Correlating negative thermal expansion and thermal conductivity in two-dimensional carbon-based materials

Area of Science:

• Condensed Matter Physics
• Materials Science
• Nanotechnology

Background:

• Negative thermal expansion (NTE) is a counterintuitive phenomenon where materials contract upon heating.
• Understanding phonon transport properties is crucial for controlling thermal conductivity (TC) in 2D carbon allotropes.
• The NTE and TC behaviors of various 2D carbon allotropes remain incompletely understood.

Purpose of the Study:

• To investigate the mechanisms governing NTE and TC in graphene, haeckelite, pentahexoctite, s-graphene, 6.6.12-graphyne, and delta-graphyne.
• To correlate structural features, such as pore size and buckling, with observed thermal properties.
• To elucidate the role of phonon modes, group velocity, lifetime, and mean free path in determining NTE and TC.

Main Methods:

• First-principles calculations were employed to analyze phonon transport properties.
• Investigated soft-phonon modes, phonon group velocity (vg), phonon lifetime (τ), and mean free path (MFP).
• Examined the influence of out-of-plane buckling and specific pore sizes on thermal characteristics.

Main Results:

• Delta-graphyne demonstrated the highest NTE and lowest TC, whereas graphene exhibited the inverse.
• Graphene showed lower anisotropic TC, while s-graphene displayed the highest anisotropic TC.
• Out-of-plane buckling was found to decrease TC by increasing phonon scattering but enhance NTE.

Conclusions:

• The study deepens the understanding of NTE and TC mechanisms in 2D carbon materials.
• Structural control, particularly pore size and buckling, significantly influences soft unit modes and thermal properties.
• These findings highlight the potential of engineered carbon allotropes for applications in nanotechnology and composites requiring tailored thermal expansion.