BaZrO3 Nanorods Research Articles

Variation of critical current and n-value of 2G HTS tapes in external magnetic fields of different orientation

The in-field orientation dependence of critical current and n-value in second generation high temperature superconductive tapes was investigated. The samples were manufactured by Metalorganic Chemical Vapour Deposition method with BaZrO3 inclusions (SuperPower Inc.) and Pulsed Laser Deposition method (Bruker HTS). For samples of each kind of fabrication techniques we observed higher critical current value in the case of external magnetic field aligned along (or nearby) c-axis direction in comparison with one aligned along ab-plane. We analysed possible reasons for this effect. Angular dependences of the critical current and n-value were investigated. The microstructure images of superconductive layer of studied samples show tilt of BaZrO3 nanorods in MOCVD sample and high density of structural defects for PLD sample.

Dopant diameter dependence of Jc(B) in doped YBCO films

In YBCO films doped with artificial pinning centers, such as BaZrO3 nanorods or BaCeO3 nanodots, the critical current density, $J_{\textrm{c}}(B \parallel c)$ , is usually described with the form $J_{\textrm{c}}\sim B^{- \alpha}$ even though the shape of the $J_{\textrm{c}}(B)$ -curve does not really allow this. In the field region just above the low field plateau, the shape of the $J_{\textrm{c}}(B)$ -curve (in log-log scale) is rounded and not straight as in undoped films. The exponent $\alpha$ is found to decrease from 0.5 to around 0.2 in BaZrO3 doped films and to 0.4 in BaCeO3 doped films. The incompatibility with the curved data and the linear fit has lead to publication of $\alpha$ -parameters which are not comparable to each other due to different fitting limits. In this paper we show that it is better to use the Dew-Hughes pinning force $F_{p}(B) = F_{p0}(B/B_{\textrm{irr}})^{p}(1 - B/B_{\textrm{irr}})^{q}$ to describe the field dependence, where $\alpha \approx 1 - p$ . We also show that the $p$ and the roundness of the curve depend on the diameter of the pinning centers, but not, e.g., temperature or dopant concentration. This is shown from measurements of differently doped thin YBCO films and from large scale Ginzburg-Landau simulations. The result should have been expected since the diameters of the dopants are roughly the same size as the coherence length and it has been shown earlier that pinning centers much smaller than the coherence length lead to $\alpha = 0.5$ and those much larger lead to $\alpha = 1$ .

Influence of the lattice strain decay on the diameter of self assembled secondary phase nanorod array in epitaxial films

A theoretical model based on an analytical solution of the elastic energy of strained lattices is developed to study the diameter of self-assembled vertically-aligned secondary phase nanorods in epitaxial films. In this model, the nanorod diameter is calculated by minimizing the energy due to the formation of the nanorods, the elastic energy of the film and nanorod lattices, and the interfacial energy on the nanorod surface. The calculated nanorod diameter is consistent with experimental measurements of BaZrO3 and BaSnO3 nanorods in YBa2Cu3O7−δ films with different nanorod densities. The primary mechanism that determines the nanorod diameter is found, for the first time, to be the lattice strain decay inside the nanorods, which depends only on the ratios of elastic constants of nanorod material and is independent of film/nanorod lattice mismatch. The discovered correlation between the nanorod diameter and the elastic properties of the secondary phase oxides can be used as a guidance in the quest of the self-assembled nanorods with different diameters in epitaxial nanocomposite films.

Strong correlation between Jc(T, H||c) and Jc(77 K, 3 T||c) in Zr-added (Gd, Y)BaCuO coated conductors at temperatures from 77 down to 20 K and fields up to 9 T

We have conducted a critical current density Jc(T, H) study over a wide temperature T from 77 down to 20 K and a magnetic field H up to 9 T on more than 50 ∼ 0.9 μm-thick REBa2Cu3O7−δ (RE = rare earth) thin films containing different concentrations of BaZrO3 (BZO). We found that, independent of the composition, there is a linear correlation between Jc(77 K, 3 T||c) and Jc(T, H||c) at T down to 20 K and H up to 9 T. Moreover, Jc(77 K, 3 T||c) is also linearly correlated to Jc(T, H||ab) below 40 K. We ascribed this linear correlation to the dominant pinning source of BZO nanorods, which act as a strong correlated pinning at T above ∼30 K and provide weak uncorrelated point pins at lower temperatures. Our result emphasizes that Jc(77 K, 3 T||c) is a key metric for metal-organic chemical vapor deposited REBa2Cu3O7−δ coated conductors.

