Temperature dependence of electric and dielectric behaviors of Ni/polyvinylidenefluoride compositesHai-Ping Xu, Zhi-Min Dang, Nai-Ci Bing, Yi-Hua Wu, and Dan-Dan Yang Citation: Journal of Applied Physics 107, 034105 (2010); doi: 10.1063/1.3289731 View online: http://dx.doi.org/10.1063/1.3289731 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/107/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Dielectric characteristics of polyvinylidene fluoride-polyaniline percolative composites up to microwavefrequencies Appl. Phys. Lett. 103, 192902 (2013); 10.1063/1.4828795 Uncovering the intrinsic permittivity of the carbonaceous phase in carbon black filled polymers from broadbanddielectric relaxation J. Appl. Phys. 109, 074107 (2011); 10.1063/1.3556431 On the question of percolation threshold in polyvinylidene fluoride/nanocrystalline nickel composites Appl. Phys. Lett. 92, 132905 (2008); 10.1063/1.2900710 Dielectric properties of upright carbon fiber filled poly(vinylidene fluoride) composite with low percolationthreshold and weak temperature dependence Appl. Phys. Lett. 91, 072912 (2007); 10.1063/1.2770664 Effects of Ba Ti O 3 on dielectric behavior of Ba Ti O 3 – Ni –polymethyl methacrylate composites Appl. Phys. Lett. 89, 132910 (2006); 10.1063/1.2354425

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Temperature dependence of electric and dielectric behaviorsof Ni/polyvinylidene fluoride composites

Hai-Ping Xu,1,a� Zhi-Min Dang,2 Nai-Ci Bing,1 Yi-Hua Wu,1 and Dan-Dan Yang1

1School of Urban Development and Environmental Engineering, Shanghai Second Polytechnic University,Shanghai 201209, People’s Republic of China2Key Laboratory of Beijing City for Preparation and Processing of Novel Polymer Materials, BeijingUniversity of Chemical Technology, Beijing 100029, People’s Republic of China

�Received 17 August 2009; accepted 12 December 2009; published online 8 February 2010�

Composites of polyvinylidene fluoride �PVDF� filled with metallic particles of nickel �Ni� wereprepared via a blending and hot-molding technique. Rescaled temperature dependence of electricand dielectric behaviors of Ni/PVDF composites were studied at wide content ranges. Results showthat there is significant positive temperature coefficient effect and giant dielectric constant as theconcentration of Ni is near the percolation threshold. The nonuniform distribution of filler particlesin PVDF host is observed from the scanning electron microscope micrograph since they areaggregated in amorphous regions of PVDF and form a network of conducting chains. Tworelaxation peak regions of dielectric constant are observed from �10 to 40 °C and from 100 to150 °C, which can be attributed to the contribution of polar effect of PVDF. The percolation theory,the thermal expansion model, and the simple concept of polarization in the capacitors areemployed to explain these experimental results. © 2010 American Institute of Physics.�doi:10.1063/1.3289731�

I. INTRODUCTION

Conductive particles filled semicrystalline polymer com-posites usually exhibit inimitable important insulator-conductor transitions, significant temperature-activatedswitching feature �either a positive temperature coefficient�PTC� effect or a negative temperature coefficient �NTC� ef-fect� when the filler content is close to the critical volumefraction referred as the “percolation threshold.”1–4 Further-more, a divergence of the real part of the dielectric constantis also observed at the percolation threshold.5–9 Research anddevelopment activities in such composites have brought suc-cessful industrial applications in self-regulating heaters,over-current and over-temperature protection devices, and di-electric materials applied in capacitors.10–12

The metal-polymer composite can possess a high PTCeffect or high dielectric constant at a low filler concentrationand thus make it possible to preserve the flexibility of thepolymer matrix. Similar composites with different metallicfillers are reported with percolative features.2 However,many properties of the composite still remain unknown, evendespite numerous studies have been carried out in the pastfew years.13–15

In this study, by employing a semicrystalline polyvi-nylidene fluoride �PVDF� with high melting temperature�Tm�165 °C� as polymer host, and nickel �Ni� as the con-ductive filler, the composites were prepared at the same pro-cessing condition. Our purpose is to give a deeper insightinto the influence of conductive filler on percolation thresh-old, PTC characteristic, and dielectric properties of polymercomposites.

