In situ formation process and mechanism of bulk MgB 2 before Mg melting
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Differential thermal analysis, as the main means of measurement, was used to prepare bulk MgB2 samples and monitor the sintering reaction process. Combined with microstructure observation by scanning electron microscopy and x-ray diffraction analysis, the formation process of MgB2 phase at the temperature before Mg melting was summarized. Additionally, a new kinetic analysis (a variant on the Flynn–Wall–Ozawa) method under nonisothermal conditions was used to determine that the reaction between Mg and B powders involves random nucleation followed by an instantaneous growth of nuclei (Avrami–Erofeev equation, n ⳱ 2), which can properly explain the in situ formation process of bulk MgB2 at the temperature before Mg melting. The value of activation energy E and the function of conversion f(␣) are obtained independently, and thereby the determination of mechanism function is not affected by the value of E. The values of E decrease from 175.418 to 160.395 kJ mol−1 with the increase of the conversion degrees (␣) from 0.1 to 0.8. However, as the conversion degrees approach 0.9, the value of E increases to 222.647 kJ mol−1, and the corresponding pre-exponential factor A is about three orders of magnitude larger than the previous ones.
I. INTRODUCTION
Since the superconductivity of magnesium diboride was discovered by Nagamatsu et al. in the beginning of 2001,1 considerable effort has been put into the synthesis of samples in the form of bulk pellet, wire tape, thin film, and single-crystal MgB2. Bulk samples are in general prepared by direct reaction of magnesium (Mg) and boron (B) powders involving high pressure and high temperature,2,3 which are not commercially viable. Lowpressure synthesis is usually done in an atmosphere of flowing argon4 or other inert gases. Hence, a great deal of effort has been made to understand the formation process and mechanism of bulk MgB2 in the protective argon atmosphere at the ambient pressure. Bulk MgB2 superconductors with great characteristics have also been prepared in many laboratories, but most of them were obtained experimentally by sintering treatment under temperature just above the melting point of Mg (about 923 K). Though bulk MgB2 can also be prepared at low temperature below the Mg melting point (about 923 K),5–7 details about in situ formation process and mechanism using Mg and B powder as the raw materials at this temperature a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0229 1840
http://journals.cambridge.org
J. Mater. Res., Vol. 23, No. 7, Jul 2008 Downloaded: 14 Mar 2015
range have not been shown in previous work, especially by the differential thermal analysis method. In this paper, both thermal and kinetic analysis (a variant on the Ozawa–Flynn–Wall) method under nonisothermal condition was used to analyze the sintering process of a bulk polycrystalline MgB2 sample in flowing protective argon atmosphere at the ambient pressure. The recorded heat flux curves were adopted to derive kinetic information for the sintering pr
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