Study On The Electromagnetic Shielding Properties Of Carbon Nanotube(Buckypaper) And Corresponding Polymeric Composites

Highly efficient electromagnetic shielding materials have become a necessary guarantee for human health,equipment operation and information security in the contemporary era of electronic.The Existing shielding materials have more or less problems s,such as large weight,high cost,low mechanical strength,large thickness,and poor shielding properties.Carbon nanotubes(CNTs)that have low density,extraordinary electrical and mechanical properties,and excellent absorbing properties can meet the requirements of broadband and highly efficienct absorption for the noval lightweight shielding materials.In this thesis,in view of the special shielding areas with harsh environment and complex shape requirements,poly(phthalazinone etherketone)(PPEK)was preferred due to excellent weather resistance,heat resistance,mechanical properties and,and good processability.PPEK/multiwall carbon nanotube(PPEK/CNT)composite films were prepared by solution blending process.Meanwhile,porous PPEK/CNT composite films were prepared by bellcore method and water vapor induced phase separation(WIPS),respectively.The shielding performance of these PPEK/CNT composite films was investigated by means of designing porous,gradient and sandwich structures.Furthermore,buckypapers(BPs)obtained from vacuum filtration and BP/Fe3O4/polyaniline(PANI)composite films obtained by in situ coprecipitation and polymerization were also prepared.The main contents and results are as follows:The microscopic morphology,porosity,electrical,mechanical,and shielding properties of PPEK/CNT composite films and porous composite films were studied.The homodisperse of a high content(15 wt%)of CNTs in the PPEK/CNT composites can be attained.The percolation threshold of PPEK/CNT composites was 2 wt%(1.24 vol%)CNT content.The electrical conductivity,mechanical strength and electromagnetic shielding effectiveness(SE)of the composite films were positively related to the CNT loading.The pore size of PPEK/CNT porous films increased,but the electrical and mechanical properties decreased with increasing of porogen content(Bellcore method)and air humidity(WIPS).The porous PPEK/CNT composite films exhibited lower mechanical and electrical properties than the ones without porous structure.But the porous composite films prepared by Bellcore method showed an improved average SE of 23.5 dB in 8?18 GHz,which was 40%higher than PPEK/CNT composite films.The gradient structure,prepared by PPEK/CNT composite films with different filler contents,had little effect on the SE,but it would contribute differently to absorption loss.Sandwich structure was designed by introducing wave-transmitting layer between PPEK/CNT composite films.When the shielding layers were fixed,the SE showed a trend of decreasing,rising,leveling off,and then decreasing again with the wave-transmitting layer thickness(d)increasing.A higher SE of shielding layers was beneficial for having thinner shielding materials with high SE.increasing the number of wave-transmitting layers was also advanced for improving the SE of sandwiched materials.The SE of sandwiched composite reached 65 dB in 8?18 GHz,which was increased by 144%compared to the composite with d=0 and attributed from the absorption loss.BPs were prepared to solve the dispersion problem of CNTs and further increase the CNTs content.The correlation between CNT dispersity and microstructure,electrical,mechanical and shielding properties of BPs was investigated in details.CNTs(CNT-A,CNT-B,and CNT-C)were dispersed homogeneously in water or ethanol after ball milling,sonication treatment.Based on the experiments,the CNT/water dispersions were far superior to CNT/ethanol dispersion in stability.Compared with BPs prepared with CNT/ethanol dispersion(BP-A-E,BP-B-E,and BP-C-E),the BP prepared with CNT/water dispersion(BP-A-W,BP-B-W and BP-C-W)showed more uniform distribution,and smaller the-most-probable pore sizes,as well as an significantly improvement in mechanical strength,electrical conductivity and SE.The conductivity,tensile strength and SE(450 ?m,101.7 dB)of BP-A-W was increased by 74%,665%and 26%,respectively,compared with those of BP-A-E.The mixing of CNTs with different length-diameter ratios showed advantages on improving the conductivity and SE of composite films.A notable SE of 48.4 dB and a high specific SE of 29300 dB·cm2/g were obtained by BP-C-W of 45 ?m thick.A comparative study was conducted aiming at the effects of multilayer and sandwich structures on the shielding properties of BPs.For multilayer structure,the SE could be adjusted from 30 to 110 dB by changing the layer numbers,but the increment rate and specific SE decreased rapidly with BP layers increasing.The sandwich structure S-BPs comprised of BPs and polypropylene were prepared by hot pressing process.The SE of S-BP increased monotonically with the d value increasing,and there was an optimal d value range(X band,0.32?0.48?,?=25 mm)to realize the maximization of SE,after which the resonance peak appeared followed by a dramatic descending of SE.The plateau period of the Ku band moved towards a smaller d value.An absorption dominated SE of above 100 dB can be achieved by constructing multilayer or sandwich structure.In comparison with multilayer structure with the same shielding components of BPs,the SE of sandwich structure with two wave-transmitting layers significantly increased by 90.7%,and the improvement of SE was mainly based on the contribution of absorption loss.BP/Fe3O4 and BP/Fe3O4/PANI composite films were prepared to improve the balance between the electrical and magnetic properties.The effects of Fe3O4 and PANI on the mechanical,electrical,magnetic,and shielding properties of BP composite films were studied in details.As the iron ion concentration was increased in the chemical deposition solution,the morphology of Fe3O4 changed from thin flake to granular state,and the Fe3O4 loading was also increased.Meanwhile,the saturation magnetization(Ms)of BP/Fe3O4 increased and the conductivity decreased with the iron ion increasing.The high Ms of 14.3 emu/g was achieved for the BP/Fe3O4 when the iron ion was 0.12 mol/L.In term of BP/Fe3O4/PANI,the PANI networks,which acted as bridges between the CNTs and Fe3O4 particles,compensated the loss of electrical propertiyes induced by Fe3O4 and enhanced the interaction between CNTs,Fe3O4 and PANI.The BP/Fe3O4/PANI composite films preserved the mechanical properties of BPs,but exhibited an enhanced megnatic property.The SE of BP/Fe3O4/PANI was improved to 36.6 dB in 5.85?18 GHz due to the magnetic loss and interfacial polarizations,which was 17.3%higher than BPs.