Broad Temperature Pinning Study of 15 mol.% Zr-Added (Gd, Y)–Ba–Cu–O MOCVD Coated Conductors

BaZrO3 (BZO) nanocolumns have long been shown to be very effective for raising the pinning force F-p of REBa2Cu3Ox (REBCO, where RE = rare earth) films at high temperatures and recently at low temperatures too. We have successfully incorporated a high density of BZO nanorods into metal organic chemical vapor deposited (MOCVD) REBCO coated conductors via Zr addition. We found that, compared to the 7.5% Zr-added coated conductor, dense BZO nanorod arrays in the 15% Zr-added conductor are effective over the whole temperature range from 77 K down to 4.2 K. We attribute the substantially enhanced J(c) at 30 K to the weak uncorrelated pinning as well as the strong correlated pinning. Meanwhile, by tripling the REBCO layer thickness to similar to 2.8 mu m, the engineering critical current density J(e) at 30 K exceeds J(e) of optimized Nb-Ti wires at 4.2 K.

Broad temperature range study of Jc and Hirr anisotropy in YBa2Cu3Ox thin films containing either Y2O3 nanoparticles or stacking faults

Industrially optimized coated conductors generate very high current densities Jc and irreversibility field Hirr by quasi-empirical additions of multiple pin types. However, their microstructural complexity makes it hard to securely explain all aspects of their properties. We here describe the properties of two specially grown pulsed laser deposited YBa2Cu3Ox thin films with simpler pinning landscapes, for which we conducted detailed Jc(T, H, θ) and Hirr(T, H, θ) characterizations from 10 K to Tc, and in magnetic fields up to 31 T. One film has a random insulating Y2O3 nanoparticle distribution, while the second was grown with many ab-plane stacking faults. As a whole, the Y2O3-containing sample shows significantly higher Jc(θ) at all temperatures, except around the ab-plane at greater than 40 K. Consistent with our earlier studies of the effect of BaZrO3 (BZO) nanorods in commercial coated conductors, we find that there is significant additional Jc at low temperatures when insulating precipitates with strain mismatch are present that we attribute to point defect pinning that can resist thermal fluctuations only below about 30 K. In addition to this significantly enhanced low temperature Jc, the Y2O3-containing thin film also exhibits significantly reduced effective Ginzburg-Landau (G-L) anisotropy parameter fits for Hirr(θ), which fall to γeff ≈ 3.6 from the more usual γeff ≈ 5 in the stacking-fault containing thin film. Of significant practical importance is our finding that the Y2O3 containing film achieved a bulk flux pinning force density Fp of 1000 GN/m3 at 16 T and 4.2 K, a value about 30% larger than the mixed BZO-RE2O3 pin coated conductors which are presently the state of the art.

High pinning performance of YBa2Cu3O7−x films added with Y2O3 nanoparticulate defects

We report the epitaxial growth and superconducting properties of Y2O3-added YBa2Cu3Ox (YBCO) films grown on SrTiO3-buffered MgO substrates by pulsed-laser deposition using surface-modified YBCO targets. Areas of Y2O3 sectors on the YBCO target were increased to 5.44% and 9.22% of the total YBCO pellet in order to find a correlation between the Y2O3 content, morphology, and the pinning properties of YBCO + Y2O3 mixed films. The maximum global pinning forces, FP, at 77 K were 14.3 GN m−3 and 1.15 GN m−3 for the Y2O3 5.44A% and 9.22A%, respectively. The 5.44A% Y2O3-added sample presents a very high value of pinning force at 77 K, approaching the value obtained in YBCO films with added BaZrO3 nanorods, but with less depression in the superconducting critical temperature, Tc. In accordance with scanning transmission electron microscopy (STEM) observations, both films present nanoparticulate Y2O3 dispersed in a YBCO matrix where Y2Ba4Cu8O16 (Y248) intergrowths were also observed. Consistent with the strong pinning theory, the size and distribution of randomly dispersed Y2O3 particles are optimal for the flux pinning of a 5.44A% Y2O3-YBCO film, while in the case of a 9.22A% film, the YBCO matrix is degraded by jam-packed Y248 intergrowth, which leads to a comparatively poor pinning performance. We further used the single-vortex dynamics model to account for vortex pinning in the samples. The 5.44A% Y2O3-YBCO film result shows good agreement with the model fit up to 4 T of the applied magnetic field.

