Thermal conductivity of polymer composites based on the length of multi-walled carbon nanotubes

The anisotropic development of thermal conductivity in polymer composites was evaluated by measuring the isotropic, in-plane and through-plane thermal conductivities of composites containing length-adjusted short and long multi-walled CNTs (MWCNTs). The thermal conductivities of the composites were relatively low irrespective of the MWCNT length due to their high contact resistance and high interfacial resistance to polymer resins, considering the high thermal conductivity of MWCNTs. The isotropic and in-plane thermal conductivities of long-MWCNT-based composites were higher than those of short-MWCNT-based ones and the trend can accurately be calculated using the modified Mori-Tanaka theory. The in-plane thermal conductivity of composites with 2 wt% long MWCNTs was increased to 1.27 W/m·K. The length of MWCNTs in polymer composites is an important physical factor in determining the anisotropic thermal conductivity and must be considered for theoretical simulations. The thermal conductivity of MWCNT polymer composites can be effectively controlled in the processing direction by adjusting the length of the MWCNT filler.     

A comparative study on the electrical,thermal and mechanical properties of ethylene–octene copolymer based composites with carbon fillers

Conducting polymer composites (CPC) were prepared with an ethylene–octene copolymer (EOC) matrix and with either carbon fibers (CFs) or multiwall carbon nanotubes (MWCNTs) as fillers. Their electrical and thermal conductivities, mechanical properties and thermal stabilities were evaluated and compared. CF/EOC composites showed percolation behavior at a lower filler level (5 wt.%) than the MWCNT/EOC composites (10 wt.%) did. Alternating current (AC) conductivity and real part of permittivity (dielectric constant) of these composites were found to be frequency-dependent. Dimensions and electrical conductivities of individual fillers have a great influence on the conductivities of the composites. CF/EOC composites possessed higher conductivity than the MWCNT-composites at all concentrations, due to the higher length and diameter of the CF filler. Both electrical and thermal conductivities were observed to increase with increasing filler level. Tensile moduli and thermal stabilities of both (CF/EOC and MWCNT/EOC) composites increase with rising filler content. Improvements in conductivities and mechanical properties were achieved without any significant increase in the hardness of the composites; therefore, they can be potentially used in pressure/strain sensors. Thermoelectric behavior of the composites was also studied. Accordingly, CF and MWCNT fillers are versatile and playing also other roles in their composites than just being conducting fillers.     

Preparation and characterization of electrospun core sheath nanofibers from multi-walled carbon nanotubes and poly(vinyl pyrrolidone)

Electrospinning is a versatile technique to prepare polymer fibers in nano to micrometer size ranges using very high electrostatic fields. Electrospun nanofibers with tunable porosity and high specific surface area have various applications, including chromatographic supports for protein separation, biomedical devices, tissue engineering and drug delivery matrices, and as key components in solar cells and supercapacitors. Unspinnable materials such as nanoparticles, nanorods, nanotubes or rigid conducting polymers can also be electrospun into fibers through co-axial electrospinning. In this study, we have prepared core-sheath nanofibers utilizing co-axial electrospinning. The core portion of these electrospun fibers consists of multi-walled carbon nanotubes and the sheath portion is poly(vinyl pyrrolidone) (PVP). Various morphologies were obtained by changing both core and sheath solution concentrations. The core-sheath nanofibers were characterized by scanning electron microscopy and transmission electron microscopy, to confirm core-sheath morphology, thermogravimetric analysis, and mechanical strength testing. The electrical conductivity of the surfaces of poly(vinyl pyrrolidone) fibers and poly(vinyl pyrrolidone)-multi-walled nanotube fibers were both 10(-15) S/m. The highest bulk conductivity observed for the poly(vinyl pyrrolidone)-multi-walled nanotube fibers was 1.2 x 10(-3) S/m.     

Electrical properties of polyvinylidene fluoride (PVDF)/multi-walled carbon nanotube (MWCNT) semi-transparent composites: Modelling of DC conductivity

Transparent conductive composites can be achieved from PVDF–MWCNT at very low concentration of MWCNT. These composites show different degree of UV–Visible radiation absorption depending on MWCNT concentration in composites. The composition dependent dielectric properties and AC conductivity were also measured for these composites. Properties like AC conductivity, dielectric constant and loss are increasing with filler concentration. The variations of DC conductivity against composition and temperature are also reported. The electrical hysteresis and electrical set are observed for PVDF–MWCNT composites when subjected to heating–cooling cycle. The validity of different theoretical models depicting percolation threshold with respect to DC conductivity was tested for these composites.     

