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We compare the temperature and field dependence of the critical current densities of high- $T_c$ superconductor materials intended for various bulk applications such as trapped-field magnets. This comprises bulk samples of YBa $_2$ Cu $_3$ O $_x$ (YBCO), MgB $_2$ , and iron-based materials, including also various versions of the YBCO compound such as melt-textured ones, infiltration-growth processed ones, and YBCO foams. Critical current densities and flux pinning forces were obtained from magnetization loops measured using Quantum Design SQUID and physical property measurement system (PPMS) systems with applied magnetic fields of up to $\pm$ 9 T. The obtained data are compared to each other with respect of the optimal cooling temperature possible using modern cryocoolers. Furthermore, we plot the temperature dependencies of the critical current densities versus the normalized temperature $t=T/T_c$ . This enables a direct judgement of the performance of the material in the trapped-field applications.

To find a cheap and efficient way to produce high-performance bulk MgB2 superconductors, a series of samples produced with different contents of carbon-coated, nanometer-sized amorphous boron was prepared to improve the magnetic properties. A single-step solid-state reaction at the optimal reaction temperature of 805 °C for 3 h in pure argon atmosphere was used. Small pieces cut from the bulk were characterized by magnetic hysteresis loop measurements in magnetic fields up to ±7 T at temperatures ranging from 5 to 35 K using a physical property measurement system. Critical current densities and flux pinning forces were calculated from the magnetization data. The pinning force data of all samples were compared using the pinning force scaling approach by Eisterer. The peak positions h 0 were found at h 0 ∼ 0.33, indicating a dominant flux pinning at the grain boundaries for samples with 50% and 30% carbon-coated boron, while the scaling reveals a peak position of 0.44 for the 10% sample that indicates pinning at point defects.

Made available in DSpace on 2021-06-25T11:12:22Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-08-01 Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Microstructural and magnetic characterizations of YBa2Cu3O7-δ nanowire fabric-like samples were performed. Fabric-like ceramic superconductors are nanoporous materials and can be applied, e.g., in magnetic shielding, since the porous morphology makes them extremely light-weight and effectively to be cooled. The samples were produced by the solution blow spinning (SBS) technique, starting from a sol-gel solution of the precursor materials in polyvinyl-alcohol. The morphology of the samples was characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction with the aim to analyze the possible current paths. The magnetic hysteresis loops revealed typical polycrystalline behavior, and field-cooling in various small applied fields shows the presence of the paramagnetic Meissner effect (PME), but in a distinctly different way as compared to melt-texured Bi2Sr2CaCu2O8+δ or Nb. Sao Paulo State University (UNESP) Campus at Ilha Solteira Experimental Physics Saarland University Sao Paulo State University (UNESP) Campus at Ilha Solteira

Flux pinning force scaling $f=F_{p}/F_{p,\max \limits }$ f = F p / F p , max vs. h = Ha/Hirr was performed on a variety of pure MgB2 samples, including a spark plasma sintered (SPS) one and a series of samples sintered at various reaction temperatures ranging between 775 and 950 ∘C. The SPS sample exhibits a well-developed scaling at all temperatures, and also the sintered samples prepared at 950 ∘C; however, the obtained peak positions of the pinning force scalings are distinctly different: The SPS sample reveals dominating pinning at grain boundaries, whereas the dominating pinning for the other one is point-pinning. All other samples studied reveal an apparent non-scaling of the pinning forces. The obtained pinning parameters are discussed in the framework of the Dew–Hughes’ pinning force scaling approach.

Bi2Sr2CaCu2O8+δ (Bi-2212) nanowires were fabricated employing electrospinning. A subsequent thermal treatment based on thermal gravity analysis is required to obtain the final crystal structure and superconductivity; however, if this preparation temperature is too high, the nanowire structure may completely collapse during the treatment, which is even more pronounced when aligning the nanowires. Therefore, Li acetate was added in various amounts to the starting composition, which enabled to produce the desired Bi-2212 phase already at about 750 °C instead of 840 °C. An X-ray and a TEM analysis proved that the resulting nanowires were single-phase Bi-2212. After the electro-spinning process, the resulting samples form fabric-like networks, and in the heat treatment, the nanowire structure was found to survive. The nanowires exhibited a polycrystalline structure with grain sizes of 20–50 nm, an average wire diameter of ∼100 nm, and a length of up to several micrometers. The resulting nanowire network samples were characterized by magnetic measurements and electric transport measurements in fields up to 9 and 4 T, respecitively.

