Raman Spectroscopy of Graphitic Foams
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Raman Spectroscopy of Graphitic Foams Eduardo B. Barros1,3, Nasser S. Demir3, Antonio G. Souza Filho1, Josué Medes Filho1, Ado Jorio2, Gene Dresselhaus4 and Mildred S. Dresselhaus3,5 1 Departamento de Física, Universidade Federal do Ceará, Fortaleza, CE, Brazil. 2 Departamento de Física, Universidade Federal de Minas Gerais, Belo-Horizonte, MG, Brazil. 3 Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA. 4 Francis Bitter Magnet Lab, Massachusetts Institute of Technology, Cambridge, MA, USA. 5 Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, USA.
ABSTRACT To better understand the very high thermal conductivity to weight ratio of the graphitic foams recently developed at the Oak Ridge National Laboratory, a Raman spectroscopy study was performed. It was also shown that the Raman scattering can be useful for the characterization of the graphitic foam, being able to evaluate the quality of the samples with respect to the density and location of lattice defects.
INTRODUCTION The recent development of a pitch-based graphitic foam obtained through the graphitization of carbon foams has attracted attention due to its potential for use in low-density thermal management devices [1]. This graphitic foam is characterized by highly aligned graphitic structures around the host cell, enclosing the large pores of the foam. With a very high thermal conductivity to weight ratio, graphitic foam is a promising material for applications, particularly in utilizing the high specific thermal conductivity for thermal management applications in the aeronautics and aerospace industries. The structure of the graphitic foams can be described in terms of a regular assembly of spherical cells surrounded by graphitic material, forming a regular tetrahedron of graphitic material with an open sphere associated with each of its vertices. Figure 1(a) shows an illustrative diagram of two junctions calculated for two different porosities (78% and 98%), defined as the ratio between the cell volume to the total volume [2]. Each junction is connected through ligaments to four other junctions in the directions of a regular tetrahedron. The graphitic foam was found to be composed by two intermixed graphitic regions, one where the planes are well aligned and the structural behavior follows that of highly aligned pyrolytic graphite (HOPG), and other regions where the planes are not well aligned, and therefore, the interaction between the graphitic planes is small, leading to properties that are similar to that of isolated graphene (2D-graphite) layers. [3] Previous high resolution TEM and polarized optical measurements have shown that the graphitic planes are aligned parallel to the axis of the ligaments [4]. Also, in these regions, the graphitic structure was found to have a low concentration of lattice defects. However, in the region of the junctions, where the differently orientated ligaments come together, the structure is disorganized, yielding a hig
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