Miguel A. Montes-Morán

Raman spectroscopy study of HM carbon fibres: effect of plasma treatment on the interfacial properties of single fibre/epoxy composites

Abstract The effect of an oxygen plasma treatment upon the structural and morphological properties of high-modulus carbon fibres has been studied by means of several characterisation techniques. Scanning electron microscopy showed that there were only minor changes of the morphology of the fibres following treatment. X-ray diffraction traces revealed that there were differences in structural parameters between the untreated fibres but no further modifications in the crystalline structure were detected after the plasma oxidation. Raman spectroscopy was used to follow the changes on the fibre surface structure following treatment. The peak positions and widths of the four main Raman bands (D, G, D′ and G′) were determined, with no significant changes observed after the surface treatment. A relationship between the width of the G band and the crystal parameter d002 was found, with the magnitudes of both decreasing as the fibre modulus increased. A reference order parameter ID/(ID+IG) ratio was calculated from the intensities of D and G bands. The treated fibres exhibited a more highly disordered surface structure that the untreated ones, as revealed by the increase of ID/(ID+IG) after the plasma oxidation.

Effects of plasma oxidation on the surface and interfacial properties of ultra-high modulus carbon fibres

Abstract An ultra-high modulus (UHM) carbon fibre was submitted to an oxygen plasma treatment. The effects of this treatment on the physical and chemical properties of the carbon surfaces were investigated by using surface characterisation techniques. SEM and STM studies were performed in order to determine the changes in the surface morphology. Observations on the nanometre scale lead to the conclusion that the plasma oxidation “cleaned” the original surfaces of carbonaceous impurities. XPS analysis of the treated fibres revealed a very significant increase of oxygen content. Single-fibre epoxy composites were prepared from as-received and plasma-treated fibres, and fragmentation tests were performed in order to characterise fibre/matrix interfacial adhesion. Raman spectroscopy has been used to map the strain along the fibre during tensile loading of the matrix, and the distribution of interfacial shear stress has been obtained. The quality of the interface improved dramatically after the surface treatment, supporting the ability of cold plasma oxidation to enhance the adhesion of UHM carbon to epoxy matrices. It is concluded that the increase of the oxygen surface content and the removing of the outermost layers may contribute in a co-operative way to the improvement on fibre/matrix adhesion.

Raman spectroscopy study of high-modulus carbon fibres: effect of plasma-treatment on the interfacial properties of single-fibre–epoxy composites: Part II: Characterisation of the fibre–matrix interface

Abstract High-modulus carbon fibres from different precursors were submitted to an oxygen plasma-treatment under similar conditions. Single-fibre epoxy composites were prepared from them, and fragmentation tests were performed in order to characterise fibre–matrix interfacial adhesion. Raman spectroscopy has been used in the present work to map the strain along the fibre during tensile loading of the matrix. The strain distributions obtained agreed well with the prediction of analytical models used conventionally to describe load transfer at interfaces. Interfacial shear stress distributions were then obtained from these distributions according to the conventional force–balance concept. The interfacial shear strength (IFSS) and frictional shear stress (τf) values were calculated to quantify the degree of fibre–matrix adhesion. It was found that both parameters increased dramatically after the surface treatment, confirming the ability of cold plasma oxidation to improve the adhesion of carbon fibre to epoxy matrices. A dependence of the IFSS on the degree of surface order, as given by the structural order parameter ID/(ID+IG), calculated from the relative intensities of the D and G bands of Raman spectra, was found. This supports the role played by the graphitic structure in fibre–matrix adhesion.

Single fibre deformation studies of poly(p-phenylene benzobisoxazole) fibres Part I Determination of Crystal Modulus

This paper constitutes the first part of a study to assess the influence of processing conditions on the final properties of poly(p-phenylene benzobisoxazole) PBO fibres. Three different samples were selected: as-spun (AS), high-modulus (HM), and ultra-high modulus (HM+) fibres. Synchrotron radiation was used to obtain single-fibre diffraction patterns. It is the first time this approach is taken to estimate the effects of deformation on the crystal properties of PBO fibres. The crystal modulus of the different types of fibre was calculated from the variation with stress of the c-spacing determined from the shift of the (005) and (006) reflections. The HM fibre was found to have the highest crystal modulus of the three fibres, with AS and HM+ PBO being lower. In comparison with tensile data, none of the fibres were found to have a Youngs modulus near to the crystal modulus value, although the HM+ fibre was closest due to its production route. These results could be compared to previous diffraction experiments, where the crystal modulus of PBO fibres were determined using fibre-bundles, assuming homogeneous stress in the bundle. Also, Raman spectroscopy experiments were carried out to examine the differences in Raman bandshift rates in response to both stress and strain. The Raman results showed both the HM and HM+ fibres to have stress-induced bandshifts of approximately −4 cm−1/GPa. The AS fibre value was significantly lower, this being attributed to the non-uniformity of the fibre cross-section. The strain-induced Raman bandshifts were found to be dependent on the tensile modulus of the fibre.

Single fibre deformation studies of poly(p-phenylene benzobisoxazole) fibres

AbstractChanges in crystal strain and crystallite orientation of three varieties of PBO fibre (namely PBO AS, HM and HM+) have been investigated during deformation from the analysis of diffraction patterns obtained across single filaments, using a synchrotron X-ray source. Crystal strain was measured from the positions of the meridional reflections and orientation calculated from azimuthal broadening of the equatorial reflections. It has been demonstrated that no difference in crystal strain across the fibre exists, with the calculated strain being equal between fibre skin and core at a given level of stress. Further skin-core crystallite orientation analysis (calculation of the orientation parameter n

Mesophase from a coal tar pitch: a Raman spectroscopy study

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Linz-Donawitz steel slag for the removal of hydrogen sulfide at room temperature.

n) proved that the AS fibre was the only PBO variety with a significant difference in orientation across the fibre, with the core region being less oriented due to the processing conditions. The skin and core orientation of all three fibres was found to improve with deformation, with the core of the AS fibre showing a significantly higher rate of improvement. This resulted in a similar level of orientation for both skin and core regions of the PBO AS fibre at high levels of stress. The fibre modulus was found to increase with the increasing initial degree of crystallite orientation. Furthermore, improvement in orientation with external stress was related to n