Aspect Ratio Of Carbon Nanotubes Research Articles

Enhancement of carbon nanotubes/kerosene nanofluids on mixed convective heat transfer in rectangular enclosures

With the objective of understanding how the mixed convective and the geometrical parameters together influence the nanofluid behavior (enhancement or deterioration) and finding the best configuration of the moving walls for engineering purposes, this paper investigates the single lid-driven mixed convective flow of carbon nanotubes/kerosene nanofluids confined in vertical and horizontal cavities with non-uniform boundary conditions applied to the vertical walls. The findings of this paper can provide crucial conclusions into the behavior and optimization of nanofluids in mixed convective flows in enclosed systems for engineering applications. The system of the governing equations is tackled by a validated in-house code, where we compared the code accuracy with previous experimental and numerical works. The governing parameters are -50 ≤ Pe ≤ 50, 0.5 ≤ A ≤ 4, and 0 ≤ φ ≤ 3%. The effective nanofluid properties are calculated using special experimental models. The outcomes revealed that the carbon nanotube effect on heat transfer depends on the aspect ratio. The greater the aspect ratio, the more pronounced the negative impact of the nanotube concentration where Nusselt number is reduced by 4.49% and 16.17% for φ = 0.01 and 0.03, respectively, in the case of Pe = 0 and A = 4. The Peclet number also has a significant impact on the heat transfer, especially for higher aspect ratios, and diminishes the critical aspect ratio where the nanofluid's behavior changes. An augmentation in the aspect ratio and Peclet number brings about an augmentation in the Nusselt number. Additionally, the Peclet number can reverse the conjugated effect of carbon nanotube volume fraction and aspect ratio on nanofluid enhancement. When Pe = 50 in C1, heat transfer rate is 1.75% and 1.11% enhanced for φ = 0.01 and 0.03, respectively, and A = 4. Moving horizontal walls exhibits higher heat transfer enhancement compared to vertical ones.

Machine learning approach to predict the mechanical properties of cementitious materials containing carbon nanotubes

This research explores the use of machine learning to predict the mechanical properties of cementitious materials enhanced with carbon nanotubes (CNTs). Specifically, the study focuses on estimating the elastic modulus and flexural strength of these novel composite materials, with the potential to significantly impact the construction industry. Seven key variables were analyzed including water-to-cement ratio, sand-to-cement ratio, curing age, CNT aspect ratio, CNT content, surfactant-to-CNT ratio, and sonication time. Artificial neural network, support vector regression, and histogram gradient boosting, were used to predict these mechanical properties. Furthermore, a user-friendly formula was extracted from the neural network model. Each model performance was evaluated, revealing the neural network to be the most effective for predicting the elastic modulus. However, the histogram gradient boosting model outperformed all others in predicting flexural strength. These findings highlight the effectiveness of the employed techniques, in accurately predicting the properties of CNT-enhanced cementitious materials. Additionally, extracting formulas from the neural network provides valuable insights into the interplay between input parameters and mechanical properties.

On the increased interfacial resistance of hydrogen in carbon nanotube arrays and its effect on gas mixture separation

We outline a surface scattering kernel for rarefied gas flows through ideally ordered nanomaterials, such as high aspect ratio carbon nanotubes. The derived model allows for a comparison of the tangential momentum accommodation coefficient, and, hence, the total effective friction, for different species of gases as a function of the particle diameter. This surface kernel is incorporated with a Fokker–Planck model as an approximation to transport of a rarefied gas through ideally ordered carbon nanotubes. The results of this analysis predict that H2 experiences higher friction in such systems in comparison with larger molecules such as CH4. The results are proposed as a potential explanation of the reduced gas transport of hydrogen gas in nanoporous systems.

Preparation and application method of CNTs dispersion solution for direct electroplating on circuit board holes

Abstract The utilization of carbon black dispersion solutions for the black hole method is common in the electronics industry due to their cost-effectiveness and stable performance. However, the slow electroplating speed caused by carbon black’s poor conductivity can impact industrial processes and time. This paper introduces a water-based dispersion solution of carbon nanotubes (CNTs) for the direct copper electroplating process on circuit board holes and its application method. The dispersion solution comprises “shortened” CNTs with a small aspect ratio, a dispersant, a pH adjuster, and other optimization agents. This black hole liquid functions as a conductive fluid with “shortened” CNTs as the conductive medium, exhibiting high dispersibility, strong through-hole capability, low resistance, and excellent adhesion. Its performance surpasses that of black hole liquids using single-carbon black as the conductive medium. The key advantage lies in avoiding the entanglement associated with conventional long aspect ratio CNTs, thereby enhancing the conductivity of the water-based dispersion solution. This facilitates the formation of a uniform conductive film through the electrostatic self-adsorption method, which is suitable for preparing a conductive film on circuit board hole walls before copper electroplating.

