[Show abstract][Hide abstract] ABSTRACT: An electromagnetic transmitter typically consists of individual components such as a waveguide, antenna, power supply, and an oscillator. In this communication we circumvent complications associated with connecting these individual components and instead combine them into a non-traditional, photonic enabled, compact transmitter device for tunable, ultrawide band (UWB) radiation. This device is a centimeter scale, continuous, thin film superconducting ring supporting a persistent super-current. An ultrafast laser pulse (required) illuminates the ring (either at a point or uniformly around the ring) and perturbs the super-current by the de-pairing and recombination of Cooper pairs. This generates a microwave pulse where both ring and laser pulse geometry dictates the radiated spectrum's shape. The transmitting device is self contained and completely isolated from conductive components that are observed to interfere with the generated signal. A rich spectrum is observed that extends beyond 30 GHz (equipment limited) and illustrates the complex super-current dynamics bridging optical, THz, and microwave wavelengths.
[Show abstract][Hide abstract] ABSTRACT: The orientation phase diagram of self-assembled BaZrO3 (BZO) nanostructures in c-oriented YBa2Cu3O (YBCO) films on flat and vicinal SrTiO3 substrates was studied experimentally with different dopant concentrations and vicinal angles and theoretically using a micromechanical model based on the theory of elasticity. The organized BZO nanostructure configuration was found to be tunable, between c-axis to ab-plane alignment, by the dopant concentration in the YBCO film matrix strained via lattice mismatched substrates. The correlation between the local strain caused by the BZO doping and the global strain on the matrix provides a unique approach for controllable growth of dopant nanostructure landscapes. In particular, a mixed phase of the c-axis-aligned nanorods and the ab-plane-aligned planar nanostructures can be obtained, leading to a three-dimensional pinning landscape with single impurity doping and much improved J
in almost all directions of applied magnetic field.
No preview · Article · Dec 2015 · Superconductor Science and Technology
[Show abstract][Hide abstract] ABSTRACT: Vortex pinning by insertion of non-superconducting defects like BZO or BSO nanorods into the YBCO matrix is an effective means to enhance pinning since they self-assemble into columnar structures that provide strong pinning along the length of the flux-line. However, only limited control of their geometry is possible by current growth methods. To meet the requirements of applications that operate in magnetic fields of varying intensity or orientation, this work studies strain-mediated self-assembly of 3D pinning landscape through theoretical modeling as well as experimental exploration to achieve controllable growth BZO or BSO nanostructures in YBCO matrix films. The microstructure of BZO- and BSO-doped YBCO thin films was studied using transmission electron microscopy and the findings indicate that it is possible to produce a controllable defect landscape and improved critical current density with respect to different orientation of the magnetic field by manipulation of the strain relationships using vicinal substrates.
No preview · Article · Jun 2015 · IEEE Transactions on Applied Superconductivity
[Show abstract][Hide abstract] ABSTRACT: Addition of second-phase nanosize defects to YBa2Cu3O7-δ (YBCO) superconductor thin films is known to enhance flux pinning and increase current densities (Jc). The addition of Y2BaCuO5 (Y211) was previously studied in (Y211/ YBCO)N multilayer structures and in Y211 + YBCO films deposited from pie-shaped targets. This research systematically studies the effect of Y211 addition in thin films deposited by pulsed laser deposition from YBCO1-xY211x(x = 0-15 vol.%) single targets, at temperatures of 785°C-840°C. Interestingly, the resulting size of Y211 particles is 20-40 nm, in contrast to 10-15 nm in previous studies of Y211 and 5-10 nm for other second-phase defect additions, and the number density is reduced. A slight increase of Jc(H, T) was achieved, compared with previous optimization studies. Results and comparisons of flux pinning, intrinsic stresses imaged by TEM, current densities, critical temperatures, and microstructures will be presented. The overall low intrinsic stress on YBCO from Y211 lattice mismatch is smaller than previously studied second-phase defect additions known, which is hypothesized to be the driving force in achieving the unusually large second-phase nanoparticle size and volume fraction thus far in YBCO thin films.
