Disorder-Induced Extremely Low Thermal Conductivity of Graphite Fluoride.

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Graphene-based materials, like graphite fluoride, are crucial functionalized nanomaterials.
• Theoretical studies explored graphene's thermal conductivity, but experimental data on fluorination effects, particularly through-plane conductivity, is scarce.

Purpose of the Study:

• To experimentally determine the through-plane and in-plane thermal conductivity of graphite fluorides.
• To investigate the impact of structural disorders on thermal transport properties.
• To analyze the effect of surface fluorination on interfacial thermal conductance.

Main Methods:

• Mechanically exfoliated graphite fluorides were used.
• Co-aligned and beam-offset time-domain thermoreflectance techniques were employed.
• Frequency-domain thermoreflectance was used for cross-validation.

Main Results:

• Graphite fluorides demonstrated exceptionally low through-plane thermal conductivity (< 0.030 W m^-1 K^-1) for thicknesses of 53-243 nm, the lowest among fully dense solids.
• In-plane thermal conductivity was measured at 4.2-5.6 W m^-1 K^-1 for a 178 nm flake, yielding a thermal conductivity anisotropy ratio exceeding 100.
• Structural disorders, including interlayer spacing variations and nm-sized fluorinated grains, were identified as causes for reduced thermal conductivity.

Conclusions:

• Structural disorder in graphite fluorides leads to unprecedentedly low thermal conductivity values.
• The findings establish new lower bounds for thermal conductivity in functionalized layered materials.
• Surface fluorination has a minimal effect on interfacial thermal conductance.