Zhongtang Xu

Design and performance of a small HTS bulk reluctance motor

HTS bulks can be used as flux-focusing elements in reluctance motors. As parts of the primary study, a small prototype reluctance motor was designed and constructed by using HTS YBCO bulks. The performance was analyzed based on magnetic field calculation. An experimental investigation was carried out on the prototype motor. Computed parameters are compared with test measurements, showing good agreement. Compared with the conventional synchronous reluctance motors, higher efficiency and smaller size make these motors very attractive for the applications, such as cryogenic pumps and flywheel drives.

High performance FeSe0.5Te0.5 thin films grown at low temperature by pulsed laser deposition

We report on fully epitaxial FeSe0.5Te0.5 (FST) thin films with high-quality grown on CaF2 (00l) substrate at a low temperature of 300{deg}C by pulsed laser deposition. The transport Jc of thin films is up to 1.36 MA/cm2 in self-field and 0.97 MA/cm2 in 9 T at 4.2 K, indicating very weak field dependence. A nearly isotropy of Jc ({gamma}= Jc(H//ab)/Jc(H//c) as low as 1.09 at 9 T is achieved in the FST thin films. Moreover, no clear amorphous interfacial layer presents between the film and the substrate probably ascribing to low temperature and low laser repetition rate grown, while the thickness of reaction layer is approximate 5 nm in many other works. The evidences of transmission electron microscopy show some lattices with lateral size of < 5 nm x 20 nm seem to be disturbed. Those location defects are thought to be responsible for nearly isotropic behavior of superconductivity.

Influence of crystalline boron powders on superconducting properties of C-doped internal Mg diffusion processed MgB2 wires

Monofilament MgB2/Fe wires or tapes were fabricated with two different purity crystalline boron powders, using internal magnesium diffusion (IMD) and in situ powder-in-tube (PIT) processes. To evaluate which method was more insensitive to the purity of the boron powders, a sensitivity factor was used. It was found that the IMD process was less sensitive to the purity of the boron powders, compared to the PIT method. Furthermore, J c values of the IMD-processed wires were higher than those of the PIT-processed samples. The reduced porosity and hence the increased density in the IMD-processed samples was thought to be the main reason. Although a number of B-rich particles remained in the MgB2 region, a layer J c as high as 4.8 × 104 A cm−2 at 4.2 K and 10 T was still obtained in the IMD-processed wire, which was comparable to that of IMD-processed wires fabricated using amorphous boron powders.

Technical analysis on the application of HTS bulk in "Permanent Magnet" motor

HTS materials can be used to develop novel permanent magnet (PM) motor. Based on the experimental and theoretical investigations, it was observed that the multi-impulse magnetization strategy should be selected for the constraints such as the temperature distributions and mechanical strength of the motor. The asymmetrical field distribution of HTS would induce torque ripples and unbalanced force. Proper cares have to be taken to avoid irreversible demagnetization. Resin impregnation and enclosing the rotor in a can/sleeve may provide the rotor enough mechanical strength to enable its high speed applications.

Angular-dependent vortex pinning mechanism and magneto-optical characterizations of FeSe0.5Te0.5 thin films grown on CaF2 substrates

Magneto-optical (MO) characterizations and the angular-dependent critical current density (J c(Θ)) of epitaxial FeSe0.5Te0.5 (FST) thin films grown on CaF2 single-crystalline substrates were performed. The MO images show typical rooftop patterns in the remanent state from which a large, homogeneous, and almost isotropic self-field J c over 2 × 106 A cm−2 at 8 K was obtained. The vortex pinning mechanism is investigated measuring the magnetic field and angular-dependent critical current density J c. The FST films exhibit small anisotropy of J c in the whole applied magnetic field range below 15 K. The Dew-Hughes model and angular scaling analyses prove that pointlike normal cores, which are distributed randomly in the FST film, dominate the pinning in the FST films on CaF2 substrates.

Superconducting Properties of 100-m Class Sr0.6K0.4Fe2As2 Tape and Pancake Coils

Iron-pnictides are hotly studied since 2008 in the superconducting materials research area, due to their special properties and unclear mechanism. Big achievement has been made in the pnictide research during the past years. For practical uses, pnictide superconductor should be fabricated in a form that can be used for different devices. In this paper, 100-m class 7-core Sr_0.6K_0.4Fe₂As₂ (Sr122) tapes have been made by using the powder-in-tube technique, which is reported for the first time. Clearly, an average <italic>J_c </italic> of 1.3 × 10⁴xa0A/cm² at 10xa0T was reached over the 115-m length, showing a high property and good uniformity of 100-m level Sr122 tapes. Using the 10-m long Sr122 tapes, two double-pancake coils were fabricated by a wind and reaction technique. No transport current could be measured for the coil made from the monofilamentary tape. However, transport <italic>I</italic>_c was obtained in the coil made from the 7-filamentary tape. The factors that affect the superconducting property of the coil were discussed in this paper.

