Carbon nanotube/epoxy composites fabricated by resin transfer molding

Carbon nanotube (CNT)/epoxy composites with controllable alignment of CNTs were fabricated by a resin transfer molding process. CNTs with loading up to 16.5 wt.% were homogenously dispersed and highly aligned in the epoxy matrix. Both mechanical and electrical properties of the CNT/epoxy composites were dramatically improved with the addition of the CNTs. The Young’s modulus and tensile strength of the composites reach 20.4 GPa and 231.5 MPa, corresponding to 716% and 160% improvement compared to pure epoxy. The electrical conductivity of the composites along the direction of the CNT alignment reaches over 1 × 10⁴ S/m.     

High-aspect ratio and horizontally oriented carbon nanotubes synthesized by RF-cCVD

Horizontally aligned long carbon nanotubes were efficiently synthesized on Si substrates by using a radio-frequency catalytic chemical vapor deposition method. The morphological as well as the growth properties of these nanotubes were systematically investigated with various analytical techniques including microscopy and Raman spectroscopy. Different reaction parameters such as temperature, type of hydrocarbon gas and catalyst amount were varied and their effects on the nanotube size, quality and alignment are reported. High-aspect ratio and horizontally oriented nanotubes were found to grow following the “tip growth” mechanism. The fast and localized heating rate produced by the RF generator helps nanotubes to separate and lift the nano-particles away from the support and hence contributes to the growth of CNTs with a very high-aspect ratio. Carbon nanotubes synthesized with methane show a better horizontal alignment compared to those synthesized with acetylene, which might be due to the flow rate of the hydrocarbon gas.     

定向碳纳米管填充橡胶复合材料的制备

采用传统的橡胶工艺制备了高取向碳纳米管(CNTs)填充5%和30%的橡胶复合材料。碳纳米管的取向可能是在球磨的优化过程中由拖曳剪切力引起的。与纯橡胶相比,碳纳米管的选择性取向使其弹性模量能都得到了提高。     

Magnetically processed carbon nanotube/epoxy nanocomposites: Morphology, thermal, and mechanical properties

The processing-structure-property relationships of multiwalled carbon nanotubes (MWNTs)/epoxy nanocomposites processed with a magnetic field have been studied. Samples were prepared by dispersing the nanotube in the epoxy and curing under an applied magnetic field. The nanocomposite morphology was characterized with Raman spectroscopy and wide angle X-ray scattering, and correlated with thermo-mechanical properties. The modulus parallel to the alignment direction, as measured by dynamic mechanical analysis, showed significant anisotropy, with a 72% increase over the neat resin, and a 24% increase over the sample tested perpendicular to the alignment direction. A modest enhancement in the coefficient of thermal expansion (CTE) parallel to the alignment direction was also observed. These enhancements were achieved even though the nanotubes were not fully aligned, as determined by Raman spectroscopy. The partial nanotube alignment is attributed to resin a gel time that is faster than the nanotube orientation dynamics. Thermal conductivity results are also presented.     

The production of large scale ultrathin aligned CNT films by combining AC electric field with liquid flow

We report a simple and versatile technique combining the use of an AC electric field with a liquid shear force to prepare ultrathin aligned CNT films on solid substrates. Multiwalled carbon nanotubes (MWCNTs), which were synthesized by a template method and acid-treated single walled carbon nanotubes (SWCNTs) were dispersed in water and used for the ultrathin film fabrication. A solid substrate was immersed in the CNT dispersions and withdrawn at constant speed under AC electric field. SEM images of the substrate showed that CNTs were aligned with the AC electric field and the withdrawal direction and formed uniform films with a thickness around 10 nm for SWCNTs and 90 nm for MWCNTs. Repeating the deposition process increases the density and size of the film while also maintaining nanometer-scale thickness. Unidirectional alignment of CNTs was also confirmed by Raman spectra and electric conductivity measurements. It was found that ultrathin films of aligned SWCNTs exhibited very high anisotropic electrical conductivity with conductivity measured parallel to the alignment direction 3.3 × 10⁵ times higher than that measured in the perpendicular direction. We demonstrate that use of the aligned ultrathin SWCNT film for a unidirectional alignment of liquid crystal.     

