Experimental procedures for assessing electrical and thermal conductivity of polyaniline

CHAPTER 13

Experimental procedures for assessing electrical and thermal conductivity of polyaniline Zahed Ahmadia, Narendra Pal Singh Chauhanb, Payam Zarrintajc,d, Aidin Bordbar Khiabanie, Mohammad Reza Saebf, Masoud Mozafarie,g,h a

Department of Chemistry, Amirkabir University of Technology, Tehran, Iran Department of Chemistry, Bhupal Nobles’ University, Udaipur, Rajasthan, India c Polymer Engineering Department, Faculty of Engineering, Urmia University, Urmia, Iran d Advanced Materials Group, Iranian Color Society (ICS), Tehran, Iran e Bioengineering Research Group, Nanotechnology and Advanced Materials Department, Materials and Energy Research Center (MERC), Tehran, Iran f Department of Resin and Additives, Institute for Color Science and Technology, Tehran, Iran g Cellular and Molecular Research Center, Iran University of Medical Sciences, Tehran, Iran h Department of Tissue Engineering & Regenerative Medicine, Faculty of Advanced Technologies in Medicine, Iran University of Medical Sciences (IUMS), Tehran, Iran b

1. Introduction Conductive polymers, based on their redox state and dopant strength, exhibit a vast range of conductivity form metallic to insulator, mostly applied as semiconductors (Fig. 1). In general, conductive polymers are not thermoformable; they are organic materials like insulating polymers. They can offer a high electrical conductivity, but do not necessarily reveal similar mechanical features in comparison with other commercially available materials. The electrical properties of PANI are to a large extent adjusted for a specific usage. Since conducting polymers like PANI have backbones of adjacent sp2 hybridized carbon centers, they provide one valence electron on each center in a pz orbital orthogonal to the other three sigma bonds, which are combined with each other to a set of orbitals of wide delocalized molecules. The presence of highly mobile electrons in the delocalized state of a “doped” condition resulting from oxidation causes removal of some of the delocalized electrons. As a result, the conjugated p-orbitals form a one-dimensional electronic band in assist, with electrons within such band becoming mobile when it is partially emptied. The band structures of a conductive polymer can be easily calculated with a tight-binding model based on the wave function set of isolated atom superposition. In principle, the reduction process is used for doping such materials for which unfilled bands are loaded by electrons. In practice, p-type semiconductors are commonly prepared by oxidative doping. PANI is composed of benzene ring and nitrogen ending in a rigid rod-like structure. PANI, due to its unique features such as suitable electrical/thermal conductivity, low

Fundamentals and Emerging Applications of Polyaniline https://doi.org/10.1016/B978-0-12-817915-4.00013-0

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Fig. 1 Conductivity range defined for conducting polymers in a brief view.

cost, and biocompatibility, has attracted a great deal of attention and is widely used in industry [1]. The properties of PANI can be altered by changing the polymerization media and polymerization route. Polymers can be prepared either by chemical [2] or electrochemical [3] oxidation of the aniline monomer and “doped” into an electrically conducting state by immersing in acid. However, only an intermediate oxidation state shows the conductive behavior [4] and the dry conducting solid state is the environmentally stable form. Nitrogen atoms are the elements connecting benzenoid and quinoid rings together with the reduced form (leucomeraldine) consisting entirely of benzenoid units, the fully oxidized (pemigraniline), and an interesting intermediate case of emeraldine with the same ratio of benzenoid/quinoid [5]. Treatment of the last state by drenching in acid causes a drastic electrical conductivity enhancement up to ca. 500 S/M. From a perspective of thermal and electrical conductivity, this chapter attempts to review methods used in this regard.

2. Electrical properties of PANI 2.1 Effect of polymerization temperature on electrical conductivity The influence of the polymerization temperature (from
50 to +50°C) on electrical conductivity of PANI was investigated by Stejskal et al. PANI was synthesized using conventional methods by using ammonium persulfate (APS) as oxidant (with equimolar with

Procedures for assessing electrical and thermal conductivity

aniline) in acidic solution below
10°C. The higher the acid concentration was, the higher the conductivity reached [6].The temperature variation exhibited negligible influence on the electrical conductivity with excess acidic concentration (1.2 M HCI). Only a decrement of electrical conductivity with polymerization temperature increment was observed for PANI samples prepared in low acidic media (0.2 M HC1). This behavior has been theoretically predicted [7]for the cases when the hopping charges from one chain to another are much faster than the life time of the charge on one chain, i.e., when the interchain transport occurs more rapidly than the intrachain one. In this sense, PANI acts as low-molecular weight charge-transfer salts [8] (intermolecular conductors) rather than conjugated polymers (intramolecular one-dimensional conductors). The observed independence of the electrical conductivity on molecular weight conforms to the theoretically predicted weak dependence of electronic properties on chain length of PANI [9]. It was also observed by prior experimental results of Lu et al. [10] that the oligoaniline octamer was found to have the same electrical conductivity (1 S/cm) as same as PANI prepared by the oxidation of aniline with APS. A detailed analysis of the conduction mechanism in PANI can be found [11]. The fact that the PANI produced under more acidic conditions has a higher electrical conductivity has been reported in the literature [11, 12]. It cannot be explained by a higher crystallinity of such samples alone; and the differences in the chain structure (including the type and degree of protonation) have to be responsible as well. Elemental analysis shows that the samples prepared in 0.2 M HC1 are protonated by chloride anions as well as by the sulfate ones (produced in the decomposition of peroxodisulfate in the course of polymerization). In samples prepared in 1.2 M HCI, only chloride counterions are present.

2.2 Influence of oxidation and protonation on electrical conductivity McManus et al. investigated the relation between PANI electrical conductivity and oxidation state. The results exhibited that (1) electrochemical inducement caused the redox-state to switch between three states, and (2) protonation induced a transitional change from insulator to conductor. A U-shaped resistivity vs. voltage curve was obtained, indicating that there were three electrochemically switchable forms. The most reduced (undoped) form is an insulator (leucoemeraldine); the intermediately oxidized form is a conductor (emeraldine); and the most oxidized form is, again, an insulator (pernigraniline). Similar to other conducting polymers, at low doping levels, PANI exhibits a doping-induced insulator-to-conductor transition; on the other hand, at higher dopant concentrations, the electrical behavior exhibits a discrepancy. Unlike other conducting polymers whose conductivity shows an increasing trend with dopant increment, PANI first reaches high conductivity at a moderate concentration of oxidation, and then it plunges down to a conductivity level in the insulator region [13]. The resistivity vs. voltage curve for electrochemical doping in electrolytes of different pH values indicated

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Fig. 2 Various redox and conductivity states of PANI.

several notable trends: (1) the bottom of the curve was a broad square at pH 1, as compared to other values, such as pH 7 in which the bottom becomes narrower and the resistivity is three orders of magnitude higher than pH 1; (2) in more protonated PANI, the electrochemical potential was transferred to more positive values. The prior measurements at low and high pH indicated that this behavior hung on beyond the range of pH values. At high pH, due to the narrow range of conductivity potential, the redox states were not distinguishable. However, at low pH, in the potential range between 0.1 and 0.8, all redox states were observed [13] (Fig. 2).

