| During carbonization processes of polyimide films, their chemical structure, crystal structure, thermal properties, morphologies of surface and cross section, and mechanical properties change violently. Therefore study on the structural evolutions and property changes are quite important for the preparation of carbon films with excellent performance.In this paper, we prepared PI films by tape casting. And in a vacuum and nitrogen conditions a series of carbon and graphite films were prepared based on these PI films via gradient heaing. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction(XRD), Raman spectrum, thermogravimetry(TG), differential scanning calorimetry(DSC), scanning electron microscopy(SEM) and high resolution transmission electron microscopy(HRTEM) were used to systemically investigate the evolutions of chemical structure, crystal structure, thermal stability, morphological microstructure and properties of the carbon and graphite films. We explore the evolution of structure and properties of polyimide-based carbon films during the carbonization process as well.The study shows that segment fracture and thermal crosslinking reaction occur first during the low temperature pyrolysis process of PI films, and thermal crosslinking reaction reforms the structure of polyimide. Structure reformation of carbon films occurs at550～600℃with imide ring decomposition and ether bond cleavage. A stable three-dimensional structure was formed by the thermal cross-linking reaction of free radicals. Radicals started restructuring and thermal condensation over700℃, and with the removal of heteroatom group from three-dimesional structure and cyclization and aromatization, polyaromatic structure of the hexagonal carbon ring was gradually formed. The polymers was further heavy condensed into a planar mesh structure and carbon microcrystalline appearanced. At last amorphous carbon structure was formed. The crystal structure of crabon films was studied by XRD and Raman spectrum, and the results indicated that the state order of carbon films prepared between550to600℃changed obviously. Structures of carbon films prepared in700to1650℃were changed and became more disordered. R value of carbon flms prepared in N2was smaller than that in vacuum, it means that the structure of these films got more orderly state. G’ peak appeared on the Raman spectra of films prepared at1800. Degree of graphitization of films prepared at2200℃increased rapidly. As further polycondensation and removal of nitrogen, a large area of conjugated carbon mesh structure was formed from nitrogen-containing heterocyclic aromatic ring systems by cyclization and aggregation at high temperatures.By synchronizing thermal analyzers we found that the thermal properties of carbon films prepared at different temperatures vary greatly. The TG curves of carbon films prepared below600℃can be divided into three phases, and the biggest weight loss rate was in the second phase:550to700℃. An endothermic peak appeared in each phase of each DSC curve. And with temperature increasing the endothermic peak in second phase gradually decreased and it instructed that chain breaking had completed below700℃. The bigger weight loss rate appeared between room temperature to110℃where there was a sharp falling in TG curves of carbon films prepared above700℃. In the second phase the weight loss rate of carbon films was slowing down. And an endothermic peak appeared in the first and third phase of DSC curve, respectively. It shows that the stable layered six-membered ring structure had not been formed in carbon films. The bigger weight loss rate appeared between room temperature to135℃where there was a fast falling in TG curves of carbon films prepared above900℃. In the second phase the weight loss rate of carbon films was slowing down. And an endothermic peak appeared in the first phase of DSC curve and it is followed by exothermic interval. The analysis shows that the carbon films prepared in N2had a high degree of thermal stability than prepared in vacuum. And the stretched films had poor thermal stability.It can be clearly seen on the surface SEM of carbon films prepared in both N2and vacuum that the grooves tended to be shallower as the temperature increasing while its width changed a little. There were more white and smaller granular objects on the carbon film surface with strip distribution of light and dark. The white granular objects shape of carbon films prepared in vacuum changed with temperature and became small straight line at1050"C. Carbon microcrystalline was clearly seen on the surface SEM of carbon films prepared in N2. The SEM micrographs of cross-section of carbon films prepared in both condition shown that the dense even structure without particles and pores transformed into rough structure with large particles and some small holes which was left by overflowing of small molecule gases. And the diameter of the holes was about2μm. It is shown in the SEM micrographs of cross-section of carbon films prepared in ultrahigh temperature that the size of carbon microcrystalline increased with temperature rising. The carbon microcrystalline piled into a state of disorder and there was a certain amount of randomness.The information in HRTEM images instructed that the structure of carbon film prepared at600℃was completely amorphous. Numerous small bending segments disorderly huddled together, but evenly distributed. In HRTEM images of carbon films prepared at700℃small stripes were arranged in parallel, the fringe spacing was about0.55nm, which means that (002) surface was regularly arranged together. The carbon crystallite area were more perfect in the carbon films prepared at1450℃, and the structure was transformed into planar mesh structure after the conversion of fused ring. Strips in the carbon films prepared at1650℃consisted of7stripes, and in the local area stripes can be observed torn and broken. In the HRTEM images of carbon films prepared at1800℃there were staggered strips stacked haphazardly together without being disconnected, which instructed that the structure of the carbon film was the graphite crystallites stack, being a "turbostratic structure". Non-carbon elements were gradually removed from aromatic rings and polymer changed into inorganic graphite film. The number of stripes within each strip of carbon film prepared at2200℃was not the same while almost there was no strip broken. Analysis is that the combination between stripes was weak, part of the stripe such as3～4stripes were split off formed a new set of stripes.Thermal conductivity of carbon films was closely related to the preparation technology. And it increased with increating of preparation temperature. The biggest change in width occurred in the carbon films prepared between550to750℃. It is because that in this temperature range C—N and C—O were broken while releasing small molecule gases such as N2, CO and CO2, and the volume and quality of the fcarbon films changed a lot. Good stability of the dimension can be obtained in the carbon films prepared above1050℃. The size in thickness increased in carbon films prepared at low temperature, but the thickness tended to be decreased in the carbon films prepared above550℃. Young’s modulus was reduced in carbon films prepared at350℃. With temperature increasing Young’s modulus increased and it increased dramatically in the carbon films prepared at550℃because of the imide ring fracture and the flexibility of the carbon films lowered. |
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