Thermodynamic studies, using phase-diagram modelling techniques, showed that - due to the high volatility of Mg - MgB2 was thermodynamically stable only under fairly high Mg over-pressures at likely in situ growth temperatures. This provided insight into the appropriate processing conditions for MgB2 thin films, including identification of the pressure–temperature region for adsorption-controlled growth. The initial MgB2 thin films were made by pulsed laser deposition, followed by in situ annealing. Cross-sectional transmission electron microscopy revealed a nanocrystalline mixture of textured MgO and MgB2, with very small grain sizes. A zero-resistance transition temperature of 34K and a zero-field critical current density of 1.3 x 106A/cm2 were obtained. The quality of the films was limited by the thermodynamic stability conditions, which favoured deposition techniques that could maintain a high flux of Mg.

Thermodynamics and Thin Film Deposition of MgB2 Superconductors. X.X.Xi, X.H.Zeng, A.Soukiassian, J.Jones, J.Hotchkiss, Y.Zhong, C.O.Brubaker, Z.K.Liu, J.Lettieri, D.G.Schlom, Y.F.Hu, E.Wertz, Q.Li, W.Tian, H.P.Sun, X.Q.Pan: Superconductor Science and Technology, 2002, 15, 451-7

Figure 14

Calculated Mg-B Phase Diagram (1atm)