Mapping carbon nanotube aspect ratio, concentration and spinning in FCCVD synthesis controlled by sulphur

Floating catalyst chemical vapor deposition (FCCVD) enables ultrafast synthesis of CNTs and other 1D nanoparticles and their direct assembly as macroscopic solids. The chalcogen growth promotor in FCCVD produces high aspect ratio CNTs that can aggregate in the gas phase and form an aerogel which can be continuously spun as macroscopic fibres or sheets. We study the role of sulphur in controlling CNT morphology and aggregation by synthesising CNTs under a wide range of S/C ratios (0.001 to 5 wt.%) and determining their diameter and length distributions, number concentration and form of aggregation. Increasing S/C ratio in this range increases mean number of CNT walls from 1 to 8, decreases mean length from 34 to 6 µm, but CNT number concentration remains approximately constant at 8 × 108#/cm3. Assuming growth within the first 3 cm of the reactor, longitudinal growth rate spans 1.5- 6.5 µm/s for the different CNT morphologies, but with similar mass throughput of 700 attogram/catalyst. This indicates the amount of carbon reaching the catalyst and solidifying as CNT remains constant regardless of the sulphur available in the catalyst, suggesting the rate limiting process is not at the catalyst/promoter interface but instead in the transport of carbonaceous active precursors to the catalyst, either due to their diffusion in the gas phase or decomposition kinetics. The CNTs produced range from polymer-like, which readily bundle and form aerogels, to rod-like that do not. We include aerogelation “phase diagrams” for different CNT concentrations, aspect ratios and CNT bending stiffness.

On the vibrational behavior of the conventional and hetero-junction carbon nanotubes

The shortcomings and insufficiency in the state-of-the-art investigations about the resonance in the hetero-junction carbon nanotubes with respect to the number of models and connection types highlight the necessity of a comprehensive study of the resonance frequency of these nanostructures for applications of these nanomaterials in nanodevices, such as nano-scale electromechanical systems (NEMS) and NEMS-based resonators. Hence in this study, 50 conventional and 73 hetero-junction middle-length (15 nm) carbon nanotubes, with armchair-armchair, zigzag-zigzag, armchair-zigzag, armchair-chiral, zigzag-chiral and chiral-chiral connections, were modeled. Then, the resonance in their structure was comprehensively studied using the finite element method to present a more reliable estimated range for the natural frequency (fundamental frequency) of hetero-junction carbon nanotubes. The analysis showed that the natural frequency of conventional carbon nanotubes lies in the range of 7.09 and 48.75 GHz. An inverse correlation between this quantity and the aspect ratios of the conventional carbon nanotubes was found, leading to proposing a mathematical relation for the prediction of such an inverse correlation. The natural frequency of hetero-junction carbon nanotubes oscillated between 4.04 and 60.20 GHz. The results revealed that the natural frequency of hetero-junction carbon nanotubes lies outside the frequency range of their constructive carbon nanotubes and majorly depends on the boundary conditions and the difference between the diameters of the connected tubes. Finally, it was observed that the existence of an armchair carbon nanotube in the structure of hetero-junction models improves their vibrational stabilities.

