Graphics & Design

A review of vapor grown carbon nanofiber/polymer conductive composites

Description
A review of vapor grown carbon nanofiber/polymer conductive composites
Published
of 21
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Related Documents
Share
Transcript
  A review of vapor grown carbon nanofiber/polymerconductive composites Mohammed H. Al-Saleh, Uttandaraman Sundararaj * Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 A R T I C L E I N F O Article history: Received 9 April 2008Accepted 17 September 2008Available online 23 September 2008A B S T R A C TVaporgrown carbon nanofiber (VGCNF)/polymerconductive composites areelegant materi-als that exhibit superior electrical, electromagnetic interference (EMI) shielding effective-ness (SE) and thermal properties compared to conventional conductive polymercomposites. This article reviews recent developments in VGCNF/polymer conductive com-posites. The article starts with a concise and general background about VGCNF production,applications, structure, dimension, and electrical, thermal and mechanical properties. Nextcomposites of VGCNF/polymer are discussed. Composite electrical, EMI SE and thermalproperties are elaborated in terms of nanofibers dispersion, distribution and aspect ratio.Special emphasis is paid to dispersion of nanofibers by melt mixing. Influence of other pro-cessing methods such as in-situ polymerization, spinning, and solution processing on finalpropertiesofVGCNF/polymercompositeisalsoreviewed.WepresentpropertiesofCNTsandCFs, which are competitive fillers to VGCNFs, and the most significant properties of theircompositescomparedtothose ofVGCNF/polymercomposites.Atthe conclusion of thearti-cle,wesummarizethemostsignificantachievementsandaddressthefuturechallengesandtasks in the area related to characterizing VGCNF aspect ratio and dispersion, determining the influence of processing methods and conditions on VGCNF/polymer composites andunderstanding the structure/property relationship in VGCNF/polymer composites.   2008 Elsevier Ltd. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.1. Applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.2. VGCNF properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 0008-6223/$ - see front matter    2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2008.09.039 Abbreviations : ABS, acrylonitrile–butadiene–styrene; ASI, Applied Sciences Inc.; CB, carbon black; CF, carbon fiber; CNT, carbonnanotubes; CNF, carbon nanofiber; CPC, conductive polymer composites; CTE, coefficient of thermal expansion; CuNW, coppernanowires; EM, electromagnetic; EMI, electromagnetic interference; ESD, electrostatic discharge; GF, glass fiber; HDPE, high densitypolyethylene; HIPS, high impact polystyrene; HRTEM, high resolution transmission electron microscopy;  I – V  , current–voltage; VGCF,vapor grown carbon fiber; VGCNF, vapor grown carbon nanofibers; LCP, liquid crystal polymer; MCP, metal coated polymer; MNW, metalnanowire; MWNT, multi-walled carbon nanotubes; PE, polyethylene; PES, poly(ether sulfone); PMMA, poly(methyl methacrylate); PP,polypropylene; PS, polystyrene; PVA, poly(vinyl alcohol); PVDF, poly(vinylidene fluoride); SE, shielding effectiveness; SEM, scanning electron microscope; SMP, shape memory polymer; SS, stainless steel; s-VGCNF, short-vapor grown carbon nanofiber; SWNT, single wallcarbon nanotube; TEM, transmission electron microscopy; VE, vinyl ester.*  Corresponding author:  Fax: +1 780 492 2881.E-mail address: u.sundararaj@ualberta.ca (U. Sundararaj). C A R B O N  47 (2009) 2  –  22 available at www.sciencedirect.comjournal homepage: www.elsevier.com/locate/carbon  2. Electrical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.1. Effect of VGCNF surface chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.2. Effect of polymer type. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.3. Effect of processing method and processing conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.3.1. Composites by melt mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.3.2. Composites by solution processing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.3.3. Composites by in-situ polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.3.4. Composite by heterocoagulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.4. Effect of VGCNF alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.5. Effect of HDPE adsorption on VGCNF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.6. Effect of volume exclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.7. Effect of using immiscible multiphase matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133. Shielding effectiveness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134. Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165. Summary and conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 1. Introduction In the last few years, conductive nanofiller/polymer compos-ites have been widely investigated in academia and industrybecause of their outstanding multifunctional properties com-pared to conventional conductive polymer composites (CPCs)[1–3]. Those composites have been mainly formulated using high aspect ratio 1D conductive nanofillers including carbonnanotubes (CNTs) [4–18], vapor grown carbon nanofibers(VGCNFs) [19–28] and more recently metal nanowires (MNWs)[29–31]. This article is intended to review in depth the pro-gress and accomplishments achieved to date in the VGCNF/polymer conductive composites field in terms of nanofibercharacterization and composite compounding and properties.VGCNF/polymer composites were recently concisely reviewedby Tibbetts and coworker [20]. The review covered a portion of the contributions published to date and did not go into detailabout thermal, electrical and electromagnetic interference(EMI) shielding effectiveness (SE) properties of the VGCNF/polymer composites. It is not the purpose of this work to re-view progress in the CNT/polymer composites field. However,we have summarized some of the most significant contribu-tions attained using CNTs for purposes of comparison or toadd more knowledge that might be transferable to VGCNFs.Interested readers in CNT/polymer composites are advisedto refer to a general review by Thostenson and coworkers[32], CNT/polymer composites reviews Breuer and Sundararaj[1] and by Moniruzzaman and Winey [4], reviews of mechan- ical reinforcement of CNTs by Coleman and coworkers [33,34]and by Miyagawa and coworkers [5], review of dispersion andalignment of CNTs in polymer by Xie and coworkers [35],CNT/polymer fibers by Ciselli and coworkers [36] and reviewsof polymer grafted-carbon nanofillers by Tsubokawa [37] andHomenick and coworkers [38].Compared to CNTs, VGCNFs have received less researchattention as nanofillers because CNTs have better mechanicalproperties (due to less microstructural defects [39]), smallerdiameter and lower density than VGCNFs. However, becauseof their availability and relatively low price, VGCNFs are anexcellent alternative for, CNTs and in addition, VGCNFs couldbe used for research purposes to build knowledge that mightbe transferable to the more expensive CNTs. Multi-walled car-bon nanotubes (MWNTs) are 2–3 times more expensive thanVGCNFs and single wall carbon nanotubes (SWNTs) are evenmore expensive. In 2007, the prices of VGCNFs, MWNTs, and90% pure SWNTs were $125/lb, $350/lb and $30,000/lb, respec-tively 1 [40]. It is forecasted that due to increase in productioncapacities, VGCNF price might drop significantly [41]. For elec-trical applications, VGCNFs are also competitive fillers withcarbon fibers (CFs) and high structure carbon black (CB), ow-ing to the lower loading of VGCNFs compared to CFs and CBrequired to achieve certain electrical conductivities.VGCNFs are produced by catalytic chemical vapor deposi-tion of a hydrocarbon (such as natural gas, propane, acety-lene, benzene, ethylene, etc.) or carbon monoxide over asurface of a metal (Fe, Ni, Au, Co) or metal alloy (such asNi–Cu, Fe–Ni) catalyst [25,42–45]. The catalyst can be depos-ited on a substrate or directly fed with the gas phase [19,46].The reaction is usually carried out in reactor operated at atemperature of 500–1500   C [19]. Pyrograf   III nanofibers (Ap-plied Sciences Inc. (ASI), Ohio, USA) have been the mostwidely investigated VGCNFs. They are produced in a