Feng Qing-rong

Deposition of MgB2 Superconducting Films on Different Metal Substrates

By a method of hybrid physical-chemical vapour deposition (HPCVD) on three metal substrates of stainless steel, copper and niobium, we deposit MgB2 superconducting films over 1 μm thickness. All of them have zero resistance temperatures TC(0)>36 K and critical current densities JC (10 K, 0 T) >106 A/cm2. Meanwhile, in the bending test, all the MgB2 superconducting films adhere strongly to the metal substrates without peeling off. Therefore, the MgB2 superconducting films supplied by the HPCVD method exhibit preferable electrical, magnetic and mechanical properties, and have potential applications in future.

Resistivity?temperature characteristics of sol gel YBa2Cu3Oy samples synthesized in flowing oxygen atmosphere

The relationship of resistivity versus synthesizing temperature of sol gel YBa2Cu3Oy samples was studied when prepared under flowing oxygen conditions. A set of high-temperature ρ–T curves was obtained for the whole process. After the sample finished the test measuring, its resistivity was ρ300=9.83 ×10−3Ω.cm at room temperature. The ρ–T curve also showed that the orthorhombic–tetragonal phase transformation of sol-gel YBa2Cu3Oy sample occurred at 581 degrees C for the sample in the rising temperature process, but at 613 degrees C in the cooling process, lower than that of the samples made by using the conventional powder metallurgy methods.

MgB2 thick films with remarkable ductility on stainless steel substrate

We fabricated several superconducting MgB2 thick films on stainless steel (SS) substrates by using hybrid physical-chemical vapor deposition (HPCVD) technique. The thickness was in the 10μm to 20μm range, and the onset critical transition temperature Tc (onset) and the width of the superconducting transition (ΔT) were about 37.8 and 1.2 K. They were dense and textured along (101) direction with high tenacity, despite the existence of a little amount of MgO and Mg. We bent the films at different degrees and studied the ductility and transport properties of these MgB2 thick films under applied force. The results demonstrated that the superconducting properties of these thick films, prepared by HPCVD, stay almost unaffected even with the films bent to a large degree with a curvature of 0.5 mm. This indicated that the superconducting wires or tapes of MgB2 with a core of SS had the advantages of avoiding rigidity and brittleness in industrial handling. The technique of HPCVD has, therefore, a high application potential.

MgB2 thick films deposited on stainless steel substrate with Tc higher than 39 K

Thick MgB2 (magnesium diborate) films, ∼10 μm, with Tc (onset) = 39.4 K and Tc (zero) = 39.2 K have been successfully grown on a stainless steel substrate using a technique called hybrid physical-chemical deposition (HPCVD). The deposition rate is high, ∼6.7 nm/s. The X-ray diffraction (XRD) indicates that it is highly (101) and c-axis oriented. The scanning electron microscope (SEM) images demonstrate that the film grown is in “island-mode”. The uniform superconducting phase in the film is shown by the M-T measurement.

A Study on the Components of MgB2 Thick Film Prepared via HPCVD

Superconducting MgB2 thick film has been prepared via hybrid physical-chemical vapor deposition method on Al2O3 (0001) substrate by using B2H6 and magnesium ingot as raw materials reacted from 730 to 830°C for 40 min under 20 to 30 kPa. Its thickness is about 40 µm. The MgB2 thick film shows Tc (onset) = 39.0 K and Tc (0) = 37.2 K. X-ray diffraction pattern shows that the film grown along (101) direction has small amount of impurities of Mg and MgO. Scanning electron microscopy and energy dispersive X-ray spectroscopy indicated that these impurities existed indeed and were Mg rich. The MgO film was formed on the surface of the MgB2 thick film to further protect the sample from oxidation. We presented a new mechanism for the formation of the thick film.

Thermally Activated Flux Motion in 2223 Phase High Temperature Superconductors

Isothermal V-I characteristics of Bi1.8Pb0.2Sr2Ca2Cu3Oy high temperature superconductors (HTSCs) have been systematically studied. Below the irreversibility temperature Tr(H), two important features were observed: 1) Both positive and negative curvatures can coexist in the same isothermal V-I curve, so it is different from the scaling behavior V ∝ exp(-(J0/J)μ). 2) The activation volume of flux motion is found temperature dependent as (1-T/Tr(H))-2ν with ν about 0.5. This behavior supports the existence of a two-dimensional secondary phase transition below Hc2(T) in this Bi-based HTSCs.

Solution Fabrication of a Superconducting MgB2 Coated Conductor on Stainless Steel

We report the solution fabrication of a MgB2 coated conductor on a stainless steel substrate. The precursor solution of Mg(BH4)2 diethyl ether is initially synthesized by refluxing the milled mixture of NaBH4 and MgCl2 in diethyl ether. Then the Mg(BH4)2 diethyl ether is spin coated on a stainless steel substrate and annealed in Mg vapor, which yields a homogeneous MgB2 coated conductor. X-ray diffraction indicates that the grown MgB2 coated conductor is polycrystalline. It has a superconducting transition temperature of 34–37 K. The slope of the upper critical field HC2 increases with decreasing temperature, and the extrapolated value of HC2(0) reaches ~28 T. The critical current density estimated by the Bean model is JC(25K, 0T)~106 A·cm−2. These parameters indicate that the solution method is potentially able to produce MgB2 coated conductors that can satisfy application purposes.

High-density MgB2 superconducting bulk samples prepared at ambient pressure

High-density MgB2 (HD-MgB2) superconducting samples (D ⩾ 2.2 g/cm3), using different sources of magnesium powder as raw material, were synthesized in ambient pressure in a rich Mg environment. The magnesium powders used in the fabrication process include nanometer-sized magnesium particles, powders from Alfa Aesar, ordinary off-the-shelf powder, and magnesium chip. The fabrication procedure involved a double-sintering process in a rich-Mg environment. A transition temperature Tc of 39 K was observed. Samples with the equally high density and matching superconducting properties were obtained as well by a triple sintering process of the MgB2 powder directly from Alfa Aesar.

Distinguish Different Factors Dominating Superconductivity in Y-Doped Bi2Sr2CaCu2Oy

We observed in Y-doped Bi2Sr2CaCu2Oy superconductors an interesting phenomenon, the Tc(onset) of the Y-doped samples is higher, but the Tc(end) is lower than that of Y-free samples. After annealing in N2 atmosphere, the Tc(end) is increased, but the Tc(onset) is hardly changed. After careful studying, by structure analysis, x-ray photoelectron spectroscopy, R - T relationship measurement, and Hall effect, we successfully distinguish the different factors that dominate the superconductivity, of which one is carrier concentration, others may possibly be structure change and charges redistribution.

Ge Film Contacts to Radio Frequency Sputtered Y-Ba-Cu-O Superconductor Film

The semiconductor-superconductor contact system of Ge/Y-Ba-Cu-O (YBCO)/SrTiO3 has been successfully fabricated by two steps of deposition. The YBCO film in the contact of Ge/YBCO did not lose its superconductivity and its Tc (on set) did not degrade as well. The top layer of Ge was polycrystalline with preferred orientation corresponding to the epitaxial structure of the underlayer of YBCO.