Mechanical properties of glassy carbon fibres derived from phenolic resin

Mechanical properties of glassy carbon fibres produced from a phenolic resin were determined by static tensile testing. These specimens are of special interest because they consist of an isotropic core surrounded by a sheath of oriented material of varying relative thickness. The chemistry of pyrolysis of the resin is summarized and the changes in mechanical properties of the fibres are discussed in terms of the pyrolysis mechanisms. The results are compared with hardness measurements made on discs produced from the same starting material. Scanning electron microscope studies revealed that the fibres have various types of flaws both in the surface and in the core. The effect of these flaws on the fibre strength is discussed by applying Griffith crack theory.     

Healable thermoset polymer composite embedded with stimuli-responsive fibres

Severe wounds in biological systems such as human skin cannot heal themselves, unless they are first stitched together. Healing of macroscopic damage in thermoset polymer composites faces a similar challenge. Stimuli-responsive shape-changing polymeric fibres with outstanding mechanical properties embedded in polymers may be able to close macro-cracks automatically upon stimulation such as heating. Here, a stimuli-responsive fibre (SRF) with outstanding mechanical properties and supercontraction capability was fabricated for the purpose of healing macroscopic damage. The SRFs and thermoplastic particles (TPs) were incorporated into regular thermosetting epoxy for repeatedly healing macroscopic damages. The system works by mimicking self-healing of biological systems such as human skin, close (stitch) then heal, i.e. close the macroscopic crack through the thermal-induced supercontraction of the SRFs, and bond the closed crack through melting and diffusing of TPs at the crack interface. The healing efficiency determined using tapered double-cantilever beam specimens was 94 per cent. The self-healing process was reasonably repeatable.     

Preparation of high-performance carbon fibres from PAN fibres modified with cobaltous chloride

Preparations of high-performance carbon fibres have been tested and the results are reported in this article. Polyacrylonitrile (PAN) fibres modified with cobaltous chloride have increased the crystal size, the crystallinity and the orientation, and also improved the tensile strength by about 15–40% and the modulus by about 10–20% of the resulting carbon fibres, which were carbonized at 1300 °C. The oxygen content, theAI value and the density for the modified PAN fibres are smaller than those for unmodified PAN fibres. Because the formation of the ladder polymer in the modified PAN fibres is slow and gradual during the stabilization process, the resulting carbon fibres have a better modulus than carbon fibres developed from unmodified PAN fibres. The activation energy of cyclization,E _a, is increased to 180.0 from 156.6 kJ mol^–1 after the modification process. The modified PAN fibres do not reduce the time required for stabilization. The carbon fibres developed from the modified PAN fibres have a greater stacking size,L _c, than those developed from unmodified PAN fibres. A higher stacking size, and therefore a higher number of crystalline carbon basal planes, is one of the reasons for the improved modulus of the carbon fibres.     

Adhesion and surface analysis of carbon fibres electrochemically oxidized in aqueous potassium nitrate