gasphase reactor operated at 1100   C. The feedstocks for thereactor are natural gas, metal catalyst (produced by decompo-sition of (Fe(CO) 5 ), hydrogen sulfide (used to disperse and acti-vate the catalyst) and ammonia. The nanofibers are producedby decomposing of hydrocarbon on the metal catalyst. Thedecomposition both nucleates and grows the nanofibers[47]. The residence time of carbon in the reactor is only fewmilliseconds. Nanofibers produced by this method have a vol-ume resistivity of about 4  ·  10  3 X cm. However, this resistiv-ity can be further decreased by graphitization [20].Depending on the feedstock, catalyst and operating condi-tions different morphologies and characteristics of VGCNFswere obtained. For example, Lee and coworkers [25] synthe-sized VGCNF using different types of catalysts (nickel–copperand pure nickel) and different feedstocks (propane, ethyleneand acetylene). VGCNF produced using propane was linear;whereas, VGCNF produced using ethylene resulted in atwisted conformation. For VGCNF produced using acetylene, 1 [cited 2007 December 31]; Available from: http://www.cheaptubesinc.com C A R B O N  47 (2009) 2  –  22  3  both twisted and helical confirmations were observed. Sur-face area and electrical conductivity of VGCNF were alsofound to depend on the catalyst and feedstock types. VGCNFproduced using propane feedstock and nickel–copper as cata-lyst had the highest surface area (348 m 2  /cm). However, thehighest conductivity was observed for VGCNF produced using ethylene as a feedstock and pure nickel as a catalyst (28.7 S/cm). 1.1. Applications EMI shielding and electrostatic discharge (ESD) protection arethe major applications for conductive polymer composites[48–51]. Surface/volume resistivity of the filled polymer deter-mines its application range. Polymer composites used for EMIshielding applications typically have a surface resistivity low-er than 10 X  /sq, whereas for the ESD applications, the opti-mum surface resistivity range is 10 6 –10 9 X  /sq. 2 Fig. 1 showssurface resistivity ranges for plastic, antistatic, static dissipa-tive, conductive and metal materials.Currently, metal coated polymers (MCP) and carbon black(CB)-filled polymers are used for EMI shielding and ESD pro-tection, respectively [52–55]. These materials have many dis-advantages and limitations. For example, MCP candelaminate and are difficult to recycle. In some cases, thereare hidden costs in coating applications that increase the pro-duction cost [54,55]. Likewise, for CB-filled polymers, the highconcentration of the CB required to achieve good electricalproperties reduces certain composite mechanical propertiesand ease of processing, while increasing the cost [56]. In addi-tion, for CB powders, sloughing is an environmental concernand could damage the packaged electronics [57–59].ConductiveVGCNF/polymercompositescanbeusedassen-sorsfororganicvapors[60].Themechanismisbasedonchang-ingthecompositeconductivitywhenitisexposedtoanorganicvapor because of swelling of the polymer matrix. Organic va-porscanbedistinguishedbasedonthecompositeconductivityaftercertaintimeofexposure.Agoodsensoristheonethatcangive different electrical conductivity for different vapors [60].VGCNFs have potential applications in automotive indus-try that could lead to better quality, lower cost, less fuel con-sumption and lower environmental emissions. Thoseapplications include: electrostatic painting of exterior panels,shielding of automotive electronics and addition of VGCNFsto tires to improve stiffness [61].VGCNFs are promising materials for batteries, where mul-tifunctional carbon materials are used as electrodes or as sup-port materials [62]. For this application, carbon materialshave many advantages over other materials such as metaloxides and sulfides in terms of cost, thermal and chemicalstability, ease of formulation in various shapes and environ-mental impact [63].In the addition to the conductivity related applications,polymers filled with VGCNFs have potential biological appli-cations. VGCNFs are more attractive than SWNTs andMWNTs for applications require incorporation of biologicalcomponents such as proteins and DNAs in the hollow