J Inorg Organomet Polym DOI 10.1007/s10904-017-0692-8

Polythiophene Based Composite with Enhanced Dielectric Performance with Basalt Yaşar Karabul1 · Mehmet Kılıç1 · Zeynep Güven Özdemir1 · Seda Erdönmez1 · Orhan İçelli1 

Received: 31 August 2017 / Accepted: 26 September 2017 © Springer Science+Business Media, LLC 2017

Abstract  In this work, Polythiophene/Basalt (PT/Basalt) composites with 4 and 8% (wt/wt) Basalt contents have been prepared by chemical method. The chemical composition of the basalt, which is a type of volcanic rock, has been determined by XRF technique. The surface morphology and chemical properties of the composites have also been analyzed by SEM and FTIR measurements. The dielectric spectra of pure PT and basalt doped PT composites have been investigated by impedance spectroscopy analysis performed within 1 Hz–40 MHz frequency region for 293–373 K temperature interval. The frequency dependences of the real and imaginary components of complex dielectric function revealed that the system can be considered as layered structure which consists of well conducting grains surrounded by poorly conducting grain boundaries and defined by Koop’s bilayer theory. Moreover, basalt doping increases 𝜀′ values which represents the charge storage ability of the dielectric material. Especially, the highest charge storage ability at room temperature has been determined for 8% basalt doped PT composite. The ac conductivity mechanism for the samples has also been determined as Correlated Barrier Hoping mechanism. Complex electrical modulus analysis also indicated non-Debye type of relaxation and temperature dependent hoping mechanism for the samples. Activation energy values determined from dc conductivity versus the inverse of the temperature curves of the samples have also revealed that as an inorganic and very cheap natural basalt doping makes the polythiophene better insulating material.

* Mehmet Kılıç [email protected] 1



Department of Physics, Faculty of Arts and Sciences, Yıldız Technical University, 34220 Istanbul, Turkey

Keywords  Polythiophene · Volcanic rock · Basalt · Dielectric material · Impedance spectroscopy

1 Introduction Since their discovery in the mid-1970s, conducting polymers (CPs) and their composites have been a significant research area. Doping of numerous inorganic materials into the CPs has increased the application areas by reducing negative aspects of the conducting polymers. Therefore, recently, focus of the researches have become a combination of semiconducting properties of CPs and the attributes of inorganic additives to provide new polymeric composites with exciting chemical and physical properties [1]. Conducting polymerrocks composites have been broadly researched, due to their interesting physiochemical attributes and possible application in devices [2]. Polythiophene (PT), polypyrrole (PPy) and polyaniline (Pani) are important representative classes of conjugated polymers for different technological applications like corrosion protection, electrochromic and electronic devices [3]. Among conducting polymers, PT is a promising conducting polymer due to its controllable and high electrical conductivity [2], its reversible electrical properties, high mobility, ease of synthesis, environmental stability [4, 5] and low cost [6]. Despite the fact that conducting polymers have many advantages, their dielectric constant are low about in the range of between 2 and 5. They have been traditionally utilized in minimal leakage capacitors due to their easy process, flexible and high dielectric strength. Innovative composites related to high dielectric constant and great dielectric breakdown strength enable high energy storage density and volume efficiency for electric energy storage applications. Improving the low dielectric constant of PT is a significant

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problem. Researchers aim to increase the dielectric constants with various additives in order to increase the usability of PT. A number of studies have been reported that the dielectric constants of PT can be increased with some additives to remove the low dielectric constant condition such as multiwalled carbon nanotube [7], tin oxide [2], and cadmium oxide nanoparticles [8]. Basalts are very widespread volcanic rocks that are quite abundant with magnesium and iron, almost located every country in the world. They include a variety of crucial elements (Si, Na, Fe, Mg, Ca, K, P, and S) and possessing very high dielectric constants [9]. From this point of view, such energy storage devices could be fabricated through combining basalts and polymers [10]. To obtain such an objective, high dielectric constant composites, basalts have been used as fillers for polymers by previous workers [11, 12]. In this study, we have used basalt as an additive material to improve the dielectric properties of PT. For this reason, PT/Basalt composites have been prepared by doping basalt to PT in different concentration (4, 8 wt%) by in-situ chemical polymerization. Structural and dielectric properties of the composites have been analyzed by SEM- FTIR techniques and impedance spectroscopy, respectively.

