Nanostructures have attracted huge interest as a rapidly growing class of materials for many applications. Several techniques have been used to characterize the size, crystal structure, elemental composition and a variety of other physical properties of nanoparticles. In several cases, there are physical properties that can be evaluated by more than one technique. Different strengths and limitations of each technique complicate the choice of the most suitable method, while often a combinatorial characterization approach is needed. In addition, given that the significance of nanoparticles in basic research and applications is constantly increasing, it is necessary that researchers from separate fields overcome the challenges in the reproducible and reliable characterization of nanomaterials, after their synthesis and further process (e.g. annealing) stages. The principal objective of this review is to summarize the present knowledge on the use, advances, advantages and weaknesses of a large number of experimental techniques that are available for the characterization of nanoparticles. Different characterization techniques are classified according to the concept/group of the technique used, the information they can provide, or the materials that they are destined for. We describe the main characteristics of the techniques and their operation principles and we give various examples of their use, presenting them in a comparative mode, when possible, in relation to the property studied in each case.

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Characterization techniques for nanoparticles: comparison and complementarity upon studying nanoparticle properties

Large-grain high-temperature superconductors of the form RE-Ba-Cu-O (where RE is a rare-earth element) can trap magnetic fields of several tesla at low temperatures, and so can be used for permanent magnet applications. The magnitude of the trapped field is proportional to the critical current density and the volume of the superconductor. Various potential engineering applications for such magnets have emerged, and some have already been commercialized. However, the range of applications is limited by poor mechanical stability and low thermal conductivity of the bulk superconductors; RE-Ba-Cu-O magnets have been found to fracture during high-field activation, owing to magnetic pressure. Here we present a post-fabrication treatment that improves the mechanical properties as well as thermal conductivity of a bulk Y-Ba-Cu-O magnet, thereby increasing its field-trapping capacity. First, resin impregnation and wrapping the materials in carbon fibre improves the mechanical properties. Second, a small hole drilled into the centre of the magnet allows impregnation of Bi-Pb-Sn-Cd alloy into the superconductor and inclusion of an aluminium wire support, which results in a significant enhancement of thermal stability and internal mechanical strength. As a result, 17.24 T could be trapped, without fracturing, in a bulk Y-Ba-Cu-O sample of 2.65 cm diameter at 29 K.

High-temperature superconductor bulk magnets that can trap magnetic fields of over 17 tesla at 29 K

The ability of large-grain (RE)Ba2Cu3O7−δ ((RE)BCO; RE = rare earth) bulk superconductors to trap magnetic fields is determined by their critical current. With high trapped fields, however, bulk samples are subject to a relatively large Lorentz force, and their performance is limited primarily by their tensile strength. Consequently, sample reinforcement is the key to performance improvement in these technologically important materials. In this work, we report a trapped field of 17.6 T, the largest reported to date, in a stack of two silver-doped GdBCO superconducting bulk samples, each 25 mm in diameter, fabricated by top-seeded melt growth and reinforced with shrink-fit stainless steel. This sample preparation technique has the advantage of being relatively straightforward and inexpensive to implement, and offers the prospect of easy access to portable, high magnetic fields without any requirement for a sustaining current source.

https://iopscience.iop.org/article/10.1088/0953-2048/27/8/082001/pdf

A trapped field of 17.6 T in melt-processed, bulk Gd-Ba-Cu-O reinforced with shrink-fit steel

Magnetic oxides show a variety of extrinsic magnetotransport phenomena: grain-boundary, tunnelling and domain-wall magnetoresistance. In view of these phenomena the role of some magnetic oxides is outstanding: these are believed to be half-metallic having only one spin-subband at the Fermi level. These fully spin-polarized oxides have great potential for applications in spin-electronic devices and have, accordingly, attracted intense research activity in recent years. This review is focused on extrinsic magnetotransport effects in ferromagnetic oxides. It consists of two parts; the second part is devoted to an overview of experimental data and theoretical models for extrinsic magnetotransport phenomena. Here a critical discussion of domain-wall scattering is given. Results on surface and interfacial magnetism in oxides are presented. Spin-polarized tunnelling in ferromagnetic junctions is reviewed and grain-boundary magnetoresistance is interpreted within a model of spin-polarized tunnelling through natural oxide barriers. The situation in ferromagnetic oxides is compared with data and models for conventional ferromagnets. The first part of the review summarizes basic material properties, especially data on the spin polarization and evidence for half-metallicity. Furthermore, intrinsic conduction mechanisms are discussed. An outlook on the further development of oxide spin-electronics concludes this review.

