Columnar Defects Research Articles

Three-Dimensional Molecular Dynamics Method for Vortex pinning by Splayed Columnar Defects

We have developed three-dimensional molecular dynamics method, in which anisotropy of a superconductor and vortex core energy are included. We have applied it to a superconductor with splayed columnar defects. We have found that under a driving force due to an external current, vortices show wandering structures, which comes from the pinning force from splayed columnar defects. Especially, for weak core energy, vortices become soft or flexible. In an isotropic superconductor, these wandering structures disappear.

Suppression of Superconductivity in Heavy-ion Irradiated 2H-NbSe2 Caused by Negative Pressure

Effects of columnar defects created by 320 MeV Au irradiation on 2H-NbSe2 single crystals with a dose equivalent matching field up to 16 T were studied. Critical temperature is found to be suppressed almost linearly at a rate of 0.07 K/T. At the same time, the lattice parameters a and c are found to be expanded at rates of 0.016%/T and 0.030%/T, respectively. Such a lattice expansion should work as negative pressure to affect Tc. By separating the effect of heavy-ion irradiation on Tc suppression through lattice expansion and disorder, it is found that Tc is suppressed more by lattice expansion rather than by disorder.

Molecular Dynamics Simulation of Chip Morphology in Nanogrinding of Monocrystalline Nickel

In this study, the nanogrinding process for single-crystal nickel was investigated using a molecular dynamics simulation. A series of simulations were conducted with different tool radii and grinding methods to explore the effects of chip morphology, friction forces, subsurface damage, and defect evolution on the nanogrinding process. The results demonstrate that the workpiece atoms at the back of the tool were affected by the forward stretching and upward elastic recovery when no chips were produced. Although the machining depth was the smallest, the normal force was the largest, and dislocation entanglement was formed. The small number of defect atoms indicates that the extent of subsurface damage was minimal. Moreover, when spherical chips were produced, a typical columnar defect was generated. The displacement vector of the chip atoms aligned with the machining direction and as the chips were removed by extrusion, the crystal structure of the chip atoms disintegrated, resulting in severe subsurface damage. By contrast, when strip chips were produced, the displacement vector of the chip atoms deviated from the substrate, dislocation blocks were formed at the initial stage of machining, and the rebound-to-depth ratio of the machined surface was the smallest.

Deuterium ion irradiation impact on the current-carrying capacity of DI-BSCCO superconducting tape

In the present work, we have irradiated the DI-BSCCO superconducting tapes with the 100 keV deuterium ions to investigate the effect of ion irradiation on their critical current (Ic). The damage simulations are carried out using the binary collision approximation method to get the spatial distribution and depth profile of the damage events in the high temperature superconducting (HTS) tape. The point defects are formed near the surface of the HTS tape. These point defects change the vortex profile in the superconducting tape. Due to the long-range interaction of vortices with each other, the Ic of the tape degrades at the 77 K and self magnetic field. The radiation dose of 2.90 MGy degrades the 44% critical current of the tape. The results of the displacement per atom (dpa) and dose deposited by the deuterium ions are used to fit an empirical relation for predicting the degradation of the Ic of the tape. We include the dpa, dose and columnar defect terms produced by the incident particles in the empirical relation. The fitted empirical relation predicts that light ion irradiation degrades the Ic in the DI-BSCCO tape at the self field. This empirical relation can also be used in neutron irradiation to predict the lifetime of the DI-BSCCO tape. The change in the Ic of the DI-BSCCO tape due to deuterium irradiation is compared with the other second-generation HTS tape irradiated with energetic radiation.

Equiaxed microstructure formation by ultrasonic assisted laser metal deposition

Laser metal deposition (LMD) provides a unique approach for surface modification and additive manufacturing. It typically leads to the formation of columnar crystal structures and defects in metals. In this study, ultrasonic vibration was applied on LMD of 316L. Contrastive studies show that columnar microstructures were observed without ultrasound, while equiaxed crystal structures were found with ultrasonic vibration. The reasons to generate the equiaxed microstructures were discussed. The results provide a feasible method for improving the quality of additive manufactured parts.