2.3 Four terminal techniques for electrical conductivity of a single conducting PANI nanotube Long et al. investigated a self-assembled camphor sulfonic acid (CSA)-doped PANI nanotube using a template-free method. The conductivity of the single PANI nanotube was high (31.4 S/cm), and the temperature dependence fit well with the threedimensional variable range hopping (VRH) model. However, the bulk conductivity of the PANI nanotube pellets was much higher (3.5  10
2 S/cm). The intensity of disorder strongly affects the electrical properties of conducting polymers. Due to the magnitude of disorder, three different regimes can be sorted out: the metallic, critical, and insulating regimes. In the insulating regime, the low-temperature resistivity ρ(T ) is related to the temperature exponentially, which depends on VRH [14]:

Procedures for assessing electrical and thermal conductivity




T0 ρðT Þ ¼ ρ0 exp T

m (1)

where the exponent m ¼ 1/4, 1/3, and 1/2 for three-dimensional (3D), two-dimensional (2D), and one-dimensional (1D) hopping, respectively; T0 is the Mott characteristic temperature, and can be obtained from the ln ρ(T )  T
1/m plot. In the critical regime, the resistivity ρ(T ) obeys the power-law metal-insulator transition [14]:
 e2pF kB T ¼ ρ0 T
β (2) ρðT Þ ¼ ђ2 Ef where 0.3 < β < 1. Due to the conductivity decrement with decreasing temperature, the PANI nanotube conductivity followed semiconductor trends. Three-dimensional VRH was a suitable model for the temperature-dependent resistance of the nanotube because of the large resistivity ratio. All this indicated that the measured PANI-CSA nanotube was in the insulating regime. However, if the structural disorder of the nanotube is reduced by tailoring the synthesis conditions, the PANI nanotube might lie in the critical or even metallic regimes. Fig. 3 shows the temperature dependence of the resistivity of the PANI-CSA nanotube pellets, in which their conductivity is lower than single nanotubes. It is evident that ln ρ(T ) is linear in T
1/2, which has been widely observed in PANI powders or films and explained by the quasi-1D VRH or the charge energy limited tunneling (CELT) model. It is obvious that the pellet’s T
1/2 dependence of ln ρ(T ) is not a characteristic of the PANI-CSA nanotube itself but the characteristic of the internanotubular contacts [14].

9

PANI-CSA nanotubes’ pellet

In r (Ω cm)

8 7 6

CSA/An=0.5

5 4

s (300 K) = 0.035 S/cm

3 0.056

0.064

0.072

T

–1/2

(K

0.080

0.088

0.096

–1/2

)

Fig. 3 Temperature dependence of the bulk resistivity of the PANI-CSA nanotube pellet; the resistance is measured by a four-probe technique; the line shows the fit to ρ(T) ¼ ρ0 exp(T0/T)1/2 [14].

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2.4 Effect of NH3 gas on the electrical conductivity of PANI blend films Gas sensors affect PANI-based sensor conductivity; hence, the PANI composite conductivity was investigated using NH3 exposure. The conductivity measurement in N2 media exhibited conductivity stability. On the other hand, the electrical conductivity decreased when introducing the NH3 gas to samples. The type of dopant, temperature, polymer matrix morphology, and solvent used were the most important factors affecting the sensing feature. Due to the porous morphology of the PANI-poly(methyl methacrylate) film, it exhibited the better performance in sensing. The percolation threshold was around 2% PANI, at which the composite with lower composition exhibited insulating behavior. 2.4.1 Response behavior of sensors Response of all sensors was measured and the results demonstrated that PANI enhancement in the composite increased the sensing. Conductivity decreased when exposing the sensor to NH3 gas and recovered toward the original value after switching back to N2 gas for all sensors. The sensing mechanism can be explained by the compensation effect [15]. When the conductive emeraldine salt is exposed to NH3 gas, the dopant is partially dedoped, which leads to the reduction of electrical conductivity. But in N2, NH3 gas volatilizes and protons restore the initial level of doping and conductivity is increased. 2.4.2 NH3 concentration dependence of the sensor response The effects of NH3 gas concentration on the sensor characteristics were examined and it was found that the threshold (limited by the measuring equipment) concentration level was 10 ppm. The change in conductivity 4 σ(σ(N2)
σ(NH3) of the sensor varies nonlinearly with the NH3 concentration. Relatively small NH3 concentrations give rise to a marked decrease in conductivity, and the conductivity approaches a plateau value at high concentrations. It was found that this sensor has large sensitivity at the low level NH3 concentration and the detection limit is <10 ppm. On the other hand, the response time became longer with decrease in the NH3 concentration (t70 ¼ 8 min at 100 ppm) [16].

2.5 Electrical conductivity of PANI/zeolite composites Three types of PANI/zeolite were prepared based on various type of zeolite with different pore size and ion exchange capacity (Y, 13X, and AlMCM41), and the effect of different factors like zeolite content and porosity on electrical behavior was investigated. 13X/PANI exhibited the highest electrical conductivity sensitivity to CO/N2 gas. Y/PANI had a better performance than AlMCM41/PANI because the AlMCM41based composite had large pore size and low ion exchange capacity. It is noteworthy that the porosity of Y and 13X was similar but the ion distribution of 13X was better. In another investigation, PANI/10MA composite properties were evaluated. The difference between the electrical conductivity of CO exposed and N2 exposed composites determined the electrical conductivity response (Δσ ¼ σ CO
σ N2[S/cm]). For

Procedures for assessing electrical and thermal conductivity

normalizing the data, the sensitivity was calculated by dividing the electrical conductivity response by the electrical conductivity of N2 (sensitivity ¼ Δσ/σ N2). Air replacement with N2 led to the observation of one order of magnitude decrement in the value conductivity. It can be premised that the air ingredients interplayed with the PANI. Exposing to CO, the conductivity was enhanced. By removing CO, the conductivity returned to the initial value, which indicated reversibility. FTIR and XRD revealed no discrepancy between the CO exposed and unexposed samples [17].

2.6 Electrical conductivity in PANI/polyurethane blends A lower percolation threshold is required for reaching a determined conductivity in miscible blends like PANI/polyurethane (PANI/PU) as compared to immiscible ones like PANI/carbon black (CB). Aging reduced the electrical conductivity of PANI/PU and the morphology changed with elapsing of time in the simultaneous presence of high moisture and temperature. After the aging process, the samples were recovered to their initial states. It was observed that the morphology along with electrical conductivity were restored to the primary values. In forming the conductive route in the insulating matrix at the percolation threshold, an abrupt increment of conductivity was observed in certain compositions [18]. Elongation of the immiscible samples caused a two-ordersof-magnitude decrement because of creating spatial dissociation among the conductive parts. On the other hand, in the miscible sample, no change was observed in conductivity because of the coherence of the conductive part and structure maintenance. In addition, this phenomenon should be of importance from the viewpoint of the development of soft conducting materials. The electrical conductivity variation as a function of aging time at 80°C and humidity of 95% for conductive components showed that the decrease of electrical conductivity of PANI was slightly more than that of CB. This was assumed to have little influence on that of the PANI/PU blend, because it was negligibly small [18].

2.7 Electrical conductivity of PANI-doped PVC-PMMA polymer blends The conductivity of a PANI-doped PVC-PMMA polymer blend (PANI/PVCPMMA) film was measured and the mechanism of conduction was clarified, with findings showing that it obeyed the Schottky and Richardson mechanisms. The result showed that the current was enhanced nonlinearly with voltage and it showed deviant behavior from the power law. Considering the observed behavior of the I-V characteristics, the existence of ohmic as well as space charge limited conduction is not possible. Ohm’s law derives from the free-electron model of a metal, in which the free electrons repeatedly collide. However, these collisions are not like billiard balls hitting other billiard balls, but occur through irregularities in the crystal structure, such as some atom impurities and atoms that are out of place during vibration, that cause a dispersion situation of electron waves. (The atoms of a perfect crystal network scatter free electron waves only in special conditions.) In the sample discussed, the components of the

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blends are almost insulators and the composites are nearly amorphous. In this situation, the structure has a large scope for irregularities and as a result ohmic conduction will not occur. Space charge limited conduction, due to the low charge carrier density of the polymer, is also not likely [19]. 2.7.1 Schottky plots The Schottky-Richardson current voltage relationship is expressed as: h
φ i s J ¼ AT 2 exp + βSR E 1=2 kT βSR is the field lowering constant given by.  1=2 e e βSR ¼ kT 4ππεo d

(3)

(4)

1=2 s and also ln J vs. E½ plots should be a As a consequence, ln J ¼ ln AT 2
φ kT + βSR E straight line with a +ve slope. Sketched plots with axes marked in this way are called Schottky plots and the linear +ve slope on Schottky plots generally characterizes the Schottky-Richardson mechanism. The applicability of the mechanism in the case under discussion is indicated by straight lines with positive slope in the Schottky plots. Moreover, it is found that a strong temperature dependence is seen in the case of the Schottky-Richardson mechanism, but in the case of the Poole-Frenkel mechanism, there is no evidence of this dependence. Thus it is very important to distinguish the temperature dependence of current density [19].

