Electrically conductive alumina–carbon nanocomposites prepared by Spark Plasma Sintering

Carbon nanotubes (CNTs) and carbon black were added to alumina to convert it into a good electrical conductor. Alumina–CNT and alumina–carbon black nanocomposites were fabricated by Spark Plasma Sintering (SPS). The electrical conductivity of alumina–CNT nanocomposites was found to be four times higher as compared to alumina–carbon black nanocomposites due to the fibrous nature and high aspect ratio of CNTs. The electrical conductivity of alumina–CNT nanocomposite increased with increasing grain size due to increasing density of CNTs at the grain boundaries. This effect was not observed for alumina–carbon black nanocomposite due to the particulate geometry of the carbon black.     

OH and COOH functionalized single walled carbon nanotubes-reinforced alumina ceramic nanocomposites

Alumina ceramics reinforced with 1 wt.% single-walled carbon nanotube (SWCNT) were fabricated via spark plasma sintering (SPS) of composite powders containing carboxyl (COOH) or hydroxyl (OH) group functionalized single-walled carbon nanotubes. The samples were SPS’ed at 1600 °C under 50 MPa pressure for holding time of 5 min and at a heating rate of 4 °C/s. The effects of CNT addition having different surface functional groups on microstructure, conductivity, density and hardness were reported. It was shown that nanotube addition decreased the grain sizeof alumina from 3.17 μm to 2.11 μm for COOH-SWCNT reinforcement and to 2.28 μm for COOH-SWCNT reinforcement. The hardness values of the composites are similar for all samples but there is 4.5 and 7.5 times increase in electrical conductivity with respect to monolithic alumina for COOH-SWCNT and OH-SWCNT, respectively. It was also shown by TEM and FEG SEM observations that transgranular fracture behaviour of alumina was changed to mostly intergranular fracture mode by the addition of both types of CNTs which may be due to location of CNTs along the grain boundaries. A significant grain size reduction in alumina is considered toresult fromthe suppressing effect of CNTs during sintering.     

Carbon Nanotube Reinforced Alumina-Based Ceramics with Novel Mechanical, Electrical, and Thermal Properties

The extraordinary mechanical, thermal, and electrical properties of single-wall carbon nanotubes (SWCNT) have prompted the development of advanced engineering materials with improved properties through the incorporation of carbon nanotubes in selected matrices. Dense SWCNT reinforced alumina nanocomposites have been fabricated by novel spark-plasma-sintering (SPS) technique. SWCNT were also successfully used to convert insulating nanoceramics to metallically conductive composites. Additionally, novel anisotropic thermal properties have been observed in the carbon nanotube composites. Such multifunctional carbon nanotube/ceramic composites with improved mechanical and electrical properties along with anisotropic thermal properties are envisaged for a wide range of applications.     

Mechano-tribological performance of Graphene/CNT reinforced alumina nanocomposites – Review and quantitative insights

CNTs and graphene have revolutionized the microstructural design of metals, polymers and ceramics, especially in structural and anti-friction applications. This article comprehensively reviews the progress in mechano-tribological performance of graphene/CNT-reinforced alumina nanocomposites against the backdrop of synergistic interplay between traditional mechanisms (toughening by crack bridging, pull-out etc.; lubrication by thin film formation) and novel phenomena unique to this class of composites (toughening by complex interlocking and slip-stick pull-out; lubrication by morphology transformation and sliding-rolling action). We have identified unprecedented correlations between nanocarbon content and enhancement in mechanical properties, and deduced the definitive ranges of inclusion for which 1D CNTs and 2D graphene trigger application-specific optimal mechanical response in alumina. The assumption of high dependence of wear behaviour on mechanical properties is quantitatively assessed to reveal that strengthening and toughening must proceed in specific proportions to minimize wear. Lastly, existing challenges, their potential solutions and exciting future research directions are discussed.     

