The inhibition of Nb3Sn grain growth in the presence of ZrO2 nanoparticles appears to be one of t... more The inhibition of Nb3Sn grain growth in the presence of ZrO2 nanoparticles appears to be one of the most promising method for pushing the critical current densities of Nb3Sn superconducting wires to levels that meet the requirements set for the Future Circular Collider. We have investigated the effect of ZrO2 nanoparticles formed by the internal oxidation of Zr on the superconducting properties and microstructure of Nb3Sn formed from Nb-1 wt%Zr, Nb-7.5 wt%Ta, Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt%Zr alloys. A monofilamentary wire configuration was used, with a 0.22 mm outer diameter Nb-alloy tube containing a core of powdered metal oxide (SnO2, CuO or MoO3) as oxygen source and successive deposits of Cu, Sn and Cu on the outer surface. As determined from inductive measurements, the layer critical current densities of the samples based on Nb alloys with internally oxidized Zr were superior to those based on Nb-7.5 wt%Ta. The samples based on Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta...
The upper critical field sets the thermodynamic limit to superconductivity. A big gap is present ... more The upper critical field sets the thermodynamic limit to superconductivity. A big gap is present between the upper-critical-field values measured in MgB2 polycrystalline bulk superconductors and those of thin films, where values as high as ~ 50 T have been achieved at 4.2 K. Filling this gap would unlock the potential of MgB2 for magnet applications. This work presents the results of an extensive experimental campaign on MgB2 bulk samples, which has been guided by a Design of Experiment. We modeled the dependence of the upper critical field on the main synthesis parameters and established a new record (~ 35 T at 4.2 K) preparing C-doped bulk samples by a non-conventional rapid-synthesis route. This value appears to be an upper boundary for the upper critical field in bulk samples. Structural disorder in films seems to act selectively on one of the two bands where superconductivity in MgB2 takes place: this enhances the upper critical field while reducing the critical temperature onl...
The high pressure (50 MPa - 2 GPa) – high temperature synthesized MgB2 bulk materials are charact... more The high pressure (50 MPa - 2 GPa) – high temperature synthesized MgB2 bulk materials are characterized by nearly theoretical density (1-2% porosity), 80-98% connectivity, extremely high critical current densities (e.g. at 20 K, in 0–1 T jc=1.3–1.0 MA·cm-2 (with 10% SiC) and jc= 0.92 – 0.73 MA·cm-2 (without doping)), large irreversibility fields (Birr(18.4 K) =15 T and Birr (0 K) = 32.5 T) and high upper critical fields (Bc2 (22 K) =15 T and Bc2(0 K) ~ 42.1 T). The transformation of grain boundary pinning to point pinning in MgB2-based materials with increasing manufacturing temperature (from 800 to 1050 oC) under pressures from 0.1 to 2 GPa correlates well with an increase in critical current density in low external magnetic fields caused by the redistribution of boron and the oxygen impurities in the material structure. As the manufacturing temperature increases, the discontinuous oxygen enriched layers transform into distinct Mg-B-O inclusions and the size and amount of inclusion...
Properties of materials synthesized from Mg and B and sintered from MgB 2 using high-pressure (2 ... more Properties of materials synthesized from Mg and B and sintered from MgB 2 using high-pressure (2 GPa) and hot-pressing (30 MPa) at 700- 1100 °C are discussed. The influence of Ta, Ti and Zr additions on the material structure and properties was studied. The average grain size of 15-37 nm was observed for materials produced under a pressure of 2 GPa. For high-pressure synthesized MgB 2 , the highest critical current densities known from literature have been obtained. Dispersed inclusions of higher borides with the stoichiometry close to MgB 12 and MgB 7 can act as pinning centers in the magnesium diboride material manufactured using high pressure and hot pressing.
high-Tc copper oxide superconductors (SC) exhibit a pseudogap state with anomalous magnetic, tran... more high-Tc copper oxide superconductors (SC) exhibit a pseudogap state with anomalous magnetic, transport, thermo-dynamic, and optical properties below a temperature, T*, which is large in comparison to the super-conducting transition temperature, Tc. The origin of the pseudogap is a challenging issue as it might eventually lead to identify the superconducting mechanism. Two major classes of theoretical models attempt to describe the pseudogap state: in the first case, it represents a precursor of the superconducting d-wave gap with preformed pairs below T*, which would acquire phase coherence below Tc. In a second approach, the pseudogap is associated either with an ordered or with a disordered phase competing with the SC state. The order parameter associated with these competing phases may involve chargeand spindensity waves, or charge currents flowing around the CuO2 square lattice, such as D-charge density wave (DDW) or orbital circulating currents (CC) as proposed by C. M. Varma [...