Effect of Ta irradiation on microstructure and current carrying properties of YBCO coated conductors with element doping

In the present work, we study the change of microstructural and current carrying properties of the YBa2Cu3O7−δ (YBCO) coated conductors (CCs) doped with mixed elements of Ta, Zr, Hf, Mn, and Sn after the irradiation of 1.9 GeV Ta ions. Magnetic property measurement system measurement was applied to study the current carrying properties of the irradiated YBCO CCs. The critical transition temperature (Tc, on) decreases by 0.5 K as the ion fluence reaches 5.0 × 1010 ions/cm2. It is revealed that for the irradiated samples with the fluence higher than 1.0 × 108 ions/cm2, their negative magnetization is enhanced pronouncedly as the applied temperature is lower than Tc, on, which implies that there are more antimagnetic phases being resistant to magnetic fields in these irradiated samples. Besides, the critical current density (Jc) and the pinning force (Fp) increase with the increase of the Ta ion fluence. Particularly for the case of the fluence more than 5.0 × 1010 ions/cm2, Jc reached 8.74 × 106 A/cm2 at 1 T and 30 K and Fp reached 8.84 × 1010 N/m3 at 1 T and 30 K. As a consequence, the Jc value of the studied YBCO CCs is improved by 4.4 times at 1 T and 30 K compared with the value of 1.98 × 106 A/cm2 of the primitive sample. Moreover, the sectional microstructures of the YBCO superconducting layer observed by a field emission transmission electron microscope show the columnar defects with the size of about 9 nm form in the doped YBCO films after the irradiation, confirming traces of continuous latent tracks exist as the evidence of effective mixed-pinning landscapes.

Smectic vortex glass

We show that in type-II superconductors a magnetic field applied transversely to correlated columnar disorder, drives a phase transition to a distinct "smectic" vortex glass (SmVG) state. SmVG is characterized by an infinitely anisotropic electrical transport, resistive (dissipationless) for current perpendicular to (along) columnar defects. Its positional order is also quite unusual, long-ranged with true Bragg peaks along columnar defects and logarithmically rough vortex lattice distortions with quasi-Bragg peaks transverse to columnar defects. For low temperatures and sufficiently weak columnar-only disorder, SmVG is a true topologically-ordered "Bragg glass", characterized by a vanishing dislocation density. At sufficiently long scales the residual ever-present point disorder converts this state to a more standard, but highly anisotropic vortex glass.

Peak Effects in 2H-NbSe2 Single Crystals Induced by Particle Irradiations

Effects of three kinds of columnar defects in NbSe2 single crystals introduced by 320 MeV Au irradiation with a dose equivalent matching field B Φ up to 4 T were studied. Through the magnetization measurement using a SQUID magnetometer, two pronounced peak effects in the external field dependence of critical current density (J c) were observed in crystals introduced with splayed and tilted columnar defects. In crystals with splayed columnar defects, J c at the peak field reaches the maximum for splay angle θ CD = ±10°, which is a little different from that found in YBa2Cu3O7 and iron-based superconductors, where optimal θ CD is equal to ±5°.

Anisotropic Critical Current Densities in Ba0.6K0.4Fe2As2 with Splayed Columnar Defects

Enhancement and non-monotonic field dependence of critical current density (Jc ), called anomalous peak effect, were observed in Ba1−x K x Fe2As2 with splayed columnar defects. In principle, anisotropy of in-plane Jc should be present in this superconductor since the introduced splayed columnar defects introduce anisotropy in the plane. This study reports in-plane anisotropy of Jc observed in 2.6 GeV U irradiated Ba0.6K0.4Fe2As2 with splay angles of ±15° and ±20°. Results show that Jc component parallel to the splay plane is larger than the perpendicular component, and the parallel component contribute to the anomalous peak effect at around B Φ/3 more.

