1. APPLICATION OF AN EXPLOSION FOR OBTAINING MAGNESIUM DIBORIDE: AMORPHOUS AND CRYSTALLINE PHASES a V.I.Mali, b O.I.Lomovskii, b G.V.Golubkova, c L.S.Dovlitova, c V.I.Zaikovskii a Lavrentyev Institute of Hydrodynamics, Siberian Division, Russian Academy of Sciences, Lavrentyev Prospekt 15, 630090 Novosibirsk, Russia b Institute of Solid State Chemistry and Mechanochemistry, Siberian Division, Russian Academy of Sciences, Kutateladze, 18, Novosibirsk, 630128, Russia c Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Lavrentyev Prospekt 5, 630090 Novosibirsk, Russia

2. Mg BB B AGO-2 planetary ball mill explosive loading with a pressure of 3 GPa Heating to 600 o С The aggregate size is ≈500 nm,- of the primary crystallites is ≈100 nm. disperse MgB 2 particles (5nm) on the Mg surface plates with a thickness up to ≈100 nm and transverse size up to 1000 nm. 50-100 µm ≤1 µm

3. Fig. 1. Changes in magnesium reflections (XRDA) versus the MA duration. Curves 1 and 2 refer to 3-min and 15-min mechanical activation. Magnesium reflections are attenuated by increasing the MA duration

4. Fig. 2. DD analysis of the Mg–2B mixture after 3-min mechanical activation with subsequent heating up to 500oС. Mg and B concentrations and Mg/B stoichiometric diagram as functions of the mass of the dissolved substance. Mechanical activation and subsequent heating to 500 0 С unambiguously showed the formation of the МgB2 phase

5. Fig. 3. DTA data for samples with different MA durations. 3 min (1), 7 min (2), 10 min (3), and 15 min (4). Exoeffects in the temperature range of 630-640 o С

6. Fig. 4. XRDA data for the sample after mechanical activation and heating up to 500 o С. Formation of the МgB2 phase

7. Fig. 5. Explosive compact of a mechanically activated mixture of magnesium and boron powders. Particles of MgO (1) and MgB2 (2). MgO particles have a rounded shape and a size of 200–1000 nm. Magnesium diboride has a morphology of plates with a thickness up to ≈100 nm and transverse size up to 1000 nm.

12. MgO particles have a rounded shape and a size of 200–1000 nm. Magnesium diboride has a morphology of plates with a thickness up to ≈100 nm and transverse size up to ≈ 1000 nm. This composition of particles is evidenced by both HRTEM (Fourier analysis) and EDX analysis. It should be noted, however, that MgB2 plates contain inclusions in the form of Mg crystallites 20–50 nm in size. TEM photographs show that these particles are incorporated into the MgB2 volume rather than located on its surface. Despite a close contact between the incorporated Mg phase and MgB2, no epitaxy of these phases was observed.

13. The number of local volumes with a stoichiometric composition of magnesium and boron increases after preliminary mechanical activation, which results in a larger number of nuclei of MgB2 crystallites formed from amorphous magnesium diboride with elevated chemical activity, which drastically speed up the process of MgB2 crystallization. In this case, MgB2 crystallization occurs at a temperature of shock compression from the amorphous phase on the crystallite nuclei. According to this model, the yield of magnesium diboride synthesized by an explosion from a mechanically activated mixture of Mg and B powders should be much greater than the yield of magnesium diboride synthesized by an explosion from a mixture of Mg and B powders in the as-received state. These results agree with the data of [8, 9] where it was shown by means of XRDA that the yield of magnesium diboride synthesized by an explosion from a mechanically activated mixture of magnesium and boron powders is nine times greater than the yields of magnesium diboride synthesized by an explosion from a mixture of magnesium and boron powders in the as- received state.

14. Thus, it was demonstrated that mechanical activation of a powder system consisting of Mg and B results in formation of amorphous magnesium diboride and nuclei of crystalline magnesium diboride, which were identified with the help of electron microscopy. Heating to limited temperatures close to the crystallization temperature allows nanometer-sized MgB2 particles to be obtained. Formation of large laminated MgB2 crystals synthesized by an explosion is caused by the fact that their growth on nuclei from the amorphous phase formed at the stage of preliminary mechanical activation of a mixture of Mg and B powders mainly occurs in a material significantly compacted by the shock wave. The morphological features and properties of such dense crystals of magnesium diboride may be of interest for engineering applications.

15. References 1.D.D.Radev, M.Marinov, V.Tumbalev, I.Radev, L.Konstantinov. Mechanically activated self-propagated high-temperature synthesis of nanometer-structured MgB2 // Physica C 418 (2005) 53–58. 2.R.A.Varin, Ch.Chiu. Synthesis of nanocrystalline magnesium diboride (MgB2) metallic superconductor by mechano-chemical reaction and post-annealing // Journal of Alloys and Compounds 407 (2006) 268–273. 3.A.Gümbel, J.Eckert, G.Fuchs, K.Nenkov, K.-H.Müller, and L.Schultz. Improved superconducting properties in nanocrystalline bulk MgB2 // Applied Physics Letters 80, No.15 (2002) 2725–2727. 4.H.Abe, M.Natio, et al., Low temperature formation of superconducting MgB2 phase from elements by mechanical alloying // Physica C 391 (2003) 211–216. 5.Y.D.Gao, J.Ding, G.V.S.Rao at al, Superconductivity of MgB2 after mechanical milling, Prys. Stat. Sol. 191, No 2, 548–554 (2002). 6.Y.D.Gao, J.Ding,* Q.Chen, G.V.S.Rao, B.V.R.Chowdari. Structure, superconductivity and magnetic properties of mechanically alloyed Mg1-xFexB2 powders with x = 0–0.4 // Acta Materialia 52 (2004) 1543–1553. 7.Malakhov V.V. Stoichiography as applied to studying composition and real structure of catalysts // J. Mol. Catal. A: Chem. 2000. V. 158. N1, P.143–148. 8.Mali V.I., Neronov V.A., Perminov V.P., Korchagin M.A., Teslenko T.S. Explosive synthesis of magnesium diboride. // Khimiya v Interesakh Ustoichivogo Razvitiya. 2005. V. 13, No. 3. pp. 451–453. 9.V.I.Mali, V.A.Neronov, V.P.Perminov, M.A.Korchagin, T.S.Teslenko. Shock synthesis of MgB2 // Shock-assisted synthesis and modification of materials. Ed. A.A.Deribas and Yu.A.Sheck. – M.: TORUS PRESS Ltd., 2006. P. 76–77.