Enhancement of short/medium-range order and thermal conductivity in ultrahard sp3 amorphous carbon by C70 precursor

At a Glance

Metadata	Details
Publication Date	2023-11-29
Journal	Nature Communications
Authors	Yuchen Shang, Mingguang Yao, Zhaodong Liu, Rong Fu, Longbiao Yan
Institutions	Shanghai University, Jilin University
Citations	21
Analysis	Full AI Review Included

Record Thermal Conductivity: A bulk sp³ amorphous carbon (AC) synthesized from a C₇₀ fullerene precursor achieved a thermal conductivity of 36.3 ± 2.2 W m^-1 K^-1, the highest reported value for any amorphous solid to date.
Enhanced Hardness: The C₇₀-derived AC exhibits superior Vickers hardness (109.8 ± 5.6 GPa) compared to AC synthesized from C₆₀, placing it in the ultrahard material class.
Structural Tuning Strategy: The use of C₇₀ (which contains five more carbon hexagons than C₆₀) successfully modified the microstructure, resulting in a higher fraction of hexagonal-diamond-like clusters and stronger short/medium-range structural order (SRO/MRO).
High Purity and Transparency: The optimal synthesis conditions yielded a nearly pure sp³-hybridized material (96.2% sp³ content) that is highly transparent and exhibits a wide optical bandgap (2.80 eV).
HPHT Synthesis: The material was synthesized using a High Pressure High Temperature (HPHT) process at optimal conditions of 30 GPa and 1100 °C.
Microstructure-Property Link: The enhanced thermal transport is directly attributed to the increased MRO, which favors the propagation of low-frequency propagons (vibrations resembling phonons) that efficiently carry heat.

Parameter	Value	Unit	Context
Optimal Synthesis Pressure	30	GPa	AC70-1 sample
Optimal Synthesis Temperature	1100	°C	AC70-1 sample
Vickers Hardness (H_v)	109.8 ± 5.6	GPa	At 9.8 N maximum load (AC70-1)
Thermal Conductivity (k)	36.3 ± 2.2	W m^-1 K^-1	C₇₀-derived AC (AC70-1)
Comparison Thermal Conductivity	26.0 ± 1.3	W m^-1 K^-1	C₆₀-derived AC (AC-3)
sp³ Hybridization Content	96.2 ± 0.9	%	Measured by EELS (AC70-1)
Optical Bandgap (E_g)	2.80	eV	Measured by UV-Vis absorption (AC70-1)
Local Ordered Region Fraction	38.1 ± 3.8	%	Total areal fraction of MRO clusters (AC70-1)
C-C Nearest Neighbor Distance (r₁)	1.55	A	Atomic Pair Distribution Function (PDF) analysis
Average C-C-C Bond Angle	108.8	°	Calculated from r₁ and r₂ peaks
Average Coordination Number	~4.05	-	Estimated from first PDF peak area

The sp³ amorphous carbon was synthesized using a large-volume press under High Pressure High Temperature (HPHT) conditions:

Precursor Preparation: Sublimed C₇₀ powders (Tokyo Chemical Industry Co., Ltd) were enclosed within a rhenium capsule, which also served as the heating element.
HPHT Assembly: A 10-MN Walker-type large-volume press was used with a 7/3 (octahedral edge length of pressure medium/truncated edge length of anvil) cell assembly.
Compression Cycle: The samples were slowly compressed to the target pressure (18 GPa to 30 GPa) over approximately 10 hours at room temperature.
Heating Cycle: The temperature was ramped up to the target range (900 °C to 1200 °C) at a rate of 100 °C min^-1 and held for 1-2 hours to induce the fullerene collapse and transformation.
Quenching and Recovery: The heating power was rapidly shut off, quenching the temperature (~500 °C s^-1). Pressure was then slowly released over approximately 15 hours to recover the bulk amorphous samples.
Structural Analysis: Microstructure was characterized using X-ray Diffraction (XRD), High-Resolution Transmission Electron Microscopy (HRTEM), and Inverse Fast Fourier Transform (FFT) analysis to quantify MRO clusters.
Property Measurement: Thermal conductivity was measured using noncontact Time-Domain Thermoreflectance (TDTR) methods, and hardness was measured using a Vickers microhardness tester.

Advanced Heat Sinks and Spreaders: The material’s record thermal conductivity (36.3 W m^-1 K^-1) makes it an exceptional candidate for thermal management solutions in high-power electronics, 5G infrastructure, and high-performance computing, surpassing conventional amorphous materials.
High-Wear Protective Coatings: The ultrahard nature (109.8 GPa) is suitable for industrial applications requiring extreme durability, such as protective layers on precision machinery, aerospace components, and specialized tooling.
Optical Windows and Lenses: The high transparency and wide bandgap (2.80 eV) allow its use in protective optical components, particularly those exposed to harsh mechanical or thermal stress, or requiring transmission in the UV spectrum.
Tailored Amorphous Materials R&D: The demonstrated strategy of using topologically distinct precursors (C₇₀ vs C₆₀) to control SRO/MRO provides a novel pathway for engineering other covalently bonded amorphous solids (like amorphous boron nitride or silicon) for specific mechanical or electronic properties.
Diamond-Like Carbon (DLC) Alternatives: This bulk sp³ AC offers superior thermal properties compared to typical DLC films, potentially enabling new applications where bulk, ultrahard, thermally conductive amorphous materials are required.