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In order to investigate the effect of the intergrowth Bi2223 phase on the transport properties of Bi2212 whiskers, we performed the electrical field vs. current density ( E – J ) measurements for Bi2212 whiskers at various temperatures and magnetic fields. Both of the irreversibility field B irr and the critical current density J c increase with the increase of the intergrowth Bi2223 content. Although the quantitative ratio of intergrowth Bi2223 is unknown, we found the possibility of the J c improvement of Bi2212 due to the intergrowth Bi2223 phase from a detailed analysis of the E – J properties.

For fabrication of sandwich-type SIS Josephson junctions of HTS, c-axis stacked layers should have smooth and uniform surface. In this regard, control of layers morphology is essential. In the present work, infinite-layer Sr x Ca 1-x CuO 2 , 0 < x < 1 thin films, to be potentially used as the insulator in SIS structures, have been grown by MOCVD at 750-800 °C. Higher temperatures promote growth of the films surface with lower roughness. Furthermore, roughness (arithmetic (Ra) and root mean square (Rms) values) are minimum for x = 0.5 and the films show a square type uniform morphology. Films are in-plane aligned, as revealed by AFM. Up to x = 0.5, films consist of infinite-layer single phase and at higher values for x, an unknown phase occurs and its amount is increasing, while the concentration of the infinite-layer phase is decreasing. Electrical resistance of the as-grown film was above 10 MΩ.

We review, based on our results, the problems and solutions for the growth of thin films and composite heterostructures emphasizing the general growth aspects and principles vs specifics for each material or heterostructure. The materials used in our examples are Bi2Sr2Ca2Cu3O10, Bi2Sr2CaCu2O8, YBa2Cu3O7, (Sr, Ca)CuO2, (Ba, Ca)CuO2, and Bi4Ti3O12. The growth method was metal organic chemical vapor deposition (MOCVD). The presented thin films or heterostructures have c- and non-c-axis orientations. We discuss the implications of the film–substrate lattice relationships, paying attention to film–substrate lattice mismatch anisotropy and to film–film lattice mismatch, which has a significant influence on the quality of the non-c-axis heterostructures. We also present growth control through the use of vicinal substrates and two-temperature (template) and interrupted growth routes allowing significant quality improvements or optimization. Other key aspects of the growth mechanism, that is, roughness, morphology, and interdiffusion, are addressed. It is concluded that the requirements for the growth of non-c-axis heterostructures are more severe than those for the c-axis ones.

Mixtures of MgB2 and metal or oxide additions with starting compositions of (MgB2)(M2O3)x, x = 0.0025, 0.005, 0.015, and (MgB2)(M)y, y = 0.01, M = Bi, Sb, were processed by spark plasma sintering (SPS). As-obtained samples are composites with high density exceeding 94% of the theoretical values. Secondary phases indicate similar reactions for samples with Bi- or Sb-based additions. However, samples show very different superconducting characteristics depending on the addition type and amount. A direct correlation with the melting temperature of the addition could not be revealed, although some aspects will be discussed. From the critical current density (Jc) and irreversibility field (Hirr) enhancement viewpoints, optimum additions are oxides for x=0.0025, 0.005. Both oxides improve Jc at high fields, but Sb2O3 is effective up to 10 K, whereas Bi2O3 is effective up to 30 K. Metal additions decrease Jc and Hirr when compared with a pristine MgB2 sample.

From DC magnetization studies in fields up to 14 T and temperatures between 20 and 100 K, we estimated the intra- and inter-grain critical current density J c of (Cu,C):1234 high- T c superconductors, in the frame of critical state models. The inter-grain J c was determined by comparing the magnetization loops of as-grown sample and of the ground sample. Finally, short comments on J c resulted from AC susceptibility measurements and of the impressive increase of intra-grain J c due to heavy-ion and neutron irradiation are presented.

Thin films of infinite-layer compound Sr1−xCaxCuO2−δ have been prepared by rf magnetron sputtering. For an optimum level of oxygen vacancy superconductivities were observed ( T c onset =42 and 50 K, T c (ρ=0) =11 and 20 K). Structural and transport data suggest that the doping mechanism is electron-type. The superconducting transitions were also confirmed by ac susceptibility. Further increase of doping showed to destroy superconductivity.

Samples of (Cu 0.6 C 0.4 )Ba 2 Ca 3 Cu 4 O y ((Cu,C)-1234) with addition of 0.1 mol LiF have been synthesized by high pressure technique. It was found that LiF promotes the formation of superconducting phases with n higher than 4 of the (Cu,C)Ba 2 Ca n −1 Cu n O y series. In order to obtain single-phase samples, for the same synthesis conditions, the oxygen content in the starting mixture (supplied by AgO) should be increased from 0.45 to 0.73 mol AgO, when using LiF. Samples with LiF have shown a certain morphology suggesting an enhanced 2D and suppressed 3D type of growth assisted by a liquid phase. T c decreased from 116.5 to 113 K for the sample with LiF, but at the same time the transition width also decreased and J c has shown a fish-tail effect.

