Electrical characteristics and pressure-sensitive response measurements of carboxyl MWNT/cement composites

In this study, electrical characteristics of pressure-sensitive carboxyl multi-walled carbon nanotube (MWNT)/cement composites with and without compressive loading were investigated. Experimental results indicate that the carboxyl MWNT/cement composites have both resistance and capacitance characteristics. Capacitance is insensitive to compressive loading, but the charging of the capacitor causes a linear increase in the measured resistance during DC measurement. The reversible pressure-sensitive responses of resistance to compressive loading can be extracted by removing the linear increase component. An AC measurement method can also be used to eliminate the effect of capacitor charging and discharging on the pressure-sensitive responses of carboxyl MWNT/cement composites.     

表面官能团化多壁碳纳米管/环氧树脂复合材料的制备及性能

本文首先将多壁碳纳米管(MWNT)进行表面化学修饰,接入羧基、胺基等官能团,采用红外光谱进行了表征.以纯化后的MWNT和表面化学修饰的MWNT作为填料,制备了MWNT /环氧树脂复合材料,研究了MWNT的加入对环氧树脂的力学性能、电学性能、热稳定性和玻璃化转变温度等的影响,并利用场发射电镜观察了胺基化MWNT在环氧树脂基体中的分散情况.     

Low electrical percolation threshold and PL quenching in solution-blended MWNT–MEH PPV nanocomposites

Pristine multiwall carbon nanotubes (MWNT) (synthesised using CVD approach) and poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH PPV) based composites were prepared using a solution blending approach by employing various nanotube weight fractions. The prepared composites have been characterised using SEM, AFM, PL spectroscopy, UV-Vis studies and I–V characterisation. Increase in MWNT concentration has been found to quench the PL spectra of the composites suggesting photoinduced electron transfer from polymer to MWNT. The increase in MWNT concentration also increases the absorption of the composites. PL quenching and increase in absorption are desirable attributes for the design of photovoltaic systems. Also, the electrical conductivities of the composites can be described by the scaling law based on percolation theory and based upon the scaling law, a low electrical percolation threshold value (0.5?wt%) has been obtained for this composite system. The value of t (critical exponent) based on percolation theory is found to be 1.11. The low value of t is attributed to the aggregation and bundling of nanotubes in the prepared composites, as is evident from SEM and AFM micrographs. The turn-on voltage is also found to be reduced in the case of polymer–nanotube composite system as compared to the pristine polymer system. Also, it has been observed that at higher weight percentages, the MWNTs form an immensely dense network and act as nanometric heat sinks, thus preventing the build up of large thermal effects, caused by the increased current in the pixels at higher voltages. Analysis of these optical and electrical properties is important before utilising the composite in organic electronics applications, in order to obtain more scientifically correct and repeatable results with fabricated devices.     

Effects of hybrid carbon fillers of polymer composite bipolar plates on the performance of direct methanol fuel cells

The effects of carbon filler type on the properties and performance of composite bipolar plates fabricated by compression molding of carbon fillers such as graphite, carbon black (CB), multi-walled carbon nanotube (MWNT), carbon fiber (CF) and powder type epoxy have been investigated. The electrical conductivity and flexural properties of the composites are increased by increasing the content of fibrous conducting fillers, e.g. MWNT and CF. On the contrary, incorporation of particulate fillers such as CB and graphite plays a significant role in enhancing the electrical conductivity but has a negative effect on the flexural properties of the composites. The current–voltage curve of the fuel cell indicates that the performance of the fuel cell is improved upon selection of an optimum amount of carbon filler in the composite bipolar plates.     

Thermo-physical properties and TEM analysis of silver based MMCs utilizing metallized multiwall-carbon nanotubes

Metal matrix composites with embedded multiwall-carbon nanotubes (MWNT) are attractive because MWNTs exhibit high intrinsic thermal conductivity. Thus to improve the thermal conductivity of a metal matrix, silver matrix composites with MWNT were prepared by “chemical” mixing, different active elements were introduced enhancing the bonding between inclusions and matrix. The evolution of the thermal conductivity and the coefficient of thermal expansion CTE as a function of the MWNT concentration and the presence of active elements cobalt, molybdenum or nickel in the silver matrix in Ag–X/MWNT composites are presented. A transition from weak to strong matrix/MWNT bonding is observed by adding active elements, the latter leading concomitantly to an increase in thermal conductivity and a decrease in CTE. The thermal conductivity was found to increase by up to 10% for a composition of 0.2 wt.% MWNT and cobalt as active element and a 6% decrease in CTE compared to a pure silver reference.     

