Quantitative structural and textural assessment of laminar pyrocarbons through Raman spectroscopy, electron diffraction and few other techniques

Abstract

In pyrocarbon materials, the width of the Raman D band (FWHM_D) is very sensitive to low energy structural defects (e.g., disorientations of the graphene layers). The correlation between the two parameters, FWHM_D and OA (as derived from selected area electron diffraction: SAED), has allowed to differentiate various pyrocarbons unambiguously. Furthermore, the optical properties of pyrocarbons, i.e., the extinction angle, the optical phase shift and the ordinary and extraordinary reflectance, have been accurately determined at 550 nm by means of the extinction curves method. These results are completed by in-plane and out-of-plane dielectric constant measurements by angular resolved EELS. Moreover, the hybridization degree of the carbon atoms has been assessed by the same technique. About 80% of the carbon atoms of the pyrocarbons have a sp² hybridization. The lack of pure sp² carbon atoms, as compared to graphite, might be explained by the presence of sp³-like line defects.

Introduction

Rough laminar pyrocarbon (RL) has been extensively studied through the past decades due to a number of successful applications. Most of the commercial C/C composite matrices, processed by chemical vapour infiltration (CVI), indeed consist of RL pyrocarbon. This low temperature pyrolytic carbon matrix exhibits the best range of properties (ablation, tribological properties, Young’s modulus, thermal diffusivity, density) among all its low temperature counterparts. Lieberman and Pierson [1] named it in the early seventies, referring to the aspect of its Maltese-cross when observed by polarized optical microscopy. The development of new densification methods of felts, like pulsed-CVI or thermal gradient CVI, led to another attractive pyrocarbon called regenerative laminar pyrocarbon (ReL) [2]. ReL indeed shares common characteristics with its famous predecessor. Like RL, ReL is a strongly anisotropic, graphitizable and high modulus pyrocarbon. However, it failed to dismiss RL because of its poorer properties, especially for tribological applications.

Two methods were developed aiming at an objective structural control of pyrocarbon matrices. First, the measurement of the extinction angle Ae was introduced by Diefendorf and Tokarsky [3]. This technique is very fast and easy as it only requires an optical microscope for metallography with two polarizers. The second method is the assessment of the orientation angle OA, measured from the selected area electronic diffraction (SAED), introduced by Bourrat et al. [4]. OA is the full width at half maximum (FWHM) of the Gaussian distribution of orientation of the graphene planes around the anisotropy plane. The two methods were linearly related [4], indicating they both quantify adequately the anisotropy¹ of the matrix. These characterization tools were extensively applied by authors who wished to understand the growth mechanism of low temperature pyrocarbons under various deposition conditions. New classifications were obtained in this way, with other designations related to the anisotropy of the material, like those proposed by Reznik and Hüttinger [5] or Le Poche et al. [6].

Since ReL pyrocarbon was still unknown through the classical chemical vapour deposition/infiltration (CVD/CVI) conditions, the well-known low temperature textural transition usually referred to as smooth laminar/rough laminar (SL/RL) was in reality a SL/ReL transition. The similar anisotropy of both RL and ReL, as assessed by their respective Ae and OA values, indeed led to a misunderstanding of the pyrocarbon texture/structure. Other recent works are also difficult to consider with the current classification. For instance, the highly textured pyrocarbon mentioned by Reznik and Hüttinger [5] can be either considered as ReL or RL. Thus, the correlation between the growth mechanisms and the CVD/CVI conditions of deposition of RL are still a subject of controversial debate, essentially owing to inappropriate tools for the characterization of the texture/structure of pyrocarbons.

Describing two distinct features (i.e., texture and structure) with only one parameter (usually Ae) is insufficient to discuss the growth mechanism of pyrocarbons adequately. The aim of the present work is to overcome inconsistencies, which may arise (e.g., the confusion between RL and ReL), by proposing other complementary techniques and new textural/structural parameters for a relevant analysis of pyrocarbons.

Six different C/C composite matrices (with rather different structural and textural features), belonging to the three main types of pyrocarbon: RL, ReL and SL, have been considered for the present study.

The first technique, which has been used for the assessment of the structural properties, is Raman microspectroscopy (RMS). Raman spectra of carbon materials are indeed very sensitive to structural defects. The FWHM value of the different bands of the first order Raman spectra of sp² carbons has been related for years to the density of in-plane structural defects in carbons [7], [8].

The second technique, considered for the texture analyses, is photo-spectroscopic microscopy. Different authors developed quantitative automatic [9] or semiautomatic [10] procedures based on polarized microscopy. The present approach is an improvement in the measurement of the extinction angle (Ae) [10]. A microspectrophotometer was used to determine precisely Ae at a given wavelength by a complete fitting of the extinction curves. This procedure also leads to another parameter characteristic of pyrocarbons, the optical phase shift (δ in nm) between the ordinary and extraordinary beams reflected parallel and perpendicular to the graphene plane. Ordinary and extraordinary reflectance can also be deduced from the same technique and compared to values determined in a more usual way. Accurate values of Ae and optical phase shift will be given and discussed for the three types of pyrocarbon studied.

Thereafter, angular resolved electron energy loss spectroscopy (EELS) of the C-K transitions have been applied to RL, ReL and SL, in order to resolve the anisotropy of their electronic properties. The EELS analysis gives both the in-plane and out-of-plane dielectric components, as well as the hybridization of the carbon atoms in the material.