Multi-walled carbon nanotubes and poly(lactic acid) nanocomposite fibrous membranes prepared by solution blow spinning

Nanocomposite fibers based on multi-walled carbon nanotubes (MWCNT) and poly(lactic acid) (PLA) were prepared by solution blow spinning (SBS). Fiber morphology was characterized by scanning electron microscopy (SEM) and optical microscopy (OM). Electrical, thermal, surface and crystalline properties of the spun fibers were evaluated, respectively, by conductivity measurements (4-point probe), thermogravimetric analyses (TGA), differential scanning calorimetry (DSC), contact angle and X-ray diffraction (XRD). OM analysis of the spun mats showed a poor dispersion of MWCNT in the matrix, however dispersion in solution was increased during spinning where droplets of PLA in solution loaded with MWCNT were pulled by the pressure drop at the nozzle, producing PLA fibers filled with MWCNT. Good electrical conductivity and hydrophobicity can be achieved at low carbon nanotube contents. When only 1 wt% MWCNT was added to low-crystalline PLA, surface conductivity of the composites increased from 5 x 10(-8) to 0.46 S/cm. Addition of MWCNT can slightly influence the degree of crystallinity of PLA fibers as studied by XRD and DSC. Thermogravimetric analyses showed that MWCNT loading can decrease the onset degradation temperature of the composites which was attributed to the catalytic effect of metallic residues in MWCNT. Moreover, it was demonstrated that hydrophilicity slightly increased with an increase in MWCNT content. These results show that solution blow spinning can also be used to produce nanocomposite fibers with many potential applications such as in sensors and biosensors.     

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.     

Coupled electro-mechanical properties of multiwall carbon nanotube/polypropylene composites for strain sensing applications

The electrical, mechanical, and coupled electro-mechanical (piezoresistive) properties of multiwall carbon nanotube/polypropylene (MWCNT/PP) composites at four MWCNT concentrations above electrical percolation (4–10 wt %) were investigated. The electrical conductivity of the composite increased monotonically from 0.77 to 15.0 S/m with the increase of MWCNT concentration. The elastic modulus also increased monotonically with increased MWCNT concentration with the concomitant reduction of ultimate strain. The coupled signal between electrical resistance and applied strain during tensile loading displayed a marked change toward higher sensitivity at the elastic-to-plastic transition zone of the polymer composite, which allowed the identification of polymer yielding by the sole monitoring of electrical resistance. Large ratios (of the order of 15–29) of normalized changes in electrical resistance over applied strain (“gage factor”) were found in the plastic zone, and such electro-mechanical sensitivity was higher for composites with lower MWCNT content.     

Electrical conductivity and shielding effectiveness of poly(trimethylene terephthalate)/multiwalled carbon nanotube composites

Poly(trimethylene terephthalate) (PTT)/multiwalled carbon nanotube (MWCNT) composites have been fabricated to evaluate the potential of PTT composites as electromagnetic interference (EMI) shielding material. The room temperature electrical conductivity, complex permittivity, and shielding effectiveness (SE) of PTT/MWCNT composites were studied in the frequency range of 8.2–12.4 GHz (X-band). The dc conductivity (σ) of composites increased with increasing MWCNT loading and a typical percolation behavior was observed at 0.48 vol% MWCNT loading. The highest EMI SE of PTT/MWCNT composites was ~23 decibel (dB) at 4.76 vol% MWCNT loading which suggest that these composites can be used as light weight EMI shielding materials. The correlation among the SE, complex permittivity, and electrical conductivity was also studied. The EMI shielding mechanism of PTT/MWCNT composites was studied by resolving the total EMI SE into absorption and reflection loss.     

