Properties of MgB 2 Fabricated by Sintering Mg and B Sandwich Structures
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1099-II04-02
Properties of MgB2 Fabricated by Sintering Mg and B Sandwich Structures Andrew H.C. Yeung, and Dickon H. L. Ng Department of Physics, The Chinese University of Hong Kong, Room 107, Science Centre North Block, The Chinese University of Hong Kong, Shatin, NT, Hong Kong, China, People's Republic of ABSTRACT Magnesium diboride (MgB2) was successfully produced in a pellet with sandwich-like structure. The sample was prepared by embedding a layer of Mg powder between layers of boron powder before they were cold-pressed and sintered in argon atmosphere at different temperatures. After sintering, the interfacial regions between the Mg and B were examined. Hexagonal platelets were observed lying along the interfacial regions in samples sintered at 700oC or above. These hexagonal platelets were confirmed to be MgB2. When the samples were sintered at higher temperatures, the lateral size of the platelets increased. At 700oC, the average size of the platelets was 0.5 µm, and it was increased to 1.5 µm in samples sintered at 900oC. In addition, the critical temperature (TC) and the magnetization were also changed from 36.6 to 37.1 K, and −0.7 to −1.8 emu/g, respectively. INTRODUCTION The discovery of superconductivity in MgB2 in 2001 has attracted remarkable attention because of its high TC, high critical current densities and fields, simple crystal structure and low production cost [1]. It has potential to be used in superconducting devices for large-scale applications [2-4]. To prepare MgB2 by powder metallurgy methods, powders of Mg and B are usually mixed and sintered at atmospheric pressure. However, because of the high vapor pressure of Mg, loss of Mg by vaporization imposes a serious problem during fabrication, and this makes it impossible to grow MgB2 crystals by stoichiometric melting [5]. Growth of MgB2 crystals by sintering at high pressure [6-9] is a common practice, however, it has a high production cost. Heating treatment in sealed Nb or stainless steel containers [10, 11] is another alternative, but Mg may react with the metal container. In our work, MgB2 crystals were fabricated by embedding Mg between B layers in a sandwich-like structure. The bulk B replaced the role of metal as a container to reduce the loss of Mg. The MgB2 phase was found on the interfacial region between Mg and B. The dependency of the size of the crystals and their superconducting properties on the sintering temperature was studied. EXPERIMENTS Magnesium powder (99%, Strem Chemicals, USA) was cold pressed under 21MPa to form a pellet 6 mm in diameter. It was then embedded between layers of B powder (95%, International Laboratory, USA) and cold pressed again to form 10 mm diameter sandwich-like pellets as shown in Figure 1.
B Mg Figure 1 A schematic diagram of the sandwich structure. The sandwich pellets were placed in an alumina boat and then positioned in a horizontal alumina tube furnace. The pellet samples were sintered at temperatures between 600oC and 900oC in argon for an hour. A control sample was also prepared by the
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