What is Graphene Powder and What Are Its Functions?

Overview: Graphene is a two-dimensional, atom-thin, hexagonal carbon allotrope, where each vertex represents a single carbon atom. It is the fundamental building block of other carbon allotropes, including graphite, charcoal, carbon nanotubes, and fullerenes. Graphene can also be considered the ultimate case of an infinitely large aromatic molecule, part of the polycyclic aromatic hydrocarbon family.

Graphene powder possesses numerous remarkable properties. Proportional to its thickness, it is about 100 times stronger than steel. It is highly efficient in conducting heat and electricity and is nearly transparent. Additionally, graphene exhibits significant nonlinear diamagnetism, even greater than that of graphite, allowing it to be suspended by NdFeB magnets. Researchers have identified several key properties in graphene, including bipolar transistor effects, ballistic transport of charges, and a wealth of quantum oscillations within the material.

Historical Context and Discovery

For decades, scientists have conducted theoretical studies on graphene. It’s possible that trace amounts of graphene were inadvertently produced over centuries through the use of graphite in pencils and other similar applications. Graphene was first observed under an electron microscope in 1962, but it was only when loaded onto metal surfaces that it was thoroughly studied. The ease of separating high-quality graphite made further research possible. This work, recognized for its pioneering experiments on the two-dimensional material graphene, earned the Nobel Prize in Physics in 2010.

Thermal Conductivity of Graphene Powder

Graphene’s thermal conductivity is an active research area due to its potential applications in thermal management. Early measurements of the thermal conductivity of suspended graphene suggested very high values, around 5300 W/m·K, compared to pyrolytic graphite’s room-temperature thermal conductivity (close to 2000 W/m·K). However, subsequent studies have questioned whether these high values were overestimated, with measurements of suspended single-layer graphene yielding values between 1500-2500 W/m·K. The wide range of reported thermal conductivities may be due to measurement uncertainties and variations in graphene quality and processing conditions.

When single-layer graphene is supported on amorphous materials, its room-temperature thermal conductivity drops to about 500-600 W/m·K due to phonon scattering by the substrate. For graphene encapsulated in amorphous oxides, the thermal conductivity is even lower. Similarly, polymer residues can reduce the thermal conductivity of suspended bilayer graphene to about 500-600 W/m·K.

Isotopic composition can significantly impact thermal conductivity. For instance, pure 12C graphene exhibits higher thermal conductivity than graphene with a 50:50 isotopic ratio or naturally occurring isotopic composition (99:1). Using the Wiedemann-Franz law, it can be shown that thermal conduction in graphene is primarily phonon-driven. However, for gate-controlled graphene bands, the Fermi energy shift caused by an applied gate bias is much larger than kBT, leading to increased electronic contributions and dominance over phonon contributions at low temperatures. Graphene’s ballistic thermal conductivity is isotropic.

Key Functions of Graphene Powder

1. Mechanical Strength: Graphene powder is extremely strong, being about 100 times stronger than steel when considering its thickness.
2. Electrical Conductivity: It efficiently conducts electricity, making it suitable for various electronic applications.
3. Thermal Conductivity: Graphene is highly effective in conducting heat, which is beneficial for thermal management applications.
4. Transparency: It is nearly transparent, allowing for potential uses in transparent electronic devices.
5. Magnetic Properties: Graphene exhibits strong diamagnetism, enabling unique applications like magnetic levitation.

Conclusion

Graphene powder’s exceptional properties make it a promising material for a wide range of applications, from electronics to thermal management and beyond. Despite the challenges in large-scale industrialization, ongoing research and development continue to unlock its potential in various fields.