Graphite–boron composite heater in a Kawai-type apparatus: the inhibitory effect of boron oxide and countermeasures

We have investigated the performance of a graphite–boron composite (GBC) with 3?wt % boron as a precursor for a boron-doped diamond heater in a Kawai-type apparatus at 15?GPa. We first tested a machinable cylinder of GBC sintered at 1000°C in Ar/H₂ gas (99:1 molar ratio). Boron oxide (B₂O₃) formed during sintering frequently hindered the GBC heater from stable operation at temperatures higher than 1400°C by producing melt throughout the heater together with oxide and/or silicates. We then rinsed the GBC heater in hydrochloric acid to remove B₂O₃. After rinsing, we succeeded in stably generating temperatures higher than 2000°C. We also improved a molding process of different-sized GBC tubes for convenient use and tested the molded GBC heater. It was free from the B₂O₃ problem. The electromotive force of the W/Re thermocouple was successfully monitored up to 2400°C.