Preparation And Properties Of Electromagnetic Wave Absorbing Gypsum-based Composites

With the continuous development of artificial intelligence technology and the arrival of5 G era,electromagnetic radiation pollution has also attracted widespread attention.Electromagnetic wave absorbing building materials are effective measures to reduce electromagnetic radiation in buildings.However,electromagnetic wave absorbing materials for military defense can not be used in civil buildings because of complex preparation process and high production cost.It is of great significance to develop a kind of electromagnetic wave absorbing building materials for civil buildings to improve the indoor electromagnetic environment and meet the requirements of some special application scenarios.At present,desulphurization gypsum is widely used as building material,which not only helps to reduce the exploitation of natural resources,but also avoids the pollution of environment and human health caused by its accumulation.Desulphurized gypsum block and desulphurized gypsum mortar used in construction are very suitable for adding absorbent to prepare electromagnetic wave absorbing building materials because they are not used as the main load-bearing structure,have large construction thickness and have no aesthetic requirements.Therefore,this thesis takes low-cost desulfurized building gypsum as the base material,takes desulfurized gypsum block and desulfurized gypsum mortar as the target application object,adds graphite or ferrite absorbing agent to prepare and studies building materials with electromagnetic wave absorption function.In this thesis,the effects of graphite type,size,content and composite thickness on the microwave absorbing properties of graphite/gypsum matrix composites were studied.Add home-made multi-layer graphite and ferrite,and study their effects on the microwave absorbing properties of composites.The related mechanism how the structure,morphology and size of the absorbent effect the absorbing properties of composites was discussed by using scanning electron microscope and X-ray diffractometer.In the practical study,the effects of the graphic types and dosage on a set of properties parameters of gypsum block and plaster mortar such as water requirement of normal consistency,setting time,flexural strength,compressive strength,water absorption rate and water resistance were discussed.In order to simulate the real electromagnetic waves absorption,the graphite/gypsum block and graphite/gypsum mortar composites were prepared into cuboid plate and their absorbing performances were tested.The study shows that the type,size and dosage of graphite will affect the absorbing property of the material.Compared with spherical graphite,flake graphite has better absorbing performance.Compared with natural flake graphite,graphene nanoplatelets have better electromagnetic wave absorption due to its loose and curly lamellar morphology as well as larger surface area caused by the thinner lamellar.The 50?m graphene nanoplatelets have larger layer and are easier to form conductive network with strong electromagnetic loss performance.when its adding content is 10 wt.%,the absorption effect is the best whose lowest RL is-9.7d B at 11 GHz,with the max attenuation coefficient,the max conductivity and the impedance matching value of nearly 1.As the thickness increases,the absorbing frequency of the material moves to the lower frequency.When the simulated thickness of 50?m graphene nanoplatelets /gypsum matrix composite with 10 wt.% dosage is 25 mm,the RL value of the composite appears lower than-5d B at the frequency band of below 6GHz.The self-made multilayer graphite has a lower cost and a larger lamellar spacing,which is more conducive to the loss of electromagnetic wave in the conductive network formed.The similar absorbing effect can be achieved at the lower dosage(8wt.%)of multilayer graphite than the 10 wt.% content of the 50?m graphite nanoplatelets,whose lowest RL is-8.6d B at12 GHz.The ferrite prepared by sol-gel method is a soft magnet with strong magnetism.The25 wt.% ferrite/gypsum matrix composite are prepared which has strong impedance matching and high attenuation coefficient of electromagnetic wave.It has a good absorbing effect in the range of 10?14GHz with the lowest RL value of-11.6d B.The study on the preparation of absorbing building materials by adding graphite into gypsum block and gypsum mortar shows that with the increase of graphite content,the water consumption and water absorption rate of gypsum block increase,and the water resistance and mechanical properties decrease.The addition of 1wt.% glass fiber can well limit the crack propagation,increase the toughness and mechanical strength of MLG/gypsum block and NFG/gypsum block,make the composites with the graphite content of 10 wt.% still meet the standard requirements.With the increase of graphite content,water consumption of gypsum mortar increases,its mechanical properties decrease and setting time shorten.The addition of0.1wt.% retarder can effectively prolong the setting time and make the gypsum mortar meet the construction requirements of machine spray plastering mortar.However,its mechanical property is a bit low and cannot meet the standard requirement.The test of electromagnetic reflectance of plate-like graphite/gypsum block and gypsum mortar by the bow method shows that the best absorption effect is attained at the addition of 8wt.% self-made multilayer graphite,when the reflection loss is less than-5d B at the frequency band below 6GHz,that is,the absorption effect is 68%.