2 Experimental 2.1 Synthesis of PT and Preparation of PT/Basalt Composites Thiophene ­(C4H4S), anhydrous iron(III) chloride ­(FeCl3) have been supplied from Merck (Germany). Chloroform ­(CHCl3) and methanol ­(CH3OH) have also been purchased from Sigma Aldrich. PT and PT/Basalt composites have been synthesized at room temperature by in situ chemical oxidative polymerization method. In the synthesis of PT, to obtain the molar ratio is 2.5, 0.02 mol thiophene has been dissolved in 70 ml chloroform by a magnetic stirring for 15 min. 0.055 mol F ­ eCl3 has also been dissolved in 80 ml chloroform, and this solution has been added into the above solution in a dropwise fashion. The resulting solution has been stirred for 24 h at room temperature. After 24 h, the solution has been filtered and the sediment has been washed with chloroform and methanol, respectively. The brown precipitate has been dried at 60 °C for 24 h. During the preparation of PT/Basalt composites, basalt solution with mass fraction of 4 and 8% have been dissolved separately in chloroform. 0.4 g basalt has been mixed with 10 ml chloroform and then added to 0.022 mol thiophene which is dissolved in 70 ml chloroform. 0.055 mol F ­ eCl3 has been dissolved in 80 ml chloroform and added drop by drop to the above prepared solution. The solution has been filtered and the resultant residuum has been washed with chloroform

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and methanol. After drying process at 60 °C for 24 h, PT/ (4%) Basalt composite has been obtained. By application the same preparation process 8% Basalt doped PT composites have been prepared with using 0.8 gr basalt. 2.2 Characterization of the Samples The chemical composition of the dopant i.e. volcanic rock used in the composites have been determined as X-Ray Fluorescence (XRF) measurement performed with the equipment model X-123SDD from Amptek, which carries an Au-target X-ray tube (30 kV, 100 µA). The chemical composition of the volcanic rock has been given in Fig. 1. As is seen from Fig. 1, the volcanic rock sample contains approximately 47% of ­SiO2 and 3% of total alkalis ­(Na2O + K2O) contents. According to TAS (Total AlkaliSilica) classification of volcanic rocks, since the sample contains more than 45 and less than 52% of ­SiO2 and less than 5% of total alkali (­ Na2O + K2O) contents, the dopant is considered as basalt [13]. The surface morphology of the samples have been determined by Scanning Electron Microscopy (SEM) measurements performed by Zeiss-EVO® LS 10 model scanning electron microscope. The micrographs of the samples have been given in Fig. 2. As is clearly observed from Fig. 2a and b, basalt displayed a rock-like morphology, while PT showed a foam-like morphology. In addition, there are significant changes with doping process in the SEM micrographs. The entrapment of basalt particles in the composites can easily be noticed in Fig. 2c and d. It can be noticed that the majority of basalt particles is covered by PT. Some free separated basalt particles can also be seen in the Fig. 2c. The basalt particles are found to be well dispersed in polymer composites.

Fig. 1  The chemical composition of volcanic rock

J Inorg Organomet Polym Fig. 2  SEM micrographs of a pure PT, b pure basalt, c PT/4% basalt and d PT/8% basalt composites