2.7.2 Richardson mechanism The Richardson current-voltage relation is expressed as: h
φ i s J ¼ AT 2 exp + βSR E 1=2 kT h
φ i J s 1=2 + β ¼ A exp E SR kT T2 h i J
φs ln 2 ¼ ln A + + βSR E 1=2 kT T J
φs ln 2 ¼ ln A + βSR E1=2
kT T

(5) (6) (7) (8)

This relation when graphed will give a straight line with a
ve slope between ln(J/T 2) vs. (1/kT). These plots are referred to as Richardson plots and the linearity of these plots is required by the Schottky-Richardson mechanism. The mentioned
ve slope is shown by the straight line graphs in the present case, and thus the Schottky-Richardson mechanism is supported by the linearity of the plots [19].

Procedures for assessing electrical and thermal conductivity

2.8 Electrical conductivity of PANI/graphite nanocomposites A facile process for the synthesis of exfoliated graphite and PANI/graphite nanocomposite was developed by Du et al. Expandable graphite was used for the graphite nanosheet preparation. In situ polymerization of the aniline monomer in the presence of graphite nanosheets was used for nanocomposite synthesis. Adding graphite to PANI along with an increment in thermal stability caused a conductivity enhancement in comparison with untouched PANI (e.g., by adding 1.5%, the conductivity of PANI, which was around 5 S/cm, reached 33.3 S/cm, which was the percolation threshold) [20]. This increase in conductivity can be ascribed to two factors: (I) formation of a conducting network within the polymer matrix at low loading because of the suitable nanoscale dispersion; (II) the interaction between the large π-conjugated structure of the quinoid rings of PANI and the aromatic structure of the graphite sheets. Based on the two factors, it can be concluded that the graphite nanosheets within the PANI matrix served as an electrically conductive bridge [20].

2.9 Electrical conductivity of PANI weakly doped with chlorocarboxylic acids Gmati et al. studied the effect on PANI of two types of dopant (dichloroacetic (DCA) and trichloroacetic (TCA) acids) at various doping rates. TCA doping resulted in better chain ordering and conductivity. In all features the direct current conductivity was coordinated to Mott’s 3D variable-range hopping (VRH) model in a suitable manner. It should be noted that comprehensive evaluation has been done on different Mott features, such as characteristic temperature T0, compression of different states at the Fermi level (N(EF)), average hopping energy (W ), and the average hopping distance (R). At high frequencies, the alternating current conductivity pursues the power law σ ac(ω,T ) ¼ A(T )ωs(T,ω). This factor, through hopping or tunneling processes, is known for charge transition in chaotic materials. It is proposed due to existing developments in the frequency exponent s with temperature that the conquering alternative current conduction mechanism is best defined by a small-polaron tunneling model [21]. Conductivity data of PANI salt samples using several conduction models were also fitted [22]. The best fits were obtained with the 3D VRH conduction model. In this model, if the interaction between charge carriers is neglected, the dc conductivity is expressed as follows: h i 1=2 1=4 σ dc ¼ BðT0 =T Þ exp
ðT0 =T Þ (9) where B is a constant depending on the distribution of localized states around the Fermi level and is given by: B ¼ N ðEF Þe2 νph

9 64α2

(10)

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where N(EF) is the density of states at the Fermi level, νph is the phonon frequency (1013 Hz), e is the electronic charge and 1/α is the decay length of the localized wavefunction. T0 is the characteristic Mott temperature, corresponding to the hopping barrier for charge carriers (also known as the pseudoactivation energy); it measures the degree of disorder present in the system, and it is given by: T0 ffi 18:11

α3 N ðEF ÞkB

(11)

The average hopping distance and the average hopping energy are given, respectively, by [23, 24]:  1=4 9 R¼ (12) 8απN ðEF ÞkB T W¼

3 4πN ðEF ÞR3

(13)

2.10 Effect of (MoO3) on the electrical conductivity of PANI Chemical oxidation procedures in the presence of molybdenum trioxide (MoO3) assist in synthesis of PANI/MoO3 composites. This process is performed via in situ polymerization of aniline. Various mixtures with different combinations have been synthesized (10, 20, 30, 40 and 50 wt%) of MoO3 in PANI. Using scanning electron micrography (SEM), the surface morphology of these mixtures was investigated. X-ray diffractometry (XRD) and infrared spectroscopy (FTIR) have the ability to characterize PANI/MoO3 combinations. Precise surveys have been done on the conductivity behavior in different ranges from 102 to 106 Hz. The direct content conductivity has also been investigated over a wide range of temperatures from 40°C to 200°C. dc conductivity supports the proposed model by Mott, which is a one-dimensional variable range hopping (1D-VRH) model. The presence of MoO3 in PANI doesn’t have the same effect on the investigated conductivity values shown in the matrix. From a scientific and technological viewpoint, the outcome of these composites is very important [24]. Some erratic materials, like polymers, have the ability to become apparent from interfacial polarization at contacts, grain boundaries, and other inhomogeneities; the presence of these disordered materials in the samples has been proven. A model of AC conductivity of insulator/conductor blends has been produced by a special network of resistancecapacitance (RC); in this situation the conducting dispersants are shown to be like resistors and also capacitors displays the dielectric of the separated matrix [25]. Here we show a complicated conductivity σ∗ of n parallel blend: X  X σ∗ ¼ αn σ ∗n (14) αn ¼ 1 and of n series components by

Procedures for assessing electrical and thermal conductivity

ðσ ∗ Þ
1 ¼

X  X  αn ¼ 1 αn α∗n
1

(15)

This equation shows that whenever αn∗ and αn are among the complicated conductivity and volume fraction of the n constituent, this process will matted in a special order. The following equation explains it in a general model [26]. X  (16) ðσ ∗ ÞV ¼ αn α∗n V ð
1  v  1Þ There are pure PANI and PANI/MoO3 blends with different weight percentages in a range of frequencies from 102 to 106 Hz. The layout of σ ac represents a function of frequency for these pure composites. One of the noticeable characteristics which is attributed to the erratic materials is the fixture of AC conductivity up to 150 Hz and higher. Polarons have a repetitive movement in a polymer chain in the amorphous region; the distance between these movements became smaller and this also supports the presence of isolated polarons in this area. At high frequencies, σ ac increases due to the contribution of polarons; an increase of σ ac for a 10 wt% increase can be observed due to the variation of σ ac with weight percent of MoO3 at 10, 100, and 1000 kHz. The reason is the solid dependence on conductivity; this dependence is due to the initial dc conductivity of the combinations. According to the results of this investigation, we understand that increasing the dc conductivity causes less reliance on frequency dependence [27]. Considering the polarization of the charge bearers and the existence of interface nanotrapping levels that communicate with the polaronic states in an effective way, for 30, 40, and 50 wt% a small rise in conductivity will be observed. One of the functions of temperature for PANI and PANI/MoO3 composites is the layout of σ dc in the temperature range from 40°C to 200°C; this function is constant up to 80°C for the dc conductivity and thereafter it increases steadily up to 200°C. This process demonstrates the behavior of semiconducting materials. With the hopping of polarons from one localized state to another localized state, an increase is displayed in the conductivity levels. Typical semiconductor behavior shows the dependency of conductivity on levels of temperature. Mott presented the one-dimensional variable range hopping (1D-VRH) model to explain this [23]. In the following we can see this model: σ ðT Þ ¼ σ 0 exp ½
ðT0 =T Þ1=2

