Xifeng Pan

Phase formation and superconducting properties of mechanically alloyed Nb3(Al1−x Ge x ) system

Mechanically alloyed Nb3(Al1−x Ge x ) compounds with nominal composition x = 0%, 10%, 15%, 20%, 25% and 30% were synthesized using high-energy ball milling. The effects of Ge content and sintering temperature on the formation of the Nb3Al superconducting phase at a relatively low temperature without the extremely high-temperature rapid heating, quenching and transformation process were studied. The results revealed that Ge doping in Nb3Al improved the formation of the A15 phase at low temperatures, enhanced the superconducting transition temperature (T c), and refined the grain structure, thus improving the overall superconducting properties. The pinning behavior was also studied for the optimized sample.

Manufacture, electromagnetic properties and microstructure of an 18-filament jelly-roll Nb3Al superconducting wire with rapid heating and quenching heat-treatment

In this paper, we have reported the manufacture of a novel simple-structured jelly-roll Nb3Al precursor long wire, and its electromagnetic properties and microstructure with different rapid heating and quenching (RHQ) heat-treatments. By comparing three processing methods, it is found that the rolling and drawing (RD) method is more suitable to the fabrication of kilometer-length Nb3Al precursor wire without annealing. Using homemade RHQ equipment, we have successfully carried out RHQ heat-treatment of Nb3Al wire samples with various heating conditions. Based on magnetization and magnetoresistivity measurements, the onset superconducting transition temperature, T c and upper critical field, H c2 (0) of optimal Nb3Al wire reach 17.9 K–18.0 K and 29.7 T, respectively. Through microstructure and composition analysis, the Nb3Al superconductor in the optimal wire displays a typical layer-to-layer structure, which comprises alternate crystalline Nb3Al and amorphous Nb layers, and the Nb3Al grains sizes are about 100 nm–300 nm. Furthermore, many small holes are dispersed in the Nb3Al superconductor due to the diffusion reaction from Al to Nb site. The work suggests the simple-structured Nb3Al precursor wire with RHQ heat-treatment is very promising for high-field application.

Preparation of Nb3Al by high-energy ball milling and superconductivity

The A15 phase superconductor Nb3Al has been considered as an alternative to Nb3Sn for high field and large scale applications. However, to prepare a stoichiometric Nb3Al with fine grain structures is very difficult. High-energy ball milling is a solid state powder processing technique and is a very useful for preparing Nb-Al alloys (Nb3Al). The effects of ball milling time and annealing temperature on the formation of Nb3Al superconducting phase have been studied. Pure Nb and Al powders with stoichiometric ratio of Nb3Al were mixed and milled, and the charging and milling were performed in an inert atmosphere. Phase formation and structural evolution during high-energy ball milling have been examined by X-ray diffraction. Al disappeared and Nb peaks broadened after about one hour of milling. With increasing milling time, the peaks of Nb became considerably broader and intensities decreased, the Nb-Al solid solution phase was extensive when milled about 3 hours. In order to obtain Nb3Al superconducting phase, a subsequent anneal was required. We have annealed the as-milled powders at 800-900°C for different times to prepared Nb3Al superconducting alloy. The results indicated that Nb3Al with small amount of impurity phase can be obtained on annealing the Nb-Al solid solution phase and the superconducting transition temperature was about 15K, but it is difficult to obtain a homogeneous Nb3Al phase by annealing the amorphous powder.

Combined Effect of Upper Critical Field and Flux Pinning on Enhancing Critical Current Density of In-situ MgB2/Fe Tapes with Various Carbon Sources Simultaneously Doped

MgB2/Fe tapes with various forms of mono- and co-doped C sources were prepared by an in-situ powder-in-tube (PIT) method to study the effect of different C sources on the transport critical current density Jc behavior of MgB2. A remarkable Jc improvement at 4.2 K under higher fields and a significant increase in the actual content of C substitution for B were achieved for the tapes with various C sources co-doped. For these co-doped tapes, both enhanced Hc2 and flux pinning properties are responsible for their Jc improvement in this work. Comparing the 4 wt % C9H11NO doped tape with the 20 wt % malic acid doped tape, the latter possesses much better in-field Jc performance at 4.2 K and 10 T and sharper field dependence of Jc, which suggests that the flux pinning plays a crucial role in determining its Jc behavior.

The microstructure and properties of J-R Nb3Al superconducting wires fabricated by diffusion method at an intermediate temperature

The precursor Nb-Al strands were fabricated by the method of Jelly-Roll. The effects of sintering temperature and time on the diffusion degree and superconducting property of the binary Nb-Al system were investigated under isothermal heat treatment at 1100-1500 °C. The results show that the Tc of superconducting wires increased with the heat-treatment temperature rising at the isothermal region. Excessive heat-treatment time led to the decrease of the properties of Nb3Al wires under a certain heat-treatment temperature. The magnetic analysis results showed that the optimal Tc of J-R Nb3Al wires was about 16.8 K at a teat-treatment temperature of 1500 °C for 3 h.

A potential route to synthesize imporous MgB2 bulks by pretreatment of B powder at ambient pressure

Imporous MgB2 bulks with a density of 1.82xa0gxa0cm−3 have been synthesized by pretreatment of B powder in an in situ solid-state reaction at ambient pressure. The results show that the MgB2 with B powder pretreatment has a significant improvement in Jc, Birr, microstructure and intergranular coupling, but with no decrease of Tc. At 20xa0K and 4xa0T, the Jc is enhanced by 4 times by the pretreatment of the B powder. For the B pretreated MgB2, the Birr at 20xa0K reaches 5xa0T and Jc at 10xa0K and 6xa0T reaches 2200xa0Axa0cm−2, compared to 4.2xa0T and 670xa0Axa0cm−2 for the B-not-pretreated MgB2. It is argued that the small amounts of highly dispersed carbon in B powder may enhance the mobility of B particles during the reaction of B and Mg, which avoids the formation of voids in the positions of Mg particles.