Influence Of Surface Physico-chemical Structures Of Polyacrylonitrile Based Carbon Fiber On Its Oxidation Behaviours

Polyacrylonitrile （PAN） based carbon fiber, whcich offers highspecific strength, specific modulus, low coefficient of thermal expansion,high thermal conductivity and excellent damping property, is mainly usedas preferred reinforcement in advanced performance composite materials.It has being playing a decisive role in the fields of aerospace engineering,automobile industry, new energy mateials, cultural entertainment andcivil construction, forming a new breed of material system and beingapplied widely in carbon fibre reinforced epoxy matrix composites （CFRP,CFRTP） and carbon fiber reinforced carbon matrix composites （C/Ccomposites）. In carbon fiber reinforced epoxy matrix composites, thecarbon fiber provides strength and stiff while the epoxy matrix transfersthe load from one failed fiber to another via the interface formed betweenthe carbon fiber and matrix. However, the applied carbon fiber withoutprevious surface modifaction contains the percentage of more than90%element carbnon and shows inert surface which is not tough enough toform an effective interface. On the other hand, in C/C composites, carbon fiber does not suffer from stress corrosion or stress failures attemperatures high than2000oC in an inter atmosphere or in vacuum,however, the carbon reacts with oxygen, burning away rapidly attemperature as low as400oC. Therefore, the necessities on the researchof oxidative behaviours including thermal oxidation and electrochemicaloxidation of PAN based carbon fiber has attracted more and moreinteresting recently, which is of great significance in the preparation ofhigh performace carbon fiber reinforced epoxy composites and theanti-ablation resistance of C/C composites.In this paper, the evolution of components （mainly as oxygencontaining functional groups） and structures （chemical structures andaggregation structures） in carbon fibers during oxidative processes arecharacterized by X-ray photoelectron spectroscopy （XPS）, Thermalgravimetric （TG）, Raman spectroscopy, X-ray diffraction （XRD）, SEM,TEM and AFM. The influences of surface physico-chemical structures onthe oxidative bahaviours of PAN based carbon fibers are discussed. Theaim of this research is to, on one hands, investigate the surface graphitestructures and unstructured components such as element iron and sodiumon the thermal oxidation behaviour, model the carbon-oxygen reactionand research thermal oxidative mechanism; On the other hand, toinvestigate the influences of surface graphite structures and morphologyon the surface properties including surface oxygen containing functional groups, surface chemical structures, surface roughness, surfacemorphology and surface wettability properties, assess the oxidationbehaviours by the interlaminar shear strength （ILSS） value of treatedcarbon fiber/epoxy composites and then research the electrochemicaloxidative mechanism. The main achievements are summaried asfollowing:Polyacrylonitrile-based carbon fibers with different graphite degreeswere oxidative ablated at500℃and600℃in air. By Thermalgravimentric （TG）, Raman spectroscopy, X-ray diffraction and SEM, themass loss, microstructure, and surface morphology of carbon fibers wereinvestigated. The mass loss of carbon fiber increases linearly withincreasing oxidative ablated time under500and600oC. The carbon fiberwith the higher graphite degree shows higher oxidative resistance, and thesurface roughness increase gradually because of chemical ablation duringthe whole oxidation. A gloss morphology appears on the surface primarilybecause of physical denudation for carbon fibers with low graphitizationdegree and then burn off according to carbon and oxygen reaction. Thecrystallite size （La） decreases significantly, while interlayer spacing (d₀₀₂)remains nearly unchanged. SEM observation suggests the two kinds ofablation mechanisms for carbon fibers with different graphite degreesindicating that CC band in sp3hybridization prefers to be attacked byoxygen molecular more than that in sp2hybridization during oxidation ablation in air.Modeling of PAN based carbon fiber oxidation in air, at25¹000oCand heating rates （β） of5,10,12,15,20and30oC/min has beenperformed. A simple thermogravimetric analysis technique has been usedto examine the oxidation process of carbon fibers with different graphitedegrees. The modeling involved development of three main empiricalequation correlating W%to T/T_0.5, T_0.5to β and（（（da）/（dt））/（（da）/（dt）））_maxto（100-W）%, where T0.5is the temperature attained when50%of thecarbon fibers gasifies, β is the heating rates, W%is the percentage ofremaining mass,（da）/（dt） is the mass loss rates while（da）/（dt）_max is themaximum value of（da）/（dt）. These equations could predict the recession of acarbon fiber component without the need to include the surface area inthe analysis.A novel method in the preparation of alkali （earth） metal or ironalloyed PAN based carbon fiber was proposed, and the affectingmechanism of element on thermal behavior of PAN based carbon fiberswas investigated. The results show that by impregnating as-spun fiberinto Fe₂（SO₄）₃/DMSO or Na₂SO₄/DMSO solution, the concentration ofelement iron or Na in carbon fiber is more than0.26%and coulddistribute evenly among the cross section of the