Ball
milling was processed under the conditions of milling for
15 min at 300 rpm and resting for 10 min to avoid generating a
high temperature during the long milling time. The milled
powders, which weighed 0.8 g, were compacted into a graphite
die of Φ10 mm inner diameter and were sintered using a spark
plasma sintering system (Sumitomo Coal Mining Co. Ltd.,
Tokyo, Japan) under vacuum condition of lower than 4 Pa at a
compressive stress of 50 MPa. A piece of carbon foil (thickness:
0.2 mm) was put between the powder and carbon die to
avoid the reaction of them during sintering at high temperature,
as well as to get the sample out of the carbon die easily after
sintering.