The FTIR spectrum of each sample has been determined by Bruker TENSOR 27 spectrometer operated in transmission mode with the spectral region from 2500 to 400 cm−1. The FTIR spectra of all samples, including pure PT have been given in Fig. 3. The FTIR spectra of all samples, including pure PT have been given in Fig. 3. In the spectrum of pure PT (Fig. 3b), the bands in the range of 600–1500 cm−1 are the fingerprint region of PT. The bands observed in 624 and 660 cm −1 assign in C–S bending mode vibration, C–S–C ring deformation respectively, which indicate the presence of thiophene monomer [14]. The vibrations at 782 and 1088 cm−1 are attributed to out of plane C–H bending and in plane C–H aromatic bending vibration. The bands at about 1192 and 1390 cm−1 are correspond to C–C stretching vibration. The stretching vibration of C=C is seen 1590 cm−1 [14, 15]. The FTIR spectrum of PT/Basalt composites which is shown Fig. 3b and c has all the characteristics peak of PT. However, there are vaguely peak shifts and intensity alterations. In the spectrum of basalt (Fig. 3a), the broad band at 940 cm −1 may be attributed to the asymmetric stretching vibration of Si–O–Si bond [16]. This band is clearly seen spectrum of PT/VBR composites. The bands at 587 and 674 cm−1 may be related to the bending vibrations of the Fe–O, the bending modes of Fe–O–H, respectively [17]. The band at 749 cm−1 also corresponds to Al–O–Al vibration which verifies the existence of ­Al2O3 in the pure basalt [18]. Fig. 3  FTIR spectra for a Basalt b pure PT, c PT+4%Basalt and d PT+8%Basalt composites

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3 Results and Discussion 3.1 Dielectric Spectra of Pure PT and PT/Basalt Composites Dielectric parameters of the samples, which have been measured in the range of 293 to 353 K temperatures within the frequency interval of 1Hz–40 MHz, have been obtained by NOVO Control Broadband Dielectric/Impedance analyzers with Quatro Cryosystem. The samples have been placed between two gold electrodes whose surfaces wholly overlap the faces of the samples. The active electrode area was 3.14 cm2. The complex dielectric function, 𝜀∗ is defined by (1) √ ′ ′′ where j equals to −1. 𝜀 and 𝜀 correspond to the real and imaginary part of complex dielectric function, respectively. While the real part of complex dielectric function

𝜀∗ = 𝜀� − j𝜀��

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corresponds to the material’s property which determines the charge separation i.e. polarization, the imaginary part of complex dielectric function represents the degree of permission of charge carriers moving in the sample. The frequency dependence of 𝜀′ for pure PT, and the basalt doped PT composites at different temperatures have been given in Fig. 4. As shown in Fig. 4, the real part of complex dielectric function of all samples decreases with increasing frequency and then saturates its lowest value at the vicinity of 200 Hz. From this point of view, it has been understood that the electric dipoles in the samples orient themselves along the electric field between 1–200 Hz frequency region. Above 200 Hz, electric dipoles in the samples cannot orient themselves in the direction of the electric field applied due to very fast variation of electric field. The dispersion curves given in Fig. 4 also revealed that basalt doping increases 𝜀′ value which represents the charge storage ability of the dielectric material. Especially, the highest charge storage ability at room temperature has

Fig. 4  The variation of the real part of complex dielectric function with frequency at various temperatures for a pure PT, b PT+4%Basalt and c PT+8%Basalt composites

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been determined for 8% basalt doped PT composite. On the other hand, regardless of the doping concentration, the real component of the complex dielectric function increases with increasing temperature but the increase in 𝜀′ is more dominant at lower frequencies. The increase in 𝜀′ with increasing temperature at low frequencies can also be interpreted as the fact that the orientional polarization is connected with the thermal motion of charge carriers. In other words, the orientation of electric dipoles becomes easier at higher temperatures. Both the frequency and temperature dependence of 𝜀′ for all samples can be explained by Koop’s phenomenological theory which suppose dielectric material as an inhomogeneous medium composed of two Maxwell–Wagner type layers [19–21]. According to Koop model, the dielectric material can be considered as consisting of well-conducting grains which are separated by poorly conducting

grain boundaries. In this context, charge carriers that are most likely electrons can move to the grain boundaries through hopping or other mechanisms such as quantum tunneling etc. at low frequencies. If the grain boundary resistance is very high for electron’s moving, electrons pile up at the grain boundaries which causes a higher degree of polarization. However, when the electric field’s frequency is increased to higher frequencies, the probability of electrons reaching the grain boundary is reduced and hence the polarization considerably decreases which manifests itself as a nearly frequency independent characteristics of 𝜀′. Moreover, the temperature and frequency dependence of 𝜀′ also implies that the dielectric materials investigated can be considered as a dielectric medium with very slight semiconducting degree. As shown in Fig. 5, the frequency dependence of 𝜀′′ show similar behavior which also confirms the Koop’s bilayer theory.

