Ten institutes from six countries participated in an international round-robin test to evaluate the critical current of a superconducting power cable made of Bi-2223 tapes. The cable design featured a two-layer inner core conductor and a single-layer outer shield conductor. The shield layer measured approximately 40 mm in diameter, and the cable length was 2.0 m. To eliminate the influence of resistive voltage drops from current transfer, voltage taps for measurement were positioned at a sufficient distance from the current terminals. The critical current was determined using the resistive method with the electric field criterion of 1.0 mu V/cm}. In addition, the n-value, an optional parameter reflecting the current-voltage (I-V) characteristics, was extracted from the I-V curve within an electric field range of 0.1-1.0 mu V/cm. A detailed uncertainty analysis was conducted for both the critical current and the n-value. Finally, this article discusses the potential for standardization of the employed resistive measurement method.

The critical current of the core and shield of a 2.1 m long, 40 mm diameter Bi-2223 HTS coaxial cable was measured as part of an International Round Robin organised by Japan involving several international laboratories. In Durham, we found using a 100 $\mu$ Vm $^{-1}$ criterion and an 8 minute trace time that I $_{c}$ of the core and shield were 4386 A and 4143 A and the n -values were 19.6 and 14.4 respectively. 8 minute trace times were found to be a practical approximation of steady state conditions, with critical current values 0.5 % and 1.2 % larger than values obtained using 2 minute trace times, and n -values not significantly different to 2 minute trace values. Two minute trace time data were compared with equivalent data from other participating labs and excellent agreement was found: I $_{c}$ was 0.6 % and 1.5 % larger for the core and shield respectively. The E -field range has a significant effect on the n -value measurements, which varied between groups. Good agreement between the participants for the shield was found for electric fields below 100 $\mu$ Vm $^{-1}$ , although larger n -values were found at higher electric fields at Durham and some other laboratories. The polarity of the current and the proximity of the return power cable were demonstrated to have negligible effect ( $< $ 0.1 % ) on the I $_{c}$ measurements. Removing conductive and ferromagnetic materials that were part of Durham's experimental set-up increased I $_{c}$ by $< $ 1 % . An intrinsic trace time dependency was found for I $_{c}$ for trace times of less than 8 minutes that we attribute to the time required for fluxons to equilibrate with the underlying pinning landscape in a changing net magnetic field.

The design and operation of a miniature strain-board for measuring the resistivity of high temperature superconducting tapes as a function of uniaxial strain ( $\epsilon$ ) are described. It was used inside a Physical Property Measurement System (PPMS) that provides access to high fields, variable temperature, and a rotator that can vary the angle of the sample with respect to the field. Resistivity measurements made using the strain-board over the strain range 0% to $-1.1$ % in-field are presented for a REBCO (REBa $_{2}$ Cu $_{3}$ O $_{7}$ , RE: Rare-earth element) tape from SuperPower with artificial pinning centres (APC). Values of the upper critical field ( $B_{c2}$ ) were extracted from the resistivity data. We found that although the temperature and angular dependence of B $_{c2}$ were similar to that found in previous studies using ac susceptibility measurements (i.e. the temperature index, $n = 1.2$ , the anisotropy constant, $\gamma = 1.4$ and the interlayer spacing $s = 12$ Å), the upper critical field we found was almost strain independent and therefore quite different. We explain these results by considering the bimodal behaviour of REBCO under strain that is found in single crystal data, and the large parallel shunt the strain-board provided in the measurements. In our resistivity measurements, $B_{c2}$ is determined by a small percolative supercurrent that preferentially flows through material with the highest critical parameters. Conversely, resistivity measurements at high currents, or ac susceptibility measurements, characterise the material with average or low critical parameters from the broad distribution of critical parameters produced in REBCO tapes under strain.

