Evaluation of thermal stability of ultrafine grained aluminium matrix composites reinforced with carbon nanotubes

The effect of carbon nanotubes on the thermal stability of ultrafine grained aluminium alloy processed by the consolidation of nano-powders obtained by mechanical alloying was evaluated via measurements of grain size and mechanical property changes upon annealing at various temperatures. It was found that the grain size of the samples containing carbon nanotubes is stable up to high temperatures and even after annealing at 450 °C (0.7T_m) no evident grain growth was observed. The limited grain boundary migration was attributed to the presence of entangled networks of carbon nanotubes located at grain boundaries and to the formation of nanoscale particles of aluminium carbide Al₄C₃. It was also revealed that carbon nanotubes decompose at a relatively low temperature of 450 °C and form fine Al₄C₃ precipitates. This transformation does not significantly affect the mechanical properties due to the nanoscale size of the carbides.     

Fabrication of magnesium based composites reinforced with carbon nanotubes having superior mechanical properties

Magnesium (Mg) composite reinforced with carbon nanotubes (CNTs) having superior mechanical properties was fabricated using both pure Mg and AZ61 Mg alloy matrix in this study. The composites were produced via powder metallurgy route containing wet process using isopropyl alcohol (IPA) based zwitterionic surfactant solution with unbundled CNTs. The produced composites were evaluated with tensile test and Vickers hardness test and analyzed by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM) equipped with energy dispersive spectroscopy (EDS) and electron back scattered diffraction (EBSD). As a result, only with AZ61 Mg alloy matrix, tensile strength of the composite was improved. In situ formed Al₂MgC₂ compounds at the interface between Mg matrix and CNTs effectively reinforced the interfacial bonding and enabled tensile loading transfer from the Mg matrix to nanotubes. Furthermore, it was clarified that the microstructures and grain orientations of the composite matrix were not significantly influenced by CNT addition.     

Preparation of porous alumina-carbon nanotube composites via direct growth of carbon nanotubes

A simple approach is reported for the in situ growth of carbon nanotube-containing porous alumina structures by a thermal pyrolysis method. The composite was created by direct on-site growth of carbon nanotubes inside the porous alumina matrix, after introducing both a catalyst (Ni(NO₃)₂) and a carbon source (camphor) into the cavities of the large matrix brick. Pyrolysis was carried out when the pre-treated brick was heated in a furnace at 850 °C under a H₂-Ar atmosphere. The resulting multi-walled carbon nanotubes with average diameters of 30-70 nm and lengths up to several micrometers are dispersed uniformly at each section of the alumina matrix. An improvement in the compression strength of the composites has been obtained, due to the inclusion of carbon nanotubes.     

Multi-walled carbon nanotubes reinforced Al2O3 nanocomposites: Mechanical properties and interfacial investigations

Well-dispersed multi-walled carbon nanotubes (CNTs) reinforced Al₂O₃ nanocomposites were successfully fabricated by hot-pressing. The resulting promising improvements in fracture toughness, by 94% and 65% with 2 and 5 wt.% CNTs addition respectively, compared with monolithic Al₂O₃, were attributed to the good dispersion of CNTs within the matrix, crack-bridging by CNTs and strong interfacial connections between the CNTs and the matrix. The interfacial phase characteristics between CNTs and Al₂O₃ were investigated via combined techniques. It is believed that a possible aluminium oxy-carbide as the primary interfacial phase was produced via a localized carbothermal reduction process. This interface phase presumably has good chemical compatibility and strong connections with both CNTs and the matrix and led nanocomposites to higher fracture toughness.     

The effect of carbon nanotubes microstructures on reinforcing properties of SWNTs/alumina composite

Alumina reinforced with 1 wt% single-wall carbon nanotubes (SWNTs) was fabricated by hot-pressing. The fracture toughness of SWNTs/Al₂O₃ composite reaches 6.40 ± 0.3 MPa m^1/2, which is twice as high as that of unreinforced alumina. Nanoindentation introduced controlled cracks and the damage were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SWNTs reinforcing mechanisms including CNT pullout, CNT fracture, CNT bridging and crack deflection were directly observed, and the relationship between carbon nanotubes microstructures in the matrix and mechanical properties was also discussed in detailed.     