DC Magnetization Relaxation and the AC Susceptibility of YBCO Films with Strong Pinning

AC susceptibility and DC magnetization relaxation measurements have been performed for YBa2Cu3O7 films containing BaZrO3 nanorods preferentially oriented along the c axis, with the external magnetic field perpendicular to the film surface. It was found that the effective vortex activation energy U e supplied by the standard analysis of the frequency dependent AC magnetic response remains very high even in the vicinity of the irreversibility line indicated by the DC relaxation. We show that the large U e values appearing in the AC signal are related to the pinning enhanced viscous drag and significant vortex velocities.

Precise Tuning of (YBa2Cu3O7‐δ)1‐x:(BaZrO3)x Thin Film Nanocomposite Structures

Self‐assembled nanocomposite films and coatings have huge potential for many functional and structural applications. However, control and manipulation of the nanostructures is still at very early stage. Here, guidelines are established for manipulating the types of composite structures that can be achieved. In order to do this, a well studied (YBa2Cu3O7‐δ)1‐x:(BaZrO3)x ‘model’ system is used. A switch from BaZrO3 nanorods in YBa2Cu3O7‐δ matrix to planar, horizontal layered plates is found with increasing x, with a transitional cross‐ply structure forming between these states at x = 0.4. The switch is related to a release in strain energy which builds up in the YBa2Cu3O7‐δ with increasing x. At x = 0.5, an unusually low strain state is observed in the planar composite structure, which is postulated to arise from a pseudo‐spinodal mechanism.

Directional pinning and anisotropy in YBa2Cu3O7−x with BaZrO3 nanorods: Intrinsic and nanorods-induced anisotropy

We present a study of the anisotropic vortex parameters as obtained from measurements of the microwave complex resistivity in the vortex state with a tilted applied magnetic field in YBa2Cu3O7−x thin films with BaZrO3 nanorods. We present the angular dependence of the vortex viscosity η, the pinning constant kp and the upper limit for the creep factor χM. We show that the directional effect of the nanorods is absent in η, which is dictated by the mass anisotropy γ. By contrast, pinning-mediated properties are strongly affected by the nanorods. It is significant that the pinning and creep affected by the nanorods is detectable also at our very high operating frequency, which implies very short-range displacements of the vortices from their equilibrium position.

The effect of lattice strain on the diameter of BaZrO3 nanorods in epitaxial YBa2Cu3O7−δ films

An elastic strain model has been applied in an effort to understand the effect of the lattice strain on the diameter of the BaZrO3 (BZO) nanorods self-assembled into aligned arrays along the c-axis in BZO-doped epitaxial YBa2Cu3O7−δ (YBCO) thin films. The calculated elastic energy of the strained BZO/YBCO composite lattice suggests that the diameter of the nanorods is approximately independent of the doping concentration of BZO as long as the density of the nanorods is sufficiently large. An experimental confirmation was carried out using transmission electron microscopy on YBCO thin films with BZO doping varying from 2% to 6% volume concentration. The diameter of the BZO nanorods was indeed found to be approximately constant in the range of 5.2–5.9 nm. The increase of the doping concentration therefore simply leads to an increase of the nanorod density, which links directly to the matching field of the effective pinning and is consistent with the transport Jc results measured for these samples.

ナノロッド導入RE123 テープ線材における相関ピンニング特性

Correlated pinning properties for GdBa2Cu3O7 (Gd123) tapes with BaHfO3 and BaZrO3 nanorods are investigated on the basis of the detailed critical current measurements in a wide temperature and magnetic field region. We found that a magnetic field dependency of Jc for the Gd123 tapes with nanorods introduced changes around the matching field B while that for the non-doped Gd123 tapes is proportional to B-0.56 in a wide field region below 20 K. These experimental results suggest that the nanorods are still effective as the c-axis correlated pinning center at low temperatures, although the Jc peaks at B//c in its angular dependency disappear below 20 K at 3 T. The cooperative model of random and correlated pinning centers indicates that the c-axis peaks in angular dependency of Jc disappear when fluctuation of the nanorods' growth direction is large. Therefore, the nanorods are effective as the c-axis correlated pinning centers even at low temperatures around 4.2 K, although their contribution becomes small.

