Multiwalled carbon nanotube/cement composites with exceptional electromagnetic interference shielding properties

Multi-walled carbon nanotube (MWCNT)/portland cement(PC) composites have been fabricated to evaluate their electromagnetic interference (EMI) shielding effectiveness (SE). The results show that they can be used for the shielding of EMI in the microwave range. The incorporation of 15 wt.% MWCNTs in the PC matrix produces a SE more than 27 dB in X-band (8.2–12.4 GHz), and this SE is found to be dominated by absorption. Furthermore, the structural analysis, surface morphology and surface interaction of MWCNTs with PC matrix have been explored using XRD, SEM and X-ray photoelectron spectroscopy technique.     

Comparative study of electromagnetic interference shielding properties of injection molded versus compression molded multi-walled carbon nanotube/polystyrene composites

This study compares electromagnetic interference (EMI) shielding properties of injection molded versus compression molded multi-walled carbon nanotube/polystyrene (MWCNT/PS) composites, i.e., properties such as EMI shielding effectiveness (EMI SE), electrical conductivity, real permittivity and imaginary permittivity. The injection molded (MWCNT-aligned) samples showed lower EMI shielding properties than compression molded (randomly distributed MWCNT) samples that was attributed to lower probability of MWCNTs contacting each other due to MWCNT alignment. The compression molded samples showed higher electrical conductivity and lower electrical percolation threshold than the injection molded samples. The compression molded samples at MWCNT concentrations of 5.00 and 20.0 wt.% showed real permittivity two times and imaginary permittivity five times greater than the injection molded samples. The EMI SE for the compression molded samples at MWCNT concentrations of 5.00 and 20.0 wt.% was 15.0 and 30.0 dB, respectively, significantly greater than EMI SE for the injection molded samples. Lower EMI SE for the injection molded samples was ascribed to lower electrical conductivity, real permittivity (polarization loss) and imaginary permittivity (Ohmic loss). Comparison of the EMI shielding properties of the compression molded versus injection molded samples confirmed that EMI shielding does not require filler connectivity; however it increases with filler connectivity.     

EMI shielding effectiveness of carbon based nanostructured polymeric materials: A comparative study

The microstructure, electromagnetic interference (EMI) shielding effectiveness (SE), DC electrical conductivity, AC electrical conductivity and complex permittivity of nanostructured polymeric materials filled with three different carbon nanofillers of different structures and intrinsic electrical properties were investigated. The nanofillers were multiwall carbon nanotubes (MWCNT), carbon nanofibers (CNF) and high structure carbon black (HS-CB) nanoparticles and the polymer was acrylonitrile-butadiene-styrene (ABS). In addition, the EMI SE mechanisms and the relation between the AC electrical conductivity in the X-band frequency range and the DC electrical conductivity were studied. The nanocomposites were fabricated by solution mixing and characterized by uniform dispersion of the nanofillers within the polymer matrix. It was found that, at the same nanofiller loading, the EMI SE, permittivity and electrical conductivity of the nanocomposites decreased in the following order: MWCNT > CNF > CB. MWCNT based nanocomposites exhibited the lowest electrical percolation threshold and the highest EMI SE owning to the higher aspect ratio and electrical conductivity of MWCNT compared to CNF and HS-CB. The AC conductivity in the X-band frequency range was found to be independent of frequency.     

Electromagnetic interference shielding properties of polymer-grafted carbon nanotube composites with high electrical resistance

Poly(methyl methacrylate) (PMMA)-grafted multiwalled CNTs were prepared, and then dispersed into additional PMMA matrix, yielding highly insulated PMMA–CNT composites. The volume resistivity of PMMA–CNT was as high as 1.3 × 10¹⁵ Ω cm even at 7.3 wt% of the CNT. The individual CNTs electrically-isolated by the grafted PMMA chains in PMMA–CNT transmitted electromagnetic (EM) waves in the frequency range of 0.001–1 GHz, whereas the percolated CNTs in a conventional composite prepared by blending PMMA with the pristine CNTs strongly shielded the EM waves. This result suggests that the intrinsic conductivity of the CNT itself in PMMA–CNT does not contribute to the EM interference (EMI) shielding in the frequency range of 0.001–1 GHz. On the other hand, PMMA–CNT exhibited EMI shielding at the higher frequency range than 1 GHz because the dielectric loss of the CNT itself was rapidly increased over 1 GHz. At 110 GHz, PMMA–CNT with 7.3 wt% of the CNT had EMI SE of as high as 29 dB (0.57 mm thickness), though is slightly lower than that of the percolated conventional composite (35 dB). Thus, it is demonstrated that the highly insulated PMMA–CNT has the good EMI shielding at extremely high frequency range (30–300 GHz).     