YBCO fabrics composed of nanowires, produced by solution blow spinning (SBS) are so brittle that the Lorentz force produced by induced currents can be strong enough to damage them. On the other hand, it is known that silver addition improves the mechanical and flux pinning properties of ceramic superconductors. Thus, in this work, we show how we successfully obtained a polymeric precursor solution containing YBCO$+$Ag salts, which can be spun by the SBS route to produce ceramic samples. Yttrium, barium, copper, and silver metal acetates, and polyvinylpyrrolidone (PVP) (in a ratio of 5:1wt [PVP:acetates]) were dissolved in a solution with 61.5 wt\% of methanol, 12 wt\% of propionic acid, and 26.5 wt\% of ammonium hydroxide, together with 6 wt\% of PVP in solution. Three different amounts of silver (10 wt\%, 20 wt\%, and 30 wt\%) were used in YBa$_2$Cu$_3$O$_{7-x}$. The TGA characterizations revealed a lowering of crystallization and partial melting temperatures by about \SI{30}{\celsius}. SEM images show that after burning out the polymer, a fabric composed of nanowires of diameters up to \SI{380}{\nano \metre} is produced. However, after the sintering process at \SI{925}{\celsius} for \SI{1}{\hour}, the nanowires shrink into a porous-like sample.

The analytic expression for the nonlinear magnetic susceptibility of a thin disk derived from Bean’s model with a uniform J C for B∥n^ predicts a peak-peak ratio of 7.1 (5.4) for the real (imaginary) components of the third harmonic χ 3 susceptibility. Our measurements show a peak-peak ratio closer to 1.1 in the real component and a noise limited lower estimate of ≈ 7 in the imaginary component. The anomalous third harmonic can be explained by the presence of a geometrical surface barrier for flux entry and exit which we have included as a small region of enhanced current density, J C,G, close to the surface of the disk. The geometrical surface current density is predicted to be approximately equal to J C,G ≈ 2B C1/μ 0 d. In 2G HTS tapes with d ≈ 1μm and B C1 ≈ 5mT this is J C,G ≈ 8GAm−2. We have measured both J C,B and J C,G using the nonlinear susceptibility of a Superpower tape without artificial pinning in fields orthogonal to the tape up to 35T. We find a geometrical surface current of J C,G ≈ 10GAm−2 and an average critical current density J C which is in good agreement with transport measurements performed on material from the same reel of tape.

We have measured the temperature and angular dependence of the upper critical field (B-C2) for three state of the art (RE)BCO HTS tapes using variable-temperature ac susceptibility measurements in applied fields up to 35 T. The three tapes measured were a Fujikura tape without artificial pinning centers and Superpower tapes with and without artificial pinning centers. We have obtained fits to our B-C2(T, theta) data using both the anisotropic Ginzburg-Landau (G-L) and Klemm's models for layered superconductors. Our calculations suggest that these tapes are three-dimensional (3-D) at all temperatures in zero field but become 2-D in magnetic fields above a crossover field of 457/gamma T, where gamma = root m(c)/m(ab) is the G-L anisotropy parameter. The values of gamma were measured and show that the 3-D-2-D crossover fields in these tapes are at least 130 T.

We present a detailed spectroscopic investigation of a thermal 87Rb atomic vapour in magnetic fields up to 0.4 T in the Voigt geometry. We fit experimental spectra with our theoretical model ElecSus and find excellent quantitative agreement, with RMS errors of ∼0.3%. We extract the magnetic field strength and the angle between the polarisation of the light and the magnetic field from the atomic signal and find excellent agreement to within ∼1% with a commercial Hall probe. Finally, we present an investigation of the relative sensitivity of this technique to variations in the field strength and angle with a view to enabling atom-based high-field vector magnetometry.