Alumina powder assisted carbon nanotubes reinforced Mg matrix composites

Mg matrix composites reinforced by carbon nanotubes (CNTs)-Al₂O₃ mixture, which was synthesized by in situ growing CNTs over Al₂O₃ particles through chemical vapor deposition (CVD) using Ni catalyst, were fabricated by means of powder metallurgy process, followed by hot-extrusion. By controlling synthesis conditions, the as-grown CNTs over Al₂O₃ particles possessed high degree of graphitization, ideal morphology, higher purity and homogeneous dispersion. Due to the ‘vehicle’ carrying effect of micrometer-level A₂O₃, CNTs were easy to be homogeneously dispersed in Mg matrix under moderate ball milling. Meanwhile, Al₂O₃ particles as catalyst carriers, together with CNTs, play the roles of synergistic reinforcements in Mg matrix. Consequently, the Mg matrix composites reinforced by CNTs-Al₂O₃ mixture exhibited remarkable mechanical properties.     

Electrical conductivity and Raman imaging of double wall carbon nanotubes in a polymer matrix

Raman spectroscopy is used to access the dispersion state of DWNTs in a PEEK polymer matrix. The interaction of the outer tube with the matrix can be determined from the line shape of the Raman G band. This allows us to distinguish regions where the nanotubes are well dispersed and regions where the nanotubes are agglomerated. The percolation threshold of the electrical conductivity of the double wall carbon nanotubes (DWNTs)/PEEK nanocomposites is found to be at 0.2-0.3 wt%. We find a maximum electrical conductivity of 3 × 10⁻² S cm⁻¹ at 2 wt% loading. We detect nanotube weight concentrations as low as 0.16 wt% by Raman spectroscopy using a yellow excitation wavelength. We compare the Raman images with transmission electron microscopy images and electrical conductivity measurements. A statistical method is used to find a quantitative measure of the DWNTs dispersion in the polymer matrix from the Raman images.     

The influence of nano/micro hybrid structure on the mechanical and self-sensing properties of carbon nanotube-microparticle reinforced epoxy matrix composite

Nano/micrometer hybrids are prepared by chemical vapor deposition growth of carbon nanotubes (CNTs) on SiC, Al₂O₃ and graphene nanoplatelet (GNP). The mechanical and self-sensing behaviors of the hybrids reinforced epoxy composites are found to be highly dependent on CNT aspect ratio (AR), organization and substrates. The CNT–GNP hybrids exhibit the most significant reinforcing effectiveness, among the three hybrids with AR1200. During tensile loading, the in situ electrical resistance of the CNT–GNP/epoxy and the CNT–SiC/epoxy composites gradually increases to a maximum value and then decreases, which is remarkably different from the monotonic increase in the CNT–Al₂O₃/epoxy composites. However, the CNT–Al₂O₃ with increased AR ⩾ 2000 endows the similar resistance change as the other two hybrids. Besides, when AR < 3200, the tensile modulus and strength of the CNT–Al₂O₃/epoxy composites gradually increase with AR. The interrelationship between the hybrid structure and the mechanical and self-sensing behaviors of the composites are analyzed.     

Processing and mechanical properties of carbon nanotube-alumina hybrid reinforced high density polyethylene composites

Carbon nanotube-alumina hybrid reinforced high density polyethylene (HDPE) matrix composites were prepared by melt processing technique. Microstructure studies verified that the nanotubes consisting of well-crystallized graphite formed a network structure with Al₂O₃ in the hybrid, which was homogeneously dispersed in the HDPE matrix composites. Mechanical measurements revealed that 5% addition of nanotube-alumina hybrid results in 100.8% and 65.7% simultaneous increases in Young's modulus and tensile strength, respectively. Fracture surface showed homogenous dispersion of nanotubes and Al₂O₃ in the HDPE matrix and presence of interlocking like phenomena between hybrid and HDPE matrix, which might contribute to the effective reinforcement of the HDPE composites.     

Chemical stability of carbon nanotubes in the 2024Al matrix

Five volume percent of carbon nanotubes and 2024Al alloy powder were mixed with ball milling method, and then the composite was fabricated at 873 K by hot pressing sintering technique. The microstructure of the composite was investigated using optical microscopy, transmission electron microscopy, and X-ray diffraction. The experimental results showed that carbon nanotubes are reacted and changed into Al₄C₃. Nano-Al₄C₃ phases with needle shape are distributed mainly on Al grain boundaries; meanwhile some of them exist within Al grains. The reaction mechanism of carbon nanotubes-Al is discussed.     

Enhancement of tensile and thermal properties of epoxy nanocomposites through chemical hybridization of carbon nanotubes and alumina

This paper presents the properties of epoxy nanocomposites, prepared using a synthesized hybrid carbon nanotube–alumina (CNT–Al₂O₃) filler, via chemical vapour deposition and a physically mixed CNT–Al₂O₃ filler, at various filler loadings (i.e., 1–5%). The tensile and thermal properties of both nanocomposites were investigated at different weight percentages of filler loading. The CNT–Al₂O₃ hybrid epoxy composites showed higher tensile and thermal properties than the CNT–Al₂O₃ physically mixed epoxy composites. This increase was associated with the homogenous dispersion of CNT–Al₂O₃ particle filler; as observed under a field emission scanning electron microscope. It was demonstrated that the CNT–Al₂O₃ hybrid epoxy composites are capable of increasing tensile strength by up to 30%, giving a tensile modulus of 39%, thermal conductivity of 20%, and a glass transition temperature value of 25%, when compared to a neat epoxy composite.     

