Fabrication of scalable,aligned and low density carbon nanotube/silicon carbide hybrid foams by polysilazane infiltration and pyrolysis

Polymer-derived ceramic (PDC) process is an attractive technique that has high ceramic yield. This versatile method allows for fabrication of porous carbon nanotube (CNT)/ silicon carbide (SiC) hybrid materials that is important high temperature structural applications. Although several forms of CNT assemblies have been used with the PDC approach, the fabricated CNT/ceramic nanocomposites were either one or two dimensional. Herein, we report, for the first time, the fabrication of a low density, three-dimensional (3D) and scalable CNT/SiC structure using PDC technique. It was synthesized by impregnating preceramic polysilazane (PSZ) into ultralow density, anisotropic, and highly aligned CNT foams, followed by thermosetting and pyrolysis processes. The ceramic phase conformally coated the CNTs. The X-ray diffraction (XRD) diffractogram confirmed the presence of β-SiC crystalline phase. The resulting hybrid foam inherited the morphology and form factor of the original CNT foam, and possessed mechanical robustness, improved electrical properties, and extraordinary thermal stability.     

Synthesis of SiC whiskers via catalytic reaction method in self-bonded SiC composites

Catalyzed by in-situ formed Fe nanoparticles (NPs), 3C–SiC whiskers were prepared from expanded graphite and Si powders after firing at 1573 K for 3 h in Argon. The density functional theory calculations revealed that Fe catalysts facilitated the formation of SiC nucleus and the epitaxial growth of SiC whiskers via reducing the bonding strength in CC dimer as well as Si–O and C–O bonds. Moreover, using SiC, expanded graphite and silicon powders as starting materials Fe-catalyzed self-bonded SiC composites were fabricated. Lots of SiC whiskers were synthesized in the as-prepared composites, leading to remarkable enhancements in high temperature mechanical behavior, oxidation resistance and cryolite resistance of the self-bonded SiC composites.     

A review on the processing technologies of carbon nanotube/silicon carbide composites

Due to the extraordinary electronic, mechanical, chemical, thermal, magnetic, and optical properties, carbon nanotube (CNT), an excellent one-dimensional nano-material, has been considered as a new filler for polymer, metal, and ceramic matrix composites with the main purpose of improving their mechanical performance, fracture behavior, and functional features. In the silicon carbide (SiC) ceramic field, there are many CNT reinforced SiC ceramic matrix composites and CNT/SiC hybrid structures, which have been investigated successfully using various of methods. This paper reviews the current status of researches and describes all different routes for effectively dispersing CNTs throughout SiC ceramic matrix, densifying composites, and synthesizing hybrid structures.     

Structure and performance of Si3N4/SiC/CNT composite fibres

Herein, a homogeneously distributed and well-orientated ceramic-CNT composite fibre (Si₃N₄/SiC/CNTs) has been prepared using carbon nanotube fibres (CNTFs) premixed with silicon powder, followed by the reaction-bonded sintering process. The SiC layers around the CNT bundles interspersed in the composite are formed during the silicon reaction stage through the contact of silicon and CNTs, and the densification of the ceramic through the further reaction-bonded silicon carbide and nitride. Due to strong interface bonding, the composite fibres exhibit the potential for CNT-based damage sensing with a tensile strength upto 225 Mpa. Furthermore, the high-volume distribution of CNT sresults in a significant enhancement of the electrical and thermal conductivities as well as photoluminescence properties. Our work provides a useful approach for thefabrication of multifunctional fibres for imaging, engineering, and other complex applications.     

Noncatalytic synthesis of carbon nanotubes, graphene and graphite on SiC

Graphene and carbon nanotubes (CNT) can be produced by vacuum decomposition of SiC, but discrepancies and conflicting data in the literature limit the use of this method for CNT synthesis. A systematic study of the effects of SiC surface morphology and carbon transport through the gas phase leads to reproducible and controlled growth of arrays of small-diameter (1–4 walls) nanotubes, which show pronounced radial breathing modes in Raman spectra, on either carbon or silicon (0 0 0 1) face of 6H SiC wafers at 1400–1900 °C. These nanotube arrays have a very high density and are catalyst-free with no internal closures. They show a higher oxidation resistance compared to CNTs produced by catalytic chemical vapor deposition (CVD). Their integration with graphite/graphene or silica layers on SiC wafers is possible in a simple 2-step process and opens new horizons in nanoscale device fabrication.     