Multiwalled carbon nanotubes–BaTiO3/silica composites with high complex permittivity and improved electromagnetic interference shielding at elevated temperature

Composites with silica matrix and mixed filler of multiwalled carbon nanotubes (MWCNTs) and BaTiO₃ powder were fabricated. Excellent uniform dispersion of MWCNTs can be obtained using a two-step mixing method. Both of the real and imaginary parts of complex permittivity increased with increasing MWCNT content and measured temperature. The electromagnetic interference (EMI) shielding results showed that the absorption mechanism is the main contribution to the total EMI shielding effectiveness (SE). Compared with the EMI SE resulting from reflection, the absorption showed more dependence on the MWCNT content, measured temperature and frequency. The total EMI SE is greater than 20 dB at 25 °C and 50 dB at 600 °C in the whole frequency range of 12.4–18 GHz with a 1.5 mm composite thickness, which suggests that the MWCNT–BaTiO₃/silica composites could be good candidates for the EMI shielding materials in the measured frequency and temperature region.     

Nanographite-reinforced carbon/carbon composites

Carbon/carbon composites (C/Cs) with nanographite platelets (NGP) filler in a matrix derived from phenolic resin were produced. Different weight concentration (0.5, 1.5, 3, 5 wt.%) NGP were introduced by spraying the NGP during the prepreg formation. The NGP-reinforced C/Cs were characterized for effect of NGP concentration on microstructure, porosity, interlaminar shear strength (ILSS), flexural, ultrasonic and vibration damping behavior. At 1.5 wt.% NGP C/C, the highest values of ILSS observed was 10.5 MPa (increased by 22%), flexure strength of 142.4 MPa (increased by 27%), flexural modulus of 59.2 GPa (increased by 68%) and porosity of 18.8% (reduced by 17.5%) in comparison to neat (without NGP) densified C/C. Ultrasonic testing revealed an average increase of 15% through the thickness Young’s modulus of NGP-C/C; (3.12 GPa at 1.5 wt.% NGP). A 20% average decrease in the damping ratio of the first four modes of vibration was observed in 1.5 wt.% NGP densified C/C. At low concentration (⩽1.5 wt.%) the NGP filled in the pores, cracks and debonded interface but at concentration higher than 1.5 wt.% NGP lost their effectiveness due to agglomeration. The required cycles for desired density/properties are projected to be less compared to neat C/C due to less porosity observed in ⩽1.5 wt.% NGP concentration C/C.     

High electromagnetic wave absorbing performance of activated hollow carbon fibers decorated with CNTs and Ni nanoparticles

Activated hollow carbon fibers (ACHFs) decorated with carbon nanotubes (CNTs) and nickel nanoparticles (CNTs–Ni–ACHFs) were prepared by thermal reduction and chemical vapor deposition technique. Microwave reflection loss, permittivity and permeability of CNTs–Ni–ACHFs composites as novel electromagnetic wave absorbents were studied in the frequency range of 2–18 GHz. It was demonstrated that CNTs–Ni–ACHFs absorbents possessed the best microwave absorbing performances whose minimum reflection loss was −43.457 dB at 13.10 GHz with a thickness of 2.0 mm, which is much better than those of Ni–ACHFs and ACHFs samples. The dielectric polarizations and magnetic loss derived from the effect of the porous structures, Ni nanoparticles, and defects in the CNTs–Ni–ACHFs composites are playing an important role for the excellent microwave absorbing performances.     

Microstructures and EMI shielding properties of composite ceramics reinforced with carbon nanowires and nanowires-nanotubes hybrid

Carbon/ceramic composites are promising candidates as electromagnetic interference (EMI) shielding materials used at various harsh environments. The aim of present work is to prepare and investigate two kinds of composite ceramics reinforced with carbon nanowires (CNWs) and nanowires-nanotubes (CNWs-CNTs) hybrid, respectively. Results indicate that CNWs is highly curved and multi-defected, and CNWs-CNTs hybrid shows the best crystal structure at an optimal catalyst concentration of 5 wt%. When CNWs accounts for 5.15 wt%, the total shielding effectiveness (SE) of CNWs/Si₃N₄ reaches 25.0 dB with absorbed SE of 21.3 dB, meaning that 99.7% incident signal can be blocked, while it reaches 25.4 dB for CNWs-CNTs/Si₃N₄ as the carbon loading only increasing to 3.91 wt%. By contrast, CNWs/Si₃N₄ exhibits better electromagnetic attenuation capability with stronger absorption, mainly due to the unique microstructure of CNWs. Both of two composite ceramics have great potential to be designed as structural and multi-functional materials.     

