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Metal–organic framework (MOF)-derived materials have gained popularity as microwave absorbing materials due to their electromagnetic dissipation advantages. In this study, a special type of MOF (ZIF-L) was used as a precursor; tannic acid was introduced for chemical etching. The resulting material obtained by in situ carbonization was a hollow structure of N-doped carbon embedded by Co nanoparticles (CoHNC), which retained the original pumpkin seed shape but had a hierarchical porous structure on the inside. By adjustment of the amount of cobalt nitrate and the carbonation temperature, the electromagnetic parameters could be altered, improving impedance matching and enabling different microwave absorption properties to be obtained. At 16.96 GHz, the material exhibits an optimal reflection loss of −45.5 dB, an effective bandwidth of 3.04 GHz, and a matching thickness of only 1.4 mm with a material filler of 30 wt %. The microwave loss mechanism is formed by the dual loss mechanism with dielectric loss as the main loss and magnetic loss as the auxiliary one. Therefore, the synthesis of the CoHNC composite through simple etching provides a useful idea for optimizing microwave absorbing materials. [ABSTRACT FROM AUTHOR]

Transition metal composites doped with heteroatomic nitrogen and having a hierarchical porous structure are considered to be an effective way to improve the electromagnetic wave absorption ability of carbon-based absorbent materials. In this work, a mixture of melamine, glucose, and nickel nitrate was used as a precursor to synthesize a multi-interface carbon nanotube network with tubular morphology by one-step catalytic pyrolysis. The absorption mechanism was further discussed in detail. The composite materials have a better complementary loss mechanism and good matching impedance capability by combining carbon and magnetic Ni nanoparticles. Notably, 6.4Ni/CNTs-900 exhibits a strong reflection loss (RL) of −49.3 dB at 8.88 GHz with a low filling content of 15% and an effective bandwidth (EAB, RL < −10 dB) of 2.96 GHz. The excellent microwave absorption properties of these materials can be attributed to the multiple scattering, interface polarization, dipole polarization, and conductivity loss induced by the unique structure of the multi-interface carbon nanotube network. This work provides a method for managing electromagnetic radiation pollution, which has great potential for efficient microwave absorption. [ABSTRACT FROM AUTHOR]

Mechanical excavation in hard or very hard rocks is still challenging in rock engineering. One plausible solution is to reduce rock strength prior to mechanical excavation to increase excavation and cost-efficiency. The present study utilizes a 6 kW microwave to irradiate rock specimens and evaluates the variation in rock strength and indentation hardness. Since the contact between the drill bits of roadheaders and the rock surface is a dynamic impact process, dynamic tests are conducted and compared with the quasi-static results. The experimental results demonstrate that short microwave irradiation (30 s) can produce substantial damage to the rock. The reduction of uniaxial compressive strength, Brazilian tensile strength, and indentation hardness under quasi-static loading can be about 39%, 43%, and 44%, respectively, and 33%, 26%, and 50%, respectively, under dynamic loading. In addition, it is found that the tensile strength is the most sensitive to microwave, while the crack density is the least sensitive. More importantly, this reduction is not linear and not monotonic to the irradiation time. The fast reduction of rock strength and indentation hardness after microwave irradiation implies the reduction in the drill-bit wear and increase in the excavation rate, and vigorously demonstrates the great potential of microwave-assisted mechanical excavation of hard or very hard rocks. Highlights: The dynamic indentation test is used to simulate the impact between drill bits and rock surfaces. Short microwave irradiation (30 s) can largely reduce the diorite strength and indentation hardness. The reduction is not linear and not monotonic to the irradiation time, regardless of loading rates. Different physical properties have different sensitivities to the microwave. Experimental results demonstrate the great potential of the microwave technique in hard rock excavation. [ABSTRACT FROM AUTHOR]