Preparation And Dielectric Properties Of MIL-125(Ti)-based Derived Carbon Materials

With the widespread use of communication devices,the problem of electromagnetic pollution is becoming increasingly serious,which not only has a negative impact on the normal growth of plants and animals,but also poses a threat to human health.Therefore,various microwave absorption materials have been studied to solve this problem.Due to their high porosity,compositional diversity,morphological variability,tunable pore structure,and large specific surface area,metal-organic frameworks（MOFs）are praised as excellent templates for MOFs-derived microwave absorption materials.In this study,Ti-MOFs were selected as precursors,and a series of MIL-125（Ti）-based derived carbon materials with varying morphologies were prepared by regulating the additional components and pyrolysis temperature.The dielectric properties of the composites were further adjusted to precisely control the microwave absorption performance.The specific studies are as follows.Using a solvothermal method,disk-shaped MIL-125（Ti）-D and truncated octahedral-shaped MIL-125（Ti）-TRO were prepared by regulating the morphology of MIL-125（Ti）through the addition of modulators.After pyrolysis,the structure of TiO₂ nanoparticles embedded in porous carbon matrix was obtained.The morphology,graphitization degree and the growth of TiO₂ nanoparticles of MIL-125（Ti）-derived carbon materials can be effectively controlled by the change of pyrolysis temperature.The rise in temperature leads to an enhancement of both the real and imaginary parts of the complex permittivity,which further impacts the microwave absorption properties of the composites.In comparison,MIL-125（Ti）-TRO derived carbon materials with truncated octahedral shape exhibit higher complex permittivity.However,the excessive complex permittivity hinders its microwave absorption properties instead.When the thickness is 2.01 mm and 2.10 mm,D/TiO₂@C-700 and TRO/TiO₂@C-700 exhibit the same effective absorption bandwidth of 3.2 GHz,ranging from9.2-12.4 GHz and 9.1-12.3 GHz,respectively.The minimum reflection loss of D/TiO₂@C-800is-23.7 d B at 9.0 GHz with a matching thickness of 1.98 mm.Based on MIL-125（Ti）-D,a series of MIL-125（Ti,Co）with different morphologies were prepared by controlling the addition amount of Co Cl₂·6H₂O through a solvothermal method.The effects of pyrolysis temperature and morphology on the dielectric properties of MIL-125（Ti,Co）-derived carbon materials were studied.With the increase of Co Cl₂·6H₂O,the thickness of MIL-125（Ti,Co）decreased and the morphology changed from disc-shaped to square.At a pyrolysis temperature of 750°C,Co/TiO₂@C（750）-2 exhibits optimal microwave absorption properties with a minimum reflection loss of-46.6 d B at 11.1 GHz and a thickness of 1.8 mm.The effective absorption bandwidth reaches 3.0 GHz（9.1-12.1 GHz）with a matching thickness of 1.9 mm.The influence of pyrolysis temperature on the microwave absorption properties of MIL-125（Ti,Co）-2 was further investigated.The results showed that the microwave absorption properties of Co/TiO₂@C（700）were significantly improved at a pyrolysis temperature of 700°C.The minimum reflection loss achieved-64.5 d B at 12.3 GHz and a thickness of 1.8 mm.The effective absorption bandwidth reaches 3.2 GHz（8.9-12.1 GHz）when the matching thickness is 2.1 mm.The enhanced microwave absorption performance can be attributed to the conductive loss caused by the porous carbon matrix and the growth of TiO₂ nanoparticles,which is not only beneficial to impedance matching,but also provides rich heterogeneous interfaces（TiO₂/C,rutile TiO₂/anatase TiO₂）to enhance the interfacial polarization.Based on MIL-125（Ti,Co）-2,g-C₃N₄/MIL-125（Ti,Co）was prepared by solvothermal method with controlled doping amount of g-C₃N₄.The effects of g-C₃N₄ and pyrolysis temperature on the complex permittivity and microwave absorption properties of CN/Co/TiO₂@C were investigated.The results indicate that g-C₃N₄ was randomly distributed within the disk-shaped MIL-125（Ti,Co）.Specifically,the complex permittivity of CN/Co/TiO₂@C decreased with increasing content of g-C₃N₄,and increased with elevating pyrolysis temperature.In contrast,CN/Co/TiO₂@C（800）-3 showed more desirable microwave absorption properties,with a minimum reflection loss of-16.1 d B at 12.2 GHz and a thickness of only 1.4mm.At a thickness of 1.6 mm,the effective absorption bandwidth reaches 2.3 GHz（9.5-11.8GHz）.The effect of pyrolysis temperature on the microwave absorption properties of g-C₃N₄/MIL-125（Ti,Co）-3-derived carbon materials was further investigated.At 700°C,CN/Co/TiO₂@C（700）exhibited excellent microwave absorption properties with a minimum reflection loss of-59.0 d B at 8.4 GHz and a matching thickness of 2.8 mm.The effective absorption bandwidth reaches 3.2 GHz（9.2-12.4 GHz）under a thickness of 2.3 mm.