Impedance Analysis, Dielectric Relaxation and Electrical Conductivity of Multiwalled Carbon

Impedance analysis, dielectric relaxation and electrical conductivity of multiwalled carbon nanotube reinforced silicon elastomer nanocomposites

Janita Saji1, Ayush Khare1, R. N. P. Chaudhary3 and *S.P.Mahapatra2

1. Department of Physics, National Institute of Technology Raipur 492010 India.

2. Department of Chemistry, National Institute of Technology Raipur 492010 India.

3. Department of Physics, ITER, SOA University Bhubaneswar 751030 India.

Abstract

Dielectric relaxation behavior of multiwalled carbon nanotube (MWCNT) reinforced silicone elastomer composites has been studied as a function of variation in filler loading in the frequency range 10-1 Hz to 106 Hz. The effect of variation in MWCNT loadings on the complex and real parts of impedance is distinctly visible, which has been explained on the basis of interfacial polarization of fillers in a heterogeneous medium and relaxation dynamics of polymer chains in the vicinity of fillers. The dielectric modulus formalism has been used to further investigate the conductivity and relaxation phenomena. The frequency dependence of ac conductivity has been investigated using Percolation theory. The phenomenon of percolation in the composites has been discussed based on the measured changes in electrical conductivity and morphology of composites at different concentrations of the MWCNT. The percolation threshold as studied by electrical conductivity occurred in the range of 4 Phr of MWCNT loading. Scanning electron photomicrographs show agglomeration of the MWCNT above 4 phr concentration and formation of a continuous network structure.

Author for Correspondence: Dr. S. P. Mahapatra, E mail: ,

Keywords: elastomer, nanocomposite, impedance, dielectric, relaxation, percolation.

1. Introduction

Many materials used in our daily life are composites, which often are made up of at least two constituents or phases. The outstanding mechanical properties of many composites, and especially the unique combination of low density with high strength and stiffness, have led not only to extensive research but also to highly developed technologies [1, 2]. Elastomeric composites are widely used because of their light weight, design flexibility, and processability. However, these composites exhibit less attractive mechanical properties such as low strength and low elastic modulus when compared with metals and ceramics. Adding micron or nanosized inorganic filler particles to reinforce the polymeric materials has been standard practice in the composite industry for decades.

The term filler in rubber technology is often misleading, implying a material that is primarily intended to reduce the cost of the costlier rubber. But the modern day fillers change one or more of these properties: optical properties and color, improve surface characteristics and dimensional stability, change thermal, magnetic and electric properties, improve mechanical properties, durability and rheology, affect chemical reactivity, biodegradability etc. the mechanical and physical properties of the composites are mostly dominated by the nature of the filler, whereas the polymer matrix determines the environmental characteristics of the composite. Therefore, the overall composite properties can be tailored to fit the desired application through proper choice of filler and matrix resin. Therefore, a judicious choice of type of the filler and its concentration in the composite will augment the overall performance of the composite [3]. A significant part of investigations in this field concerns carbon based composites using (CNTs) [4], carbon black [5], graphite [6] as filler. These composites find applications in electronics and antistatic devices, electro-magnetic shielding, gas sensing etc. [7]. CNTs have exceptional electrical, mechanical, electrochemical, chemical, thermal and thermo electric properties. Since the discovery of CNTs by Lijima [8] in 1991, many potential applications have been proposed that exploit these extraordinary properties, such as nanoelectronics, chemical and physical sensors, biosensors, actuators, composites, integrated circuit manufacturing and scanning probes [9]. CNTs have high aspect ratio (length to radius ratio), high conductivity and an added advantage of achieving percolation at lower concentration than spherical fillers, which make them an excellent choice for electrically conducting composites. There are some studies on the applicability of carbon nanotube as reinforcing fillers in rubber, such as natural rubber [10], fluoro elastomer [11], styrene butadiene rubber [12], epoxy [13]. Studies have shown that in these nanocomposites carbon nanotubes exhibit the electrical capability of acting as metallic-like conductors or having characteristics of a semiconductor depending on the chirality of the graphite lattice. Compared with traditional materials, conductive nanocomposites have several advantages including extra-low threshold of particle content, no degradation of mechanical properties, light weight, flexibility, ability to absorb impact energy and magnetic resistance, and tunable conductivity [14]. Multiwalled carbon nanotube (MWCNTs) have several potential advantages in industrial applications because their production is less complex and more cost effective for their requirement of low loading to achieve comparable composites properties [15]. MWCNTs with extra ordinary mechanical and electric properties have the potential application as the reinforced material in polymer composites [16]. However, it is generally difficult to disperse MWCNTs in polymer matrices because they have large surface area possess strong Vander Wall forces, which result in significant aggregation [17]. The composites demand uniform dispersion of the MWCNTs in the polymer matrix without their aggregation and good interfacial adhesion between the MWCNTs and polymer matrix. Special functional groups have been generally introduced to MWCNTs for the well dispersion in the polymer matrix as well as good adhesion with the polymer matrix. Mixing CNTs with rubber normally ends up in a poor dispersion of CNTs because the attractive Vander Waals bonds between the outer planes of neighboring nanotubes result in agglomeration of the elastomeric materials.

