Machinable boron-doped diamond as a practical heating element in multi-anvil apparatuses

Abstract

Being refractory and X-ray transparent, a boron-doped diamond (BDD) heater is considered an ideal heating element in a multi-anvil apparatus under diamond-stable pressures. However, the extremely high hardness of diamond makes it difficult to manufacture a BDD tube, which, in turn, hinders the wide application of BDD heaters in multi-anvil apparatuses. Here, I sintered a machinable BDD (MBDD) from a mixture of BDD powder and pitch (CnH2n+2) by its annealing in Ar at 1273 K for 5 h. The BDD powder was bound by a small amount of graphite (<10 wt. %) during the sintering process. Tubes (such as 1.2/0.7/4.0 mm in outer/inner diameter/length) can be manufactured from the MBDD block using a lathe or a computer numerical control machine. Due to the low content of graphite in MBDD, the graphite–diamond conversion has a small effect on heating performance. The MBDD heater shows a comparable performance in ultrahigh temperature generation with a high-pressure synthesized BDD heater by generating a temperature higher than 3300 K and melted Al2O3 under a pressure of 15 GPa. With good heating performance and excellent machinability, MBDD is a practical heating element in multi-anvil apparatuses. The achievement of stable temperature generation over 3300 K by the MBDD heater enables various measurements on the physicochemical properties of melts under the Earth’s mantle conditions.

Simultaneous generation of ultrahigh pressure and temperature to 50 GPa and 3300 K in multi-anvil apparatus

“…Interestingly, the thermocouple reading seemed to recover after cooling to 2673 K. The power-temperature relationships for heating and cooling below 1800 K are consistent with each other. The power-temperature curve shows a bend during cooling, which is commonly observed in BDD heaters, 14,[18][19][20][21] but shows approximately a linear shape during the heating. For a conservative estimate, we estimated the highest temperature at the maximum applied power (1154 W) to be 3300 K by linear extrapolation of the power-temperature relationship obtained up to 1800 K.…”

“…For example, a BDD heater synthesized at high pressure and temperature expanded the temperature range to 4000 K at 15 GPa. 18 In addition, the adoption of BDD synthesized by the chemical vapor deposition (CVD) method 19,20 and the development of machinable BDD block 21 made it possible to use BDD as a routine heating element in multi-anvil apparatus. Owing to its high x-ray transparency, BDD also shows a great advantage for in situ multi-anvil experiments.…”