Type II carbon fibres (PAN-based) have been electrochemically oxidized in aqueous potassium nitrate to varying electron charge densities (0–4294 Cg-1). The fibres were subsequently characterized by angle-resolved X-ray photoelectron spectroscopy (ARXPS) and ion scattering spectroscopy (ISS) and the results were correlated to acid/base surface titrations and BET surface area measurements. Relative to the as-received fibres (commercially treated, but unsized), the electrochemical treatments increased the ARXPS O/C atomic ratios by approximately 50%–100% and the concentration of oxidized carbon became more uniform within the XPS sampling depth (10 nm). At the same time, the number of acidic functions titrated by sodium hydroxide rose from about 2.6 eq g-1 to 1078 eq g-1, and the BET surface area increased from 0.67 m2 g-1 to 2.9 m2 g-1. A large portion of the increase in acidic groups was due to the increased fibre oxidation below the XPS sampling depth. The surface densities of acidic functions (functions/nm2) determined from NaOH uptake and nitrogen BET surface area experiments were far larger than is physically possible. Thus, it is postulated that aqueous NaOH solutions are able to access a much larger surface region than can be measured by nitrogen BET. A model involving subsurface pores, voids, crevasses, etc., which become available via swelling during solvation in aqueous NaOH, but are at least partially closed off when dry (during BET measurements), was proposed. The quantity of acidic functions introduced (detected by NaOH) was directly proportional to the extent of oxidation as referenced to the electron charge density (C g-1). The ISS spectra suggest that the surface density of carbon/oxygen groups was also increased. Single-fibre fragmentation tests (using an epoxy resin matrix) revealed that in most cases the interfacial shear strengths (IFSS) increased with increasing ARXPS O/C atomic ratio probably due to enhanced fibre/matrix chemical bonding and/or mechanical interlocking. As the extent of the electrochemical oxidation progressed above 1500 C g-1 the IFSS of single filament specimens then began to decrease. This was due to a continuing decrease in filament tensile strength as the extent of electro-oxidation increased. The critical transfer length, Lc, also decreased from 0.36 mm to 0.18 mm as the extent of electro-oxidation proceeded. Electrochemically oxidized fibres were compared to nitric acid-oxidized fibres in terms of acidic groups generated, BET surface areas, acidic group surface densities, dye adsorption with methylene blue and the role of aqueous NaOH in exposing some of the microstructure created by oxidative processes. © 1998 Kluwer Academic Publishers     

Development of carbon fibres from pitches modified with polymers

Polymethylmethacrylate (PMMA), polystyrene (PS) and polycarbonate (PC) were incorporated into the pitch with an objective to increase the tensile strength and tensile modulus of the pitch fibres (green) and to retain or to improve the mechanical properties of the carbonised fibres. It was found PMMA addition to the pitch increases the strength or flexibility of the green fibres but results in the formation of highly porous carbon fibres. The incorporation of PS and PC improves the tensile strength and tensile modulus both at the green and carbonised levels. The enhancement of mechanical properties is better in the case of carbon fibres derived from PC modified pitches.     

Improved fire retardancy of thermoset composites modified with carbon nanofibers

Abstract

Multifunctional thermoset composites were made from polyester resin, glass fiber mats and carbon nanofiber sheets (CNS). Their flaming behavior was investigated with cone calorimeter under well-controlled combustion conditions. The heat release rate was lowered by pre-planting carbon nanofiber sheets on the sample surface with the total fiber content of only 0.38 wt.%. Electron microscopy showed that carbon nanofiber sheet was partly burned and charred materials were formed on the combusting surface. Both the nanofibers and charred materials acted as an excellent insulator and/or mass transport barrier, improving the fire retardancy of the composite. This behavior agrees well with the general mechanism of fire retardancy in various nanoparticle-thermoplastic composites.     

Some aspects of interface adhesion of electrolytically oxidized carbon fibres in an epoxy-resin matrix

A comprehensive investigation of the adhesion at the interface of a carbon fibre in an epoxy resin was made. The fibre surfaces were modified, to increase their adhesion to resin, by an electrolytic surface treatment which was applied at various current densities. Subsequent changes in the fibre properties relating to possible mechanical, physical and chemical contributions to adhesion were monitored. Tensile tests on single fibres indicated that the treatment altered the strengths of the fibres, which were found to have their highest values and to be least variable at an optimum adhesion level. A method was developed to estimate the strength of the fibres in the resin, this confirmed the single-fibre data. A novel method of labelling the acidic sites by producing adsorption isotherms was developed to identify surface functionality. Surface acidity correlated well with adhesion levels. Single-fibre pull-out tests, modelled using a new combination failure criterion and fragmentation tests, indicated that the optimum adhesion level for this fibre/resin system was achieved with an electrolytic treatment at 25 C m^–2. The principal effects of this treatment were considered to be due to chemical modification of the fibre surface coupled with the removal of a loosely adherent surface layer.     

