Thermal conductivity of MWCNT/epoxy composites: The effects of length,alignment and functionalization

Carbon nanotubes (CNTs) show great promise to improve composite electrical and thermal conductivity due to their exceptional high intrinsic conductance performance. In this research, long multi-walled carbon nanotubes (long-MWCNTs) and its thin sheet of entangled nanotubes were used to make composites to achieve higher electrical and thermal conductivity. Compared to short-MWCNT sheet/epoxy composites, at room temperature, long-MWCNT samples showed improved thermal conductivity up to 55 W/mK. The temperature dependence of thermal conductivity was in agreement with κ ∝ Tⁿ (n = 1.9–2.3) below 150 K and saturated around room temperature due to Umklapp scattering. Samples with the improved CNT degree of alignment by mechanically stretching can enhance the room temperature thermal conductivity to over 100 W/mK. However, functionalization of CNTs to improve the interfacial bonding resulted in damaging the CNT walls and decreasing the electrical and thermal conductivity of the composites.     

Effect of particle size,heating rate and CNT addition on densification,microstructure and mechanical properties of B4C ceramics

Monolithic B₄C ceramics and B₄C–CNT composites were prepared by spark plasma sintering (SPS). The influence of particle size, heating rate, and CNT addition on sintering behavior, microstructure and mechanical properties were studied. Two different B₄C powders were used to examine the effect of particle size. The effect of heating rate on monolithic B₄C was investigated by applying three different heating rates (75, 150 and 225 °C/min). Moreover, in order to evaluate the effect of CNT addition, B₄C–CNT (0.5–3 mass%) composites were also produced. Fully dense monolithic B₄C ceramics were obtained by using heating rate of 75 °C/min. Vickers hardness value increased with increasing CNT content, and B₄C–CNT composite with 3 mass% CNTs had the highest hardness value of 32.8 GPa. Addition of CNTs and increase in heating rate had a positive effect on the fracture toughness and the highest fracture toughness value, 5.9 MPa m^1/2, was achieved in composite with 3 mass% CNTs.     

The hierarchical structure and properties of multifunctional carbon nanotube fibre composites

The axial mechanical, electrical and thermal properties of carbon nanotubes (CNTs) can be exploited macroscopically by assembling them parallel to each other into a fibre during their synthesis by chemical vapour deposition. Multifunctional composites with high volume fraction of CNT fibres are then made by direct polymer infiltration of an array of aligned fibres. The fibres have a very high surface area, causing the polymer to infiltrate them and resulting in a hierarchical composite structure. The electrical and thermal conductivities of CNT/epoxy composites are shown to be superior to those of equivalent specimens with T300 carbon fibre (CF) which is widely used in industry. From measurements of longitudinal coefficient of thermal expansion (CTE) of the composites we show that the CTE of CNT fibres is approximately ?1.6 × 10^?6 K^?1, similar to in-plane graphite. The combination of electrical, thermal and mechanical properties of CNT fibre composites demonstrates their potential for multifunctionality.     

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.     

Improved thermal interface property of carbon nanotube–Cu composite based on supercritical fluid deposition

In this paper, we present a new synthesis method of carbon nanotubes (CNTs)-copper (Cu) composite on a silicon substrate using combination of supercritical fluid deposition (SCFD) and electrochemical plating (ECP) process. Deposition of a Cu layer onto CNTs is carried out under supercritical condition, and the CNTs–Cu composite with high-density Cu is synthesized by additional ECP process. The Cu layer deposited by SCFD functions as a seed layer for ECP, and spaces between neighboring CNTs are filled by Cu. The measured density of the CNTs–Cu composite is 8.2 ± 0.3 g/cm³, and the volume percentage of voids is 3–6%. The evaluated thermal resistance including the thermal interface resistance and bulk resistance of the composite is as low as 28.4 mm² K W⁻¹ at a contact pressure of 0.2 MPa. A CNT brush formed on the composite surface can reduce the thermal resistance to be 68.4 mm² K W⁻¹ at a contact pressure of 0.25 MPa. The CNTs–Cu composite shows the ability applicable to many microelectronics applications as a thermal interface material.     

Improved mechanical performance of solution-processed MWCNT/Ag nanoparticle composite films with oxygen-pressure-controlled annealing

A method for the synthesis of solution process-based MWCNT/Ag nanoparticle composite thin films as electrode or interconnect materials for flexible electronic devices is presented. The method produces homogeneously-dispersed CNT networks and increases the density of the Ag matrix, which are major factors in determining the mechanical performance of CNT/Ag films. By introducing nanometer-sized Ag particles as a matrix material, the agglomeration of CNTs is suppressed. In addition, the generation of pores during the synthesis procedure is effectively restrained by oxygen-pressure-controlled annealing. The elastic modulus of the pristine Ag films was observed to increase by 34% by adding 5 wt% CNTs. An improvement in the fatigue resistance of the CNTs under cyclic tensile deformation was confirmed. The normalized resistance change ((R ? R_o)/R_o) of the Ag films containing 5 wt% CNTs after fatigue testing was reduced by about 27% compared to that of the pristine Ag films. For industrial application the process has the advantage of relatively low-temperature processing without any high pressure compaction compared to the conventional powder metallurgy techniques normally used.     