The detrimental effects of grain boundaries have long been considered responsible for the low critical current densities (J(c)) in high temperature superconductors. In this paper, we apply the quantitative approach used to identify the cause of the 'weak-link' grain boundary behaviour in YBa2Cu3O7 (Wang et al 2017 Supercond. Sci Technol. 30 104001), to the Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10 materials that we have fabricated. Magnetic and transport measurements are used to characterise the grain and grain boundary properties of micro-and nanocrystalline materials. Magnetisation measurements on all nanocrystalline materials show non-Bean-like behaviour and are consistent with surface pinning. Bi2Sr2CaCu2O8: our microcrystalline material has very low grain boundary resistivity (rho(GB)), which is similar to that of the grains(rho(G)) such that rho(GB) approximate to rho(G) = 2 x 10(-5) Omega m (assuming a grain boundary thickness (d) of 1 nm) equivalent to an areal resistivity of rho(G) = 2 x 10(-14) Omega m(2). The transport J(c) values are consistent with well-connected grains and very weak grain boundary pinning. However, unlike low temperature superconductors (LTS) in which decreasing grain size increases the pinning along the grain boundary channels, any increase in pinning produced by making the grains in our Bi2Sr2CaCu2O8 materials nanocrystalline was completely offset by a decrease in the depairing current density of the grain boundaries caused by their high resistivity. We suggest a different approach to increasing J(c) from that used in LTS materials, namely incorporating additional strong grain and grain boundary pinning sites in microcrystalline materials to produce high J(c) values. Bi2Sr2Ca2Cu3O10: both our micro-and nanocrystalline samples have rho(GB)/rho(G) of at least 10(3). This causes strong suppression of J(c) across the grain boundaries, which explains the low transport J(c) values we find experimentally. Our calculations show that low J(c) in untextured polycrystalline Bi2Sr2Ca2Cu3O10 material is to be expected and the significant effort in the community in texturing samples and removing grain boundaries altogether is well-founded.

We have investigated the applied magnetic field, temperature, and self-field dependence of the critical current density of a Nb-Ti strand produced for the ITER poloidal field conductor. Measurements were made on a standard ITER barrel in magnetic fields from 4.0 to 8.0 T and temperatures from 3.5 to 6.0 K. We investigated the effect of self-field by changing the direction of the transport current, resulting in an inward or outward Lorentz force acting on the strand. At 4.2 K and 5 T, the difference in the measured J(C) between the two Lorentz force polarities was about 1%. From this low value, we conclude that self-field effects in the Nb-Ti strand are about 40% of those expected using standard self-field calculations for fully transposed multifilamentary strands, consistent with theoretical considerations for annular filament transposition.

In this paper, a round robin test of residual resistance ratio (RRR) is performed for Nb3Sn composite superconductors prepared by an internal tin method by six institutes with the international standard test method described in IEC 61788-4. It was found that uncertainty mainly resulted from determination of the cryogenic resistance from the intersection of two straight lines drawn to fit the voltage versus temperature curve around the resistive transition. The measurement clarified that RRR can be measured with expanded uncertainty not larger than 5% with the coverage factor 2 by using this test method.

We have measured the superconducting properties of the titanium alloy Ti-6Al-4V (Ti-64) as supplied and following two of the heat treatment schedules used for the Nb3Sn strands in the ITER tokamak. The Ti-64 alloy is the standard choice in the superconducting community for the barrels used tomake critical current (IC) measurements in high magnetic fields at cryogenic temperatures. Ti-64, which has a two-phase alpha + beta microstructure and contains vanadium (n.b. T-C (V) similar to 5.4 K), is superconducting at 4.2 K in fields up to 3 T. We have also measured Ti-6Al-2Sn-4Zr-2Mo- 0.2Si (Ti-6242), which is in the near alpha phase and contains tin (n.b. T-C (Sn) similar to 3.7 K). The critical temperature of Ti-6242 is 2.38 K, which is lower than the 5.12 K of Ti-64. Hence Ti-6242 is a better choice of barrel material for I-C measurements required at 4.2 K in low fields up to 3 T, because it remains in the normal state.