Effect of Ta irradiation on microstructures and current carrying properties of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7–</sub><sub><italic>δ</italic></sub>

High-temperature superconducting coated conductors (HTS CCs), which are based on epitaxial growth and biaxial texture technology of REBCO thin films, show excellent physical properties including high transition temperature T c,on, high irreversible field H irr, as well as high critical current density J c. However, YBa2Cu3O7– δ (YBCO) has limited applications in a high magnetic field because of low critical current. Previous studies have shown that building smaller artificial pinning centers (APCs) is an effective way to increase the J c of HTS CCs. Compared with the traditional chemical doping route, the particle irradiation method will result in various types and sizes of defects with uniform distribution by controlling the type, energy and dose of particles and keeping the original chemical ratio of the material. 1.2 µm YBCO superconducting layer was deposited on the LaMnO3 substrate of the samples by metal organic deposition (MOD) method. YBCO thin films were irradiated with 1.9 GeV Ta ions at room temperature and vacuum conditions. The microstructure of irradiated samples was systematically observed using a field emission transmission electron microscope (FETEM). The result shows that Ta ions do produce continuous columnar defects with uniform distribution in the superconducting layer of the YBCO films, and the size of the defects is about 10 nm. The basic superconducting parameters, such as T c,on and J c, were measured using magnetic property measurement system (MPMS). The result shows the T c,on of the pristine sample was 91.3 K. When the irradiation fluence was small, the T c,on of the doped sample was basically unchanged. As the fluence reached 5.0×1010 ions/cm2, the T c,on decreased by 0.6 K. In addition, the superconducting antimagnetic phases did not change much until the fluence exceeded 1.0×108 ions/cm2. After the fluence exceeded 1.0×108 ions/cm2, the superconducting antimagnetic phases increased obviously. They increased with the decrease of temperature as well. The result shows that J c increased obviously with the increase of fluence for the samples. In normal situation, the current carrying properties of the samples irradiated in the low field should decrease compared with that before irradiation, but the J c of the samples increased in the low field because of the increase of superconducting antimagnetic phases in the samples after irradiation. At 30 K and 1 T, the J c increased by 3 times of the original, and at 77 K, the J c also improved by 2.7 times in the same field. To analyze the pinning state in this process, the exponential relationship model of J c∝ H – β was used to fit the data of the samples at 30 K and 77 K. The result shows that the doped samples with 5.0×1010 ions/cm2 had the higher value of H * and β , which suggests the generation of new effective pinning centers due to the Ta ion implantation. The normalized pinning force f p = F p/ F p,max of samples irradiated with different fluence was analyzed as a function of the magnetic field h = H / H max by using Higuchi model. The fitting result shows that Ta ion irradiation does affect the pinning behavior of doped YBCO films. Further study is needed to find out why Ta ions have such effects on the YBCO films.

Modification of Critical Current Density Anisotropy in High-Tc Superconductors by Using Heavy-Ion Irradiations

The critical current density Jc, which is a maximum value of zero-resistivity current density, is required to exhibit not only larger value but also lower anisotropy in a magnetic field B for applications of high-Tc superconductors. Heavy-ion irradiation introduces nanometer-scale irradiation tracks, i.e., columnar defects (CDs) into high-Tc superconducting materials, which can modify both the absolute value and the anisotropy of Jc in a controlled manner: the unique structures of CDs, which significantly affect the Jc properties, are engineered by adjusting the irradiation conditions such as the irradiation energy and the incident direction. This paper reviews the modifications of the Jc anisotropy in high-Tc superconductors using CDs installed by heavy-ion irradiations. The direction-dispersion of CDs, which is tuned by the combination of the plural irradiation directions, can provide a variety of the magnetic field angular variations of Jc in high-Tc superconductors: CDs crossing at ±θi relative to the c-axis of YBa2Cu3Oy films induce a broad peak of Jc centered at B || c for θi < ±45°, whereas the crossing angle of θi ≥ ±45° cause not a Jc peak centered at B || c but two peaks of Jc at the irradiation angles. The anisotropy of Jc can also modified by tuning the continuity of CDs: short segmented CDs formed by heavy-ion irradiation with relatively low energy are more effective to improve Jc in a wide magnetic field angular region. The modifications of the Jc anisotropy are discussed on the basis of both structures of CDs and flux line structures depending on the magnetic field directions.

