Zili Zhang,Min Tian,Lin Ma,Min Liu,Hongli Suo,Qiuliang Wang
We have established a novel reaction process using Mg vapour and B powder (a gas–solid reaction) to synthesize MgB2 powder with a low oxygen content. For the first time, we tried to eliminate the oxygen in Mg powder with a specially designed furnace tube that can keep the sample under an inert atmosphere throughout the entire synthesis process. The duration of heat treatment has a conspicuous effect on the phase formation of MgB2 which differs greatly from the conventional solid state reaction method. At short durations such as 1 h, unreacted B powder is ubiquitous in the sample. Full formation of MgB2 is achieved at 3 h. At longer times such as 4 h, MgB2 decomposes to form MgB4. It is deduced that the main reason for the unstable MgB2 phase formation in the gas–solid reaction is the Mg vapour present during the entire process. A feasible chemical reaction model of the novel gas–solid reaction has been established. This work provides an important foundation to obtain almost oxygen-free MgB2, which can finally resolve the mystery of the effect of oxygen on superconductivity in this system.
Organic acid doping can enhance the current-carrying capacity of MgB 2 superconductors but may induce cracking and fracture due to microstructural degradation. In this work, MgB 2 bulks with different malic acid doping levels were investigated by combining experimental characterization with cohesive zone model-based RVE simulations to clarify the competition between flux-pinning enhancement and porosity-induced degradation. Increasing malic acid content promoted carbon substitution and lattice distortion, as indicated by the contraction of the MgB 2 lattice parameter a from 3.083 to 3.074 Å and the increase in lattice strain from 0.00202 to 0.00263. The 10 wt% sample exhibited the highest magnetic critical current density among the doped samples, reaching 1.9 × 10 4 A cm -2 at 20 K and 2 T. However, the porosity increased from approximately 6-7% at low and intermediate doping levels to 11% at 10 wt%, leading to stronger stress concentration and intergranular damage. The grain-boundary damage variable D GB decreased to 0.273 at 5 wt% but increased to 0.436 at 10 wt%, indicating a transition from second-phase-assisted crack suppression to porosity-dominated degradation. These findings provide a theoretical basis for managing sintering-induced damage in MgB 2 superconductors, highlighting the balance between performance enhancement and structural integrity during organic doping.
Suo Hongli,Yu Chenfei,Zhang Zili,Zhou Benzhe,Wang Lei,Liu Jianhua,Wang Qiuliang
Magnesium diboride (MgB 2 ) bulks, as a superconducting material with a relatively high superconducting transition temperature, low raw material cost, absence of weak-link effects, and flexible fabrication routes, have shown great potential for applications in various fields. Among the available fabrication methods, the gas-solid method has attracted considerable attention due to its ability to significantly reduce the MgO impurity content in the samples. However, cracking is prone to occur during the gas-solid reaction process, which adversely affects the structural integrity of the bulks. The introduction of molds can effectively suppress crack formation and improve the shaping quality of MgB 2 bulks. Nevertheless, the use of molds also reduces the diffusion rate of Mg vapor, resulting in a slower reaction process and consequently lower production efficiency. In this work, the optimal sintering time for MgB 2 bulk fabrication via the mold-assisted gas-solid method is optimized, thereby reducing experimental cost. A series of MgB 2 bulk samples with sintering times ranging from 12 h to 72 h were fabricated using the mold-assisted gas-solid method. The results show that all samples are dominated by the MgB 2 phase with a small amount of MgO impurity. With increasing sintering time, the grain size gradually increases, mechanical properties such as microhardness are significantly enhanced, and electrical resistivity decreases. Meanwhile, the A f , J c , F p , and H irr are all improved. Nano-CT results reveal that Mg vapor diffuses from the exterior to the interior of the boron compact and reacts progressively. The internal defects evolve from dispersed pores at the early stage of sintering to planar cracks at later stages. Among all samples, the one sintered for 60 h exhibits the optimal comprehensive performance, indicating that 60 h is the optimal sintering time for the mold-assisted gas-solid fabrication of MgB 2 bulks. This work provides important experimental evidence and theoretical guidance for further optimization of the fabrication process, microstructural control, and performance enhancement of MgB 2 bulk materials.
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