Papers by Keyword: MgB₂

Abstract: Considerable progress to synthesize magnesium diboride (MgB₂) films by electron-beam annealing has been made. A series of MgB₂ films with a T_c higher than 35 K had been fabricated. MgB₂ film superconductivity is affected by electron-beam accelerating voltage, beam current, annealing duration, beam spot size and Mg/B ratio. In order to fabricate better MgB₂ films, these influence factors must be intensively studied. In this paper, the influence of e-beam current on superconductivity was investigated with an accelerating voltage of 32 kV, an annealing time of 0.26 s and different annealing beam currents of 9.9 mA, 10.7 mA, 12.8 mA, 13.3 mA and 14.0 mA. The results show the MgB₂ film with 12.8 mA has the highest T_c and the densest structure. Its zero-field J_c at 15K has reached 3.2×10⁶ A/cm². The variation of the film superconducting properties with the beam currents was grasped, which will lay the foundation for the fabrication of high quality MgB₂ thin films

Abstract: It is well known that SiC doping in superconducting MgB₂ improves the upper critical magnetic field (B_c2) and the critical current density (J_c) under high magnetic field. However, the relationship between SiC doping and the flux pinning mechanism has not been clarified. In this study, several MgB₂ samples with and without SiC doping were prepared by the conventional in situ powder-in-tube method. The critical current densities and the force-displacement characteristics of fluxoids in samples were investigated by an ac inductive measurement (Campbell’s method). The Labusch parameter (α_L) and the interaction distance (d_i) were estimated from the obtained force-displacement profile. It was found that SiC doping enhances the values of α_L, but does not change the characteristics of the magnetic field dependence of α_L apparently. Namely, α_L vs. B^3/2 characteristics in the pure samples and SiC doped samples are almost the same. Such a result of α_L properties implies that the pinning mechanism in the SiC doped samples could be consistent with the conventional pinning theory. On the other hand, d_i, which is considered to be proportional to the size of pinning potential, decreases rapidly with increasing magnetic field, especially in the pure samples. For high magnetic field region, the variations of d_i were deduced to be caused by flux creep. The depth of pinning potential, U₀, was estimated by using the values of α_L and d_i. The values of U₀ give evidence of that SiC doping can prevent the flux bundles moving to another pinning center under high magnetic field.

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: In this paper, we have prepared a series of Fe sheathed MgB₂ tapes with SiC, Malic acid and C₉H₁₁NO mono-and co-doping by in-situ powder-in-tube (PIT) method, and studied the effects of various C sources co-doping on transport critical current density and flux pinning of MgB₂/Fe tapes. The results suggest by various C sources co-doping, the amount of C substitution for boron largely increase, comparing to SiC, Malic acid or C₉H₁₁NO mono-doped sample. For MgB₂/Fe tapes with 5 mol% SiC+10 wt% Malic acid and 4 wt% C₉H₁₁NO+10 wt% Malic acid co-doping, the transport J_c at 4.2 K and 10 T are 14.8 and 13.5 kA/cm², respectively, which are clearly higher than the J_c of these dopants mono-doped samples. Furthermore, it is found that at the 4 wt.% C₉H₁₁NO doped MgB₂, the poor in-field J_c should be attributing to the cracks at grains boundary, which results in the bad grains-connectivity.

Abstract: A series of MgB₂ bulks with different porosity have been successfully prepared by in-situ solid-state reaction at ambient pressure with a toluene-treatment boron powder. The mass density reaches 1.86 g cm^-3 at the most compact MgB₂ bulk, which has an imporous microstructure, excellent grains coupling, clean grain boundaries and nanosized grains (100~200 nm). Our results reveal that the porosity of MgB₂ can be significantly depressed by tuning the residual toluene content and heat treatment temperature in this way. Due to the degradation of porosity and thus the enhancement on grains connectivity and grains boundary pinning, the critical current density, J_c and irreversible fields Birr of MgB₂ are significantly improved. For the best sample, at 20K, 4T and 10K, 6T, the J_c reach 2400 and 3700 A cm^-2, which is higher than normal porous sample by a factor of 20 and 8, respectively; and the Birr at 20 K reaches 5.10 T.

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 present the fabrication, structural, and transport properties of MgB2-based ceramic composites with magnetic nanospheres fabricated by spark plasma sintering. The nanospheres are either carbon–encapsulated iron or iron oxide. The former nanospheres have been prepared by laser pyrolysis whereas the iron oxide was obtained by the pyrolysis of the polysiloxane-based copolymers embedded into MgB2 matrix during the sintering process. The structural data show the shrinkage of the a-axis lattice constant as a result of the partial carbon substitution for boron. However, the transport data suggest that carbon diffusion is limited to the outer layer of the MgB2 grains in both cases.

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: Superconductor samples Mg(B_1-2x(SiC)_x)₂ (x=0, 5%, 10%) are synthesized from nano SiC, Mg and amorphous boron powders by microwave direct synthesis in a short time. Powder X-ray diffraction (XRD) analysis indicates that the phases of the synthesis sample are MgB₂ (major phase) and a small amount of MgO and Mg₂Si. The main peaks of MgB₂, (100), (101), (002) and (110) are shift to the higher diffraction angle position and the width of half height of the diffraction plane is broaden for the SiC doping Mg(B_1-2x(SiC)_x)₂, which show that the B positions of MgB₂ are partly substituted and the grains of MgB₂ are fine. Scanning electron microscope (SEM) observation shows that the MgB₂ grain size is very small and the sample is tightness (compact). The onset superconducting transition temperature of the Mg(B_1-2x(SiC)_x)₂ (x=0, 5%, 10%) samples measured by magnetization measurement are about 37.6 K, 37.0 K, 36.8 K respectively. The critical current density J_c are calculated according to the Bean model from the magnetization hysteresis loop of the slab Mg(B_1-2x(SiC)_x)₂ (x=0, 5%, 10%) samples. The critical current density Jc of nano SiC doping Mg(B_1-2x(SiC)_x)₂ samples are greatly enhanced. In higher external magnetic field, the J_c of 10% SiC doped sample is the highest; in lower external magnetic field, the Jc of 5% SiC doped sample is the highest; while in the whole external magnetic field, the J_c of undoped sample is the lowest.