Construction Of Electrically Conductive Networks Of MXene And Their Polymer Composites For Electromagnetic Interference Shielding

With the innovation of communication technology,all kinds of portable devices update fast and stride forward to the direction of miniaturization,flexibility and high integration.Therefore,electromagnetic interference(EMI)shielding materials need to possess customized architectures and deformability to meet the requirements of the next-generation devices.Transition metal carbides or nitrides(MXenes),especially Ti3C2Tx,become an ideal candidate for highly efficient EMI shielding due to their unique layered structures,highly electrical conductivity and ease of fabrication.However,the agglomeration and uneven dispersion of two-dimensional nanosheets in composites affect the construction of the highly conductive networks of MXene.Besides,the shapes of MXene EMI shielding materials are generally in films or bulks,which is difficult to directly assembling in electronic devices.This dissertation focuses on the construction of MXene electrically conductive networks and their composites for EMI shielding.On the one hand,the uneven dispersion of nanosheets in composites is solved by preforming conductive networks and then coating polymers.On the other hand,the conductive networks of MXene are elaborate by three-dimensional(3D)printing to customize stereoscopic frames and extrusion printing to prepare flexible,programmable meshes.The precise and complex exteriors of MXene EMI shielding materials are achieved.Besides,the concept of electromagnetic thermochromic composites is first proposed by combining MXene frames with thermochromic polymers.This material exhibits obvious color change from blue to red under the stimulation of high-intensity electromagnetic field and provides a new method for the visualization of electromagnetic waves.The conclusions of this dissertation are as follows:(1)For the studies on EMI shielding performances of directional-freezing MXene aerogels and their polydimethylsiloxane(PDMS)coated foams,in order to solve the agglomeration of MXene nanosheets in the polymer matrix,a method of preforming conductive networks and then coating it with the polymer is developed to prepare MXene/PDMS composite foam with lightweight,high conductivity,and durability.First,with the help of sodium alginate(SA),the MXene aerogels are prepared by directional freeze-drying.The abundant hydrogen bonds between SA and MXene connect the nanosheets to form aerogels with oriented porous structures.The aerogels have the highest conductivity of 2211 S m-1 and the EMI shielding efficiency(SE)of up to 70.5 dB.Then,a thin layer of PDMS is coated on the surface of the aerogel by vacuum-assisted impregnation to reinforce the MXene networks.When the MXene content is only 6.1 wt%,the foam exhibits an EMI SE of 53.9 dB,as well as a retention rate of SE is about 90%after 500 compression-release cycles.This composite form can be used as an EMI shielding gasket to prevent the leakage of electromagnetic waves.(2)For the preparation of MXene EMI shielding frames by direct ink writing(DIW)and their electromagnetic wave-induced thermochromic application,the existing preparing methods of 3D EMI shielding products are difficult to form complex shapes.Here,the 3D printing method is adopted to customize macroscopic morphology.The 3D MXene conductive networks are constructed by improving the contact interface of the printed filaments through ionic crosslinking.In detail,the rigid AlOOH nanoparticles are added into the MXene paste to prepare a free-standing printing ink with suitable viscoelasticity.Then,the customized stereo MXene frames are fabricated by DIW.The printed frames are immersed in the AlCl3/HCl solution to cross-link MXene nanosheets and remove dielectric AlOOH nanoparticles.Finally,the lightweight and robust MXene frames are obtained after freeze-drying.The printed MXene frames can achieve a conductivity of 5323 S m-1 and a low density of 72.4 mg cm-3.The EMI SE can be flexibly adjusted from 25 to 80 dB by changing printing parameters like the composition of the ink,the number of printing layers and the spacing of filaments.More interestingly,the electromagnetic thermochromic material is first prepared by combining MXene frames with thermochromic t-PDMS.Under the stimulation of a 50 W electromagnetic field,the MXene electromagnetic thermochromic material shows a visible color change from blue to red in 3 s,which paves a new way for visualization of electromagnetic waves.(3)For the extrusion printing flexible,ultrathin MXene EMI shielding meshes and their Joule heating applications,the MXene meshes with complex patterns are prepared by extrusion printing.Here,the additive-free MXene inks are prepared with large few-layered MXene sheets,which utilizes the character that MXene flakes with high aspect ratio are easy to produce highly viscoelastic suspension with low concentration.The extruding speed and the printing speed are coordinated to ensure the smooth printing of complex patterns.Then,the printed meshes are impregnated in the hydrochloric acid solution to adjust the internal lamellar spacing of MXene sheets.After natural drying at room temperature for 24 h,the free-standing,ultra-thin,highly conductive MXene meshes are obtained.The infiltration of protonic acid significantly improves the mechanical properties and electrical conductivity of printed meshes.The mesh with a filament spacing of 1 mm and a thickness of 9.5 ?m achieves an excellent EMI SE of 50 dB in the whole X band and the EMI SE can be easily adjusted by printing parameters.Besides,the acid-treated meshes have the good energy conversion efficiency and stable Joule heating capacity,exhibiting a saturation temperature of 58? at the voltage of 1 V.These flexible,ultrathin,patterned MXene meshes have a wide prospect in flexible EMI shielding and thermal management.