Electrochemical Reduction Synthesis Of Micro-nano Dendritic Iron Series Materialsand Their Electromagnetic Absorption Performances

Due to the specially morphologic characteristics and physical-chemical properties, the hierarchical micro-nano dendritic structural materials have been widely studied by more and more scientisits. In particular, their large surface area and high magnetic anisotropy make them show a bright future in electromagnetic wave absorption field. The mainly synthetic methods of dendritic mateirals are hydrothermal, electrochemical and microwave assisted method. However, these methods have too many limitations and disadvantages, such as bad homogeneity of morphology, size and low productivity.In this work, we established a novel and facile electrochemical reduction method, which could provide symmetrical distribution ring electric field and effectively prevent redox reaction, to rapidly synthesize dendritic structural iron-based materials with uniform morphology and size. The effects of current indensity, reaction time and samples’ crystalline structure on the morphology type and size of dendritic samples were studied by Scanning Electron Microscope（SEM）, Transmission Electron Microscope（TEM）, High Resolution Transmission Electron Microscope（HRTEM）, Selected Area Electron Diffraction（SAED） and X-ray Diffraction（XRD）. Based on the experimental results and concentration gradient theory, the formation mechanism of dendritic materials was proposed. All of the dendritic samples’ electromagnetic absorption（EMA） properties were characterized by Vibrating Sample Magnetometer（VSM） and Vector Network Analyzer（VNA）, and their reflection loss mechanism was also discussed. Moreover, we also studied the effects of rare earth（RE） ions in the electrolyte on the morphology and dendritic structure of α-Fe, the control mechanism of RE ions also was discussed. Meanwhile, the EMA performances of different RE-added dendritic α-Fe samples were studied. Finally, the dendritic Fe S₂ was also prepared via thermal sulfidation of as-synthsized dendritic α-Fe. Not only the EMA properties were studied, but also the electrochemical properties of dendritic Fe S₂ were researched by cyclic voltammetry, differential capacity and constant current charge/discharge cycling test.The body-centered cubic crystal structural leaf-like shape α-Fe and hexagonal close packing crystal structural grain-like shape ε-Co were synthesized by this electrochemical reduction method. The study results suggest that higher current density and shorter reaction time help to get the smaller and finer dendritic samples. We proposed that the formation of dendritic structural materials is attributed to the concentration gradient near the electrode surface, electric field distribution and samples’ crystalline structure. During the reaction, fast reduction reaction under high current density leads to the formation of cation concentration gradient which set the condition for the growth of dendritic samples. Concentration gradient leads to the formation of growing region and growing inhibition region. The growing region stablely and continuously converts into the growing inhibition region, which makes all the dendritic samples exhibit uniform morphology and size. The samples’ crystalline structure and electric field distribution decide the types of dendritic samples. Finally, this growth mechanism is further confirmed by the studies of Co Fe alloys.All the dendritic α-Fe, ε-Co and Co Fe alloys show good EMA performances. Dendritic ε-Co shows the minimum Reflection loss（RL） value of-58.0 d B. Co_0.9Fe_0.1 shows the widest absorption band, up to 8.4 GHz in which RL <-10 d B. The good EMA performances are mainly attributed to not only their large surface area that could enhance the free electron polarization on the interface and further enhance the dielectric loss, but also the high magnetic anisotropy that could make the response frequency of natural resonance shift to the high-frequency range and enhance the magnetic loss at there. In addition, the plentiful crystal defects and grain bouduaries in dendritic structure also affect the polarization effect and changes domain and magnetic response. The effective complementarities between dielectric loss and magnetic loss are another important reason for dendritic materials’ good EMA properties. Coating Si O₂ on dendritic α-Fe could enhance the dielectric loss and complementarity, but against the magnetic loss.The morphology and dendritic structure of dendritic α-Fe are adjusted by adding RE ions in electrolyte. From the results, it can be known that RE ions adsorbed on the surface of electrode and samples could increase the polarization potential, suppress growth rate and promote nucleation rate. So the dendritic samples with smaller size and finer structure are synthesized When the concentration of RE additive reaches 2.0 g·L-1, the multi-grain-boundary dendritic α-Fe consisted by lots of nanoparticles are synthesized. In all of the RE-added samples, the α-Fe synthesized in the electrolyte containing 1.5 g·L-1 Ce additive possesses the minimum RL of-68.2 d B, and the α-Fe synthesized in the electrolyte containing 2.0 g·L-1 La additive shows the widest absorption band of 6.1 GHz（RL <-10 d B）. Their enhanced EMA performances are mainly attributed to larger surface area and plentiful grain boundaries.The dendritic Fe S₂ prepared under 450℃ shows a good EMA performance. Its minimum RL is low to-56.0 d B; absorption band width in which RL<-20 d B is 3.0 GHz. Dendritic Fe S₂ also exhibits a good electricchemistry properties as cathode for Li-ion batteries. Its reversible capacity could keep at 350 m A·hg-1 after 150 cycles at 0.2C in electrolyte containing 10 wt.% FEC. The good cycling properties are due to its large surface area and stability of dendritic structure. Furthermore, the unstable Li2-x Fe S₂ intermediate phase was observed by HRTEM and Electron Energy Loss Spectroscopy in Scanning Transmission Electron Microscopy（STEM-EELS） when studying the charging/discharging mechanism of dendritic Fe S₂.