Synthesis,morphology and physical properties of multi-walled carbon nanotube/biphenyl liquid crystalline epoxy composites

We have developed multi-walled carbon nanotube/liquid crystalline epoxy composites and studied the effects of incorporation carbon nanotubes (CNTs) on the morphology, thermal and mechanical properties of the composites. The CNTs are functionalized by liquid crystalline (LC) 4,4′-bis(2,3-epoxypropoxy) biphenyl (BP) epoxy resin for the ease of dispersion and the formation of long range ordered structure. The epoxy functionalized CNT (ef-CNT) were dispersed in the LC BP epoxy resin that can be thermal cured with an equivalent of 4,4′-diamino-diphenylsulfone to form composite. The curing process was monitored by polarized optical microscopy. The results indicate the LC resin was aligned along the CNTs to form fiber with dendritic structure initially then further on to obtain micro-sized spherical crystalline along with fibrous crystalline. With homogeneous dispersion and strong interaction between nanotubes and matrix, the composite containing 2.00 wt.% ef-CNT exhibits excellent thermal and mechanical properties. When the amount of ef-CNT exceeds 2.00 wt.%, vitrification stage of curing is fast reached, which lowers the degree of conversion. As compared with the neat resin, the composite containing 2.00 wt.% ef-CNT increases the glass transition temperature by 70.0 °C, the decomposition temperature by 13.8 °C, the storage modulus by 40.9%, and the microhardness by 63.3%.     

Effect of oxyfluorination on electromagnetic interference shielding of polypyrrole-coated multi-walled carbon nanotubes

The polypyrrole-coated multi-walled carbon nanotubes (MWCNTs) were prepared by in situ chemical oxidative polymerization of pyrrole on the surface of MWCNTs for the novel electromagnetic interference (EMI) shielding materials. The oxyfluorination treatment on MWCNTs introduced the hydrophilic functional groups resulting in well distribution and higher interfacial affinity between polypyrrole (PPy) and MWCNTs. The PPy phases formed on MWCNTs were observed by SEM. The thickness of PPy on the surface of MWCNTs decreased as increasing the hydrophilic groups on MWCNTs by the oxyfluorination treatment. The PPy-coated MWCNT composites showed the remarkable increases in permittivity, permeability, and EMI shielding efficiency (SE). The EMI SE of PPy-coated MWCNTs increased up about 28.6 dB mainly based on the absorption mechanism.     

Mechanical and electrical properties of carbon nanotube buckypaper reinforced silicon carbide nanocomposites

The nanocomposite was produced via phenolic resin infiltrating into a carbon nanotube (CNT) buckypaper preform containing B₄C fillers and amorphous Si particles followed by an in-situ reaction between resin-derived carbon and Si to form SiC matrix. The buckypaper preform combined with the in-situ reaction avoided the phase segregation and increased significantly the volume fraction of CNTs. The nanocomposites prepared by this new process were dense with the open porosities less than 6%. A suitable CNT–SiC bonding was achieved by creating a B₄C modified interphase layer between CNTs and SiC. The hardness increased from 2.83 to 8.58 GPa, and the indentation fracture toughness was estimated to increase from 2.80 to 9.96 MPa m^1/2, respectively, by the reinforcing effect of B₄C. These nanocomposites became much more electrically conductive with high loading level of CNTs. The in-plane electrical resistivity decreased from 124 to 74.4 μΩ m by introducing B₄C fillers.     

Singly dispersed carbon nanotube/aluminum composites fabricated by powder metallurgy combined with friction stir processing

Carbon nanotube (CNT)/aluminum composites were fabricated by a combination of powder metallurgy and subsequent friction stir processing (FSP). Microstructural observations indicated that the CNTs were singly dispersed in the composites. The CNTs tended to be dispersed along grain boundaries resulting in a much finer grain size. Although the CNTs were shortened and some Al₄C₃ formed in the matrix, the layer structures of the CNTs were well retained. Raman spectroscopy also showed that the damage to CNTs during FSP was not severe. Compared to that of unreinforced Al, the yield strength of 1 wt.% and 3 wt.% CNT/Al composites increased by 23.9% and 45.0%, respectively. A strength equation relating with load transfer and the grain refinement was proposed to describe the increase of the yield strength of the composites.     