Thermophysical Properties of SWCNT Nanofluid

AbstractThe current paper has the objective of exploring the thermophysical properties (TPPs) of single walled carbon nanotube (SWCNT) nanofluids (NFs) for heat transfer applications. The effect of dispersing the nanotubes in conventional heat transfer fluids (HTFs) has been investigated. Carbon nanotubes (CNTs) have exceptionally high thermal conductivities. Using the effective medium theory, density, specific heat, thermal conductivity, and viscosity has been evaluated as functions of volume fractions. The base fluids considered in this work are water, ethylene glycol, engine oil, and kerosene oil as these are the commonly used HTFs for industrial applications. Theoretical predictions from this study show an increase in density, thermal conductivity, and viscosity with volume fraction while there is a decrease in specific heat with volume fraction. The effect of the aspect ratio (AR) of CNTs on the thermal conductivity of NF is also investigated. For low volume fractions of SWCNT, the thermal conductivity has been found to increase with AR. Further, this study has been extended for a real‐world application of heat transfer using SWCNT. In this paper, a numerical investigation of heat transfer in a square enclosure with differentially heated vertical walls, filled with SWCNT‐water NF under natural convection is carried out. Due to low concentrations of SWCNT in water, single phase flow is assumed. Velocity and temperature distributions are obtained as solutions to the energy and Navier–Stoke's equations. A comparison of the temperature and velocity profile has been represented by simulating the system over a range of Rayleigh numbers.

Construction of high aspect ratio carbon nanotube networks decorated with MoS2 nanoflowers for superior microwave absorption properties

Construction of high aspect ratio carbon nanotube networks decorated with MoS₂ nanoflowers for superior microwave absorption properties

Tribological properties of CNT-filled epoxy-carbon fabric composites: Optimization and modelling by machine learning

Polymer matrix composites reinforced with fibers/fillers are extensively used in several tribological components of automotive and boating applications. The mechanical performance of polymer composites improves by incorporating nanofillers as secondary reinforcement. The present research work fabricated carbon fabric-reinforced epoxy composites using the hand layup. The carbon fabric-reinforced polymer composites were fabricated with 0.1 wt%, 0.2 wt%, and 0.5 wt% of carbon nanotubes (CNT) fillers as secondary reinforcement. Tribological properties of carbon fabric-reinforced epoxy composites filled with CNT have been carried out using a pin‐on‐disc method. Adding fillers significantly improves the tribological behaviour of the carbon fabric-reinforced epoxy composites by reducing wear rate and coefficient of friction. The large surface area of interaction due to the higher aspect ratio of CNT shows improved adhesion between epoxy matrix and carbon fabrics. It improves the various mechanical and tribological characteristics of composites—also, an analysis of worn surfaces is carried out to analyze the wear mechanisms using scanning electronic microscopy. The research employs a combination of experimental analyses and machine learning (ML) techniques to explore the wear resistance, hardness, and predictive modeling of volume loss in the composites. The hyperparameter fine-tuning of ML algorithms, including Random Forest (RF), k-Nearest Neighbors (KNN), and XGBoost, demonstrates superior predictive capabilities, particularly with RF. The study bridges material science, ML, and practical applications, contributing valuable insights for developing advanced composite materials.

Random distribution of interphase characteristics on the overall electro-mechanical properties of CNT piezo nanocomposite: Micromechanical modeling and Monte Carlo simulation

A phenomenological study is carried out to speculate the statistical impacts of the CNT/polymer interphase on the overall electro-elastic behavior of piezo-polymer nanocomposites by presenting a full-field micromechanical model. The nanocomposite system consists of carbon nanotube (CNT) and PVDF. Various statistical distributions, including Weibull, log-normal, normal, beta, and uniform distributions on the thickness and strength of the interphase are carefully assessed. The results are compared with experimental data, and satisfactory agreements are reported. It is found that, compared to the random distribution of the interphase strength, the statistical distribution of the interphase thickness has more effect on the overall electro-elastic properties. For example, for the effective longitudinal modulus, the overall coefficients of variation are 14 %, 13 %, 13.56, and 10 %, respectively, for the normal, Weibull, beta, and uniform distributions of the thickness compared with the measured experimental values. Also, the effects of the CNT content, aspect ratio, and orientation on the effective electro-elastic properties by considering the various random distributions are fully examined. Moreover, using the Monte Carlo simulation, the probability of not meeting design specification (failure probability) is evaluated at the random distributions of the interphase strength and thickness to identify the optimum CNT content for which the values of the overall properties are maximum. It is obtained that the failure probabilities are different for 5–8 % CNT volume fraction in the distributions of the thickness, and for only 5 VF% CNT in the strength distributions. For other values of the CNT content, the failure probabilities are independent of the distribution of the interphase strength and thickness.

