Vortex Pinning Properties of Dense Ti-Doped MgB2 Bulks Sintered at Different Temperatures

We have studied the trapped field by field-cooled magnetization and microstructure of the Ti-doped MgB
₂
bulks fabricated at 700°C by an in situ hot isostatic pressing method, which were compared with those at 900°C. The maximum trapped field B
_T
of 3.0 T at 19 K was realized at the Ti-doping level of 10%-20%. In spite of the lower filling factor of 700°C Ti-doped bulks, i.e., f ~80%, the obtained B
_T
= 3.0 T was 1.3 times larger than that of the nondoped bulk (f ~ 90%) and was comparable with those for the 900°C Ti-doped bulks (f ~ 90%) with the same Ti contents. The quantitative analysis of the chemical composition revealed that fine particles of higher borides, i.e., MgB
_y
(y > 2), existed in the MgB
₂
matrix. The low sintering temperature of 700°C and the Ti doping strongly suppressed the grain growth of the MgB
₂
matrix. The increase in both the grain boundaries of MgB
₂
and the nonsuperconducting fine particles enhanced the trapped field of the present 700°C Ti-doped MgB2 bulks.