Microstructures And Electromagnetic Absorption Properties Of MXenes-based Absorbers

MXenes,which are a new series of two-dimensional materials,possess multilayered structure,high specific surface area,hydrophilic surface,abundant surface groups and other controllable chemical and physical properties,leading to the excellent designability of dielectric properties and showing significant potential in EM absorbers.At present,the research on their dielectric properties is still at an early stage,and their EM absorption performance is not able to meet the requirements of high-performance EM absorbers:high absorption intensity,broad effective absorption bandwidth,low filler loading and thin matching thickness.Therefore,it is very necessary to conduct further exploration and optimization of their dielectric properties.In this thesis,Ti₃C₂T_X and Ti₂CT_X MXenes are chosen as the research object,and their intrinsic dielectric properties are explored in detail.The dielectric properties and EM absorption properties of MXenes materials are optimized by phase transformation or interface engineering which include the introduction of conductors?CNTs?and semiconductors?SiCnws?from the angle of microstructure.And a variety of EM absorption materials are successfully synthesized.In addition,by researching the effects of structure evolution and phase transition on EM absorption properties,the controllable EM parameters can be reached.The following results and conclusions are made:1.Ti₃C₂T_X/CNTs composites modified with in situ grown carbon nanotubes are synthesized by combination of freeze-drying and catalytic chemical vapor deposition processes.Moreover,the mechanism of the effect of structure evolution and phase transition on EM wave absorption properties are studied.1)The multilayered structure of Ti₃C₂T_X/CNTs composites remain unchanged but the but be less surface groups after thermal treatment at 800°C.The defective CNTs with diameters ranging from 9 to 45 nm display a typical hollow structure and uniformly distribute on the interlayers and surfaces of Ti₃C₂T_X MXenes flakes.And the obtained Ti₃C₂T_X/CNTs composites possess abundant heterogeneous interface and porous microstructure.2)Ti₃C₂T_X/CNTs composites exhibit excellent EM absorption properties in the frequency range of 2?18 GHz.With the adjustment of thickness from 1.55 to 5 mm,the effective absorption bandwidth can reach up to 14.54 GHz?3.46?18 GHz?,and the minimum reflection coefficient reaches up to-53 dB.3)The excellent EM absorption properties of Ti₃C₂T_X/CNTs composites can be ascribed to the reasons as follow:the longer distance that EM entered pass through provide by hierarchically porous structure;enhanced interfacial and dipolar polarization caused by heterogeneous interface,defects and residual functional groups;stronger conductivity loss caused by CNTs and amorphous carbon phases.2.Ti₂CT_X MXenes and its derivatives are prepared using CO₂ as an oxidant via simple heat treatment at different temperatures.Moreover,the mechanism of the effect of structure evolution and phase transition on EM wave absorption properties are studied.1)The carbon layers are completely exfoliated using CO₂ as an oxidant after thermal treatment at 800°C,forming a highly ordered carbon-supported sandwich-like structure composed of well-aligned two-dimensional carbon sheets with embedded TiO₂ nanoparticles.This is the first study to ever report the complete oxidation of MXenes materials.2)C/TiO₂/Wax composites exhibit excellent EM absorption properties in the frequency range of 2?18 GHz.Especially,when the mass ratio of carbon is only about 2.8 wt%,the effective absorption bandwidth can reach up to 4.7 GHz?covering the whole X-band?,and the minimum reflection coefficient reaches up to-50.3 dB.3)The excellent EM absorption properties of C/TiO₂/Wax composites can be ascribed to the reasons as follow:enhanced polarization behavior caused by abundant heterogeneous interface and defects;stronger conductivity loss caused by exfoliated carbon phase;enhanced EM scatting caused by hierarchically porous structure.3.Ultralight,ordered lamellar and free-standing f-Ti₃C₂T_X-based hybrids foams are synthesized by combination of self-assembly and bidirectional freezing processes.Moreover,the mechanism of the effect of structure evolution and phase transition on EM wave absorption properties are studied.1)Two-dimensional and flexible f-Ti₃C₂T_X with negative charge wrap the one-dimensional rob-like SiCnws with positive charge to form a whole network structure.And then the ordered lamellar f-Ti₃C₂T_X/SiCnws hybrids foams are constructed via a controlled directional ice-template freeze-drying process.f-Ti₃C₂T_X/SiCnws hybrids foams possess abundant heterogeneous interface and porous microstructure.2)The f-Ti₃C₂T_X/SiCnws hybrids foams exhibit excellent EM absorption properties in the frequency range of 8.2?12.4 GHz?X-band?.The effective absorption bandwidth is always able to cover the whole X band,when the sample thicknesses of f-Ti₃C₂T_X/SiCnws hybrid foams distribute in any value between 3.5 to 3.8 mm,and the minimum reflection coefficient can reach-55.7 dB at a low density of only about0.029 g cm^-3.More importantly,the EM absorption properties of few-layered Ti₃C₂T_X MXenes is reported for the first time in this thesis.3)The excellent EM absorption properties of f-Ti₃C₂T_X/SiCnws hybrids foams can be ascribed to the reasons as follow:well-matched impedance characteristic;the establishment of multiple polarization mechanism;stronger conductivity loss caused by conductive f-Ti₃C₂T_X sheets;enhanced EM scatting caused by hierarchically porous structure.