Anisotropy and directional pinning in YBa2Cu3O7−x with BaZrO3 nanorods

Measurements of anisotropic transport properties (dc and high-frequency regime) of driven vortex matter in YBa2Cu3O7−x with elongated strong-pinning sites (c-axis aligned, self-assembled BaZrO3 nanorods) are used to demonstrate that the effective-mass angular scaling takes place only in intrinsic physical quantities (flux-flow resistivity), and not in pinning-related Labusch parameter and critical currents. Comparison of the dynamics at different time scales shows evidence for a transition of the vortex matter toward a Mott phase, driven by the presence of nanorods. The strong pinning in dc arises partially from a dynamic effect.

The Effect of $\hbox{BaZrO}_{3}$ Nanorods on the Normal State Properties and on the Anomaly in the Even Transverse Resistivity in YBCO Thin Films

Nanoscale insulating rods grow in YBa2Cu3O6 +x (YBCO) thin films when they are made by pulsed laser deposition from BaZrO3 (BZO) doped targets. The longitudinal and transverse resistivities of BZO doped YBCO thin films were measured to investigate the effect of the BZO nanorods as a small-scale inhomogeneity to the anomalies sometimes observed in transverse resistivity. The BZO nanorods were seen to reduce the oxygen content of the films only at high BZO doping levels. Anomalies in the even transverse resistivity in zero field were observed but no systematic correlation with the BZO doping was seen. The origin of the anomaly is attributed to larger scale inhomogeneities present in the samples than the BZO nanorods.

Interactive Growth Effects of Rare‐Earth Nanoparticles on Nanorod Formation in YBa2Cu3Ox Thin Films

AbstractThe controlled growth of self‐assembled second‐phase nanostructures has been shown to be an essential tool for enhancing properties of several composite oxide thin film systems. Here, the role of Y2O3 nanoparticles on the growth of BaZrO3 (BZO) nanorods is investigated in order to understand the mechanisms governing their self‐assembly in YBa2Cu3O7–x (YBCO) thin films and to more fully control the resulting defect landscape. By examining the microstructure and current‐carrying capacity of BZO‐doped YBCO films, it is shown that the nanorod growth dynamics are significantly enhanced when compared to films double‐doped with BZO and Y2O3 nanoparticles. The average nanorod length and associated critical current densities are found to increase at a significantly higher rate in the absence of Y2O3 nanoparticles when the growth temperature is increased. Using microstructural data from transmission electron microscopy studies and the response in critical current density, the interactive effects of multiple dopants that must be considered to fully control the defect landscape in oxide thin films are shown.

Inhibition of the detrimental double vortex-kink formation in thick YBa2Cu3O7 films with BaZrO3 nanorods

We investigated the temperature (T) variation of the normalized magnetization relaxation rate S and of the corresponding normalized vortex-creep activation energy U* = T/S for YBa2Cu3O7 films containing BaZrO3 nanorods, with the external magnetic field H oriented perpendicular to the film surface. It was found that by increasing the film thickness and using nanodot decorated substrates the high-T S(T) maximum appearing at low H is substituted by a minimum in S(T). As revealed by the analysis of the current density dependence of U*, this behaviour is due to the inhibition of vortex excitations involving double vortex-kinks and superkinks formation in the investigated thick films, owing to the large nanorod splay and pinning energy dispersion.

Directional Vortex Pinning at Microwave Frequency in YBa2Cu3O7−x Thin Films with BaZrO3 Nanorods

We investigate the effect of the anisotropy and of the directional pinning in YBa2Cu3O7−x films grown by pulsed laser ablation from targets containing BaZrO3 at 5 mol%. BaZrO3 inclusions self-assemble to give nanorods oriented along the c-axis, thus giving a preferential direction for vortex pinning. The directionality of vortex response is studied at high ac frequency with the complex microwave response at 48 GHz, as a function of the applied field and of the angle θ between the field and the c-axis. The complex microwave response does not exhibit any angular scaling, suggesting that the structural anisotropy of YBa2Cu3O7−x is supplemented by at least another preferred orientation. The pinning parameter r shows evidence of directional pinning, effective in a wide range of angles around the c-axis (thus ascribed to BZO nanocolumns).

