Effect of MWCNT alignment on mechanical and self-monitoring properties of extruded PET–MWCNT nanocomposites

Self-monitoring aligned MWCNT loaded PET composites, with different CNT content, were prepared via twin-screw extrusion starting from a PET/MWCNT masterbatch, and fully characterized. All electrically conductive samples showed self-monitoring ability, i.e. a variation in electrical resistance as a function of stress. Moreover, the insertion of MWCNTs resulted in mechanical reinforcement with respect to neat PET. It was found that both self-monitoring behavior and mechanical performance are directly related to MWCNT content and to the direction of applied stress with respect to CNT orientation. In particular, too high MWCNT content decreased sensitivity at low strain, whereas a minimum MWCNT content was required to insure ohmic conductivity.     

Correlation between dispersion state and electrical conductivity of MWCNTs/PP composites prepared by melt blending

Non-conductive polymers filled with conductive carbon nanotubes (CNTs) often do not show detectable conductivity due to poor dispersion of carbon nanotubes in the polymer matrix and the lack of conductive networks formed from CNTs. In this work, we attempted two ways to improve the dispersion of multi-walled carbon nanotubes (MWCNTs) in a polypropylene (PP) matrix: chemical modification of MWCNTs and addition of a master batch as a compatibilizer, followed by melt blending using a micro-compounder. The relationship between the dispersion state of MWCNTs and the electrical conductivity of the CNTs/PP composites have been investigated by controlling several factors such as CNTs modification, compatibilization by a master batch, melt mixing, and post-heat treatment. The enhanced interfacial adhesion between the CNTs and the polymer could improve the dispersion of CNTs but it could also reduce the electrical conductivity of the composites. Meanwhile, it is interestingly found that the post-heat treatment could increase the conductivity remarkably due to the connection of CNTs into networks. Thus, it is concluded that the balance between dispersion of CNTs and the formation of conductive networks plays an important role in enhancing the electrical conductivity of composites.     

Superior mechanical and electrical properties of multiwall carbon nanotube reinforced acrylonitrile butadiene styrene high performance composites

In-house synthesized multiwall carbon nanotubes (MWCNTs) have been dispersed in acrylonitrile butadiene styrene (ABS) using a micro twin-screw extruder with back flow channel. The electrical and mechanical properties of MWCNTs in ABS with different wt% have been studied. Incorporation of only 3 wt. % MWCNTs in ABS leads to significant enhancement in the tensile strength (up to 69.4 MPa) which was equivalent to 29% increase over pure ABS. The effect of MWCNTs on the structural behaviour of ABS under tensile loading showed a ductile to brittle transition with increase concentration of MWCNTs. The results of enhanced mechanical properties were well supported by micro Raman spectroscopic and scanning electron microscopic studies. In addition to the mechanical properties, electrical conductivity of these composites increased from 10⁻¹² to 10⁻⁵ Scm⁻¹ showing an improvement of ∼7 orders of magnitude. Due to significant improvement in the electrical conductivity, EMI shielding effectiveness of the composites is achieved up to −39 dB for 10 wt. % loaded MWCNTs/ABS indicating the usefulness of this material for EMI shielding in the Ku-band. The mechanism of improvement in EMI shielding effectiveness is discussed by resolving their contribution in absorption and reflection loss. This material can be used as high-strength EMI shielding material.     

Effect of MWCNTs/ZnO inorganic fillers on the electrical,mechanical and thermal properties of SiR-based composites

Rubber insulation materials were widely used in the fields of electrical and electronic engineering, especially, which have excellent nonlinear electrical conductivity and can be employed to homogenize the electric field distribution of cable accessories. To enable the rubber materials, such as silicon rubber (SiR), to possess excellent nonlinear electrical conductivity has been a hot issue. In this paper, MWCNTs/ZnO inorganic fillers were prepared by mixing a small amount of multi-wall carbon nanotubes (MWCNTs) with zinc oxide (ZnO) nanosheets, and MWCNTs/ZnO/SiR composites were prepared. The macroscopical properties results show that the nonlinear electrical conductivity characteristics can be induced by filling appropriate content of MWCNTs/ZnO fillers, and the threshold field strength corresponding to the nonlinear conductivity gradually decreases with the increase of MWCNTs filling content, which further decreases with the increase of measured temperature. The COMSOL simulation results also verify that MWCNTs/ZnO/SiR composite with nonlinear conductivity can effectively reduce the electric field strength at the stress cone of cable accessories. In addition, the thermal conductivity and tensile strength for MWCNTs/ZnO/SiR composite are also improved comparing to pristine SiR. This work demonstrates MWCNTs/ZnO/SiR composites possess outstanding overall properties and have good potential to be used in the cable accessory.

