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

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.

We attempted to generate ultrahigh pressure and temperature simultaneously using a multi-anvil apparatus by combining the technologies of ultrahigh-pressure generation using sintered diamond (SD) anvils, which can reach 120 GPa, and ultrahigh-temperature generation using a boron-doped diamond (BDD) heater, which can reach 4000 K. Along with this strategy, we successfully generated a temperature of 3300 K and a pressure of above 50 GPa simultaneously. Although the high hardness of BDD significantly prevents high-pressure generation at low temperatures, its high-temperature softening allows for effective pressure generation at temperatures above 1200 K. High temperature also enhances high-pressure generation because of the thermal pressure. We expect to generate even higher pressure in the future by combining SD anvils and a BDD heater with advanced multi-anvil technology.

A strip-type boron-doped diamond heater synthesized by chemical vapor deposition for large-volume presses.

We have developed a high-pressure furnace assembly with a commercially available chemical-vapor-deposition synthesized boron-doped diamond heater consisting of four strips for large-volume multi-anvil presses (LVPs). This assembly successfully generated temperatures up to 2990 K at 15 GPa. It also has highly reproducible power-temperature relations, enabling us to estimate temperature from power reliably. It can be used for experiments above 9 GPa and is particularly useful for synchrotron x-ray experiments because of the x-ray transparency. It is also competitive in price. This technique is, thus, practical in various LVP experiments in the diamond-stability field.

Formation of bridgmanite-enriched layer at the top lower-mantle during magma ocean solidification.

Thermochemical heterogeneities detected today in the Earth's mantle could arise from ongoing partial melting in different mantle regions. A major open question, however, is the level of chemical stratification inherited from an early magma-ocean (MO) solidification. Here we show that the MO crystallized homogeneously in the deep mantle, but with chemical fractionation at depths around 1000 km and in the upper mantle. Our arguments are based on accurate measurements of the viscosity of melts with forsterite, enstatite and diopside compositions up to ~30 GPa and more than 3000 K at synchrotron X-ray facilities. Fractional solidification would induce the formation of a bridgmanite-enriched layer at ~1000 km depth. This layer may have resisted to mantle mixing by convection and cause the reported viscosity peak and anomalous dynamic impedance. On the other hand, fractional solidification in the upper mantle would have favored the formation of the first crust.

Boron-MgO composite as an X-ray transparent pressure medium in the multi-anvil apparatus.

X-ray transparent materials are very beneficial for in situ X-ray experiments in the multi-anvil apparatus. We sintered machinable blocks of boron-MgO composites at 800-1000 °C under atmospheric pressure from a mixture of amorphous boron and brucite or Mg(OH)2. The machinability of composite blocks improved with an increase in the brucite content in the starting material; a brucite content higher than 15 wt. % showed reasonable machinability in forming various shapes such as octahedron, cylinder, and sleeve. We confirmed the feasibility of the boron-MgO pressure medium by successfully generating lower mantle pressure (>23 GPa); its pressure generation efficiency is comparable to that of a Cr2O3 doped MgO pressure medium. The boron-MgO composite is expected to be an excellent thermal insulator owing to the extremely low thermal conductivity of amorphous boron; we confirmed its better thermal insulation performance through a comparative heating test with a zirconia sleeve in a Cr2O3 doped MgO pressure medium. Constituting light elements, the boron-MgO composite has high X-ray transparency, which enables us to conduct various cutting edge X-ray measurements in the large volume multi-anvil apparatus.

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.