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

Preparation and crystal structure of members of the solid solution phase Ba5Ru2-xAl1+x-yCuyO11 with x = 0.378, y = 0.085 and x = 0.5, y = 0

Preparation of layered manganites: oxygen content as a variable parameter of the bilayer compound

We report on the temperature dependence of the trapped field in neutron irradiated YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// bulk melt-textured materials. The field trapping capability of these materials at low temperatures is limited by their mechanical properties. We observe that samples without reinforcement usually break during activation at temperatures of around 50 K. Two reinforcement techniques were tested. Resin impregnation is found to be less effective than stainless steel bandaging to compensate for the magnetic stress generated during activation. The resin impregnated samples crack during activation at temperatures of around 45 K. Stainless steel bandaging improves the mechanical properties of the YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// samples and trapped fields of up to 13.3 T were achieved at 33 K.

Temperature dependence of the trapped field and mechanical properties of neutron irradiated and reinforced YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// bulk superconductors

On the base of phase diagram investigations the influence of oxygen partial pressure and reacting admixtures on the texturing process of YBa 2 Cu 3 O 7-δ is estimated. Methods are derived and tested which provide processing at decreased temperatures.

Phase diagram investigations for lowering the temperature of melt texturing of YBCO

We present a detailed joint experimental and theoretical investigation of the valence band electronic structure of single crystals of the model cuprate Ba2Cu3O4Cl2. This oxychloride system possesses a Cu3O4 plane which can be regarded as a superposition of two subsystems: a Cu AO 2 cuprate plane and an extra Cu B site, and thus represents an ideal trial system for assessing the impact of deviations from the CuO2 plane stoichiometry upon the electronic structure in cuprate materials. From polarization-dependent, k-resolved photoemission measurements the dispersion relations and symmetry of a number of the observed valence band features are determined and compared with the results of band structure calculations carried out within the LSDA1U formalism, which include a detailed analysis of the character and symmetry of the individual bands. Upon electron removal, the extra copper site makes its presence felt via the formation of a second Zhang-Rice singlet located on the Cu B subsystem, in addition to that originating from the CuAO2 subsystem. The main valence band edge at ;2 eV binding energy is shown to be mainly due to bands involving combinations of O 2px ,y orbitals, some of which exhibit essentially pure O 2p character. These low-lying oxygen bands are different in origin from the nonmixing O 2p states observed, for example, at (p ,p) in Sr2CuO2Cl2, as the orbital combination responsible for the latter is involved in Ba2Cu3O4Cl2 in the formation of the Zhang-Rice singlet state on the CuB sublattice. @S0163-1829~98!00717-6#

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G. Krabbes

Papers

Preparation and crystal structure of members of the solid solution phase Ba5Ru2-xAl1+x-yCuyO11 with x = 0.378, y = 0.085 and x = 0.5, y = 0

Preparation of layered manganites: oxygen content as a variable parameter of the bilayer compound

Temperature dependence of the trapped field and mechanical properties of neutron irradiated and reinforced YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// bulk superconductors

Phase diagram investigations for lowering the temperature of melt texturing of YBCO

Electronic structure of the Cu3O4 plane of Ba2Cu3O4Cl2: experiment and theory