'Beautiful' unconventional synthesis and processing technologies of superconductors and some other materials

Abstract

Superconducting materials have contributed significantly to the development of modern materials science and engineering. Specific technological solutions for their synthesis and processing helped in understanding the principles and approaches to the design, fabrication and application of many other materials. In this review, we explore the bidirectional relationship between the general and particular synthesis concepts. The analysis is mostly based on our studies where some unconventional technologies were applied to different superconductors and some other materials. These technologies include spray-frozen freeze-drying, fast pyrolysis, field-assisted sintering (or spark plasma sintering), nanoblasting, processing in high magnetic fields, methods of control of supersaturation and migration during film growth, and mechanical treatments of composite wires. The analysis provides future research directions and some key elements to define the concept of 'beautiful' technology in materials science. It also reconfirms the key position and importance of superconductors in the development of new materials and unconventional synthesis approaches.

Keywords: Ag; La0.8Sr0.2Ga0.9Mg0.1O3; La1−xSrxMnO3; MgB2; MgO; MoO3; Nb3Sn; Sr–Ca–Cu–O; Tl–Ba–Ca–Cu–O; Y–Ba–Cu–O; cables; coated conductors; composites; critical current density; nanodots; nanopowders; pinning centers; precipitates; superconductors; thin films; whiskers.

Figure 1.

Spray-frozen freeze-drying (SFFD) method: a solution is sprayed through an air nozzle into a liquid nitrogen bath, and water from the produced solid frozen particles is sublimated. In the next processing steps, dried powders are thermally decomposed to oxides.

Figure 2.

Schematics of (a) fast pyrolysis (FP), (b) field-assisted sintering technique (FAST) and (c) continuous field-assisted sintering or rolling. P denotes applied pressure.

Figure 3.

(a) Real part of ac magnetic susceptibility plotted versus temperature for SFFD BSCCO powders thermally decomposed with fast pyrolysis. The time of heating at reaction temperature increases in the sequence 0, 5, 10, 20, 30, 60, 90, 240 and 1080 min in the arrow direction. Bi-2223 was detected after 5 min of heating. (b) Pinning force by Kramer scaling for pure MgB₂ and for MgB₂ (MB) with additions of 5 wt.% SiC (MBSC) or B₄C (MBBC) sintered by FAST. For the MgB₂ sample, there is a deviation from the usual linear behavior indicating mixed pinning states. (c) Scanning electron microscopy (SEM) image (left) of high-density Fe–MgB₂ metal–ceramic sandwich produced by FAST. In the right image, large and oriented iron-rich MgB₂ grains (indicated by arrow) can be seen at the metal–ceramic interface.

Figure 4.

(a) Chemical structure of the explosive agent cyclotrimethylenetrinitramine used in the NB processing. (b, c) Magnetic moment measured versus temperature in zero-field-cooling and field-cooling arrangements for La_0.8Sr_0.2Ga_0.9Mg_0.1O₃–Ce samples produced by NB+FAST. Note the paramagnetic behavior in the sample (b) when the particle size is 30 nm (maximum FAST temperature of 1350 °C) and the magnetoresistive one for the sample (c) when the particle size is 14 nm (1250 °C). The ferromagnetic M-H hysteresis of the sample (c) is shown in panel (d).

Figure 5.

(a) Superconducting transition (zero field cooling) for Bi-2212 whiskers grown in 0 and 7 T. A smaller transition width is obtained for the whiskers grown in a magnetic field, where the transition widths are defined between T_c and the temperatures indicated with arrows for each curve. (b) SEM image of the whiskers grown in a field of 7 T (orientation of the field is indicated) and (c) SEM image of the whiskers grown in zero field.

Figure 6.

CuO obtained during the growth of Bi-2212 HTS whiskers in zero field and in a magnetic field of 10 T. Note the change from 2D and 3D growths in zero field (SEM images on the left) to 1D structures (right) of cuboid (top) or ribbon like (bottom) morphology.

Figure 7.

Relationship between superconductors or other materials, magnetic field generation and unconventional thermomagnetic synthesis/processing technologies.

Figure 8.

Schematics of a conventional mesa (a), and of a twisted Josephson junction fabricated from whiskers (b). Also indicated in (b) are HTS lattice parameters, the layered structure composed of alternative superconducting (S) and normal (insulating, I) blocks and the current path through the S-I-S layers.

Figure 9.

A composite hierarchical tree like structure of α-MoO₃ whiskers on a sillimanite fiber (silicon-aluminium oxide, SAO; see SEM image) and the growth through the vapor transport. Lattice parameters are indicated for MoO₃.

Figure 10.

Self-assembled Au nanodots (atomic force microscopy, AFM, image) grown by PLD on a substrate and used as a template for further growth of MgO array of nanowires (SEM image to the right).

Figure 11.

Self-assembled Ag nanodots grown by rf sputtering on (001)SrTiO₃ single-crystal substrate with terraces and natural Ca nanodots precipitated on the surface of the (001)MgO substrate.

Figure 12.

Critical current density plotted versus magnetic field for Tl-1223 HTS (Tl–Ba–Ca–Cu–O and Tl–Ba–Sr–Ca–Cu–O) grown on (001)SrTiO₃ substrates with and without nanodots (NDs). Arrows indicate the enhancement of J_c.

Figure 14.

(a) Optical microscopy image of the surface of a SmBa₂Cu₃O₇ film grown by MOCVD. (b) Schematic drawing showing the principle of ‘double migration length’ for the precipitate removal in the growth of a film on a substrate with artificial steps. (c) and (d): Optical images of the Bi-2223 films grown on substrates containing artificial steps of the respective widths of 60 and 20 μm. For the 60 μm step, 10 μm-wide regions at the step edge (dashes in panel c) are free of precipitates. At the step width of 20 μm, the entire step is free of precipitates and the condition of ‘double migration length’ is fulfilled. (e) and (f) show substrates with steps (60 and 20 μm) indicated by circles. Substrate (f) was used for the growth of precipitate-free HTS thin films. On these films successful fabrication of intrinsic Josephson junctions with mesa geometry was demonstrated in [47].

Figure 13.

AFM image of the parallel Ag nanowalls (wires) grown by rf sputtering on (100)SrTiO₃ with terraces.

Figure 15.

AFM images of Bi-2223 MOCVD thin films grown by (a) conventional continuous deposition and (b) interrupted growth. Height profiles along the indicated lines are presented in the bottom panels.

Figure 16.

Arrangement of pulleys for release of the residual strain in the composite Nb₃Sn superconducting wires.

Figure 17.

Mechanical treatments of (a) Nb₃Sn wire produced by Furukawa Electric Co. Ltd, Japan. (b) and (c): The experimental arrangements for torsion and bending on short samples, respectively (1, press to fix the wire; 2, quartz tube; 3, Nb₃Sn wire; 4, holder). (d) Critical current, I_c, plotted versus applied magnetic field, B, for the following wires: as-reacted, after cycles of alternate bending (15 times with an optimum bending strain ε=0.8%, B n15(0.8)), alternate torsion (15 times with an optimum torsion angle ϕ=60°, T n15/60°) and alternate torsion and bending (6 torsion and 9 bending steps in the following sequence 3×[(ϕ=60° ×n=2)+ (ε=0.8% ×n=2)+(ε=0.2% ×n=1)], T+B n15). The enhancement of I_c after mechanical processing is indicated by the arrow.