fiber. The influence ofelement on thermal stability of carbon fiber in air is related to theelement’s characterisitic and heat treatment temperature of the specimen experienced. Element Na is chemical activity and serves as a catalyst incarbon-oxygen reaction, however, iron atom with vacant outer shell couldaccept other electron from C1s and O1s on carbon fiber surface duringthermal oxidation. After the heat treatment at temperature of1700℃,carbon fiber alloyed with iron shows a higher anti-oxidation property thanthat of carbon fiber experienced a heat treatment temperature of2200℃.Meanwhile, the thermal stability of carbon fiber increases with theincreasing concentration of element iron in fibers, however, when theconcentration of iron is more than49μg/g the thermal stability decreaseslinearly. XPS results suggest that the iron alloying decreased the Fermienergy of the surface state of carbon fibers leading to a higheranti-oxidation property.Electrochemical oxidation was employed to investigate the surfaceproperties including surface oxygen functional groups, surface chemicalstructure, surface topography and the surface wettability properties oncarbon fiber surface. By means of X-ray photoelectron spectroscopy（XPS）, active oxygen containing functional groups were identified. TheO/C ratio increases apparently, approaches to a saturated value graduallyand could be divided into two regions such as a rapid increase region（0¹.78A/m²） and a plateau region （1.78².76A/m²）. The surfacechemistry analysis showed that the interlaminar shear strength （ILSS）value of carbon fiber/epoxy composite could be improved by24.70%. Raman spectroscopy investigated the carbon structure of carbon fibersurface in terms of order/disorder in the graphite structure and the resultsrevealed that the relative percentage of graphitization carbon in the formof sp²hybridization increases above a current density of1.39A/m². Theincreasing non-polar graphitization carbon on carbon fiber surfacedecreased the surface energy. As a result, both the surface free energy （γ_s^T）and polar component （γ_s^p）, which were investigated by Dynamic ContactAngle Analysis-DCAA, show decrements as the decreasing R value. TheILSS value begins to decrease when the current density is higher than2.07A/m². It is found that ILSS value has no direct relationship with thenature and surface density of the oxygen containing functional groups orwith the carbon structure. It is the surface free energy （γ_s^T）, especially thepolar component （γ_s^p） that plays a critical role in affecting the interfacialadhesion of carbon fiber/epoxy composites. The ILSS value changes withthe increasing current density and could be divided into three regionscorrespongding to the region of chemical interaction （Ⅰ）, anchor force（Ⅱ） and matrix damage （Ⅲ）. When the failure in the tensile strengthdecreases more than17.9%, the ILSS value deteriorates.The influence of surface morphology of carbon fiber on theelectrochemical oxidation was investigated by means of XPS, SEM andtensile test. The results show that carbon fiber with rough surface ispreferred to be oxidative etched, and the surface roughness increases as increasing current density, while carbon fiber with smooth surface showsa better morphology resistance during electrochemical oxidation. Asincreasing current density, the O/C ratios on carbon fiber surface increase,although the O/C ratios on treated carbon fibers with rough surface arelower that with smooth surface under the same condition, indicatingsurface morphology may contribute to the oxidative process. Thedeconvolution results obtained from C1s shows that the surfacemorphology affect the relative amount of C=O group, although has littleinfluemce on the relative amount of C-OH and COOR groups. The tensilestrength and tensile modulus of carbon fiber with rough surface isbenifical to be improved by17.3%and5.8%, respectively. Moreover, theILSS value shows that the rough surface also plays an important role inimproving the interfacial adhesion of carbon fiber/epoxy composites,because increased roughness can supply large contact area between thereinforcement and matrix, and thus providing stronger mechnicalinterlocking on the interface of composites, indicating the chemicalbonding between oxygen containing functional groups on the fibersurface and resin is not the governing factor.The influence of graphite structure of carbon fiber on theelectrochemical oxidation was investigated by means of XPS, SEM,Dynamic Contact Angle Analysis-DCAA and the ILSS test. The resultsshow that carbon fibers with developed graphite structures are inactive during electrochemical oxidation and the O/C ration or oxygen functionalgroups become saturated soon. The increasing percentages of oxygencontaining functional groups for carbon fibers with R value lower than0.68mainly attributes to the increasing of hydroxyl group. The dynamiccontact angle of electrochemical oxidized carbon fibers measured forboth deionized water and diiodomethane decrease as increasing R value,and the surface wettability properties especially the polar component （γ_s^p）of surface free energy decrease. The ILSS test is employed to assess theinfluence of graphite structure on the interfacial adhesion of carbonfiber/epoxy composite, it is found that ILSS value decrease as increasingR value on carbon fiber surface.