Magnesium diboride($MgB_2$) is a recently introduced metal type superconductor of which transition temperature is 39K. Even though it shows much smaller transition temperature than HTS (high temperature superconductor), it has some advantages in real application due to its low anisotropy, large coherence length, high critical current density, and transparent current flow in grain boundary.
Although many concerns have been taken to it, research actions were restricted to a few research groups because of two reasons as followed. Firstly, to prepare the dense $MgB_2$ bulk for its application to semiconductor devices, sintering equipments in which ultrahigh pressure (100MPa to 10GPa) can be applied at high temperature are essential due to its strong covalent bonding and strong decomposition to $MgB_4$. Secondly, $MgB_2$ powder is so expensive. Therefore, in the present study, the low pressure-assisted densification and combustion-assisted synthesis methods of $MgB_2$ particles were developed, and then superconductivity of prepared $MgB_2$ bulks was evaluated so as to investigate the applicability of the developed techniques.
Low Pressure-Assisted Densification
Magnesium diboride($MgB_2$) bulks were fabricated by uniaxial hot pressing mixed powders of commercial $MgB_2$ and Mg under pressure of 25 MPa at 1030℃ for 3 hrs. The commercial $MgB_2$ powder contained not only $MgB_2$ but also considerable amount of MgO phase. The Mg added in the mixed powder was introduced to prevent $MgB_2$ from decomposition to $MgB_4$ because the added Mg increases vapor pressure of Mg during hot pressing.
All the hot pressed bodies were discs-shaped with π = 20mm and t = 1.5 to 2mm and their bulk density values were in the range of 2.61 to 2.63g/㎤. The $MgB_2$ bulk prepared by hot pressing pure commercial $MgB_2$ powder was composed of MgO, $MgB_2$, and $MgB_4$ phases. The $MgB_4$ phase was originated from decomposition of $MgB_2$ to $MgB_4$ and Mg vapor. In the $MgB_2$ bulks prepared by hot pressing Mg-$MgB_2$ mixed powders, the $MgB_4$ content decreased as the initial added amount of Mg increased, because the increment of Mg vapor pressure by Mg addition can prevent the $MgB_2$ from decomposition. When the Mg content was 10wt% or more, the hot pressed bulks didn’t contain $MgB_4$ to be MgO-$MgB_4$ mixtures.
The hot-pressed $MgB_2$ bulks showed superconducting transition behavior at temperatures of 37 to 39K. Magnetization vs. magnetic field curves showed that the prepared $MgB_2$ superconductors were type Ⅱ superconductors. The critical current density $(J_c)$ was about $9×10^{4} Acm^{-2}$ at 20K and 1T. This means that the hot pressed $MgB_2$ bulks show superconducting behavior comparable to the $MgB_2$ superconductor prepared by the ultrahigh pressure processes reported by others.
Combustion-Assisted Particle Synthesis
Magnesium diboride($MgB_2$) powders were synthesized by simple calcination of Mg-B mixed powder at Ar atmosphere. During the calcination, the Mg-B mixed powders were directly combusted to be $MgB_2$ powder in a narrow processing window. The Mg-B mixed powder was a 1.33Mg-2B mixture, because Mg easily evaporates on heating.
The mixed powders calcined at 850 to 870℃ were composed of $MgB_2$ and small amount of MgO. The synthesized powders have similar particle size and agglomeration state to the commercial powder.
The synthesized $MgB_2$ powder was hot pressed with addition of 30wt% of Mg at 1030℃ for 3 hrs under 25MPa. The hot-pressed bulk was composed of both $MgB_2$ and MgO phases and has 2.57g/㎤ of bulk density. This means that the synthesized $MgB_2$ powder shows similar densification behavior to the commercial powder.
The hot pressed $MgB_2$ bulk obviously showed typical superconducting behavior. Its magnetization vs. magnetic field curve showed that the prepared $MgB_2$ bulk was a type Ⅱ superconductor. Its critical current density $(J_c)$ was about $2×10^{4} Acm^{-2}$ at 20K and 1T. This means that its superconducting behavior is comparable to the $MgB_2$ superconductors prepared from the commercial powder, and the synthesized $MgB_2$ powder has high potential in real application due to its simple processing and high production rate.