No preview · Article · Jun 2015 · IEEE Transactions on Applied Superconductivity
[Show abstract][Hide abstract] ABSTRACT: For high current superconductors in high magnet fields with currents in the order of 50 kA, single ReBCO coated conductors must be assembled in a cable. The geometry of such a cable is mostly such that combined torsion, axial and transverse loading states are anticipated in the tapes and tape joints. The resulting strain distribution, caused by different thermal contraction and electromagnetic forces, will affect the critical current of the tapes. Tape performance when subjected to torsion, tensile and transverse loading is the key to understanding limitations for the composite cable performance. The individual tape material components can be deformed, not only elastically but also plastically under these loads. A set of experimental setups, as well as a convenient and accurate method of stress–strain state modeling based on the finite element method have been developed. Systematic measurements on single ReBCO tapes are carried out combining axial tension and torsion as well as transverse loading. Then the behavior of a single tape subjected to the various applied loads is simulated in the model. This paper presents the results of experimental tests and detailed FE modeling of the 3D stress–strain state in a single ReBCO tape under different loads, taking into account the temperature dependence and the elastic-plastic properties of the tape materials, starting from the initial tape processing conditions during its manufacture up to magnet operating conditions. Furthermore a comparison of the simulations with experiments is presented with special attention for the critical force, the threshold where the tape performance becomes irreversibly degraded. We verified the influence of tape surface profile non-uniformity and copper stabilizer thickness on the critical force. The FE models appear to describe the tape experiments adequately and can thus be used as a solid basis for optimization of various cabling concepts.
Full-text · Article · May 2015 · Superconductor Science and Technology
[Show abstract][Hide abstract] ABSTRACT: The discovery of high temperature superconductors (HTS) and the expected applications in the field of ultrafast opto-electronics has created a unique opportunity where the technology has the potential to bridge the frequency gap from infrared to microwave. A pulsed ultrafast laser impinging on a HTS thin film grown using yttrium barium copper oxide (YBCO) excites transient electron dynamics to generate radiation that spans from the terahertz to the microwave regime. The radiation phenomena were demonstrated by making transient photo-excitation measurements using an ultrafast laser to induce non-equilibrium quasi-particle dynamics. The photo-response from a laser of an average power of 1 W and a pulse duration greater than 120 fs (808 nm wavelength) incident on charged YBa2Cu2O7-δ (YBCO) thin film at superconductive temperatures was measured using a series of microwave antennas. From the observed nanosecond response time of the transient pulse, we were able to extract frequency band structure in the GHz regime that was dependent on the incident beam diameter, pulse duration, power, and the physical structure of the YBCO thin film. The electron-phonon energy relaxation time is known to be on the order of a picosecond. However, by manipulating the resistive and kinetic inductive response of the material we demonstrate the ability to generate wideband microwave frequencies with a transient response on the order of the nanosecond time scale. Quasi-particle dynamics and the temporal response were analyzed using the Rothwarf-Taylor rate equations.
[Show abstract][Hide abstract] ABSTRACT: 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.
Full-text · Article · Mar 2014 · Superconductor Science and Technology
[Show abstract][Hide abstract] ABSTRACT: We injected a dc near-critical current through a yttrium-barium-copper oxide (YBCO) superconductor microbridge in the presence of a high pulsed ac magnetic field (10 MT/s up to 20 T) parallel to the dc current flow-the so-called Lorentz force free configuration. A transmitted RF signal probed the YBCO mixed state that followed the ac magnetic field cycles. Based on inflection points on this modulated RF signal, we found where the YBCO switched from a superconducting to normal (S-N) state. Injecting a dc near-critical current does not affect the S-N switch time or the S-N field point, at least 2 degrees below the critical temperature, i.e., T-c. Rather, the injected dc current only suppresses the RF signal's magnitude across its duration. At 5 degrees below T-c, injecting a current does modify the S-N transition point and shorten the switch time. Applications for cryotron-like switches in superconducting magnetic energy storage devices are discussed.
