Preparation and microwave absorption properties of ferrite/reed charcoal composites

Electromagnetic pollution has become a new type of environmental source of pollution, second only to water pollution and air pollution. The key to solving electromagnetic pollution problems lies in designing high-performance absorbing materials. Ferrite is a typical magnetic loss-absorbing material with advantages such as strong absorption, low cost, nontoxicity, and simple preparation. However, when using ferrite alone as an absorbing material, the impedance matching characteristics are poor, and there are problems such as narrow absorption frequency band, high density, and poor thermal stability, which limit its practical application in the field of electromagnetic wave absorption. In order to solve the problems of high density and narrow absorption bandwidth of ferrite-absorbing materials, carbon-based materials are combined with ferrite materials to prepare absorbing materials. The magnetic loss of ferrite is combined with the dielectric loss of carbon materials to achieve complementary advantages and improve the absorption performance of the materials.

Ferrite/reed charcoal (Ferrite/RC) composites were prepared by room temperature impregnation and high-temperature in-situ growth methods using reed stems as raw materials. The electromagnetic properties and absorption performance of the composites were controlled by adjusting the carbonization temperature. Firstly, prepare 60 ml of 0.01 mol/L iron nitrate solution in the beaker. Then, soak 2 g of reed stalks in the solution at room temperature for 20 hours and transfer them to an electric hot air drying oven at 60 ℃ until completely dry, to obtain a mixed precursor. Place the dried mixed precursor in a corundum boat and transfer it to a tube furnace. Heat it up at a rate of 5 ℃/min in a nitrogen atmosphere to the preset temperature (650 ℃, 670 ℃, 690 ℃), hold it for 2 hours, and then cool it down at the same rate to 100 ℃. After natural cooling to room temperature, take out the sample, and the Ferrite/RC composites are obtained. Use field emission scanning electron microscopy (SEM) to observe the microstructure and morphology of the sample. Analyze the crystal structure of the sample using an X-ray diffractometer (XRD). Analyze the relative content of carbon components and ferrites using a thermos-gravimetric analyzer (TG). Study the static magnetic properties of materials using a vibrating sample magnetometer (VSM). Using Vector Network Analyzer (VNA) to Obtain Electromagnetic Parameters of samples in the frequency range of 2-18 GHz().

① The XRD results show that the characteristic diffraction peaks of FeO (JCPDS No.74-0748) at 18.3 °, 30.1 °, 35.5 °, 43.1 °, 57.0 °, and 62.5 °can be observed in all samples. Almost all diffraction peaks are sharp and the half- width is small, indicating that FeO has been successfully loaded into the reed charcoal and the resulting composites have good crystallinity. When the carbonization temperature is 690 ℃, a diffraction peak of Fe (100) crystal plane (JCPDS 50-1275) appeared at 44 °. This may be because, at higher temperatures, Fe is reduced by carbon or pyrolysis gas to form elemental iron. ② The TG test shows that the relative content of carbon components in the sample is about 24%, and the proportion of ferrite is about 74%, with a ratio of about 1:3. ③ The SEM results indicate that the composites exhibit a typical honeycomb structure surrounded by hexagonal charcoal walls with some pores of 1-3μm, and a hollow straight pipeline structure inside, with a pipeline radius of about 15-30μm. The charcoal wall of the reed stems has reached the nanometer level, and the surface of the charcoal wall is not smooth and wrinkled. The FeO particles are densely and orderly fixed in the reed stem charcoal skeleton. ④ The TEM results show that the ferrite nanoparticles were uniformly distributed in the charcoal skeleton of the reed stem, with a relatively uniform particle size and a diameter of about 20-100 nm. The high-resolution TEM image shows a stripe spacing of 0.25 nm, corresponding to the (311) plane of Fe3O4, which is consistent with the XRD results. ⑤ The VSM result shows that the saturation magnetization strengths of FRC-650, FRC-670, and FRC-690 are 1.37 emu/g, 1.57 emu/g, and 2.34 emu/g, respectively, with coercivity of 67.98Oe, 60.48Oe, and 59.83Oe, respectively. ⑥ The VNA tests indicate that FRC-650 has the minimum reflection loss () of -45.4 dB at 4 mm, and it has the maximum effective absorption bandwidth (EAB) with 4.3 GHz when the thickness is 2 mm. The overall of FRC-670 increases with decreasing thickness, reaching -45.7 dB at a thickness of only 1.7 mm, corresponding to an EAB of 3.4 GHz (14.7-18 GHz). The EAB is the widest at a thickness of 2 mm, reaching 5.7 GHz (12.1-17.8 GHz) with an of -32.1 dB. FRC-690 exhibits optimal absorption performance at a thickness of 1.5 mm, with corresponding of -18 dB and an EAB of 4.8 GHz, respectively.Conclusions: The Ferrite/RC composites retained the natural three-dimensional honeycomb network structures of the reed stalks, and FeO and iron nanoparticles were uniformly distributed in the charcoal wall and pores of the reed stem; Raising the carbonization temperature (650~690℃) can increase the conductivity and dielectric loss ability of composites, but excessive temperature can lead to impedance mismatch of the material and reduce its electromagnetic attenuation ability. The composites prepared at a carbonization temperature of 670℃ exhibit the best absorption performance, with a reflection loss of -45.7 dB at a thickness of only 1.7 mm and an effective absorption bandwidth of 5.7 GHz (12.1-17.8 GHz) at a thickness of 2 mm, which is attributed to the good conductivity loss, polarization relaxation, and the synergistic effect of electrical and magnetic losses of composite materials.