RESUMO
Transition metal oxides have been widely used in microwave-absorbing materials, but how to improve impedance matching is still an urgent problem. Therefore, we introduced urea as a polymer carbon source into a three-dimensional porous structure modified by Co3O4 nanoparticles and explored the influence of different heat treatment temperatures on the wave absorption properties of the composite. The nanomaterials, when calcined at a temperature of 450 °C, exhibited excellent microwave absorption capabilities. Specifically, at an optimized thickness of 9 mm, they achieved a minimum reflection loss (RLmin) of -97.3 dB, accompanied by an effective absorption bandwidth (EAB) of 9.83 GHz that comprehensively covered both the S and Ku frequency bands. On the other hand, with a thickness of 3 mm, the RLmin was recorded as -17.9 dB, with an EAB of 5.53 GHz. This excellent performance is attributed to the multi-facial polarization and multiple reflections induced by the magnetic loss capability of Co3O4 nanoparticles, the electrical conductivity of C, and the unique three-dimensional structure of diatomite. For the future development of bio-based microwave absorption, this work provides a methodology and strategy.
RESUMO
Microwave irradiation is one of the most effective strategies to accelerate the curing of resin. However, the mechanism is still unclear. The debates mainly focus on how to quantitatively evaluate the 'non-thermal' effect of the microwave. In this work, the non-thermal effect on DGEBA with amine (D230) and anhydride (MHHPA) hardeners respectively was evaluated via an isothermal microwave curing reactor. The 'thermal effect' caused by the microwave was peeled off accurately. Iso-conversional kinetic analysis was performed based on the reaction extent from the real time FTIR spectrum, and the apparent activation energy (E a) was calculated for the quantitative evaluation of the microwave non-thermal effect. The influence caused by the polarized functional groups, reaction temperature and the transition state were explored, and the microwave non-thermal effect on the mechanical performance of the cured thermosets was evaluated. Results indicated that the microwave non-thermal effect was able to reduce the E a and accelerate the curing speed of epoxy resin. At the same time, the final curing extent was increased when the non-thermal effect of microwave was induced leading to enhanced mechanical and thermal performance.
RESUMO
A magnetically separatable catalyst Fe3O4@SiO2@PEI@Au (gold) nanoparticle was successfully constructed by a novel regional selective photoreduction method. Based on the photolysis mechanism of a type II photoinitiator, through controlling the distribution of polyethylene imine (PEI), Au nanoparticles about 10 nm, which are only on the surface of the Fe3O4@SiO2@PEI nanoparticle, could be photoreduced due to the PEI acting as a coordinating agent, capping agent, and photoreducing agent simultaneously. The small size Au nanoparticles endow the catalyst with a high catalytic performance toward the reduction of 4-nitroaniline to 4-aminophenol by NaBH4. In addition, magnetic Fe3O4@SiO2@PEI@Au nanoparticles could easily be recovered and could be reused at least six times still keeping catalytic efficiency higher than 95%, which contributes to their high stability and magnetization. Furthermore, compared to another reported approach, this method showed great regional selectivity of reducing metal nanoparticles by controlling the distribution of the PEI. Taking advantage of the regional selectivity of the photoreducing method could also be used to fabricate other metal nanoparticles as catalysts for various reactions.