Dielectric,electromagnetic absorption and interference shielding properties of porous yttria-stabilized zirconia/silicon carbide composites

SiC was infiltrated into porous yttria-stabilized zirconia (YSZ) felt by chemical vapor infiltration (CVI), and continuous SiC matrix layer was formed around YSZ fibre. When 86.9 wt.% SiC is introduced into the porous YSZ felt, the mean values of the real part of the permittivity and dielectric loss tangent of porous YSZ felt increase from 1.16 and 0.007 to 8.2 and 1.31, respectively. The electromagnetic interference (EMI) shielding efficiency (SE) increases from 0.069 dB to 16.2 dB over the frequencies ranging from 8.2 GHz to 12.4 GHz. The reflection loss of the composites with a thickness of 5 mm at 8–18 GHz is smaller than ?6.5 dB, and the bandwidth below ?10 dB is 5 GHz at room temperature, which increases to 5.9 GHz at 800 °C. The considerable increases in EMI SE and microwave absorption properties are attributed to the formation of continuous SiC matrix layer composed of SiC nanocrystals in the porous YSZ felt, which is beneficial for the production of induced electric current and the enhancement of dielectric loss.     

Fabrication and microwave absorption properties of magnetite nanoparticle–carbon nanotube–hollow carbon fiber composites

Absorbents with “tree-like” structures, which were composed of hollow porous carbon fibers (HPCFs) acting as “trunk” structures, carbon nanotubes (CNTs) as “branch” structures and magnetite (Fe₃O₄) nanoparticles playing the role of “fruit” structures were prepared by chemical vapor deposition technique and chemical reaction. Microwave reflection loss, permittivity and permeability of Fe₃O₄–CNTs–HPCFs composites were investigated in the frequency range of 2–18 GHz. It was proven that prepared absorbents possessed the excellent electromagnetic wave absorbing performances. The bandwidth with a reflection loss less than −15 dB covers a wide frequency range from 10.2 to 18 GHz with the thickness of 1.5–3.0 mm, and the minimum reflection loss is −50.9 dB at 14.03 GHz with a 2.5 mm thick sample layer. Microwave absorbing mechanism of the Fe₃O₄–CNTs–HPCFs composites is concluded as dielectric polarization and the synergetic interactions exist between Fe₃O₄ and CNTs–HPCFs.     

Low temperature growth of carbon nanotubes on Si substrates in high vacuum

Carbon nanotube (CNT) growth was carried out on SiO₂/Si substrates using an alcohol gas source in a high vacuum without any carbon decomposition processes. In the Raman spectra of the grown CNTs, both the G/Si peak intensity ratio and G/D peak intensity ratio indicated that the optimum growth temperature became lower as the pressure decreased. By reducing the pressure to 1 × 10^− 4 Pa, CNTs could be grown at 400 °C, and the G/D ratio was about 16, indicating that the quality of the grown CNTs was good, taking into account the low growth pressure. In addition, the Raman spectra in the radial breathing mode (RBM) region showed that the diameter distribution of the grown CNTs was dependent on both the growth pressure and temperature, and the relative intensity of the RBM peaks from small-diameter CNTs increased as the growth pressure and/or temperature was reduced.     

Electromagnetic wave absorption properties of graphene modified with carbon nanotube/poly(dimethyl siloxane) composites

By using a catalytic growth procedure, carbon nanotubes (CNTs) are in situ formed on reduced graphene oxide (RGO) sheet at 600 °C. CNTs growing on RGO planes through covalent C–C bond possess lower interfacial contact electrical resistance. As a hybrid structure, the CNTs/graphene (CNT/G) are well dispersed into poly (dimethyl siloxane). The hybrid combining electrically lossy CNTs and RGO, which disperses in electrically insulating matrix, constructs an electromagnetic wave (EM) absorbing material with ternary hierarchical architecture. The interfacial polarization in heterogeneous interface plays an important role in absorbing EM power. When the filler loading is 5 wt.% and thickness of absorber is 2.75 mm, the minimum value of reflection coefficient and the corresponding frequency are −55 dB and 10.1 GHz, and the effective absorption bandwidth reaches 3.5 GHz. Therefore, combining the CNTs and graphene sheet into three-dimensional structures produces CNT/G hybrids that can be considered as an effective route to design light weight and high-performance EM absorbing material, while the effective EM absorption frequency can be designed.     

