HIGH-PRESSURE SYNTHESIS OF MgB_2-BASED MATERIAL WITH HIGH CRITICAL CURRENTS

摘要

Additions of Ta, Ti, Zr and nano-SiC can increase j_c of the HPS MgB_2. Additions of Ti and Zr to MgB_2 provoke the highest increase of j_c. The main effect of Ta, Ti and Zr is most likely to be due to the absorption of impurity hydrogen at low synthesis temperatures to form TaH, Ta_2H, TiH_2, ZrH_2, which prevents harmful MgH_2 impurity phase from appearing and may prevent hydrogen from being introduced into the material structure. The amount of MgH_2 can be decreased by an increase of the synthesis temperature but in this case, due to the lower amount of Mg-B inclusions, the SC characteristics are decreased. The mechanism of Ti and Zr influence on the increase of critical current density of magnesium diboride differ from that reported in the literature (it is not due to TiB_2 or ZrB_2 formation). The appearance of ZrB_2 phase in the samples high-pressure synthesized at temperature higher than 900℃ does not provoke the increase in critical current density. SiC does not absorb hydrogen. Therefore, this may be the reason why samples synthesized at lower temperatures have lower SC characteristics. A decrease in the amount of Mg-B inclusions in the samples with nano-SiC (20-30 nm) added that have been synthesized at higher temperatures seems to be replenished by the presence of SiC. SiC can be introduced into MgB_2 structure to improve pinning. Nano-SiC (20-30 nm) addition provokes the highest j_c in a MgB_2-based material in the fields above 8 T at 10 K.