High Vortex Depinning Temperatures in YBCO Films with BZO Nanorods

The Bose glass theory for the vortex matter in superconductors with correlated disorder predicts the depinning of vortices due to the renormalization of the vortex pinning barriers by thermal fluctuations. For YB2Cu3O7 (YBCO) in external magnetic fields H oriented along the columnar pins generated by various techniques theoretical estimates give a depinning temperature Tdp very close to the critical temperature Tc (Tdp∼0.95Tc), whereas the results of standard magnetization relaxation experiments are repeatedly interpreted in terms of a much lower Tdp (∼0.5Tc). We investigated the temperature T variation of the normalized magnetization relaxation rate S for YBCO thin films containing BaZrO3 (BZO) nanorods preferentially oriented along the c axis, with H along the nanorods. The nonmonotonous S(T) variation below the matching field observed up to close to Tc does not support a low Tdp. The often considered S(T) maximum occurring at relatively low T (which was connected to a disappointing Tdp) is related to the occurrence of thermomagnetic instabilities. We show that the accommodation of vortices to the columnar pins in the presence of the T dependent macroscopic currents induced in the sample is signaled by a pronounced S(T) deep located at high T, in agreement with a Tdp close to Tc. By increasing the film thickness and using the substrate decoration the BZO nanorods splay out, leading to the inhibition of (detrimental) vortex excitations involving double vortex kink or superkink formation, characteristic for high-quality thin films and single crystals with columnar pins along the c axis.

Transmission electron microscopy study of GdBa2Cu3O7−x containing nano-sized BaMO3 (M: Hf, Zr, Sn) rods fabricated by pulsed laser deposition

Three types of superconducting GdBa2Cu3O7−x (GdBCO) layers containing rods of either BaHfO3 (BHO), BaZrO3 (BZO), or BaSnO3 (BSO) were fabricated by pulsed laser deposition on Hastelloy substrates with a CeO2 based textured buffer layer. The critical currents (J c) values of the GdBCO layers containing those nano-rods are enhanced compared with those of pristine GdBCO layer in high magnetic fields. In order to investigate the relationships between their superconductive properties and their nanostructures, they were characterized in detail by transmission electron microscopy (TEM). TEM is the only method for direct observation of these nano-rods in the GdBCO grains. The GdBCO layers were mainly composed of c-axis oriented GdBCO grains containing numerous nano-sized rods. The crystal orientation relationships between the GdBCO and the nano-rods were as follows; (001)GdBCO//(001)nano-rods and (100)GdBCO//(100) nano-rods. The average diameters of the BHO and the BZO nano-rods were 4.5 and 5.6 nm, respectively. The BSO nano-rods were thicker than other rods. These nano-rods in the central region of the c-axis oriented GdBCO grains were aligned parallel to the c-axis of the GdBCO, while nano-rods in the outer region of the c-axis oriented grains were tilted away from the c-axis. With increase in the thickness of the GdBCO layers, the ratio of the BZO or the BSO nano-rods aligned parallel to the c-axis to those tilted away from the c-axis decreased, so that the J c-B-θ profiles of the thicker GdBCO layers containing the BZO or the BSO nano-rods became flatter. The BHO nano-rods were homogeneously distributed throughout the GdBCO, and their average length of was less than that of the other nano-rods. The homogeneous distribution and short length of the BHO nano-rods enhanced the J c values of the GdBCO layers containing them in high magnetic fields. The J c-B-θ profiles of the GdBCO layers containing the BHO were independent of the layer thickness. From these results, we will discuss about the morphologies and distributions of suitable vortex pinning for applications of GdBCO coated conductor in high magnetic fields.

Non-monotonous temperature variation of the magnetization relaxation rate in YBCO films with BZO nanorods below the matching field

Standard zero-field cooling (zfc) magnetization relaxation measurements over a large temperature T interval were performed for YB2Cu3O7 films with BaZrO3 nanorods parallel to the c axis having the matching field BΦ∼2.3T. A rich non-monotonous T dependence of the normalized magnetization relaxation rate S at low external magnetic fields H (oriented along the nanorods) was observed. For H=2kOe, for example, S(T) clearly exhibits two maxima, located around T=30K and T=62K, as well as a pronounced S(T) deep at T∼77K. It is shown that the first S(T) maximum may have an extrinsic origin, related to the occurrence of thermo-magnetic instabilities in the low-T domain, the second one is due to a crossover between the non-glassy vortex creep regime involving double vortex kink formation and variable range vortex hopping, whereas the S(T) deep at high T is generated by the accommodation of nearly isolated vortices to the nanorods in the presence of the T dependent macroscopic current induced in the specimen.