Tilted elastic lines with columnar and point disorder, non-Hermitian quantum mechanics, and spiked random matrices: Pinning and localization

We revisit the problem of an elastic line (such as a vortex line in a superconductor) subject to both columnar disorder and point disorder in dimension d=1+1. Upon applying a transverse field, a delocalization transition is expected, beyond which the line is tilted macroscopically. We investigate this transition in the fixed tilt angle ensemble and within a "one-way" model where backward jumps are neglected. From recent results about directed polymers in the mathematics literature, and their connections to random matrix theory, we find that for a single line and a single strong defect this transition in the presence of point disorder coincides with the Baik-Ben Arous-Péché (BBP) transition for the appearance of outliers in the spectrum of a perturbed random matrix in the Gaussian unitary ensemble. This transition is conveniently described in the polymer picture by a variational calculation. In the delocalized phase, the ground state energy exhibits Tracy-Widom fluctuations. In the localized phase we show, using the variational calculation, that the fluctuations of the occupation length along the columnar defect are described by f_{KPZ}, a distribution which appears ubiquitously in the Kardar-Parisi-Zhang universality class. We then consider a smooth density of columnar defect energies. Depending on how this density vanishes at its lower edge we find either (i) a delocalized phase only or (ii) a localized phase with a delocalization transition. We analyze this transition which is an infinite-rank extension of the BBP transition. The fluctuations of the ground state energy of a single elastic line in the localized phase (for fixed columnar defect energies) are described by a Fredholm determinant based on a new kernel, closely related to the kernel describing the largest real eigenvalues of the real Ginibre ensemble. The case of many columns and many nonintersecting lines, relevant for the study of the Bose glass phase, is also analyzed. The ground state energy is obtained using free probability and the Burgers equation. Connections with recent results on the generalized Rosenzweig-Porter model suggest that the localization of many polymers occurs gradually upon increasing their lengths.

Origin analysis and the elimination of the columnar crystal defects in the 3C-SiC/Si (1 0 0) heteroepitaxial layers by the modified two-step carbonization process

New columnar crystal defects were formed on the surface of 3C-SiC epitaxial growth on Si (100) substrates by a modified two-step carbonization process. The density of columnar crystal defects was reduced with the increase in the C/Si mole ratio. However, the defects could not be completely removed, even after increasing the C/Si mole ratio from 0.8 to 2.0. The size of the columnar crystal defects in 3C-SiC epi-layer growth in the SiH4-C3H8-H2 system was around 4 μm, and was unaffected by the C/Si mole ratio. The FIB and TEM investigations showed that the columnar crystal defects originated from the supersaturation of Si vapor. The columnar crystal defect was also present along 〈002〉 orientation and inclined along [111] direction at an angle of about 54.7°. HCl was used to inhibit the homogeneous gas-phase nucleation of Si, which was regarded as the formation introducing a factor of columnar defects. When the Cl/Si mole ratio was increased to 5.0, columnar crystal nucleation was completely suppressed.

