Critical Current Density Jc Research Articles

Comparison of the AC losses of BSCCO and YBCO conductors by means of numericalanalysis

This paper presents a quantitative comparison of the AC loss performance of two BSCCOand two YBCO conductors, characterized by the same self-field critical current of 150 A. Wecompare a 37-filamentary BSCCO tape, a 16-filamentary BSCCO square wire, a standardYBCO tape, and a stack of four narrower YBCO tapes. The comparison is made using anumerical technique, based on the finite-element method, which employs a non-linearE–J relation and a dependence of the local critical current densityJc on the local magnetic field. For the simulation of YBCO conductors, a new ‘shell-region’model is utilized. This overcomes the geometry and mesh problems typical ofsuperconductors with very high geometric aspect ratio. Different AC working conditions aresimulated: self-field, applied external field, and combined transport current and externalfield of varying orientation. We outline the advantages of using BSCCO or YBCOconductors for the different applications. Various magnetic field and current density profilesare investigated in order to illustrate the reasons for the loss difference in the fourconductors. Particular attention is drawn to the YBCO tape and the YBCO stack, whoseAC loss characteristics are less well known than those of BSCCO conductors.

Effects of solid solutions on the superconducting properties of Gd–Ba–Cu–Osuperconductors

The superconducting properties of the nominalGd1+2xBa2+xCu3+xO7−δ (x = 0, 0.1, 0.2, 0.3, 0.4) single-domain bulks which consist ofGdBa2Cu3O7−δ (Gd123),Gd2BaCuO5 (Gd211)and Gd1+yBa2−yCu3O7−δ, have been investigated through magnetization and resistivity measurements. With increasing Gd211content x, the trapped magnetic field reaches a maximum for the samplex = 0.2. The second peak in the critical current densityJc versusmagnetic field B curve shifts toward the high magnetic regime for the samplex = 0.2 compared withthat for the sample x = 0, but decreases to low magnetic field for the samplesx>0.2. Two superconducting transitions in resistivityρab indicate thatthe low Tc clusters,i.e. Gd1+yBa2−yCu3O7−δ solid solutions,are interspersed in the GdBa2Cu3O7−δ superconducting matrix. The δTc pinning resulting from the compositional fluctuations should be responsible for the enhancement ofJc and the trappedfield for the sample x = 0.2.

Diffusion studies and critical current in superconducting Nb–Ti–Ta artificial pinning center wire

The diffusion between Nb–20%Ta (wt %) and pure Ti is studied at temperatures of 973, 1023, and 1073K, for duration times among 25 and 121h in an artificial pinning center (APC) wire composed of a Ti core surrounded by a Nb–20%Ta layer. The produced diffusion layer is a ternary alloy with superconducting properties, such as critical field Bc2 and critical current density JC, which intrinsically depend on the layer composition. Measurements of layer morphology and composition were performed, and the results show a preferential diffusion of Nb and Ta into Ti. There is a slight diffusion of Ti into Nb through grain boundaries. The presence of Ta also slows down the diffusion of Nb in Ti if compared to the couple formed by pure Nb and Ti. Regarding the mechanical properties of the composite wire, the use of lower temperatures to form the ternary phase is desirable in order to avoid a larger portion of the diffusion layer rich in Ti that favorites α-Ti precipitations that are detrimental to the wire ductility. The best JC value was obtained for the sample heat treated at 973K. The improvement of the flux-line pinning was associated with a sharp change of the diffusion layer composition rather than pinning by normal layer interfaces, suggesting a new source of pinning in this kind of material. Nb–Ti–Ta ternary alloys have the potential to be used in superconducting magnets when fields above 12T are required.

Observations and model of a new ac-loss valley in a YBCO superconductor

We report on measurements of the ac susceptibility of sinteredYBCO pellets, with ZnO added, as a function of temperatureT, for several different ac magnetic field amplitudes,Hac, in the presence of static bias magnetic fieldHb directedalong Hac. Foreach choice of Hac, the imaginary part of the ac susceptibility versusT traces a peak,denoted χ′′max, centred at atemperature denoted Tmax. We find that χ′′max versus both Hac and Tmax traces a valley. Calculations of hysteresis losses, exploiting the critical state model where the critical currentdensity jc = α(T)/Bn, reproduce the experimental data quite well.

