Impact Energy Absorption Analysis of Spark Plasma Sintered Al2O3 Reinforced Bulk Multiwalled Carbon Nanotube Compacts Using Nanoindentation

For utilizing the outstanding energy absorbing capacity of highly elastic carbon nanotube (CNT), bulk multiwalled CNT (MWCNT) structure containing 15 wt% alumina (Al₂O₃) was fabricated using spark plasma sintering at 1600°C for 10 min under 50 MPa. The compacted mass was ~85% dense having morphologically stable MWCNTs. Microindentation studies up to 9.81 N indicated outstanding elastic recovery of the bulk structure leaving only a diffused indentation mark at indenter‐specimen interaction zone. Quantitative estimation of elastic response behavior of the fabricated structure using instrumented nanoindentation in 10–300 mN load range indicated promising applicability of Al₂O₃/MWCNT compact structure as energy absorbing material.     

Perfect High‐Temperature Plasticity Realized in Multiwalled Carbon Nanotube‐Concentrated α‐Al2O3 Hybrid

We investigate the high‐temperature compressive deformation behavior of a novel, fully dense and structurally uniform, 20 vol% multiwalled carbon nanotube (MWCNT)–α‐Al₂O₃ matrix hybrid, which has a strong room‐temperature interfacial shear resistance (ISR) and a unique MWCNT‐concentrated grain‐boundary (GB) structure. We realized a perfect plastic deformation at 1400°C and a rather high initial strain rate of 10^?4 s^?1 by a low ~30 MPa flow stress, which is contrary to the strain hardening response of fine‐grain monolithic Al₂O₃. This unique performance in CNT–ceramic system in compression is explained as follows: the concentrated network of individual MWCNTs perfectly withstands the high‐temperature and shear/compressive forces, and strongly preserves the nanostructure of Al₂O₃ matrix by preventing the dynamic grain growth, even during a large ~44% deformation. Furthermore, the presence of large amount of radially soft/elastic, highly energy‐absorbing MWCNTs in the GB and specially multiple junction areas, and a potentially weak 1400°C‐ISR, could greatly facilitate the GB sliding process (despite the hybrid's strong room‐temperature ISR), as evidenced by the formation of some submicrometer‐scale MWCNT aggregates in GB area, the equiaxed grains and dislocation‐free nanostructure of the deformed hybrid. The results presented here could be attractive for the ceramic forming industry and could be regarded as a reference for oxide systems in which, the GB areas are occupied with soft/elastic, highly energy‐absorbing nanostructures.     

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.     

Processing YAG/α‐Al2O3 composites via reactive sintering Y2O3/Al2O3 NP mixtures. A superior alternative to bottom up processing using atomically mixed YAlOx NPs

This effort contrasts “bottom‐up” processing of YAG/α‐Al₂O₃ composites where both elements (as 40‐50 nm APSs nanopowders) are present at close to atomic mixing with reactive sintering where ball‐milled mixtures of the individual nanopowders (40‐50 nm APSs) give uniform elemental mixing at length scales closer to 100‐800 nm with correspondingly much longer diffusion distances. In contrast to expectations, densification with control of final grain sizes is best effected using reactive sintering. Thus, reactive sintering to densities ≥95% occurs at only 1500°C with final grain sizes of ≈1000 nm for all samples. In contrast “bottom up” processing to ≥95% densities is only achieved at 1600°C, and with final grain sizes of 1700 nm. The reason for this unexpected behavior is that YAG phase forms early in the bottom up approach greatly inhibiting diffusion promoted densification. In contrast, in reactive sintering, YAG is prevented from forming because of the longer diffusion distances such that densification occurs prior to full conversion of the Y₂O₃ component to YAG. The found hardness values are statistically superior to literature values for composites near the known eutectic composition. In an accompanying paper, the addition of a third component reverses this behavior.     

Mechanical and Dielectric Properties of SiCf/AlPO4 Composites with Multi‐Walled Carbon Nanotubes

SiC_f/AlPO₄ composites were prepared by laminating method. The slurry comprising Al(H₂PO₄)₃ solution, multi‐walled carbon nanotubes (MWCNTs), and Al₂O₃ powder was used as matrix precursor. The complex permittivities of the composites were measured in the X‐band (8.2–12.4 GHz). The reflection loss was calculated using the transmission line theory. It was found that the addition of 2.5 wt% MWCNTs noticeably improved the absorption property of 3 mm thick composites with the assistance of 15 wt% Al₂O₃, which also exhibited an obvious toughened fracture behavior with 190 MPa fracture strength.     