Great improvement in adhesion and uniformity of carbon nanotube field emitters through reactive nanometer-scale SiC fillers

We developed highly adhesive uniform field emitters (FEs) from carbon nanotube (CNT) pastes by reacting nanometer-scale silicon carbide (SiC) fillers on a Kovar substrate at a high temperature in vacuum. The reaction of SiC on Kovar results in significant morphological changes at the substrate–composite interface along with moderate Si diffusion into the substrate, enhancing adhesion of CNT FEs to the substrate greatly. Furthermore, a post surface-treatment after the reaction of SiC fillers results in very uniform CNT FEs over an entire emitter pattern of several hundred micrometers. The strongly enhanced adhesion and uniformity of the CNT FEs, in turn, give stable and reliable field emission even at a high current density. The applicability of the SiC/CNT FEs was evaluated in super-miniature X-ray tubes where any detachment of materials including outgassing from the inner side of the tubes should be avoided. The smallest super-miniature CNT X-ray tube to date, with an outer diameter of 2.0 mm, shows good operation with X-ray imaging at an anode voltage of above 25 kV, confirming almost no outgassing and strong substrate adhesion of the CNT FEs.     

One-step growth of graphene–carbon nanotube hybrid materials by chemical vapor deposition

Graphene–carbon nanotube (CNT) hybrid materials were synthesized by simple one-step chemical vapor deposition (CVD) using ethanol as precursor. On a copper foil decorated with silicon nanparticles (Si NPs), a graphene film grows uniformly on the substrate while CNTs sprout out from Si NPs to form a network on top. The density of CNTs can be controlled by the CVD growth temperature. As measured by scanning and transmission electron microscopy, the obtained CNTs exhibit bamboo-like multiple-wall structures. Electrical characterization shows that the graphene–CNT hybrids exhibit p-type field-effect characteristics and a significantly higher conductivity compared to a CVD grown pure graphene film.     

Noncovalent functionalization of carbon nanotubes with maleimide polymers applicable to high-melting polymer-based composites

As novel carbon nanotube (CNT) dispersants are effective not only for obtaining stable CNT-dispersed solutions but also for high-melting polymer/CNT composites, we synthesized maleimide polymers (MIPs) using N-substituted maleimide for imparting physical adsorption on the CNT surfaces and high heat resistance. The MIPs showed strong physical adsorption on various CNT surfaces and good solubility in a wide variety of organic solvents, and acted as excellent CNT dispersants in these substances. The MIPs on the CNT surfaces were very stable at high temperatures (?∼300 °C) required for melt mixing using high-melting polymers. The addition of MIP-adsorbed CNTs (CNT/MIPs) to poly(1,4-phenylenesulfide) (PPS) as a high-melting polymer was, therefore, very effective for dispersing CNTs and improving the physical properties of the resulting PPS/CNT/MIP composites, in comparison with the PPS/CNT composites. Even at a low CNT loading (1 vol%), the storage modulus of the PPS/CNT/MIP composites increased drastically. Furthermore, thermal conductivity of the PPS/CNT/MIP composites also improved, in comparison with the PPS/CNT composites. These results are considered to be due to an increase of interactions between the CNT and PPS matrices, caused by the stable formation of MIPs on the CNT surfaces.     

Anisotropic compressive properties of porous CNT/SiC composites produced by direct matrix infiltration of CNT aerogel

Carbon nanotube‐reinforced silicon carbide composites (CNT/SiC) produced by direct infiltration of matrix into a porous CNT arrays have been demonstrated to possess a unique microstructure and excellent micro‐mechanical properties. However, the thickness of the array preforms is usually very small, typically less than 2 mm. Therefore, fabrication of macroscopic CNT/SiC composites by chemical vapor infiltration (CVI) process requires that the nanoscale fillers could form macroscopic architectures with an open pore network. Here, this study reports an experimental strategy for the fabrication of SiC matrix composites reinforced by CNT based on an ice‐segregation‐induced self‐assembly (ISISA) technique. Macroscopic CNT aerogel with well‐defined macroporous network was produced by ISISA technique and was subsequently infiltrated by SiC in a CVI reactor. After five CVI cycles, the porosity of as‐fabricated composites was 11.6±0.3% and the machined specimens exhibited lamellar structure with parallel lamellaes intersected at discrete angles. By observed, there are in fact five different representative anisotropic macrostructures, the compressive strengths of these five different loading modes with respect to lamella orientation were 933±55, 619±34, 200±45, 199±21, and 297±41 MPa, respectively, and the failure mechanisms were attributed to the anisotropic nature of the macrostructures. Energy dissipation toughening mechanism at the nanoscale such as CNT pull‐out was observed and the phase composition of the fabricated materials included β‐SiC, CNT, and SiO₂.     