SiC-based multilayered composites containing short carbon fibres obtained by tape casting

Silicon carbide multilayered composites containing short carbon fibres were prepared by tape casting followed by pressureless sintering. The dispersion of fibres into the SiC slurry was studied either by ultrasonics or by mechanical agitation, choosing the best solvent-dispersant couple that was compatible with the requirements of the tape casting technique. The effect of sintering temperature was studied, using carbon and boron as sintering aids, and mechanical properties were measured on composites containing from 5 vol.% to 25 vol.% of short carbon fibres. The results show that carbon fibres inhibit the complete densification of the sintered samples, and mechanical properties suffer from the high residual porosity. Since tape casting demonstrated to be a good technique to align the fibres along the casting direction, different architectures were tested for the composites, studying the effect of fibres alignment direction on the mechanical properties.     

Improved fire retardancy of thermoset composites modified with carbon nanofibers

Multifunctional thermoset composites were made from polyester resin, glass fiber mats and carbon nanofiber sheets (CNS). Their flaming behavior was investigated with cone calorimeter under well-controlled combustion conditions. The heat release rate was lowered by pre-planting carbon nanofiber sheets on the sample surface with the total fiber content of only 0.38 wt.%. Electron microscopy showed that carbon nanofiber sheet was partly burned and charred materials were formed on the combusting surface. Both the nanofibers and charred materials acted as an excellent insulator and/or mass transport barrier, improving the fire retardancy of the composite. This behavior agrees well with the general mechanism of fire retardancy in various nanoparticle-thermoplastic composites.     

Ceramic fibres from polymer precursor containing Si-O-Ti bonds

By the reaction between organosilicon compounds and titanium alkoxides, polymers (atom ratio Ti/Si = 0.045 to 0.12) as the precursors for ceramic fibre were synthesized. Titanium in the precursors was bonded to organosilicon compounds in the Si-O-Ti bond. The synthesis process of the fibre is melt-spinning of the precursor, curing of the precursor fibre, and its heat-treatment. Curing was achieved by cross-linking of the polymer by the oxidation of Si-H bonds or hydrolysis of Ti-OR bonds in the precursor. The formation mechanism of the precursors and their pyrolysis process were examined in an inert gas atmosphere and in a vacuum. It was shown that the Si-O-Ti bond in the precursors is cleaved by pyrolysis of the Ti-O bond, so that TiC is formed. The tensile strength of the ceramic fibre obtained, is relatively high even by the heat-treatment at temperatures higher than 1200° C. Further, the Young's modulus is little changed.     

Ceramic fibres from polymer precursor containing Si-O-Ti bonds

Polycarbosilanes containing titanium alkoxide as pendant groups (atom ratio Ti/Si = 0.07 and 0.15) were synthesized. These polymers were melt-spun and then heat-treated in a vacuum, in oxygen or ammonia gas flow, resulting in Si-Ti-C-O, Si-Ti-0 and Si-Ti-O-N fibres, respectively. The pyrolysis process of the polymer is discussed in connection with the mechanical properties and the structure of the fibre. At high heat-treatment temperatures, -SiC and TiC (in Si-Ti-C-O fibre), anatase (in Si-Ti-O fibre) and TiN (in Si-Ti-O-N fibre) crystallized, which may be closely related to the decomposition of the Si-O-Ti bond in the fibre.     

Preparation of NbN fibres by nitridation of sol-gel derived Nb2O5 fibres

Nbn fibres were prepared by nitriding sol-gel-derived Nb₂O₅ fibres. Precursor niobium-oxide-gel fibres were obtained by spinning viscous sots prepared through acid-catalysed hydrolysis of niobium penta-ethoxide, Nb(OC₂H₅)₅. It was found that the spinnability of the sols depended on the water/alkoxide and the acid/alkoxide ratios. The niobium-oxide-gel fibres thus obtained were converted to niobium nitride by heat-treatment in a NH₃ gas above 800°C. Characterization of these fibres was carried out using X-ray powder diffraction, Auger electron spectroscopy and scanning electron microscopy. The nitride fibres obtained at 1050°C were mostly of stoichiometric -NbN phase, and they showed a transition to a superconducting state at around 11 K.     