We have obtained by Spark Plasma Sintering (SPS), dense samples of MgB2 added with Ho2O3. Starting composition was (MgB2)0.975(HoO1.5)0.025 and we used addition powders with an average particle size below and above 100 nm. For Mg, pristine and added MgB2 samples we measured potentiodynamic polarization curves in Phosphate Buffered Saline (PBS) solution media at room temperature. MgB2 based composites show corrosion/ degradation effects. This behavior is in principle similar to Mg based alloys in the same media. Our work suggests that the different morphologies and phase compositions of the SPS-ed samples influence the interaction with corrosion medium; hence additions can play an important role in controlling the corrosion rate. Pristine MgB2 show a significant improvement of the corrosion resistance, if compared with Mg. The best corrosion resistance is obtained for pristine MgB2, followed by MgB2 with nano-Ho2O3 and μ-Ho2O3 additions.

We present the magnetic and transport properties of superconducting composites fabricated by admixing carbon-encapsulated Fe nanospheres and MgB2 powder. The addition of nanoparticles is expected to enhance the critical current density by carbon-doping the MgB2 matrix and by providing artificial pinning sites. Three samples with estimated amounts of 0.35, 0.6, and 1.0 wt. % metallic Fe were prepared using the spark plasma sintering technique. The average size of these nanoparticles is comparable to the superconducting coherence length of MgB2 at approximately 5 nm. We found that the additions do not significantly alter the critical temperature which is very high, close to that of the pure MgB2 samples. We have also observed improved current densities, as high as 1100 kA/cm2 for the samples with 0.35 wt. % metallic Fe at 5 K and 1 T. A core-shell model for explaining the transport data is presented. The field and temperature dependence of the reduced pinning force is described in terms of pinning on grain ...

We investigated the relaxation of the irreversible magnetization in a series of 200?nm thick YBa2Cu3O7/PrBa2Cu3O7 [(YBCO)n/(PrBCO)m] superlattices, where the thickness m of the nonsuperconducting PrBCO layer (measured in unit cells) was kept to m = 4 (sufficient to decouple the superconducting YBCO layers), whereas the thickness n of the YBCO layer was varied between 2 and 20 unit cells. The analysis of standard zero-field-cooling dc magnetization relaxation data obtained in the low temperature T region with the applied magnetic field H oriented along the c axis reveals the occurrence of a crossover elastic (collective) vortex creep at low T?plastic vortex creep at high T, generated by the T dependent macroscopic currents induced in the sample during measurements. For thin superlattices (n < 20) the creep crossover temperature , and Tcr decreases linearly with increasing ln(H) for a fixed n. This crossover represents an alternative to the elastic vortex glass behavior reported for superlattices, as well as to 'quantum vortex creep' at unexpectedly high T inferred for thin films. We also discuss the absence of an increase of the magnetically determined critical current density with decreasing YBCO thickness in our superlattices, which apparently contradicts the collective pinning theories.

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.

We investigated the influence of fullerene (C60) addition on the superconducting parameters of MgB2 bulk samples obtained by spark plasma sintering (SPS). It was found that 1.5?wt% C60 addition leads to the reduction of the critical temperature by a few K, whereas the magnetically determined critical current density increases by approximately one order of magnitude in an applied magnetic field H = 70?kOe at temperature T = 10?K. This indicates substitution of B by C in C60 added MgB2 processed by SPS, contrary to some data reported in the literature obtained using conventional methods. For our relatively large specimens, the occurrence of macroscale flux jumps was observed over a wide (H, T) domain. The relaxation of the irreversible magnetization suggests that the macroscale flux jumps transform into microscale flux jumps with increasing H and T.

The relaxation of the irreversible magnetization in optimally doped YBCO films with natural and artificial pinning centres was measured in zero-field cooling conditions using SQUID magnetometry. The external magnetic field H was oriented along the c axis. An appropriate method for the determination of the characteristic vortex pinning energy from the normalized vortex-creep activation energy is discussed. This is based on the existence of a crossover elastic (collective) vortex creep at low temperatures T – plastic vortex creep at high T, caused by the T dependent macroscopic currents induced in the sample during magnetization measurements.