     

Effect of Moisture Content on Thermal Properties of Porous Building Materials

The thermal conductivity and specific heat capacity of characteristic types of porous building materials are determined in the whole range of moisture content from dry to fully water-saturated state. A transient pulse technique is used in the experiments, in order to avoid the influence of moisture transport on measured data. The investigated specimens include cement composites, ceramics, plasters, and thermal insulation boards. The effect of moisture-induced changes in thermal conductivity and specific heat capacity on the energy performance of selected building envelopes containing the studied materials is then analyzed using computational modeling of coupled heat and moisture transport. The results show an increased moisture content as a substantial negative factor affecting both thermal properties of materials and energy balance of envelopes, which underlines the necessity to use moisture-dependent thermal parameters of building materials in energy-related calculations.     

Simultaneous global and local strain sensing in SWCNT-epoxy composites by Raman and impedance spectroscopy

Polymer composites can be benefited in many ways through the addition of carbon nanotubes (CNT). For instance, CNT can build up a percolated network within the polymer matrix, which results in a composite material with electrical conductivity and piezoresistive characteristics. This has very important implications for the realization of self-stress sensing structural composites. Moreover, the remarkable optical and transport properties of CNT permit to obtain information about the stress state of the composite at different scales. In the present work, the local and global stress response of SWCNT-epoxy composites is characterised by simultaneous Raman spectroscopic and electrical measurements on nanocomposite specimens submitted to different levels of surface strain. Both the Raman G′-band resonance frequency and the electrical resistance of the composite are found to change monotonically with strain until an inflection point is reached at ∼1.5% strain. Increased sensitivity of the piezoresistive network and lower load transfer efficiency occur beyond this strain level, and are considered to be the result of CNT slippage from the polymer. The reversibility of the stress sensitivity of the composites is verified by performing cyclic loading tests. Hysteresis loop are found to develop earlier on the Raman curves as in the resistance curves, which indicates that even at low strain levels, permanent damage is induced in the vicinity of carbon nanotubes. The use of Raman spectroscopy in combination with electrical methods provides a further insight on the stress sensing capabilities of CNT and the factors which affect the sensitivity and reproducibility of this behaviour.     

Electrical conductivity and mechanical properties of multiwalled carbon nanotube-reinforced polypropylene nanocomposites

In this paper, the electrical conductivity and mechanical properties such as elastic modulus of multiwalled carbon nanotubes (MWCNTs) reinforced polypropylene (PP) nanocomposites were investigated both experimentally and theoretically. MWCNT-PP nanocomposites samples were produced using injection mold at different injection velocities. The range of the CNT fillers is from 0 up to 12?wt%. The influence of the injection velocity and the volume fraction of CNTs on both electrical conductivity and mechanical properties of the nanocomposites were studied. The injection speed showed some effect on the electrical conductivity, but no significant influence on the mechanical properties such as elastic modulus and stress-strain relations of the composites under tensile loading. Parallel to the experimental investigation, for electrical conductivity, a percolation theory was applied to study the electrical conductivity of the nanocomposite system in terms of content of nanotubes. Both Kirkpatrick (Rev Mod Phys 45:574?C588, 1973) and McLachlan et?al. (J Polym Sci B 43:3273?C3287, 2005) models were used to determine the transition from low conductivity to high conductivity in which designates as percolation threshold. It was found that the percolation threshold of CNT/PP composites is close to 3.8?wt%. For mechanical properties of the system, several micromechanical models were applied to elucidate the elastic properties of the nanocomposites. The results indicate that the interphase between the CNT and the polymers plays an important role in determining the elastic modulus of the system.     