Amorphous carbon fibres from linear low density polyethylene

Carbon fibres with good mechanical properties have been produced from linear low density polyethylene (LLDPE). The melt-spun LLDPE fibres were made infusible by treatment with chlorosulphonic acid. The cross-linked fibres were pyrolysed at temperatures between 600 and 1100 °C under tension, in a nitrogen atmosphere, within 5 min. Carbon fibres prepared at 900 °C had a tensile strength of 1.15 GPa and a Young's modulus of 60 GPa. The elongation at break was extremely high, up to 3%. The carbon yield of the process was 72 to 75%.     

High-temperature compatibility of carbon fibres with nickel

A study has been made of the elevated temperature degradation of a number of carbon fibre types coated with nickel by a variety of methods (electroless, electrolytic, carbonyl and physical vapour deposition). At high temperatures, Ni-coated fibres undergo a transformation of structure to crystalline graphite with a consequent loss of strength and elastic modulus. Resistance to this recrystallization is related to the fibre type and structure and increases in the order HTS PAN-based, HM PAN-based, HM rayon-based. For PAN-based fibres the resistance increases with the degree of structural order and orientation. The recrystallization of HTS fibres is consistent with a simple model of dissolution and reprecipitation controlled by diffusion of carbon in nickel. To explain the higher stability of HM fibres an additional factor must be introduced. For example, their behaviour can be explained in terms of a highly stable surface layer between about 0.1 and 0.5m thick. Rapid recrystallization occurs when the nickel breaks through this layer e.g. by dissolution. The recrystallization was not greatly affected by the type of nickel coating but the recrystallization temperature of HM fibres was considerably reduced by a small proportion of air in the heat-treatment atmosphere. HTS fibres were not affected in this way but the fibres were severely weakened through surface attack by both air and hydrogen at temperatures well below the recrystallization temperature.     

ESCA studies of carbon fibres: Part II—Surface reactions of carbon fibres with epoxides

The reactions between epoxides and Courtaulds AS carbon fibre surfaces have been studied by the ESCA technique. The carbon fibres were oxidized in a mixture of H₂SO₄ and HNO₃, and then refluxed in epoxide. The formation of epoxide overlayers on the carbon fibres is confirmed. The average epoxide depths (in Å) have been calculated from the ESCA data. The fractions of the carbon fibre surfaces covered by epoxide monolayers are also estimated. The epoxides are assumed to be covalently bonded to the fibre surfaces by ether and ester links.     

Luminescent graphene quantum dots from oxidized multi-walled carbon nanotubes

Fluorescent nano-graphene quantum dots (GQD) were isolated from oxidized carbon nanotube suspension with the aid of cysteamine. The oxidized GQD were thiol functionalized with cysteamine in presence of dicyclohexylcarbodiimide as coupling agent. The GQD chemistry and morphology were characterized by means of X-ray photoelectron spectroscopy and atomic force microscopy. The thiolated graphene quantum dots exhibit an intense luminescence (quantum yield around 10%) in the visible range with an excitation wavelength-dependent fluorescence.     

Ex-cellulose carbon fibres with improved mechanical properties

Ex-rayon carbon yarns have been prepared according to an original route characterized by a fast pyrolysis step in the presence of an organosilicon compound. The evolution of the mechanical and physicochemical properties of the fibres throughout the transformation of cellulose into carbon showed that the organosilicon additive was necessary to obtain carbon yarns exhibiting satisfactory mechanical behaviour and to improve significantly the fracture properties of the ex-rayon carbon fibres.