High-performance FeSe0.5Te0.5 thin films fabricated on less-well-textured flexible coated conductor templates

We report on the transport properties of FeSe0.5Te0.5 (FST) thin films fabricated on less-well-textured flexible coated conductor templates with LaMnO3 (LMO) as buffer layers using pulsed laser deposition. The LMO buffer layers exhibit large in-plane misalignment of ~7.72°, which is unfavorable for cuprate-coated conductors due to the high grain boundaries. The FST thin films show a superconducting transition temperature of 16.8 K, higher than that of bulk materials due to the compressive strain between LMO and FST. Atomic force microscopy observations reveal that island-like features appear at the surfaces of both LMO and FST, confirming the island growth mode. A self-field transport critical-current density of up to 0.43 MA cm−2 at 4.2 K has been observed in FST thin films, which is much higher than that in powder-in-tube processed FST tapes. The films are capable of carrying current densities of over 105 A cm−2 in the whole applied magnetic field up to 9 T, showing great potential for high-field applications. The results indicate that, for FST, highly textured metal tapes are not needed to produce coated conductors with high performance, which is of great advantage over cuprate-coated conductors.

Vortex pinning properties in Co-doped BaFe2As2 thin films with a high critical current density over 2 MA cm-2 at 9 T

Similar to other high-temperature superconductors, the pinning centers largely determine the critical current density (J c) in Co-doped BaFe2As2 (Ba122:Co). Therefore, understanding the vortex pinning mechanism of high-J c Ba122:Co thin films is important for the applications of Ba122:Co. Herein we report the pinning of the naturally grown defects in Ba122:Co thin films with a high critical current density. The transport J c of thin films is up to 2.6 MA cm−2 in 9 T at 4.2 K, which is the highest value in iron-based superconductors. Microstructure investigations reveal a high density of ab-planar defects (stacking faults) and localized vertical defects present in the sample. The Dew–Hughes mode analyses prove that pinning centers by surface defects and by point defects are responsible for H//ab and H//c, respectively. Therefore, the high J c in strong magnetic field for both H//ab and H//c are related to surface and point defects which act as the pinning centers in Ba122:Co films.

Transport properties of ultrathin BaFe1.84Co0.16As2 superconducting nanowires

Superconducting nanowire single-photon detectors (SNSPDs) have an absolute advantage over other types of single photon detectors except the low operating temperature. Therefore, many efforts have been devoted to find high-temperature superconducting materials that are suitable for preparing SNSPDs. Copper-based and MgB2 ultra-thin superconducting nanowires have been already reported. However, the transport properties of iron-based ultra-thin superconducting nanowires have not been studied. In this work, a 10 nm thick 200 nm wide 30 {mu}m long high quality superconducting nanowire was fabricated from ultrathin BaFe1.84Co0.16As2 films by a lift-off process. The precursor BaFe1.84Co0.16As2 film with a thickness of 10 nm and root-mean-square roughness of 1 nm was grown on CaF2 substrates by pulsed laser deposition. The nanowire shows a high superconducting critical temperature Tzero c=20 K with a narrow transition width of Delta T=2.5 K and exhibits a high critical current density Jc of 1.8E7 A/cm2 at 10 K. These results of ultrathin BaFe1.84Co0.16As2 nanowire will attract interest in electronic applications, including SNSPDs

Optimization of Deposition Conditions to Grow High-Quality Fe–Se–Te Thin Films

High-performance Fe-Se-Te (FST) films are favorable for revealing the relationships between the crystal structure and the superconducting properties of the material because of their simple structures. Therefore, it is particularly important to prepare high-quality “11” thin films. Herein, we focused on the thickness and deposition temperature dependence of T_c^zero and found that the thickness of about 200 nm and the deposition temperature of 300 °C are the optimal parameters for growing high-quality FST films on CaF₂ and SrTiO3 substrates, which exhibit T_c^zero of 18.8K and 17 K, respectively. Magnetic J_c values of FST films grown on CaF₂ and SrTiO₃ are both larger than 2 × 10⁶ A·cm^-2 at 4.2 K and self-field, significantly higher than that of FST single crystals.