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Register a new userNoncured Graphene Thermal Interface Materials: Minimizing the Thermal Contact Resistance
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We report on experimental investigation of thermal contact resistance of the noncuring graphene thermal interface materials with the surfaces characterized by different degree of roughness. It is found that the thermal contact resistance depends on the graphene loading non-monotonically, achieving its minimum at the loading fraction of ~15 wt.%. Increasing the surface roughness by ~1 micrometer results in approximately the factor of x2 increase in the thermal contact resistance for this graphene loading. The obtained dependences of the thermal conductivity, thermal contact resistance, and the total thermal resistance of the thermal interface material layer on the graphene loading and surface roughness indicate the need for optimization of the loading fraction for specific materials and roughness of the connecting surfaces. Our results are important for developing graphene technologies for thermal management of high-power-density electronics.
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This study explores the potentialities of Scanning Thermal Microscopy (SThM) technique as a tool for measuring thermal transporting properties of carbon-derived materials issued from thermal conversion of organic polymers, such as the most commonly known polyimide (PI), Kapton. For quantitative measurements, the Null Point SThM (NP-SThM) technique is used in order to avoid unwanted effects as the parasitic heat flows through the air and the probe cantilever. Kapton HN films were pyrolysed in an inert atmosphere at temperatures up to 1200{deg}C to produce carbon-based residues with varying degree of conversion to free sp2 disordered carbon. The thermal conductivity of carbon materials ranges from 0.2 to 2 Wm-1K-1 depending on the temperature of the carbonization process (varied between 500{deg}C and 1200{deg}C). In order to validate the applicability of NP-SThM approach to these materials, the results were compared to those obtained with the three more traditional techniques, namely photo-thermal radiometry, flash laser analysis and micro-Raman thermometry. It was found that NP SThM data are in excellent agreement with previous work using more traditional techniques. We used the NP-SThM technique to differentiate structural heterogeneities or imperfections at the surface of the pyrolysed Kapton on the basis of measured local thermal conductivity.
The increase in the temperature of photovoltaic (PV) solar cells affects negatively their power conversion efficiency and decreases their lifetime. The negative effects are particularly pronounced in concentrator solar cells. Therefore, it is crucial to limit the PV cell temperature by effectively removing the excess heat. Conventional thermal phase change materials (PCMs) and thermal interface materials (TIMs) do not possess the thermal conductivity values sufficient for thermal management of the next generation of PV cells. In this paper, we report the results of investigation of the increased efficiency of PV cells with the use of graphene-enhanced TIMs. Graphene reveals the highest values of the intrinsic thermal conductivity. It was also shown that the thermal conductivity of composites can be increased via utilization of graphene fillers. We prepared TIMs with up to 6% of graphene designed specifically for PV cell application. The solar cells were tested using the solar simulation module. It was found that the drop in the output voltage of the solar panel under two-sun concentrated illumination can be reduced from 19% to 6% when graphene-enhanced TIMs are used. The proposed method can recover up to 75% of the power loss in solar cells.
We compute the thermal conductance between two nanoparticles in contact based on the Molecular Dynamics technique. The contact is generated by letting both particles stick together under van der Waals attractions. The thermal conductance is derived from the fluctuation-dissipation theorem and the time fluctuations of the exchanged power. We show that the conductance is proportional to the atoms involved in the thermal interaction. In the case of silica, the atomic contribution to the thermal conductance is in the range of 0.5 to 3 nW.K-1. This result fits to theoretical predictions based on characteristic times of the temperature fluctuation. The order of magnitude of the contact conductance is 1 mu W.K-1 when the cross section ranges from 1 to 10nm2.
We study interface thermal resistance (ITR) in a system consisting of two dissimilar anharmonic lattices exemplified by Fermi-Pasta-Ulam (FPU) model and Frenkel-Kontorova (FK) model. It is found that the ITR is asymmetric, namely, it depends on how the temperature gradient is applied. The dependence of the ITR on the coupling constant, temperature, temperature difference, and system size are studied. Possible applications in nanoscale heat management and control are discussed.
We report a systematic study of the contact resistance present at the interface between a metal (Ti) and graphene layers of different, known thickness. By comparing devices fabricated on 11 graphene flakes we demonstrate that the contact resistance is quantitatively the same for single-, bi-, and tri-layer graphene ($sim800 pm 200 Omega mu m$), and is in all cases independent of gate voltage and temperature. We argue that the observed behavior is due to charge transfer from the metal, causing the Fermi level in the graphene region under the contacts to shift far away from the charge neutrality point.
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