Fabrication and characterization of Al-matrix composites reinforced with amino-functionalized carbon nanotubes

We report the fabrication of Al-matrix composites reinforced with amino-functionalized carbon nanotubes (fCNTs) using powder metallurgy process. Functionalization of the nanotubes was carried out by ball milling multiwalled carbon nanotubes (MWCNTs) in the presence of ammonium bicarbonate. It has been found that the mechanical properties of Al-fCNT composites were much superior to the composites fabricated using non-functionalized or acid functionalized carbon nanotubes. The enhancement in mechanical properties in these composites are attributed mainly to the better and homogeneous dispersion of fCNT in Al matrix as compared to non-functionalized or acid functionalized carbon nanotubes and the formation of a strong interfacial bonding between fCNT and Al matrix leading to an efficient load transfer from Al matrix to fCNT following high-resolution transmission electron microscopy.     

Study on the phase change thermal storage performance of palmitic acid/carbon nanotubes composites

After oxidization with the mixed acid of H₂SO₄ and HNO₃, carbon nanotubes (CNTs) were further grafted by γ-(2,3-epoxypropoxy)propyltrimethoxysilane. Then, CNTs, oxidized CNTs and grafted CNTs were respectively dispersed into palmitic acid (PA) matrix to prepare phase change composites at a mass ratio of 1/100. According to the comparative analysis results of the FT-IR spectra, morphology, dispersion and latent heat of the three composites, it is discovered that the composite with grafted CNTs showed the greatest dispersion and tube–matrix adhesion, and thus gained the highest latent heat. It is worth mentioning that the composite with grafted CNTs demonstrated a more favorable latent heat and a 34.1% enhancement of thermal conductivity in comparison with PA. Moreover, the composite with grafted CNTs kept homogeneous after 100 times of melting and freezing, and the retention rate of latent heat can be as high as 98.5%.     

Structural characterization of a mechanically milled carbon nanotube/aluminum mixture

The structural evolution of carbon nanotubes (CNTs) during mechanical milling was investigated using SEM, TEM, XRD, XPS and Raman spectroscopy. The study showed that milling of the CNTs alone introduces defects but preserves the tubular structure. When milling the CNTs with aluminum (Al) powder in order to produce a composite, Raman spectroscopy has shown that most of the nanotubes are destroyed. During sintering of the CNT/Al milled mixture, the carbon atoms available from the destruction of the nanotubes react with the Al to form aluminum carbide (Al₄C₃). The effect of milling on the Al matrix was also studied.     

Synthesis of zirconia nanoparticles on carbon nanotubes and their potential for enhancing the fracture toughness of alumina ceramics

Nanoparticles of zirconia (ZrO₂) were in situ synthesized on the surface of carbon nanotubes by means of liquid phase reactions and a proper heat treatment process. The size of the nanoparticles could be controlled by the amount of zirconium source materials in a solution and its reaction times. In this study, the size of the nanoparticles ranged from several nanometers to twenty nanometers. It was particularly noted that the synthesized zirconia possessed a cubic structure (c-phase) which generally existed as a stable form of zirconia crystals at high temperatures (above 2370 °C) as well as a form of zirconia that could be used for enhancing the fracture toughness of alumina ceramics. Experimental results showed that the mechanical properties of alumina ceramics mixed with in situ synthesized nanoparticles on the surface of carbon nanotubes were much better than that of pristine nanotubes or zirconia nanoparticles alone. The existence of the nanoparticles on the surface of nanotubes results in improving the dispersion and bonding properties of the nanotubes in alumina matrix environment. The fracture toughness of CNT/ZrO₂ alumina ceramics was also improved by the mechanism of bridging effect.     