A review of high-performance carbon nanotube-based carbon fibers

With the growing importance of high-performance carbon fibers (CFs), researches have been conducted in many applications such as aerospace, automobile and battery. Since conventional CFs which were made from polyacrylonitrile, pitch and cellulose display either high tensile strength or high modulus properties due to structural limitations, it has been a challenge to develop CFs with both tensile strength and modulus with high conductivity. Therefore, various studies have been conducted to obtain high-performance multifunctional CFs. Among them, 1-dimensional carbon nanotubes (CNTs) have been used commonly to make CFs because of high mechanical and conducting properties. In this review, the recent development of CFs was introduced briefly, and CNT-based composite CFs were introduced. Many efforts are being made to create high-performance CFs by combining various carbon nanomaterials and polymers, which can have potential to be utilized in aerospace, defense and other industries. The those fibers may be nextgeneration high-performance fibers due to both high strength and high modulus as well as high conducting properties. The challenges and outlook for commercialization of CNT-based CFs are addressed in terms of aspect ratio of CNTs, solvent recycling, and mass-production.

Piezoresistive Characteristics of Carbon Nanotube Polymer Composites with Different Filler Content and Aspect Ratio for Pressure Sensors

Polymer composites enriched with conductive fillers hold immense potential for flexible pressure sensors, because of the remarkable piezoresistive effect they possess. Research on polymer composite pressure sensors has been actively ongoing because of their flexibility and high electrical resistance performance. This paper presents a comprehensive comparison of the piezoresistive characteristics of conductive composites for pressure sensors, focusing on the influence of carbon nanotube (CNT) content and aspect ratio. Polymer composites with conductive fillers, specifically CNTs, have demonstrated significant potential for pressure sensing applications based on the piezoresistive effect. By systematically varying the CNT concentration and aspect ratio, we investigated the impact of these parameters on the piezoresistive behavior of the composites. A pressure in the range of 0-200 kPa was applied to the conductive composite, and resistance change due to pressing was measured. The best performing samples were evaluated in 150 cycle tests to verify repeatability and durability. Experimental analysis and characterization revealed the intricate relationship between CNT content, aspect ratio, and the resulting piezoresistive properties. Through this study, we aim to enhance understanding of how CNT concentration and aspect ratio influence the performance of CNT/PDMS composites as pressure sensors, thereby facilitating the development of optimized sensing materials for various pressure sensing applications.

Mechanical, microstructural, and fretting wear behaviour of Al2O3–ZrO2-CNT based bimodal composite coatings

In current work, bimodal thermal barrier coatings (TBC) were deposited via atmospheric plasma spraying (APS) using Al2O3 as matrix, reinforced with 20 wt% of ZrO2 and 4 wt% of carbon nanotubes (CNT) over Inconel 718 substrate. The objective is to study the effect of bimodality and reinforcements on mechanical and tribological behaviour of Al2O3 based thermal barrier coatings (TBC). Al2O3–ZrO2, Al2O3–ZrO2-CNT and micro-Al2O3 coatings named as AZ, AZC and μ-Al2O3, respectively. Presence of partially melted and solid-state sintered nanostructured region (NR) with fully melted and re-solidified microstructured regions (MR) showed bimodal microstructure. Reinforcements leads to enhance nano-hardness and plasticity index from (12.8 GPa, 0.12) for μ-Al2O3 to (14.7 GPa, 0.17) for MR of AZ and (14.1 GPa, 0.17) for AZC, which is attributed to higher amount of α-Al2O3 and t-ZrO2 present after coating deposition. Nanostructured regions reduce shear stress generation under 9.8 N load during fretting wear in AZ (293.8 MPa) and AZC (298.7 MPa), resulting in a more uniform surface and reduced wear rate. Additionally, the higher aspect ratio and lubrication effect of CNTs in AZC generates least energy dissipation (92.9 J) and wear rate (0.9 × 10−6 mm3N−1m−1). SEM and EDS analysis of worn surfaces in coatings and counter body revealed abrasive, adhesive wear is dominant in μ-Al2O3 than AZ and AZC coatings due to presence of larger wear debris and more material transfer from coating to Si3N4 body than AZ and AZC. Overall, the synergistic effect of bimodality and reinforcements leads to improved mechanical property, and reduced energy dissipation and shear stress generation during wear that makes APS-AZC coating a potential material for wear and high temperature environment.

