Evolution Mechanism Of Composition, Structure And Mechanical Properties Of Carbon Fiber During High Temperature Heat Treatment

High-modulus carbon fibers are widely used in the astronauticalengineering and missile system due to its high performance. It is important toknow exactly the correlation of composition, structure and mechanicalproperties of carbon fibers heat treated at high temperature (1500-2500℃).In this paper, we studied the evolution rules of the compositon, structure andmechanical properties, and also the correlation of these factors.In high temperature heat treatment process, nitrogen cracked and the Ncontent decreased with increasing heat treatment temperatures. The amountof cracked C is little, the carbon content increased due to the decreasing of N.After heat treatment at temperatures above1900°C the nitrogen content wasnegligible (below detection limits). The hydrogen content can also beneglected, and the samples were essentially all carbon.N content tends to decline in exponent gradually with temperature and heating time. The pyrolysis of nitrogen obeys time temperature equivalenceprinciple and higher temperature is more effective than increasing heatingtime for the cracking of nitrogent. There are several kinds of existing form ofN in the carbon fibers, N in the zones of amorphous carbon or in the edgewith single bonds such as–NH-are easily to crack, but N in the inner ofgraphite crystallite or in the edge with form of double bonds such as=N-aredifficult to crack.During high temperature heat treatment, N escapes with form of N2.The pyrolysis of N can leave pores, at the same tme, the pressure in thecarbon fibers due to the formation of gas, which possibly make the poreslarger. When the rate of pyrolysis is more than that of escape, the pressurebecame larger and larger and induced the expanstion of fibers, which called"puffing". The diameter of fibers increased and density decrease because of"puffing".Based on the results of Raman spectra, the degree of graphitization ofcarbon fibers improved with high temperature heat treatment. There are threemicrostructures of graphite single-layer in the graphitization process:(1)graphite single-layer with N-substituted heterocycles (2) graphitesingle-layer with vacancies caused by denitrogenation (3) inner-defect-freegraphite single-layer. Denitrogenation induced vacancies, and vacanciesdiminished by graphitization transition.Within carbon fibers, C-C bonds at the edge of the graphite crystallites (bond energy<400KJ/mol) are weaker than those inside the hexagonalgraphitic network (bond energy>500KJ/mol). Chemical bonds involvingheteroatoms, such as C-N (bond energy <350KJ/mol) or C-H (bond energy<430KJ/mol) are also weaker. During heat treatment, these weaker bondsinitially break to form highly active dangling bonds. Those crystallites withdangling bonds grow by the dangling bonds either reacting to each other orwith agraphitic carbon atoms to form more stable structure as soon as theyare generated. Different bonding ways lead to crystallites with differentprofiles.These dangling bonds react with each other or with agraphitic carbon toinduce graphite crystallite growth. The graphite crystallite size becomeslarger with higher temperature and their distribution morphology changes.Crystallites with a small size are dispersed throughout fibers that are heattreated at lower temperatures, and the crystallites begin to interlink with oneanother to form a network structure with increasing crystallite size. Thecrystallites combine each other and form a transfixtion struture above2100°C and the internal stress induces the weaker interlinking points tobreak, re-dispersing the crystallites.The tensile strength decreased rapidly with increasing treatmenttemperature for fibers with a dispersed crystallite structure. The decrease wasslower for fibers with a network structure, because the cross-linking pointsbetween the crystallites can effectively prevent crack propagation. The decrease was again more rapid for fibers with a transfixion structure, due tothe destruction of cross-linking. The tensile modulus increased slowly for thedispersed and network structures, but rapidly for the transfixion structure.