Superconducting composites of MgB 2 with Additions obtained by Spark Plasma Sintering
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Superconducting composites of MgB2 with Additions obtained by Spark Plasma Sintering P. Badica1, G. Aldica1, M. Burdusel1 1
National Institute of Materials Physics, str. Atomistilor 105 bis, Magurele, 077125, Romania
ABSTRACT Sb2O3, Sb-metal, Bi2O3 or Bi-metal, powders were mixed with MgB2 powder. Starting compositions were ((MgB2)(M2O3)x, x = 0.0025, 0.005, 0.015, and (MgB2)(M)y, y = 0.01, M = Sb, Bi. Mixtures were processed by Spark Plasma Sintering (SPS) technique. As obtained composite samples show high density, above 94% of the theoretical density. While the secondary phases indicate on similar reactions, samples show different behavior vs. addition type and amount. This does not directly correlate with the melting temperature of the addition. From the critical current density (Jc) and irreversibility field (Hirr) enhancement viewpoints, optimum additions are oxides for x=0.025, 0.005. Both oxides are improving Jc at high fields, but Sb2O3 is effective up to 10 K, while Bi2O3 is up to 30 K. Metal additions are decreasing Jc and Hirr when compared to pristine MgB2 sample. Key words: MgB2, Bi2O3, Sb2O3, Bi, Sb, spark plasma sintering, critical current density, irreversibility field INTRODUCTION MgB2 bulks are produced by in-situ and ex-situ routes. In the in-situ route Mg and B or their compounds are reacted to obtain the MgB2 bulk. In the ex-situ method MgB2 powder is processed into a bulk. Each method has its advantages and disadvantages. At present, preference is given to the in-situ method because the achieved critical current densities Jc are higher. However, for mass production, ex-situ method is also of interest. To further improve Jc, different additions were introduced. In most cases, research was performed for the in-situ route. Recently, we have shown that for the ex-situ route different additions may be effective [1-4], but often we noted that between the in-situ and the ex-situ routes the results might be different due to very different processing conditions of the two routes (e.g. low temperatures for the in-situ and high temperatures for ex-situ). It results that search within the ex-situ route of efficient additions, deserves further attention. In this work, samples were obtained by ex-situ spark plasma sintering (SPS) on mixtures of MgB2 and Bi2O3, Sb2O3, Bi-metal or Sb-metal additions. Spark plasma sintering applies a uniaxial pressure and a pulsed electrical current on a graphite die system. Powder to be sintered is placed in the die between the punches. SPS has the advantage of high heating and cooling rates. In the case of MgB2 this can avoid Mg evaporation and can preserve a low particle size in the bulk. Low particle size means a high density of grain boundaries in the volume. It was established that the grain boundaries in MgB2 are excellent vortex pinning sites enhancing the critical current density [5]. Literature also notes [6] non-thermal specific effects for SPS taking place at the grain boundaries. Additions from this work have very different melting points of 271.5 °C for Bi, 817
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