The influence of CNTs on the thermoelectric properties of a CNT/Bi2Te3 composite

CNT/Bi₂Te₃ composites were prepared from composite powders in which CNTs were implanted in the Bi₂Te₃ matrix powders by a novel chemical route. It was found that the fabricated composite had a microstructure of a homogeneous dispersion of CNTs in the Bi₂Te₃ matrix due to interfacial bonding agents of oxygen atoms attaching to the surface of CNTs. The dimensionless figure of merit (ZT) of the composite shows significantly increased values compared to those of pure binary Bi₂Te₃ in the temperature range of 298–498 K and a maximum ZT of 0.85 was obtained at 473 K. It is considered that the improved thermoelectric performance of the composite mainly originated from thermal conductivity that was reduced by active phonon-scattering at the CNT/Bi₂Te₃ interface.     

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.     

Electromagnetic interference shielding properties of polymer-grafted carbon nanotube composites with high electrical resistance

Poly(methyl methacrylate) (PMMA)-grafted multiwalled CNTs were prepared, and then dispersed into additional PMMA matrix, yielding highly insulated PMMA–CNT composites. The volume resistivity of PMMA–CNT was as high as 1.3 × 10¹⁵ Ω cm even at 7.3 wt% of the CNT. The individual CNTs electrically-isolated by the grafted PMMA chains in PMMA–CNT transmitted electromagnetic (EM) waves in the frequency range of 0.001–1 GHz, whereas the percolated CNTs in a conventional composite prepared by blending PMMA with the pristine CNTs strongly shielded the EM waves. This result suggests that the intrinsic conductivity of the CNT itself in PMMA–CNT does not contribute to the EM interference (EMI) shielding in the frequency range of 0.001–1 GHz. On the other hand, PMMA–CNT exhibited EMI shielding at the higher frequency range than 1 GHz because the dielectric loss of the CNT itself was rapidly increased over 1 GHz. At 110 GHz, PMMA–CNT with 7.3 wt% of the CNT had EMI SE of as high as 29 dB (0.57 mm thickness), though is slightly lower than that of the percolated conventional composite (35 dB). Thus, it is demonstrated that the highly insulated PMMA–CNT has the good EMI shielding at extremely high frequency range (30–300 GHz).     

Multifunctional bioceramic-based composites reinforced with silica-coated carbon nanotube core-shell structures

Carbon nanotube (CNT) possesses eminent mechanical properties and has been widely utilized to toughen bioceramics. Major challenges associated with CNT-reinforced bioceramics include the inhomogeneous dispersion of CNTs and the insufficient interfacial strength between the two phases. To address such issues, this research describes the first use of silica-coated CNT (S-CNT) core-shell structures to reinforce bioceramics using hydroxyapatite (HA) as a representative matrix. HA-based composites with 0.1–2 wt% S-CNT are sintered by spark plasma sintering to investigate their mechanical and biological properties. It is found that when 1 wt% raw CNT (R-CNT) is added, very limited increases in fracture toughness (K_IC) is observed. By contrast, the incorporation of 1 wt% S-CNT increased the K_IC of HA by 101.7%. This is attributed to more homogeneously dispersed fillers and stronger interfacial strengths. MG63 cells cultured on the 1 wt% S-CNT/HA pellets are found to proliferate faster and possess significantly higher alkaline phosphatase activities than those grown on the HA compacts reinforced with 1 wt% R-CNT, probably by virtue of the released Si ions from the SiO2 shell. Therefore, the S-CNT core-shell structures can improve both mechanical and biological properties of HA more effectively than the conventionally used R-CNTs. The current study also presents a novel and effective approach to the enhancement of many other biomedical and structural materials through S-CNT incorporation.     

Improvements on the thermal shock behaviour of MgO–spinel composite refractories by incorporation of zircon–3 mol% Y2O3

The effects of incorporations of zircon–3 mol% Y₂O₃ (Y) into MgO–spinel (M–S) compositions to improve mechanical properties and thermal shock behaviour were investigated. Mechanical properties were measured, R ? R_st parameters were calculated, and thermal shock tests were performed. Microstructural features were examined using SEM and XRD. By adding zircon + Y to M–S: (i) up to ～2 and ～3-fold improvements were achieved in mechanical properties and R ? R_st parameters, (ii) there were improvements up to ～2-fold in strength data measured after performing thermal shock tests at 1000 °C. Parameters improving mechanical properties and thermal shock behaviour of M–S–(zircon + Y) materials are given as follows: (i) interlinking and arresting or deviation of microcracks when reaching the ZrO₂–Y₂O₃ grains or pores, increases in (ii) K_Ic, (iii) critical defect size and (iv) bulk density, (v) formation of forsterite phase, (vi) coexistence of intergranular and transgranular fractures, and (vii) reduction in MgO grain size, leading to longer service life.     

