The effect of boron powder on the microstructure of MgB₂filaments prepared by the modified internal magnesium diffusion technique

Abstract

We have analyzed the microstructure of MgB2 prepared by the modified internal magnesium diffusion technique resulting in filaments without a central hole. Three different amorphous boron powders with a nominal purity of 98.5%–99.0% and a different particle size were used. It was shown that the real purity of the powders was different from the nominal purity, and the microstructure as well as the critical current densities of the MgB2 were affected by using boron. Larger B particles with a size of 400–500 nm led to the presence of large B-rich grains inside the MgB2 matrix. For one wire series, boron powder containing residual chlorine from the boron preparation process was used. It was found that just a small quantity of chlorine in the B powder caused significant MgB2 filament inhomogeneity, an apparent decrease in the critical current density and high reactivity with air. We estimated the ideal packing density for achieving a large volume of dense MgB2.

Current densities and strain tolerances of filamentary MgB2 wires made by an internal Mg diffusion process

“…Figure 1(c) shows the engineering current densities of filamentary IMD wires 1 and 4 compared with two commercial MgB 2 wires made by the PIT process, see table 2 [1, 2]. The same slope of J e (B) is observed for both C-doped wires 1 and HyT-30, but J e of wire 1 at 5 T is ∼1.5 times MgO phase created by the IMD process [24]. It should be noted that a uniform Mg/B ratio in the as-deformed filaments is very important for homogeneous long-length IMD wires, which it is not so crucial for PIT wires where a mixture of powders are deformed.…”

Homogeneity of SiC distribution in IMD MgB2 wires

“…It is well known that the nature of the raw powders plays an important role on the formation of the final component. Purity and particle size of boron powder are identified as the main two factors [18][19][20]. A previous work of our group has shown that the boron powder purity has a more strong influence on the properties of MgB 2 wires than the particle size [21].…”

Size reduction of boron particles by high-power ultrasound for optimization of bulk MgB2

“…Several research groups have been trying to improve trapped field and critical current density in MgB 2 bulks. Most of the trials were related to optimization of synthesis parameters [12,13], novel synthesis techniques [14][15][16], refining the raw precursors, doping, additions [17,18], and fabrication of films [19]. The commonly used synthesis method for bulk MgB 2 is sintering.…”

“…Anyway, it does not hinder massive production of bulk MgB 2 material. Different techniques and synthesis methods were also practiced such as spark plasma sintering [24][25][26], diffusion method [16], infiltration growth [27] and chemical routes (combustion, pyrolysis, precipitation etc.). All these methods have their advantages and disadvantages.…”