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The two most common types of MgB 2 conductor fabrication technique – in-situ and ex-situ – show increasing conflicts concerning the connectivity, an effective current-carrying cross-sectional area. An in-situ reaction yields a strong intergrain coupling with a low packing factor, while an ex-situ process using pre-reacted MgB 2 yields tightly packed grains, however, their coupling is much weaker. We studied the normal-state resistivity and microstructure of ex-situ MgB 2 bulks synthesized with varied heating conditions under ambient pressure. The samples heated at moderately high temperatures of ∼900 °C for a long period showed an increased packing factor, a larger intergrain contact area and a significantly decreased resistivity, all of which indicate the solid-state self-sintering of MgB 2 . Consequently the connectivity of the sintered ex-situ samples exceeded the typical connectivity range 5–15% of the in-situ samples. Our results show self-sintering develops the superior connectivity potential of ex-situ MgB 2 , though its intergrain coupling is not yet fulfilled, to provide a strong possibility of twice or even much higher connectivity in optimally sintered ex-situ MgB 2 than in in-situ MgB 2 .
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Yamamoto et al. (2012) studied this question.
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