely dispersed in the SGD123 matrix. Therefore, it is assumed that Co has a remarkable effect for reduction of the size for SGD211.

The effects of Nano-Boron (98% B - 2% C ; Pavezyum) addition on (Bi,Pb)2Sr2Ca2Cu3O10 superconducting polycrystalline samples prepared by conventional solid state reaction method in bulk forms have been investigated. Samples were prepared and then characterized by X-ray diffraction, SEM microstructural analysis, DC electrical, AC susceptibility measurements, M-H and Microhardness. X-ray diffraction analysis shows that both (Bi, Pb)-2223 and Bi-2212 phases coexist in the studied samples. I–V characterization and AC susceptibility measurements reveal that Nano-Boron addition increases critical current density. The enhancement of the Jc may be caused by improvement of the grain connectivity with Nano-Boron additions.

This work has been supported by Turkish Research and Scientific Council (TUBITAK) under grant contract No: 113F150. We also acknowledge The Republic of Turkey, Ministry of Development under the Project Number 2010K120520.

This work was supported by Slovak Research and Development Agency, APVV-0494-11 and LPP-0078-07, Slovak Grant Agency for Science, VEGA 2/0120/14, VEGA 2/0173/13, project CENTE II, R&D Operational Program by ERDF, ITMS code 26240120019. These results are subject of patent application No. 50057-2014.

YBCO-like Nd123 and Gd1212 superconductors in the normal state, in visible and infrared regime, have been analysed in this work.
Superconductors, and HTSC in particular, rivest indeed a great interest in the rapidly spreading and highly innovative research field of MetaMaterials (MM) and Photonics. HTSC are plasmonic materials at nonzero temperature and present very attractive features, as charge carriers variety and negative dielectric constant, that cannot be found in more conventional materials. The use of nanostructured devices in the high frequency regime is, indeed, strongly limited by increasing resistivity of metals that causes high dissipative losses, thwarting MM properties. The use of HTSC materials, instead of gold or other metals, provides a viable route to approach this problem. High Nd123 Tc and coexistence, in Gd1212, of magnetic order and superconductivity, make them very interesting for MM use.
But investigation on HTSC optical parameters provides also interesting insight on materials. Normal state investigation, besides a more precise calibration for low temperature study, allows the observation of the SC pseudogap, that affects HTSC properties up to high temperatures, and is detectable by optical analysis.
Materials were prepared as TSG bulk samples (pellets and bars).
FTIR analysis, performed in the mid-infrared (MIR) and in the far-infrared (FIR) show articulated peak structures with k < 250 cm-1 and a plateau for higher k, revealing a pseudogap of about 17 meV. Moreover a band structure, correlated to charge carrier and spin stripes, is observed.
Ellipsometric measurements were performed on the samples in near infrared and visible ranges. Both materials present a strong light absorption, leading to very feeble signal detection. An hybrid model, combining both Lorentz oscillator and Drude components, was used to fit and analyse data. We expect that, in the SC regime, more components will be needed to model the presence of two conducting fluids.

Copper-nickel alloy or copper-manganese alloy is used as the matrix to reduce the AC loss of the NbTi superconducting wire. But during the manufacturing process that alloy is easily work hardening which could reduce the critical current. In present paper, NbTi/Cu10Ni/Cu superconducting wire with 12240 filaments and copper-to-superconductor ratio of 1.6 was prepared using high homogenous Nb47Ti alloy bar, Cu10Ni alloy and oxygen-free copper firstly. The effect of heat treatment and filament diameter on the critical current density of NbTi/Cu10Ni/Cu superconducting wire was investigated. The result showed that when the number of heat treatment (2 times or 4 times) is the same, prolong the final heat treatment time from 15 h to 80 h, critical current density changed little. In addition, the critical current density of wire heat treatment for 2 times or 4 times was almost the same while it was 311 A/mm2(4.2k,5T) lower than that of heat treatment for 3 times. When the heat treatment is same, the critical current density was decreased from 2492 A/mm2(4.2k,5T) to 2081A/mm2(4.2k,5T) with the filament diameter from 4.1μm to 1.6μm. Testing results show that the acquired maximum critical current desity for the
NbTi/Cu10Ni/Cu wire can reach 2659 A/mm2(4.2k,5T)(criterion:0.1μV/cm) and the
AC loss is only 35.6mJ/cm3 (@ 4.2 K, ± 3T).
Key words: NbTi/Cu10Ni/Cu superconducting wire, critical current density, heat treatment, AC loss