Synthesis of glass fiber‐multiwall carbon nanotube hybrid structures for high‐performance conductive composites

The conductive polyamide 66 (PA66)/carbon nanotube (CNT) composites reinforced with glass fiber‐multiwall CNT (GF‐MWCNT) hybrids were prepared by melt mixing. Electrostactic adsorption was utilized for the deposition of MWCNTs on the surfaces of glass fibers (GFs) to construct hybrid reinforcement with high‐electrical conductivity. The fabricated PA66/CNT composites reinforced with GF‐MWCNT hybrids showed enhanced electrical conductivity and mechanical properties as compared to those of PA66/CNT or PA66/GF/CNT composites. A significant reduction in percolation threshold was found for PA66/GF‐MWCNT/CNT composite (only 0.70 vol%). The morphological investigation demonstrated that MWCNT coating on the surfaces of the GFs improved load transfer between the GFs and the matrix. The presence of MWCNTs in the matrix‐rich interfacial regions enhanced the tensile modulus of the composite by about 10% than that of PA66/GF/CNT composite at the same CNT loading, which shows a promising route to build up high‐performance conductive composites. POLYM. COMPOS. 34:1313–1320, 2013. © 2013 Society of Plastics Engineers     

The Band Structure of Polycrystalline Al2O3 and Its Influence on Transport Phenomena

The electronic (band) structure of polycrystalline Al₂O₃, in particular the density of near‐band edge grain‐boundary localized states, plays a significant role in a host of high‐temperature phenomena, including sintering, high‐temperature creep, oxygen permeability in dense “dry” Al₂O₃ ceramics, and Al₂O₃ scale formation on Al₂O₃ scale‐forming alloys. All these phenomena involve creation or annihilation of charged point defects (vacancies and/or interstitials) at grain boundaries and interfaces, and must of necessity involve electrons and holes. Thus, the density of states associated with grain boundaries in Al₂O₃ assume great importance, and has been calculated using DFT for both nominally undoped and Y‐doped Σ7 bi‐crystal boundaries. These quantum mechanical calculations must be taken into account when considering why Y₂O₃ segregation to Al₂O₃ grain boundaries is so effective in enhancing high‐temperature creep resistance of polycrystalline Al₂O₃, and in understanding the reactive element effect in Al₂O₃ scale‐forming alloys. Finally, a case will be made that grain‐boundary diffusion is mediated by the migration of a class of grain‐boundary ledge defects called disconnections, which are characterized by a step height h and a Burgers vector b.     

Siloxane Core-Modified Organo Soluble Novel Polyimide/Multi-walled Carbon Nanotube Nanocomposites

Siloxane core-modified polyimides (PI)/multi-walled carbon nanotubes (MWCNTs) nanocomposites were designed and developed. The confinement of segmental motion of the PI chain due to high dispersion uniformity of the MWCNTs caused the increment in the glass transition temperature (T _g ) and remarkable elevated thermal stability. All the PIs could afford better solubility with high dielectric constants of 3.5–5.9 and low water absorptions of 0.42–0.68%. These PIs emitted a strong blue fluorescent at 380 nm, assigning the structural confirmation of PI matrix. Heterogeneous morphology of MWCNT/PI nanocomposites was evidenced by SEM and TEM images, resulted from the incorporation of MWCNT.     

The effect of Al2O3 doped multi-walled carbon nanotubes on the thermal conductivity of Al2O3/epoxy terminated poly(dimethylsiloxane) composites

Al₂O₃ doped multi-walled carbon nanotubes (MWCNT) were synthesized as a conducting additive in alumina–epoxy terminated poly(dimethylsiloxane) (ETDS). The addition of Al₂O₃ doped MWCNT improved the thermal conductivity of the composites, which was a function of the Al₂O₃ loading. The mechanisms underlying this enhanced conductivity were examined in the context of the Hashin–Shtrikman (HS) boundaries and interconnectivity. The measured thermal conductivity revealed more enhanced thermal conductivity than expected by analytical predictions at a fixed micro Al₂O₃ concentration. Further analytical investigations showed that the addition of Al₂O₃ doped MWCNT affected the interconnectivity between the conducting particles because of their high aspect ratios. Overall, Al₂O₃ doped MWCNT may be useful for establishing three-dimensional heat conducting percolating networks in a matrix that affect the thermal conductivity of a composite.     

