Synthesis
Magnesium diboride can be synthesized by several routes. The simplest is by high temperature reaction between boron and magnesium powders. Formation begins at 650 °C; however, since magnesium metal melts at 652 °C, the reaction mechanism is considered to be moderated by magnesium vapor diffusion across boron grain boundaries. At conventional reaction temperatures, sintering is minimal, although enough grain recrystallization occurs to permit Josephson quantum tunnelling between grains.
Superconducting magnesium diboride wire can be produced through the powder-in-tube (PIT) process. In the in situ variant, a mixture of boron and magnesium is poured into a metal tube, which is reduced in diameter by conventional wire drawing. The wire is then heated to the reaction temperature to form MgB2 inside. In the ex situ variant, the tube is filled with MgB2 powder, reduced in diameter, and sintered at 800 to 1000 °C. In both cases, later hot isostatic pressing at approximately 950 °C further improves the properties.
In 2003, a new and easy in situ technique for the synthesis of MgB2 was presented by Giunchi et al. (Edison S.p.A.). This new technique employs reactive liquid infiltration of magnesium inside a granular preform of boron powders and was called Mg-RLI technique. The method allowed to manufacture both high density (more than 90% of the theoretical density for MgB2) bulk materials and special hollow fibers. The process of Reactive Mg Liquid Infiltration in a boron preform to obtain MgB2 has been a subject of patent applications by Edison S.p.A. (Italy).
Hybrid physical-chemical vapor deposition (HPCVD) has been the most effective technique for depositing magnesium diboride (MgB2) thin films. The surfaces of MgB2 films deposited by other technologies are usually rough and non-stoichiometric. In contrast, the HPCVD system can grow high-quality in situ pure MgB2 films with smooth surfaces, which are required to make reproducible uniform Josephson junctions, the fundamental element of superconducting circuits.
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