Singly dispersed carbon nanotube/aluminum composites fabricated by powder metallurgy combined with friction stir processing

Carbon nanotube (CNT)/aluminum composites were fabricated by a combination of powder metallurgy and subsequent friction stir processing (FSP). Microstructural observations indicated that the CNTs were singly dispersed in the composites. The CNTs tended to be dispersed along grain boundaries resulting in a much finer grain size. Although the CNTs were shortened and some Al₄C₃ formed in the matrix, the layer structures of the CNTs were well retained. Raman spectroscopy also showed that the damage to CNTs during FSP was not severe. Compared to that of unreinforced Al, the yield strength of 1 wt.% and 3 wt.% CNT/Al composites increased by 23.9% and 45.0%, respectively. A strength equation relating with load transfer and the grain refinement was proposed to describe the increase of the yield strength of the composites.     

Preparation and electrochemical performance of SnO2@carbon nanotube core–shell structure composites as anode material for lithium-ion batteries

Carbon nanotube-encapsulated SnO₂ (SnO₂@CNT) core–shell composite anode materials are prepared by chemical activation of carbon nanotubes (CNTs) and wet chemical filling. The results of X-ray diffraction and transmission electron microscopy measurements indicate that SnO₂ is filled into the interior hollow core of CNTs and exists as small nanoparticles with diameter of about 6 nm. The SnO₂@CNT composites exhibit enhanced electrochemical performance at various current densities when used as the anode material for lithium-ion batteries. At 0.2 mA cm^?2 (0.1C), the sample containing wt. 65% of SnO₂ displays a reversible specific capacity of 829.5 mAh g^?1 and maintains 627.8 mAh g^?1 after 50 cycles. When the current density is 1.0, 2.0, and 4.0 mA cm^?2 (about 0.5, 1.0, and 2.0C), the composite electrode still exhibits capacity retention of 563, 507 and 380 mAh g^?1, respectively. The capacity retention of our SnO₂@CNT composites is much higher than previously reported values for a SnO₂/CNT composite with the same filling yield. The excellent lithium storage and rate capacity performance of SnO₂@CNT core–shell composites make it a promising anode material for lithium-ion batteries.     

Grafting of epoxy chains onto graphene oxide for epoxy composites with improved mechanical and thermal properties

Epoxy composites filled with both graphene oxide (GO) and diglycidyl ether of bisphenol-A functionalized GO (DGEBA–f–GO) sheets were prepared at different filler loading levels. The correlations between surface modification, morphology, dispersion/exfoliation and interfacial interaction of sheets and the corresponding mechanical and thermal properties of the composites were systematically investigated. The surface functionalization of DGEBA layer was found to effectively improve the compatibility and dispersion of GO sheets in epoxy matrix. The tensile test indicated that the DGEBA–f–GO/epoxy composites showed higher tensile modulus and strength than either the neat epoxy or the GO/epoxy composites. For epoxy composite with 0.25 wt% DGEBA–f–GO, the tensile modulus and strength increased from 3.15 ± 0.11 to 3.56 ± 0.08 GPa (∼13%) and 52.98 ± 5.82 to 92.94 ± 5.03 MPa (∼75%), respectively, compared to the neat epoxy resin. Furthermore, enhanced quasi-static fracture toughness (K_IC) was measured in case of the surface functionalization. The GO and DGEBA–f–GO at 0.25 wt% loading produced ∼26% and ∼41% improvements in K_IC values of epoxy composites, respectively. Fracture surface analysis revealed improved interfacial interaction between DGEBA–f–GO and matrix. Moreover, increased glass transition temperature and thermal stability of the DGEBA–f–GO/epoxy composites were also observed in the dynamic mechanical properties and thermo-gravimetric analysis compared to those of the GO/epoxy composites.     

Preparation and characterization of manganese oxide/CNT composites as supercapacitive materials

Manganese oxide was synthesized and dispersed on carbon nanotube (CNT) matrix by thermally decomposing manganese nitrates. CNTs used in this paper were grown directly on graphite disk by chemical vapor deposition technique. The capacitive behavior of manganese oxide/CNT composites was investigated by cyclic voltammetry and galvanostatic charge–discharge method in 1 M Na₂SO₄ aqueous solutions. When the loading mass of MnO₂ is 36.9 μg cm^− 2, the specific capacitance of manganese oxide/CNT composite (based on MnO₂) at the charge–discharge current density of 1 mA cm^− 2 equals 568 F g^− 1. Additionally, excellent charge–discharge cycle stability (ca. 88% value of specific capacitance remained after 2500 charge–discharge cycles) and power characteristics of the manganese oxide/CNT composite electrode can be observed. The effect of loading mass of MnO₂ on specific capacitance of the electrode has also been investigated.     