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2013M2A8A1035822)

Ex-situ spark plasma sintering (SPS) was used to obtain dense MgB₂ based-bulk and tapes with the starting composition (MgB₂)_0.975+(SiC)_0.025+Te_0.01. SPS-ed tapes were previously prepared by powder-in-tube technique.

The bulk samples have a mass density higher than 97 % of the theoretical value. The critical temperature, T_{c, onset}, is 38.5 K but the transition is rather wide with a midpoint T_{c, midpoint}, is 35.5 K. X-ray diffraction data shows well crystalized MgB₂ but we also identified impurity phases like MgO, MgB₄, MgTe and Mg₂Si. Traces of W₂B were also detected most probably originating in the raw MgB₂ commercial powder. The critical current density J_c (20 K, 5 T) is 3.5 × 10² A cm^-2 and J_c (5 K, 9 T) = 2 × 10³ A cm^-2. These values are higher than for pristine MgB₂ or for MgB₂ separately doped with either Te or SiC.

The tapes show, in addition, the presence of the Fe₂B phase at the Fe-MgB₂ interface. The critical temperatures T_{c, onset} and T_{c, midpoint} are slightly lower, 38 K and 35 K, respectively. For the co-doped tapes, the critical current density at high magnetic fields is below the values of the co-added bulk sample, but exceeds the values for the tape with undoped MgB₂. At low fields the critical current density of the co-added tape is slightly lower than for the undoped tape but twice higher than for the bulk sample. Other similarities and differences between the bulk samples and tapes with similar compositions are revealed and discussed.

Authors acknowledge the project 214/2014 BENZISUPRA and the Core Program PN09-450, 2015, Romanian Ministry of National Education. Authors thank Dr. V. Mihalache for technical assistance with XRD measurements and Dr. Zhichao Guo and Dr. Zhiyong Liu for help with mechanical processing of the wires.

The compound Ge2C6H10O7 was added to MgB2. Starting compositions were [(MgB2)1-x + (Ge2C6H10O7)x], x=0.0007, 0.0014, 0.0025, 0.005, 0.015. Powder mixtures were processed by Spark Plasma Sintering at 1150°C for 3 min and under a uniaxial pressure of 95MPa. All added samples have high density, above 93 %, and a critical onset temperature of 37.5-39.5 K. For the optimum starting composition, x=0.0014, critical current density at high fields measured in the temperature range of 5-25K shows significantly higher values than for the pristine MgB2 sample. At 20K the irreversibility field for a criterion of 103A/cm2 is about 1T higher than for the pristine sample. Remarkable is also that at low fields and at 5-25K Jc-H curve of the optimum doped sample is below, but very close, to the curve for the pristine sample

This work was performed within Partnership program in the priority domains - PN II, funded by MEN-UEFISCDI, project No. 214/2014 BENZISUPRA.

The fracture strength of a high packing ratio MgB₂ bulk processed by Spark Plasma Sintering was evaluated at 77 K through bending tests of specimens cut from the bulk. The fracture strength of the high packing ratio MgB₂ bulk processed by Spark Plasma Sintering was improved in comparison with those of other MgB₂ bulks obtained by conventional route. The bending strength of the spark plasma sintered MgB₂ bulk at 77 K was higher than that at room temperature, which is mainly due to the decrease of inter-atomic distance by cooling. In this study, the bending tests were carried out at 77 K because the bending tests at 77 K can be easily carried out by using a liquid nitrogen bath. However, the critical temperature of MgB₂ is lower than 77 K. Thus the fracture strength at very low temperature will be estimated from the bending test results at 77 K and room temperature.