Extrinsic magnetotransport phenomena in ferromagnetic oxides

This paper describes the present status of high temperature superconductors (HTS) and of bulk superconducting magnet devices, their use in bearings, in flywheel energy storage systems (FESS) and linear transport magnetic levitation (Maglev) systems. We report and review the concepts of multi-seeded REBCO bulk superconductor fabrication. The multi-grain bulks increase the averaged trapped magnetic flux density up to 40% compared to single-grain assembly in large-scale applications. HTS magnetic bearings with permanent magnet (PM) excitation were studied and scaled up to maximum forces of 10 kN axially and 4.5 kN radially. We examine the technology of the high-gradient magnetic bearing concept and verify it experimentally. A large HTS bearing is tested for stabilizing a 600 kg rotor of a 5 kWh/250 kW flywheel system. The flywheel rotor tests show the requirement for additional damping. Our compact flywheel system is compared with similar HTS–FESS projects. A small-scale compact YBCO bearing with in situ Stirling cryocooler is constructed and investigated for mobile applications. Next we show a successfully developed modular linear Maglev system for magnetic train operation. Each module levitates 0.25t at 10 mm distance during one-day operation without refilling LN2. More than 30 vacuum cryostats containing multi-seeded YBCO blocks are fabricated and are tested now in Germany, China and Brazil.

http://iopscience.iop.org/article/10.1088/0953-2048/25/1/014007/pdf

Superconductor bearings, flywheels and transportation

A series of solid solution powders with the compositions of (Nd 0.33 Sm 0.67 ) 1+ x Ba 2− x Cu 3 O 7− δ ( x =0, 0.05, 0.1, 0.2, 0.3) were synthesized in air and the superconductivity and phase stability were investigated. Single-phase powders were obtained and the maximum solid solubility limit x was revealed as 0.2 by quenching from 1040°C for x =0, 0.05, 0.1 and 1030°C for x =0.2 and 0.3 to room temperature in air. The unusual two-kink superconducting transitions were reported and discussed in the framework of phase separation. The post-annealing in Ar displayed a dramatic suppression of solid solubility limits. The precipitate due to the decomposition in Ar treatments may act as additional pinning centers. These results demonstrated that Ar isothermal treatments can act as a key measure to process light rare-earth 123 superconductors with desired properties for applications.

On the superconductivity and phase stability of (Nd0.33Sm0.67)1+xBa2-xCu3O7-δ

Trapped magnetic fields and levitation forces at 77 K have been studied in bulk melt-textured YBCO with single grains of 26 mm, doped with different content of Li and Ni. Li dopant results in higher trapped field than Ni indicating that Li additions are more effective in increasing pinning capabilities.

Trapped field and levitation force in melt-textured YBCO doped with Ni and Li

We report single crystal growth of the spin-ladder compound La 2 Cu 2 O 5 (4-leg) and spin-ladder-like compound CaCu 2 O 3 (2-leg). Crystal growth of La 2 Cu 2 O 5 has been carried out by the self-flux method using a modified slow cooling technique. Single crystals of CaCu 2 O 3 have been grown by the travelling solvent floating zone method under oxygen pressure (∼5 bar). An optimum solvent for growth of CaCu 2 O 3 was found to be Ca 0.8 Sr 0.2 Cu 2 O 3 . The grown crystals were characterized for their purity and structure by using optical polarized microscope, energy dispersive X-ray spectroscopy, and X-ray diffraction methods.

Crystal growth of spin-ladder compound La2Cu2O5 and ladder-like compound CaCu2O3

Nd1+xBa2−xCu3O7−δ (Nd-123) superconductors were melt-processed in air atmosphere starting from an initial precursor consisting of Nd2O3, BaCuO2 and CuO, or Nd-123 and BaCuO2, respectively, to stabilise the process to proceed in equilibrium with compositions along the tie line Nd-123–Nd-422, both with its stoichiometric compositions. The Nd:Ba ratio was stabilised close to 1:2 and high Tc values up to 96 K were achieved. As a result, Jc values of 30 kA/cm2 (0 T) and an expressed peak effect of 10 kA/cm2 arises at an applied external magnetic field of 2 T at 77 K.

Composition-controlled melt processing of Nd1+xBa2−xCu3O7−δ-based bulk material in air

The microstructure of single-grain, melt-processed (MP) YBa2Cu3O7/Y2BaCuO5 (Y-123/Y-211) samples (YBCO) containing varying amounts of depleted uranium (DU) and Pt has been studied. Only partial refinement of the Y-211 particle size was observed in Pt-free samples, which generally contained both small and large Y-211 particles. Small Y-211 particles in these samples are pushed extensively in the c-growth sector (c-GS) and all Y-211 particles (small and large) coarsen with the distance from the seed and with increasing DU concentration. Samples fabricated with Pt contained only very fine Y-211 particles, which were generally pushed strongly in the c-GS. In this case the size of the Y-211 particles did not vary significantly with distance from the seed. U- and U/Pt-containing sub-micron sized particles present in the melt-processed YBCO microstructure were not pushed during solidification, although their arrangement within the structure of the sample was influenced clearly by the growth process. The so-called cyclic growth was observed in the c-GS at the highest DU concentration (0.8 wt%). In these samples, this growth pattern is associated with the formation of a liquid phase rich in U and Y at the growth front. The cyclic growth mechanism was modified by the addition of Pt. Crystals of Y2Ba4UCuOx with Ba3YUOx phase inclusions were observed to be present in the U/Y-rich melt.

G. Krabbes

Papers

On the superconductivity and phase stability of (Nd0.33Sm0.67)1+xBa2-xCu3O7-δ

Trapped field and levitation force in melt-textured YBCO doped with Ni and Li

Crystal growth of spin-ladder compound La2Cu2O5 and ladder-like compound CaCu2O3

Composition-controlled melt processing of Nd1+xBa2−xCu3O7−δ-based bulk material in air

The influence of the addition of depleted uranium on particle pushing in melt-processed, bulk Y–Ba–Cu–O