Synthesis,microstructure and electrical conductivity of carbon nanotube–alumina nanocomposites

Carbon nanotube–alumina (CNT–Al₂O₃) nanocomposites have been synthesized by direct growth of carbon nanotubes on alumina by chemical vapor deposition (CVD) and the as-grown nanocomposites were densified by spark plasma sintering (SPS). Surface morphology analysis shows that the CNTs and CNT bundles are very well distributed between the matrix grains creating a web of CNTs as a consequence of their in situ synthesis. Even after the SPS treatment, the CNTs in the composite material are still intact. Experimental result shows that the electrical conductivity of the composites increases with the CNT content and falls in the range of the conductivity of semiconductors. The nanocomposite with highest CNT content has electrical conductivity of 3336 S/m at near room temperature, which is about 13 orders of magnitude increase over that of pure alumina.     

Reaction sintering of alumina/mullite nanocomposites from nano-sized starting powders

Alumina/mullite ceramic nanocomposites were prepared by the mixtures of nano-sized starting powders of alumina with silica and alumina with silicon carbide. Silica from deliberate addition and as the product of silicon carbide oxidation reacted completely with alumina to form mullite. Silica from direct addition segregated at the grain boundary and intergranular mullite was formed whereas silica from oxidation was surrounded by alumina matrix and intragranular mullite was formed after reaction sintering. The most significant difference was fracture behaviour where intragranular mullite nanoparticles promoted transgranular fracture in alumina matrix due to thermal mismatch around nanoparticles and intergranular mullite nanoparticles gave rise to intergranular fracture similar to pure alumina. Wear resistance of the nanocomposites was better than that of alumina. Pull-out formation in the nanocomposites was less and pull-out size was also smaller. Fracture toughness of the nanocomposites was significantly higher than that of alumina.     

The bulk polymerisation of N-vinylcarbazole in the presence of both multi- and single-walled carbon nanotubes: A comparative study

The bulk polymerisation of N-vinylcarbazole (NVC) at an elevated temperature in the presence of both multi- and single-walled carbon nanotubes (CNTs) leads to the formation of two different types of composite materials, the morphology and properties of which were characterised by a field emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis, and electrical property measurements. The efficiency of CNTs to initiate the NVC polymerisation was investigated using both multi-walled CNTs (MWCNTs) and single-walled CNTs (SWCNTs). The focus was on three major aspects: the degree of polymerisation, the morphology and the properties of the resulting nanocomposite materials. Results showed that SWCNTs were more efficient in initiating NVC polymerisation than MWCNTs, and the morphology of resultant nanocomposites revealed wrapping and grafting of some poly(N-vinylcarbazole) (PNVC) chains on the SWCNT surfaces. The morphology of the PNVC/MWCNT nanocomposites showed only homogeneous wrapping of the outer surfaces of MWCNTs by PNVC chains. The direct current (dc) electrical conductivity of pure PNVC improved dramatically in the presence of both MWCNTs and SWCNTs, however, the extent of improvement is higher in the case of PNVC/MWCNT nanocomposites.     

Nonisothermal melt crystallization kinetics of syndiotactic polypropylene/alumina nanocomposites

Nonisothermal melt crystallization kinetics of syndiotactic polypropylene (sPP)/alumina nanocomposites were investigated via differential scanning calorimetry. The addition of alumina nanoparticles significantly increases the number of nuclei and promotes the crystallization rate of sPP. Nonisothermal melt crystallization kinetics was analyzed by fitting the experimental data to a Nakamura model using Matlab. The average values of Avrami exponent n are 1.7 for both sPP and sPP/Al₂O₃ nanocomposites during slow cooling, which implies a two‐dimensional growth is the predominant mechanism of crystallization following a heterogeneous nucleation. The two nanocomposites give n values equal to 2.3 during faster cooling, indicating that the main growth type taking place for sPP/alumina nanocomposites is also the two‐dimensional growth. The subsequent melting behavior shows that the presence of alumina nanoparticles changed both the cold crystallization and the recrystallization of sPP. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Preparation-microstructure-property relationships in double-walled carbon nanotubes/alumina composites

Double-walled carbon nanotube/alumina composite powders with low carbon contents (2–3 wt.%) are prepared using three different methods and densified by spark plasma sintering. The mechanical properties and electrical conductivity are investigated and correlated with the microstructure of the dense materials. Samples prepared by in situ synthesis of carbon nanotubes (CNTs) in impregnated submicronic alumina are highly homogeneous and present the higher electrical conductivity (2.2–3.5 S cm⁻¹) but carbon films at grain boundaries induce a poor cohesion of the materials. Composites prepared by mixing using moderate sonication of as-prepared double-walled CNTs and lyophilisation, with little damage to the CNTs, have a fracture strength higher (+30%) and a fracture toughness similar (5.6 vs 5.4   MPa m^1/2) to alumina with a similar submicronic grain size. This is correlated with crack-bridging by CNTs on a large scale, despite a lack of homogeneity of the CNT distribution.     