Electrical conductivity enhancement of polymer/multiwalled carbon nanotube (MWCNT) composites by thermally-induced defunctionalization of MWCNTs

We report a thermally-induced increase of electrical conductivity of polymer/multiwalled carbon nanotube (MWCNT) composites using Diels-Alder-adduct-modified MWCNTs as additives. Thermal treatments of the composites induce the defunctionalization of the modified MWCNTs through retro-DA reaction, consequently to recover the electrical conductivity of MWCNTs and to increase the conductivity of PVDF/MWCNT composites. For the composites possessing 0.5 wt % of MWCNTs, thermal treatment increases the electrical conductivity from 2 × 10(-12) S cm(-1) to 4 × 10(-8) S cm(-1) and significantly reduces the value of percolation threshold. Meanwhile, the thermal treatment does not alter the mechanical properties of the composites.     

Hygro-Thermo-Electric Properties of Carbon Nanotube Epoxy Nanocomposites with Agglomeration Effects

In the present study, the effective electric, thermal, and moisture properties of carbon nanotube (CNT) epoxy composites are derived by considering the agglomeration effect of CNT concentrations in the epoxy matrix. In this direction, the Voigt and Reuss homogenization method is adopted in the derivations. It is well known from experiments that the CNT thermal and electrical conductivities and the epoxy hygro-thermal expansion coefficients have significant effects on the behavior of CNT nanocomposites. Moreover, it has been experimentally proved that the agglomeration of CNTs in the matrix with high and low concentrations of the CNTs certainly affects the resistivity and, hence, the thermal expansion properties. Therefore, the effective elastic, thermal, electrical, and moisture properties for the randomly distributed CNTs in the matrix has been derived in terms of the agglomeration volume fractions of CNTs. In the effective relations, a single agglomeration parameter is considered to be active for a given potential. The results of variation in the hygro-electro-thermal properties due to change in CNT volume fraction as well as agglomeration parameters have been presented. The results and observation show that CNT agglomeration has a strong influence on the effective hygro-thermo-electric properties of the nanocomposites.     

The effect of geometric factor of carbon nanofillers on the electrical conductivity and electromagnetic interference shielding properties of poly(trimethylene terephthalate) composites: a comparative study

In this study, the effects of filler geometry on the electrical conductivity and electromagnetic interference (EMI) shielding properties of poly(trimethylene terephthalate) (PTT) composites filled with graphene nanosheets (GNSs), carbon nanotubes (CNTs), and GNS–CNT hybrid nanofillers have been investigated. The GNSs, CNTs, and hybrid GNS–CNT were well dispersed in the PTT matrix using a simple coagulation process. GNSs were prepared from graphene oxide (GO) through hydrazine reduction, and thermal reduction of GO at two different temperatures of 1050 and 1500 °C. PTT filled with different aspect ratios and oxygen functional groups of GNS were also prepared in order to compare the electrical conductivity and EMI shielding properties. The aspect ratios of GNSs and CNTs were estimated by using an ellipsoid model. Percolation scaling laws were applied to the magnitudes of conductivity to reveal the percolation network and filler dispersion. The percolation exponent of the PTT/GNS composites was larger than that of the PTT/CNT composites. The percolated filler–filler network at which the percolation exponent changed was correlated with the filler geometric structure. GNS–CNT hybrid nanofillers formed a complex double brush structure in the PTT/GNS–CNT composites. The geometric structure, aspect ratio, and intrinsic conductivity of carbon nanofillers affected the electrical percolation threshold and EMI shielding efficiency of the composites.     

MWCNTs改善WS2/三元乙丙橡胶复合材料的非线性电导特性与热导性能

通过在一定量的纳米WS₂中添加极少量的多壁碳纳米管（MWCNTs）,形成MWCNTs-WS₂复配填料,采用双辊开炼机将三元乙丙橡胶（EPDM）与不同配比的复配填料混合制备了不同MWCNTs含量的MWCNTs-WS₂/EPDM复合材料。并研究了极少量的MWCNTs添加对MWCNTs-WS₂/EPDM复合材料非线性电导性能、直流击穿性能和导热性能的影响。结果表明,极少量的MWCNTs对MWCNTs-WS₂/EPDM复合材料在25℃时的非线性电导特性起到明显的增强作用,且随着MWCNTs含量的增加,复合材料非线性电导特征有明显的规律性变化;由于MWCNTs自身的高电导率和电导正温度系数效应,MWCNTs-WS₂/EPDM复合材料电导率随电场强度的变化趋势在80℃时不再表现非线性特征。另外,极少量的MWCNTs对MWCNTs-WS₂/EPDM复合材料的热导率有明显地改善。      

