IET NanodielectricsElectrical and Thermal Properties of SurfacePassivated Carbon Nanotube/PolyvinylideneFluoride CompositesNDE-2018-0010.R1 | Research ArticleSubmitted on: 27-06-2018Submitted by: Sheng-Guo Lu, Dandan Li, Caihong Lei, Yingxiu Ouyang, Zhihong Cai, Yongmao Deng, Ruijie XuKeywords: CARBON NANOTUBES, COMPOSITE MATERIALS, ELECTRICAL INSULATIONReView by River Valley Technologies IET Nanodielectrics2018/06/27 15:39:34 IET Review Copy Only 1Answers to the Reviewer CommentsComments to the AuthorIn this paper, the authors have reported the electrical and thermalProperties of Surface Passivated Carbon Nanotube and PolyvinylideneFluoride Composites. The composite material shows the changes ofelectrical resistivities, whereas thermal conductivities does not changesignificantly. They have studied electrical properties using few existingmethods like Maxwell-Wagner polarization, universal law, and Gerhardt’smethod.
The work is experimental and time taking. The paper is well written.Further improvement is needed.Thanks!1. Why author have chosen MWCNT with poly(vinylidene fluoride)? Pleasejustify.Answer (Abbreviated as A below)?It is well known that the MWCNT is a goodelectrical and thermal conductor, and will enhance the electrical and thermalconductivity if the polymer matrix in added this kind of material.
Here the surfacepassivated MWCNT was used because our objective is to procure a composite thathas a lower electrical conductivity, but a higher thermal conductivity. In addition,the MWCNT is usually cheaper than SWCNT in the commercial market. Forpoly(vinylidene fluoride) (PVDF), this is a well-known piezoelectric polymer, andalso a commercially cheap material.2. Explanation also needed for SWCNT, Mixed CNT and Graphene Nanoribbonlike nanomaterial.
A: SWCNT or mixed CNT and graphene nanoribbon can also be incorporated intoPVDF to form the composite which has the similar properties as MWCNT/PVDF.Due to the reason that the MWCNT is cheaper, and also more convenient to bepurchased, we chose MWCNT. In principle, they are much similar.
3. What is the electron mean free path of MWCNT they have analyzed in theirexperiment?A: We didn’t investigate the electron mean free path of MWCNT. But accordingto Kyriakou et al’s work, the inelastic mean free path of electron in MWCNT witha diameter larger than 20 nm will be in the range of ~ 1 to 10 nm.
The diameter ofthe MWCNT used in our work is 45 nm. This means that the electron will belocalized in the MWCNT, to produce the electronic transportation.4.
In Fig.3 the labels are not clear please improve it.ReView by River Valley Technologies IET Nanodielectrics2018/06/27 15:39:34 IET Review Copy Only 2A: Thanks. We have improved the labels in Fig. 3.
5. References are not well organized. Improvement is needed.A: OK. The references have been organized again.
Thanks!6. In Fig.4 the MWCNT content vs. resistivity plot two different line ishighlighted one is red and other is dotted blue.
Please mention thetwo types.A: OK. We have mentioned them in the context.7.
“For simplicity, tau corresponds the maximum in the tauldistribution.” please improve the tau1.A?OK. The tau1 has been improved.
8. typo mistake z’-z”’ in page-5 1st column.A: The typo has been corrected.9. Like equation (3) and (4) the Eqn.
(1) and (2) total impedance shouldbe function of angular frequency.A: Yes. The total impedance is a function of angular frequency, since thecapacitor is appeared in the equivalent circuit.10.It will be good if the experimental real and imaginary impedancepresented in a table format including RC value.A: We have the data of experimental real and imaginary impedance, which are thesame as the curves presented.