Superconducting SiC and B4C-doped MgB2 samples containing up to three layers of pure Fe have been prepared by Field-Assisted-Sintering Technique (FAST). The adhesion between the Fe and doped-MgB2 layers is excellent due to the enhanced local interdiffusion and increased solubility of Fe into MgB2 grains that are typical for the non-equilibrium processes. Magnetic measurements showed the largest irreversibility, hence, the largest critical current density in the B4C-doped sandwich with one Fe foil. Additionally, the drawback of flux jumps at low temperatures is almost absent for one Fe-layer sandwiches. However, the irreversibility is smaller than in pure MgB2 and SiC-doped MgB2 samples prepared by the same technique, but without the insertion of Fe foil. When the number of Fe foils is increased to three and an excess of 5 %wt. Mg is added, the SiC-doped sandwiches show almost similar irreversibility values and field dependence (at 5 K and at 20 K) as the pristine MgB2 samples without Fe. But, the flux jump is recovered at the same level found for the pristine MgB2 without Fe. An explanation in terms of doping dependence of the ratio between the thermal and magnetic diffusion constants is proposed.

It is shown that x-ray microtomography (XRT) is a powerful technique to observe on extended volumes the morphology, alignment and local bulk density in MgB2 tapes, thin films and especially bulk specimens. For the bulk samples obtained from mechanically milled powders for different conditions, several types of the 3D microstructural patterns were observed. These significant differences between the samples were not revealed by SEM. Microtomography can be a useful additional technique to characterize the samples and to obtain unique information, e.g. on local bulk density, but it cannot replace the existing methods: advantages and limitations are discussed. It is concluded that the method is promising and further developments are expected to be helpful for a better understanding of the complex processing-microstructure-properties relationship in the MgB2 superconductor.

Materials may show anisotropic properties on different crystal directions and this is also the case of High Temperature Superconductors (HTS). To take advantage of the materials anisotropy one concept of interest is 'orientation engineering' in thin films. This can be realized through the control of the film-substrate lattice relationship. Some examples in this regard are presented in this work and through comparative analysis we try to evaluate the viability of this approach and of he entcountered problems. It is expected that in the future this approach will generate new nano composite materials with new properties and effects leading to development of new devices with new or improved functionality.

Superconducting c-axis thin films of Bi2Sr2Ca2Cu3O10 (Bi-2223) were grown by MOCVD on (001) and (110) MgO substrates. Films on both substrates show, as revealed by AFM investigations, similar morphology composed of regular rectangular grains and relatively low roughness of about two or three half c-axis unit cell order. AFM images also suggest that films might have in-plane epitaxy. The films have zero resistance critical temperatures of 95.1K for the (001) MgO substrate and 75K for the (110) MgO substrate, respectively. Considering very large films-substrate lattice mismatch between (110) MgO and (ab)-plane of the superconducting phase the growth of indicated films is a surprise.

Field-assisted-sintering technique (FAST) has been employed to obtain polycrystalline MgB2 samples, pristine and doped with 5 mol % SiC and B4C, using commercial MgB2 powder. The mass density of the samples is around 90% of the theoretical value. The response in magnetic field shows that the upper critical field and the irreversibility field are depressed by comparison with pristine MgB2. The latter has an upper critical field Hc2=14.37 T close to the values reported for the single crystals. Scaling analysis of the field dependence of the pinning force suggests that both addition of SiC and B4C stabilizes two well defined pinning regimes in FAST-processed MgB2. At low temperatures, T< 24 K (H≤5T), the pinning occurs at the grain boundaries whereas at higher temperatures there is a mixed pinning. These effects are supposed to be the combined result of relatively low level of C diffusion into the lattice of MgB2, specific features in formation of the grain boundaries during FAST processing and the influence of the added materials on morphology. At high fields (above 8.5T) and 4.2K, doped samples are superior to the pristine one from the critical current density viewpoint.

We have developed practical multifilamentary Nb3Sn wires with CuNb composite reinforcement (CuNb/Nb3Sn). In order to fabricate a CuNb/Nb3Sn superconducting magnet by a react-and-wind method, the bending strain effect was investigated in detail for CuNb/Nb3Sn wires. We found that the repeated bending treatment enhances Tc from 17.4 to 17.9 K and Bc2 from 24.0 to 25.3 T at 4.2 K for CuNb/Nb3Sn wire. As a result, the repeated bending treatment for CuNb/Nb3Sn wire outstandingly enhances the critical current in high magnetic fields. A Nb3Sn filament was prepared by chemically solving practical multifilamentary CuNb/Nb3Sn wires. After removing Cu stabilizer, CuNb reinforcement, Nb barrier, and Bronze, Bc2 of a Nb3Sn filament was measured, and the Bc2 value of 25.9 T at 4.2 K was obtained. This means that the bending treatment for CuNb/Nb3Sn wires extremely reduces the residual strain close to the strain free state. To decrease the residual strain, it is important to control the three-dimensional strain distribution of CuNb/Nb3Sn wires. We intended to apply the bending effect to the cabling process of CuNb/Nb3Sn strands by a react-and-wind method. High strength Nb3Sn cables consisting of CuNb/Nb3Sn strands and stainless steel reinforcement strands were developed for a performance test of a next phase superconductor with a large critical current and a strong mechanical property at a high magnetic field of 20 T.

