1A-M-O2 Sep 7 - Afternoon (4:30-7:00 PM)
Materials - MgB2
4:30 - 5:00 Enhanced trapped field in MgB2 bulk magnets by tuning grain boundary pinning|
YAMAMOTO Akiyasu1, SUGINO Sho2, KISHIO Kohji2, ATSUSHI Ishihara3, AKASAKA Tomoyuki3, TOMITA Masaru3
1The University of Tokyo, JST-PRESTO, Japan, 2The University of Tokyo, Japan, 3Railway Technical Research Institute, Japan
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MgB2 has several attractive natures as superconducting bulk magnet, such as low cost of materials, light weight, weak-link-free homogeneous current flow on a bulk scale and great flexibility of magnet shape designing . In the present study, the effects of the grain size, lattice strain, and microstructure on the trapped field properties of ball-milled MgB2 bulks were studied. A decrease in the in-plane lattice parameters and transition temperature and an increase in the in-plane XRD peak widths occurred with milling as a result of grain refinement and carbon substitution. Microstructural analysis revealed that sub-micron-size fine MgB2 grains were well connected, resulting in increased grain boundary density. The trapped field of the magnetized bulk samples exhibited a large improvement with milling and reached 3.72 T at 5 K, which is the highest among MgB2 bulks prepared using pressureless synthesis. Excess milling resulted in degradation of the trapped field because of Tc reduction. The increased grain boundary density and electron scattering are considered to contribute to the high trapped field through quantitatively and qualitatively enhancing the grain boundary flux pinning.
 A. Yamamoto, A. Ishihara, M. Tomita, K. Kishio: Appl. Phys. Lett. 105, 032601 1-4 (2014)
This work was partially supported by JST-PRESTO and JSPS under grant Nos. 23246110 and 22860019.
5:00 - 5:15 Synthesis of amorphous and nanostructured boron suitable for superconducting power application through various manufacturing technique|
BOVONE Gianmarco1, VIGNOLO Maurizio1, MATERA Davide1, BERNINI Cristina1, MALAGOLI Andrea1, SIRI Antonio2
1CNR-SPIN, Italy, 2UniversitÓ degli Studi di Genova, Italy
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In order to enhance MgB2 superconducting properties, to make it more attractive for power application, suitable boron precursor has to be optimized. On this idea, we developed a new synthesis procedure for boron powder, based on the lyophilisation of a frozen boron oxide solution. Lyophilised boron oxide powder is used to synthesize elemental boron by reacting with Mg at high temperature (magnesio-thermic reaction), as following Moissan process, the procedure used for large-scale production of boron. The initial steps of solubilisation and freeze-drying are necessary to obtain nanometrical powder (with average grain size = 60 nm) and to introduce dopant in boron lattice. In order to establish the presence of some impurities into the lab-made B precursor some EDS analysis, ICP-OES and XRD on B have been performed.
Purified boron powders have been used for the manufacturing of mono-filamentary MgB2 wire, by following different procedures, in order to evaluate improvement in superconducting properties. MgB2 conductors were manufactured using the same B precursor through four different techniques: typical ex-situ and in-situ, via MgB4 (also called mixed technique: ½ ex situ and ½ in situ) and Reactive liquid infiltration. A systematic study on the effect of manufacturing procedure and final heat-treatment are presented. This includes magnetic and transport critical current density (at different temperatures), critical and irreversibility field, residual resistivity and critical temperature. Detailed morphology and X-ray analysis of powders from superconducting cross-section are shown to evaluate the effects of voids and defects on wires performances
5:15 - 5:30 Influence of impurity hydrogen and oxygen, Ti-addition on the consolidation and superconducting properties of MgB2.|
PRIKHNA Tatyana1, EISTERER Michael2, GOLDACKER Wilfried3, WEBER Harald W.2, GAWALEK Wolfgang4, SHAPOVALOV Andrii1, MOSHCHIL Viktor1, SVERDUN Vladimir1, KOZYREV Artem1, SOKOLOVSKIY Vladimir5, BELOGOLOVSKIY Mikhael6, SHATERNIK Anton1
1Institute for Superhard Materials of the NASU, Ukraine, 2Atominstitut, Vienna University of Technology, Austria, 3Karlsruhe Institute of Technology (KIT), Germany, 4Magnetworld AG, Germany, 5Ben-Gurion University of the Negev, Israel, 6G.V.Kurdyumov Institute for Metal Physics of the National Academy of Sciences of Ukraine, Ukraine