ReView by River Valley Technologies IET Nanodielectrics2018/06/27 15:39:34 IET Review Copy Only 31Electrical and Thermal Properties of Surface Passivated CarbonNanotube/Polyvinylidene Fluoride CompositesYingxiu Ouyang;, Yongmao Deng;, Dandan Li;, Zhihong Cai, Ruijie Xu, Caihong Lei, and ShengguoLu*Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, School of Materials and Energy,Guangdong University and Technology, Guangzhou, 510006 China*[email protected];Equally contribute authorsAbstract: The composites reported here were prepared via a melt-rolling mixing process, in which surface passivatedmulti-wall carbon nanotube and poly(vinylidene fluoride) (PVDF) were used as the filler and the matrix, respectively. Xraydiffraction (XRD), precision inductance-capacitance-resistance (LCR), and laser flash thermal-diffusivity analyseswere employed to characterize the structural, electrical and thermal properties.
Results indicate that through the surfacepassivation of carbon nanotubes, the electrical resistivities of the composites were greatly enhanced, while the thermalconductivities do not change significantly. The electrical properties were discussed in terms of the comparison with theMaxwell-Wagner polarization, universal law, and Gerhardt’s method.1.
IntroductionMulti-wall carbon nanotube (MWCNT) is a kind ofmaterial having high mechanical strength and sound electricaland thermal conductivities. MWCNT has been regarded as anideal filler for advanced composites due to its excellentmechanical and electrical properties 1-3. In addition, singlewallcarbon nanotube (SWCNT) 4, mixed CNT 5 andgraphene nanoribbon 6 are also good fillers due to theirgood electrical and thermal properties as MWCNT.Poly(vinylidene fluoride) (PVDF) and its copolymers orterpolymers are important functional materials, which havebeen widely used as ferroelectric memories, dielectric energystorages, various sensors and mechanical actuators because oftheir good dielectric, ferroelectric, piezoelectric andelectromechanical properties 7-9. Thus it is better to formcomposites to get multiple functionalities for the versatileapplications. Recently, a large number of investigations havebeen carried out in order to enhance the functional propertiesof MWCNT filled composites 10-12.
In general, the electrical conductivity has a closerelationship with the thermal conductivity. In some situations,e.g., thermal management pipes in an electrical vehicle 13,however, the good thermal conductivity is demanded but inthe meantime the electrical conduction should be avoided, i.
e.,good electrical insulation is needed. Fortunately, the surfacepassivation of the MWCNTs may lead to a great reduction ofthe electrical conductivity on the surfaces of MWCNTs, thusthe electrical conductivity of the composites using passivatedMWCNT as a filler will be much lower even if they contactone another.
Recently, there are quite a lot of investigationson the oxidation and functionalization of the surface ofMWCNTs 14-19. The oxidation using acid, e.g.
nitric acidor sulphuric acid, results in the formation of surfacecarboxylic groups in terms of rapid formation of carbonylgroups, and then transformation into phenol, lactones,anhydrides, or carboxylic groups 15. These groups might bedecomposed by heat treatment, releasing CO and/or CO2.Partial groups, e.g. carboxylic acids, may be removed whenheat treated at 400 ?C, while almost all the groups will becleaned when heat treated at 900 ?C 14.
Although thegroups are removed, it was found that the resistivities of theMWCNT/polymer composites increase quite a lot. Thismeans that the heat treatment at 900 ?C could not remove allthe groups completely, and the surface structures have beenreally changed. In this work, MWCNTs are passivated usingH2SO4/HNO3, then MWCNT/PVDF composites are preparedby a melt-rolling mixing method. The structural, electricaland thermal properties were characterized. It is found that theinsulation resistance is greatly enhanced while the thermalconductivity keeps almost the same.
2. Experimental procedures2.1. Passivation of MWCNTFor the preparation of oxidized MWCNT, Firstly, theMWCNT were oxidized by H2SO4 and HNO3 (3:1 in volume)at 90 ?C for 3 h and 2 h, respectively. Then, the oxidizedCNTs, tetrahydrofuran (KH550) and N,N’-Dicyclohexylcarbodie (DCC) were mixed in tetrahydrofuran(THF) at 60 ?C for 1 h to get the silicone grafted CNTs.