The transmission electron backscatter diffraction (t-EBSD) technique has proven to be an indispensable tool for the analysis of microstructures of superconducting samples, both high-Tc samples (YBa2Cu3Oy, Bi2Sr2CaCu2O8) as well as MgB2 or iron-based materials. The knowledge of the grain boundary properties (misorientation, length, width) is essential for the further optimization of sample performance. Any addition of secondary phase(s) to improve the flux pinning properties is required to be of nanometer dimensions, so the higher achievable resolution and the better imaging properties are important to obtain reasonably high image quality to enable automated orientation mapping. The orientation maps reveal not only the location and the shape of the inclusions within the superconducting matrix or at the grain boundaries but also their influence on the surrounding superconducting matrix, which also plays an important role in flux pinning. In the case of sintered MgB2 bulk samples, the demand for higher critical current densities leads to MgB2 grains in the 100-nm range, which is already difficult to be studied by means of conventional EBSD. Furthermore, t-EBSD is useful for the analysis of specific microstructures of unconventional superconductors like superconducting foams or superconducting nanowire networks.

YBa2Cu3O y (YBCO) foam samples show an open, porous foam structure, which may have benefits for many applications of high-Tc superconductors. As the basic material of these foams is a pseudo-single crystalline material with the directional growth initiated by a seed crystal similar to standard melt-textured samples, the texture of YBCO is a very important parameter. Therefore, we analysed the local texture and grain orientation of the individual struts forming the foam by means of atomic force microscopy (AFM) and electron backscatter diffraction (EBSD). Due to the processing route starting with Y2BaCuO5 (211), a two-phase analysis must be performed, so a high surface quality is necessary to enable an automated EBSD scan. Good quality Kikuchi patterns were obtained from both the YBCO and 211 phases. We found an inhomogeneous distribution of the residual 211 particles, which are mainly randomly oriented and have sizes ranging between 200 nm and 15 μm. In contrast to this, the YBCO matrix shows a dominating orientation with cracks with a typical distance of 1–10 μm. Furthermore, the analysis of strut cross-sections reveals that the entire strut is converted to the YBCO phase.

Self-organized nanostripes are observed in various light-rare-earth (LRE)-based 123-type high-Tc superconductors in the as-grown state by means of AFM and STM topography measurements. These observations indicate that the nanostripes are formed during the crystal growth process. The necessary oxygenation step to obtain superconducting samples causes the formation of twin boundaries and thus, deforms the original arrangement of the nanostripes.

Crystallographic orientations of thin films and bulk, melt-textured YBa 2 Cu 3 O x high- T c superconductors with embedded Y 2 Ba 4 Cu M O x ( M = Nb, Zr, Ag) nanoparticles are studied by electron backscatter diffraction. The Y 2 BaCuO 5 particles exhibit no preferred orientation but have a strong negative influence on the matrix orientation. In contrast, the nanoparticles are not disturbing the texture of the YBCO matrix. For the bulk samples, depending on the preparation route, a different particle orientation with respect to the matrix is obtained. Untextured M -2411 nanoparticles are formed by solid state reaction during the melt process by adding oxides (Nb 2 O 5 or Y 2 O 3 ) to the precursor powder. Pre-formed Y 2 Ba 4 Cu M O x particles added to the precursor in the form of pre-reacted nanopowder, exhibit a dominant single orientation related to the surrounding YBCO matrix. In the case of thin films prepared using laser ablation with a bulk YBCO sample with embedded nanoparticles as a target, the nanoparticles are transferred into the thin film, but the transferred M -2411 particles are found to be randomly oriented. Nevertheless, these films exhibit increased flux pinning properties.

