Elsevier

Micron

Volume 89, October 2016, Pages 34-42
Micron

Microstructural investigations of carbon foams derived from modified coal-tar pitch

https://doi.org/10.1016/j.micron.2016.07.006Get rights and content

Highlights

  • The microstructure of coal-tar pitch-derived carbon foams is examined.

  • Microspheres containing sp-bonded species are observed at 1000 °C.

  • Above 1000 °C the microspheres experience transformations in a non-aromatic pathway.

  • Microspheres are identical with the sp2-sp3 foams matrices at 2000 °C.

  • The compressive strength depends on the concentration of sp-bonded carbon atoms.

Abstract

This work reports the microstructural evaluation of carbon foams derived from coal-tar pitch precursors treated with H2SO4 and HNO3 and finally annealed at 1000 °C and 2000 °C. Our experimental investigations combine scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) imaging, X-ray photoelectron spectroscopy (XPS) and micro-spot near-edge X-ray absorption fine structure (μ-NEXAFS) spectroscopy. This set of complementary techniques provides detailed structural and chemical information of the surface and the bulk of the carbon foams. The high-resolution microscopy data indicate the formation of carbonaceous amorphous microspheres (average diameters of 0.28 ± 0.01 μm) embedded in the partially graphitized carbon foam matrix at 1000 °C. The microspheres are enriched with sp-bonded species and their microstructural characteristics depend on the reagent (nitric vs. sulfuric acid) used for pitch treatment. A complete chemical transformation of the microspheres at temperatures >1000 °C occurs and at 2000 °C they are spectroscopically identical with the bulk material (sp2- and sp3-hybridised forms of carbon). The microstructure-property relationship is exemplified by the compressive strength measurements. These results allow a better description of coal-tar pitch-derived carbon foams at the atomic level, and may account for a better understanding of the processes during graphitization step.

Introduction

During the last two decades carbon foam materials have received much attention due to their advantageous properties, such as low density, high mechanical strength, high thermal conductivity and low coefficient of thermal expansion (Inagaki et al., 2015, Gul and Yardim, 2015). Carbon foams are cellular structures that consist of randomly distributed pores with typical sizes between 100 and 500 μm. These unique properties, which mainly depend on the precursors’ features and synthesis conditions, make carbon foams high performance engineering materials, and determine their many potential applications in numerous industries. Carbon foams can be produced as materials with thermal insulating properties, and could be applied as construction materials for airplanes, rockets and thermal management systems (Gallego and Klett, 2003, Klett et al., 2000). Carbon foams are promising radar absorbers, however these materials can be manufactured also with desired electrical resistance, dielectric constant and radar reflection coefficient, suitable for advanced radar antenna construction (Fang et al., 2007; Yang et al., 2004). Carbon foams are relatively inert and stable even at high temperatures and radiation—they are suitable for nuclear shields and rods for nuclear reactors (Gallego et al., 2006). The inertness and mechanical strength of carbon foams determine the suitability of these materials for bone surgery materials, prosthetics and tooth implants (Mathieu et al., 2006).

Ford firstly prepared carbon foams by the pyrolysis of thermosetting polymer in 1964 (Ford, 1964). From there on, carbon foams were successfully produced by using coal, polyimide, melamine, resorcinol/formaldehyde, biomass materials like cork and olive stones, as well as from mesophase pitches as alternative precursors (Inagaki et al., 2015, Gul and Yardim, 2015, Kim et al., 2015, Nagel et al., 2014, Szeluga et al., 2015).

The effect of the precursor on the structure and properties of the obtained carbon foams is of great significance and it is under vast investigations (Klett et al., 2000). When coal-derived pitches are used as precursors, a preliminary treatment is usually required before the foaming process, in order to adjust the viscosity and plastic properties of the pitch that would allow an effective foaming process (Duk et al., 1986, Petrova et al., 2005). Recently, a relatively simple and low cost method was developed for making mesophase-pitch-based carbon foams at low pressure and fast heating rate during the foaming process without a stabilization treatment (Tsyntsarski et al., 2010). Carbon foams with an anisotropic texture and high mechanical strength were produced using precursors obtained after thermo-oxidation treatment of commercial coal-tar pitch with mineral acids (Tsyntsarski et al., 2010). These carbon foams exhibit outstanding mechanical properties as well as a good performance as catalysts supports (Tsyntsarski et al., 2010, Velasco et al., 2010).

The microstructure of carbon foam materials is of key interest, since it determines their functional performances. Thus, a comprehensive characterization of the physical and chemical properties of carbon foams is required in order to obtain information on structure-property relationships. The goal of this work is to yield new insights into the micro- and nanoscale properties of carbon foams derived from commercial coal-tar pitch precursors treated with H2SO4 and HNO3 and finally annealed at 1000 °C and 2000 °C. Herein, we report on the investigation of the latter carbon foams with emphasis on the correlation of morphological and electronic properties using scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and micro-spot near-edge X-ray absorption fine structure (μ-NEXAFS) spectroscopy. The obtained microstructural information is important for tailoring the properties of the coal-tar pitch precursor in order to achieve high performance carbon foam materials.

Section snippets

Materials and methods

The preparation of foaming precursors has been explained in details elsewhere (Tsyntsarski et al., 2010). In summary, the thermo-oxidation treatment of commercial coal-tar pitch with H2SO4 and HNO3 was conducted at 120 °C with continuous stirring. The modified samples were heated up to 350 °C at atmospheric pressure and after that up to 580 °C in a N2 atmosphere, at pressure up to 1 MPa. The resultant “green” foams were calcinated at 1000 °C in N2 atmosphere to increase the strength and further

Results and discussion

Fig. 1 describes the morphology of CFPS and CFPN samples. As shown, the carbon foams are characterized by a reticular vitreous structure with foam cells of size in the range of 200–350 μm. The presence of cracks in the matrices of all materials is also observed, characteristic for well-developed CFP structure. The X-ray diffraction (XRD) analysis of the materials revealed that the CFPs are partially graphitic with interlayer distances between the graphitic carbon sheets ranging from 0.3469 nm for

Conclusions

The following conclusions can be drawn from our combined microscopy and spectroscopy survey. First, the microscopy observations revealed that the thermo-oxidation modification of coal-tar pitch and subsequent foaming procedure and heating at 1000 °C lead to the formation of amorphous carbonaceous microspheres embedded in the partially graphitized carbon foam matrix. These microspheres are mainly composed of sp-bonded species and, furthermore, their presence depends on the modifier (higher

Acknowledgements

The work was done under the frame of MPNS COST Action MP 1306. The PolLux end station was financed by the German Minister für Bildung und Forschung (BMBF) through contracts 05KS4WE1/6 and 05KS7WE1.

References (36)

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