The Synthesis and Performance of MgB 2
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The Synthesis and Performance of MgB2 R. L. Meng, B. Lorenz, Y. Y. Xue, D. Pham, J. Cmaidalka, J. K. Meen, Y. Y. Sun, and C. W. Chu* Department of Physics and Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5002, U.S.A. * also at Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, U.S.A.; and Hong Kong University of Science and Technology, Hong Kong ABSTRACT We have studied the kinetics of the chemical reaction between Mg and B by differential thermal analysis. There are two exothermal peaks observed at 500 and 650 °C. We speculate that the first exothermal peak is mainly related to the chemical reaction between Mg and oxygen, forming MgO. The second exothermal peak, which coincides with the melting point of Mg, clearly indicates the chemical reaction between Mg and B. The effect of synthesis conditions and defects on the transport property of MgB2 has been investigated. A correlation between the microstrain, the lattice parameters, and the Mg concentration were observed and are discussed.
The relatively high transition temperature, small anisotropy, and negligible grain boundary effects on the supercurrent flow of the newly discovered MgB2 suggest that the compound may hold great promise for both large- and small-current applications. Unfortunately, control of the density, microstructure, electrical connectivity, and surface morphology of the MgB2 samples, which define the performance of the superconducting devices, remains a challenge. We have therefore examined the influence of various synthesis conditions and identified the crucial parameters affecting the above properties. The microstrain and lattice parameters have been found to be an effective method to measure the quality of the sample. The results are presented and discussed in this paper. Ceramic MgB2 samples were prepared using the solid-state reaction method. Small Mg chips (99.8% pure) and B powder (99.7% pure) were sealed inside a Ta tube in vacuum. The sealed Ta ampoule was then enclosed in a quartz tube. The assembly was heated to 900-950 °C and was held at this temperature for 2 h, followed by furnace cooling. The structure was determined by X-ray powder diffraction (XRD) using a Rigaku DMAX-IIIB diffractometer. The refinement was performed using a Reitan-94 program. The resistivity was measured by the standard 4-probe method using a R-700 ac bridge. The grain morphology and particle sizes were measured by electron microprobe analysis (EMPA) at an accelerating voltage of 15 KV and 30 nA beam current using a JEOL JXA 8600 electron microprobe with attached Wavelength Dispersive Spectrometers (WDS). High quality MgB2 with Tc near 39 K and high residual resistivity ratios (RRR) = ρ(300 K/50 K) is usually obtained by higher synthetic temperature [1]. However, films [2] fabricated near 600 °C have Tc of about 25 K and the residual resistance ratios (RRR) were close to or even less than one. The origins of these non-ideal properties are not yet known with certainty. The differential thermal analysis (D
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