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The consolidation of MgB2 at 2 GPa or 30-50 MPa in contact with BN allows obtaining high critical current densities, jc (above 106 A/cm2 at 20 K and 0-1 T), irreversibility fields (Birr(18.5 K)=15 T, Birr(0 K)=32.5 T), upper critical fields (Bc2 (22 K)=15 T, Bc2(0 K)=42.1 T) and trapped magnetic fields (1.9 T at 20 K for a sample of 30 mm in diameter and 10 mm in height). The materials demonstrated 73-98 % connectivity and a shielding fraction of 75-100%. They are promising for SC bearings, fault current limiters, electromotors, levitation, transport, etc. As it was shown, the distribution of impurity oxygen (which is usually present in MgB2 due to the high affinity of Mg to O) affects jc of MgB2 to a high extent due to formation of oxygen-enriched Mg-B-O nanolayers or nanoinclusions. The presence of Ti can influence the process of oxygen redistribution in MgB2. Auger studies showed that some O can be solved in the MgB2 matrix of the samples which demonstrate extremely high jc. The calculation of the density of states (DOS) in Mg(B1-xOx)2 for x=0.125, 0.25, 0.5 showed that all compounds are conductors with a non-zero DOS at the Fermi level. The DOS is even somewhat higher for x=0.25 than that in MgB2. The presence of a high amount of H does not allow dense materials manufacturing, lead to the formation of MgH2 that correlates with a decrease of jc. Besides, Mg(BH4)2 or Mg(BH4)2(NH3)x (isolators) may form during synthesis process. The production of crack-free samples of large size (above 30 mm) under pressure below 1100 oC without Ti is practically impossible. Ti can absorb H or act as catalyst by decreasing the activation energy and temperature for H release from MgH2, Mg(BH4)2 or Mg(BH4)2(NH3)x during consolidation. The release of H sometimes results in bubbles in the surface layer of just prepared MgB2, the characteristic smell of ammonia or the decomposition of bulk MgB2 into powder within one week. The promotion of H release is one of the effects caused by Ti in MgB2. Very important in view of the jc variation is the segregation of O in presence of Ti and thus “cleaning” of the MgB2 matrix from impurity oxygen (confirmed by SEM EDS analyses). In addition Ti influences the formation in MgB2 of higher amounts of MgBx (x>4) inclusions and separate Mg-B-O inclusions (instead of nanolayers even at low temperatures, 800oC ) which may act as pinning centers.
5:30 - 5:45 High-performance MgB2 superconductors: role of Mg-B-O nano-scaled inhomegeneties|
SHAPOVALOV Andrii1, PRIKHNA Tatyana1, BELOGOLOVSKII Mikhail2, SEIDEL Paul3
1Institute for Superhard Materials of the NASU, Ukraine, 2Institute for Metal Physics, NASU, Ukraine, 3Institut fŘr Festk÷rperphysik, FSU Jena, Germany
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Whereas MgB2 is nominally an oxygen-free compound, oxygen-rich Mg-B-O inhomogeneties are always present in the bulk compounds as inevitable impurity phases due to the high affinity of magnesium to oxygen supplied by the gaseous oxygen in the protective Ar gas and/or boron oxides in the starting materials. In this contribution, we provide new arguments supporting the statement that, contrary to the generally accepted opinion, nano-scaled Mg-B-O inhomogeneties play a positive role for attaining high critical current densities not only as pinning centers but also as connecting bridges between superconducting MgB2 grains.
MgB2 bulk samples with enhanced critical current densities have been successfully fabricated using the hot pressing process and characterized. It was found that Mg-B-O nano-layers and/or nano-scaled inclusions have been observed in all samples independently on the preparation method, pressure (0.1 MPa - 2 GPa), and temperature (600-1100o C) . In order to get systematic insight into electronic properties of oxygen-rich MgB2 compounds, we have performed a comparative first-principles study of the electronic band structure of three Mg(B1-xOx)2 compounds with a hexagonal structure. In contrast to thermal-equilibrium compounds of the Mg-B-O system as Mg3B2O6 and Mg2B2O5  which arte dielectrics, the energy band structure of metastable compounds Mg(B1-xOx)2 is metallic, similar to MgB2. The maximum density of electronic states at the Fermi level is reached for the Mg(B0.75O0.25)2 compound . This finding suggests that superconducting transport across two comparatively large MgB2 grains linked by a Mg-B-O bridge occurs due to the proximity effect which induces superconducting correlations in, probably, normal oxygen-rich conducting voids.