Electrical and dielectric properties of polypropylene nanocomposites based on carbon nanotubes and barium titanate nanoparticles

Functional polypropylene (PP) nanocomposites were prepared by melt compounding with multiwalled carbon nanotubes (MWNT) as the electrically conductive component and barium titanate (BT) spherical nanoparticles as the ferroelectric component. To make PP electrically conductive, more than 3 wt.% MWNT is required. Surface modification of either MWNT or BT with titanate coupling agent further improves the electrical conductivity of the PP/MWNT/BT ternary nanocomposites. Interestingly, by modifying both MWNT and BT, 2 wt.% MWNT are sufficient to make the ternary nanocomposite electrically conductive. In addition, the incorporation of MWNT greatly increases the dielectric permittivity of PP/BT nanocomposites. However, to retain a low dielectric loss, the MWNT loading should be slightly less than the percolation threshold of the nanocomposites. The improved electrical conductivity and dielectric properties make the ternary nanocomposites attractive in practical applications.     

Syndiotactic polystyrene/multi-walled carbon nanotube nanocomposites: Polymorphism,thermal properties,electrical conductivity,and rheological properties

Syndiotactic polystyrene (sPS)/multi-walled carbon nanotube (MWNT) nanocomposites were prepared using a melt mixing technique. SEM images demonstrated the fine dispersion of MWNTs through the sPS matrix. DSC results illustrated that the MWNTs facilitated the nucleation of sPS (up to ca. 12.2 °C increase), but retarded the subsequent crystals growth. Based on the Avrami analysis, the dimension of sPS crystals growth in the composites decreased because of the effects of extensive nucleation and the formation of a nanoconfined/constrained environment from the MWNTs. XRD data confirmed that the presence of MWNTs facilitated the formation of β-form sPS crystals. The thermal stability of sPS improved to approximately 31 °C at 3 phr of MWNT loading. A rheological percolation threshold between an MWNT loading of 0.5 and 1 phr was determined by measuring the rheological properties of the samples. The incorporation of MWNTs reduced the electrical resistivity of sPS by 10 orders of magnitude.     

MWNT的分散性及其对复合水泥基材料力学性能的影响

以4种表面活性剂（SAA）（十二烷基硫酸钠（SDS）、曲拉通x100（Tx100）、十六烷基三甲基溴化铵（CTAB）、阿拉伯胶（GA））分别作为多壁纳米碳管（MWNT）分散剂,结合静置及离心分离观察法评价MWNT在水中的分散性;混合浇筑了4组经不同SAA分散、MWNT掺量均为0.21%（相对于水泥质量）的MWNT/水泥基复合材料（MWNT/CCs）,同时制备了空白试件（PCP）,测试了各组试件的抗折强度σf及抗压强度σc,并观察了用GA分散的MWNT/CC及PCP试样的显微结构.静置3 h后,4种MWNT悬浮液黑度均匀,但在5 000 r/min离心分离后显示：GA对MWNT分散效果最好,离心90 min才开始分层;SDS效果最差,离心10 min就开始分层.相比PCP,用GA分散的MWNT/CC的σf、σc提高幅度最大,分别达31.5%、23.6%,表明MWNT在基体中能较好地发挥纤维拔出和桥联作用;TEM及SEM图也显示MWNT在水及相应水化产物中分布较均匀,MWNT与基体界面结合良好.     