Understanding the role of carbon nanotubes in low carbon sulfoaluminate cement-based composite

Carbon nanotubes (CNTs) have excellent mechanical properties and outstanding aspect ratios, which could remarkably impact the concrete's properties. This research investigates the influence of four different aspect ratios of CNTs with two dosages (i.e., 0.09% and 0.15%, by weight of cement) on hydration, macro performance, and microstructure of low-carbon sulfoaluminate cement-based composite. The experimental results show that adding different aspect ratios of CNTs can enhance the macro performance of the composite. The SM2 (short-CNTs with 0.15% content) group with a small aspect ratio (50–100) has better flexural strength and compressive strength than M0 (plain group), which is increased by 32.7% and 37.0%, respectively, the hydration degree of C4A3Š (ye'elimite) and C2S (belit) are increased by 19.9% and 57.3% than that of the M0 group. At low dosages (0.09%), LM1 (long-CNTs with an aspect ration of 250–1500) and LM2 (long-CNTs with an aspect ration of 125–750) improved the flexural strength of UHPC more than SM1 (short-CNTs with an aspect ration of 100–200). However, LM1 (long-CNTs with a large aspect ratio of 250–1500) has a better enhancement at a dosage of 0.09% than 0.15%. CNTs mainly accelerated the hydration process of the matrix and promoted the generation of more AFt, AH3. AFt is a needle-like fiber, and the increase in content is beneficial to improve the toughness and flexural strength of the matrix. The increase in AH3 content can improve the compressive strength of the matrix. The AH3 phase can be filled into the pores of the matrix, thereby increasing the compatibility of the matrix and improving the pore structure. The increase in the content of the two hydration products can synergistically enhance the mechanical properties of the matrix. AH3 is closely connected to CNTs, which can improve the bond strength between the matrix and CNTs. It is observed that AFt is closely associated with AH3, filling the pores of the matrix, and the nanoscale AFt, AH3, and CNTs work together to improve the strength of the matrix, which differs from the effect of CNTs in OPC. This research prepares UHPC from low-carbon cement, which effectively enhances the life cycle of concrete materials and provides a new idea for reducing carbon emissions.

Finite element modeling of the effective creep compliance of carbon nanotube-polymer nanocomposites: A critical microstructure-level investigation

A micromechanical model implemented with the finite element method (FEM) using the representative volume element (RVE) approach is developed to investigate the creep compliance behavior of carbon nanotube (CNT)-polymer nanocomposites. Effects of the CNT waviness, orientation, volume fraction and aspect ratio on the creep compliance of the nanocomposite are examined. Contribution of the interphase region formed due to the interaction between CNTs and matrix to the nanocomposite creep behavior is considered. The nanocomposite creep behavior is affiliate on the waviness and orientation of CNTs. Increasing the volume fraction and length of CNT and interphase thickness improves the nanocomposite creep performance.

96 Longitudinal Profiling of Carbon Nanotube-Induced Sporadic Mesothelioma Development: Defining the Adverse Outcome Pathway for Disease Progression

Abstract Exposure to asbestos causes pathological changes in the pleural cavity, including malignant mesothelioma (MPM). Length-dependent retention of asbestos fibres in the pleural cavity is crucial for disease development. Chronic inflammation induced by biopersistent asbestos fibres plays a key role in carcinogenesis. Engineered CarbonNanoTubes (CNT) are similar to long-fibre asbestos (LFA) in terms of their high aspect-ratio and thus may pose an asbestos-like inhalation hazard, with evidence from our laboratory and others showing the carcinogenic potential of Long-CNT. Asbestos-induced MPM is diagnosed most commonly at late stage; as a result, little is known about the genetic or epigenetic aberrations occurring in pre-malignant pleural lesions and whether they align with deletions/mutations commonly found in end-stage asbestos-exposed patient tumours. Using a mouse model of direct injection into the pleural cavity, we compared the molecular changes that occur at the mesothelium following injection with an occupationally-relevant dose of Long-CNT or LFA over 20-months. We show a common molecular signature induced by Long-CNT and LFA throughout disease progression eventually leading to the development of sporadic MPM. DNA methylation, gene expression profiles, as well as specific genes are similarly altered in the presence of Long-CNT and LFA, at matched longitudinal exposure times. For the first time, these data uncover the Adverse Outcome Pathway (AOP) from initial Long-CNT exposure through to induction of pre-neoplastic lesions and the development of sporadic pleural mesothelioma, replicating the pathogenesis of end-stage asbestos-induced human disease. This reinforces concerns that high aspect-ratio CNT may pose an asbestos-like hazard, including sporadic malignant mesothelioma.

