Microstructure of MgB2 superconducting wire prepared by internal magnesium diffusion and in-situ powder-in-tube processes – Secondary phase intergrain nanolayers as an oxygen content indicator

• We compared microstructure of undoped MgB2 wires made by in-situ PIT and IMD.
• The main differences were in MgB2 density and volume ratio of amorphous MgO.
• Thin oriented layers of secondary phases were observed between MgB2 plate grains.
• Only MgO or only Mg oriented layers were in the PIT or IMD samples, resp.
• High MgB2 density and low MgO2 content caused higher Jc of IMD wire.

We analyzed microstructure of two undoped MgB2 single core wires prepared by two different technologies – in-situ powder-in-tube (PIT) technology and internal magnesium diffusion (IMD) processes. Their microstructure showed mainly different densities of superconducting material, corresponding content of pores and the volume ratio of amorphous MgO phase. We observed a special microstructural feature – thin oriented secondary phase layers on grain boundaries of plate-shaped MgB2 grains. The significant decrease of oxygen content in IMD wire was reflected in the presence of thin oriented Mg layers while in PIT wire thin oriented MgO layers were observed. The microstructure difference is reflected in increased critical current density Jc of the IMD wire in comparison to the PIT one.