Speaker
Description
An infiltration and reaction (IR) process realizes the dense MgB$_2$ bulk without the external physical pressure. The trapped field, $B_{\rm T}$, of MgB$_2$ bulk prepared by the IR process was 2.4 T at 15.9 K, which was as high as that of MgB$_2$ bulk prepared by in-situ hot isostatic pressing method [1]. However, we found quite a large amount of unreacted micrometric B particles due to the incomplete diffusion of liquid Mg into a B precursor [2]. Therefore, overcoming this problem should give a further enhanced $B_{\rm T}$ for the IR-processed MgB$_2$ bulks. In this paper, we report the refining effects of B powder on the formation of MgB$_2$ and the trapped field properties. Crystalline B powder was refined by ball-milling at various rotation speeds up to 600 rpm for 1 h. The Mg pellet was placed on the B pellet in a stainless steel container and heat-treated at 900 $^\circ$C for 24 h. The $B_{\rm T}$ of the IR-processed MgB$_2$ bulk increased with reducing the grain size of starting B powder, which originated mainly from the increase of the grain boundaries in the MgB$_2$ bulks. Moreover, the volume fraction of MgB$_2$ using ball-milled B powder is larger than that of MgB$_2$ using as-purchased B powder, which also contributes to the enhancement of $B_{\rm T}$. We also attempted to fabricate the IR-processed MgB$_2$ bulks using the ball-milled amorphous B powders, however, the MgB$_2$ bulks were not created. We discuss the relationship of the creation of MgB$_2$ with the grain size and crystallinity of B.
References
[1] T. Naito et al., Supercond. Sci. Technol. 29 (2016) 115003.
[2] A. Ogino et al., IEEE Trans. Appl. Supercond. 27 (2016) 6800905.
