Semi-quantitative analysis of the structural evolution of mesophase pitch-based carbon foams by Raman and FTIR spectroscopy

Abstract

Graphitized carbon foams (GFms) were prepared using mesophase pitch (MP) as a raw material by foaming (450 °C), pre-oxidation (320 °C), carbonization (1 000 °C) and graphitization (2 800 °C). The differences in structure and properties of GFms prepared from different MP precursors pretreated by ball milling or liquid phase extraction were investigated and compared, and semi-quantitative calculations were conducted on the Raman and FTIR spectra of samples at each preparation stage. Semi-quantitative spectroscopic analysis provided detailed information on the structure and chemical composition changes of the MP and GFm derived from it. Combined with microscopic observations, the change from precursor to GFm was analyzed. The results showed that ball milling concentrated the distribution of aromatic molecules in the pitch, which contributed to uniform foaming to give a GFm with a uniform pore distribution and good properties. Liquid phase extraction helped remove light components while retaining large aromatics to form graphitic planes with the largest average size during post-treatment to produce a GFm with the highest degree of graphitization and the fewest open pores, giving the best compression resistance (2.47 MPa), the highest thermal conductivity (64.47 W/(m·K)) and the lowest electrical resistance (13.02 μΩ·m). Characterization combining semi-quantitative spectroscopic analysis with microscopic observations allowed us to control the preparation of the MP-derived GFms.

1. Introduction

Carbon foam, a light carbon material characterized by its sponge-like interconnected porous structure, exhibits remarkable properties including low density, high thermal stability and impact resistance. Furthermore, its customizable thermal and electrical conductivity holds great potential for applications in structural materials, thermal management, electrochemistry and energy storage, catalyst supports, etc.[1–4] Carbon foam can be synthesized from different carbonaceous precursors, polymers such as phenolic resin and polyurethane, various pitches including coal tar pitch, petroleum pitch and naphthalene pitch, as well as biomass materials[5]. Among them, pitch-based carbon foam can form a highly ordered graphite structure after graphitization, exhibiting excellent properties, which has received extensive attention from academia and industry[5].

The unique thermal, electrical and mechanical properties of carbon foams are mainly attributed to the highly aligned or oriented graphitic structure along the pore walls[6]. The pore structure and graphite crystallite size are greatly influenced by the process control, thus investigating the evolution of foam carbon in the whole preparation process is curical. In previous studies, researchers mainly applied electron microscopy and polarized optical microscopy to visually illustrate the evolution of pore structure and graphite orientation[2,7,8], or qualitative Raman and FTIR spectroscopic analysis to indirectly infer changes in fine structure and chemical composition. Nevertheless, it is challenging to provide effective guidance for the controllable synthesis of carbon foam solely through this qualitative characterization. In this study, we conducted further semi-quantitative spectral analysis through peak fitting and calculation of specific aromatic parameters. This approach allowed us to obtain key information about the content and size of polycyclic aromatic hydrocarbons and aliphatic hydrocarbon side chains, thereby simplifying the characterization and analysis of carbon foam through microscopic verification.

Plastic properties have an important impact on the foaming behavior of pitch precursors, and normally, high dilatation and fluidity could induce low-density foams[9]. Therefore, different precursor pretreatments were often applied, such as ball milling and liquid phase extraction. The purpose of ball milling is to achieve uniform particle size for uniform softening. Additionally, the high-energy mechanochemical facilitate the conversion of carbonaceous molecules, thereby adjusting pitch fluidity[10]. In liquid phase extraction, the light components that are rich in transferable hydrogen can be removed by solvent dissolution and volatilization, to stabilize free radicals generated by pyrolysis, and fluidity can be decreased due to larger condensed aromatic molecules extracted[9].