Thickness-, alignment- and defect-tunable growth of carbon nanotube arrays using designed mechanical loads

We demonstrate the thickness-, morphology-, and defect-tunable growth and simultaneous integration of aligned carbon nanotube (CNT) arrays using a novel microscale platform. This platform consists of a micromechanical spring of desired stiffness, which applies a precise vertical load to a vertically aligned CNT array during its growth by chemical vapor deposition (CVD). The micromechanical spring is strained by the extrusive growth force output from the aligned CNT array during its growth and, at the same time, exerts a mechanical restoring force against the buckling resistance of the CNTs. This application of a designed vertical load on the CNTs allows modulation of the thickness and degree of alignment of the CNT array, as well as the structural quality of the individual CNTs. Consequently, the electrical resistance between two opposing CNT arrays can be tuned by adjusting the vertical load. In addition, their sensing responsiveness toward chemical species can also be enhanced by applying larger vertical load on the CNTs. In contrast to conventional growth methods for producing aligned CNT arrays, our approach offers an efficient way for the growth engineering and on-chip integration of aligned CNT arrays in a single step of the CVD.     

Simultaneous dispersion and alignment of carbon nanotubes in epoxy resin through chronoamperometry

A novel technique of dispersion, stretching and alignment of carbon nanotubes (CNTs) in an epoxy resin has been developed based on chronoamperometry. The procedure is to apply an electrical field in non-cured CNT/epoxy mixtures, using an electrode of large surface. CNT movements can be promoted by their intrinsic polarisation or by the introduction of electrical charges over their surfaces. The efficiency of the process can be monitored through the electrical current passing through the system as a function of time. Different experimental conditions, such as the applied voltage, have been analysed. The composites have been characterised through their thermal, mechanical and electrical behaviour. They show high dispersion degree of nanotubes, which are found to be stretched and untangled. CNTs show alignment along a preferred direction in the epoxy matrix, which affects to their electrical conductivity.     

Enhanced dielectric permittivity of BaTiO3/epoxy resin composites by particle alignment

Barium titanate (BaTiO₃)/epoxy resin composites with a novel structure, in which the BaTiO₃ particles were directionally aligned in the epoxy resin matrix, were fabricated using the ice-templating method. The effects of the filler particle alignment and the filler fraction on the dielectric permittivity as well as the dielectric loss of the composites were studied. The results show that the aligning filler particles can significantly improve the dielectric permittivity while maintaining the dielectric loss compared with the traditional composite structure (homogeneously distributed). Due to the feasibility of the enhancement of the dielectric properties of the composites, the particle alignment that is achieved via the ice-templating method can be used in the field of high energy density capacitors.     

Processing and properties of polymer composites containing aligned functionalized carbon nanofibers

AC electric field was used to align functionalized carbon nanofibers (CNFs), carboxylic acid-functionalized CNFs (O-CNFs) and amine-functionalized CNFs (A-CNFs), in an epoxy resin. The resulting composites were characterized for dispersion and alignment structure as well as for their mechanical and electrical properties in the CNF alignment direction. Optical images of the composites revealed uniform distribution and alignment of the CNFs in the direction of the electric field. Due to the similarity in the alignment structure, it was observed that alignment of the functionalized CNFs was independent of the functional groups attached to the CNFs. Compression tests (parallel to the direction of the aligned A-CNFs) of A-CNF/epoxy composites showed an increase of 19% in compressive modulus and 9% in compressive strength at a CNF concentration of 4.5 wt.%, with respect to the neat composite. Electrical resistivity of composites measured parallel to the direction of aligned CNFs (containing up to 4.5 wt.% O-CNFs and A-CNFs) were found to be approximately three orders of magnitude lower than composites with non-aligned CNFs. The electrical resistivity percolation threshold for composites with aligned O-CNFs and A-CNFs occurred at approximately 0.75 wt.%. Discussion regarding the contribution of CNF type towards the mechanical and electrical properties is also presented.     