II. EXPERIMENT

A. Materials and experimental procedures

The semicrystalline polymer PVDF �FR903, the melt in-dex is 2.0 g/10 min� powder was supplied by Shanghai 3FNew Materials Ltd. The Ni particles used as conductive fillerwere provided by VAS Laboratory Supplies Ltd. of Chinawith the average size of about 50 nm and the surface area of60–70 mm2 g−1. Prior to use, the surface of Ni particles wasembellished with 1.0 wt % �-aminopropyl triethoxysilane�KH550� coupling agent to improve the dispersion and affin-ity of Ni particles in the PVDF host. The coupling agent wasfirst dissolved in ethanol solvent and mixed with Ni particlesin a ball mill for 2 h, and then the Ni particles were treated at120 °C in a vacuum oven for 1 h after vaporizing the sol-vent.

The samples were prepared by a solution method. ThePVDF was first dissolved in an organic solvent �N,N-dimethylformamide�, then mixed with the treated Ni par-ticles and a few assistant additives in a ball mill to achieve areasonably uniform dispersion. The assistant additives incomposites were 0.5 wt % Tetra-�methylene-�-�3,5–di-tert-butyl-4–hydroxyphenyl�-propionate�methane �antioxidant-LH� and 3.0 wt % flame retardant Al�OH�3, respectively. Themixture films were obtained by vaporizing the solvent. Thesamples with diameter of 12 mm and thickness of 1 mmwere molded by a hot press at about 200 °C and 18 MPa for10 min and slowly cooled to room temperature under thepressure.

B. Characterization

Microstructures of fractured surfaces of composites wereobserved by scanning electron microscope �SEM� �HITACHI

a�Author to whom correspondence should be addressed. Electronic mail:hpxu@eed.sspu.cn.

JOURNAL OF APPLIED PHYSICS 107, 034105 �2010�

0021-8979/2010/107�3�/034105/5/$30.00 © 2010 American Institute of Physics107, 034105-1

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S-4700�. The samples were gold sputtered in vacuum prior toobservation. The electrical measurements were carried out inthe thickness direction of the samples. The two sides of thesamples were deposited with thin gold electrode to reducethe contact resistance. The volume resistivity of the sampleswas recorded consecutively by employing both a digital mul-timeter if the resistance was lower than 2�107 � and aZC-36 type megger if the resistance was higher than 2�107 � with increase in the temperature. The alternatingcurrent dielectric properties were measured by an impedanceanalyzer �Agilent 4294A� in frequency ranges of102–107 Hz.

III. RESULTS AND DISCUSSION

A. Electrical percolation of Ni/PVDF composites

Figure 1 shows the dependence of conductivity on thevolume fraction of Ni particles at room temperature. Theconductivities of composites are calculated from the resis-tance and the dimension of the specimens. A reversed zigzagdrop in conductivity against the filler content is observed,which is generally attributed to the percolation phenomenon.According to the percolation theory, the predicted percola-tion threshold for individual spherical conductive particlesdistributed randomly in the matrix is in the range of 16%volume content of fillers.16,17 In our case, the percolationthreshold value for metallic Ni particles loaded in semicrys-talline polymer PVDF prepared by the solution method iscoincident with this value.

B. The melting behavior of composite near thepercolation

Before the resistivity dependence of composites on tem-perature is discussed, the influence of Ni filler on meltingbehavior and crystallization property of PVDF are examinedby differential scanning calorimetry �DSC� measurementssince the PTC phenomenon resulted from the thermal expan-sion of polymer near the melting temperature. The dynamicDSC curves of the melting behaviors of pure PVDF and thecomposite are shown in Fig. 2. The corresponding DSC datawere listed in Table I. It is found that the intrinsic thermalproperty of PVDF is hardly affected by the introduction offiller. That is, the composite displays an obvious endothermicpeak at about 165 °C, which is the same with the melting

temperature of PVDF. It is also discovered that the crystal-linity estimated from the DSC curves is diminished by theintroduction of filler. It is generally believed that the perfec-tion of crystalline regions of pure PVDF is destroyed be-cause of the interaction between PVDF and filler.

C. PTC effect of Ni/PVDF composites

Figure 3 shows the resistivity of composites as the func-tion of temperature. The resistivity increased slowly withrising temperature below 150 °C and it increased abruptlyby four orders of magnitude at the temperature range from150 to 170 °C �so-called PTC effect�. This critical tempera-ture point about 170 °C mainly depends on the melting tem-perature of PVDF according to the DSC curves as shown inFig. 2. When temperature is over the melting point of PVDF,the resistivity of the composites begins to decrease, indicat-ing a NTC effect.