Silicone rubbers have shown various applications due to their good chemical resistance, thermal stability and elasticity [18-20]. However, carbon atoms are not compatible with silicon atoms and carbon materials of conductive fillers; especially MWCNT inevitably causes the self-aggregation because of its high Vander Waals force, surface area and high aspect ratio. Moreover, it is insoluble in any organic solvents due to the pure carbon element and its stable structure [21]. The systematic study of the properties of silicon rubber particularly filled with multi walled carbon nanotube has not received much attention till today. The objective of the present work is to study the relaxation behavior of MWCNT reinforced silicone elastomer as a function of frequency (0.1 to 106 Hz). The effect of variation MWCNT loading on dielectric characteristics like real and complex part of impedance, dielectric loss tangent, dielectric permittivity, dielectric modulus and electrical conductivity has been studied.

2. Experimental

2.1. Materials

Silicone Elastomer (Mooney viscosity ML1+4 at 100°C =60) was procured from Bayer AG. Multiwall carbon nanotube (MWCNT) of purity ≥ 95% were purchased from Nanoshel LLC, USA. Before usage, the nanotubes were treated with acid mixture under ultrasonication to remove amorphous carbon and metallic impurities. Other chemicals like zinc oxide specific gravity of 5.4, stearic Acid and di cumyl peroxide used as curing agent were chemically pure grade procured from standard suppliers.

2.2. Compounding and sample preparation

The rubber was compounded with the ingredients according to the formulation of the mixes (Table 1). Compounding was done in a brabender plastograph at 60 rpm followed by laboratory size (325x150 mm) two roll mixing mill at a friction ratio of 1:1.25 according to ASTM D 3182 standards while carefully controlling the temperature, nip gap, time of mixing, and uniform cutting operation. The temperature range for mixing was 65–70°C. After mixing the elastomer compositions were molded in an electrically heated Moore hydraulic press at a pressure of 10MPa and at 160oC using moulding conditions determined by Monsanto Rheometer (R-100) according to ASTM D2084 and ASTM D5289 procedures.

2.3. Testing

2.3.1. Dielectric relaxation spectra

Dielectric relaxation spectra of the composites were obtained by a Hioki 3530 HiTester LCR meter in the frequency range of 10-1 Hz to 106 Hz using aluminum foil as blocking electrode. The dielectric characteristics have been observed as a function of frequency. Electrical conductivity () has been evaluated from dielectric data in accordance with the relation:

(1)

where is (is frequency), is permittivity of the vacuum and dielectric constant or relative permittivity

(2)

where is the observed capacitance of the sample and is vacuum capacitance of the cell and is calculated using the expression (where is area of the sample and is thickness of the sample) and is the dielectric loss tangent

2.3.2. Scanning electron microscopy (SEM)

The surface morphology of the CNT/SiR nanocomposites was imaged using a scanning electron microscope (Britain Cambridge MK 3-250, USA). The samples were mounted on the sample studs by means of double–sided adhesive tapes. A thin layer of gold was sputtered on the sample surface prior to the Scanning electron microscopy (SEM) measurements. The SEM measurements were performed at an accelerating voltage of 10kV. The average size of dispersed CNT filler was measured on the photomicrographs using the Roentec Scan vision software, produced by Hitachi Crop., Japan.

2.3.3 Raman spectra

Raman spectra were recorded with a Jobin Yvon micro-Raman LabRam system in a backscattering geometry using 488 nm laser excitation wavelengths. The laser beam was focused on the sample with the aid of an optical microscope.

3. Results and Discussion

3.1 Complex impedance analysis

The dielectric properties of silicon rubber nanocomposites filled with MWCNT have been investigated using complex impedance spectroscopy (CIS). CIS is an important tool to analyze the electrical properties of the composites in view of its capability of correlating the samples electrical behavior with its microstructure, filler loading and dispersion.