Static buckling analysis of single walled carbon nanotube

In this paper, the different structures of single walled carbon nanotube like zigzag, armchair and chiral are modeled. The modeling is done by considering the carbon atoms as key points and bond between the carbon atoms as lines. Buckling analysis of different structures of carbon nanotube performed using finite element analysis using Ansys software. It has been shown that the ability of carbon nanotube to withstand for the axial compressive force is considerably affected by aspect ratio and structure of Single walled carbon nanotubes (SWCNTs) and also the effect of different chiral angle for constant diameter and length are discussed.

Vibration and buckling analysis of agglomerated CNT composite plates via isogeometric analysis using non-polynomial shear deformation theory

The present study employs the non-polynomial shear deformation theory within the framework of isogeometric analysis to study the vibration and buckling behavior of carbon nanotubes reinforced composite plates. Owing to the higher aspect ratio of carbon nanotubes, they are prone to agglomeration in matrix. Therefore, to analyze the effect of carbon nanotubes agglomeration in composite plates, a two-parameter micromechanics model is used to determine the effective mechanical properties of the composite plate. Current study opts isogeometric analysis as numerical method, as it can represents the complex shape geometries, and provides higher-order element continuity. Higher-order element continuity is desirable for implementation of non-polynomial shear deformation theory. By the virtue of non-uniform rational B-splines, isogeometric analysis turns out to be better analysis numerical method for complex and shell structures. Governing isogeometric element equations are derived using Hamilton's principle. A detailed parametric study is also performed to investigate the effect of degree of agglomeration, thickness to length/radius ratio, different boundary conditions, and complex shape cutouts in a composite plate. The presented formulation is validated across analytical and finite element method solutions with available literature.

Similarity conditions for predicting the effect of carbon nano-tubes on the non-linear frequency of laminated doubly curved shells

This paper presents a study of the relationship, based on structural similitude, for predicting the effect of the Carbon Nano-Tubes (CNTs) on the non-linear frequency of cross-ply laminated doubly curved shells. The frequency equation is used directly, to establish the similarity conditions between the two similar systems. The applicability of the small-scale model has recently been demonstrated for predicting the vibration and buckling responses of laminated composite shells and panels. In this study, limitations of previous studies were omitted. Unlike previous researches, it is not necessary to change all geometric dimensions proportionally with each other. The results show that, the scaling law is introduced can predict the results for prototypes with excellent accuracy. The effects of distorted models with relaxations in the CNTs weight percentage, large deflection and the stacking sequence are investigated. According to the results are obtained the models with different CNTs weight percentage, aspect ratio and stacking sequences than the prototypes can be used to predict the non-linear frequency of the prototypes accurately.

Effect of aspect ratio, bulk density, and production date of multi-walled carbon nanotubes on the electrical conductivity of polypropylene and polycarbonate/multi-walled carbon nanotube nanocomposites

Effects of aspect ratio (AR), bulk density (BD), and fabrication date of multi-walled carbon nanotubes (MWCNTs) on the dispersion quality, electrical properties, and rheological properties were investigated for the nanocomposites of polypropylene and polycarbonate matrix. Six different MWCNTs with various AR and BD were adopted. Among six MWCNTs, two pairs have the same grade names but were produced on different dates. They are two Nanocyl NC7000s and two Hanwha CM130s. The two Nanocyl NC7000s produced on different dates showed different dispersion states and electrical resistivity for both PC and PP based nanocomposites. Characterizations of MWCNTs reveal that the length and diameter of two Nanocyl NC7000s are almost the same regardless of the production date, but the BD values of the two MWCNTs are significantly different. The same is noted for the two Hanwha CM130s. Therefore, the different electrical resistivity of the two nanocomposites using two MWCNTs of the same grade name but different production dates was attributed to the different BD. When comparing two nanocomposites with two MWCNTs (one has slightly higher AR and BD than the other), the nanocomposites with MWCNTs of lower AR and lower BD showed better dispersion and higher electrical conductivity. By various investigations with six MWCNTs, we concluded that BD is a very crucial factor in determining the electrical properties of polymer/MWCNT nanocomposites.