(17)

T0 ¼ 8α=ZN ðEF Þ kB

(18)

In this equation, α stands for the duration of the localization, N(EF) stands for the density of states at the Fermi level, kB the Boltzmann constant, and Z the amount of the closest adjacent chains. There is a transmission mechanism, which may be because of the important pattern of the polymer combination at the joining of the nanosheets that shapes the trapping levels. The existing interactions between the electron-phonon also influence this process [28]. For PANI, one can observe an increase in σ dc upon

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addition of 10 wt% and also a decrease for 20 wt% of MoO3; this observation is due to the variation in σ dc with weight percentages of MoO3 in PANI at 100, 150, and 200°C. In fact, because of the hopping mechanism of charge bearers from one localized manner to another one, and also sectional blocking of charge carriers in a specific order. Moreover, a light rise can be seen in conductivity for 30, 40 and 50 wt%. The AC conductivity behavior is supported by the outcome of this investigation.

3. Thermal conductivity of PANI 3.1 Application of 3-omega method The 3-omega method has become a readily accepted technique for direct thermal conductivity measurement, because it is highly accurate and can be specifically used to study thin film properties. The technique involves the diffusion of heat into a medium from a periodically oscillating heat source located on the surface [29]. Thermal information on the underlying medium can be obtained from the amplitude of thermal oscillations of the heater [30]. More precise and accurate measurements on the level of thermal conductivity in thin films and other materials have been provided by 3-omega procedures. But it is observed that, with films being soft and conductive, in the process of preparing attributes conventional methodologies, and as a result the measurement technique will be a challenging task, perhaps unreachable most of the time. There are different corrections in the process of preparing attributes to use this technique for soft conducting films. In this chapter we propose techniques like the application of shadow masks for metal heater deposition and also a procedure to prepare the low-temperature insulating films demanded between film and heater. Some of the functions of temperature have been measured and an enhancement of chemical vapor deposited SiO2 is displayed; this shows thick (5 μm) and ultrathin (110 nm) films of PANI as well as a thin (300 nm) film of low-temperature plasma. However, SiO2 film was not used as a soft material compared to other applications of silicon dioxide. As the outcome of the investigation, the SiO2 film displays a thermal conductivity less than others’ more disorders will happen in the film information because of the low temperature processing conditions. Through increasing the temperature, an increase in thermal conductivity with temperature in the PANI films is displayed. Because of the importance of phonon boundary dispersing in the ultrathin film, the thick film thermal conductivity has more value corresponding to the thin film [31]. Fig. 4 displays the outcomes of the measurements of thermal conductivity of the 300-nm thick SiO2 film compared to other results from the same films of different thicknesses over the existing temperature range. The level of unreliability of the measurement is calculated by using the Kline-McClintock uncertainty method; the calculated level was not greater than 8% [32]. In order to prepare the cross-plane thermal conductivity of the film, a 25-μm wide heater was utilized. It should be mentioned that experimental

Procedures for assessing electrical and thermal conductivity

Fig. 4 Thermal conductivity of PECVD silicon dioxide as a function of temperature. The data reported in this work (diamonds) are for a low temperature (150°C) PECVD deposition of the SiO2 films. Data from Lee and Cahill of PECVD deposition at 300°C are shown for comparison [31].

limitations do not let us study the films in a temperature range beyond 160–300 K. Lee and Cahill obtained values for the 92- and 32-nm films, which were a little less than the thermal conductivity of a 190-nm film; all were provided at an operating temperature of 300°C, comparable to the explained outcomes; this demonstrates that the 300-nm a-SiO2 film deposited at 150°C shows thermal conductivities in the determined temperature range. There is a noticeable difference between the actual structure of the 300-nm SiO2 film deposited via PECVD at 150°C from that deposited at a more general processing temperature of 300°C. In fact, at lower temperatures there is likely to be an increase in the scatterings in the thin film and/or the presence of microvoids in the structural makeup. Due to the existence of hydrogen in the precursor gas, the content of the scatterings may have received more effects (tetraethyl orthosilicate, TEOS). More phonon scattering in the sample will occur due to these factors. Therefore, although the investigated film has a greater width than the 190-nm film and thus would be likely to show a thermal conductivity a little more than that of the 190-nm film, due to the given information we see a reduction because of the additional scatterings. Also, it is mentioned that a 3-omega measurement of thermal conductivity of the oxide film includes interfacial thermal resistance between the oxide and silicon substrate, though this was estimated to be small by others [33]. Fig. 5 shows that increasing temperature will increase the thermal conductivities of the thin and thick PANI. For the thin film, the level of unreliability in the measurement

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Fundamentals and emerging applications of polyaniline

Fig. 5 Thermal conductivity of a 110-nm thin film of PANI (closed diamonds) and a 5-μm thick film of PANI (open diamonds) as compared with previous work. Quality, preparation method, and thickness can significantly influence the magnitude of thermal conductivity. The increasing trend with temperature is largely indicative of increasing heat capacity [31].

was not more than 9% of the measured values; while for the thick film the level of unreliability was not more than 5%. The investigation on this subject shows that with thick PANI films [34] the doping level does not have any special effect on the thermal conductivity at normal room temperature; this stability shows that the distribution of electrons toward thermal conductivity is at a low level, and the transition of phonons is bold. As mentioned, the process of synthesizing CSA-doped PANI is possible through the usage of m-cresol as a solvent, which is exhibited as a tiny crystalline in scattering [35] different material have imperfections which are expected to have a special role in specifying the phonon mean free path, which is not dependent on the temperature. Thus because of the behavior of the heat capacity, the temperature dependence of thermal conductivity develops; this behavior is demonstrated to develop the same points regarding temperature as in polymeric materials like polyethylene [36], polyimide [37], and PMMA [38], which are comparable in these particular temperature levels. For this work, two films in the same situation have been investigated. The comparison of the two films showed the PANI thin film displayed thermal conductivity at all temperatures at a lower level. The dispersion of the phonon boundary is not the cause of the reduction. In fact, we can use the phonon mean free path kinetic theory. In this theory, the compression specific heat sound velocity (estimated using the elastic modulus and accounting for the presence of dopants) and a measured thermal conductivity less than 10 nm is used. From the evidence, it is obvious that heat capacity of ultrathin polymeric

Procedures for assessing electrical and thermal conductivity

films (nanometer/angstrom scale) can be affected by two important happenings: the first one is the compression of the cross-link of the polymer network (the chain stiffness), and the other one is the relation between polymer and substrate [39]. The reduction in thermal conductivity of the 110-nm thin film can be influenced by these two mechanisms.

