Grain Growth Behavior of Aluminum Oxide Reinforced with Carbon Nanotube During Plasma Spraying and PostSpray Consolidation

Grain-boundary mobility of the plasma sprayed aluminum oxide (Al₂O₃)–carbon nanotube (CNT) composites is evaluated in the current work. Grain mobility is evaluated from the grain growth within the spray-dried particles and thermal history experienced during high-temperature plasma processing. CNTs form an interfacial grain boundary layer during thermal exposure, limiting the grain growth of plasma-sprayed coatings. Consequent hot isostatic pressing (HIPing) of CNT-reinforced Al₂O₃ at 1773 K shows differences in grain growth kinetics, degree of densification, and pore shrinkage. Densification of HIPed coatings is observed to be dictated by CNTs, phase transformation, initial grain size, and time of thermal processing. CNTs have shown to impede the Al₂O₃ grain growth by serving as grain pinning obstacles. Impediment of grain-boundary mobility with variation of CNT content, and time and temperature of the heat treatment of aluminum oxide (Al₂O₃)–CNT nanocomposite is addressed in detail.     

Electrophoretic deposition of carbon nanotube–ceramic nanocomposites

The purpose of this paper is to present an up-to-date comprehensive overview of current research progress in the development of carbon nanotube (CNT)–ceramic nanocomposites by electrophoretic deposition (EPD). Micron-sized and nanoscale ceramic particles have been combined with CNTs, both multiwalled and single-walled, using EPD for a variety of functional, structural and biomedical applications. Systems reviewed include SiO₂/CNT, TiO₂/CNT, MnO₂/CNT, Fe₃O₄/CNT, hydroxyapatite (HA)/CNT and bioactive glass/CNT. EPD has been shown to be a very convenient method to manipulate and arrange CNTs from well dispersed suspensions onto conductive substrates. CNT–ceramic composite layers of thickness in the range <1–50 μm have been produced. Sequential EPD of layered nanocomposites as well as electrophoretic co-deposition from diphasic suspensions have been investigated. A critical step for the success of EPD is the prior functionalization of CNTs, usually by their treatment in acid solutions, in order to create functional groups on CNT surfaces so that they can be dispersed uniformly in solvents, for example water or organic media. The preparation and characterisation of stable CNT and CNT/ceramic particle suspensions as well as relevant EPD mechanisms are discussed. Key processing stages, including functionalization of CNTs, tailoring zeta potential of CNTs and ceramic particles in suspension as well as specific EPD parameters, such as deposition voltage and time, are discussed in terms of their influence on the quality of the developed CNT/ceramic nanocomposites. The analysis of the literature confirms that EPD is the technique of choice for the development of complex CNT–ceramic nanocomposite layers and coatings of high structural homogeneity and reproducible properties. Potential and realised applications of the resulting CNT–ceramic composite coatings are highlighted, including fuel cell and supercapacitor electrodes, field emission devices, bioelectrodes, photocatalytic films, sensors as well as a wide range of functional, structural and bioactive coatings.     

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.     

Enhanced electrical properties of polycarbonate/carbon nanotube nanocomposites prepared by a supercritical carbon dioxide aided melt blending method

Polycarbonate/carbon nanotube (CNT) nanocomposites were generated using a supercritical carbon dioxide (scCO₂) aided melt blending method, yielding nanocomposites with enhanced electrical properties and improved dispersion while maintaining the aspect ratio of the as-received CNTs. Baytubes^® C 150 P CNTs were benignly deagglomerated with scCO₂ resulting in 5 fold (5X), 10X and 15X decreases in bulk density from the as-received CNTs. This was followed by melt compounding with polycarbonate to generate the CNT nanocomposites. Electrical percolation thresholds were realized at CNT loading levels as low as 0.83 wt% for composites prepared with 15X CNT using the scCO₂ aided melt blending method. By comparison, a concentration of 1.5 wt% was required without scCO₂ processing. Optical microscopy, transmission electron microscopy, and rheology were used to investigate the dispersion and mechanical network of CNTs in the nanocomposites. The dispersion of CNTs generally improved with scCO₂ processing compared to direct melt blending, but was significantly worse than that of twin screw melt compounded nanocomposites reported in the literature. A rheologically percolated network was observed near the electrical percolation of the nanocomposites. The importance of maintaining longer carbon nanotubes during nanocomposite processing rather than focusing on dispersion alone is highlighted in the current efforts.     

