Diamond formation from methane hydrate under the internal conditions of giant icy planets

At a Glance

Metadata	Details
Publication Date	2021-04-14
Journal	Scientific Reports
Authors	Hirokazu Kadobayashi, Satoka Ohnishi, Hiroaki Ohfuji, Yoshitaka Yamamoto, Michihiro Muraoka
Institutions	National Institute of Advanced Industrial Science and Technology, Ehime University
Citations	21
Analysis	Full AI Review Included

Milder Synthesis Conditions: Diamond formation from methane hydrate (C-O-H system) was achieved at significantly milder High-Pressure/High-Temperature (HPHT) conditions, initiating at approximately 1600 K and 13-45 GPa.
Water Influence: The presence of water, acting as a chemical solvent, accelerates the molecular dissociation and polymerization of methane, lowering the energy barrier required for diamond nucleation compared to pure C-H systems.
Nanoparticle Product: The resulting recovered material consists of ultrafine diamond nanoparticles, exhibiting grain sizes ranging from 50 nm to 350 nm.
Methodology: Experiments utilized a CO₂ Laser-Heated Diamond Anvil Cell (CO₂-LHDAC) for static heating (up to 3800 K and 45 GPa), coupled with in situ Synchrotron X-ray Diffraction (XRD) and Raman spectroscopy.
Rapid Kinetics: Diamond formation was observed to proceed rapidly (instantaneous blackening of the sample) once the critical temperature of ~1600 K was reached.
Planetary Relevance: These conditions overlap with the predicted isentropes of Uranus and Neptune, suggesting that diamond precipitation and accumulation can occur in the upper icy mantles of these giant planets.

Parameter	Value	Unit	Context
Maximum Pressure Achieved	45.0	GPa	CO₂-LHDAC experiments.
Maximum Temperature Achieved	3800	K	CO₂-LHDAC experiments.
Diamond Formation Onset (T)	~1600	K	Minimum temperature for rapid diamond formation in C-O-H system.
Diamond Formation Pressure Range	13-45	GPa	Conditions where diamond was observed in the C-O-H system.
Starting Material	Methane Hydrate Phase I (MH-I)	N/A	Homogeneous water-methane sample (CH₄-H₂O).
Water:Methane Molar Ratio	5.75-6.05	to 1	Composition of the synthesized MH-I.
Product Grain Size	50 to 350	nm	Size range of recovered diamond nanoparticles (FE-SEM).
Static Heating Duration	10-120	min	Reaction time provided by LHDAC (contrasts with nanosecond dynamic compression).
Diamond Raman Shift	1331	cm^-1	Characteristic peak of recovered diamond at ambient conditions.
Synchrotron X-ray Wavelength	0.04150	nm	Used for in situ XRD measurements at SPring-8.
Previous C-H System Onset (T)	2000-3000	K	Reported range for methane-only LHDAC experiments (10-80 GPa).
Previous C-H System Onset (P)	> 150	GPa	Required pressure for dynamic laser compression (polystyrene).

Starting Material Synthesis: Methane hydrate phase I (MH-I) was synthesized via the conventional ice-gas interface reaction method at 8 MPa and 269 K, ensuring a homogeneous molecular-level mixture of water and methane.
Sample Loading: Powdered MH-I and ruby pressure markers were loaded into a symmetric Diamond Anvil Cell (DAC) using a Rhenium gasket (pre-indented to ~50 µm thickness).
HPHT Generation: Samples were pressurized and then heated using a CO₂ Laser-Heated DAC (CO₂-LHDAC) system, achieving static conditions up to 45 GPa and 3800 K.
Temperature Measurement: Temperatures were determined by measuring the thermal radiation spectra (10-12 µm area) and fitting the data to the grey-body radiation formula.
In Situ Phase Analysis (XRD): Synchrotron X-ray Diffraction (XRD) was performed at SPring-8 (BL10XU) to identify crystalline phases, confirming the presence of MH-III, Ice VII, and the nucleation of diamond (111 peak).
In Situ Chemical Analysis (Raman): Raman spectroscopy was used to monitor molecular changes, identifying the decomposition of MH-III, the formation of solid methane (Phase B), heavier hydrocarbons, and hydrogen-related materials (e.g., hydrogen hydrate, CH₄-H₂ vdW compounds).
Microstructure Characterization: HPHT products were recovered to ambient conditions and analyzed using Field-Emission Scanning Electron Microscopy (FE-SEM) to observe the morphology and measure the grain size of the resulting diamond nanoparticles.

Nanodiamond Production: The demonstrated low-temperature synthesis route provides a potential pathway for the scalable production of ultrafine diamond nanoparticles (50-350 nm) suitable for high-precision polishing, advanced lubricants, and quantum sensing applications.
Catalysis and Solvents: The finding that water acts as a chemical solvent to promote methane dissociation under HPHT conditions may inform the design of new catalytic processes utilizing complex C-O-H feedstocks for carbon material synthesis.
Energy Materials: Understanding the high-pressure chemistry of hydrocarbons and water is critical for modeling and potentially utilizing carbon sequestration processes or developing materials stable under extreme geothermal conditions.
Advanced Feedstock Utilization: This method suggests that feedstocks containing oxygen (e.g., alcohols, biomass derivatives, or other hydrates) could be viable, lower-cost precursors for diamond synthesis, expanding beyond traditional pure methane (CH₄) or graphite sources.
Thermal Management Composites: The ability to produce small, uniform diamond grains is valuable for creating high-thermal-conductivity composites used in electronics and high-power RF devices.