3.2 Thermal conductivity stability of PANI prepared in various acids Prokes and Stejskal prepared PANI under various conditions and in different media. At enhanced temperature, samples protonated with methane sulfonic or hydrofluoric acid exhibited better conductivity [40]. Usually, temperature increment causes a conductivity decrement. The main reason for the decrease is deprotonation of PANI at the molecular level. The changes in the macromolecular structure, i.e., the degradation of the PANI backbones, occur at a much higher temperature, above 400°C [41], and the decrease in molecular weight of PANI caused by chain scission is not the main factor affecting the conductivity [42]. 3.2.1 Aging process Conductivity σ explains the electrical process of conducting polymers better than resistivity, (which was used more commonly in the past), through aging results that are explained in the following text in terms of the former quantity. In order to find the stability differences in different principals, they are conveniently presented as doublelogarithmic plots of conductivity vs. time. These plots always exhibit a downward curvature as the aging proceeds. Occasionally, they become sigmoidal when the samples last for a sufficiently long aging time and the conductivity approaches values typical of nonconducting polymers. A more detailed analysis, however, shows that for short aging times ta, during which the conductivity falls to about half its original value, σ 0, it holds that σ 0
σ ðta Þ∝ ta ½

(19)

The subsequent aging can usually be described by the proportionality, log ðσ=σ 0 Þ ¼
ðta =τÞ1=2

(20)

The parameter t characterizes the rate of conductivity decrement. We can see that such a plot for PANI hydrochloride is a nearly straight line. The diffusion models proposed by Sixou et al. for polypyrrole [43] and by Wolter et al. for PANI [44] assume that the deprotonation of a conducting polymer, which is responsible for the conductivity loss, is controlled by a diffusion-like process that starts at the surface of the conducting grains. It is assumed that the size of metallic grains or islands, composed of well-ordered chains, is reduced during the thermal aging. The resulting regions formed by disordered deprotonated chains have a lower conductivity [45]. This is in accordance with the

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observation that the aging of thin PANI films is faster than that of compressed pellets [46]. Besides the deprotonation, additional changes in the polymer structure, like oxidative degradation, aromatic-ring substitution, and cross-linking, are likely to induce an irreversible decrease in the conductivity [47]. 3.2.2 Conductivity stability of PANI For a relative comparison of the thermal stability of conductivity, a simple parameter has been introduced, a ratio of the conductivity after 5 h of aging at 173 1°C to the original conductivity at the beginning of aging, σ 5/σ 0. Similar parameters have also been introduced for longer aging times of 25, 125, and 500 h. All samples could withstand 5 h of aging at elevated temperature but, despite repeated attempts, many of them could not survive 25 h at 173 1°C, even though, from a conductivity point of view, they performed well over short times. This means that they were disqualified because of the deterioration of their mechanical rather than their electrical properties [44]. The samples prepared at 20°C were amorphous and had a lower molecular weight than semicrystalline samples prepared at
50°C. It was earlier observed that differences in molecular weight and crystallinity have only a small effect on the conductivity. The thermal stability of conductivity of PANI hydrochloride is better for the sample prepared at reduced temperature, but this is rather an exception to the rule. With most of the samples, no marked improvement of this type has been observed. This is rather surprising because one would expect that the improved mechanical properties of high molecular weight, semicrystalline PANI would have a beneficial influence on the thermal stability of the conductivity [46]. This does not seem to be the case.

3.3 Thermal conductivity of PANI nanofibers A direct, straightforward method is introduced to synthesize PANI nanofibers and nanofluids of PANI nanofibers in DI water. Moreover, these synthesized nanofluids are analyzed in terms of thermal conductivity at a broad range of temperature (10–80°C) using a transient plane source method. In analyzing the PANI nanofibers, potassium biiodate is utilized as an oxidant and ultrasonic irradiation for nucleation. SEM graphs show a small nanofiber structure with average diameter of 80 nm and length of 2 μm. The impact of particle loading is furthermore observed on the thermal conductivities of synthesized nanofluids. Thermal conduction behavior in nanofluids has been noticeably boosted because of more crystallinity and morphological uniformity of reinforced nanofibers [48]. In order to find out the heat transfer specifications of PANI nanofibers in DI-water, the two parameters, thermal conductivity and thermal conductivity enhancement ratio (Knf/Kbf), are calculated for samples having various concentrations at different temperatures. To maximize the homogeneity of nanofluids, a tiny amount of sodium dodecylsulfate and weak ultrasonic irradiation was utilized to uniformly disperse the nanofibers in the liquid. As already mentioned, the method of calculating the thermal conductivity of

Procedures for assessing electrical and thermal conductivity

these samples was TPS. The plots of thermal conductivity enhancement vs. temperature depict that for 0.08 vol% particle loading, no significant change can be seen in sample thermal conductivity at 20°C, compared with distilled water with thermal conductivity of 0.62 W/m K at 20°C [48]. Compared with water, at 0.08, 0.16, and 0.24 vol% loadings, nanofluid samples had greater thermal conductivities at various temperatures. The thermal conductivity variation of nanofluids with the volume percent loading of PANI nanofibers depicts that all these improvements in thermal conductivity are as a result of PANI nanofiber suspension in water. To check the accuracy of the equipment, a trend line is used having 4% bars based on the conductivity improvement information. All the data were totally within the desired accuracy range of the equipment. Results elucidate the ratio of effective thermal conductivities of the nanofluid (Knf) compared with the base fluid (Kbf) at equal temperatures [48]. The nanofluid experiments were carried out using a thin layer of electrical insulation coating to shroud the nickel strip, a technique suggested by Nagasaka and Nagashima [49]. In the case of solids, Assael et al. [50]shows the significance of utilizing a third solid material between the hot wire and the desired solid, to significantly decrease contact thermal resistances. In our experiment we used Kapton film over the nickel coil to insulate the sensor electrically. To reduce the effects of thin Kapton film on thermal conductivity, the initial data points obtained from the equipment are not considered. However, the thermal wave impact of the current instrumental set-up cannot be eliminated, as mentioned by various researchers. Vadasz et al. [51] suggested the thermal wave impact only from studying several plain experimental situations. However, we were able to significantly minimize the impact of thermal waves by conducting a suitable measurement time period, as presented by Vadasz et al. in our experimentation. Therefore, particle thermal convection by significant temperature variation between sensor and the sample can be minimized. In the lights of the points mentioned, it is possible to claim that more exact thermal conductivity data is presented here for nanofluids.

4. Thermoelectric properties of PANI Polymers have several applicable advantages when used for thermoelectric materials in comparison with inorganic semiconductors; these include suitable prices, light weight, as well as flexibility. Although numerous research studies have focused on increasing electrical conductivity of the conducting polymers, that may cause malfunctions in thermoplastic applications. Little research exists that has suggested conducting polymers being utilized as thermoelectric materials; this is due to the fact that conducting polymers 2 possess an obviously low thermoelectric dimensionless figure of merit ZT ¼ σακ T , where α is the Seebeck coefficient (V K
1), σ and κ are the electrical conductivity (S/cm), and the thermal conductivity (W cm
1 K
1), respectively, and T is the absolute temperature. Mateeva et al. have presented the relationship between σ and α of light doping PANI and polypyrrole. They discovered that the behavior of the electrical conductivity logarithm

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with respect to the Seebeck coefficient is linear. Because the ZT values for the experimented materials were small, the Seebeck coefficient increased as the doping level was reduced; however the conductivity decreased. These values were also measured for various doped PANI samples at several temperatures, including room temperature. The impacts of preparing methods and temperature on thermoelectric properties were investigated. The results depict that the electrical conductivity and the Seebeck coefficient of PANI showed high levels of dependency on the preparation conditions and temperature. The electrical conductivity increased and the Seebeck coefficient decreased as PANI molecular weight increased. Redoping by organic acid and HCl caused an increase in both electrical conductivity and Seebeck coefficient of PANI, and therefore the ZT value. The electrical conductivity boosts and the Seebeck coefficient is reduced as the temperature increases for T < Td (dedoping temperature). The opposite effects are observed when T > Td. The thermal conductivity is lower and independent of the sample preparation methods [52].

4.1 Thermoelectric properties of PANI/graphite composites After preparing HClO4-doped PANI/graphite composites using ball milling and cold pressing, the thermoelectric behavior was analyzed with respect to graphite content. Thermal conductivity rises with increasing graphite concentration, whereas the electric conductivity (σ) as well as the Seebeck coefficient (S) rises significantly, which results in noticeable improvement in the plot of merit for the composites. The ZT of the composite with graphite concentration of 50 was evaluated to be at least 10,000 times greater in comparison to the HClO4-doped PANI without graphite (1.13  10
7). The study presents a new method to enhance thermoelectric behavior of conducting polymers. In terms of stability, the results suggest that the composites were stable in the temperature range of 303–453 K and their Seebeck coefficient as well as electric conductivity at the same temperature range were measured; the results suggest that the electrical conductivity rises significantly from 1.23  102 to 1.2  104 S/cm from increasing the graphite concentration. The behavior of the Seebeck coefficient with respect to graphite content also increased from
0.82 to 18.66 μV/K with graphite concentration from 0 to 50 wt% at 393 K. At the same temperature and graphite content range, the thermoelectric power factors increased from 8.30  10
11 to 4.18  10
6 W/ m K2. The thermal conductivity also showed the same increasing behavior from 0.29 to 1.20 W/km2, which is nearly four times, for graphite concentrations of 0 and 50 wt %, respectively [53].

4.2 Thermoelectric properties of hydrochloric acid-doped PANI The impacts of HCl-doping content on thermoelectric properties have been investigated at temperatures between 303 and 423 K. The results depict that increasing the HCl-

Procedures for assessing electrical and thermal conductivity

DCS (mW/mg)

TG (%) 100 –5.03%

5 –12.66%

80 –25.10%

4

586.6°C

60

3

40

2

20

1 86.8°C

0

0 100

200

300

400 Temperature (°C)

500

600

700

Fig. 6 TGA and DSC curves of the doped PANI prepared at HCl concentration of 1.0 M [54].

doping concentration causes the electrical conductivity and thermoelectric pattern-ofmerit ZT to first increase and then decrease, whereas the Seebeck coefficient showed the opposite behavior [54]. By means of DSC-TG analysis, the doped PANI was analyzed in terms of stability at HCl content of 1.0 M, with the results depicted in Fig. 6. A three-step mass loss can be observed for the powder. The first one occurs in the range of 70–140°C showing a 5.03% decrease in weight. The second begins at about 190°C and continues to about 390°C. The third happens in the temperature range of 400–700°C, associated with the breaking down of PANI backbones [54]. 4.2.1 Thermoelectric properties The trends of electrical conductivity of different doped PANI samples were plotted against temperature on Fig. 7. It can be observed from Fig. 7 that the undoped PANI changes from an insulator to a semiconductor or even to a conductor by increasing doping level. The experimental results show this conversion in conductivity. At 0.25 M HCl concentration, the sample analysis shows extremely low electrical conductivity with a nonmetallic character (dσ/dt > 0). More than this concentration, the electrical conductivity of all the samples will increase up to 100 S/m, as well as showing a normal metallic temperature dependence (dσ/dT < 0). Furthermore, by adding HCl content, the electrical conductivity will increase first and then reduce until it reaches the maximum 601.5 S/m at 303 K and 188.4 S/m at 423 K with HCl content at 1.25 M. Polarons are

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Fig. 7 The temperature dependence of the electrical conductivity of various HCl-doped PANI samples [55].

the main carriers for conductive PANI, and they can jump between chains as well as transferring along the chain. By increasing HCl concentration, more H+ ions will be there to bond with ]Nd, that will lead to the formation of more polarons, causing the electrical conductivity to improve [55]. The Granular Metal Island Model [56] has been utilized by previous researchers to explain the electrical properties of conductive polymers. For PANI, the electrical conductivity can increase to 200–300 S/cm in the metal islands; however, between these metal islands, carriers have to transfer through a hopping or tunneling mechanism, resulting in a low conductivity and a nonmetallic character of the whole material. In the present study, the temperature dependence of the electrical conductivity of all the samples prepared with HCl concentration of more than 0.25 M depicts a metallic sign in the whole test temperature range. Moreover, the near-linear relationship between the electrical conductivity and 1/T is also compatible with that of 3-D metallic conduction, confirming the metallic nature of PANI.

4.3 Thermoelectric properties of Bi0.5Sb1.5Te3/PANI hybrids Thermoelectric hybrid materials consisting of Bi0.5Sb1.5Te3 and 1–7 wt% PANI have been prepared by mechanical blending and cold pressing. It is found that the Seebeck coefficients of the hybrid materials are about 10% lower than the Bi0.5Sb1.5Te3 sample. It is shown that the power factors of the hybrid materials decrease significantly with the increase of the polymer content, mainly due to the decline of electrical conductivities [57].

Procedures for assessing electrical and thermal conductivity

The PANI sample is a p-type semiconducting polymer with a maximum Seebeck coefficient of about 6 μV/K at 360 K. The electric conductivity of the PANI sample is about 2000 Ω m
1 at 300 K and 2600 Ω m
1 at 370 K. Both Seebeck coefficient and electric conductivity are much lower than any Bi2Te3-based thermoelectric alloys. However, considering the low thermal conductivity of PANI, being about one-third or even smaller of that of Bi2Te3-based thermoelectric alloys, PANI additives would still be helpful to improve the thermoelectric properties of the alloys [58]. The transport properties of Bi0.5Sb1.5Te3 and the Bi0.5Sb1.5Te3/PANI models with various concentrations of PANI additives elucidated that the average Seebeck coefficients in the measuring temperature range are reduced from about 185 μV/K for the Bi0.5Sb1.5Te3 sample to 173 μV/K for the hybrid sample with only 1 wt% PANI. However, continuing to increase polymer content in the hybrid materials will not cause a significant decrease of the Seebeck coefficients. By adding the PANI concentration of hybrids, the electric conductivities are reduced more dramatically in comparison with the Seebeck coefficients. The average electric conductivities in the measuring temperature range are reduced approximately from 9400 Ω m
1 of the Bi0.5Sb1.5Te3 sample to 7400, 6000, 5000, and 4000 Ω m
1 for the hybrid samples with 1, 3, 5, and 7 wt% PANI, respectively. The significant reduction of the electrical conductivity coefficients is the main source of the decrease of the power factor α2σ for the hybrid samples. Compared to the Bi0.5Sb1.5Te3 sample, the power factors reduced to about 70%, 50%, 45%, and 35% for the hybrid samples with 1, 3, 5, and 7 wt% PANI, respectively, which is merely due to the significant reduction of the electric conductivities of the hybrid samples [59].

4.4 Effect of graphite oxide on the thermoelectric properties of PANI Graphite oxide (GO)/ordered PANI composites have been synthesized using in situ polymerization. Exfoliated GO played the template role for forming the highly ordered crystallinity structure during PANI polymerization. Enhanced carrier mobility of PANI/ GO (which is confirmed by Hall measurement) resulted in higher electrical conductivity and Seebeck coefficient than pristine PANI. Because of the π-π stacking, H-bond, and electrostatic forces, a strong interaction between GO and PANI is formed. In the GO/ PANI composites, temperature increment causes conductivity enhancement, which it is similar to semiconductors [60]. The electrical conductivity increases with increasing GO content up to 30 wt%. This behavior can be explained by the template effect of graphene oxide created by the exfoliation of GO. With the aid of graphene oxide, conductive PANI has a preferably oriented structure, and the oriented polymer chains allow the carrier to move easily. GO with higher content can supply more templates for PANI. When the GO content exceeds a certain value, for example 30 wt%, a decrement is observed due to the

247

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Fundamentals and emerging applications of polyaniline

insulation of GO. At 90°C, 40% GO/PANI exhibits a slightly higher σ than that of 30% GO/PANI. The negligible increase value in σ is within measurement error range, and it is not enough large to break the conclusion that σ decreases when GO content exceeds 30 wt%. The thermal conductivity of the GO/PANI composite is basically similar to pure PANI. The carrier mobility of conductive polymers is much lower than those of metals because of the amorphous nature of conductive polymers. For conductive polymers, the charge carrier contribution to the thermal conductivity is generally small, while the phonon contribution is dominant [60]. Numerous interfaces in the composites may act as effective scattering centers of phonons. Phonons are highly scattered. Thermal transport in composites is impeded while the electrical conductivity can be maintained by a transport mechanism such as the hopping. This is consistent with the low thermal conductivity and high electrical conductivity obtained experimentally for GO/PANI composites. Both a constant κ and an increased σ contribute to the enhanced thermoelectric performance with a maximum ZT of 4.86  10
4. The thermoelectric properties of the GO/PANI blend, which is prepared by mixing under ultrasonic agitation 1 part GO and 9 parts PANI powders polymerized without GO, were also measured. The GO/PANI blend has a lower σ and S than those of GO/PANI composite with the same GO content, respectively. This may further verify the template effect of graphene oxide on PANI. The GO/PANI blend has slightly higher S than PANI, which could be due to the relatively higher intrinsic S of GO compared with PANI. Meng et al. [61] showed that the enhancement of thermoelectric properties of the CNT/PANI composites is due to the sized dependent energy-filtering impact as a result of the nanostructured PANI coating layer surrounding the CNTs. This may be an additional explanation for the enhanced thermoelectric performance of the GO/PANI composite, because the PANI coating layer formed on the graphene oxide. The graphene oxide, being an electron acceptor, and aniline, being an electron donor, form a kind of weak charge transfer complex [62]. In GO/PANI composites, PANI deposits in an orderly manner on the surface of the graphene oxide sheet through strong interactions between them. These interactions include electrostatic forces, hydrogen bonding, and π-π stacking from the surface of the graphene oxide and conjugated structure of PANI [63]. The strong interactions may induce free anilines to be adsorbed on the surface of graphene oxide, which as a support template could supply a large number of active sites for PANI to nucleate. Free anilines are more competitive to be bound with the surface of graphene oxide than water molecules. Thus the PANI deposited densely on the surface of the graphene oxide [64]. The chain packing of PANI in the composite is ordered due to the template effect of graphene oxide; however, it is disordered in the absence of GO. There is some spacing between molecular chains of PANI. The hopping distance of carriers is fixed at a certain

Procedures for assessing electrical and thermal conductivity

temperature. Under the guidance of the template graphene oxide, the PANI molecular chains are closely packed and ordered. The carrier hopping may occur more easily due to the decreased chain spacing; therefore the carrier mobility is increased [65].

4.5 Thermoelectric studies of poly(o-anisidine) functionalized multiwall carbon nanotube composites The method of adsorption of poly(o-anisidine) (POAS) on the surface of multiwall carbon nanotubes (MWCNTs) by the solution method is utilized to create a POAS/silverized MWCNTs (POAS-MWCNTs/Ag) composite. It was assessed whether or not it is possible to metallize MWCNTs and use a polymer as a substitute for PANI to synthesize MWCNTs/conducting polymer nanocomposites by a one-step easy mixing of solutions. The resulting composite was analyzed using SEM, TGA, FTIR, and X-ray spectroscopy. The electrical resistivity of the composites was investigated and a noticeably higher conductivity, compared with pure POAS and MWCNTs, was observed of 5 S/cm. The results presented an effective method for synthesis of POAS-MWCNTs/Ag composites, maintaining acceptable electrical conductivity and excellent thermal stability at high temperature [66]. It can be clearly seen from the TGA curve of the composite that, up to 150°C, only 5% weight loss occurred, which may be the result of removal of the external H2O molecules that are present at the surface of the composite [67]. A 7% weight loss from 150°C to 250°C may be because of the minor decomposition of the organic part of the material. Further continuous loss of mass approximately between 250°C and 900°C can be due to the low conversion of inorganic metal into metal oxides (Fig. 8). 2.5

100

2.0

90

Weight (%)

1.5 80 1.0 70 0.5 60

0.0

50 0

200

400

600

800

–0.5 1000

Temperature (°C)

Fig. 8 Thermogravimetric analysis of POAS-MWCNTs/Ag nanocomposite [66].

Deriv. weight (%/min)

TGA

249

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Fundamentals and emerging applications of polyaniline

4.5.1 Electrical resistivity behavior of POAS-MWCNTs/Ag The four-probe method for semiconductors was utilized to analyze the electrical resistivity of the pellets of silver nanoparticles embedded in POAS functionalized CNT composite samples, using the conductivity of the samples. This may be the most efficient method, as it overcomes the problems occurring in conventional methods of resistivity measurement. The current-voltage data measurements at rising temperatures for the calculation of electrical conductivity of the composite sample were analyzed for determination of electrical resistivity using the following equation: ρ ρ ¼
0  (21) W G7 S where ρ is corrected resistivity (Ω/cm), ρ0 ¼ uncorrected resistivity (Ω/cm), G7(W/S) is the correction factor used for the case of a nonconducting bottom surface, which is a function of W, the thickness of the sample under test (cm) and S, the probe spacing (cm); i.e.,

 W 2S G7 ¼ ln 2 (22) S W V  2π S I 1 σ¼ ρ

ρ0 ¼

(23) (24)

where σ is electrical conductivity in S/cm. The conduction specifications are affected by the percolation trend of the conducting phase and the reverse of the resistivity. The electrical conductivity of the composite, because of oxidized POAS, remained in its conductive state with silver nanoparticle embedded POAS counterions in excess. The behavior of electrical resistivity (ρ) of the composite samples was analyzed by adding temperatures from 35°C to 195°C. Resistivity is reduced by adding the temperature, meaning that electrical conductivities of the samples increase with increasing the temperature, and values on the order of 0.2 Ω/cm were found in the boundaries of the conductor and semiconductor region. To evaluate the dependency behavior of electrical resistivity on temperature, plots of log ρ vs. 1000/T (K) were created using the Arrhenius equation, similarly to other semiconductors [68]. The initial resistivity is in
ve because at room temperature the material may be working as a p-type of semiconductor, and as we increase the temperature, the resistivity decreases and reached +ve value, meaning that it achieves the n-type of conductivity from 70°C onwards. Four-probe-in-line dc electrical resistivity measurements were utilized to investigate the thermal stability of the composite material (HCl treated) associated with dc electrical

Procedures for assessing electrical and thermal conductivity

resistivity retention under cyclic conditions at different temperatures to 195°C. The results suggested the stability of the electrical resistivity in each cycle’s repeated heating/cooling cycle, which confirms the fact that the dc electrical resistivity of the composites is sufficiently stable under ambient temperature conditions. The electrical resistivity is reduced (with the increase of conductivity) with temperatures up to 135°C, which can be the result of the loss of dopant and the chemical reaction of dopant with the material. The conductivity (5 S/cm) of the silver embedded POAS-MWCNTs composite synthesized is higher in comparison with the POAS made using chemical polymerization with ammonium persulfate as the oxidant at 0°C (2.2  10
3 S/cm) [69], and for the conductivity of the POAS LS film it was shown to vary between 0.1 and 10
9 S/cm [70]. The enhanced conductivity could be because of the electrondonating property of the MWCNTs. The conductivity of the composite from silverized polymerization goes up with rising temperature up to 130°C; as elucidated by reduction of resistivity up to this stage, it is due to the reduction of bandgap in the composite. This increase in conductivity with increase in temperature is the characteristic of “thermal activated behavior” [71]. The improved conductivity may be the result of the enhancing of the efficiency of charge transfer between the composite chains and the dopant (Ag) by increasing the temperature [72].

4.6 Thermoelectric properties of PANI/silver composites PANI/metallic silver composites are chemically synthesized in terms of their electrical, thermal, and thermoelectric properties and the results are discussed with respect to their morphological features as determined by SEM. The metal-intrinsic conducting polymer power factor (PF) and figure of merit (ZT) are interpreted from measurements of the electrical conductivity (σ), Seebeck coefficient (S), thermal diffusivity (α), thermal effusivity (e), and thermal conductivity (κ). It is depicted that the PANI/Ag thermoelectric (TE) properties are much more enhanced in comparison with pure PANI. Furthermore, the results suggested the inherent dependency of the metal/PANI properties on the volume concentration of silver and morphology of the composites. It is found that electrical percolation occurs for samples having 16 vol% Ag concentrations with no noticeable thermal percolation. The composite with 26 vol% Ag loading shows the best thermoelectric performance (PF  3 μW m
1 K
2; ZT  0.5  10
3). The overall improved TE behavior of the PANI/Ag composites is the result of an enhanced electrical conductivity with a concomitant limitation in the rise of thermal transport via scattering [73]. 4.6.1 Electrical properties Fig. 9A shows the behavior of the electrical conductivity (σ) with respect to silver volume fraction of the composites. It can be observed that σ has an increasing trend with respect to the addition of Ag amounts in the samples. This dependency behavior of the electrical conductivity on filler volume content can be justified by the critical power law defined as:

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Fundamentals and emerging applications of polyaniline

σ 0 ¼ σ 0 ðØ
Øc Þt

(25)

where t is the critical exponent, σ 0 the electrical conductivity prefactor, f the volume fraction of silver in the sample, and fc the percolation threshold. A percolation limit of Øc ¼ 16.2 vol% Ag is calculated using curve fitting of the various parameters, which exhibits acceptable consistency with the theoretical 3D model of Scher and Zallen [74]. Moreover, the calculated t ¼ 2.3 totally backs up a three-dimensional transport process. 4.6.2 Seebeck coefficient Utilizing the defining Seebeck (S) coefficient equation as: 4V (26) 4T values of S were acquired by measuring the TE voltages (4 V ) behavior with respect to temperature gradient (4T ) at T ¼ 30°C. The plots of 4V vs. 4T were curved with slopes proportional to the S constants. The positive S values calculated elucidate that the composites are of p-type materials, i.e., hole carriers are the dominant contributors to the S coefficients [74]. The TE power factor (PF) can be obtained from these values of electrical conductivity σ, using the formula: SðT Þ ¼

PF ¼ S2 σ

(27)

It is observed from Fig. 9B that σ exhibits a decreasing trend with respect to increasing Ag content. This behavior usually suggests a higher doping level, which consequently leads to a higher concentration of charge carriers.

1E7 1,000,000 100,000 10,000 1000 100

Percolation threshold @ 16.2%

10 1

Electrical conductivity s Fit Eq. (3)

0.1

100 1

1E-4 1E-6 30 20 10

0.01

(A)

(C)

0.01

S (µV K–1)

PF (µW m–1 K–2)

4.6.3 Thermal properties Using the photothermal radiometry set-up described earlier, the influence of silver content on the thermal parameters is investigated. Fig. 10A displays the thermal diffusivity (α)

s (S m–1)

252

0 0

20

40

60

vol % Ag

80

100

(B)

0

20

40

60

80

100

vol % Ag

Fig. 9 Electrical conductivity (A) and Seebeck S coefficient (B) and power factor (PF) (C) as a function of volume fraction of Ag in PANI/Ag composites [73].

Procedures for assessing electrical and thermal conductivity

Fig. 10 Thermal (A) effusivity e and diffusivity α, and (B) conductivity κ as a function of volume fraction of Ag in PANI/Ag composites [73].

and effusivity (e) of the composites as a function of vol% Ag. It can be seen that (α) and (e) increase with increasing Ag content. Knowing α and e, the thermal conductivity, κ can be calculated according to the equation: κ ¼ e  α1=2

(28)

The evolution of κ vs. volume fraction of silver is shown in Fig. 10B. From a physical point of view, κ Ag (thermal conductivity of silver) and κPANI (thermal conductivity of PANI) are the upper and lower limits of the effective thermal conductivity of the composite (κc). Then as a first approach to assess the evolution of the thermal conductivity of the composite, its thermal properties have been modeled, using a weighted geometric mean, as follows: ð1
γ Þ

κc ¼ κγPANI κ Ag , 0  γ  1

(29)

where γ is the volume fraction of Ag. As shown in Fig. 10B a good agreement is obtained between the experimental data points and the fit (solid line), providing a value of κPANI ¼ 0.26 W m
1 K
1 and κAg ¼ 429.3 W m
1 K
1, which is consistent with data reported in the literature for PANI [75] and silver [76]. This result demonstrates that the behavior of the thermal conductivity (κc) of PANI/Ag hybrids is in contrast with the electrical transport measurements for which a percolation phenomenon is observed with increasing silver content.

5. Conclusion Conducting polymers, as compared to the inorganic semiconductors, have several attractive features for applications like thermoelectric materials. These polymers have several positive points including low cost, low weight, and flexibility, all attractive for PANI, which is air stable and easy to polymerize. The electrical conductivity of PANI is not dependent to the polymerization temperature, and also it does not have any dependence

253

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Fundamentals and emerging applications of polyaniline

on molecular weight. Principals with acidic medium situations have a higher electrical conductivity. An investigation was carried out in to compare between electrical conductivity of a miscible PANI/PU blend aging in high levels of humidity and temperature and immiscible PANI/SIS composites and CB/PU. The aging investigation was undertaken to explain the mechanism of electrical aging for the blends. Through passing time, the miscible blend exhibits phase detachments with homogeneous morphology; this produces a reduction of conductivity due to the discontinuity of the conducting pathways. Comprehensive investigations were carried out of an easy process for the synthesis of graphite nanosheets and PANI/graphite nanocomposites through microwave irradiation and sonication of the synthesized expandable graphite. The measurements of electrical conductivity displayed a large increase in conductivity of the final PANI/graphite nanocomposites in comparison to pure PANI. An in situ polymerization procedure synthesized PANI doped with dichloroacetic (DCA) and trichloroacetic (TCA) acids. Increasing the doping level for both salts increased the conductivity and PANI-TCA was recognized to be more conductive than PANI-DCA. The 3D Mott VRH model represents the charge transition in dc in both salts in a suitable way. PANI composites with different weight percentages of MoO3 in PANI were synthesized by chemical oxidative polymerization of the monomer aniline. In PANI, conductivity displays a deep dependence on the weight percent of MoO3. Due to polaron hopping from one localized manner to another one at higher temperatures, conductivity (σ dc) developed. The application of the 3-omega procedure was used to measure thermal conductivity of a thin (300 nm) low-temperature PECVD deposited SiO2 thin film, a thin PANI film (110 nm), and a thick PANI film (5 μm) at different temperatures. The synthesis of PANI nanofibers has been carried out and, as a result, the nanofluids of various nanofiber concentrations (0.08, 0.16, and 0.24 vol%) with DI-water were provided. Temperature-dependent behavior of heat transfer fluids is noticed as they endure a high level of temperature in the process of decreasing the temperature systems in various heat sources; 80°C temperature is the suitable level of temperature for a maximum thermal conductivity development rate in synthesized nanofluids at 140% with 0.24 vol% of nanofibers. By using chemical oxidative polymerization, a group of hydrochloric aciddoped PANIs was synthesized. Comprehensive investigations have been carried out on the thermoelectric application of the as-provided PANI in various levels of HCl-doping. The thermal conductivity of PANI has no sensitivity to doping conditions. The rise of HCl-doping level leads to an attitude increase first and after that a reduction in ZT and the maximum value; π-type Bi0.5Sb1.5Te3 alloy powders and HClO4-doped PANI added ingredients, which are the thermoelectric hybrid materials, have been provided by mechanical compositions and cold pressing. The reductions of the power factors are due to decrease of the electric conductivities of the hybrid materials to the range of only about 30%–70% of the Bi0.5Sb1.5Te3 principals. Through a light in situ polymerization, thermoelectric attributes

Procedures for assessing electrical and thermal conductivity

of GO/PANI composites can be synthesized. In the Hall measurement, it is explained that the raised thermoelectric attributes of PANI mainly stemmed from increased carrier dynamism. Considering phonon dispersion, compared to pure PANI, nearly stable thermal conductivity was considered for GO/PANI blends.

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