Morphology,mechanical properties and electromagnetic shielding effectiveness of poly(styrene‐b‐ethylene‐ran‐butylene‐b‐styrene)/carbon nanotube nanocomposites: effects of maleic anhydride,carbon nanotube loading and processing method

Nanocomposites based on poly(styrene‐b‐ethylene‐ran‐butylene‐b‐styrene) (SEBS) and carbon nanotubes (CNTs) (SEBS/CNT) as well as SEBS grafted with maleic anhydride (SEBS‐MA)/CNT were successfully prepared for electromagnetic shielding applications. Both SEBS/CNT and SEBS‐MA/CNT nanocomposites were prepared by melt compounding and were post‐processed using two different techniques: tape extrusion and compression moulding. The different nanocomposites were characterized by Raman spectroscopy and rheological analysis. Their mechanical properties, electrical properties (10^-2–10⁵ Hz) and electromagnetic shielding effectiveness (8.2–12.4 GHz) were also evaluated. The results showed that the CNT loading amount, the presence of MA in the matrix and the shaping technique used strongly influence the final morphologies and properties of the nanocomposites. Whilst the nanocomposite containing 8 wt% CNTs prepared by compression moulding presented the highest electromagnetic shielding effectiveness (with a value of 56.73 dB, which corresponds to an attenuation of 99.9996% of the incident radiation), the nanocomposite containing 5 wt% CNTs prepared by tape extrusion presented the best balance between electromagnetic and mechanical properties and was a good candidate to be used as an efficient flexible electromagnetic interference shielding material. © 2018 Society of Chemical Industry     

On the manufacturing and electrical and mechanical properties of ultra-high wt.% fraction aligned MWCNT and randomly oriented CNT epoxy composites

The effect of CNT orientation on electrical and mechanical properties is presented on the example of an ultra-high filler loaded multi-walled carbon nanotube (68 wt.% MWCNTs) epoxy-based nanocomposite. A novel manufacturing method based on hot-press infiltration through a semi-permeable membrane allows to obtain both, nanocomposites with aligned and randomly oriented CNTs (APNCs and RPNCs) over a broad filler loading range of ≈10–68 wt.%. APNCs are based on low-defected, mm-long aligned MWCNT arrays grown in chemical vapour deposition (CVD) process. Electrical conductivity and mechanical properties were measured parallel and perpendicular to the direction of CNTs. RPNCs are based on both, aligned mm-long MWCNTs and randomly oriented commercial μm-long and entangled MWCNTs (Baytube C150P, and exemplarily Arkema Graphistrength C100). The piezoresistive strain sensing capability of these high-wt.% APNCs and RPNCs had been investigated towards the influence of CNT orientations. For the highest CNT fraction of 68 wt.% of unidirectional aligned CNTs a Young’s modulus of E_|| ≈ 36 GPa and maximum electrical conductivity of σ_|| ≈ 37·10⁴ S/m were achieved.     

Mechanical properties and electrical conductivity in a carbon nanotube reinforced silicon nitride composite

The influence of carbon nanotubes (CNTs) addition on basic mechanical, thermal and electrical properties of the multiwall carbon nanotube (MWCNT) reinforced silicon nitride composites has been investigated. Silicon nitride based composites with different amounts (1 or 3 wt%) of carbon nanotubes have been prepared by hot isostatic pressing. The fracture toughness was measured by indentation fracture and indentation strength methods and the thermal shock resistance by indentation method. The hardness values decreased from 16.2 to 10.1 GPa and the fracture toughness slightly decreased by CNTs addition from 6.3 to 5.9 MPa m^1/2. The addition of 1 wt% CNTs enhanced the thermal shock resistance of the composite, however by the increased CNTs addition to 3 wt% the thermal shock resistance decreased. The electrical conductivity was significantly improved by CNTs addition (2 S/m in 3% Si₃N₄/CNT nanocomposite).     

Enhanced phase stability in hydroxylapatite/zirconia composites with hot isostatic pressing

Hydroxylapatite (HA) composites with pure zirconia (ZrO₂), and 3 and 8% Y₂O₃ doped ZrO₂ were pressure-less sintered in air and hot isostatically pressed (under 120 MPa gas pressure) at 1100 °C for 2 h. The reactions and phase transformations were monitored by X-ray diffraction, thermal analysis, and Raman spectroscopy. HA/pure ZrO₂ composites were not thermally stable in air sintering; HA dissociated into α and β tricalcium phosphate while monoclinic ZrO₂ was transformed into tetragonal and cubic phases. No decomposition in HA or phase transformation in ZrO₂ were observed in hydroxylapatite/3% Y₂O₃ doped ZrO₂ or HA/8% Y₂O₃ doped ZrO₂ composites. On the other hand, HA and ZrO₂ phases in hot isostatically pressed composites remained stable. The highest densification was found in a composite initially containing 10% monoclinic ZrO₂ among the composites sintered in air. The densification of the composites decreased at lower sintering temperatures and higher ZrO₂ contents upon air-sintering. The HIPped composites were densified to about 99.5% of theoretical densities in all mixing ratios. The reactivity between ZrO₂ and HA was dependent on the amount of air in the sintering environment. Hot isostatic pressing with very limited retained air was proved to be a very convenient method to insure both phase stability and full densification during the production of hydroxylapatite zirconia composites.     