The microstructures of composite xBaTiO3–(1−x)(Ni0.5Zn0.5)Fe2O4 (BT–NZF) multiferroics with various mixing ratios (x = 0.50, 0.60 and 0.70) are investigated by means of electron backscatter diffraction (EBSD) and magnetic force microscopy (MFM). The EBSD measurements reveal a change in the texture of the ferrite and the BaTiO3 grains upon increasing the ferrite content in the sample. The sample with x = 0.70 exhibits the best ferrite texture, where only some directions are present. Furthermore, the resulting grain sizes vary from several µm (x = 0.50) to about 100 nm in the sample with x = 0.70. The MFM images reveal the presence of magnetic domains being extended over several adjacent grains, which according to the EBSD data may comprise different crystallographic orientations. In this way, we can explain the differences in the magnetic contrast obtained.

By means of electron backscatter diffraction (EBSD), we analyse the crystallographic orientation of electroplated magnetite thin films on Si/copper substrates. Varying the voltage during the electroplating procedure, the resulting surface properties are differing considerably. While a high voltage produces larger but individual grains on the surface, the surfaces become smoother on decreasing voltage. Good quality Kikuchi patterns could be obtained from all samples; even on individual grains, where the surface and the edges could be measured. The spatial resolution of the EBSD measurement could be increased to about 10 nm; thus enabling a detailed analysis of single magnetite grains. The thin film samples are polycrystalline and do not exhibit a preferred orientation. EBSD reveals that the grain size changes depending on the processing conditions, while the detected misorientation angles stay similar.

The analysis of the achieved texture is of great importance for the performance of ferrite materials, either bulk or thin films. The recently developed electron backscatter diffraction (EBSD) technique enables a spatially resolved study of the crystallographic orientations by means of recording of Kikuchi patterns. To our knowledge, such a thorough EBSD analysis was not yet performed in any oxidic magnetic material, and only very recently on magnetite thin films by us. A good surface polishing/cleaning is essential for this analysis, as the method requires an undisturbed surface area for a high image quality (IQ). This information is recorded to each measured Kikuchi pattern, together with a parameter describing the quality of indexation. Here, the spatially highly resolved EBSD mappings provide additional information as compared to the standard analysis techniques, which can contribute to an optimization of the growth process. Furthermore, an analysis of the grain aspect ratio is possible which provides further insight to the microstructural dependence of the magnetic properties of ferrites.

The electron backscatter diffraction (EBSD) technique enables an advanced analysis of anisotropic materials like ferrites. Here, the spatially highly resolved EBSD mappings provide additional information as compared to the standard analysis techniques, which can contribute to an optimization of the growth process. Furthermore, an analysis of the grain aspect ratio is possible which provides further insight to the microstructural dependence of the magnetic properties of ferrites.

By means of electron backscatter diffraction (EBSD), we studied the grain orientation of cross-sections of magnetite (Fe 3 O 4 ) thin films grown on (001) oriented MgO substrates. Earlier studies showed that the magnetic properties of the magnetite films are dominated by the presence of so-called anti-phase boundaries (APBs). The use of cross-sections of thin-films enables to study the growth mechanism in a more direct way. The EBSD maps reveal that the first magnetite layer on the MgO substrate is comprised of many small, misoriented magnetite grains. Subsequent layers show the desired (001) orientation with a ±5° misorientation. The misori-entations found in the cross-section of the film resemble those obtained in EBSD measurements at the sample surface.

Thin-films of Ni x Zn 1-x Fe 2 O 4 [(Ni,Zn)-ferrite] are grown by means of RF sputtering on Si(100) and (111) substrates, corresponding to the orientation of Si cantilevers for AFM/MFM measurements. We find that the ferrite can be sputtered directly onto the Si surfaces, but an additional annealing step is required to obtain a purely polycrystalline, soft magnetic film. The microstructure of the films is investigated employing transmission electron microscopy, electron backscatter diffraction and magnetic force microscopy.

