Yonggen Lu

Graphene oxide: the mechanisms of oxidation and exfoliation

Graphene oxides (GOs) with large sheets and more perfect aromatic structure were prepared by a novel modified Hummers method. We demonstrated that the graphite did not need to be oxidized to such a deep degree as described in Hummers method because the space distance increased little when the oxidation proceeded to a certain extent and the obtained graphite oxides (GTOs) could be fully exfoliated to single layers with high thermal stability. The oxidation mechanism and chemical structure model of GO were proposed by analyzing the evolution of the functional groups with oxidation proceeded based on thermogravimetric analysis, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. The layer spacing calculated by molecular dynamics simulations coincided with the X-ray diffraction results. Furthermore, the size distribution and thickness of GOs were also studied. The results confirmed that the GOs prepared by the modified method were fully exfoliated to uniform single layers, and this method may be important for efficient exfoliation of GTO to GO and large-scale production of graphene.

A comparison of the effect of hot stretching on microstructures and properties of polyacrylonitrile and rayon-based carbon fibers

The effect of a different stretching stress at different heat treatment temperatures (HTT) on the structure and the mechanical properties of polyacrylonitrile (PAN)- and rayon-based carbon fibers was studied. The tensile strength increases first and then decreases with increasing stretching stress, whereas the Young’s modulus of the fibers continuously increases. The behavior of PAN- and rayon-based carbon fibers is similar with increasing stretching stress, but the tensile strength of PAN fiber decreased while that of rayon fiber increased with increasing HTT, what is more, the latter have a considerable lower tensile strength and modulus for equivalent processing conditions. The structure of the fibers was investigated with X-ray diffraction. A continuous change toward a nanostructure with a higher order was observed, which explains the increase in the Young’s modulus. For more complex dependence of the tensile strength on the processing conditions, a quantitative model to describe the effect of stretching stress at different HTT on preferred orientation degree and shear modulus is proposed. From the critical stress fracture of carbon fiber analysis, we can understand the different changes of tensile strength of both type fibers with stretching stress at different HTT.

Effect of carbonization temperature on microwave absorbing properties of polyacrylonitrile-based carbon fibers

ABSTRACT The pre-oxidized fibers were carbonized at the temperature ranging from 400 to 1300 °C for 1 h. The microwave absorption properties of carbon fibers (CFs) were examined in the frequency range of 2–18 GHz. It is found that the reflection loss characteristics are highly sensitive to the carbonization temperature. At a thickness of 2 mm, the CFs obtained at 710 °C exhibit the best microwave absorbing ability with a maximum reflection loss of −22.9 dB at 15 GHz, and a bandwidth exceeding −10 dB in the range 12.4–18 GHz. Results indicate that dielectric loss in cooperation with better matched characteristic impedance results in the excellent microwave absorption of CFs. Low temperature makes ϵ′ and ϵ″ too small to consume the energy of microwave, while over high temperature makes ϵ′ and ϵ″ too large to transmit the microwave into the CFs.

Effect of sizing on interfacial adhesion property of glass fiber-reinforced polyurethane composites:

This research was undertaken to investigate the influence of the sizing components, principally coupling agents and film formers, of glass fiber on the interface of reinforced polyurethane composites. The contents of film-forming agents and coupling agents were, respectively, checked by acetone extraction and heat treatment. The categories and chemical structures of the film-forming agents, which mainly exist in the extracting solutions, were investigated by nuclear magnetic resonance spectroscopy. The coupling agents, which mainly existed on the extracted glass fiber surfaces, were analyzed by X-ray photoelectron spectroscopy. Then, the interfacial adhesion properties between glass fibers of untreated, extracted and heat-treated and polyurethane resin were measured with micro-droplet test. The results show that the interfacial shear strength of the glass fiber-reinforced polyurethane composite mainly depends on the coupling agents, in which the N-containing groups play a dominant role. The film formers also take some effects, in which the hydroxyl groups have the greatest contribution.