Graphene nanoplate and multiwall carbon nanotube–embedded polycarbonate hybrid composites: High electromagnetic interference shielding with low percolation threshold

This study has reported the preparation of polycarbonate (PC)/graphene nanoplate (GNP)/multiwall carbon nanotube (MWCNT) hybrid composite by simple melt mixing method of PC with GNP and MWCNT at 330°C above the processing temperature of the PC (processing temperature is 280°C) followed by compression molding. Through optimizing the ratio of (GNP/MWCNT) in the composites, high electromagnetic interference shielding effectiveness (EMI SE) value (∼21.6 dB) was achieved at low (4 wt%) loading of (GNP/MWCNT) and electrical conductivity of ≈6.84 × 10⁻⁵ S.cm⁻¹ was achieved at 0.3 wt% (GNP/MWCNT) loading with low percolation threshold (≈0.072 wt%). The high temperature melt mixing of PC with nanofillers lowers the melt viscosity of the PC that has helped for better dispersion of the GNPs and MWCNTs in the PC matrix and plays a key factor for achieving high EMI shielding value and high electrical conductivity with low percolation threshold than ever reported in PC/MWCNT or PC/graphene composites. With this method, the formation of continuous conducting interconnected GNP‐CNT‐GNP or CNT‐GNP‐CNT network structure in the matrix polymer and strong π–π interaction between the electron rich phenyl rings and oxygen atom of PC chain, GNP, and MWCNT could be possible throughout the composites. POLYM. COMPOS., 37:2058–2069, 2016. © 2015 Society of Plastics Engineers     

Fabrication and mechanical properties of Al2O3–TiC ceramic composites synergistically reinforced with multi-walled carbon nanotubes and graphene nanoplates

In this study, Al₂O₃–TiC composites synergistically reinforced with multi-walled carbon nanotubes (MWCNTs) and graphene nanoplates (GNPs) were prepared via spark plasma sintering (SPS). The effects of the MWCNT and GNP contents on the phase composition, mechanical properties, fracture mode, and toughening mechanism of the composites were systematically investigated. The experimental results indicated that the composite grains became more refined with the addition of MWCNTs and GNPs. The nanocomposites presented high compactness and excellent mechanical properties. The composite with 0.8 wt% MWCNTs and 0.2 wt% GNPs presented the best properties of all analysed specimens, and its relative density, hardness, and fracture toughness were 97.3%, 18.38 ± 0.6 GPa, and 9.40 ± 1.6 MPa m^1/2, respectively. The crack deflection, bridging, branching, and drawing effects of MWCNTs and GNPs were the main toughening mechanisms of Al₂O₃–TiC composites synergistically reinforced with MWCNTs and GNPs.     

Carbon nanotube/boehmite-derived alumina ceramics obtained by hydrothermal synthesis and spark plasma sintering (SPS)

Preparation, structure and properties of hydrothermally treated carbon nanotube/boehmite (CNT/γ-AlOOH) and densification with spark plasma sintering of Al₂O₃ and CNT/Al₂O₃ nanocomposites were investigated. Hydrothermal synthesis was employed to produce CNT/boehmite from an aluminum acetate (Al(OH)(C₂H₃O₂)₂) and multiwall-CNTs mixture (200 °C/2 h.). TEM observations revealed that the size of the cubic shape boehmite particles lies around 40 nm and the presence of the interaction between surface functionalized CNTs and boehmite particles acts to form ‘nanocomposite particles’. Al₂O₃ and CNT/Al₂O₃ compact bodies were formed by means of spark plasma sintering (SPS) at 1600 °C for 5 min using an applied pressure of 50MPa resulting in the formation of stable α-Al₂O₃ phase and CNT–alumina compacts with nearly full density. It was also found that CNTs tend to locate along the alumina grain boundaries and therefore inhibit the grain coarsening and cause inter-granular fracture mode. The DC conductivity measurements reveal that the DC conductivity of CNT/Al₂O₃ is 10^?4 S/m which indicate that there is a 4 orders of magnitude increase in conductivity compared to monolithic Al₂O₃. The results of the microhardness tests indicate a slight increase in hardness for CNT/Al₂O₃ (28.35 GPa for Al₂O₃ and 28.57 GPa for CNT/Al₂O₃).     

Effect of ball milling and solid solution treatment on the microstructure and interfacial behaviors of ZrMgMo3O12p/2024Al composites

