Quantifying the Dispersion in Graphene Nanocomposites Through Thermal Diffusivity Mapping

The efficient and even dispersion of nanoparticles in the fabrication of polymer nanocomposite is considered an industry-wide challenge. The dispersion quality in these systems is paramount to achieving a material with effective properties, but the ability to quantify these dispersions at the macroscopic level is an arduous task. Now, a team of Researchers from the University of Manchester, UK, have developed a quantitative dispersion characterization method, which utilizes non-contact infrared thermography mapping techniques to measure the thermal diffusivity of a graphene nanocomposite and its corresponding dispersion index.

Current ‘gold standard’ methods to measure a dispersion rely on qualitative transmission electronic microscopy (TEM), but is often cumbersome and struggles to quantify the dispersion at the macroscopic level, and is restricted to small samples.

The development of a new method and model to quantify a dispersion is a difficult task and one that needs to take into acoount many considerations. Any method to study the dispersion, at the very least, requires the systematic study of the loading, particle size, agglomeration and interfacial interactions within the nanocomposite.

The Manchester-based Researchers have employed an infrared thermography diffusivity measurement and mapping method on a nanocomposite composed of graphene nanoplatelets (GNPs) and a polymer matrix consisting of a low-viscosity bisphenol-A epoxy resin and a cycloaliphatic polyamine curing agent, which were sheared mixed together (SilverSon L5MPA).

Thermal diffusivity is the rate of heat conduction through a material and assumes that the material is homogeneous and isotropic, and the heat flow is one-dimensional and there is no heat loss. The method is non-destructive and mapping of the thermal diffusivity requires a calculation for each pixel.

Due to the complex nature of nanocomposites, the thermal conductivity of a nanocomposite is reliant on the elastic vibrations of the lattice and the heat flux between nanoparticles and its matrix. As the structure scatters the phonons (the acoustic particles responsible for thermal conductivity), the thermal transport is dependent upon the loading weight, and so the density, heat capacity and thermal diffusivity all had to be taken into account whilst attempting to map the dispersity of the nanocomposite.

The Researchers used a combination of two flash lamps (heat source) to heat the front face of the sample and an infrared (IR) camera (Thermosensorik GmbH) to record the thermal radiation, or the rear surface of each pixel when in transmission mode. Each image that the Researchers took consisted of 400 × 400 pixels with a resolution of 150-200 µm. The Researchers also used differential scanning calorimetry (DSC, Q100) to measure the heat capacity of the sample.

One of the major advantages of this method, is its ability to provide an accurate picture of the GNPs in dispersion at the macroscale; using large samples; and being less cumbersome with a low cost and effort; whilst measuring the thermal properties of the system.

Whilst the resolution reached 200 µm in this study, it is thought that this could be improved upon by using a high magnification lens. Further advances in the resolution have been touted to offer a way to characterize nanocomposite at the microscale.

The Researchers identified a dispersion index to quantify the dispersion of graphene molecules within the polymer matrix. The Researchers observed a linear increase in the thermal conductivity for the composites featuring up to 10 wt% GNP. Post-dispersion treatment was also applied to align the GNPs using an electric field and showed an improvement in the directional thermal conductivity by up to 400% when using 5 wt% GNPs.

One of the key challenges ahead is to reduce the interfacial thermal resistance in order to achieve a superior thermal conductivity. Further research is also required to explore the mechanism associated with the thermal transport across the interface. But the Researchers have provided an alternative to the norm which allows for dispersion quantification on a much larger scale.

In addition to this discovery, the Researchers have also deduced that the anisotropic behavior of the graphene/polymer nanocomposites makes it a suitable candidate in de-icing and lighting strike protection applications for the aviation industry.

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“Thermal Diffusivity Mapping of Graphene Based Polymer Nanocomposites”- Gresil M., et al, Scientific Reports, 2017.

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