Elsevier

Carbon

Volume 40, Issue 9, August 2002, Pages 1475-1486
Carbon

Chemical, microstructural and thermal analyses of a naphthalene-derived mesophase pitch

https://doi.org/10.1016/S0008-6223(01)00320-7Get rights and content

Abstract

A detailed characterisation of a synthetic naphthalene-derived mesophase pitch, in its as-received state and during pyrolysis, has been performed. The study has been conducted by means of various techniques and with a particular attention to Raman microspectroscopy. The Raman spectra show features in common with the naphthalene precursor, i.e., a broad and complex band at 1150–1500 cm−1 and a multicomponent G band at 1600 cm−1. These features correspond to the vibration modes of the molecules of the pitch and more especially to the non-aromatic C–C bonds involved in alkyl groups, aryl–aryl bonds or naphthenic rings. The pyrolysis of the pitch into coke takes place within a narrow temperature range (480–500 °C) through the elimination of hydrogen and light alkanes resulting from the breaking of homolytic C–H bonds and naphthenic cycles, respectively. This process initiates a swelling of the pitch. The analysis of the Raman features shows that the structure of the pitch is only slightly affected within this temperature range. Conversely, significant structural changes of the material (as shown by the vanishing of the multicomponent bands at 1600 and 1150–1500 cm−1) are evidenced beyond 750 °C, simultaneously with a hydrogen release and an increase of the true density. This phenomenon corresponds to the extension of the graphene layers of the coke and the formation of a distorted carbon network.

Introduction

Carbon–carbon composites (C–C) exhibit a large variety of valuable properties for structural applications, such as high strength and stiffness at high temperature, a low density, a low thermal expansion, a wide range of thermal conductivity, a high ablation–abrasion resistance and a good friction behaviour. Initially developed for a high technology field, i.e., for military and space (rocket nozzles, exhaust cones, etc.) [1], new generation C–C composites are being developed for civil applications, implying a significantly larger production. C–C brakes for aircrafts represent the most important part of the currently produced C–C composites. A wider application range for such materials is still difficult to achieve because of their high manufacturing cost. Much research has been devoted to the development of faster, cheaper and effective processes. Chemical vapour deposition (CVD) is generally used to process high performance C–C composites [2]. An interesting alternative is the impregnation and pyrolysis of a liquid precursor, such as pitch-based for instance. Pitch precursors are either derived from the distillation of coal tar or petroleum. They are converted into carbon matrix according to appropriate heat treatments usually requiring severe conditions (e.g., very high pressures, typically 70–100 MPa) [3].

From 1987, on the basis of Mochida’s work, Mitsubishi Gas Chemical (MGC) and Mitsubishi Oil (MO) [4], [5], [6], [7] have simultaneously developed a new generation of synthetic pitches derived from polyaromatic systems. In addition to the advantage of a better control of the composition, these synthetic pitches are fully anisotropic, they are characterised by a relatively low viscosity and a high coke yield (beyond 80 wt.%). One should therefore consider such a precursor for the densification of three-dimensional fibrous textures. This study mainly focuses on a 100% anisotropic commercial precursor synthesised from naphthalene, the ara24r from MGC [4], [5]. The physical and the chemical properties of the as received ara24r precursor have been first investigated and compared to other commercial pitches. The mechanism of the low-pressure pyrolysis of the pitch and its conversion into carbon has been subsequently studied to precisely define the optimal conditions for the C–C composite processing.

Section snippets

Experimental procedures

The precursor was characterised by polarised light optical microscopy. The observations were carried-out in the reflection mode on polished solid pitch samples embedded in epoxy resin. The specimens were prepared from as-received pitch granules or from solidified samples after melting at 370 °C. The microscope (Leitz) was equipped with crossed polarisers and a tint λ plate to clearly evidence the anisotropic character of the pitch, from the interference colours resulting from the birefringence

General properties

The synthetic mesophase pitch was provided by MGC under the reference ara24r (batch number 6T10). The properties of the pitch are shown in Table 1. The chemical composition, as assessed by elementary analysis, is presented in Table 2. They are compared with the properties of two other synthetic mesophase pitches synthesised by MGC from methylnaphthalene (5T14 from MGC) or alkylbenzene (MSP285) and two isotropic pitches, either coal tar (V pitch, from HGD, Centre de Pyrolyse de Marienau, France)

Conclusion

The various characterisations of the ara24r pitch, in the as-received state and during pyrolysis, have provided detailed information on the structure, composition and thermal behaviour of the pitch. These properties are helpful to consider the application of this pitch as a carbon matrix precursor. All the analyses and the pyrolysis studies of the pitch, either performed simultaneously (TGA, DTA, mass spectrometry, etc.) or in parallel (elemental analyses, Raman spectroscopy, etc.) were

Acknowledgements

This work has been supported through a grant given by CNRS and Snecma-Moteur to M.D. The authors are indebted to Professor I. Mochida for his assistance and for valuable discussions as well as to Mitsubishi Gas Chemical for providing different batches of aromatic pitch.

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