Core-shell morphology and characterization of carbon nanotube nanowires click coupled with polypyrrole

Core-shell nanowires having multiwalled carbon nanotubes (MWNT) as a core and polypyrrole (PPy) as a shell were synthesized using Cu(I)-catalyzed azide-alkyne cycloaddition click chemistry. According to transmission electron microscopy measurements, the uniform PPy layers of 10-20 nm in thickness were formed well on the MWNT's surface. In particular 'grafting from' click coupling was more effective in obtaining uniform and stable core-shell nanowires as well as in the reaction yield, compared to 'grafting to' click coupling. This is due to chemical bond formation between PPy and MWNT in equal intervals along the longitudinal direction of the MWNT, achieved by 'grafting from' click coupling. As a result, the core-shell nanowires were very stable even in the sonication of nanowires and showed an enhanced electrical conductivity of 80 S cm(-1), due to the synergetic interaction between MWNTs and PPy, which is higher than the conductivity of pure MWNTs and pure PPy. In addition, the core-shell nanowires could show better NO2 gas sensing properties compared to pure MWNTs and pure PPy as well as MWNT/PPy composites prepared by in situ polymerization. The synthesized core-shell nanowires would play an important role in preparing electrical and sensing devices.     

Rheological and mechanical properties of polypropylene prepared with multi-walled carbon nanotube masterbatch

In this study, the effects of polypropylene-grafted maleic-anhydride-treated multi-walled carbon nanotubes (PP-MWNTs) on the viscoelastic behaviors and mechanical properties of a polypropylene-(PP)-based composite system were examined. The PP-MWNT/PP composites were prepared via melt mixing with a 3:1 ratio of PP-g-MA and acid-treated MWNTs at 220 degrees C. The surface characteristics of the PP-MWNTs were confirmed via Fourier transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM). The viscoelastic behavior and mechanical properties of the PP-MWNT/PP composites were confirmed using a rheometer and an ultimate testing machine (UTM). The storage and loss moduli increased with increasing PP-MWNT content. The critical intensity stress factor (K(IC)) of the PP-MWNT/PP composites at high filler loading was also higher than that of the MWNT/PP composites. In conclusion, the viscoelastic behavior and mechanical properties of MWNT/PP can be improved by grafting MWNTs to PP-g-MA.     

Structure-property relationship in polyethylene reinforced by polyethylene-grafted multi-walled carbon nanotubes

Polyethylene-grafted multiwalled carbon nanotubes (PE-g-MWNT) were used to reinforce polyethylene (PE). The nanocomposites possessed not only improved stiffness and strength, but also increased ductility and toughness. The effects on the structure and morphology of composites due to pristine multiwalled carbon nanotubes (MWNT) and PE-g-MWNT were studied and compared using small angle X-ray scattering (SAXS), wide angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The SAXS long period, crystalline layer thickness and crystallinity of polymer lamellar stacks were found to decrease significantly in MWNT composites, while the decreases were much smaller in PE-g-MWNT composites. PE-g-MWNT allowed a more efficient and unhindered crystallization at a lamellar level, while MWNT disrupted the order of lamellar stacks, probably because of their tendency to aggregate. The SAXS crystallinity and the mechanical properties of the composites showed similar trends as a function of MWNT content. This suggested that the improvement of the interfacial strength between polymer and carbon nanotubes was a result of synergistic effects of better dispersion of the filler, better stress transfer, due to the grafting of polymer and MWNT, and the nucleation of a crystalline phase around MWNT. The latter effect was confirmed by measurements of kinetics of non-isothermal crystallization.     

Measurement of the critical aspect ratio and interfacial shear strength in MWNT/polymer composites

This paper presents a bulk composite method for determining the critical aspect ratio and relative interfacial shear stress (ISS) for multiwalled carbon nanotube (MWNT)/polymer composites. Through a modified pullout test and fragmentation test, it was found that the critical aspect ratio was 300 and decreased by a factor of 3 due to surface modification, and that MWNTs at an angle of greater than 60° to the loading direction failed in bending instead of pulling out of the matrix. Finite element analysis was used to determine the critical bending shear strength and MWNT modulus. The obtained bending shear strength was used in a mechanics model developed to provide bounds for the ISS in the experimental composite system. The calculated ISS for as-received nanotube falls between 4.8 and 13.7 MPa, and for surface treated nanotube falls in the range of 11.1 and 38.3 MPa. These values are consistent with the ISS reported for carbon fiber/polymer composites and also show that the ISS almost triples due to chemical modification of the MWNT surface.     

