Yongchang Liu

Low-temperature synthesis of MgB2 superconductors

Abstract MgB2 superconductors have the outstanding potential to be integrated into diverse commercial applications. However, the critical current density Jc in MgB2 is still smaller compared with expectations for an optimised material in these applications. Various flux pinning mechanisms are introduced into MgB2 superconductors to improve Jc by different methods, including irradiation, chemical doping and ball milling. On the other hand, these methods mainly focused on the enhancement of pinning, but always neglect or even worsen the grain connectivity, which limits the further improvement in Jc. Recently, the low-temperature synthesised MgB2 exhibits improved Jc due to the increases in both the grain connectivity and flux pinning. Ball milling pre-treatment of original powders, usage of different Mg-based precursors and the additions of different metal or alloys are employed to enhance the sintering efficiency of MgB2 at low temperature. Among them, the minor metal or alloy additions are proved to be the most convenient, effective and inexpensive way in accelerating the fabrication of MgB2 superconductors at low temperature. Combining the advantages of metal-activated sintering and carbon doping, it is also proposed that the low-cost MgB2 superconductors with further improved Jc will be rapidly synthesised at low temperature with metal and carbon-based chemical co-doping.

Investigation of phase composition and nanoscale microstructure of high-energy ball-milled MgCu sample

The ball milling technique has been successfully applied to the synthesis of various materials such as equilibrium intermetallic phases, amorphous compounds, nanocrystalline materials, or metastable crystalline phases. However, how the phase composition and nanoscale microstructure evolute during ball milling in various materials is still controversial due to the complex mechanism of ball milling, especially in the field of solid-state amorphization caused by ball milling. In the present work, the phase evolution during the high-energy ball milling process of the Mg and Cu (atomic ratio is 1:1) mixed powder was investigated. It was found that Mg firstly reacts with Cu, forming the Mg2Cu alloy in the primary stage of ball milling. As the milling time increases, the diffracted peaks of Mg2Cu and Cu gradually disappear, and only a broad halo peak can be observed in the X-ray diffraction pattern of the final 18-h milled sample. As for this halo peak, lots of previous studies suggested that it originated from the amorphous phase formed during the ball milling. Here, a different opinion that this halo peak results from the very small size of crystals is proposed: As the ball milling time increases, the sizes of Mg2Cu and Cu crystals become smaller and smaller, so the diffracted peaks of Mg2Cu and Cu become broader and broader and result in their overlap between 39° and 45°, at last forming the amorphous-like halo peak. In order to determine the origin of this halo peak, microstructure observation and annealing experiment on the milled sample were carried out. In the transmission electron microscopy dark-field image of the milled sample, lots of very small nanocrystals (below 20u2009nm) identified as Mg2Cu and Cu were found. Moreover, in the differential scanning calorimetry curve of the milled sample during the annealing process, no obvious exothermic peak corresponding to the crystallization of amorphous phase is observed. All the above results confirm that the broad halo diffracted peak in the milled MgCu sample is attributed to the overlap of the broadened peaks of the very small Mg2Cu and Cu nanocrystalline phase, not the MgCu amorphous phase. The whole milling process of MgCu can be described as follows: Mg+Cu→Mg2Cu+Cu→Mg2Cunanocrystal+Cunanocrystal.

The synthesis of lamellar nano MgB2 grains with nanoimpurities, flux pinning centers and their significantly improved critical current density

MgB(2) superconductors with unique microstructures were rapidly fabricated at low temperatures, and exhibited significantly improved critical current density (J(c)). According to the microstructure observations, the prepared samples consisted of lamellar nano MgB(2) grains with many embedded nanoimpurities (about 10 nm). The formation of these lamellar nano MgB(2) grains is associated with the presence of a local Mg-Cu liquid at sintering temperatures as low as 575 °C. The ball milling treatment of the original powders also plays a positive role in the growth of lamellar grains. Based on an analysis of the relationship between resistivity and temperature, the lamellar nano MgB(2) grains in the prepared sample possess better grain connectivity than the typical morphology of MgB(2) samples prepared by traditional high-temperature sintering. Furthermore, the presence of many nano MgB(2) grain boundaries and nano impurities in the prepared sample can obviously increase the flux pinning centers in accordance with the analysis of flux pinning behavior. Both factors mentioned above contribute to the significant improvement in J(c) from low field to relative high field. The method developed in the present work is an effective and low-cost way to further enhance J(c) in MgB(2) superconductors across a wide range of applied magnetic fields without using expensive nanometer-sized dopants.

Influence of Premilling Time on the Sintering Process and Superconductive Properties of FeSe

A facile combinative method with mechanical alloying and solid-state reaction was adopted to produce tetragonal β-FeSe using Fe and Se powders. The powder mixtures were premilled for different times and then sintered at 750 ° C with a one-step method. The effect of synthesis parameters, microstructures, sintering process, and superconducting properties on FeSe bulks was well investigated. It indicated that mechanical alloying played an important role in the sintering process and morphologies of the FeSe bulk samples. The sintering process for the 50-h premilled sample changed a lot. From the differential thermal analysis measurements, the band of iron selenides splits into two peaks, of which the former exceeds the peak for Se fusion. In addition, the premilled sample for 20 h exhibited fine grains, whereas the 50-h sample exhibited fine stratified crystals. Meanwhile, the lattice parameters and the volume fraction of β-FeSe increased with the milling time. However, the slight fluctuation in lattice parameters and volume fraction led to slight variation in transition temperature Tc.