Self-field effects upon the critical current density of flat superconducting strips

We develop a general theory to account self-consistently forself-field effects upon the average transport critical current densityJc of a flat type-II superconducting strip in the mixed state when the bulkpinning is characterized by a field-dependent depinning critical current densityJp(B),where B is the local magnetic flux density. We first consider the possibility of both bulkand edge-pinning contributions but conclude that bulk pinning dominates overgeometrical edge-barrier effects in state-of-the-art YBCO films and prototypesecond-generation coated conductors. We apply our theory using the Kim model,JpK(B) = JpK(0)/(1+|B|/B0), as an example.We calculate Jc(Ba) as a function of a perpendicular applied magnetic inductionBa and showhow Jc(Ba) is relatedto JpK(B). We findthat Jc(Ba) is verynearly equal to JpK(Ba) when Ba≥Ba*,where Ba* isthe value of Ba that makes the net flux density zero at the strip’s edge. However,Jc(Ba) is suppressedrelative to JpK(Ba) atlow fields when Ba<Ba*, with the largest suppression occurring whenBa*/B0 is of order unity or larger.

Spin-current-induced magnetization reversal in magnetic nanowires with constrictions

We have performed experiments on current-induced domain-wall motion (CIDWM) in the case of the domain walls (DW) trapped within the nanoscale constrictions in patterned NiFe structures. Direct observation of current-induced magnetization reversal was achieved and critical current densities jc were measured in the presence of easy-axis magnetic fields. The direction of CIDWM was found to be along the direction of the electron motion in absence of an applied magnetic field and in the direction of the field when in the presence of even relatively weak fields. Data for the field dependence of jc for both uniform and fast rising pulses suggest that the current, regardless of polarity, assists in the depinning of the DW. Only for the dc case does the data strongly reveal the influence of the electron pressure in promoting or hindering DW motion.

The effect of reversible flux motion on the estimation of critical current density in a thinsuperconductor using a third harmonic voltage method

A significant overestimation of the critical current densityJc is foundfor a measuring method of induced third harmonic voltage for thin superconductors of thickness lessthan 1 µm. This is speculated to be caused by a reversible motion of flux lines in a pinningpotential which reduces the nonlinear properties such as AC losses or the thirdharmonic voltage. It is known that the effect of reversible flux motion is prominentfor superconductors with a thickness comparable to or smaller than the pinningcorrelation length of the order of a sub-micrometre, the so-called Campbell’s ACpenetration depth. In this paper the effect of reversible flux motion on the estimation ofJc by this method is preliminarily investigated by using a simplified Campbell’s model for theforce–displacement characteristics of flux lines.

Enhanced superconducting properties of bicrystallineYBa2Cu3Ox and alkali metals under pressure

Nearly hydrostatic pressures are found to enhance the critical current densityJc through singlegrain boundaries in YBa2Cu3Ox rings and to induce superconductivity in Li. Whereas Li becomes superconducting above20 GPa at temperatures as high as 15 K, no superconductivity was observed above 4 K in Nato 65 GPa nor in K above 4 K to 43.5 GPa or above 1.5 K to 35 GPa.

Nonlinear approximation for limiting current-carrying capacity of Ag-sheathedBi2Sr2CaCu2O8 conductors

The thermal runaway phenomenon inAg/Bi2Sr2CaCu2O8 composite conductors is studied for the nonlinear temperatureand magnetic field dependences of their critical current densityJc(T,B) and matrixresistivity ρm(T,B) (nonlinear approximation). The influence of the applied magnetic field, heat transfercoefficient, matrix resistivity and volume fraction of the composite superconductor on thestatic thermal and electric states were investigated. The used model is basedon steady heat and current balance equations. It allows us to investigate thenon-isothermal voltage–current characteristic of composite superconductors, that rootsthe static description of their operating regimes and allows us to determine thelimiting quantities of the thermal runaway parameters. It is found that there existstable jumps of the electric field, current and temperature without the transitionof the superconducting composite into the normal state. The presence of thesestatic states is a result of the additional stable branches on the voltage–currentcharacteristics, which appear according to the variation of the differential resistivity of acomposite. The latter may have wide range fields due to the coupled thermo-magneticand current-sharing mechanisms changing the quantities and . This variation takes place when the applied magnetic field or operating parameters of acomposite exceed some characteristic values. As a result, the non-monotonic temperaturedependence of the Joule heat release will take place. Besides, it is shown that thestable voltage (without thermal runaway) may exist when the temperature of theAg/Bi2Sr2CaCu2O8 composite stably increases from the coolant temperature up to the critical temperature of asuperconductor during current charging. The thermal runaway parameters as a function ofthe applied magnetic field are numerically derived accounting for the additional stablebranches of the voltage–current characteristic.

Overdamped Nb∕Al–AlOx∕Nb Josephson junctions

We report the fabrication and characterization of overdamped Nb∕Al–AlOx∕Nb superconductor–insulator–superconductor Josephson junction whose fabrication process derives from that of the well-known hysteretic junctions. These junctions are an intermediate state between the superconductor–normal metal–superconductor and the superconductor–insulator–superconductor Josephson junctions. Stable and reproducible nonhysteretic current–voltage characteristics are obtained with a proper choice of the fabrication parameters. We have measured critical current densities JC from 103 up to 2×104A∕cm2, with characteristic voltages from 80 to nearly 450μV. The junctions are stable against time and repeated thermal cycling.