No preview · Article · Feb 2014 · IEEE Transactions on Applied Superconductivity
[Show abstract][Hide abstract] ABSTRACT: Striation and selective electroplating of copper stabilizer layer has been proven to be an effective way of reducing the hysteretic AC loss of second generation high temperature superconducting (2G-HTS) tapes. This method consists of a top down laser material removal (ablation) followed by post-oxygenation to create resistive oxide layer on the grooves and selective electroplating of a thick Cu stabilizer layer on the filaments. In such a filamentization technique, scalability, the degree of coupling loss contribution and critical current (Ic) degradations are the main concerns. In this study, we have investigated the post-oxygenation temperature effect on the Ic degradation and coupling loss contribution. The results showed that post-oxygenation temperature has immense effect on the Ic degradation and also alters the coupling loss contribution to the total magnetization AC losses. The coupling loss contribution was found to be less in samples oxygenated at higher temperatures; however, the degradation of Ic was found to be higher. In addition, preliminary experiments were conducted to scale-up this newly-developed technique. Initially, 10 m long fully-filamentized tape with thick Cu stabilizer layer was fabricated and at least 5 times reduction in AC loss was observed.
Full-text · Article · Jan 2014 · IEEE Transactions on Applied Superconductivity
[Show abstract][Hide abstract] ABSTRACT: Multifilament fully stabilized second generation superconductor tapes have been fabricated with filaments widths as small as 180 μm. Each superconducting filament is copper stabilized, with copper thickness up to 30 μm, without compromising the beneficial effect of striations on magnetization AC losses. This has been accomplished by a combination of laser scribing (ablation), post-ablation oxidation, and subsequent selective electroplating of copper. Twelve millimeters wide superconductor tapes divided into 48 filaments with 10 μm thick copper stabilizer is found to meet an AC loss target of 1 W/kA/m in applied alternating magnetic field of 0.075 T at 100 Hz. This technique has a potential to be transferred to a large scale manufacturing of stabilized, low loss multifilament coated conductors.
Full-text · Article · Dec 2013 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: In this paper, we have searched for superconductivity by measuring ohmic resistivity as a function of temperature in amorphous carbon films deposited by pulsed laser deposition and doped by ion implantation with sulfur and phosphorus ions. The doping concentrations were varied from 0.0003 to 4 Vol% for sulfur and 0.0003 to 1 Vol% for phosphorus. Previous efforts have studied doping of carbon-family materials such as highly oriented pyrolytic graphite, diamond-like carbon, and graphite/graphene, which have yielded critical temperatures lower than 20 K. In this study, amorphous carbon films doped with 2.55×1012 ions/cm2 phosphorus concentration showed a distinct change from semiconducting to metallic behavior and a dramatic 10 000-fold decrease in resistivity below 100 K as compared to undoped films. Sulfur-doped films with 2.55 × 1012 ions/cm2 concentration showed up to a 100-fold decrease in resistivity below 100 K as compared to undoped samples. While evidence of superconductivity was not observed, significant improvements in conductivity were noted below 100 K.
No preview · Article · Jun 2013 · IEEE Transactions on Applied Superconductivity
[Show abstract][Hide abstract] ABSTRACT: Addition of nanophase defects to YBa2Cu3O7 superconductor thin films enhances flux pinning, resulting in an increase in transport current densities (J(ct)). While previous studies focused on single-phase additions, the addition of several phases simultaneously has shown strong improvements by combining different flux pinning mechanisms. This paper further explores the effect of mixed phase nanoparticle pinning, with the addition of insulating, nonreactive phases of BaSnO3 and Y2O3. Processing parameters vary the BaSnO3 concentration of 3, 5, and 10 vol.%, while maintaining Y2O3 constant at 3 vol.%. Pulsed laser deposition produces films on LaAlO3 and SrTiO3 substrates at deposition temperatures of 750-815 degrees C. Current density is measured for fields ranging from H = 0 to 9 T with H // c, and temperatures from 5 to 77 K, providing a detailed picture of pinning effects. Optimized results of flux pinning, magnetic current densities J(cm) (H, T), critical transition temperatures (T-c), lattice parameters, and microstructures are presented.
No preview · Article · Jun 2013 · IEEE Transactions on Applied Superconductivity
[Show abstract][Hide abstract] ABSTRACT: We present a design and details of construction of two calorimetric systems that allow us to measure the total loss in high temperature superconducting coils or linear samples carrying alternating current while exposed to a strong alternating magnetic field. This measurement technique is based on the boil-off of liquid nitrogen. The first system is designed to measure ac losses in superconducting coils in self-field generated by AC transport current. The second system contains a permanent magnet rotor and simulates the environment of an electric motor or generator. The sensitivity of the system is such that it can measure low losses from a few milliwatts to several hundred milliwatts, in either a static or dynamic magnetic field.