Afterthat, the grafted CNTs were hydrolyzed in ethanol. Finally,the hydrolyzed CNTs, and DCC (ratio 3:1:1) were reacted inTHF at 60 ?C for 2 h. At last the obtained filler was dried ina vacuum oven at 90 ?C for 6 h 20.
2.2. Preparation of MWCNT/PVDF compositesThe MWCNT was prepared via a chemical vapordeposition method. As shown in Fig. 1, the MWCNT has adiameter of c.
a. 45 nm, a length larger than 1 ?m, and aspecific surface area of 90 – 120 m2/g. For the preparation ofoxidized MWCNT/PVDF composites, the PVDF was firstlydissolved in acetone, then the passivated MWCNTs wereadded and stirred for 5 ? 10 h. After that, the solution waspoured into a glass utensil, stirred at 60 ?C for 5 ? 15 h, anddried at 80 ?C for 8 h in an oven. Furthermore, the compositewas weighed with the weight ratios of MWCNT/PVDF ofReView by River Valley Technologies IET Nanodielectrics2018/06/27 15:39:34 IET Review Copy Only 421:60, 1:40, 1:20 and 1:15 respectively (specified as Samples1,2, 3, and 4, respectively) for passivated MWCNT/PVDFcomposites, and 1:10, 2:10, 3:10, and 4:10 respectively(specified as Samples 5, 6, 7, and 8, respectively) for pristineMWCNT/PVDF composites. Above mentionedcompositions were then melt mixed using a two-roll mixingmachine at 175 ?C for several times, then the rough compositeplates were put into a hot presser and pressed at 180 ?C for 10min, and then cooled down to room temperature with themachine, and dwelled for 5 min at room temperature.
2.3. Electrical and Thermal CharacterizationThe large piece of composite obtained from hotpressingwas cut into pieces with 10?10?0.8 mm3 in size. Auof 10?10 mm2 in area was sputtered on both surfaces of thesample (SBC-12, KYKY Technology). The electricalproperties were measured at room temperature using aprecision LCR meter (HP 4284A) with a temperaturechamber (homemade) in which the temperature could bevaried from -150 to 150 ?C.
The same samples were also used to measure theirthermal conductivities at room temperature using a thermalconductivity apparatus (LFA447 Nanoflash). During themeasurement, a sample with larger size (10 mm in diameterand 2 mm in thickness) is needed for precise characterizationof the thermal behavior. Three points of each sample weremeasured, and the averaged value was calculated and used asthe sample’s thermal conductivity.3.
Results and discussion3.1. XRD and SEM characterizationFigure 1 shows the scanning electron microscopy(SEM) image of MWCNT. One can see that the CNTs areuniform in diameter, and demonstrate multiwallmorphologies.
The detailed dimensions and the specificsurface areas are mentioned in the last section.Figure 2 demonstrates the XRD patterns forMWCNT/PVDF composites with MWCNT/PVDF ratios of1:60, 1:40, 1:20 and 1:15 respectively. One can see that withthe increasing MWCNT content, the (002) peak of MWCNTFig.1. SEM image of MWCNTFig.
2. XRD patterns for MWCNT/PVDF composites ofSamples 1, 2, 3, and 4, respectivelyat 26.2? becomes larger, while the peak at 26.8? from ? phaseof PVDF (021) is depressed. In addition, the (020) peak of ?phase of PVDF is divided into two peaks, i.
e., (100) and (020),after the incorporation of MWCNT, these two peaks becomemore distinct with the increase of MWCNT content. The(110)/(200) peak of ? phase does not change significantly forfour composite samples.3.2. Electrical propertiesThe real and imaginary parts of impedance as afunction of frequency for the four composites (Samples 1, 2,3 and 4) measured at room temperature are shown in Figs.