coreof the fiber, because they have much larger hollow core diam-eter [64]. In addition to the above mentioned applications,VGCNFs could be used in many applications that previouslyrequired CB or conventional CFs. 1.2. VGCNF properties Dimension, structure, electrical, mechanical and thermalproperties of the VGCNF depends on the production tech-nique and post-treatment methods [65]. Table 1 summarizes some typical properties of VGCNFs, CFs, MWNTs and SWNTs.VGCNFs are high aspect ratio nanofibers. They have largerdiameters than CNTs, but smaller than CFs. The length of VGCNFs is comparable to that of CNTs, but shorter than thatof CFs [1,20]. VGCNFs are hollow core nanofibers comprise of asingle layer, as shown in Fig. 2, or a double layer, as shown inFig. 3, of graphite planes stacked parallel or at a certain anglefrom the fiber axis [70,71]. The stacked planes are nested witheach other and have different structures including bamboo-like, parallel and cup-stacked [32,70,72,73]. Fig. 4 is a high res- olution transmission electron microscopy (HRTEM) image of aside-wall of a VGCNF (the inset is a schematic illustrating thestructure of cup-stacked VGCNF). The nanofiber is clearlyseen to have a hollow core surrounded by concentric cup-stacked planes. This type of structure has a large number of reactive edges both inside and outside the nanofiber [74]. Par-allel layers in single layer VGCNFs were also observed using HRTEM [70]. The d-spacing of the graphene sheets was re-ported as 0.34 nm (the same as that in MWNTs and graphiteplatelets).Uchida and coworkers [71] observed the two differentmorphologies of VGCNF graphite layers, namely: single layerand double layer. Fig. 5 depicts schematic sketches demon-strating the structure of a single layer VGCNF and a doublelayer VGCNF. The inset is a HRTEM showing the presenceof loops of about 3–5 graphite sheets at the inner and outer 2 [cited 2005 July 25]; Available from: wwwesd.org  Fig. 1 – Classification of materials according to their surfaceresistivity and application ranges. (Note: [cited 2007 July];Available from: http://www.rtpcompany.com/products/ conductive/index.htm.) 4  C A R B O N  47 (2009) 2  –  22  surfaces of the single layer VGCNF. The inner and outerdiameters of the single layer VGCNF are 25 and 60 nm,respectively, while for the double layer VGCNF, the innerand outer diameters are 20 and 83 nm, respectively. In addi-tion of having double layers and larger diameter, the graph-ene planes in the outer layer of the double layer VGCNF wereparallel to the fiber axis; whereas, both the single layerVGCNF and the inside layer of the double layer have a trun-cated cone morphology.Because of their high electrical conductivity, VGCNFs arefavorable fillers to formulate CPCs. As-produced VGCNFs areusually covered with layers of amorphous carbon that de-graded the conductivity of the nanofibers. Post treatment isrequired in order to remove those layers of less conductivecarbon (increasing crystallinity). Endo and coworkers [62]measured the volume electrical conductivity of short-VGCNF(s-VGCNF); 100–200 nm in diameter and 10–20  l m in length;using a four-point method. They found that the intrinsic vol-ume resistivity of nanofibers decreased to 10  3 and 10  4 X  cmby heat treating the fibers at 1200   C (carbonization) and2800   C (graphitization), respectively. Compared to normal va-por grown carbon fiber (VGCF), which is several microns indiameter, VGCNFs have higher volume resistivity due to theirlower crystallinity [62]. Volume resistivity of normal VGCFafter graphitization is 6  ·  10  5 X  cm [66].With a thermal conductivity value of 1950 W/(m K),VGCNFs have the highest thermal conductivity among allcommercial CFs 3,4 [41]. Experimentally measured thermalconductivity of MWNTis around 3000 W/(m K) [69]. This valueis much lower than the 6600 W/(m K) theoretically estimatedfor SWNTusing molecular dynamics simulations [69,75]. 