Properties of Waterborne Polyurethane/CNT Nanocomposite Adhesives: Effect of Countercations

Two series of waterborne polyurethane (WBPU)/carbon nanotube (CNT) nanocomposites were prepared with various CNT contents (0–1.50 wt%). We used a metal-hydroxide (copper hydroxide, Cu(OH)₂) and amine (triethylamine, TEA) as the countercation in the nanocomposites. The interaction of the countercations with the CNTs in the nanocomposite was characterized by TEM, and the interaction effects on the properties, such as the glass transition temperature (T_g), storage modulus, tensile strength, Young's modulus and adhesive strength, were investigated. The CNTs were homogeneously (optimum) dispersed at concentrations of up to 1.25 and 1.00 wt% for the metal-hydroxide and amine series, respectively. At the optimum CNT content, the tensile strength and adhesive strength were maximized in each series. However, the adhesive strength of the WBPU/CNT nanocomposite with the metal-hydroxide countercation was less affected than with the amine-countercation after immersing the adhesive bonded nylon fabrics in water (for up to 48 h).     

Magnetic Fe2O3/CNT nanocomposites: Characterization and photocatalytic application towards the degradation of Rose Bengal dye

In this work, hybrid nanocomposite materials for the wastewater treatment via photocatalysis have been developed by combining multi walled carbon nanotubes (MWCNT) and hematite (α-Fe₂O₃).A straightforward strategy via sonication method has been used to prepare theα-Fe₂O₃/CNT nanocomposites with varying CNT content (5%, 7.5%, and 10%)and characterized by X-ray Diffractometer (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), and UV–Vis. spectrophotometer. XPS spectra was used to identify the defects/oxygen vacancies in the α-Fe₂O₃ lattice. TEM revealed the well deposition of α-Fe₂O₃ nanoparticles on the CNT surface. α-Fe₂O₃/CNT 10% nanocomposites have higher photocatalytic activity with 87% degradation of Rose Bengal dye in 90 min. The increased photocatalytic activity can be attributed to the synergistic contribution of α-Fe₂O₃ and CNTs, which inhibits photo-generated charge carrier recombination and the formation of highly active radical species (OH^• radicals, and O₂^• radicals) on the surface of CNTs. This research may be useful not only for understanding the photocatalytic mechanism, but also for developing efficient photocatalysts for the organic pollutant degradation.     

Fabrication of a flower-like Cu(OH)2 nanoarchitecture and its composite with CNTs for use as a supercapacitor electrode

Fabrication of nanostructured electro-active materials with an ordered organization improved the overall performance of supercapacitor devices (SCDs). In this spirit, we developed Cu(OH)₂ nano-flakes that were statistically ordered to resemble flowers. To increase the specific capacitance and kinetics of the electroactive sample, we employed ultra-sonication to fabricate a Cu(OH)₂ nanocomposite with conductive and capacitive carbon nanotubes (CNTs). The textural and functional group analyses of the wet-chemically produced samples were completed using the XRD and FTIR techniques. I–V, FESEM, and EDX measurements Analyses of pure Cu(OH)₂ and its CNT-based nanocomposites were conducted to evaluate the materials' electrical conductivity, morphology, and chemistry, respectively. The electrochemical characteristics of the as-prepared material's electrodes were investigated, and the CNT-based nanocomposite electrode demonstrated an outstanding specific capacity (Csp) and a promising rate of performance. Our CNT-based nanocomposite had a Cs of 733 Fg^-1 at 1 Ag^-1 and dropped 8.7% after 4 × 10³ cycles. The higher electrochemical properties of the nanocomposite are governed by the nano-flakes-like architecture of the Cu (OH)₂ and the more conductive CNT matrix. According to the obtained findings, our manufactured Cu(OH)₂/CNT based electrode has great promise for practical applications in next-generation supercapacitor, which are known to be very efficient.     