. T.A. Prikhna, M. Eisterer, H.W. Weber et al // Supercond. Sci. Techn. –2014. - V. 27.– N 4. – Р. 044013.
. Z. Bai, L. Shen, Q. Wu et al // Physical Review B. – 2013. – V. 87. – N 1. – P. 014114.
 A.Shapovalov, T. Prikhna, V. Boutko et al // Annual report 2014 of IFK FSU Jena, ed. T.Fritz. - 2014 .-P.86. (http://www.ifk.uni-jena.de/Jahresberichte.html)
5:45 - 6:00 Feasibility study for an MgB2 superconducting magnetic cloak|
GIUNCHI Giovanni1, BARZI Emanuela2
1Materials Science Consultant, Italy, 2FermiLab, United States
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The magnetic shielding capability of bulk MgB2 hollow cylinders can be fruitful combined with an external paramagnetic sheath, to tailor the shape of the external magnetic flux lines. By appropriate selection of the external sheath permeability and thickness it is possible to leave the magnetic flux lines unaltered by the shield (cloaking effect).Several measurements have been performed of the flux lines distribution around a metallic sheated MgB2 bulk cylinder, subjected to a transversal magnetic field up to 5 T, at 4.2 K, finding an optimazed sheathing material and thickness to realize the cloaking effect. The MgB2 material of the superconducting shield is also optimized to avoid low temperature flux jumps, without losing its shielding capability.
6:00 - 6:15 Influence of grain size of ex-situ MgB2 powder on the properties of MgB2 bulk samples|
H─▀LER Wolfgang1, SCHEITER Juliane1, KOKAL Imre2, ACAR Selcuk2, SOMER Mehmet3
1Leibniz Institute for Solid State and Materials Research, Germany, 2Pavezyum Chemicals, Turkey, 3Koc University, Turkey
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Improving the superconducting properties of bulk samples and wires prepared with ex-situ is a challenging task. For the preparation of MgB2 bulk samples two different qualities of ex-situ MgB2 powder from Pavezyum Chemicals were used, a standard batch with a mean grain size of some micrometers and a nano-MgB2 powder with a grain size in the range of some 10 nm. Additionally to the powders in the as-delivered state, a part of the powder was milled in a planetary ball mill shortly before hot pressing of the bulk samples at 700°C.
The structural properties were characterized by XRD and SEM, resistance and magnetization measurements were used for the estimation of the superconducting properties. Some samples were additionally annealed at 850°C.
All bulk samples are superconducting. The milled powders show the better electrical properties which (especially the upper critical field Bc2 and the critical current density jc) were further improved by post annealing at 850°C. The highest jc was measured at samples based on nano-MgB2 powder. The influence of grain size and fresh reactive grain surfaces produced by milling is discussed.
The work was partially supported by German Bundesministerium für Bildung und Forschung (FKZ 03SF0480C)
6:15 - 6:30 Large critical currents in bulk MgB2 material with MgB4 addition|
MIRYALA Muralidhar1, JO Ishiwata1, DIKO Pavel2, KOBLISCHKA Michael R3, INOUE Kazuhiro1, MURAKAMI Masato1
1Shibaura Institute of Technology, Japan, 2Slovak Academy of Science, Slovakia, 3University des Saarlandes, Germany
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The objective of this work is to improve the flux pinning performance of bulk MgB2 superconductors by dispersing nanometer-sized MgB4 particles. The high-quality MgB4 material was produced, and the residual unwanted MgO rest in the powder was removed by HNO3 (1M) leaching and used the MgB4 as a flux pinning medium, which has not been tested up to date. We made several MgB2 bulks with various contents of MgB4: 0, 1, 2, 3, 5, and 10 wt%, sintered at 775 oC for 3h in Ar atmosphere. We studied the x-ray diffraction, scanning electron microscopy, atomic force microscopy, superconducting transition temperature, and magnetic performance at 10 to 25 K. The magnetization measurements showed that the critical current density at low magnetic fields increased dramatically with an addition of 1wt% but decreased at higher content of MgB4. Further, in all studied MgB4-added samples the critical current densities are high as compared to the MgB2 bulk material without MgB4. The MgB2 sample with 1wt% of MgB4 processed at 775 oC for 3 h showed the critical current density (Jc) of 325 kA cm-2 at 20 K in self field, which is twice higher than that of the pure sample. Finally, the microstructural changes and the flux pinning properties upon addition of MgB4 will be discussed regarding the improved performance of the bulk MgB2 material.