Highly Dispersive Carbon Nanotube/Alumina Composites and their Electrospun Nanofibers

Noncovalent functionalization approach was used to make negatively charged carbon nanotubes (CNTs) suspension in water medium. Stable alumina/CNT sol solution was consequently formed by simple titration process, which allowed high dispersibility of CNTs in final composite powders and final CNT/alumina (CA) nanocomposites. Moreover, these CA composite powders were transformed to continuous composite fibers via electrospinning technique. Very significant shear stresses exerted on alumina were well recorded and reflected by shifts of X-ray diffraction peaks. The as-fabricated CA fibers possess a specific surface area 30 times larger than that of pure poly(acrylic acid) fibers. The axial alignment of CNTs observed in the composite fibers is promising for developing textured ceramics, and also for the study of the effect of orientation of fillers on the mechanical and functional properties.     

Electrical conductivity and transparency of polymer hybrid nanocomposites based on poly(trimethylene terephthalate) containing single walled carbon nanotubes and expanded graphite

Single‐walled carbon nanotubes (SWCNT)/expanded graphite (EG)/poly(trimethylene terephthalate) (PTT) hybrid nanocomposites were prepared via in situ polymerization. Raman spectroscopy and scanning electron microscopy (SEM) were employed to determine both, purity and morphology of the nanofillers and the dispersion of nanotubes and nanosheets. The electrical and optical properties of thin polymer films based on both “single” nanocomposites and hybrid nanocomposites were studied. For PTT/SWCNT nanocomposites, results confirmed that films optical transmittance decreases as the concentration of SWCNT increases, attaining almost no optical transmittance for 0.3 wt % of nanofiller. Conversely, the electrical conductivity of nanocomposites was found to increase by increasing the nanofiller amount and the σ_dc values indicate that percolation occurs at a very low SWCNT content (around 0.05 wt %). In the case of PTT/SWCNT + EG nanocomposites, when the content of SWCNT is 0.05%, the hybrid system presents lower conductivity than that corresponding to the “single” nanocomposite. The incorporation of additional EG to the PTT/SWCNT nanocomposite has a small effect on the electrical conductivity but inhibits the transparency of the system. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44370.     

Effect of particle morphology on viscoelastic and flexural properties of epoxy–alumina polymer nanocomposites

Nanocomposites were synthesised by dispersing two different types of alumina nanoparticles in epoxy matrix by ultrasonication. Alumina nanoparticles of two shapes, rod and spherical were selected to investigate the effect of particle morphology on viscoelastic and flexural properties of nanocomposites. Specific surface area of both the selected nanoparticles was kept in the similar range. Good dispersion of nanoparticles was observed through transmission electron microscopy. The addition of nanoparticles in epoxy had significant enhancement in the viscoelastic properties and moderate improvement in flexural properties of composites. Composites having alumina nanorods showed higher improvement both in storage modulus as well as in flexural properties in comparison to composites having spherical alumina nanoparticles. Efficacy of Mori-Tanaka method was explored in modelling storage modulus of nanocomposites. Assorted size of alumina nanorods based on particle size distribution was used to model composites with nanorods to see the effect of size assortment on storage modulus.     