Fabrication and mechanical/conductive properties of multi-walled carbon nanotube (MWNT) reinforced carbon matrix composites

Multi-walled carbon nanotube (MWNT) reinforced carbon matrix (MWNT/C) composites have been explored using mesophase pitch as carbon matrix precursor in the present work. Results show that carbon nanotubes (CNTs)can enhance the mechanical properties of carbon matrix significantly. The maximal increment of the bending strength and stiffness of the composites, compared with the carbon matrix, are 147% and 400%, respectively. Whereas the highest in-plane thermal conductivity of the composites is 86 W m^− 1 K^− 1 which much lower than that of carbon matrix (253 W m^− 1 K^− 1).At the same time the electrical resistivity of the composites is much higher than that of matrix. It is implicated that CNTs seem to play the role of thermal/electrical barrier in the composites. FSEM micrograph of the fracture surface for the composites shows that the presence of CNTs restrains the crystallite growth of carbon matrix, which is one of factors that improve mechanical properties and decrease the conductive properties of the composites. The defects and curved shape of CNTs are also the affecting factors on the conductive properties of the composites.     

Liquid sensing properties of melt processed polypropylene/poly(ε-caprolactone) blends containing multiwalled carbon nanotubes

The sensing properties of polypropylene (PP)/poly(ε-caprolactone) (PCL) blends containing multiwalled carbon nanotubes (MWNT) were studied in terms of their electrical resistance change in presence of liquids (solvents). The preparation of co-continuous blends based on the double percolation concept was done by melt mixing of electrically conductive PCL composites containing 3 wt.% MWNT and neat PP in ratios of 30:70, 40:60, and 50:50. The electrical resistance change of the PCL-MWNT composites and blends was monitored in a solvent immersion/drying cycle. Various solvents, such as n-hexane, ethanol, methanol, water, toluene, chloroform, and tetrahydrofuran were successfully detected, yielding different responses and reversibility of the resistance changes.PP and PCL were tested separately for solvent sorption using ethanol and n-hexane, both showing a low sorption of n-hexane. Ethanol sorption was large for PCL and almost absent for PP. The 50/50 blend composites with 3 wt.% MWNT in the PCL phase presented larger resistance changes for n-hexane, showing larger sensing ability for this solvent compared to PCL composites with 1 and 3 wt.% loadings. The opposite response was observed for immersion in ethanol where the PCL-MWNT composites showed larger changes than the blends. As the ratio of the conductive PCL phase over PP in the blend composition (i.e. the overall MWNT content) decreased, larger resistance changes were observed. The liquid sensing properties of compression-moulded discs and melt-drawn filaments were compared indicating higher responses for the discs.     

Broad Microwave Dielectric Property of Single-walled Carbon Nanotube Composites

Microwave dielectric measurements over the broad bandwith of 10 MHz to 20 GHz were conducted on composites comprising bundles of single-walled carbon nanotubes （SWNT） embedded in an epoxy matrix, in comparison to the nano-graphite and MWNT. It is found that both relative real and imaginary permittivity of the nanocomposites are strong functions of the SWNT concentration, showing large, wide dielectric and electrical response. Distinct resonance around 1.5 GHz is observed at high SWNT concentrations, accompa- nied by the downshift of the resonance frequency with increasing concentration. Largely, the SWNT-epoxy composites share the behavior of the MWNT owing to structural similarity, much more effective than the nano-graphite. The remarkable, broadband dielectric and electrical properties of the nanotubes acquired in the work originate from their unique seamless graphene architectures, modeled by two major contributions, dielectric relaxation/resonance and electronic conduction, which is substantiated by the agreement between theoretical analysis and experimental results. The carbon nanotube composites are prospective for microwave applications and offer experimental evidence for fundamental studies in low-dimensional systems.     

Melt mixed composites of poly(ethylene-co-methacrylic acid) ionomers and multiwall carbon nanotubes: influence of specific interactions

Multiwall carbon nanotubes (MWNT) were melt-mixed with poly(ethylene-co-methacrylic acid) ionomers (Surlyn) using twin screw microcompounder. The specific interactions existing between the Na+ moieties in Surlyn and the pi electron clouds of MWNT were supported by FTIR and Raman spectroscopic analysis. SAXS scattering patterns were found to be progressively broadened in presence of MWNT in Surlyn/MWNT composites. Morphological investigations revealed selective clustering of MWNT in the vicinity of the ionic domains in Surlyn. Further, the domain size of the ionic clusters was found to increase with increasing MWNT content disrupting the ionic pairs apart in the ionic domain. The melt rheological response of Surlyn was significantly affected in presence of MWNT and was profoundly dependent on the ionic clusters. The state of dispersion of MWNT was assessed by AC electrical conductivity measurements. The associated percolation threshold was observed between 1.5-2 wt% of MWNT.     