Permalloy (Ni 81 Fe 19 ) films and foils were fabricated by means of electroplating. Two different types of samples were produced, (i) a patterned permalloy film on a Si wafer (type 1) and (ii) a NiFe foil (type 2), which was separated from the substrate after deposition. The samples were characterized employing magnetic force microscopy (MFM), transmission electron microscopy (TEM) and electron-backscatter diffraction (EBSD). On the foil type sample, MFM reveals a maze domain pattern, and on the structured samples (type 1), a Landau domain pattern is obtained. EBSD enables the identification of the permalloy Kikuchi pattern without the effect of a substrate. The analysis reveals randomly oriented grains with a narrow size distribution.

Using automated orientation imaging, the grain orientations and texture of monofilamentary, Ag-sheathed (Pb,Bi) 2 Sr 2 Ca 2 Cu 3 O x (Bi-2223) tape is analysed in detail by means of electron backscatter diffraction (EBSD). The achieved high image quality of the Kikuchi patterns enables multi-phase scans including Bi-2223, Bi 2 Sr 2 CaCu 2 O x , Bi 2 Sr 2 CuO x , (Sr,Ca) 14 Cu 24 O 41 and Ag to be performed. Two areas are selected for the EBSD analysis, one close to the silver sheath, the other located in the center of the sample. The grain orientation maps are presented for each phase separately allowing a new insight into the microtexture of Ag-sheathed Bi-2223 tapes. Furthermore, the EBSD analysis provides the possibility for a misorientation angle analysis within each individual phase.

The local orientation of particles embedded within melt-textured Y–Ba–Cu–O (YBCO) samples containing depleted uranium oxide, Y 2 O 3 , Nb 2 O 5 , and Y 2 Ba 4 CuMO x (with M = Nb, Zr) have been studied via automated electron backscatter diffraction (EBSD) analysis. The different morphologies of the samples are discussed based on the EBSD scans of a-growth sector areas. We conclude that the formation of small, nanoscale particles does not disturb the growth of the superconducting YBCO matrix. The presence of large, micrometre-sized particles, however, may cause severe changes in the YBCO growth morphology.

Local orientations of superconducting YBa2Cu3O x (123) and embedded Y2BaCuO5 (211) particles within bulk, melt-textured 123 samples are studied on a local scale by means of electron backscatter diffraction (EBSD). The embedded Y2BaCuO5 (211) particles do not show any preferred texture, but their presence within the 123-matrix alters the growth of the 123-phase, independent of the orientation. EBSD enables to analyse the interactions between the 123- and 211-phases in detail.

The texture and phase distribution of melt-textured YBa 2 Cu 3 O x (YBCO) was studied by means of automated electron backscatter diffraction (EBSD) analysis as a function of the position in the bulk pellet of 4 cm diameter. A total of five samples was cut from the pellet; four samples from the surface of the bulk with different distances to the seed crystal, and one cut in vertical direction in the middle of the pellet. The melt-textured YBCO samples require a two-phase analysis to be performed, so a high surface quality is necessary to enable an automated EBSD scan. Good quality Kikuchi patterns are obtained from both the 1 2 3 and 2 1 1 phases. We found an inhomogeneous distribution of the 2 1 1 particles. Whereas the samples cut from the surface contain a large amount of 2 1 1 particles, in the samples of the vertical direction only traces of 2 1 1 particles are found. Furthermore, we measured the misorientation angle distribution of all samples. The data are presented in form of phase mappings, misorientation distribution functions and pole figures.

By means of automated electron backscatter diffraction (EBSD) analysis, we studied the local orientations of embedded Y 2 BaCuO 5 (2 1 1) particles within melt-textured YBCO samples, and also the orientations of embedded YBCO particles in directional solidified 211 samples. On both systems, we obtained high-quality Kikuchi patterns, allowing the automated mapping of the crystal orientations and a multi-phase analysis. In melt-textured YBCO with (0 0 1) orientation, we find that the embedded 211 particles do not have any preferred orientation, but the maps also reveal that at certain orientations of the 211 particles the YBCO growth is not altered. In directional solidified 211 samples, where the 211 is mainly oriented in (0 0 1) direction, the embedded YBCO particles show only some specific orientations.

