Deposition Mechanism And Morphology Of PyC And Their Effects On Properties Of Rayon-based Carbon Fiber

The main purpose of this paper was to design an appropriate process to enhance properties of RCF(tensile strength primarily, anti-oxidation performance secondly and crystallinity etc. ), by means of investigations on theories. The technological parameters of chemical vapor deposition (CVD) and precursor pyrolysis coating were studied respectively. Moreover, a thorough research was given to the deposition mechanism of CVD-PyC(Pyrolytic carbon) and influences of the two processes on RCF properties, comparison between two processes was also discussed.Deposition mechanism of PyC was studied firstly in this paper. Spherical PyC was made by control of technical conditions. It is approved that condensation-polymerization mechanism exists in the CVD process by analyzing the intermediates qualitatively. Temperature influences on morphology and deposition process of PyC are explained by means of thermodynamics and the nuclei-growth principle of crystals. Results show that improving temperature can reduce nucleation energy and facilitate nucleation. PyC particles fuse easily and transit to layer structure on 1000℃. Depositing velocity is controlled by surface reaction course when temperature is less than 1000 ℃,while it is controlled by gas diffusion course when more than 1000℃ .And the sediment morphologies during the two courses are different from each other greatly.Tensile strength of RCF vary greatly when treated by different CVD technology conditions (such as volume ratio of n-heptane/tetrachloride temperature, carrier flux, velocity of gas flow etc. ). When volume ration of n-C7H16/CCl4 is 4: 1 and carrier flux is 30ml/min, tensile strength of RCF deposited on 1100℃ can be enhanced 5 percent , however it's of unfavorable stability and recurrence and tensile strength has different decrease on other conditions. This is because PyC deposited on RCF surface can inducenew breakpoints easily, which result in decrease of tensile strength. But tensile strength can retain above 90 percent and improvement of anti-oxidation is distinct when certain conditions (with the volume ration of n-C7H16/CCl4 4: 1 and carrier flux 30ml/min, 1100 癈 deposited) are controlled.Effects of different conditions (such as volume ratio of impregnants, pyrolysis temperature) on properties of RCF are studied by precursor pyrolysis coating while n-C7Hi6/CCl4 mixture used as pyrolysis reagent. The SEM, X-ray and calculated results of show that line density and surface flaw of RCF are decreased after treatment, and crystallinity increases slightly. Meanwhile small molecules (for instance hydrogen chloride and chlorine) do certain eroding effects on RCF, which helps to improve tensile strength to some extent, however eroding effects will be intensified with improvement of temperature, so as that tensile strength of RCF will decrease. When the volume ratio of n-C7H16 /CCl4 is 2:3 and pyrolysis of the mixture occur at 900-1100, the maximal increment of tensile strength of RCF is about 10-17% . And the strength distribution is evaluated by Weibull statistic theory as well. Results show that Weibull modulus m increases, which means the distribution of RCF tensile strength is incentive and dispersing degree reduces. These prove that pyrolysis coating of n-C7H16 /CCl4 is a practical method to enhance tensile strength of RCF. Tests on oxidation resistance also shows that anti-oxidation property isn't changed basically, improves slightly some extent.Comparisons between two methods show that precursor pyrolysis coating is a good way to enhance tensile strength. While CVD is hard to control and not apt to be used to improve tensile strength but it's can be utilized to boost anti-oxidation performance of RCF.