The mechanical properties and thermal conductivity of carbon/carbon composites with the fiber/matrix interface modified by silicon carbide nanofibers

For modification, silicon carbide nanofibers (SiCNFs) are uniformly dispersed on the fiber surface of the unidirectional carbon preform. The modified unidirectional carbon preform was then densified to obtain SiCNF–C/C composites by chemical vapor deposition (CVD). The microstructure of SiCNF–C/C composites was investigated. The mechanical properties and thermal conductivity of the modified composites were analyzed as well. Results show that PyC preferentially deposits on the surface of SiCNFs with high degree of order. The interface between carbon fibers and matrix has high texture, resulting in a good bonding between them. The mechanical properties of C/C composites are adjusted. After modification, the fracture mode is changed and the flexural strength is enhanced, especially in vertical direction. The thermal conductivity of modified composites is also enhanced in both vertical and parallel directions.     

Electromagnetic interference shielding behavior of poly(trimethylene terephthalate)/multi-walled carbon nanotube composites

Poly(trimethylene terephthalate) [PTT]/multiwalled carbon nanotube [MWCNT] composites having varying amounts of MWCNTs were fabricated with an aim to investigate the potential of such composites as an effective light weight electromagnetic interference (EMI) shielding material in the frequency range of 12.4-18 GHz (Ku-band). PTT/MWCNT composite with shielding effectiveness (SE) of 36-42 dB was obtained at 10% (w/w) MWCNT loading. Shielding mechanism was studied by resolving the total SE into absorption (SE_A) and reflection loss (SE_R). PTT/MWCNT composite showed absorption dominated shielding; thus it can be used as microwave, radar absorbing and stealth material. The effect of MWCNT loadings on electrical conductivity (σ) and dielectric properties of PTT and the correlation among conductivity, tan δ, absorption loss and reflection loss were also studied.     

Polyacrylonitrile nanofibers prepared using coaxial electrospinning with LiCl solution as sheath fluid

A modified coaxial electrospinning process including an electrolyte solution as sheath fluid was used for preparing high quality polymer nanofibers. A series of polyacrylonitrile (PAN) nanofibers were fabricated utilizing a coaxial electrospinning containing LiCl in N, N-dimethylacetamide (DMAc) as the sheath fluid. FESEM results demonstrated that the sheath LiCl solutions have a significant influence on the quality of PAN nanofibers. Nanofibers with smaller diameters, smoother surfaces and uniform structures were successfully prepared. The diameters of nanofibers were controlled by adjusting the conductivity of the sheath fluid over a suitable range and this was determined by varying LiCl concentrations. The influence of the effect of LiCl on the formation of PAN fibers is discussed and it is concluded that coaxial electrospinning with electrolyte solutions is a convenient and facile process for achieving high quality polymer nanofibers.     

Multiwall carbon-nanotube-doped ion conducting polymer electrolyte for electrochemical application

Electrical, structural and optical properties of a composite containing a polymer electrolyte (namely polyethylene oxide complexed with sodium iodide) and multiwall carbon nanotube (MWCNT) are reported. The films of these composites were ‘solution casted’ using the viscous solution of polyethylene oxide (PEO) complexed with sodium iodide (NaI) in desired ratios and characterised using various techniques. The conductivity versus composition plot in PEO:NaI shows conductivity maxima at 12?wt% NaI concentration while in MWCNTs doped polymer electrolyte it occurs at 40?wt% MWCNTs concentration. The surface morphology by scanning electron microscopy (SEM) shows the enhancement in amorphous reason by MWCNTs doping which is a well-known favourable condition for conductivity enhancement. The differential scanning calorimetry shows that dispersal of MWCNTs reduces the crystallinity of polymer electrolyte that is well-supported by our polarised optical micrographs and SEM measurements.     

Three-dimensional carbon nanotube based polymer composites for thermal management

In this study, the porous multiwall carbon nanotube (MWCNT) foams possessing three-dimensional (3D) scaffold structures have been introduced into polydimethylsiloxane (PDMS) for enhancing the overall thermal conductivity (TC). This unique interconnected structure of freeze-dried MWCNT foams can provide thermally conductive pathways leading to higher TC. The TC of 3D MWCNT and PDMS composites can reach 0.82 W/m K, which is about 455% that of pure PDMS, and 300% higher than that of composites prepared from traditional blending process. The obtained polymer composites not only exhibit superior mechanical properties but also dimensional stability. To evaluate the performance of thermal management, the LED modulus incorporated with the 3D MWCNT/PDMS composite as heat sink is also fabricated. The composites display much faster and higher temperature rise than the pristine PDMS matrix, suggestive of its better thermal dissipation. These results imply that the as-developed 3D-MWCNT/PDMS composite can be a good candidate in thermal interface for thermal management of electronic devices.     