6:30 - 6:45 Processing of homogeneous bulk MgB2 by an infiltration and growth process|
BHAGURKAR Ashutosh1, YAMAMOTO Akiyasu2, BABU N Hari1, DENNIS Anthony3, DURELL John3, CARDWELL David3
1Brunel University, United Kingdom, 2University of Tokyo, Japan, 3University of Cambridge, United Kingdom
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This study reports the processing of dense, superconducting MgB2 (32 mm Dia, 6 mm thick, ρ ≈ 2.4 g/cm3) by an infiltration and growth technique. The process, which involves infiltration of liquid magnesium into a pre-defined boron precursor pellet, is relatively simple, results in a hard, dense structure and has the potential to fabricate complex shapes. X-ray diffraction confirms the presence of the MgB2 phase with residual magnesium content (≈18%) in the fully processed samples, whereas low temperature processed (700 ºC) samples show large fraction of Mg2B25 intermediate phase. The resulting MgB2 bulk samples exhibit sharp superconducting transitions at 37.8 K and have critical current densities of upto 380 kA/cm2 in self-field at 5K. Trapped field measurements on centre of top and bottom surfaces of single bulk show ≈2.2 T (5 K) field with <1% variation, confirming very good homogeneity in the bulk of the sample. SEM images show a bimodal particle size distribution with 20–50 nm sized fine precipitates, which are potential flux pinning sites, in the inter particle region.
6:45 - 7:00 High magnetic field generated by MgB2 bulk prepared by Spark Plasma Sintering|
BERGER KÚvin1, KOBLISCHKA Michael2, DOUINE Bruno1, NOUDEM Jacques3, BERNSTEIN Pierre4, HAUET Thomas5, L╔V╩QUE Jean1
1GREEN - University of Lorraine, France, 2Dep. of Experimental Physics - Saarland University, Germany, 3CRISMAT, CNRS-ENSICAEN and LUSAC - Caen University, France, 4CRISMAT, CNRS-ENSICAEN - Caen University, France, 5IJL - Lorraine University, France
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Theoretically, MgB2 bulk samples have very low density of 2.64 g/cm3, compared to REBaCuO samples which are around 6.36 g/cm3, and NdFeB permanent magnets at around 7.60 g/cm3. Practically, this ratio higher than 2-3 is maintained. From the applications point of view, this advantage must be taken into consideration and, the generation of strong magnetic flux densities using MgB2 should be investigated.
In this contribution, we have studied the magnetic properties of samples with 20 mm and 30 mm in diameter and a height of 9.7 mm. These samples were processed by the fast spark plasma sintering machine that is able to produce dense and high quality MgB2 samples.
Experiments were carried out both on small pieces of the samples using a SQUID-VSM and on big samples performing trapped field measurements in a Field Cooling process. At temperatures between 10 K to 30 K, the results show a strong dependence of the magnetic behavior of the big samples to the applied magnetic field sweep rate, while nothing particular appeared on the small samples.
As the magnetic flux density produced by the MgB2 bulks is directly linked to the potential of the applications, we also report the field produced on the surface of one single MgB2 sample of 20 mm diameter, the field produced inside two MgB2 samples of 20 mm in diameter, and finally, the field produced on the surface of one single 30 mm MgB2 sample at various temperatures. A generation of magnetic flux density up to 4.8 T @ 10 K and 3.9 T @ 20 K inside a set of 2 MgB2 samples of 20 mm was observed. The dependence of the flux jumps on the magnetic field sweep rate observed in big samples will be also investigated in detail.
GRMN - Magnetism Network for the Greater Region