Electrical conductivity of drying cement paste

Previous research has shown that electrical measurements can be used to monitor moisture movement inside concrete. The interpretation of these measurements is frequently based on empirical relationships between moisture changes and electrical properties of concrete. As such, these empirical relationships can limit the application of the electrical measurements to a specific material or exposure history. To facilitate the development of a general method that is applicable to a concrete member in service, this paper characterizes the electrical conduction in cement paste subjected to drying (desorption) and moisture absorption. The paper quantifies how the electrical conductivity is dependent on the volume and connectivity of the moisture inside the pores and the conductivity of pore solution. This paper also presents a procedure to quantify the contribution of the surface (solid-pore) conduction on the overall conductivity of the cement paste. The results of this investigation contribute to the development of an embedded relative humidity sensor that can be used to monitor changes in the internal humidity of concrete during its service life.     

Highly conductive carbon nanotube buckypapers with improved doping stability via conjugational cross-linking

Carbon nanotube (CNT) sheets or buckypapers have demonstrated promising electrical conductivity and mechanical performance. However, their electrical conductivity is still far below the requirements for engineering applications, such as using as a substitute for copper mesh, which is currently used in composite aircraft structures for lightning strike protection. In this study, different CNT buckypapers were stretched to increase their alignment, and then subjected to conjugational cross-linking via chemical functionalization. The conjugationally cross-linked buckypapers (CCL-BPs) demonstrated higher electrical conductivity of up to 6200?S?cm( - 1), which is more than one order increase compared to the pristine buckypapers. The CCL-BPs also showed excellent doping stability in over 300?h in atmosphere and were resistant to degradation at elevated temperatures. The tensile strength of the stretched CCL-BPs reached 220?MPa, which is about three times that of pristine buckypapers. We attribute these property improvements to the effective and stable conjugational cross-links of CNTs, which can simultaneously improve the electrical conductivity, doping stability and mechanical properties. Specifically, the electrical conductivity increase resulted from improving the CNT alignment and inter-tube electron transport capability. The conjugational cross-links provide effective 3D conductive paths to increase the mobility of electrons among individual nanotubes. The stable covalent bonding also enhances the thermal stability and load transfer. The significant electrical and mechanical property improvement renders buckypaper a multifunctional material for various applications, such as conducting composites, battery electrodes, capacitors, etc.     

Fabrication and effective thermal conductivity of multi-walled carbon nanotubes reinforced Cu matrix composites for heat sink applications

A novel particles-compositing method was used for the first time to disperse different contents of multi-walled carbon nanotubes (CNTs) in micron sized copper powders, which were subsequently consolidated into CNT/Cu composites by spark plasma sintering (SPS). Microstructural observations showed that the homogeneous distribution of CNTs and dense composites could be obtained for 0–10 vol.% CNT contents. The CNT clusters were appeared in the powder mixture with 15 vol.% CNTs, which resulted in an insufficient densification of the composites. The effective thermal conductivity of the composites was analyzed both theoretically and experimentally. The addition of CNTs showed no enhancement in overall thermal conductivity of the composites due to the interface thermal resistance associated with the low phase contrast of CNT to copper and the random tube orientation. Besides, the composite containing 15 vol.% CNTs led to a rather low thermal conductivity due possiblely to the combined effect of unfavorable factors induced by the presence of CNT clusters, i.e. large porosity, lower effective conductivity of CNT clusters themselves and reduction of SPS cleaning effect. The CNT/Cu composites may be a promising thermal management material for heat sink applications.     

Electric field effects on CNTs/vinyl ester suspensions and the resulting electrical and thermal composite properties

In this study, electrical conductivity of a vinyl ester based composite containing low content (0.05, 0.1 and 0.3 wt.%) of double and multi-walled carbon nanotubes with and without amine functional groups (DWCNTs, MWCNTs, DWCNT-NH₂ and MWCNT-NH₂) was investigated. The composite with pristine MWCNTs was found to exhibit the highest electrical conductivity. Experiments aimed to induce an aligned conductive network with application of an alternating current (AC) electric field during cure were carried out on the resin suspensions with MWCNTs. Formation of electric anisotropy within the composite was verified. Light microscopy (LM), scanning electron (SEM) and transmission electron microscopy (TEM) were conducted to visualize dispersion state and the extent of alignment of MWCNTs within the polymer cured with and without application of the electric field. To gain a better understanding of electric field induced effects, glass transition temperature (T_g) of the composites was measured via Differential Scanning Calorimetry (DSC). It was determined that at 0.05 wt.% loading rate of MWCNTs, the composites, cured with application of the AC electric field, possessed a higher T_g than the composites cured without application of the AC electric field.     

