Synthesis And Microwave Electromagnetic Properties Of Nanocrystalline Iron-based Flakes

The nanocrystalline ferromagnetic metal and alloy flakes have gained a considerable attention because of their higher saturation magnetization and large shape anisotropy can exceed the Snoek's limit and have higher permeability in the GHz frequency range. Hence it can be expected to be good microwave absorption filler in GHz frequency range. However, the large permittivity owe to their large surface areas of flakes makes them unfavorable to wave impedance matching and deteriorate the microwave absorbing ability. At the same time, permeability decreased rapidly in higher frequency because of eddy current loss and natural resonance. Based on this, we conducted intensive study on how to reduce the permittivity while maintaining high permeability and improve high frequency permeability.First, the influence of wet-milling time on the morphology and microwave electromagnetic parameters of Fe flakes were investigated. The results show that the width of Fe flakes gradually becomes larger and the thickness of Fe flakes gradually decreases along with the wet-milling time extended to 36 h. With the wet-milling time further prolonging to 48 h, Fe flakes fractured, resulting in the reduced width and little changed thickness of about 220 nm. The aspect ratio of Fe flakes got a maximum value of 41.2 at 36 h. Electromagnetic parameters show that the real part (ε') and imaginary part (ε") of the relative complex permittivity increase with the wet-milling time. Low-frequency magnetic permeability (μ') and magnetic loss (μ") both have maximum values at 36 h, which were found to be in qualitative agreement with aspect ratio, whereas high-frequency magnetic permeability show oppsite variation law. Compared with spherical Fe powders, Fe flakes show a single natural resonance band which is shifted to high frequencies and bandwidth decreases with prolonging wet-milling time in the 2-18 GHz.Then, Nanocrystalline Fe flakes were prepared by two-step ball milling in order to decrease the permittivity but to keep their high permeability. At the initial stage of wet-milling process, Fe powders were flattened faster because of the grain refinement caused by the first-step dry-milling, with the wet-milling time further prolonging, Fe flakes fractured, resulting in the reduced width due to the high internal strain which is also owing to dry-milling. Consequently, Fe flakes with large aspect ratio and small width, which show high permeability and low permittivity are obtained. The epoxy resin-based microwave absorbing materials containing this Fe flakes exhibit reflection loss of less than-10 dB in 9.4-17.9 GHz frequency range with a peak of-14.6 dB when the thickness is 1.5 mm and the volume ratio of Fe flakes in the materials is 20%.Finally, the influence of the adulteration of Ni on the morphology and electromagnetic parameters of Fe flakes were studied by the numbers. The width of Fe flakes is getting smaller and more fragments were obtained with the increase of Ni content. Electromagnetic parameters show that the complex permittivity increase and low-frequency magnetic permeability decrease with increasing Ni content, but high-frequency magnetic permeability incresae with increasing Ni content, and the resonant frequency shift to the low frequency with increasing Ni content. As to the FeNi flakey powders with same component, morphology has a big influence on the microwave electromagnetic parameters. Compared with Fe flakes, Fe85Ni15 alloy flakes improve the real permeability only in 13.6-18 GHz range because of the heterogeneity of morphology. While the flakes obtained by wet-milling 10 h dry-milled Fe85Ni15 mixture show good magnetic dispersion properties, which can increase the real permeability in 5.52-18 GHz range. The epoxy resin-based microwave absorbing materials containing this Fe85Ni15 flakes exhibit reflection loss of less than-10 dB in larger frequency range with a peak of-16.8 dB, and reflection loss decrease in 12-18 GHz frequency range when the thickness is 1.5 mm and the volume ratio of Fe flakes in the materials is 15%.