Carbon nanotube film/epoxy composites with high strength and toughness

A strong and tough carbon nanotube (CNT)/epoxy composite was fabricated by resin solution impregnation process based on floating catalyst chemical vapor deposition (CVD)–growth CNT films, which had a tensile strength and toughness of 405 MPa and 122 J/g, respectively, and good damping properties as well. Evolution of the composite structure revealed that the CNTs aligned along the tension direction with decreasing orientation angle, and the CNT bundle size enlarged during the tensile test process, which contributed to efficient load transfer among the composite network. Results showed that the proper resin content could bring benefit for strong connections and dense packing of CNTs/bundles, but excessive resin content was unfavorable for improving mechanical properties and conductivities of the nanocomposite. In addition, the resin in CNT film/epoxy composites had a lower crosslink density than that in a neat epoxy system, which endowed the CNT composites with large deformation capability. POLYM. COMPOS., 38:588–596, 2017. © 2015 Society of Plastics Engineers     

Preparation of Grain-Oriented Sr0.5Ba0.5Nb2O6 Ferroelectric Ceramics by Magnetic Alignment

The properties of polycrystalline ceramics are strongly influenced by their crystallographic texture. In this study, highly grain-oriented tungsten bronze structure ferroelectric ceramics, Sr_0.5Ba_0.5Nb₂O₆, were successfully fabricated by magnetic alignment and gelcasting techniques using only the conventional solid-state-synthesized starting powder. Spherical Sr_0.5Ba_0.5Nb₂O₆ particles were aligned according to their anisotropic magnetic property in 40 vol% slurry in a 10 T magnetic field, and then in situ locked by polymerization via a gelcasting technique for 30 min. A 〈00 l 〉-axis orientation perpendicular to the magnetic field direction ( B ) was obviously observed in the green compact and sintered sample. The sintered Sr_0.5Ba_0.5Nb₂O₆ sample contained equiaxial grains and reached 98% theoretical density. Compared with the sample with randomly oriented grains, the magnetically aligned sample showed an enhanced with dielectric constant in the ⊥ B direction (1100 versus 750 at room temperature and 4300 versus 2800 at Curie temperature). This new method is readily applicable to other ceramics with tungsten bronze structure, and is expected to facilitate mass preparation of large and dense grain-oriented ceramic materials.     

Microstructural Control of Colloidal‐Based Ceramics by Directional Solidification Under Weak Magnetic Fields

The use of weak magnetic fields to control the microstructural evolution of colloidal‐based systems in conjunction with directional solidification is demonstrated as a convenient processing route to fabricate anisotropic ceramic scaffolds with complex microarchitectures. A variety of graded and aligned microstructures were formed by applying external static magnetic fields oriented radially, axially, and transversely with respect to the solidification direction of freezing slurries containing micro/nanoparticles of ZrO₂ and Fe₃O₄. The graded structures, formed by the radial and axial fields, resemble core–shell architectures composed of dense outer perimeters surrounding porous inner cores. The aligned structures, formed by transverse fields, exhibit two modes of microstructural alignment: lamellar walls aligned by the growing ice crystals and mineral bridges aligned by the magnetic fields. The alignment of mineral bridges that connect adjacent lamellae, provide these scaffolds enhanced strength and stiffness when compressed parallel to their orientation (parallel to the direction of the magnetic field).     