Room temperature mechanical treatment consisting of multiple torsion loadings (named pre-torsion) was applied to Nb3Sn composite wires with different architecture to reduce thermal residual compressive strain experienced by the superconducting Nb3Sn filaments. Due to this effect all investigated wires have shown enhancement of the critical current density, Ic up to 56% at 15T and 4.2K. Enhancement of Ic was larger for the reinforced wires than for the wires without reinforcement suggesting that reinforcement is useful in strain relaxation during pre-torsion. The best results were obtained when the position of the reinforcement was located in the outer region of the wire. Pre-torsion is similar or more efficient than cycles of bending loadings (named pre-bending). For pre-torsion important parameter is rotation angle per length of the wire and optimum conditions for short wires are valid for long wires. Uniformity of Ic along the length of a 99cm wire after pre-torsion was within ±2.1A.

We measured directly by neutron diffraction the axial and lateral residual strains for the prebent and the as-reacted CuNb∕Nb3Sn wires at room temperature and at 7K, in order to investigate the change of the residual strain with prebending treatment. In the axial direction of the wire, the residual strain was changed with 0.20% to the tensile side when measured at both temperatures, while in the lateral direction, the change was 0.08% and 0.03% to the compressive side for the measurements at RT and 7K, respectively. From the obtained data, we estimated the deviatoric strain. At 7K, the value is 0.40% for the as-reacted wires and it reduces to 0.19% when the prebending is applied with a strain epb=0.8%. These results suggest that the reduction of the residual strain in the axial direction as well as in the lateral direction, i.e., of the deviatoric strain is responsible for the observed enhancement in the superconducting properties of the prebent Nb3Sn wires. In addition, we succeeded in the quantitative e...

Magnetization in DC magnetic fields and at different temperatures have been measured on the Li2Pd3B compound. This material was recently found to show superconductivity at 7-8K. Critical fields Hc1(0) and Hc2(0) have been determined to be 135Oe and 4T, respectively. Critical current density, scaling of the pinning force within the Kramer model and the irreversibility field data are presented. Several superconductivity parameters were deduced: x(csi)=9.1 nm, l(lamda)=194nm and k=21. The material resembles other boride superconductors from the investigated points of view.

Poster; International audience; The advantage of the very low density of MgB2 bulk samples must be taken into consideration and, the generation of strong magnetic flux densities using MgB2 should be investigated. It was shown in [1] that the organometallic addition to MgB2 of Ge2C6H10O7 greatly enhances the pinning forces, leading to higher critical current density Jc and irreversibility field Hirr in the added sample compared to pristine MgB2, Fig. 1(a). All the samples were produced by ex-situ Spark Plasma Sintering. In this contribution, we have studied the trapped magnetic field in samples of 20 mm in diameter and 4 mm high, but also in a stack of 6 of these samples leading to a total height of 24 mm. Field Cooling experiments were carried out at temperatures from 12 K to 30 K. The trapped magnetic field inside the stack of 6 samples and at the center of the surface are reported, Fig. 1(b). According to the obtained values of trapped magnetic field greater than 5 T at 12 K, and 4 T at 20 K, to their very low density and to their ease of manufacturing, doped bulk MgB2 with Ge2C6H10O7 are really interesting for the intended applications.Fig. 1: Irreversibility field Hirr of a pristine MgB2 and of a doped MgB2 (a), and the measured trapped magnetic field at the center of the surface of a stack of 6 samples of 20 mm diameter & 4 mm high (b).References1. D. Batalu, G. Aldica, S. Popa, L. Miu, M. Enculescu, R. F. Negrea, I. Pasuk, and P. Badica, High magnetic field enhancement of the critical current density by Ge, GeO2 and Ge2C6H10O7 additions to MgB2, Scripta Mater. 82 61 (2014).

The effect of intermediate grounding and sintering environment (air or sintered powder) on the superconducting characteristics of Bi(Pb)-Sr-Ca-Cu-O bulk ceramics obtained by spray-frozen freeze drying technique was investigated by X-ray diffraction, electric and magnetic measurements. Intermediate grounding leads to higher scattering coefficients of the superconducting characteristics. This result does not depend on sintering environment. Improved results are obtained when sintering is performed by immersing pellets “in powder” rather than placing them “on powder”. The powder used as sintering environment was pyrolized precursor powder calcinated in air for 100 h.