The Structure Influence Of Carbon Materials On The Growth Of Transition Metal Carbides In A Molten Salt Media

Carbide coatings on the surface of carbon fibers were prepared in a molten salt medium by using of PAN-based carbon fibers with different structures as the reaction carbon source and IVB-VB transition metals as the metal source.The forming behavior of the corresponding carbides derived from different metal substances on the surface of carbon fibers with different structures was studied,and the reaction mechanism and the growth process between of carbide coating on the carbon material structure were proposed.In order to further clarify this mechanism,the pitch-based carbon fibers with special morphology and various crystal orientation were selected as the reaction carbon source and template.Based on the morphological and crystal structural characterization of the seleted carbon fibers,as well as the growth status and the quality of the carbide coating on the fiber surface,the correlation between the carbon material structure and the growth of the carbide coating was further explored.Micro-scale carbon materials with a representative structure were utilized as the carbon source to perform the comparative experiments so as to further verify the affecting mechanism of carbon material structure on the formation and growth of carbides.The main conclusions of the research work are as follows:1.Using Nb and Hf as metal source,PAN-based carbon fibers as the carbon source and reaction template,the NbC and HfC coated carbon fibers were prepared by thermal reacting in molten salt media,respectively.The results show that the NbC and HfC continuous coatings are formed at temperatures ranging from 950¹000°C and 850⁹50°C,respectively.The morphology of the carbide coating on the carbon fiber surface is related to the reaction temperature,the holding time,the molar ratio of the reactants,and the molten salt system.As the reaction temperature rises and the holding time extends,the grain size of the carbide particles increases,and the thickness of the coating also increases.The growth process of NbC and HfC coatings shows the characteristics of hetero-epitaxial growth.The carbide coating with obvious orientation features shows better mechanical stability,leading to the formation of a thicker coating with continuous integrity character.2.By evaluating of the continuous integrity of the carbide coating formed on the surface of T700 carbon fibers by using the IVB-VB transition metal elements such as Ti,Ta,Nb,Hf and Zr,the order of coating quality is found to be in a sequence of TiC(d₂₀₀=0.216 nm),TaC(d₂₀₀=0.223 nm),NbC(d₂₀₀=0.224 nm),HfC(d₂₀₀=0.232 nm),and ZrC(d₂₀₀=0.235 nm).It is found that the coating quality has a significant correlation with the matching degree of the d₁₀₀ value of the carbon?100?crystal plane(d₁₀₀=0.213 nm)and the d₂₀₀ value of the carbide?200?crystal plane.The calculation based on the“edge-to-edge matching”crystallographic model shows that the carbides are epitaxial grown along the carbon?100?crystal plane in the carbon material,with a growth direction of<110>crystal orientation.The lattice mismatch of the d₂₀₀ values of?200?crystal plane of different types of carbides with the d₁₀₀ value of the carbon?100?crystal plane is consistent with the order of the continuous integrity of the above-mentioned coatings.From the perspective of crystallography,it is confirmed that the lattice mismatch degree of the carbide d₂₀₀ and carbon d₁₀₀ values affects the nucleation of carbides,and lattice mismatch dislocations produced during the subsequent epitaxial growth will affect the morphology and structure of the resultant coating,ultimately influencing the continuous integrity of the carbide coating formed on the surface of the carbon fibers.The transition metal elements such as Ti,Ta,Nb,Hf and Zr metals reacted with carbon fibers to form the corresponding carbide coatings on the surfaces of T300 and T700 carbon fibers.It was found that the coating quality of the same metal carbide on the former surface was better than that of the latter.This indicates that the orientation of the graphite crystallites in the carbon material may have a significant effect on the nucleation and diffusion rate of the carbide coating,and thus affects the continuous integrity of the coating.3.In order to further clarify the relationship between the crystal orientation of carbon materials and the nucleation and growth of carbides,three types of 1000°C carbonized and 3000°C graphitized fibers were used as carbon sources,i.e.rectangular cross-sectional mesophase pitch-based ribbon fibers?marked with RI-1K and RI-3K?with carbon layers parallel to the main plane of the fibers,circular cross-sectional mesophase pitch-based carbon fibers?labeled as RO-1K and RO-3K?with a radial orientation of carbon layers,and isotropic pitch-based carbon fibers?marked with SC-1K and SC-3K?.TiC and TaC coated carbon fibers were prepared in molten salts using Ti and Ta as metal sources,respectively.The morphology of the TaC coating produced by the reaction of the RI-1K carbon fibers with the Ta metal perfectly reproduces the fold structure of the fiber surface.This manifests that the carbide coating follows the template growth of the carbon material,Which indicates that the carbide growth process is based on metal atoms passing through the carbide nucleus and diffusing along the carbon sheets of carbon fibers to form a carbide coating.The morphology and thickness of the coating on the RI-3K carbon fiber surface and the RO-3K wedge-shaped opening are closely related to the orientation direction of the graphite crystallites.The grain size of coating formed on the wedge-shaped fracture surface due to the internal stress is larger,however,the coating thickness is thinner than that of RI-3K,indicating that the diffusion-formation rate of carbides in the direction perpendicular to the graphite layers is smaller than that along the?002?carbon layer.The crystallite size of graphite microcrystals in SC-1K carbon fibers is small and the curved carbon layer is disorderly oriented,which results in the difficulty of nucleation of carbides.At the same time,the small size and misorientation of the carbon layers within the carbon fibers lead to decrease the diffusion growth rate of carbides.Consequently,it is hard to form continuous TiC and TaC coatings.However,in comparison with SC-1K carbon fibers,the graphitized SC-3K carbon fibers with relatively larger crystallite size and straighter carbon layer is benefited for the formation and growth of TiC and TaC coatings with better quality.It indicates that carbon crystallite size is too small to form of a continuous coating.4.In order to further study and verify the effects of carbon material structure on the nucleation and growth of carbides,micro-scale carbon materials?i.e.carbon black,carbon nanotubes,and graphene?with representative structures were used as carbon sources to prepare the corresponding carbides by reacting with Ti and Ta metals in the molten salts.The curved carbon nanotubes with a fish-bone shaped graphite layer structure reacted completely with Ta to form TaC fibers.By adjusting the ratio of the carbon source and the metal,the reaction degree between the straight carbon nanotubes with a tubostratic structure and the metal could be controlled,therefore tantalum carbide fibers or tantalum carbide coated carbon nanotubes were controllable obtained.On the other hand,the carbon layers of the straight multi-wall carbon nanotubes after graphitization treatment are concentrically arranged,leading to that their crystal structure lacks a certain crystal size of 100 crystal plane which is suitable for carbide nucleation?i.e.La is extremely small?,thus carbides are hard to be nucleated and the carbide coationgs is not easy to be formed.The reduced graphene oxide was used as a carbon source to react with Ti metal,and only a few regions reacted to form carbides.There is no suitable accumulation of carbon?100?crystal plane in the monolayer or few-layer graphene for carbide nucleation,so that the formation of carbides is more difficult.And smaller Lc?stacking thickness of002 carbon layer?of few-layer graphene cannot satisfy and support the nucleus and epitaxial growth of carbide grains,which shows that the epitaxial growth of the carbides depends on the stacking thickness of the graphite sheets.