Owing to their superior mechanical and physical properties, carbon nanotubes seem to hold a great promise as an ideal reinforcing material for composites of high-strength and low-density. In most of the experimental results up to date, however, only modest improvements in the strength and stiffness have been achieved by incorporating carbon nanotubes in polymers. In the present paper, the stiffening effect of carbon nanotubes is quantitatively investigated by micromechanics methods. Especially, the effects of the extensively observed waviness and agglomeration of carbon nanotubes are examined theoretically. The Mori-Tanaka effective-field method is first employed to calculate the effective elastic moduli of composites with aligned or randomly oriented straight nanotubes. Then, a novel micromechanics model is developed to consider the waviness or curviness effect of nanotubes, which are assumed to have a helical shape. Finally, the influence of nanotube agglomeration on the effective stiffness is analyzed. Analytical expressions are derived for the effective elastic stiffness of carbon nanotube-reinforced composites with the effects of waviness and agglomeration. It is found that these two mechanisms may reduce the stiffening effect of nanotubes significantly. The present study not only provides the relationship between the effective properties and the morphology of carbon nanotube-reinforced composites, but also may be useful for improving and tailoring the mechanical properties of nanotube composites.

-Encapsulation and immobilization technology is important for the food processing and bioengineering industries. Chitosan is a natural polysaccharide prepared by the N-deacetylation of chitin. It has been widely used in food and bioengineering industries, including the encapsulation of active food ingredients, in enzyme immobilization, and as a carrier for controlled drug delivery, due to its significant biological and chemical properties such as biodegradability, biocompatibility, bioactivity, and polycationicity. In this work, chitosan nanoparticles and nanofibers used to encapsulate bioactive substances and immobilize enzymes were reviewed. Preparation of chitosan nanoparticles and nanofibers, including the work achieved in our group on chitosan nanoparticles for enzyme immobilization, were also introduced. Some problems encountered with nano-structured chitosan carriers for bioactive substance encapsulation and enzyme immobilization were discussed, together with the future prospects of such systems.

HIGHLIGHTS• Hierarchically porous Fe-Co/N-doped carbon/rGO (Fe-Co/NC/rGO) composites were successfully prepared. Macropores, mesopores, and micropores coexisted in the composites.• Hierarchically porous Fe-Co/NC/rGO showed effective bandwidth of 9.29 GHz.ABSTRACT Developing lightweight and broadband microwave absorbers for dealing with serious electromagnetic radiation pollution is a great challenge. Here, a novel Fe-Co/N-doped carbon/reduced graphene oxide (Fe-Co/NC/rGO) composite with hierarchically porous structure was designed and synthetized by in situ growth of Fe-doped Cobased metal organic frameworks (Co-MOF) on the sheets of porous cocoon-like rGO followed by calcination. The Fe-Co/NC composites are homogeneously distributed on the sheets of porous rGO. The Fe-Co/NC/rGO composite with multiple components (Fe/Co/NC/rGO) causes magnetic loss, dielectric loss, resistance loss, interfacial polarization, and good impedance matching. The hierarchically porous structure of the Fe-Co/NC/rGO enhances the multiple reflections and scattering of microwaves. Compared with the Co/NC and Fe-Co/NC, the hierarchically porous Fe-Co/NC/rGO composite exhibits much better microwave absorption performances due to the rational composition and porous structural design. Its minimum reflection loss (RL min ) reaches − 43.26 dB at 11.28 GHz with a thickness of 2.5 mm, and the effective absorption frequency (RL ≤ − 10 dB) is up to 9.12 with the same thickness of 2.5 mm. Moreover, the widest effective bandwidth of 9.29 GHz occurs at a thickness of 2.63 mm. This work provides a lightweight and broadband microwave absorbing material while offering a new idea to design excellent microwave absorbers with multicomponent and hierarchically porous structures.

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γ-Fe2O3 nanotube/porous reduced graphene oxide (rGO) composites were prepared using a controllable method. Uniform γ-Fe2O3 nanotubes with a diameter of about 85 nm and a length of about 230 nm are well distributed between porous rGO sheets. Compared with the γ-Fe2O3 nanorod/porous rGO and γ-Fe2O3 nanotube/nonporous rGO, the γ-Fe2O3 nanotube/porous rGO composites with both unique hollow and porous structure show an advantage for the attenuation of microwaves. The appropriate dielectric loss and magnetic loss result in a good impedance matching. The minimum reflection loss of γ-Fe2O3 nanotube/porous rGO composite reaches −34.20 dB with a thickness of 2.0 mm, and the absorption bandwidth is 4.59 GHz. These results reveal that the synthesis of hollow and porous composites is a promising way for getting lightweight microwave materials with high performance.