In this study, effect of Turkish Raki on superconducting properties of bulk MgB2 fabricated by an novel preparation technique using nano boron. The amount of Mg and B were calculated according to stoichiometric ratio of Mg:B, the quantity of B was kept as calculated value. The amount of Mg was determined to be double amount of B quantity without considering the calculated value. Appropriate amounts of elemental magnesium (Mg) powder and amorphous nano boron (B) powder were mixed in a ball-milling for 10 minutes at 400 rpm. This mixture was transferred in a chromium tube which both ends tightly closed and put in a preheated muffle furnace at 1000 oC for 15 min to allow the rapid reaction of Mg with B. Six bulk samples were pressed into rectangle shape with the sizes of 10 mm and 15 mm under the pressure of 15 tons. These samples were kept for 1h, 4h, 8h, 10h and 13h in the 4.5 ml Raki and after that, all samples were heat treated at 1000 oC for 15 min under Ar atmosphere. Tc was measured by AC susceptibility in the different applied AC field and found to be around 37 K. VSM measurements indicated that diffusion of Raki has changed the structure of MgB2 and result increase of density and enhanced magnetization values. Finally, it clearly seen that Turkish Raki significantly has affected electric, magnetic and mechanical properties in the positive meaning.

This work has been supported by Turkish Research and Scientific Council (TUBITAK) under grant contract No: 113F205. We also acknowledge The Republic of Turkey, Ministry of Development under the Project Number 2010K120520.

We have investigated the influence of Boron doping on the final quality of superconducting ceramics Bi-2212 bulk samples using Bi₂Sr₂Ca₁Cu₂O_8±y stoichiometry. Different annealing temperatures and different amount of doping have been investigated. Structural, electrical and magnetic characterizations of the samples are studied. Structural analysis of samples have been performed by means of X-ray powder diffraction and scanning electron microscopy. The critical transition temperatures, T_c, have been determined by AC susceptibility and resistivity versus temperature. In this study we have focused on susceptibility measurements to investigate the effect boron of doping on the superconducting and magnetic properties of bulk samples. The critical transition temperature, grain size, phase purity, lattice parameter, surface morphology and resistivity values of the prepared bulk samples are compared with each other.

This work has been supported by Turkish Research and Scientific Council (TUBITAK) under grant contract No: 113F150. We also acknowledge The Republic of Turkey, Ministry of Development under the Project Number 2010K120520.

Artificial promotion of self-sintering was studied to clarify the factors associated with formation of grain boundaries with higher intergranular connectivity in ex-situ MgB2 bulks and wires. Starting MgB2 powders with different magnesium precursor powder size and carbon additive in the form of B4C powder were prepared and used to synthesize sintered MgB2 samples to investigate the influences of grain size, interface impurities and intragrain lattice defects on self-sintering of MgB2. Laboratory-made high purity MgB2 powder with less oxidized surface and carbon-substituted MgB2 powder were effective to stimulate self-sintering at 900°C by increasing the specific surface area and enhancing the driving force of mass transfer. Microstructural analyses showed that ex-situ MgB2 samples synthesized from such pretreated powders demonstrated increased sintered grain boundary area with the evolution of intergrain necks. Record high connectivity ~30%, which is among the highest in pressureless MgB2 samples, was achieved by a sintering period of 1 h [1], which is much shorter than our conventional sintering time [2,3]. On the other hand, MgB2 grains rather maintained their small initial grain size during the high-temperature processing [4]. As a consequent, high engineering critical current density exceeding 10 kA/cm2 at 20 K under 3 T was obtained for wires owing to the strong inter-grain coupling and small grain size, i.e. dense and coherent grain boundaries. Our results show that self-sintering is a simple but strongly effective way to extract the superior current carrying potential of high bulk density ex-situ MgB2.

References:
1) S. Mizutani et al., Supercond. Sci. Technol. 27, 114001 (2014).
2) H. Tanaka et al., Supercond. Sci. Technol. 25, 115022 (2012).
3) A. Yamamoto et al., Jpn. J. Appl. Phys. 51, 010105 (2012).
4) Y. Shimada et al., IEEE Trans. Appl. Supercond. 25, 6801105 (2015).

This work was partially supported by JST-PRESTO and by JSPS under grant Nos. 24656368 and 25286027. The microstructural analysis at UT was conducted at the Center for Nano Lithography & Analysis, UT, and the microstructural analysis at KU was supported by the Nanotechnology Support Project, both of which are supported by the MEXT, Japan.