Summation of Flux Pinning by Columnar Defects Tilted at Different Angles in YBCO Thin Films

Angular behaviors of critical current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and n-values at 77 K, 1 T were investigated in YBCO films with columnar defects (CDs) tilted at small angles to the ab-plane before and after the additional introduction of CDs at different angles, where CDs were installed by heavy-ion irradiations. The enhanced J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> at B || ab by the initial irradiation at θ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> = ±85° relative to the c-axis was reduced by the additional irradiation at θ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> = ±75°, whereas the J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> was increased at the intermediate angle between the two irradiation angles. The addition of CDs at θ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> = 0° also lowered the J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> around B || ab, which were enhanced by the initial irradiation angles of θ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> = ±80°. The n-values, by contrast, were hardly lowered in any direction of magnetic field by the second irradiation: the n-value seems to be determined by a simple sum of the pinning interactions of CDs tilted at different angles. These results suggest that the flux pinning is improved in each direction of the tilted CDs, without impeding the flux pinning in the other CD direction each other. The additional introduction of CDs, however, induces both the reduction of the superconducting percolation path and the irradiation damage to the ab-correlated PCs, resulting in the reduction of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> .

Influence of columnar defects on magnetic relaxation of microwave nonlinearity in superconducting YBCO resonator devices

Intermodulation distortion (IMD) of microwave signals incident upon a superconducting YBa2Cu3O7−x thin film sample on a LaAlO3 substrate is shown to relax as the sample magnetization also relaxes, revealing the signature of fluxon decay in the microwave distortion. The sample contained a microchannel of columnar defects that was introduced using heavy ion beam irradiation. After exposure to and then removal of a low strength static magnetic field, IMD relaxed due to thermal activation of fluxons over the surface barrier. This process influenced second and third order IMD differently, visible in the differing functional form of the IMD in time and the temperature dependence of the relaxation function. The presence of columnar defects in the test sample suppressed thermal activation by pinning of fluxons, notably with complete suppression of relaxation in third order IMD at low temperatures. Higher order IMD was found to relax with the same functional form as the second and third order IMD of the same parity.

Combined effect of flux pinning by three-directional columnar defects in a field-angular region of high-Tc superconductors

We investigated the combined effect of flux pinning by three-directional columnar defects (CDs) in a wide field-angular range of YBa2Cu3Oy thin films, where CDs parallel to the c-axis and CDs crossing at ±θi (θi = 45°, 60° and 80°) relative to the c-axis were installed by 200 MeV Xe-ion irradiations. The three-directional CDs with θi ≤ ±60° form a field angular curve of critical current density Jc with a broad peak centered at magnetic field (B) parallel to the c-axis, whereas the Jc shows no peaks around the crossing angles of ±θi. When the crossing angle is expanded to θi= ±80°, a drastic change in the field- angular behavior of Jc is induced: there is not a large enhancement of Jc around B || c, but two large peaks of Jc emerge at the two irradiation angles of θi= ±80°. We also demonstrated that the combined configuration consisting of CDs at θi = 0°, ±45°, ±60° and ±80° provides a uniform enhancement of Jc over a wide angular-region.

Realization of structural evolution in grain boundary, solute redistribution and improved mechanical properties by adding TiCnps in wire and arc additive manufacturing 2219 aluminium alloy

Non-equilibrium solidification and experienced thermal conditions of deposited 2219 aluminium alloy during wire and arc additive manufacturing (WAAM) result in the grain boundary segregation, which makes it difficult to avoid the appearance of columnar grain. In this work, columnar crystal defects and grain boundary segregation were eliminated by adding TiC nanoparticles (TiCnps) to WAAM-deposited 2219 aluminium alloy. The effects of TiCnps addition on the solidification dynamics, particles distribution, grain boundary structure, solute redistribution, and mechanical properties were investigated. The solidification dynamics of the droplet induced by mesoscopic TiC particles was assessed and the result showed that the addition of c. 80 nm TiCnps could reduce the solid–liquid growth rate (R), besides, the critical movement rate (Rc) of 80 nm TiCnps was much lower than that of R, i.e. TiCnps were inclined to be distributed inside the grains as effective nucleation particles. Micro-pools with multi-site nucleation inhibited grain boundary segregation and increased Cu concentration in Al-matrix. Such a variation in Cu atom concentration promoted the precipitation of the fine spot-like θ′-CuAl2 phase with a diameter of c. 500 nm, besides, the dendritic θ-CuAl2 phase along the grain boundary was all transformed into α-Al+θ-CuAl2 phases with a semi-coherent state. The fine spot-like phase and TiCnps possessed a strong interfacial bonding with the Al-matrix, thereby the mechanical properties of the deposited 2219 aluminium alloy were strengthened. Eventually, the deposited 2219 aluminium alloy with the addition of TiCnps exhibited excellent mechanical properties, i.e. the tensile strength of the samples were around 380 MPa, and the elongation could be up to 21.4%.