We compare the results of an X-ray based pole figure texture analysis with the local texture analysis by means of electron-backscatter diffraction (EBSD) analysis. As samples, we employ two different melt-textured YBCO samples; one fully processed and one without oxygen treatment. To enable the direct comparison of the two techniques, we employ the [1 0 3] pole figures. We find a clear coincidence between the results obtained by the two measurement techniques on our samples, however, the EBSD results are much more detailed, yielding the local grain orientation distribution and quantitative results of the grain or subgrain misorientation angles. Therefore, the EBSD measurements give information not accessible to the X-ray pole figure analysis. The surface preparation procedure is essential to enable the automated EBSD mapping as high image quality Kikuchi patterns are required. The polishing procedures are discussed in detail.

We have studied the microstructures of subgrains in large single-domain RE–Ba–Cu–O (RE=Y, Nd, Sm) bulk superconductors through microscopic observation and microarea X-ray pole figure analyses. The misorientation angle of subgrains increases as the crystal growth proceeds, and thus more crystal defects such as dislocations are seen at the later stage of the growth. Electron backscatter diffraction technique was successfully utilized for characterizing the subgrain boundaries. Electric and magnetic measurements for several samples containing different subgrain geometries have been performed. The results of I – V and resistivity measurements indicated that subgrains act as weak-links especially in a high magnetic field region.

The electron backscatter diffraction (EBSD) technique was applied to polycrystalline YBa 2 Cu 3 O 7− δ ceramics. To obtain good quality EBSD Kikuchi patterns on the ceramic superconductors, a new sample polishing route was established. Samples with and without KClO 3 -doping were investigated; and the crystallographic orientations of the grains were analyzed using pole figures and EBSD orientation mapping. On the polycrystalline KClO 3 -doped YBa 2 Cu 3 O 7− δ samples we found two major orientations (0 0 1) and (1 0 0) normal to the sample surface, while the undoped sample did not exhibit any orientation maxima. We ascribe this observation to the effect of a liquid phase due to the KClO 3 -additive during the preparation process.

The influence of KClO 3 addition on the magnetic properties and microstructure of YBCO HTSC materials with nominal composition (A): Y 1 Ba 2− x K x Cu 3 O y , (B): Y 1−0.2 x Ba 2−0.2 x K x Cu 3 O y , (C): Y 1 Ba 2 Cu 3.5− x K x O y ( x =0–0.75) were investigated. Magneto-optic imaging of flux distributions was employed to study the grain connectivity in the doped and undoped samples in a direct way. The obtained flux patterns taken at various temperatures reveal that KClO 3 -doped samples ( x =0.30) exhibit flux distributions being close to those being observed in melt-textured superconductors. This demonstrates that the grain connectivity in the doped samples is considerably improved as compared to pure ones, which is an important issue for the fabrication of coated conductors of YBCO.

DC susceptibility as a function of temperature was measured in both field-cooled cooling and warming modes in magnetic fields in the range of 10 mT ⩽μ 0 H a ⩽7 T on a variety of YBa2Cu3O7−δ samples, comprising different single crystals, melt-processed bulks and polycrystalline samples, with and without KClO3 addition. The Meissner curves measured at a field of 1 mT reveal that all the samples are of a single phase. However, with increasing applied fields, the superconducting transitions of the single crystals and the melt-processed samples develop kinks, and above 4 T, even a secondary transition can be observed. At low temperatures, the field-cooled moment increases in all samples, except the KClO3-doped samples, where a clear decrease is observed. These features are discussed in detail. Further, we discuss the correlation between the kinks in the DC susceptibility and the appearance of the fishtail shape of magnetization hysteresis loops.

