Nayandeep K. Mahanta

Thermal conductivity of graphene and graphene oxide nanoplatelets

The superior thermal transport in graphene has been a topic of great interest to the scientific community, for graphene is envisioned to be important in numerous applications such as thermal management of electronics. While single layer graphene exhibits high thermal conductivity, molecular and lattice dynamics simulations reveal that even in the presence of one or few additional layers, thermal conductivity can be significantly reduced. In fact, with increasing number of layers, thermal conductivity is expected to eventually approach the value of bulk graphite. The interlayer spacing is also known to have a significant influence on thermal conductivity, for it is the combination of the number of layers and the spacing between them that truly is responsible for the thermal conductivity of a multi-layer graphene platelet. Here, we report the experimentally obtained thermal conductivities for nanoplatelets of graphene oxide and reduced graphene exfoliated to differing degrees. Results show that the thermal conductivity measured for reduced graphene platelets with ~ 30 to 45 layers approaches the value of bulk graphite. The thermal conductivity of oxygen intercalated graphene nanoplatelets with ~ 3 layers and 7% oxygen is higher than bulk graphite with similar interlayer spacing. Despite the increased interlayer spacing and presence of the oxygen atoms, which typically enhances phonon scattering, the high value of thermal conductivity can be attributed to the increase in the interlayer coupling due to covalent interactions provided by the oxygen atoms.

The thermal flash technique: The inconsequential effect of contact resistance and the characterization of carbon nanotube clusters

This article presents a comprehensive mathematical treatment of the theory behind the thermal flash technique used to measure the thermal diffusivity of nanostructures. Analytical expressions predicting the temperature and its rate of change for various combinations of sample length and diffusivity confirmed that the presence of contact resistance between the heat sink/source or within a cluster of materials does not influence the measurement. Measurements on multi-walled carbon nanotube clusters provide further experimental evidence supporting the claim that contact resistance is inconsequential to this technique and yield a thermal conductivity of 2665 W/m K, which corresponds to an isolated nanotube and not the overall cluster.

Thermal conductivity measurements on individual vapor-grown carbon nanofibers and graphene nanoplatelets

The thermal flash technique was utilized for measuring the thermal conductivity of vapor-grown carbon nanofibers and graphene nanoplatelets. The vapor-grown carbon nanofibers with stacked-cone morphology and heat treated to 1100 °C and 3000 °C were measured to have thermal conductivities of 1130 W/m K and 1715 W/m K, respectively. The physical dimensions of the constitutive cones determining the mean free path due to static phonon scattering were estimated to be ∼128 nm and ∼176 nm for the low and high heat treatment temperatures, respectively. Static scattering lengths shorter than the Umklapp scattering length indicate ballistic transport within individual cones and limit the thermal conductivities of the nanofibers. Additionally, nanoplatelets of few-layer oxygen intercalated graphene and multi-layer reduced graphene exhibited thermal conductivities of 776 W/m K and 2275 W/m K, respectively. The lower thermal conductivity of few-layer (∼3 layers) graphene is attributed to the presence of intercalating ...

Development of the thermal flash method for characterization of carbon nanofibers

The transient thermal flash technique, originally developed for testing low thermal diffusivity micro/nanofibers, was implemented for measuring the thermal conductivity of vapor-grown carbon nanofibers. The present technique uses a microfabricated strip of gold, which acts both as a heater and a temperature sensor. The modifications were validated against commercially available carbon fibers (Pyrograf® – I from Applied Sciences, Inc. and Mitsubishi K13D2U) and the results obtained were seen to match values previously reported in the literature. The carbon nanofibers reported in this article were also obtained from Applied Sciences, Inc. and are known as PR-25, belonging to the Pyrograf® – III family of nanofibers. The thermal conductivities calculated based on the experimentally determined values of diffusivity along with the specific heat capacity and density of graphite were around 1100 W/m-K and 1700 W/m-K, respectively for the nanofibers heat treated to 1100 °C and 3000 °C.Copyright