Papers by Author: Qing Rong Feng

Abstract: A novel method to fabricate Magnesium diboride ( MgB₂) film by electron beam annealing was presented. The MgB₂ thin films could be prepared in a second or sub-second without any toxic diborane gas, extra Mg vapor or argon gas. The method has the advantages of short formation time and high efficiency. Based on this, the relationship between MgB₂ conductivity and electron beam annealing duration was investigated experimentally with an accelerating voltage of 40 × 10³ V, a beam current of 3×10³ A and different annealing durations of 0.1796s, 1860s, 0.2108s, 0.2200s and 0.2332s. The experimental results showed the MgB₂ film with 0.2200s has the highest T_c^onset , the most dense structure and the strongest diamagnetic signal. Its zero-field Jc at 5 K was 5.0 × 10⁶ A/cm². Also the variation of the film superconducting properties with the annealing duration was found, it will provide an important reference for the preparation of high-quality MgB₂ thin films.

Abstract: Ultrathin MgB₂ film is essential for the fabrication of MgB₂ superconducting single photon detectors (SSPDs). In this paper, we prepared 20nm and 10nm MgB₂ film using ex-situ annealing of Mg-B multilayer method. The precursor films were prepared by electron beam evaporation. A flowing Mg vapour and H₂ was introduced in the annealing process to keep MgB₂ thin film thermodynamically stable. The annealing temperature was between 680 and 740 and annealing time was 1-10min. 20nm MgB₂ films on MgO(111) substrates had the critical temperature (T_c) of 32.2K. The films grew along c-axes direction. As the substrate changed to SiC(001) and Al₂O₃(001), T_c decreased to 30.3K and 10.2K respectively. For 10nm MgB₂ film on SiC(001) substrate, T_c was 24.2K. The self-field critical current density for 10nm and 20nm film on SiC(001) substrate was 2.1×10⁶A/cm² and 2.3×10⁶A/cm², respectively. AFM image showed that the film had a flat surface with mean roughness of 0.899nm for 10nm MgB₂ film.

Abstract: Niobium with the transition temperature of 9 K is the usual material for TESLA superconducting accelerate cavity. The cavity must work at the temperature range of of 2~1.8 K, which would consume large energy. In addition, the low upper critical field is one of reasons causing the problem of accelerate cavities Q-slop. Comparing with Nb material, MgB₂ has several advantages in this application, such as higher Tc of ~39 K, higher upper critical field and the possible lower microwave loss, which would help increasing accelerating gradient and saving operation cost. Because of these benefit, more and more focus is put on this style of accelerating cavity, MgB₂-thick-film/metal-cavity-body. Using Hybrid Physical-Chemical Vapor Deposition method, the micronmeter-thick MgB₂ film on Nb substrate was prepared in the present investigation. The experimental results showed that Tc(0) reached 38.5 K and the Hc₂ was about 20 T. In order to test the films fracture toughness the film as the inner surface was bended. It was found that at small bending angle the influence on the superconducting properties was little. When the bend radius increased to 5 mm, some cracks smaller than 1 micron occured on the film surface. However, the film still attached to the Nb substrate and the Tc was as high as ~33.5 K, showing that the MgB₂ film fabricated by HPCVD had a good compact state with the Nb substrate and good mechanical toughness. These results indicate that the clean MgB₂ thick film has a better feature in the application of superconducting accelerate cavity.

Abstract: We have fabricated MgB2 thick films on SiC substrates growing along c axis by using hybrid physical–chemical vapor deposition (HPCVD) technique. The thickness was 8μm. Electric measurement showed that the Tc (onset) was 41.4K, and the transition width was 0.5K, the residual resistance ratio (RRR) was near 7. Magnetic measurement showed that the critical current density was 1.7×106A/cm2 at 5K in a self field.

Abstract: We have fabricated several superconducting MgB₂ thin films on stainless steel substrates by using hybrid physical-chemical vapor deposition (HPCVD) in pure argon atmosphere. These films were observed by scanning electron microscopes (SEM) and used the energy dispersive X-ray spectroscopy (EDX) to make elements analyses. The film thickness is about 800~1000 nm. There were some cracks on the film surface when the film is bent by different angle. The number of cracks and their width increased with the increasing bending angle. Nevertheless, the films were attached to the substrates firmly. It concludes that the superconducting MgB₂ thin films have great ductility and adhesion to the stainless steel substrates. We found in these films many granules about tens of nanometers in size. These nano-granules can balance both the inner structure and the surface activity of the MgB₂ crystal. This might be an important reason for the ductility observed with the superconducting thin films. The exact explanation depends on further research.

Abstract: The relationship of the resistivity versus synthesizing temperature of sol-gel YBa2Cu3O7-x samples, with Tc
91K, was studied while synthesizing in flowing oxygen atmosphere. A set of high temperature  -T curves were obtained for the whole process. After four rounds of synthesizing, the resistivity of the sample was =1.00×10-3cm at room temperature. The -T curve of the last round also showed that the orthorhombic to tetragonal phase transformation of the sample occurred around 600oC that is lower than the YBa2Cu3O7-x sample prepared in ambient atmosphere. Other measurements such as X-ray diffraction, SEM measurement, low temperature R-T and M-T measurement were also taken to get more information of these samples.

Abstract: We have successfully synthesized MgB2 thick films on 4H-SiC substrate by hybrid physical-chemical deposition (HPCVD). They have TC above 40 K, X-ray diffraction (XRD) shows the polycrystal bulk like structure. And JC estimated using magnetic hysteresis loops (MHLs) and Bean model is 7.4 MA/cm2 in self field, at 5 K.

Abstract: Poly-crystalline MgB2 films have been grown on niobium substrate by using the hybrid physical-chemical vapor deposition (HPCVD) with a thickness about 1.0 μm, which have good transport performance with Tc(0) ranged from 38.5 K to 38.9 K, ,T about 0.1-0.2 K. Films are composed of regular plate-shape hexagonal MgB2 crystallites and have lattice constants approaching the bulk values. For this sample, the bending test has been also performed, and it can be found that the film adhered on the substrate firmly after bending to 120º.

Abstract: Superconducting MgB2 films have been fabricated on sapphire substrate by the method of hybrid physical-chemical vapor deposition (HPCVD). The film thickness ranges from 75 nm to 3 μm. The investigation on the physical properties, such as the transition temperature, TC, the transition width,
T, and the critical current density, JC, has indicated that the optimum film thickness is about 1 μm. At this thickness, Tc(onset) = 40.3 K and 0Tc = 0.15 K. At T = 5 K without external field, JC has been estimated as 5.6×105 A/cm2according to the Bean model. The film quality begins to degrade as the thickness exceeds 1 μm. This work has demonstrated that HPCVD is a simple but effective technique for the application in the production of MgB2 wires.

Abstract: The thick MgB2 films have been successfully grown on the Cu substrate by the technique of hybrid physical-chemical vapor deposition (HPCVD). The films are about 2 ~ 3 μm and quite dense. They possess the Tc (onset), as high as 37-38 K, and sharp transition ~ 0.8 K. X-ray diffraction (XRD) indicates their polycrystalline character. The upper critical field at T=0K, HC2(0), is extrapolated as 15.3T. The controlled growth of MgB2 film on Cu substrate opens a new route in the preparation of MgB2 tape materials.