Large enhancement of the in-field critical current density of YBCO coated conductors due to composite pinning landscape

YBa2Cu3O7-based coated conductors (CCs) achieve the highest critical current densities (J c) of any known superconductor and are a key technology for applications such as rotatory machines, high-field magnets and power transmission. Incorporation of nano-sized non-superconducting second phases as additional vortex pinning centers has been considered the most amenable route to further enhance J c at an industrial scale, and has been successfully used in commercial CCs. The resulting pinning landscape is quite complex, with both synergistic and competing interactions among the various types of defects. Particle irradiation, on the other hand, allows for a controlled post-processing incorporation of a well-defined defect morphology. We have previously shown that irradiation with protons and other light ions can further enhance the in-field J c in commercial state-of-the-art CCs. Here we develop a combined irradiation process that increases J c above values previously achieved by irradiating with only one species. Our new approach involves sequentially irradiating with 250 MeV Au ions and 4 MeV protons. For example, at T∼ 27 K (liquid neon) and µ 0 H∼ 4 T, a region of interest for rotatory machines applications, we obtain J c ∼ 5 MA cm−2, which is about 40% higher than the values produced by the individual irradiations. Finally, we conclude that this is due to the synergistic pinning effects of the introduced splayed, non-uniform columnar defects and small clusters.

Experimental evaluation of direct current magnetron sputtered and high-power impulse magnetron sputtered Cr coatings on SiC for light water reactor applications

Two types of physical vapor deposition Cr coatings were deposited on chemical vapor deposited SiC coupons using conventional direct current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) to mitigate the corrosion of SiC, an advanced fuel and structural material in light water nuclear reactor cores. Coating microstructure was characterized using scanning electron microscopy and scanning transmission election microscopy. X-ray diffraction analysis was conducted for phase identification and qualitative analysis of residual stresses. The mechanical integrity of the coatings was evaluated using scratch testing, micro-indentation, and nano-indentation tests. The microstructure of the DCMS coating consisted of fine columnar grains with nanoscale intercolumnar channels, while the HiPIMS microstructure was denser and free of any columnar defects. The DCMS coating showed a small tensile residual stress, while the HiPIMS coating was under compressive stress. Both types of coatings showed good mechanical integrity in terms of ductility and adhesion to the substrate. Corrosion performance of the coatings was assessed with a 30-day high temperature water autoclave test. Both types of coatings formed a protective Cr2O3 surface layer 20-30 nm thick during the autoclave test. In the DCMS coating, the columnar defects provided permeation pathways for water penetration resulting in oxide formation in the intercolumnar regions in the interior of the coating. Overall, both types of Cr coatings offer promise for the mitigation of hydrothermal corrosion of SiC in light water reactor operating environments.

Stable states of vortex-like magnetic formations in inhomogeneous magnetically uniaxial films and their behavior in a longitudinal magnetic field

This paper looks into the magnetization reversal processes of uniaxial ferromagnetic films with spatially modulated material parameters. It has been established that four types of vortex-like inhomogeneities can form on columnar defects of a certain type in such films, with these inhomogeneities having a nontrivial configuration of magnetic moments and differing in their topological parameters. It has been shown that in a magnetic field, their structure transforms into two or three stages, such transformation depending both on the topological charge and on the polarity of their core. A diagram of their stable states has been constructed, and a range of material parameters has been determined within which they can exist only in the presence of a magnetic field. The methods for producing such films have been considered.