ZrMgMo₃O₁₂ is a negative expansion material, while 2024Al alloy is a positive expansion material. The difference in thermal expansion coefficients between them will cause thermal mismatch stress at the interface in ZrMgMo₃O_12p/2024Al composites. Therefore, the interface behaviors of ZrMgMo₃O₁₂–Al determine the properties of ZrMgMo₃O_12p/2024Al composites to a great extent. The effects of ball milling and solid solution treatment on the microstructure and interface behaviors of 10 vol% ZrMgMo₃O_12p/2024Al composites were studied to improve the reticular microstructure of ZrMgMo₃O₁₂ distributed at the grain boundary of the α-Al matrix. The results showed that with increasing milling energy, the microstructure of the composites changed from reticular to equiaxed, and the distribution of ZrMgMo₃O₁₂ reinforcements in the matrix was more uniform. The content and size of the primary phase Al₂Cu decreased with increasing solid solution treatment time. In addition, only ZrMgMo₃O₁₂–Al, Al₂Cu–Al₁₂Mo and Al–Al₁₂Mo interfaces can be observed, but it is difficult to observe the interfaces of ZrMgMo₃O₁₂–Al₁₂Mo in the composites milled for 12 h and solution treated for 24 h, which is related to the decomposition mechanism of ZrMgMo₃O₁₂. The decomposition mechanism is as follows: Al atoms from the α-Al matrix captured O atoms from ZrMgMo₃O₁₂ to form Al₂O₃. ZrMgMo₃O₁₂ simultaneously released Mo, Zr and Mg atoms. Mo atoms were enriched and nucleated in situ and precipitated with Al atoms to form the intermetallic compound Al₁₂Mo, while Zr and Mg atoms entered the α-Al matrix to form a solid solution.     

Fabrication and characterization of homogeneous composites of polypropylene and multiwalled carbon nanotubes

The polypropylene‐grafted multiwalled carbon nanotubes (PP‐MWCNTs) were produced from the reaction of PP containing the hydroxyl groups and MWCNTs having 2‐bromoisobutyryl groups. The PP‐MWCNTs had a significantly rougher surface than the original MWCNTs. PP‐MWCNTs had PP layers of thickness 10–15 nm on the outer walls of the MWCNTs. PP/PP‐MWCNT composites and PP/MWCNT composites were prepared by solution mixing in o‐xylene. Unlike PP/MWCNT composites, PP‐MWCNTs were homogeneously dispersed in the PP matrix. As a consequence, the thermal stability and conductivity of PP/PP‐MWCNT composites were dramatically improved even if only 1 wt % of PP‐MWNTs was added to the PP matrix. The good miscibility of PP and PP‐MWCNTs plays a critical role in the formation of the homogeneous composites and leads the high thermal stability and conductivity. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Diameter- and length-dependent self-organizations of multi-walled carbon nanotubes on spherical alumina microparticles

Multi-scale hybrid structures of multi-walled carbon nanotubes (MWCNTs) and micrometric alumina particles (μAl₂O₃) have been produced. The hybrid structures are obtained by in situ grafting carbon nanotubes (CNTs) on spherical μAl₂O₃ particles using an easy chemical vapor deposition method, without any pre-patterned catalyst treatment. The study of the influence of temperature and hydrogen ratio shows three regular hybrid structures defined according to CNT arrangement on μAl₂O₃. Furthermore, the organization modes demonstrate that the hybrid structures are strongly dependent on the diameter, length and area number density of CNTs. This dependency has been explained using a proposed nano-cantilever beam model. It evaluates the maximum deflection of one CNT due to weak van der Waals interactions. The model analysis shows that the MWCNT organizations are a result of varying competitive interactions between MWCNT rigidity and their attractive forces on the large curvature surface of μAl₂O₃. In addition, the influence of specific characteristics of μAl₂O₃ on hybrid structures is also discussed.     

Polystyrene composites containing crosslinked polystyrene‐multiwalled carbon nanotube balls

Crosslinked polystyrene‐multiwalled carbon nanotube (PS‐MWCNT) balls, which act as conductive microfillers, were prepared by the in situ suspension polymerization of styrene with MWCNTs and divinyl benzene (DVB) as a crosslinking agent. The diameters of the synthesized crosslinked PS‐MWCNT balls ranged from 10 to 100 μm and their electrical conductivity was about 7.7 × 10^?3 S/cm. The morphology of the crosslinked PS‐MWCNT balls was observed by scanning electron microscopy and transmission electron microscopy. The change in the chemical structure of the MWCNTs was confirmed by Raman spectroscopy and Fourier transform infrared spectroscopy. The mechanical and electrical properties of the PS/crosslinked PS‐MWCNT ball composites were investigated. It was found that the tensile strength, ultimate strain, Young's modulus, and impact strength of the PS matrix were enhanced by the incorporation of the crosslinked PS‐MWCNT balls. In addition, the mechanical properties of the PS/crosslinked PS‐MWCNT ball composites were better than those of the PS/pristine MWCNT composites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Thermal conductivities of YSZ/Al2O3 composites

The thermal conductivities/diffusivities of YSZ/Al₂O₃ composites have been investigated by a laser flash technique. The thermal conductivity of the composite increases with an increase in the Al₂O₃ volume fraction, and it can be fitted well to the Maxwell theoretical model. The consistency of the thermal conductivities of the composites with the predicted values indicates the absence of obvious interfacial thermal resistances in the composites. The negligible thermal resistance effect from the YSZ and Al₂O₃ grain boundaries is due to the much lower phonon mean free path compared with the grain size in the composite. The low Kapitza resistance of the YSZ/Al₂O₃ interface is discussed in terms of the “clean” and coherent nature of the YSZ/Al₂O₃ interface, together with the small difference between the elastic properties of YSZ and Al₂O₃.     

