The effect of ZrO2 additions on the properties of MgB2 was studied using in situ and ex situ processes. The in situ process was performed by introducing ZrO2 from the milling tools into MgB2 throughout the planetary ball milling, whereas the ex situ process was accomplished by mixing ZrO2 from the milling tools with MgB2 by hand grinding in a mortar. A detectable amount of ZrO2 was present in MgB2 after 4h of milling during the in situ process and its content increased with milling time as expected. The 400h milled powder was partially amorphized and exhibited the formation of a minority ZrB2 phase. For milling up to 100 h, diamagnetism of MgB2 was significantly reduced while TC remained unchanged. Superconductivity was totally destroyed after 148h of milling. The loss of superconductivity was attributed to the effect of disordering induced by mechanical milling. As a result of in situ ZrO2 addition, the initial TC and crystal structure of MgB2 could not be restored upon annealing. With increasing milling time, the expansion of lattice parameters in both the a-axis and c-axis may be due to possible substitution of Mg or B by Zr. The result from the magnetic measurement shows that JC of MgB2 was deteriorated by in situ ZrO2 addition. On the other hand, ex situ ZrO2 addition with annealing did not degrade the TC of MgB2.

Influence of in situ and ex situ ZrO2 Addition on the Properties of MgB2. S.K.Chen, B.A.Glowacki, J.L.MacManus-Driscoll, M.E.Vickers, M.Majoros: Superconductor Science and Technology, 2004, 17, 243-8