Mechanical,electrical and thermal properties of aligned carbon nanotube/polyimide composites

Carbon nanotubes (CNTs) have high strength and modulus, large aspect ratio, and good electrical and thermal conductivities, which make them attractive for fabricating composite. The poly(biphenyl dianhydride-p-phenylenediamine) (BPDA/PDA) polyimide has good mechanical and thermal performances and is herein used as matrix in unidirectional carbon nanotube composites for the first time. The strength and modulus of the composite increase by 2.73 and 12 times over pure BPDA–PDA polyimide, while its electrical conductivity reaches to 183 S/cm, which is 10¹⁸ times over pure polyimide. The composite has excellent high temperature resistance, and its thermal conductivity is beyond what has been achieved in previous studies. The improved properties of the composites are due to the long CNT length, high level of CNT alignment, high CNT volume fraction and good CNT dispersion in polyimide matrix. The composite is promising for applications that require high strength, lightweight, or high electrical and thermal conductivities.     

碳纳米管/聚丙烯腈基复合材料的热导率研究

采用原位聚合法制备了碳纳米管/聚丙烯腈(CNT/PAN)复合材料,用MDSC的测试方法研究了复合材料的热性能,并由此推导了复合材料的热导率.应用Cheng-Vachon、Nielsen-Lewis和Okamoto-Ishida 3种导热理论模型对CNT/PAN复合材料的热导率进行估算.对比实验测试与导热理论模型的计算结果,考虑到碳纳米管在聚合物基体中的分散和取向情况,得出Nielsen-Lewis理论在低填充含量及室温条件下可以较准确地估算无规分散的CNT/PAN复合材料体系的热导率.     

Carbon nanotubes dispersed polymer nanocomposites: mechanical,electrical, thermal properties and surface morphology

The various properties and surface morphology of the carbon nanotubes (CNTs) dispersed polydimethyl siloxane (PDMS) matrix were studied to determine their usefulness in various applications. The tensile strength, Young’s modulus and electrical breakdown strength of CNT/polymer composites were 0.35 MPa, 1.2 MPa and 8.1 kV, respectively. The thermal conductivity and dielectric constant for the material having 4.28 wt% CNT were 0.225 W m^?1 K^?1 and 2.329, respectively. The CNT/polymer composites are promising functional composites with improved mechanical and electrical properties. The scanning electron microscope analysis of surface morphology of PDMS/CNT composite showed that the rough surface texture on nanocomposite has large surface area with circular pores. The Fourier transform infrared spectroscopy showed the functional groups present in polymer nanocomposite.     

On the contribution of carbon nanotube deformation to piezoresistivity of carbon nanotube/polymer composites

The change in electrical resistance due to mechanical deformation of carbon nanotube (CNT)/polymer composites can be rationalized in terms of two effects: (i) changes in the composite electrical resistivity due to changes in the CNT network configuration and (ii) deformation of the CNTs themselves. The contribution of CNT dimensional changes (ii) to the piezoresistivity of CNT/polymer composites is investigated here. An analytical model based exclusively on dimensional changes which describes the CNT change of electrical resistance in terms of its mechanical deformation is proposed. A micromechanics approach and finite element analysis are performed to correlate the macroscale composite strain to the individual CNT strain. The CNT change of electrical resistance is quantified for different matrix elastic moduli and CNT weight fractions. The CNT/polymer composite is also modeled as an effective continuum material in terms of both its electrical and mechanical responses so that the effect of dimensional changes on the global piezoresistivity can be investigated. Based on the modeling predictions and previous experimental results, it is estimated that the CNT change of resistance due to the macroscale composite strain is marginal (∼5%) compared to the total composite change of resistance commonly measured in the laboratory, suggesting that the dominant effect in the piezoresistivity of CNT/polymer composites is the change in the CNT network configuration.