Denghua Li

Polyacrylonitrile/lignin sulfonate blend fiber for low-cost carbon fiber

Polyacrylonitrile/lignin sulfonate (PAN/LS) blend fibers were spun via a wet spinning process. The fiber structure, mechanical properties and thermal stability of the precursor fibers were studied by FT-IR, SEM, tensile tester, and TG-DSC. Results indicated that there was no chemical crosslinking between PAN and LS during the process of wet spinning. PAN and LS had good compatibility in the blend fibers. LS could weaken the skin of the blend fibers and reduce the fiber structure defects. The increase of dope concentration could improve the fiber structure and mechanical properties. LS blending with PAN could reduce fiber weight loss in the thermal stabilization process, and most importantly the precursor fibers could be stabilized rapidly without fiber fusion. Through polymer blending and wet spinning, this study provided a promising way to prepare a precursor fiber for carbon fiber.

A cellular automaton model for austenite to ferrite transformation in carbon steel under non-equilibrium interface conditions

The isothermal decomposition of austenite into ferrite is investigated using a two-dimensional cellular automaton (CA) algorithm. This CA model provides a simple solution for the difficult moving boundary problem that governs the ferrite grain growth. In this model, the growth of ferrite grains is controlled by both the austenite–ferrite (γ–α) interface mobility and the carbon diffusion in the retained austenite. The competition between the γ–α interface dynamics and the carbon diffusion in the austenite results in a non-equilibrium γ–α interface condition. In order to predict the growth kinetics of ferrite grains, the carbon diffusion coupled with the γ–α interface dynamics is resolved numerically. The driving force for the γ–α interface mobility is calculated using a regular solute sub-lattice model. The kinetics of ferrite transformation predicted by this CA method is compared with that of a JMA model and experiments in the literature. The simulation provides an insight into the carbon diffusion in austenite and the microstructure evolution during transformation. Meanwhile, the results also show that the γ–α interface under the present non-equilibrium condition is numerically stable in two dimensions and the simulated morphology of the ferrite grains is an almost equiaxed polygon.

A finite element analysis of strain induced transformation rolling and an experimental study on the grain refinement potential of severe undercooling thermo-mechanical treatment

A finite element analysis was performed to study the strain induced transformation rolling process through which ultra-fine-grain steels were produced. Within the surface layer of the specimen, where ultra-fined grains were found, high strain and severe undercooling were found to coexist during hot deformation. It is proposed in this paper that the ultra-refinement was caused by the deformation of austenite in a severely undercooled state. To verify the grain refinement potential of severe undercooling thermomechanical treatment, hot compression experiments were done on a Gleeble1500 thermo-mechanical simulator. Severe undercooling was obtained by cooling the specimens at very high cooling rate before deformation. Metallographic observations showed that ultrafine grains could be obtained at a true strain of 0.9 provided that undercooling was higher than 200 K

Structural heterogeneity and its influence on the tensile fracture of PAN-based carbon fibers

This communication reported an intuitive and convenient approach to detect the structural heterogeneity of carbon fibers by using Raman spectroscopy. As indicated by both the linear and mapping modes, the skin–core difference was enhanced with the rising temperature during the graphitization. This enhanced structural heterogeneity was observed to strongly influence the tensile fracture mode and the tensile properties of the graphitized fibers.

Structural evolution during the graphitization of polyacrylonitrile-based carbon fiber as revealed by small-angle X-ray scattering

On the basis of a Debye–Bueche correlation length analysis, the small-angle X-ray scattering (SAXS) intensity components due to different scatterers within polyacrylonitrile-based carbon fiber were determined and analyzed separately. According to Guiniers law and other related methods, an intensity component indicating a relatively large scatterer was ascribed to the amorphous structure within the boundaries of fibrils. Results indicated that the amorphous regions decreased in dimension and finally transformed completely into voids as the heat treatment temperature rose to 2773 K. The general trend for microvoids was a systematic change from many small voids to a few large voids, while the local density fluctuation within the samples weakened and finally faded away. In conclusion, the graphitization process of carbon fibers as revealed by SAXS is a systematic evolution from a quasi-two-phase system (fibril, amorphous region and microvoid within the fibril) of high-strength carbon fiber to the true two-phase structure (crystallite and microvoid) of high-modulus graphite fiber.

A discussion of the formation of ultra-fine grains in low-carbon steels based on FE analysis

A FE analysis was performed to study the formation of ultra-fine grains in a new hot rolling process. It was verified in this paper that ultra-fine grains were caused by high level undercooling and high level strain. The level of undercooling and the level of strain of the strip during this new rolling process were studied quantitatively with FE analysis. It was found that the critical undercooling to achieve ultra-fine grain was about 190 K. Compression experiments were done with a schedule involving rapid cooling before deformation to achieve high level undercooling. Ultra-fine grains were achieved in compressed experiments and the critical undercooling to obtain ultra-fine grains was about 200 K, similar to that calculated by FE analysis

Polyacrylonitrile-based turbostratic graphite-like carbon wrapped silicon nanoparticles: a new-type anode material for lithium ion battery

We investigated a new-type anode material for silicon (Si) nanoparticles wrapped in polyacrylonitrile (PAN)-based turbostratic graphite-like carbon, and discovered an interesting phenomenon that the battery capacity increased rather than decreased with the increase of cycling numbers. The carbonaceous matrix formed by PAN-based turbostratic graphite-like carbon could accommodate the volume change of Si nanoparticles and make the pulverized Si nanoparticles to keep good contact with the working electrode, thus to enable the full lithiation of Si and improve the battery capacity. As a result, the capacity of the anode material reached 680 mA h g−1, about twice as that of the commercial anode material, with only 9.8 w% silicon content. We also provided a new anode material preparation method that was easy to be industrialized, including using the PAN precursor and the preoxidation process to ensure the high carbon yield and the relatively high graphitization degree of the anode material. Some findings in this study are helpful to more clearly understand the lithiation–delithiation process of Si-based anode materials, as well as the basic strategy of increasing the Si-based anode material capacity.