Structural and Synthetic Modification of Graphitic Foams and Dendritic Graphitic Foams for Thermal Management

Abstract

Porous graphitic foam structures have been explored for applications ranging from electrochemical energy storage devices, [1][2][3][4] packaging and thermal interface materials, [5] reinforced polymer composites, [6] and flexible electronics. [7,8] Because of the high basal-plane thermal conductivity of graphite, porous graphitic foams have been used for thermal management, [9,10] such as providing an interconnected open cellular network to transfer heat from low-thermal conductivity phase change materials (PCMs) filled in the pore space for thermal energy storage. [11][12][13] By overcoming the high interface thermal resistance problem that limits the effective thermal conductivity ðκ e Þ of van der Waals networks of graphitic nanostructures, [11] the 3D continuous porous foam structures achieve considerably higher κ e than the van der Waals networks. Compared with highdensity carbon foams grown by other methods, [14][15][16][17] relatively low-density graphitic foam (GF) grown by chemical vapor deposition (CVD) on reticular nickel (Ni) foam templates is suitable for applications such as PCM thermal energy storage, where the graphite volume fraction (ϕ g ) needs to be controlled to provide both sufficient thermal transport enhancement and only a small reduction of the volume fraction of the energy-storing PCM.However, the thermal performance enhancement by CVD GF grown on reticular Ni foam is limited due to the relatively large pore size, which results in a large distance and resistance for heat flow from the functional materials inside the pore space to the high-thermal conductivity graphitic matrix grown on the sacrificial catalyst template. The low specific surface area associated with the large pores also limits the ϕ g and κ e . If the pore size can be reduced without altering the struct thickness and solid thermal conductivity (κ s ) of the graphite struts, the κ e of the GF is expected to increase together with an increase of the ϕ g . One approach to reducing the pore size is to use sintered Ni powder bed or Ni powder sintered in reticular Ni foam to replace the reticular Ni foam as the catalyst for CVD growth of powder-derived graphite foam (PGF) and hybrid GF-PGF (HGF) structures, respectively. [18] In another approach, carbon nanotubes (CNTs) have been grown on the strut walls of GF to fill the pore space and increase the specific surface area. [13,19] The κ e of these hybrid CNT-GF structures with ϕ g increased to 1.8% is about 2.4 times the value reported for GF with ϕ g of 0.45%. Although the obtained κ e of CNT-GF is still lower than PGF with ϕ g as high as 8%, the high specific surface provided by the additional CNTs yielded enhanced thermal performance, such as improved interfacial thermal transport and additional heterogeneous nucleation sites to suppress subcooling of composite PCMs for thermal energy storage. [13] However, the CNT-GF growth process requires two-step CVD reactions: one for GF growth on the reticular Ni foam template and the other for CNT growth with iron catalyst deposited on ...