3 [cited 2008 April 4]; Available from: http://www.apsci.com/ngm-pyro1.html 4 [cited 2008 April 04]; Available from: http://www.electrovac.com Table 1 – Typical properties of VGCNF, SWNT, MWNT and CF Property VGCNF a SWNT b MWNT b CF c Diameter (nm) 50–200 0.6–1.8 5–50 7300Length ( l m) 50–100 3200Aspect ratio 250–2000 100–10,000 100–10000 440Density (g/cm 3 ) 2   1.3 d  1.75 e 1.74Thermal conductivity (W/m K) 1950 3000–6000 f  3000–6000 f  20Electrical resistivity ( X  cm) 1  ·  10  4 1  ·  10  3 –1  ·  10  4 2  ·  10  3 –1  ·  10  4 1.7  ·  10  3 Tensile strength (GPa) 2.92 50–500 g  10–60 g  3.8Tensile modulus (GPa) 240 1500 1000 227a From Refs. [1,20,62,66].b From Ref. [2].c Properties of Akzo Nobel Fortafil 243 PAN-based fibers [67].d From Ref. [33].e From Ref. [68].f From Ref. [69].g From Ref. [35]. Fig. 2 – TEM micrograph of VGCNF [20].Fig. 3 – TEM micrograph of a double layer VGCNF [70]. C A R B O N  47 (2009) 2  –  22  5  One of the majordrawbacks of VGCNFs is their poor tensileproperties compared to those of CNTs. Before talking aboutstress transfer, adhesion, etc., knowing intrinsic mechanicalproperties of VGCNFs is very crucial since it determines theultimate properties that can be achieved when using themas mechanical reinforcing fillers. VGCNF and VGCF tensileproperties depends on the fiber diameter [62,76]. For example,tensile strength of graphitized s-VGCNF having a diameter of 100 and 300 nm are 2.2 GPa and 1.77 GPa, respectively. Thedependence of tensile properties on nanofiber diameter mayreveal a change in nanofiber morphology (crystallinity, graph-ene planes orientation) with increasing diameter (for exam-ple, changing from single layer to double layer) and/orincrease in defects with increasing fiber diameter.No direct measurements of Pyrograf   III nanofibersmechanical properties have been conducted. The 2.92 GPatensile strength and 237 GPa tensile modulus usually usedin literature are for 7.5  l m in diameter VGCF. Patton andcoworkers [77] estimated the lower limit of the tensile modu-lus and tensile strength of Pyrograf   III nanofibers based onthe rule of mixtures and experimentally measured mechani-cal properties of 15.5 vol% VGCNF reinforced epoxy. Their cal-culations revealed that the lower limit of tensile modulus andtensile strength are in the range of 88–166 GPa and 1.7–3.38 GPa, respectively. 2. Electrical properties A critical filler loading must be incorporated to transfer thecomposite from the insulative state into the conductive state.At this critical concentration, which is known as the percola-tion threshold, the electrical conductivity of the compositesuddenly increases by several orders of magnitude. Often, atthe percolation threshold, the filler forms a continuous net-work inside the polymer matrix, and increasing in the fillerloading further usually has little effect on the composite elec-trical resistivity, as shown in Fig. 6.However, if a remarkable decrease in the composite’s elec-trical resistivity is noticed with increasing the filler loading above the percolation threshold, this means that the threedimensional conductive network has not yet been formed atthe percolation concentration, and thus the composite con-ductivity is due to tunneling in addition to direct contact be-tween the particles. In some cases, tunneling could be thedominant mechanism. Tunneling conduction occurs whenthe distance between the filler particles are close enough,roughly less than 10 nm [78]. Investigating the current–volt-age ( I – V  ) relationship gives an indication whether the com-posite conductivity is due to tunneling or direct contactbetween the particles [79,80]. Linear  I – V   relationship, Ohm’slaw, indicates that direct contact between the filler particlesis the dominant conduction mechanism. However, tunneling mechanism is the dominant mechanism for composites char-acterized by power law  I – V   relation [80,81]. Fig. 4 – HRTEM image shows a side-wall of a VGCNF havinga cup-stacked structure. The inset is a schematic illustratesthe cup-stacked structure [74].Fig. 5 – Schematic illustrate the structure of (a) a single layer VGCNFand (b) a double layer VGCNF (c) a HRTEM showing loops(two loops have been enclosed in white ellipse as a guide for the eye) on the side-wall of a single layer VGCNF [70]. 6  C A R B O N  47 (2009) 2  –  22
Search
Similar documents
View more...
Related Search
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks