Block copolymer self-assembly-directed synthesis of mesoporous gyroidal superconductors

Abstract

Superconductors with periodically ordered mesoporous structures are expected to have properties very different from those of their bulk counterparts. Systematic studies of such phenomena to date are sparse, however, because of a lack of versatile synthetic approaches to such materials. We demonstrate the formation of three-dimensionally continuous gyroidal mesoporous niobium nitride (NbN) superconductors from chiral ABC triblock terpolymer self-assembly-directed sol-gel-derived niobium oxide with subsequent thermal processing in air and ammonia gas. Superconducting materials exhibit a critical temperature (T c) of about 7 to 8 K, a flux exclusion of about 5% compared to a dense NbN solid, and an estimated critical current density (J c) of 440 A cm(-2) at 100 Oe and 2.5 K. We expect block copolymer self-assembly-directed mesoporous superconductors to provide interesting subjects for mesostructure-superconductivity correlation studies.

Fig. 1. G^A structure and sample/structure evolution from initial compounds to final NbN superconductors.

(A) G^A before and after processing, with the unit cell indicated by the black cube. (B) (Top) Chemical structures of compounds and (bottom) schematic of synthesis and processing steps with photographs of the final materials. Block terpolymers (ISO) are combined with the Nb₂O₅ sol-gel precursors in a common solvent. Hybrid block copolymer/Nb₂O₅ G^A structures are generated by solvent evaporation–induced self-assembly. After calcination in air, the mesoporous Nb₂O₅ G^As are transformed to NbN G^As in a two-step nitriding process. Scale bars in all photographs represent 1 cm. NH₃, ammonia.

Fig. 2. Materials characterization by x-ray scattering.

(A and B) SAXS patterns of samples derived from ISO-64k (A) and ISO-86k (B) at various processing stages. From bottom to top: ISO/oxide hybrids; samples calcined at 450°C in air; samples nitrided at 700°C; and sample nitrided at 850°/865°C. Observed (solid) and expected (dashed) peak positions for the G^A structure are indicated by ticks above each curve. Curves for the ISO-86k–derived samples were integrated using a selected angular range as a result of the significant orientation of the mesostructure. (C) Powder XRD patterns of samples at various processing stages. From bottom to top: Sample calcined at 450°C in air; sample nitrided at 700°C; sample nitrided at 850°C; and sample nitrided at 865°C. All patterns are from samples derived from ISO-64k, except for the top trace, which is from a sample derived from ISO-86k. Bottom tick marks indicate expected peak positions and relative intensities for a cubic rock salt NbN pattern (Powder Diffraction File card 04-008-5125).

Fig. 3. Materials characterization by N₂ sorption and SEM.

(A) Pore size distributions from N₂ sorption measurements for ISO-64k–derived samples at various processing stages. (B to F) SEM images of mesoporous samples at different processing stages. ISO-64k–derived gyroidal NbN (B) after nitriding at 700°C and (C) after nitriding at 850°C. (D) ISO-86k–derived gyroidal Nb₂O₅ after calcination at 450°C in air. ISO-86k–derived gyroidal NbN (E) after nitriding at700°C and (F) after nitriding at 865°C.

Fig. 4. Magnetization and electrical resistance of superconducting gyroids.

(A) Temperature-dependent magnetization from 2.5 to 10 K for ISO-64k–derived NbN films in an applied field of 200 Oe, with their long axis oriented either parallel (top; ||) or perpendicular (bottom; ┴) to the applied field (see insets for the different geometries tested). (B) Temperature-dependent magnetization from 2.5 to 10 K for ISO-86k–derived NbN films in an applied field of 100 Oe, with their long axis oriented perpendicular to the field. (C) Temperature-dependent four-point electrical resistance of ISO-86k–derived NbN films showing a drop beginning at approximately 7 K.