Aluminum oxide particles/silicon carbide whiskers’ synergistic effect on thermal conductivity of high-density polyethylene composites

Aluminum oxide (Al₂O₃) particles and silicon carbide (SiC) whiskers improved the thermal conductivity of high-density polyethylene (HDPE). To improve the dispersion of inorganic fillers in the matrix, 5 wt% of maleic anhydride-modified polyethylene was added into HDPE as a compatibilizer, and the hybrid matrix was denoted as mHDPE. The thermal conductivity, heat resistance, and tensile properties of resulting HDPE composites were characterized. The results showed that the thermal conductivity reached its maximum value of 0.8876 W/(m K) at 1/4 weight ratio of Al₂O₃/SiC, which was 110.3, 54.8, and 8.8% higher than that of pure HDPE, mHDPE/Al₂O₃, and mHDPE/SiC composites, in the order given, indicating that hybrid fillers have synergistic effect on the thermal conductivity of HDPE composites. Moreover, they also have a synergistic effect on the heat resistance and Young’s modulus. As the SiC content increases, the heat resistance of the composites increases at first and then falls, and the maximum VST is reached at an Al₂O₃/SiC weight ratio of 3/2, which is 5.4 °C higher than that of HDPE. The maximum Young’s modulus of the composites (1160 MPa) is obtained at an Al₂O₃/SiC weight ratio of 1/4, and the yield strength increases gradually as the SiC whiskers’ content increases.     

Broadband electromagnetic shielding performance of carbon nanotube-carbon fibre/silicon carbide cross-scale laminated composites

A carbon nanotube-carbon fibre/silicon carbide (CNT-CF/SiC) laminated composite, with a density of 1.61 g/cm³, thickness of 2.7–3.0 mm and conductivity of 6.10 S/cm, was prepared by densifying a single layer with boron-modified phenolic resin and then welding it with resin-derived carbon layer by layer. This laminated composite was alternately composed of a relatively dense CNT buckypaper/SiC composite layer and a relatively porous three-dimensional needled CF felt/SiC composite layer. The CF felt with a laminated constructure produced a laminated substructure nested within the layers. Expanded graphite with laminated structures produced laminated substructures nested within the interfaces. The average total shielding efficiency values of the composites with 5 layers (CNT-CF/SiC-5), 4 layers and a CNT buckypaper/SiC composite layer on the top surface, and 4 layers and a CF felt/SiC composite layer on the top surface were 45.14, 37.70 and 38.85 dB, respectively, throughout the X-band and were 52.31, 45.56 and 43.54 dB, respectively, throughout the Ku-band. The transmission coefficient of CNT-CF/SiC-5 was as low as 10^?5?10^?6 orders of magnitude over the entire frequency range of 8.2–18 GHz except for very few frequency points. The optimal number of layers for this multilevel and multiscale laminated composite is believed to be 5.     

Carbon nanotube reinforced pyrocarbon matrix composites with high coefficient of thermal expansion for self-adapting ultra-high-temperature ceramic coatings

The mismatch in the coefficients of thermal expansion (CTE) of the carbon fiber reinforced pyrocarbon (Cf/C) composites and their thermal barrier coatings (TBCs) has significantly restricted the service life of Cf/C composites in high-temperature environments. Owing to the high CTE of TBCs, it is vital to find a material with similar mechanical properties and higher CTE than Cf/C composites. In this work, carbon nanotube reinforced pyrocarbon (Ct/C) nanocomposites with high CTEs were prepared to self-adapt to the TBCs. Different CTEs (～4.0–6.5 × 10^?6/°C) were obtained by varying the carbon nanotube (CNT) content of the Ct/C composites. Owing to the decreased mismatch in the CTEs, no cracks were formed in the TBCs (SiC and HfB₂-SiC-HfC coatings) deposited on the Ct/C composites. After heat treatment at 2100 °C, several wide cracks were found in the TBCs on the Cf/C composite, whereas the TBCs on the Ct/C composites were intact without cracks. We found that the CTE-tunable Ct/C composites can self-adapt to different TBCs, protecting the composites from oxidation at high temperatures.     

