Synthesis of boron-doped diamond and its application as a heating material in a multi-anvil high-pressure apparatus

We developed methods to use synthesized boron-doped diamond (BDD) as a heater in a multi-anvil high-pressure apparatus. The synthesized BDD heater could stably generate an ultra-high temperature without the issues (anomalous melt, pressure drop, and instability of heating) arising from oxidation of boron into boron oxide and graphite-diamond conversion. We synthesized BDD blocks and tubes with boron contents of 0.5-3.0 wt. % from a mixture of graphite and amorphous boron at 15 GPa and 2000 °C. The electrical conductivity of BDD increased with increasing boron content. The stability of the heater and heating reproducibility were confirmed through repeated cycles of heating and cooling. Temperatures as high as ∼3700 °C were successfully generated at higher than 10 GPa using the BDD heater. The effect of the BDD heater on the pressure-generation efficiency was evaluated using MgO pressure scale by in situ X-ray diffraction study at the SPring-8 synchrotron. The pressure-generation efficiency was lower than that using a graphite-boron composite heater up to 1500 tons. The achievement of stable temperature generation above 3000 °C enables melting experiments of silicates and determination of some physical properties (such as viscosity) of silicate melts under the Earth's lower mantle conditions.