Fig. 5  The variation of the imaginary part of complex dielectric function with frequency at various temperatures for a pure PT, b PT+4%Basalt and c PT+8%Basalt composites

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3.2 Electrical Conductivity of the Samples The angular frequency and temperature dependences of alternative current (ac) conductivity, 𝜎ac, of the samples have been investigated. The ac conductivity of the samples have been calculated by Eq. (2)

𝜎ac =

Z�A d

(2)

where d is the thickness of the sample and A is the active electrode area [22]. The ln 𝜎ac = f (ln 𝜔) curves of each sample for different temperatures have been given in Fig. 6. According to Fig. 6, angular frequency dependence of ac conductivity curves for pure PT have two distinct conductivity behaviors: dc domain which covers both the low and mid frequency regions and dispersive domain for the high frequency band. On the other hand, 4 and 8% basalt doped PT samples displayed frequency dependent behavior for the

whole frequency region with different slopes for the low and high frequency regions. 𝜎dc values of all samples at each temperature operated have been determined by extrapolating ac conductivity spectra at zero frequency in Fig. 6 and listed in the Table 1. As shown in Table 1, dc conductivity values increase with both increasing basalt content and temperature. In this respect, the temperature dependence of conductivity also confirms the slight semiconducting property of the samples. In addition, a linear relationship between the inverse of temperature and ln 𝜎dc is valid for all samples (See Fig. 7). From this point of view, the temperature dependence of dc conductivity can be defined by Arrhenius equation:

ln 𝜎dc = 𝜎0 e−Ea ∕ kB T

where 𝜎0 is a pre-exponential factor, Ea is activation energy, kB is Boltzmann constant (kB = 8.617 × 10−5 eV∕ K ), and T is the temperature in Kelvin scale. Activation energy for each samples have been calculated from the slope of the linear

Fig. 6  ln 𝜎ac = f (ln 𝜔) curves for a pure PT, b PT+4%Basalt and c PT+8%Basalt composites

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(3)

J Inorg Organomet Polym Table 1  dc conductivity and frequency exponent values of all samples including pure PT for various temperatures. SI and SII denote the low and high frequency regions

Temp. (K)

Sample PT

PT/4.0 wt% Basalt

293 313 333 353

PT/8.0 wt% Basalt

𝜎dc (Ωm)

sI

sII

𝜎dc (Ωm)

sI

sII

𝜎dc (Ωm)−1

sI

sII

1.678 × 10−7 1.874 × 10−7 2.335 × 10−7 3.028 × 10−7

0.028 0.045 0.061 0.088

1.114 1.055 0.956 0.838

2.504 × 10−7 7.753 × 10−7 1.384 × 10−6 2.448 × 10−6

0.247 0.243 0.219 0.198

0.493 0.327 0.321 0.313

2.686 × 10−7 2.260 × 10−6 4.417 × 10−6 8.546 × 10−6

0.252 0.255 0.244 0.232

0.891 0.473 0.488 0.407

−1

−1

Fig. 7  ln 𝜎dc = f (1∕ T) curves for pure PT, PT+4%Basalt and PT+8%Basalt composites

