High Critical Current Coated Conductors

One of the important critical needs that came out of the DOE’s coated conductor workshop was to develop a high throughput and economic deposition process for YBCO. Metal-organic chemical vapor deposition (MOCVD) technique, the most critical steps in high technical micro fabrications, has been widely employed in semiconductor industry for various thin film growth. SuperPower has demonstrated that (Y,Gd)BCO films can be deposited rapid with world record performance. In addition to high critical current density with increased film thickness, flux pinning properties of REBCO films needs to be improved to meet the DOE requirements for various electric-power equipments. We have shown that doping with Zr can result in BZO nanocolumns, but at substantially reduced deposition rate. The primary purpose of this subtask is to develop high current density MOCVD-REBCO coated conductors based on the ion-beam assisted (IBAD)-MgO deposition process. Another purpose of this subtask is to investigate HTS conductor design optimization (maximize Je) with emphasis on stability and protection issues, and ac loss for REBCO coated conductors

Additive manufacturing of soft magnets for electrical machines—a review

With growing interest in electrification from clean energy technologies, such as wind power and use of pure electric powertrains in various applications, the demand for next-generation, high-performance magnetic materials has risen significantly. Electrical machine design for these applications is facing challenges in terms of meeting very demanding metrics for power densities and conversion efficiencies, thereby motivating the exploration of advanced materials and manufacturing for the next generation of lightweight ultraefficient electric machines. Additive manufacturing (AM), a layer-by-layer three dimensional (3D) printing technology, opens up new venues of improvements for industrial manufacturing of electrical machines via near-net shape printing of complex geometries, reduction of parts count and production lead time, and conservation of expensive critical materials such as rare-earth magnets as well as nanocrystalline and amorphous soft magnetic composites, allowing their use in only critical regions required by desired properties of the printed parts. The magnetic, electrical, thermal, and mechanical properties of the magnetic materials are also greatly influenced by the selection of the AM method. Among the seven major American Standard Testing and Materials-defined standard modes of 3D printing, selective laser melting, fused deposition modeling, and binder jetting technology dominate the AM processing of soft magnetic materials and their integration in electrical machines. In this work, the state of the art in printability and performance characteristics of soft magnetic materials for electric machines is summarized and discussed. The prospects of soft magnetic materials selection in terms of price, printability, weight, and performance of the electrical machines are also discussed. This review highlights the current status of AM of large electrical machines, AM process selection guidelines, hybrid printing technologies, and the associated opportunities and challenges. An emphasis is put on multimaterial processing that is essential for electrical machines. Hybrid printing technologies that combine multiple AM processes with adequate automation and enable simultaneous multimaterials dispensing, real-time quality control, postprocessing, and surface finish with integrated subtractive computer numeric control machining are the requirements for progressing toward the end-user electrical machines

Soft x-ray spectroscopy measurements of the p-like density of states of B in MgB2 and evidence for surface boron oxides on exposed surfaces

Soft X-ray absorption and fluorescence measurements are reported for the K-edge of B in MgB2. The measurements confirm a high density of B pxy(sigma)-states at the Fermi edge and extending to approximately 0.9 eV above the edge. A strong resonance is observed in elastic scattering through a core-exciton derived from out-of-plane pz(pi*)-states. Another strong resonance, observed in both elastic and inelastic spectra, is identified as a product of surface boron oxides.Comment: 7 pages total, 4 figures, submitted to Phys. Rev. Let

Preparation of YBCO Films on CeO 2 -Buffered (001) YSZ Substrates by a Non-Fluorine MOD Method

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66369/1/j.1551-2916.2004.01669.x.pd

Growth mechanism of superconducting MgB2 films prepared by various methods

The growth mechanisms of MgB2 films obtained by different methods on various substrates are compared via a detailed cross-sectional scanning electron microscopy (SEM) study. The analyzed films include (a) samples obtained by an ex-situ post-anneal at 900 degree of e-beam evaporated boron in the presence of an Mg vapor (exhibiting bulk-like Tc0 about 38.8 K), (b) samples obtained by the same ex-situ 900 degree anneal of pulsed laser deposition (PLD)-grown Mg+B precursors (exhibiting Tc0 ~ 25 K), and (c) films obtained by a low-temperature (600 - 630 degree) in-situ anneal of PLD-grown Mg+B precursors (with Tc0 about 24 K). A significant oxygen contamination was also present in films obtained from a PLD-grown precursors. On the other hand, it is clearly observed that the films obtained by the high-temperature reaction of e-beam evaporated B with Mg vapor are formed by the nucleation of independent MgB2 grains at the film surface, indicating that this approach may not be suitable to obtain smooth and (possibly) epitaxial films.Comment: 1 table, 4 figures, 17 references, 16 pages PDF fil

Effective Vortex Pinning in MgB2 thin films

We discuss pinning properties of MgB2 thin films grown by pulsed-laser deposition (PLD) and by electron-beam (EB) evaporation. Two mechanisms are identified that contribute most effectively to the pinning of vortices in randomly oriented films. The EB process produces low defected crystallites with small grain size providing enhanced pinning at grain boundaries without degradation of Tc. The PLD process produces films with structural disorder on a scale less that the coherence length that further improves pinning, but also depresses Tc

Retention of Two-Band Superconductivity in Highly Carbon-Doped MgB2

Tunneling data on MgB_{1.8}C_{0.2} show a reduction in the energy gap of the pi-bands by a factor of two from undoped MgB2 that is consistent with the Tc reduction, but inconsistent with the expectations of the dirty limit. Dirty-limit theory for undoped MgB2 predicts a single gap about three times larger than measured and a reduced Tc comparable to that measured. Our heavily-doped samples exhibit a uniform dispersion of C suggestive of significantly enhanced scattering, and we conclude that the retention of two-band superconductivity in these samples is caused by a selective suppression of interband scattering.Comment: 4 pages, 4 figures; added one figure, added one reference, minor changes to the text, manuscript accepted for publication as a Phys. Rev. B Rapid Communicatio

Electron transport, penetration depth and upper critical magnetic field of ZrB12 and MgB2

We report on the synthesis and measurements of the temperature dependence of resistivity, R(T), the penetration depth, l(T), and upper critical magnetic field, Hc2(T), for polycrystalline samples of dodecaboride ZrB12 and diboride MgB2. We conclude that ZrB12 as well as MgB2 behave like simple metals in the normal state with usual Bloch-Gruneisen temperature dependence of resistivity and with rather low resistive Debye temperature, TR=280 K, for ZrB12 (as compared to MgB2 with TR=900 K). The R(T) and l(T) dependencies of ZrB12 reveal a superconducting transition at Tc=6.0 K. Although a clear exponential l(T)dependence in MgB2 thin films and ceramic pellets was observed at low temperatures, this dependence was almost linear for ZrB12 below Tc/2. These features indicate s-wave pairing state in MgB2, whereas a d-wave pairing state is possible in ZrB12. A fit to the data gives a reduced energy gap 2D(0)/kTc=1.6 for MgB2 films and pellets, in good agreement with published data for 3D \pi - sheets of the Fermi surface. Contrary to conventional theories we found a linear temperature dependence of Hc2(T) for ZrB12 (Hc2(0)=0.15 T).Comment: 8 pages, 10 figures, submitted to JET