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.     

Efficient fabrication of carbon/silicon carbide composite for electromagnetic interference shielding applications

Ceramic matrix composites are typically prepared by a costly, time-consuming process under severe conditions. Herein, a cost-effective C/SiC composite was fabricated from a silicon gel-derived source by Joule heating. The β-SiC phase was generated via carbothermal reduction, and the carbon fabric showed a well-developed graphitic structure, promoting its thermal and anti-oxidation stabilities. Owing to the excellent dielectric loss in carbon fabric, SiC and SiO₂ as well as the micropore structure of the ceramic matrix, the absolute electromagnetic interference shielding (EMI) effectiveness (SSE/t) reached 948.18 dB?cm²?g^-1 in the X-band, exhibiting an excellent EMI SE. After oxidation at 1000 °C for 10 h in the air, the SSE/t of the composite was only reduced to 846.02 dB?cm²?g^-1. The C/SiC composite promises the efficient fabrication of high-temperature resistant materials for electromagnetic shielding applications.     

Tailoring carbon nanotube/matrix interface to optimize mechanical properties of multiscale composites

Pyrolytic carbon (PyC) was deposited on surfaces of carbon nanotubes (CNT) which were grown on carbon fibers to optimize the interfacial bonding between CNT/Matrix. The PyC protected CNT effectively and weakened CNT/Matrix interfacial strength, leading to long pull-out of CNT compared to brittle fracture of uncoated CNT. The well-protected CNT have more effective contributions to the improvement of mechanical properties. A “fiber-PyC/SiC-(CNT + PyC)-(CNT + SiC)” structure was formed using this process.     

Role of carbon nanotubes on mechanical properties of SiC-B4C-Ni hybrid composites fabricated by reactive spark plasma sintering

The present study was conducted to investigate the microstructural and mechanical properties of SiC-45 vol%B₄C-10 vol%Ni and SiC-45 vol% B₄C-10 vol%Ni-5vol% CNT in-situ composites fabricated through reactive spark plasma sintering (SPS) method at 1650 °C for 5 min. The phase and microstructural characterization of the composites were evaluated utilizing x-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). Ultimately, microhardness, flexural strength, and fracture toughness of the composites were measured. Densification behavior of the samples revealed that the initial shrinkage of the composites took place at about 750 °C, which is related to the reaction between Ni and SiC phases. XRD results confirmed the formation of Ni₂Si phase. The relative density of 98.3 ± 0.32% was achieved for the SiC-B₄C-10Ni sample; this value was 97.8 ± 0.62% for the SiC-B₄C-10Ni-5CNT sample. The flexural strength of 362 ± 23 MPa was achieved for the SiC-B₄C-10Ni sample; this value was 369 ± 15 MPa for the SiC-B₄C-10Ni-5CNT sample. The comparison between the fabricated composites concerning the fracture toughness indicated further fracture toughness of the SiC-B₄C-10Ni-5CNT sample (5.82 ± 0.32 MPa m^1/2) than that of the SiC-B₄C-10Ni sample (3.35 ± 0.56 MPa m^1/2). Formation of micro-cracks, crack path deflection, and CNT bridging was the main toughening mechanisms in the SiC-B₄C-10Ni-5CNT sample.     

Study on the toughening mechanism of in-situ synthesis (TixZr1−x)B2 in solid-state sintered SiC composite ceramics

High-dense SiC-(Ti_xZr_1?x)B₂ composite ceramics were fabricated by in-situ synthesis of (Ti_xZr_1?x)B₂ solid solution using solid-state spark plasma sintering (SPS). 64 vol% SiC, 20 vol% ZrB₂, 15 vol% TiB₂, and 1 vol% graphite powders are chosen as raw materials. The composite ceramics has the relative density of 99.97 %, the Vickers hardness of 24.71 GPa, the flexure strength of 435 MPa and the fracture toughness of 8.05 MPa ? m^1/2. Compared with the single-phase SiC ceramics and SiC-TiB₂ composite ceramics, the fracture toughness of SiC-(Ti_xZr_1?x)B₂ composite ceramics increased by 242.6 % and 53.6 %, respectively. A shell-core structure is found in the SiC-(Ti_xZr_1?x)B₂ composite ceramics, in which (Ti_xZr_1?x)B₂ solid solution is the core and fine SiC grain is the shell. The results show that the toughening effect of solid-state sintered SiC-(Ti_xZr_1?x)B₂ composite ceramics is attributed to the shell-core structure.     

