Comparative analysis of MgB2 superconducting wire for various cooling treatments through a sintering process

This paper presents a comparative analysis of MgB2 superconducting wire for various cooling treatments. The study investigates the effect of different cooling rates on the phase transformation, hardness, microstructure, and properties of the MgB2 wires. The samples were prepared by the powder-in-tube method using a mixture of magnesium and amorphous-boron powders through the sintering and hot rolling process The wires were cooled down at three different conditions, followed by characterization using X-ray diffraction, scanning electron microscopy, and hardness measurements. The results show that the cooling rate after sintering significantly impacted the microstructure and hardness strength of the MgB2 wires. A slower cooling rate led to a denser microstructure and higher hardness properties, while a faster cooling rate resulted in a more porous microstructure and lower hardness. However, it can be recommended further deformation for smaller wire sizes. These findings could provide insights into optimizing the sintering process to produce high-performance MgB2.


Introduction
Superconducting materials have revolutionized various fields of science and technology, including but not limited to medical imaging, transportation, and energy storage.Among the various superconducting materials, magnesium diboride (MgB2) has emerged as a promising candidate for high-temperature superconducting (HTS) applications due to its relatively high critical temperature (Tc) of approximately 39 K and simple crystal structure [1].The potential of MgB2-based superconductors has led to extensive research on their synthesis and properties, especially in the form of wires that can be used for practical applications [2].
One of the critical factors affecting the properties of MgB2 wires is the cooling treatment used during the sintering process.Sintering is a process of heating a powdered material below its melting point to form a solid mass by bonding the particles together.In the case of MgB2 wires, sintering is an essential step in the fabrication process, where the powdered MgB2 is compacted into a wire shape and heated to high temperatures to form a solid mass [3].
Various cooling treatments, including furnace cooling, quenching, and slow cooling, have been employed to optimize the properties of MgB2 wires.Furnace cooling involves allowing the material to cool down naturally inside the furnace after sintering.Quenching, on the other hand, involves rapid cooling of the material using a coolant such as liquid nitrogen or helium.Slow cooling involves allowing the material to cool down slowly in the air or under controlled conditions.
Several studies have investigated the effect of cooling treatment on the properties of MgB2 wires.For instance, Shcherbakova et al. reported that to obtain the best superconducting properties for pure 1291 (2023) 012019 IOP Publishing doi:10.1088/1757-899X/1291/1/012019 2 and MgB2 wires with SiC addition, cooling rates must be considered and will affect to mechanical properties [4][5] [6].The objective of this research is to study the effect of the cooling method after the hot rolling process to form a single-phase superconducting MgB2 with minimal oxides to characterize its microstructure and hardness properties for the wire drawing process.

Experimental
The MgB2 wires were fabricated using the sinter and hot rolling method.The starting materials used were magnesium (Mg) and boron (B) powders with a purity of 99.9%.The Mg and B powders were weighed in the stoichiometric ratio of Mg:B=1:2, mixed thoroughly, and then put into the SUS316 tube using a hydraulic The tubes were then sintered in a vacuum furnace at 790°C for 2 hours to obtain a dense MgB2 precursor.The lower reaction temperature is crucial because it reduces the likelihood of powder-barrier reactions [7].
The sintered precursor was then hot-rolled into a wire using a rolling mill at a temperature of 500°C and a reduction ratio of 50% [8].The wire was then cooled using three different methods: normalizing, full anneal, and water quenching.For the normalizing method, the wire was allowed to cool in the air to room temperature.For the full anneal method, the wire was slowly cooled in the furnace to room temperature over a period of 24 hours.For the water quench method, the wire was rapidly cooled by immersion in water immediately after hot rolling.
The microstructure of the fabricated MgB2 wire was analyzed using scanning electron microscopy (SEM).The wire samples were sputter-coated with gold to avoid charging effects during SEM analysis.The grain size, texture, and distribution of MgB2 grains were investigated.The phase transformation of the MgB2 wire was measured using an X-Ray diffraction with a Cu source.The mechanical properties of the wire, such as hardness strength were also determined using a Rockwell testing machine.

Results
The XRD analysis revealed that the MgB2 wire has a hexagonal crystal structure with lattice parameters a = 3.084 Å and c = 3.523 Å.The SEM analysis showed that the MgB2 wire has a uniform microstructure with an average grain size of approximately 1 µm.

Figure 1. XRD diffraction analysis
The XRD analysis results are consistent with the previous studies on MgB2 wires, which also showed a hexagonal crystal structure.The SEM analysis showed that the MgB2 wire has a uniform microstructure with a small grain size, which is important for high critical current density [9].The Vickers hardness test result showed that the MgB2 wire has excellent mechanical properties, which is crucial for its application in various devices [10].The results indicate that the cooling treatment significantly influenced the hardness of the MgB2 samples.The highest average hardness was observed in the full anneal treatment, followed by normalizing, and lastly, the water quench treatment.This process allows for the formation of larger grains and reduces internal stresses, which results in a higher hardness value.In the case of the MgB2 samples, the full anneal treatment resulted in an average hardness of 373.5 HV, demonstrating the effectiveness of this cooling treatment in enhancing the material's hardness.In comparison, the normalizing treatment involves heating the material and then allowing it to be cooled in air.This process results in a more uniform microstructure and moderate hardness values.The MgB2 samples subjected to normalizing displayed an average hardness of 366.0 HV, slightly lower than the full anneal treatment but still relatively high.The water quench treatment, on the other hand, involves rapidly cooling the material by immersing it in water.Although this method is effective in producing a fine grain structure and increasing hardness in some materials [11]- [13], the MgB2 samples exhibited the lowest average hardness value of 360.0 HV.The rapid cooling may have introduced internal stresses and generated a less uniform microstructure, leading to reduced hardness in comparison to the other treatments.
In summary, the full anneal treatment resulted in the highest hardness values among the MgB2 samples, followed by normalizing and water quenching.These findings suggest that the choice of cooling treatment can significantly impact the hardness of MgB2 materials, and understanding the effects of each method can aid in optimizing the desired material properties for specific applications.

Conclusion
The characterization of MgB2 wire using XRD, SEM, and hardness tests revealed that the wire has a single phase of MgB2, uniform microstructure, and excellent mechanical properties.These properties make the MgB2 wire a promising candidate for various applications, such as superconducting magnets, power transmission cables, and thin film/wire superconductors.

Figure 2 .
Figure 2. SEM image using backscattered and secondary electron (a) normalizing (b) full anneal (c) water quench