Low temperature growth mechanisms of vertically aligned carbon nanofibers and nanotubes by radio frequency-plasma enhanced chemical vapor deposition

Using radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD), carbon nanofibers (CNFs) and carbon nanotubes (CNTs) were synthesized at low temperature. Base growth vertical turbostratic CNFs were grown using a sputtered 8 nm Ni thin film catalyst on Si substrates at 140 °C. Tip growth vertical platelet nanofibers were grown using a Ni nanocatalyst in 8 nm Ni films on TiN/Si at 180 °C. Using a Ni catalyst on glass substrate at 180 °C a transformation of the structure from CNFs to CNTs was observed. By adding hydrogen, tip growth vertical multi-walled carbon nanotubes were produced at 180 °C using FeNi nanocatalyst in 8 nm FeNi films on glass substrates. Compared to the most widely used thermal CVD method, in which the synthesis temperature was 400–850 °C, RF-PECVD had a huge advantage in low temperature growth and control of other deposition parameters. Despite significant progress in CNT synthesis by PECVD, the low temperature growth mechanisms are not clearly understood. Here, low temperature growth mechanisms of CNFs and CNTs in RF-PECVD are discussed based on plasma physics and chemistry, catalyst, substrate characteristics, temperature, and type of gas.     

Improved graphitization and electrical conductivity of suspended carbon nanofibers derived from carbon nanotube/polyacrylonitrile composites by directed electrospinning

Single suspended carbon nanofibers on carbon micro-structures were fabricated by directed electrospinning and subsequent pyrolysis at 900 °C of carbon nanotube/polyacrylonitrile (CNT/PAN) composite material. The electrical conductivity of the nanofibers was measured at different weight fractions of CNTs. It was found that the conductivity increased almost two orders of magnitude upon adding 0.5 wt.% CNTs. The correlation between the extent of graphitization and electrical properties of the composite nanofiber was examined by various structural characterization techniques, and the presence of graphitic regions in pyrolyzed CNT/PAN nanofibers was observed that were not present in pure PAN-derived carbon. The influence of fabrication technique on the ordering of carbon sheets in electrospun nanofibers was examined and a templating effect by CNTs that leads to enhanced graphitization is suggested.     

Electromagnetic interference shielding and dielectric properties of SiCf/SiC composites containing pyrolytic carbon interphase

The SiC_f/SiC composites containing various thickness of pyrolytic carbon (PyC) interphase were prepared and their properties were investigated for electromagnetic interference (EMI) shielding applications in the frequency of 8.2–12.4 GHz. The composites containing 310 nm thickness (3.3 vol%) PyC interphase show an about 25 dB shielding effectiveness in the whole frequency band. Interestingly, the contribution of reflection to the EMI shielding effectiveness increases and the contribution of absorption decreases as the PyC interphase thickness increases.     

Influence of wall number and surface functionalization of carbon nanotubes on their antioxidant behavior in high density polyethylene

Carbon nanotubes (CNTs) are extensively incorporated as reinforcement into polymeric materials due to their extraordinary properties. The antioxidant ability of CNTs in high density polyethylene (HDPE) was studied. Single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), and hydroxylated multi-walled carbon nanotubes (MWCNTs-OH) were involved to investigate the influence of wall number and surface functionalization of CNTs on their antioxidant behavior in HDPE. Based on measurements of the oxidation induction temperature and oxidation induction time of CNT/HDPE composites, it is found that the antioxidant ability of the three kinds of CNTs is in the following order: MWCNTs-OH > MWCNTs > SWCNTs. The antioxidant ability and mechanism of CNTs are further examined by electron spin resonance spectra and Raman spectra. It is observed that the antioxidant behavior of CNTs obeys a free radical scavenging mechanism. The order of the radical scavenging efficiency and the defect concentration for CNTs are in good agreement with that of their antioxidant ability in HDPE. With more walls and surface hydroxyl groups, the CNTs have more structural defects and exhibit higher antioxidant ability. The study raises the possibility that CNTs can improve antioxidant properties as well as mechanical properties of polymer matrix.