Effect of Graphene Layer Thickness and Mechanical Compliance on Interfacial Heat Flow and Thermal Conduction in Solid–Liquid Phase Change Materials

Solid–liquid phase change materials (PCMs) are attractive candidates for thermal energy storage and electronics cooling applications but have limited applicability in state-of-the-art technologies due to their low intrinsic thermal conductivities. Recent efforts to incorporate graphene and multilayer graphene into PCMs have led to the development of thermal energy storage materials with remarkable values of bulk thermal conductivity. However, the full potential of graphene as a filler material for the thermal enhancement of PCMs remains unrealized, largely due to an incomplete understanding of the physical mechanisms that govern thermal transport within graphene-based nanocomposites. In this work, we show that the number of graphene layers (<i>n</i>) within an individual graphene nanoparticle has a significant effect on the bulk thermal conductivity of an organic PCM. Results indicate that the bulk thermal conductivity of PCMs can be tuned by over an order of magnitude simply by adjusting the number of graphene layers (<i>n</i>) from <i>n</i> = 3 to 44. Using scanning electron microscopy in tandem with nanoscale analytical techniques, the physical mechanisms that govern heat flow within a graphene nanocomposite PCM are found to be nearly independent of the intrinsic thermal conductivity of the graphene nanoparticle itself and are instead found to be dependent on the mechanical compliance of the graphene nanoparticles. These findings are critical for the design and development of PCMs that are capable of cooling next-generation electronics and storing heat effectively in medium-to-large-scale energy systems, including solar–thermal power plants and building heating and cooling systems

Quantification of the Impact of Embedded Graphite Nanofibers on the Transient Thermal Response of Paraffin Phase Change Material Exposed to High Heat Fluxes

Paraffin phase change material (PCM) is enhanced with suspended graphite nanofibers at high loading levels. The loading levels reach in excess of 10% by weight. The thermal effects of the nanofiber loading level, the PCM module design, and the applied power density on the transient thermal response of the system are examined. A strong effect of nanofiber loading level on thermal performance is found, including a suppression of Rayleigh-Benard convection currents at high loading levels. Increases in nanofiber loading level also result in lowered heating rates and greater thermal control of the heated base. Increases in power density are found to result in higher heating rates, and increases in mass lead to lower operating temperatures. The design of the module is found to have a strong effect on thermal performance

Strained Polymer Thermal Conductivity Enhancement Counteracted by Additional Off-Axis Strain

Thin-film (5-20 μm) polymer dielectrics are a critical component in high energy density capacitors. Understanding thermal transport in these materials is critical to addressing thermally enabled dielectric breakdown, a primary failure mechanism. Here, we measure the anisotropy in thermal conductivity for strained polymer films using frequency-domain thermoreflectance (FDTR), which provides us with unique sensitivity to thermal conductivity in the cross-plane and radial directions using a wide range of imposed modulation frequencies. We find that the anisotropy in the thermal conductivity is significantly enhanced by in-plane polymer alignment with strain in polypropylene films. Interestingly, this enhancement is then reduced by the application of additional strain in the orthogonal direction. We use insights from molecular dynamics simulations and Raman spectroscopy to understand the physical mechanism for this reduction

Molecular Tuning of the Vibrational Thermal Transport Mechanisms in Fullerene Derivative Solutions

Control over the thermal conductance from excited molecules into an external environment is essential for the development of customized photothermal therapies and chemical processes. This control could be achieved through molecule tuning of the chemical moieties in fullerene derivatives. For example, the thermal transport properties in the fullerene derivatives indene-C₆₀ monoadduct (ICMA), indene-C₆₀ bisadduct (ICBA), [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM), [6,6]-phenyl C₆₁ butyric acid butyl ester (PCBB), and [6,6]-phenyl C₆₁ butyric acid octyl ester (PCBO) could be tuned by choosing a functional group such that its intrinsic vibrational density of states bridge that of the parent molecule and a liquid. However, this effect has never been experimentally realized for molecular interfaces in liquid suspensions. Using the pump–probe technique time domain thermotransmittance, we measure the vibrational relaxation times of photoexcited fullerene derivatives in solutions and calculate an effective thermal boundary conductance from the opto-thermally excited molecule into the liquid. We relate the thermal boundary conductance to the vibrational modes of the functional groups using density of states calculations from molecular dynamics. Our findings indicate that the attachment of an ester group to a C₆₀ molecule, such as in PCBM, PCBB, and PCBO, provides low-frequency modes which facilitate thermal coupling with the liquid. This offers a channel for heat flow in addition to direct coupling between the buckyball and the liquid. In contrast, the attachment of indene rings to C₆₀ does not supply the same low-frequency modes and, thus, does not generate the same enhancement in thermal boundary conductance. Understanding how chemical functionalization of C₆₀ affects the vibrational thermal transport in molecule/liquid systems allows the thermal boundary conductance to be manipulated and adapted for medical and chemical applications