Longjian Xie

Semiconductor diamond heater in the Kawai multianvil apparatus: an innovation to generate the lower mantle geotherm

Semiconductor diamond is considered the best heater material to generate ultra-high temperatures in a Kawai cell. In two pioneering studies, a mixture of graphite and amorphous boron (or boron carbide, B4C) was converted to semiconductor diamond in the diamond stability field and was confirmed to generate 2000°C and 3500°C, respectively. Following these works, we synthesized a homemade boron-doped graphite block with fine machinability. With this technical breakthrough, we developed a semiconductor diamond heater in a smaller Kawai-type cell assembly. Here, we report the procedure for making machinable boron-doped graphite, and the performance of the material as a heater in a Kawai cell at 15 GPa using tungsten carbide anvils and at ∼50 GPa using sintered diamond anvils. Furthermore, we present a finite element simulation of the temperature distribution generated by a semiconductor diamond heater, which is much more homogeneous than that generated by a metal heater.

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

ABSTRACT 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.

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 Earths lower mantle conditions.

Single crystal elasticity of gold up to ∼20 GPa: Bulk modulus anomaly and implication for a primary pressure scale

We measured the elasticity of single crystal gold (Au) and its lattice parameters under high pressure using inelastic X-ray scattering (IXS). The elastic moduli were obtained at five pressure points between 0 and 20 GPa. The pressure variation of the bulk modulus displays anomalous behavior, being nearly constant up to ~5 GPa, and then steeply increasing at higher pressure. A similar anomaly is observed in first-principles calculations. An absolute pressure scale was derived by direct numerical integration of the bulk modulus over volume change. This yields a scale that gives slightly lower pressure values than those of previous work, about 5–10% lower at ~20 GPa.

Boron-doped diamond as a new heating element for internal-resistive heated diamond-anvil cell

ABSTRACT We have developed an internal-resistive heated diamond-anvil cell (IHDAC) with a new resistance heater – boron-doped diamond (BDD) – along with an optimized design of the cell assembly, including a composite gasket. Our proposed technique is capable of heating a silicate/oxide material with (1) long-term stability (>1 h at 2500 K) and (2) uniform radial temperature distribution (±35 K at 2500 K across a 40-µm area), which are clear advantages over the conventional laser-heated and internal-heated DACs. In addition, the achieved temperature in this study was greater than 3500 K, which mostly covers the possible geotherm of the entire lower mantle. In situ X-ray diffraction (XRD) measurement and ex situ chemical analyses confirmed that weak XRD intensity from the BDD heater and chemical inertness (no boron diffusion into silicate samples). This newly developed IHDAC with a BDD heater can be used to determine the phase diagrams of mantle materials with high precision and be used in lower-mantle petrology.