Lightweight Graphene-based Electromagnetic Interference Shielding Materials: Fabrication And Characterization

The high aspect ratio and excellent conductivity of graphene make itself either an ideal nanoscale functional filler in the fabrication of lightweight polymer-/carbon-based composites for electromagnetic interference(EMI)shielding or a basic building block for fabricating high-performance and multifunctional graphene foams(GFs).Although great progress has been made,there are still some chal enges,including how to facilely and efficiently fabricate high-performance polymer/graphene foams,carbon/graphene foams(termed as carbon foams)and graphene foams,and how to significantly improve the EMI shielding effectiveness(SE)and even microwave-absorbing(MA)ability just by special structure designs.To address these issues,some perspective and systematical researches have been conducted in this thesis and the details are summerized as follows:(1)Aromatic polyimide(PI)foams with reduced graphene oxide(rGO)content as high as 16 wt%were fabricated by the combination of in situ polymerization and nonsolvent induced phase separation.Consequently,the high content of rGO endowed PI foam with a sufficient EMI SE,not only meeting the commercial standard of 20 dB,but also reducing the sample thickness from≥2 mm for traditional polymeric shield ing foams to 0.8 mm.It was also found that rGO could act as a radical scavenger during the degradation of PI,and hence dramatically improved the 5 wt%weight loss temperature of our PI foam to 581°C.Even with the high rGO content of 16 wt%,the PI foam was still flexible and showed good mechanical performance.(2)Thermoplastic polyurethane(TPU)composite foam with rGO content as high as 20 wt%were prepared by water vapor induced phase separation.Afterwards,TPU/rGO foam composites were fabricated by the random stacking of several layers of TPU/rGO foams together,and the gradient concentration of rGO could be realized by the arrangement of monolayer TPU/rGO foams in different orders,which was proved to be a facile approach to enhance the MA property of polymeric shield ing foams,instead of their EMI SE.The reason was believed that the gradient structure can help to improve the impedance matching between mateirals and air,which consequently enhances the MA performance of the composites.Further sandwiching a wave-transmitting layer between two pieces of TPU/rGO foams formed a sandwich structure,by which the SE of the two samples could be significantly improved.The SE increment was assigned to the enhanced reflection loss induced by constructive interference,and largely dependent on the thickness of the interlayer.(3)Ultrathin carbon foams were fabricated by the carbonization and graphitiza t io n of PI/rGO composites,a carbon precursor.The results indicated that rGO was able to stabilize the porous structure of PI foams by suppressing the shrinkage of the foams during their pyrolysis process,and simultaneously accelerated the graphitiza t io n process of PI foam via the accumulation of internal stress.As a result,the highest SE of carbon foams in a thickness only of 24μm reached to 24 dB over the frequency range of 8-12 GHz(X band),about 7 dB higher than that of the non-foamed counterpart.The reason was due to the enhanced microwave absorption of foam material via the multiple reflections in the foam structure.Moreover,the carbon foam also exhibited ultrahigh thermal stability,mainly because of its high thermal-decomposit io n temperature of~600°C under air atmosphere.(4)High-performance GFs were efficiently fabricated by immediately sintering low-temperature exfoliated graphene powder at a temperature over 500°C.The one-pot sintering process not only integrated two-dimensional(2D)graphene sheets into3D GF,but also accelerated the structural integrity of graphene by inducing its deoxygenation and repairing the defects.More importantly,the whole sintering process could be finished within hours,usually less than 12 h,which is more efficient than conventional hydrothermal and templating methods.The resultant GFs with ultrali ght density of 11-34 mg/cm~3 are one sort of multifunctional materials,which possess both exceptional EMI SE of 34-39 dB and extraordinary adsorption capacities(weight gain of 4700%-12000%)for organic pollutants.