Enhancement of Critical Current Density in Graphite DopedMgB2 Wires

Graphite doped MgB2−xCx (x = 0.00, 0.05, 0.10)wires were fabricated via the in situ powder-in-tube method inflowing argon by using low carbon steel tubes as the sheathmaterials.With the increase of graphite concentration, the amount of unreactedgraphite in the core area increases, and the average grain size ofMgB2 decreases. It is found that the critical current densityJc can be significantly improved by graphite doping. TheMgB2 wire with x = 0.05 exhibits the best Jcvalue of 16710 A/cm2 at 6 K, 4.5 T, but theMgB1.9C0.1 wire has the highest Jc value of2060 A/cm2 at 6 K, 8 T. It is suggested that the enhancement ofJc is due to not only the improvement of the microstructurefeatures but also the introduction of pinning centres.

Nanometre-sized inhomogeneity inhigh-Jc Bi2Sr2CaCu2O8+δ superconductors

We have performed atomic-scale high-resolution scanning tunnelling microscopyand spectroscopy measurements on the cleaved surface of single crystalBi2Sr2CaCuO8+δ superconductors with high critical current densityJc. The samples exhibited rugged structure about 15 nm in period, larger thanthe modulation of the BiO layer, which corresponded well to the energy gapdistribution at 77 K. The presence of inhomogeneity from a nanometre to amicrometre scale, in the energy gap distribution, the structural modulation andthe chemical composition fluctuation, may play an important role in improvingJc valuesin the Bi2Sr2CaCuO8+δ crystals.

Hybrid liquid phase epitaxy processes forYBa2Cu3O7 film growth

A number of liquid phase epitaxy (LPE) related growth methods have been investigated.These hybrid-LPE processes enable high rate ‘liquid assisted’ growth of epitaxialYBa2Cu3O7 films without the many disadvantages of classical LPE. Growth occurs by diffusive transport ofY through a thin liquid flux layer. This layer may be pre-deposited onto the substrate byvarious means including vacuum and non-vacuum techniques, or deposited at the growthtemperature. The composition of the liquid layer is maintained during film growth by feedingYBa2Cu3O7, or the separate components, either from the vapour or by a powder route. Growth rates up to10 nm s−1 have been demonstrated.Deposition of c-axisoriented epitaxial YBa2Cu3O7 is reported on both seeded and non-seeded substrates; the process is tolerant of a high substrate mismatch.Films 1–2 µm thick with K and a critical current densityJc>2 MA cm−2 have been grown on a range of single crystal substrates as well as on buffered texturedmetallic tapes. The mechanism of nucleation and growth from a thin liquid layer isdescribed within the general theoretical framework of crystal growth. Particular features ofthe growth are the short time constant for equilibration of transients in the depositionconditions, the wide range of relative supersaturation spanned by the process, anddominance of interface kinetic effects compared to volume diffusion in the liquid flux.

Critical current density of MgB2 thin films and the effect of interface pinning

Preferentially oriented MgB2 thin films with c-axis normal to the surface have been prepared and characterized for microstructure andtransport properties. The magnetic field dependence of superconducting critical current densityJc has been determined from the magnetization hysteresis(M–H) loops at various temperatures using the Bean’s critical state model. HighJc ofthese films show their potential for applications. We have also measured the angular dependences ofJc. The angular dependence is seen to be in agreement with theanisotropic Ginzburg–Landau model except that at angles close to theab plane, increased pinning due to film–substrate interaction is observed. The angularrange where interface pinning is effective has been determined by measurement ofasymmetry in dissipation on reversal of current for fields applied at angles close to theab plane.

Matching in YBCO nanobridges due to surface barrier effects

The critical current density jc of YBa2Cu3O7−δ (YBCO) nanobridges (width ≈200 nm) is measured as a function of the magnetic flux density B, applied perpendicular to the YBCO-film plane. The resulting jc-vs-B characteristics, when analyzed in detail, exhibit a periodic modulation, which can be attributed to an enhanced stability of specific flux-line configurations. Such an increased stability is found for magnetic fields leading to a vortex lattice being commensurable with the width of the bridge and, thus, with the surface barriers at its edges. The magnitude of this new type of matching with perpendicular field orientation appears to be significantly smaller than what has been found in previously reported cases of parallel fields.