3.1.1 Real part of complex impedance

Figure 1 shows variation of real part of complex impedance () as a function frequency at increasing MWCNT loadings. Irrespective of the MWCNT concentrations, shows a monotonous decrease with increasing frequency and curves almost merge for all MWCNT loadings in the higher frequency region (Hz). However in the low frequency region value is gradually decreasing with MWCNT loading. The decrease in with MWCNT loading and frequency indicates the possibility of increasing conductivity with MWCNT loading and frequency (A more detailed study of conductivity is given in subsequent sections of the manuscript). The merging of curves in the higher frequency region may be attributed to release of space charge.

3.1.2 Imaginary part of complex impedance

Figure 2 represents the variation of Imaginary part of complex impedance () with frequency. The impedance loss spectra have features such as decrease in the height of the peak and shift in peak towards higher frequencies with increasing MWCNT concentrations. This confirms better capacitive nature and decrease in resistance of the composites with MWCNT loading. The frequencies at which peak occurred are at 17.75, 22.65, 61.619 and 606.2798 Hz for unfilled and 2, 4, 6 phr of MWCNT loading respectively. This increase in frequency can be explained on the basis of the mechanical and viscoelastic properties of cross-linked and reinforced multiphase polymeric materials. The addition of the MWCNT particles has a significant effect on the dielectric behavior of the sample. Filler particles in the matrix acquires induction charges in presence of the applied external field, polarization effects take place so called Maxwell-Wagner-Sillar’s polarizations. Below the critical concentration of the filler loading, the inter-particle distance is large enough so that neighboring local fields apparently do not interact. Thus dielectric factor in this region increases slowly. But as the filler loading increases, the Maxwell-Wagner-Sillar’s effect increases due to reduction in the inter-aggregate distance giving rise to dielectric properties.

The data has been fitted to different spectral functions commonly used such as Debye, Cole-Cole, Cole-Davidson, Havriliak-Negami, Frohlich [22]. The best fit was obtained using a Havriliak-Negami function, superimposed with Frohlic function to account the effect of conductivity.

The spectral function can be expressed as:

(3)

where denotes Havriliak-Negami function form and denotes Frohlic function, denotes the angular frequency, is the DC conductivity and is a constant.

3.1.3 Nyquist plots

Figure 3 shows the Nyquist plot (the relationship between imaginary part of impedance () and real part of impedance ()) of silicon rubber composites as a function of increasing MWCNT concentrations. It can be observed that increasing MWCNT in the composite has a sizable effect on the dielectric properties of this system at all frequencies. From the figure it can also be observed that irrespective of the filler loadings, the plots yield good semicircles indicating the occurrence of polarization with a single relaxation time taking place, i.e. a local mode process dominated. However, at higher filler loadings (6 phr), the semicircles did not reach the origin and had a small negative intercept on the axis indicating build up of ions at the interphase between the filler and polymer matrix[23].

Several attempts have been made to interpret the impedance spectroscopy of polymer-filler systems using the resistance-capacitance parallel circuit (RC) model. In a Nyquist plot for a polymer composite system the real axis represents bulk resistivity () and the imaginary axis represents, which is given by

(4)

where is the bulk capacitance. In a Nyquist plot, increase in represents poor conductivity. Figure 3 shows that with increase in MWCNT loading, value decreases or in other words the composites become more capacitive in nature, which are calculated from the intercept of the semicircular arc on the Z’ axis in Nyquist plot and tabulated in Table 2.

With increasing MWCNT loading the distance between the aggregates reduces. This gap can be approximated by a parallel plate capacitor with an area (A), separation distance (), and capacitance (C), where is the dielectric constant of the polymer. Each filler aggregate has a resistance (Ra), the resistance within the aggregate. The impedance of a composite can be written as

(5)

The respective imaginary and real parts of impedance can be expresses as

and (6)

and the dielectric loss tangent can be expressed as

(7)

From above equations the relationship between and is

(8)

Therefore a plot of and will give a half circle which has the center at and radius of.

Wang et al.[24] proposed that since the circular curve of the vs occurs only for the parallel resistor circuit, the above analysis can be used to confirm the existence of the capacitor effect. The capacitor effect also confirms that the gap between the nanotubes controls the electron conduction via non-Ohmic contacts between the filler aggregates. The variation in the values of radius and centre of the half circle can also be used as a measure of the gaps in between filler aggregates. Using the above equations, the centre has been calculated and tabulated in Table 2. It can be observed that increasing MWCNT loadings the radius reduces and the centre shifts to lower values. It can also be observed that with increasing nanotube concentrations the area under the curve in the Nyquist plot is decreasing. The intensity of this decrease is more pronounced at higher loadings of MWCNT when compared to lower loadings. This can be explained on the basis of “space charge” phenomenon in heterogeneous systems. Filled polymers are multi-component systems that have complex molecular, supra-molecular and topological structures, which determine their ultimate properties. These structures are formed during compounding and processing.