Fig. 8  Temperature dependences of s parameters of pure PT, PT+4%Basalt and PT+8%Basalt composites

fit given in Fig. 7. The related activation energies have also given in Fig. 7. Both the increase in dc conductivity with increasing temperature and the low activation energies for all samples indicates the thermally activated nearest neighbor hopping mechanism [23, 24]. In addition, as shown in Fig. 7, activation energy enhances with increasing basalt doping in the composites. This situation can be explained by the fact that the conduction process getting harder by basalt additive which results obtaining better insulating material. The angular frequency dependences of conductivity, 𝜎ac , at higher frequencies for all samples have also been investigated. As is known, the angular frequency dependence of ac conductivity is defined by

dependency [25]. To define the dominant ac conductivity mechanism of each sample for the high frequency band at each temperature, the frequency exponent (s) values have been calculated by the slope of ln 𝜎ac = f (ln 𝜔) curves given in Fig. 6 and listed in Table 1. The temperature dependences of frequency exponent of the samples have been shown in Fig. 8. As shown in Fig. 7, the ac-conductivity of each sample increases with increasing frequency and temperature. This behavior of ac-conductivity has been observed in amorphous materials and several oxides [26]. The decreasing trend of s parameter with increasing temperature (see Fig. 8) implies that the pre-dominant conduction mechanism for the samples is Correlated Barrier Hoping (CBH) mechanism [27]. According to CBH model [28, 29] the conduction is due to charge carrier transfer over the barrier between two sites which have their own potential wells. The temperature dependence of the s parameter is given by

𝜎ac = A𝜔s

(4) where A and s are frequency independent constant and frequency exponent, respectively. In accordance with Jonscher’s power law, s takes values between zero and one. Jonscher’s universal power law enables to determine the conductivity process of materials based on different mechanisms depending on frequency exponent value and its temperature

s=1−

6kB T Wm

(5)

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Fig. 9  The Cole–Cole plots of samples in complex electrical modulus plane for different temperatures

where kBis the Boltzmann constant, T is the temperature and Wm is the maximum barrier height. According to Eq. (1), s decreases with increasing frequency. According to Fig. 8, the temperature dependence of the s parameter for the composites have a plateau for the temperatures higher than approximately 313 K. The maximum barrier height values have been calculated for the composites and listed in a table shown in Fig. 8. As is seen, at 313 K there is a decrease in the barrier heights for 4 and 8% basalt doped PT composites. The decrease of barrier height with increasing temperature can be attributed to the thermally activated hoping conduction mechanism [30]. However, if the temperature is continuously increased, the maximum barrier height does not considerably change. This behavior can be interpreted that the temperature do not affect the hopping conduction mechanism significantly above 313 K due to increasing oxide additive in PT. 3.3 Complex Modulus Analysis of the Samples Complex electrical modulus analysis has also been utilized for the investigation of electrical properties of the samples. ( ) The complex electrical modulus spectrum i.e. M �� = f M �

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curves for the samples at different temperatures have been given in Fig. 9. ( ) As shown in Fig. 9, M �� = f M � curves are asymmetric and depressed semicircular arcs center are below ( whose ) the real axis. Hence, M �� = f M � curves indicate nonDebye type of relaxation. In addition, it has been clearly observed that completing the semicircle in complex electrical modulus plane become harder with( increasing temperature. The general ) tendency of M �� = f M � curves also indicates the temperature dependent hopping mechanism for charge transport [27] which is in good agreement with our previous results. On the other hand, while the M″ decreases with increasing temperature for pure PT, M″ increases with increasing temperature for the PT/basalt composites. This behavior of the PT/basalt composites can be attributed to increase of grain boundary resistance with increasing temperature. This tendency also manifests itself as a significant increase in the dielectric loss with increasing temperature for the composites.

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4 Conclusions Polythiophene/Basalt composites synthesized by chemical reactions have been investigated by impedance spectroscopy method. The surface morphology and chemical properties of the composites have also been analyzed by SEM and FTIR measurements. The frequency dependences of the real and imaginary component of complex dielectric function indicated that the composites have layered structure defined by Koop’s phemenological model. From technological point of view, the highest dielectric constant i.e. charge storage ability has been obtained for 8% basalt additive at room temperature. The ac and dc conductivity investigations also revealed that as a natural and low cost basalt additive improves the insulating performance of the Polythiophene for technological applications. Acknowledgements  This work has also been supported by Yildiz Technical University Scientific Research Projects Coordination Department under Project number: 2015-01-01-GEP03.

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