Growth of mechanically fixed and isolated vertically aligned carbon nanotubes and nanofibers by DC plasma-enhanced hot filament chemical vapor deposition

Vertically aligned, mechanically isolated, multiwalled carbon nanotubes (MWCNTs) and nanofibers (MWCNFs) were grown using an array of catalyst nickel nanowires embedded in an anodic aluminum oxide (AAO) nanopore template using DC plasma-enhanced hot filament chemical vapor deposition (HFCVD). The nickel nanowire array, prepared by electrodeposition of nickel into the pores of a commercially available AAO membrane, acts as a template for CNT and CNF growth. It also provides both a mechanical “fixed support” boundary condition and enforces sufficient spatial separation of the CNT/CNFs from each other to enable reliable and well-controlled mechanical testing of individual vertically aligned CNT/CNFs. In contrast with other AAO-templated growth methods, no post-growth etching of the AAO is required, since the CNTs/CNFs grow out of the pores and remain vertically aligned. A mixture of hydrogen and methane was used for the growth, with hydrogen acting as a dilution and source gas for the DC plasma, and methane as the carbon source. A negative bias was applied to the sample mount to generate the DC plasma. The filaments provided the necessary heat for dissociation of molecular species, and also heat the sample itself significantly. Both of these effects assist the CNT/CNF growth. Minimal heating came from the low-power plasma. However, the associated DC field was essential for the vertical alignment of the CNTs and CNFs. Scanning electron, transmission electron, and atomic force microscopy confirm that the CNT/CNFs are composed of graphitic layers, and form a vertically aligned, relatively uniform, and dense array across the AAO template. A significant number of the structures grown are indeed high quality nanotubes, as opposed to more defective nanofibers that are often predominant in other growth methods. This method has the advantage of being scalable and consuming less power than other techniques that grow vertically aligned CNTs/CNFs.     

Directed assembly of carbon nanocones into wires with an epoxy coating in thin films by a combination of electric field alignment and subsequent pyrolysis

We report on a nanoparticle manipulation method for assembling carbon nanocones and disks (CNCs) into molecular wires that create a regular two-dimensional network inside thin films. This technique includes electric field induced assembly and electric field orientation (dielectrophoresis), curing and pyrolysis. First, CNCs are dispersed near to the individual particle scale in a two component epoxy adhesive containing phenolic resin. Second, a thin layer (?10 μm) of this dispersion is spread onto interdigitated metal electrodes (spacing between 10 and 100 μm) on a glass substrate (area of several cm²). CNC wires are assembled and aligned by an alternating electric field (∼1 kHz, ∼1 kV/cm) yielding an epoxy film with uniaxially aligned CNC pathways (diameter 1–5 μm) in-plane. Third, the aligned film is cured by heating, which leads to a solid film where the wire alignment is maintained within the cross-linked polymer matrix. Finally, most of the cured epoxy is removed from in between the CNC wires by further heating (pyrolysis), which results in a network of aligned, separated wires with a CNC interior and polymer covering. This procedure provides a general concept for forming aligned and stable networks of CNC wires over large surfaces.     

Improved structural and mechanical performance of iron oxide scaffolds freeze cast under oscillating magnetic fields

Manufacturing processes yielding stronger, yet lighter structures are sought for in many industries and scientific applications. Freeze casting is a fabrication process that offers a way to achieve these strong, lightweight structures, but only in a single direction (the direction of the templating-ice growth). Applying a uniform magnetic field to these structures allows for increased strength in an additional direction, thus allowing for them to be applied in a variety of complex loading environments. Using a Helmholtz coil, it is possible to apply weak, uniform fields in any direction, magnitude, or frequency. Previous research using Helmholtz coils has shown that an applied field can increase strength through microstructural alignment, but the limited field strength reduces the applicability of these materials. To mitigate this, an oscillating field (i.e., a stronger magnetic field in a single direction with a weaker alternating field in an orthogonal direction) of various magnitudes of oscillation during the fabrication of freeze-cast materials was applied using Helmholtz coils. These oscillating magnetic fields led to an increase of strength of up to 2.5x compared to materials fabricated with either no applied field or a non-oscillating applied field due to increased alignment and thickness of the lamellar walls. This demonstrates that increased material response can be induced through the application of an oscillating field without increasing the maximum magnetic field strength.     