Although we can use misorientation angle to distinguish the grain boundaries that can carry high critical current density (J(c)) in high temperature superconductors (HTS) from those that cannot, there is no established normal state property equivalent. In this paper, we explore the superconducting and normal state properties of the grains and grain boundaries of polycrystalline YBa2Cu3O7 (-) (x) (YBCO) using complementary magnetisation and transport measurements, and calculate how resistive grain boundaries must be to limit J(c) in polycrystalline superconductors. The average resistivity of the grain boundaries, rho(GB), in our micro-and nanocrystalline YBCO are 0.12 Omega m and 8.2 Omega m, values which are much higher than that of the grains(rho(G)) and leads to huge rho(GB)/rho(G) values of 2 x 10(3) and 1.6 x 10(5) respectively. We find that the grain boundaries in our polycrystalline YBCO are sufficiently resistive that we can expect the transport J(c) to be several tens of orders of magnitude below the potential current density of the grains in our YBCO samples, as is found experimentally. Calculations presented show that increasing J(c) values by similar to 2 orders of magnitude in high fields is still possible in all state-of-the-art technological high-field superconductors. We conclude: grain boundary engineering is unlikely to improve J(c) sufficiently in randomly aligned polycrystalline YBCO, to make it technologically useful for high-field applications; in low temperature superconducting intermetallics, such as Nb3Sn, large increases in J(c) may be achieved by completely removing the grain boundaries from these materials and, as is the case for thin films of Nb, Ba(FeCo)(2)As-2 and HTS materials, by incorporating additional artificial pinning.

The European Union contributes around 20% of the cable-in-conduit conductor lengths needed for the ITER toroidal field (TF) magnet coils. For that purpose, 97 tons of Nb 3 Sn superconducting strand have been fabricated over five years, the production being completed in 2014. This superconducting strand has been manufactured by two companies, namely, Bruker EAS (Germany) and OST (USA), through the bronze route and the internal tin diffusion, respectively. This paper reports the outcomes of this strand mass production and of the strand characterization as performed by the suppliers and cross-checked on a regular basis by Durham University.

In this paper, we present data for two nanocrystalline YBa2Cu3O7-x (YBCO) samples which both exhibit Josephson-like Colossal Resistive Switching (JCRS) in voltage-current (V-I) traces from 4.2 K up to room temperature, in magnetic fields up to 8 T. We report Josephson-like hysteresis for both positive and negative current that has not been observed before in colossal resistive switching materials. Non-zero resistance was measured in transport measurements at all temperatures. At low temperatures (< 90 K), we also observed the usual properties for YBCO including weak superconducting and paramagnetic behavior, measured using ac susceptibility and magnetization measurements. The resistivity of these nanocrystalline samples is 3 orders of magnitude higher than standard polycrystalline materials at 300 K and the temperature dependence semiconductor-like. We cannot rule out the possibility that these materials contain a superconducting component responsible for the JCRS behavior at room temperature.

The development of high-temperature superconducting (HTS) wires is now at a stage where long lengths of high quality are commercially available, and of these, (Re)BCO coated conductors show the most promise for practical applications. One of the most crucial aspects of coil and device modeling is providing accurate data for the anisotropy of the critical current density J c (B, θ) of the superconductor. In this paper, the in-field critical current density characteristics J c (B, θ) of two commercial HTS coated conductor samples are experimentally measured, and based on these data, an engineering formula is introduced to represent this electromagnetic behavior as the input data for numerical modeling. However, due to the complex nature of this behavior and the large number of variables involved, the computational speed of the model can be extremely slow. Therefore, a two-variable direct interpolation method is introduced, which completely avoids any complex data fitting for J c (B, θ) and expresses the anisotropic behavior in the model directly and accurately with a significant improvement in computational speed. The two techniques are validated and compared using numerical models based on the H-formulation by calculating the self-field and in-field dc critical currents and the ac loss for a single coated conductor.