Catalytic synthesis of SiC nanowires in an open system

SiC nanowires (NWs) are usually synthesized in a closed vacuum reaction system which limits the yield of SiC NWs. In this work, SiC NWs and carbon nanotubes were synthesized in an open tube furnace at 1550°C with Si powder as silicon sources, ethanol as carbon sources and ferrocene as catalyst. The as-synthesized products were ultralong β-SiC NWs with the diameter about 80-100 nm and the length up to several tens micrometers. The diameter of the carbon nanotubes was about 20-30 nm. The carbon nanotube yarns about 20 cm in length were obtained at the end of the tube furnace. The growth mechanism of SiC NWs and carbon nanotubes were proposed. Compared with the traditional synthetic techniques in the high vacuum closed system, the novel synthesis method in the open system provided a new approach to the synthesis of SiC NWs.     

Development of microreactors made of vertically aligned carbon nanotube films

Microreactors consisting of multiwalled carbon nanotube (CNT) microchannels have been developed. Vertically aligned CNT films with negative pattern shapes of microchannels are grown on silicon oxide films, providing CNT microchannels. Polymethyl methacrylate plates are placed on the CNT microchannels for the flow experiments. Since CNTs are hydrophobic and the silicon oxide film is hydrophilic, fluids can flow in the silicon oxide regions in the CNT microreactors. Fermat’s spiral and Y-junction type multiwalled CNT microreactors were synthesized.     

Field emission properties from aligned carbon nanotube films with tetrahedral amorphous carbon coatings

Tetrahedral amorphous carbon (ta-C) film was coated on aligned carbon nanotube (CNT) films via filtered cathodic vacuum arc (FCVA) technique. Field electron emission properties of the CNT films and the ta-C/CNT films were measured in an ultra high vacuum system. The I–V measurements show that, with a thin ta-C film coating, the threshold electric field (E_thr) of CNTs can be significantly decreased from 5.74 V/μm to 2.94 V/μm, while thick ta-C film coating increased the E_thr of CNTs to around 8.20 V/μm. In addition, the field emission current density of CNT films reached 14.9 mA/cm² at 6 V/μm, while for CNTs film coated with thin ta-C film only 3.1 V/μm of applied electric field is required to reach equal amount of current density. It is suggested that different field emission mechanisms should be responsible for the distinction in field emission features of CNT films with different thickness of ta-C coating.     

Synthesis and mechanical properties of carbon nanotube/diamond-like carbon composite films

Diamond-like carbon (DLC) coatings were successfully deposited on carbon nanotube (CNT) films with CNT densities of 1 × 10⁹/cm², 3 × 10⁹/cm², and 7 × 10⁹/cm² by a radio frequency plasma-enhanced chemical vapor deposition (CVD). The new composite films consisting of CNT/DLC were synthesized to improve the mechanical properties of DLC coatings especially for toughness. To compare those of the CNT/DLC composite films, the deposition of a DLC coating on a silicon oxide substrate was also carried out. A dynamic ultra micro hardness tester and a ball-on-disk type friction tester were used to investigate the mechanical properties of the CNT/DLC composite films. A scanning electron microscopic (SEM) image of the indentation region of the CNT/DLC composite film showed a triangle shape of the indenter, however, chippings of the DLC coating were observed in the indentation region. This result suggests the improvement of the toughness of the CNT/DLC composite films. The elastic modulus and dynamic hardness of the CNT/DLC composite films decreased linearly with the increase of their CNT density. Friction coefficients of all the CNT/DLC composite films were close to that of the DLC coating.     