Electron backscatter diffraction (EBSD) analysis is applied to [0 0 1] oriented magnetite thin films grown on MgO substrates. A high image quality of the Kikuchi patterns was achieved enabling multi-phase scans. Several types of magnetite thin films were analyzed; one as-grown and the others after different annealing steps in oxygen atmosphere. From the EBSD mappings, we learn that the optimum orientation in [0 0 1]-direction is not yet achieved for the as-grown sample, but develops upon oxygen treatment. Furthermore, the distribution of misorientation angles within the investigated area (=1 grain) is found to change during the annealing steps. After 3 min of annealing, most of the misorientations around 30°–40° have vanished, and some islands with high misorientation angles remain, which may play a role as antiferromagnetic pinning centers.

Melt-textured YBCO samples with embedded nanosize particles of Y 2 Ba 4 CuNbO x (Nb-2411) have been studied by means of electron backscatter diffraction (EBSD). The microstructure of these samples shows some distinct and unusual features, not observed in melt-textured bulk samples of other compositions. The resulting texture of the superconducting YBCO matrix is found to be excellent. Y 2 BaCuO 5 (Y-211) particles, which are also present in these samples, exhibit no preferred orientation, as is the case generally in melt-textured YBCO samples. The embedded Y 2 Ba 4 CuNbO x particles, in contrast, exhibit only one orientation within the YBCO matrix. Furthermore, we find in the bulk pellets distinct areas with a high second phase particle concentration, adjacent to regions without secondary phase particles.

The crystallographic orientation of magnetite (Fe3O4) thin films was measured using electron backscatter diffraction (EBSD). Misorientation boundaries appear in maps of angular misorientation data. The distribution of misorientation angles changes after annealing the samples in air at 250degC. Most small-angle misorientations (

Nanostripes are observed in melt-textured and single-crystalline samples of the ternary light rare earth (LRE)-compound (Nd 0.33 Eu 0.33 Gd 0.33 )Ba 2 Cu 3 O x (NEG) by means of atomic force microscopy, scanning tunnelling microscopy at ambient conditions, combined with transmission electron microscopy and electron backscatter diffraction. This enables the observation of several important features: The nanostripes are formed by chains of nanoclusters, representing the LRE/Ba substitution. The dimensions of the nanostripes are similar for both types of NEG samples. The periodicity of the nanostripes is found to range between 40 and 60 nm; the shape of the nanoclusters is elliptic with a major axis length between 300 and 500 nm and a minor axis length of about 30–150 nm. The stripes are filling effectively the space in between the twin boundaries. Concerning the flux pinning, the nanoclusters are the important pinning sites, not the nanostripes themselves.

The influence of KClO 3 addition on the microstructure and magnetic properties of YBCO were investigated. The AC complex susceptibility χ AC = χ′ − iχ″ was recorded as a function of temperature and concentration of KClO 3 addition at various AC magnetic fields up to 2 mT. DC magnetization as function of temperature was measured by means of a SQUID magnetometer in field-cooled cooling and field-cooled warming (FCW) modes in magnetic fields in the range 0 ≤ μ 0 H a ≤ 7T.

International audience; The current flow and the flux pinning properties on struts of superconducting YBa2Cu3Ox (YBCO) foams are analyzed in detail in the temperature range 60 K < T < Tc. For this purpose, magnetization loops were measured taken from various positions of a 5 × 2 × 2 cm3 large foam sample prepared at RWTH Aachen. From these data, the critical current densities, jc, and the flux pinning forces, Fp = jc × B, were calculated and pinning force scaling diagrams Fp/Fp,max vs. h = Ha/Hirr were established. The scaling in the temperature range 60 K < T < 90 K was found to be well developed for all samples with peak positions, h0, above 0.4, which is an indication of δTc-pinning. From microstructure analysis of the foam struts employing the electron backscatter diffraction technique (EBSD) [1] it is known that the struts are fully converted into the 123-phase with embedded 211 particles being relatively small. These small 211 particles and their specific distribution within the samples play an important role to explain the observations. The variation of Fp depending on the position of the struts in the original sample directly reveals the changes of the texture as found employing EBSD.[1] M.R. Koblischka, A. Koblischka-Veneva, E. S. Reddy, G. Schmitz, J. Adv. Ceram. 3, 317 (2014).