Preparation and characterization of conductive carbon nanotube-polyurethane foam composites

Electrical, thermal, and morphological properties of the polyurethane foam (PUF)/multiwall carbon nanotube (MWCNT) composites were investigated with the MWCNT content. Electrical conductivity of the PUF/MWCNT composites increased rapidly from 0 to 0.23 S/cm at 0.1 php MWCNT content, then, the electrical conductivity did not change significantly with the increase of MWCNT content up to 0.5 phr because of the aggregation of the MWCNT when the amount of MWCNT was large (0.5 php). The PUF/MWCNT composite having low MWCNT contents (0.01, 0.05, and 0.1 php) showed lower thermal conductivity than that of the PUF/MWCNT composite having higher content (0.5 php). This is maybe due to that the PUF with the lower MWCNT contents (0.01, 0.05, and 0.1 php) showed smaller cell size than that of the PUF with the higher content of MWCNT (0.5 php). From the results of thermal conductivity and cell size of the PUF/MWCNT composites, it is suggested that reduction in cell size of the composite affects lowering the thermal conductivity of the PUF/MWCNT composites. Also, small amount (0.01, 0.05, and 0.1 php) of MWCNT may contribute to decrease the thermal conductivity of the PUF/MWCNT composites.     

Silver coated vapor-grown-carbon nanofibers for effective reinforcement of polypropylene-polyaniline

This work utilizes a modification of our process of polymer entrapment in silver to deposit silver crystals on carbon nanofibers at different relative concentrations. The experimental procedure and the characteristics of silver coated nanofibers are presented in detail. The resulting nanofibers are then melt-mixed with a polypropylene-polyaniline blend to form a uniform dispersion that is finally extruded to produce continuous monofilament composites of high axial orientation. The reinforcement effect of the silver coated nanofibers, manifested in the mechanical properties of the monofilament composites, is 3-5 folds higher than that of the pristine nanofibers due to the improved stress transfer mechanism of the former. Additional attractive properties of the new system may result from its anisotropic crystalline structure, enhanced thermal stability, potential electrical conductivity and antibacterial behavior.     

PAN-based carbon fibers/PMMA composites: thermal, dielectric, and DC electrical properties

The study deals with thermal, dielectric, and DC electrical properties of polyacrylonitrile (PAN)-based carbon fibers/poly(methyl methacrylate) composites. The polymer composites contain 0, 5, 10, 20 and 30 wt.% PAN-based carbon fibers. The thermal conductivity was studied as a function of filler content and temperature. It was found that the thermal conductivity is enhanced by addition of carbon fibers concentration and temperature. The dielectric properties were determined using impedance measurements. The results showed that the dielectric constant and dielectric loss are decreased with frequency, and increased with both temperature and fibers content. The DC electrical conductivity, temperature coefficient of resistance, and activation energy were studied as a function of fibers concentration in the temperature ranges 30–110?°C. It was found that the composites exhibit negative temperature coefficient of resistivity and enhancement of electrical conductivity with increasing temperature and carbon fibers concentration. The observed increase in the DC conductivity was explained according to the approach of conductive paths and connections between the carbon fibers.     

Synthesis and characterization of conductive polypyrrole/multi-walled carbon nanotubes composites with improved solubility and conductivity

High conductivity and solubility of polypyrrole (PPy)/multi-walled carbon nanotubes (MWCNT) composites has been successfully synthesized by in situ chemical oxidation polymerization using various concentrations of cationic polyelectrolyte poly(styrenesulfonate) (PSS) and ammonium peroxodisulfate (APS). Raman spectroscopy, FTIR, EPR, FESEM and HRTEM were used to characterize their structure and morphology. These images of FESEM and HRTEM showed that the fabricated PPy/MWCNT composites are one-dimensional core-shell structures with the average thickness of the PPy/MWCNT composites without PSS is about 250 nm and considerably decreases to 100–150 nm by adding the PSS content. The results of Raman spectrum, FTIR and UV–Vis indicate the synthesized PPy/MWCNT composites are in the doped state. The conductivities of PPy/MWCNT composites synthesized with the weight ratio of PSS/pyrrole monomer at 0.5 are about two times of magnitude higher than that of PPy/MWCNT composites without PSS. These results are perhaps due to the part of cationic electrolyte served as a dopant can be incorporated to the PPy structure to improve the conductivity of fabricated PPy/MWCNT composites.     