Structure and properties of melt-texturedYBa2Cu3O7−δ, high pressure–high temperature treated and oxygenated under evaluated oxygenpressure

MT-YBCO samples oxygenated under controlled oxygen pressure exhibited at 77 K a critical current densityjc = 85 kA cm−2 in zero fieldand more than 10 kA cm−2 up to 5 T field when the external magnetic field was perpendicular to theab-plane ofY123, and a jc = 23 kA cm−2 in zero field and jc close to 1 kA cm−2 in 10 T field when the magnetic field was perpendicular to thec-axisof Y123. The microstructure of these samples contained an unusually high density of twins (about30 twins µm−1) as well as a lot of stacking faults around Y211 inclusions. Using quasi-hydrostatichigh pressure–high temperature (HP–HT) treatment we may vary the twin anddislocation densities in the material by changing the sample orientation in highpressure apparatus, while the oxygen content of Y123 phase as well as the latticeparameters remain unchanged. The microstructure of the material in the casewhere the highest pressure has been applied in the direction perpendicular to theab-plane of Y123 is characterized by a very low twin density, perfect dislocations steppedalong directions and small faulted loops corresponding to CuO intercalating in the matrix. For this materialjc = 10 and 8 kA cm−2 in zero field were observed (when the external magnetic field was perpendicular to theab-plane andperpendicular to the c-axis of Y123, respectively). High pressure–high temperature treatment causes an increasein the material density (up to near the theoretical one), microhardness and fracturetoughness.

Effect of TaC Microadditions on the Microstructure and Properties of (Bi,Pb)2Sr2Ca2Cu3O10 + xSuperconducting Ceramics

Information-theoretical approaches are used to assess the effect of nanocrystalline TaC microadditions on the thermodynamic conditions of microstructure evolution in (Bi,Pb)2Sr2Ca2Cu3O10 + x superconducting ceramics. Optimization analysis of property–structure–composition relationships indicates that the introduction of TaC makes it possible to raise the midpoint transition temperature Tc of the ceramics by 4 K and their 77-K critical current density jc by 1000 A/cm2 . It is shown using information interpretation of multifractal formalism that the optimal TaC content is about 0.14 wt %. At this doping level, the expected increase in Tc is 8 K, and that in jc is 1300 A/cm2 . The crystallite and pore substructures in Bi-2223 ceramics are shown to be correlated.

Simulation of the critical current density and its dependence on geometrical factors inRABiTS based coated conductors

The conductor width dependence of the critical current densityJc in the grain boundary network occurring in RABiTS based coated conductors is simulatedusing experimentally obtained electron backscattering diffraction (EBSD) maps and theexponential dependence of the critical current density on the misorientation angle. It isfound that a conductor width of around 20 grains (of average size) is sufficient to pass 90%of the maximum current density, independent of texture quality and grain aspect ratio inthe current direction. The aspect ratio does however influence the absolute value ofJc, givinghigher Jc values for higher aspect ratios. These results are in good agreement withJc simulations based on purely statistical grain boundary distributions.

Nondestructive inductive measurement of local critical current densities in Bi-2223 thickfilms

The local critical current densityJc of Bi-2223 thick films (thickness mm) was nondestructively measured using an ac inductive technique previously developed for measuringJc of large bulk superconductors (Mawatari et al 2003 Appl. Phys. Lett. 83 3972).In this method an ac magnetic field is generated by an ac drive current(I0cosωt) in a small flat coil placed just above the superconducting specimen. Whenthe magnetic fluxes penetrate the superconductor, a third-harmonic voltage(V3cos3ωt) is generated inthe coil, and V3 canthen be calculated as V3 = (G3ω/Jc)I02, where G3 is a coil coefficient calculated from the size and configuration of the coil. Thus,Jc can be measured in the surface region (typically down to mm beneath the surface) of the bulk superconductor by measuringV3 as a functionof I0. In themeasurement of Jc of Bi-2223 thick films, in regions with smallI0,V3 followed the expected behaviour, , and thus the surface Jc can be measured from the slope of theV3–I0 curves.With larger I0,V3 gradually deviated from this behaviour. With further increase inI0,V3 suddenly changed its curvature, reaching a plateau and then increasing again, due tomagnetic-field penetration below the bottom of the film. The threshold currentIth at the change in curvature is nearly proportional toJcd, and thusthe total Jc can be estimated as the average critical current density for the entire thickness. The distribution ofJc in a large superconducting thick film can be measured by scanning the measuring coil overthe entire sample surface.

The mechanism of nonlinear microwave behaviour inhigh-Tc superconducting devices

First, this paper gives an overview of possible origins and mechanisms that might beresponsible for experimentally observed nonlinear effects. Focusing on well-known physicalloss mechanisms with nonlinear characteristics, their impact on microwave properties andtheir relevance in microwave devices are discussed. Secondly, an experimental investigationof the power handling capability of coplanar resonators in additionally applied dc magneticfields in zero field cooled, field cooled, and field sweep experiments is presented. Theseexperiments demonstrate that the microwave power handling capability of optimizedhigh-Tc thin film devices is limited by a maximum current density, andsuggest that the limitation is caused by the dc critical current densityJc of the superconducting material.