Mechanical properties and multiscale characterization of nanofiber–alumina/epoxy nanocomposites

In contrast to the bulk of published nanocomposite studies, in this study we investigated the mechanical properties of alumina/epoxy nanocomposites manufactured with nanofillers having a fiber or whisker morphology. The article describes how ultrasonic dispersion and in situ polymerization were used to incorporate these 2–4 nm diameter fibers (with aspect ratios of 25–50) into a two‐part epoxy resin (Epon 826/Epicure 9551). The use of untreated and surface‐modified nanoparticles is contrasted, and improvements in both the tensile strength and modulus were observed at low filler loadings. Microstructural characterization of the nanocomposites via multiscale digital image analysis was used to interpret the mechanical properties and was found to be useful for direct comparison with other nanocomposites. In addition, superior performance was demonstrated through comparisons with numerous nanocomposites with nanoparticle reinforcements ranging from carbon nanofibers to spherical alumina particles. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

The effect of nanoalumina silanisation in tetraglycidylether epoxy adhesive

The efficiency of different surface modifications on alumina nanoparticles on both filler dispersion and the final properties of the resulting adhesive nanocomposites have been investigated. A tetraglycidyldiaminodiphenylmethane (TGDDM) epoxy resin and three sample series of nanocomposites were prepared via in-situ incorporation of alumina nanoparticles into the reactor. The alumina/TGDDM nanocomposites were prepared individually using neat or non-treated alumina nanoparticles and two kinds of silane-grafted alumina nanoparticles, i.e., APS-treated alumina and GPS-treated alumina. The presence of different alumina nanoparticles in the epoxy matrices resulted in different states of nanofiller dispersion as revealed in SEM and AFM micrographs. It was elucidated that the silane treatment on alumina nanoparticles is crucial for the desired dispersion in the epoxy matrix. Besides, the appropriate filler dispersion resulted in improved thermal resistance and high degree of cure, especially for the adhesive nanocomposite containing APS-treated alumina nanoparticles. In adhesion tests, the shear strength was improved in both nanocomposites containing silane-grafted alumina with more pronounced values for the nanocomposite containing APS-treated alumina nanoparticles. The shear strength reached from 6.6 MPa for the neat epoxy adhesive to 10.2 MPa for the adhesive nanocomposite containing 5 wt % APS-treated alumina nanoparticles mainly due to high levels of dispersion of the high modulus alumina nanoparticles and effective interfacial interactions with the epoxy matrix. The adhesive peel strength of alumina/TGDDM nanocomposites showed a similar trend as in shear strength with more pronounced variations. A noticeable increase in the peel strength of the nanocomposites containing silane-grafted alumina nanoparticles appeared to correlate with greater levels of crack deflection and hence dissipation of fracture energy as observed in SEM pictures.     

Nano-functionalization of carbon-bonded alumina using graphene oxide and MWCNTs

This paper focused on the nano-functionalization of carbon-bonded alumina using graphene oxide (GO) and multi-walled carbon nanotubes (MWCNTs). GO was prepared according to the modified Tour's method. The rheology of suspensions containing GO, CNTs and both substances were analyzed and discussed. Xanthan proved to be a suitable stabilizer for the three systems. The spraying process of the suspensions was investigated with the aid of a high-speed camera. Al₂O₃-C filters and flat samples were spray coated, for investigations in contact with steel. The ceramic foam filters as well as the starting suspensions were analyzed by several analytic techniques to investigate the microstructure and other properties. Moreover, the hot stage microscope was used to study the steel/coating wetting behavior during operation at high temperatures. The results showed that the formulation containing both GO and CNTs delivered the best performance in contact with the melt. Similar coatings based on these nano-sized materials may offer an innovative route to improve purification of steel melts by filtration.     

Advanced carbon nanotube/polymer composite infrared sensors

We demonstrate that both single-walled carbon nanotube (SWCNT) types and nanotube-matrix polymer-nanotube (CNT-P-CNT) junctions have profound impact on electro-optical properties of SWCNT/polymer composites. Composite IR sensors based on CoMoCAT^®-produced SWCNTs (SWCNTs_CoMoCAT) significantly outperform those based on HiPco^®-produced SWCNTs (SWCNTs_HiPco). Higher semiconducting nanotube concentration in a SWCNT material is critical to enhance the photo effect of IR light on SWCNT/polymer nanocomposites, whereas CNT-P-CNT junctions play a dominant role in the thermal effect of IR light on supported SWCNT/polymer composite films.     