MWCNT/PEDOT复合材料的微观结构和热电性能

热电转换技术能将大量的废弃热能转换为电能以重新利用,是一种绿色能源转换技术,可以有效提高能源利用效率,缓解煤炭、石油等主要化石类能源过度开采、使用带来的能源危机及环境污染问题,因此受到科研工作者的广泛关注,是近年来的研究热点。基于此,本文以电子型导电高聚物中机能较优的聚(3, 4-乙烯二氧噻吩)(PEDOT)作为研究主体,通过化学原位氧化聚合将多壁碳纳米管(MWCNT)复合到载体中得到MWCNT/PEDOT复合材料。利用XRD、拉曼、TEM及正电子湮没寿命(PAL)等方法对MWCNT/PEDOT复合材料的形貌和微观结构进行了系统研究,研究表明:当MWCNT含量高于24.9wt%时,复合材料中出现MWCNT团聚现象,其分散性变差。同时,MWCNT/PEDOT复合材料的热电性能测试结果显示,未掺杂PEDOT的电导率仅为7.5 S·m?1,而MWCNT含量为30.1wt%时,该复合材料的电导率高达566.59 S·m?1,提高近76倍。同时,30.1wt%MWCNT/PEDOT的功率因子(814.3×10?4 μW·(m·K2)?1)相对于未掺杂PEDOT(14.5×10?4 μW·(m·K2)?1)提高约56倍,这主要是由于PEDOT分子链与MWCNT掺杂物间π-π相互作用及MWCNT的高导电性。随着MWCNT含量的增加,PAL测试结果中第一寿命成分τ₁(即正电子在材料中湮没的第一寿命成分)的下降证实了该复合材料中MWCNT与PEDOT间界面变小或者界面间相互作用减弱,导致其热导率相对于未掺杂PEDOT有一定的上升,但远远低于功率因子的升高。最终,该MWCNT/PEDOT复合材料的热电优值(即热电材料Z_T值)由0.015×10?4升至0.45×10?4,增加了约30倍。结果表明:掺杂的高电导率MWCNT能够极大地提高PEDOT类电子型导电聚合物的热电性能。      

Electrical conductive and graphitizable polymer nanofibers grafted on graphene nanosheets: Improving electrical conductivity and flame retardancy of polypropylene

Polyaniline (PANI) nanofibers grafted reduced graphene oxide (PANI–RGO) is prepared using the “grafting-from” strategy and then is incorporated into polypropylene (PP) matrix by way of the master batch-based melt mixing method. Grafted PANI nanofibers can improve the dispersion and electrical conductivity of reduced graphene oxide (RGO). The electrical conductivity of the modified RGO and its composites is not impaired by the grafted polymer, due to the conductive characteristics of PANI. The barrier action of PANI–RGO can greatly inhibit the release of flammable pyrolysis products of PP. PANI–RGO exhibits a marked flame retardancy effect on PP. The smoke release of the composites is slightly retarded. Transmission electron microscopy image and Raman spectrum of the char residue for PANI–RGO based composite indicate the formation of carbon nanofibers during combustion. The in situ formed carbon nanofibers on graphene nanosheets can enhance barrier performance against heat and mass transfer, resulting in enhanced flame retardancy.     

The electrical behavior of laminated conductive polymer composite at low temperatures

The study deals with the electrical characteristics of laminated conductive polymer composites consisting of epoxy and carbon fibers with different concentration. The composites contain 7, 17.5, and 25.2 wt% carbon fibers (10, 25, and 36 layers of carbon fibers), respectively. The DC electrical conductivity was studied as a function of filler concentration in low temperature range 25–275 K. It was found that the composites exhibit negative temperature coefficient of resistivity (TCR) and electrical conductivity enhancement with temperature and carbon fibers concentration. The semiconducting behavior of the observed electrical conductivity is characterized by two different regions: high temperature range where the conductivity increases gradually (thermal process) and low temperature range where the conductivity increases with a less rate (Motts hopping process) with increasing of temperature.