Graphene nanoplatelets: Thermal diffusivity and thermal conductivity by the flash method

Abstract

The present work deals with the measurement of thermo-physical properties of a freestanding sheet of graphene (thermal diffusivity and thermal conductivity), and their dependence on sample density as result of uniform mechanical compression. Thermal diffusivity of graphene nano-platelets (thin slabs) was measured by the pulse flash method. Obtained response data were processed with a specifically developed least square data processing algorithm. GNP specific heat was assumed from literature and thermal conductivity derived from thermal diffusivity, specific heat and density. Obtained results show a significant difference with respect to other porous media: the thermal diffusivity decreases as the density increases, while thermal conductivity increases for low and high densities, and remain fairly constant for the intermediate range. This can be explained by the very high thermal conductivity values reached by the nano-layers of graphene and the peculiar arrangement of platelets during the compression applied to the samples to get the desired density. Due to very high thermal conductivity of graphene layers, the obtained results show that thermal conductivity of conglomerates increases when there is an air reduction due to compression, and consequent density increases, with the number of contact points between platelets also increased. In the intermediate range (250 ≤ ρ ≤ 700 kg·m-3) the folding of platelets reduces density, without increasing the contact points of platelets, so thermal conductivity can slightly decrease.

Salt templated and graphene nanoplatelets draped copper (GNP-draped-Cu) composites for dramatic improvements in pool boiling heat transfer

“…Higher wickability of the coatings allows a continuous liquid supply to the nucleation cavities during boiling, thereby delaying the vapor layer formation and enhancing the critical heat flux of the coatings. Several reports 38 – 40 have shown that thermal conductivity of a few layered graphene (2–4 layers) is in the range of ~ 2,300 W/m K to ~ 3,000 W/m K (as against 3,000 W/m K to 5,000 W/m K for a single layer of graphene). And reduces to ~ 2000 W/m K with increase in the number of deposited graphene layers (greater than 4 layers).…”

Transmittance and Reflectance Effects during Thermal Diffusivity Measurements of GNP Samples with the Flash Method

“…In [6] a better description of the real flash signal was adopted, approximating the pulse with two overlapping exponentials, each with its own time constant:Sfalse(τfalse)=A[expfalse(−R1τfalse)−expfalse(−R2τfalse)] where A represents the signal intensity connected with the supplied energy, while R 1 , and R 2 are the inverses of the two time constants, the first linked to the lamp filament temperature increase, and the second to the time constant of the flash capacitor discharge. The way to determine R 1 and R 2 is described in [6].…”

“…Using Equation (2) as input, in the way described by the Green function method [16], the solution is as follows (see Appendix A of [6]):Tfalse(L,τfalse)=A′{1−exp(−R1τ)R1−exp(−R2τ)−1R2+2true∑n=1∞[false(−1false)n×(exp(R1τ)−exp(−(nπL)2ατ)(nπL)2α−R1−exp(R2τ)−exp(−(nπL)2ατ)(nπL)2α−R…”

“…In order to find out this relation, a type K flat thermocouple (homemade) was applied to the measured surface of the sample during one specific test, and an asymptotic temperature increase of 22 °C was measured, corresponding to a pyrometer signal increase of 0.226 V. Thus, a sensitivity of the pyrometer of 10.3·mV⋅°C −1 is deduced. The linearity between the detector output (in V) and the temperature increase (in °C) leads to write, as in [6]:Sfalse[normalVfalse]=c+d·t false[°normalCfalse]…”

Direct Measurement of Temperature Diffusivity of Nanocellulose-Doped Biodegradable Composite Films

“…High-temperature diffusivity , defined by the thermal conductivity [W/(m·K)], mass density [kg/m ] and specific heat capacity [J/(kg·K)], is a necessary trait for optimised polymeric materials and their composites. As graphene has a very high in-plane 3000 W/(m·K) conductivity, it is a potential filler in polymeric and epoxy hosts for increased thermal and electrical conductivity; the out-of-plane conductivity of graphene-stack is only 5 W/(m·K) [ 4 ]. Composites that use graphene as the filler, it was previously shown that the thermal conductivity is increasing linearly with its weight percentage (up to 30 wt% before saturation), and in the case of NC-paper with 10 wt% of graphene, reached high 25 W/(m·K) values [ 5 ].…”