The chemical behaviour of carbon fibres in magnesium base Mg-Al alloys

The chemical interaction between carbon fibres and Mg-rich Mg-Al alloys was studied at 723–1273 K using optical metallography, scanning electron microscopy and electron probe microanalysis. In a first stage, carbon fibres were heated at 723, 1000 and 1273 K with Mg-Al alloys of different compositions. Two carbide phases were identified at 1000 and 1273 K: an Al₄C₃ type phase with up to 6 wt.%Mg present in solid solution and a new ternary carbide with the chemical formula Al₂MgC₂ existing under two hexagonal crystalline varieties. In a second stage, the solid-liquid phase equilibria in the Al-C-Mg ternary system were experimentally established at 1000 K. At that temperature, all the Mg-Al alloys containing more than 19 ± 2 wt.%Al were observed to be in equilibrium with the Al₄C₃ type phase whereas Mg-Al alloys containing from 0.6 to 19 wt.%Al were found in equilibrium with the ternary carbide Al₂MgC₂. As for the Mg-Al alloys with an Al content lower than or equal to 0.6 ± 0.2 wt.%, they appeared to be in equilibrium with carbon. These thermodynamic principles being established, the extent, morphology and composition of the reaction zones formed in out of equilibrium conditions at the interface between carbon fibres and Mg-rich Mg-Al alloys were characterized. Attempts were made to determine the influence of different factors such as the fibre nature, the alloy composition, the heating time and the heating temperature on the formation and growth of the ternary carbide Al₂MgC₂. The observed changes were interpreted in terms of reaction mechanism and kinetics.     

Properties of magnesium alloys reinforced with nanoparticles and carbon nanotubes: a review

Magnesium alloys suffer from only moderate high-temperature strength and creep resistance. Aluminium-free magnesium alloys for sand casting or alloys containing aluminium with expensive additional alloying elements may be in use, but only microparticle or microfibre-reinforced magnesium alloys really exhibit satisfactory creep strengths at temperatures up to 250 °C. Reinforcing magnesium alloys with ceramic nanoparticles could be a solution for preserving a low density while increasing the high-temperature performance. When produced using melting processes, nanoparticle-reinforced magnesium composites are expected to enjoy strengthening due to the grain refinement described in the Hall–Petch relation. When an isotropic distribution of nanoparticles is achieved, the composites are additionally expected to be Orowan-strengthened. In this review, a variety of ceramic materials, such as SiC, Al₂O₃, Y₂O₃, SiO₂ and carbon nanotubes were investigated for reinforcement. Pure magnesium and various magnesium alloys were chosen as the matrix material and both powder metallurgical (PM) and melting processes were used for production of the composites. The mechanical properties of the composites were generally enhanced, compared to an unreinforced alloy; not only at room temperature, but also at elevated temperatures. In some cases an increase in strength in combination with increased ductility was also identified.     

Preparation of ceria nanoparticles supported on carbon nanotubes

By chemical reaction of CeCl₃ and NaOH on carbon nanotube solution and subsequent heat treatment, ceria nanoparticles supported on carbon nanotubes were prepared. The processing parameters affecting the size of ceria particles were discussed. The particles were characterized by XRD and TEM. XRD patterns revealed that the particles exhibited CaF₂-type crystal structure. The TEM micrograph showed that the mean sizes of ceria particles were about 6 nm.     

Micro-structural and tensile strength analyses on the magnesium matrix composites reinforced with coated carbon fiber

Magnesium matrix composites reinforced with SiO₂ coated carbon fibers have been investigated, with an emphasis given on the relation between the material strength and interfacial microstructure. The composites were studied as a function of aluminium (Al) content that is varied between 0 and 9 wt%. The obtained results indicate that the reactivity at the C/Mg–Al interface of the composite can be controlled by varying the Al content. The low Al content in C/Mg–1Al has been completely dissolved in the matrix with no segregation even after solidification, leading to the best mechanical performance. If the Al content is increased to ≥3 wt% (composites such as C/AZ31 and C/AZ91), the SiO₂ coatings are fully depleted due to an extensive formation of carbides at the interface. The precipitates are further identified as Al₂MgC₂ phase that is similar to binary carbide Al₄C₃. SiO₂ coating on the fiber layer prior to fabrication of composite is found to be a promising way to suppress the carbide formation and enable the use of Mg–Al matrix with appropriate Al content.     

Multi-scale electrical response of silicon nitride/multi-walled carbon nanotubes composites

Dense silicon nitride (Si₃N₄) composites with various amounts (0-8.6 vol%) of multi-walled carbon nanotubes (MWCNTs) are electrically characterised by combining macroscopic dc-ac and nanoscale conductive scanning force microscopy (C-SFM) measurements. In this way, a coherent picture of the dominant charge transport mechanisms in Si₃N₄/MWCNTs composites is presented. A raise of more than 10 orders of magnitude in the electrical dc conductivity compared to the blank specimen is measured for MWCNTs contents above 0.9 vol%. Semiconductor and metallic-like behaviours are observed depending on both the temperature and the MWCNTs content. Macroscopic measurements are further supported at the nanoscale by means of C-SFM. The metallic-type conduction is associated to charge transporting along the nanotube shells, whereas the semiconductor behaviour is linked to hopping conduction across nanotube-nanotube contacts and across intrinsic defect clusters within the nanotubes.