Precursor Directed Synthesis Of Cobalt Nanosheets With Microwave Absorption Performances

With the development and progress of society,more and more electronic devices are being applied to people's production and life.The electromagnetic waves generated by these electronic devices can interfere with the normal information transmission of other devices,reduce production efficiency,and even damage the central nervous system of the human body,resulting in poor blood circulation and threatening people's health.At the same time,in modern warfare,the development of reconnaissance and detection technology has brought great challenges to the survival environment of weapons.Therefore,it is imperative to develop high-performance stealth materials.To this end,many research scholars have also conducted in-depth research on various absorbing materials.In this paper,magnetic metal cobalt,a typical absorbing material,is taken as an example.Through a series of experiments,the effects of process conditions on the electromagnetic parameters and absorbing properties of cobalt nanosheets are explored.(1)Hexagonal cobalt hydroxide nanosheets with two crystal phases of ? and ? are synthesized by a homogeneous precipitation method.Among them,?-phase cobalt hydroxide is dark green,microscopically petal-like,with a radial dimension of about 3~5 ?m and a thickness of about 100 nm,and the uniformity of the sample is not easy to regulate;while the ? phase is pink,and the whole is a regular hexagonal sheet structure with a flat surface and uniform size.The radial dimension is about 2~3 ?m and the thickness is about 150 nm.The aspect ratio of the ? phase is larger than that of the ? phase.(2)The prepared ? and ? cobalt hydroxide precursors are reduced by a hydrothermal reduction method.The reduced products at 400 °C,500 °C and 600 °C were compared to investigate the effects of different temperatures on the electromagnetic parameters of the materials.The results show that the sample after the reduction of the ? precursor phase can maintain the sheet morphology well,with a radial dimension of about 3 ?m and a thickness about 120 nm.After the reduction of the ?-phase precursor,porous cobalt nanosheets were obtained.The sample reduced at 500 °C had the best electromagnetic absorbing properties with a minimum reflection loss of-45 dB and a corresponding effective bandwidth of 8.4 GHz.The large aspect ratio of the sheet structure increases the anisotropy and permeability,with breaking through the snoke limit,and the porous structure can effectively adjust its permittivity to keep good electromagnetic absorbing properties.When the ? phase precursor is reduced at 400 °C,500 °C,and 600 °C,the sample changes from the petal-like shape of the precursor to the granular shape.Compared with the ? phase sample,the absorbing performance of the ? phase precursor is inferior after they reduced at the same temperature.(3)In order to improve the reduction efficiency,different concentrations of ?-phase precursors were reduced by the method of microwave hydrogen plasma reduction.Compared with hydrogen reduction,hydrogen plasma with higher energy not only processes higher efficiency than hydrogen,but also can enhance the magnetocrystalline anisotropy of cobalt nanosheets.As the concentration of the precursor increases gradually,the products gradually change from ? phase to ? phase.Due to the large ratio of the height and the width,the ? phase products cannot keep their complete hexagonal morphology and display the breakages.The minimum reflection loss of cobalt nanosheets reduced with the precursor concentration of 5 mmol/L is-70.7 dB,which is higher than that of cobalt nanosheets obtained by thermal reduction.(4)The nano-silica was coated on cobalt nanosheets by sol-gel method to form a coreshell structure,thereby reducing the dielectric constant and improving the electromagnetic absorbing properties of the composite structure.Compared with the uncoated silica cobalt nanosheets,the reduced conductivity exhibits a significant decrease in the permittivity,while the minimum reflection loss,effective bandwidth and bandwidth adjustment capability are significantly improved.After coated with a series of concentration gradients of TEOS,the results show that the cobalt nanosheets coated the TEOS at a concentration of 3.725 ml/L can achieve the minimum reflection loss of-53 dB.Especially,the effective bandwidth reaches 5.1 GHz,while the minimum reflection loss remains at a very low value.Additionally,the silica coated on cobalt nanosheets can separate contact between internal cobalt core and the air,and then enhances its own oxidation resistance.