Mechanical characterisation and hydrothermal degradation of Al2O3-15 vol% ZrO2 nanocomposites consolidated by two-step sintering

The objective of this work was to study two-step sintering as a means of controlling the microstructure of Al₂O₃ matrix nanocomposites containing nanometric inclusions of ZrO₂ 15% by volume and evaluate its hydrothermal degradation as function of time and its mechanical properties. Powders of Al₂O₃ and ZrO₂ were prepared and compacted by means of isostatic pressing and sintering in different heating cycles. The results showed that two-step sintering allowed a more efficient microstructural control than single-step sintering, resulting in good mechanical properties. The studied nanocomposites showed excellent resistance to hydrothermal degradation compared to commercial ZrO₂ and ZrO₂ TZ-3Y-E.     

Preparation and properties of β-CaSiO3/ZrO2 (3Y) nanocomposites

β-CaSiO₃/ZrO₂ (3Y) nanocomposites were successfully fabricated by spark plasma sintering (SPS) technique. The addition of nanocrystalline ZrO₂ could inhibit the phase transition of micrometer sized CaSiO₃ and increase its phase transitional temperature. The relative densities of the dense β-CaSiO₃/ZrO₂ nanocomposites were above 98%. Nanocrystalline ZrO₂ has formed a network structure in the nanocomposites, which could improve the mechanical properties of nanocomposites. The fracture strength and fracture toughness of the nanocomposites were as high as 395 MPa and 4.08 MPa m^1/2, respectively. The nanocomposites showed good in vitro bioactivity with hydroxyapatite (HA) layer formation on the ZrO₂ network of the nanocomposites in simulated body fluid. The spark plasma sintered β-CaSiO₃/ZrO₂ (3Y) nanocomposite may provide a new bone graft for load bearing applications.     

Controlling the sizes of in-situ TiC nanoparticles for high-performance TiC/Al–Cu nanocomposites

TiC/Al–Cu nanocomposites were fabricated in Al–Ti–C powder systems using carbon black, a mixture of C and carbon nanotubes (C + CNTs), and CNTs via an in-situ method involving combustion synthesis and hot pressing. As the carbon source changed from pure carbon black to the C + CNT mixture and pure CNTs, the size of the TiC nanoparticles decreased gradually. The nanocomposites synthesized based on the C + CNT mixture exhibited the most uniform dispersion of TiC nanoparticles. The 30 vol% TiC/Al–Cu nanocomposite prepared from the C + CNT mixture had the best comprehensive mechanical properties (yield strength (411 MPa), compressive strength (712 MPa), fracture strain (17.2%), hardness (206.8 HV), and wear resistance under the experimental conditions due to having the most uniformly dispersed TiC nanoparticles. The wear mechanism was a combination of plastic deformation, abrasion, and adhesion. This method may be a low-cost and convenient means to control the sizes of in-situ TiC nanoparticles and prepare high-performance TiC/Al–Cu nanocomposites.     

The nanocomposites of carbon nanotube with Sb and SnSb0.5 as Li-ion battery anodes

Nanocomposites of carbon nanotubes (CNTs) with Sb and SnSb_0.5 particles were prepared by chemical reduction of SnCl₂ and SbCl₃ precursors in the presence of CNTs. SEM and TEM imaging showed that the Sb and SnSb_0.5 particles are uniformly deposited on the CNT exterior and in the CNT web. These CNT-metal composites are active anode materials for lithium ion batteries, showing improved cyclability compared to unsupported Sb and Sn-Sb particles and higher reversible specific capacities than CNTs. The reversible capacities were as high as 462 mAh/g for CNT-36 wt.% Sb and 518 mAh/g for CNT-56 wt.% SnSb_0.5. After 30 cycles, the capacity was 62.1% of the initial capacity for the former and 67.2% of the initial capacity for the later. In comparison Sb and SnSb_0.5 could only retain 17.7 and 23.5%, respectively of their initial capacities in the same number of cycles. The improvement in cyclability may be attributed to the nanoscale dimension of the metal particles and CNTs’ role as a buffer in relieving the mechanical stress induced by specific volume changes in electrochemical lithium insertion and extraction reactions.     

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.     