3(a), (b), (c) and (d). The figure shown on the right of eachfigure shows the conductivity ? as a function of angularfrequency ?. Except for Sample 4, the profiles for other threesamples are similar to Debye relaxations in the real –imaginary permittivity plot for conventional dielectrics.
ForSample 4, one can find out that the static resistance (~ realpart at the lowest frequency 20 Hz) greatly drops from 143.3k? (Sample 1), 154.2 k? (Sample 2), to ~ 36.1 k? (Sample3), and then ~ 5.4 k? (Sample 4). On the contrary, imaginarypart at the lowest frequency 20 Hz rises up from ~ 1.
1 k?(Sample 1), 1.5 k? (Sample 2), to ~ 3.2 k? (Sample 3), andthen 53.2 k? (Sample 4).
The large variation of real andimaginary parts of impedance at the lowest frequency 20 Hzfor 4 samples strongly suggests that there might be amechanism that makes contribut ion to the electricalproperties of Sample 4, but it is still unclear right now. Forthe right one of each figure, all the samples demonstrate apower law of the conductivity versus angular frequency,which is the typical feature of so-called “universal law”proposed by A. K. Jonscher for large ranges of frequency,time and temperature, insensitive to the many materialproperties 21. The common basis for the universal dielectricresponse was regarded as the presence of interactions arisingfrom the close proximity of atoms and molecules and theabrupt or discontinuous nature of the dipolar or charge carriertransitions between their preferred orientations or positions.The second feature comes from the Maxwell-Wagner (M-W)polarization 22,23, which is usually referred to in theReView by River Valley Technologies IET Nanodielectrics2018/06/27 15:39:34 IET Review Copy Only 53Fig.
3. (left) Real and imaginary parts of impedance as a function of frequency and (right) conductivity as a function of angularfrequency for MWCNT/PVDF composites of Samples (a) 1,(b) 2, (c) 3, and (d) 4 at room temperature.inhomogeneous or multiphase systems, for instance, thecomposites used here. The mechanism of M-W polarizationis due to the charge accumulation in the interface between twokinds of materials or phases, which show quite differentpermittivities and conductivities. Since two phases aresterically in series, the M-W polarization could be equivalentto two series of impedances which consist of a resistor inparallel to a capacitor, respectively (see inset of Fig. 4).
Hence, the relaxation behavior of the M-W polarization isactually due to the frequency dependence of resistance andcapacitance of the MWCNT/PVDF composite in response tothe a.c. electrical signal 21.Based on above discussion, the total impedance can beexpressed in the following form 24,Z?(?) =1?1?1+???1+ R2 (1)However, the electrical response (imaginary – real impedanceplot) of the composites shows a semicircle profile, whichcould be illustrated using a modified equation 25,?(?) = ?2 +?11 + (???)? (2)Here R2 and R1 are the electrical resistances of MWCNT andPVDF, respectively, ? is the effective time constantcorresponding to the maximum in the ?l distribution( ?l=R1C1~??0/?, where ?0 is the permittivity of free space, ?and ? are the local permittivity and the local conductivity ofthe PVDF, respectively.
?l is the Maxwell relaxation time inthe MWCNT-PVDF structure. Owing to the randomdistribution of MWCNT throughout the PVDF matrix, theparameters R1 and C1 are random quantities as well, therefore?l is also a random distribution. For simplicity, ? correspondsto the maximum in the ?l distribution. ?=2?/fmax, here fmax isthe frequency at which Z”(f) shows the maximum.) Theparameter ? illustrates the deviation from the ideal semicircleprofile, i.e.
, the Z’-Z” semicircle moves downward withrespect to the real resistance axis.Furthermore, the experimental real and imaginaryimpedances could be obtained using the following equations26,?? (?)= ?2 + ?11 + (??)? sin (?2(1 ? ?))1 + 2(??)? sin (?2(1 ? ?)) + (??)2?(3)?”(?) = ?1(??)? cos (?2(1 ? ?))1 + 2(??)? sin (?2(1 ? ?)) + (??)2?(4)Eliminating the ?? term, one can obtain the equation of acircle 23,(?? ?2?2+?12)2+ (??? ??12tan (?2?))2=(?12sec (?2?))2(5)Here h=1-?. Then the experimental data can be fitted usingEq. (5).