Sliding wear behaviour of alumina/nickel nanocomposites processed by a conventional sintering route

The wear resistance of Al₂O₃/2.5 vol.% Ni nanocomposites sintered by a conventional route was studied under ball-on-disk dry sliding conditions and compared with the same nanocomposites but consolidated by spark plasma sintering, together with alumina obtained by the same technique and by hot pressing. The results showed an improvement of about 0.5, 1 and 2 orders of magnitude, respectively. Thus, alumina/Ni nanocomposites processed by conventional route can compete, in cost and wear performance, with nanomaterials obtained by more sophisticated techniques.     

Morphology of syndiotactic polypropylene/alumina nanocomposites

The effect of the addition of 50 nm spherical alumina nanoparticles with hydrophilic or hydrophobic surfaces on the morphology of syndiotactic polypropylene (sPP) was investigated. The filler content in the nanocomposites was 3 wt%. Polarized Optical Microscopy and Small Angle Light Scattering (SALS) studies showed that sPP and the nanocomposites form hedrites. The addition of alumina nanoparticles significantly increased the number of hedrites. A higher number of nucleation sites in the nanocomposites promote a higher crystallization rate, and thus hedritic growth was stopped at the early stage of crystallization. Quantitative evaluations by SALS analysis show that the object size is decreased by not only the crystallization conditions but also the presence of Al₂O₃ nanoparticles. A small amount of Al₂O₃ nanoparticles did not noticeably affect the crystallinity of sPP, but increased the melting point. Transmission Electron Microscopy images showed that the lamellar thickness did not change significantly with the incorporation of nanoparticles. The lamella thickness, however, depends on cooling rates. X-ray diffraction characterization indicated that the sPP and the nanocomposites were crystallized in disordered “form I”. The incorporation of alumina nanoparticles had a small effect on the crystal structure of syndiotactic polypropylene.     

High-crystallization polyoxymethylene modification on carbon nanotubes with assistance of supercritical carbon dioxide: Molecular interactions and their thermal stability

As a typical engineering plastic and high-crystallization polymer, polyoxymethylene (POM) has been successfully wrapped on single-walled carbon nanotubes (SWCNTs) using a simple supercritical carbon dioxide (SC CO₂) antisolvent-induced polymer epitaxy method. The characterization results of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) reveal that the SWCNTs are coated by laminar POM with the thicknesses of a few nanometers. The polymer adsorption on CNTs via multiple weak molecular interactions of CH groups with CNTs has been identified with FTIR and Raman spectroscopy. The experimental results indicate that the decorating degree of POM on the surface of CNTs increases significantly with the increase of SC CO₂ pressure, and accordingly the dispersion of SWCNT modified by POM at higher pressure are more excellent than that of obtained at lower pressure. Further the processing stability of POM/CNTs composites are investigated by differential scanning calorimetry and thermogravimetric analysis. The experimental results obtained show that their thermal stability behavior is closely related to surface properties of CNTs. Apparently, the composites with POM-decorating SWCNTs as the filler shows higher melting points compared to the POM composites with pristine SWCNTs as the filler. Therefore, we anticipate this work may lead to a controllable method making use of peculiar properties of SC CO₂ to help to fabricate the functional CNTs-based nanocomposites containing highly crystalline thermoplastic materials such as POM.     

Rheology and dynamic mechanical analysis of bisphenol E cyanate ester/alumina nanocomposites

Alumina nanoparticles were functionalized with 3‐glycidyloxypropyl trimethoxysilane for compatibility with a low viscosity bisphenol E cyanate ester (BECy) resin. The functionalized alumina nanoparticles were characterized with Fourier transform infrared and thermogravimetric analysis. The alumina nanoparticles, which increase the viscosity of the BECy/alumina suspension, show a concurrent catalytic effect on the cure of the BECy resin, as indicated by reduced gelation times under isothermal cure conditions. Transmission electron microscopy micrographs reveal that most of the alumina nanoparticles are well dispersed in the BECy matrix, but a small fraction of particles formed agglomerates. The thermal‐mechanical properties of cured BECy composites reinforced with either bare alumina or functionalized alumina are evaluated by dynamic mechanical analysis. The storage modulus increases with both bare and functionalized alumina loading. Although the glass transition temperatures (T_g) of bare and functionalized alumina/BECy nanocomposites decrease with increasing filler content, the reduction in T_g is less severe when the alumina nanoparticles are first functionalized. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers