Fabrication of composites with excellent mechanical properties based on cubic boron nitride reinforced with carbon nanotubes

Composites consisting of cubic boron nitride (cBN) as a matrix and carbon nanotubes (CNTs) as reinforcing additives were fabricated by high-temperature and high-pressure sintering (HTHP). Microstructures, mechanical properties, fracture modes and toughening mechanisms of these composites were investigated. Composites exhibited excellent bending strength, wear resistance, and fracture toughness. Fracture toughness of composites reached 7.02 MPa·m^1/2. Comparing to pure cBN matrix, bending strength improved from 475.27 to 600.15 MPa, and wear resistance increased by 43.23%. Such improvements of mechanical properties were mainly attributed to pullout and bridging reinforcements by CNTs. CNTs incorporation also changed fracture mode from inter-to trans-granular.     

Reinforcement of epoxy resins with multi-walled carbon nanotubes for enhancing cryogenic mechanical properties

Epoxy resins are widely applied in cryogenic engineering and their cryogenic mechanical properties as important parameters have to be improved to meet the high requirements by cryogenic engineering applications. Carbon nanotubes (CNTs) are regarded as exceptional reinforcements for polymers. However, poor carbon nanotube (CNT)–polymer interfacial bonding leads to the unexpected low reinforcing efficiency. This paper presents a study on the cryogenic mechanical properties of multi-walled carbon nanotube reinforced epoxy nanocomposites, which are prepared by adding multi-walled carbon nanotubes (MWCNTs) to diglycidyl ether of bisphenol-F epoxy via the ultrasonic technique. When the temperature decreases from room temperature to liquid nitrogen temperature (77 K), a strong CNT–epoxy interfacial bonding is observed due to the thermal contraction of epoxy matrix because of the big differences in thermal expansion coefficients of epoxy and MWCNTs, resulting in a higher reinforcing efficiency. Moreover, synthetic sequence leads to selective dispersion of MWCNTs in the brittle primary phase but not in the soft second phase in the two phase epoxy matrix. Consequently, the cryogenic tensile strength, Young's modulus, failure strain and impact strength at 77 K are all enhanced by the addition of MWCNTs at appropriate contents. The results suggest that CNTs are promising reinforcements for enhancing the cryogenic mechanical properties of epoxy resins that have potential applications in cryogenic engineering areas.     

Enhanced electrical conductivity of nylon 6 composite using polyaniline-coated multi-walled carbon nanotubes as additives

Aggregation in polymer composites is one of the major obstacles in the carbon nanotubes (CNTs) applications. Authentic CNTs are known to have very good electrical conductivity and mechanical strengths. Surface functionalization can avoid aggregation and help dispersion of CNTs, but reduces CNT’s electrical conductivities and mechanical strengths dramatically. It needs a good way to resolve the above dilemma situation; i.e., poor dispersion-good conductivity vs. good dispersion-poor conductivity. Herein, we demonstrate that in-situ polymerized polyaniline (PANI)-coated CNTs have good polymer matrix compatibility, and are superior electrically conductive fillers to nylon 6 composites. In this report, multi-walled CNTs (MWCNTs) were surface-modified with poly(acrylic acids) (PAA), followed by further coating with PANI. The electrical conductivity of (PANI-MWCNTs)-nylon 6 composite thin film was increased from 10⁻¹² to 7.3 × 10⁻⁵ S/cm in the presence of 1 wt% PANI-coated MWCNTs prepared by physical mixing of PANI and PAA-grafted MWCNTs. When in-situ polymerized PANI-coated MWCNTs were added, the electrical conductivity of MWCNTs-nylon 6 composite was further enhanced by 3 orders to be 3.4 × 10⁻² S/cm at the same 1 wt% loading of MWCNTs. Both Fourier-transformed infrared and uv-visible absorption spectra indicate that there exist very strong site-specific charge transfer interactions between the quinoid rings of PANI and MWCNTs, which results in the superior electrical conductivity of MWCNT-nylon 6 composite.     

Mechanical behavior of phenolic-based composites reinforced with multi-walled carbon nanotubes

Two types of carbon nanotubes (CNTs), the network multi-walled nanotubes (MWNTs) and the dispersed MWNTs, were used for fabricating MWNTs/phenolic composites. The MWNTs were synthesized using the floating catalyst method through the chemical vapor deposition process. The effects of the MWNT content on the mechanical properties of the composites were investigated. Modified Halpin-Tsai equation was proposed to evaluate the Young’s modulus and tensile strength of the MWNTs/phenolic composites by adopting an orientation factor and an exponential shape factor in the equation. It is found that the results obtained from the modified Halpin-Tsai equation on tensile strengths and Young’s moduli fit successfully the experimental ones. The tensile fracture surfaces of MWNTs/phenolic composites were examined using field emission scanning electron microscope to study the failure morphologies of the MWNTs/phenolic composites.     

Microstructure and mechanical properties of a metakaolinite-based geopolymer nanocomposite reinforced with carbon nanotubes

The preparation and mechanical behavior of metakaolin-based geopolymer nanocomposite reinforced with multi-walled carbon nanotubes are presented in this study. In this work, Multiwall carbon nanotubes (MWCNTs) were added to the metakaolin-based geopolymer paste at 0, 0.5, or 1 wt% concentration. For each specimen, the mechanical properties were tested at the age of 7, 14 and 28 days. TEM and FESEM were employed to evaluate the dispersion quality of MWCNTs within the metakaolin geopolymer matrix and determine their strengthening mechanism. The test results showed that the addition of about 0.5 wt% MWCNTs increased the compressive and flexural strength by as much as 32% and 28%, respectively. Based on these results, the MWCNTs can act as effective bridges to minimize and limit the propagation of micro cracks through the metakaolin-based geopolymer nanocomposite under the conditions of homogenous dispersion and good bonding between the MWCNTs and the surrounding metakaolin-based geopolymer paste.     

Processing and characterization of epoxy nanocomposites reinforced by cup-stacked carbon nanotubes

The effect of the dispersion, ozone treatment and concentration of cup-stacked carbon nanotubes on mechanical, electrical and thermal properties of the epoxy/CSCNT nanocomposites were investigated. Ozone treatment of carbon fibers was found to increase the surface oxygen concentration, thereby causing the contact angle between water, epoxy resin and carbon fiber to be decreased. Thus, the tensile strength, modulus and the coefficient friction of carbon fiber reinforced epoxy resin were improved. Moreover, the dispersion of fibers in polymer was increased and the electrical resistivity was decreased with the addition of filler content. The dynamic mechanical behavior of the nanocomposite sheets was studied. The storage modulus of the polymer was increased by the incorporation of CSCNTs. But the glass transition temperature decreased with increasing fiber loading for the ozone treated fiber composites. The ozone treatment did affect the morphology, mechanical and physical properties of the CSCNT.     

Effect of multi-walled carbon nanotubes on microstructure and fracture properties of carbon fiber-reinforced ZrB2-based ceramic composite

Multi-walled carbon nanotubes (MWCNTs) were used to optimize the microstructure and improve the fracture properties of hot-pressed carbon fiber-reinforced ZrB₂-based ultra-high temperature ceramic composites. Microstructure analysis indicated that the introduction of MWCNTs effectively reduced the carbon fiber degradation and prevented fiber-matrix interfacial reaction during processing. Due to the presence of MWCNTs, the matrix contained fine ZrB₂ grains and in-situ formed nano-sized SiC/ZrC grains. The fracture properties were evaluated using the single edge-notched beam (SENB) test. The fracture toughness and work of fracture of the C_f/ZrB₂-based composite with MWCNTs were 7.0±0.4 MPa m^1/2 and 379±34 J/m², respectively, representing increases of 59% and 87% compared to those without MWCNTs. The excellent fracture properties are attributed to the moderate interfacial bonding between the fibers and matrix, which favour the toughening mechanisms, such as fiber bridging, fiber pull-out and crack deflection at interfaces.     

Capacitance properties of multi-walled carbon nanotubes modified by activation and ammoxidation

Catalytic multi-walled carbon nanotubes were modified by KOH activation at 800 °C and/or ammoxidation at 350 °C, and the effect of these treatments on the physicochemical and electrochemical properties was investigated. Whereas texture is moderately changed by ammoxidation, the chemical composition is significantly modified due to the formation of various nitrogen containing groups. The influence of nitrogenated functionality (pyridine, pyridone, NH) on charge accumulation is considered in full electrochemical capacitors, as well as in positive and negative electrodes separately, using acidic (4 mol L⁻¹ H₂SO₄) and alkaline (7 mol L⁻¹ KOH) electrolytes. The presence of nitrogen in the carbon network, especially in the form of pyridone/pyrrolic (N5) and/or pyridine (N6) groups, affects the electron density and enhances the charge affinity of the carbon material. It seems that the nitrogen groups improve particularly the capacitance performance of the negative electrode operating in alkaline medium. Besides the nitrogenated groups, the oxygenated functionality plays also an important role for the ammoxidized nanotubes. Generally, a few-fold increase of capacitance was observed in the N-enriched carbon nanotubular samples. Apart of this capacitance improvement, the presence of nitrogen in the carbon network limits significantly the leakage current and diminishes the self-discharge of supercapacitors.     

Synthesis and characterization of multi-walled carbon nanotubes reinforced polyamide 6 via in situ polymerization

Polyamide 6 (PA6)/carbon nanotubes (PA6/CNTs) composites have been prepared by in situ polymerization of ε-caprolactam in the presence of pristine and carboxylated multi-walled carbon nanotubes (MWNT and MWNTCOOH). Viscosity measurements show that adding 0.5 wt% of carbon nanotubes (CNTs) does not affect the molecular weight of PA6. Compared with pure PA6, the yield strength of PA6/CNTs composites loaded with 0.5 wt% CNTs is almost unchanged, and the tensile strength is increased slightly. Dynamic mechanical analysis (DMA) demonstrates that both the storage modulus (E′) and glass transition temperature (T_g) of the PA6/CNTs composites increase, particularly for PA6/MWNTCOOH, indicating there is some chemical bonding between PA6 and MWNTCOOH. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and ultra small-angle X-ray scattering (USAXS) show that MWNT and MWNTCOOH are well dispersed in PA6 matrix. Comparison of the USAXS data with a stiff-rod model and wormlike rod model reveals that the CNTs are quite flexible, regardless the degree of chemical modification. Due to the flexibility of CNTs, mechanical properties of the PA6/CNTs composites are marginally enhanced.     

Effect of zirconia addition amount in glaze on mechanical properties of porcelain slabs

Applying toughened glaze layer on porcelain slabs can improve the fracture toughness of slabs and greatly reduce the production cost. In this study, porcelain slabs glaze with high toughness was fabricated by the processes of impregnation glazing and single firing method, using opaque frits, kaolin clay as the main raw materials, zirconia as an additive, and the effect of the addition amount of zirconia in glaze on fracture toughness of porcelain slabs was investigated. The results showed that the type and content of crystal phase of the glaze were greatly influenced by the addition amount of zirconia. Meanwhile, compared with the base glaze, the hardness and fracture toughness of the sample with zirconia glaze were significantly improved. Porcelain slabs with 10 wt% zirconia in glaze, sintering at 1200 °C, exhibited higher quality glaze and outstanding properties, including a water absorption of 1.95%, a Vickers hardness of 6.36 GPa, and a fracture toughness of 2.71 MPa m^1/2. The toughening mechanism of the glaze layer was as follows: a large number of zirconium silicate grains with high hardness were generated by the reaction of added zirconia with silica in the glass phase, which increased the content of crystal phase and then prevented the propagation of cracks; moreover during the martensitic transformation of the tetragonal zirconia grains, the volume and shear strain were generated to offset the stress field generated by the crack tip, thus toughening the material.     

Fabrication and mechanical properties of multi-walled carbon nanotubes doped AlN ceramics prepared by spark plasma sintering

In this study, multi-walled carbon nanotubes (MWCNTs) are uniformly dispersed in aluminium nitride (AlN) powders, and the MWCNTs-doped AlN ceramics are sintered at 1500 °C with a holding time of 5 min by spark plasma sintering using Y₂O₃ as the sintering additive. The effects of the MWCNTs content on the microstructure and mechanical properties of the as-obtained ceramic composites are investigated. The results reveal that many submicron pores are generated when protecting the structure of the CNTs, thereby reducing the density of the AlN ceramic. However, the gradual filling of the grain gap may compensate for the strengthening after CNT doping. The relative density and hardness reach the maximum values of 89.6% of the theoretical density and 7.0 ± 0.3 GPa, respectively, at the doping amount of 2.5 wt%.     

Processing and properties of syntactic foams reinforced with carbon nanotubes

This article presents synthesis and mechanical characterization of carbon nanotube (CNT)‐reinforced syntactic foams. Following a dispersion approach (comprising ultrasonic, calendering, and vacuum centrifugal mixing), single‐ and multi‐walled functionalized CNTs (FCNTs) were incorporated into two foam composites containing various commercially available microballoon grades (S38HS, S60HS, and H50 from 3M). The FCNT‐reinforced composites were tested for compressive strength and apparent shear strength before and after hot/wet conditioning. The results showed that the FCNT‐reinforced composites' mechanical properties depended on the vacuum pressure used during processing. Compared with pristine and commercially available syntactic foam (EC‐3500 from 3M), the FCNT‐reinforced composites processed at high vacuum (0.2 kPa) showed significant increase in compressive strength and apparent shear strength before and after hot/wet conditioning. Dynamic mechanical analysis showed an increase of about 22°C in glass transition temperature for composites processed at high vacuum with 0.5 wt % FCNT and 45 wt % S38HS–5 wt % S60HS microballoons. Thermogravimetric analysis indicated water absorption and lower decomposition temperature for the FCNT‐reinforced composite mixed at atmospheric pressure, whereas no significant change was observed for the compound processed at high vacuum. Fracture analysis showed matrix failure for the composite processed at high vacuum and microballoon crushing for the composite mixed at atmospheric pressure. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

High temperature mechanical properties of zirconia metastable t'-Phase degraded yttria stabilized zirconia

Air plasma sprayed (APS) 8 wt%-yttria stabilized zirconia (8YSZ) with metastable tetragonal prime phase (t′) has been widely applied as thermal barrier coatings (TBCs) for gas turbine blades because of its outstanding mechanical properties at high temperatures. In the present research, a carefully designed process was used to prepare 8YSZ samples with different phase composition (t′, t and c) simulating the phase degradation of the material during operation conditions. High temperature (1000–1200 °C) bending strength, elastic modulus, and thermal expansion coefficient were measured, which exhibit strong dependence on the phase degradation during heat treatment. Effect of the phase composition on high temperature thermo-mechanical properties and the enhancement of the bending strength have been discussed, providing a new perspective for further improvements.     

Etching effects of ethanol on multi-walled carbon nanotubes

The morphology and microstructure of multi-walled carbon nanotubes (MWCNTs) were modified using ethanol as a mild gas reactant. The etching by OH radicals and deposition of C radicals on the carbon nanotubes were considered to be responsible for the modification of the MWCNT structures and the formation of new carbon nanostructures. The effects of etching and deposition on the MWCNTs were confirmed by using methanol as another gas reactant; this molecule has a higher ratio of hydroxyl radicals to carbon atoms than ethanol. In addition, water vapor, containing no carbon atoms in the molecule, was also applied to etch the MWCNTs as a weak oxidant which resulted in stronger etching effects on the MWCNTs than methanol and ethanol.     