The results of the irreversibility field and critical temperature change induced by the 1-3 at % additions of the cerium and neodymium oxide to the carbon encapsulated boron magnesium diboride cylindrical sample is presented. The cylindrical samples are made by the CIP cold isostatic pressing of the milled substrates at 0,8 GPa pressure. The cylindrical samples were paced in the Nb foil, the packed of the wrapped samples were then placed inside stainless steel hermetic container and HIP-ed at 1GPa argon gas pressure at 740 oC for 40 minutes. The samples had cylindrical shapes with dimensions of 6x6 [mm]. For the comparison of the HIP treated and normal pressure annealed samples the analogical types of the samples have been prepared without the high pressure annealing. All the samples have been prepared, annealed and microstructurally analyzed at the Institute of High Pressure Physics in Warsaw. The measurements of the critical temperature and the hysteresis loop were done at International Laboratory of High Magnetic Fields and Low Temperatures Wroclaw, using a vibrating magnetometer (VSM) up to 14T. The critical current density (Jc) and pining force density Fp as a function of the technology parameters are presented and the results are analyzed especially on the application function, of such annealing, especially for middle magnetic field region and especially for ex-situ cylindrical barrier of dual core patented wire in the future applications. The XRD analysis of the obtained material were also made by the Tele @ Radio Research Institute and the results are presented. The important influences of the high density (cerium or neodymium) borides inclusion randomly inserted in the MgB2 grains borders was observed, thus in the additive high density of materials obtained by the HIP treatment results in extremely high pinning density at ca 5-7 T magnetic field region. The critical current densities were calculated from the Bean’s model.

Since the discovery of the MgB₂ as a superconducting material in 2001, many groups around the world work on an attempt to improve the transport properties for technologic application, due to the properties and to the low price of the precursor elements, as well as the possibility to apply the material in magnets and electronic devices, operating with cryocoolers. High energy ball milling is a useful technique to enhance the superconducting properties of the MgB₂, increasing the grain connectivity and densification. Doping is another way to change the intrinsic superconducting properties, as the critical temperature and upper critical magnetic field. Carbon is the most useful doping element to enhance the critical current densities in magnesium diborides due to the enhancement of the upper critical field. In this work the influence of stearic acid (as a carbon source) addition is studied, in an attempt to verify the influence of the stearic acid addition in the MgB₂ crystalline structure and the superconducting behavior of the material. Samples were prepared using a high energy ball mill with different compositions, in a glove-box with argon controlled atmosphere. Several heat treatment profiles were performed using Hot Isostatic Press. X-ray and scanning electron microscopy were used. VSM analysis was used to perform measurements of the magnetic moment versus temperature to determine the critical temperatures, and magnetic moment versus applied magnetic field, which were used to estimate the critical current densities of the samples. As a result, the superconducting matrix was dopped with carbon due to the stearic acid addition, as well as the superconducting properties could be improved.

The authors wish to thank USP, CNPq, CAPES, and FAPESP for the financial support. DRJ is a CNPq researcher.

To understand the route to increase trapped magnetic field of MgB2 bulk magnets, the effects of impurity doping on the microstructure and trapped field properties were systematically studied. Disk-shaped MgB2 bulk magnets (30 mm in diameter, 10 mm in thickness) were fabricated from Mg, B and dopant (B4C, SiC, CaCO3, CaC2 and CaO) powders by using in-situ method. The mixed powders were pressed into pellet and heated at 850˚C for 3 h under Ar atmosphere. Small pieces with dimension of ~1x1x0.5 mm3 were cut from the bulks for investigation of structural and superconducting properties. A decrease in the in-plane lattice parameter and transition temperature and an increase in the in-plane XRD peak widths occurred with doping as a result of carbon substitution. Microstructural analysis revealed that sub-micron-size fine MgB2 grains were well connected. The trapped field at the surface of the magnetized bulk samples exhibited a large improvement by doping and reached >3 T at 5 K.

This work was partially supported by JST-PRESTO and JSPS under grant Nos. 23246110 and 22860019.