The Relationships of Structure to Properties in Graphite Fibers. Part 2

160

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 sp 2 hybridization. The lack of pure sp 2 carbon atoms, as compared to graphite, might be explained by the presence of sp 3 -like line defects.

Section: Hybridization Of Pyrocarbonsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Vallerot¹,

Bourrat²,

Mouchon³

et al. 2006

160

“…Measurements were performed on polished cross-sections under polarized light as described by Diefendorf and Tokarsky [16]. The extinction angle, Ae, is indirectly a measurement of the reflecting power of the carbon.…”

Section: Characterizationmentioning

confidence: 99%

LPCVD and characterization of boron-containing pyrocarbon materials

Jacques

Guette

Bourrat

et al. 1996

146

Pyrocarbon materials containing various amounts of boron have been prepared by LPCVD from BCl 3 -C 3 H 8 -H 2 precursor mixtures. The growth rate is increased with respect to pure pyrocarbon deposition. By increasing BCl 3 / (C 3 H 8 + BCl 3 ) ratio up to 85%, the incorporation of boron can reach 33 at.%. A small amount of boron (e.g. 8 at.%) highly enhances the anisotropy of pyrocarbon, as evidenced by optical microscopy, X-ray diffraction and transmission electron microscopy (selected area diffraction and lattice fringes techniques). X-ray photoelectron spectroscopy has shown that a large fraction of the boron atoms are included by substitution in the carbon layers, the remaining boron atoms belongs to a boron-rich amorphous part of the material. As boron content increases beyond 8 at.%, the anisotropy of the boron-rich pyrocarbon decreases, due to the limited growth and stacking of the carbon layers. Also, amorphous boron-rich regions are more and more abundant as the total amount of boron increases. The oxidation resistance of the C(B) materials is better than that of pure pyrocarbon. It is mainly due to the improvement of the structural organization for the low boron content materials and to the coating of the whole material with a stable boron oxide for materials with a higher boron content.

“…In 1971, Diefendorf and Tokarsky [5,6] introduced the use of a measurement, commonly carried out in petrography [7]: the determination of the extinction angle of carbons, using polarized light microscopy, as a quantitative way to measure their anisotropy. It is a probe of the defects of the crystal lattice, over a selected area and at a specific wavelength.…”

Section: Introductionmentioning

confidence: 99%

A quantitative, space-resolved method for optical anisotropy estimation in bulk carbons

Gillard

Couégnat

Caty

et al. 2015

A B S T R A C TThis paper presents an innovative and quantitative method for characterization of bulk carbons using polarized light microscopy. It yields maps of transverse preferential orientation and quantitative transverse anisotropy. It can be utilized on any region observable with an optical microscope, at any attainable scale. It requires a set of micrographs from the same region of a carbon-based sample at different combinations of polarizer and analyzer positions. Applications of the technique to several actual carbon-based materials are presented and discussed.