The results are shown in Fig. 5. The ? values obtainedare 0.
57, 0.40, 0.13, and 0.59 for Samples 1 to 4.
In the rangeof experimental errors, the ? value has a big variance fromsample to sample, indicating that except for the impact ofCNT content, the structure and defects also have impact onthe electrical properties, which make the “polarization time”distribution (for the M-W polarization) quite different. Thefactors affecting the dielectric properties include the mainphase PVDF, filled phase MWCNT, the interface betweenthe PVDF and MWCNT, and the defects existed during thesample preparation. For our samples, the MWCNT content isabove the percolation threshold (1.5 wt.
% 27), thus theimpact of conductivity would be significant, but for thesurface passivated MWCNT filled composites, the resistivityhas been observed to increase 14. Due to lots of factorsaffecting the electrical conductivity of the MWCNT/PVDFcomposite, e.g., Bondi et al.
did the first-principles densityfunctiontheory calculations for the electrical conductivity ofoxygen-deficient tantalum pentoxide and found that thereduction and oxidation reactions may effectively impact theReView by River Valley Technologies IET Nanodielectrics2018/06/27 15:39:34 IET Review Copy Only 64Fig. 4. Static resistivity as a function of MWCNT content fordifferent MWCNT/PVDF ratios. The blue dots areexperimental data.
The red solid line is guided for eyes.Fig. 5.
Fitted real and imaginary impedances using amodified Cole-Cole model.conductivity in terms of the donor activation and deactivation28 , the semicircles move down the Y-axis, a modifiedrelationship (Eq. (2)) should be used. The variation of ? or hindicates that the imhomogeneities, interfaces and defects coexistin the composites, which affect the real and imaginaryimpedances as a function of frequency. In general, themodified Z’-Z” plot means that the contribution to theimpedance by the two phases, i.
e., a conducting phase?aninsulating phase and the interface, in the MWCNT filledcomposite, make the Z’ – Z” curve a small departure from theideal semicircle, in which the defects or imhomogeneitiesmight make the composite demonstrate complex “timeconstant distribution” (? ~ RC).For the two-phase system, Gerhardt also developed amethod using complex permittivity or dielectric constant,complex impedance, complex admittance, complex electricmodulus and dielectric loss or dissipation factor to distinguishthe localized and non-localized conductivities 29,30. Theoccurrence of peak of the imaginary part in the real -imaginary part plot is mathematically associating with asemicircle (see Fig. 3). The dissipation factor versusfrequency profile at low frequencies is usually associatedwith the space charges, or the long-range conductivity, whichhas been illustrated in Jonscher’s “university law” of thedielectric response 21. Thus the semicircle or modifiedsemicircle that we used to illustrate the impedance behaviorsis actually the same as the description of imaginary part (M”,tan?, and Z”) as a function of frequency in Gerhardt’smethod.3.3. Thermal conductivity propertiesFigure 6 shows the thermal conductivity of thecomposite as a function of MWCNT content in volumetricfraction. As can be seen from the Fig. 6, the thermalconductivity exhibits basically a linear relationship with theMWCNT content. The effective media approximation (EMA)is usually used to fit the experimental result of thermalconductivity versus filler content, e.g., Bruggeman 31,Maxwell Garnett models in dilute ceramic composites 32,Bruggeman model in binary dispersed composites 33. Thefitted results, however, was quite larger than the experimentaldata when using the EMA theory. The reason is probably dueto the interface thermal resistance across the filler and thematrix. Fortunately, Nan et al. obtained a formula for the highthermal conductivity MWCNT filled composite for a smallloading of MWCNT (