[B þ C] compacts were prepared by uniaxially pressing
0.122 g (B ¼ 0.100 g and C ¼ 0.022 g) of the mixed powder at
480 MPa into a rectangular shape (1.99 3.05 14.17 mm3
) using a
die in air. In an Ar-filled glove box (O2 and H2O < 1 ppm), a [B þ C]
compact and approximately 0.2 g of Na (Nippon Soda, 99.95% purity)
were placed in a boron nitride (BN) crucible (Showa Denko,
99.5%, 8.5 mm outer diameter, 6.5 mm inner diameter, 18 mm
depth), which was then set in a stainless-steel tube with one end
welded shut (SUS316, 12.7 mm outer diameter, 10.5 mm inner
diameter, 80 mm height) and sealed with a SUS 316 cap, as schematically
illustrated in F