Zongqing Ma

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.

Influence of ball-milled amorphous B powders on the sintering process and superconductive properties of MgB2

The sintering process of MgB2 superconductors with milled amorphous B as precursor powder was investigated using thermal analysis, and the corresponding superconductive properties were measured. It was found that the presence of a B2O3 layer on the surface of milled B particles could effectively hinder the solid–solid reaction between Mg and B during the sintering process. Since the formation of MgB2 phase mainly completes after Mg melting, the size and homogeneity of MgB2 grains mainly depends on that of the B particles. As a result, the milling treatment of B powder could result in the formation of refined and homogeneous MgB2 grains, which are responsible for the excellent Jc performance in the prepared sample with briefly milled B powder. However, as the milling time was increased, the B2O3 impurity in the B original powder increased and resulted in an obvious increase of MgO impurity in the prepared sample. These excess MgO impurities led to a significant decrease in Jc at low fields in the sample prepared with the heavily milled B powder. On the other hand, they could also serve as effective flux pinning centers due to their nanometer size and lead to an improvement in the Jc of the samples prepared with the heavily milled B powder.

Mechanism analysis for the enhanced electromagnetic properties in nano-SiC-doped MgB2 based on the discussion of the sintering process

Combined with the thermal analysis and phase identification, the sintering process of nano-SiC-doped MgB2 samples was systemically investigated. Accordingly, a new consideration for the mechanism of enhanced electromagnetic properties of nano-SiC-doped MgB2 is proposed, which is more consistent with the observed experimental results of nano-SiC-doped MgB2 samples sintered at different temperatures and has many advantages over the previous model in explaining the experimental observations.

MgB2 superconductors with abnormally improved Jc sintered after autoxidation of milled original powders

An autoxidation treatment of short-time milled original powders was introduced for the synthesis of MgB2 superconductors and the critical current density Jc of the sintered MgB2 bulks was measured. It is unusually found that the undoped MgB2 bulks sintered with those autoxided milled original powder exhibit abnormally excellent Jc (above 1×104 A cm−2 even at 3.5 T, 20 K). Combined with the investigation of sintering process, it was found that the autoxidation treatment of the milled powders affects the subsequent sintering process dramatically and finally leads to the formation of MgB2 nanocrystalline with lots of dislocation and self-generated MgO nanoinclusions embedded in them. This unique microstructure brought up a significant improvement of Jc at high fields. Besides, the formation mechanism of this unique microstructure during the sintering process was also discussed in detail. It suggested that the MgO preformed by the reaction between Mg and B2O3 in the interface between Mg particles and B partic...

The accelerated formation of MgB2 phase with high critical current density by Cu and SiC multidoping during the low-temperature sintering process

Combined with the thermal analysis and phase identification, it is found that during the low-temperature sintering process of the SiC-doped MgB2 samples, the Cu addition can improve both the reaction between Mg and B and the reaction between Mg and SiC and thus accelerate the formation of the MgB2 phase with effective C substitution for B. Accordingly, we successfully synthesize MgB2 bulks with excellent critical current density (Jc) by proper amount of Cu and SiC multidoping sintered at 575 °C for only 5 h.

The acceleration of low-temperature sintering of MgB2 bulks with high critical density by minor Sn doping

In the present study, MgB2 bulk was successfully synthesized by Sn-activated sintering at 600 °C for just 5 h, exhibiting a dramatic decrease in the sintering time compared to that required for sintering an undoped sample at the same temperature. According to the thermal analysis, a local Mg–Sn liquid formed first during the sintering process and then this prompted the diffusion of Mg into B. As a result, the solid–solid reaction between Mg and B was accelerated and the phase formation of MgB2 was completely achieved at low temperature. The complete formation of the MgB2 phase results in a high Jc of the Sn-doped samples sintered at low temperature. Besides this, the small MgB2 grains and nanoimpurities in the sintered samples also contributed to the good performance as regards Jc.

Evaluation of persistent-mode operation in a superconducting MgB2 coil in solid nitrogen

We report the fabrication of a magnesium diboride (MgB2) coil and evaluate its persistent-mode operation in a system cooled by a cryocooler with solid nitrogen (SN2) as a cooling medium. The main purpose of SN2 was to increase enthalpy of the cold mass. For this work, an in situ processed carbon-doped MgB2 wire was used. The coil was wound on a stainless steel former in a single layer (22 turns), with an inner diameter of 109 mm and height of 20 mm without any insulation. The two ends of the coil were then joined to make a persistent-current switch to obtain the persistent-current mode. After a heat treatment, the whole coil was installed in the SN2 chamber. During operation, the resultant total circuit resistance was estimated to be <7.4 × 10−14 Ω at 19.5 K ± 1.5 K, which meets the technical requirement for magnetic resonance imaging application.

Variation of pinning mechanism and enhancement of critical current density in MgB2 bulk containing self-generated coherent MgB4 impurity

Bulk MgB2, with self-generated MgB4 pinning centers, have experienced two-step sintering process, initially at 750 °C and then 900–1000 °C. On the contrary to the widely accepted point that MgB4 deteriorates superconductivity, it was found that MgB4 played a significant role in enhancing critical current density. The precipitation pattern of MgB4 was studied from the lattice scale images. It was observed that the initial coherent relation between the MgB4 and the matrix was destroyed to become semi-coherent and even incoherent as the second-step sintering temperature increased. Owing to the lattice distortion caused by the elastic accommodation of the coherent interface, the small-sized MgB4 particles controlled by the sintering temperature, and the fine grain connectivity affected by the porosity, the critical current density was improved over the entire magnetic field. Finally, the dominating pinning mechanism within the crystal was confirmed to be Δκ pinning in the two-step sintered MgB2 sample, where ...

Effect of microstructure variation on the corrosion behavior of high-strength low-alloy steel in 3.5wt% NaCl solution

The effect of microstructure variation on the corrosion behavior of high-strength low-alloy (HSLA) steel was investigated. The protective property of the corrosion product layer was also explored. Experimental results reveal that the type of microstructure has significant effect on the corrosion resistance of HSLA steel. The measurement results of weight loss, potentiodynamic polarization curves, and electrochemical impedance spectroscopy indicate that the steel with acicular ferrite microstructure exhibits the lowest corrosion rate. Martensite exhibits a reduced corrosion resistance compared with polygonal ferrite. It is found that the surface of the acicular ferrite specimen uniformly covered by corrosion products is seemingly denser and more compact than those of the other two microstructures, and can provide some amount of protection to the steel; thus, the charge transfer resistance and modulus values of the acicular ferrite specimen are the largest. However, corrosion products on martensite and polygonal ferrite are generally loose, porous, and defective, and can provide minor protectiveness; thus, the charge transfer resistance values for polygonal ferrite and martensite are lower.

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 20 nm) 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.