It has often been admitted that the PTC effect resultingfrom a disturbance in the continuity of the conducting net-work is principally due to the significant volume expansionduring the transformation of the crystalline phase to theamorphous phase of the semicrystalline polymers.18,19 Thetransformation of PVDF from crystal phase into amorphousphase produced a significant volume expansion, which in-creases the interparticle gaps of the conductive particles andreduces the number of conductive pathways, resulting in adramatic increase in the resistivity. However, the volume ex-pansion theory was not able to explain the NTC effect abovethe melting point of PVDF, when the volume of the matrixcontinued to increase with temperature. Some researcherssuggested that the NTC effect might ascribed to the forma-tion of new conductive pathways or the reparation of thedisconnected conductive chains resulting from the dispersionof filler particles agglomeration when the viscosity of thepolymer is sufficiently low at high temperature.19–22 Wethink the dynamic factor �such as van der Waals interactionbetween Ni particles� is also the reason to induce the NTCeffect, which resulting in reaggregation of Ni particles whenthe viscosity of PVDF decreased over the melting tempera-ture. It has been reported that the crosslinked or electronbeam radiation polymer composites show a faintish NTCeffect.23–25 So it is confirmed that the increased viscosity of

FIG. 1. �Color online� Variation in the resistivity with Ni concentration ofthe Ni/PVDF composites. The curve shows that the percolation thresholdvalue for metallic Ni particles loaded in PVDF is about fNi=0.16.

FIG. 2. �Color online� Dynamic DSC thermograms of pure PVDF and theNi/PVDF composite at fNi=0.20. The arrows point the peak melting tem-perature of pure PVDF and the Ni/PVDF composite at fNi=0.20,respectively.

034105-2 Xu et al. J. Appl. Phys. 107, 034105 �2010�

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polymer plays an important role on reducing the mobility ofthe filler particles in the composites. Nevertheless, this ex-planation still remains in some problems and cannot explainexperimental results clearly, and the mechanism behind theNTC effect has not been well established till now.

The properties of PTC composites are also estimated bythe PTC intensity, which is defined as the ratio of the maxi-mal resistivity ��max� to the resistivity at room temperature��RT�, as PTC intensity=�max /�RT. As shown in the inset ofFig. 3, the PTC intensity of the composites changes with thecontent of Ni, and the highest PTC intensity appears at fNi

=0.23, which is slightly higher than the percolation thresh-old. As a result, the content of Ni for the desired Ni/PVDFcomposites serving as PTC materials should be a bit higherthan the upper limit value estimated by the percolation re-gion.

Because of the commercial significance of such atemperature-activated switching feature, the PTC materialscan be utilized in a wide range of industrial applications,such as self-regulating heaters, current protection devices,microswitch sensors, and other outdoor equipments.

D. Dielectric behaviors of Ni/PVDF composites

High dielectric constant polymer matrix composites havebeen paid more attention due to their wide range of applica-tions in high charge-storage capacitors and electric energystorage devices. PVDF and its copolymer are familiar poly-mers with high dielectric constant, so the study on its dielec-tric properties is very important.26 PVDF has a relative highdielectric constant value ��10�.27 Fig. 4 shows the depen-

dences of dielectric constant and loss tangent of Ni/PVDFcomposites on frequency at different concentration of Nifiller. It can be seen that the dielectric constant of all studiedcomposites decrease with increase in frequency, and the di-electric constants increase with increase in the filler concen-tration. The giant enhancement of the dielectric constantfrom 30 to 60 in the composites near percolation threshold isalso observed. At lower Ni content �fNi�0.16�, the changeof dielectric constant of Ni/PVDF composites is less. AtfNi=0.16, the dielectric constant is about 30 and it is over 60when the volume fraction of Ni is 0.20, which is close to sixtimes higher than pure PVDF. The giant dielectric constantoriginated from the Maxwell–Wagner–Sillars effect atpercolation.28 In addition, as shown in Fig. 4�b�, the dielec-tric losses of all composites decrease with frequency to alowest value, then increase with frequency.

E. Temperature dependence of dielectric propertiesof Ni/PVDF composites

The temperature dependences of the dielectric constantof pure PVDF and Ni/PVDF �fNi=0.16� composite areshown in Fig. 5. It can be seen in Fig. 5�a� that there are twoobvious peak regions from �10 to 40 °C and from 100 to150 °C indicated as � and �, respectively. It can also beobserved that the constant always becomes higher with thetemperature increasing. The two regions can be attributed to

TABLE I. Thermal properties of PVDF and its composites derived from DSC data

SampleDensity�g /cm3�

Fusion Heat�J/g�

Peak melting temp.�°C�

Melting region

Melting temp. regionCrystallinitya

�%�Lower peak

�°C�Upper peak

�°C�

Pure PVDF 1.79 37.08 163.5 151.6 169.8 15.1 35.42fNi=0.20 8.88 13.55 165.0 157.3 168.1 10.1 12.94

aCalculated from the ratio of the measured fusion heat Hf to fusion heat Hf0 of a 100% crystalline

PVDF�Hf0=104.7 J /g�.