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.     

Optimization of the microwave absorptivity of SiCf/Resin composites in the GHz range

The effects of structural factors on the electromagnetic wave absorption properties of SiC fibre reinforced resin composites (SiC_f/Resin) were investigated. Transmission line theory was used to calculate the reflection loss and to tap the potential of SiC fibres as broadband wave absorbents. The structure of the SiC_f/Resin composite was optimized based on a double-layered laminate containing high-resistance SiC fibres (H-SiC_f, ρ = 6.5 × 10⁵ Ω cm) in the upper layer and low-resistance SiC fibres (L-SiC_f, ρ = 109.7 Ω cm) in the bottom layer. The calculation suggests that to achieve a high absorptivity better than ?10 dB, the permittivity of the L-SiC_f/Resin bottom layer must be enhanced to quite a high value with a specific frequency dispersion degree. The desired permittivity was realized by controllable addition of carbon black into L-SiC_f/Resin. Under the optimized thickness combination, the reflection loss of the double-layered composite could be lower than ?13.3 dB in the whole X and Ku bands.     

Influence of SiC microstructure on its corrosion behavior in molten FLiNaK salt

The influence of the microstructure on the corrosion rate of three monolithic SiC samples in FLiNaK salt at 900 °C for 250 h was studied. The SiC samples, labeled as SiC-1, SiC-2, and SiC-3, had corrosion rates of 0.137, 0.020, and 0.043 mg/cm²h, respectively. Compared with grain size and the presence of special grain boundaries (i.e., Σ3), the content of high-angle grain boundaries (HAGBs) appeared to have the strongest influence on the corrosion rate of SiC in FLiNaK salt, since the corrosion rate increased six times as the concentration of high-angle grain boundaries increased from 19 to 32% for SiC-2 and SiC-1, respectively. These results stress the importance of controlling the content of HAGBs during the production process of SiC.     

The effect of the SiC content on the high duration erosion behavior of SiC/ZrB2– SiC/ZrB2 functionally gradient coating produced by shielding shrouded plasma spray method

In this research, the effect of the ZrB₂ middle layer and SiC Weight percentage on the erosion behavior of SiC/ZrB₂– SiC/ZrB₂ functionally gradient coating were investigated. For this purpose, SiC gradient coating was prepared via the reactive melt infiltration method (RMI). Afterwards ZrB₂–SiC layers containing 10, 20 and 30 wt% SiC and, ZrB₂ as the outer layer were applied on SiC coated graphite via solid shielding shrouded plasma spraying (SSPS). To investigate the erosion resistance of the coating, the specimens were subjected to oxy-acetylene and propane flame. The results showed that by applying the ZrB₂–SiC layer between SiC and ZrB₂ coating, due to the gradual change of the coefficient of thermal expansion mismatch and reduction of thermal stresses, erosion resistance improves, so that the coating with 20 wt% SiC with mass and linear erosion rate, ?0.072 × 10^?4 g.cm^?2.s^?1, 0.0166 μm s^?1 respectively had the best erosion resistance under oxy propane flame.In the oxyacetylene flame test, a similar result was obtained to the oxy propane test and the SiC/ZrB₂-20% wt. SiC/ZrB₂ coating had the lowest erosion rate.     