Melt processed superconductors containing mechanically drilled holes parallel to the c-axis were ... more Melt processed superconductors containing mechanically drilled holes parallel to the c-axis were investigated by means of Hall scans of the trapped field distribution and the magnetoscan technique. We show that the remnant flux profiles are affected by the perforation in different ways. The location of the holes can be resolved by field mapping at activation fields below that for complete
IEEE Transactions on Applied Superconductivity, 2012
The second generation (2G) HTS conductors, the YBaCuO Coated Conductors, show very exciting perfo... more The second generation (2G) HTS conductors, the YBaCuO Coated Conductors, show very exciting performances in terms of critical currents under very high fields whereas their mechanical properties, the key issues for very high field magnets, are outstanding for the IBAD (Ion Beam Assisted Deposition) route. Preliminary measurements have shown critical currents Ic above 500 A at 15 T and 4
The inhibition of Nb3Sn grain growth in the presence of ZrO2 nanoparticles appears to be one of t... more The inhibition of Nb3Sn grain growth in the presence of ZrO2 nanoparticles appears to be one of the most promising method for pushing the critical current densities of Nb3Sn superconducting wires to levels that meet the requirements set for the Future Circular Collider. We have investigated the effect of ZrO2 nanoparticles formed by the internal oxidation of Zr on the superconducting properties and microstructure of Nb3Sn formed from Nb-1 wt%Zr, Nb-7.5 wt%Ta, Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt%Zr alloys. A monofilamentary wire configuration was used, with a 0.22 mm outer diameter Nb-alloy tube containing a core of powdered metal oxide (SnO2, CuO or MoO3) as oxygen source and successive deposits of Cu, Sn and Cu on the outer surface. As determined from inductive measurements, the layer critical current densities of the samples based on Nb alloys with internally oxidized Zr were superior to those based on Nb-7.5 wt%Ta. The samples based on Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta...
The upper critical field sets the thermodynamic limit to superconductivity. A big gap is present ... more The upper critical field sets the thermodynamic limit to superconductivity. A big gap is present between the upper-critical-field values measured in MgB2 polycrystalline bulk superconductors and those of thin films, where values as high as ~ 50 T have been achieved at 4.2 K. Filling this gap would unlock the potential of MgB2 for magnet applications. This work presents the results of an extensive experimental campaign on MgB2 bulk samples, which has been guided by a Design of Experiment. We modeled the dependence of the upper critical field on the main synthesis parameters and established a new record (~ 35 T at 4.2 K) preparing C-doped bulk samples by a non-conventional rapid-synthesis route. This value appears to be an upper boundary for the upper critical field in bulk samples. Structural disorder in films seems to act selectively on one of the two bands where superconductivity in MgB2 takes place: this enhances the upper critical field while reducing the critical temperature onl...
The high pressure (50 MPa - 2 GPa) – high temperature synthesized MgB2 bulk materials are charact... more The high pressure (50 MPa - 2 GPa) – high temperature synthesized MgB2 bulk materials are characterized by nearly theoretical density (1-2% porosity), 80-98% connectivity, extremely high critical current densities (e.g. at 20 K, in 0–1 T jc=1.3–1.0 MA·cm-2 (with 10% SiC) and jc= 0.92 – 0.73 MA·cm-2 (without doping)), large irreversibility fields (Birr(18.4 K) =15 T and Birr (0 K) = 32.5 T) and high upper critical fields (Bc2 (22 K) =15 T and Bc2(0 K) ~ 42.1 T). The transformation of grain boundary pinning to point pinning in MgB2-based materials with increasing manufacturing temperature (from 800 to 1050 oC) under pressures from 0.1 to 2 GPa correlates well with an increase in critical current density in low external magnetic fields caused by the redistribution of boron and the oxygen impurities in the material structure. As the manufacturing temperature increases, the discontinuous oxygen enriched layers transform into distinct Mg-B-O inclusions and the size and amount of inclusion...
Properties of materials synthesized from Mg and B and sintered from MgB 2 using high-pressure (2 ... more Properties of materials synthesized from Mg and B and sintered from MgB 2 using high-pressure (2 GPa) and hot-pressing (30 MPa) at 700- 1100 °C are discussed. The influence of Ta, Ti and Zr additions on the material structure and properties was studied. The average grain size of 15-37 nm was observed for materials produced under a pressure of 2 GPa. For high-pressure synthesized MgB 2 , the highest critical current densities known from literature have been obtained. Dispersed inclusions of higher borides with the stoichiometry close to MgB 12 and MgB 7 can act as pinning centers in the magnesium diboride material manufactured using high pressure and hot pressing.
high-Tc copper oxide superconductors (SC) exhibit a pseudogap state with anomalous magnetic, tran... more high-Tc copper oxide superconductors (SC) exhibit a pseudogap state with anomalous magnetic, transport, thermo-dynamic, and optical properties below a temperature, T*, which is large in comparison to the super-conducting transition temperature, Tc. The origin of the pseudogap is a challenging issue as it might eventually lead to identify the superconducting mechanism. Two major classes of theoretical models attempt to describe the pseudogap state: in the first case, it represents a precursor of the superconducting d-wave gap with preformed pairs below T*, which would acquire phase coherence below Tc. In a second approach, the pseudogap is associated either with an ordered or with a disordered phase competing with the SC state. The order parameter associated with these competing phases may involve chargeand spindensity waves, or charge currents flowing around the CuO2 square lattice, such as D-charge density wave (DDW) or orbital circulating currents (CC) as proposed by C. M. Varma [...
Melt processed superconductors containing mechanically drilled holes parallel to the c-axis were ... more Melt processed superconductors containing mechanically drilled holes parallel to the c-axis were investigated by means of Hall scans of the trapped field distribution and the magnetoscan technique. We show that the remnant flux profiles are affected by the perforation in different ways. The location of the holes can be resolved by field mapping at activation fields below that for complete
IEEE Transactions on Applied Superconductivity, 2012
The second generation (2G) HTS conductors, the YBaCuO Coated Conductors, show very exciting perfo... more The second generation (2G) HTS conductors, the YBaCuO Coated Conductors, show very exciting performances in terms of critical currents under very high fields whereas their mechanical properties, the key issues for very high field magnets, are outstanding for the IBAD (Ion Beam Assisted Deposition) route. Preliminary measurements have shown critical currents Ic above 500 A at 15 T and 4
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