| Summary: | In this study, boron powder with different particle sizes was purified by both chemical and heat treatment methods. The reduction in the particle size can improve the chemical purification with no effort on the heat treatment. The superconducting properties of the powder drastically changed even with only a partial elimination of oxygen. On the one hand, less oxygen content resulted in high <i>T</i><sub>c</sub> and <i>J</i><sub>c</sub> values under the low magnetic field, and most importantly, a significant improvement in the superconducting connectivity (<i>A</i><sub>f</sub> value). On the other hand, the degradation of <i>J</i><sub>c</sub> under a high field and a change in the pinning mechanism were also found, along with decreasing oxygen. This result indicated that oxygen, probably MgO, might act as the pinning center and as an obstacle for the supercurrent in MgB<sub>2</sub> at the same time. This work paves the way for obtaining pure oxygen-free MgB<sub>2</sub> and understanding the real effect of oxygen in MgB<sub>2</sub>.
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