Plasma sprayed graphene/carbon nanotube reinforced lanthanum-cerate hybrid composite coating

Lanthanum cerate (LC: La₂Ce₂O₇) is a potential material for thermal barrier coating, whose improved toughness is a crucial necessity for the pathway of its industrialization. Herein, we demonstrated a promising approach to develop graphene/carbon nanotube hybrid composite coating using a large throughput and atmospheric plasma spraying method. Graphene nanoplatelets (GNP: 1 wt %) and carbon nanotube (CNT: 0.5 wt %) reinforced lanthanum cerate (LCGC) hybrid composite coatings were deposited on the Inconel substrate. Addition of 1 wt % GNP and 0.5 wt % CNT in LC matrix has significantly increased its relative density, hardness, and elastic modulus up to 97.2%, 2–3 folds, 3–4 folds, respectively. An impressive improvement of indentation toughness (8.04 ± 0.2 MPa m^0.5) was observed on LCGC coating, which is ～8 times higher comparing the LC coating. The toughening was attributed to the factors: such as the distribution of GNPs and CNTs in the LC matrix, synergistic toughening offered by the GNPs and CNTs; (i) GNP/CNT pull-out, (ii) crack bridging and arresting, (iii) splat sandwiching, mechanical interlocking, etc. Finally, this improved toughness offered an exceptional thermal shock performance up to 1721 cycles at 1800 °C, without any major failure on the coating. Therefore, the GNP and CNT-reinforced LC hybrid composite coating can be recommended to open a path for turbine industries.     

Iron nanoparticles in aligned arrays of pure and nitrogen-doped carbon nanotubes

Arrays of aligned carbon nanotubes (CNTs) and nitrogen-doped carbon (CN_x) nanotubes have been grown on silicon substrates as the result of thermolysis of ferrocene/toluene and ferrocene/acetonitrile mixture. The microstructure of materials was studied by transmission and scanning electron microscopy, and X-ray diffraction was used to control the carbon and iron forms. The composition and properties of iron nanoparticles developed in the CNT and CN_x nanotube samples were determined from Mössbauer spectroscopy data. The total iron content in CN_x nanotubes was found to be considerably higher than that in CNTs. Three forms of iron nanoparticles α-Fe, γ-Fe, and Fe₃C were detected in CNTs and only two last of them in CN_x nanotubes. In the interior of CNT channels the α-Fe and Fe₃C nanoparticles were observed to be coupled by a strong exchange interaction and to exhibit magnetic behavior at room temperature.     

Transformation of silicon nanoparticles on a carbon nanotube heater into hollow graphitic nanocapsules via silicon carbide

The changes in the structure and composition of silicon (Si) nanoparticles supported on a multiwall carbon nanotube (MWCNT) during Joule heating of the MWCNT were studied by in situ high-resolution transmission electron microscopy. The Si nanoparticles reacted with the outer layers of the MWCNT to form silicon carbide (SiC) nanoparticles with increasing temperature. At temperatures of up to approximately 1900 K, silicon atoms were entirely sublimated from the SiC nanoparticles, and the remaining carbon atoms formed hollow carbon nanocapsules consisting of multilayered graphene shells on the MWCNT.     

In-situ synthesis and textural evolution of the novel carbonaceous SiC/mullite aerogel via polymer-derived ceramics route

A novel carbonaceous SiC/mullite composite aerogel is derived from catechol-formaldehyde/silica/alumina hybrid aerogel (CF/SiO₂/AlOOH) via polymer-derived ceramics route (PDCR). The effects of the reactants concentrations on the physicochemical properties of the carbonaceous SiO₂/Al₂O₃ aerogel and SiC/mullite aerogel are investigated. The mechanism of the textural and structural evolution for the novel carbonaceous SiC/mullite is further discussed based on the experimental results. Smaller reactants concentration is favorable to formation of mullite. Reactants concentration of 25% is selected as the optimal condition in considering of the mullite formation and bulk densities of the preceramic aerogels. Spherical large silica particles are also produced during heat treatment, and amorphous silica is remained after this reaction. With further heat treatment at 1400 °C, silicon carbide and mullite coexist in the aerogel matrix. The mullite addition decreases the temperature of SiC formation, when compared with the conventional methods. However, after heat treatment at 1450 °C, the amount of mullite begins to decrease due to the further reaction between carbon and mullite, forming more silicon carbide and alumina. The carbonaceous SiC/mullite can be transferred to SiC/mullite binary aerogel after carbon combustion under air atmosphere. The carbonaceous SiC/mullite has a composition of SiC (31%), mullite (19.1%), SiO₂ (14.4%), and carbon (35%). It also possesses a 6.531 nm average pore diameter, high surface area (69.61 m²/g), and BJH desorption pore volume (0.1744 cm³/g). The oxidation resistance of the carbonaceous SiC/mullite is improved for 85 °C when compared with the carbon based aerogel.