An effective approach to reinforced closed-cell Al-alloy foams with multiwalled carbon nanotubes

Exploring the reinforcing role of carbon nanotubes to obtain materials (polymers, metals, ceramics) with enhanced properties has been often attempted, but the success is strongly limited by the dispersing degree of carbon nanotubes. Here we report on an innovative colloidal approach to disperse the carbon nanotubes in the powders mixture of the precursor materials in order to profit from their reinforcing potential and obtain a new class of closed-cell metal foams. The feasibility of the proposed approach was demonstrated for aluminium foams reinforced with multi-walled carbon nanotubes. These nanocomposite metal foams synergistically combine the remarkable properties of both metal foams and carbon nanotubes. The results indicate that the tubular structure of carbon nanotubes is preserved throughout the entire the process. The carbon nanotubes are individually dispersed, stretched and randomly aligned in the aluminium-matrix of these closed-cell foams, thus potentiating their homogeneous 3D reinforcing role. Accordingly, the Vickers micro-hardness of the closed-cell foams was greatly enhanced.     

Facile synthesis of silicon dioxide nanoparticles decorated multi-walled carbon nanotubes with graphitization and carboxylation for electrochemical detection of gallic acid

We proposed an efficient and scalable ultrasound-assisted approach for the synthesis of functionally integrated nanohybrid of silicon dioxide (SiO₂) nanoparticles and multi-walled carbon nanotubes with graphitization and carboxylation (GCMCN), which was employed to modify the glassy carbon electrode (GCE) for the fabrication of GCMCN@SiO₂/GCE sensor. Graphitization of GCMCN contributed to the reduction of defect density and enhancement of electrical conductivity, and carboxylation of GCMCN improved the dispersion degree of carbon nanomaterial due to the hydrophilicity of carboxyl groups. SiO₂ nanoparticles possessed abundant binding active sites for target analytes due to the surface hydroxyl groups or silanol groups, which were beneficial for the enrichment of gallic acid (GA) molecules. For the functionally integrated GCMCN@SiO₂ nanocomposite, the interconnected conductive networks of GCMCN presented more efficient charge transport channels, which recompensed the non-conductive property of SiO₂ nanoparticles. Based on the functional collaboration of GCMCN and SiO₂ nanoparticles, the fabricated GCMCN@SiO₂/GCE sensor presented good GA detection property (GA concentration: 0.01–15 μM, LOD value: 1.99 nM). The proposed sensor exhibited acceptable repeatability, reproducibility, and selectivity. Moreover, the good practicability performance could be effectuated at the GCMCN@SiO₂/GCE sensor for the quantitative analysis of GA in black tea and green tea samples.     

The effect of electrolytic oxidation on the electrochemical properties of multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWNTs) were electrochemically oxidized by a constant-potential electrolysis method and then investigated in detail using scanning electron microscope, transmission electron microscope, FT-IR, electrical impedance spectroscopy, and cyclic voltammetry. The FT-IR spectra showed that the amount of hydroxyl generated on the surface of MWNTs increased with increasing the electrochemical oxidation time of MWNTs. The CV results, being conducted in nitrobenzene solution, showed that the nitrobenzene reduction current increased with the increase in oxidation time of the MWNTs within the first 60 min of electrolysis. An electrical equivalent circuit model for electrical impedance spectroscopy was further established to analyze the surface capacitance and resistance of the MWNTs, and the model results showed that the capacitance of the oxidized MWNTs increased greatly while the charge transfer resistance decreased, suggesting electrochemical oxidized MWNTs modified pyrolytic carbon electrode being an effective electrochemical sensor for nitrobenzene determination.