Continuous growth of carbon nanotubes on the surface of carbon fibers for enhanced electromagnetic wave absorption properties

As electromagnetic wave (EMW) pollution has become a serious problem in daily life, lightweight, efficient, and mass-produced EMW-absorbing materials are urgently needed. Herein, we developed a novel method for the continuous growth of carbon nanotubes (CNTs) on the surface of polyacrylonitrile (PAN)-based carbon fibers (CFs) by chemical vapor deposition (CVD), which can be applied to mass production. The obtained CF/CNT composites demonstrate outstanding EMW absorption capability, exhibiting a -58.75 dB reflection loss (RL) at a thickness of 1.54 mm. An effective absorption bandwidth (RL < -10 dB) of 4.24 GHz (13.76–18.00 GHz) was achieved at a thickness as low as 1.25 mm, which almost covers the entire Ku band. The excellent EMW-absorbing performance can be attributed to the 3D conductive network constructed by the CNT forest, which effectively promotes multiple reflections and scattering, and further favors dipole and interface polarizations. The mechanical properties of CF, CF-electrochemical anodic oxidation (EAO), and CF/CNT composites were examined, the results showed that the single-filament tensile strength of CF/CNT@0.07 and CF/CNT@0.09 was effectively improved. Our work suggests that the novel CF/CNT composite is a promising material for EMW absorption and strength enhancement owing to its light weight, high strength, low thickness, and good scale-up ability.     

Electrochemical insertion of lithium into a doped diamond film grown on carbon felt substrates

This paper summarizes the preliminary results obtained from lithium electrochemical intercalation into boron-doped diamond films grown on carbon felt (BDD/CF electrode). BDD films have been grown by Hot Filaments Chemical Vapor Deposition (HFCVD) and have been characterized by Scanning Electron Microscopy (SEM) and Raman Scattering spectroscopy. BDD/CF composite electrodes, which contain a diamond layer, lead to higher conductivity and smaller grain sizes. In turn, they are richer in boundary or sp² sites, and present a reversible specific capacity that is much larger than that of the substrate alone, indicating that the diamond layer effectively participates in lithium storage. Diamond layers displaying boron doping levels of 10¹⁹ and 10²¹ part cm^− 3 provide a specific capacity of 160 and 370 mA h g^− 1, respectively, which is associated with lithium storage.     

The hierarchical structure and properties of multifunctional carbon nanotube fibre composites

The axial mechanical, electrical and thermal properties of carbon nanotubes (CNTs) can be exploited macroscopically by assembling them parallel to each other into a fibre during their synthesis by chemical vapour deposition. Multifunctional composites with high volume fraction of CNT fibres are then made by direct polymer infiltration of an array of aligned fibres. The fibres have a very high surface area, causing the polymer to infiltrate them and resulting in a hierarchical composite structure. The electrical and thermal conductivities of CNT/epoxy composites are shown to be superior to those of equivalent specimens with T300 carbon fibre (CF) which is widely used in industry. From measurements of longitudinal coefficient of thermal expansion (CTE) of the composites we show that the CTE of CNT fibres is approximately ?1.6 × 10^?6 K^?1, similar to in-plane graphite. The combination of electrical, thermal and mechanical properties of CNT fibre composites demonstrates their potential for multifunctionality.     

Removal of natural organic matter in water using functionalised carbon nanotube buckypaper

A simple, surfactant-free assembly process was used to prepare multi-wall carbon nanotube (CNT) buckypapers using a highly efficient purification, sonication, and filtration process. To achieve effective dispersion of CNT into ethanol, a minimum 5-min sonication time was required. Here, we fabricated a buckypaper with pore size of 41 ± 10 nm and porosity of 72.9% with a 10-min sonication. The as-prepared buckypaper was used as a membrane for humic acid (HA) removal from water. During purification process, carboxylic and hydroxylic functional groups were introduced onto the CNT surface. The functional groups increased the hydrophilicity of the CNTs and improved the removal efficiency of HA by the buckypaper. The buckypaper prepared from purified CNTs exhibited excellent removal of HA (>93%) and a long lifetime for filtration.     

