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Ultrahigh-temperature–pressure experiments are crucial for understanding the physical and chemical properties of matter. The recent development of boron-doped diamond (BDD) heaters has made such melting experiments possible in large-volume presses. However, estimates of temperatures above 2600 K and of the temperature distributions inside BDD heaters are not well constrained, owing to the lack of a suitable thermometer. Here, we establish a three-dimensional finite element model as a virtual thermometer to estimate the temperature and temperature field above 2600 K. The advantage of this virtual thermometer over those proposed in previous studies is that it considers both alternating and direct current heating modes, the actual sizes of cell assemblies after compression, the effects of the electrode, thermocouple and anvil, and the heat dissipation by the pressure-transmitting medium. The virtual thermometer reproduces the power–temperature relationships of ultrahigh-temperature–pressure experiments below 2600 K at press loads of 2.8–7.9 MN (∼19 to 28 GPa) within experimental uncertainties. The temperatures above 2600 K predicted by our virtual thermometer are within the uncertainty of those extrapolated from power–temperature relationships below 2600 K. Furthermore, our model shows that the temperature distribution inside a BDD heater (19–26 K/mm along the radial direction and <83 K/mm along the longitudinal direction) is more homogeneous than those inside conventional heaters such as graphite or LaCrO3 heaters (100–200 K/mm). Our study thus provides a reliable virtual thermometer for ultrahigh-temperature experiments using BDD heaters in Earth and material sciences. [ABSTRACT FROM AUTHOR]

As evidenced by isotope geochemistry, the persistence of primitive reservoirs indicates that the earth's lower mantle is likely to be heterogeneous. Such heterogeneity could be a legacy from magma‐ocean (MO) solidification. The viscosity of MO is a key parameter to constrain the solidification type of MO. Here we directly measure the viscosity of peridotite (an analog of MO composition) melt at the pressure‐temperature conditions of the deep mantle, using the in situ falling sphere method. The viscosity of peridotite melt along liquidus is in the range of 38–17 mPa s at pressures from 7 to 25 GPa, which is 0.9–0.4 times of the estimation based on the viscosity of endmember compositions. Low viscosity favors fractional solidification and chemically layering of the early mantle at least to the top lower mantle, which could be a source of heterogeneity for the present mantle. Plain Language Summary: The earth experiences a large‐scale melting and forms a deep magma ocean in its early history. The viscosity of peridotite melt is a key parameter for understanding the solidification type of magma ocean, which leads to the primitive mantle structure. This study directly measured the viscosity of peridotite melt to deep mantle conditions and revealed that peridotite melt has a lower value of viscosity than expected. The low viscosity of peridotite melt suggests a fractional solidification of the magma ocean, which supports a heterogeneous primitive mantle. Key Points: The viscosity of peridotite melt has been measured down to lower‐mantle conditions by in situ falling sphere viscometryPeridotite melt has a lower viscosity than expected from the viscosity of the endmembersThe low viscosity of peridotite melt suggests a fractional solidification in the magma ocean, supporting a heterogeneous primitive mantle [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We tested boron-doped diamond (BDD) synthesized by chemical vapor deposition (CVD) as a heating element in a multi-anvil apparatus. We succeeded in manufacturing BDD into a tubular shape by laser cutting and electric discharging machining. The BDD tube shaped by the electric discharging machining was contaminated by discharging electrode materials (Mo and W), which affected the heating performance. The laser-cut BDD tube has a clean surface and, therefore, had a good heating performance. We succeeded in generating temperature as high as 2670 K at a pressure around 30 GPa with laser-cut heater. Heating reproducibility was confirmed through repeated heating and cooling cycles. The recovered sample shows that a higher temperature generation above 2670 K was prevented by eutectic melting of ZrO2 thermal insulator and Al2O3 sample. Owing to the commercial availability with a reasonable price, CVD–BDD heaters are more practical than a high-pressure synthesized BDD heaters for wide applications. [ABSTRACT FROM AUTHOR]

TiC-MgO composite was developed as a heating element for X-ray study in the multi-anvil high pressure apparatus. We synthesized TiC-MgO blocks (50–70 wt.% of TiC) by compression in a cold isostatic press followed by baking in a gas flow furnace. Heaters of tubular shape were manufactured from the synthesized blocks either by lathe or numerically controlled milling machine. The so-produced heating elements have been proved to generate temperatures up to 2250 K at 10 GPa, condition where classical graphite heaters are not suitable anymore due to graphite-diamond transition. These new heaters have been successfully used for in situ X-ray radiography and diffraction measurements on liquid Fe alloys, exploiting excellent X-ray transparency. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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/H2gas (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 B2O3problem. The electromotive force of the W/Re thermocouple was successfully monitored up to 2400°C. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

Publisher's Note: "Boron-MgO composite as an X-ray transparent pressure medium in the multi-anvil apparatus" [Rev. Sci. Instrum. This article was originally published online on 23 April 2020. All online versions of the article were corrected on 8 May 2020. [Extracted from the article]

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.