FIG. 3. �Color online� Temperature dependence of resistivity of Ni/PVDFcomposites at fNi=0.20, 0.23, and 0.27, respectively. Inset in Fig. 3 is thePTC intensity of Ni/PVDF composites at fNi=0.20, 0.23, and 0.27,respectively.

FIG. 4. �Color online� Dependence of �a� dielectric constant and �b� losstangent of Ni/PVDF composites on frequency at room temperature, in whichNi particles content is fNi=0.08, 0.12, 0.16, and 0.20, respectively. Theright-hand insets show the dielectric constant and the loss tangent, respec-tively, of the four composites at frequency 104 Hz.

034105-3 Xu et al. J. Appl. Phys. 107, 034105 �2010�

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the contribution of polar effect of PVDF. The low-temperature constant peak corresponding to � relaxation isprobably associated with the glass transition.29–31 It has beensuggested that this process is due to rotational motions ofdipolar groups in the amorphous regions of the PVDF poly-mer. But these rotational motions of dipolar groups are oftenfrozen so that they cannot catch up with the frequencychange of the electric field. This is charge with the low di-electric constant at the � relaxation process. The high-temperature peak corresponding to � relaxation has been as-sociated with the annealing process in the crystalline regions,which is due to molecular motions that alter the dipole di-rection only along the applied electrical field because thedipolar units are unfrozen at high temperature, and they canmove smoothly. Therefore, we can always observe the di-electric constant at high temperature is larger than that at lowone. In addition, the dielectric constant of the Ni/PVDF com-posite as shown in Fig. 5�b� also presents two peak regions.In comparison with the pure PVDF, the constant of the Ni/PVDF composite is always higher both at the same tempera-ture and frenquency.

Figure 6 shows the dependence of the dielectric constantof Ni/PVDF composites on temperature and volume contentof Ni at 104 Hz. We can see that two relaxation peak regionsfrom �10 to 40 °C and from 100 to 150 °C, which arecorresponding to the � and � relaxations of PVDF, respec-tively. The peak of constant is more outstanding with in-crease in the filler content.

F. Microstructures of Ni/PVDF composite

Figure 7 is the SEM micrograph of the Ni/PVDF com-posite, which shows that the Ni particles consisting of large

agglomerations in the PVDF host. There is no distinct inter-face between Ni particles and the polymer because the mag-netism of Ni particles acts as a good consistent agent for Niin PVDF host.

The nonuniform distribution of filler particles in PVDFhost is expected since they has been preferentially rejectedfrom the crystal phases during the crystallization of PVDF,thus the filler particles are always aggregated in the amor-phous phases and form a network of conducting chains. Asthe composites are heated and their viscosities are reducedwith rising the temperature, an enhanced redistribution pro-cess is taking place toward a more uniform distribution per-mitting more and more particles or aggregations to diffuseinto the melting crystal phases, which can result in an in-crease in the average distance among particles or aggrega-tions and destroy partially the original conductive network.

IV. CONCLUSIONS

The maximal PTC effect, whose intensity was about fourorders of magnitude, was presented in Ni/PVDF compositecontaining Ni filler slightly higher than the percolationthreshold. The dielectric constants of all studied compositesdecrease with increase in frequency and the dielectric con-stants increase with increase in the filler concentration. Tworelaxation peak regions of dielectric constant are observedfrom �10 to 40 °C and from 100 to 150 °C, which can beattributed to the contribution of polar effect of PVDF. The

FIG. 5. �Color online� Temperature dependence of the dielectric constant of�a� pure PVDF and �b� the Ni/PVDF composite at fNi=0.16 at selectedfrequencies as indicated.

FIG. 6. �Color online� Temperature dependence of dielectric constant of theNi/PVDF composites at 104 Hz, in which Ni particles content is fNi=0.08,0.12, 0.16, and 0.20, respectively.

FIG. 7. �Color online� SEM micrograph of the fractured surface of Ni/PVDF composite at fNi=0.27.

034105-4 Xu et al. J. Appl. Phys. 107, 034105 �2010�

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nonuniform distribution of filler in PVDF host was observedfrom the SEM micrograph since they are aggregated inamorphous regions and formed a network of conductingchains. It was believed that the homogenization diffusion offiller particles would result in an increase in conductivityduring the melting process and the volume expansion ofPVDF host.

ACKNOWLEDGMENTS

This work was financially supported by the Basic KeyResearch Programs of Science and Technology CommissionFoundation of Shanghai City �Grant No. 09JC1406700�, In-novation Program of Shanghai Municipal Education Com-mission �Grant No. 10ZZ132�, The Key Discipline Construc-tion Program of Shanghai City �Grant No. P1701�, and NSFof China �Grant No. 50677002�.

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