Increased critical current density in Nb–Ti wires having Nb artificial pinning centers

Abstract

Artificial pinning center ͑APC͒ wires containing Nb 47 wt.% Ti with 24 vol.% of round Nb pins have produced very high critical current densities (J c ) which are attributed to a sharply defined, nanometer-scale Nb-pin array. By reducing both the number of warm extrusion steps from four to three and the temperature of the third extrusion from 650°C to 250°C, the degree of pin-matrix interdiffusion has been reduced and J c values at all applied magnetic fields increased by 25-45% over those for a previous composite of almost identical design. The best wire achieved the very high J c ͑5 T, 4.2 K͒ value of 4600 A/mm 2 . These results underscore the importance of the thermomechanical treatment in determining the maximum flux pinning properties of APC Nb-Ti wires. © 1997 American Institute of Physics. ͓S0003-6951͑97͒02407-8͔The critical current density (J c ) of conventionally processed Nb 45-55 wt. % Ti wire is limited by the amount of normal conducting ␣-Ti precipitate that can be developed during thermomechanical treatment of the ␤-Nb-Ti alloy. Optimum flux pinning is achieved when the ␣-Ti precipitates are drawn to 1-2 nm in thickness. 1 J c increases linearly with the volume fraction of ␣-Ti, 2 but there appears to be a kinetic and/or thermodynamic limit of ϳ25 vol. % of ␣-Ti in Nb 47 wt. % Ti. In order to overcome this apparent limit to the conventional process, artificial pinning center ͑APC͒ designs 3 have been implemented which allow, in principle, for complete control over the pinning center composition, spatial arrangement, and volume fraction. Nb has been the most widely studied APC material, primarily because Nb is mechanically similar to Nb-Ti and because Nb-pin composites have produced the highest J c values thus far. Most notably, an APC composite using a multilayer-sandwich design of Nb 50 wt. % Ti matrix and 28 vol. % of Nb pins 4 achieved J c ͑5 T, 4.2 K͒ of 4250 A/mm 2 , distinctly higher than the maximum J c (ϳ3700 A/mm 2 ) achieved conventionally.While the thermomechanical processing steps 5,6 and the resulting flux pinning nanostructures 1,5 of high J c conventional Nb-Ti wire are firmly established, the same is not true for APC wires. Consequently, it is not surprising that a significant range in J c ͑H͒ is found for APC composites of nominally the same composition but manufactured via different processing routes. For instance, three groups have manufactured and characterized APC composites of Nb 47 wt. % Ti with 24-25 vol. % of nominally round 7,8 or planar 9 Nb pins. They produced maximum J c ͑5 T͒ values of 2400, 3200, and 2800 A/mm 2 , respectively. Although hard evidence is lacking, it seems likely that the range in J c originates from differences in the flux pinning nanostructures. As recent work by Jablonski et al. 10,11 has shown the detailed nanostructure of the pins exerts an important effect on the attainable J c , independent of the vol. % of pin and its composition.A very important difference between conventional and APC Nb-Ti lies in the evolution of the flux pinning nanostructure...

Ultrahigh critical current densities, the vortex phase diagram, and the effect of granularity of the stoichiometric high- Tc superconductor CaKFe4As4

“…Other models that take into account the field variation of the critical current density and the vortex dynamics were also developed by Kim in Ref. 33 , (Ba,K)Fe2As2 34 , Fe(Se,Te) 35 , LiFeAs 36 ) and conventional superconductors (MgB2 37-39 , Nb3Sn 39 , NbTi 40 ) in magnetic field of (c) 0 T and (d) 3 T for H||c.…”

“…The temperature dependence of critical current density, Jc, extrapolated at (a) µ0H = 0 T and (b) µ0H= 15 T of the single crystal in the two orientations in magnetic field and of the powder sample of CaKFe4As4. Comparison between the temperature dependence of the critical current density Jc (calculated using the Bean model 20 ) in single crystals of CaKFe4As4 with those reported for different iron-based superconductors (SmFeAs(O,F)33 , (Ba,K)Fe2As234 , Fe(Se,Te)35 , LiFeAs36 ) and conventional superconductors (MgB2 37-39 , Nb3Sn 39 , NbTi40 ) in magnetic field of (c) 0 T and (d) 3 T for H||c.…”

Review of the superconducting properties of MgB2

“…Figure 27 shows data of the critical current versus applied magnetic field, J c (H), for bulk MgB 2 samples, taken at different temperatures: 5, 10, 15, 20, 25 and 30 K. For comparison, the J c (H) data for Nb-Ti (Heussner et al 1997) and Nb 3 Sn (Kim and Stephen 1969) (bulk, film, wire and powder). Data are obtained from Fuchs et al (2001), Finnemore et al (2001), Thompson et al (2001, Wen et al (2001aWen et al ( , 2001b, Che et al (2001), Patnaik et al (2001), Zhao et al (2001b), Eom et al (2001), Ferdeghini et al (2001), Wang et al (2001c) and Bugoslavsky et al (2001a).…”

“…Joshi et al 2001a, Dhalle et al 2001, Bugoslavski et al 2001c, Takano et al 2001. The data for Nb-Ti (Heussner et al 1997) and Nb 3 Sn (Kim and Stephen 1969) (Dhalle et al 2001, Takano et al 2001, Bugoslavsky et al 2001a. The data for Nb-Ti (Heussner et al 1997) and Nb 3 Sn (Kim and Stephen 1969) The data is taken from Canfield et al (2001), Che et al (2001), Goldacker et al (2001), Glowacki et al (2001), Jin et al (2001b), Kumakura et al (2001), Soltanian et al (2001), Song et al (2001), Suo et al (2001 and Wang et al (2001c).…”

STM Studies of the Proximity Effect in Superconducting Wires with Artificial Pinning Centers

“…This idea has already proven itself useful, since wires of this type have higher critical currents (at various magnetic fields) than those manufactured with conventional processing. Details of the fabrication procedures and critical currents vs. fields of these wires are brought elsewhere [9,10]. This technological success stimulates further study of the microscopic behavior of these wires, in order to model their PE and continue to optimize them.…”