Study on the infrared property of polyaniline/multi-wall carbon nanotube composite

Composites of polyaniline (PANI) and multi-wall carbon nanotube (MWNT) were synthesized by in situ polymerization with different MWNT content. The composites were characterized by Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The SEM photos indicated that a mass of MWNT was enchased into PANI matrix when the content of MWNT was low. With the MWNT content increases, the surface of MWNT was covered with PANI and formed the core-shell structure. From the FT-IR spectroscopy of the composites, it can be confirmed that there was interaction between PANI and multi-wall carbon nanotube. The composites had better thermal stability than pure PANI. Infrared emission property of the composites was analyzed by an IR-II infrared emissivity instrument and an infrared camera. It was found that infrared emission of the composites was lower than pure PANI in all wavelength range and infrared emissivity value was related to the content of MWNT in the composites.     

Electrochemical fabrication of polyaniline/multi-walled carbon nanotube composite films for electrooxidation of methanol

Polyaniline (PANI)/multi-walled carbon nanotubes (MWNTs) composite films were fabricated by electropolymerization of aniline containing well-dissolved MWNTs. The films can be used as catalyst supports for electro-oxidation of methanol. Cyclic voltammogram and Chronoamperogram results show that platinum particles deposited on PANI/MWNT composite films exhibit higher electrocatalytic activity towards methanol oxidation than that deposited on pure PANI films. The porous structure and electrical conductivity of PANI films has been significantly changed by introduction of MWNTs, higher surface areas of PANI/MWNT composites has been achieved therefore. It favors for platinum particles to be highly dispersed on the PANI/MWNT composite films and the better electrocatalytic activity of Pt/PANI/MWNT electrode is induced consequently.     

Synthesis,characterisation and performance of piezo-resistive cementitious nanocomposites

Smart material reinforced non-destructive structural health monitoring technique has been evolving as the most predominated route for assessing the performance of the civil structures. In the present study, multiwalled nanotubes (MWNT) were suitably incorporated into the cement matrix, which act as actively embedded sensor for monitoring real-time flaws in structures. Initially, the stable homogenous MWNT dispersion was prepared by using ionic surfactant technique with high-intensity ultrasonic agitation process. Since, a suitable and adequate synthesis procedure to incorporate MWNT in cement matrix is essential, but complicated, the role of amplitude and frequency of sonication on dispersion of nanotubes was categorically evaluated. Further, this paper focuses to find out the effect of surfactant on MWNT dispersion by using the UV Visible spectroscopy and by evaluating the effective hydro-dynamic diameter. Based on micromechanics based analytical model, the influence of the interface layer thickness and geometrical configuration of nanotubes on the electrical conductivity of cement nano-composite are also analyzed. Further, the electrical conductivity of MWNT incorporated cement system, as developed in the present study, is measured using four probe method. Piezo-resistivity of the oven dried samples is measured to evaluate the change in potential drop under cyclic loading regime. It is found that the efficiency of the piezo-resistive strain sensors greatly depends on synthesis process and the circuit system. Appropriately proportioned and properly synthesized MWNTs incorporated in cement matrix were capable of providing consistent and steady response under the variable external stress. Thus, the material can be used as embedded sensor for health monitoring and identifying initiation of any damage in reinforced concrete structure.     

Aligned carbon nanotube-polystyrene composites prepared by in situ polymerisation of stacked layers

In situ polymerisation provides a route to polystyrene (PS) matrix composites reinforced with aligned multi-walled carbon nanotubes (MWNTs). As shown, fully densified composites can be prepared; by varying the number of layers of aligned MWNT arrays, desired thickness of the composite can be manufactured. These aligned composites have characteristic anisotropic electrical and thermal properties.