Full-text · Article · Jun 2013 · IEEE Transactions on Applied Superconductivity
[Show abstract][Hide abstract] ABSTRACT: The 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.
[Show abstract][Hide abstract] ABSTRACT: We report the data on magnetization losses and critical current of
multifilament copper stabilized coated conductors. Eight centimeters long
samples of copper stabilized YBa2Cu3O7-x (YBCO) coated conductors manufactured
commercially were subdivided into superconducting filaments by near-IR laser
micromachining. The width of the superconducting stripes was varied from 0.2 mm
to 0.04 mm. Some of the samples were striated leaving superconducting bridges
for current sharing between the filaments. The AC losses were measured at
different sweep rates of the magnetic field up to 14 T/s. We will present the
results for the hysteresis and coupling losses and discuss the means to reduce
the coupling loss by changing the processing parameters of micromachining and
by post-ablation treatment.
Full-text · Article · Dec 2012 · IEEE Transactions on Applied Superconductivity
[Show abstract][Hide abstract] ABSTRACT: This work reports a detailed structural study by synchrotron x-ray diffraction of several sets of thick YBa2Cu3O7 layers. The samples represent recent advances in flux-pinning design, containing various concentrations of artificial pinning centers: (i) BaZrO3 nanorods, (ii) BaZrO3 nanoparticles, and (iii) Y2O3 nanoparticles. A statistical analysis was performed in order to separate the effects of defect-induced and intrinsic pinning. We report a statistically significant correlation between the orthorhombic distortion of the YBCO matrix and the pinning strength. Our result implies that the in-plane ordering of oxygen ions in the chain positions accounts for approximately 60% of the pinning force. The strain-induced pinning mechanism analysis, based on the Eshelby model of elastically strained composites, predicts that small YBCO grain size is a critical component of a strong pinning architecture that can enable critical current density values approaching the depairing limit.
Full-text · Article · Sep 2012 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: With research in the area of superconductivity growing, it is no
surprise that new efforts are being made to induce superconductivity or
increase transition temperatures (Tc) in carbon given its
many allotropic forms. Promising results have been published for boron
doping in diamond films, and phosphorus doping in highly oriented
pyrolytic graphite (HOPG) films show hints of superconductivity..
Following these examples in the literature, we have begun studies to
explore superconductivity in thin film carbon samples doped with
different elements. Carbon thin films are prepared by pulsed laser
deposition (PLD) on amorphous SiO2/Si and single-crystal
substrates. Doping is achieved by depositing from
(C1-xMx) single-targets with M = B4C
and BN, and also by ion implantation into pure-carbon films. Previous
research had indicated that Boron in HOPG did not elicit superconducting
properties, but we aim to explore that also in thin film carbon and see
if there needs to be a higher doping in the sample if trends were able
to be seen in diamond films. Higher onset temperatures, Tc ,
and current densities, Jc, are hoped to be achieved with
doping of the thin film carbon with different elements.
[Show abstract][Hide abstract] ABSTRACT: Textured metallic substrate based HTS coated conductors with the YBCO/CeO2/YSZ/CeO2/Ni architecture have already been shown to exhibit high current densities. The CeO2 seed layer can effectively minimize the formation of NiO during the initial deposition on Ni and the CeO2 cap layer provides good lattice matching to the subsequent YBCO layer. However, there are reports of cracks developing in the CeO2 seed layer after a thicker growth due to a lattice mismatch with Ni, which can lead to poor performance of the YBCO conductor. The present work explores an alternate approach by using yttrium oxide not only as the seed layer but also as the cap layer in place of CeO2. In the literature, yttrium oxide films grown on nickel by electron beam evaporation processes were found to be dense and crack-free with good epitaxy. This is likely the first report of using Y2O3 as a seed as well as a cap layer within the YBCO coated conductor architecture on specimens being fabricated in a single chamber. Pulsed laser deposition was used to perform deposition of all layers. Preliminary experiments resulted in specimens with current densities of more than 1 MA/cm2 at 77K in self field. Characterization of samples was accomplished using x-ray diffraction, both resistive and ac susceptibility derived Tc, and Jc transport measurements.