Effect of load volume on power absorption and temperature evolution during radio-frequency heating of meat cubes: A computational study

During radio frequency (RF) processing, the size of sample between RF electrodes has certain effect on power absorption and heating rates. Hence, certain load sizes might be required for effective RF processes for temperature evolution. Therefore, the objective of this study was to evaluate the effect of sample size on power absorption and heating rate during RF heating. For this purpose, a 3-dimensional multi-physics model was used for various load volumes in two configurations. In the first configuration, distance between RF electrodes was fixed while air gap between sample's surfaces and electrodes was fixed in the second configuration. The smaller the load volume, the larger the air gap and the slower the heating rate of sample due to the behavior of electric field in the first case. The smallest volume in the second case, however, was heated much faster via the deflection of electric field by top–bottom edges increasing net electric field in the sample with the effect of shorter air gap distance. The results indicated that the sample load volume is rather important, and it might be possible to obtain optimal tuning of RF cavities to allow a high heating efficiency by changing the distance between electrodes.     

Morphological manipulation of carbon nanotube/polycarbonate/polyethylene composites by dynamic injection packing molding

Multiwalled carbon nanotubes (CNTs) filled polymer composite based on polycarbonate (PC) and polyethylene (PE) was fabricated by shear controlled orientation in injection molding (dynamic samples) and conventional injection molding (static samples). The morphological observation by scanning electronic microscope (SEM) indicated that PC phase in situ generated more and finer microfibrils in the dynamic samples than in the static ones, and the CNTs predominantly localized in the PC microfibrils without obvious migration to PE matrix and also aligned along the microfibrils. With such unique morphology, the tensile properties of the dynamic samples were simultaneously considerably increased compared to their complementary samples, especially in the presence of 0.5 wt% of CNTs, which indicates both stretch alignment of CNTs and molecule orientation can bring out a significant reinforcement on PE. Furthermore, the static samples displayed double yielding points in on the stress-strain curves, and interestingly, a small quantity of CNTs in PC fibrils strengthened this phenomenon.     

Fluffy carbon nanotubes produced by shearing vertically aligned carbon nanotube arrays

Fluffy carbon nanotubes (CNTs), which are cotton-like macroscopic structures, are obtained by simple high-speed shearing of vertically aligned CNT (VACNT) arrays. The fluffy CNTs are composed of CNT bundles with a diameter of several micrometers, and have an extremely low apparent density of 3-10 g/L. A requisite for their formation is the alignment of CNTs in the initial array. The shear between the rotor and the arrays tears the arrays along the axial direction and this results in their dispersion into low density fluffy CNTs.     

Optimization of plasma-enhanced chemical vapor deposition parameters for the growth of individual vertical carbon nanotubes as field emitters

In this article plasma enhanced growth of single vertical carbon nanotubes (CNTs) from individual nickel catalyst dots is studied, aiming at the fabrication of CNT field emitters. It is found that the growth of individual CNTs differs from that of CNT forests grown from unpatterned catalyst films, an effect that can be attributed to the difference in catalyst volumes. In the context of growth parameters the influence of temperature, growth time, catalyst volume, pressure and power is characterized. After determining the growth behavior, an individual CNT of desired geometry is fabricated on a conducting lead. The CNT is electrically characterized in terms of its field emission behavior and stable emission currents and its work function is determined to Φ = 5.4 ± 0.2 eV.     

Synthesis and field emission properties of vertically aligned carbon nanotube arrays on copper

We report the synthesis of periodic arrays of carbon nanotubes (CNTs) with different densities on copper substrate by employing nanosphere lithography (NSL) and plasma enhanced chemical vapor deposition. At a growth pressure of 8 torr and temperature of 520 °C, vertically aligned bamboo-like CNTs were formed with a catalyst particle on the tip. Electrical properties of CNTs with different densities were investigated for the possible applications in field emission (FE). The investigation of FE properties reveals a strong dependence on the density of CNTs. Experimental results show that NSL patterned low density CNTs exhibit better field emission properties as compared to the high density CNTs. Low-density CNTs exhibit lower turn-on and threshold electric fields, and a higher field enhancement factor. The high density of CNTs results in the deterioration of the FE properties due to the screening of the electric field by the neighboring CNTs.