Effect of SiC interphase on the mechanical,high-temperature dielectric and high-temperature microwave absorption properties of the SiCf/SiC/Mu composites

In this study, SiC interphase was prepared via a precursor infiltration-pyrolysis process, and effects of dipping concentrations on the mechanical, high-temperature dielectric and microwave absorption properties of the SiC_f/SiC/Mu composites had been investigated. Results indicated that different dipping concentrations influenced ultimate interfacial morphology. The SiC interphase prepared with 5 wt% PCS/xylene solution was smooth and homogeneous, and no bridging between the fiber monofilament could be observed. At the same time, SiC interphase prepared with 5 wt% PCS/xylene solution had significantly improved mechanical properties of the composite. In particular, the flexural strength of the composite prepared with 5 wt% PCS/xylene solution reached 281 MPa. Both ε′ and ε′′ of the SiC_f/SiC/Mu composites were enhanced after preparing SiC interphase at room temperature. The SiC_f/SiC/Mu composite prepared with 5 wt% PCS/xylene solution showed the maximum dielectric loss value of 0.38 at 10 GHz. Under the dual action of polarization mechanism and conductance loss, both ε′ and ε′′ of the SiC_f/SiC/Mu composites enhanced as the temperature increased. At 700 °C, the corresponding bandwidth (RL ≤ ?5 dB) of SiC_f/SiC/Mu composites prepared with 5 wt% PCS/xylene solution can reach 3.3 GHz at 2.6 mm. The SiC_f/SiC/Mu composite with SiC interphase prepared with 5 wt% PCS/xylene solution is expected to be an excellent structural-functional material.     

Fabrication and mechanical properties of laminated HfC–SiC/BN ceramics

Laminated HfC–SiC/BN ceramics were successfully fabricated by tape casting and hot pressing. Fully dense HfC–SiC ultra-high temperature ceramics with homogeneous structure were obtained. The introduction of the weak BN layer resulted in a slight decrease of the flexural strength but significantly improved the fracture toughness compared with monolithic HfC–SiC ceramics. The fracture toughness of laminated HfC–SiC/BN ceramics in the parallel direction peaked at 8.06 ± 0.46 MPa m^1/2, which increased by 115% than that of monolithic HfC–SiC ceramics. The composites showed non-catastrophic fracture behaviors in both parallel and perpendicular directions. It indicates that laminated structure design is a promising approach to obtain full density HfC–SiC ceramics with high fracture toughness.     

High Hardness and Ductile Mosaic SiC/SiO2 Composite by Spark Plasma Sintering

SiC powder was coated with SiO₂ layer by chemical vapor deposition, and the SiC(core)/SiO₂(shell) composite powder was consolidated to a SiC/SiO₂ composite with a mosaic microstructure by spark plasma sintering (SPS) at 1923 K for 1.8 ks. The SiC(core)/SiO₂(shell) powder with a 80–100 nm thick SiO₂ layer resulted in a SiC/SiO₂ composite with a relative density of 97% and hardness and fracture toughness of 17.1 GPa and 8.4 MPa m^1/2, respectively.     

Comparative study of EMI shielding effectiveness for carbon fiber pultruded polypropylene/poly(lactic acid)/multiwall CNT composites prepared by injection molding versus screw extrusion

The electrical conductivity and electromagnetic interference (EMI) shielding effectiveness of the composites of polypropylene/poly(lactic acid) (PP/PLA) (70/30, wt %) with single filler of multiwall carbon nanotube (CNT) or hybrid fillers of nickel‐coated carbon fiber (CF) and CNT were investigated. For the single filler composite, higher electrical conductivity was observed when the PP‐g‐maleic anhydride was added as a compatibilizer between the PP and PLA. For the composite of the PP/PLA (70/30)/CF (20 phr)/CNT (5 phr), the composite prepared by injection molding observed a higher EMI shielding effectiveness of 50.5 dB than the composite prepared by screw extrusion (32.3 dB), demonstrating an EMI shielding effectiveness increase of 49.8%. The higher values in EMI shielding effectiveness and electrical conductivity of the PP/PLA/CF (20 phr)/CNT (5 phr) composite seemed mainly because of the increased CF length when the composites were prepared using injection molding machine, compared with the composites prepared by screw extrusion. This result suggests that the fiber length of the conductive filler is an important factor in obtaining higher values of electrical conductivity and EMI shielding effectiveness of the PP/PLA/CF/CNT composites. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45222.     

