Preparation And Properties Of C_f/Polyurethane Foam Broadband Microwave Absorbing Composites

In the 21st century,the problem of electromagnetic wave pollution in human life is increasing day by day.In military affairs,people also need to reduce the reflection of electromagnetic waves to realize anti-reconnaissance.Therefore,the research on absorbing stealth materials has become one of the current research focuses.In this paper,polyurethane rigid foam is used as matrix,and short carbon fiber,carbonyl iron powder and polycrystalline iron fiber are used as wave absorbers to prepare broadband wave absorbing composites.After a series of optimization studies,the three-layer C_f/polyurethane foam broadband absorbing composite material which can effectively absorb in the whole frequency range from 2 to 18 GHz was obtained.By testing the influence of the content of various wave absorbers in the range of1?9 wt%on the mechanical properties and thermal properties of polyurethane foam composite samples,it was found that the density of polyurethane samples containing carbonyl iron powder increased slowly with the increase of carbonyl iron powder content,and its compressive strength first increased and then decreased with the increase of carbonyl iron powder content,reaching the maximum value of 563.44 k Pa at wt=7%,and the specific strength changed slightly,fluctuating around 5.17 kg·m^-3,whilethe density,compressive strength and specific strength of samples containing carbon fiber or polycrystalline iron fiber increase obviously with the increase of the content of absorbing agent.Carbon fiber and polycrystalline iron fiber can improve oxygen index,while carbonyl iron powder can reduce oxygen index.The thermal decomposition of each group of samples includes two stages,in which the main decomposition reaction takes place at 190?390?,reaching the peak temperature of decomposition reaction between 300??32,0 and the initial temperature of decomposition reaction reaches the maximum when the content of absorbing agent is3?5 wt%.Single absorber absorbing materials with short carbon fiber content of 0.1?0.7wt%and carbonyl iron powder content of 10?30 wt%were prepared,and their absorbing performance was tested in the frequency range of 2?18 GHz.It was found that the effective absorption bandwidth of single absorber samples containing short carbon fiber with reflectivity less than-10d B was above 1.75 GHz.However,there is no reflectivity less than-10d B in the single wave absorber samples containing carbonyl iron powder.Combined with the test results of single absorbing material,short carbon fiber with 0.3 wt%and 0.4 wt%content and carbonyl iron powder with different contents were added into the matrix as composite absorbing agent,and their absorbing performance was tested.It was found that the effective absorption bandwidth was obviously widened,and the effective absorption bandwidth with reflectivity less than-10 d B in the frequency range of 2?18 GHz was all above8.72GHz.According to the test results of composite wave absorbers,double-layer wave absorbing materials were prepared and their wave absorbing properties were tested.It is found that the effective absorption bandwidth is further widened,and the effective absorption bandwidth with reflectivity less than-10 d B is above 13.42 GHz in the frequency range of 2?18 GHz.According to the absorbing test results of the double-layer absorbing material,a surface impedance matching layer of polycrystalline iron fiber is added on the surface of the double-layer absorbing material under the condition that the total thickness of the material is unchanged.The impedance matching characteristics of the material are improved,and it is found that the effective absorption bandwidth of the material with reflectivity less than-10 d B is widened,and the effective absorption bandwidth is above 13.98 GHz in the frequency range of 2?18 GHz,and the E₁ and E₃ samples even reach the effective absorption in the whole frequency band.