Study On The Preparation And Electromagnetic Wave Absorption Performace Of Fe₃O₄/Geopolymer Composites

With the development of radio technology,electromagnetic(EM)waves as a medium have been widely used in information communication,radar detection,electronic countermeasures and so on.However,the radiation and interference produced by EM waves seriously affect people's normal lives.Particularly,radar detection threatens the survival of weapons and equipment.Because the materials with EM wave absorbing functions can transform EM waves into heat energy or other forms of energy consumption,the development of building materials with absorbing functions is of great significance.At present,cement-based building composites have achieved preliminary results in EM wave absorbing applications because of their high popularity and simple preparation methods.However,high energy consumption and carbon emissions in the cement manufacturing process restrict their wide application.In addition,the present work only adopted a method that simple mixing of different types of absorbents with cement matrices,lacking the consideration of the distribution of adsorbents in the cement matrix as well as the interaction between adsorbents and the cement matrix.Based on the above background,a series of different Fe₃O₄/geopolymer(GP)composites were prepared by using environmentally friendly GP instead of traditional cement as the matrix and low-cost Fe₃O₄ as the absorbents in this study.The effects of the Fe₃O₄ structures,sizes,dispersion states and interfacial interactions with the matrix on the EM wave absorbing properties were analyzed by studying the EM wave absorbing properties of Fe₃O₄/GP composites in the 2.0-18.0 GHz range.It is expected that this study will provide theoretical support,a database and an experimental basis for the research,development and engineering application of building materials.The main research results are summarized as follows:1.Based on the consideration of chemical compatibility between Fe₃O₄ and GP,GP composites filled with different fillers,Fe₃O₄ and Si O₂-coated Fe₃O₄(Si O₂@Fe₃O₄)with the diameter of?10 nm,were prepared.The effect of Si O₂ coating on the dispersion of Fe₃O₄ in the GP matrix,thereby affecting the absorption performance of GP composites was studied.When the filling amount was Fe₃O₄ is 0.5 vol%,the absorption performance of Si O₂@Fe₃O₄/GP was better than that of Fe₃O₄/GP.The RL_minof Si O₂@Fe₃O₄/GP reached-43.6 d B(2.3 mm)at 15.3 GHz,and the EAB was expanded to 7.0 GHz(2.5 mm).This is mainly because the chemical reaction between the Si O₂layer and GP alkaline activator promoted the dispersion of Fe₃O₄ in the GP matrix.The better the dispersion of Fe₃O₄was,the larger the area of interaction with the EM wave were achieved.As a result,the interface polarization loss and magnetic resonance loss increased,and thus the capability of the material to attenuate EM wave was improved.2.Solid Fe₃O₄(S-Fe₃O₄)and hollow Fe₃O₄ microspheres(H-Fe₃O₄)with sizes of?400 nm were synthesized by a hydrothermal reaction method.S-Fe₃O₄/GP and H-Fe₃O₄/GP composites were then successfully prepared.When the filling amount of Fe₃O₄ microspheres was 0.5 vol%,the RL_min of H-Fe₃O₄/GP at 14.0 GHz was-36.3 d B(2.3 mm),and the EAB was widened to 4.8 GHz(2.3 mm).Due to the unique hollow structure of H-Fe₃O₄,the multiple scattering and reflection of EM waves inside H-Fe₃O₄/GP were increased,contributing to better EM wave absorbing performance compared with S-Fe₃O₄/GP.To investigate the effect of the Si O₂ coating on the EM wave absorbing performance,the GP composites filled with Si O₂@S-Fe₃O₄ and Si O₂@H-Fe₃O₄ were prepared.It was found that with the increase in Fe₃O₄particle size,the dispersion effect of Si O₂ on Fe₃O₄ in the GP matrix weakened.Furthermore,the presence of Si O₂ increases the interaction between Fe₃O₄ and the GP matrix,leading to the easier transmission of EM wave between Fe₃O₄ and GP.As a result,the interface polarization effect weakened,and thus degrading the EM was absorption performance of the GP composites.3.Medium and large sizes of H-Fe₃O₄(named as MH-Fe₃O₄ and LH-Fe₃O₄)microspheres with diameters of?700 nm and?1?m were prepared by changing the hydrothermal reaction conditions.MH-Fe₃O₄/GP and LH-Fe₃O₄/GP composites were then successfully prepared by combining the MH-Fe₃O₄ and LH-Fe₃O₄ microspheres with GP.Compared with the H-Fe₃O₄/GP composites prepared in the previous chapter,the size effects of the absorbent on the EM wave absorption properties of the GP composites were investigated.When the weight ratio of the effective absorbing component Fe₃O₄ was the same as 1.15 wt%,the RL_min of the H-Fe₃O₄/GP,MH-Fe₃O₄/GP and LH-Fe₃O₄/GP composites reached-36.3 d B at 14.0 GHz(2.3 mm),-42.3d B at 15.2 GHz(2.2 mm)and-34.8 d B at 17.6 GHz(2.2 mm),respectively.With the increase in the size of the hollow microspheres,the frequency of the RL_min moved from14.0 GHz to 17.6 GHz,and the EAB narrowed from 4.8 GHz to 3.9 GHz and then to2.1 GHz.The RL_min was correlated with the shell thickness ratio of H-Fe₃O₄microspheres,and the low shell thickness ratio could enhance the interface polarization and exchange resonance ability,resulting in high EM wave attenuation.As the size of H-Fe₃O₄ increased,the distance between absorbers in GP matrix became larger,and thus the response to EM waves required higher energy,resulting in the right shift of frequency.The change in EAB was the result of the combination of impedance matching and the attenuation coefficient.