Thermodynamics and regeneration studies of CO2 adsorption on multiwalled carbon nanotubes

Multiwalled carbon nanotubes (CNTs) were fabricated and modified by 3-aminopropyl-triethoxysilane (APTS) solutions to study thermodynamics and regeneration of CO₂ adsorption from gas streams. The CO₂ adsorption capacities of CNTs and CNT(APTS) decreased with temperature indicating the exothermic nature of adsorption process while the thermodynamic analysis gave low isosteric heats of adsorption, which are typical for physical adsorption. The cyclic CO₂ adsorption on CNT(APTS) showed that the adsorbed CO₂ could be effectively desorbed via thermal treatment at 120 °C for 25 min while the adsorbed CO₂ due to physical interaction could be effectively desorbed via vacuum suction at 0.145 atm for 30 min. If a combination of thermal and vacuum desorption was conducted at 120 °C and 0.145 atm, the time for effectively desorbing CO₂ could be further shortened to 5 min. The adsorption capacities and the physicochemical properties of CNT(APTS) were preserved during 20 cycles of adsorption and regeneration. These results suggest that the CNT(APTS) can be stably employed in prolonged cyclic operation and they are thus possibly cost-effective sorbents for CO₂ capture from flue gases.     

Thermoelectric properties of carbon nanotube reinforced cement-based composites fabricated by compression shear

Carbon nanotubes (CNTs) with weight percent of 5.0%, 10.0% and 15.0% were added into the cement matrix to fabricate CNT reinforced cement-based composites (CNTs/CC) by mixing and dry compression shear methods. Seebeck coefficient, electrical conductivity and thermal conductivity of the as-received CNTs/CC were measured and analyzed in detail. The CNTs/CC exhibits the thermoelectric behavior of p-type semiconductor. CNTs were dispersed uniformly in cement matrix by compression shear stress, which promoted a relatively high electrical conductivity (0.818 S/cm) and Seebeck coefficient (57.98 μV/°C) of CNTs/CC. Combining with their lower thermal conductivity ranged from 0.734 to 0.947 W m^?1 K^?1, the CNTs/CC shows the highest thermoelectric figure of merit (ZT) has reached 9.33 × 10^?5, Which is benefit to the applications in large-scale energy harvesting in the buildings and pavements with low cost in the future cities.     

The effect of carbon nanotubes on the sintering behaviour of zirconia

The sintering behaviour and activation energy of Y₂O₃ partially stabilised ZrO₂ and ZrO₂–CNT (0.5 and 2 vol%) composites was determined using spark plasma sintering (SPS) under isothermal conditions. The sintering activation energy for the Y₂O₃ partially stabilised ZrO₂ was found to be 456 kJ/mol. The addition of 2 vol% CNTs reduced the sintering activation energy to 172 kJ/mol. The significant reduction of the activation energy with the addition of only 2 vol% CNTs is attributed to the formation of a percolating network of CNTs providing a lower energy diffusion pathway. The sintering mechanism was found to be grain boundary diffusion for all samples suggesting that the presence of CNTs does not change the sintering mechanism but does lower the activation energy for the rate limiting step in the sintering process.     

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.     

Physical properties of carbon nanotube sheets drawn from nanotube arrays

We report mechanical, thermal, and electrical properties of novel sheet materials composed of multiwalled carbon nanotubes, drawn from a CNT array. At low loading there is some slippage of CNTs but at higher loading tensile strength σ₀ = 7.9 MPa and Young’s modulus E = 310 MPa. The room-temperature thermal conductivity of the CNT sheet was 2.5 ± 0.5 W m^?1 K^?1, giving a thermal conductivity to density ratio of κ/ρ = 65 W m^?1 K^?1 g^?1 cm³. The heat capacity shows 1D behavior for T > 40 K, and 2D or 3D behavior at lower temperatures. The room-temperature specific heat was 0.83 J g^?1 K^?1. The iV curves above 10 K have Ohmic behavior while the iV curve at T = 2 K is non-Ohmic, and a model to explain both ranges is presented. Negative magnetoresistance was found, increasing in magnitude with decreasing temperature (?15% at T = 2 K and B = 9 T). The tensile strength, Young’s modulus and electrical conductivity of the CNT sheet are low, in comparison with other CNT materials, likely due to defects. Thermal conductivity is dominantly phononic but interfacial resistance between MWCNTs prevents the thermal conductivity from being higher.