Direct current (DC) characterization of high temperature superconducting (HTS) coils is important for HTS applications, such as electric machines, superconducting magnetic energy storage (SMES) and transformers. In this paper, DC characterization of a circular, epoxy-impregnated HTS coil made from YBCO coated conductor for use as a prototype axial flux HTS electric machine is presented. Multiple voltage taps were utilized within the coil during measurement to help provide further detailed information on its DC behavior as a function of length. Based on the experimental results, there exist regions of non-uniformity along the length of superconductor in the coil, resulting in non-ideal superconducting properties of the coil. By studying the current-voltage (I-V) curves across different regions, it is found that a decreasing n-value and critical current exists in the non-uniform parts of the HTS coil.

The fabrication and processing by solid-state heat-treatment, mechanical ball milling and hot isostatic pressing of microcrystalline and nanocrystalline niobium carbonitride is reported. This material is subjected to a number of characterisation measurements including x-ray diffraction, resistivity, ac-susceptibility, dc-extraction and heat capacity. The resultant measurement data are used to assess the adequacy of the material’s processing and quality with respect to the fundamental superconducting characteristics, transition temperature, T_c, upper critical magnetic field, B_c2, and critical current density, J_c. It is shown that a substantial increase in B_c2 from ~ 11 T (in the microcrystalline material) to ~ 21 T (in the nanocrystalline material) has been produced. A fortyfold increase in J_c from 1.8 x 107 Am^(-2) (in microcrystalline material measured at 3 T and 6 K) to 7.4 x 108 Am^(-2) (in nanocrystalline material measured at 3 T and 5.9 K) has also been produced. These substantial increases have been made with only a 32 % reduction in T_c from ~17.6 K to ~ 11.9 K, well above the temperature of liquid helium. The accurate large quantity metrology of 10,000 Nb3Sn samples for the International Thermonuclear Experimental Reactor toroidal field coils is also reported and an overview analysis of the data provided. In particular, all seven measurement types; critical current, hysteresis loss, residual resistivity ratio, diameter, chromium plating thickness, twist pitch and copper to non-copper volume ratio are discussed in relation to the accuracy with which they were performed. The methodology in performing the heat-treatments and measurements is discussed and the detail of the necessary equipment set up is given. The results from some additional experiments that deal with the effect of heat-treatment cleanliness and sample geometry on various measurement types is provided.

We provide evidence that a single mechanism-flux flow along channels-can explain the functional form of the critical current density (J(c)) in the low-temperature superconductor Nb3Sn and in the high-temperature superconductors (HTS) YBa2Cu3O7-delta (YBCO) and (Bi, Pb)(2)Sr2Can-1CunOx (BiSCCO) in low and high magnetic fields. In this paper, we show that standard flux pinning theories, used for the past four decades to describe J(c) in low-temperature superconductors (LTS), cannot explain the strain dependence of J(c) in YBCO because J(c) is a function of strain but the average superconducting properties are not. We conclude that in the polycrystalline samples presented here, the channels are grain boundaries that are narrow and metallic in Nb3Sn and YBCO but wide and semiconducting in BiSCCO. In Nb3Sn, strain alters J(c) by changing the superconducting properties of the grains, whereas in YBCO, strain alters J(c) by changing the properties of the grain boundaries.

Niobium carbonitride's superior radiation tolerance , coupled with the recently reported increase in its upper critical magnetic field when made nanocrystalline, increases its potential importance in future technological high-field superconductor applications. The maximum transition temperature for the composition NbC0.3N0.7 is ~17.8 K and its upper critical magnetic field is ~11 T; this increases to ~12 T for the composition NbC0.2N0.8,. Using solid-state processing, we have fabricated microcrystalline bulk niobium carbonitride with a transition temperature of ~ 7.6 K. A comprehensive characterization of this material, which includes susceptibility, resistivity, magnetization, heat capacity and XRD measurements is provided. Comparisons between the heat-treated material and the same material subjected to hot isostatic pressing are made so that the values of the intrinsic fundamental properties can be identified and their sensitivities to different fabrication processes determined.