A theoretical evaluation of the effects of carbon nanotube entanglement and bundling on the structural and mechanical properties of buckypaper

Structural formation mechanisms of carbon nanotube (CNT) buckypaper and their effects on its mechanical properties are studied with numerical simulations. A bond swap algorithm, resulting from coupling the molecular dynamics and Monte Carlo methods, has been developed to equilibrate initial structures of buckypaper, generated by a random walk approach. Entanglement and bundling mechanisms are found to affect major structural features of buckypaper. Both mechanisms are evaluated quantitatively by calculating the entanglement network and pore size of buckypaper. Compared with (8,8)-(12,12) double-walled CNT, the structure of (5,5) single-walled CNT buckypaper is mainly dominated by entanglement, due to its smaller adhesion energy. We show that the pore size of modeled buckypaper, containing both types of CNTs, can be tuned from 7 nm to 50 nm by increasing the double-walled CNT content from 0 wt% to 100 wt%, due to the transformation from entanglement-dominated to bundling-dominated structures. Such an observation agrees exceptionally well with experimental results. Both entanglement and bundling mechanisms are also found to play important roles in the mechanical properties of buckypaper. The findings open a way to tailor both structural and mechanical properties of buckypaper, such as Young’s modulus or Poisson’s ratio, by using different CNTs and their mixtures.     

Microwave absorbing properties of a radar absorbing structure composed of carbon nanotube papers/glass fabric composites

In this study, carbon nanotube papers were employed in fabricating thin and broadband radar absorbing structures (RAS). Different concentrations of the CNT papers have been made by using a vacuum filtration method, with 20 × 20 cm in size and 21-27 μm in thickness. An epoxy resin was added into the CNT paper and then cured to become a composite with 1-5 wt.% of CNTs and 83-309 μm in thickness. The complex permittivity and permeability (ε′, ε″, μ′, μ″) of the CNT paper composites were measured using the transmission/reflection method in the frequency range of 2-18 GHz. The results reveal that the real (ε′) and imaginary (ε″) parts of the complex permittivity are increased with the CNT concentration. The ε′ of 5 wt.% CNT sample reaches 323 at 2 GHz and then decreases to 49.0 at 18 GHz. The ε″ reaches 321 at 2 GHz and decreases to 26.0 at 18 GHz. The CNT paper composite combined with a glass fabric composite used for a dielectric spacer is fabricated for an innovative RAS and the reflection loss is measured using the arch method in a microwave anechoic chamber. The results show that the 5 wt.% CNT paper composite/glass fabric composite absorbers attain maximum reflection loss of −13.3 dB at 12.0 GHz, −13.8 dB  at 10.0 GHz, and −16.0 dB at 7.5 GHz for spacer thickness of 1.5, 2.0, and 3.0 mm, respectively.     

Erosive wear behavior of spark plasma-sintered SiC–TaC composites

Spark plasma sintering of SiC-10, 20, or 30 wt% TaC composites was performed at 1800°C. Microstructures of sintered composites revealed uniform dispersion of TaC particles in SiC matrix. With the increase in TaC content, hardness decreased from 25.75 to 23.30 GPa and fracture toughness increased from 3.48 to 3.85 MPa m^1/2. Erosion testing was performed to evaluate the potential of sintered composites at room temperature and 400°C by a stream of SiC particles impinging at different angles (30°, 60°, or 90°). The erosion rate varied from 25 to 166 mm³/kg, with change in TaC content, impingement angle, or temperature. The erosion rate increased as the angle of impingement and temperature increased, but reduced when the TaC concentration increased. Worn surfaces revealed that the material was dominantly removed via fracture of SiC grains and TaC particles pull-out. SiC-30 wt% TaC composites exhibited superior erosive wear resistance at low impingement angle and room temperature.