Pressureless sintering of carbon nanotube–Al2O3 composites

Alumina ceramics reinforced with 1, 3, or 5 vol.% multi-walled carbon nanotubes (CNTs) were densified by pressureless sintering. Commercial CNTs were purified by acid treatment and then dispersed in water at pH 12. The dispersed CNTs were mixed with Al₂O₃ powder, which was also dispersed in water at pH 12. The mixture was freeze dried to prevent segregation by differential sedimentation during solvent evaporation. Cylindrical pellets were formed by uniaxial pressing and then densified by heating in flowing argon. The resulting pellets had relative densities as high as ～99% after sintering at 1500 °C for 2 h. Higher temperatures or longer times resulted in lower densities and weight loss due to degradation of the CNTs by reaction with the Al₂O₃. A CNT/Al₂O₃ composite containing 1 vol.% CNT had a higher flexure strength (～540 MPa) than pure Al₂O₃ densified under similar conditions (～400 MPa). Improved fracture toughness of CNT–Al₂O₃ composites was attributed to CNT pullout. This study has shown, for the first time, that CNT/Al₂O₃ composites can be densified by pressureless sintering without damage to the CNTs.     

Dispersed ZrO2 nanoparticles in MCM‐48 with high adsorption activity

A new two‐step synthesis of ZrO₂‐MCM nanocomposites using the gel combustion technique was accomplished; the resulting material had a high‐surface area and showed very high adsorption activity. The deposition of 2–5 nm ZrO₂ particles over MCM was achieved using gel combustion technique with glycine as a fuel, and the formation of nanocomposites was confirmed using transmission electron microscopy. The composites were also characterized by XRD, SEM, FTIR and N₂ adsorption‐desorption analysis. The nanocomposites were tested for the adsorption of cationic dyes. High rates of adsorption and large dye uptake were observed over the nanocomposites. The rate of adsorption over the nanocomposites was higher than that observed for physical ZrO₂‐MCM mixtures and commercial activated carbon. The nanocomposite with 10 wt % ZrO₂ showed the highest rate of adsorption owing to the synergistic effects of ZrO₂ surface groups, smaller particle size, fine dispersion and high‐surface area of the composite. © 2012 American Institute of Chemical Engineers AIChE J, 58: 2987–2996, 2012     

The influence of MgO,Y2O3 and ZrO2 additions on densification and grain growth of submicrometre alumina sintered by SPS and HIP

Spark plasma sintering followed by hot isostatic pressing was applied for preparation of polycrystalline alumina with submicron grain size. The effect of additives known to influence both densification and grain growth of alumina, such as MgO, ZrO₂ and Y₂O₃ on microstructure development was studied. In the reference undoped alumina the SPS resulted in some microstructure refinement in comparison to conventionally sintered materials. Relative density >99% was achieved at temperatures >1200 °C, but high temperatures led to rapid grain growth. Addition of 500 ppm of MgO, ZrO₂ and Y₂O₃ led, under the same sintering conditions, to microstructure refinement, but inhibited densification. Doped materials with mean grain size <400 nm were prepared, but the relative density did not exceed 97.9%. Subsequent hot isostatic pressing (HIP) at 1200 and 1250 °C led to quick attainment of full density followed by rapid grain growth. The temperature of 1250 °C was required for complete densification of Y₂O₃ and ZrO₂-doped polycrystalline alumina by HIP (relative density >99.8%), and resulted in fully dense opaque materials with mean grain size<500 nm.     

Structural,mechanical and tribological properties of Cu–ZrO2/GNPs hybrid nanocomposites

Hybrid Cu–ZrO₂/GNPs nanocomposites were successfully produced using powder metallurgy technique. The effect of GNPs mass fraction, 0, 0.5, 1 and 1.5%, on mechanical and tribological properties of the produced hybrid nanocomposite was studied while maintaining ZrO₂ mass fraction constant at 5%. High energy ball milling was applied for mixing powders and compaction and sintering were applied for consolidation. The morphological analysis of the produced powder showed acceleration of Cu particles fracture during ball milling with the addition of GNPs up to 0.5% with noticeable reduction of agglomeration size. Moreover, the crystallite size of Cu–5%ZrO₂/0.5%GNPs hybrid nanocomposites revealed smaller crystallite size, 142 nm, compared to 300 nm for Cu–5%ZrO₂ nanocomposite. Additionally, the hybrid nanocomposite with 0.5% GNPs shows homogeneous distribution of both reinforcement phases in the sintered samples. This improved nano and micro structure of Cu–5%ZrO₂/0.5%GNPs nanocomposites revealed higher hardness, 169.3 HV, compared to 65.5 HV for Cu–5%ZrO₂ nanocomposite. The wear rate is decreased in this composite while it increased with increasing GNPs content more than 0.5%. The coefficient of friction is decreased as well for this hybrid nanocomposite and remain constant with increasing GNPs content more than 0.5%.