Q. Y. Chen

Design of superconducting magnetic bearings with high levitating force for flywheel energy storage systems

Hybrid superconducting magnetic bearing (SMB), using YBCO high temperature superconductors (HTS) coupled with permanent magnets, has been implemented into a flywheel energy storage (FES) system prototype. The hybrid SMB design uses permanent magnets to levitate the rotor weighing 19 kg and superconductors to stabilize the inherently unstable magnet-magnet interactions. The SMB bearings are brought into action under convenient field-cooled conditions. Spin-down performance of the constructed FES prototype was tested under a moderate vacuum.<<ETX>>

Absence of ferromagnetism in Co-doped ZnO prepared by thermal diffusion of Co atoms

Thermal diffusion was used to dope Co atoms into ZnO single crystal. Particle-induced x-ray emission combined with channeling technique, x-ray diffraction, and ultraviolet-visible light absorption show that Co-diffused ZnO is pure single-phase alloy. The temperature variation of magnetization can be fitted by a linear combination of the Curie law and the Curie-Weiss law with negative Curie-Weiss temperature (−175K). In the high temperature region, the Co-doped ZnO layer is paramagnetic. In the low temperature region, antiferromagnetism and paramagnetism coexist. Evidence of ferromagnetic hysteresis behavior was not observed down to 5K.

Enhancement of tensile stress near a hole in superconducting trapped-field magnets

The mechanical stress caused by flux pinning in a cylindrical superconductor with a concentric hole is investigated theoretically. Exact expressions for the radial and hoop stress are derived using the critical-state model. Stress profiles during a magnetization process often used to activate high-Tc superconductors as strong trapped-field magnets are presented and analyzed in detail. It is shown that due to the hole the tensile hoop stress is enhanced by a factor of 2 or more, depending on the hole diameter. The dramatic increase in cracking probability is emphasized.

Epitaxial GaN nanorods free from strain and luminescent defects

Raman spectroscopy, cathodoluminescence imaging, and electron backscatter diffraction have been used to characterize the GaN nanorods as compared to their supporting matrix. The nanorods are strain free, distinguished from the mechanically and thermally stressed matrix that bears the brunt of all lattice mismatch and thermal strain, strain relaxation, and the related defect generation. This thus allows the loosely attached nanorods to grow to measurable perfection in electronic and crystal structures. The nanorods are crystallographically aligned with the matrix as well as the substrate.

InN nanotips as excellent field emitters

Unidirectional single crystalline InN nanoemitters were fabricated on the silicon (111) substrate via ion etching. These InN nanoemitters showed excellent field emission properties with the threshold field as low as 0.9V∕μm based on the criterion of 1μA∕cm2 field emission current density. This superior property is ascribed to the double enhancement of (1) the geometrical factor of the InN nanostructures and (2) the inherently high carrier concentration of the degenerate InN semiconductor with surface electron accumulation layer induced downward band bending effect that significantly reduced the effective electron tunneling barrier even under very low external field.

Buffer controlled GaN nanorods growth on Si(111) substrates by plasma-assisted molecular beam epitaxya)

Size, shape, and density of self-assembled GaN nanorods grown on Si(111) substrates by plasma-assisted molecular beam epitaxy were successfully controlled by inserting a GaN buffer layer. The structure of the GaN buffer layer plays a vital role in the nanorod growth. Only a broken buffer layer with a suitable opening size can grow nanorods. Evolution of the nanorod is traced to the initial growth stage. Crystal seed grown at the wall of the opening in the buffer layer initiates the beginning of the nanorod, and a self-catalytic vapor-liquid-solid process, triggered by the nanocapillary condensation effect, enhances the GaN nanorod growth. Furthermore, the nanorod density can be largely controlled by using the beam equivalent pressure of the N∕Ga ratio. Other GaN nanostructures grown at different growth conditions are also discussed in details.

Segregation effects in Ag-Si composites

Abstract Ag-Si composites prepared by co-sputtering the components onto sapphire substrates are found to result in the Ag rapidly segregating to the Si surface if both sputtering sources are shut off at the same time. If the Si source is allowed on for a period of time after shutting off the Ag source this segregation does not occur and the Ag is uniformly distributed throughout the Ag-Si composite. Though strain considerations or a “kick-out” mechanism may be used to explain this segregating effect, it is not clear why the Ag Si/Si interface prevents Ag segregation at the free Si-vacuum surface, though the αSi overlayer may provide a sufficient diffusion barrier to prevent the Ag from segregating.

Magnetic and microstructure studies of boron-enriched (Nd/sub 0.95/La/sub 0.05/)/sub 11/Fe/sub 76.5

The magnetic properties and microstructures of rare earth lean and boron-rich (Nd/sub 0.95/La/sub 0.05/)/sub 11/ Fe/sub 76.5-x/CoxTi/sub 2/B/sub 10.5/ (x=0-15) melt-spun ribbons have been investigated. Two magnetic phases, namely /spl alpha/-Fe and R/sub 2/Fe/sub 14/B, were found, by thermal magnetic analysis (TMA), in all the ribbons studied. High intrinsic coercivity (iHc) and maximum energy product [(BH)max] values in the range of 14-18.7 kOe and 13.9-18.3 MGOe, respectively, have been achieved in these nanocomposites. Among compositions studied, the Co-substitutions, x/spl les/10, were found to be effective in increasing the Br and (BH)max, while keeping a high value of iHc. It was evidenced from transmission electron microscopy that the homogeneity, fine grain size, and high volume fraction of the R/sub 2/Fe/sub 14/B phase led to an increase in the iHc and maximum energy product of nanocomposites studied. The optimum magnetic properties of Br=9.4 kG, iHc=16.1 kOe and (BH)max=18.3 MGOe have been obtained on (Nd/sub 0.95/La/sub 0.05/)/sub 11/Fe/sub bal./Co/sub 10/Ti/sub 2/B/sub 10.5/ ribbons after an optimum treatment.

/ -/sub x/Co/sub x/Ti/sub 2/B/sub 10.5/ (x=0-15) melt-spun ribbons

One-dimensional nitride based diluted magnetic semiconductors were grown by plasma-assisted molecular beam epitaxy. Delta-doping technique was adopted to dope GaN nanorods with Mn. The structural and magnetic properties were investigated. The GaMnN nanorods with a single crystalline structure and with Ga sites substituted by Mn atoms were verified by high-resolution x-ray diffraction and Raman scattering, respectively. Secondary phases were not observed by high-resolution x-ray diffraction and high-resolution transmission electron microscopy. In addition, the magnetic hysteresis curves show that the Mn delta-doped GaN nanorods are ferromagnetic above room temperature. The magnetization with magnetic field perpendicular to GaN c-axis saturates easier than the one with field parallel to GaN c-axis.

Above room-temperature ferromagnetism of Mn delta-doped GaN nanorods

In 1993 Tang proposed [1] that vortex avalanches should produce a self organized critical state in superconductors, but conclusive evidence for this has heretofore been lacking. In the present paper, we report extensive micro-Hall probe data from the vortex dynamics in superconducting niobium, where a broad distribution of avalanche sizes scaling as a power-law for more than two decades is found. The measurements are combined with magneto-optical imaging, and show that over a widely varying magnetic landscape the scaling behaviour does not change, hence establishing that the dynamics of superconducting vortices is a SOC phenomenon.