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In order to elucidate the mechanism of enhancement of heat transfer in polymer composites, in this work, we investigated two types of polymer-carbon filler composites. This investigation was made using scanning thermal microscopy (SThM) with the Wollaston microprobe operated in active mode as a function of the carbon filler weight fraction within the polymer matrix. Samples consist of high-density polyethylene (HDPE) filled with 50 µm expanded graphite (EG) and polyvinylidene difluoride (PVDF) containing multiwall carbon nanotubes (MWCNTs). For HDPE/EG samples, SThM images allow the detection of zones with a thermal conductance larger than that of the matrix for the highest studied filler concentration. These zones correspond to EG filler agglomerations within the polymer and explain the observed enhancement of the thermal conductivity k of the HDPE/EG composite. For PVDF/MWCNTs samples, it is found that k increases from 0.25 W m−1 K−1 for pristine PVDF to 0.37 W m−1 K−1 for PVDF nanocomposites filled with 8 wt. % MWCNTs. This k variation vs filler concentration is found in good correspondence with that of the β phase relative percentage in the PVDF nanocomposites. This suggests that the observed heat transfer enhancement is rather due to the formation of β phase for PVDF/MWCNTs samples, resulting from the addition of MWCNTs than the addition of MWCNTs itself. Thus, tuning the thermophysical properties of polymer-based nanocomposites can establish new design laws to confer them specific thermal properties. [ABSTRACT FROM AUTHOR]

The main purpose of this study is to focus the light for the first time on the recognized re-dispersion ability of the sonicated MWCNTs within low viscous mineral oil. For that, pure MWCNTs, without any special treatment, were dispersed in mineral oil by sonication without using any stabilization method. After full agglomeration, it was realized that agitating, shaking or even blowing compressed air on the agglomerated nanolubricant is able to break down the aggregation of the MWCNTs. Microscopic video is presented to clarify the behavior of the tubes during the re-dispersion process. Quantitatively, particle size distribution (PSD) was employed to evaluate the re-dispersion quality. The viscosity and thermal conductivity of both recently sonicated and re-dispersed samples were also investigated with accurate measurement setups at four different weight concentrations (0.05, 0.1, 0.2 and 0.3 wt.%). It was observed that the sonicated MWCNTs are separated, saturated, lubricated, and surrounded by oil layers, which not only ease its motion within the lubrication oil, but also give them the ability of detaching by applying any slight force. Moreover, the PSD results with the equal values of the viscosity and thermal conductivity, obtained within the uncertainty of the measurements, confirm that the MWCNTs were re-dispersed to the same quality of the recently sonicated ones by just shaking. [ABSTRACT FROM AUTHOR]

In this study, ethylene-propylene-diene monomer (EPDM) composites reinforced with graphene nanoplatelets (GnPs) and carbon black (CB) fillers and their hybrid combinations of various contents were prepared and examined. The size effect (lateral size and specific surface area) of GnPs and filler hybridization effects between GnPs and CB on the electrical and thermal conductivity of the EPDM composites were examined. The lateral size of GnPs is a more decisive factor in the thermal conductivity of composites than their specific specific surface area, and GnPs with a higher lateral size give better thermal conductivity. The addition of GnPs with a high lateral size and low specific specific surface area results in an almost one-fold increase in thermal conductivity. After a certain filler concentration, a synergistic effect of GnPs and CB on the thermal conductivity can be observed. Up to 20 phr GnPs, the specific surface area or lateral size of GnPs has a slight effect on the electrical conductivity of EPDM/GnPs composites. Higher electrical conductivity was achieved when hybrid fillers, 50 phr CB and 7 phr GnPs with higher lateral size were used for composite preparation. The morphology of the composite samples and the dispersion of all used fillers and hybrids were examined using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). [Display omitted] • GnP fillers with the same surface area and thickness but different lateral size have been used in EPDM composites. • The lateral size of GnPs is a more decisive factor in thermal conductivity of composites than their specific surface area. • GnPs with a higher lateral size give better thermal conductivity. • After a certain filler concentration, it can be observed to a synergic effect of GnPs and CB on the thermal conductivity. • The size of GnPs has a slight effect on the electrical conductivity of EPDM/GnPs nanocomposites. [ABSTRACT FROM AUTHOR]