Microstructure and mechanical properties of Al2O3–YSZ spherical polycrystalline composites

The mechanical behavior and microstructure of highly densified, spherically shaped, polycrystalline Al₂O₃–YSZ composites, processed from pseudoboehmite powders by sol–gel is reported here. Processing was carried out by combining nanometric sized α-Al₂O₃ (120 nm) seeds and YSZ particles of tetragonal structure. The YSZ particles were homogeneously distributed in a coarse-grained matrix of alumina, both inside grains and along grain boundaries. Fracture surfaces, achieved by impact tests showed toughening effects of the zirconia particles. The tetragonality of the YSZ phase stability even after fracture events and fracture toughness measurements by Vickers indentation, where the crack tip interacts with YSZ particles, are all provided and discussed. The local mechanical properties, such as elastic modulus, indentation hardness and the onset of plastic deformation or fracture contact pressure of both YSZ particles and the Al₂O₃ matrix were quantified by nanoindentation. Evidence of coercive contact pressure was observed in YSZ from indentation stress–strain curves.     

Phase Evolution in the Transformation of Atomically Mixed Versus Ball‐Milled Mixtures of Nanopowders in the Formation of Composite MO·3Al2O3 Spinels: Bottom‐Up Processing is Not Always Optimal

Liquid‐feed flame spray pyrolysis (LF‐FSP) provides atomically homogeneous mixed metal powders with 30–40 nm average particle sizes, often producing kinetic phases due to the high quench rate As produced LF‐FSP Al₂O₃‐rich spinels, such as MgO·3Al₂O₃, form an Al₂O₃‐rich metastable single‐phase spinel. On heating, the powders phase separate to form MAl₂O₄ and α‐Al₂O₃. Compacts of MO·3Al₂O₃ (M = Co, Ni, Mg) were produced and sintered to evaluate the final duplex microstructure. The same composition was also approached from stoichiometric LF‐FSP MAl₂O₄ nanopowders ball‐milled with Al₂O₃ nanopowders in an attempt to evaluate how the initial length scale of mixing affected the final microstructure. Contrary to traditional sintering, we observe two distinct mechanisms. At 1000°C–1200°C, cation diffusion appears to control densification as a consequence of high vacancy concentrations and atomic mixing where traditionally expected site inversion plays less of a factor given the high quench rates. The second mechanism follows α‐Al₂O₃ exsolution and densification occurs via oxygen diffusion and α‐Al₂O₃ grain growth. When sintering the duplex MAl₂O₄/α‐Al₂O₃ compacts to at least 95% theoretical density, we find final microstructures that do not reflect the initial degrees of mixing. That is, the atomically mixed MgO·3Al₂O₃ does not does not offer an advantage over the submicron length scale of mixing in the ball‐milled samples.     

Preparation and thermal,electrical, and morphological properties of multiwalled carbon nanotubeand epoxy composites

Multiwalled carbon nanotube (MWCNT)/epoxy composites are prepared, and the characteristics and morphological properties are studied. Scanning electron microscopy microphotographs show that MWCNTs are dispersed on the nanoscale in the epoxy resin. The glass‐transition temperature (T_g) of MWCNT/epoxy composites is dramatically increased with the addition of 0.5 wt % MWCNT. The T_g increases from 167°C for neat epoxy to 189°C for 0.5 wt % CNT/epoxy. The surface resistivity and bulk resistivity are decreased when MWCNT is added to the epoxy resins. The surface resistivity of CNT/epoxy composites decreases from 4.92 × 10¹² Ω for neat epoxy to 3.03 × 10⁹ Ω for 1 wt % MWCNT/epoxy. The bulk resistivity decreases from 8.21 × 10¹⁶ Ω cm for neat epoxy to 6.72 × 10⁸ Ω cm for 1 wt % MWCNT/epoxy. The dielectric constant increases from 3.5 for neat epoxy to 5.5 for 1 wt % MWCNT/epoxy. However, the coefficient of thermal expansion is not affected when the MWCNT content is less than 0.5 wt %. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1272–1278, 2007